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Synthesis, structural and optical properties of gold nanoparticle-graphene-selenocysteine composite bismuth ultrathin film electrode and its application to Pb(II) and Cd(II) determination
⁎Corresponding author at: Chemistry Department, Northern Border University, Faculty of Science, Arar, Saudi Arabia. Tel.: +20 (88)2294980 (home), +20 1004688146, +966 563935228 (mobile). ahmed73chem@scinv.au.edu.eg (Ahmed Farouk Al-Hossainy)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The transparency of the gold nanoparticle-graphene-selenocysteine composite is 90–98% in the visible range. The optical and transport energy gaps were estimated as 1.15 eV and 2.25 eV respectively. A new electrochemical method using gold nanoparticle-graphene-selenocysteine modified bismuth film glassy carbon electrode, was applied to improve the performance of the simultaneous determining trace cadmium and lead in square wave anodic stripping voltammetry with a detection limit of 0.08, and 0.05 ppb for metal ions, with a high correlation coefficient of 0.9811 and 0.99, respectively. Finally, the gold nanoparticle-graphene-selenocysteine modified bismuth film glassy carbon electrode was successfully applied for the determination of cadmium, and lead in environmental samples and the results were successful.
Keywords
Gold nanoparticle-graphene-selenocysteine
Bismuth ultrathin film glassy carbon electrode
Optical properties
Stripping voltammetry and environmental samples
1 Introduction
Recently, graphene/graphene oxide has been considered as “rising star” carbon material because of superior mechanical strength (Freitag et al., 2009 and Vinod et al., 2015) and high heat conductance. Graphene used in electrochemistry is produced by a chemical exfoliation method from the reduction of graphene oxide that is formed by using a Hummers method. However, because of van der Waals and π–π stacking interactions among individual graphene sheets, the as-reduced graphene sheets from graphene oxide tend to form irreversible agglomerates and even restack to form graphite when graphene dispersion solutions are dried (Veerapandian et al., 2012).
In addition, gold-graphene nanoparticles (AuNPs) have gained considerable attention as labels in electroanalytical methods based on affinity reactions, such as immunosensors and immunoassays. In particular, inorganic semiconductor sensors and electrodes, which combine square wave anodic stripping voltammetry with porous bismuth thin film electrode (Yasemin et al., 2011 and Mehmet et al., 2014), offer wide applications in environmental and biological samples. Gold-graphene nanoparticles (AuNPs) have been the most extensively studied due to their unique optical, chemical, electrical, and catalytic properties. In addition, AuNPs can provide good affinity for covalent bonds with biological samples (Mehmet et al., 2014, 2015; Sensors and Actuators B 2014).
On the other hand, bismuth film (BiFE) electrode for the determination of toxic metals in environmental samples is an environmentally friendly electrode with negligible toxicity. It was introduced in SWASV method, exhibiting an impressive electroanalytical performance, which was compared favorably to its mercury analogue (Sandra et al., 2015). The ability of BiFE to form intermetallic alloys with different trace heavy metals, as well as its insensitivity toward dissolved oxygen is just some of remarkable electrochemical features of bismuth-based electrodes that stay behind their widespread use (Economou, 2005). There were numerous efforts to improve the sensitivity of the glassy carbon electrode (GCE) working electrodes, including the expansion of the sensor surface via the synthesis of mesoporous bismuth-based materials (Chen et al., 2013), the enlargement of the substrate electrode surface before modification with bismuth (Zhaomeng et al., 2013) and the incorporation of micro/nanoparticles of Au-GN-SeCys nanoparticles (Dandan et al., 2014). Under the optimum experimental conditions, the SWASV technique was applied to solid GCE, and on the working electrode surface the introduced Bi(III) ions and target trace heavy elements have been deposited together via electrochemical reduction to form coordinate-component alloys of low-temperature melting. The sensitivity of the modified GCE can improve significantly with strong adsorptive ability toward the metal ions in environmental samples and facilitate the nucleation of toxic heavy elements during the optimum experimental conditions of SWASV, without, removal of dissolved oxygen in the medium for analysis (Xu et al., 2013). SWASV using chemically modified electrode has been proved to show significant selectivity toward some amino acids such as, aspartic acid (Jinfen et al., 2012) and cysteine (Wenrong et al., 2001), which have either electrostatic interaction or complexation with metal ions and promote the preconcentration of the latter on the electrode surface. The interaction of amino acid with cadmium and lead ions in groundwater was investigated by square wave stripping voltammetry (SWSV) technique, and the adsorption of amino acids on modifying porous gold electrodes was studied by electrochemical reductive desorption in 0.5 M potassium hydroxide (Tribidasari et al., 2015). Detection limit of toxic metals’ concentrations (i.e., the ratio of amino acids to bound cadmium and lead ions are 2:1) represent an attractive features of the developed electrochemical sensor. At higher concentrations cadmium or lead metals have been bonding with adsorbed amino acid forming cadmium or lead sulfide on the electrode surface (Meijiao et al., 2013; Lian et al., 2014 and Metters et al., 2014).
The objective of this work was to propose a new nanocomposite containing gold-graphene nanoparticles (AuNPs) and selenocysteine was prepared and characterized by UV–vis spectrophotometry, Fourier transform infrared (FT-IR) spectroscopy spectra, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Modified graphene and porous bismuth film glassy carbon electrodes were induced to a lower charge-transfer resistance than GCE. The surface area of GCE was enlarged resulting in comparatively wide electrochemical potential window and increases the loading amount of complex with amino acids. The formation complexes between the carboxyl group in selenocysteine amino acid and porous bismuth gold nanoparticles (AuNPs) have been used as the selective ligand for cadmium and lead ions. In addition, the porous thin film bismuth has been widely recognized as a powerful technique for the determination of Cd(II) and Pb(II) ions, and due to the formation of complex Au-GN-SeCys, the sensitivity of the microsensor has been increasing the sensitive area of the working electrode. In other words, the bismuth film-modified Au-GN-SeCys presented an advantageous and high-performance platform for the sensing of Cd(II) and Pb(II) in groundwater, soil and Alhagi maurorum plants.
2 Experimental methods
2.1 Preparation of gold nanoparticle-graphene-selenocysteine composite (Au-GN-SeCys) suspension
Se-(Methyl) selenocysteine hydrochloride (PN M6680) (95% (TCL) was purchased from Sigma–Aldrich (USA). Monolayer Graphene Powder Chemical method (CAS NO. 7440-44-0) was obtained from Nanjing CJNANO Tech Co., Ltd. (China). HAuCl4 was obtained from Shanghai Arkin Chemical Technology Co., Ltd. (China).
Graphene suspension was prepared by adding 2 mL deionized water to 1.4 mg graphene. A solution of HAuCl4 (0.2 ml, 1%) was added to graphene suspension, and heated to boiling. Then 0.4 mL, 0.1 mol L−1 trisodium citrate (Na3C6H5O7) was added to this solution drop by drop under vigorous stirring for 20 min. The suspension Au-GN composites were separated by centrifugation and washed with deionized water. 2 mL of Se-(Methyl) selenocysteine hydrochloride solution was added to 2 ml of suspension precipitate with stirring for 15 h at room temperature under nitrogen. Next, the nano-composites were centrifuged at 1 × 104 rpm for 20 min and washed with deionized water. Next, the Au-GN-SeCys composites were re-dispersed in 1 mL deionized water and the suspension was stored in a brown glass bottle at 4 °C for further use. Finally, the Au-GN-SeCys suspension (3 (L) was dropped on the glassy carbon electrode surface and allowed to dry at room temperature. The prepared electrode was marked as an Au-GN-SeCys/GCE and preserved in a refrigerator at 4 °C before use.
2.2 Experimental procedure
Infrared spectra were recorded on a Perkin–Elmer FT-IR type 1650 spectrophotometer in wavenumber region 4000–400 cm−1. The spectra were recorded as KBr pellets. The absorbance, the transmittance and reflectance spectra were recorded by using SHIMADZU UV-3600 UV–vis NIR spectrophotometer at room temperature.
Thin film of Au-GN-SeCys composites was prepared by thermal evaporation technique using a high vacuum coating unit (Edwards E 306 A, England). The vacuum during the deposition process is about 1 × 10−4 Pa. The films were deposited onto clean optical flat glass and quartz substrates for structural and optical measurements, respectively. These substrates were carefully cleaned by chromic acid for 15 min and then rinsed by deionized water. The Au-GN-SeCys composite was sublimated from quartz crucible heated gradually by molybdenum boat shaped filament. The deposition rate and the film thickness were measured during the evaporation using a quartz crystal thickness monitor (Model TM-350 MAXTEK, Inc., USA) attached to the coating system. The grown film thickness of Au-GN-SeCys composites is 100 nm.
The square wave anodic stripping voltammetry (SWASV) measurements (EG&G PAR Model 273A potentiostat with 250/270 research electrochemistry software version 4.0, manufactured by Princeton Applied Research Corporation by using a glassy carbon electrode (GCE), 3 mm in diameter) were performed in a 10 mL electrochemical cell, containing (0.12 mol L−1, pH = 4.8) sodium acetate–acetic acid buffer solutions (SAB), 275 ppb, Bi(NO3)3. 5H2O, 20 ppb of Cd(II) and Pb(II). Then, the cleaned Au-GN-SeCys/GCE was immersed in the 275 ppb of Bi(III) at −1.0 V for 14 min under stirring where Bi(III) and the target metals were simultaneously deposited on the surface of the electrode. The voltammogram was recorded between −1.0 V and −0.4 V by applying SWASV with a frequency of 15 Hz, an amplitude of 25 mV. All measurements were performed at room temperature (25 ± 2 °C) in air atmosphere.
2.3 Samples and quality assurance
One gram of each soil sample from S2 and S3 was prepared directly in deionized water for one hour. The final filtrate of soil samples S2 and S3 was transferred into a 100 mL measuring flask. The second filtrate samples S4 and S5 were digested with 5 mL of mixed concentrated acid and evaporated to dryness (Benjamin et al., 2013). Also, one gram of each Alhagi maurorum plant sample of S6, S7 and S8 after drying and grinding was dissolved in 5 mL mixed concentrated acid and evaporated to dryness. All analytical tools were soaked in 2 M nitric acid, washed three times with deionized water and finally soaked in 0.1 M hydrochloric acid until being ready for use.
Under optimum experimental condition of SWASV (0.12 mol L−1, pH 4.8 SAB containing 275 ppb Bi(III), deposition potential: −1.00 V, deposition time: 14 min, amplitude: 0.025 V, increment potential: 0.004 V) Au-GN-SeCys composites for determining ultra-trace Cd(II) and Pb(II) ions in reference material (Whole Milk Powder 8435 WMP8435) have been studied. The Whole Milk Powder 8435 (WMP8435) from the National Institute of Standard and Technology in Canada was analyzed for cadmium (0.46 ppm) and lead (0.46 ppm) metals.
3 Results and discussion
3.1 Characterization of Au-GN-SeCys composites
FT-IR spectra of selenocysteine, have a structure similar to those of cysteine, but with an atom of selenium taking the place of the usual sulfur, forming a selenol group which is deprotonated at physiological pH. Fig. 1a shows the FT-IR spectra of the cysteine, selenocysteine, Au-GN-SeCys composite and their thin film of Au-GN-SeCys from 400 to 4000 cm−1, respectively. The characteristic peaks of cysteine had a very broad band at 3025–3000 cm−1 corresponding to the amino group stretching vibration, and the bands at 1635–1620 and 1401–1385 cm−1 due to the asymmetric and symmetric stretching vibration of carboxylic group, respectively. Besides, the stretching bands of the C–N at 1055–1030 cm−1, the stretching band of S–H at 2550 cm−1 was very weak and the stretching band of Se–H at 2517 cm−1 was very broad. The above absorption bands could also be found in the spectrum of Au-GN-SeCys, indicating that the prepared nanocomposite was composed of selenocysteine.
FT-IR spectrum (a), XRD (b), and (c) SEM image of thin film Au-GN-SeCys composite.
The carboxylic groups at the graphene surface were responsible for a previous attachment of Au(III) (HAuCl4) in solution owing to electrostatic interactions. The addition of the reducing agent, citrate ion, to the precursor solution could promote the subsequent reduction of Au(III), enabling the growth of gold nanoparticles at the graphene surface (Goncalves et al., 2009). It is known that the Se–Au interaction could result in the disappearance of the band of Se–H at 2517 cm−1 in the spectra of selenocysteine–Au-NPs aggregates (Wang et al., 2008). This band was still observed in the spectra of Au-GN-SeCys, suggesting that most selenocysteine molecules might be directly immobilized on graphene due to electrostatic interaction and hydrogen bond.
The thin film X-ray diffraction studies were employed using an X-ray diffraction (XRD, Philips X’ Pert Pro MRD) advance diffractometer with Cu Kα (λ = 1.5418 Å) radiation. The thin film Au-GN-SeCys was scanned in the 2 angles from 4° to 80° at a scan rate of 1°/min. Fig. 1b shows the XRD profile of the Au-GN-SeCys thin film. The positions of the peaks were found to be in good agreement with the literature data available of amino acid in JCPDS file no.: 06-230 (Organic Index, 2002).
The characteristic peak at 25.285° (2θ) corresponds to amino acid. This study confirms the γ-phase of amino acid. The reflection peaks corresponding to different crystal (h k l) planes in the recorded XRD profile were indexed and the data obtained from the XRD spectrum, such as angle 2θ, d value, h k l, peak intensity, full width half maximum value (FWHM) and size (nm) of every prominent peak in the spectrum are tabulated in Table 1. The sharp and strong peaks in the XRD profile confirm the good crystallinity of the Au-GN-SeCys thin film. From the XRD data, the lattice parameters of the Au-GN-SeCys thin film have a monoclinic crystal system and space group P21/c with lattice parameters: a = 11.705 Å, b = 14.927 Å, c = 9.89 Å, α = 90.00°, β = 105.65° and γ = 90.00° respectively.
Data/(h k l)
Selenocysteine
Au
GN
(1 0 1)
(
)
(2 0 0)
(1 0 2)
(2 0 1)
(1 1 1)
(2 0 0)
(2 2 0)
(3 1 1)
(1 1 0)
d-spacing (Å)
2.8472
3.4797
1.5367
1.5786
1.4380
1.6276
1.3458
1.6581
1.2547
1.4258
2θ
21.965
25.285
29.448
36.125
39.437
38.484
44.485
65.363
78.851
47.765
Intensity
1438.0
2007.0
842.00
942.00
823.00
1542.0
796.00
823.00
391.00
796.00
FWHM
0.1247
0.1687
0.1548
0.1483
0.1469
0.1378
0.1158
0.1592
0.1115
0.1119
D (nm)
65.319
48.362
53.105
56.395
57.489
61.111
74.178
59.335
92.613
77.705
∊ microstrain
0.1307
0.1536
0.1205
0.0929
0.0839
0.0807
0.0579
0.0507
0.0277
0.0517
δ × 10−4 (nm)−2
2.3441
4.2762
3.5459
3.1446
3.0257
2.6776
1.8174
2.8404
1.1659
1.6562
The broadening of the X-ray diffraction peak can be attributed to the contribution of different effects such as broadening due to smaller crystallite size (D) and lattice microstrain (∊) present in the material. The average crystallite size, D, and ∊ were calculated using the Scherrer and Williamson-Hall equations as follows:
The morphology and chemical composition of the film Au-GN-SeCys composite surface were investigated and verified by scanning electron microscopy (SEM) image, as shown in Fig. 1c. From Fig. 1c, it can be observed that large amounts of gold nanoparticles were well-dispersed on the surface of the GCE.
The spectral distribution of A(λ), T(λ) and R(λ) of Au-GN-SeCys nanostructure thin film measured at the normal incidence in the wavelength range 250–1200 nm for a thickness of 100 nm is illustrated in Fig. 2 at room temperature. The absorption coefficient (α), absorption index (k), and the refractive index (n), were calculated by the following relationships (El-Nahass et al., 2003, 2012):
The spectral dependence of the normal incidence transmittance, A(λ), T(λ) and reflectance R(λ) for Au-GN-SeCys thin film (100 nm).
Using the spectrophotometric measurements of transmittance and reflectance on the basis of the thin homogenous absorbing film onto thick non-absorbing quartz substrates, the values of n(λ) and k(λ) with the help of Eqs. (3) and (4) can be calculated respectively. Fig. 3 shows the spectral dependencies of both n(λ) and k(λ) in the wavelength range 250–1200 nm of Au-GN-SeCys composites thin films, and both n(λ) and k(λ) were calculated from the measured values of T(λ) and R(λ) at normal light incidence with a film thickness equal to 100 nm. Since, n(λ) and k(λ) have the two broad peaks in the wavelength range 350 nm ⩽ λ ⩽ 1000 nm, however the maximum values of n(λ) and k(λ) are 0.307 and 2.05 at wavelengths 450 nm and 825 nm, respectively. However, the peak in the refractive index corresponds to the fundamental energy gap of the Au-GN-SeCys composite film. The extinction index k(λ) starts to decrease with increasing λ above 350 nm which is a characteristic property of the existence of free carriers (Zeyada et al., 2012).
The spectral dependence of the refractive index n(λ) and the absorption index k(λ) of Au-GN-SeCys composites thin film.
During this treatment, the absorption coefficient (α) is related to the photon energy (hν) according to a power-law behavior of Au-GN-SeCys composites thin film (Agilan et al., 2007, and Wemple and DiDomenco, 1971) which is given by
Plot (αhν)1/2 as a function of photon energy of Au-GN-SeCys composites thin film.
3.2 Simultaneous determination of cadmium and lead, metals in situ plating bismuth film-Au-GN-SeCys/GCE
A supporting electrolyte SAB (0.12 mol L−1, pH 4.8) containing 20 ppb of Cd(II) and Pb(II) and 275 ppb Bi(III) was used in the stripping voltammograms of SWASV technique. The anodic stripping peaks of Cd(II) at −0.7905 V and Pb(II) at −0.5302 V were presented in situ plating bismuth film-modified GCE. The peak height enhancement of bismuth film-modified Au-GN-SeCys/GCE is 57.47 and 28.07 μA for Cd(II) and Pb(II) six times higher than that of the bismuth film-modified GCE, respectively. Fig. 5 shows the SWASV analytical characteristics of GCE, Au-GN/GCE, Au-GN-Cys/GCE and Au-GN-SeCys/GCE for Cd(II) and Pb(II) determination in reference material (WMP8435).
SWASV curves at GCE, Au-GN/GCE, Au-GN-Cys/GCE and Au-GN-SeCys/GCE in 0.12 mol L−1, pH 4.8 SAB containing 20 ppb Cd(II), Pb(II) ions and 275 ppb Bi(III). Deposition potential: −1.00 V, deposition time: 14 min, amplitude: 0.025 V, increment potential: 0.004 V.
In the meantime, the peak potentials of Cd(II) and Pb(II) ions on the nanocomposite-modified Au-GN-SeCys/GCE electrode were positively shifted. The improvement in stripping peak signals of Cd(II) and Pb(II) on Au-GN-SeCys/GCE can be attributed to three aspects. First, a selenium element in amino acid is used in biosensor, electronics, semiconductor and stable complexes, which was beneficial to the deposition of Cd(II) and Pb(II) on surface of modified electrode and high-performance platform of current peak. Second, the special characteristics of graphene on GCE had great role of surface area and electric conductivity. Finally, [C6H5O7]−3 ions around Au NPs had a high deposition of Cd(II) and Pb(II) on the electrode surface. Our experimental results indicate that selenocysteine and Au-GN could provide an obvious synergistic effect for the deposition of metals since selenocysteine played an important role in the metal binding.
3.3 Optimization of experimental conditions at the porous thin film of Bi(III)/Au-GN-SeCys/GCE
In order to obtain the best voltammetric behavior of the situ plating bismuth film-modified Au-GN-SeCys toward Cd(II) and Pb(II), many parameters including selenocysteine concentration, pH of SAB solution, concentration of bismuth, preconcentration time, preconcentration potential, and operational parameters of SWASV, which could have an influence on Cd(II) and Pb(II) deposition and reduction on modified electrode surface, were examined in detail.
Fig. 6 illustrates the effect of selenocysteine concentration on anodic stripping voltammograms (SWASV) of a solution containing 20 ppb Cd(II) and Pb(II) in pH 4.8 SAB containing 275 ppb Bi(III) at the Au-GN-SeCys using SWASV, with typical instrumental settings for each mode. It is obvious that both the peak currents of Cd(II) and Pb(II) ions increased up to 0.12 mol L−1 selenocysteine. At higher concentrations of selenocysteine, the Cd(II) and Pb(II) peak current continued to decrease but at a slower rate. Thus, 0.12 mol L−1 selenocysteine was used in the preparation process of Au-GN-SeCys.
The plot of cysteine concentration vs. the stripping peak current of 20 ppb Cd(II) and Pb(II) at Au-GN-SeCys/GCE in 0.12 mol L−1 pH 4.8 SAB containing 275 ppb Bi(III).
The supporting electrolyte and pH can greatly affect the voltammetric response of the sensor. The influence of pH by using an SAB buffer solution (0.12 mol/L) on the anodic stripping peak current of Cd(II) and Pb(II) was studied in the pH range of 3.7–5.6. The value 4.8 ⩽ optimum pH ⩾ 4.8, the anodic stripping peak current for Cd(II) and Pb(II) was decreased.
Fig. 7 shows the effect of pH of SAB on the electrochemical responses of Cd(II) and Pb(II). The stripping peak currents of Cd(II) and Pb(II) increased by increasing pH in the range 3.80–4.80 (maximum point), due to competition between the protons and the metal ions from binding to Au-GN-SeCys and also the isoelectric point of selenocysteine is 5.47 (Bettelheim et al., 2010), while, the current at 4.80–5.60 pH decreased in the anodic peak currents due to the hydrolysis of cations. From the above results, the 4.8 value of pH was employed in all the subsequent experiments.
The effect of pH on the stripping peak current of 20 ppb Cd(II) and Pb(II) at Au-GN-SeCys/GCE in 0.12 mol L−1, pH 4.8 SAB containing 275 ppb Bi(III).
The concentration of bismuth (25–500 ppp) was playing an important role in thickness of alloy film and film topography, when, the stripping peak of Cd(II) and Pb(II) was the highest at the Bi/Au-GN-SeCys/GCE in 0.12 mol L−1 pH 4.8 SAB, with a deposition potential of −1.00 V, deposition time of 14 min, amplitude of 0.025 V, and increment potential of 0.004 V. As shown in Fig. 8, the concentration of Bi(III) from 25 to 275 ppb increased with the increasing stripping peak currents of (10.07–42.48 μA) Cd(II) and (7.00–33.08 μA) Pb(II) ions, indicating that the electrode sensitivity is obviously improved by increasing the Bi(III) film thickness of the electrode. However, the Bi(III) concentration increases (275–500 ppb) with gradually decreasing the stripping peak currents of both metal ions, which is probably due to a layer of bismuth metals partially prevented the conductive surface of the modifying porous electrode (Cao et al., 2008). Therefore, the results indicated that the highest peak current responses were obtained at concentration of bismuth 275 ppb for cadmium(II) and lead(II) ion, which was chosen for the following experiments.
SWASV peak currents of 20 ppb Cd(II) and Pb(II) with respect to varied Bi(III) concentrations, measured by Au-GN-SeCys/GCE in 0.12 mol L−1, pH 4.8 SAB.
Fig. 9 shows the influence of the variation of the accumulation potential on the stripping peak current, examined over the range −0.90 to −1.20 V with a 20 ppb Cd(II) and Pb(II) with respect to varied Bi(III) concentrations, measured by Au-GN-SeCys/GCE in 0.12 mol L−1, pH 4.8 SAB. Obviously, the stripping peak current signals of Cd(II) and Pb(II) were a notable increasing trend with the negative shift of deposition potential in the potential range of −0.90 to −1.00 V (the maximal peak height). However, the peak current decreases upon changing the potential over −1.20 V (Krolicka et al., 2006 and Liu et al., 2008). An optimum Eacc of −1.00 V was selected for further experiments.
The effect of preconcentration potential on the stripping peak current of 20 ppb Cd(II) and Pb(II) at Au-GN-SeCys/GCE in 0.12 mol L−1, pH 4.8 SAB containing 275 ppb Bi(III).
The influence of accumulation time on the sensitivity of the stripping peak current of 20 ppb Cd(II) and Pb(II) in the range of 5–18 min is shown in Fig. 10. It was found that the stripping peak current of the Cd(II) and Pb(II) at Au-GN-SeCys/GCE increases linearly with increasing accumulation time from 5 to 18 min. In this study an accumulated time of 14 min was chosen, because it gave an adequate sensitivity to determine Cd(II) and Pb(II) in these samples.
The effect of deposition time on the stripping peak current of 20 ppb Cd(II) and Pb(II) at Au-GN-SeCys/GCE in 0.12 mol L−1 pH 4.8 SAB containing 275 ppb Bi(III).
3.4 Analytical performance of the electrochemical sensor
The analytical performances of the prepared Au-GN-SeCys/GCE microsensors were explored by quantifying the standard solutions (WMP8435) with the metal ion concentrations, and the corresponding SWASV waves are presented in Fig. 11 under the optimum conditions. The peak currents of SWASV were proportional to the concentrations of Cd(II) and Pb(II) from 0.5 to 100 ppb, and the detection limit (S/N = 6) was 0.08, and 0.05 ppb for metal ions, respectively. The linear regression equations were ICd(ppb) = 1.2439CCd(ppb) + 13.073 and Ipb(ppb) = 0.5049CPb(ppb) + 11.185, with correlation coefficients of 0.9848 and 0.98, respectively.
SWASV and the calibration curves at Au-GN-SeCys/GCE in 0.12 mol L−1 pH 4.8 SAB containing 275 ppb Bi(III), Cd(II) and Pb(II) with different concentrations, from 1 to 10: 0.5, 5, 10, 15, 20, 25, 30, 50, 75 and 100 ppb. Deposition potential: −1.00 V, deposition time: 14 min, amplitude: 25 mV, increment potential: 0.004 V. Results are presented as mean ± SD (error bar) of sixplicate experiments.
The features demonstrate that the Au-GN-SeCys/GCE microsensors with an excellent analytical performance toward Cd(II) and Pb(II) could be used for efficient determination of two metal ions in WMP8435 and environmental samples. The detection limits for Cd(II) and Pb(II) were calculated to be 0.08 ppb, and 0.05 ppb, respectively, based on a signal-to-noise ratio equal to 6 (S/N = 6). The Cd(II) and Pb(II) detection performance of the proposed sensor was compared with other previously reported bismuth film-modified electrodes and the results are listed in Table 2. In addition, the developed electrochemical sensor had low determination limits and wide linear ranges, which may be attributed to the synergistic effect of graphene, Au NPs and Se-(Methyl) selenocysteine hydrochloride on the deposition of the Cd(II) and Pb(II) ions. DL: detection limit (ppb), LR; Linear range (ppb), PANI-MES: polyaniline-2-mercaptoethanesulfonate, BiBE: bismuth bulk electrode, SWNTs: single-walled carbon nanotubes, Bi-np-SPCP: Bi nano powder–screen printed carbon paste.
Electrode
LR
DL
Interferences
References
Cd(II)
Pb(II)
Cd(II)
Pb(II)
Bi–Nafion–GC
Analytical technique SWASV
2–60
6–80
2.00
2.00
Al(III), As(III), Fe(III), Cu(II) Mn(III), Ni(II), Zn(II)
Georgia (2006)
Bi/Nafion/PANI-MES/GCE
0.1–20
0.1–30
0.040
0.05
K(I), Na(I), Ca(II), Mg(II), Co(II), Ni(II), Zn(II), Ag(I), Fe(II), Fe(III), Cu(II), Hg(II)
Chen (2011)
Bi–Nafion–medical stone–Graphite
2–11
2–12
0.47
0.07
Li(I), K(I), Na(I), Ca(II), Mg(II), Ba(II), Al(III), Mn(II), Zn(II), Co(II), Ni(II), Cu(II), Ag(I)
Hongbo (2011)
BiBE
10–100
10–100
0.054
0.093
Zn(II), Pb(II), Cd(II)
Kristie (2010)
Bi–Nafion–2,2′-bipyridyl–GC
0.1–225
0.2–414
0.12
0.077
NH4 (I), Li(I), Na(I), K(I), Cs(I), Rb(I), Mg(II), Ca(II), Sr(II), Ba(II), Al(III), Ag(I), Mn(II), Ce(IV), Cu(II), Co(II), Ni(II), Fe(II), Fe(III), Hg(II)
Ferenc (2008)
Hg–Bi/SWNTs/GCE
0.5–11
0.5–11
0.076
0.18
Cu(II), Al(III), Ni(II), Fe(III), Sn(II), Ta(V), Cr(III), In(III), Rh(III), Ru(III)
Ruizhuo (2011)
Bi-np-SPCP Bi
–
–
0.15
0.07
–
Gyoung (2007)
Au-GN-Cys/GCE
0.5–40
0.5–40
0.100
0.050
Co(II), Fe(III), Ni(II), Cr(III),Zn(II), Cu(II), In(III), Sn(II)
Lian (2014)
Au-GN-SeCys/GCE
0.5–50
0.5–50
0.080
0.050
Fe(III), Cu(II), Ni(II), Co(II), Ca(II), Zn(II), Cr(III)
This study
3.5 Interferences
The main interferences of iron(III), copper(II), nickel(II), cobalt(II) calcium(II), zinc(II) and chromium(III) solution had been studied on a standard solution containing 20 ppb Cd(II) and Pb(II). Thus, these seven ions were chosen as interfering ions for investigation of the sensor selectivity. The selectivity of the Au-GN-SeCys/GCE was evaluated by SWASV measurements in 20 ppb Cd(II) and Pb(II) solution spiked with 100 fold excess of Fe(III), Cu(II), Ni(II), Co(II), Ca(II), Zn(II) and Cr(III). As shown in Table 3, the relative signal change was calculated from the current with and without 100-fold interfering ion. The absolute values of relative signal changes were varied from 1.463% to 7.978%, which suggests a satisfying selectivity for the simultaneous determination of Cd(II) and Pb(II) ions. In conclusion, the responses of the new electrochemical sensor have been a decrease of 95% after 15 repetitive simultaneous determinations of Cd(II) and Pb(II), and the original responses were retained after 30 repetitive measurements above 90%. It indicates a satisfying durability of the developed sensor.
Interference ions
Peak current (μA)
Relative signal change (%)
Cd(II)
Pb(II)
Cd(II)
Pb(II)
No interference ions
37.59
22.06
Fe(III)
35.11
20.3
6.597499
7.978241
Cu(II)
36.54
20.97
2.793296
4.94107
Ni(II)
37.04
21.21
1.463155
3.853128
Co(II)
36.85
21.39
1.968609
3.037171
Ca(II)
35.14
20.58
6.517691
6.708976
Zn(II)
35.79
21.58
4.788508
2.175884
Cr(III)
36.23
20.88
3.617984
5.349048
3.6 Analysis of environmental samples
The results of Cd(II) and Pb(II) concentrations in groundwater, soil and Alhagi maurorum plant are determined by adsorptive stripping voltammetry and are presented in Fig. 12 and 13. The levels of the mean cadmium concentrations in groundwater, soil and Alhagi maurorum plant of all samples in different location are 0.696 ± 0.0124, 0.516 ± 0.0175, 0.596 ± 0.009, 0.709 ± 0.005, 0.684 ± 0.005 and 0.787 ± 0.007 ppm, respectively, as shown in Fig. 12. The results of the studied lead mean in groundwater, soil and Alhagi maurorum plant are 0.104 ± 0.01, 0.093 ± 0.005, 0.0112 ± 0.012, 0.166 ± 0.0465, 0.138 ± 0.0216 and 0.164 ± 0.021 ppm, respectively, Fig. 13.
Cadmium concentrations (ppm) in groundwater, soil and Alhagi maurorum plant measured by SWASV.
Lead concentrations (ppm) in groundwater, soil and Alhagi maurorum plant measured by SWASV.
In conclusion overall results of biological samples from Sakakah and Arar cities exhibited significantly (p < 0.01) higher values of Cd(II) and Pb(II) in their biological samples than in those of the other areas. Biological sample controls from Zalom exhibited the lowest value of studied heavy metals in their biological samples, which reveals that Zalom environment is free from pollution with heavy metals Cd(II) and Pb(II) contaminations. Biological heavy metal concentrations differ according to several factors; one of them is the living location of the subject, which differs from one area to another and from one city to another in the same country.
The data obtained are well correlated with those obtained by parallel tests using (AAS), and (ICP-AES) for Cd(II) (0.729 ± 0.0079, and 0.714 ± 0.0042 ppm) and Pb(II) (0.09502 ± 0.0217, and 0.09431 ± 0.0204, respectively) in roots of Alhagi maurorum plant samples and are nearly in agreement with those obtained using SWASV of the same elements (0.722 ± 0.004 ppm) for Cd(II) and (0.09473 ± 0.0193 ppm), for Pb(II), respectively. It confirms the application potential of the developed sensor for real water samples.
4 Conclusion
In this work, a novel Au-GN-SeCys nanostructure thin film has been synthesized by using conventional thermal evaporation and the prepared films are characterized as well adherent and nearly a homogeneous film. The Ft-IR spectra obtained for Au-GN-SeCys nanostructure thin film confirm that the materials are nanocrystalline average crystallite size of 50 nm by using Scherrer’s formula. The absorption bands in the UV–vis region are generally interpreted in terms of π to π∗ transitions between bonding and antibonding molecular orbital. The optical constants of Au-GN-SeCys were calculated. The allowed transitions were found to be indirect transitions.
Carbon-based nanocomposite composed of Au-GN-SeCys nanostructure thin film has been successfully synthesized for the simultaneous determination of Cd(II) and Pb(II) in biological samples. The applicability and suitability of Bi/Au-GN-SeCys/GCE for the determination of Cd(II) and Pb(II) were demonstrated. Modifying electrode Bi/Au-GN-SeCys/GCE had played an important role in electrochemical activity, high sensitivity and low detection limit for the determination of trace Cd(II) and Pb(II) in reference material (WMP8435), when, compared with GCE and Au-GN/GCE, Au-GN-Cys/GCE in aqueous solution based on SWASV technique. In addition, the materials involved in this experiment such as gold nanoparticle, graphene and selenocysteine were nontoxic or bio-compatible, which may provide an environment friendly sensor. A novel thin film Bi/ Au-GN-SeCys/GCE technique indicates that Au-GN-SeCys may find its wide application in the determination of trace and heavy elements due to its characteristics such as excellent electric conductivity and obvious synergistic effect for the metal cation deposition.
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