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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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