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Karlovits et al. (2021). “Cyclic voltammetry tests,” BioResources 16(2), 3916-3933. 3916
Comparison of Cyclic Voltammetry Measurements of
Paper-Based Screen Printed Electrodes via Proprietary
and Open Source Potentiostats
Igor Karlovits,a Urška Kavčič,a Špela Trafela,b and Kristina Žužek Rožman b,c
The use of screen-printed electrodes in different monitoring applications,
e.g., polluted water, biotechnology, agriculture, industrial process control,
and other applications, are continuously being developed. New cheap and
open-source potentiostats have been recently emerging, in addition to the
commercial and proprietary solutions on the market. In this study, paper-
based, screen-printed electrodes were utilised as an alternative solution
for ceramic-based electrodes and were tested with two potentiostats
(proprietary and low-cost open-source running on wireless 64 bit Linux
system installed on Raspberry Pi 3+). Unique paper substrates made from
invasive plant papers and one commercial product were used for screen
electrode printing. Ink layer thicknesses variations and mechanical
grinding were applied, and cyclic voltammetry measurements were
conducted. The variation in cyclic voltammetry measurements could be
attributed to two sources: the potentiostats showed differences in their
sensibility and signal values, and paper surface and structure also
contributed to differences. Simultaneously, the additional processing
steps, e.g., mechanical grinding, introduced additional measurement
variations and differences in the measurement process.
Keywords: Screen-printed electrodes; Paper properties; Cyclic voltammetry
Contact information: a: Pulp and Paper Institute, Bogišićeva 8, Ljubjana 1000 Slovenia; b: Jožef Stefan
Institute, Department for Nanostructured Materials, Jamova cesta 39, Ljubljana SI-1000 Slovenia; c: Jožef
Stefan International Postgraduate School, Jamova cesta 39, Ljubljana SI-1000 Slovenia; *Corresponding
author: igor.karlovits@icp-lj.si
INTRODUCTION
Electrochemistry research is helpful in many different applications, ranging from
life sciences to environmental and medical sciences. Cyclic voltammetry (CV) is a widely
used electrochemical measurement method that provides values of redox reactions
energetics, electron transfer dynamics, and rates of coupled chemical reactions. The
measurement consists of scanning the applied potential on the working electrode and
measuring the resulting current flow. The working electrode's potential is cycled between
two potential limits, inducing the oxidation and reduction of electroactive substances
dissolved in the sample or absorbed at the working electrode surface (Foster et al. 2019).
Cyclic voltammetry is a powerful but straightforward generic method for the
electrochemical characterization of trace amounts of substances in water and deposits on
conductive surfaces. Over the past few decades, screen-printed electrodes (SPEs), which
are used as inexpensive electrochemical substrates, have experienced significant changes
in their format and printing materials. Screen-printed electrodes have been successfully
used for the rapid in situ studying of environmental toxins due to their favourable material
properties, e.g., biodegradability, simplicity, and rapid reactions. The electrodes for
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Karlovits et al. (2021). “Cyclic voltammetry tests,” BioResources 16(2), 3916-3933. 3917
portable devices have been primarily produced on ceramic surfaces; however, several
studies have used paper substrates for electrode printing (Oh and Chow 2015; Yáñez-
Sedeño et al. 2020). The interaction of paper substrates, which have porous hydrophilic
surfaces, has not been thoroughly studied in terms of the papers' properties and their
influence with different electronics setups. The knowledge obtained from these interactions
can be valuable in improving further usage of disposable and recyclable electrodes in
fieldwork applications, especially in the environmental protection field. Besides using
paper-like substrates for single-use measurement electrodes, e.g., in fieldwork, there has
also been a noticeable trend in developing open-source potentiostats in the last few years.
Since potentiostats have been primarily expensive benchtop versions for decades, this new
development with the advent of microelectronics enables new applications and mobility.
The influencing factors, of course, are the potentiostat elements and the sensing systems
(printed electrodes). The potentiostats differ in their usage of signal microprocessors,
voltage amplifiers, current to voltage converters, operational amplifiers, and other elements
necessary to convert the signals into CV readings. With the ability to connect through
different networking protocols (internet of things (IoT), 4G, WiFi), the use of CV can be
expanded in terms of fieldwork, which is vital for sustainability preservation. The use of
cheap screen-printed electrodes on low-cost paper substrates in conjunction with entirely
open-source potentiostats that can connect through different network systems can lead to
new applications. Besides, this combination can provide easy access for students and
individuals to use CV or other electrochemical techniques in their research and
measurements. According to a study by Dryden and Wheeler (2015), the circuits of the
open potentiostats fall into four categories: (1) tiny instruments intended for implantation
or wearable use; (2) low-cost instruments developed to feature extreme cost savings or
field-portability (with a lower level of performance); (3) multiplexed instruments designed
and developed for specialised applications involving arrays of electrodes; or (4) bench-
scale instruments constructed around "virtual instrument" frameworks (which are useful
for prototyping, but are expensive and lack robustness and portability).
Some of the solutions that can be prototyped or bought and upgraded include the
Rodeostat (Long 2020), built as a potentiostat shield around the teensy 3.2 development
board and has online P.C. Linux software packages with open-source documentation and
hardware libraries. Recently, more than one cheap open source potentiostat has been built
around this board. Glasscott et al. (2020) designed and developed SweepStat, capable of
performing voltammetry amperometry. The device had the additional benefit of using
ultramicroelectrodes (UME). The authors reported that cyclic voltammetry and
amperometry measurements at macrodisk electrodes and UMEs offer high-quality data that
corresponds to literature reported values, commercially available electrochemical
workstations finite-element simulation packages. Another solution is the Dstat, which is
built around the ATxmega 256A3U microcontroller and its additional circuitry (Dryden
and Wheeler 2015). The authors have also tested the electrochemical performance of DStat
with CheapStat (another open-source solution) and the performance of PalmSens via
potassium hexacyanoferrate(III) and 4-aminophenol measurements. The three potentiostats
produced the expected double peak-shaped cyclic voltammograms and similar values for
the square wave voltammograms; however, there were individual measurement variations
in the measurement data between the instruments.
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Karlovits et al. (2021). “Cyclic voltammetry tests,” BioResources 16(2), 3916-3933. 3918
The DStat and EmStat displayed almost identical voltammograms, characterised by
smooth noise-free responses; variations in individual electrodes likely caused the slight
differences between them. In contrast, the CheapStat potentiostat voltammograms were
considerably different; the cyclic voltammogram had vast peak separation and reduced
peak heights, and its square wave voltammogram peak was asymmetric, with substantial
background current. Inspired and built with the Dstat, the ABE-Stat is capable of
conducting routine electrochemical analyses, including cyclic voltammetry (CV),
differential pulse voltammetry (DPV), and high impedance potentiometric measurements,
as well as having wireless options through an Android interface (Jenkins et al. 2019). It
has been claimed as the first fully open-source potentiostat capable of evaluating
electrochemical impedance spectroscopy (EIS) across a broad frequency spectrum (0.1 Hz
to 100 kHz) user-selectable amplitude and bias. The novelty, compared to DStat, was the
incorporation of a network analyser chip (AD5933, Analog Devices, Norwood, MA) and
reconfiguration of the control amplifier network to enable EIS across the spectrum (0.1 Hz
to 100 kHz), with arbitrary bias and any of the four pre-programmed A.C. amplitudes.
Single CV and DPV scans were recorded for a three-electrode cell in an electrolyte
solution containing equimolar concentrations of K4[Fe(CN)6], and K3[Fe(CN)6] were
performed to validate the voltammetric functions of the ABE-Stat. Cyclic voltammograms
resulted in similar measurements obtained with a reference instrument with larger redox
current peaks for bare gold wire electrodes than electrodes coated in polyethyleneimine.
Also, DPV scans resulted in expected characteristic shapes of redox current curves, with
substantial correction for the background currents from dielectric charging at the boundary
layers and more prominent current peaks on bare wires. All the performed scans were
symmetric, showing the high reversibility of the selected redox reaction.
The PSoC-Stat is another solution where an open-source potentiostat was made
with a commercially available integrated circuit (I.C.), which does not need external
electronic components to perform electrochemical experiments (Lopin and Lopin 2018).
This result was achieved by using the Programmable System's capabilities on a Chip
(PSoC®) by Cypress Semiconductor, which integrates all of the necessary electrical
components. This step eliminates the process of having to make a printed circuit board and
soldering on electronic components. A graphical user interface (GUI) was developed in the
python programming language to control the instrument. However, the authors point out
some limitations regarding the noise optimisation of the offset voltage. This model has
been tested against EmStat3 (a commercially available potentiostat) by using potassium
ferricyanide (K3[Fe(CN)6]) and an additional test of lead determination in water and
glucose measurements. Performing cyclic voltammetry on a five mM ferricyanide solution
with 1 M KCl with a scan rate of 10 mV/s showed the characteristic oxidation and reduction
peaks. There were slight differences between the traces, which could be due to
electrochemical variations and temporal changes in the electrodes.
Two other solutions further upgrade the possibilities; enabling the use of mobile
phones together with the potentiostat, e.g., UWED (universal wireless electrochemistry
detector) (Ainle et al. 2018) and KAUStat (Ahmad et al. 2019), which the authors declare
as a wireless wearable open-source potentiostats for electrochemistry measurements. The
UWED has an RFduino microcontroller which contains a 32-bit ARM processor, memory,
and peripheral components, i.e., a BLE for communication, a I2 C bus for interfacing with
DAC, digital outputs for switches, and a 10-bit ADC with a conversion time of 70 μs for
the measurement of the input signals. The authors used a commercial screen-printed three-
electrode cell (DRP-110CNT-GNP, DropSens, Llanera, Spain) for performing C.A., SWV,
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and CV on ten μM to 10 mM of ferricyanide. In a comparison experiment, the cyclic
voltammograms (C.V.s) of ferricyanide at three different concentrations, 10.0 mM, 0.1
mM (scan rate of 100 mV/s), and 1.0 mM (scan rates of 20 mV/s to 300 mV/s), were
recorded; the UWED and the commercial potentiostat (Autolab) had similar peak shapes,
peak potentials, and peak currents. The authors of KAUStat demonstrated the efficiency of
the KAUSTat with three solutions: (1) a phosphate buffer saline (PBS) solution containing
137 mM of sodium chloride (NaCl); (2) a sodium hydroxide (NaOH) solution; and (3) a
hexacyanoferrate probe (5 mM [Fe(CN)6]3-/4- and 0.1 M KCl) solution. All the CV
measurements were conducted using platinum (Pt) as the counter, Ag/AgCl as a reference,
and glassy carbon electrodes (GCE) as the working electrodes. In the PBS buffer, both the
CV curves of KAUSTat and Emstat were comparable 80% of the chosen measurement
period) set by the application. The difference in the clock timing explained the slight
difference in peak heights. All of the comparisons between the open-source and newly
developed potentiostats were primarily made with laboratory type electrodes or
commercially available non-paper-based (ceramic type electrodes).
Paper Properties for Screen-Printed Electrodes (SPEs)
The other electrochemistry reading element is the electrode, especially if they are
printed on substrates like paper. Due to its properties, paper has been a suitable and usable
substrate for the past’s printed electrodes. For example, different biosensor applications
have been used to analyse water samples with different kind of papers holders in a study
by Peixoto et al. (2019), where cellulose filter paper, cellulose chromatography paper,
nitrocellulose membrane, and printing paper were described and compared. In an article by
Honeychurch and Hart (2003), similar paper substrates used in biosensing applications
were accentuated and reviewed. All these reviewed articles primarily described the paper
substrates' material composition with no further details on the absorption or chemical
interaction with the inks. Besides, the paper layers needed to be modified or pre-treated for
some applications, e.g., with a spin coating to achieve additional functional property
(Honeychurch and Hart 2003). The use of invasive plant papers for these purposes has not
been widely studied except by Kavčič and Karlovits (2020). The combination of ink
printing techniques and the influence of the paper is well known (Brodnjak et al. 2017),
but newly developed materials (paper substrates) need to be tested due to the inherent
influence of the fibre composition and surface sizing or coating, which can influence the
absorption and ink spreading mechanism.
When printing electronics, to achieve excellent and accurate results, the screen-
printed electrodes must have consistent quality regarding their ink film thickness and
formation on the paper, as well as other characteristics; these are governed by the substrate
ink interaction, after which the functional part of the process (the electrochemistry part)
performs the reaction part. In this research, the authors have tested invasive plant species-
based papers as electrode substrates, which were analysed via a commercially available
potentiostat using the Windows operating system on P.C., and an open-source potentiostat
was operated via a portable Raspberry Pi 3+ system running Linux. The portability of the
Raspberry Pi system combined with the WiFi connection protocol enabled low costs,
complete portability, and effortless transfer of technology into real-world applications
outside the laboratory with a good signal reading quality.
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EXPERIMENTAL
Methods and Materials
The screen-printed electrode (SPEs) samples were printed on three different
printing materials; two were produced from invasive plant species, and one was
commercially available cardboard.
Fallopia japonica (Japanese knotweed) (Pulp and Paper Institute, Ljubljana,
Slovenia) is an invasive plant, which was made into cardboard from their fibres (JK 240
g/m2). Solidago gigantea (giant goldenrod) and Solidago canadensis (Canadian goldenrod)
are also invasive species made into cardboard from their fibres (Pulp and Paper Institute,
Ljubljana, Slovenia) (GCG 240 g/m2). Excellent top brown (ETB), which is commercially
available cardboard (MM Karton, Vienna, Austria), in which 250 g/m2, made from 95%
virgin fibres (chemical and mechanical pulp as well as 5% of the surface treatment), were
used as the SPE carriers.
A handheld roughness tester (TR200) was used, and the average roughness (Ra),
with a 0.8 mm cut-off length, was determined. For ink vehicle absorption, the authors used
the K.N. ink absorption test based on the SCAN-P 70:09 ink absorbency test. The authors
also used an internal test to determine the paper substrates' micro-and macro-porosity,
which governs the ink vehicle absorption. Also, the authors performed surface free energy
measurements with water and formamide using a Fibrodat 1100 instrument.
To be compatible with the chosen commercial potentiostat measurement systems,
screen-printed working electrodes (WE) with a 4 mm diameter were developed. A semi-
automatic screen-printing system (GTO EVO 570) was used for printing, and the electrodes
were screen-printed to achieve additional functions in three distinct layers with three
different printing inks. In the first printing run, electrode connections and a quasi-reference
electrode (RE) were printed with silver printing ink. The working (WE) and auxiliary
electrodes (A.E.), printed with carbon printing ink, were added. Finally, dielectric printing
ink was overprinted to determine the testing area. The working electrodes were also printed
in one and two ink layer depositions. As an additional step, the authors mechanically
grounded the working electrodes, as this was shown to be beneficial and to improve the
electrochemical response (Pravda et al. 2001; Cumba et al. 2016; Lee et al. 2016; Kavčič
and Karlovits 2020). The printed electrodes and the processing steps are presented in Fig.
1.
Fig. 1. Screen-printed electrode parts and processing steps: (a) one ink layer deposition; (b) two
ink layer deposition; (c) grinding of the working electrode
(a)
(b)
(c)
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Grinding removes the binder surface and opens up the silver particles in the ink
layer. The grinding was performed with a fixed mechanical setup, and the same grinding
stone was used for all samples. The technical parameters of the printing process and the
used printing inks are presented in Table 1.
Table 1. Printing and Curing Conditions Used for Sample Printing
Ink Type
Ink Info
Mesh
Count
(l/cm)
Curing Conditions
Ink
Layers
Time
(s)
Temperature
(°C)
Silver
printing ink
SC
CRSN 2442, SunTronic Silver 280
(SunChemical, USA)
120
90
90
1x
Carbon
printing ink
PE
PE-C200 Carbon resistive ink
(Applied Ink Solutions, USA)
77
180
120
1x and
2x
Dielectric
printing ink
Dielectric
Dielectric Grey dielectric paste
D2070423P5 (SunChemical, USA)
90
180
120
1x
The samples' electrochemical behaviour was characterised after the samples were
printed using a DropSens μStat 300 Bipotentiostat (DropSens, Oviedo, Spain). For the
cyclic voltammetry measurements, 100 μL of 2.5 mM potassium ferricyanide K3[Fe(CN)6]
(Merck, Darmstadt, Germany) in 0.1 M KCl was used, and the cyclic voltammograms of
six to ten samples were calculated for each paper form. The following cyclic voltammetry
parameters were used for the measurements: Ebegin 0.15 V; Evtx1 -0.3 V; Evtx2 0.5 V;
Estep 0.002 V; Scan rate 0.05 V/s; and the scan number was 1.
The other open-source potentiostat was the I.O. Rodeo (RSTAT-01) potentiostat, a
fully functional open-source (hardware and software) potentiostat designed as an
expansion board for the popular Teensy 3.2 development board. The current measurement
range of the used open source potentiostat was: ± 1 uA, 10 uA, 100 uA, and 1000 uA, while
the output voltage range is ± 1 V, 2 V, 5 V, and 10 V vs an Ag quasi-reference electrode.
For the readings of the cyclic voltammetry measurements, there are two solutions (web-
based and local computer-based solution). The open-source software was adjusted for a
Linux 64-bit system and was installed on a Raspberry Pi 3+ system. The data collection
were monitored through a web application available at the address
http://stuff.iorodeo.com/apps/rodeostat/ (I.O. Rodeo Blog 2020). The authors conducted
the measurements at values determined as having good cycling values (closed loops). After
establishing this value as optimal instead of OCP and extremes, it was used for all other
samples on this type of potentiostat based on trial and error.
RESULTS AND DISCUSSION
Ink deposition and the theory of ink spreading or ink slumping in screen printing
can be divided into several parameters which can influence the printing. This study tested
one of the high-value parameters which influence the fast-spreading mechanism and two
of the slow-spreading mechanisms. The surface tension and surface free energy induces
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rapid spreading of the applied ink; when the surface tension is low, the ink viscosity is high
(Abbott 2018).
Surface Roughness
The surface roughness values obtained via stylus profilometry were as follows:
Japanese knotweed - the base paper had an average surface roughness value of 4.28 µm,
and the working electrode had a Ra value of 2.53 µm; the goldenrod (giant and Canadian)
had Ra = 6.06 µm, the working electrode had Ra = 2.36 µm, the ETB sample had Ra = 2.97
µm, and the printed working electrode had Ra = 1.42 µm. From these results, it can be seen
that the invasive based papers (Japanese knotweed, giant goldenrod, and Canadian
goldenrod) had higher surface roughness values due to a lack of coating and calendaring;
after ink levelling was performed, the electrodes had similar roughness values. The ETB
board had lower initial values since it was a smoother substrate, and thus the working
electrode also had lower roughness values after printing and drying.
Surface Free Energy of the Substrates
The paper samples' surface energy was measured using the harmonic contact angle
method with two testing liquids (water and formamide) on a Fibrodat 1100 contact angle
measurement device. The ETB carboard had a surface energy of 56.24 mJ/m2, the Japanese
knotweed (J.K.) sample had surface energy of 78.3 mJ/m2, and the giant/Canadian
goldenrod (GCG) sample had surface energy of 53.15 mJ/m2. All of the paper was in a
suitable range that enables excellent printability. The differences in surface energy are
important also regarding potential problems with adhesion, which is dependent on the
surface energy of the printing inks.
Ink Absorbency
The ink oil absorbency test was carried out via a SCAN- P 70:09 ink absorbency
test, in which a porosimetric ink (IGT Testing Systems, Almere, Netherlands) was applied
to the paper surface, and after 2 min the excess ink was wiped off. The ink absorbency
value was calculated from the measured reflectance values of the stained papers and the
paper's intrinsic reflectance value via a spectrophotometer. The results indicated that the
ETB cardboard had a low absorbency value (69.08%), while the J.K. sample had an
absorbency value of 88.62%, and the GCG sample had an absorbency value of 87.20%.
The two paper types made from invasive plant species had similar oil absorbency values,
while the ETB sample had more closed surfaces than the other samples; therefore, the ink
vehicle was expected to hold out longer on the ETB paper surface.
Micro- and Macro-porosity
As the micropores (in this article defined as under 2 µm) primarily absorb the ink
vehicle and the liquids, e.g., water, are primarily absorbed by macropores, the porosity
parameters were determined using the micro-macro porosity test method described in
(Karlovits et al. 2018). The differences in the paper samples' porosity could influence the
screen-printing ink deposition, while the macro-porosity could govern the absorption
during the paper substrate's measurement. The micro- and macro-porosity values are
unitless and are presented in Table 2. The micro-porosity governs the ink resin filtration
and the ink levelling of the deposited conductive inks.
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Table 2. Micro- and Macro-porosity Values
Sample Name
Micro-porosity (y)
Macro-porosity (x)
ETB
6.91
7.45
JK
11.01
32.62
GCG
15.48
10.08
As shown in Table 2, the ETB had a low micro-and macro-porosity index, which
correlates with the ink absorbency value, while the J.K. and GCG samples had relatively
similar micro-porosity values and significant differences in the macropores. These results
mean that it is expected that the oil-like components will absorb more quickly in the J.K.
and GCG paper samples (in addition to high ink absorbency values), while water and
similar liquids will penetrate the J.K. sample in more significant amounts due to a greater
number of open larger pores on the surface.
Cyclic Voltammetry Results
Cyclic voltammetry was applied to the different screen-printed electrodes,
produced with J.K., GCG, and ETB materials, using a [Fe(CN)63-/4- redox standard, which
exhibits a conjugated pair of peaks corresponding to the one-electron transfer of the redox
couple, i.e., [Fe(CN)63-/[Fe(CN)64- (Cardoso et al. 2007; Rodriguez et al. 2009; Galicia et
al. 2014). The cyclic voltammogram responses are shown in Figs. 2 through 7.
Fig. 2. The cyclic voltammetry results of the J.K. -, GCG-, and ETB-based papers with two
different ink film thicknesses (all observed with a Dropsens commercial potentiostat)
A couple of well-defined redox peaks were observed at each cyclic voltammogram
with distinct cathodic peak currents (Ired) and distinct anodic peak current (Iox) values. The
cathodic peak currents (Ired) with negative values were observed at the potential of Ered,
which can be attributed to the reduction reaction: [Fe(CN)63- + e- → [Fe(CN)64-. The
Potential vs. Ag quasi reference electrode (V)
Current (µA)
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anodic peak currents (Iox) were reached when the surface concentration of [Fe(CN)64- was
depleted, causing the anodic peak current (Iox) (Kissinger and Heineman 1983; Pečko 2014;
Trafela 2020). The anodic peak currents (Iox) with positive values were observed at the
potential of Eox, which can be attributed to the oxidation reaction: [Fe(CN)64- →
[Fe(CN)63- + e- (Cinti et al. 2017; Elgrishi et al. 2018).
First, cyclic voltammetry was applied to the 1x and 2x layer J.K., GCG, and ETB
screen-printed electrodes. These cyclic voltammograms are shown in Fig. 2 and
demonstrate one-electron redox behaviour, i.e., the oxidation of [Fe(CN)63- to [Fe(CN)64-
in the cathodic region and the reduction of [Fe(CN)64- to [Fe(CN)63- in the anodic region,
at a scan rate of 50 mV/s.
The redox peak potentials of the [Fe(CN)63-/4- system were not appreciably
different between the 1x and 2x ink printed layers. The anodic peak (Eox)/cathodic peak
(Ered) potentials were 0.15 V/0.05 V for ETB 1x and ETB 2x, 0.17 V/0.07 V for JK 1x and
JK 2x, and 0.2 V/0.1 V for GCG 1x and GCG 2x, respectively. The peak-to-peak separation
(ΔE = Eox-Ered) was calculated using 50 mV/s as the scan rate; the resulting peak separation
was comparable among all the investigated electrodes. The results were as follows:
approximately 100 mV for ETB 1x and ETB 2x, approximately 1.1 mV for J.K. 1x and
J.K. 2x, and approximately 1.2 mV for GCG 1x and GCG 2x. As stated by Cinti et al.
(2017), ΔE provides a qualitative estimation of the electron transfer rate due to the
[Fe(CN)63-/[Fe(CN)64- redox process at the electrode surface. Hence, it appears evident
that using different inks (ETB, J.K., or GCG) does not drastically influence the electrode
reaction kinetics (Cinti et al. 2017; Elgrishi et al. 2018). The oxidation peak currents of the
redox behaviour of Fe3+ to Fe2+ obtained with Dropsens commercial potentiostat were
approximately 44.5, 44.5, 51.6, 51.6, 25.4, and 23.8 µA for the JK 1X, JK 2X, GCG 1X,
GCG 2X, ETB 1X, and ETB 2X, respectively. The current responses for the reduction peak
were -40.4, -40.4, -43.7, -43.7, -34.1, and -34.4 µA for the JK 1X, JK 2X, GCG 1X, GCG
2X, ETB 1X, and ETB 2X, respectively. The results generally showed that the smallest
current intensity for the oxidation and reduction process in the ETB (1x and 2x) paper
samples was due to the smoothest surface and the smallest electrode area with the lowest
oil ink absorbency and micro-and macro-porosity. Once the roughness of the paper
increased, the output current intensity also increased. The highest current intensity for the
oxidation and reduction process was observed in the GCG (1x and 2x) samples, i.e., the
paper with the highest surface roughness, ink absorbency, and micro-porosity.
Furthermore, the additional reduction peak at -0.28 V and the oxidation peak at 0.02 V
were observed in the ETB samples (ETB 1x and 2x), indicating a complex reaction
mechanism, implying a molecular hydrogen interaction (cathodic adsorption and anodic
desorption) with C-based ink.
Also, the CV behaviour of all three paper samples, with both ink film thicknesses,
was studied using Rodeostat (an open-source potentiostat). Figure 3 shows the CVs of the
1x and 2x layer J.K., GCG, and ETB screen-printed electrodes obtained from a [Fe(CN)63-
/4- redox system at a scan rate of 50 mV/s. The differences in the anodic-cathodic peak
potentials (ΔE = Eox-Ered) were approximately 100 mV for all six samples, which indicated
similar electrode reaction kinetics. Furthermore, the oxidation peak currents observed with
open-source potentiostat are approximately 41.5, 42.0, 38.8, 34.0, 34.4, and 28.9 µA for
the JK 1X, JK 2X, GCG 1X, GCG 2X, ETB 1X, and ETB 2X, respectively. The current
responses for the reduction peaks were -40.4, -39.2, -40.3, -34.9, -32.8, and -33.1 µA for
the JK 1X, JK 2X, GCG 1X, GCG 2X, ETB 1X, and ETB 2X, respectively. With the
differences in the current intensities for the oxidation and reduction process being minor
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between the JK, GCG substrates, and ETB films compared to the CV measurements
observed with a commercial potentiostat.
Fig. 3. The cyclic voltammetry results of the J.K. -, GCG-, and ETB-based papers with two
different ink film thicknesses (all observed with an open-source potentiostat)
One of the possible sources of the CPU unit's differences was the fact that the digital
signal processor of the open-source potentiostat is not as powerful as the commercial one.
As seen from Fig. 3, an apex was observed at a starting potential -0.3 V, which most
probably represented a complete reduction reaction of [Fe(CN)63- to [Fe(CN)64- at the
surface of the working electrode, since the highest current values were observed. However,
in the case of the commercial potentiostat (as shown in Fig. 2), the surface species
([Fe(CN)63-) were reduced via potential scanning towards -0.3 V in a cathodic scan, which
was later oxidised in the reversed/anodic scan.
Furthermore, the influence of grinding was tested to study electrode sensitivity. The
CV profiles obtained from the [Fe(CN)63-/4- system with the commercial potentiostat are
presented in Fig. 4 (one ink layer) and Fig. 5 (two ink layers). The CV profiles obtained
from the [Fe(CN)63-/4- system with the open-source potentiostat are presented in Fig. 6 for
(one ink layer) and Fig. 7 (two ink layers).
As shown in Figs. 4 and 5, the redox peak potentials and the current values of the
ground samples from the [Fe(CN)63-/4- system were not appreciably different between 1 or
2 layers of printed ink. The oxidation peak currents for the ground samples JK 1X G, JK
2X G, GCG 1X G, GCG 2X G, ETB 1X G, and ETB 2X G were observed with DropSens
commercial potentiostat as follows: 38.0, 36.7, 59.3, 59.4, 32.6, and 32.7, respectively. The
current responses for the reduction peaks obtained with commercial potentiostat were -
Potential vs. Ag quasi reference electrode (V)
Current (µA)
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Karlovits et al. (2021). “Cyclic voltammetry tests,” BioResources 16(2), 3916-3933. 3926
36.6, -34.0, -54.9, -54.1, -33.6, and -33.6 µA for the JK 1X G, JK 2X G, GCG 1X G, GCG
2X G, ETB 1X G, and ETB 2X G, respectively.
Fig. 4. The cyclic voltammetry results of the J.K., GCG, and ETB-based papers with one layer-
ink thickness and one layer-ink thickness with additional grounding (Dropsens commercial
potentiostat)
Fig. 5. The cyclic voltammetry results of the J.K., GCG, and ETB-based papers with two layer-
ink thickness and two layer-ink thickness with additional grounding (Dropsens commercial
potentiostat)
Potential vs. Ag quasi reference electrode (V)
Potential vs. Ag quasi reference electrode (V)
Current (µA)
Current (µA)
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Karlovits et al. (2021). “Cyclic voltammetry tests,” BioResources 16(2), 3916-3933. 3927
The difference between the anodic peak and cathodic peak potentials (ΔE = Eox -
Ered) did not change after the grinding process for any of the samples (ETB 1x G, ETB 2x
G, J.K. 1x G, J.K. 2x G, GCG 1x G, and GCG 2x G) (Arduini et al. 2012). However, the
CV measurements observed using a commercial potentiostat (as shown in Figs. 4 and 5)
indicated that the grinding process changed the electrodes’ surfaces, i.e. electrode areas
reflected in the CVs as changes in the peak current values and a shift of the potential peak
values. The grinding process caused an increase in the peak intensity of the anodic and
cathodic currents for the GCG 1x G, GCG 2x G, ETB 1x G, and ETB 2x G samples. The
increase in the peak intensity can be ascribed to the increased electrode surface areas of the
GCG and ETB-based electrodes, as the grinding process increased the roughness of
surfaces. The mechanical grinding process polished away the outer-ink-layer ridges of the
printed ink, thus increasing the micro-level surface roughness of the inner-ink-layer's
carbon particles (Arduini et al. 2012; Kavčič and Karlovits 2020).
In contrast, the peak intensity of the anodic and cathodic currents for the JK 1x G
and JK 2x G samples decreased after the grinding process. While the grinding process was
performed manually with the same setup as the ETB and GCG electrode samples, the
reason for a lower electrochemical response, i.e., the current values, could be either of the
following. The J.K. ink-printed working electrodes had the highest surface roughness
before the grinding process compared to the ETB and GCC samples. Hence, the JK-based
working electrode samples' mechanical grinding could level out some of the macro-level
roughness and consequently decrease the anodic and cathodic current values (Foster et al.
2019). As the peak intensity of the anodic and cathodic currents for the J.K. 1x and 2x
based electrode samples (J.K. 1x G and J.K. 2x G) decreased after undergoing the grinding
process, the use of samples that were not ground, i.e., the J.K. 1x and J.K. 2x samples,
could make the same platform more suitable to be used in the development of
electroanalytical devices (Wong et al. 2012).
The grinding process shifts the oxidation and reduction peak potentials, which may
arise from experimental error due to small changes in the Cl- ions concentration in the
electrolytes (Harris et al. 2013). Even a tiny change in the Cl- concentration of the testing
solution has been known to shift the electrode potential, as the reference electrode on the
screen-printed electrodes is a quasi-reference and not a standard reference, i.e., a controlled
inner filling solution and a salt bridge, e.g., Ag/AgCl (Cinti et al. 2017). Hariss et al. (2013)
suggested that the separation of the quasi-reference electrode via a membrane or coating
could be a solution for removing the quasi-reference electrode potential shift effect.
Furthermore, from Figs. 4 and 5, the additional reduction peaks at -0.28 (Fig. 4, ETB 1X),
-0.22 V (Fig. 4, ETB 1X G), -0.28 (Fig. 5, ETB 2X), and -0.22 V (Fig. 5, ETB 2X G), and
the additional oxidation peaks at 0.01 V (Fig. 4, ETB 1X), 0.05 V (Fig. 4, ETB 1X G), 0.01
(Fig. 5, ETB 2X), and 0.05 V (Fig. 5, ETB 2X G) were observed in the ETB samples. The
additional reduction and oxidation peaks in the CV curves indicate an additional redox
process occurring, most probably due to cathodic hydrogen adsorption and the anodic
hydrogen desorption, on the surface of the ETB electrodes (Mushrin et al. 2010; Stevens
and Dahn 2003) . The presence of the additional reduction peak for hydrogen adsorption
and oxidation peak for hydrogen desorption on the JK and GCG electrodes was not
observed, which may suggest a significantly high energy barrier for those processes.
Alternatively, the effect could be due to the unwillingness of the carbon support to accept
the H-atoms in the case of JK-/GCG-based inks (Mushrin et al. 2010; Stevens and Dahn,
2003). Besides, it can be seen from Table 3 that the ratio of the peak current at the anode
to the peak current at the cathode response (Iox/Ired) on the ETB electrodes diverge from 1,
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Karlovits et al. (2021). “Cyclic voltammetry tests,” BioResources 16(2), 3916-3933. 3928
indicating a quasi-reversible or irreversible electron transfer in [Fe(CN)63-/4- which can be
attributed to interfering (adsorption/desorption) reaction.
The CV measurements observed using the open-source potentiostat (as shown in
Figs. 6 and 7) showed more significant overall differences in the current intensities for both
the oxidation and reduction processes, which are due to different digital signal processing
steps in the potentiostats. The oxidation/reduction peak currents obtained with open-source
potentiostat were 50.5/-47.7, 44.2/-45.9, 42.7/-45.2, 52.7/-45.5, 27.5/-30.2, and 25.2/-31.4
µA for the JK 1X G, JK 2X G, GCG 1X G, GCG 2X G, ETB 1X G, and ETB 2X G,
respectively. As shown in Figs. 6 and 7, the grinding process increases the electrochemical
response, i.e., increases the current.
Fig. 6. The cyclic voltammetry results of the J.K., GCG, and ETB-based papers with one layer-
ink thickness and one layer-ink thickness with additional grounding (open-source potentiostat)
Fig. 7. The cyclic voltammetry results of the J.K., GCG, and ETB-based papers with two layer-
ink thickness and two layer-ink thickness with additional grounding (open-source potentiostat)
Potential vs. Ag quasi reference electrode (V)
Potential vs. Ag quasi reference electrode (V)
Current (µA)
Current (µA)
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Karlovits et al. (2021). “Cyclic voltammetry tests,” BioResources 16(2), 3916-3933. 3929
Also, the grinding process causes an increase in the peak intensity of the anodic and
cathodic currents for the GCG 1x G, GCG 2x G, J.K. 1x G, and J.K. 2x G samples. The
increase in the peak intensity can be ascribed to the increased surface roughness and with
it connected to larger electrode area of the GCG and JK-based electrodes; the mechanical
grinding process polishes away the outer-ink-layer ridges of the printed ink and increases
the surface area of the carbon particles present in the inner-ink-layer (Rodriguez et al. 2009;
Kavčič and Karlovits 2020). However, the anodic and cathodic currents' peak intensity for
the ETB 1x G and ETB 2x G samples decreased after undergoing the grinding process. The
observed decrease in the electrochemical response can be attributed to the polishing
process, i.e., the grinding process probably levelled out some of the macro-level surface
roughness and consequently decreased the anodic and cathodic current (Kavčič and
Karlovits 2020).
To explore the reversibility of the electron transfer process, the peak current ratio
(Table 3), i.e., Iox/Ired, rather than the individual peaks current values was compared. It was
found that the JK and GCG-based electrode reactions can be referred to as a reversible
electron transfer reaction as the Iox/Ired for Commercial Potentiostat is 1±0.18 and Iox/Ired
for Open Source Potentiostat is 1±0.05. For the ETB electrodes, the electron transfer
processes between [Fe(CN)63- and [Fe(CN)64- reactions are referred to as a quasi-
reversible or irreversible electron transfer reaction. As already described, the ETB
electrodes are showing different CV behaviour in [Fe(CN)63-/4- electrolyte in comparison
to JK and GCG electrodes, as the additional reduction peak at -0.28 V and the oxidation
peak at 0.02 V were observed, indicating more complex electron transfer reaction
mechanisms.
Furthermore, the CV measurements made with the open-source potentiostat (as
shown in Figs. 6 and 7) indicated that the grinding process caused a slight shift in the anodic
and cathodic peak potentials values (towards the positive values). The observed shift is
attributed to experimental error, i.e., the small changes in Cl- ion concentration in the
electrolytes, as explained above (Harris et al. 2013). The differences in the anodic-cathodic
peak potentials are presented in Table 3.
Table 3. Anodic and Cathodic Peak Potential Difference
Sample
Name
Commercial
Potentiostat
(Iox/Ired)
Open
Source
Potentiostat
(Iox/Ired)
Commercial
Potentiostat
(ΔE)
Open Source
Potentiostat (ΔE)
Absolute
Difference (mV)
JK 1X
1.10
1.02
84.71
87
2.29
GCG 1X
1.18
0.97
96.14
82,46
13.68
ETB 1X
0.74
1.05
62.30
80.64
18.34
JK 2X
1.10
1.04
85.05
87.49
2.44
GCG 2X
1.18
0.97
95.4
75,31
20.1
ETB 2X
0.69
0.87
61.79
78.94
17.15
JK 1X G
1.04
1.05
75.07
107.96
32.89
GCG 1X G
1.08
0.95
114.33
101.05
12.83
ETB 1X G
0.97
0.92
72.75
82.08
9.33
JK 2X G
1.08
0.96
71.73
105.26
33.53
GCG 2X G
1.09
0.95
113.54
92.49
21.05
ETB 2X G
0.97
0.80
79.52
85.39
5.87
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Karlovits et al. (2021). “Cyclic voltammetry tests,” BioResources 16(2), 3916-3933. 3930
Table 3 includes the calculated peak current ratio (Iox/Ired) for Commercial
(Dropsens) and Open Source Potentiostats (Rodeostat). Table 3 also shows that the open-
source potentiostat had a more significant overall anodic and cathodic peak potential
difference for almost all samples, and the grinding process further increased the difference
between the potential difference measurements for the two potentiostats.
CONCLUSIONS
1. By using unique paper-based substrates made from invasive plant fibre papers, SPEs
were manufactured. The measured data on mobile commercial (black box) and open-
source potentiostats, one with wireless options and one with a Linux operating system,
found that the tested potentiostats yielded different measured values.
2. While the voltammetry shapes were similar, the commercial potentiostat showed more
significant variations in the measurements due to the paper samples’ properties; when
measured on the open-source potentiostat, these differences, while similar, were
smaller. This difference was probably due to the electrical setup and hardware
components employed in the potentiostats.
3. Another variable factor was the use of different kind of paper substrates. The surface
and absorption differences of the substrates were also observed in the potentiostat
measurements. These differences have not been thoroughly studied in terms of
electrochemical measurements. This study's findings indicate that the paper sample
with the lowest surface roughness and porosity and a surface free energy of 56.24
mJ/m2 had the slightest variations and changes due to additional grinding. The other
two papers carriers made from invasive plant species had greater surface roughness
values and higher porosity values (macro and micro) with a massive difference in
surface energy.
4. The papers’ surface roughness influenced the peak values and resulted in different
values when the ink film for the working electrode was varied. With improved surface
smoothness, which would close the macropores, the invasive plant papers could be used
as viable disposable and more sustainable electrodes carrier.
5. These results indicate that the variation in the cyclic voltammetry measurements is
twofold, i.e., the potentiostats have differences in their sensibility and signal values. At
the same time, the properties of the paper samples (especially surface roughness and
surface free energy), as well as additional processing steps, e.g., mechanical grinding
(which can change the peak intensity of the anodic and cathodic peak potentials), can
introduce additional measurement variations and differences in the measurement
process.
ACKNOWLEDGMENTS
The authors are grateful for the Slovenian Research Agency's support for providing
funding (No. J2-8182).
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Article submitted: December 2, 2020; Peer review completed: March 20, 2021; Revised
version received and accepted: April 11, 2021; Published: April 15, 2021.
DOI: 10.15376/biores.16.2.3916-3933
