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The consumption of reactive dyes in the batik industry has led to a severe concern in monitoring the heavy metal level in wastewater. Due to the necessity of implementing a wastewater monitoring system in the batik factory, a Heavy Metal potentiostat (HMstat) was designed. The main goal of this study is to understand the optimal design concept of the potentiostat function in order to investigate the losses of accuracy in measurement using off-the-shelf devices. Through lab-scale design, the HMstat comprises of an analog potentiostat read-out circuit component (PRCC) and a digital control signal component (CSC). The PRCC is based on easy to use components integrated with a NI-myRIO controller in a CSC. Here, the myRIO was equipped with built-in analog to digital converter (ADC) and digital to analog converter (DAC) components. In this paper, the accuracy test and detection of cadmium(II) (Cd 2+) and lead(II) (Pb 2+) were conducted using the HMstat. The results were compared with the Rodeostat (an open source potentiostat available on the online market). The accuracy of the HMStat was higher than 95% and within the precision rate of the components used. The HMstat was able to detect Cd 2+ and Pb 2+ at −0.25 and −0.3 V, respectively. Similar potential peaks were obtained using Rodeostat (Cd 2+ at −0.25 V and Pb 2+ at −0.3 V).
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applied
sciences
Article
Development of Heavy Metal Potentiostat for
Batik Industry
Siti Nur Hanisah Umar 1, Mohammad Nishat Akhtar 1, Elmi Abu Bakar 1, *,
Noorfazreena M. Kamaruddin 1and Abdul Rahim Othman 2
1School of Aerospace Engineering, Engineering Campus, Universiti Sains Malaysia,
Nibong Tebal 14600, Penang, Malaysia; snhanisahumar@gmail.com (S.N.H.U.); nishat@usm.my (M.N.A.);
fazreena@usm.my (N.M.K.)
2Department of Mechanical Engineering, Universiti Teknologi PETRONAS,
Seri Iskandar 32610, Perak, Malaysia; rahim.othman@utp.edu.my
*Correspondence: meelmi@usm.my
Received: 31 August 2020; Accepted: 5 October 2020; Published: 4 November 2020


Abstract:
The consumption of reactive dyes in the batik industry has led to a severe concern in
monitoring the heavy metal level in wastewater. Due to the necessity of implementing a wastewater
monitoring system in the batik factory, a Heavy Metal potentiostat (HMstat) was designed. The main
goal of this study is to understand the optimal design concept of the potentiostat function in order
to investigate the losses of accuracy in measurement using o-the-shelf devices. Through lab-scale
design, the HMstat comprises of an analog potentiostat read-out circuit component (PRCC) and a
digital control signal component (CSC). The PRCC is based on easy to use components integrated
with a NI-myRIO controller in a CSC. Here, the myRIO was equipped with built-in analog to digital
converter (ADC) and digital to analog converter (DAC) components. In this paper, the accuracy test
and detection of cadmium(II) (Cd
2+
) and lead(II) (Pb
2+
) were conducted using the HMstat. The results
were compared with the Rodeostat (an open source potentiostat available on the online market).
The accuracy of the HMStat was higher than 95% and within the precision rate of the components
used. The HMstat was able to detect Cd
2+
and Pb
2+
at
0.25 and
0.3 V, respectively. Similar potential
peaks were obtained using Rodeostat (Cd2+at 0.25 V and Pb2+at 0.3 V).
Keywords: potentiostat; heavy metal; cadmium; lead; batik; Rodeostat
1. Introduction and background
Batik is a traditional handmade textile craft in the cottage industry [
1
]. This industry has
contributed positively to economic growth, especially in Kelantan and Terengganu in Malaysia, and has
also become one of the main attractions of foreign and local tourists [
2
]. Batik factories are known to
generate a large amount of wastewater included wax, resin, sodium, silicate, and dyes. The presence of
dyes is one of the main concerns in wastewater [
3
]. Among all types of dyes, reactive dyes are preferred
due to their convenience, transparency, and brilliant color along with ease of textile fastening [4].
Five dierent types of reactive dyes were studied in [
5
], and the results reported that each reactive
dye contains heavy metal elements of cadmium (Cd), lead (Pb), arsenic (As), zinc (Zn), chromium (Cr),
cobalt (Co), and copper (Cu). Amongst all, Zn and Cr are essential elements, and small doses are
required by living organisms to maintain various biochemical and physiological functions [
6
9
],
while the others are non-essential elements which are highly toxic and harmful to human health and
the environment, even at low concentrations [6,7].
According to the Environmental Quality Act (EQA 1974), the permissible limit for industrial
euent discharge based on standard A (applicable to discharge into any inland waters within catchment
Appl. Sci. 2020,10, 7804; doi:10.3390/app10217804 www.mdpi.com/journal/applsci
Appl. Sci. 2020,10, 7804 2 of 12
areas) for Cd is 0.01 ppm, As and Cr are 0.05 ppm, Pb is 0.1 ppm, Cu is 0.2 ppm, and Zn is 1.0 ppm [
10
].
However, as most of the batik factories were build-up with improper or without a waste management
system, the level of heavy metals discharged was not determined [
11
]. This happened due to the lack
of awareness regarding the hazardous eect of heavy metal elements toward the environment and
especially concerning human health [11].
Determination of the heavy metal level before being discharged to the environmental water is
crucial. Today, various heavy metal detection techniques have been established, including inductive
coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry
(ICP-AES), inductively coupled plasma-optical emission spectrometry (ICP-OES), atomic absorption
spectrometry (AAS), atomic emission spectrometry (AES) [
12
], X-ray Fluorescence Spectrometry (XRF),
cold vapor atomic absorption spectrometer (CVAAS), neutron activation analysis (NAA), and flameless
atomic absorption spectrophotometry (FAAS) [
12
14
]. These techniques are highly sensitive and
selective [
14
]. However, these techniques are inconvenient for fieldwork application, such as in a batik
factory, because instruments are relatively expensive, bulky, require complex operational procedures,
and long detection times [1214].
Nowadays, electrochemical techniques hold great potential to realize the on-field detection of
heavy metal elements in the batik factory. This technique has the advantages of low-cost, ease of
operation, fast analysis, and is also well suited to fabricate on a small circuit for portable and on-field
application [
12
15
]. Until now, numerous research has been done in the development of electrochemical
devices for various applications, which are known as potentiostats [1623].
One of the potentiostat developments is an Arduino-based potentiostat, fabricated using
inexpensive components which were capable of performing simple electrochemical experiments
and were suitable for an undergraduate teaching level [
17
19
]. For such experiments, the results
and data were recorded in real-time on a Windows-based computing system via a USB interface [
17
].
However, a potentiostat integrated with Raspberry Pi eliminated the requirement of an external
computing system. This standalone device carried out the electrochemical experiments, and the results
were displayed on an LCD touch panel [
20
]. Both Arduino- and Raspberry Pi-based potentiostats did
not possess a built-in analog to digital converter (ADC), and digital to analog converter (DAC) and the
requirement for external ADC and DAC made the design more complicated.
Many previous works have reported on the capability of a self-fabricated potentiostat to detect
heavy metal elements. A potentiostat named CheapStat [
21
] was capable of detecting the arsenic (As)
level in lake water. The test was conducted using a gold disk working electrode (WE), a platinum
counter electrode (CE), and a silver/silver-chloride (Ag/AgCl) reference electrode (RE). A universal
mobile electrochemical detector (UMED) [
16
] is a potentiostat which is compatible with a mobile phone
and is capable of detecting cadmium (Cd), lead (Pb), and zinc (Zn). A screen-printed electrode (SPE)
was used with UWED, which consists of carbon ink modified by carbon nanotubes as a WE, carbon ink
as a CE, and Ag/AgCl ink as a RE. A programmable system on chip (PSoC) [
22
] is a commercially
available integrated circuit (IC) that does not require any external electronic components to perform
electrochemical experiments. The PSoC demonstrated the detection of Pb using gold wire as a WE
and Ag/AgCl as a CE and RE. The latest work reported was a MiniStat [
23
], in which the design was
developed to be suited for general electrochemical analysis in the smaller form factor. The MiniStat was
tested to detect copper (Cu) using a 3 mm glassy carbon WE, a platinum wire as a CE, and Ag/AgCl
as a RE. However, the potentiostats mentioned previously [
16
,
21
,
23
] demand the user to fabricate
a printed circuit board (PCB) and to solder the electronic components onto the board. Since these
potentiostats were designed with small surface-mounted integrated circuits (ICs), they required more
skilled technicians and sophisticated tools to solder together, as compared to through-hole connections.
This creates hindrances to chemists and other scientists who want to use a potentiostat.
Besides, the detecting heavy metal as reported in [
21
24
] used a three separated electrodes
approach which is bulky and not suited for on-field application. The incorporation of a screen-printed
electrode (SPE) eliminated the classical bulky electrodes and at the same time, enabled reduction in the
Appl. Sci. 2020,10, 7804 3 of 12
sample used [
25
]. Although there are several reports working on modified SPE and WE [
16
,
25
28
],
since heavy metals have defined redox potential, the selectivity toward specific heavy metal ions still
can be achieved by bare electrodes and unmodified SPE [29].
Therefore, in the requirement of a heavy metal feature-based device for monitoring heavy metal
levels in the batik industry, a Heavy Metal Potentiostat device (HMstat) incorporated with SPE
was designed. The design of HMstat was based on the idea to simplify the potentiostat design by
implementing a controller equipped with built-in DAC and ADC. With this, the development of
electronic components in the potentiostat focused more on the main part, which is the potential control
and current measurement part. Besides, the construction of the electronic component in HMstat
implemented through-hole technology, which eliminates the hindrance for users who are less skilled in
electronic areas in designing a potentiostat. Moreover, implementing through-hole technology enabled
the design to be easily adjusted or replaced when necessary. This study will also demonstrate the
capability of HMstat to implement performance tests and heavy metal measurements.
2. Methodology
2.1. Design of HMstat
The HMstat consists of two main components, which comprise of the digital control signal
component (CSC) and the electronic component, which is the analog potentiostat read-out circuit
component (PRCC), as shown in Figure 1. The function of the CSC is for parameter control,
signal generation, acquisition, and processing. The CSC is based on NI myRIO, which is equipped
with built-in ADC and DAC provided with a bipolar input/output voltage channel up to
±
10 V.
These features allow the reduction in stages in PRCC from nine steps (from previous work [
19
]) to three
steps (for this current work; refer to Figure 2). The stages in PRCC can be categorized into two parts,
which are the potential control part (PCP) and the current measurement part (CMP). There are two
stages in PCP; the first stage consists of a summing inverting amplifier (AO
SUM
), and the second stage
is a voltage follower (AO
F
). The function of the PCP is to apply and control the interfacial potential
at the WE through CE with the consideration of feedback potential from the RE through AO
F
(the
function of AO
F
is to limit any current that might otherwise flow through RE [
30
]). Thus, the applied
potential input to the electrochemical cell can be expressed using Equation (1):
Vin =R2
R1
Vap, (1)
where
Vap
is the applied potential generated by CSC;
R1
and
R2
are resistor 1 and resistor 2, respectively.
Since R1=R2=10 k, thus ideally, Vin applied to the electrochemical cell will be equal to the Vap.
Here, the CMP is comprised of a transimpedance amplifier (OA
TIA
) with the function to covert
the small current changed from the electrochemical cell to a voltage signal. The correlation between
the measured current change and the output voltage from OA
TIA
can be expressed using Equation (2):
Iout =Vout
Rgain
, (2)
where
Iout
is the measured output current,
Vout
is the output voltage, and
Rgain
is the gain resistor of
OATIA with a resistance value of 10 k.
Appl. Sci. 2020,10, 7804 4 of 12
Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 13
Figure 1. (a) The HMstat consists of a control signal component (CSC) and potentiostat read-out
circuit component (PRCC) connected to the electrochemical cell (consisting of a screen-printed
electrode gold (SPGE)) and (b) overall connection of HMstat.
Figure 1.
(
a
) The HMstat consists of a control signal component (CSC) and potentiostat read-out circuit
component (PRCC) connected to the electrochemical cell (consisting of a screen-printed electrode gold
(SPGE)) and (b) overall connection of HMstat.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 13
Figure 2. Block diagram of the potentiostat read-out circuit component (PRCC) comprised of (a)
potential control part (PCP) and (b) current measurement part (CMP), connected to a (c) dummy cell
(for accuracy and noise measurement).
2.2. Signal Generation and Processing
The HMstat has been designed in such a way that it can perform two types of measurement.
One is the measurement for performance tests, and the second is for heavy metal measurement. Three
different types of signals were designed for the performance test, which were the constant potential,
ramp potential, and square wave potential. For heavy metal measurement, a square wave anodic
stripping voltammetry (SWASV) signal was designed. Basically, the SWASV signal is the
incorporation of the constant, ramp, and square wave potential signal.
These signals were generated and configured in the CSC using graphical programming
language. The development of the graphical programs is depicted in the flowchart, as illustrated in
Figure 3. Based on the figure, once the program has been turned on, the user must choose to perform
the performance test or heavy metal measurement test. If the performance test is chosen, the user
must set the performance test parameters and then, press the “Start performance test” button. The
generated 𝑉 is applied to the PRCC by a mini system port (MSP) through analog output zero (AO0)
and then, the performance test is implemented. At the same time, the data acquisition of 𝑉() and
𝑉() is to be done from the PRCC through MSP analog input zero (AI0) and one (AI1), respectively.
Once the test is finished, the data are saved as xlsx file. In addition, the test can be aborted at any
Figure 2.
Block diagram of the potentiostat read-out circuit component (PRCC) comprised of (
a
) potential
control part (PCP) and (
b
) current measurement part (CMP), connected to a (
c
) dummy cell (for accuracy
and noise measurement).
Appl. Sci. 2020,10, 7804 5 of 12
2.2. Signal Generation and Processing
The HMstat has been designed in such a way that it can perform two types of measurement.
One is the measurement for performance tests, and the second is for heavy metal measurement.
Three dierent types of signals were designed for the performance test, which were the constant
potential, ramp potential, and square wave potential. For heavy metal measurement, a square wave
anodic stripping voltammetry (SWASV) signal was designed. Basically, the SWASV signal is the
incorporation of the constant, ramp, and square wave potential signal.
These signals were generated and configured in the CSC using graphical programming language.
The development of the graphical programs is depicted in the flowchart, as illustrated in Figure 3.
Based on the figure, once the program has been turned on, the user must choose to perform the
performance test or heavy metal measurement test. If the performance test is chosen, the user must set
the performance test parameters and then, press the “Start performance test” button. The generated
Vap
is applied to the PRCC by a mini system port (MSP) through analog output zero (AO0) and then,
the performance test is implemented. At the same time, the data acquisition of
Vin(m)
and
Vout(m)
is
to be done from the PRCC through MSP analog input zero (AI0) and one (AI1), respectively. Once
the test is finished, the data are saved as xlsx file. In addition, the test can be aborted at any moment
by pressing the “Stop test” button. Meanwhile, if heavy metal measurement is chosen, the user will
undergo a similar sequence as done for the performance test.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 13
moment by pressing the “Stop test” button. Meanwhile, if heavy metal measurement is chosen, the
user will undergo a similar sequence as done for the performance test.
Figure 3. Flow chart of graphical program for signal generation (apply potential, 𝑉) and acquisition
(measured input potential, 𝑉() and measured output potential, 𝑉()).
2.3. Accuracy and Noise Measurement of HMstat
The HMstat performance test was conducted to evaluate: (1) the capability of the PCP to
accurately apply the desired potential to WE, (2) the accuracy of the CMP to measure the operating
current 𝐼, and (3) the noise current measurement of the CMP under operating current 𝐼. To assess
the performance of the HMstat, a dummy cell was designed and connected to the CE, WE, and RE of
the HMstat. The dummy cell (refer to Figure 2c) is an electronic circuit used to replicate the primary
electrochemical cell with a known operating current 𝐼.
As mentioned earlier, three types of 𝑉 (constant, ramp, and square wave potential) were used
for the HMstat performance test. Under a different kind of 𝑉, the accuracy of the PCP and CMP
was measured based on the percentage error (PE), as given in the following equations:
𝑐𝑐𝑢𝑟𝑎𝑐𝑦 𝑜𝑓 𝑃𝐶𝑃 =100 − 𝑃𝐸 (3)
𝐴
𝑐𝑐𝑢𝑟𝑎𝑐𝑦 𝑜𝑓 𝐶𝑀𝑃 = 100 − 𝑃𝐸 (4)
where 𝑃𝐸 and 𝑃𝐸 are the percentage error of the PCP and CMP, respectively, calculated
using Equations (5) and (6).
Figure 3.
Flow chart of graphical program for signal generation (apply potential,
Vin
) and acquisition
(measured input potential, Vin(m)and measured output potential, Vout(m)).
Appl. Sci. 2020,10, 7804 6 of 12
2.3. Accuracy and Noise Measurement of HMstat
The HMstat performance test was conducted to evaluate: (1) the capability of the PCP to accurately
apply the desired potential to WE, (2) the accuracy of the CMP to measure the operating current
Iw
, and (3) the noise current measurement of the CMP under operating current
Iw
. To assess the
performance of the HMstat, a dummy cell was designed and connected to the CE, WE, and RE of
the HMstat. The dummy cell (refer to Figure 2c) is an electronic circuit used to replicate the primary
electrochemical cell with a known operating current Iw.
As mentioned earlier, three types of
Vap
(constant, ramp, and square wave potential) were used
for the HMstat performance test. Under a dierent kind of
Vap
, the accuracy of the PCP and CMP was
measured based on the percentage error (PE), as given in the following equations:
Accuracy o f PCP =100 PEPCP (3)
Accuracy o f CMP =100 PECMP (4)
where
PEPCP
and
PECMP
are the percentage error of the PCP and CMP, respectively, calculated using
Equations (5) and (6).
PEPCP =
Vin(m)Vap
Vap
×100, (5)
PECMP =
Iout Iw
Iw
×100, (6)
where
Vin(m)
is the measured
Vin
taken through AI0,
Iout
is the measured output current taken through
AI1, and Iwis the current flow through Rwr.
Here, the noise current measurement of the CMP was taken as the standard deviation of the
data [22,30,31].
The performance of the HMstat was then compared to another available potentiostat in the online
market, which is the Rodeostat (Rstat). The availability of the Rstat, which was tested under dierent
types of Vap, was shown in Table 1.
Table 1. Type of potential applied to the HMstat and Rstat.
Type of Vap Vap (V) Test Part Parameter HMstat Rodeostat
Constant 0.5 PCP Vin ×
CMP Iout √ √
+0.5 PCP Vin ×
CMP Iout √ √
Ramp 1 to +1PCP Vin ×
CMP Iout √ √
SWV 1 to +1PCP Vin ×
CMP Iout ×
tested; ×not tested due to the data unavailability.
2.4. Detection of Heavy Metal Using Square Wave Anodic Stripping Voltammetry Method
In this study, the capability of the HMstat to detect heavy metal elements was demonstrated.
The working solution of 10 ppm of Pb and Cd was prepared by diluting a standard solution of Pb and
Cd (1000 ppm) in 0.1 M acetate buer. The screen-printed gold electrodes (SPGEs) were purchased
from DropSens (Spain). Each strip contained three electrodes printed on the same planar platform.
The three electrodes were a 4 mm diameter gold disk-shaped working electrode (WE), a gold counter
electrode (CE), and a silver pseudo-reference electrode (RE). All detection was performed by placing
Appl. Sci. 2020,10, 7804 7 of 12
100
µ
L solution on the three-electrode strip. It is to be noted that all the potential applied throughout
this work refers to silver pseudo-RE.
The experiment was conducted using a square wave anodic stripping voltammetry method
(SWASV), in which the SWASV signal was generated by the CSC. There were two main steps in the
SWASV method. First was the deposition step and second was the stripping step. The mechanism of the
heavy metal ion during the deposition and stripping steps was illustrated in Figure 4. The deposition
step is where the negative potential is applied to the WE. The purpose of the deposition step was to
reduce the heavy metal ion in the working solution onto the electrode surface. The reduction will occur
if the applied deposition potential is more negative than the reduction potential of heavy metal [
32
].
This step was followed by the stripping step. In the stripping step, the deposited heavy metal ion
on the electrode surface is reoxidized and dissolved into the working solution [
32
]. The reoxidation
tends to occur when the applied potential matches the oxidation potential of each heavy metal, so that
the measured current indicates a dierent peak for each heavy metal species [
16
]. The details of the
parameters used in the steps for heavy metal ion detection were listed in Table 2.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13
Figure 4. Heavy metal (HM) ion during (1) deposition and (2) stripping step on screen-printed gold
electrode (SPGE) connected to potentiostat read-out circuit component (PRCC). RE, WE, and CE are
reference electrode, working electrode, and counter electrode, respectively.
Table 2. Experimental parameters.
Deposition Step
Potential 0.9 V
Time 120 s
Stripping Step
Initial potential 0.7 V
Final potential 0.0 V
Modulation amplitude 50 mV
Step amplitude 50 mV
frequency 10 Hz
3. Results and Discussion
3.1. Accuracy and Noise Measurement
Based on Equations (5) and (6), the percentage error (PE) of the PCP and CMP of the HMstat,
and the CMP of Rstat was shown in Figure 5. The PE obtained by the HMstat for both PCP and CMP
was less than 5% for every types of 𝑉. Generally, the lowest error was observed when the 𝑉 was
at the constant potential. The highest error was obtained for the ramp potential followed by SWV
potential. The constant potential produced significantly lower error compared to the ramp and SWV.
This was due to the fluctuated signal of the ramp and SWV, which may have led to higher noise
generation. Moreover, these patterns were also observed in the PE of the CMP for Rstat. However,
the PE obtained for the CMP of the Rstat was higher compared to HMstat for all types of 𝑉.
Figure 4.
Heavy metal (HM) ion during (1) deposition and (2) stripping step on screen-printed gold
electrode (SPGE) connected to potentiostat read-out circuit component (PRCC). RE, WE, and CE are
reference electrode, working electrode, and counter electrode, respectively.
Table 2. Experimental parameters.
Deposition Step
Potential 0.9 V
Time 120 s
Stripping Step
Initial potential 0.7 V
Final potential 0.0 V
Modulation amplitude 50 mV
Step amplitude 50 mV
frequency 10 Hz
Appl. Sci. 2020,10, 7804 8 of 12
3. Results and Discussion
3.1. Accuracy and Noise Measurement
Based on Equations (5) and (6), the percentage error (PE) of the PCP and CMP of the HMstat, and
the CMP of Rstat was shown in Figure 5. The PE obtained by the HMstat for both PCP and CMP was
less than 5% for every types of
Vap
. Generally, the lowest error was observed when the
Vap
was at the
constant potential. The highest error was obtained for the ramp potential followed by SWV potential.
The constant potential produced significantly lower error compared to the ramp and SWV. This was
due to the fluctuated signal of the ramp and SWV, which may have led to higher noise generation.
Moreover, these patterns were also observed in the PE of the CMP for Rstat. However, the PE obtained
for the CMP of the Rstat was higher compared to HMstat for all types of Vap.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 13
Figure 5. Accuracy of the current measurement part (CMP) for HMstat and Rstat.
Table 3 shows the performance HMstat and Rstat, including the lists of PE, accuracy, and noise
measurement. The accuracy was measured based on Equations (3) and (4) and the noise measurement
was determined based on the standard deviation of the data. The measurement of the accuracy was
related to PE, and the accuracy of HMstat was higher than 95% which is within the precision rate of
the components used. Moreover, HMstat showed better accuracy compared to Rstat in a similar
pattern as in PE. The degradation of accuracy might have been caused by a higher noise current, as
𝑉 changed from constant potential to ramp potential, and to SWV. The lowest noise current was
obtained for constant 𝑉 (5 nA for both 𝑉 of 0.5V and +0.5V). The noise current increased to 6
nA when ramp 𝑉 was applied. The highest noise current of 7 nA has been obtained for SWV 𝑉.
This was due to the fluctuated signal of ramp, and SWV, which may have led to higher noise
generation.
Table 3. Performance of HMstat and Rodeostat (Rstat) in term of percentage error (PE), accuracy, and
noise current measurement (SD).
Type of 𝑽𝒂𝒑 𝑽𝒂𝒑 (V) Potentiostat PE (%) Accuracy (%) SD (µA)
PCP CMP PCP CMP SD
Constant
0.5 HMstat 2.575 1.445 97.425 98.555 5.215
Rstat × 1.961 × 98.039 6.934 µ
+0.5 HMstat 3.373 1.567 96.627 98.433 5.337
Rstat × 4.063 × 95.937 14.364 µ
Ramp 1 to +1 HMstat 2.470 4.121 97.530 95.879 6.789
Rstat × 8.998 × 91.002 20.664 µ
SWV 1 to +1 HMstat 4.899 4.219 95.101 95.781 7.287
Rstat × × × × ×
× not tested due to the data not being able to be obtained.
3.2. Heavy Metal Detection Test
Figure 6a shows the stripping peak of 10 ppm Cd2+ and 10 ppm Pb2+. Based on the figure, the
Cd2+ and Pb2+ stripping peaks were observed at 0.30 and 0.25 V, respectively, and the peaks were
located quite close to each other. This was due to the fact that detection was done using a gold-based
electrode. With regard to this, there were several studies which reported that closer stripping peaks
were observed for Cd2+ and Pb2+ under gold-based electrodes and the presence of both metals in the
solution caused overlapping of the stripping peaks [33–35]. Moreover, the presence of Cd2+ in Pb2+
0
1
2
3
4
5
6
7
8
9
10
-0.5 0.5 -1 to +1 -1 to +1
Constant Constant Ramp SWV
Percentage error, PE (%)
Apply potential, Vap (V)
PCP of HMstat
CMP of HMstat
CMP of Rstat
Figure 5. Accuracy of the current measurement part (CMP) for HMstat and Rstat.
Table 3shows the performance HMstat and Rstat, including the lists of PE, accuracy, and noise
measurement. The accuracy was measured based on Equations (3) and (4) and the noise measurement
was determined based on the standard deviation of the data. The measurement of the accuracy was
related to PE, and the accuracy of HMstat was higher than 95% which is within the precision rate
of the components used. Moreover, HMstat showed better accuracy compared to Rstat in a similar
pattern as in PE. The degradation of accuracy might have been caused by a higher noise current, as
Vap
changed from constant potential to ramp potential, and to SWV. The lowest noise current was obtained
for constant
Vap
(5 nA for both
Vap
of
0.5 V and +0.5 V). The noise current increased to 6 nA when
ramp
Vap
was applied. The highest noise current of 7 nA has been obtained for SWV
Vap
. This was
due to the fluctuated signal of ramp, and SWV, which may have led to higher noise generation.
Table 3.
Performance of HMstat and Rodeostat (Rstat) in term of percentage error (PE), accuracy, and
noise current measurement (SD).
Type of Vap Vap (V) Potentiostat PE (%) Accuracy (%) SD (µA)
PCP CMP PCP CMP SD
Constant 0.5 HMstat 2.575 1.445 97.425 98.555 5.215
Rstat ×1.961 ×98.039 6.934 µ
+0.5 HMstat 3.373 1.567 96.627 98.433 5.337
Rstat ×4.063 ×95.937 14.364 µ
Ramp 1 to +1HMstat 2.470 4.121 97.530 95.879 6.789
Rstat ×8.998 ×91.002 20.664 µ
SWV 1 to +1HMstat 4.899 4.219 95.101 95.781 7.287
Rstat ×××××
×not tested due to the data not being able to be obtained.
Appl. Sci. 2020,10, 7804 9 of 12
3.2. Heavy Metal Detection Test
Figure 6a shows the stripping peak of 10 ppm Cd
2+
and 10 ppm Pb
2+
. Based on the figure, the
Cd
2+
and Pb
2+
stripping peaks were observed at
0.30 and
0.25 V, respectively, and the peaks were
located quite close to each other. This was due to the fact that detection was done using a gold-based
electrode. With regard to this, there were several studies which reported that closer stripping peaks
were observed for Cd
2+
and Pb
2+
under gold-based electrodes and the presence of both metals in
the solution caused overlapping of the stripping peaks [
33
35
]. Moreover, the presence of Cd
2+
in
Pb
2+
measurements will aect the stripping intensity of Pb
2+
and vice versa [
33
]. Figure 6b shows
the stripping peaks of Cd
2+
and Pb
2+
extracted from [
34
] were close to each other and simultaneous
detection of Pb2+and Cd2+will cause overlapping of the stripping peaks.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 13
measurements will affect the stripping intensity of Pb2+ and vice versa [33]. Figure 6b shows the
stripping peaks of Cd2+ and Pb2+ extracted from [34] were close to each other and simultaneous
detection of Pb2+ and Cd2+ will cause overlapping of the stripping peaks.
Figure 6a also shows that the current intensity of the Cd2+ stripping peak versus Pb2+ was
significantly smaller, in which the peak current of Cd2+ was 74 µA and the peak current of Pb2+ was
148 µA. This indicated that the HMstat has higher sensitivity to detect Pb2+ compared to Cd2+. Similar
results were also observed in [35] and as shown in Figure 6b [34], where the peak current of Pb2+
obtained was more elevated than Cd2+.
The same experimental procedures were repeated using Rstat. The detection results of HMstat
were compared with Rstat. Referring to Figure 7, using Rstat, the stripping peak of Cd2+ and Pb2+
were located at 0.25 and 0.3 V, respectively. These results were the same when compared to
HMstat. As expected, the peak current of the Pb2+ using Rstat and HMstat was quite similar (136 and
148 µA for Rstat and HMstat, respectively). However, the peak current of Cd2+ (297 µA) using Rstat
showed significantly higher value when compared to the result obtained by the HMstat. It was also
observed that using Rstat, the peak current of Cd2+ was higher when compared to the Pb peak, which
indicated that Rstat has a higher sensitivity to detect Cd2+ when compared to Pb2+. This result is in
contradiction with the result obtained in [35] and in Figure 6b [34].
Figure 6. (a) Stripping peak of Cd2+ and Pb2+ using HMstat incorporated with a screen-printed gold
electrode (SPGE). Experimental condition: deposition potential 0.6 V; deposition time 120 s;
modulation and step amplitude 50 mV and frequency 10 Hz. In 0.1 M acetate buffer. (b) Stripping
peak of Cd2+ (at 0.195 V) and Pb2+ (at 0.18 V) under gold-based electrode, extracted from previous
research [34].
Figure 7. Comparison detection peak between HMstat and Rodeostat (Rstat) for (a) Cd2+ and (b) Pb2+.
Using the same experimental condition as in Figure 6.
Based on the above results, the stripping peak of simultaneous detection of Cd2+ and Pb2+ under
gold-based electrode using both HMstat and Rstat interfered with each other’s peaks. This can be
0
50
100
150
200
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0
Current, I (µA)
Potential, V (V)
(a) Baseline
Cd2+
Pb2+
0.3
0.4
0.5
0.6
0.7
-0.5 -0.4 -0.3 -0.2 -0.1
Current, I (A)
Potential, V (V)
(b)
Cd2+
Pb2+
Cd2+ + Pb2+
0
100
200
300
400
500
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0
Current, I (µA)
Potential, V (V)
(a) Baseline
Hmstat
Rstat
0
100
200
300
400
500
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0
Current, I (µA)
Potential, V (V)
(b) Baseline
Hmstat
Rstat
Figure 6.
(
a
) Stripping peak of Cd
2+
and Pb
2+
using HMstat incorporated with a screen-printed
gold electrode (SPGE). Experimental condition: deposition potential
0.6 V; deposition time 120 s;
modulation and step amplitude 50 mV and frequency 10 Hz. In 0.1 M acetate buer. (
b
) Stripping
peak of Cd
2+
(at
0.195 V) and Pb
2+
(at
0.18 V) under gold-based electrode, extracted from previous
research [34].
Figure 6a also shows that the current intensity of the Cd
2+
stripping peak versus Pb
2+
was
significantly smaller, in which the peak current of Cd
2+
was 74
µ
A and the peak current of Pb
2+
was 148
µ
A. This indicated that the HMstat has higher sensitivity to detect Pb
2+
compared to Cd
2+
.
Similar results were also observed in [
35
] and as shown in Figure 6b [
34
], where the peak current of
Pb2+obtained was more elevated than Cd2+.
The same experimental procedures were repeated using Rstat. The detection results of HMstat
were compared with Rstat. Referring to Figure 7, using Rstat, the stripping peak of Cd
2+
and Pb
2+
were located at
0.25 and
0.3 V, respectively. These results were the same when compared to HMstat.
As expected, the peak current of the Pb
2+
using Rstat and HMstat was quite similar (136 and 148
µ
A
for Rstat and HMstat, respectively). However, the peak current of Cd
2+
(297
µ
A) using Rstat showed
significantly higher value when compared to the result obtained by the HMstat. It was also observed
that using Rstat, the peak current of Cd
2+
was higher when compared to the Pb peak, which indicated
that Rstat has a higher sensitivity to detect Cd
2+
when compared to Pb
2+
. This result is in contradiction
with the result obtained in [35] and in Figure 6b [34].
Based on the above results, the stripping peak of simultaneous detection of Cd
2+
and Pb
2+
under
gold-based electrode using both HMstat and Rstat interfered with each other’s peaks. This can be
overcome by electrode modification to improve anti-interference of the electrode by diminishing either
one stripping peak [
36
], or by implementing a chemometrics method such as back-propagation artificial
neural network (BP-ANN) using the formation of a prediction model for detection Cd
2+
and Pb
2+
[
37
].
Appl. Sci. 2020,10, 7804 10 of 12
Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 13
measurements will affect the stripping intensity of Pb2+ and vice versa [33]. Figure 6b shows the
stripping peaks of Cd2+ and Pb2+ extracted from [34] were close to each other and simultaneous
detection of Pb2+ and Cd2+ will cause overlapping of the stripping peaks.
Figure 6a also shows that the current intensity of the Cd2+ stripping peak versus Pb2+ was
significantly smaller, in which the peak current of Cd2+ was 74 µA and the peak current of Pb2+ was
148 µA. This indicated that the HMstat has higher sensitivity to detect Pb2+ compared to Cd2+. Similar
results were also observed in [35] and as shown in Figure 6b [34], where the peak current of Pb2+
obtained was more elevated than Cd2+.
The same experimental procedures were repeated using Rstat. The detection results of HMstat
were compared with Rstat. Referring to Figure 7, using Rstat, the stripping peak of Cd2+ and Pb2+
were located at 0.25 and 0.3 V, respectively. These results were the same when compared to
HMstat. As expected, the peak current of the Pb2+ using Rstat and HMstat was quite similar (136 and
148 µA for Rstat and HMstat, respectively). However, the peak current of Cd2+ (297 µA) using Rstat
showed significantly higher value when compared to the result obtained by the HMstat. It was also
observed that using Rstat, the peak current of Cd2+ was higher when compared to the Pb peak, which
indicated that Rstat has a higher sensitivity to detect Cd2+ when compared to Pb2+. This result is in
contradiction with the result obtained in [35] and in Figure 6b [34].
Figure 6. (a) Stripping peak of Cd2+ and Pb2+ using HMstat incorporated with a screen-printed gold
electrode (SPGE). Experimental condition: deposition potential 0.6 V; deposition time 120 s;
modulation and step amplitude 50 mV and frequency 10 Hz. In 0.1 M acetate buffer. (b) Stripping
peak of Cd2+ (at 0.195 V) and Pb2+ (at 0.18 V) under gold-based electrode, extracted from previous
research [34].
Figure 7. Comparison detection peak between HMstat and Rodeostat (Rstat) for (a) Cd2+ and (b) Pb2+.
Using the same experimental condition as in Figure 6.
Based on the above results, the stripping peak of simultaneous detection of Cd2+ and Pb2+ under
gold-based electrode using both HMstat and Rstat interfered with each other’s peaks. This can be
0
50
100
150
200
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0
Current, I (µA)
Potential, V (V)
(a) Baseline
Cd2+
Pb2+
0.3
0.4
0.5
0.6
0.7
-0.5 -0.4 -0.3 -0.2 -0.1
Current, I (A)
Potential, V (V)
(b)
Cd2+
Pb2+
Cd2+ + Pb2+
0
100
200
300
400
500
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0
Current, I (µA)
Potential, V (V)
(a) Baseline
Hmstat
Rstat
0
100
200
300
400
500
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0
Current, I (µA)
Potential, V (V)
(b) Baseline
Hmstat
Rstat
Figure 7.
Comparison detection peak between HMstat and Rodeostat (Rstat) for (
a
) Cd
2+
and (
b
) Pb
2+
.
Using the same experimental condition as in Figure 6.
4. Conclusions
The novel idea to design and optimize the performance of the potentiostat apparatus called
HMstat is based on easy to use components integrated with a NI myRIO equipped with built-in ADC
and DAC units; thus, making the overall design of the HMstat less complicated and easy to handle
and operate. With respect to the methodological function of the proposed apparatus pertaining to
its reliability, noise measurement, accuracy tests, and heavy metal detection tests were conducted.
The noise measurement of the HMstat was lower than 7 nA, and the accuracy of the HMstat was
higher than 95%, which indicated that the HMstat is within the precision rate of the component used.
The detection result showed that the HMstat was capable of detecting Cd
2+
and Pb
2+
at a stripping peak
of 0.25 and
0.3 V, respectively. Through investigation, the significant potential peak of Cd
2+
and Pb
2+
had agreement with slight overlapping under a gold-based electrode, as shown in the above result.
Furthermore, the simultaneous detection of Cd
2+
and Pb
2+
or detection of a single element of
Cd
2+
in the presence of Pb
2+
or vice versa will be more challenging in the near future, as the locations
of the stripping peaks of Cd
2+
and Pb
2+
were close to each other and tend to overlap and aect each
other. It was also observed that the HMstat had higher sensitivity to detect Pb
2+
compared to Cd
2+
.
This was based on the intensity current of the Pb
2+
stripping peak, which was significantly higher
compared to Cd2+.
Author Contributions:
Conceptualization, S.N.H.U. and E.A.B.; Formal analysis, M.N.A.; Funding acquisition,
E.A.B. and A.R.O.; Methodology, S.N.H.U., E.A.B. and N.M.K.; Project administration, E.A.B.; Supervision, M.N.A.
and N.M.K.; Writing—original draft, S.N.H.U.; Writing—review and editing, M.N.A. and A.R.O. All authors have
read and agreed to the published version of the manuscript.
Funding:
The authors would like to acknowledge the RU Top-Down research grant (1001/PAERO/870052),
(1001/PAERO/6740041) provided by the Research Creativity and Management Oce, Universiti Sains Malaysia to
support this research. The authors would also like to acknowledge the Research Management Oce of Universiti
Teknologi Petronas and its industrial grant (015MD0-052) for supporting this research.
Acknowledgments:
The proposed experiments have been carried out in the School of Aerospace Engineering of
Universiti Sains Malaysia. The authors would also like to acknowledge the Ministry of Education (MOE) Malaysia
for supporting S.N.H.U. under Universiti Sains Malaysia Academic StaTraining Scheme (ASTS).
Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
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