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: p.kanyong@ulster.ac.uk

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.

References1. Goyer RA (1995) Nutrition and metal toxicity; Am J Clin Nutr 611: 640S-650S.2. Patrick L (2006) Lead toxicity, a review of the literature part I: exposure, evaluation

and treatement; Altern Med Rev 11: 2-22.3. Goyer RA (1997) Toxic and essential metal interactions; Ann Rev Nutr 17: 37-50.4. Gaetke LM, Chow CK (2003) Copper toxicity, oxidative stress and antioxidant

nutrients; Toxicol 189:147-163.5. Bulut Y, Tez Z (2007) Removal of heavy metals from aqueous solution by sawdust

adsorption; J Environ Sci 19:160-166.6. Bailey SE, Olin TJ, Bricka RM, Adrian DD (199) A review of potentially low-cost

sorbents for heavy metals; Water Res 33:2469-2479.7. Kurmar SR, Agrawal M, Marshall F (2007) Heavy metal contamination of soil and

vegetables in surburban areas of Varranasi, India; Ecotoxicol Environ Saf 66:258-266.

8. Ghaedi M, Ahmadi F, Shokrollahi A (2007) simultaneous preconcentration and determination of copper, nickel, cobalt and lead ions content by flame atomic absorption spectrometry; J Hazard Mater 142:272-278.

9. Ammann AA (2002) Speciation of heavy metals in environment water by ion chromatography coupled to ICP-MS; Anal Bioanal Chem 372:448-452.

17

10. Welch CW, Compton RG (2006) The use of nanoparticles in electroanalysis: a review; Anal Bioanal Chem 384:601-619.

11. Fei Y, Lv ZY, Wang AJ, Chen YH, Chen JR, Feng JJ (2014) Simultaneous determination of trace levels of lead (II) and copper (II) by square wave stripping voltammetry using a glassy carbon electrode modified with hierarchical gold dendrites; Microchim Acta, 181:389-394.

12. Wang L, Chen Z, Megharaj M, Naidu R (2014) Anodic stripping voltammetric determination of traces of Pb (II) and Cd (II) using a glassy carbon electrode modified with bismuth nanoparticles; Microchim Acta, 181:1199-1206.

13. Hart JP, Wring SA (1994) Screen-printed voltammetric and amperometric electrochemical sensors for decentralized testing; Electroanal 6:617-624.

14. Bernalte E, Sánchez CM, Gil EP (2011) Determination of mercury in ambient water samples by anodic stripping voltammetry on screen-printed gold electrodes; Anal Chim Acta 689:60-64.

15. Rueda-Holgado F, Bernalte E, Paomo-Martin M, Calvo-Blácquez L, Cereceda-Balic F, Pinilla-Gil E (2012) Miniaturized voltammetric stripping on screen printed gold electrodes for field determination of copper in atmospheric deposition; Talanata 101:435-439.

16. Laschi S, Palchetti I, Mascini M (2006) Gold-based screen-printed sensor for detection of trace lead; Sensor Actuator B Chem 114: 460-465.

17. Kanyong P, Rawlinson S, Davis J (2016) Simultaneous electrochemical determination of dopamine and 5-hydroxyindoleacetic acid in urine using a screen-printed graphite electrode modified with gold nanoparticles; Anal Bioanal Chem 1-9, DOI 10.1007/s00216-016-9351-0.

18. Fisher LM, Tenje M, Heiskanen AR, Masuda N, Castillo J, Bentien A, et al (2009) Gold cleaning methods for electrochemical detection applications; Microelectron Eng 86:1282–5.

19. Renedo OD, Martinez MJA (2007) Anodic stripping voltammetry of antimony using gold nanoparticle-modified carbon screen-printed electrodes; Anal Chim Acta 589:255-260.

20. Wang J, Tian B, Nascimento VB, Angnes L (1998) Performance of screen-printed carbon electrodes fabricated from different carbon inks; Electrochim Acta 43:3459-3465.

21. Gosser DK. Cyclic voltammetry; simulation and analysis of reaction mechanisms, VCH, New York, 1993.

22. Suni II (2008) Impedance methods for electrochemical sensors using nanomaterials; Trends Anal Chem 27:604–11.

18

23. Bard AJ, Faulkner LR. Electrochemical methods; fundamentals and applications. Wiley, New York (2001).

24. Echabaane M, Rouis A, Bonnamour I, Ben Quada H (2013) Electrical and electrochemical properties of the MEH-PPV and MEH-PPV doped calix[4]arene derivative layers for the detection of Cu2+ and Na+ ions; Measurement 46:2411-2422.

25. Kuralay F, Erdem A, Abaci S, Ozyoruk (2013) Electrochemical characterization of redox polymer modified electrode developed for monitoring of adenine; Colloid Surface B 105:1-6.

26. Nolan MA, Kounaves SP (1999) Microfabricated array of iridium microdisks as a substrate for direct determination of Cu2+ or Hg2+ using square-wave anodic stripping voltammetry; Anal Chem 71: 3567-3573.

27. Luther III GW, Swartz CB, Ullman WJ (1988) Direct determination of iodide in seawater by cathodic stripping square wave voltammetry; Anal Chem 60:1721-1724.

28. Wan H, Sun Q, Li H, Sun F, Hu N, Wang P (2015) Screen-printed gold electrode with gold nanoparticles modification for simultaneous electrochemical determination of lead and copper; Sensor Actuator B Chem 209: 336-342.

19