electrical characterization of gold and platinum thin film
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Clemson UniversityTigerPrints
All Theses Theses
12-2015
Electrical Characterization of Gold and PlatinumThin Film Electrodes With Polyaniline ModifiedSurfacesJohn AggasClemson University, jraggas@clemson.edu
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Recommended CitationAggas, John, "Electrical Characterization of Gold and Platinum Thin Film Electrodes With Polyaniline Modified Surfaces" (2015). AllTheses. 2259.https://tigerprints.clemson.edu/all_theses/2259
ELECTRICAL CHARACTERIZATION OF GOLD AND PLATINUM THIN FILM
ELECTRODES WITH POLYANILINE MODIFIED SURFACES
A Thesis
Presented to
the Graduate School of
Clemson University
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Electrical Engineering
by
John Richard Aggas
December 2015
Accepted by:
Dr. Anthony Guiseppi-Elie, Committee Chair
Dr. Rod Harrell
Dr. Daniel Noneaker
ii
ABSTRACT
Recent studies into soft organic electronics have burgeoned as a result of
discoveries of conducting polymers such as polyaniline, polythiophene, and polypyrrole.
However, in order to make these conducting polymers suitable for in vivo soft organic
electronics, they must be developed so that they can be biocompatible and provide
accurate sensing. Chitosan, a naturally occurring polymer structure found in
exoskeletons of crustaceans, has been studied for its biocompatible properties.
Composites of polyaniline (PAn), an intrinsically conductive polymer (ICP) and
chitosan (Chi), a biopolymer, were developed and applied to gold and platinum Thin
Film Electrode (TFE) devices. Electropolymerization and drop cast deposition were
utilized to modify TFEs with a thin film of PAn or PAn-Chi composite. The impedance
response over a spectrum of frequencies was studied for blank control TFEs, platinized
TFEs, and platinized TFEs with various polyaniline coatings. Impedance measurements
were taken in dry environments, DI Water, and in buffers such as PBS, and HEPES.
Current-Voltage (I-V) characterization was used to study the current response and SEM
imaging was used to study the surface topography. Resistance was measured for PAn
modified unplatinized gold TFEs with varying amounts of incorporated chitosan.
Impedance measurements of control and platinized TFEs yielded results similar to
a low pass filter. Due to the conductive nature of polyaniline, the impedance of TFEs
decreased substantially after poylaniline deposition. Measured resistance values for
polyaniline and chitosan composites on TFEs revealed a window of concentrations of
incorporated chitosan to lower resistance.
iii
ACKNOWLEDGMENTS
I would first like to sincerely thank Dr. Anthony Guiseppi-Elie, who gave me the
opportunity to pursue my Masters of Science under his guidance. I feel very fortunate to
work for a professor who cares so much for his students. His genuine kindness,
motivational techniques, and knowledge of a wide variety of topics have allowed me to
branch out of my traditional electrical engineering background. His love for
bioengineering has sparked a new interest in learning for me, which I plan to continue as
I pursue a PhD. I look forward to researching with him in the future and solving real life
problems together.
My thesis committee was also a fundamental aspect of my success. Professor
Rod Harrell and Professor Daniel Noneaker were both excellent committee members.
Their notes on drafts of my thesis helped to make my final paper an excellent product.
I would also like to thank Dr. Christian Kotanen for his continued support and
leadership over the past year. His hard work and helpful demeanor over the time we have
worked together has been crucial for the completion of this thesis as well as lab work in
general.
I was also fortunate enough to have worked with some excellent co-workers
throughout my time at Clemson. Guneet Bedi, who showed me the ins and outs of the
C3B Lab when I joined, was an excellent guide. Stephanie Kubecka, who worked with
me for the bulk of the summer of 2015, was a vital asset for data collection and
representation.
iv
TABLE OF CONTENTS
Page
TITLE PAGE .................................................................................................................... i
ABSTRACT ..................................................................................................................... ii
ACKNOWLEDGMENTS .............................................................................................. iii
LIST OF TABLES .......................................................................................................... vi
LIST OF FIGURES ....................................................................................................... vii
1. INTRODUCTION ............................................................................................... 1
1.1 Overview of Organic Electronics............................................................. 1
1.1.1 Polyaniline ...................................................................................... 2
1.1.2 Chitosan .......................................................................................... 4
1.1.3 Electroconductive Hydrogels .......................................................... 4
1.2 Overview of Thin Film Electrodes .......................................................... 5
1.2.1 Electrode Surface Modification ...................................................... 7
1.2.2 Electrode Environment Modification ............................................. 8
1.3 Research Goals......................................................................................... 9
2. MATERIALS AND EXPERIMENTAL METHODS ....................................... 10
2.1 Materials ................................................................................................ 10
2.2 Methods.................................................................................................. 12
2.2.1 Electrode Preparation .................................................................... 11
2.2.2 Electrode Surface Modification .................................................... 12
2.2.3 Electrode Environment Modification ........................................... 16
2.2.4 Characterization Procedures ......................................................... 16
3. RESULTS AND DISCUSSION ........................................................................ 19
3.1 Impedance Characterization of Thin Film Electrodes ........................... 19
3.1.1 Electrodes without Environmental Modification .......................... 19
3.1.2 Electrodes with Environmental Modification ............................... 21
3.2 Equivalent Circuits Modeling ................................................................ 23
3.2.1 Equivalent Circuit Data................................................................. 24
v
Table of Contents (Continued) Page
3.3 IV Characterization of Thin Film Electrodes ......................................... 26
3.4 Resistance Characterization of Thin Film Electrodes
with Chitosan Coating ...................................................................... 29
3.5 SEM Characterization of Thin Film Electrodes ..................................... 31
3.5.1 Blank Control (No Alterations) TFEs .......................................... 31
3.5.2 Platinized TFEs ............................................................................. 32
3.5.3 Platinized/PAn Deposition TFEs .................................................. 33
4. Conclusions .................................................................................................. 35
4.1 Summary ................................................................................................ 35
4.2 Future Work ........................................................................................... 36
APPENDICES ............................................................................................................... 37
A: Equivalent Circuit Graphs............................................................................ 37
BIBLIOGRAPHY .......................................................................................................... 40
vi
LIST OF TABLES
Table Page
3.1 TFE-Au impedance magnitude measurements at various frequencies ........ 22
3.2 TFE-Pt impedance magnitude measurements at various frequencies.......... 22
3.3 Equivalent circuit parameters for TFE-Au .................................................. 24
3.4 Equivalent circuit parameters for TFE-Pt .................................................... 24
3.5 TFE-Au resistivity and conductivity from I-V characterization .................. 28
3.6 TFE-Pt resistivity and conductivity from I-V characterization ................... 28
vii
LIST OF FIGURES
Figure Page
1.1 Oxidation states of polyaniline ...................................................................... 3
1.2 Cyclic voltammogram and measured conductivity
of states of polyaniline .................................................................................. 4
1.3 TFE-Pt and labeled dimensions ..................................................................... 6
1.4 SEM image of the gap between semicircular electrodes ............................... 7
2.1 Fabricated TFE (Pt shown) sample connected to a 4-pin electrode
wand with epoxy-lined heat shrink tubing for insulation ........................... 11
2.2 Experimental setup for platinization ............................................................ 13
2.3 Platinized TFE-Pt ......................................................................................... 13
2.4 Various amounts of chitosan placed on semicircular
electrodes ..................................................................................................... 14
2.5 Platinized/PAn TFE-Pt directly after polyaniline
deposition and 48 hours later ....................................................................... 15
2.6 Randles circuit ............................................................................................. 17
3.1 TFE-Au impedance measurements without environment
modification ................................................................................................ 20
3.2 TFE-Pt impedance measurements without environment
modification ................................................................................................. 20
3.3 TFE-Au impedance measurements including measurements
in DI Water .................................................................................................. 21
3.4 TFE-Pt impedance measurements including measurements
in PBS ......................................................................................................... 22
3.5 TFE-Au and TFE-Pt after impedance measurements in
buffers .......................................................................................................... 23
viii
List of Figures (Continued) Page
3.6 TFE-Au and TFE-Pt Rs values .................................................................... 25
3.7 TFE-Au and TFE-Pt Rp values .................................................................... 25
3.8 I-V Characterization of Blank Control Gold and
Platinum TFEs ............................................................................................. 27
3.9 I-V Characterization of platinized Gold and
Platinum TFEs ............................................................................................. 27
3.10 I-V Characterization of platinized/PAn Gold and
Platinum TFEs ............................................................................................. 27
3.11 Electropolymerization of Analine and meta-amino
aniline to form polyaniline on a gold TFE ................................................... 30
3.12 An example of highly porous chitosan scaffolding
(Soleri et al., 2015)...................................................................................... 31
3.13 Blank TFE-Au SEM .................................................................................... 32
3.14 Blank TFE-Pt SEM ...................................................................................... 32
3.15 Platinized TFE-Au SEM .............................................................................. 33
3.16 Platinized TFE-Pt SEM................................................................................ 33
3.17 Platinized/PAn TFE-Au SEM ...................................................................... 34
3.18 Platinized/PAn TFE-Pt SEM ....................................................................... 34
3.19 Fractal PAn Structures ................................................................................. 34
1
CHAPTER ONE
INTRODUCTION
1.1 Overview of Organic Soft Electronics
As applications of electronics become more common in every aspect of human
life, from the mobile phones in our pockets to satellites that orbit the earth, functional
integration into biomedical applications is a logical next step. If a functional electronic
device is to be successfully directly implanted into a human, a new approach to
traditional electronics must be employed. Traditional electronic hardware is not directly
suitable for integration into the soft tissue and organs of the human body. Silicon devices
governed by Moore’s law have been documented at sizes less than 20 nm (Mohapatra,
Pradhan, Sahu, Singh, & Panda, 2014). Even though there has been great success with
shrinking the size of these devices, the oxidative instability and the rigid nature of silicon
does not make it ideal for direct contact in the human body. Soft electronics, which are
much more flexible than traditional electronics and match the modulus of human tissue,
are being studied as a replacement for silicon (Chae & Lee, 2014). Soft organic
electronics, as opposed to traditional metals and periodic semiconductors, are materials
developed using organic polymers that are capable of electrical conduction based on their
state of oxidation/reduction (Malliaras, 2013). The 1970’s marked the discovery of
intrinsically conducting polymers (ICP’s), which quickly became a widely studied area
for several different applications (Bhadra, Khastgir, Singha, & Lee, 2009). There are a
wide range of devices used in biomedicine that may use organic electronic interfaces,
such as deep brain stimulating electrodes, implantable microanalytical systems,
2
intraocular implants, and cochlear implants. The goal of utilizing organic electronic
interfaces is to provide a more reliable ABIO-BIO interface as well as to enable
functional electronic circuits at the interface.
Organic electronics, or electronics utilizing carbon based materials, are composed
of 3 groups: small molecular materials, fullerenes and nanotubes, and polymers. Today,
several polymers such as polypyrrole, polythiophene, polyphenylene, and polyaniline are
studied for their conductive properties (Balint, Cassidy, & Cartmell, 2014).
These organic electronics, which allow for functionality to be designed and
tailored are not without drawbacks. These materials traditionally have low carrier
mobility, which can limit the switching speed of organic electronic devices (Salleo,
2015). Switching speed and the issue of biocompatibility can be addressed by using
biopolymers such as chitosan. Also, the use of conductive polymers such as polyaniline
allow for decreased resistance.
1.1.1 Polyaniline
Polyaniline is one of the most widely studied conductive polymers in modern
science due to its low cost, simple synthesis, and high conductivity (Ćirić-Marjanović,
2013). Compared to other ICP’s, polyaniline exhibits a lower conductivity but is highly
stable and readily processed (Bhadra et al., 2009). Applications of polyaniline range
from microelectronic devices, displays, lightweight battery electrodes, anticorrosive
coatings, and sensors. Polyaniline exists in several forms, depending on the degree of
oxidation or extent of protonation (Bober, Humpolíček, Pacherník, Stejskal, & Lindfors,
2015). Several molecular structures are shown in Figure 1.1. Studies of polyaniline
3
nanofibers have shown that they assemble into porous and high surface area electrodes
(Jeon, Kwon, & Lutkenhaus, 2015). Deposition of polyaniline nanofibers onto and
across co-planar Thin Film Electrodes will result in lower measured electrochemical
impedance (Sarac, Ates, & Kilic, 2008). The magnitude of that lowering will depend
upon the oxidation state of the polymer. From Figure 1.1, the most conductive state of
polyaniline is the emeraldine salt (ES). This green polyaniline exhibits a conductivity of
𝜎 ≈ 100 S cm−1. This conductivity is much higher than that of most polymers
(<10−9 S cm−1) but lower than metals (>104 S cm−1) (Stejskal & Gilbert, 2002). Figure
1.2 further explains why polyaniline is of great interest in organic electronics. The green
dots show a measure of the conductivity during the application of the sweeping voltage of
the potentiostat. This clearly distinguishes the non-conductive states and the highly
conductive state of the polymer (between 0.2 V and 0.7 V).
Figure 1.1 Oxidation states of polyaniline
4
Figure 1.2 Cyclic voltammogram and measured conductivity of states of
polyaniline
1.1.2 Chitosan
Recent bio implantable devices have utilized chitosan, a polysaccharide derived
from chitin (Kim et al., 2014). Chitosan is the second most plentiful biopolymer found
on Earth, and is known for its ability in sorption of divalent metal ions from aqueous
solutions (Soleri et al., 2015). In hydrogel form, chitosan is used in several fields such as
food packaging, wastewater treatment, and sensing (Thanpitcha, Sirivat, Jamieson, &
Rujiravanit, 2006). Blending intrinsically conductive polymers and bio-derived
hydrogels such as chitosan has becoming increasingly popular in biosensor research
5
because such composites exhibit enhanced conductivity as well as biocompatibility
(Thanpitcha et al., 2006).
1.1.3 Electroconductive Hydrogels
Hybrid polymers (composites, blends, interpenetrating networks or alloys)
combining these two types of polymers, intrinsically conductive polymers and natural
biopolymers, give rise to a new class of responsive polymers called electroconductive
hydrogels (ECHs). The research group of Prof. Guiseppi-Elie has pioneered these novel
polymers that have potential to be used as coatings on deep brain stimulating electrodes,
electro-stimulated controlled release devices, implantable bioanalytical biosensors, as soft
electronic circuits and as coatings to control the ABO-BIO interface. Conductive
polymers have been successfully polymerized inside hydrogel networks (Balint et al.,
2014). The creation of electroactive hydrogels combines redox switching abilities of
intrinsically conductive polymers with the ion mobility of hydrogels (Justin & Guiseppi-
Elie, 2009).
1.2 Overview of Thin Film Electrodes
The Thin Film Electrodes used in this body of work are laser cut gold (TFE-Au)
and platinum (TFE-Pt) sheets mounted on a flexible, polyester plastic backing. These
Thin Film Electrodes allow for a high throughput manufacturing of low cost devices that
may be produced in sheets or rolls and processed in a continuous operation. This allows
for quick device synthesis and investigation into conductivity and impedance under
several test conditions.
6
Geometry
The dimensions of the TFEs used in this work are shown in Figure 1.3. The
device (2.0 cm long x 1.0 cm wide x 0.05 cm thick) is separated into 4 electrodes, each of
gold (Au) or platinum (Pt), and each with its own bonding pad. Each inner bonding pad
is connected to its respective semicircular electrode, while the outer bonding pads are
connected to the outer electrodes. An adhesive-backed silicone layer separates the
bonding pads from their respective electrodes.
Figure 1.3 TFE-Pt and labeled dimensions
The laser cut gap separating the 4 electrodes is approximately ~50 μm in width.
Figure 1.4 shows an SEM Image of the gap between the semicircular electrodes of the
TFE-Pt.
Outer electrodes
Semicircular electrodes
7
Figure 1.4 SEM image of the gap between semicircular electrodes
Design
These TFEs are designed for testing point to point conductivity and impedance
responses of thin polymer films subjected to environmental changes. The gaps between
the electrodes can be bridged by deposition of polyaniline nanofiber suspension.
1.2.1 Electrode Surface Modification
Platinization
Before deposition of polyaniline on a TFE device, an important preprocessing
step is platinization. Platinization, or the process of depositing a thin film of platinum on
an electrode, is a very useful process for altering surface properties of electrodes.
Platinization increases the surface roughness of an electrode. A rough surface is
advantageous because it increases the working surface area of the electrode, which can
~50 μm
8
allow for greater sensitivity in biosensors (Silpa, 2007). Rougher surface areas are also
amenable for greater adhesion of deposited polymers (Vidal, Garcia-ruiz, & Castillo,
2000), and finally, platinized surfaces may be electrocatalytic (Qin & Guo, 2012).
1.2.2 Electrode Environment Modification
When developing biologically responsive circuit elements, it is important to
understand that these devices must be biocompatible and be able to work in a variety of
environments. Several different buffers are commonly used in experiments to resist
changes in hydrogen ion concentration and emulate the physiological environment. This
experiment explores 3 test environments:
DI Water
Deionized water is essentially water with mineral ions such as sodium, calcium,
iron, copper, chloride, and bromide removed. Without dissolved solids in the solution,
DI water tends to react with metals with which it contacts (Ahmed & Reifsnider, 2011).
It has a conductivity of 5.5 ∗ 10−8 S cm−1 when degassed but falls to 7.5 ∗ 10−5 S cm−1
when it contains dissolved CO2.
PBS
Phosphate-buffered saline (PBS) is commonly used in biosensor experiments
because its osmolarity and ion concentrations match those found in the human body. The
salt ions contained in the buffer keep the salt ions inside of a cell balanced while
maintaining a physiologically relevant pH.
9
HEPES
HEPES, which is one of Good’s buffers, is a zwittertonic buffer commonly used
in biosensor experiments. HEPES is a popular buffer because it maintains pH regardless
of changes caused by cellular respiration much better than other buffers (Baicu & Taylor,
2002).
1.3 Research Goals
The goal of this research is to development of a reliable abiotic-biotic interface
using Thin Film Electrodes with polyaniline and chitosan. This body of work will
discuss the materials and methods used to fabricate Thin Film Electrode testing devices,
platinization of these devices, polyaniline and chitosan deposition, and device testing
processes. The application of conductive polyaniline nanofibers and chitosan hydrogels
onto and across co-planar Thin Film Electrodes will result in lower measured
electrochemical impedance which can be used as a reliable abiotic-biotic interface.
10
CHAPTER TWO
MATERIALS AND EXPERIMENTAL METHODS
2.1 Materials
Gold and platinum Thin Film Electrodes (TFE-Au, TFE-Pt) with 2 inner
semicircular electrodes and 2 outer electrodes were purchased from ABTECH Scientific,
Inc. (Richmond, VA). The inner semicircular electrodes share a diameter of 7.00 mm
and a combined area of 0.385 cm2. The semicircular electrodes were separated by a laser
cut gap 50 μm in width. Printed circuit board wands used to attach the TFEs to various
instruments were also purchased from ABTECH Scientific, Inc. A 20 mL platinizing
solution was prepared using 0.1 M KCl, 2.0 mM H2PtCl6, and 1.0 mM Pb(C2H3O2)
(Aldrich). A polyaniline (2mM) nano-fiber suspension was synthesized by the group of
Dr. Jodie Lutkenhaus of the Department of Chemical Engineering at Texas A&M
University. J. Mater. Chem. A 12/2014; 3(7). DOI:10.1039/C4TA04697H. Chitosan was
supplied by Aber Technologies (France) and its molecular weight (125,000 g∙mol-1) was
determined using size exclusion chromatography (SEC) coupled with light scattering and
refractometry as previously reported (Ruiz, Sastre, Zikan, & Guibal, 2001). The degree of
deacetylation was determined by Fourier Transform Infrared (FTIR) spectroscopy and
was reported to be 87% (Guibal, Larkin, Vincent, & Tobin, 1999). Phosphate Buffered
Saline (PBS) was prepared from tablets (MP Biomedicals, Fisher) dissolved in ultrapure
DI water to create a solution of 10 mM concentration with a pH of 7.4. A 15 mM HEPES
buffer solution (CAS: 7365-45-9) was used with a pH of 7.25. De-ionized, ultrapure
11
water with a pH of 7.0 (CAS: 7732-18-5) was prepared by purifying reverse osmosis
water through a Milli-Q® plus (Millipore Inc.) ultrapure water system.
2.2 Methods
2.2.1 Electrode Preparation
The Thin Film Electrodes were connected to the 4-pin electrode wand first by
gluing (cyanoacrylate) the chip to the seat of the wand and electrically connected using
conductive copper tape. Pieces of the copper tape were approximately 1.25 mm x 0.15
mm to allow for the 4 gold plated conductors on the wand to attach to the 4 bonding pads
of the TFE. The connections of the copper tape to the bonding pads of the TFE as well as
the gold plated conductors of the wand were then covered for protection using 1.5 mm of
epoxy-lined heat shrink tubing. A completed TFE testing device is shown in Figure 2.1.
Figure 2.1 Fabricated TFE (Pt shown) sample connected to a 4-pin electrode wand with
epoxy-lined heat shrink tubing for insulation
The TFEs were cleaned following the standard operating procedure for cleaning
electrodes made by ABTECH Scientific, Inc. (Guiseppi-Elie, 1995). This process
12
involves washing the electrode in isopropyl alcohol for 10 seconds, followed quickly by
blow drying with nitrogen gas for no less than 20 seconds until there were no visible
liquid droplets. The electrodes were then UV cleaned (Model 13550, Boekel Industries,
Inc. Feasterville, Pennsylvania) for 20 minutes.
2.2.2 Electrode Surface Modification for AC Impedance Measurements
Electrode modifications were performed in order to examine the effects of surface
chemistry and topography on electrical properties. The surface modifications used in this
experiment include no modification (control), chitosan deposition, platinization, and
polyaniline deposition.
Platinization
After cleaning, the surfaces of the TFEs were notably smooth. In order to
increase surface roughness, a platinization process was used. The platinizing solution
used was placed in a plastic beaker inside of a Faraday cage. The process of platinization
involves the use of the Model 273 potentiostat/galvanostat (EG&G, Princeton, NJ). The
lead to the working electrode was attached to the two semicircular electrodes on the TFE.
The TFE, a platinum mesh counter electrode, and a Ag/AgCl (3 M KCl) reference
electrode were placed in the platinizing solution at RT. All potentials reported are with
respect to the Ag/AgCl reference unless otherwise noted. Figure 2.2 shows the
experimental setup for platinization. The electrochemical platinization was completed by
applying a potential of -0.1mV until a total charge of 38.5 mC was reached, which
allowed for an electric flux density of 100 mC/cm2 on the two semicircular electrodes.
After this process, the device was immediately taken out of the platinization solution,
13
rinsed in DI water and gas dried using nitrogen. Figure 2.3 shows a TFE after
platinization.
Figure 2.2 Experimental setup for platinization
Figure 2.3 Platinized TFE-Pt
(Ag/AgCl)
14
Chitosan Foam Deposition using Lyophilization
Chitosan solutions (2 mg/mL) were prepared in DI water. Various amounts of
chitosan solution ranging from 10-40 µL were drop casted onto the 2 center semicircular
electrodes of the TFE. These are shown in Figure 2.4. The chitosan was allowed to dry
overnight and immediately put into a freezer at -80 for 3 hours. Lyopholization, or
freeze-drying, involves removal of a solvent directly from the solid phase to vapor phase.
It has been reported to keep the structure of porous nano-materials intact (Mandal, Cliff,
& Pancrazio, 2015). Lyophilization was performed on the chitosan at -50 at 1000 Pa
after drying overnight. The foam resulting from this procedure was placed in 1.0M
NaOH at 3.85 for 3 hours and finally rinsed with a 50/50 mix of EtOH and water.
Figure 2.4 Various amounts of chitosan drop-casted onto the center semicircular
electrodes of TFEs
15
Polyaniline Deposition
In this experiment, polyaniline was deposited using 2 different methods. The first
method was applied to TFEs which had been first platinized. After platinization, a layer
of 15 μL of polyaniline (2 mM) was drop casted on top of the 2 semicircular electrodes.
The devices were then allowed to air dry at 20 for 48 hours. This process allows for
the liquid to evaporate, leaving behind the polyaniline nano-fibers on the TFE. Figure
2.5 shows a platinized/PAn TFE-Pt directly after polyaniline deposition and the dried
TFE 48 hours later.
Figure 2.5 Platinized/PAn TFE-Pt directly after polyaniline deposition and 48 hours later
The second method of polyaniline deposition was applied to TFEs which were
first drop casted with chitosan foam. The polyaniline was electropolymerized onto the
TFE with an applied voltage of 800 mV at various electropolymerization times (0, 5, 10,
16
20, and 25 seconds). Various times were studied in order to determine the effect of
electropolymerization times on resistance response.
2.2.3 Electrode Environment Modification for AC Impedance Measurements
Electrodes were interrogated in multiple solutions to measure the environmental
effect on impedimetric response. As the applications for these electrodes cover a variety
of fields, multiple environments must be characterized to ensure versatility. A small
amount of each solution (environment) was poured into a cleaned storage container for
each TFE up to the plastic covering between the electrodes and the connective bonding
pads. The environments used in this experiment include air (control), Phosphate
Buffered Saline (PBS), HEPES, and DI water.
PBS tablets from MP Biomedicals were mixed into ultrapure DI water to create a
solution of 10 mM concentration with a pH of 7.4. A 15 mM HEPES solution (CAS:
7365-45-9) was used with a pH of 7.25. De-ionized, ultrapure water (CAS: 7732-18-5)
was also used with a pH of 7.0 (Milipore System).
2.2.4 Characterization Procedures
Several methods were used in order to characterize and study the TFEs used.
Impedance spectroscopy was used to study the impedance response at a wide range of
frequencies in order to determine the energy storage and dissipation properties of TFEs.
Equivalent circuit values, derived from equivalent circuit models, were calculated from
the AC impedance measurements. Current-Voltage (I-V) characterization was used to
study the relationship between the current through the electrodes over a range of DC
voltages applied. DC resistance characterization was used to study the resistance of TFEs
17
coated with chitosan and electropolymerized with polyaniline. Finally, SEM imaging
was used in order to examine surface topography and composition of the electrodes with
different surface modifications.
AC Impedance Characterization
AC impedance characterization was completed using the Schlumberger Model
1060A Impedance/Gain-Phase Analyzer. Impedance spectra were taken from an AC
signal of 50 mV peak-to-peak applied over a range of 0.1 Hz to 1.0 MHz. The device
under tests was housed in a Faraday cage during data acquisition.
Equivalent Circuits
Equivalent circuits were calculated from the spectral data collected from AC
impedance characterization. The ZView 2 computer program was used to observe the
results and calculate equivalent low pass Randles R(R-CPE) circuits as well as determine
the goodness of fit (R2) to the data collected. The Randles circuit is shown in Figure 2.6.
Figure 2.6 Randles circuit
The resistance Rs acts as an electrolyte resistance, and the resistance Rp acts as
the faradaic reaction resistance (Rashid, Sabir, Rahim, & Waware, 2014). The constant
phase element (CPE) in Zview 2 is defined by a CPE-T and a CPE-P value. If a CPE-P
value is close to 1, the constant phase element essentially acts as a capacitor. If a CPE-P
18
value is close to -1, the constant phase element essentially acts as an inductor (Orazem,
Jorcin, Pébère, & Tribollet, 2006).
Current Voltage (I-V) Characterization
I-V characterization was completed using the Keithley Model 4200
Semiconductor Characterization System. A voltage sweep from 0 V to 0.8 V to 0 V was
used with a step size of 0.05 V. In order to protect the TFE, a compliance (max current
allowed before system shutoff) of 20 mA was used.
Resistance Characterization of Chitosan Coated TFEs
The resistance was measured on TFEs that were coated with chitosan and then
electropolymerized with polyaniline at various lengths of time using the Keysight
Technologies 34410A digital multimeter.
SEM Characterization
SEM characterization was completed using the Hitachi S4800 SEM and the
Hitachi SU6600 VPSEM. Images were taken at magnifications as low as x30 and as high
as x10k. Applied voltages ranged from 10 kV to 20 kV. VPSEM images were taken at a
pressure of 30 Pa.
19
CHAPTER THREE
RESULTS AND DISCUSSION
3.1 Impedance Characterization of Thin Film Electrodes
3.1.1 Impedance Measurements without Environment Modification
As shown in Figure 3.1 and Figure 3.2, both TFE-Au and TFE-Pt exhibit similar
impedance response for the blank control and the platinized electrodes. The low
frequency (< 10−1 Hz) values were around 2 ∗ 105 Ohms. This confirmed that
platinization, while effective in coating the electrodes of the device surface, did not
bridge the 50 micron gap between the electrodes nor did it change the gap separation
sufficient to alter the gap impedance. Platinization did however reduce the high
frequency (>104 Hz). After deposition of a polyaniline layer, TFE-Au and TFE-Pt
exhibited lower impedance. At 0.1 Hz, TFE-Au exhibited a drop from 223 kΩ (Blank
Control) to 1.37 Ω (platinized/PAn). At 0.1 Hz, TFE-Pt exhibited a drop from 165 kΩ
(Blank Control) to 16.5 Ω (platinized/PAn). Table 3.1 and Table 3.2 show an in depth
comparison of the measured impedances following application of PAn nanofibers.
20
Figure 3.1 TFE-Au impedance measurements without environment modification
Figure 3.2 TFE-Pt impedance measurements without environment modification
21
3.1.2 Impedance Measurements with Environment Modification
As shown in Figure 3.3 and Figure 3.4, platinized/PAn TFE-Au and TFE-Pt both
failed in buffer. The metal of the TFEs appeared to delaminate off of their plastic
backing as soon as impedance measurements began in buffer, which explains the
unusually large increase in impedance magnitude. The likely cause of this failure has to
do with the lack of an adhesion promoting metal such as Cr, Ti or TiW between the
plastic backing and the electrode material. Figure 3.5 shows the failed TFE-Au and TFE-
Pt after impedance measurements in buffer. Reliable measurements in any of the three
test environments was not possible.
Figure 3.3 TFE-Au impedance measurements including measurement in DI water
22
Figure 3.4 TFE-Pt impedance measurements including measurement in PBS
TFE-Au Impedance Magnitude Measurements at Various Frequencies (Ω)
𝟏𝟎−𝟏 Hz 𝟏𝟎𝟎 Hz 𝟏𝟎𝟏 Hz 𝟏𝟎𝟐 Hz 𝟏𝟎𝟑Hz 𝟏𝟎𝟒Hz 𝟏𝟎𝟓 Hz
Blank As Is 223000 390000 265000 221000 20400 2100 269
Platinized 129000 254000 281000 173000 18700 1790 237
Platinized/PAn 1.37 1.13 1.24 1.35 1.45 1.50 1.52
Table 3.1 TFE-Au impedance magnitude measurements at various frequencies
TFE-Pt Impedance Magnitude Measurements at Various Frequencies (Ω)
𝟏𝟎−𝟏 Hz 𝟏𝟎𝟎 Hz 𝟏𝟎𝟏 Hz 𝟏𝟎𝟐 Hz 𝟏𝟎𝟑Hz 𝟏𝟎𝟒Hz 𝟏𝟎𝟓 Hz
Blank As Is 165000 279000 284000 283000 20000 1840 187
Platinized 258000 403000 249000 242000 23100 2200 229
Platinized/PAn 16.5 10.4 23.8 39.9 40.5 55.3 79.4
Table 3.2 TFE-Pt impedance magnitude measurements at various frequencies
23
Figure 3.5 TFE-Au and TFE-Pt after impedance measurements in buffers
3.2 Equivalent Circuit Modeling
Table 3.3 and Table 3.4 show the equivalent circuit parameters calculated for gold
and platinum TFEs under the various conditions. The data calculated closely follows the
measured impedances and acts as a low pass filter. Comparisons of measured impedance
with its respective simulated equivalent circuit are shown in Appendix A. Current
research in equivalent circuits of electrodes with polyaniline deposition shows that the
low pass circuit model provides the best fit for experimental data (Sarac et al., 2008).
The first value to notice in both of the tables is the CPE-P value. In theory, CPE
should not be greater than 1, however the optimization software used does not take this
limitation into account. Since all of the values of CPE-P are ~1 and all of the values of
CPE-T are ~0, the constant phase element essentially acts as a capacitor. Current
24
research in equivalent circuits has yielded CPE-P and CPE-T values very similar to those
shown in this work (Afzal, Akhtar, Nadeem, & Hassan, 2010).
The platinization surface modification technique decreased Rs and Rp parameters
for both gold and platinum TFEs. The addition of PAn further decreased the values of Rs
and Rp compared to the control TFEs. This drastic decrease correlates with the measured
decrease in impedance for platinized/PAn TFEs. Figure 3.6 and Figure 3.7 illustrate the
drastic decrease for both resistance values.
3.2.1 Equivalent Circuit Data
TFE-Au Equivalent Circuit Parameters
𝐑𝐬(Ω) 𝐑𝐩 (Ω) CPE-T CPE-P Chi-Square Sum of
Squares
Blank Control 1590 244000 2.01E-09 1.15 1.84 49.6
Platinized 921 234000 3.56E-09 1.10 1.38 37.2
Platinized/PAn 1.26 0.563 2.58E-08 1.34 1.94E05 2.70E07
Table 3.3 Equivalent circuit parameters for TFE-Au
TFE-Pt Equivalent Circuit Parameters
𝐑𝐬(Ω) 𝐑𝐩 (Ω) CPE-T CPE-P Chi-Square Sum of
Squares
Blank Control 1270 242000 9.14E-10 1.24 1.61 43.5
Platinized 1190 236000 1.45E-09 1.17 4.34 117
Platinized/PAn 26.7 0.003 3.37E-09 0.963 1110 1.21E05
Table 3.4 Equivalent circuit parameters for TFE-Pt
25
Figure 3.6 TFE-Au and TFE-Pt Rs values
Figure 3.7 TFE-Au and TFE-Pt Rp values
In most of the equivalent circuits shown in the tables, the sum of squares is
relatively small, indicating a good fit. However, the platinized/PAn TFEs have a high
sum of squares. This is likely due to the fact that this data does not fit a low pass filter
model. Appendix A-3 and A-6 show that for both TFE-Au and TFE-Pt, the impedance
26
response is roughly constant over the entire spectrum of measured frequencies. These
measurements demonstrate that the application of PAn decreases the impedance and
capacitive element (Afzal et al., 2010).
3.3 IV Characterization of Thin Film Electrodes
From Figure 3.8 and Figure 3.9, the both gold and platinum blank control and
platinized TFEs exhibit similar I-V curves. Using Ohm’s Law (V=IR) and a linear
(y=mx+b) trendline in Microsoft Excel, the approximate resistance (inverse slope) and R2
(correlation coefficient) were calculated. Resistivity (ρ) and conductivity (σ) were
calculated from the resistance values. In ohmic materials, resistivity (Ω-cm) is a function
of the resistance, sample cross sectional area, and length:
𝜌 =𝑅𝐴
𝑙(1)
The conductivity (S/cm) is calculated as the inverse of resistivity:
𝜎 =1
𝜌(2)
These values are shown in Table 3.5 and Table 3.6.
27
Figure 3.8 I-V Characterization of blank control TFE-Au and TFE-Pt
Figure 3.9 I-V Characterization of platinized TFE-Au and TFE-Pt
28
Figure 3.10 I-V Characterization of Platinized/PAn TFE-Au and TFE-Pt
TFE-Au Resistance
(Ω) R Squared
Resistivity
(Ω-cm)
Conductivity
(S/cm)
Blank Control 5.13E+11 0.838 2.82E+11 3.55E-12
Platinized 1.98E+11 0.949 1.08E+11 9.26E-12
Platinized/PAn 102 0.973 56.1 1.78E-02
Table 3.5 TFE-Au resistivity and conductivity from I-V characterization
TFE-Pt Resistance
(Ω) R Squared
Resistivity
(Ω-cm)
Conductivity
(S/cm)
Blank Control 4.85E+11 0.614 2.66E+11 3.75E-12
Platinized 3.36E+11 0.462 1.85E+11 5.41E-12
Platinized/PAn 3.57E+11 0.578 1.96E+11 5.10E-12
Table 3.6 TFE-Pt resistivity and conductivity from I-V characterization
From the tables above, the platinization process has been shown to slightly
increase the overall conductivity of the TFEs. The increased surface area as a result of
platinization allows for an increase in the time for the surface to saturate with ions and
decreasing the resistance of the TFE (Fernandez, Peixoto, & Ramirez-Fernendez, 2000).
29
The addition of PAn has increased the conductivity of TFE-Au by several orders of
magnitude (1.78 ∗ 10−2 S/cm). Current research in gold electrodes with PAn deposition
have yielded similar results of 5 ∗ 10−2 S/cm (Elhalawany, Elmelegy, & Nayfeh, 2015).
These results are substantial, but are still much less than the conductivity of pure gold
(4.10 ∗ 105 S/cm).
The platinized/PAn TFE-Pt shows very similar resistance and conductivity to the
platinized TFE-Pt. This is likely due to the PAn not correctly bridging the gap between
the electrodes. This phenomena was observed several times throughout the study. SEM
images of TFE-Au and TFE-Pt after platinization, shown in Figure 3.15 and Figure 3.16,
give evidence that the surface roughness for TFE-Pt is much less than TFE-Au. This
lower roughness will not allow for as much PAn to bond to the surface, which can lead to
the electrodes not being properly connected by PAn branches.
3.4 Resistance Characterization of Thin Film Electrodes with Chitosan Coating
Figure 3.11 shows the resistance measured on TFEs that were drop casted with
various amounts (0-40 µL) of chitosan and then electropolymerized with polyaniline.
30
1
10
100
1000
10000
100000
1000000
10000000
100000000
1E+09
1E+10
0 5 10 15 20 25 30
Re
sist
ance
(R
) [Ω
]
Electropolymerization Time (t) [s]
Unplatinized Electrodes with Polyanaline
0 μL 10 μL 20 μL30μL 40 μL
Figure 3.11 Electropolymerization of Analine and meta-amino aniline to form polyaniline
on a gold TFE
Volumes ranging from 20-30 µL of chitosan resulted in a decrease in resistance
(~16 Ω). Less than 20 µL and more than 30 µL of chitosan appeared to show a much
higher resistance (~500 kΩ). Figure 3.12 shows an SEM image of a chitosan scaffold.
These scaffolds are very porous structures, which allow for polymers such as polyaniline
to grow and interweave to create an interpenetrating network (Nwe, Furuike, & Tamura,
2009). A lack of chitosan will result in a poor surface for polyaniline to interweave. The
lack of scaffolding fails to create an electrical link between the electrodes on the TFE.
Too much chitosan, however, appears to result in a network that the polyaniline will not
31
be able to fully penetrate and interweave through. The polyaniline will not reach the
surface of the electrode, and in turn not allow for optimal conduction.
Figure 3.12 An example of highly porous chitosan scaffolding (Soleri et al., 2015)
3.5 SEM Characterization of Thin Film Electrodes
3.5.1 Blank Control (No Alterations) TFEs
Figure 3.13 and Figure 3.14 show SEM images of the blank control TFE-Au
and TFE-Pt, respectively. At x50k it can be seen that these control electrodes are very
smooth. This lack of a rough surface does not allow for polymers to bond easily (Vidal et
al., 2000).
1 2 3
32
Figure 3.13 Blank TFE-Au SEM Figure 3.14 Blank TFE-Pt SEM
3.5.2 Platinized TFEs
Figure 3.15 and Figure 3.16 show SEM images of the Platinized TFE-Au and
TFE-Pt, respectively. At x50k it can be seen that the surface of each electrode has been
changed drastically. The Platinized TFE-Au shows a mean roughness of 132.8 nm, while
the Platinized TFE-Pt shows a mean roughness of 106.8 nm. The surface area of these
TFEs have been increased greatly compared the their blank counterparts, which allowed
for a greater adhesion of PAn to the electrodes. This increased adhesion will also lead to
a signifigant increase in biosensor sensitivity (Vidal et al., 2000).
33
Figure 3.15 Platinized TFE-Pt SEM Figure 3.16 Platinized TFE-Pt SEM
3.5.3 Platinized/PAn Deposition TFEs
Figure 3.17 and Figure 3.18 show SEM images of the Platinized/PAn TFE-Au and
Platinized/PAn TFE-Pt, respectively. At x500, it is possible to see the gap between the 2
semicircular electrodes being bridged by polyaniline. These fractal branch-like structures
are a commonly seen when depositing PAn by potentiodynamic diffusion controlled
electropolymerization. An example from other current research is shown in Figure 3.19,
which shows the similarity of the branch like structures observed.
34
Figure 3.17 Platinized/PAn TFE-Au SEM Figure 3.18 Platinized/PAn TFE-Pt SEM
Figure 3.19 Fractal PAn structures (Bhattacharjya & Mukhopadhyay, 2012)
These polyaniline structures allow the gaps between the electrodes to be joined,
which allow for lower overall impedance when measured. The polyaniline structures
formed on the gold TFE are far more branchlike and dense that those of the platinum
TFE. The more extensive branchlike structures observed on TFE-Au may explain why
the TFE-Au exhibited a lower impedance than the TFE-Pt.
35
CHAPTER FOUR
CONCLUSIONS
4.1 Summary
The impedance response of TFE-Au and TFE-Pt were measured for blank control
TFEs, platinized TFEs, and platinized/PAn TFEs. The impedance of the platinized TFEs
did not differ substantially from the blank control TFEs. The platinized/PAn TFEs
exhibited substantially lower impedance over the entire spectrum of frequencies,
particularly for TFE-Au. Impedance measurements in buffer were unsuccessful due to
delamination of the electrode metal.
Scanning Electron Microscopy has given a close look at each type of electrode
with and without surface modification. The flat surface of the control TFEs were made
rough by platinization. The platinized TFE-Au shows a mean roughness of 132.8 nm,
while the platinized TFE-Pt shows a mean roughness of 106.8 nm.
In addition, the resistance values obtained from electropolymerizing TFEs with
PAn are dependent on the thickness of the chitosan foam deposited. Electrodes with
smaller amounts of chitosan were not linked correcly, resulting in high resistances.
Electrodes with too much chitosan also resulted in high resistances due to the thickness of
the chitosan foam layer. Electrodes with 20-30 µL of chitosan showed a substantial
decrease in resistance.
The gold thin fim electrodes used in this study provided a much more reliable
surface for platinization and polyaniline deposition. The impedance response of gold thin
film electrodes was also much more predictable than that of platinum. With this
36
knowledge, the gold thin film electrodes can be used as a suitable surface in which to
deposit polyaniline and chitosan in order to build biologically responsive organic circuits.
4.2 Future Work
Future work will involve further testing on soft organic electronics, in order to
pinpoint the cause of TFEs failing in buffer. Further research into methods of applying a
chitosan/polyaniline composite to TFEs will allow an understanding of how an organic
resistor-capacitor pair can be designed and modified. After this, research will involve
integration of horseradish peroxidase (HRP) and glucose oxidase (GOx) to render
biologically responsive composites. These results will be used to complete an enzyme
nanofiber composite for use in a biologically responsive organic circuit.
37
APPENDICES
Appendix A
Equivalent Circuit Graphs
Figure A-1: Blank TFE-Au Impedance and equivalent circuit fit
Figure A-2: Platinized TFE-Au impedance and equivalent circuit fit
38
Figure A-3: Platinized/PAn TFE-Au impedance and equivalent circuit fit
Figure A-4: Blank TFE-Pt impedance and equivalent circuit fit
39
Figure A-5: Platinized TFE-Pt impedance and equivalent circuit fit
Figure A-6: Platinized/PAn TFE-Pt impedance and equivalent circuit fit
40
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