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PORTABLE ELECTROCHEMICAL SENSING PLATFORM: FROM HARDWARE TO SOFTWARE
KARTHK GANGADHARA
Presented to the Faculty of the Graduate School of
The University of Texas at Arlington in Partial Fulfillment
of the Requirements
for the Degree of
MASTER OF SCIENCE IN COMPUTER SCIENCE
THE UNIVERSITY OF TEXAS AT ARLINGTON
MAY 2020
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Copyright © by Karthik Gangadhara 2020
All Rights Reserved
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Acknowledgements
Firstly, I would like to thank my advisor Dr. Sungyong Jung for giving me this opportunity.
I have learnt a lot of valuable things from him. When I joined his lab in December 2018, I had no
idea of how to conduct research, he taught me various aspects of research by being extremely
patient and motivated me from time to time. I will be indebted to him for the rest of my life.
I would also like to thank my committee members Dr. Jean Gao, and Dr. Dajiang Zhu for
their consent and time to serve on my thesis defense committee and evaluate my work.
I take this opportunity to thank all current and previous members of my lab, ISCS – Special
thanks to Ms. Hyusim Park, without your guidance, help and support this work would have been
impossible. I would also like to thank Mr. Manu Chilukuri, Ms. Shanthala Lakshminarayana, Mr.
Younghun Park, Ms. Pallavi Vinubhai Bharoliya and Ms. Ruthya Chikkaputte Gowda. I had a
wonderful time working with you all and I will always cherish it.
Finally, I would like to thank my parents and my sisters for being my constant source of
motivation and for all the love and support.
April 30, 2020
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Abstract
PORTABLE ELECTROCHEMICAL SENSING PLATFORM: FROM HARDWARE TO SOFTWARE
Karthik Gangadhara, M.S
The University of Texas at Arlington, 2020
Supervising Professor: Sungyong Jung
Recent advances in the electrochemical biosensors is increasing the popularity of the
point of care devices since the electrochemical sensor can provide low cost, portability,
detectability, experimental simplicity, and capacity to provide real time monitoring. The point of
care devices has been used in various biomedical applications such as blood glucose monitors,
pregnancy tests, HIV tests, hemoglobin level tests etc. However, the existing portable devices are
limited to a specific sensing mechanism due to the inability to include the various electrochemical
sensing techniques into a compact formfactor. Thus, there is a need for miniaturized all-in-one
electrochemical sensing platform for the point of care devices.
In this research, we present a portable electrochemical sensing platform designed and
implemented from hardware to software for the point-of-care device to accommodate widely
used electrochemical sensing mechanism including amperometry, voltammetry. To control the
systems interfacing with the smart-devices and computers as well as displaying the obtained data,
an android application and a graphical user interface were developed in open-source
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programming using commercial-off-shelf components which consists of a microcontroller unit
with readout circuit, a multiplexer, and wired communication unit.
The proposed electrochemical sensing platform accommodates electrochemical
techniques such as linear sweep voltammetry, cyclic voltammetry, amperometry, anode strip
voltammetry, chrono amperometry and square wave voltammetry.
We performed experiments on proposed system. The Potentiostat dummy cell array tests
was performed for the aqueous phase detection and test results showed the linear response,
which is important in electrochemical sensing system.
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Table of Contents
Acknowledgements ............................................................................................................. 3
Abstract ............................................................................................................................... 4
List of Figures ...................................................................................................................... 9
List of Tables .......................................................................................................................11
Chapter 1 Introduction ..................................................................................................... 12
Chapter 2 Background....................................................................................................... 17
2.1 Electrochemistry ..................................................................................................... 17
2.2. Electrochemical Cell ............................................................................................... 18
2.2.1 Galvanic cell (Voltaic Cell) ................................................................................ 19
2.2.2 Electrolytic Cell ................................................................................................. 19
2.3. Electrochemical Sensors ........................................................................................ 24
2.3.1 Potentiometric sensors .................................................................................... 24
2.3.2 Conductometric sensors .................................................................................. 27
2.3.3 Voltammetric sensors ...................................................................................... 29
2.3.4 Electrochemical Impedance Spectroscopy (EIS) .............................................. 40
2.4. Measurement Instrumentation ............................................................................. 42
2.4.1 Potentiostat and Galvanostat .......................................................................... 42
2.4.2 Potentiostat/galvanostat operation ................................................................. 43
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2.4.3 Measurement Software ................................................................................... 45
2.4.4 Applications of Potentiostat/Galvanostat ........................................................ 46
2.5 Existing Works : Portable Electrochemical sensing systems ................................... 47
2.5.1 The universal Wireless electrochemical detector (UWED): ............................. 47
2.5.2 Universal mobile electrochemical detector (UMED): ...................................... 48
2.5.3 Dstat: ................................................................................................................ 48
Chapter 3 Proposed System .............................................................................................. 51
3.1. Hardware Design and Implementation .................................................................. 52
3.2. Software Design and Implementation ................................................................... 56
3.2.1. Firmware Design ............................................................................................. 56
3.2.2. Graphical User Interface Design (GUI) ............................................................ 61
3.3. Test Setup and Result ............................................................................................. 67
3.3.1 Cyclic Voltammetry .......................................................................................... 69
3.3.2 Square wave Voltammetry ............................................................................... 71
3.3.3. Linear Sweep Voltammetry ............................................................................. 72
3.3. 4. Amperometry ................................................................................................. 74
3.3.5 Anode Stripping Voltammetry ......................................................................... 76
3.3.6. Chrono Amperometry ..................................................................................... 78
Chapter 4. Conclusion ...................................................................................................... 81
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References ......................................................................................................................... 83
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List of Figures
Figure 2.1. Hydrofluoric acid Redox Reaction ................................................................... 18
Figure 2.2. Electrolytic cell ............................................................................................... 20
Figure 2.3. Electrolytic cell Redox Reaction. ..................................................................... 21
Figure 2.4. Simple schematic of two electrode configuration where EA is the applied
voltage and C and W are the counter and working electrodes, respectively. .............................. 22
Figure 2.5. Simple schematic of three electrode configuration where EA is the applied
voltage and C, W, and R are the working and counter and reference electrodes, respectively. .. 23
Figure 2.6 Simple schematic of two electrode configuration for potentiometry ............. 25
Figure 2.7 Cyclic voltammetry Input and output plot ....................................................... 31
Figure 2.8 Anode stripping voltammetry Input and output plot ...................................... 33
Figure 2.9 Linear Sweep voltammetry Input and output plot .......................................... 34
Figure 2.10 Square wave voltammetry Input and output plot ........................................ 35
Figure 2.11 Chrono Amperometry Input and output plot ................................................ 37
Figure 2.12 Amperometry Input and output plot with respect to time. .......................... 39
Figure 2.13. Experimental EIS system set up using three electrode mode....................... 41
Figure 2.14. Schematic representation of a three-electrode controlled potential
apparatus. X1 on an amplifier indicates it is a unity gain amplifier ............................................. 44
Figure 3.1. Block diagram for PSoC based Electrochemical sensing platform .................. 52
Figure 3.2. PSoC based Electrochemical sensing PCB board top and bottom layout ....... 54
Figure 3.3. PSoC electrochemical sensing PCB board unpopulated front side ................. 55
Figure 3.4. PSoC electrochemical sensing PCB board unpopulated front side ................. 56
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Figure 3.5. PSoC electrochemical sensing PCB board populated front and back ............. 56
Figure 3.6 Voltage Control Circuit ..................................................................................... 58
Figure 3.7 Current Measuring Circuit ................................................................................ 60
Figure 3.8 Other peripheral components ......................................................................... 60
Figure 3.9 Firmware Flowchart. ........................................................................................ 62
Figure 3.10 Python Environment and associated packages. ............................................. 64
Figure 3.11 GUI initialization, control, and data acquisition flowchart. ........................... 66
Figure 3.12 PSoC-Stat Graphical User Interface ................................................................ 67
Figure 3.13. Updated GUI with SWV, LSV and Chrono Amperometry techniques ........... 68
Figure 3.14. PCB board electronic measurement using dummy cell. ............................... 69
Figure 3.15. Potentiostat 50K Dummy Cell ....................................................................... 70
Figure 3.16. PCB board electronic measurement plot. ..................................................... 71
Figure 3.17. PCB board electronic measurement plot for SWV. ....................................... 73
Figure 3.18. PCB board electronic measurement plot for LSV. ......................................... 74
Figure 3.19. PCB board electronic measurement plot for amperometry. ........................ 76
Figure 3.20. anode stripping voltammetry dummy cell test plot. .................................... 78
Figure 3.21. chrono amperometry result plot for ±900mV .............................................. 79
Figure 3.22. chrono amperometry result plot for +900mV to 0V .................................... 80
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List of Tables
Table 2.1. Comparison of different potentiostat .............................................................. 50
Table 3.1. Part List ............................................................................................................. 55
Table 3.2. sample of CV response with dummy cell. ........................................................ 72
Table 3.3. sample of SWV response with dummy cell. ..................................................... 74
Table 3.4. sample of LSV response with dummy cell. ....................................................... 75
Table 3.5. sample of Amperometric response with dummy cell. ..................................... 76
Table 3.6. sample of ASV response with dummy cell. ...................................................... 78
Table 3.7. chronoamperometry result with dummy cell for pulse ±900mV. .................... 80
Table 3.8. chronoamperometry result with dummy cell for pulse +900mV to 0V.. .......... 81
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Chapter 1
Introduction
The impact of the sensors and sensing system is enormous and electrochemical sensors
are one of the most active areas of analytical research in many fields, including clinical and
environmental analysis as they offers a tremendous promise for scaling down sensing systems,
with features that include high sensitivity, inherent miniaturization, low cost, low-power
requirements, and high compatibility with advanced micromachining and microfabrication
technologies.
Electrochemical sensors are devices that converts chemical energy into electrical energy
and gives information about the composition in real time with the help of transducers. In this
way, the chemical energy of the selective interaction between the chemical species and the
sensor is transduced into an analytically useful signal. They are the largest and oldest group of
chemical sensors. They use simplest procedures and instrumentation. They attract great interest
nowadays because they are easy to miniaturize and integrate into automatic systems, without
compromising analytical characteristics.
Based on the type electrical magnitude used for the transduction of the recognition
incident the electrochemical sensors are classified into following families: [1]. Potentiometry,
Conductometry, Impedometry and Voltammetry/Amperometry. The potentiometry measures
changes of membrane potential, conductometry measures changes of conductance, the change
of current for electrochemical reaction with applied voltage in case of voltammetry and current
for the applied constant voltage in case of amperometry.
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Among the four, analytical chemists routinely use voltammetric techniques for the
quantitative determination of a variety of dissolved inorganic and organic substances. The
biological chemist uses voltammetric technique for variety of purposes like study of redox
reactions in various media, absorption processes on surfaces, reaction mechanisms and the
electron transfer and its kinetics, and transport, speciation, and thermodynamic properties of
solved species. Voltammetric methods are also applied to the determination of compounds of
pharmaceutical interest and for the analysis of complex mixtures[2].
This is mainly because in voltammetry input signals are varied therefore It can detect
multiple chemicals species. It is simple and easy to conduct and has various sub techniques
depending on the type of signal used as input, the most used waveforms are linear scan,
differential pulse, and triangular and square wave. The cyclic voltammetry and square wave
voltammetry techniques are more sensitive compared to EIS.
The main devices used to electrochemistry is the potentiostat and galvanostat [3]. The
potentiostat involves controlling the voltage while measuring the current passing between them,
whereas galvanostat controls the current while measuring the voltage across the electrodes. For
this minimum 2-electrode configuration is required. In potentiostat, the electrode must pass a
current while maintaining the standard voltage between them for the accurate measurements.
Because a current passing through an electrode causes change in interstitial potential, a voltage
change occurs that interferes with keeping the voltage between the electrode’s constant, a third
electrode is habitually used for a 3-electrode configuration. In the 3-electrode configuration, the
chemical reactions occur on the working electrode surface while the reference electrode provides
a stable reference potential, and not should not pass current, and an auxiliary electrode provides
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enough current to the solution to keep the potential between the reference and working
electrode at the desired voltage[4].
In the recent years various portable potentiostat designs related to discrete
integrated circuits and printed circuit board have reported. The printed circuit boards usually
include a chipset or a microcontroller with the analog devices to create potentiostat or
galvanostat [5][6][7]. Most of the potentiostats are limited to around 0.1±0.2 mA of current and
±1.5 V voltage this is due to as they have mostly been designed for analytical chemistry purposes
and to avoid the electrolysis of water. The most common electrochemical techniques used in
these devices are cyclic voltammetry, amperometry and potentiometry. There are some which
include pulse based voltammetric techniques such as square wave voltammetry [4][6][13] and
differential pulse voltammetry. There exist wireless communication incorporated modules like
uMed and UWED design making them a good fit for remote healthcare applications [14].
The existing commercial portable devices are limited to a specific sensing mechanism.
There are Electrochemical systems which includes all the electrochemical sensing techniques to
provide a wide range detection and measurement capabilities. Unfortunately, not many products
can be considered as portable. This is due to the inability to include the various electrochemical
sensing techniques into a compact formfactor. Most of commercial products are heavy and big.
The product should have small size, light weight and be budget friendly.
This demand stimulates progress aimed at development of novel portable Electrochemical
sensing platform from hardware to software to accommodate all the voltammetric
electrochemical sensing mechanisms namely, cyclic voltammetry, linear sweep voltammetry,
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anode stripping voltammetry, amperometry, chronoamperometry and lastly, square wave
voltammetry with custom built open-source software programs to control the systems.
Our proposed system includes programable system on chip (PSoC) which has built in
analog components such as ADC, TIA and opamp which can be used to build potentiostat without
using any external analog components, which significantly reduces the size of the board with good
form factor making it good fit for the portable electrochemical sensing applications and also of
low cost device compared to the above mentioned papers.
To control the various techniques of the device, a graphical user interface (GUI) was
developed in the python programming language. The GUI provides various options to users. The
users can select different electrochemical technique by selecting the appropriate notebook[16].
The data is displayed using the matplotlib library. The GUI also provides features to save the
experimental result into a excel file. The experimental controls such as selecting the voltage range,
current range, adjusting the experimental duration are some of the GUI added functionalities.
This dissertation is organized as follows.
Chapter 1 introduces to the need for portable electrochemical sensing/monitoring
platform. Current devices to perform electrochemical sensing is described in brief. It also covers
the research objective and motivations.
Chapter 2 covers the background of electrochemistry and electrochemical sensing
techniques. It also surveys in detail current potentiostat circuit architectures used for various
sensing techniques with advantages and drawbacks.
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Chapter 3 presents the proposed architecture for portable electrochemical sensing
platform from hardware to software. This chapter presents all the theory with required analysis
showing advantages of the present architecture over others. Applications build with the proposed
system and their experimental results and measurement results are presented to validate the
circuit performance.
Chapter 4 concludes the dissertation by summarizing the work and detailing the
advantages of the work.
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Chapter 2
Background
2.1 Electrochemistry
Electrochemistry is the study of movement of electrons during the chemical reactions. It
was discovered by Faraday. Electro chemical reactions produces or consumes free electrons
with two main processes know as oxidation-reduction (“redox”), the once where electrons
exchange. Example of a redox reaction,
H𝟐 + F𝟐 → 2HF (1)
Which can be written as follows:
Oxidation H𝟐 → 2H + +2e − (2)
Reduction F2 + 2e−→ 2F− (3)
Overall reaction H2 + F2 → 2H + 2F − (4)
As shown in the above equation, the Hydrogen loses two electrons to undergo oxidation,
it is called as reducing agent or reductant. Fluorine gains two electrons to undergo reduction is
known as oxidizing agent or oxidant. The redox reaction is shown in Figure 2.1 below. The
Electrochemical methods are a class of techniques which study an analyte by measuring the
voltage or current in an electrochemical cell [17][18][19]][20].
Electrochemical reactions occur in a solution containing dissolved ions knows as
electrolyte. The electrolyte aids the current flow. Strong electrolytes with solvent-electrolyte
combinations were chosen in electrochemistry. A few volts of interfacial potential differences are
corresponding to a thin layer of electrolyte attached to the electrode surface. The interfacial
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potential difference is very small. This create a potential gradient across the electrode/electrolyte
surface. The interfacial potential difference, E, of an electrode can be calculated using the Nernst
equation [19]:
𝐸 = 𝐸° +𝑅𝑇
𝑛𝐹ln (𝐶𝑜/𝐶R) (5)
where 𝐸° is the standard potential of the electrode, R is the molar gas constant, T is
temperature, F is Faraday's constant and Co and CR are the concentration of the oxidized and
reduced forms of the species, respectively [22].
Figure 2.1. Hydrofluoric acid Redox Reaction[63]
2.2. Electrochemical Cell
Electrochemical cells are broadly divided into two sub-types, galvanic and electrolytic. In
galvanic cells the electrodes are connected via wire. The reactions start spontaneously. while in
electrolytic cell a potential more than cell operating potential is required to be applied to lead
the electrochemical process. Both galvanic and electrolytic cells will consist of two electrodes (an
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anode and a cathode), which can be made of the same or different metals, and an electrolyte in
which the two electrodes are immersed.
2.2.1 Galvanic cell (Voltaic Cell)
Galvanic cells are used in DC electrical power. The basic galvanostat consists of a semi-
porous membrane separating the electrolyte and others includes two half cells bridged with a
salt bridge. The salt bridge contains an inert electrolyte like potassium sulfate whose ions will
diffuse into the separate half-cells to balance the building charges at the electrodes. A Galvanic
Cell induces a spontaneous redox reaction to create a flow of electrical charges, or electricity.
Non-rechargeable batteries are examples of Galvanic cells. A Reaction is spontaneous when the
change in Gibb’s energy[15], ∆G is < 0. Electrons flow from the anode. [23][24][25][27].
2.2.2 Electrolytic Cell
An electrolytic cell is a cell which requires an outside electrical source to initiate the redox
reaction. Electrolysis is the process of how the electric energy drives the non-spontaneous
reactions. Whereas the galvanic cell used a redox reaction to make electrons flow, the electrolytic
cell uses the source of electricity to cause the redox reaction. In an electrolytic cell, electrons are
forced to flow in the opposite direction. Since the direction is reversed of the voltaic cell, the E0
cell for electrolytic cell is negative. Also, to force the electrons to flow in the opposite direction,
the electromotive force that connects the two electrode-the battery must be larger than the
magnitude of cell potential E0. This additional requirement of voltage is called
overpotential[23][26][25][27]. Electrolytic cell for the example shown in Figure 2.2.
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An Electrolytic cell is one kind of battery that requires an outside electrical source to drive
the non-spontaneous redox reaction. Rechargeable batteries act as Electrolytic cells when they
are being recharged. A reaction is non-spontaneous when Gibbs free energy, ∆G is > 0. Must
supply electrons to the cathode to drive the reduction, so cathode is negative. Must remove
electrons from the anode to drive the oxidation, so anode is positive.
Figure 2.2. Electrolytic cell [27].
To understand the redox reaction, Solve the Redox equation.
Oxidation 𝐶𝑢(𝑆) ⟶ 𝐶𝑢2+(𝑎𝑞) + 2𝑒− (6)
Reduction 𝐴𝑔2+(𝑎𝑞) + 2𝑒− ⟶ 𝐴𝑔(𝑠) (7)
The most common form of Electrolytic cell is the rechargeable battery or electroplating.
The reaction is redox reaction is shown pictorially in Figure 2.3.
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Figure 2.3. Electrolytic cell Redox Reaction [63].
The Electrolytic cells used for the electrochemical analysis comes with different electrode
configuration. The two electrode and three electrode configurations are explained here.
2.2.2.1 Two Electrode Configurations
This configuration consists of a Working Electrode where the chemistry of interest occurs
and a Counter Electrode which acts as the other half of the cell. The applied potential (EA) is
measured between the working and counter electrode and the resulting current is measured in
the working or counter electrode lead as in Figure 2.4.
The counter electrode in the two-electrode setup serves two functions. It completes the
circuit allowing charge to flow through the cell, and it also maintains a constant interfacial
potential, regardless of current. maintaining both requirements is an impossible task under most
conditions. In a two-electrode system, it is very difficult to maintain a constant counter electrode
potential (eC) while current is flowing. This is due to Interfacial potential changes when the
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current changes. This fact, along with a lack of compensation for the voltage drop across the
solution (iRS) leads to poor control of the working electrode potential (eW) with a two-electrode
system. The roles of passing current and maintaining a reference voltage are better served by
two separate electrodes [35].
Figure 2.4. Simple schematic of two electrode configuration where EA is the applied voltage and
C and W are the counter and working electrodes, respectively.
2.2.2.2 Three Electrode Configurations
The three electrode system remedies many of the issues of the two-electrode
configuration. The three-electrode system consists of a working electrode, counter electrode,
and reference electrode. The reference electrode’s role is to act as a reference in measuring and
controlling the working electrode potential, without passing any current. The reference electrode
should have a constant electrochemical potential at low current density. Additionally, since the
reference electrode passes negligible current, the iR drop between the reference and working
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electrode (iRU) is often very small. Thus, with the three-electrode system, the reference potential
is much more stable, and there is compensation for iR drop across the solution. This translates
into superior control over working electrode potential. The most common lab reference
electrodes are the Saturated Calomel Electrode and the Ag/AgCl electrode. Setup is illustrated in
the Figure 2.5.
In the three-electrode configuration, the only role of the counter electrode is to pass all
the current needed to balance the current observed at the working electrode. The counter
electrode will often swing to extreme potentials to accomplish this task [36].
Figure 2.5. Simple schematic of three electrode configuration where EA is the applied voltage
and C, W, and R are the working and counter and reference electrodes, respectively.
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2.3. Electrochemical Sensors
The Electrochemical sensors are the devices that gives information of the composition of
the system by undergoing oxidation reduction process. Electrochemical sensors usually consists
of two basic components, a chemical (molecular) recognition system which is the main part of
the sensor and a physiochemical transducer which is a device that converts the chemical response
into a signal(current, potential, or concentration) that can be detected by modern electrical
instruments. Electrochemical sensors are broadly classified into four categories [20]:
2.3.1 Potentiometric sensors
Potentiometric sensors, in which the open circuit electrochemical cell potential is
measured, which according to the Nernst equation[19] is proportional to the logarithm of the
concentration of the chemical species of interest. Potentiometric sensors are commonly used
when the concentration of chemical specie to be measured changes over several orders of
magnitude (e.g., in the case of pH measurements). In cases, where the concentration changes
over only one or two orders of magnitude (e.g., in the case of glucose concentration in
physiological fluids), a sensor with a linear relationship between response and concentration is
preferable (such as Amperometric sensors) rather than logarithmic response.
Potentiometry is one of the methods of electroanalytical chemistry. It is usually employed
to find the concentration of a solute in solution. In potentiometric measurements, the potential
between two electrodes is measured using a high impedance voltmeter.
Use of a high impedance voltmeter is important because it ensures that current flow is
negligible. Since there is no net current, there are no net electrochemical reactions, hence the
25
system is in equilibrium. At its most fundamental level, a potentiometer consists of two electrodes
inserted in two solutions connected by a salt bridge (see Figure 2.6 below). The voltmeter is
attached to the electrodes to measure the potential difference between them. [21][22].
Figure 2.6. Simple schematic of two electrode configuration for potentiometry[64].
The reference electrode potential is known, and the other electrode is the test electrode
whose potential needs to be find out. Test electrodes are dipped in its own ionic solution whose
concentration is to be detected, or a carbon rod electrode sitting a solution which contains the
ions of interest in two different oxidation states. The Nernst equation[13] is used to find out the
concentration of the test solution.
For example, to find the concentration of Ag+ in a silver nitrate solution, reference
electrode should consist of silver metal in a know 0.1 M silver nitrate solution. The test
26
electrode is silver in silver nitrate solution of wish to find the concentration. Since no current
flows the cell is at equilibrium.
Ag+ Ag (8)
The potential measured by the voltmeter, Ecell is related to the reference electrode potential and
test electrode potential as follows:
𝐸𝑐𝑒𝑙𝑙 = 𝐸𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 − 𝐸𝑡𝑒𝑠𝑡 (9)
Now, Ereference and Etest can both be expanded using the Nernst Equation:
𝐸𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 = 𝐸𝐴𝑔/𝐴𝑔+0 −
𝑅𝑇𝑙𝑛(𝐾𝑒𝑞)
𝑛𝐹 (10)
Therefore, at temperature of 298 K:
𝐸𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 = 𝐸𝐴𝑔/𝐴𝑔+0 − 0.059𝑙𝑜𝑔10(1/[𝐴𝑔𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒
+ ]) (11)
Similarly,
𝐸𝑡𝑒𝑠𝑡 = 𝐸𝐴𝑔/𝐴𝑔+0 − 0.059𝑙𝑜𝑔10(1/[𝐴𝑔𝑡𝑒𝑠𝑡
+ ]) (12)
Now we can substitute equation 11 to equation 12 into equation 9, we can also assume
[𝐴𝑔𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒+ ] is constant. So, after rearranging, we get:
[𝐴𝑔𝑡𝑒𝑠𝑡+ ] = 10−[(𝐸𝑐𝑒𝑙𝑙 + 0.059)/0.059] (13)
The potential reading from the voltmeter is Ecell .which can used to calculate the concentration
of the silver Ions in the test solution.
The application of potentiometry are as follows. To determine the cell potential as
described above. Potentiometric titration is used for all types of volumetric analysis: acid base,
precipitimetry[51], complexometry[50] and redox. It is used when it is not easy or impossible to
detect the end point by ordinary visual methods i.e. For high colored or turbid solutions. For very
dilute solutions and when there is no available indicator[27].
27
2.3.2 Conductometric sensors
Conductometric sensors, in which the conductance of the electrochemical cell is
measured by an alternating current bridge method. In this kind of sensor mostly the resistive
component of the impedance of the chemical specie is measured. The measurement is performed
at one fixed frequency [30].
Conductometry is the measurement of the electrical conductivity of a solution. The
conductance is defined as the current flow through the conductor. The unit for the conductance
is Siemens (S) which is the reciprocal of Ohm's (Ω−1). It is mainly used for the determination of
the physico-chemical properties of the compounds. The main principle involved in this method is
that the movement of the ions creates the electrical conductivity. The movement of the ions is
mainly depended on the concentration of the ions.
𝐴+𝐵− + 𝐶+𝐷− ⟶ 𝐴𝐷 + 𝐶+𝐵− (14)
where A+B− is the solution of strong electrolyte; C+D− is the solution of the reagent. Here the ionic
concentration of A+ is determined by reacting the electrolyte solution with the reagent solution
so that the A+ ions are replaced by the C+ ions. This replacement of the ions with the other ions
shows the conductance increase or decrease. This is done mainly by the replacement of the
hydrogen ion with another cation.
The theory is mainly based on Ohm's law which states that the current (I) is directly
proportional to the electromotive force (E) and inversely proportional to the resistance (R) of the
conductor:
𝐼 = 𝐸/𝑅 (15)
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The conductance is defined as the reciprocal of the resistance. The resistance is expressed
by the following equation:
𝑅 = 𝜌𝑙/𝑎 (16)
where 𝜌 is the resistivity; 𝑙 is the length; 𝑎 is the cross-sectional area of the homogenous
material. Therefore,
𝐶 = 1/𝑅 = 𝑘/𝑙𝑎 (17)
where K is the conductivity; l is the length; a is the cross-sectional area of the homogenous
material. Then the molar conductivity is defined as the conductivity due to 1 mole and it is
expressed by the following formula:
𝛬 = 1,000𝑘/𝐶 (18)
where K is the conductivity; C is the concentration of the solution in mol/l. The sample solution
is placed on the cell which is composed of platinum electrodes. These are calibrated with the help
of known conductivity of the solution, for example, standard potassium chloride solution. Cell
constant is defined as the conductivity of the cell.
𝑐𝑒𝑙𝑙 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = 0.002765 / 𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 𝑐𝑜𝑛𝑑𝑐𝑢𝑡𝑖𝑣𝑖𝑡𝑦 (19)
The conductometry is used for determination of Sulphur dioxide in air pollution studies.
Determination of soap in oil. Determination of accelerators in rubber. Determination of total soap
in latex. Specific conductance of water. To check water solubility in rivers and lakes. To determine
alkalinity of fresh water, salinity of sea water. To determination of deuterium ion concentration
in water – deuterium mixture. Food microbiology for tracing micro-organisms and tracing the
antibiotics[30].
29
2.3.3 Voltammetric sensors
In Voltammetric sensors, the voltage is applied across the electrodes of the electro-
chemical cell and the current flowing between them is measured. The measured current is due
to the response of chemical species of interest. By applying linear, square, or cyclic voltammetry,
improved selectivity and sensitivity is achieved.
In voltammetry, the applied voltage and the redox current follows a well define law. The
applied potential controls the concentration of the oxidation and reduction species at the
electrode (CO and CR) and the rate of the reaction(k0) as described by Nernst[25] or Butler–Volmer
equations[33], respectively. When diffusion is involved the material flux between the electrode
and the solution controls the redox current based on Fick’s law[34]. The interplay between these
processes is responsible for the characteristic features observed in the Volta gram of various
techniques. For a reversible electrochemical reaction described by below equation.
O + 𝑛𝑒− ⟺ R (20)
The application of potential E forces the respective concentrations O and R at the surface of the
electrode to the ratio according to Nernst equation:
𝐸 = 𝐸0 − 𝑅𝑇
𝑛𝐹ln (
𝐶𝑅0
𝐶𝑂0) (21)
Where,
R – molar gas constant (8.3144 J mol–1K–1),
T – Absolute Temperature (K),
n – Number of electrons transferred,
F – Faraday constant (96,485 C/equiv), and
E0 – The standard reduction potential.
30
When the applied potential to the electrode is changed, the ratio of the concentration
changes satisfying equation 13. The variables for current, potentials and concentration can be
expressed using Butler-Volmer equation:
𝑖
𝑛𝐹𝐴= 𝑘0𝐶𝑂𝑒−𝛼𝜃 − 𝐶𝑅𝑒(1−𝛼)𝜃 (23)
Where,
𝜃 = 𝑛𝐹(𝐸−𝐸0)
𝑅𝑇 (24)
K0 – heterogeneous rate constant,
𝛼 – transfer coefficient,
A – Area of the electrode.
Depending on the type of signal used the voltammetric techniques are further classified
into subcategories as follows:
2.3.3.1 Cyclic Voltammetry(CV)
Cyclic voltammetry (CV) has become an important and widely used electroanalytical
technique in many areas of chemistry. It is widely used for the study of redox processes, for
understanding reaction intermediates, and for obtaining stability of reaction products. This
technique is based on varying the applied potential at a working electrode in both forward and
reverse directions (at some scan rate) while monitoring the current [32].
For example, the initial scan could be in the negative direction to the switching potential.
At that point, the scan would be reversed and run in the positive direction. One full cycle analysis
31
Input is provided in the Figure 2.7 shows the potential applied to between the WE and RE for the
cyclic voltammetry measurement and the resultant response.
Figure 2.7 Cyclic voltammetry Input and output plot
The important parameters in a cyclic voltammogram are the peak potentials (Epc , Epa) and
peak currents (ipc , ipa) of the cathodic and anodic peaks, respectively. If the electron transfer
process is fast compared with other processes such as diffusion, the reaction is said to be
electrochemically reversible, and the peak separation is
𝐸𝑝 = |𝐸𝑝𝑎 − 𝐸𝑝𝑐| = 2.303 𝑅𝑇/𝑛𝐹 (25)
For reversible reaction at 250C with electrons 𝐸𝑝 should be 0.0592/n V, or about 60
mV for one electron. Irreversibility due to a slow electron transfer rate results in 𝐸𝑝 > 0.0592/n
V, greater, 70 mV for a one-electron reaction.
The reduction potential (E0) for a reversible couple is given by,
𝐸0 = 𝐸𝑝𝑎+𝐸𝑝𝑐
2 (26)
For a reversible reaction, the concentration is related to peak current by the Randles–
Sevcik [25] expression (at 25 °C):
32
𝑖𝑝 = 2.686 × 105𝑛3/2𝐴𝑐0𝐷1/2𝑣1/2 (27)
Where,
ip – peak current in amps,
D – the diffusion coefficient (cm2 s–1),
A – Area of the electrode (cm2),
c0 – Concentration in mol cm–3, and
n – Scan rate in Vs–1.
The Applications of cyclic voltammetry are as follows. Determination of band gap of
semiconductor. To determine the oxidation state of central metal atom in many metal complexes.
To determine electron transfer where it is one or two electron transfer. It provides supplementary
fluorescence spectrometry.
2.3.3.2 Anode Stripping Voltammetry(ASV)
Anode stripping voltammetry (ASV) is most widely used for trace metal detection. Anodic
Stripping Voltammetry is a method to demonstrate the presence of multiple metals in water.
There are two steps. First, the analyte species in the sample solution is concentrated onto or into
a working electrode. During the second step, the precontracted analyte is measured or stirred
from the electrode by the application of potential scan. Figure 2.8 shows the ASV with cyclic
voltammetry scan. A is the cleaning step and B is the electroplating and equilibrium step, C is the
stripping step [26].
33
The resultant plot for the Anode stripping voltammetry is shown in the Figure 2.8. The
response is like the one obtained for the cyclic voltammetry with exceptional sensitivity due to
the preconcentration step.
Figure 2.8 Anode stripping voltammetry Input and output plot.
Advantages and applications are as follows. ASV allows researchers to detect multiple
types of dissolved metal in one experiment. Researching water quality using ASV is also fast and
accurate. An ASV test is very easy to perform on site by taking a sample of the water and testing
it with the potentiostat linked to a laptop or tablet.
The metal film formed during the reduction step concentrates the metal particles at the
electrode, so the detection of very low concentrations (ppb range) of metal ions is possible.
Anodic Stripping Voltammetry is widely used for testing drinking water quality, surface water
and sewage or wastewater – water that leaves the treatment plant before being discharged into
surface water. [35].
34
2.3.3.3 Linear Sweep Voltammetry(LSV)
In linear sweep voltammetry (LSV) a fixed potential range is employed much like potential
step measurements. However, in LSV the voltage is scanned from a lower limit to an upper limit
as shown Figure 2.9 below.
Figure 2.9 Linear Sweep voltammetry Input and output plot
The voltage scan rate (v) is calculated from the slope of the line. Clearly by changing the
time taken to sweep the range we alter the scan rate. The characteristics of the linear sweep
voltammogram recorded depend on several factors including the rate of the electron transfer
reaction(s), The chemical reactivity of the electroactive species, The voltage scan rate. Typical
response plot for the LSV is shown in Figure 2.9.
LSV mainly used in irreversible reactions. one example,[37] linear voltammetry was used
to examine direct methane production via a biocathode. Since the production of methane from
CO2 is an irreversible reaction.
35
2.3.3.4 Square wave voltammetry(SWV)
The excitation signal in SWV consists of a symmetrical square-wave pulse of Fixed
amplitude superimposed on a staircase waveform of fixed step height, where the forward pulse
of the square wave coincides with the staircase step. The net current is obtained by taking the
difference between the forward and reverse currents (Ifor – Irev) and is centered on the redox
potential. Typical Square wave voltammetric Input plot is shown in the Figure 2.10 below.
The typical response plot for the square wave voltammetry for a redox reaction is shown
in Figure 2.10. The SWV has several advantages and is used in determination of some species in
trance levels and in electrochemical detection in HPLC [32][38].
Figure 2.10 Square wave voltammetry Input and output plot
The scan rate of the square wave voltammetry is inversely dependent upon the time per
step, τ:
𝑆𝑐𝑎𝑛 𝑟𝑎𝑡𝑒 (𝑚𝑉
𝑠) =
𝐸𝑠𝑡𝑒𝑝(𝑚𝑣)
τ(s) (28)
Where,
Estep – Amplitude of the step voltage,
36
τ – Time per step.
During the scan, the current is recorded at the end of the forward pulse and at the end of the
reverse pulse, meaning it is sampled twice per cycle. Waiting till the end of the pulse to sample
the current avoids involving the charging current.
The frequency, f, used in square-wave voltammetric experiments is generally from about
1 to 125 Hz. Such a high f means that square-wave voltammetry is usually much faster than other
pulsed experiments.
Applications are as follows: SWV is very sensitive, often allowing direct analyses at the
ppb (parts per billion) level and even the low ppt (parts per trillion) level when used in a stripping
mode. SWV requires less time per sweep than older techniques such as differential pulse
polarography. A SWV sweep can often be recorded in less than ten seconds, in contrast with a
differential pulse polarogram that typically requires more than two minutes for data acquisition.
The square-wave frequency can be used to differentiate between processes with fast and slow
kinetics. In some cases, kinetically fast processes can be measured without interference from
slower processes that occur in the same potential range.
Other techniques, such as cyclic voltammetry, are generally preferred over SWV for
mechanistic and kinetic studies. However, square-wave voltammetry’s sensitivity allows
mechanistic and kinetic measurements in solutions that are too dilute for more conventional
study.
SWV is generally performed on a stationary solid electrode or a hanging mercury drop
electrode. The SWV script in the Pulse Voltammetry software provides for mercury-drop
37
generation, solution de-aeration, and experiment sequencing suitable for the most common
applications for square-wave voltammetry.
2.3.3.5 Chrono Amperometry
In chronoamperometry electrochemical analytical technique, the potential of the working
electrode is stepped, and the resulting faradaic current is measured as a function of time. As
shown in the Figure 2.11 the applied potential is instantaneously jumped from one value to
another.
Figure 2.11 Chrono Amperometry Input and output plot
The resultant current is measured. The current rises instantaneously after the changes in
the voltage and then begins to drop as a function of time. Chronoamperometry is a time-
dependent technique where a square-wave potential is applied to the working electrode. The
current of the electrode, measured as a function of time, fluctuates according to the diffusion of
an analyte from the bulk solution toward the sensor surface.
38
Chronoamperometry can therefore be used to measure current–time dependence for the
diffusion-controlled process occurring at an electrode. It is a sensitive technique which does not
require labeling of the analyte or bioreceptor and has been applied in many studies
independently or alongside other electrochemical techniques such as CV[40].
The characteristic shape of the resulting chronoamperogram can be represented by the
Cottrell equation[31]
𝑖𝑡 = 𝑛𝐹𝐴𝐶0𝐷1/2
𝜋1/2𝑡1/2 (29)
Where,
n – The moles of electrons involved in the reaction.
F – The Faraday constant.
A – The area of the electrode (cm2),
C0 – The concentration of the analyte in the bulk solution (mol./dm3),
D – The diffusion coefficient (cm2/s) and
t – Time (s).
Consequently, i will be proportional to t− 1/2.
It is used in wide range of applications such as in measuring the concentration by
measuring I vs conc. at any fixed time. It can analyze the shape of the current-time curve to study
coupled chemical reactions. There are better ways to do both with more modern techniques
Chronoamperometry is important because it is a fundamental method on which other techniques
are based.
39
2.3.3.6 Amperometric sensors
Amperometry is a special type of voltammetry, where the voltage applied across the
electrodes of the electrochemical cell is kept constant while its current is measured. The
measured current is a function of the concentration of the electroactive species. The response
will have high linearity if the current is limited by the rate of the mass transfer and not by the rate
of charge transfer. Typical voltage Input and current output plots are illustrated in Figure 2.12.
below [41].
Figure 2.12 Amperometry Input and output plot with respect to time.
The Magnitude of the measured current is proportional to the concentration of the
oxidized and reduced analyte. The Amperometric methods is highly dependent on the applied
potential. However, the electrode material and the composition of the supporting electrolyte can
also influence it. This steady state diffusion current is the direct measure of the concentration of
the electroactive species in the solution.
Some of the applications are, Amperometric titrations are used for redox;
precipitation[59] and complexometric titrations[58] of the reducible inorganic or organic ions. For
40
conventional acid-base titrations it is not useful. In the determination of moisture by Karl fisher
reagent, the end point is located by amperometry. Factors like surface area completely disappear
from Amperometric titrations.
2.3.4 Electrochemical Impedance Spectroscopy (EIS)
Electrochemical Impedance Spectroscopy (EIS) is an electrochemical technique to
measure the impedance of a system in dependence of the AC potentials frequency. EIS systems
characterize the time response of chemical systems using low amplitude alternating current (AC)
voltages over a range of frequencies. Using an electrode setup consisting of a working, reference,
and counter electrodes a known AC voltage is passed from the working electrode through an
electrolytic solution and into the counter electrode. Quantitative measurements are produced by
the EIS and enable the evaluation of small-scale chemical mechanisms at the electrode interface
and within the electrolytic solution. Therefore, EIS is useful in determining a wide range of
dielectric and electrical properties of components in research fields studying batteries, corrosion,
etc.
The Figure 2.13 shows the experimental setup for EIS system with 3 electrodes. An
electrochemical cell is used to for the chemical reaction and is connected to the electrochemical
spectrometer to obtain the electrical response of an electrolytic solution. EIS systems are
operated using computer programs specifically designed for EIS testing.
41
Figure 2.13. Experimental EIS system set up using three electrode mode[55].
When the voltage is passed through the electrolytic solution, from the WE to RE, its acts
as resistors to impede electron flow. The resulting voltage recorded by the RE is used to determine
the electrolyte resistance by comparing the input and output voltage. The excitation signal is
expressed as a function of time, has the form
𝐸𝑡 = 𝐸𝑜 sin (𝜔𝑡) (30)
Where, 𝐸𝑡 is the potential at time t, 𝐸0 is the amplitude of the signal and 𝜔 is the radial
frequency. The frequency f and radial frequency 𝜔 is related as follows
𝜔 = 2𝜋𝑓 (31)
In a linear system, the response signal, It , is shifted in phase and has a different amplitude than I0.
𝐼𝑡 = 𝐼0 sin (𝜔𝑡 + 𝜙) (32)
An expression analogous to Ohm’s law allows us to calculate the impedance of the system as:
𝑍 =𝐸𝑡
𝐼𝑡=
𝐸0 sin (𝜔𝑡)
𝐼0 sin (𝜔𝑡+𝜙)= 𝑍0
sin (𝜔𝑡)
sin (𝜔𝑡+𝜙) (33)
42
EIS is used in corrosion for rate determination, inhibitor and coatings, passive layer
Investigations. In Coatings evaluation used for dielectric measurements and corrosion protection.
In batteries to determine state-of-charge, materials selection, electrode design. In electrode
deposition for bath formulation, surface pretreatment, deposition mechanism, deposit
characterization. In Electro-Organic synthesis to determine reaction mechanism. In
Semiconductors for photovoltaic work and dopant distributions[56][57].
2.4. Measurement Instrumentation
The basic components of a modern electroanalytical systems are a
potentiostat/galvanostat, computer, and the electrochemical cell. In some cases, the
potentiostat/galvanostat and computer are bundled into one package, whereas in other systems
the computer and the A/D and D/A converters and microcontroller are separate, and the
potentiostat can operate independently. Potentiostats and galvanostats are electrochemical
instruments used in electrochemistry, battery and fuel cell testing, corrosion control,
voltammetry, biomedical research, surface imaging, and related applications.
2.4.1 Potentiostat and Galvanostat
Potentiostats are used to keep the potential (voltage) between a working electrode and a
reference electrode at a constant value. With potentiostats, the signal from the reference
electrode is fed through an impedance transformer which amplifies the input current but leaves
the voltage unchanged. An input resistor protects the buffer from static loads, and a capacitor
43
reduces the intrinsic noise. Like potentiostats, galvanostats function as electronic amplifiers with
low feedback. A simple galvanostat produces a constant voltage with a resistor connected in
series. To force the flow of a near-constant current through a load, this resistor needs to function
at considerably higher levels than the load resistor. A more complex galvanostat can feed a
constant current ranging from a few picoamperes (pA) to several amperes (A).
2.4.2 Potentiostat/galvanostat operation
A basic potentiostat can be modeled as an electronic circuit consisting of four components: the
electrometer, the I/E converter, the control amplifier, and the signal. The Figure 2.14 is the
schematic representation of 3 electrode controlled potential apparatus. The working of various
parts are as follows:
2.4.2.1 Electrometer
The electrometer circuit measures the voltage difference between the working and the reference
electrode. It acts as a feedback signal within the potentiostat, and I is the voltage signal that is
measured and displayed to the user. Its output serves two purposes. An ideal electrometer has
infinite impendence and zero current. The reference electrode does pass a very small amount of
current. Current through the reference electrode can change its potential, but this current is
usually so close to zero that the change is negligible.
The capacitance of the electrometer and the resistance of the reference electrode form
an RC circuit. If the RC time constant is too large it can limit the effective bandwidth of the
electrometer. The electrometer bandwidth must be higher than the bandwidth of all other
components in the potentiostat.
44
Figure 2.14. Schematic representation of a three-electrode controlled potential apparatus. X1
on an amplifier indicates it is a unity gain amplifier[44].
2.4.2.2 The I/E Converter
The current to voltage converter measures the cell current. The cell current is forced
through a current measurement resistor, Rm . The resulting voltage across this resistor is a
measure of cell current. During an experiment, cell current can change by several orders of
magnitude. Such a wide range of current cannot be accurately measured by a single resistor.
45
Modern potentiostats have several Rm resistors and an “I/E auto ranging” algorithm that selects
the appropriate resistor and switches it into the I/E circuit under computer control.
The bandwidth of the I/E converter depends strongly on its sensitivity. Unwanted
capacitance in the I/E converter along with Rm forms an RC circuit. To measure small currents,
Rm must be sufficiently large. This larger resistance, however, increases the RC time constant of
the circuit limits the I/E bandwidth. For instance, no potentiostat can measure 10nA at 100 kHz.
2.4.2.3 The Control Amplifier
The control amplifier compares the measured cell voltage to the desired cell voltage and drives
current into the cell to force these voltages to be the same. The control amplifier works on the
principle of negative feedback. The measured voltage enters the amplifier in the negative or
inverting input. Therefore, a positive perturbation in the measured voltage creates a decrease in
the control amplifier output, which counteracts the initial change.
The potentiostat simplified schematic in Figure 2.14 becomes a galvanostat when the
feedback is switched from the cell voltage signal to the cell current signal. The instrument then
controls the cell current rather than the cell voltage. The electrometer output can still be used to
measure the cell voltage.
2.4.3 Measurement Software
For the galvanostat/potentiostat controlling is software’s are essential without software,
is of no use. With dedicated software one can control the instrument, apply the experimental
settings, and see the measurement data in many graphical representations. Most of the software
always includes free use on multiple computers and free updates. The commercial
46
potentiostat/galvanostat are sold including its own easy-to-use intuitive software, which allows
the user to execute experiments immediately without any required training. Potentiostats and
galvanostats differ in terms of form factor and may include integral software that runs under the
Microsoft Windows operating system (OS). Software features may include cyclic and linear
voltammetry, normal and differential pulse voltammetry, user-definable programming, and
chronoamperometry and chronopotentiometry. Potentiostat software with potentiostatic
polarization, galvanostatic polarization, and potentiodynamic polarization is also available[47].
The software’s such as graphical user interfaces can provide various features such as
Automatic data saving during and/or after a measurement, scripting and equivalent circuit
fittings, saving all the available curves and plots, measurement data and methods to a single file.
And can also provide dynamic feedback on method parameters.
2.4.4 Applications of Potentiostat/Galvanostat
The potentiostat are used in controlled potential electrolysis for the synthesis of organic
and inorganic chemicals when control of the electrode potential allows a high degree of selectivity
to be obtained as compared to conventional chemical approaches. output drive or output power
of the instrument are key things to consider while selecting potentiostat for such applications.
Some major uses of the potentiostat related to processes in metal finishing are as follows.
Metal deposition - techniques involving the potentiostat are widely used to investigate
fundamental aspects of electrocrystallisation. Electropolishing - this technique is used extensively
to produce stress-free, highly polished surfaces on many metals and their alloys. Coatings - the
corrosion of aluminum, magnesium[38].
47
Galvanostats are used in battery charger research works. They are used to check the
battery longevity, charging and discharging characteristics. They are also used as battery chargers.
To determine the resilience of a certain protective layer by applying an electron current on the
layer, and to measure how high the potential of the layer is and how it will behave under different
circumstances galvanostats are used [61][47].
2.5 Existing Works : Portable Electrochemical sensing systems
Recently there have been many potentiostat solutions designed with discrete Integrated
circuits (ICs) connected to Printed Circuit Board (PCBs) to perform electrochemical analysis
[6][7][8]. The typical potentiostat performs with voltage range ±1.5 V and current range of 0.1mA
to 0.2mA. The main concerns of these designs were low cost, small size, and good performance.
The universal Wireless electrochemical detector (UWED) [48], Universal mobile electrochemical
detector (UMED)[49], Dstat[50] and Cheapstat[51] are the most recent papers.
2.5.1 The universal Wireless electrochemical detector (UWED):
This paper demonstrated application for the most used electrochemical techniques of
potentiometry, chronoamperometry, cyclic voltammetry, and square-wave voltammetry. The
working voltage range is about ± 1.5 and can measure current up to ± 0.18 mA current. The board
make use of 10-bit analog to digital converter (ADC) and 0.05mV resolution digital to analog
converter (DAC). It provides both wired and wireless communication mechanism.
The UWED communicates with a smartphone or a tablet using the wireless Bluetooth Low
Energy (BLE) protocol. A host program in the smartphone receives the input parameters of the
48
experiment from the user, communicates the experimental protocol to the UWED, receives the
raw data as the result of the experiment from the UWED, visualizes the data for the user, stores
the data, and sends the results to the Cloud[48].
2.5.2 Universal mobile electrochemical detector (UMED):
This paper describes an inexpensive, handheld device that couples the most common
forms of electrochemical analysis directly to “the cloud” using any mobile phone, for use in
resource-limited settings. The electrochemical methods that demonstrated are
chronoamperometry, cyclic voltammetry, differential pulse voltammetry, square wave
voltammetry, and potentiometry each with different uses. The combination of these
electrochemical capabilities in an affordable, handheld format that is compatible with any mobile
phone or network worldwide guarantees that sophisticated diagnostic testing can be performed
by users with a broad spectrum of needs, resources, and levels of technical expertise. It is
hardware specifications consist of ± 2 voltage range and ± 0.156 mA current range with up to 16
bit of ADC resolution. [49].
2.5.3 Dstat:
The Dstat is inexpensive, freely modifiable open-source analytical instruments that both
complement and compete with commercial instruments. It demonstrated Its capabilities for low-
current voltammetry in laboratory settings and found its performance to be comparable to that
of a compact commercial potentiostat as well as being superior to that of a previously reported
open-source instrument. DStat was also demonstrated to be useful for potentiometry (mostly
49
unique among lab-built potentiostats) with performance comparable to that of a commercial pH
meter. It also provides Integration with other devices through hardware or the custom-made
software as intermediary. Compared to other Potentiostats Dstat can measure up to 10mA
current. The opamp used in Dstat were selected for criteria to have low noise 45 𝑛𝑉/√𝐻𝑧 and an
offset of 0.2 mV to optimize the noise and offset voltage, which is better compared to many open
source potentiostats [50].
There are also many reports of single chip potentiostats manufactured with
complementary metal-oxide-semiconductor (CMOS) and related technologies [52][53][54].
Table 2.1. Comparison of different potentiostat
The above Table 2.1 shows the comparison of a list of different potentiostat proposed over
the last decade. Among them Dstat can measure up to 10mA current. Almost all the potentiostat
can perform most common electrochemical techniques such as Cyclic Voltammetry,
Amperometry and Potentiometry. The universal Wireless electrochemical detector (UWED),
Universal mobile electrochemical detector (UMED), Dstat include square wave voltammetry and
differential pulse voltammetry with above techniques. The UMED and UMED also includes
wireless protocols for remote operations.
Potentiostat Voltage (V)
Current (mA)
ADC Resolution
DAC Resolution
(mV)
Communication Protocol
Sampling Rate
UWED ± 1.5 ±0.18 10 bits 0.05 Wired and Wireless 0.1
UMED ± 2 0.156 16 bits 0.05 Wired and Wireless 0.8
Dstat ± 1.5 10 24 bits 0.046 Wired 30
Cheapstat ± 0.99 0.05 12 bits 1 Wired 2
PSoCStat ± 2 ±0.1 12 bits 1 Wired 50
50
In our research, we present a portable electrochemical sensing platform designed and
implemented from hardware to software for the point-of-care device to accommodate widely
used electrochemical sensing mechanism including amperometry, voltammetry. To control the
systems interfacing with the computers as well as displaying the obtained data, a graphical user
interface was developed in open-source programming language such as python. The low-power
compact hardware was designed and fabricated using commercial-off-shelf components which
consists of a microcontroller unit with readout circuit, a multiplexer, and wired communication
unit.
The proposed electrochemical sensing platform accommodates electrochemical
techniques such as linear sweep voltammetry, cyclic voltammetry, amperometry, anode strip
voltammetry, chrono amperometry and square wave voltammetry. The custom designed python
graphical user interface provides user options to select and perform each of the voltammetric
experiments. The GUI was developed using python 2.7 environment provides various functional
control over the sensing system.
The sensing system is design consists of a programmable system on chip unit which
includes a CPU and other analog components(ADC, DAC and TIA) to build the on-chip potentiostat
which help in reducing the external analog components inclusion compared to the various above
mentioned board. This also helps in achieving a good formfactor and developing small and
portable sensing system. Since the used components are a few including the MCU and the
multiplexer, the cost of the board is also very low.
51
Chapter 3
Proposed System
The purpose of the proposed system is to describe the design and characterization of a
portable electrochemical sensing platform from hardware to software. The proposed system
includes the voltammetric technique such as cyclic voltammetry, linear sweep voltammetry,
anode stripping voltammetry, amperometry, chrono amperometry and square wave voltammetry.
This system interfaces with a graphical user interface for receiving the experimental parameters
and experimental protocols. This approach provides a better form factor, simplified design and
reduced size and cost of the hardware. The small size is suited for the portable remote
electrochemical analysis. The systems operating ranges are (± 2 V, ± 100 uA) which are sufficient
for most of the electrochemical analysis in aqueous solution.
Figure 3.1. Block diagram for PSoC based Electrochemical sensing platform.
The Figure 3.1 shows the block diagram for the PSoC based Electrochemical sensing
platform. The sensor plugged into the board; the board is connected to computer via USB cable.
A host program in the computer controls the experiments performed by MCU, processes,
visualizes, and stores the data in the computer. The trans impedance amplifier and analog to
digital convertor block in MCU are used to interpret the data from the sensor. Communication
part, it has built in USB communication protocol for the data transmission. User can get the data
52
with the PC using the GUI. It will be discussed more in detail in later chapter. For the commercial
sensor connection part, 3 pin sockets are used.
3.1. Hardware Design and Implementation
Most open source potentiostats cover the voltage ranges of ± 1.5 V, and voltages more
than this results in electrolysis of water, making larger voltages less useful in aqueous solutions.
And most of the potentiostats are limited to around 0.1±0.2 mA of current as they have mostly
been designed for analytical chemistry purposes.
To do this we use a Programmable System on a Chip (PSoC), a single IC microcontroller
that includes with the CPU core, an array of other digital and analog electronic components. The
electrochemical sensing system board is designed to include the PSoC 5LP CY8C5888LTI-LP097
MCU. Five pin SWD interface is used to program the MCU. microUSB is used for Wired
communications, respectively.
There are some tradeoffs for the convenience of using a single chip potentiostat such as,
the Opamps cannot be selected to optimize noise or the offset voltage. For example the Opamps
used in the Dstat were selected for this criteria and have a noise of 6.5 𝑛𝑉√𝐻𝑧 and an offset of
< 0.2 mV, in comparison the Opamps of the PSoC 5LP have a noise of 45 𝑛𝑉√𝐻𝑧 and an offset of
up to 12 mV (for the reference and working electrodes combined). Also, the inputs of the PSoC
5LP are programmable general-purpose input-output pins having leakage current of up to 2nA
while the DStat Opamps, with their dedicated inputs designed for low currents, only have an input
bias current of 200 fA.
53
Due to use of on chip components and the size of the board is significantly reduced and it
is design as a good fit for remote healthcare applications.
The Figure 3.2 shows the PCB layout front and back side of it, respectively. The total size
of the board is 48mmX37mm. The Figure 3.3 and 3.4 are the front and back sides of the PCB board
unpopulated and the Figure 3.5 is the PCB board populated with the components completely. For
the commercial sensor connection 3 pin sockets are used.
Figure 3.2. PSoC based Electrochemical sensing PCB board top and bottom layout.
The list of the major components used are listed in the Table 3.1 below.
54
Table 3.1. Part List
Item Part Number Manufacturer
Multiplexer TMUX1208SVR Texas Instrument
MCU CY8C5888LTI-LP097 Cypress
Figure 3.3. PSoC electrochemical sensing PCB board unpopulated front side
55
Figure 3.4. PSoC electrochemical sensing PCB board unpopulated back side.
Figure 3.5. PSoC electrochemical sensing PCB board populated front and back
56
3.2. Software Design and Implementation
The software portion for the PSoC electrochemical sensing system consists of two main
sections. Which are, Firmware design and Graphical user Interface (GUI) design. The potentiostat
is designed in the Firmware design section. The controlling software is designed in the GUI
section.
3.2.1. Firmware Design
The Firmware for the liquid based electrochemical sensing system is developed in PSoC
4.2 developed by Cypress Semiconductor. The liquid based electrochemical system consists of
PSoC 5LP (part number CY8CKIT-059), which has integrated analog components into a single chip
with an ARM Cortex-M3 CPU. The potentiostat designed for the system make use of the built-in
components such as Transimpedance amplifier, VDAC, 12-bit ADC and DAC modules. The detailed
Firmware made in the TopDesign window of PSoC Creator, which will configure the analog portion
of the PSoC 5LP chip.
3.2.1.1. Voltage Control Circuit
The voltage between the Reference Electrode and the Working Electrode is controlled
using the circuit shown in Figure 3.6. The PSoC 5LP MCU consists of two types of voltage digital
to analog convertors. One is low resolution 8-bit VDAC and another one is the 12-bit dithering
digital to analog convertor(DVDAC). The voltage control circuit make use of both the modules
but one at a time. The built in opamp is used as a buffer to which voltage from selected
VDAC/DVDAC module will be provided. To select a voltage source an analog mux is used. The cons
57
with DVDAC is the noise it generates, to avoid this an external capacitor is used. The graphical
user interface used to provide user option for the voltage source selection.
Figure 3.6 Voltage Control Circuit
The output of the opamp is connected to the counter electrode. Counter electrode and
reference electrodes are connected via analog mux for selecting two or three electrode
configurations. The current through the counter electrode will flow until the voltage across the
reference electrode is same as the voltage source. The control functionalities for the voltage
control unit are provided in graphical user interface.
3.2.1.2. Current Measuring Circuit
The PSoC 5LP MCU consists of 3 types of Analog to digital converters. Successive
approximation ADC (SAR ADC), Delta sigma ADC and Sequencing SAR ADC. All are of 12-bit
58
resolution. In this circuit built in Trans impedance amplifier is used to convert the current from
the working electrode into voltage. The trans impedance amplifier high input impedance makes
the working electrode current to pass through the feedback resistor. The drop across the resistor
is then digitized using the delta sigma ADC. The TIA of the PSoC MCU uses switch capacitors to
produce the feedback resistance values. This range varies from 20 kiloohms to 1 Mega ohm(8
resistor values).The device is limited to 100uA as the minimum value of the resistance can be used
it 20 kiloohms. Due to use of switched capacitors the measured values of the TIA sometimes vary,
to rectify this an 8-bit IDAC is used. The know values of current is passed to the TIA and the
respective ADC readings are then matched with the expected values. If there is deviation the
correction offset is used to correct the reading. Figure 3.7 shows the arrangement of the various
current measuring circuit components.
Figure 3.7 Current Measuring Circuit
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3.2.1.3. Peripheral Components
Figure 3.8 Other peripheral components.
Figure 3.8 shows the other peripherals used by the device. The data read operation from
the ADC and the DAC is controlled using a pulse width modulator. The PWM generates pulse that
is used to trigger the interrupt service routines of the ADC/DAC. The pulse width modulator
period is decided by the user, which will be provided as an input to the Firmware from the GUI.
An EEPROM is used to store the user preferences such as the number of electrodes to use, the
type of voltage source to use. The Full speed USB component is used for data transmission
between the firmware and the GUI.
3.2.1.4 Firmware Workflow
In the Firmware design once the required components are placed, the next step is to
initialize these components and their respective interrupt routines. The flowchart of the main
program and its functional calling based on commands are shown in the Figure. 3.9. below.
There are two main parts. At first when the firmware starts, the components such as
Analog to digital converter (ADC), Digital to analog converter (DAC), Trans-impedance amplifier
60
(TIA), PWD (Pulse width modulator) are initialized and put into sleep mode to save the power
consumption. The main program then invokes into a forever loop where it will constantly check
for the received USB commands. If the USB commands received, then the program will jump into
the respective program to perform next set of operations. This workflow is shown in the first of
the three flowcharts shown in the Figure 3.9.
Figure 3.9 Firmware Flowchart.
61
The first set of the received commands are the one to set the look up table for the
potentiostat input voltage. Depending on the different type of voltammetric sensing technique,
the lookup table created varies. For example, cyclic voltammetry involves creating of a triangular
wave points, the square wave voltammetry involves creating of a square wave look up points for
the voltage based on the user provided start and the stop voltage values. Once these lookup
tables were created the main program returns to its loop to check for the next set of USB
commands. This part of the programming flow is shown in the third flowchart of the Figure 3.9.
The main set of the commands are the one to run each experiment and then to return the
received ADC response. Once the GUI command to run these experiments is received, depending
on the experiments respective hardware components will be awaken and the DAC voltage for the
electrode is set using the already created experiment specific lookup table. Once the DAC values
are set, the program return the “Done” command to the GUI. The GUI after receiving the “Done”
command will send another command to read the respective ADC value for the Current value
readings. This part of the programming flow is shown in the second flowchart in the Figure 3.9.
There are other set of programs which handles the interrupt service routines and
hardware configuration which are not included here. But are essential part of the Firmware.
3.2.2. Graphical User Interface Design (GUI)
The electrochemical sensing platform communicates with computer via Universal Serial
Bus (USB) and by sending and receiving the commands in the string format. The wired
communication is provided using the inbuilt USB or COM port communication protocol.
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The Graphical User Interface (GUI) for wired control protocols is designed using the
python software language, version 2.7. We followed the GUI provided by the open source
potentiostat for the further functional enhancement and modification for the portable sensing
platform. The user can select different electrochemical sensing techniques by selecting the
appropriate notebook. The main programming language and its libraries are illustrated in the
Figure 3.10.
Figure 3.10 Python Environment and associated packages.
The GUI development consists of python environment version 2.7. The Visual Studio Code
IDE, developed by Microsoft for which provides support for debugging, embedded Git control
63
and GitHub, syntax highlighting intelligent code completion, snippets, and code refactoring. The
main framework of the GUI is built using the tkinter python library. The data is displayed using
the matplotlib library. USB communication is done using pyUSB and the libusb-win32, WinUSB
driver libraries. The source code is bundled has been compiled into an executable file. The
packaging of the python script into an executable file is accomplished with the Pyinstaller
package. Other supporting libraries such time, CSV are used for the synchronization and data
saving options. All these libraries and their association are shown in the Figure 3.10. On broadly
speaking, the GUI consists of two set of the programs. The initialization and then the device
control and data acquisition programs.
3.2.2.1 GUI initialization
The GUI initialization involves establishing the connection with the board via USB
communication protocol. The GUI will send a pre-defined command “I” the PSoC electrochemical
sensing board and it will wait for the handshake signal from it. The PSoC electrochemical sensing
board on receiving the command “I” will return with the USB setup string which will be used by
the GUI to set the connection button with connected string in green color to indicate the
connection is established. If the connection establishment is resulted in fail due to some reason,
then the indicator button is set to red color and with the string “unable to connect”.
On establishing the communication, the GUI will then send the individual voltammetric
sensing technique lookup table commands and electrode settings, the DAC source settings and
wait for the completion response from the board. Once this established the GUI will enter the
stage where it will wait for the user to select the next operation. The other voltage or current
64
Figure 3.11 GUI initialization, control, and data acquisition flowchart.
65
settings or running the experiment are the next set up of commands. This programming
flow of the GUI is shown in the first flowchart which is also called as GUI initialization flowchart
of Figure 3.11.
3.2.2.2 GUI device control and data Acquisition:
In this section of GUI functions, based on the user selected electrochemical sensing
technique, on selecting “Run”, a command identifier for the techniques and the sensor count is
sent to the sensing system board. The board on receiving the command will provide the
respective voltage response to the electrode and will return the completion message.
On receiving the completion message the GUI will then send another command to
continuously receive the data point for the electrodes. These read values are used to plot the
data and on user request these points are exported to CSV data file. While running these
commands any error occurred will be handled from the python exception handling side.
66
3.2.2.3. Finished Application
Figure 3.12 PSoC-Stat Graphical User Interface
The graphical user interface (GUI) provided by the PSoC-Stat [48] is shown in the figure
3.12. The User Interface provide measuring capabilities for the cyclic voltammetry, amperometry
and anode stripping voltammetry.
The user interface has been modified with new windows for the PSoC electrochemical
system to include the added electrochemical technique measurements. New windows for the
square wave voltammetry, Linear sweep voltammetry and Chrono amperometry are added.
Functional modification to run each electrochemical sensing techniques are added. The
Application with added windows is shown in figure 3.13.
67
Figure 3.13. Updated GUI with SWV, LSV and Chrono Amperometry techniques.
3.3. Test Setup and Result
To test the board with the electrochemical techniques we used the dummy cell test setup
shown in the Figure 3.14. The customized sensor board is placed in its case to isolate it from any
other electrical contact.
In the experiment we used the a 50K Potentiostat dummy cell [47] described in Figure
3.15. The dummy cell consists of the 3 terminals, one for each of WE, CE, and RE. These terminals
connected to their respective slot. The board USB is connected to the computer via a USB cable
to control the device through the graphical user Interface. Once the board is connected, on stating
the GUI, connection is established between the board and the GUI via USB communication
protocol.
68
Figure 3.14. PCB board electronic measurement using dummy cell.
Figure 3.15. Potentiostat 50K Dummy Cell
69
To compare the performance of our PCB, we performed cyclic voltammetry electrically
with commercial sensor board using the Potentiostat 50k Dummy Cell. The PSoC-Stat cyclic
voltammetry performed on 50K Potentiostat Dummy cell[47] run from 900 mV to -900 mV with
a scan of 10mV/s.
The Dummy cell consists of 50k resistor connected between the counter and reference
electrode. The working electrode connected to the other side of the reference electrode. The
Dummy cell will produce current at working electrode proportional to the supplied ADC voltage
when connected to the device.
The experimental result produced current between the Working electrode (WE) and
Counter electrode (CE) in the range of -18uA to +18uA. The resistance calculated using the applied
voltage and measured current is matched with the resistor connected between counter and
reference electrode.
3.3.1 Cyclic Voltammetry
The Potentiostat dummy cell experiment is performed on cyclic voltammetry with the
voltage range 900mv to -900mv which produced a current between the working electrode and
the counter electrode in the range of 18uA to 18uA. The result of the experiment is interpreted
using the executable file.
The Figure 3.16. shows the electronic measurement performed on the bord using dummy
cell. The x-axis consists of the ADC voltage applied, the y-axis contains the resultant current
measured across the WE and the RE.
70
Figure 3.16. PCB board electronic measurement plot for CV.
Table 3.2. sample of CV response with dummy cell.
Voltage(mv) current(uA) Resistance (KΩ)
-896 -17.7774 50.4011
-880 -17.2576 50.99208
-864 -17.3096 49.91461
-848 -16.8937 50.19618
-832 -16.5299 50.33318
-816 -16.166 50.47635
-800 -16.166 49.48661
-784 -15.5422 50.44324
-768 -15.3343 50.08381
-752 -14.7625 50.93985
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The resultant current of both the PSoC-Stat and the PCB board generated with respect to
the provided ADC voltage. The dummy cell resistance is calculate using the Experimental data
listed in the table 3.2. The average resistance measured for the board is 50.326 KΩ. which is
almost in agreement with the dummy cell resistance 50 KΩ.
3.3.2 Square wave Voltammetry
For the Square wave voltammetry, a square wave pulse signal is applied between the
working electrode and the reference electrode. Resultant current is measured between the
working electrode and counter electrode.
The dummy cell experiment is performed on square wave voltammetry with the voltage
range pulse generated in the range of -900mv to 900mv over the default DAC voltage(0V) which
produced a current between the working electrode and the counter electrode in the range of -
12uA to 12uA. The result of the experiment is interpreted using the GUI.
72
Figure 3.17. Commercial sensor board electronic measurement plot for SWV.
The Figure 3.17 shows the electronic measurement performed on the board using the
dummy cell. The x-axis consists of the time duration, the y-axis contains the current measured
across the WE and CE.
Table 3.3. sample of SWV response with dummy cell.
Voltage(mv) PSoC 5LP current(uA)
-0.585282306 -11.70564611
-0.577714 -11.55428
-0.590327843 -11.80655686
-0.582759537 -11.65519074
-0.570145694 -11.40291389
-0.587805074 -11.75610149
-0.585282306 -11.70564611
-0.562577389 -11.25154777
-0.577714 -11.55428
-0.590327843 -11.80655686
3.3.3. Linear Sweep Voltammetry
The Potentiostat dummy cell experiment is performed on Linear sweep voltammetry with
the voltage range 900mv to -900mv which produced a current between the working electrode
and the counter electrode in the range of 18uA to 18uA. The electrical measurement is performed
the result of the experiment is interpreted using the GUI. The Figure 3.18 shows the electronic
measurement performed on the commercial sensor board using the dummy cell. The x-axis
73
consists of the ADC voltage applied, the y-axis contains the current measured across the WE and
RE.
Figure 3.18. PCB board electronic measurement plot for LSV.
Table 3.4. sample of LSV response with dummy cell.
Voltage(mv) PSoC 5LP current(uA)
Computed Resistance (KΩ)
-890 -17.2065 51.7246
-889 -17.8644 49.7637
-888 -17.6114 50.4218
-887 -17.409 50.9506
-886 -17.4596 50.7457
-885 -17.7632 49.8221
-884 -17.4596 50.6311
-883 -17.3078 51.0174
-882 -17.5608 50.225
-881 -17.409 50.6060
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The resultant current of the PCB board generated with respect to the provided ADC
voltage. The Comparison of both the plot is shown in Table 3.4. Response obtained are almost
identical indicates both the boards are producing same characteristics average current
50.5908Kohm.
3.3. 4. Amperometry
The Potentiostat dummy cell experiment is performed on amperometry with the supply
voltage set to 500mv, the circuit produced a constant current of 10mv as shown in the Figure 3.21.
The result of the experiment is interpreted using the executable file.
The Figure 3.21. shows the electronic measurement performed on the bord using dummy
cell. The x-axis consists of the time duration in seconds, the y-axis contains the resultant current
measured.
75
Figure 3.19. PCB board electronic measurement plot for amperometry.
Table 3.5. sample of Amperometric response with dummy cell.
Time(sec) Current Measured (uA)
Resistance of dummy cell (KΩ)
Voltage computed from the result (mv)
0.001 10.03683 50 0.501842
0.002 10.34098 50 0.517049
0.003 10.08753 50 0.504376
0.004 9.986143 50 0.499307
0.005 10.08753 50 0.504376
0.006 10.13822 50 0.506911
0.007 10.29029 50 0.514514
0.008 10.49305 50 0.524653
0.009 10.08753 50 0.504376
0.01 10.49305 50 0.524653
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The resultant current of the PCB board generated with respect to the provided ADC
voltage. The voltage calculated using the measured current is listed in the table 3.5. The average
current measured for the board is 10.23uA for the supply voltage of 500mV. which is almost in
agreement with the computed value for the dummy cell resistance 50 KΩ and the supplied voltage
500mv.
3.3.5 Anode Stripping Voltammetry
The Potentiostat dummy cell experiment is performed on anode stripping voltammetry
with the voltage range 900mv to -900mv which produced a current between the working
electrode and the counter electrode in the range of -18uA to 18uA. The result of the experiment
is interpreted using the executable file.
The Anode stripping voltammetry consists of two parts. First, the calibration of the circuit.
The Figure 3.22. shows the initial calibration performed on the board though GUI. Once the
calibration is complete the second step, cyclic voltammetry is performed with dummy cell. The x-
axis consists of the ADC voltage applied, the y-axis contains the resultant current measured across
the WE and the RE.
The resultant current of the board generated with respect to the provided ADC voltage.
The dummy cell resistance is calculated using the Experimental data listed in the table 3.6. The
average resistance measured for the board is 50.716 KΩ. which is almost in agreement with the
dummy cell resistance 50 KΩ.
77
Figure 3.20. anode stripping voltammetry dummy cell test plot.
Table 3.6. sample of ASV response with dummy cell.
Voltage(mv) current(uA) Computed Resistance (K-Ohm)
-896 -18.31500134 48.92164533
-880 -17.50772856 50.26351632
-864 -17.45727401 49.4922632
-848 -17.05363762 49.72546146
-832 -16.65000122 49.9699663
-816 -16.34727393 49.91657959
-800 -16.09500118 49.70487364
-784 -15.79227389 49.64452907
-768 -15.69136479 48.94411738
-752 -15.03545565 50.01511212
78
3.3.6. Chrono Amperometry
To perform chronoamperometry we generated two types pulse signal and checked the
response with the Potentiostat dummy cell with 50K resistance.
The first pulse generated was with voltage range 900mv to -900mv which produced a
current between the working electrode and the counter electrode in the range of -18uA to 18uA.
The result of the experiment is interpreted using the executable file.
The Figure 3.23. shows the result for +900mv to -900mv pulse. The test performed on the
board through GUI. The x-axis consists of the time duration in seconds, the y-axis contains the
resultant current measured across the WE and the RE.
Figure 3.21. chrono amperometry result plot for ±900mV
The same experiment is performed with the pulse generated from +900mv to 0. During
the first half of the duration the pulse value is held at +900mv and during the remaining half, it
79
was held at 0v. The result is shown in the Figure 3.24. and the sample of the experimental data is
provided in the table 3.7 and 3.8.
Figure 3.22. chrono amperometry result plot for +900mV to 0V
Table 3.7. chronoamperometry result with dummy cell for pulse ±900mV.
Time(sec) Current(uA) Computed voltage (mV)
1 17.95603 897.8014
2 17.69504 884.7519
3 17.85163 892.5816
4 17.69504 884.7519
5 18.00823 900.4113
6 17.85163 892.5816
7 17.59064 879.5322
8 17.79943 889.9717
9 17.53845 876.9223
10 17.85163 892.5816
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The resultant current of the board generated with respect to the provided ADC voltage.
The pulse voltage is calculated suing the dummy cell resistance 50 KΩ and the read current value
was ±900mV as expected.
Table 3.8. chronoamperometry result with dummy cell for pulse +900mV to 0V.
Time(sec) Current(uA) Computed Voltage (mV)
54 17.23575 0.861787
55 17.9066 0.89533
56 17.75179 0.887589
57 17.9066 0.89533
58 17.64858 0.882429
59 17.75179 0.887589
60 -0.15481 -0.00774
61 -0.15481 -0.00774
62 -0.0516 -0.00258
63 0.051604 0.00258
The table 3.8 shows the resultant current of the board generated with respect to the
provided +900mv to 0V pulse signal. The pulse voltage is calculated suing the dummy cell
resistance 50 KΩ and the read current value was ±900mV as expected.
81
Chapter 4.
Conclusion
In this work, a Portable Electrochemical sensing system from hardware to software for
voltammetric sensing mechanism is proposed. The board is designed to work with voltage range
± 2V and current ± 0.1 mA range, respectively. The electrochemical sensing techniques included
are cyclic voltammetry, anode stripping voltammetry, amperometry, linear sweep voltammetry,
square wave voltammetry and chrono amperometry. The system is developed in 3 stages. They
are hardware design, firmware design and graphical user interface design.
In the hardware design, the PCB board is designed for the typical electroanalytical
specification, i.e. the voltage range within ± 2V in as excess of this results in electrolysis of water,
making larger voltages less useful in aqueous solutions and current range ± 0.1 mA as have mostly
been designed for analytical chemistry purposes. We used 3 commercial multiplexers to provide
eight sensors measurement at a time which is not implemented in any of the existing opensource
potentiostats. We used PSoC MCU which includes most of the analog components helps in
reducing the size of the board significantly to provide portability. The MicroUSB for the wired
communication protocol.
In the firmware design, the 3 electrode potentiostat is designed using the PSoC 5LP
Internal analog components such as TIA, ADC and DAC and the amplifier. This helped in reducing
the size but also has its disadvantage in not able to reduce the noise significantly compared to
the potentiostats designed using the discrete analog components(Opamp and TIA).
82
Finally, the graphical user interface is designed and to control the board using USB
communication with opensource python 2.7 version programming language. To control the
systems interfacing with the computers as well as to display the obtained data, a graphical user
interface is developed.
We performed experiments on PSoC based electrochemical system. The Potentiostat
dummy cell array tests was performed for the aqueous phase detection and test results showed
the linear response, which is important in electrochemical sensing system.
The existing papers on electrochemical sensor implementation includes some of main
voltammetric techniques. In our board, most of the voltammetric techniques are included. The
sensing system was developed with programmable system on chip which eliminates the
requirement of any of the external analog component for the potentiostat implementation. This
Significantly reduced cost factors and made us to reduce the board size to 50mmX40mm. This
helps to achieve the portability and cost friendly (MCU and MUX).
For future work, it is intended to include other electrochemical sensing mechanisms such
as Potentiometry, Conductometry, Electrochemical Impedance spectroscopy to form a more
versatile, compact, cheaper, and minimally invasive solution to existing methods of commercially
available universal electrochemical sensing platform. The development of an android applications
to provide both GUI and to control and collect response in both wired and wireless
communication protocols.
83
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