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Gold nanoparticle modified screen-printed carbon arrays for the simultaneous electrochemical analysis of lead and copper in water
Prosper Kanyong,1* Sean Rawlinson,1 James Davis1
1School of Engineering, Ulster University, Jordanstown, BT37 0QB, United Kingdom*Corresponding author: [email protected]
Abstract
The development of disposable screen-printed carbon arrays modified with gold
nanoparticles (AuNPs) is described. The AuNP-modified screen-printed carbon arrays,
designated as AuNP-SPCE arrays, were characterized by cyclic voltammetry and
electrochemical impedance spectroscopy. The AuNP-SPCE display excellent electrocatalytic
activity towards lead and copper. Two well-defined and fully resolved anodic stripping peaks,
at 20 mV for Pb(II) and at 370 mV for Cu(II), both vs. Ag/AgCl, can be seen. Square wave
anodic stripping voltammetry was used to simultaneously analyze Pb (II) and Cu(II) in their
binary mixtures in tap water. The linear working range for Pb(II) extends from 10 μg.L-1 to
100 μg.L-1 with a sensitivity of 5.942 μA.μg-1.L.cm-2. The respective data for Cu(II) are a
working range from 10 μg.L-1 to 150 μg.L-1 with a sensitivity of 3.52 μA.μg-1.L.cm-2. The
limits of detection (based on 3x the baseline noise) are 2.1 ng.L-1 and 1.4 ng.L-1, respectively.
In our perception, this array is particularly attractive because Pb(II) and Cu(II) can be
determined at rather low working potentials which makes the method fairly selective in that it
is not significantly interfered by other electroactive species that require higher reduction
potentials.
Keywords: Nanomaterial-based sensing, Screen-printed electrode, Heavy metal analysis,
Disposable sensor, Hexacyanoferrate, Cyclic voltammetry, Square wave anodic stripping
voltammetry, Electrochemical impedance spectroscopy, Electrode coverage, Dual sensing
1
1. Introduction
Lead (Pb) and copper (Cu) are toxic heavy metals that are not essential for human nutrition;
thus, their quantitative determination is of great relevance in the healthcare industry [1]. For
example, excessive intake of Pb is known to be associated with skeletal, renal, hematological,
neurobehavioral and neurological diseases as well as the impairment of cognitive
development in infants and children [1,2]. Copper toxicity can cause hemolytic crisis,
jaundice, liver cirrhosis and hepatitis [3,4]. Additionally, these heavy metals are often
retained in the ecosystem when they are discharged through activities such as mining and
casting and can bio-accumulate in the human body via the food chain [5-7]. Therefore, there
is the need for the development of analytical tools that allows for rapid and sensitive
detection of these heavy metals.
Currently, Pb and Cu are routinely analyzed by Inductively Coupled Plasma Mass
Spectroscopy (ICP-MS) and Atomic Absorption Spectroscopy (AAS) [8,9]. However, these
methods are time-consuming, laborious, expensive, require the use of skilled personnel, and
involve complicated operation procedures; thus, rendering them unsuitable for on-site and
decentralized sensing. Owing to the electroactive nature of Pb and Cu, the use of
electrochemical techniques, particularly at nano-sized surfaces, are most attractive for their
analysis because they are rapid in response, simple, relatively inexpensive, sensitive and
selective while permitting the exclusion of sample pre-treatment procedures [10]. The use of
nano-sized surfaces provides additional benefits by enhancing conductivity and charge-
transfer processes [11, 12]; thus improving the overall analytical performance of such
surfaces.
2
Due to their large effective surface area and superior electrocatalytic capabilities, AuNPs
have been extensively used to modify electrode surfaces [10-12]. The technology of screen-
printing offer the possibility of achieving further miniaturization of such modified electrodes;
thus making them portable, low-cost, simple to operate, reliable and relatively inexpensive to
manufacture. The latter point allows them to be disposable, which is clearly required for on-
site and decentralized testing [13-16].
In this study, the use of an in-house manufactured screen-printed carbon arrays modified with
AuNPs for the simultaneous analysis of Pb(II) and Cu(II), in conjunction with cyclic
voltammetry, electrochemical impedance spectroscopy (EIS) and square wave anodic
stripping voltammetry (SWASV), is described. Systematic experiments were carried out to
determine the optimum deposition time and potential for the electrodeposition of AuNPs and
stripping voltammetric analysis. The surface coverage of the AuNPs and reproducibility of
the sensors were also investigated. Lastly, SWASV was employed for the quantification of
both Pb(II) and Cu(II) in water. Details of the sensor fabrication and characterization are
described and discussed.
2. Experimental 2.1 Apparatus and reagents
Electrochemical experiments were conducted using VSP-300 Multichannel Potentiostat/
Galvanostat/EIS (Bio-Logic Science Instruments, France, http://www.bio-logic.info/) with a
standard three-electrode configuration. Electrochemical impedance spectroscopy in 5.0 mM
potassium ferrocyanide ([Fe(CN)6]3-/[Fe(CN)6]4-) was carried out at open circuit within the
frequency range 200 kHz - 0.1 Hz at an applied potential of 0.259 V. Unless otherwise
stated, SWASVwere recorded with deposition potential, deposition time, equilibrium time,
3
step increment, pulse amplitude and frequency of -0.5 V, 120 s, 15 s, 5 mV, 25 mV and 25
Hz, respectively. The SPCE were printed using DEK 240 Manual Screen Printing Machine
and a Stainless Steel Screen Mesh (DEK: 1615503, ASM Assembly Systems,
http://www.dek.com/) onto a valox substrate. Valox substrate was purchased from Cardillac
Plastics, UK (http://www.cadillacplastic.co.uk/). A Ag/AgCl (1.0 M KCl) reference electrode
was used throughout. The working electrode was either the bare SPCE or AuNP-SPCE array
with a platinum wire as the counter electrode. Copper standard solution was purchased from
BDH Chemicals, UK. Lead standard solution, gold (III) chloride solution (HAuCl4; 30 wt. %
in dilute HCl), sulfuric acid and nitric acid were purchased from Sigma Aldrich, UK
(http://www.sigmaaldrich.com/united-kingdom.html). Water was obtained from a tap at
NIBEC, Ulster University. All other chemicals were of analytical grade and used without
further purification.
2.2 Fabrication of SPCE and AuNP-SPCE array
The base unmodified SPCE transducer was prepared using graphite ink (GEM Product code:
C205010697) and arrays of the electrodes were screen-printed onto valox substrate (Scheme
1) and cured at 70 °C for 90 min. Thereafter, a polymer dielectric material (GEM Product
Code: D2071120P1, http://www.gwent.org/home.html) was screen-printed onto the cured
electrodes, to define the working area (5.03 x 105 μm2), and cured at 70 °C for 30 min. Prior
to modification with AuNPs, the SPCE was anodized by applying a potential of +1.6 V for 60
s vs. Ag/AgCl in 5.0 mL 0.1 M NaOH solution, under unstirred conditions. Electrodeposition
of AuNPs onto the SPCE was carried out from 5.0 mL of 6.0 mM HAuCl4 in 0.1 M HNO3
(prepared in doubly distilled water and degassed in N2 for 10 min) [17], at different
deposition potentials (from -0.1 to -4.0 V) vs. Ag/AgCl and with different deposition time
ranging from 10 s to 600 s. Prior to use, the AuNP-SPCE array was cleaned by cycling the
4
electrode potential from -0.1 V to 0.6 V vs Ag/AgCl at 0.1 V.s-1 in 0.1 M H2SO4 solution until
stable cyclic voltammograms were obtained (~5 cycles) [18]. All solutions were degassed
with N2 for about 10 min before each measurement.
Scheme 1: Screen-printing of carbon arrays and gold nanoparticles (AuNPs) modification
procedure.
3. Results and discussion3.1 Electrodeposition of AuNPs onto SPCE array
The electrodeposition of AuNPs is relevant due to the sensitive role it plays in heavy metal
analysis and quantification. Different electrodeposition time can significantly affect the
quantity, size and electrical transport properties of any AuNPs deposited onto SPCE [19].
Therefore, there is the need to obtain the optimum deposition time and potential for the
preparation of the AuNP-SPCE arrays. After the AuNP-SPCE arrays were prepared, they
were tested by cyclic voltammetry (CV) in [Fe(CN)6]3-/[Fe(CN)6]4- solution (Figure 1). Both
the oxidation and reduction peak currents for [Fe(CN)6]3-/[Fe(CN)6]4- couple increased from
10 s deposition time up to 500 s at an applied potential of -0.4 V (Figure 1A). However,
subsequent increases in deposition time (Figure 1B) did not result in an increase in both the
anodic and cathodic peak currents; thus, 500 s was considered to be the optimum deposition
time.
5
Figure 1:(A) Cyclic voltammograms recorded using AuNP-SPCE array prepared with
different deposition time (from 10 s to 600 s) at -0.4 V; (B) Plot of anodic and cathodic peak
currents for [Fe(CN)6]3-/[Fe(CN)6]4- vs. deposition time; (C) Cyclic voltammograms recorded
using AuNP-SPCE array prepared with different deposition potential (from -0.1 to -0.7 V) for
500 s. All voltammograms were recorded in Britton-Robinson buffer (BRB, pH 7.0)
containing 5.0 mM [Fe(CN)6]3-/[Fe(CN)6]4- and 0.1 M KCl at 50 mV.s-1 scan rate.
Figure 1C shows cyclic voltammograms of AuNP-SPCE array obtained with different
deposition potentials for 500 s while the effect of deposition potential on both the anodic and
cathodic peak currents for [Fe(CN)6]3-/[Fe(CN)6]4- is shown in Figure 1D. In general, the peak
currents had optimum values when -0.4 V was used to fabricate the AuNP-SPCE array; thus,
-0.4 V was considered to be the optimum deposition potential. Consequently, -0.4 V and 500
s were used to fabricate subsequent AuNP-SPCE array.
6
3.2 Characterization of SPCE and AuNP-SPCE array3.2.1 Cyclic voltammetry
The electrochemical behavior of the SPCEs towards [Fe(CN)6]3-/[Fe(CN)6]4- redox couple
was examined by cyclic voltammetry. Figure 2A shows cyclic voltammograms recorded in
BRB (pH 7.0) containing 5.0 mM [Fe(CN)6]3-/[Fe(CN)6]4- and 0.1 M KCl at 50 mV.s-1 scan
rate using the bare SPCE and AuNPs-SPCE array obtained at -0.4 V for 500 s.
As expected, in comparison to what occurred on the bare SPCE, AuNP-SPCE array exhibited
a characteristic increase of the anodic and cathodic peak currents for [Fe(CN)6]3-/[Fe(CN)6]4-,
thus confirming the successful modification process. Higher peak currents and a smaller
peak-to-peak potential separation (ΔEp) were observed at the AuNP-SPCE array (Ipa =
5.51μA, Ipc = 3.72 μA; ΔEp= ~57 mV) when compared with the bare SPCE (Ipa = 0.53 μA, Ipc=
0.34 μA; ΔEp= ~130 mV). This can be attributed to the higher electrochemical activity of
AuNPs which allowed the increase of the active area of the electrode. The presence of the
AuNPs produced slight shifts of the anodic and cathodic peak potential to less positive
values, giving rise to a smaller peak-to-peak separation (ΔEp = ~57 mV), which corresponds
to the 57 mV values expected for one-electron transfer reactions [20]. This more than ten-fold
increase in both the anodic and cathodic peak currents of [Fe(CN)6]3-/[Fe(CN)6]4- can be
attributed to the electrocatalytic effect of the AuNPs.
7
Figure 2: Cyclic voltammograms recorded using (a) SPCE and (b) AuNP-SPCE array in 5.0
mM [Fe(CN)6]3-/[Fe(CN)6]4- in BRB (pH 7.0) containing 0.1 M KCl at 50 mV.s -1 scan rate;
(B) Cyclic voltammograms recorded using AuNP-SPCE array in 5.0 mM
[Fe(CN)6]3-/[Fe(CN)6]4- in BRB (pH 7.0) containing 0.1 M KCl at (a) 50, (b) 100, (c) 150, (d)
200, (e) 250 and (f) 300 mV.s-1 scan rates. Insert; peak current vs. square root of scan rate;
(C) log Ipa vs. log v.
8
The effect of scan rate (v) on the voltammetric behavior of AuNP-SPCE array was also
examined by CV (Figure 2B). At the scan rates investigated (50.0 to 300.0 mV.s -1), a plot of
the square root of the scan rate vs. the anodic and cathodic peak currents (Ip) exhibited a
linear relationship (Figure 2B), which is typical of a diffusion-controlled process [21]. A
linear relationship was also observed when both log Ipa and log Ipc were plotted against log v
(Figure 2C) with slope values of 0.499 and 0.514, respectively. These slope values are
comparable with the theoretically expected value of 0.5 for purely diffusion controlled
currents [21]; thus, confirming that the electrochemical process is diffusion-controlled.
3.2.2 Electrochemical impedance analysis
Electrochemical impedance spectroscopy (EIS) is a versatile technique that provides relevant
information about the barrier properties of electrode surfaces [22, 23]; thus, EIS was used to
evaluate the barrier properties of the bare SPCE and AuNP-SPCE array. Figure 3A illustrates
experimental Nyquist spectra for SPCE and AuNP-SPCE array including the equivalent
Randle’s circuit used to extract the impedance circuit fitted data in Table 1. The spectrum
associated with SPCE consists of a semicircle part in the high frequency region and a linear
part in the low frequency region, corresponding to the electron transfer and diffusion
processes, respectively. The diameter of the semicircle represents the charge-transfer
resistance (RCT) at the surface of the electrode. At the bare SPCE, a semicircle with a larger
diameter was obtained and an RCT value for [Fe(CN)6]3-/[Fe(CN)6]4-redox process was found
to be ~7209 Ω. However, on the AuNP-SPCE array, the diameter of semicircle was nearly
nonexistent and RCT was significantly reduced to ~3.71 x 10-6 Ω. This change is attributed to
the high electrical transport properties of AuNPs which facilitated the [Fe(CN)6]3-/[Fe(CN)6]4-
redox process. These impedance results are in agreement with the results obtained from CV
9
measurements; thus, confirming the successful electrodeposition of AuNPs onto the bare
SPCE.
Figure 3: (A) Nyquist plots observed for electrochemical impedance measurements at SPCE
and AuNP-SPCE array in BRB (pH 7.0) containing 5.0 mM ([Fe(CN)6]3-/[Fe(CN)6]4-) and 0.1
M KCl. Insert is the Randle’s equivalent circuit used for data fitting (R s; Resistance of the
electrolyte solution, Zw; Warburg impedance, Cdl; Double layer capacitance) and; (B)
Determination of the coverage rate of the gold nanoparticles.
Table 1: Impedance circuit fitted parameters for SPCE and AuNP-SPCE array
Parameter SPCE AuNP-SPCE Array
Rs (Ω)
Cdl (μF)
RCT (Ω)
Zw (Ω.s-½)
1776
0.5213
7209
26472
1742
0.664
3.71 x 10-6
15087
10
3.2.3 Determination of the coverage rate of AuNPs on SPCE
The determination of the coverage (θ) rate may provide information regarding the surface
morphology of the AuNP-SPCE array as well as the porosity of the AuNP-layer [24]. The
coverage of a compact layer can be determined by EIS through the expression:
θ=1−¿) (1)
Figure 1B represents the impedance of the real part of the bare SPCE and AuNP-SPCE array
as a function of the inverse of the square root of the sinusoidal excitation pulsation. The
extrapolation of the linear zone to the high frequencies provides the sum of RCT, the
resistance of AuNP-layer (RAuNP) and Rs. The latter resistances (RAuNPs and Rs) are generally
considered negligible when compared with the first one (RCT); thus, the calculated fractional
coverage area of the AuNP-layer was found to be ~91.16%. This high coverage values
indicate that the AuNP-layer is dense, compact and with few pores [24, 25].
3.3 Electrochemical behavior of the electrodes towards Pb(II) and Cu(II)
Square wave anodic stripping voltammograms were recorded in BRB (pH 7.0) containing a
binary mixture of Pb(II) and Cu(II) (300 μg.L-1 each) using the bare SPCE and AuNP-SPCE
array (Figure 4A). The stripping voltammogram of the bare SPCE in Figure 4A shows a low
current response and a single stripping peak within the potential window employed.
However, two well-defined, fully resolved stripping peaks were found at ~0.02 V and ~0.37
V on the AuNP-SPCE array, corresponding to Pb(II) and Cu(II), respectively.
11
Figure 4: (A) Anodic stripping voltammograms of Pb(II) and Cu(II) at bare SPCE and AuNP-
SPCE array; (B) Anodic stripping voltammograms at AuNP-SPCE array prepared using
different deposition time from 50 s to 300 s. Insert; anodic stripping peak currents vs.
deposition time; (C) Anodic stripping voltammograms of AuNP-SPCE array prepared using
different deposition potential from -0.1 V to -0.6 V. Insert; anodic stripping peak currents vs.
deposition potential.
12
3.4 Optimization of deposition time and potential for Pb(II) and Cu(II) analysis
The deposition time and potential are known to be relevant parameters that affect anodic
stripping voltammetric analysis of heavy metals [26-28]; thus, these two parameters were
thoroughly investigated in order to obtain their optimum values for Pb(II) and Cu(II)analysis.
The deposition potential was investigated from -0.1 V to -0.6 V in BRB containing binary
mixture of Pb(II) and Cu(II) (100 μg.L-1 each) (Figure 4B). There was a gradual increase in
the stripping peak of Pb(II) from -0.1 V up to -0.5 V (insert of Figure 4B) while the peak
currents for Cu(II) was slightly reduced. With the deposition potential moving slightly
positive and the peak current of Pb (II) significantly reduced at -0.6 V, the stripping currents
of Pb(II) and Cu(II) at -0.5 V was chosen as the optimum deposition potential for further
measurements.
The deposition time for the anodic stripping of Pb(II) and Cu(II) was also carried out from 50
s to 300 s at -0.5 V in BRB containing Pb(II) and Cu(II) (100 μg.L -1 each) (Figure 4C). Two
well-defined stripping peaks associated with Pb(II) and Cu(II), which increased with
increasing deposition time, were observed. As shown in the insert of Figure 4C, the stripping
current for Pb(II) increased linearly with increasing deposition time from 50 s up to 250 s.
Even though, the stripping peaks of Cu(II) increased from 50 s to 300 s, the increase was not
linear after 250 s, which can be attributed to the saturation of the surface of the AuNP-SPCE
array. Consequently, 250 s was chosen as the optimum deposition time.
3.5 SWASV analysis of Pb(II) and Cu(II) at AuNP-SPCE array in tap water
Using the optimized parameters of -0.5 V and 250 s, the quantitative analysis of binary
mixtures of Pb(II) and Cu(II) was carried out with the method of standard addition in tap
13
water. Prior to this, baseline measurements were made and this confirmed the absence of both
Pb(II) and Cu(II) in the water. Anodic stripping voltammograms for different binary
concentrations of Pb(II) and Cu(II) ranging from 10 to 300 μg.L-1 were carried out (Figure
5A) and two characteristic stripping potentials for Pb(II) and Cu(II) were observed at ~0.02 V
and ~0.37 V, respectively. The anodic stripping currents increased with increasing binary
concentrations of both Pb(II) and Cu(II) (Figure 5B). In the range of 10 - 100 μg.L-1, the
stripping currents for Pb(II) was linear with a calibration equation of IPb (μA) = 0.0299 [Pb]
(μg.L-1) + 4.9796, R2 = 0.9681 and a sensitivity of 0.03 μA.μg-1.L (5.94 μA.μg-1.L.cm-2).
However, there was a decrease in the sensitivity of the sensor towards Pb(II) when the
concentration was increased beyond 100 μg.L-1. On the other hand, the stripping currents for
Cu(II) exhibited good linearity from 10 μg.L-1 to 150 μg.L-1; with a calibration equation of ICu
(μA) = 0.0177 [Cu] (μg.L-1) + 8.0359, R2 = 0.9854 and a sensitivity of 0.02 μA.μg-1.L (3.52
μA.μg-1.L.cm-2). The calculated limits of detection for Pb(II) and Cu(II) (based on 3x the
baseline noise) were found to be 2.1 ng.L-1and 1.4 ng.L-1, respectively; these were deemed to
be satisfactory for routine analysis of Pb(II) and Cu(II) in water.
14
Figure 5: (A) Anodic stripping voltammograms recorded for different binary mixtures of
Pb(II) and Cu(II) (10, 20, 40, 60, 80, 100, 150, 200, 250, 300 μg.L) in tap water; (B) Plot of
anodic stripping peak current vs. concentration; (C) Ten replicated stripping anodic
voltammograms recorded for a binary mixture of Pb(II) and Cu(II) (300 μg.L each) in BRB
(pH 7.0); (D) Corresponding anodic stripping peak currents obtained from (C) vs. replicate
number. All measurements were carried out using AuNP-SPCE array.
15
The reproducibility of the AuNP-SPCE array including batch-to-batch modification with
AuNPs were also investigated. Figure 5C shows ten repetitive anodic stripping
voltammograms recorded for a binary mixture of Pb(II) and Cu(II) (300 μg.L -1 each) in BRB
(pH 7.0). Their corresponding anodic stripping peaks for the repeats are shown in Figure 5D
with relative standard deviations of 9.2 % and 5.3% for Pb(II) and Cu(II), respectively. The
batch-to-batch reproducibility of the fabrication procedure was also investigated. As shown in
Figure 6, the three AuNP-SPCE array tested exhibited slightly different peak current
responses to the same sample solution with standard deviations of 4.3 % and 5.8 % for Pb(II)
and Cu(II), respectively. These standard deviation values from both the repetitive
measurements and the three different AuNP-SPCE array indicate a highly reproducible
procedure.
Figure 6: Three replicated anodic stripping voltammograms for three AuNP-SPCE array in
BRB (pH 7.0) containing a binary mixture of Pb(II) and Cu(II) (300 μg.L-1 each).
16
4. Conclusions
An in-house manufactured screen-printed carbon array, modified with gold nanoparticle has
been demonstrated as a useful sensing tool for the analysis Pb(II) and Cu(II) by square wave
anodic stripping voltammetry. Potential applicability in the analysis of these two heavy
metals in water was demonstrated with stable, accurate results obtained; which holds a great
promise for application in routine analysis of these species. In future, similar sensors would
be developed for the analysis of Pb(II) and Cu(II) in environmental samples.
Conflict of interest
The authors declare no conflict of interest.
Acknowledgements
The authors would like to thank the Department of Employment and Learning Ireland (Grant
No.: USI035) and the National Institutes of Health (Grant No.: 5R01ES003154-30) for the
funding.
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