University of Southern California Dissertations and Theses

The patterning of polymer thin films on porous substrates via initiated chemical vapor deposition

(USC Thesis Other)

The patterning of polymer thin films on porous substrates via initiated chemical vapor deposition

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content

The Patterning of Polymer Thin Films on Porous Substrates via Initiated
Chemical Vapor Deposition

Doctoral Dissertation
Philip Kwong
May, 2015

University of Southern California
Mork Family Department of Chemical Engineering and Materials Science
Los Angeles, CA, USA

ii

Committee Members
Dr. Malancha Gupta
Dr. Ellis Meng
Dr. Noah Malmstadt

iii

Executive Summary
The patterning of polymer coatings on porous substrates cannot be easily achieved via either
solution phase or vapor phase methods, and represents a technical hurdle for a number of
potential applications. This dissertation describes the patterning of polymer coatings deposited
onto porous substrates using the initiated chemical vapor deposition (iCVD) process. The
patterning of polymer coatings in the iCVD process is achieved by using transition metal salts to
quench radical species and is discussed primarily for the development of paper-based
microfluidic devices, although this technique can also be extended to a number of other
applications.
This report is divided into six sections. The introduction will provide a broad overview of
chemical vapor deposition (CVD) processes and discuss the application of CVD techniques for
the development of diagnostic platforms. The second section will discuss original research
showing the general utility of polymer coatings incorporated into paper-based microfluidic
platforms by demonstrating an improvement in the separation of small molecules using ionizable
coatings. The third section will provide a broad screening of the ability of transition metal salts
to prevent the deposition of polymer, while the fourth section will demonstrate that deposition is
prevented by quenching of the radical species via a reduction of the transition metal salt.
Additionally, the fourth section will quantify this prevention of deposition and discuss how to
optimize the deposition parameters in order to improve the fidelity of the pattern. In the fifth
section, numerous methods of patterning transition metal salts onto chromatography paper are
presented and the utility of these patterning techniques for paper-based microfluidic devices is
demonstrated through the development of fluoropolymer barriers that allow these platforms to be
iv

compatible with organic solvents. The final section will provide an overview of this work and
discuss future challenges in the field.
v

General Acknowledgements
The author would like to extend a special acknowledgement to individuals whose assistance was
vital towards the completion of this dissertation. The author thanks the entire Gupta lab group,
and notes specifically co-authors Cristofer Flowers, Benny Chen, and Scott Seidel. The author
would like to thank the members of both the qualifying and dissertation committees for their
valuable time and input. Additionally, the author thanks the multitude of mentors encountered
over the author’s entire professional career, specifically noting Dr. Malancha Gupta, Dr. Jeff
Downey, and Dr. Chris Kozak.

vi

Table of Contents
Executive Summary ....................................................................................................................... iii
General Acknowledgements ........................................................................................................... v
Table of Contents ........................................................................................................................... vi
List of Figures .............................................................................................................................. viii
List of Tables ................................................................................................................................ xii
1.0 Introduction ............................................................................................................................... 1
1.1 Chemical Vapor Deposition .................................................................................................. 1
1.1.1 Overview ........................................................................................................................ 1
1.1.2 Initiated Chemical Vapor Deposition ............................................................................ 4
1.2 Diagnostic Applications ........................................................................................................ 7
1.3 Patterning in CVD Techniques ........................................................................................... 12
1.4 Project Summary ................................................................................................................. 16
1.5 References ........................................................................................................................... 17
2.0 Vapor Phase Deposition of Functional Polymers onto Paper-Based Microfluidic
Devices for Advanced Unit Operations ........................................................................................ 24
2.1 Abstract ............................................................................................................................... 24
2.2 Introduction ......................................................................................................................... 25
2.3 Experimental ....................................................................................................................... 29
2.4 Results and Discussion ....................................................................................................... 33
2.5 Conclusions ......................................................................................................................... 44
2.6 Acknowledgements ............................................................................................................. 45
2.7 Supporting Information ....................................................................................................... 46
2.8 References ........................................................................................................................... 49
3.0 Directed Deposition of Functional Polymers onto Porous Substrates Using Metal
Salt Inhibitors ................................................................................................................................ 53
vii

3.1 Abstract ............................................................................................................................... 53
3.2 Introduction ......................................................................................................................... 54
3.3 Experimental ....................................................................................................................... 58
3.4 Results and Discussion ....................................................................................................... 62
3.5 Conclusions ......................................................................................................................... 75
3.6 Acknowledgments ............................................................................................................... 76
3.7 Supporting Information ....................................................................................................... 77
3.8 References ........................................................................................................................... 78
4.0 The Effect of Transition Metal Salts on the Initiated Chemical Vapor Deposition
of Polymer Thin Films .................................................................................................................. 81
4.1 Abstract ............................................................................................................................... 81
4.2 Introduction ......................................................................................................................... 82
4.3 Experimental ....................................................................................................................... 85
4.4 Results and Discussion ....................................................................................................... 89
4.5 Conclusions ......................................................................................................................... 99
4.6 Acknowledgements ........................................................................................................... 100
4.7 References ......................................................................................................................... 101
5.0 Patterned Fluoropolymer Barriers for Containment of Organic Solvents within
Paper-Based Microfluidic Devices ............................................................................................. 103
5.1 Abstract ............................................................................................................................. 103
5.2 Introduction ....................................................................................................................... 104
5.3 Experimental ..................................................................................................................... 108
5.4 Results and Discussion ..................................................................................................... 113
5.5 Conclusions ....................................................................................................................... 125
5.6 Acknowledgements ........................................................................................................... 126
5.7 Supporting Information ..................................................................................................... 127
5.8 References ......................................................................................................................... 133
6.0 Conclusions and Future Work .............................................................................................. 136

viii

List of Figures
Figure 1-1 - The Gorham process. Paracyclophanes are pyrolyzed to form para-
xylylenes which resonate with a biradical conformation. The xylylenes polymerize to
form Parylene.
1
............................................................................................................................... 2
Figure 1-2 - Reaction pathways in plasma-enhanced chemical vapor deposition.
Reproduced from reference 14 with permission from Taylor & Francis Group LLC. ................... 3
Figure 1-3 - General schematic of atomic layer deposition and/or molecular layer
deposition demonstrating self-limiting chemistry. Reproduced from reference 24 with
permission. ...................................................................................................................................... 4
Figure 1-4 - Typical reactor setup for an initiated chemical vapor deposition system
and traditional free-radical polymerization mechanism.
31
.............................................................. 6
Figure 1-5 - Desirable qualities for diagnostic platforms outlined by the World Health
Organization.
45
................................................................................................................................ 8
Figure 1-6 - Microfluidic card developed to perform diagnostics that incorporates
multiple functions. Reproduced from reference 62 with permission from The Royal
Society of Chemistry....................................................................................................................... 9
Figure 1-7 - Paper-based microfluidic device for the detection of glucose and total
protein concentration. Reproduced from reference 55 with permission. ..................................... 11
Figure 1-8 - Method of patterning the CVD of Parylene by shadow masking.
Reproduced from reference 69 with permission. .......................................................................... 13
Figure 1-9 - a) Patterning using an electron-beam active resist material to pattern a
CVD coated film. b) Direct patterning of an electron-beam active film. Reproduced
from reference 75 with permission. .............................................................................................. 14
Figure 1-10 - Patterning of ALD using self-assembled monolayers (SAM) to
deactivate the surface and limit deposition. Reproduced from reference 77 with
permission from The Royal Society of Chemistry. ...................................................................... 15
Figure 2-1 - FTIR spectra of a) xPMAA and b) xPDMAEMA, high-resolution carbon
1s XPS spectra of c) PMAA and xPMAA, and XPS survey spectra of d) PDMAEMA
and xPDMAEMA.......................................................................................................................... 34
ix

Figure 2-2 - Schematic of the grafting process. Cross-linked polymer is deposited
onto the paper fibers followed by deposition of homopolymer, which results in
grafting. Additional ungrafted homopolymer is removed by repeatedly washing the
paper in water. ............................................................................................................................... 36
Figure 2-3 - SEM images of a) uncoated paper, b) paper coated with gPMAA, and c)
paper coated with gPDMAEMA. .................................................................................................. 38
Figure 2-4 - R
f
values of a) toluidine blue O, b) crystal violet, c) tartrazine, d) ponceau
S, e) methyl orange, and f) brilliant blue G on uncoated paper channels (blue
diamond), paper channels coated with gPMAA (green triangle), and paper channels
coated with gPDMAEMA (red square). ....................................................................................... 39
Figure 2-5 - a) Applied mixture of toluidine blue O and ponceau S on an uncoated
channel and separation on b) an uncoated channel, c) a channel coated with gPMAA,
and d) a channel coated with gPDMAEMA. Arrow depicts direction of fluid flow. .................. 41
Figure 2-6 - a) Deposition and patterning of a UV-responsive switch onto
microfluidic channels coated with gPMAA. The switch is patterned by masking a
portion of the channel during exposure. b) Activation of the switch and subsequent
separation of toluidine blue O and ponceau S. ............................................................................. 43
Figure 3-1 - A) A physical mask with holes was placed on top of porous
chromatography paper and placed into the iCVD chamber. B) After iCVD deposition
of PPFDA, dyed water was repelled over the entire surface of the paper, indicating
that the physical mask cannot be used to control the location of growth onto porous
substrates. Scale bars represent 1 cm. ........................................................................................... 56
Figure 3-2 - Contact angles of water on untreated paper and paper treated with a 2.0
M solution of FeCl
3
after iCVD deposition of PPFDA, PPFM, and PoNMBA. Without
the use of the salt, polymerization occurs on the untreated paper and the surface
becomes hydrophobic. When the inhibiting salt is used, polymerization does not occur
and the paper remains hydrophilic and wets. ................................................................................ 63
Figure 3-3 - A) Schematic of the experimental setup. The inhibition capability of
different transition metal salts was combinatorially screened by dividing the porous
x

chromatography paper into individual square compartments. B) Chemical structures of
the monomer precursors used in this study. .................................................................................. 66
Figure 3-4 - Wetting behavior after deposition of PPFM using 2.0 M salt solutions.
The square compartments measure 1.34 cm x 1.34 cm and the dotted gray circles
indicate the location of beaded water droplets. Polymerization occurs on the untreated
paper surrounding the square compartments. ............................................................................... 67
Figure 3-5 - A,B) XPS survey scans of the top and bottom of chromatography paper
after PoNBMA deposition, respectively. The presence of the nitrogen 1s peak
indicates PoNBMA deposition. C,D) XPS survey scans of the top and bottom of paper
treated with a 2.0 M solution of FeCl
3
after PoNBMA deposition, respectively. The
lack of a nitrogen peak in both spectra verifies that inhibition occurs uniformly
through the depth of the paper. ..................................................................................................... 69
Figure 3-6 - SEM images of A,B) plain chromatography paper, C,D) paper after
deposition of PoNBMA, E,F) paper treated with a 2.0 M solution of FeCl
3
, and G,H)
paper treated with a 2.0 M solution of FeCl
3
after PoNBMA inhibition. The addition
of polymer and salt does not change the porosity or morphology of the paper. ........................... 70
Figure 3-7 - Inhibition through the depth of the paper and along the edges varies with
deposition time as shown by PPFDA deposited on paper treated with different
concentrations of CuCl
2
solutions after A,B) 15 minutes of deposition and C,D) 60
minutes of deposition. ................................................................................................................... 74
Figure 3-S1 - High resolution X-ray photoelectron spectroscopy scans of the a) Cu 2p
peak and b) Cl 2s peak of CuCl
2
and the c) Cu 2p peak and d) N 1s peak of Cu(NO
3
)
2
.
The bottom scan is before PPFM deposition and the top scan is after PPFM
deposition. ..................................................................................................................................... 77
Figure 4-1 - a) P4VP surface coverage after deposition onto silicon wafer, CuCl
2
, and
FeCl
3
as a function of deposition time; b) High-resolution XPS spectra of the Cu 2p3
region showing conversion of Cu(II) to Cu(I) upon deposition. .................................................. 91
Figure 4-2 - a) PPFDA surface coverage after deposition onto silicon wafer, CuCl
2
,
and FeCl
3
as a function of deposition time; b) High-resolution XPS spectra of the Cu
2p3 region showing conversion of Cu(II) to Cu(I) upon deposition. ........................................... 92
xi

Figure 4-3 - High-resolution XPS spectra of the Cu 2p3 region of CuCl
2
before and
after exposure to reactive species showing conversion of Cu(II) to Cu(I) upon
exposure to propagating radicals. ................................................................................................. 94
Figure 4-4 - Effect of filament temperature on the surface coverage of PPFDA after
deposition onto CuCl
2
. .................................................................................................................. 96
Figure 4-5 - a) Screen printed CuCl
2
on chromatography paper, b) paper after
deposition of P(4VP-co-DVB) and removal of CuCl
2
, and c) polymer pattern after
dyeing with ponceau S. ................................................................................................................. 98
Figure 5-1 - a) Schematic representation of the device fabrication process. CuCl
2
is
applied to the chromatography paper by painting, spray coating, or photolithography,
yielding selective deposition of PPFDA by iCVD in the areas free of CuCl
2
. The salt is
then removed to yield the final device. b) Scanning electron micrographs of
chromatography paper before and after deposition of 440 nm of PPFDA. ................................ 115
Figure 5-2 - XPS spectra showing the chemical composition of a) the barrier regions
and b) the channel regions of devices with different thicknesses of deposited PPFDA. ............ 118
Figure 5-3 - Images of paper-based microfluidic devices after applying organic
solvents containing dye for visualization. Fluoropolymer barriers a) are unable to
contain hexane with a 10 nm thick PPFDA coating, b) are able to successfully contain
hexane with a 50 nm thick PPFDA coating, and c) have non-uniform wetting with a
640 nm thick PPFDA coating. d) Fluoropolymer barriers made with a 50 nm thick
PPFDA coating succeed at containing a wide variety of organic solvents whereas
traditional wax barriers fail. ........................................................................................................ 119
Figure 5-4 - Graphs plotting retardation factors of a) Sudan Black B and Nile Red in
solvent blends of water and methanol on cellulose patterned with fluoropolymer
barrier coatings (inset schematic represents Rf values of Sudan Black B and Nile Red),
and b) Sudan Black B in hexane on channels coated with copolymers composed of
4VP and EGDMA on devices patterned with fluoropolymer barrier coatings. .......................... 123
Figure 5-S1 - Image defining the variables used to perform resolution analysis on
intended isosceles triangle patterns ............................................................................................. 127

xii

List of Tables
Table 2-1 - Contact angle and lateral flow rate data of uncoated and coated paper.
Grafted coatings demonstrate an increased hydrophilicity relative to cross-linked
coatings. ........................................................................................................................................ 36
Table 2-S1 - Retardation factors (R
f
) and standard deviations (S.D.) for each dye on
unmodified paper, paper coated with gPMAA and paper coated with gPDMAEMA.
S.D. values were determined experimentally from triplicate measurements. ............................... 46
Table 2-S2 - Capacity factors (k’) and standard deviations (S.D.) for each dye on
unmodified paper, paper coated with gPMAA and paper coated with gPDMAEMA.
S.D. values were determined from the standard deviations of the R
f
values using
standard error propagation. ........................................................................................................... 47
Table 3-1 - Reaction conditions. .................................................................................................. 60
Table 3-2 - Summary of the effects of the metal salts on the inhibition through the
depth of the paper and along the edges for PPFDA, PPFM, and PoNBMA. “I”
indicates that polymerization was inhibited while “P” indicates that polymerization
occurred, “T” indicates the top of the paper, and “B” indicates the bottom of the paper.
The shaded regions indicate uniform inhibition through the depth and along the edges. ............ 67
Table 3-3 - Summary of the inhibition of PPFDA deposition through the depth of the
paper and along the edges with varying monomer source temperature (2a-2c) and
deposition time (3a-3c). “I” indicates that deposition was inhibited, “P” indicates
polymer deposition occurred, “T” indicates the top of the paper, and “B” indicates the
bottom of the paper. The shaded regions indicate uniform inhibition through the depth
and along the edges. ...................................................................................................................... 72
Table 5-1 - iCVD experimental conditions ................................................................................ 111
Table 5-2 - Weight fraction of PPFDA in the barrier and channel regions of the paper
devices......................................................................................................................................... 119
Table 5-3 - Water contact angles on both the top and bottom of the barrier and
channel regions of the paper devices. ......................................................................................... 120
xiii

Table 5-4 - Mole fraction of EGDMA in copolymer coating and corresponding R
f
value of Sudan Black B with hexane as the mobile phase. ......................................................... 124
Table 5-S1 - Dimensions of the intended regions, patterned regions, and deviations
(∆ ) for painting............................................................................................................................ 127
Table 5-S2 - Dimensions of the intended regions, patterned regions, and deviations
(∆ ) for spray coating. .................................................................................................................. 128
Table 5-S3 - Dimensions of the intended regions, patterned regions, and deviations
(∆ ) for photolithography. ............................................................................................................ 129
1

1.0 Introduction
1.1 Chemical Vapor Deposition
1.1.1 Overview
Chemical vapor deposition (CVD) is a process in which precursor gases react on the surface of a
desired substrate in order to fabricate solid thin film materials. CVD techniques remove the use
of solvents from the coating process in order to eliminate surface tension effects, and reduce
solvent waste and solvent-substrate chemical incompatibilities. The processing conditions are
controlled such that the precursor gases have relatively low sticking coefficients and only adsorb
onto the substrate and do not condense or deposit (as solid precursor material). The systems are
generally operated under vacuum to help maintain this condition, although a vacuum is not
always required. The low sticking coefficients allow these processes to deposit materials
conformally on a variety of complex substrates. Furthermore, the lack of surface tension effects
makes CVD techniques a particularly attractive option for the coating of non-planar substrates or
substrates with small features. A variety of CVD techniques have been developed to deposit
both organic and inorganic materials, and a few of the more common techniques will be
discussed briefly in this section.
The deposition of Parylene via the Gorham process
1
is one of the oldest CVD techniques and
relies on the pyrolysis of paracyclophanes (Figure 1-1).
2
The intermediate para-xylylenes
resonate with a biradical state
3
and subsequently react with other xylylenes to form Parylene.
While the chemistry of the polymer backbone is restricted, the functionality of the polymer is
often controlled by substituting moieties on the benzene ring of the paracyclophane
precursors.
4,5,6
The diverse range of functionalities attainable in Parylene coatings has allowed it
2

to be used in radiation applications
7
and as thin film transistors.
8
A number of devices that
contain Parylene have also been approved by the Federal Drug Administration (FDA), and this
has also led to an extensive amount of research regarding Parylene’s use in biomedical
applications.
9,10,11

Figure 1-1 - The Gorham process. Paracyclophanes are pyrolyzed to form para-xylylenes which
resonate with a biradical conformation. The xylylenes polymerize to form Parylene.
1

Plasma-enhanced CVD (PECVD) utilizes high energy plasmas to radicalize precursor gases in
order to initiate deposition, and is applicable to an extremely wide variety of chemistries,
including for the deposition of both organic and inorganic materials.
12,13
The use of plasmas to
radicalize the precursors can lead to complex reaction pathways, cross-linking, and loss of
functionality (Figure 1-2);
14,15
however these side-reactions can be mitigated by pulsation of the
plasma source or by reduction of the plasma power.
16,17
The broad range of chemistries that are
compatible with PECVD have allowed PECVD coatings to be used in a wide range of
applications, including as barrier layers,
18,19
bioconjugation platforms,
20
and antireflection
3

coatings.
21
The broad applicability of PECVD coatings has led to additional research into the
ability to perform PECVD at ambient pressures for large area coatings.
22,23

Figure 1-2 - Reaction pathways in plasma-enhanced chemical vapor deposition. Reproduced
from reference 14 with permission from Taylor & Francis Group LLC.

Atomic layer deposition (ALD) and molecular layer deposition (MLD) are CVD techniques
whose distinguishing feature is that they utilize self-limiting chemistries.
24,25
ALD generally
refers to the synthesis of inorganic coatings, such as metals or metal oxides, while MLD
generally refers to the synthesis of organic or hybrid organic-inorganic coatings, such as
polymers.
26
In both ALD and MLD, precursor gases (at least 2) are alternately exposed to a
4

substrate in order to generate the final film (Figure 1-3). Different precursor gases react with one
another but not with themselves, thereby allowing only a single layer of precursor to be
deposited upon each cycle, hence the term “self-limiting”. The self-limiting nature of these
processes provides excellent uniformity and precise control over the thickness of the film at the
expense of slower deposition rates as compared to other CVD processes.
27
Additionally, a
reactive substrate or activation of the surface of the substrate is often needed to deposit the first
monolayer.
28
Despite the need for activated surfaces, the advantages of these techniques have
allowed them to be used extensively in industrial processes, such as for semiconductor
fabrication.
29,30

Figure 1-3 - General schematic of atomic layer deposition and/or molecular layer deposition
demonstrating self-limiting chemistry. Reproduced from reference 24 with permission.

1.1.2 Initiated Chemical Vapor Deposition
The initiated chemical vapor deposition (iCVD) process uses a free-radical initiator to
polymerize unsaturated monomers in order to deposit organic polymer films.
31,32
A heated
5

filament array is often used to thermally decompose the initiator (Figure 1-4) although systems
that are radicalized upon exposure to ultra-violet light have also been demonstrated.
33
The
polymerization mechanism is believed to follow a free-radical mechanism that is analogous to
solution-phase free-radical polymerizations, and this leads to a high retention of the monomer
functionality with minimal side reactions. The iCVD process does not require a specific
substrate chemistry in order to deposit thin films, and a variety of functional polymers have been
synthesized, including click-active,
34
thermo-responsive,
35
photo-sensitive,
36
and anti-microbial
films.
37
Copolymer and cross-linked polymer films can also be generated by coflowing
precursor vapors, allowing for a high degree of tunability with respect to the composition of the
resulting films. These advantages have resulted in the iCVD process being used to deposit
conformal thin films for applications on electrospun fiber mats,
38
nanotubes,
39
and membranes.
40

Additionally, the iCVD process can be scaled up for large roll-to-roll fabrication.
41

6

Figure 1-4 - Typical reactor setup for an initiated chemical vapor deposition system and
traditional free-radical polymerization mechanism.
31

7

1.2 Diagnostic Applications
There has been significant attention given towards diagnostic applications both for point-of-care
(POC) applications and for resource-limited settings.
42,43
This is because successful diagnosis is
the first step towards effective treatment of many diseases and infections.
44
In addition, the
effective treatment of diseases and infection will aid significantly to mitigate possible
transmissions and outbreaks. While POC diagnostics have applications in both the developed
and the developing world, diagnostics in the developing world are currently much less
established.
The World Health Organization (WHO) has used the acronym “ASSURED” to help define
desirable qualities for the research and development of diagnostics in resource-limited settings
(Figure 1-5).
45
One of the most promising technologies for POC and resource-limited
diagnostics are lab-on-a-chip (LOC) and microfluidic devices, which decrease the size and scale
of various processes.
46,47,48
These devices satisfy a number of the qualities outlined by the
“ASSURED” acronym. For instance, the miniaturization of diagnostic functions reduces the
amount of equipment required to perform an assay (improving cost and deliverability to end-
users), allows low volumes of sample to be used to facilitate rapid testing, and their small size
and lower power requirements (relative to centralized technologies used in hospitals) allow them
to be operated in resource-limited settings.

8

Affordable
Sensitive
Specific
User-friendly
Robust and rapid
Equipment-free
Deliverable to end-users

Figure 1-5 - Desirable qualities for diagnostic platforms outlined by the World Health
Organization.
45

A number of different materials for platforms have been used for LOC and microfluidic devices,
including polymers such as poly(dimethylsiloxane)
49,50
(PDMS) and poly(methyl
methacrylate)
51,52
(PMMA), glass,
53,54
and paper.
55,56
These devices have been used for a variety
of diagnostics, including for the detection of cervical cancer,
57
Escherichia Coli,
50
and antibodies
such as sheep immunoglobulin M.
58
In addition, other analytical functions, including mixing
59

and separations by electrophoresis
60
or diffusion
61
have been demonstrated. Practical systems
that combine multiple functions such as filtration, reagent distribution, dilution, and detection
into a single device for practical analysis of biologically relevant mixtures have also been
demonstrated (Figure 1-6).
62

9

Figure 1-6 - Microfluidic card developed to perform diagnostics that incorporates multiple
functions. Reproduced from reference 62 with permission from The Royal Society of
Chemistry.

Paper-based microfluidic platforms have attracted significant attention recently because of a
number of advantages over other non-porous platforms (such as polymer or glass), including:
55,63

1. low cost (paper-based microfluidic devices are estimated to cost only a few cents per
device to manufacture);
2. low toxicity (paper is compatible with biological fluids and is biodegradable);
3. ease of use (flow is achieved through capillary action, reducing or eliminating the need
for equipment, trained operations personnel, and the requirement of power for operation);
4. ease of transport (devices are thin and light-weight reducing storage and transport
concerns); and
5. scalability (the paper and printing industries are well-developed allowing for easy
commercialization).
10

In a typical paper-based microfluidic device, the sample liquid is passively wicked through the
pores of the paper by capillary action. In order to direct the flow of liquids within paper-based
microfluidic devices, hydrophobic barriers are patterned within the paper. Initial barriers were
composed of hydrophobic photoresists patterned using standard photolithographic techniques
55

or PDMS patterned by printing.
64
However, wax printing emerged as the dominant method of
patterning hydrophobic barriers because of its low cost, low toxicity, rapid pattern formation, and
ease of scalability.
65
Wax patterns are directly printed onto paper using a commercially available
printing technology, and heated in an oven or on a hot plate to reflow the wax in order to
penetrate the depth of the paper.
Additionally, paper presents a uniform white background which allowed initial bioassays to
utilize colorimetric responses.
55
An example of a paper-based microfluidic device which
simultaneously detects glucose and total protein concentration is shown in Figure 1-7.
Colorimetric responses also allow paper-based microfluidic devices to be easily combined with
telemedicine to further reduce the requirement of trained healthcare personnel to be present
during analysis.
66
In addition, handheld detectors have been developed that allow for simple
quantification of a colorimetric response for on-site determination of analyte concentrations.
67

Other researchers have also developed methods for performing electrochemical quantification of
analytes on paper as a method for detection.
56,68
However, challenges associated with the
manipulation of passive fluid flow have limited the examples of paper-based microfluidics with
advanced functionality (beyond detection/quantification). The development of additional
operational functionality would expand the current capabilities of paper-based microfluidic
devices to be competitive with other non-porous platforms.
11

Figure 1-7 - Paper-based microfluidic device for the detection of glucose and total protein
concentration. Reproduced from reference 55 with permission.

The wide functionality of polymer coatings makes them an excellent potential option to
introduce additional operational functionality into paper-based microfluidic platforms. However,
the complex architecture of paper and the incompatibility of paper and wax barriers with organic
solvents make applying polymer coatings by solution phase techniques challenging.
Additionally, solution phase techniques may alter the morphology of the paper, due to the
blocking of pores with polymer or the wrinkling of the substrate during drying, which could
significantly affect the capillary flow properties. The ability of CVD techniques to conformally
coat complex substrates without altering the substrate morphology makes them an excellent tool
for applying polymer coatings to paper-based microfluidic devices to expand their operational
functionality.
12

1.3 Patterning in CVD Techniques
In order to utilize polymer coatings to expand the operational functionality of paper-based
microfluidic devices, it is important to be able to pattern the polymer into desired areas. While
the conformal nature of CVD techniques makes them an excellent technique to functionalize
porous substrates, it also makes patterning of the polymer area extremely challenging.
Shadow masking has been used in conjunction with a variety of CVD techniques, including the
deposition of Parylene (Figure 1-8),
69
PECVD,
70
ALD,
71
and iCVD.
72
However, in order for
shadow masking to be successful the distance between the mask and the substrate must be
extremely small (10’s of nanometers for ALD).
71,73
This is often not possible on nonplanar and
porous substrates, which results in the diffusion and subsequent deposition of precursors
underneath the intended mask region.
73

13

Figure 1-8 - Method of patterning the CVD of Parylene by shadow masking. Reproduced from
reference 69 with permission.

Electron beam patterning is another useful technique to pattern thin film deposition and has been
used with a variety of CVD techniques.
74,75,76,77
In this technique, there are two methods of
generating a pattern: 1) resist materials patterned by the electron-beam can be used to control
further etching in order to generate the desired pattern (Figure 1-9a), or 2) the thin film can be
directly patterned by etching areas as directed by the electron beam if the film material is
electron-beam active (Figure 1-9b). In both cases, however, the patterning of the thin film or the
resist requires direct line-of-sight between the substrate and the electron beam. Similar to
shadow masking, this limitation precludes its use on many nonplanar and porous substrates.
14

Figure 1-9 - a) Patterning using an electron-beam active resist material to pattern a CVD coated
film. b) Direct patterning of an electron-beam active film. Reproduced from reference 75 with
permission.

Tuning of the substrate surface chemistry has also been used to form patterns. In this technique,
a specific surface chemistry which limits the deposition of the precursor is used to generate
patterns (Figure 1-10). Unsurprisingly, because of the requirement of an activated substrate for
ALD and MLD, tuning of the surface chemistry has been studied extensively for these
techniques.
77,78,79,80
Patterning of the initial deactivating surface chemistry is often done by
micro-contact printing or by some form of etching, which again can be challenging on complex
15

and porous substrates. More recently, it has been demonstrated that the deposition of Parylene
can also be limited by specific surface chemistries.
81,82,83
As the deposition of Parylene does not
rely on a specific surface chemistry, the cause of this limited deposition has been attributed to
multiple factors, such as a deactivation of the intermediate via complexation,
82
or quenching of
the radical form of the intermediate.
84

Figure 1-10 - Patterning of ALD using self-assembled monolayers (SAM) to deactivate the
surface and limit deposition. Reproduced from reference 77 with permission from The Royal
Society of Chemistry.

16

1.4 Project Summary
The goal of this work is to develop a technique to pattern polymer deposition that is amicable
towards porous substrates such as paper, as well as to demonstrate the utility of polymer coatings
in order to expand on the advanced operational functionality available to paper-based
microfluidic platforms. In this work, we will focus on the use of the iCVD process for the
functionalization of the paper substrates because of iCVD’s low operating substrate temperature
(which is important for paper substrates as they may oxidize at higher temperatures), high
retention of monomer functionality, wide array of polymers that can be deposited, lack of
required specific surface chemistry, and relatively fast deposition rates. This patterning
technique could allow for a wide variety of advanced operational functionality to be incorporated
into paper-based microfluidic platforms, and could also be more generally extended to help
alleviate challenges associated with patterning in CVD processes for other applications.

17

1.5 References

1 Gorham, W. F. J. Polym. Sci. Part A-1 1966, 4, 3027-3039.
2 Tan, C. P.; Craighead, H. G. Materials, 2010, 3, 1803-1832.
3 Hiberty, P. C.; Karafiloglou, P. Theor. Chim. Acta., 1982, 61, 171-177.
4 Lahann, J.; Balcells, M.; Lu, H.; Rodon, T.; Jensen, K. F.; Langer, R. Anal. Chem., 2003,
75, 2117-2122.
5 Lahann, J.; Balcells, M.; Rodon, T.; Lee, J.; Choi, I. S.; Jensen, K. F.; Langer, R.
Langmuir, 2002, 18, 3632-3638.
6 Bier, A. K.; Bognitzki, M.; Mogk, J.; Greiner, A. Macromolecules, 2012, 45, 1151-1157.
7 Spivack, M. A. Rev. Sci. Instrum., 1970, 41, 1614-1616.
8 Dimitrakopoulos, C. D.; Mascaro, D. J. IBM J. Res. & Dev., 2001, 45, 11-27.
9 Kuo, J. T.W.; Kim, B. J.; Hara, S. A.; Lee, C. D.; Gutierrez, C. A.; Hoang, T. Q.; Meng,
E. Lab Chip, 2013, 13, 554-561.
10 Hassler, C.; von Metzen, R. P.; Ruther, P.; Stieglitz, T. J. Biomed. Mater. Res.B: Appl.
Biomater., 2010, 93, 266-274
11 Winslow, B. D.; Christensen, M. B.; Yang, W.-K.; Solzbacher, F.; Tresco, P. A.
Biomater., 2010, 31, 9163-9172.
12 Martinu, L.; Poitras, D. J. Vac. Sci. Technol. A, 2000, 18, 2619-2645.
13 Vasudev, M. C.; Anderson, K. D.; Bunning, T. J.; Tsukruk, V. V.; Naik, R. R. ACS Appl.
Mater. Interfaces, 2013, 5, 3983-3994.
14 Yasuda, H.; Luminous Chemical Vapor Deposition and Interface Engineering, Marcel
Dekker, New York, 2005, p. 64.
18

15 Coclite, A. M.; Ozaydin-Ince, G.; Palumbo, F.; Milella, A.; Gleason, K. K. Plasma
Processes Polym., 2010, 7, 561-570.
16 Mackie, N. M.; Dalleska, N. F.; Castner, D. G.; Fisher, E. R. Chem. Mater., 1997, 9, 349-
362.
17 Burkey, D. D.; Gleason, K. K. Chem. Vap. Deposition, 2003, 9, 65-71.
18 Park, B. L.; Ko, D.-H.; Kim, Y. S.; Ha, J. M.; Park, Y. W.; Lee, S. I.; Lee, H.-D.; Lee, M.
B.; Chung, U. I.; Koh, Y. B.; Lee, M. Y. J. Electron. Mater., 1997, 26, L1-L5.
19 Chatham, H., Surf. Coat. Technol., 1996, 78, 1-9.
20 Coad, B. R.; Vasilev, K.; Diener, K. R.; Hayball, J. D.; Short, R. D.; Griesser, H. J.
Langmuir, 2012, 28, 2710-2717.
21 Chen, Z.; Sana, P.; Salami, J.; Rohatgi, A. IEEE Trans. Electron Devices, 1993, 40,
1161-1165.
22 Agemi, M.; Kayama, K.; Noborisaka, M.; Tachimoto, Y.; Shirakura, A.; Suzuki, T. Surf.
Coat. Technol., 2011, 206, 2025-2029.
23 Hilt, F.; Boscher, N. D.; Duday, D.; Desbenoit, N.; Levalois-Grützmacher, J.; Choquet, P.
ACS Appl. Mater. Interfaces, 2014, 6, 18418-18422.
24 George, S. M. Chem. Rev., 2010, 110, 111-131.
25 Leskelä, M.; Ritala, M. Thin Solid Films, 2002, 409, 138-146.
26 Dameron, A. A.; Seghete, D.; Burton, B. B.; Davidson, S. D.; Cavanagh, A. S.; Bertrand,
J. A.; George, S. M. Chem. Mater., 2008, 20, 3315-3326.
27 Peng, Q.; Sun, X.-Y.; Spagnola, J. C.; Hyde, G. K.; Spontak, R. J.; Parsons, G. N. Nano
Lett., 2007, 7, 719-722.
19

28 George, S. M.; Yoon, B.; Dameron, A. A. Acc. Chem. Res., 2009, 42, 498-508.
29 Sneh, O.; Clark-Phelps, R. B.; Londergan, A. R.; Winkler, J.; Seidel, T. E. Thin Solid
Films, 2002, 402, 248-261.
30 Ye, P. D.; Yang, B.; Ng, K. K.; Bude, J.; Wilk, G. D.; Halder, S.; Hwang, J. Appl. Phys.
Lett., 2005, 86, 063501.
31 Tenhaeff, W. E.; Gleason, K. K. Adv. Funct. Mater., 2008, 18, 979-992.
32 Seidel, S.; Riche, C.; Gupta, M. Chemical Vapor Deposition of Polymer Films
(Encyclopedia of Polymer Science and Technology, 2011).
33 Montero, L.; Baxamusa, S. H.; Borros, S.; Gleason, K. K. Chem. Mater., 2009, 21, 399-
403.
34 O’Shaughnessy, W. S.; Marí-Buyé, N.; Borrόs, S.; Gleason, K. K. Macromol. Rapid.
Commun., 2007, 28, 1877-1882.
35 Alf, M. E.; Godfrin, P. D.; Hatton, T. A.; Gleason, K. K. Macromol. Rapid Commun.,
2010, 31, 2166-2172.
36 Haller, P. D.; Flowers, C. A.; Gupta, M. Soft Matter, 2011, 7, 2428-2432.
37 Martin, T. P.; Kooi, S. E.; Chang, S. H.; Sedransk, K. L.; Gleason, K. K. Biomaterials,
2007, 28, 909-915.
38 Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules, 2005, 38,
9742-9748.
39 Lau, K. K. S.; Bico, J.; Teo, K. B. K; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.;
McKinley, G. H.; Gleason, K. K. Nano Lett., 2003, 3, 1701-1705.
40 Gupta, M.; Gleason, K. K. Thin Solid Films, 2009, 517, 3547-3550.
20

41 Gupta, M.; Gleason, K. K. Thin Solid Films, 2006, 515, 1579-1584.
42 Urdea, M.; Penny, L. A.; Olmsted, S. S.; Giovanni, M. Y.; Kaspar, P.; Shepherd, A.;
Wilson, P.; Dahl, C. A.; Buchsbaum, S.; Moeller, G.; Hay Burgess, D. C. Nature, 2006,
444, 73-79.
43 Chin, C. D.; Linder, V.; Sia, S. K. Lab Chip, 2012, 12, 2118-2134.
44 Daar, A. S.; Thorsteinsdόttir, H.; Martin, D. K.; Smith, A. C.; Nast, S.; Singer, P. A.
Nature Genet., 2002, 32, 229-232.
45 UNDP/World Bank/Who Special Programme for Research and Training in Tropical
Diseases. Sexually Transmitted Diseases Diagnostics Initiative
http://www.who.int/std_diagnostics/index.htm
46 Yager, P.; Edwards, T.; Fu, E.; Helton, K.; Nelson, K.; Tam, M. R.; Weigl, B. H. Nature,
2006, 442, 412-418.
47 Yager, P.; Domingo, G. J.; Gerdes, J. Annu. Rev. Biomed. Eng., 2008, 10, 107-144.
48 Chin, C. D.; Linder, V.; Sia, S. K. Lab Chip, 2007, 7, 41-57.
49 Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Anal. Chem., 1998,
70, 4974-4984.
50 Varshney, M.; Li, Y.; Srinivasan, B.; Tung, S. Sens. Actuators, B, 2007, 128, 99-107.
51 Ko, J. S.; Yoon, H. C.; Yang, H.; Pyo, H.-B.; Chung, K. H.; Kim, S. J.; Kim, Y. T. Lab
Chip, 2003, 3, 106-113.
52 Bi, H.; Meng, S.; Li, Y.; Guo, K.; Chen, Y.; Kong, J.; Yang, P.; Zhong, W.; Liu, B. Lab
Chip, 2006, 6, 769-775.
21

53 Herr, A. E.; Hatch, A. V.; Throckmorton, D. J.; Tran, H. M.; Brennan, J. S.; Giannobile,
W. V.; Singh, A. K. PNAS, 2007, 104, 5268-5273.
54 Holden, M. A.; Jung, S.-Y.; Cremer, P. S. Anal. Chem., 2004, 76, 1838-1843.
55 Martinez, A. W.; Phillips, S. T.; Butte, M. J.; Whitesides, G. M. Angew. Chem. Int. Ed.,
2007, 46, 1318-1320.
56 Dungchai, W.; Chailapakul, O.; Henry, C. S. Anal. Chem., 2009, 81, 5821-5826.
57 Du, Z.; Colls, N.; Cheng, K. H.; Vaughn, M. W.; Gollahon, L. Biosens. Bioelectron.,
2006, 21, 1991-1995.
58 Eteshola, E.; Leckband, D. Sens. Actuators, B, 2001, 72, 129-133.
59 Johnson, T. J.; Ross, D.; Locascio, L. E. Anal. Chem., 2002, 74, 45-51.
60 Lagally, E. T.; Simpson, P. C.; Mathies, R. A. Sens. Actuators, B, 2000, 63, 138-146.
61 Hatch, A.; Kamholz, A. E.; Hawkins, K. R.; Munson, M. S.; Schilling, E. A.; Weigl, B.
H.; Yager, P. Nature Biotechnol., 2001, 19, 461-465.
62 Lafleur, L.; Stevens, D.; McKenzie, K.; Ramachandran, S.; Spicar-Mihalic, P.; Singhal,
M.; Arjyal, A.; Osborn, J.; Kauffman, P.; Yager, P.; Lutz, B. Lab Chip, 2012, 12, 1119-
1127.
63 Martinez, A. W.; Phillips, S. T.; Whitesides, G. M. Anal. Chem., 2010, 82, 3-10.
64 Bruzewicz, D. A.; Reches, M.; Whitesides, G. M. Anal. Chem., 2008, 80, 3387-3392.
65 Lu, Y.; Shi, W.; Jiang, L.; Qin, J.; Lin, B. Electrophoresis, 2009, 30, 1497-1500.
66 Martinez, A. W.; Phillips, S. T.; Carrilho, E.; Thomas III, S. W.; Sindi, H.; Whitesides,
G. M. Anal. Chem., 2008, 80, 3699 – 3707.
22

67 Ellerbee, A. K.; Phillips, S. T.; Siegel, A. C.; Mirica, K. A.; Martinez, A. W.; Striehl, P.;
Jain, N.; Prentiss, M.; Whitesides, G. M. Anal. Chem., 2009, 81, 8447-8452.
68 Carvalhal, R. F.; Simão Kfouri, M.; de Oliveira Piazetta, M. H.; Gobbi, A. L.; Kubota, L.
T. Anal. Chem., 2010, 82, 1162-1165.
69 Chen, H.-Y.; Lahann, J. Adv. Mater., 2007, 19, 3801-3808.
70 Bullett, N. A.; Short, R. D.; O’Leary, T.; Beck, A. J.; Douglas, C. W. I.; Cambray-
Deakin, M.; Fletcher, I. W.; Roberts, A.; Blomfield, C. Surf. Interface Anal., 2001, 31,
1074-1076.
71 Langston, M. C.; Usui, T.; Prinz, F. B. J. Vac. Sci. Technol. A, 2012, 30, 01A153.
72 Seidel, S.; Gupta, M. J. Vac. Sci. Technol. A, 2014, 32, 041514.
73 Sweet III, W. J.; Oldham, C. J.; Parsons, G. N. ACS Appl. Mater. Interfaces, 2014, 6,
9280-9289.
74 Lewis, H. G. P.; Weibel, G. L.; Ober, C. K.; Gleason, K. K. Chem. Vap. Deposition,
2001, 7, 195-197.
75 Im, S. G.; Kim, B. S.; Lee, L. H.; Tenhaeff, W. E.; Hammond, P. T.; Gleason, K. K.
Macromol. Rapid Commun., 2008, 29, 1648-1654.
76 Mancini, D. P.; Gehoski, K. A.; Ainley, E.; Nordquist, K. J.; Resnick. D. J.; Bailey, T. C.;
Sreenivasan, S. V.; Ekerdt, J. G.; Willson, C. G. J. Vac. Sci. Technol. B, 2002, 20, 2896-
2901.
77 Mackus, A. J. M.; Bol, A. A.; Kessels, W. M. M. Nanoscale, 2014, 6, 10941-10960.
78 Chen, R.; Kim, H.; McIntyre, P. C.; Porter, D. W.; Bent, S. F. Appl. Phys. Lett., 2005, 86,
191910.
23

79 Park, K. J.; Doub, J. M.; Gougousi, T.; Parsons, G. N. Appl. Phys. Lett., 2005, 86,
051903.
80 Avila, J. R.; DeMarco, E. J.; Emery, J. D.; Farha, O. K.; Pellin, M. J.; Hupp. J. T.;
Martinson, A. B. F. ACS Appl. Mater. Interfaces, 2014, 6, 11891-11898.
81 Vaeth, K. M.; Jensen, K. F. Adv. Mater., 1999, 11, 814-820.
82 Vaeth, K. M.; Jensen, K. F. Chem. Mater., 2000, 12, 1305-1313.
83 Chen, H.-Y.; Lai, J. H.; Jiang, X.; Lahann, J. Adv. Mater., 2008, 20, 3474-3480.
84 Bobrowski, M. Comput. Theor. Chem., 2012, 991, 56-65.
24

2.0 Vapor Phase Deposition of Functional Polymers onto Paper-Based
Microfluidic Devices for Advanced Unit Operations
Citation: Philip Kwong and Malancha Gupta, Anal. Chem., 2012, 84, 10129-10135

2.1 Abstract
Paper-based microfluidic devices have recently received significant attention as a potential
platform for low-cost diagnostic assays. However, the number of advanced unit operations, such
as separation of analytes and fluid manipulation, that can be applied to these devices has been
limited. Here, we use a vapor phase polymerization process to sequentially deposit functional
polymer coatings onto paper-based microfluidic devices to integrate multiple advanced unit
operations while retaining the fibrous morphology necessary to generate capillary-driven flow.
A hybrid grafting process was used to apply hydrophilic polymer coatings with a high surface
concentration of ionizable groups onto the surface of the paper fibers in order to passively
separate analytes, which allowed a multi-component mixture to be separated into its anionic and
cationic components. Additionally, a UV-responsive polymer was sequentially deposited to act
as a responsive switch to control the path of fluid within the devices. This work extends the
advanced unit operations available for paper-based microfluidics and allows for more complex
diagnostics. In addition, the vapor phase polymerization process is substrate-independent and
therefore these functional coatings can be applied to other textured materials such as membranes,
filters, and fabrics.

25

2.2 Introduction
Microfluidic devices have received significant attention for applications in diagnostics, primarily
in resource-limited settings, because they require low volumes of reagents and are easy to
transport and store.
85,86,87
While traditional microfluidic devices have been fabricated from
polymers such as poly(dimethyl siloxane)
88,89
and poly(methyl methacrylate),
90,91
paper-based
microfluidic devices have recently emerged as an excellent platform for diagnostic applications
due to its numerous advantages, including low-cost of fabrication and minimal equipment
requirements.
92,93
Paper-based microfluidic devices are easily fabricated by printing and melting
hydrophobic wax into hydrophilic porous chromatography paper.
93,94
A number of recent papers
have highlighted the current and potential advantages and capabilities of paper-based
microfluidic devices.
95,96,97

Despite the numerous advantages of paper-based microfluidic devices, one potential drawback is
that the development of advanced unit operations, such as separation of analytes and fluid
manipulation, that are ubiquitous in polymer-based microfluidics has been challenging,
98,99
and
only a few systems have been demonstrated. For example, Carvalhal et al.
100
separated uric acid
from ascorbic acid by altering the pH of the diagnostic solution to reduce the solubility of uric
acid which led to chromatographic separation on unmodified cellulose while Yang et al.
101
spotted antibodies onto paper to agglutinate red blood cells in order to separate plasma from
whole blood. Both of these techniques are highly specific and cannot be universally extended to
other systems. More generally, Osborn et al.
98
separated analytes of different molecular weights
by allowing the smaller molecular weight analytes to diffuse perpendicular to the direction of
flow into an adjacent co-flowing stream. However, the design of this system required a
26

significant increase in the size of the paper-based microfluidic device, the device could only
isolate the lower molecular weight component, and large molecular weight differences were
required. Switches have been recently used to control the path of fluid within paper-based
microfluidic devices. For example, Martinez et al.
102
and Li et al.
103
fabricated switches that
were activated by physically connecting different hydrophilic paper regions, however this
approach can be tedious or lead to non-reproducible flow profiles. Chen et al.
104
fabricated
switches by using surfactants to reduce the surface tension of the diagnostic solution to allow the
fluid to permeate hydrophobic barriers. However, operation of these devices required more
complex designs with multiple inlets.
It is difficult to incorporate advanced unit operations onto paper-based microfluidic devices
because flow through these devices relies on capillary action rather than external pumping and
therefore any modification of the paper substrate must not affect the fibrous morphology.
92
The
integration of multiple unit operations can be further complicated by the need to sequentially
modify the paper substrate for each function. Here, we use initiated chemical vapor deposition
(iCVD) to deposit polymer coatings onto paper-based microfluidic devices to integrate multiple
advanced unit operations onto a single device. The iCVD process is a solvent-free process that
allows for deposition of thin conformal polymer coatings onto non-planar substrates such as
electrospun fiber mats
105,106
and membranes
107
because of the lack of surface tension effects such
as dewetting and clogging.
108
The use of the iCVD process allows us to modify the surface
properties of paper-based microfluidic devices while retaining the fibrous morphology necessary
to generate capillary-driven flow and therefore multiple unit operations can be integrated onto
one device, the size of the device remains unaltered, and the amount of additional equipment
27

required is minimized. Furthermore, the iCVD process allows for the patterning of polymer
coatings onto chromatography paper,
109,110
which is useful for the incorporation of additional
functionality. In this paper, we used iCVD to deposit acidic poly(methacrylic acid) (PMAA) and
basic poly(dimethylaminoethyl methacrylate) (PDMAEMA) as ion-exchange coatings to
separate analytes on the device. In contrast to the separation techniques described above, ion-
exchange chromatography can be generally applied to a wide variety of systems such as for
protein separation
111,112
and water purification.
113,114
While thin-layer chromatography (TLC) is
often used to separate analytes because of its compatibility with organic and corrosive solvents
and ability to achieve high resolution,
115
the use of inflexible TLC plates is not well suited for the
incorporation of other unit operations, such as three-dimensional multiplexing,
116
thus limiting
its potential for lab-on-a-chip applications. Additionally, TLC plates are more expensive than
chromatography paper, making the design of low-cost diagnostic TLC systems less achievable.
We fabricated a single paper-based microfluidic device with multiple integrated unit operations
by sequentially depositing poly(o-nitrobenzyl methacrylate) (PoNBMA) as a UV-responsive
switch. This device can be used for multi-step processes, such as assays which require
incubation.
104
The ion-exchange coatings were composed of two layers: a cross-linked layer that
is insoluble in aqueous solvents and a grafted homopolymer layer that improves the wettability
and increases the number of ionizable groups on the surface. This allowed for selective
separation of cationic and anionic analytes from a multi-component mixture. The hydrophobic
PoNBMA switch was activated by exposure to UV-light, which renders the polymer hydrophilic,
allowing aqueous fluid to pass through the polymer region. Our iCVD modification process
advances the current capabilities of paper-based microfluidics by allowing for the analysis of
more complex mixtures and the performance of assays that require separations or external
28

control over the path of fluids. Additionally, the use of ionizable polymer coatings enables the
development of other advanced pH-responsive unit operations to control the release of specific
analytes, which is useful for signal amplification and multi-step processes.
98,117

29

2.3 Experimental
Methacrylic acid (MAA) (Aldrich, 99 %), dimethylaminoethyl methacrylate (DMAEMA)
(Aldrich, 98 %), ethylene glycol dimethacrylate (EGDMA) (Aldrich, 98 %), o-nitrobenzyl
methacrylate (oNBMA) (Polysciences, 95 %), t-butyl peroxide (TBPO) (Aldrich, 98 %),
toluidine blue O (Aldrich, 80 %), crystal violet (Aldrich, 90 %), methyl orange (Aldrich, 85 %),
ponceau S (Aldrich, 75 %), tartrazine (Aldrich, microscopy grade), brilliant blue G (Aldrich,
microscopy grade), and buffers at pH 4, pH 6, pH 8, and pH 10 (BDH, ACS grade) were used as
received without further purification.

Polymer depositions were performed in a custom-designed pancake-shaped iCVD vacuum
chamber (GVD corporation), as described previously.
110
The pressure was maintained using a
rotary vane vacuum pump (Edwards E2M40) and controlled using a throttle valve (MKS 153D).
The initiator flow rate was controlled using a mass flow controller (MKS 1479A) and the
monomer flow rate was controlled by heating the monomer source at a constant temperature. A
nichrome filament array (Omega Engineering, 80 %/20 % Ni/Cr) was resistively heated to 270
°C to decompose the initiator into radicals. The reactor stage was kept at a constant temperature
using a back-side recirculating heat exchanger. The thickness of the polymer coatings was
measured on reference silicon wafers using a 633 nm helium-neon laser interferometry system
and determined to be approximately 100 nm for both the gPMAA and gPDMAEMA coatings.
For the deposition of poly(methacrylic acid-co-ethylene glycol dimethacrylate)-g-
poly(methacrylic acid) (gPMAA), we first deposited poly(methacrylic acid-co-ethylene glycol
dimethacrylate) (xPMAA) for 9 minutes followed immediately by deposition of poly(methacrylic
acid) (PMAA) for 7 minutes. To deposit the xPMAA layer, the pressure of the reactor was
30

maintained at 250 mTorr, the initiator flow rate was 0.6 sccm, the MAA was kept at room
temperature and had a flow rate of 13.4 sccm, the EGDMA was heated to 55 °C and had a flow
rate of 0.5 sccm, and the stage was maintained at 25 °C. To deposit the PMAA layer, the
conditions described above were used without EGDMA. Residual ungrafted PMAA was
removed by repeatedly washing samples in deionized water. For Fourier transform infrared
(FTIR) spectroscopy analysis, depositions of xPMAA were extended to 20 minutes to achieve
thicker samples for analysis.
For the deposition of poly(dimethylaminoethyl methacrylate-co-ethylene glycol dimethacrylate)-
g-poly(dimethylaminoethyl methacrylate) (gPDMAEMA), we first deposited
poly(dimethylaminoethyl methacrylate-co-ethylene glycol dimethacrylate) (xPDMAEMA) for 10
minutes followed immediately by deposition of poly(dimethylaminoethyl methacrylate)
(PDMAEMA) for 10 minutes. To deposit the xPDMAEMA layer, the pressure of the reactor
was maintained at 150 mTorr, the initiator flow rate was 0.6 sccm, the DMAEMA was kept at
room temperature and had a flow rate of 6.0 sccm, the EGDMA was heated to 40 °C and had a
flow rate of 0.3 sccm, and the stage was maintained at 22 °C. To deposit the PDMAEMA layer,
the conditions described above were used without EGDMA. Residual ungrafted PDMAEMA
was removed by repeatedly washing samples in deionized water. For FTIR analysis, depositions
of xPDMAEMA were extended to 30 minutes to achieve thicker samples for analysis.
For the deposition of poly(o-nitrobenzyl methacrylate) (PoNBMA), the pressure of the reactor
was maintained at 60 mTorr, the initiator flow rate was 0.6 sccm, the oNBMA was kept at 80 °C
and had a flow rate of 0.1 sccm, and the stage was maintained at 30 °C. The deposition
proceeded for 5 minutes.
31

FTIR was performed using a Thermo Scientific Nicolet iS10 spectrometer. X-ray photoelectron
spectroscopy (XPS) was performed using a Surface Science Instruments M-Probe spectrometer
with a monochromatic Al Kα X-ray source. High-resolution scans of the C 1s region were
acquired with a resolution of 0.065 eV. Low-resolution survey scans were acquired at binding
energies between 1 and 1000 eV with a resolution of 1 eV. Data analysis was performed using
the ESCA25 Analysis Application (V5.01.04) software.
Paper-based microfluidic devices were produced by printing wax (Xerox Phaser 8560N) onto
chromatography paper (Whatman, grade 1) and subsequently melting the wax through the depth
of the paper in an oven at 180 °C for three minutes.
93,94
The thickness of the paper was
approximately 180 μm. The inner dimensions of the channels were 50.0 mm in length and 5.0
mm in width prior to melting of the wax and 48.8 mm in length and 3.6 mm in width after
melting of the wax. Static contact angles were measured on paper coated with polymer using a
contact angle goniometer (Ramé-Hart model 290-F1). The contact angle of a 5 μL drop of
deionized water was measured at its equilibrium value, which was normally achieved within 3
minutes. Lateral flow rates were measured by suspending paper channels in a saturated water
environment to prevent evaporative loss. Both static contact angles and lateral flow rates were
performed in triplicate. The morphology of unmodified paper and the paper coated with polymer
was analyzed using a JSM 7001F scanning electron microscope.
Analysis of the separation ability of paper devices coated with gPMAA and gPDMAEMA was
performed by spotting a 0.5 mg/mL buffered solution of dye at the pH of interest onto the
channels and allowing them to dry under ambient conditions. The samples were then placed
with one end in a reservoir of buffer solution in a saturated water environment such that the
32

buffer solution wicked vertically upward through the paper channel. The migration distance of
the dye relative to the migration distance of the buffer, known as the retardation factor (R
f
), was
used to quantify the separation ability of each polymer coating. Determination of the R
f
values
on paper channels without wax barriers yielded similar values to those measured on channels
with wax barriers. Therefore, channels with wax barriers were used as this is currently the most
common method for making paper-based microfluidic devices, and ensures that our work can be
extended to more complex devices where wax barriers are required to direct fluid flow into
multiple paths. Separation analysis of each dye was performed in triplicate. The R
f
and capacity
factor (k’=(1-R
f
)/R
f
) values are tabulated in Table 2-S1 and Table 2-S2 in the Supporting
Information, respectively. Multi-component separations were performed by combining 0.5
mg/mL solutions of toluidine blue O and ponceau S at pH 4. The solution was spotted and
separation was analyzed as described above.
For the fabrication of a UV-responsive PoNBMA switch, PoNBMA was first deposited
conformally over the paper, as described above. The paper channels were masked and the
remaining channel was exposed to UV-light for three hours (UVP, 254 nm, 6 W), washed in
deionized water, and dried in atmosphere. To activate the switch, the entire channel was exposed
to UV-light for one hour, which rendered the polymer zone hydrophilic.

33

2.4 Results and Discussion
The iCVD process was used to deposit acidic PMAA and basic PDMAEMA onto paper-based
microfluidic devices to act as ion-exchange coatings and enhance separations. The carboxylic
acid groups of PMAA are readily deprotonated in basic solutions which generates anionic
moieties, while the tertiary amine groups of PDMAEMA become protonated in acidic solutions
which generates cationic moieties. For practical applications, it is important to prevent the
dissolution of PMAA and PDMAEMA in aqueous solutions. Therefore, the coatings were cross-
linked by the addition of ethylene glycol dimethacrylate (EGDMA) during the deposition process
to generate insoluble poly(methacrylic acid-co-ethylene glycol dimethacrylate) (xPMAA) and
poly(dimethylaminoethyl methacrylate-co-ethylene glycol dimethacrylate) (xPDMAEMA). The
flow rate of the EGDMA cross-linker was varied to try to minimize the amount of EGDMA in
the polymer coatings in order to increase the number of ionizable groups on the surface.
EGDMA flow rates lower than 0.5 sccm for xPMAA and 0.3 sccm for xPDMAEMA led to
polymer films that dissolved in aqueous solutions. Fourier transform infrared (FTIR)
spectroscopy was used to confirm the chemical structure of the insoluble xPMAA and
xPDMAEMA coatings, as shown in Figure 2-1. The spectrum of xPMAA (Figure 2-1a) was
consistent with incorporation of both methacrylic acid (MAA) and EGDMA into the polymer
coating. The broad band at 3500 cm
-1
is attributed to the hydroxyl moiety of MAA, the weak
absorbance at 1623 cm
-1
is attributed to the vinyl stretching vibration from partially unreacted
EGDMA units, and the weak absorbances at 2975 cm
-1
and 2950 cm
-1
, and the strong absorbance
at 1720 cm
-1
are attributed to the stretching vibrations of the aliphatic and carbonyl moieties,
respectively, of both MAA and EGDMA.
118,119
Similarly, the spectrum of xPDMAEMA (Figure
2-1b) was consistent with incorporation of both dimethylaminoethyl methacrylate (DMAEMA)
34

and EGDMA into the polymer coating. The weak absorbance at 2760 cm
-1
is attributed to the
tertiary amine of DMAEMA, the weak absorbance at 1623 cm
-1
is attributed to the vinyl
stretching vibration from partially unreacted EGDMA units, and the weak absorbances at 2975
cm
-1
and 2950 cm
-1
, and the strong absorbance at 1726 cm
-1
are attributed to the stretching
vibrations of the aliphatic and carbonyl moieties, respectively, of both DMAEMA and
EGDMA.
120,121

Figure 2-1 - FTIR spectra of a) xPMAA and b) xPDMAEMA, high-resolution carbon 1s XPS
spectra of c) PMAA and xPMAA, and XPS survey spectra of d) PDMAEMA and xPDMAEMA.

X-ray photoelectron spectroscopy (XPS) further confirmed the structure of the polymer coatings
and was used to determine the relative amount of EGDMA in both the xPMAA and
xPDMAEMA coatings. Figure 2-1c shows the high-resolution carbon 1s spectra of PMAA and
35

xPMAA. For PMAA, the carbon environments at 282 eV and 286 eV are attributed to carbon
bound only to other carbon (C-C) or hydrogen (C-H), and carboxyl carbon (COOH),
respectively.
122
For xPMAA, the ester carbon (COOR) of EGDMA overlaps with the carboxyl
carbon of PMAA at 286 eV, while the carbon singly bound to oxygen (C-O) of EGDMA appears
at 284 eV.
122
The ratio of MAA to EGDMA in the xPMAA coating was determined by
comparing the relative area of the carboxyl and ester carbon to the carbon singly bound to
oxygen and was found to be 0.6. Figure 2-1d shows the survey spectra of PDMAEMA and
xPDMAEMA which is characterized by the carbon 1s peak at 284 eV, the nitrogen 1s peak at
399 eV, and the oxygen 1s peak at 531 eV. The PDMAEMA coating had experimental atomic
ratios of carbon, nitrogen, and oxygen of 71 %, 6 %, and 23 %, respectively, which is in
relatively close agreement with the theoretical values of 73 %, 9 %, and 18 %, respectively.
Conversely, the xPDMAEMA coating had atomic ratios of 73 %, 2 %, and 25 %, for carbon,
nitrogen, and oxygen, respectively. The ratio of DMAEMA to EGDMA in the xPDMAEMA
coating was determined by comparing the relative atomic ratios of nitrogen to oxygen and was
found to be 0.5.
While cross-linking rendered the polymer films insoluble in aqueous solutions, it also led to an
increase in the hydrophobicity of the paper surface as observed by an increase in the contact
angle as summarized in Table 2-1. The high contact angle on paper limited the lateral flow rate
of water through the paper and limits the practicality of xPMAA and xPDMAEMA for paper-
based microfluidic applications. In an effort to produce more hydrophilic polymer coatings,
PMAA and PDMAEMA homopolymers were grafted onto their respective cross-linked polymers
using a process previously developed by Ye et al. to graft antimicrobial iCVD polymer
36

coatings.
123
They hypothesize that grafting occurs by polymerization to unterminated radicals on
the surface of the cross-linked layer. In order to produce hydrophilic coatings, PMAA and
PDMAEMA were deposited immediately after deposition of their respective cross-linked
copolymers leading to the fabrication of poly(methacrylic acid-co-ethylene glycol
dimethacrylate)-g-poly(methacrylic acid) (gPMAA) and poly(dimethylaminoethyl methacrylate-
co-ethylene glycol dimethacrylate)-g-poly(dimethylaminoethyl methacrylate) (gPDMAEMA),
respectively, as shown schematically in Figure 2-2. Additional ungrafted PMAA and
PDMAEMA chains were removed by repeatedly washing the paper in water.
Table 2-1 - Contact angle and lateral flow rate data of uncoated and coated paper. Grafted
coatings demonstrate an increased hydrophilicity relative to cross-linked coatings.
Polymer Coating Contact Angle (°) Lateral Flow Rate (mm/min)
Uncoated 0 12.8 ± 2.0
xPMAA 102 ± 5 0
gPMAA 0 6.4 ± 0.8
xPDMAEMA 96 ± 5 0
gPDMAEMA 0 2.5 ± 0.3

Figure 2-2 - Schematic of the grafting process. Cross-linked polymer is deposited onto the
paper fibers followed by deposition of homopolymer, which results in grafting. Additional
ungrafted homopolymer is removed by repeatedly washing the paper in water.
37

The gPMAA coating contained a significantly higher amount of MAA (MAA/EGDMA ~6.0) on
the surface compared to xPMAA (MAA/EGDMA ~0.6) as determined by comparing the relative
area of the carboxyl and ester carbon to the carbon singly bound to oxygen in the C 1s XPS
spectrum, which is consistent with grafting of PMAA to xPMAA. Similarly, the gPDMAEMA
coating (atomic ratios of carbon, nitrogen, and oxygen of 67 %, 5 %, and 28 %, respectively, as
determined by XPS) had a higher DMAEMA to EGDMA ratio (DMAEMA/EGDMA ~2.2) than
the xPDMAEMA coating (DMAEMA/EGDMA ~0.5), which is also indicative of successful
grafting. Analysis of the paper coated with gPDMAEMA indicated nitrogen was present in
similar atomic ratios on both the top and bottom sides of the paper, demonstrating that the
polymer conformally coated through the depth of the paper. The grafting process increased the
hydrophilicity of the polymer coatings relative to the cross-linked polymers as demonstrated by a
decrease in the static contact angle and increase in the lateral flow rate of water through the
coated paper (Table 2-1). While the lateral flow rates of water through channels coated with
either gPMAA or gPDMAEMA were lower than through uncoated paper, the flow rates were
sufficient for practical paper-based microfluidic applications. In addition to increasing the
hydrophilicity of the polymer coatings, the grafting process also increased the surface
concentration of ionizable MAA and DMAEMA moieties which should lead to improved
separation. The scanning electron microscopy images shown in Figure 2-3 demonstrate that the
gPMAA and gPDMAEMA coatings did not occlude the pores allowing for the retention of the
fibrous morphology that is important to generate capillary driven flow.
38

Figure 2-3 - SEM images of a) uncoated paper, b) paper coated with gPMAA, and c) paper
coated with gPDMAEMA.

The ability of paper-based microfluidic devices coated with either gPMAA or gPDMAEMA to
separate analytes based on electrostatic interactions was studied as a function of the pH of the
diagnostic solution using organic dyes as model analytes. The degree of separation was
determined by the relative ratio of the migration distance of the dye to the migration distance of
the solvent front, known as the retardation factor (R
f
). The value of R
f
on coated paper channels
was compared to uncoated paper channels to account for any changes in dye solubility as a
function of pH, and the results are shown in Figure 2-4. It should be noted that cellulose displays
electronegative hydroxyl groups although significant deprotonation is not expected at pH values
less than 10.
124

39

Figure 2-4 - R
f
values of a) toluidine blue O, b) crystal violet, c) tartrazine, d) ponceau S, e)
methyl orange, and f) brilliant blue G on uncoated paper channels (blue diamond), paper
channels coated with gPMAA (green triangle), and paper channels coated with gPDMAEMA
(red square).

Toluidine blue O and crystal violet were used as model cationic analytes. The paper channels
coated with gPMAA demonstrated improved separation (lower R
f
values) relative to the
uncoated channels, leading to almost complete partitioning of the analytes to the stationary
phase. This is attributed to an increased electrostatic attraction between the cationic dyes and the
electronegative carboxylic acid groups of gPMAA. The effect of pH on the separation ability of
gPMAA is negligible despite PMAA having a pK
a
value of approximately 5.5.
125
This may be
due to partial ionization of the gPMAA at pH 4
126
as well as increased hydrogen bonding
effects
127,128
as the degree of ionization of the gPMAA decreases. Alternatively, the paper
40

channels coated with gPDMAEMA generally demonstrated slightly worse separation (higher R
f

values) relative to the uncoated channels due to the gPDMAEMA coatings bearing less
electronegative functionality than cellulose. Tartrazine and ponceau S were used as model
anionic analytes. The channels coated with gPDMAEMA exhibited significantly improved
separation of the anionic analytes at acidic pH values, but comparable or poorer separation at
basic pH values. This is attributed to protonation of the gPDMAEMA (pK
a
value of ~8)
129
only
at more acidic pH values which significantly increases the electrostatic interaction between the
cationic gPDMAEMA and the anionic analytes. Conversely, the paper channels coated with
gPMAA demonstrated comparable separation of the anionic analytes relative to the uncoated
channels due to the fact that both gPMAA and cellulose do not display significant electropositive
functional groups. Methyl orange and brilliant blue G were used to model amphoteric analytes
that contain both acidic and basic functional groups. The channels coated with gPDMAEMA
demonstrated moderately improved separation. This suggests that these amphoteric analytes
display an overall anionic character which is attributed to the presence of the highly
electronegative sulfonic acid groups. Similar to the separation of the anionic analytes, the
separation of these amphoteric analytes on the channels coated with gPDMAEMA is pH-
dependent with improved separation at more acidic pH values. The two dyes behaved differently
on channels coated with gPMAA, with the separation of methyl orange on channels coated with
gPMAA being comparable to separation on uncoated channels and separation of brilliant blue G
on channels coated with gPMAA being significantly reduced at more basic pH values. It is
possible that amphoteric analytes which display more electropositive character will show
reduced separation on gPDMAEMA coatings and improved separation on gPMAA coatings.
41

We demonstrated the applicability of these coatings for paper-based microfluidic applications by
separating analytes from a multi-component mixture. Figure 2-5 shows the effect of gPMAA
and gPDMAEMA coatings on the separation of a mixture of cationic toluidine blue O (blue) and
anionic ponceau S (red) at pH 4. Initially, the applied dye is purple due to a combination of the
individual dyes (Figure 2-5a). On uncoated paper channels (Figure 2-5b), the two dyes move at
approximately the same speed and the dye area remains purple. On paper channels coated with
gPMAA (Figure 2-5c), the polymer coating separates the cationic toluidine blue O, resulting in
the red ponceau S flowing upwards from the toluidine blue O. On paper channels coated with
gPDMAEMA (Figure 2-5d), the reverse case is observed where the cationic polymer separates
the anionic ponceau S resulting in the toluidine blue O flowing upwards from the stationary red
ponceau S. This demonstrates the ability to selectively separate either anionic or cationic
analytes from a mixture of components, and could easily be extended to biologically relevant
systems.

Figure 2-5 - a) Applied mixture of toluidine blue O and ponceau S on an uncoated channel and
separation on b) an uncoated channel, c) a channel coated with gPMAA, and d) a channel coated
with gPDMAEMA. Arrow depicts direction of fluid flow.

42

The iCVD process allows us to apply polymer coatings without altering the morphology of the
paper substrate and therefore multiple coatings can be deposited to integrate different unit
operations onto a single device. We demonstrated this by sequentially applying coatings of
gPMAA and PoNBMA onto a paper channel to integrate separation of analytes and fluid
manipulation, respectively, as shown in Figure 2-6. PoNBMA is a hydrophobic polymer that is
converted to hydrophilic PMAA upon exposure to UV-light and has previously been deposited
onto chromatography paper using the iCVD process.
109
The response of PoNBMA to UV-light
allows for both patterning and real-time operation of the switch. The PoNBMA switch was
patterned by exposure to UV-light through a mask and washing in deionized water, revealing the
underlying gPMAA coating, shown schematically in Figure 2-6a. The remaining hydrophobic
PoNBMA region restricts aqueous solutions to the inlet portion of the channel only. The switch
can be activated by exposing the PoNBMA region to UV-light allowing for aqueous fluids to
travel up the length of the channel and subsequently allow analytes to be separated by the ion-
exchange coating. While the response time of the PoNBMA switch is approximately one hour,
this can be reduced by using a higher intensity UV-light. Figure 2-6b demonstrates the
controlled separation of toluidine blue O and ponceau S on gPMAA after activation of the
switch.
43

Figure 2-6 - a) Deposition and patterning of a UV-responsive switch onto microfluidic channels
coated with gPMAA. The switch is patterned by masking a portion of the channel during
exposure. b) Activation of the switch and subsequent separation of toluidine blue O and ponceau
S.

44

2.5 Conclusions
The iCVD process was used to conformally deposit functional polymer coatings onto paper-
based microfluidic devices to integrate multiple unit operations while retaining the morphology
of the paper. Ionizable polymer coatings to enhance separations were composed of a layer of
cross-linked polymer and a layer of homopolymer which was grafted onto the cross-linked layer.
The grafting process was found to be necessary to increase the hydrophilicity of the polymer
coatings in order to allow them to be used in paper-based microfluidic applications and also to
increase the surface concentration of the ionizable groups to enhance separation.
Chromatography paper coated with gPMAA and gPDMAEMA improved the separation of
cationic and anionic analytes, respectively, relative to uncoated chromatography paper and the
ability to separate analytes from a multi-component mixture was demonstrated. The ability to
integrate multiple unit operations was demonstrated by sequentially patterning a UV-responsive
switch onto the channels coated with ion-exchange coatings, allowing for control of the path of
fluids. This work extends the available advanced unit operations for paper-based microfluidics
and allows for more complex diagnostics and for external manipulation of fluids. Additionally,
the pH-responsive nature of the ionizable coatings suggests that these coatings can also be used
to control the release of analytes for signal amplification and multi-step processes in paper-based
microfluidic applications.
98,117

45

2.6 Acknowledgements
This work was supported by the National Science Foundation Division of Civil, Mechanical, and
Manufacturing Innovation Award Number 1069328, the Natural Sciences and Engineering
Research Council of Canada Scholarship (P.K.), and the Alfred Mann Institute at the University
of Southern California (P.K.). We thank the Molecular Materials Research Center of the
Beckman Institute of the California Institute of Technology for use of their XPS.

46

2.7 Supporting Information
Table 2-S1 - Retardation factors (R
f
) and standard deviations (S.D.) for each dye on unmodified
paper, paper coated with gPMAA and paper coated with gPDMAEMA. S.D. values were
determined experimentally from triplicate measurements.
pH 4 pH 6 pH 8 pH 10
R
f
S.D. R
f
S.D. R
f
S.D. R
f
S.D.
Unmodified
Cationic Dyes
Toluidine Blue O 0.23 0.01 0.20 0.01 0.13 0.00 0.06 0.00
Crystal Violet 0.04 0.01 0.14 0.01 0.18 0.01 0.19 0.1
Anionic Dyes
Tartrazine 0.78 0.00 0.45 0.03 0.48 0.01 0.89 0.02
Ponceau S 0.31 0.00 0.22 0.00 0.28 0.01 0.34 0.02
Amphoteric Dyes
Methyl Orange 0.50 0.01 0.32 0.02 0.30 0.01 0.35 0.00
Brilliant Blue G 0.21 0.00 0.14 0.01 0.16 0.03 0.18 0.01
gPMAA
Cationic Dyes
Toluidine Blue O 0.05 0.01 0.01 0.00 0.01 0.00 0.02 0.01
Crystal Violet 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00
Anionic Dyes
Tartrazine 0.79 0.02 0.55 0.02 0.59 0.04 0.75 0.05
Ponceau S 0.43 0.01 0.33 0.01 0.27 0.01 0.38 0.01
Amphoteric Dyes
Methyl Orange 0.33 0.00 0.37 0.01 0.30 0.01 0.36 0.02
Brilliant Blue G 0.07 0.00 0.36 0.01 0.82 0.07 1.00 0.00
gPDMAEMA
Cationic Dyes
Toluidine Blue O 0.29 0.01 0.23 0.00 0.17 0.01 0.05 0.00
Crystal Violet 0.11 0.00 0.22 0.00 0.22 0.00 0.07 0.01
Anionic Dyes
47

Tartrazine 0.15 0.00 0.14 0.01 0.42 0.01 0.88 0.09
Ponceau S 0.02 0.01 0.04 0.00 0.10 0.02 0.54 0.02
Amphoteric Dyes
Methyl Orange 0.22 0.01 0.17 0.00 0.19 0.01 0.38 0.00
Brilliant Blue G 0.00 0.00 0.00 0.01 0.02 0.01 0.16 0.01

Table 2-S2 - Capacity factors (k’) and standard deviations (S.D.) for each dye on unmodified
paper, paper coated with gPMAA and paper coated with gPDMAEMA. S.D. values were
determined from the standard deviations of the R
f
values using standard error propagation.
130

pH 4 pH 6 pH 8 pH 10
k’ S.D. k’ S.D. k’ S.D. k’ S.D.
Unmodified
Cationic Dyes
Toluidine Blue O 3.26 0.21 4.07 0.17 6.79 0.37 14.71 1.43
Crystal Violet 25.96 13.73 6.37 0.64 4.69 0.52 4.13 0.34
Anionic Dyes
Tartrazine 0.28 0.00 1.23 0.10 1.10 0.05 0.12 0.00
Ponceau S 2.19 0.00 3.60 0.04 2.63 0.20 1.94 0.14
Amphoteric Dyes
Methyl Orange 1.02 0.03 2.13 0.17 2.36 0.10 1.82 0.03
Brilliant Blue G 3.66 0.07 5.93 0.65 5.15 1.24 4.67 0.27
gPMAA
Cationic Dyes
Toluidine Blue O 20.81 5.12 88.82 33.63 85.75 29.63 42.03 14.47
Crystal Violet --- --- --- --- 66.65 31.49 --- ---
Anionic Dyes
Tartrazine 0.27 0.01 0.80 0.04 0.69 0.06 0.34 0.03
Ponceau S 1.34 0.04 2.07 0.13 2.71 0.11 1.66 0.05
Amphoteric Dyes
Methyl Orange 2.07 0.04 1.68 0.06 2.30 0.14 1.82 0.16
Brilliant Blue G 13.92 0.75 1.74 0.07 0.22 0.03 0.00 0.00
48

gPDMAEMA
Cationic Dyes
Toluidine Blue O 2.44 0.11 3.28 0.05 4.97 0.36 20.00 0.82
Crystal Violet 8.24 0.40 3.46 0.08 3.50 0.03 13.86 1.48
Anionic Dyes
Tartrazine 5.68 0.05 6.12 0.45 1.36 0.03 0.13 0.02
Ponceau S 53.41 21.44 23.25 1.89 8.83 2.28 0.85 0.04
Amphoteric Dyes
Methyl Orange 3.52 0.30 5.03 0.05 4.38 0.28 1.64 0.03
Brilliant Blue G --- --- 290.75 712.19 61.18 43.57 5.29 0.54

49

2.8 References

85 Urdea, M.; Penny, L. A.; Olmsted, S. S.; Giovanni, M. Y.; Kaspar, P.; Shepherd, A.;
Wilson, P.; Dahl, C. A.; Buchsbaum, S.; Moeller, G.; Hay Burgess, D. C. Nature, 2006,
S1, 73-79.
86 Chin, C. D.; Linder, V.; Sia, S. K. Lab Chip, 2007, 7, 41-57
87 Yager, P.; Edwards, T.; Fu, E.; Helton, K.; Nelson, K.; Tam, M. R.; Weigl, B. H. Nature,
2006, 442, 412-418.
88 Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Anal. Chem., 1998,
70, 4974-4984.
89 Varshney, M.; Li, Y.; Srinivasan, B.; Tung, S. Sensors and Actuators B, 2007, 128, 99-
107.
90 Ko, J. S.; Yoon, H. C.; Yang, H.; Pyo, H.-B.; Chung, K. H.; Kim, S. J.; Kim, Y. T. Lab
Chip, 2003, 3, 106-113.
91 Bi, H.; Meng, S.; Li, Y.; Guo, K.; Chen, Y.; Kong, J.; Yang, P.; Zhong, W.; Liu, B. Lab
Chip, 2006, 6, 769-775.
92 Martinez, A. W.; Phillips, S. T.; Butte, M. J.; Whitesides, G. M. Angew. Chem. Int. Ed.,
2007, 46, 1318-1320.
93 Lu, Y.; Shi, W.; Jiang, L.; Qin, J.; Lin, B. Electrophoresis, 2009, 30, 1497-1500.
94 Carrilho, E.; Martinez, A. W.; Whitesides, G. M. Anal. Chem., 2009, 81, 7091-7095.
95 Pelton, R. TrAC, Trends Anal. Chem., 2009, 28, 925-942.
96 Martinez, A. W.; Phillips, S. T.; Whitesides, G. M.; Carrilho, E. Anal. Chem., 2010, 82,
3-10.
50

97 Coltro, W. K. T.; de Jesus, D. P.; da Silva, J. A. F.; do Lago, C. L.; Carrilho, E.
Electrophoresis, 2010, 31, 2487-2498.
98 Osborn, J. L.; Lutz, B.; Fu, E.; Kauffman, P.; Steves, D. Y.; Yager, P. Lab Chip, 2010,
10, 2659-2665.
99 Focke, M.; Kosse, D.; Müller, C.; Reinecke, H.; Zengerle, R.; von Stetten, F. Lab Chip,
2010, 10, 1365-1386.
100 Carvalhal, R. F.; Kfouri, M. S.; de Oliveira Piazetta, M. H.; Gobbi, A. L.; Kubota, L. T.
Anal. Chem., 2010, 82, 1162-1165.
101 Yang, X.; Forouzan, O.; Brown, T. P.; Shevkoplyas, S. S. Lab Chip, 2012, 12, 274-280.
102 Martinez, A. W.; Phillips, S. T.; Nie, Z.; Cheng, C.-M.; Carrilho, E.; Wiley, B. J.;
Whitesides, G. M. Lab Chip, 2010, 10, 2499-2504.
103 Li, X.; Tian, J.; Nguyen, T.; Shen, W. Anal. Chem., 2008, 80, 9131-9134.
104 Chen, H.; Cogswell, J.; Anagnostopoulos, C.; Faghri, M. Lab Chip, 2012, 12, 2909-2913.
105 Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38,
9742-9748.
106 Ma, M.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K. K.; Cohen, R. E.; Rubner, M. F.;
Rutledge, G. C. Adv. Mater., 2007, 19, 255-259.
107 Gupta, M.; Kapur, V.; Pinkerton, N. M.; Gleason, K. K. Chem. Mater., 2008, 20, 1646-
1651.
108 Tenhaeff, W. E.; Gleason, K. K. Adv. Funct. Mater., 2008, 18, 979-992.
109 Haller, P. D.; Flowers, C. A.; Gupta, M. Soft Matter, 2011, 7, 2428-2432.
110 Kwong, P.; Flowers, C. A.; Gupta, M. Langmuir, 2011, 27, 10634-10641.
51

111 Andersen, T.; Pepaj, M.; Trones, R.; Lundanes, E.; Greibrokk, T. J. Chromatogr. A,
2004, 1025, 217-226.
112 Zeng, X.; Ruckenstein, E. J. Membr. Sci., 1998, 148, 195-205.
113 Weidlich, C.; Mangold, K.-M.; Jüttner, K. Electrochim. Acta, 2001, 47, 741-745.
114 Oldani, M.; Killer, E.; Miquel, A.; Schock, G. J. Membr. Sci., 1992, 75, 265-275.
115 Fuchs, B.; Süß, R.; Teuber, K.; Eibisch, M.; Schiller, J., J. Chromatogr A, 2011, 1218,
2754-2774.
116 Martinez, A. W.; Phillips, S. T.; Whitesides, G. M., PNAS, 2008, 105, 19606-19611.
117 Fu, E.; Kauffman, P.; Lutz, B.; Yager, P. Sens. Actuators, B, 2010, 149, 325-328.
118 Tang, E.; Cheng, G.; Ma, X.; Pang, X.; Zhao, Q. Applied Surface Science, 2006, 252,
5227-5232.
119 Ye, Y.; Mao, Y.; Wang, H.; Ren, Z. J. Mater. Chem., 2012, 22, 2449-2455.
120 Yang, J.-M.; Jong, Y.-J.; Hsu, K.-Y. J. Biomed. Mater. Res., 1997, 35, 175-180.
121 Yang, R.; Xu, J.; Ozaydin-Ince, G.; Wong, S. Y.; Gleason, K. K. Chem. Mater., 2011, 23,
1263-1272.
122 Zuyderhoff, E. M.; Dekeyser, C. M.; Rouxhet, P. G.; Dupont-Gillain, C. C. J. Colloid
Interface Sci., 2008, 319, 63-71.
123 Ye, Y.; Song, Q.; Mao, Y. J. Mater. Chem., 2011, 21, 13188-13194.
124 Saric, S. P.; Schofield, R. K. Proc. R. Soc. London, Ser. A, 1946, 185, 431-447.
125 Chao, GT.; Deng, HX.; Huang, Q.; Jia, WJ.; Huang, WX.; Gu, YC.; Tan, HP.; Fan, LY.;
Liu, CB.; Huang, AL.; Lei, K.; Gong, CY.; Tu, MJ.; Qian, ZY. J. Polym. Res., 2006, 13,
349-355.
52

126 Yu, S.; Chow, G. M., J. Mater. Chem., 2004, 14, 2781-2786.
127 Ohno, H.; Matsuda, H.; Tsuchida, E. Makromol. Chem., 1981, 182, 2267-2275.
128 Kharlampieva, E.; Sukhishvili, S. A., Macromolecules, 2003, 36, 9950-9956.
129 Patrickios, C. S.; Hertler, W. R.; Abbott, N. L.; Hatton, T. A. Macromolecules, 1994, 27,
930-937.
130 Drosg, M. Dealing with Uncertainties: A Guide to Error Analysis, 2nd Ed.; Springer,
2009.
53

3.0 Directed Deposition of Functional Polymers onto Porous Substrates Using
Metal Salt Inhibitors
Citation: Philip Kwong, Cristofer A. Flowers, and Malancha Gupta, Langmuir, 2011, 27, 10634-
10641

3.1 Abstract
This paper demonstrates the ability to control the location of polymer deposition onto porous
substrates using vapor phase polymerization in combination with metal salt inhibitors. Functional
polymers such as hydrophobic poly(1H,1H,2H,2H-perfluorodecyl acrylate), click-active
poly(pentafluorophenyl methacrylate), and light-responsive poly(ortho-nitrobenzyl methacrylate)
were patterned onto porous hydrophilic substrates using metal salts. A combinatorial screening
approach was used to determine the effects of different transition metal salts and reaction
parameters on the patterning process. It was found that CuCl
2
and Cu(NO
3
)
2
were effective at
uniformly inhibiting the deposition of all three polymers through the depth of the porous
substrate and along the entire cross-section. This study offers a new and convenient method to
selectively deposit a wide variety of functional polymers onto porous materials and will enable
the production of next-generation multifunctional paper-based microfluidic devices, polymeric
photonic crystals, and filtration membranes.

54

3.2 Introduction
The development of next-generation microfluidic devices,
131,132
biological sensors,
132,133
and
photonic devices
134,135
requires the ability to selectively deposit functional polymers onto porous
materials. For example, deposition of temperature-responsive and click-active polymers onto
filtration membranes and paper-based microfluidic devices will enhance their current
capabilities. Patterning nonplanar surfaces is much more challenging than patterning flat
surfaces. Current patterning techniques often involve top-down processing techniques such as
photolithography which requires specific light-sensitive chemical moieties
131
and reactive
etching which requires multiple processing steps.
134,135
In this paper, we introduce a novel and
versatile bottom-up patterning technique that uses solventless initiated chemical vapor deposition
(iCVD) in combination with transition metal salt inhibitors.
The iCVD process is a one-step, solventless, substrate-independent process that can be used to
deposit a wide variety of organic polymer coatings. In the iCVD process, vapors of monomer
and initiator are flown into a vacuum reactor where a heated filament array decomposes the
initiator into free-radicals. The free-radicals and monomer molecules adsorb onto the surface of
a cooled substrate where polymerization occurs via a free-radical mechanism. The advantages of
using iCVD over liquid phase polymerization are: (i) iCVD is an environmentally benign process
because it does not require the use of organic solvents; (ii) iCVD does not suffer from surface
tension problems such as dewetting and clogging
136,137,138
and therefore can be used to coat
complex structures such as carbon nanotubes,
139
membranes,
140
and electrospun fiber mats;
137,141

(iii) iCVD can be used to deposit a wide range of polymers that exhibit functionalities such as
55

hydrophobicity
142
, chemical reactivity
143
and photo-responsiveness;
144
and (iv) iCVD can be
scaled up for large-scale roll-to-roll processing.
145

Although the iCVD technique has been used to pattern polymers onto flat surfaces using
colloidal lithography
146
, electron-beam lithography,
147,148
and capillary force lithography,
149

these techniques are not amenable to nonplanar substrates and a general technique to pattern
polymers onto porous materials has not yet been demonstrated. Unlike other vapor deposition
techniques, the iCVD process is not a line-of-sight process and deposition occurs in all
directions. Figure 3-1 shows our attempt to pattern the deposition of hydrophobic
poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) onto porous chromatography paper using
a physical mask. The PPFDA deposited onto the entire sheet indicating that we could not restrict
the deposition to regions directly exposed to the vapor. The monomer precursor and initiator
radicals diffuse isotropically throughout the porous paper due to their large mean free path.
Since physical masking did not work to pattern deposition onto porous materials, we developed
an alternative method that involved photolithographic patterning.
144
We used iCVD to deposit
light-sensitive poly(ortho-nitrobenzyl methacrylate) (PoNBMA) onto porous chromatography
paper and then selectively removed the PoNBMA by cleaving the nitrobenzyl moieties using
ultra-violet light and dissolving the exposed polymer in pH 8 buffer. This top-down patterning
method can only be used with light-sensitive polymers such as PoNBMA. In this paper, we
introduce a bottom-up patterning method that can be used to control the location of deposition of
a wide variety of acrylate-based polymers irrespective of their chemical composition.
56

Figure 3-1 - A) A physical mask with holes was placed on top of porous chromatography paper
and placed into the iCVD chamber. B) After iCVD deposition of PPFDA, dyed water was
repelled over the entire surface of the paper, indicating that the physical mask cannot be used to
control the location of growth onto porous substrates. Scale bars represent 1 cm.

Metal compounds have been known to inhibit polymerization in a wide range of systems. In a
1952 patent, Taylor demonstrated that nitrites of alkali and alkaline earth metals inhibited the
polymerization of viscous monomer and polymer mixtures of both methyl methacrylate and
styrene.
150
Inskip and Patane inhibited the solution phase polymerization of a variety of
unsaturated hydrocarbons, hydrocarbyl acids, and hydrocarbyl esters using cobalt(III), nickel(II),
and manganese(II) N-nitrosophenylhydroxylamine compounds.
151
More recently, Jensen and
coworkers used transition metals and transition metal salts to inhibit the vapor phase deposition
57

of Parylene-based polymers to produce spatially controlled two-dimensional polymer coatings on
flat surfaces.
152,153,154,155
The vapor phase deposition of Parylene occurs via pyrolytic
polymerization where di-p-xylylene precursors are cleaved at high temperatures to form reactive
quinonoid intermediates, which self-react to form Parylene.
156
Jensen and coworkers proposed a
mechanism in which the inhibiting metal reversibly complexes to the quinonoid intermediate;
this causes deactivation of the intermediate and thereby prevents it from participating in
initiation or propagation reactions.
In this paper, we demonstrate that transition metal salt inhibitors can be used in conjunction with
the iCVD process to pattern functional acrylate-based polymers onto porous substrates.
Specifically, we show that we can pattern PPFDA, PoNBMA, and poly(pentafluorophenyl
methacrylate) (PPFM) onto porous chromatography paper. Furthermore, we use a combinatorial
screening process for high-throughput examination of the effect of the cation and the anion of the
metal salt, the monomer, the concentration of the transition metal salt, the monomer flow rate,
and the deposition time on the inhibition process. The polymers patterned herein are useful in a
broad range of applications as they have very diverse functionalities. The hydrophobic nature of
PPFDA can be exploited to make water-repellent and self-cleaning surfaces.
157,158
The highly
reactive ester group of PPFM can be used to covalently attach biological molecules such as
immunoglobulin G
159
and biotin ligands.
160

58

3.3 Experimental
Iron(III) chloride (FeCl
3
) (Aldrich, 97%), iron(III) nitrate (Fe(NO
3
)
3
) nonahydrate (Aldrich,
>98%), copper(II) chloride (CuCl
2
) dihydrate (Aldrich, reagent grade), copper(II) nitrate
(Cu(NO
3
)
2
) hydrate (Aldrich, 99.999%), ruthenium(III) chloride (RuCl
3
) hydrate (Aldrich,
reagent grade), cobalt(II) chloride (CoCl
2
) (Aldrich, 97%), 1H,1H,2H,2H-perfluorodecyl acrylate
(PFDA) (Aldrich, 97%), ortho-nitrobenzyl methacrylate (oNBMA) (Polysciences, 95%),
pentafluorophenyl methacrylate (PFM) (Synquest, 97%), and tert-butyl peroxide (TBPO)
(Aldrich, 98%), were used as received without further purification. Aqueous solutions of FeCl
3,
Fe(NO
3
)
3
, CuCl
2
, Cu(NO
3
)
2
, RuCl
3
, and CoCl
2
were prepared by dissolving the metal salts in
deionized water.
A commercially available wax printer (Xerox Phaser 8560N color printer) was used to print 1.45
cm x 1.45 cm squares onto Whatman No. 1 chromatography paper (VWR) as described by Lu et
al.
161
and Carrilho et al.
162
The printed lines had a width of 0.56 ± 0.01 mm. In order to
generate individual compartments, the paper was heated for 10 minutes at approximately 160°C
using a hot plate (VWR). The wax melted through the depth of the paper (approximately 180
μm) and spread isotropically. The width of the line increased to 1.74 ± 0.06 mm and 1.57 mm ±
0.07 mm on the top and bottom of the paper, respectively. The final inner dimensions of the
melted square compartments on the top measured 1.34 cm x 1.34 cm. Twenty-five μL of metal
salt solution was applied to each square compartment to ensure complete liquid coverage. The
solutions were applied by placing one 5 μL droplet in the center of the square and one 5 μL
droplet at each corner of the square. This method generated a more uniform salt distribution
compared to placing 25 μL of solution directly in the center of the square. Samples were
59

allowed to dry for a minimum of one hour under ambient conditions prior to placement into the
iCVD chamber.
The paper samples were taped onto the inside stage of a custom designed pancake-shaped
reaction chamber (GVD Corporation, 25.0 cm in diameter, 4.8 cm in height). The pressure in the
reaction chamber was maintained using a rotary vane vacuum pump (Edwards E2M40) and
controlled using a throttle valve (MKS 153D). The pressure in the reaction chamber was
monitored using a capacitance manometer (Baratron 622A01TDE). The reactor stage was kept at
a constant temperature using a back-side recirculating heat exchanger. The top of the reaction
chamber was composed of removable quartz glass that allowed for sample loading and visual
inspection during deposition. The quartz top was covered with aluminum foil during the
deposition of PoNBMA to prevent ambient light from entering the reactor. A stainless steel
monomer source jar was located 23.5 cm from the edge of the reaction chamber and was
opposite to the vacuum pump. The monomer source jar was maintained at a constant temperature
using an external electrical heating jacket (Watlow 0903C-14) and was fed into the reaction
chamber through externally heated (Watlow 0934C-57) stainless steel piping (12.7 mm
diameter). A bellows sealed valve (12.7 mm diameter) was used to isolate the monomer source
jar from the reaction chamber. The monomer flow rate was controlled by varying the source
temperature and was determined based on the rate of the increase in pressure in the chamber. The
TBPO source jar was located 42.0 cm from the edge of the reaction chamber and was opposite to
the vacuum pump. For all depositions, TBPO was kept at room temperature and fed into the
reaction chamber through stainless steel piping (6.35 mm diameter). The TBPO flow rate was
controlled using a mass flow controller (MKS 1479A) to achieve a flow rate of approximately
60

1.0 sccm. A nichrome filament array (Omega Engineering, 80%/20% Ni/Cr) was resistively
heated between 200°C and 225°C as measured by a thermocouple (Omega Engineering, K-type)
to decompose the TBPO initiator into radicals. The distance between the filament array and the
substrate was kept constant at 32 mm. The specific reactor conditions for the experiments are
summarized in Table 3-1. Contact angle goniometry (Ramé-hart Model 290-F1) was used to
study the surface properties of the substrates. High resolution images of the chromatography
paper were taken using a JEOL-6610 low vacuum scanning electron microscope.
Table 3-1 - Reaction conditions.
Metal Screening Series
Polymer
Stage
Temp.
(°C)
Monomer
Source
Temp. (°C)
Monomer
Flow Rate
(sccm)
Initiator
Flow Rate
(sccm)
Total
Pressure
(mTorr)
Deposition
Time
(min.)
1a PPFDA 40.0 50.0 0.7 1.0 110 15
1b PPFM 30.0 23.0 1.7 1.0 200 15
1c PoNBMA 22.0 85.0 0.05 1.0 120 120
Monomer Flow Rate Series
Polymer
Stage
Temp.
(°C)
Monomer
Source
Temp. (°C)
Monomer
Flow Rate
(sccm)
Initiator
Flow Rate
(sccm)
Total
Pressure
(mTorr)
Deposition
Time
(min.)
2a PPFDA 40.0 42.5 0.4 1.0 110 15
2b PPFDA 40.0 50.0 0.7 1.0 110 15
2c PPFDA 40.0 60.0 0.9 1.0 110 15
Deposition Time Series
Polymer
Stage
Temp.
(°C)
Monomer
Source
Temp. (°C)
Monomer
Flow Rate
(sccm)
Initiator
Flow Rate
(sccm)
Total
Pressure
(mTorr)
Deposition
Time
(min.)
3a PPFDA 40.0 50.0 0.7 1.0 110 30
3b PPFDA 40.0 50.0 0.7 1.0 110 45
3c PPFDA 40.0 50.0 0.7 1.0 110 60

61

X-ray photoelectron spectroscopy (XPS) was performed using a Surface Science Instruments M-
Probe spectrometer with a monochromatic Al Kα X-ray source. Low resolution survey spectra
for analysis of PoNBMA on paper were acquired between binding energies of 1-1000 eV with a
resolution of 1 eV. High resolution spectra for analysis of the metal salts were acquired with a
resolution of 0.065 eV. Data analysis was performed using the ESCA25 Analysis Application
(V5.01.04) software. Metal salts were drop-casted onto silicon wafers for XPS analysis of the
inhibition mechanism. Circular patterns were drawn onto silicon wafers using a permanent
marker (black Sharpie) in order to prevent the solution from spreading. Twenty μL of 250 mM
aqueous metal salt solutions were pipetted onto the center of the circular patterns. The substrate
was heated to 75°C to evaporate the water, resulting in dried adhered salt. PPFM was deposited
onto the substrate using the reactor condition 1b in Table 3-1 with the deposition time increased
to 50 minutes.

62

3.4 Results and Discussion
We examined the deposition of poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA),
poly(pentafluorophenyl methacrylate) (PPFM) and poly(ortho-nitrobenzyl methacrylate)
(PoNBMA) onto porous chromatography paper. Chromatography paper is naturally hydrophilic,
however the paper becomes hydrophobic when PPFDA, PPFM, or PoNBMA is uniformly
deposited over the entire surface. We can therefore use visual analysis of the contact angle of
water to determine if polymerization has occurred or been inhibited. Figure 3-2 shows an
example of our analysis method using the reaction conditions described in Table 3-1 (conditions
1a-1c) and a 2.0 M aqueous solution of FeCl
3
as the inhibiting transition metal salt. Without the
use of the inhibiting salt, polymerization occurs on the chromatography paper and the surface
becomes hydrophobic. If the inhibiting salt is used, polymerization does not occur and the paper
remains hydrophilic and wets.
63

Figure 3-2 - Contact angles of water on untreated paper and paper treated with a 2.0 M solution
of FeCl
3
after iCVD deposition of PPFDA, PPFM, and PoNMBA. Without the use of the salt,
polymerization occurs on the untreated paper and the surface becomes hydrophobic. When the
inhibiting salt is used, polymerization does not occur and the paper remains hydrophilic and
wets.

The inhibition capability of different transition metal salts was combinatorially screened by
dividing the porous chromatography paper into individual square compartments and treating
each compartment with 2.0 M aqueous solutions of FeCl
3
, Fe(NO
3
)
3
, CuCl
2
, Cu(NO
3
)
2
, RuCl
3

and CoCl
2
. The individual compartments were fabricated by melting wax barriers through the
depth of the paper as described in detail in the experimental section. Figure 3-3 shows a
schematic of the setup and the structure of the monomer precursors. Our combinatorial approach
ensured that the reaction conditions were identical when comparing the effects of different
64

transition metal salts. We compared the wetting behavior through the depth of the paper and
along the edges in order to test the uniformity of the inhibition process. Table 3-2 shows the
inhibiting behavior of each metal salt at the center and along the edges on both the top and the
bottom of the paper with respect to PPFDA, PPFM, and PoNBMA. An entry of “I” indicates that
the deposition of polymer was inhibited and an entry of “P” indicates that deposition occurred.
When deposition occurred, water did not penetrate into the paper. An entry of “I” was given
when water penetrated into the paper irrespective of the rate of penetration. The rate of
penetration was generally faster for squares that exhibited uniform inhibition. The shaded
regions in Table 3-2 indicate uniform inhibition through the depth of the paper and along the
edges. CuCl
2
and Cu(NO
3
)
2
were able to uniformly inhibit the deposition of all three polymers.
Figure 3-4 shows the wetting behavior at the top center of the square after deposition of PPFM.
CuCl
2
, Cu(NO
3
)
2
, and CoCl
2
were able to uniformly inhibit polymerization whereas Fe(NO
3
)
3

and RuCl
3
did not. Although FeCl
3
was able to inhibit polymerization at the top center of the
square, the inhibition was not uniform. The cation appears to affect the ability of a metal
compound to inhibit polymer deposition since metal compounds with similar oxidation states and
anions did not behave similarly. For example, CuCl
2
uniformly inhibited the deposition of
PPFDA while polymer growth occurred in the presence of CoCl
2
,

and

FeCl
3
uniformly inhibited
the deposition of PoNBMA while polymer growth occurred in the presence of RuCl
3
. Of the salts
tested, RuCl
3
was the least effective at uniformly inhibiting polymerization. Some salts were
effective at uniformly inhibiting polymerization of only certain polymers. For example,
Fe(NO
3
)
3
was able to uniformly inhibit the deposition of PPFDA and PoNBMA but not PPFM
whereas CoCl
2
was only able to inhibit the deposition of PPFM. These results suggest that
specific combinations of cation and monomer result in inhibition which is consistent with the
65

inhibition of Parylene-based polymers in which there are strong effects depending on the metal
cation and precursor molecule.
154,163
For example, Lahann and coworkers found that the
deposition of reactive vinyl-containing Parylene-based polymers could be inhibited, while the
deposition of Parylene-based polymers that contain oxygen and nitrogen could not be
inhibited.
163

It is important to note that several salts such as CuCl
2
and Cu(NO
3
)
2
could uniformly
inhibit the deposition of all three of our polymers which indicates that we have found a general
technique to pattern iCVD polymers irrespective of their chemical functionality.

66

Figure 3-3 - A) Schematic of the experimental setup. The inhibition capability of different
transition metal salts was combinatorially screened by dividing the porous chromatography paper
into individual square compartments. B) Chemical structures of the monomer precursors used in
this study.

67

Table 3-2 - Summary of the effects of the metal salts on the inhibition through the depth of the
paper and along the edges for PPFDA, PPFM, and PoNBMA. “I” indicates that polymerization
was inhibited while “P” indicates that polymerization occurred, “T” indicates the top of the
paper, and “B” indicates the bottom of the paper. The shaded regions indicate uniform inhibition
through the depth and along the edges.

Polymer

Location
Metal Salt
Cu(NO
3
)
2
CuCl
2
Fe(NO
3
)
3
FeCl
3
CoCl
2
RuCl
3

T B T B T B T B T B T B
PPFDA
(1a)
Center I I I I I I I I P P P I
Edge I I I I I I P P P P I I
PPFM
(1b)
Center I I I I P P I I I I P P
Edge I I I I P P P I I I P P
PoNBMA
(1c)
Center I I I I I I I I I I P I
Edge I I I I I I I I P I I I

Figure 3-4 - Wetting behavior after deposition of PPFM using 2.0 M salt solutions. The square
compartments measure 1.34 cm x 1.34 cm and the dotted gray circles indicate the location of
beaded water droplets. Polymerization occurs on the untreated paper surrounding the square
compartments.

68

X-ray photoelectron spectroscopy (XPS) was used to examine the chemical composition on the
chromatography paper in order to confirm inhibition. Figures 3-5A and 3-5B show the XPS
survey scans of the top and the bottom of chromatography paper after PoNBMA deposition. The
atomic percentages calculated from the XPS scans are 70.6% carbon, 5.0% nitrogen, and 24.4%
oxygen on the top and 68.4% carbon, 4.0% nitrogen, and 27.6% oxygen on the bottom. These
values are in close agreement to the theoretical atomic percentages calculated from the chemical
formula of the oNBMA monomer (68.75% carbon, 6.25% nitrogen, and 25.00% oxygen),
confirming that the PoNBMA coating is uniform through the depth of the paper. Figures 3-5C
and 3-5D show the XPS survey scans of the top and the bottom of chromatography paper treated
with a 2.0 M solution of FeCl
3
after PoNBMA deposition. The lack of a nitrogen peak in both
spectra verifies that inhibition occurs uniformly through the depth of the paper. Figure 3-6 shows
the scanning electron microscopy images of plain chromatography paper, paper after deposition
of PoNBMA, paper treated with a 2.0 M solution of FeCl
3
, and paper treated with a 2.0 M
solution of FeCl
3
after PoNBMA inhibition. There is no variation amongst the images hence the
addition of polymer and salt does not change the porosity or morphology of the paper.
69

Figure 3-5 - A,B) XPS survey scans of the top and bottom of chromatography paper after
PoNBMA deposition, respectively. The presence of the nitrogen 1s peak indicates PoNBMA
deposition. C,D) XPS survey scans of the top and bottom of paper treated with a 2.0 M solution
of FeCl
3
after PoNBMA deposition, respectively. The lack of a nitrogen peak in both spectra
verifies that inhibition occurs uniformly through the depth of the paper.

70

Figure 3-6 - SEM images of A,B) plain chromatography paper, C,D) paper after deposition of
PoNBMA, E,F) paper treated with a 2.0 M solution of FeCl
3
, and G,H) paper treated with a 2.0
M solution of FeCl
3
after PoNBMA inhibition. The addition of polymer and salt does not change
the porosity or morphology of the paper.

We chose to use CuCl
2
to further study the effect of the concentration of the inhibiting salt and
the reactor conditions on the inhibition process because of the ability of CuCl
2
to uniformly
inhibit the deposition of all three polymers through the depth of the paper and across the cross-
section. The individual compartments of the paper substrate were treated with different
concentrations of CuCl
2
solutions and the inhibition of PPFDA was studied as a function of the
71

monomer flow rate and the deposition time. In the iCVD process, it has been established that the
rate of polymer deposition is dependent on the concentration of monomer at the surface of the
substrate.
164,165
The monomer surface concentration is monotonically related to the ratio of the
monomer partial pressure, P
m
, to the saturation pressure, P
sat
. In order to determine whether
increasing the monomer surface concentration can lead to a loss of inhibition due to passivation
of the CuCl
2
molecules, the monomer source temperature was varied between 42.5°C and 60.0°C
(experiments 2a-2c in Table 3-3), which varied the value of P
m
/P
sat
from 0.19 to 0.31 For a 15
minute deposition, it was found that the minimum concentration of CuCl
2
required to uniformly
inhibit the deposition of PPFDA was 2.0 M for all values of P
m
/P
sat
. The inability of
compartments with lower concentrations of CuCl
2
to inhibit polymer deposition is likely due to
rapid passivation of the small number of CuCl
2
molecules.

72

Table 3-3 - Summary of the inhibition of PPFDA deposition through the depth of the paper and
along the edges with varying monomer source temperature (2a-2c) and deposition time (3a-3c).
“I” indicates that deposition was inhibited, “P” indicates polymer deposition occurred, “T”
indicates the top of the paper, and “B” indicates the bottom of the paper. The shaded regions
indicate uniform inhibition through the depth and along the edges.
Dep.
Time
(min.)
Monomer
Source
Temp
(°C)
P
m
/
P
Sat
Location
CuCl
2
Solution Concentration (M)
0 0.5 1.0 1.5 2.0 2.5
T B T B T B T B T B T B
15
(2a)
42.5 0.19
Center P P P P P P P I I I I I
Edge P P P I I I I I I I I I
15
(2b)
50.0 0.27
Center P P P P P P P I I I I I
Edge P P P P P P I I I I I I
15
(2c)
60.0 0.31
Center P P P P P P P P I I I I
Edge P P P P P P P P I I I I
30
(3a)
50.0 0.27
Center P P P P P P P P I I I I
Edge P P P P P P P P I I I I
45
(3b)
50.0 0.27
Center P P P P P P P P I I I I
Edge P P P P P P P P I P I I
60
(3c)
50.0 0.27
Center P P P P P P P P I I I I
Edge P P P P P P P P I P I P

For Parylene-based polymers, it was found that the deposition of polymer could not be inhibited
indefinitely and extended deposition times led to polymer growth.
154
In the proposed Parylene
mechanism, loss of inhibition resulted from secondary adsorption on top of the deactivated
precursor species that occupied available metal sites. It was hypothesized that a layer of
deactivated precursor molecules effectively shielded additional quinonoid intermediates from the
73

inhibiting metal. We studied the mechanism associated with inhibition in the iCVD process by
using XPS to examine the chemical composition of the salts before and after deposition. If
inhibition occurs though chemical reactions with the metal salts, we would expect a shift in the
location or a change in the shape of the peaks associated with the metal cation or anion. Figure 3-
S1 in the Supplemental Information shows examples of high resolution scans of the Cu 2p and Cl
2s peaks of CuCl
2
and Cu 2p and N 1s peaks of Cu(NO
3
)
2
before and after PPFM deposition.
There is no significant change in the location or shape of the peaks before and after deposition
indicating that inhibition most likely occurs through physical means and not covalent
interactions. The monomer most likely forms a reversible complex with the salt. Since the
deposition of all three acrylates can be inhibited, the complex is likely formed between the metal
salt and the acrylate moiety as opposed to the pendant side group.
To test if extended deposition times affected inhibition in the iCVD process, the deposition time
was increased from 15 minutes to 60 minutes (experiments 3a-3c in Table 3-3). As the duration
of deposition increased, higher concentrations of CuCl
2
were required to uniformly inhibit
polymer deposition. Figure 3-7 shows that solutions of at least 2.0 M CuCl
2
were required to
uniformly inhibit the deposition of PPFDA during a deposition of 15 minutes, whereas even
CuCl
2
solutions as high as 2.5 M were unable to uniformly inhibit deposition after 60 minutes as
shown by polymerization along the edges of the squares. The loss of uniform inhibition after
extended deposition times is likely due to the passivation of CuCl
2
by accumulation of
deactivated species which shield additional precursor molecules from the inhibiting metal salt.
74

Figure 3-7 - Inhibition through the depth of the paper and along the edges varies with deposition
time as shown by PPFDA deposited on paper treated with different concentrations of CuCl
2

solutions after A,B) 15 minutes of deposition and C,D) 60 minutes of deposition.

75

3.5 Conclusions
We have demonstrated a general method to control the location of polymer deposition onto
three-dimensional porous substrates using metal salt inhibitors. The generality of our process
was confirmed by demonstrating that the deposition of PPFDA, PPFM, and PoNBMA could be
inhibited by several transition metal salts such as CuCl
2
and Cu(NO
3
)
2
. The inhibition process
lost uniformity after extended deposition times likely due to the passivation of the salt by
accumulation of deactivated precursor molecules. Our study offers a new and convenient
method to deposit hydrophobic, click-active, and light-responsive polymers onto specific regions
of porous materials. In our study, the inhibiting metal salt was contained within compartments in
order to combinatorially screen the impact of reaction conditions, however inkjet printing will be
used to print the transition metal salts for future applications.
166,167
Future work involves using
our patterning process to develop multifunctional paper-based microfluidic devices and filtration
membranes.

76

3.6 Acknowledgments
This work was supported by the National Science Foundation Division of Civil, Mechanical, and
Manufacturing Innovation Award Number 1069328 and the National Sciences and Engineering
Research Council of Canada Scholarship (P.K.).

77

3.7 Supporting Information

Figure 3-S1 - High resolution X-ray photoelectron spectroscopy scans of the a) Cu 2p peak and
b) Cl 2s peak of CuCl
2
and the c) Cu 2p peak and d) N 1s peak of Cu(NO
3
)
2
. The bottom scan is
before PPFM deposition and the top scan is after PPFM deposition.

78

3.8 References

131 Lee, J.-T.; George, M. C.; Moore, J. S.; Braun, P. V. J. Am. Chem. Soc. 2009, 131,
11294-11295.
132 Martinez, A. W.; Phillips, S. T.; Butte, M. J.; Whitesides, G. M. Angew. Chem. Int. Ed.
2007, 46, 1318-1320.
133 Dungchai, W.; Chailapakul, O.; Henry, C. S. Anal. Chem. 2009, 81, 5821-5826.
134 Arsenault, A. C.; Clark, T. J.; von Freymann, G.; Cademartiri, L.; Sapienza, R.;
Bertolotti, J.; Vekris, E.; Wong, S.; Kitaev, V.; Manners, I.; Wang, R. Z.; John, S.;
Wiersma, D.; Ozin, G. A. Nat. Mater. 2006, 5, 179-184.
135 Li, Y. Y.; Kollengode, V. S.; Sailor, M. J. Adv. Mater. 2005, 17, 1249-1251.
136 Tenhaeff, W. E.; Gleason, K. K. Adv. Funct. Mater. 2008, 18, 979-992.
137 Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromol. 2005, 38, 9742-
9748.
138 Gupta, M.; Kapur, V.; Pinkerton, N. M.; Gleason, K. K. Chem. Mater. 2008, 20, 1646-
1651.
139 Lau, K. K. S.; Bico, J.; Teo, K. B. K; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.;
McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701-1705.
140 Gupta, M.; Gleason, K. K. Thin Solid Films 2009, 517, 3547-3550.
141 Ma. M.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K. K.; Cohen, R. E.; Rubner, M. F.;
Rutledge, G. C. Adv. Mater. 2007, 19, 255-259.
142 Gupta, M.; Gleason, K. K. Langmuir 2006, 22, 10047-10052.
79

143 O’Shaughnessy, W. S.; Marí-Buyé, N.; Borrós, S.; Gleason, K. K. Macromol. Rapid
Commun. 2007, 28, 1877-1882.
144 Haller, P. D.; Flowers, C. A.; Gupta, M. Soft Matter 2011, 7, 2428-2432.
145 Gupta, M.; Gleason, K. K. Thin Solid Films 2006, 515, 1579-1584.
146 Trujillo, N. J.; Baxamusa, S.; Gleason, K. K. Thin Solid Films 2009, 517, 3615-3618.
147 Im, S. G.; Kim, B. S.; Lee, L. H.; Tenhaeff, W. E.; Hammond, P. T.; Gleason, K. K.
Macromol. Rapid Commun. 2008, 29, 1648-1654.
148 Mao, Y.; Felix, N. M.; Nguyen, P. T.; Ober, C. K; Gleason, K. K. J. Vac. Sci. Technol. B
2004, 22, 2473-2478.
149 Im, S. G.; Bong, K. W.; Kim, B.-S.; Baxamusa, S. H.; Hammond, P. T.; Doyle, P. S.;
Gleason, K. K. J. Am. Chem. Soc. 2008, 130, 14424-14425.
150 Taylor, R. S. U.S. Patent 2,607,081, August 19, 1952.
151 Inskip, E. B.; Patane, J. M. U.S. Patent 4,638,079, January 20, 1987.
152 Vaeth, K. M.; Jensen, K. F. U.S. Patent 5,869,135, February 9, 1999.
153 Vaeth, K. M.; Jensen, K. F. Adv. Mater. 1999, 11, 814-820.
154 Vaeth, K. M.; Jensen, K. F. Chem. Mater. 2000, 12, 1305-1313.
155 Vaeth, K. M.; Jackman, R. J.; Black, A. J.; Whitesides, G. M.; Jensen, K. F. Langmuir
2000, 16, 8495-8500.
156 Gorham, W. F. J. Polym. Sci. Part A-1 1966, 4, 3027-3039.
157 Churchley, D.; Rees, G. D.; Barbu, E.; Nevell, T. G.; Tsibouklis, J. Int. J. Pharm. 2008,
352, 44-49.
80

158 Kumar, V.; Pulpytel, J.; Rauscher, H.; Mannelli, I.; Rossi, F.; Arefi-Khonsari, F. Plasma
Processes Polym. 2010, 7, 926-938.
159 Francesch, L.; Borrós, S.; Knoll, W.; Förch, R. Langmuir 2007, 23, 3927-3931.
160 Francesch, L.; Garreta, E.; Balcells, M.; Edelman, E. R.; Borrós, S. Plasma Processes
Polym. 2005, 2, 605-611.
161 Lu, Y.; Shi, W.; Jiang, L.; Qin, J.; Lin, B. Electrophoresis 2009, 30, 1497-1500.
162 Carrilho, E.; Martinez, A. W.; Whitesides, G. M. Anal. Chem. 2009, 81, 7091-7095.
163 Chen, H.-Y.; Lai, J. H.; Jiang, X.; Lahann, J. Adv. Mater. 2008, 20, 3474-3480.
164 Lau, K. K. S.; Gleason, K. K. Macromol. 2006, 39, 3688-3694.
165 Lau, K. K. S.; Gleason, K. K. Macromol. 2006, 39, 3695-3703.
166 Reddington, E.; Sapienza, A.; Gurau, B.; Viswanathan, R.; Sarangapani, S.; Smotkin, E.
S.; Mallouk, T. E. Science 1998, 280, 1735-1737.
167 Thomas III, S. W.; Chiechi, R. C.; LaFratta, C. N.; Webb, M. R.; Lee, A.; Wiley, B. J.;
Zakin, M. R.; Walt, D. R.; Whitesides, G. M., Proc. Natl. Acad. Sci. 2009, 106, 9147-
9150.

81

4.0 The Effect of Transition Metal Salts on the Initiated Chemical Vapor
Deposition of Polymer Thin Films
Citation: Philip Kwong, Scott Seidel, and Malancha Gupta, J. Vac. Sci. Technol., A, submitted.

4.1 Abstract
In this work, the effect of transition metal salts on the initiated chemical vapor deposition of
polymer thin films was studied using X-ray photoelectron spectroscopy. The polymerizations of
4-vinyl pyridine and 1H,1H,2H,2H-perfluorodecyl acrylate were studied using copper(II)
chloride (CuCl
2
) and iron(III) chloride (FeCl
3
) as the transition metal salts. It was found that the
surface coverages of both poly(4-vinyl pyridine) (P4VP) and poly(1H,1H,2H,2H-perfluorodecyl
acrylate) (PPFDA) were decreased on CuCl
2
, while the surface coverage of only P4VP was
decreased on FeCl
3
. The decreased polymer surface coverage was found to be due to quenching
of the propagating radicals by the salt, which led to a reduction of the oxidation state of the
metal. The identification of this reaction mechanism allowed for tuning of the effectiveness of
the salts to decrease the polymer surface coverage through the adjustment of the processing
parameters, such as the filament temperature. Additionally, we demonstrated that the ability of
transition metal salts to decrease the polymer surface coverage could be extended to the
fabrication of patterned cross-linked coatings, which is important for many practical applications
such as sensors and microelectronics.

82

4.2 Introduction
Vapor phase polymerization processes have a number of advantages when compared to solution-
phase processes, including an absence of surface tension effects and chemical compatibility
requirements between the substrate and the solvent.
168,169
For these reasons, vapor phase
polymerization processes have been widely used to conformally coat a variety of geometrically
complex and chemically sensitive substrates. For example, silica microtoroids were coated with
poly(N-isopropyl acrylamide) for humidity detection,
170
implantable neural probes were coated
with Parylene to enhance tissue integration,
171
and paper-based microfluidic devices were coated
with pH-responsive coatings for the separation of small molecules.
172
However, the conformal
nature of chemical vapor deposition processes can make the patterning of deposition onto
complex substrates challenging.
173
While patterning techniques such as physical masking
174
and
e-beam lithography
175
are compatible with planar substrates, these techniques cannot be applied
to geometrically complex or porous substrates.
Transition metal salts have been extensively used in solution phase polymerization processes to
prevent, slow, or alter the pathway of polymerization reactions. The reactivity of transition
metal salts in these systems generally arises from their ability to participate in redox processes
with radical species. For example, copper(II) chloride (CuCl
2
) was used to prevent the
polymerization of methyl methacrylate
176
and both prevent and slow the polymerization of
acrylonitrile
177
in dimethyl formamide by quenching propagating radicals via the reduction of the
copper(II) chloride to copper(I) chloride. Similarly, iron(III) chloride (FeCl
3
) was used to
prevent the polymerization of styrene, acrylonitrile, and methacrylonitrile in dimethyl
formamide.
178,179
By preventing polymerization in bulk solutions, transition metal salts have
83

been used to improve the grafting of polymers such as poly(N-vinyl pyrrolidone), poly(2-
hydroxyethyl methacrylate), and poly(acrylamide) to rubbers.
180
Additionally, by using
appropriate ligands, transition metal salts can be used to reversibly trap radicals to facilitate
controlled polymerizations such as atom transfer radical polymerization.
181,182

More recently, transition metal salts have been used in vapor phase polymerization processes in
order to pattern the deposition of thin films on both planar and porous substrates by selectively
decreasing the amount of polymer that deposits on the transition metal salt regions of the
substrates.
173,183,184,185
For example, Vaeth et al. used metals and transition metal salts to
fabricate patterns on planar substrates by selectively preventing the deposition of Parylene-C and
Parylene-N.
183,184
They attributed the prevention of deposition to the deactivation of the
adsorbed p-xylylene species, which suppressed both initiation and propagation reactions, while
Bobrowski recently proposed, based on computational experiments, that the transition metal salts
may also quench the radical Parylene chains via a redox process.
186
Vaeth et al. found that
polymer deposition eventually occurred, which they attributed to the adsorption of multilayers
that shielded additional precursors from the metal or metal salt. In our own work, we
demonstrated that transition metal salts could be used to selectively decrease the amount of
various acrylate based polymers that deposit in the initiated chemical vapor deposition (iCVD)
process.
173
The iCVD process is a solventless vapor phase polymerization process in which
initiator and monomer vapors are simultaneously introduced into a vacuum chamber. A heated
filament array thermally decomposes the initiator into radicals, which react with the monomer to
form propagating radicals. These radicals then react with additional adsorbed monomer to form
conformal polymer films. We recently used transition metal salts in the iCVD process to pattern
84

fluoropolymer barriers onto chromatography paper for the development of paper-based
microfluidic platforms that are compatible with organic solvents.
185
Additional research into the
effect of transition metal salts in the iCVD process is important for providing a better
understanding of how to optimize this patterning technique over a range of processing conditions
for the development of devices such as sensors
187
and microelectronics.
183

In this work, we determine the chemical interactions between the transition metal salts and the
reactive species present during the iCVD process. The polymerizations of 4-vinyl pyridine and
1H,1H,2H,2H-perfluorodecyl acrylate are studied using copper(II) chloride (CuCl
2
) and iron(III)
chloride (FeCl
3
) as the transition metal salts. X-ray photoelectron spectroscopy (XPS) is used to
quantify the relative amount of polymer that is deposited on the transition metal salt surfaces and
to determine if there are any changes in the oxidation state of the metal as a result of the
deposition process. We identify the specific species that interacts with the salt by systematically
exposing the transition metal salt to each individual reactive species. Our identification of the
reaction mechanism allows for tuning of the effectiveness of the salts to decrease the polymer
surface coverage through the adjustment of the processing parameters, such as the filament
temperature. Additionally, we extend our work toward practical applications by demonstrating
that transition metal salts can be used to pattern the deposition of cross-linked polymer coatings.

85

4.3 Experimental
Copper(II) chloride dihydrate (Aldrich, reagent grade), iron(III) chloride (Aldrich, 97%),
methanol (Macron, absolute), 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (Synquest
Laboratories, 97%), 4-vinyl pyridine (4VP) (Aldrich, 95%), divinyl benzene (DVB) (Aldrich,
80%), di-tert-butyl peroxide (TBPO) (Aldrich, 98%) ponceau S (Aldrich, 75%), pH 4 buffer
(BDH, ACS grade), and chromatography paper (Whatman No. 1) were used as received without
further purification.
Transition metal salt solutions (2.0 M in methanol) were spun coat onto silicon wafers at 6000
RPM (Laurell Technologies, WS40BZ-6NPT). The solutions were consecutively spun coat onto
the same wafer three times to ensure sufficient coverage of the wafer. The silicon wafer was
roughened with Scotch Brite and plasma-treated (BD20-AC Electro-Technic Products, Inc.) prior
to spin coating to reduce dewetting of the salt solution. The spun coat silicon wafer samples
were stored in a vacuum desiccator prior to deposition to reduce exposure to ambient air and
moisture. Sample surfaces generally showed less than 5 % silicon as determined by X-ray
photoelectron spectroscopy (XPS) indicating that the surface was sufficiently covered with salt.
Polymer depositions were performed in a pancake-shaped (250 mm diameter, 48 mm height)
custom-built iCVD vacuum chamber (GVD Corp.), as described previously.
173
Pressure was
achieved by a rotary vane vacuum pump (Edwards E2M40) and maintained by a throttle valve
(MKS 153D) and capacitance manometer (MKS Baratron 622A01TDE). The substrate
temperature was maintained at 25 °C for all experiments using a backside recirculating chiller.
TBPO was introduced through a mass flow controller at a flow rate of 2.3 sccm for all
experiments. A nichrome filament array (Omega Engineering, 80%/20% Ni/Cr) was heated
86

during the deposition process in order to thermally decompose the initiator. For the analysis of
the effect of CuCl
2
and FeCl
3
on the deposition of poly(4-vinyl pyridine) (P4VP) and
poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) as a function of deposition time, the
filament array was heated to 250 °C. In order to deposit polymer, both the monomer and
initiator were first introduced into the chamber at the desired flow rates for one minute in order
to stabilize the flow rates and the gas composition in the chamber prior to heating of the filament
array. For the deposition of P4VP, the 4VP monomer was heated to 23 °C to achieve a flow rate
of 24.7 sccm and the pressure was maintained at 400 mTorr. For the deposition of PPFDA, the
PFDA monomer was heated to 47 °C to achieve a flow rate of 0.6 sccm and the pressure was
maintained at 50 mTorr. For the exposure of the transition metal salts to individual reactive
species in the absence of deposition, the desired gas was introduced into the chamber for 20
minutes at 400 mTorr. For samples requiring exposure to multiple reactive species, samples
were pumped under full vacuum for ten minutes between exposures to remove gaseous species
from the vacuum chamber. For the deposition of poly(4-vinyl pyridine-co-divinyl benzene)
(P(4VP-co-DVB)), the filament array was heated to 250 °C, the 4VP monomer was heated to 23
°C to achieve a flow rate of 24.7 sccm, the DVB monomer was heated to 23 °C to achieve a flow
rate of 0.4 sccm, the pressure was maintained at 400 mTorr, and the deposition occurred for 10
minutes. Successful cross-linking was determined by visual inspection of the insolubility in
water of the films deposited onto silicon wafer.
After deposition, samples were stored and transported under vacuum in order to limit oxidation
of the samples prior to analysis by XPS. XPS was performed using a Surface Science
Instruments M-Probe spectrometer with a monochromatic Al Kα X-ray source. High-resolution
87

scans for peak analysis were acquired with a resolution number of two and a step size of 0.065
eV. Low-resolution scans for the determination of atomic composition were acquired with a
resolution number of four and a step size of 0.1 eV. Data analysis was performed using the
ESCA25 Analysis Application (V5.01.04) software. Quantification of the surface coverage of
the polymer was calculated as the percentage of nitrogen or fluorine for P4VP or PPFDA,
respectively, compared to the maximum value attainable. The maximum value attainable was
measured using reference homopolymer films (of at least 100 nm in thickness) deposited on
native silicon wafers. The 4VP monomer adsorbed onto native CuCl
2
and was visible by XPS.
High-resolution scans of the N 1s region were used to differentiate between the adsorbed
monomer (~ 400 eV) and the deposited polymer (~ 399 eV). Atomic compositions were
determined from an average of at least three separate depositions with error bars representing one
standard deviation. Trend lines showing the increase in PPFDA surface coverage as a function
of the polymer film thickness on reference silicon wafers at different filament temperatures
represent a linear regression of the data up to approximately 90 % surface coverage, after which
the data begins to plateau.
For the patterning of P(4VP-co-DVB) thin films, a 4.0 M solution of CuCl
2
in methanol was
screen printed onto chromatography paper by painting the solution using a standard 3/0 round
paint brush (Princeton Art & Brush Co.) through a mask generated from a transparency film
(3M, CG3460) and a cutting plotter (Graphtec, CE6000-40). Following the deposition, the
chromatography paper was rinsed in deionized water to remove the CuCl
2
, and soaked in a 0.5
mg/mL solution of ponceau S in pH 4 buffer for one hour to dye the polymer regions. The
88

samples were then allowed to soak in deionized water for 16 hours to remove excess ponceau S
and then dried under ambient conditions.
Film thickness measurements on reference silicon wafers were performed using a variable
wavelength spectroscopic ellipsometer (V-VASE J.A. Woolam Co.) at an angle of incidence of
70 ° and wavelengths of 830 nm, 632.8 nm, and 405 nm. Deposition rates were determined from
the average of at least 8 samples fabricated at varying deposition times.

89

4.4 Results and Discussion
The effect of transition metal salts on the deposition of polymer in the iCVD process was studied
by XPS using CuCl
2
and FeCl
3
as model salts. We chose CuCl
2
as the main salt for our study
because when compared to the reduction of other transition metal salts, the reduction of Cu(II) to
Cu(I) is easily identifiable by XPS.
188,189
FeCl
3
was included to demonstrate that the effect of
transition metal salts on polymer deposition was not restricted to CuCl
2
. Transition metal salt
solutions were spun coat onto silicon wafers prior to deposition. Quantification of the polymer
surface coverage was calculated as the percentage of nitrogen or fluorine, as determined from
low-resolution XPS scans, for P4VP or PPFDA, respectively, compared to the maximum value
attainable. The maximum value attainable was measured using reference homopolymer films (of
at least 100 nm in thickness) deposited on native silicon wafers. A surface coverage value of 100
% is reached when the polymer thickness on the transition metal salt surface is at least 5 nm,
which prevents the spectrometer from detecting the underlying salt. P4VP and PPFDA were
chosen as model polymers because nitrogen and fluorine are generally not present in adventitious
contaminants, which increases the reliability of the analysis as compared to the examination of
the carbon or oxygen environments. In the iCVD process, the ratio of the partial pressure of the
monomer to its saturation pressure (P
m
/P
sat
) is an important process variable since it affects the
rate and degree of polymerization.
190
Thus, for the examination of the effect of CuCl
2
and FeCl
3

on the polymer surface coverage after the deposition of P4VP and PPFDA, process conditions
were selected with a moderate P
m
/P
sat
value of 0.2 for both cases. This resulted in deposition
rates of approximately 4 nm/min for P4VP and 6 nm/min for PPFDA, as measured on native
silicon wafers.
90

Figure 1a shows the surface coverage of P4VP on CuCl
2
and FeCl
3
substrates as a function of the
deposition time with a native silicon wafer included as a reference. The surface coverage of
P4VP was decreased by both CuCl
2
and FeCl
3
as values were less than on the native silicon
wafer. On CuCl
2
, polymer could not be detected during the first two minutes, while the surface
coverage on FeCl
3
was decreased less significantly such that polymer was always detectable.
The high-resolution Cu 2p3 XPS spectra of native CuCl
2
, as shown in Figure 1b, showed
predominantly Cu(II) on the surface with a main line at 934.5 eV and a higher energy shake-up
peak, as expected.
189
As the deposition time was increased from 0 to 10 minutes, the copper was
converted from primarily Cu(II) to a mixture of Cu(II) and Cu(I). This was observed by the
appearance of a new peak at 932.5 eV as well as a decrease in the areal ratio of the shake-up
peak, which arises only from Cu(II) species, to the main line. Figure 2a shows the surface
coverage of PPFDA after deposition onto CuCl
2
, FeCl
3
, and native silicon wafer substrates as a
function of the deposition time. In this case, the surface coverage of PPFDA was decreased on
CuCl
2
while FeCl
3
did not appear to significantly affect the deposition. The high-resolution XPS
spectra of the Cu 2p3 region in Figure 2b again showed the formation of a Cu(I) species. These
results demonstrate that the surface coverage of both acrylate based and non-acrylate based
polymers can be decreased and that this occurs via a reduction of the transition metal salt.
91

Figure 4-1 - a) P4VP surface coverage after deposition onto silicon wafer, CuCl
2
, and FeCl
3
as a
function of deposition time; b) High-resolution XPS spectra of the Cu 2p3 region showing
conversion of Cu(II) to Cu(I) upon deposition.

92

Figure 4-2 - a) PPFDA surface coverage after deposition onto silicon wafer, CuCl
2
, and FeCl
3
as
a function of deposition time; b) High-resolution XPS spectra of the Cu 2p3 region showing
conversion of Cu(II) to Cu(I) upon deposition.

In order to identify which species was reducing the transition metal salt, we exposed CuCl
2
to
each of the following reactive species that were present during the deposition of P4VP: di-tert-
butyl peroxide (TBPO) initiator, 4VP monomer, TPBO radicals, and propagating radicals. We
subsequently examined the oxidation state of the copper. Given that a significant amount of the
CuCl
2
was reduced within 10 minutes of deposition of P4VP, the CuCl
2
was exposed to each
93

reactive species for 20 minutes to ensure that the reduction of the oxidation state of the metal or
any other chemical change would be detectable by XPS. The filament was deactivated to
prevent any radicalization of the precursors when the system was exposed to TBPO initiator and
4VP monomer. Conversely, the filament was activated in order to thermally decompose the
initiator when the system was exposed to TBPO radicals. The high-resolution Cu 2p3 XPS
spectra of the CuCl
2
after exposure to TBPO, 4VP monomer, and TBPO radicals, as shown in
Figure 3, were similar to native CuCl
2
and indicate that the copper remained primarily in its
Cu(II) oxidation state. This suggests that none of these three species caused the reduction of the
CuCl
2
and therefore are likely not directly involved in the mechanism responsible for the
decreased polymer surface coverage.
The interaction between the propagating radicals and the salt in the absence of significant
polymer deposition was studied by first exposing CuCl
2
to 4VP monomer, which adsorbed to the
salt surface. We subsequently evacuated the chamber to remove monomer from the gas phase
and then exposed the substrate to TBPO radicals, which reacted with the adsorbed 4VP monomer
to generate propagating radicals. After the exposure of the CuCl
2
to propagating radicals, the
high-resolution Cu 2p3 XPS spectrum in Figure 3 showed the formation of Cu(I). While other
species such as TBPO and 4VP monomer were also present during the exposure of the salt to the
propagating radicals, we conclude that the reduction of the transition metal salt is due to its
interaction with the propagating radicals since none of the other species affected the oxidation
state of the metal. This result is similar to what has been reported in solution phase
polymerization systems in which transition metal salts quench primarily carbon-centered
radicals.
191,192
Therefore, the mechanism responsible for decreasing the polymer surface
94

coverage occurs by the quenching of the propagating radicals by the transition metal salt via an
electron transfer reaction, which results in the reduction of the transition metal salt. We
hypothesize that eventually the transition metal salt becomes passivated via this redox process,
allowing polymer deposition to occur. For the deposition of P4VP and PPFDA, complete
prevention of deposition was only observed for a short period of time (if at all), which may be
due to the lack of mobility of the non-solubilized salt in our system. For example, once a region
of the CuCl
2
surface is reduced, that area would no longer be able to quench propagating radicals
leading to deposition despite the presence of Cu(II) elsewhere on the surface.

Figure 4-3 - High-resolution XPS spectra of the Cu 2p3 region of CuCl
2
before and after
exposure to reactive species showing conversion of Cu(II) to Cu(I) upon exposure to propagating
radicals.

Since the ability of the transition metal salts to decrease the polymer surface coverage is due to
quenching of the propagating radicals, we hypothesized that it was possible to tune the
effectiveness of the transition metal salts to decrease the polymer surface by adjusting the
filament temperature in order to control radical formation. The filament temperature is
95

responsible for the thermal decomposition of the initiator and a decrease in the temperature by 30
°C results in a decrease in the rate of thermal decomposition of TBPO by approximately a factor
of 10.
193
Tuning the effectiveness of the transition metal salts would allow for control over the
amount of polymer that could be deposited on a substrate relative to a region containing a
transition metal salt, which is a useful design parameter in patterning applications. Adjusting the
filament temperature resulted in deposition rates for PPFDA of 6 nm/min, 6 nm/min, and 2
nm/min on native silicon wafers at filament temperatures of 250 °C, 220 °C, and 190 °C,
respectively. The minimal dependence of deposition rate on the filament temperature at high
filament temperatures is consistent with what has been reported previously in iCVD
194
and other
inorganic CVD systems.
195
At high filament temperatures, the system is considered to be in a
mass transport limited regime in which the deposition kinetics are dominated by the transfer of
gas phase radicals from the filament array to the substrate surface, and thus not sensitive to the
rate of TBPO thermal decomposition. Conversely, at lower filament temperatures, the system is
considered to be in a reaction-kinetics limited regime, in which the deposition kinetics are
dominated by the decomposition of the initiator, and hence the deposition rate is significantly
affected. Figure 4 shows the effect of varying the filament temperature on the surface coverage
of PPFDA after deposition onto CuCl
2.
The data points are plotted versus the polymer film
thickness on native silicon wafers in order to account for the different deposition rates at each
filament temperature. The trend lines show the increase in polymer surface coverage as a
function of the polymer film thickness on native silicon wafer. The differences between the
mass transport limited and reaction-kinetics limited regimes directly impact the ability of CuCl
2

to decrease the polymer surface coverage. Lowering the filament temperature from 250 °C to
220 °C only had a small effect on the ability of CuCl
2
to decrease the surface coverage of
96

PPFDA as the surface coverage trended slightly towards lower values. When the filament
temperature was lowered to 190 °C, it further enhanced the effectiveness of CuCl
2
as the
polymer surface coverage trended more significantly towards lower values. Thus, by adjusting
the rate of the thermal decomposition of initiator, the effectiveness of the transition metal salts
could be tuned. Furthermore, lowering the filament temperature allowed for an increased
amount of polymer to be deposited on a native region of a substrate relative to a region
containing transition metal salt, which could improve pattern fidelity.

Figure 4-4 - Effect of filament temperature on the surface coverage of PPFDA after deposition
onto CuCl
2
.

In order to further extend this work toward practical applications, we examined the ability to use
transition metal salts to fabricate a cross-linked polymer pattern on porous chromatography
paper, as shown in Figure 5. Cross-linking of the polymer film is useful in order to prevent
dissolution of the polymer pattern. A solution of CuCl
2
was screen printed onto chromatography
paper (Figure 5a) and subsequently coated with poly(4-vinyl pyridine-co-divinyl benzene)
97

(P(4VP-co-DVB)) for 10 minutes at a filament temperature of 250 °C. The addition of a small
amount of divinyl benzene (DVB) during the deposition process acted to cross-link the film and
we hypothesized that the addition of DVB would not significantly affect the ability of the
transition metal salt to decrease the polymer surface coverage because of its low P
m
/P
sat
value of
0.01. The data from Figure 1a showed that only a small amount of polymer would be deposited
on the CuCl
2
region at these deposition conditions and therefore the salt could be easily washed
away (Figure 5b). The remaining polymer was dyed with ponceau S which allowed for
visualization of the resulting pattern (Figure 5c). The ability of ponceau S to preferentially dye
the P(4VP-co-DVB) film is due to electrostatic and hydrogen bonding forces. The resulting
image clearly shows that the addition of a cross-linker did not preclude the transition metal salt
from being able to selectively prevent the deposition of polymer. This allowed preferential
deposition to occur in the areas free of salt in order to generate the desired pattern.
98

Figure 4-5 - a) Screen printed CuCl
2
on chromatography paper, b) paper after deposition of
P(4VP-co-DVB) and removal of CuCl
2
, and c) polymer pattern after dyeing with ponceau S.

99

4.5 Conclusions
In this work, XPS was used to study the effect of transition metal salts in the iCVD process. The
polymer surface coverage was decreased on both CuCl
2
and FeCl
3
substrates and a reduction of
the metal oxidation state occurred during the deposition of both acrylate based and non-acrylate
based polymers. This reduction was shown to be caused by the exposure of the transition metal
salts to propagating radicals, which led to quenching of the polymerization reaction.
Identification of the reactive species allowed the effectiveness of the transition metal salts to be
tuned by the adjustment of the processing parameters, such as the filament temperature.
Additionally, the ability of transition metal salts to decrease the surface coverage of the polymer
was extended to the development cross-linked patterns. This work underlines the importance of
surface chemistry in polymer CVD systems and helps to provide design parameters for the
fabrication of patterned thin films via the use of transition metal salts in the iCVD process.

100

4.6 Acknowledgements
This work was supported by the National Science Foundation Division of Civil, Mechanical, and
Manufacturing Innovation Award Number 1069328, the Natural Sciences and Engineering
Research Council of Canada Scholarship (P.K.), the Alfred Mann Institute at the University of
Southern California (P.K.), and the National Science Foundation Graduate Research Fellowship
under Grant No. DGE-0937362 (S.S.). We thank the Molecular Materials Research Center of
the Beckman Institute of the California Institute of Technology for use of their XPS.

101

4.7 References

168 Seidel, S.; Riche, C.; Gupta, M. Chemical Vapor Deposition of Polymer Films
(Encyclopedia of Polymer Science and Technology, 2011).
169 Lei, Y.; Wang, W.; Yu, H.; Luo, Y.; Li, T.; Jin, Y.; Zhang, H.; Li, Z. J. Micromech.
Microeng. 2009, 19, 035013.
170 Mehrabani, S.; Kwong, P.; Gupta, M.; Armani, A. M. Appl. Phys. Lett. 2013, 102,
241101.
171 Kuo, J. T.W.; Kim, B. J.; Hara, S. A.; Lee, C. D.; Gutierrez, C. A.; Hoang, T. Q.; Meng,
E. Lab Chip, 2013, 13, 554-561.
172 Kwong, P.; Gupta, M. Anal. Chem., 2012, 84, 10129-10135.
173 Kwong. P.; Flowers, C. A.; Gupta, M. Langmuir, 2011, 27, 10634-10641.
174 Chen, H.-Y.; Lahann, J. Adv. Mater., 2007, 19, 3801-3808.
175 Mao, Y.; Felix, N. M.; Nguyen, P. T.; Ober, C. K.; Gleason, K. K. J. Vac. Sci. Technol.
B, 2004, 22, 2473-2478.
176 Bengough, W. I.; Fairservice, W. H. Trans. Faraday Soc., 1965, 61, 1206-1215.
177 Bengough, W. I.; Fairservice, W. H. Trans. Faraday Soc., 1967, 63, 382-391.
178 Bamford, C. H.; Jenkins, A. D.; Johnston, R. Proc. Roy. Soc. London, Ser. A,
Mathematical and Physical Sciences, 1957, 239, 214-229.
179 Bamford, C. H.; Jenkins, A. D.; Johnston, R. Trans. Faraday Soc., 1962, 58, 1212-1225.
180 Katbab, A. A.; Burford, R. P.; Garnett, J. L. Radiat. Phys. Chem., 1992, 39, 293-302.
181 Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc., 1995, 117, 5614-5615.
182 Matyjaszewski, K.; Xia, J. Chem. Rev., 2001, 101, 2921-2990.
102

183 Vaeth, K. M.; Jensen, K. F. Adv. Mater., 1999, 11, 814-820.
184 Vaeth, K. M.; Jensen, K. F. Chem. Mater., 2000, 12, 1305-1313.
185 Chen, B.; Kwong, P.; Gupta, M. ACS Appl. Mater. Interfaces, 2013, 5, 12701-12707.
186 Bobrowski, M. Comput. Theor. Chem., 2012, 991, 56-65.
187 Nock, V.; Blaikie, R. J.; David, T. Lab Chip, 2008, 8, 1300-1307.
188 Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; McIntyre, N. S. Surf. Interface Anal.,
2004, 36, 1564-1574.
189 Biesinger, M. C.; Lau, L. W.M.; Gerson, A. R.; Smart, R. St.C. Appl. Surf. Sci., 2010,
257, 887-898.

190 Lau, K. K. S.; Gleason, K. K. Macromolecules, 2006, 39, 3688-3694.
191 Bamford, C. H.; Jenkins, A. D.; Johnston, R. Nature, 1956, 177, 992-993.

192 Moad, G.; Solomon, D. H. The Chemistry of Radical Polymerization, 2nd ed. (Elsevier
Ltd., Oxford, United Kingdom, 2006), pp. 273.
193 Shaw, D. H.; Pritchard, H. O. Can. J. Chem., 1968, 46, 2721-2724.
194 Ozaydin-Ince, G.; Gleason, K. K. J. Vac. Sci. Technol. A, 2009, 27, 1135-1143.
195 Wu, T. H. T.; Rosler, R. S.; Lamartine, B. C.; Gregory, R. B.; Tompkins, H. G. J. Vac.
Sci. Technol. B, 1988, 6, 1707-1713.
103

5.0 Patterned Fluoropolymer Barriers for Containment of Organic Solvents
within Paper-Based Microfluidic Devices
Citation: Benny Chen, Philip Kwong, and Malancha Gupta, ACS Appl. Mater. Interfaces, 2013,
5, 12701-12707

5.1 Abstract
In this paper, we demonstrate for the first time the ability to pattern lipophobic fluoropolymer
barriers for the incorporation of pure organic solvents as operating liquids within paper-based
microfluidic devices. Our fabrication method involves replacing traditional wax barriers with
fluoropolymer coatings by combining initiated chemical vapor deposition with inhibiting
transition metal salts to pattern the polymer. Multiple techniques for patterning the transition
metal salt are tested including painting, spray coating, and selective wetting through the use of a
photoresist. The efficacy of the barrier coatings to contain organic solvents is found to be
dependent on the conformality of the polymer deposited around the paper fibers. We demonstrate
examples of the benefits provided by the containment of organic solvents in paper-based
microfluidic applications including the ability to tune the separation of analytes by varying the
operating solvent and by modifying the channel region of the devices with additional polymer
coatings. The work exhibited in this paper has the potential to significantly expand the
applications of paper-based microfluidics to include detection of water insoluble analytes.
Additionally, the generality of the patterning process allows this technique to be extended to
other applications that may require the use of patterned hydrophobic and lipophobic regions,
such as biosensing, chemical detection, and optics.
104

5.2 Introduction
Paper-based microfluidic devices are an attractive option for rapid low-cost diagnostics due to
numerous advantages including portability,
196
ease of use,
197
and limited requirements for
operation.
198
Recent developments have expanded the available operations for paper-based
microfluidic platforms to include mixing,
199
separation of analytes,
200,201
biosensing,
202
and fluid
manipulation using integrated valves and fluidic diodes.
203
Paper-based microfluidic devices
operate using capillary action to drive liquid flow which is typically directed by wax
barriers.
204,205
Attempts at using other media to direct the flow of liquids such as hydrophobic
photoresists,
206
polymer yarns,
207
and silk
208
have also been explored. However, these methods
use hydrophobic barriers that are vulnerable to effects such as dissolution, solvent diffusion, or
swelling by organic solvents that ultimately restricts the possible operating liquids to aqueous
solvents.
The use of organic solvents within paper-based assays has been challenging due to the difficulty
of creating lipophobic barriers.
209
Expanding the operating liquids used in paper-based
microfluidic devices to include organic solvents could unlock several advantages over traditional
aqueous solvents. For example, utilizing organic solvents could expand the applications of these
devices for the processing and analysis of water insoluble chemicals, including pharmaceutical
drugs
210
and chemical warfare agents and pesticides such as organophosphates.
211,212,213
These
assays may require sequential or timed multi-step reactions, interfacial reactions, or separation of
other hydrophobic contaminants. By utilizing lipophobic barriers that are compatible with
organic solvents, these assays could potentially be translated onto paper-based microfluidic
devices by directing and manipulating liquid flow. The use of organic solvents would also allow
105

for additional modification and tuning of both the mobile and stationary phases, yielding highly
adaptable platforms.
Fluoropolymers have been known to exhibit desirable properties as a barrier material, including
lipophobicity,
214,215
as well as excellent mechanical, thermal, and chemical stability.
216,217
For
example, fluoropolymers have been used as the bulk material to replace poly(dimethylsiloxane)
(PDMS) in order to fabricate chemically robust microfluidic devices.
218,219,220
Additionally, we
have recently shown that initiated chemical vapor deposition (iCVD) can be used to deposit
fluoropolymer coatings of poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) onto PDMS
microfluidic devices to act as lipophobic barriers that prevent the diffusion of organic solvents.
221

The patterning of fluoropolymers has led to advancements in fields such as biosensing,
217
pH-
sensing,
222
and optical device fabrication.
223
However, the current techniques for patterning
fluoropolymers, such as directed plasma exposure
224
or ion bombardament,
217
and microcontact
printing,
222
are generally limited to planar substrates. In this paper, we demonstrate several
methods for patterning thin conformal PPFDA coatings onto porous substrates in order to replace
traditional wax barriers used in paper-based microfluidic devices, allowing for the containment
of both aqueous and organic solvents without dissolution or swelling of the barrier. The
fluoropolymer was patterned using iCVD in combination with transition metal salts, which are
able to selectively inhibit the deposition of polymer.
225
The iCVD technique is a solvent-free
coating process that uses a heated filament to decompose initiator molecules into radical species
that react with monomer molecules to begin free radical polymerization. The solventless nature
of the iCVD process alleviates common issues caused by solvents during polymer processing
such as dewetting and clogging, and has been previously used to conformally coat a variety of
106

non-planar substrates, including electrospun fiber mats
226,227
and silicon nanotrenches.
228
In
addition, the iCVD technique allows for sequential deposition of conformal, functional polymer
coatings onto porous materials.
201
We evaluate the resolution of the following three patterning
methods for the application of the transition metal salt inhibitor: painting, spray coating, and
selective wetting through the use of a photoresist. Previous attempts at patterning polymers onto
porous substrates via iCVD using physical masking have proven unsuccessful due to the large
mean-free path of the reactant vapors.
225
While other patterning methods have been successfully
combined with iCVD, such as electron-beam lithography
229,230
and capillary force lithography,
231

these methods are generally limited to planar substrates. In contrast, the use of transition metal
salts allows us to pattern fluoropolymer barrier coatings through the depth of porous
chromatography paper by selectively inhibiting the deposition of polymer.
We demonstrate that the fluoropolymer barrier coatings can be used to improve separation
processes within paper-based microfluidic devices. The fluoropolymer barrier coatings enable
the modification of the mobile phase using various organic solvents to tune the separation of
water insoluble analytes. Additionally, the use of organic operating liquids allows for
modification of the stationary phase with organic polymer coatings that are incompatible with
aqueous systems due to their hydrophobicity. The use of polymer coatings to modify stationary
phases in column chromatography has shown an abundance of utility, such as the ability to
facilitate hydrophobic interaction chromatography
232,233
and size-based separation,
234,235
but has
only recently been demonstrated for aqueous-based paper-based microfluidic applications using
electrostatic interactions.
201
In this paper, we use the iCVD process to modify the channels of our
devices with polymer coatings to further tune the separation of analytes using interactions such
107

as hydrogen bonding and π-stacking. The iCVD process is ideal for modifying paper-based
microfluidic devices due to its ability to conformally coat porous substrates without
compromising their morphology
201,225,236
since maintaining the porous nature of the paper is
essential due to its critical role in facilitating capillary-force driven flow.

108

5.3 Experimental
1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) (SynQuest, 97 %), di-tert-butyl peroxide (Sigma
Aldrich, 98 %), 4-vinyl pyridine (Sigma Aldrich, 95 %), ethylene glycol dimethacrylate (Sigma
Aldrich, 98 %), ortho-nitrobenzyl methacrylate (oNBMA) (Polysciences, 95 %), copper(II)
chloride (Sigma Aldrich, reagent Grade), Grade 1 chromatography paper (Whatman), Grade 3
chromatography paper (Whatman), pH 8 buffer (BDH, pH 8 ± 0.02), acetone (Macron, 99.5 %),
acetonitrile (Mallinckrodt, 99.5 %), butanol (Mallinckrodt, 99.4 %), chloroform (Mallinckrodt,
99.9 %), cyclohexane (EMD, 99.99 %) diethyl ether (BDH, 99 %), dimethyl formamide (EMD,
99.8 %), dimethyl sulfoxide (Mallinckrodt, 99.8 %), ethanol (Koptec, 200 proof), ethyl acetate
(Mallinckrodt, 99.5 %), hexane (EMD, 98.5 %), isopropanol (Macron, ACS Grade), methanol
(Macron, absolute), tetrahydrofuran (Mallinckrodt, 99.0 %), toluene (J. T. Baker, 99.7 %), blue
food coloring (McCormick), Sudan Black B (Sigma Aldrich, Biological Stain Commission
certified), Nile Red (Sigma Aldrich, microscopy Grade), sulfuric acid (EMD, ACS Grade), and
sodium hydroxide (Mallinckrodt, 98.8 %) were used as received without further purification.
To fabricate PPFDA barrier coatings, CuCl
2
was applied to chromatography paper and
subsequently coated in a custom designed iCVD reactor chamber (GVD Corp, 250 mm diameter,
48 mm height). The deposition of PPFDA was performed at a constant pressure of 40 mTorr
while the samples were maintained at 30 °C using a backside recirculating chiller. The flow rates
of the monomer, PFDA, and initiator, di-tert-butyl peroxide, were 0.4 and 3.9 sccm, respectively.
During the deposition, a nichrome filament array (80 % Ni, 20 % Cr, Omega Engineering) inside
the reactor was resistively heated to 250 °C to decompose the initiator molecules into free
radicals. Polymerization occurred on the surface of the substrate via free radical polymerization.
109

Polymer film thickness measurements on silicon wafers were obtained using a 633 nm helium-
neon laser interferometer (Industrial Fiber Optics). Scanning electron microscopy (JEOL-7001)
was used to confirm the conformal nature of the polymer coatings after deposition of 440 nm of
PPFDA. Gold was sputtered onto the samples prior to imaging to prevent charging.
The CuCl
2
was applied prior to deposition of PPFDA using painting, spray coating, or selective
wetting through use of a hydrophobic photoresist. Following the deposition of PPFDA, the CuCl
-
2
salt was removed by washing the samples with water, followed by methanol while the samples
were still wet. The subsequent methanol wash was used to aid in removing the CuCl
2
salt, and
also served to alleviate wrinkling effects that occurred during the evaporation of water. The
samples were then allowed to dry in ambient conditions prior to analysis. The resolution of the
patterning technique for each method of salt application was determined by comparing the width
of an isosceles triangular mask with a base of 5 mm and a height of 50 mm to that of a final
device dyed with a 50:1 by volume mixture of methanol and blue food coloring for visualization
at nine equally distributed intervals from 1 mm to 5 mm from the apex along a line perpendicular
to the base (Figure 5-S1). The reported deviation is an average of these nine measurements
across three samples per method with ± values representing one standard deviation.
Painted samples were patterned by applying approximately 80 µ L/cm
2
of a 1:1 by volume
mixture composed of diethyl ether and 4 M CuCl
2
in methanol onto chromatography paper using
a standard 3/0 round paintbrush (Princeton Art & Brush Co.). A dark outline of the intended
channel area was situated below the paper to act as a guide to be traced. Spray coated samples
were patterned by spraying a 4:1 by volume mixture composed of diethyl ether and 2 M CuCl
2
in
methanol

50 times over physically masked chromatography paper using a household hand-
110

operated spray bottle (FamilyMaid). After every 10 sprays, the samples were dried with a heat
gun to evaporate excess methanol and diethyl ether. The selective wetting through the use of a
hydrophobic photoresist was performed by first coating chromatography paper with
approximately 25 nm of the photoresist PoNBMA using the iCVD process with a constant
pressure of 50 mTorr while the samples were maintained at 20 °C. The flow rates of the
monomer, oNBMA, and initiator, di-tert-butyl peroxide, were 0.05 sccm and 0.7 sccm,
respectively. After the photoresist was deposited, the paper was selectively exposed to 365 nm
UV light (UVP, UVL-21) through a mask for 90 minutes. Afterwards, the sample was
submerged in pH 8 buffer for 60 min followed by a water rinse to remove the buffer and
photoresist from the exposed areas. The paper was allowed to dry under ambient conditions, after
which 40 µ L/cm
2
of a 4 M aqueous solution of CuCl
2
was pipetted onto the exposed area.
Contact angle values for the unpatterned paper and the barrier and channel regions were
measured with a goniometer (Ramé-Hart Model 290-F1) using 5 μL water droplets. Triplicate
measurements were taken for each contact angle value. All unpatterned PPFDA paper samples
were coated with 1 μm of polymer using the same reaction conditions as used for the barrier
coatings.
In order to compare the fluoropolymer barriers to traditional wax barriers, wax toner was printed
onto chromatography paper using a Xerox Phaser 8560N printer and subsequently melted
through the depth of the paper using an oven set at approximately 180 °C for 3 minutes as
described in previous studies.
204,205
Channels measuring 1 cm by 5 cm with PPFDA barriers were
made by applying the CuCl
2
using the painting method. The evaluation of the ability of PPFDA
barriers and traditional wax barriers to contain various solvents was performed by pipetting
111

approximately 150 μL of a 0.2 mg/mL solution of Sudan Black B in each solvent into the
channels followed by visual detection of whether the solvent bled through the barrier or
exhibited irregular wetting behavior within the channel. Sudan Black B was dissolved in the
solvents to provide greater contrast between the wetted and non-wetted areas.
The channel regions of devices measuring 1 cm by 5 cm were modified with polymer coatings
by pre-coating chromatography paper with copolymers composed of 4-vinyl pyridine and
ethylene glycol dimethacrylate using the iCVD process. For all depositions, the di-tert-butyl
peroxide flow rate was 0.7 sccm, the filament temperature was 250 °C, and the samples were
kept at 20 °C. The reaction conditions for each polymer coating are summarized in Table 5-1.
PPFDA barriers were then subsequently patterned onto the coated paper as described above by
applying CuCl
2
using the painting method.
Table 5-1 - iCVD experimental conditions
Flow Rate of
EGDMA (sccm)
Flow Rate of
4VP (sccm)
Pressure
(mTorr)
Mole Fraction of
EGDMA in Coating
0.2 7.8 500 0.04
0.2 5.8 325 0.53
0.2 3.5 200 0.64
0.2 0.0 50 1.00

The R
f
values were determined by spotting a 0.4 % by weight methanolic solution of either
Sudan Black B or Nile Red 1 cm from the bottom edge of the device. After the analyte dried, the
device was inserted vertically into a glass chamber filled with solvent to a height of
approximately 0.5 cm. The chamber was then immediately covered to reduce evaporation. When
112

the mobile phase travelled at least 2 cm, the sample was removed from the chamber and the
distances travelled by both the mobile phase and the analyte were measured to calculate the R
f
value. The reported R
f
values were an average of triplicate measurements using a new device for
each measurement.

X-ray photoelectron spectroscopy (XPS) was performed using a Surface Science Instruments M-
Probe spectrometer with a monochromatic Al Kα X-ray source. Survey spectra were averaged
over 5 scans and were acquired at binding energies between 1 and 1000 eV with a resolution of 1
eV. Data analysis was performed using the ESCA25 Analysis Application software (V5.01.04).
The relative amount of PPFDA on the surface of the paper devices was determined by comparing
the relative carbon to fluorine atomic ratios, according to Equation 5-S1. The mole fraction of
EGDMA in the copolymer coating was determined by comparing the relative carbon to nitrogen
atomic ratios, according to Equation 5-S2, as measured on a reference silicon wafer.

113

5.4 Results and Discussion
Paper-based microfluidic channels were fabricated by patterning fluoropolymer PPFDA barrier
coatings onto Grade 1 Whatman chromatography paper using the iCVD process in combination
with copper(II) chloride (CuCl
2
), as shown in Figure 5-1a. A solution of CuCl
2,
which acted to
inhibit polymer deposition,
225
was applied using three different techniques: painting, spray
coating, and selective wetting through the use of a hydrophobic photoresist. After the transition
metal salt was applied, the sample was coated using the iCVD process with 50 nm of PPFDA (as
measured on a reference silicon wafer using in-situ interferometry). After the deposition of the
PPFDA barrier coating, the channels were washed with water and methanol to remove the CuCl
2
salt. The PPFDA coating was conformal around the paper fibers and did not occlude the pores of
the paper, as shown in the scanning electron micrographs of the paper before and after deposition
(Figure 5-1b). The resolution of each salt application technique was evaluated by comparing the
dimensions of a resultant fluoropolymer pattern to an intended design. A tapered triangular
channel was used as the intended design in order to evaluate the resolution as a function of
channel width. For all three methods of salt application, the resultant fluoropolymer pattern was
larger than the intended design and the deviation was found to be independent of channel width
over the range tested. Painting was performed by brushing a solution of CuCl
2
in methanol and
diethyl ether

onto the paper. Methanol was selected to ensure the dissolution of the CuCl
2
salt,
while diethyl ether was selected to provide a fast evaporation rate for greater control over the
application of the salt. Even while demonstrating extreme care when painting, this technique was
the least consistent, yielding an average deviation of 0.80 ± 0.26 mm (Table 5-S1) between the
intended channel width and the actual channel width. When attempting to pattern large areas or
multiple devices simultaneously, a technique such as spray coating is preferable since it is
114

capable of rapidly applying salt solution over large areas. We tested the resolution of this
technique by spraying a solution of CuCl
2
in methanol and diethyl ether

onto the paper through a
physical mask. The resultant deviation between the intended channel width and the actual
channel width was found to be 0.46 ± 0.10 mm (Table 5-S2), which is a significant improvement
when compared to painting. Although the spray coating technique was able to produce samples
quickly and with improved resolution, it should be noted that this technique required
significantly more CuCl
2
due to the loss of the solution onto the mask and to the surrounding
area. For applications that require higher resolution, a photolithographic approach can be used. In
this method, patterns were produced by first conformally coating the paper fibers with the
hydrophobic photoresist poly(ortho-nitrobenzyl methacrylate) (PoNBMA) using the iCVD
process.
236
The photoresist was then selectively exposed to UV light to generate patterned
hydrophilic areas, which were then wet with an aqueous solution of CuCl
2
. The conformal nature
of the iCVD process ensured that the patterned photoresist did not occlude the pores of the paper,
allowing for the subsequent fluoropolymer deposition to infiltrate the entire thickness of the
paper. Using this photolithographic process, we were able to produce patterns that deviated from
the intended channel width by 0.33 ± 0.10 mm (Table 5-S3). While the deviations in the painting
and spray coating methods are likely the result of the CuCl
2
solution bleeding outside the
intended area, the use of a hydrophobic photoresist prevents bleeding. The deviation associated
with the use of a photoresist is instead attributed to undercutting of the photoresist during
exposure. It is important to note that other patterning techniques for applying the salt solutions
may yield better resolution or scalability, such as inkjet printing,
237
however this method requires
surfactants and optimization to prevent clogging of printer heads. Since the painting technique
115

required less CuCl
2
than spray coating and is less cumbersome than photolithography, we used
the painting method for the remainder of our studies.

Figure 5-1 - a) Schematic representation of the device fabrication process. CuCl
2
is applied to
the chromatography paper by painting, spray coating, or photolithography, yielding selective
deposition of PPFDA by iCVD in the areas free of CuCl
2
. The salt is then removed to yield the
final device. b) Scanning electron micrographs of chromatography paper before and after
deposition of 440 nm of PPFDA.

116

For the deposition of the PPFDA barrier coatings, two criteria must be met to effectively
fabricate the devices: 1) the PPFDA coating in the barrier region must be conformal around the
paper fibers, otherwise the uncoated areas may cause solvent to bleed through the barrier; and 2)
the deposition of PPFDA in the channel region must be inhibited in order to prevent
unpredictable wetting and decreased device performance. While the first criterion is more easily
satisfied at higher coating thicknesses, the second criterion is more easily satisfied at lower
coating thicknesses since the ability of transition metal salts to inhibit the deposition of iCVD
coatings has been shown to decrease as greater amounts of polymer are deposited. This decrease
in inhibition was hypothesized to be caused by the formation of a layer of deactivated precursor
molecules that shields the transition metal salt from deactivating additional precursor
molecules.
225
In order to systematically determine the PPFDA coating thicknesses that fulfilled
both criteria, X-ray photoelectron spectroscopy (XPS) was used to analyze the amount of
PPFDA in both the barrier region (Figure 5-2a) and in the channel region (Figure 5-2b) as a
function of the coating thickness as measured on a reference silicon wafer. The XPS spectra of
the paper surface showed an increasing fluorine intensity at 686 eV with increasing coating
thickness in both the barrier and channel regions which is indicative of an increasing amount of
PPFDA on the surface. XPS probes approximately the top 5 nm of the surface and therefore the
weight fraction of the PPFDA on the surface relative to cellulose can be estimated by comparing
the fluorine to carbon ratio on the surface of the devices to a surface of homopolymer PPFDA
(Table 5-2). If the coating was too thin, areas of cellulose remained exposed as indicated by a
low PPFDA weight fraction. For example, when 10 nm of coating was deposited, the PPFDA
weight fraction within the barriers was only 0.68, resulting in barriers that were unable to contain
hexane within the channel (Figure 5-3a). This XPS data also indicates that the thickness of
117

PPFDA on the paper was lower than that measured on the reference silicon wafer, which is likely
due to the fibrous nature of the paper. When the coating thickness was increased to 50 nm or
more, the PPFDA weight fraction within the barriers remained steady at approximately 0.98-0.99
indicating at least 5 nm of conformal coating, yielding barriers that could contain hexane (Figure
5-3b). However, as we deposited thicker coatings, the transition metal salt was unable to
conformally inhibit the deposition of polymer, which resulted in a significant increase in the
PPFDA weight fraction within the channel. As a result, 640 nm thick coatings showed non-
uniform wetting of hexane within the channel region (Figure 5-3c). In order to further evaluate
the amount of PPFDA in the barrier and channel regions and the corresponding wetting
properties, we used contact angle goniometry to measure the static contact angles of water on the
top and bottom sides of our samples (Table 5-3). The contact angles on both regions approached
the contact angle on a control sample of PPFDA (1 µ m thick) deposited on unpatterned paper
(154.1° ± 2.7°), which was consistent with the XPS data. The top and bottom sides of the paper
exhibited similar contact angles even at low deposition thicknesses, confirming that the PPFDA
coating was conformal through the depth of the Grade 1 Whatman chromatography paper (180
μm thick). We also measured the water contact angle after deposition of PPFDA onto Grade 3
Whatman chromatography paper (360 μm thick), and found that the contact angles on the top
and bottom sides of the paper were identical within error. These results demonstrated that the
deposition of PPFDA was highly conformal despite the tortuous nature of the paper and
generated uniform wetting properties.
118

Figure 5-2 - XPS spectra showing the chemical composition of a) the barrier regions and b) the
channel regions of devices with different thicknesses of deposited PPFDA.

250 350 450 550 650 750
Binding Energy (eV)
a)
C
250 350 450 550 650 750
Binding Energy (eV)
F O
b)
440 nm PPFDA
50 nm PPFDA
10 nm PPFDA
Unmodified chromatography paper
640 nm PPFDA
440 nm PPFDA
50 nm PPFDA
10 nm PPFDA
Unmodified chromatography paper
640 nm PPFDA
119

Table 5-2 - Weight fraction of PPFDA in the barrier and channel regions of the paper devices.
Polymer Thickness on
Reference Silicon Wafer (nm)
Weight Fraction of PPFDA
in Barrier Regions
Weight Fraction of PPFDA
in Channel Regions
10 0.68 0.03
50 0.99 0.08
440 0.98 0.17
640 0.99 0.85

Figure 5-3 - Images of paper-based microfluidic devices after applying organic solvents
containing dye for visualization. Fluoropolymer barriers a) are unable to contain hexane with a
10 nm thick PPFDA coating, b) are able to successfully contain hexane with a 50 nm thick
PPFDA coating, and c) have non-uniform wetting with a 640 nm thick PPFDA coating. d)
Fluoropolymer barriers made with a 50 nm thick PPFDA coating succeed at containing a wide
variety of organic solvents whereas traditional wax barriers fail.

120

Table 5-3 - Water contact angles on both the top and bottom of the barrier and channel regions
of the paper devices.
Polymer Thickness on
Reference Silicon Wafer (nm)
Contact Angle in
Barrier Regions (°)
Contact Angle in
Channel Regions (°)
10
Top 124.5 ± 0.9 0.0 ± 0.0
Bottom 125.9 ± 4.0 0.0 ± 0.0
50
Top 148.1 ± 1.2 0.0 ± 0.0
Bottom 145.4 ± 3.0 0.0 ± 0.0
440
Top 147.3 ± 1.0 106.0 ± 7.4
Bottom 149.9 ± 4.9 110.9 ± 4.1
640
Top 154.0 ± 1.2 154.2 ± 0.6
Bottom 155.6 ± 2.6 154.9 ± 2.9

Based on our observations above, we deposited approximately 50 nm of PPFDA on all further
paper-based microfluidic devices to effectively contain organic solvents. Although the channel
regions contained a small amount of PPFDA, the resultant devices exhibited the same solvent
wicking behavior as uncoated paper, indicating that this low amount of PPFDA did not
significantly affect device performance. A comparison between the ability of the PPFDA barriers
and traditional wax barriers to contain organic solvents with contrasting polarities (hexane, ethyl
acetate, acetone, and methanol) is shown in Figure 5-3d. In all cases, the tested solvents were
able to penetrate the traditional wax barriers, leading to undesirable flows. Conversely, the
PPFDA barriers were able to successfully contain this wide variety of solvents. A full list of
tested solvents can be found in Table 5-S4.
The ability to use organic solvents in paper-based microfluidic devices allows for a variety of
operations that are difficult or impossible with aqueous solvents, such as the chromatographic
121

separation of lipophilic analytes. A common metric used to describe chromatographic separation
is the retardation factor (R
f
), which is defined as the distance travelled by the analyte relative to
the distance travelled by the mobile phase. Higher R
f
values exist when there is greater affinity
between the analyte and the mobile phase, while lower R
f
values exist when there is greater
affinity between the analyte and the stationary phase. Thus, we can tune the R
f
value of analytes
by modifying either the mobile or stationary phase of the system, allowing for separation of a
multi-component system. A simple demonstration of how the mobile phase can be tailored to
yield specific degrees of separation is shown in Figure 5-4a where various compositions of
methanol and water were used to control the R
f
values of the lipophilic dyes Sudan Black B and
Nile Red. An inset schematic in Figure 5-4a depicts the relative R
f
values of Sudan Black B and
Nile Red as the mobile phase is varied. As the relative amount of methanol decreased, the R
f

value of Nile Red decreased more rapidly than Sudan Black B, resulting in a larger separation
between the analytes. This observation can be explained by the greater affinity between Sudan
Black B and methanol compared to the affinity between Nile Red and methanol. Although two
lipophilic dyes were examined as model analytes, the ability to use organic solvents to affect the
separation of a mixture of analytes is applicable to a wide variety of systems. Additionally,
channels can be modified with functional polymer coatings that may be incompatible with
aqueous systems due to their hydrophobicity in order to further tune the R
f
value of analytes. For
example, we modified our channels using iCVD by conformally pre-coating chromatography
paper with copolymers composed of 4-vinyl pyridine (4VP) and ethylene glycol dimethacrylate
(EGDMA) prior to depositing the fluoropolymer barriers in order to tune the R
f
value of Sudan
Black B in hexane. The mole fraction of EGDMA in the copolymer coating was systematically
varied between 0.04 and 1.00, as determined by XPS (Table 5-4). In addition to providing
122

functionality, the EGDMA cross-linker also prevented dissolution of the copolymer coatings.
The lowest R
f
value of Sudan Black B was measured on an unmodified cellulose channel (0.22 ±
0.02), which we hypothesize to be due to the ability of cellulose to act as both a proton donor
(OH--N) and acceptor (O--HN), resulting in greater affinity to the analyte Sudan Black B. The
effect of the EGDMA mole fraction in the copolymer coating on the R
f
values of Sudan Black B
is shown in Figure 5-4b. When the stationary phase was modified with copolymer coatings
composed mostly of 4VP (EGDMA mole fraction of 0.04), the R
f
value increased slightly, which
is likely due to weaker hydrogen bonding interactions between the 4VP moieties and Sudan
Black B (N--HN) compared to cellulose and Sudan Black B, as well the inability of 4VP
moieties to act as a proton donor. However, the weaker interactions may be offset by the
presence of π-stacking interactions, leading to only a small net change in the R
f
value. As the
mole fraction of EGDMA increased, the R
f
value monotonically increased, which we attribute to
decreased attraction between Sudan Black B and the copolymer coating due to a reduction of π-
stacking interactions. The effect of the copolymer on the R
f
value of Nile Red showed a similar
trend and thus resulted in minimal separation of the two dyes. Nevertheless, these results show
that modifying the stationary phase with polymer coatings allows the R
f
value of analytes to be
tuned over a wide range, which can potentially improve the separation of multi-component
systems.
123

Figure 5-4 - Graphs plotting retardation factors of a) Sudan Black B and Nile Red in solvent
blends of water and methanol on cellulose patterned with fluoropolymer barrier coatings (inset
schematic represents R
f
values of Sudan Black B and Nile Red), and b) Sudan Black B in hexane
on channels coated with copolymers composed of 4VP and EGDMA on devices patterned with
fluoropolymer barrier coatings.

0.00
0.20
0.40
0.60
0.80
1.00
0.00 0.20 0.40 0.60 0.80 1.00
R
f
H
2
O wt. Fraction in Methanol
Sudan Black B
Nile Red
a)
b)
0.00
0.20
0.40
0.60
0.80
1.00
0.00 0.20 0.40 0.60 0.80 1.00
R
f
EGDMA Mole Fraction
124

Table 5-4 - Mole fraction of EGDMA in copolymer coating and corresponding R
f
value of
Sudan Black B with hexane as the mobile phase.
Mole Fraction of EGDMA R
f

0.04 0.28 ± 0.01
0.53 0.50 ± 0.02
0.64 0.71 ± 0.02
1.00 0.87 ± 0.01

125

5.5 Conclusions
We have demonstrated the ability to use organic solvents within paper-based microfluidic
devices by patterning fluoropolymer barriers using iCVD in conjunction with transition metal
salts that inhibit polymer deposition. The patterning resolution of three different methods of
applying the salt were compared, and it was revealed that selective wetting through use of a
hydrophobic photoresist yielded the highest resolution. XPS was used to determine the amount
of fluoropolymer coating necessary for optimal performance of the device. The efficacy of the
fluoropolymer barrier coatings relied on the conformality of the coating around the paper fibers.
However, depositing excess polymer jeopardized the ability of the transition metal salt to inhibit
polymer deposition within the intended channel regions. We demonstrated the utility of using
organic solvents in paper-based microfluidic applications by separating lipophilic dyes by
controlling the composition of the operating solvent and by application of organic polymer
coatings within the channels. Although we provided examples of applications using simple
straight channels, the generality of our technique can be applied to more complex devices and
can broaden the range of available applications for paper-based microfluidic devices by
expanding the possible operating liquids. Additionally, our technique has the potential to
advance current fields that employ the use of hydrophobic or lipophobic regions such as
biosensing, chemical detection, and optics.

126

5.6 Acknowledgements
This work was supported by the National Science Foundation Division of Civil, Mechanical, and
Manufacturing Innovation Award Number 1069328, the Natural Sciences and Engineering
Research Council of Canada Scholarship (P.K.), and the Alfred Mann Institute at the University
of Southern California (P.K.). We thank the Molecular Materials Research Center of the
Beckman Institute of the California Institute of Technology for use of their XPS. Grade 3
Whatman chromatography paper was kindly provided by GE Healthcare.

127

5.7 Supporting Information

Figure 5-S1 - Image defining the variables used to perform resolution analysis on intended
isosceles triangle patterns

Table 5-S1 - Dimensions of the intended regions, patterned regions, and deviations (∆ ) for
painting.
Painting

Sample 1 Sample 2 Sample 3
Distance
From
Apex
(mm)
Intended
Width
(mm)
Pattern
Width
(mm)
Δ
1

(mm)
Pattern
Width
(mm)
Δ
2

(mm)
Pattern
Width
(mm)
Δ
3
(mm)
1.00 0.29 1.07 0.78 1.52 1.23 1.16 0.87
1.50 0.37 1.17 0.80 1.50 1.13 1.15 0.78
2.00 0.43 1.15 0.72 1.31 0.88 1.49 1.06
2.50 0.50 1.73 1.23 1.57 1.07 1.16 0.66
3.00 0.57 1.50 0.93 1.56 0.99 1.04 0.47
3.50 0.65 1.81 1.16 1.61 0.96 1.07 0.42
4.00 0.73 1.37 0.64 1.66 0.93 1.20 0.47
4.50 0.80 1.41 0.61 1.60 0.80 1.32 0.52
5.00 0.89 1.45 0.56 1.57 0.68 1.17 0.28
Average Δ: 0.80 Standard Deviation Δ: 0.26
apex
1 mm from apex 5 mm from apex
intended
width
patterned
width
128

Table 5-S2 - Dimensions of the intended regions, patterned regions, and deviations (∆ ) for spray
coating.
Spray Coating

Sample 1 Sample 2 Sample 3
Distance
From
Apex
(mm)
Intended
Width
(mm)
Pattern
Width
(mm)
Δ
1

(mm)
Pattern
Width
(mm)
Δ
2

(mm)
Pattern
Width
(mm)
Δ
3
(mm)
1.00 0.29 0.73 0.44 0.54 0.25 0.65 0.36
1.50 0.37 0.84 0.47 0.77 0.40 0.76 0.39
2.00 0.43 0.80 0.37 0.96 0.53 0.84 0.41
2.50 0.50 0.95 0.45 1.11 0.61 1.03 0.53
3.00 0.57 0.93 0.36 1.17 0.60 1.04 0.47
3.50 0.65 1.04 0.39 1.19 0.54 0.96 0.31
4.00 0.73 1.25 0.52 1.30 0.57 1.18 0.45
4.50 0.80 1.39 0.59 1.35 0.55 1.15 0.35
5.00 0.89 1.47 0.58 1.40 0.51 1.38 0.49
Average Δ: 0.46 Standard Deviation Δ: 0.10

129

Table 5-S3 - Dimensions of the intended regions, patterned regions, and deviations (∆ ) for
photolithography.
Photolithography

Sample 1 Sample 2 Sample 3
Distance
From
Apex
(mm)
Intended
Width
(mm)
Pattern
Width
(mm)
Δ
1

(mm)
Pattern
Width
(mm)
Δ
2

(mm)
Pattern
Width
(mm)
Δ
3
(mm)
1.00 0.29 0.42 0.13 0.73 0.44 0.62 0.33
1.50 0.37 0.61 0.24 0.82 0.45 0.66 0.29
2.00 0.43 0.67 0.24 0.90 0.47 0.68 0.25
2.50 0.50 0.79 0.29 0.92 0.42 0.80 0.30
3.00 0.57 0.81 0.24 0.97 0.40 0.83 0.26
3.50 0.65 0.85 0.20 1.09 0.44 1.02 0.37
4.00 0.73 0.91 0.18 1.17 0.44 1.13 0.40
4.50 0.80 1.01 0.21 1.25 0.45 1.09 0.29
5.00 0.89 1.35 0.46 1.34 0.45 1.26 0.37
Average Δ: 0.33 Standard Deviation Δ: 0.10

130

Table 5-S4. Ability of wax barriers and fluoropolymer barriers to contain solvents.
Solvent
Contained by
Wax Barriers
Contained by
Fluoropolymer Barriers
Acetone No Yes
Acetonitrile Yes Yes
Aqueous sulfuric acid (5 M) Yes Yes
Aqueous sodium hydroxide (5 M) Yes Yes
Butanol No Yes
Chloroform No Yes
Cyclohexane No Yes
Diethyl ether No Yes
Dimethyl formamide Yes Yes
Dimethyl sulfoxide Yes Yes
Ethanol No Yes
Ethyl acetate No Yes
Hexane No Yes
Isopropanol No Yes
Methanol No Yes
Tetrahydrofuran No Yes
Toluene No Yes
Water No Yes

Determination of relative amount of PPFDA
The relative amount of PPFDA on the surface of the paper devices was determined by comparing
the relative carbon to fluorine atomic ratios, according to Equation 5-S1. This calculation
determines the weight fraction of PPFDA assuming cellulose (monomer unit: (C
6
H
10
O
5
)) makes
up the remaining portion of the surface, where #C and #F represent the atomic percentages of
carbon and fluorine in the sample, respectively, and #C
PPFDA
and #F
PPFDA
represent the atomic
131

percentages of carbon and fluorine in the reference homopolymer PPFDA, as measured on paper
coated with more than 2 μm of polymer to ensure at least 5 nm of coverage. The measured
experimental ratios of carbon, oxygen, and fluorine in the reference PPFDA sample were 38 %, 6
%, and 56 %, which are in close agreement with the theoretical values of 40.625 %, 6.25 %, and
53.125 %, respectively. The weight fraction of PPFDA is determined by subtracting the weight
fraction of cellulose, represented by the fractional term in Equation 5-S1, from unity. The
numerator represents the relative weight of cellulose. The relative amount of carbon associated
with PPFDA is subtracted from the total atomic percentage of carbon and divided by six to
account for the number of carbons per cellulose moiety, and is then multiplied by the molecular
weight of a cellulose moiety (162.14). The denominator represents the total relative weight of
both cellulose and PPFDA, where the factor of 13 accounts for the number of carbons per PFDA
moiety, and the value 518.17 is the molecular weight of a PFDA moiety.

Determination of copolymer composition
The mole fraction of EGDMA in the copolymer coating was determined by comparing the
relative carbon to nitrogen atomic ratios, according to Equation 5-S2, as measured on a reference
silicon wafer. #C and #N represent the atomic percentages of carbon and nitrogen in the sample,
respectively, and #C
P4VP
and #N
P4VP
represent the atomic percentages of carbon and nitrogen in
homopolymer P4VP, as measured experimentally. The measured experimental ratios of carbon
% =1−
162.14×
#−
#

#

×#
6
162.14×
#−
#

#

×#
6
+518.17×
#

#

×#
13
(S1)
132

and nitrogen in the homopolymer sample of P4VP were 90 %, and 10 %, which were in close
agreement with the theoretical values of 87.5 % and 12.5 %, respectively. The numerator
represents the relative number of EGDMA moieties; the relative amount of carbon associated
with 4VP is subtracted from the total atomic percentage of carbon and divided by 10 to account
for the number of carbons per EGDMA moiety. The denominator represents the total number of
both EGDMA and 4VP moieties, where the factor of 7 accounts for the number of carbons per
4VP moiety.

% =
#−
#

#

×#

10

#−
#

#

×#

10
+
#

#

×#

7
(S2)
133

5.8 References

196 Nie, Z.; Nijhuis, C. A.; Gong, J.; Chen, X.; Kumachev, A.; Martinez, A. W.;
Narovlyansky, M.; Whitesides, G. M. Lab Chip 2010, 10, 477-483.
197 Martinez, A. W.; Phillips, S. T.; Whitesides, G. M. Proc. Natl. Acad. Sci. U. S. A. 2008,
105, 19606-19611.
198 Martinez, A. W.; Phillips, S. T.; Whitesides, G. M.; Carrilho, E. Anal. Chem. 2010, 82, 3-
10.
199 Osborn, J. L.; Lutz, B.; Fu, E.; Kauffman, P.; Stevens, D. Y.; Yager, P. Lab Chip 2010,
10, 2659-2665.
200 Yang, X.; Forouzan, O.; Brown, T. P.; Shevkoplyas, S. S. Lab Chip 2012, 12, 274-280.
201 Kwong, P.; Gupta, M. Anal. Chem. 2012, 84, 10129-10135.
202 Su, S.; Ali, M. M.; Filipe, C. D. M.; Li, Y.; Pelton, R. Biomacromolecules 2008, 9, 935-
941.
203 Chen, H.; Cogswell, J.; Constantine, A.; Faghri, M. Lab Chip 2012, 12, 2909-2913.
204 Lu, Y.; Shi, W.; Jiang, L.; Qin, J.; Lin, B. Electrophoresis 2009, 30, 1497-1500.
205 Carrilho, E.; Martinez, A. W.; Whitesides, G. M. Anal. Chem. 2009, 81, 7091-7095.
206 Martinez, A. W.; Phillips, S. T.; Carrilho, E.; Thomas, S. W.; Sindi, H.; Whitesides, G.
M. Anal. Chem. 2008, 80, 3699-3707.
207 Owens, T. L.; Leisen, J.; Beckham, H. W.; Breedveld, V. ACS Appl. Mater. Interfaces
2011, 3, 3796-3803.
208 Bhandari, P.; Narahari, T.; Dendukuri, D. Lab Chip 2011, 11, 2493-2499.
209 You, I.; Yun, N.; Lee, H. ChemPhysChem 2013, 14, 471-481.
134

210 World Health Organization Basic Tests for Pharmaceutical Dosage Forms; 1991.
211 Han, S.; Xue, Z.; Wang, Z.; Wen, T. B. Chem. Commun. 2010, 46, 8413-8415.
212 O’Brien, R. D. J. Biol. Chem. 1956, 219, 927-931.
213 Kim, J.-W.; Rainina, E. I.; Mulbry, W. W.; Engler, C. R.; Wild, J. R. Biotechnol. Prog.
2002, 18, 429-436.
214 Yabu, H.; Takebayashi, M.; Tanaka, M.; Shimomura, M. Langmuir 2005, 21, 3235-3237.
215 Mao, J.; Ni, P.; Mai, Y.; Yan, D. Langmuir 2007, 23, 5127-5134.
216 Patri, M.; Hande, V. R.; Phadnis, S.; Deb, P. C. Polym. Adv. Technol. 2004, 15, 622-627.
217 Jung, J.-C.; Hwang, I.-T.; Kuk, I.-S.; Choi, J.-H.; Oh, B.-K.; Lee, Y.-M. ACS Appl.
Mater. Interfaces 2013, 5, 2155-2160.
218 Rolland, J. P.; Van Dam, R. M.; Schorzman, D. A.; Quake, S. R.; DeSimone, J. M. J. Am.
Chem. Soc. 2004, 126, 2322-2323.
219 Taberham, A.; Kraft, M.; Mowlem, M.; Morgan, H. J. Micromech. Microeng, 2008, 18,
064011.
220 Begolo, S.; Colas, G.; Viovy, J.-L.; Malaquin, L. Lab Chip 2011, 11, 508-512.
221 Riche, C. T.; Marin, B. C.; Malmstadt, N.; Gupta, M. Lab Chip 2011, 11, 3049-3052.
222 Prissanaroon, W.; Brack, N.; Pigram, P. J.; Hale, P.; Kappen, P.; Liesegang, J. Synth.
Met. 2005, 154, 105-108.
223 Tamura, H.; Kojima, R.; Usui, H. Appl. Opt. 2003, 42, 4008-4015.
224 Vargo, T. G.; Calvert, J. M.; Wynne, K. J.; Avlyanov, J. K.; MacDiarmid, A. G.; Rubner,
M. F. Supramol. Sci. 1995, 2, 169-174.
225 Kwong, P.; Flowers, C. A.; Gupta, M. Langmuir 2011, 27, 10634-10641.
135

226 Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38,
9742-9748.
227 Ma, M.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K. K.; Cohen, R. E.; Rubner, M. F.;
Rutledge, G. C. Adv. Mater. 2007, 19, 255-259.
228 Tenhaeff, W. E.; McIntosh, L. D.; Gleason, K. K. Adv. Funct. Mater. 2010, 20, 1144-
1151.
229 Im, S. G.; Kim, B.-S.; Lee, L. H.; Tenhaeff, W. E.; Hammond, P. T.; Gleason, K. K.
Macromol. Rapid Commun. 2008, 29, 1648-1654.
230 Mao, Y.; Felix, N. M.; Nguyen, P. T.; Ober, C. K.; Gleason, K. K. J. Vac. Sci. Technol. B
2004, 22, 2473-2478.
231 Im, S. G.; Bong, K. W.; Kim, B.-S.; Baxamusa, S. H.; Hammond, P. T.; Doyle, P. S.;
Gleason, K. K. J. Am. Chem. Soc. 2008, 130, 14424-14425.
232 Chang, J.-P.; El Rassi, Z.; Horváth, C. J. Chromatogr. 1985, 319, 396-399.
233 Hjertén, S.; Mohammad, J.; Eriksson, K.-O.; Liao, J.-L. Chromatographia 1991, 31, 85-
94.
234 Desilets, C. P.; Rounds, M. A.; Regnier, F. E. J. Chromatogr. 1991, 544, 25-39.
235 Sing, Y. L. K.; Kroviarski, Y.; Cochet, S.; Dhermy, D.; Bertrand, O. J. Chromatogr.
1992, 598, 181-187.
236 Haller, P. D.; Flowers, C. A.; Gupta, M. Soft Matter 2011, 7, 2428-2432.
237 Thomas, S. W.; Chiechi, R. C.; LaFratta, C. N.; Webb, M. R.; Lee, A.; Wiley, B. J.;
Zakin, M. R.; Walt, D. R.; Whitesides, G. M. Proc. Natl. Acad. Sci. U. S. A. 2009, 106,
9147-9150.
136

6.0 Conclusions and Future Work
The ability to pattern polymer coatings on porous substrates has been a significant technological
hurdle for a wide range of applications, including the development of paper-based diagnostics.
In this work, we have demonstrated how the initiated chemical vapor deposition (iCVD) process
can be applied to solve this issue by using transition metal salts to prevent deposition in select
areas. The transition metal salts quench the radical species via reduction of the oxidation state of
the transition metal, resulting in an increased amount of deposition in the areas free of salt. This
preferential deposition is temporary; however, the deposition conditions can be adjusted to
optimize the fidelity of the pattern. The ability to fabricate patterns is restricted by the
requirement that the transition metal salt must first be patterned onto the desired substrate.
Consequently, we have demonstrated several methods of patterning the transition metal salt on
chromatography paper. The utility of this patterning technique has been demonstrated by the
development of additional functionality for paper-based microfluidic platforms.
CVD techniques have started receiving recognition for their potential applicability towards
paper-based diagnostics.
238
However, in order for this patterning technique to be more broadly
adopted towards paper-based diagnostics as well as additional applications, a number of
challenges still need to be solved. Perhaps most importantly is the ability to generate transition
metal salt patterns on other substrates. Our patterning of transition metal salts on
chromatography paper relied on the paper being hydrophilic such that a salt solution would
adsorb throughout the depth of the paper in a relatively uniform manner; patterning of the salt is
likely to be more difficult on hydrophobic and non-absorbing substrates due to dewetting of the
salt solution. Additionally, the ability to generate salt patterns with small features is an area that
137

requires improvement. This has been so far limited by the hygroscopic nature of the salts, which
leads to the absorption of water, spreading of the salts on the substrate, and subsequent loss of
pattern resolution. Additional research regarding the optimization of a transition metal salt ink
formulation, including an appropriate selection of a salt, would help to solve these challenges.

238 Cate, D. M.; Adkins, J. A.; Mettakoonpitak, J.; Henry, C. S. Anal. Chem., 2015, 87, 19-
41.

Abstract (if available)

Abstract The patterning of polymer coatings on porous substrates cannot be easily achieved via either solution phase or vapor phase methods, and represents a technical hurdle for a number of potential applications. This dissertation describes the patterning of polymer coatings deposited onto porous substrates using the initiated chemical vapor deposition (iCVD) process. The patterning of polymer coatings in the iCVD process is achieved by using transition metal salts to quench radical species and is discussed primarily for the development of paper-based microfluidic devices, although this technique can also be extended to a number of other applications. ❧ This report is divided into six sections. The introduction will provide a broad overview of chemical vapor deposition (CVD) processes and discuss the application of CVD techniques for the development of diagnostic platforms. The second section will discuss original research showing the general utility of polymer coatings incorporated into paper-based microfluidic platforms by demonstrating an improvement in the separation of small molecules using ionizable coatings. The third section will provide a broad screening of the ability of transition metal salts to prevent the deposition of polymer, while the fourth section will demonstrate that deposition is prevented by quenching of the radical species via a reduction of the transition metal salt. Additionally, the fourth section will quantify this prevention of deposition and discuss how to optimize the deposition parameters in order to improve the fidelity of the pattern. In the fifth section, numerous methods of patterning transition metal salts onto chromatography paper are presented and the utility of these patterning techniques for paper-based microfluidic devices is demonstrated through the development of fluoropolymer barriers that allow these platforms to be compatible with organic solvents. The final section will provide an overview of this work and discuss future challenges in the field.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Scale-up of vapor-phase deposition of polymers: towards large-scale processing

PDF

Fabrication of polymer films on liquid substrates via initiated chemical vapor deposition: controlling morphology and composition

PDF

Vapor phase deposition of dense and porous polymer coatings and membranes for increased sustainability and practical applications

PDF

Selective deposition of polymer coatings onto structured surfaces

PDF

Understanding structural changes of polymer films during vapor phase deposition

PDF

Controlling polymer film patterning, morphologies, and chemistry using vapor deposition

PDF

Formation of polymer gels, films, and particles via initiated chemical vapor deposition onto liquid substrates

PDF

Simultaneous monomer deposition and polymerization at low substrate temperatures for the formation of porous polymer membranes

PDF

Vapor phase deposition of polymers in the presence of low vapor pressure liquids

PDF

Novel processing of liquid substrates via initiated chemical vapor depostion

PDF

Inorganic/organic hybrid materials and grafted coatings via vapor phase deposition processes

PDF

Fabrication of functional porous membranes via polymerization of solid monomer by a vapor-phase initiator

PDF

Fabrication of silicon-based membranes via vapor-phase deposition and pyrolysis of organosilicon polymers

PDF

Rapid microfluidic prototyping for biological applications via stereolithographic 3-D printing: a journey

PDF

Synthesis of high-quality nanoparticles using microfluidic platforms

PDF

Chemical vapor deposition of graphene and two-dimensional materials: synthesis, characterization, and applications

PDF

Fabrication and study of organic and inorganic optoelectronics using a vapor phase deposition (VPD)

PDF

Expanded functionality and scalability of modular fluidic and instrumentation components

PDF

Real time surface analysis of complex oxide thin films during pulsed laser deposition

PDF

Strategies for improving mechanical and biochemical interfaces between medical implants and tissue

Asset Metadata

Creator Kwong, Philip (author)

Core Title The patterning of polymer thin films on porous substrates via initiated chemical vapor deposition

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Chemical Engineering

Publication Date 03/23/2015

Defense Date 02/26/2015

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag coatings,CVD,microfluidics,OAI-PMH Harvest,patterning,polymer,thin films

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Gupta, Malancha (committee chair), Malmstadt, Noah (committee member), Meng, Ellis (committee member)

Creator Email cold_storage99@hotmail.com,philipkw@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-540320

Unique identifier UC11297615

Identifier etd-KwongPhili-3239.pdf (filename),usctheses-c3-540320 (legacy record id)

Legacy Identifier etd-KwongPhili-3239.pdf

Dmrecord 540320

Document Type Dissertation

Format application/pdf (imt)

Rights Kwong, Philip

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA