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Reduced graphene oxide decorated with tin nanoparticles through electrodeposition for simultaneous determination of trace heavy metals
Accepted Manuscript Title: Reduced graphene oxide decorated with tin nanoparticles through electrodeposition for simultaneous determination of trace heavy metals Author: Pui Mun Lee Zhong Chen Lin Li Erjia Liu PII: DOI: Reference:
S0013-4686(15)01215-3 http://dx.doi.org/doi:10.1016/j.electacta.2015.05.092 EA 25025
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
7-4-2015 11-5-2015 16-5-2015
Please cite this article as: Pui Mun Lee, Zhong Chen, Lin Li, Erjia Liu, Reduced graphene oxide decorated with tin nanoparticles through electrodeposition for simultaneous determination of trace heavy metals, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.05.092 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Reduced graphene oxide decorated with tin nanoparticles through electrodeposition for simultaneous determination of trace heavy metals
Pui Mun Lee1,2,3,4, Zhong Chen3, Lin Li4, Erjia Liu4†
1
Interdisplinary Graduate School, Nanyang Technological University 50 Nanyang Avenue, Singapore 639798
2
Environmental Chemistry and Materials Group (ECMG), Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University 1 Cleantech Loop, Singapore 637141 3
School of Materials Science and Engineering, Nanyang Technological University 50 Nanyang Avenue, Singapore 639798
4
School of Mechanical and Aerospace Engineering, Nanyang Technological University 50 Nanyang Avenue, Singapore 639798
Highlights
► A simple fabrication method of graphene decorated with tin nanoparticles was developed. ► A mechanism for the formation of graphene-tin nanocomposite was proposed. ► The graphenetin nanocomposite electrode developed was able to detect heavy metals at 10-10 M level concentration. Abstract Reduced graphene oxide (RGO) decorated with tin nanoparticles (SnNPs) was electrodeposited on glassy carbon sheet (GCS), i.e. G-Sn/GCS, with a drop-casting method followed by constant potential electroreduction. Raman spectroscopic analysis on the graphitic structure of the GSn/GCS confirmed a good intercalation of SnNPs in the RGO matrix. Field emission scanning electron microscopic measurements illustrated a uniform distribution of the SnNPs on the RGO sheets. The G-Sn/GCS showed a better electroanalytical performance than the bare GCS and RGO/GCS in the simultaneous determination of divalent cadmium ions (Cd2+), lead ions (Pb2+) and copper ions (Cu2+) using square wave anodic stripping voltammetry. The electroanalytical measurements using the G-Sn/GCS were optimized at -1 V for 150 s in a 0.1 M acetate buffer solution (pH 5). The G-Sn/GCS demonstrated a highly linear behavior in the detection of Cd2+, Pb2+ and Cu2+ in the concentration range of 10 nM to 100 nM with detection limits of 0.63 nM, 0.60 nM and 0.52 nM (S/N=3), respectively. A mechanism for the formation of the G-Sn nanocomposite was proposed in this paper.
Keywords: Graphene-tin nanocomposite; Glassy carbon sheet; Electrodeposition; Trace cadmium, lead and copper; Simultaneous detection
†
Corresponding author:
Tel.: +65-67905504 Fax: +65-67924062 E-mail: [email protected] 1. Introduction Industrial activities, such as mining, construction and metal processing have caused the increasing contamination of natural water by heavy metals. Heavy metals, such as copper (Cu), lead (Pb) and cadmium (Cd) are harmful to human. Hence, detection of these heavy metals at trace level is essential due to their high toxicity to biota. Compared to conventional detection methods, such as atomic absorption spectrometry (AAS), inductive coupled plasma-atomic emission spectrometry (ICP-AES) and inductive coupled plasma-mass spectrometry (ICP-MS), electrochemical methods provide shorter processing time, lower cost and portability [1-3]. Two main electrochemical techniques used in trace metal detection are potentiometry using an ion-selective electrode (ISE) and stripping voltammetry [4]. There are two versions of ISE, i.e., liquid-state electrode and solid-state membrane electrode. A solid-state membrane electrode that is usually constructed with poly(vinyl chloride) (PVC) membrane is comparably easy to implement and stable [5]. Several studies have shown that PVC membrane electrodes that have been modified with ligands can perform satisfactorily in detection of trace mercury ions (Hg2+) [5-8] , lead ions (Pb2+) [3,9] and copper ions (Cu2+) [10,11] in wide ranges of concentrations and pH values within a very short processing time. However, the ISE measurements have performed non-ideally in the solutions with very low analyte concentrations in the presence of interfering ions [4]. Meanwhile, stripping voltammetry can achieve lower
LODs than any other analytical techniques, but it could only perform ideally at specific pH levels [4, 12]. In the past, hanging mercury drop electrodes (HMDEs) have been widely used as working electrodes in electrochemical stripping analysis due to their wide cathodic potential range and renewing ability [13]. However, the toxicity of mercury has restricted the popularity of HMDE. At a later stage, bismuth (Bi) film electrodes have been introduced as a potential replacement of HMDEs because of their good detection ability with lower toxicity [14]. Recently, thin film electrodes have been reported for the usage in the determination of divalent Chromium, Cd and Zinc ions [15,16]. Thin film electrodes are attractive because they are stable, non-toxic and inexpensive. The sensitivity of tin film electrodes is around 10 nM that is fairly low [17]. In the last decade, graphene has become one of the most exciting research subjects because it possesses several extraordinary properties, such as high electrical conductivity, thermal conductivity and chemical inertness [18, 19]. Hence, it has been developed for applications in supercapacitor [20], biosensor [21], fuel cell [22], and electrode for tracing heavy metals [23,24]. Graphene can also be functionalized with metal nanoparticles, such as silver [25], gold [26, 27] and Bi nanoparticles [28] to enhance its sensitivity and stability in detection of trace heavy metals. For example, a reduced graphene oxide (RGO)/Bi nanocomposite electrode can detect as low as 24.9 nM of Cd ions (Cd2+) [28]. In this work, RGO was functionalized with tin nanoparticles (SnNPs) and drop-cast on glassy carbon sheets (GCS) followed by electrochemical reduction to form the G-Sn/GCS electrodes. The G-Sn/GCS electrodes were used for the simultaneous detection of Cd2+, Pb2+ and Cu2+ ions contained in acetate buffer solutions (ABS). The results showed that the G-Sn/GCS
had good stability, high sensitivity and good repeatability in heavy metal tracing. A mechanism for the formation of the G-Sn nanocomposite was proposed to provide a new insight to the reader. 2. Experimental 2.1 Reagents and chemicals All chemicals employed in this work were purchased from Sigma-Aldrich with analytical grade and used without further purification. 0.1 M ABS solutions of different pH were prepared by mixing sodium acetate with glacial acetic acid. A solution containing 0.05 M of tin ions (Sn2+) was prepared by mixing tin (II) chloride dehydrate (SnCl2.2H2O) with 1 M of HCl. Stock solutions containing Cd2+, Pb2+ and Cu2+ of 0.05 M each were prepared by mixing cadmium (II) nitrate, lead (II) nitrate and copper (II) nitrate with deionized (DI) water. A solution containing 0.25 M of sodium chloride (NaCl) was prepared by diluting NaCl in DI water.
2.2 Sample preparation GO was prepared by Hummers’ method [29]. At first, 0.5 mg/ml of the GO was dispersed in DI water, followed by the addition of Sn2+ to make two solutions with the ratios of Sn2+ to GO being 0.1 and 0.3, respectively. The mixture was then bubbled with nitrogen gas for 30 min to disperse and deoxygenate the mixture. The bare GCS samples with a size of 1 cm x 3 cm were polished, followed by washing with acetone, ethanol and DI water for 10 min each sequentially. After that, the GCS samples were covered by a polyimide tape with an only 6 mm diameter area of each sample surface exposed, on which 30 µl of the prepared Sn2+-GO mixture was drop-cast. The GCS samples with
the drop-cast Sn2+-GO mixture were put into a vacuum chamber to dry overnight. Next, the dried Sn2+-GO/GCS samples were electrochemically reduced in the 0.25 M NaCl solution at -1 V for 15 min to form the G-Sn/GCS samples, followed by rinsing with DI water and drying with compressed air. The preparation procedure is schematically shown in Fig. 1. The surface morphology of the samples was measured using field emission scanning electron microscopy (FESEM, JEOL JSM 7600F, JEOL Ltd., Tokyo, Japan) operated with 2 kV. The bonding structure of the samples was analyzed by using Raman spectroscopy (RS, Renishaw 1000, Renishaw, UK) with 633 nm laser light.
2.3 Electrochemical analysis Electrochemical experiments were performed using electrochemical potentiostat (Gamry Interface 1000, Gamry Instruments, USA) with a platinum mesh as the counter electrode, silver/silver chloride (Ag/AgCl) as the reference electrode and the fabricated samples as the working electrode. Square Wave Anodic Stripping Voltammetry (SWASV) measurements were applied to study the performance of the electrodes. The three electrodes were dipped into a 0.1 M ABS (pH 5) containing predetermined concentrations of target Cd2+, Pb2+ and Cu2+ ions. In an SWASV measurement, a preconcentration potential of -1.0 V was applied to the working electrode for 150 s with continuous magnetic stirring. After that, a 30 s quiet time was performed to stabilize the solution. Then, an SWASV scanning was performed from -1 V to 0.2 V. In real sample measurements using the G-Sn/GCS, tap water samples were adjusted to pH 5 using glacial acetic acid and sodium acetate, followed by SWASV measurements. After that, Cd2+,
Pb2+ and Cu2+ of two different concentrations each (i.e. 35 nM and 70 nM) were added into the tap water samples to conduct the recovery tests of the G-Sn/GCS.
3. Results and discussion 3.1
Characterization of electrodes Fig. 2 presents the FESEM micrographs of the G-Sn/GCS, where the SnNPs are
uniformly distributed on the RGO surface with the average particle size of about 50 nm. Furthermore, the wrinkled surface of the RGO increases the total surface area for analytes to react. RS is a powerful tool for characterization of carbon products due to their high Raman intensities of conjugated and double C=C bonds [30]. A spectral range of 1000 to 4000 cm-1 is used to investigate the graphitic structures of the GO/GCS, Sn2+-GO/GCS and G-Sn/GCS. Fig. 3 illustrates the Raman spectra of the GO/GCS, Sn2+-GO/GCS and G-Sn/GCS. The spectra of the Sn2+-GO/GCS and G-Sn/GCS consist of three prominent characteristic peaks that are D band (~1350 cm-1), G band (~ 1582 cm-1) and 2D band (~2700 cm-1). The D band represents structural defects, while the G band indicates sp2 hybridized carbon-based materials. The degree of structural defects could be obtained from the D/G intensity ratio (ID/IG) [31]. Table 1 shows that the ID/IG ratio of the GO slightly increases from about 1.07 to 1.14 after the addition of Sn2+, implying the increased structural defects due to the mildly reduced GO to partially reduced GO (GO’). After electroreduction, ID/IG of the Sn2+-GO/GCS further increases from about 1.14 to 1.69, indicating the further increased structural defects due to the intercalation of the SnNPs in the RGO matrix. Furthermore, the D and G bands are getting narrower from the GO/GCS, Sn2+-
GO/GCS to the G-Sn/GCS, as presented in Table 1, proving the restoration of the graphitic structure [32]. The 2D band is the second-order of the D band, which is very sensitive to the stacking order of the graphene sheets [33]. Usually, single layer graphene and graphite possess a strong 2D peak. However, the presence of the stacking order between graphene planes due to strong interactions between adjacent planes leads to a change in the 2D band from one peak to two peaks [32], as shown in Fig. 3.
3.2 Proposed mechanism When Sn2+ is added into the GO suspension, redox reactions will take place. Sn2+, as a reducing agent, will partially reduce GO to GO’, while Sn2+ will be oxidized to Sn4+, as shown in Eq. 1. As GO’ is mildly reduced, it may still maintain its amphiphilic properties. In addition, HCl, a by-product, will lower the pH and the GO’ dispersion will remain stable at this lowered pH level (Table S1). Hence, GO’ could still disperse uniformly in the solution without agglomeration as shown in Fig. 4. At the same time, Sn4+ are drawn towards GO’ by electrostatic interactions. When electroreduction is performed, GO’ will be fully reduced to RGO. Further, Sn4+ will react with GO’ and be reduced to G-Sn as shown in Eq. 2. SnCl2 + GO+ H2O Sn4+ + GO’
3.3
Sn4+ + GO’ + HCl
G-Sn
Comparison of electrodes
(1) (2)
Fig. 5 exhibits the SWASV responses of Cd2+, Pb2+ and Cu2+ simultaneously on the bare GCS, RGO/GCS and G-Sn/GCS electrodes. The preconditioning process is carried out at -1 V for 150 s in the 0.1 M ABS (pH 5) solution containing 50 nM of Cd2+, Pb2+ and Cu2+ without deaeration. There are very weak peaks measured with the bare GCS and RGO/GCS electrodes, but higher current peaks for the three target metal ions were measured with the G-Sn/GCS. The three peaks corresponding to Cd2+, Pb2+ and Cu2+ can be identified at the potentials of -0.79 V, 0.58 V and -0.09 V, respectively. The enhanced signals from the G-Sn/GCS may be due to its strong absorptive ability towards the three metal ions and the increased total surface area of the electrode. In addition, it may be because of the intercalation between SnNPs and RGO preventing the RGO from agglomeration and hence improving the electric conductivity of the GSn/GCS.
3.4
Optimization of the experimental parameters Fig. 6 shows the effect of Sn2+:GO ratio on the detection of Pb2+. The preconditioning
process is carried out at -1 V for 150 s in the 0.1 M ABS (pH 5) solution containing 50 nM of Pb2+ without deaeration. Curve b, which represents Sn2+:GO ratio of 0.1, shows a distinct and strong peak of Pb2+. However, for curve a, which represents Sn2+:GO ratio of 0.3, a weaker peak current of Pb2+ is observed, which is due to the poor reduction of GO when the Sn2+:GO ratio is high. When the concentration of Sn2+ is increased, the GO is further reduced to the RGO. ID/IG of Sn2+:GO ratio of 0.3 is about 1.396, while ID/IG of Sn2+:GO ratio of 0.1 is about 1.14 (Fig. S1), indicating that a higher concentration of Sn2+ leads to further reduction. As the RGO is hydrophobic, it will not disperse uniformly in the solution. In addition, the by-product HCl
lowers the pH of the solution (Table S1), causing the destabilization of the GO suspension, followed by the aggregation of the GO sheets [34]. Hence, the aggregated GO sheets can be only partially reduced to the RGO, where certain areas of the sample surface remain the brownish GO (Fig. S2). In addition, ID/IG of Sn2+:GO ratio of 0.3 after electroreduction is about 1.47, which is lower than that of Sn2+:GO ratio of 0.1 (Fig. S3), indicating a less effectiveness in electroreduction, which may be due to the aggregated GO sheets. Selection of a proper pH value is important to optimize the performance of the working electrode. Fig. 7 shows the performance of the G-Sn/GCS on the detection of Cd2+, Pb2+ and Cu2+ at different pH levels. From pH 3-5, the SWASV responses on the detection of Cd2+, Pb2+ and Cu2+ are enhanced. It is attributed to hydrogen evolution that reduces the electrode surface reactivity, which, otherwise, would occur at a low pH. On the other hand, when pH is increased to about 6, the SWASV intensity is slightly reduced possibly due to the hydrolysis of metal ions that, otherwise, would happen when pH is increased. Hence, pH 5 is chosen as an optimum pH value. The effect of the preconcentration potential on the detection of trace Cd2+, Pb2+ and Cu2+ is studied in the range of -0.8 V to -1.2 V. As the potential is more negative, more analytes are reduced. Hence, the SWASV current is increasing as shown in Fig. 8. However, after the optimal point, at -1 V, the current intensity drops, which is mainly due to the hydrogen evolution that deteriorates the surface activity of the working electrode. Hence, -1 V is selected as an optimal preconcentration potential. Fig. 9 exhibits the SWASV current responses to Cd2+, Pb2+ and Cu2+ in the range of preconcentration time of 60 s to 240 s. As expected, when the preconcentration time is increased,
the current intensity is enhanced. However, a prolonged preconcentration time tends to cause the surface saturation at a high metal ion concentration. Further, as the preconcentration time is increased up to 180 s, the standard deviation of the current, especially the SWASV response to Pb2+, is higher. Hence, to compromise between high sensitivity, low detection limit and high reproducibility, 150 s is selected as an optimal preconcentration time.
3.5 Electrochemical responses of G-Sn/GCS toward Cd2+, Pb2+ and Cu2+ Fig. 10(a-c) illustrate the SWASV responses measured with the G-Sn/GCS to Cd2+, Pb2+ and Cu2+ individually with the concentrations varying from 10 nM to 100 nM in the 0.1 M ABS (pH 5) at -1 V for 150 s. The peaks obtained are well-defined and show good linear relationships with the concentrations of Cd2+, Pb2+ and Cu2+, respectively (Fig. 10(d)). The linear regression equation for Cd2+ is i(µA) = 289.16[Cd2+](µM) - 0.3279, with the correlation coefficient of 0.9967. The lowest detection limit (LOD) is calculated using the equation C=3Sbl/S, where Sbl is the standard deviation of blank measurements and S refers to the sensitivity of the calibration curves [35. 36]. The LOD of Cd2+ obtained is about 0.63 nM with a relative standard deviation of about 6.1%. The linear regression equations for Pb2+ and Cu2+ are i(µA) = 367.73[Pb2+](µM) – 3.6869 and i(µA) = 145.36[Cu2+](µM) – 0.0752, with the correlation coefficients of 0.9955 and 0.9983, respectively. The LODs of Pb2+ and Cu2+ obtained are about 0.60 nM and 0.52 nM with the relative standard deviations of about 7.4% and 2.5%, respectively. The obtained LODs using the G-Sn/GCS developed in this work are compared with the previously reported results measured using other electrodes modified with carbon nanotubes (CNT), nitrogen doped microporous carbon (NMC), gold nanoparticles (GNPs), RGO, bismuth
(BI) and polyaniline (PANI) with the summary of the comparison illustrated in Table 2. From Table 2, the LODs of Cd2+ and Pb2+ from our work are the lowest though the LOD of Cu2+ is slightly higher than the CNT thread electrode. Hence, the G-Sn/GCS has been proved to be an effective working electrode for heavy metal detection. Fig. 11(a) demonstrates the SWASV responses measured with the G-Sn/GCS for the simultaneous detection of Cd2+, Pb2+ and Cu2+ with the concentrations varying from 10 nM to 100 nM in the 0.1 M ABS (pH 5) at -1 V for 150 s. The stripping peak current shows good linear relationships with respect to the concentrations of Cd2+, Pb2+ and Cu2+ (Fig. 11(b)). The linear regression equations for Cd2+, Pb2+ and Cu2+ are i(μA) = 43.572[Cd2+](μM) - 0.2321, i(μA) = 173[Pb2+](μM) + 2.0638 and i(μA) = 86.217[Cu2+](μM) + 1.5109, with the correlation coefficients of about 0.9967, 0.9967 and 0.9952, respectively. However, compared to the SWASV results from the individual trace metals, the sensitivities of these three simultaneous analytes are decreased significantly. For example, the sensitivity of Pb2+ in the individual analysis is about 367.73 µA/µM that is lowered to about 173 µA/µM in the simultaneous analysis. This is mainly due to the competition of the analytes towards the limited active sites on the electrode. Moreover, there are two current peaks observed at about -0.46 V and -0.05 V, which may be due to the formation of intermetallic compounds such as Pb-Cu [43] and Cd-Cu [25] during the deposition stage, which has suppressed the intensities of the Cd2+, Pb2+ and Cu2+ peaks. Without adding Cu2+ in the solution, the peaks of Cd2+ and Pb2+ are less likely affected, as shown in Fig. 12. Nevertheless, the peak current of Cd2+ remains low, which may be due to its poorer absorptivity on the electrode surface, as compared to Pb2+ and Cu2+. The LODs of Cd2+, Pb2+ and Cu2+ obtained are about 7.56 nM, 6.77 nM and 5.62 nM, with comparably high relative
standard deviations of about 11%, 39% and 16%, respectively. The formation of the intermetallic compounds severely affects the stability and reproducibility of the electrode. Furthermore, in order to investigate the feasibility of the proposed method in real sample application, the proposed electrode was used for the simultaneous detection of the three targeted metals in tap water samples. From the SWASV measurements on the tap water samples using the G-Sn/GCS developed in this work, no stripping peaks of the targeted metals are observed. However, after adding Cd2+, Pb2+ and Cu2+ of two different concentrations each (i.e. 35 nM and 70 nM) into the tap water samples, obvious stripping peaks are observed. The results of the recovery tests summarized in Table 3 indicate that the G-Sn/GCS provides an excellent platform for the simultaneous detection of trace heavy metals in real samples.
Conclusions In this work, G-Sn/GCS electrodes were developed through drop-casting and electroreduction of RGO decorated with SnNPs on GCS for the simultaneous electrochemical detection of Cd2+, Pb2+ and Cu2+. RS analysis illustrated that a good intercalation of the SnNPs in the RGO matrix, with strong interactions between adjacent RGO-SnNPs sheets was achieved. A low Sn2+:GO ratio during fabrication was essential to prevent the aggregation of GO that can possibly disrupt the reduction process of the GO. The modified G-Sn/GCS electrodes presented a highly linear behavior for the voltammetric stripping peak current measurement and a high sensitivity for the detection of Cd2+, Pb2+ and Cu2+ individually and simultaneously. The individual detection showed a good stability with the LODs of Cd2+, Pb2+ and Cu2+ of about 0.63 nM, 0.60 nM and 0.52 nM, respectively. However, the stability of the simultaneous detection
was relatively lower due to the formation of some intermetallic compounds, such as Cd-Cu and Pb-Cu. As a result, the LODs of Cd2+, Pb2+ and Cu2+ were reduced to about 7.56 nM, 6.77 nM and 5.62 nM, respectively, in the simultaneous detection. Nevertheless, the obtained LODs above mentioned were still very well below the values stipulated by the World Health Organization (WHO) [44-46]. In addition, the G-Sn/GCS demonstrated good recoveries of the targeted metals in tap water samples, proving that the G-Sn/GCS developed in this work could be a promising device for the simultaneous electroanalysis of trace heavy metals in real samples including environmental and biological samples.
Acknowledgement P.M. Lee was grateful for the PhD scholarship from the Interdisplinary Graduate School (IGS) and the Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University, Singapore. References [1] E.P. Achterberg, C. Braungardt, Stripping voltammetry for the determination of trace metal speciation and in-situ measurements of trace metal distributions in marine waters, Analytica chimica acta, 400 (1999) 381-397. [2] Z. Wang, H. Guo, E. Liu, G. Yang, N. Khun, Bismuth/polyaniline/glassy carbon electrodes prepared with different protocols for stripping voltammetric determination of trace Cd and Pb in solutions having surfactants, Electroanalysis, 22 (2010) 209-215.
[3] A. Jain, V. Gupta, L. Singh, J. Raisoni, A comparative study of Pb 2+ selective sensors based on derivatized tetrapyrazole and calix [4] arene receptors, Electrochimica Acta, 51 (2006) 25472553. [4] R.J. Brown, M.J. Milton, Analytical techniques for trace element analysis: an overview, TrAC Trends in Analytical Chemistry, 24 (2005) 266-274. [5] V.K. Gupta, S. Chandra, H. Lang, A highly selective mercury electrode based on a diamine donor ligand, Talanta, 66 (2005) 575-580. [6] V.K. Gupta, S. Jain, U. Khurana, A PVC‐based pentathia‐15‐crown‐5 membrane potentiometric sensor for mercury (II), Electroanalysis, 9 (1997) 478-480. [7] V.K. Gupta, B. Sethi, R. Sharma, S. Agarwal, A. Bharti, Mercury selective potentiometric sensor based on low rim functionalized thiacalix [4]-arene as a cationic receptor, Journal of Molecular Liquids, 177 (2013) 114-118. [8] V. Gupta, A. Singh, M. Al Khayat, B. Gupta, Neutral carriers based polymeric membrane electrodes for selective determination of mercury (II), Analytica chimica acta, 590 (2007) 81-90. [9] V. Gupta, A. Jain, P. Kumar, PVC-based membranes of N, N′-dibenzyl-1, 4, 10, 13tetraoxa-7, 16-diazacyclooctadecane as Pb (II)-selective sensor, Sensors and Actuators B: Chemical, 120 (2006) 259-265. [10] V.K. Gupta, R. Prasad, A. Kumar, Preparation of ethambutol–copper (II) complex and fabrication of PVC based membrane potentiometric sensor for copper, Talanta, 60 (2003) 149160.
[11] V.K. Gupta, L. Singh, R. Singh, N. Upadhyay, S. Kaur, B. Sethi, A novel copper (II) selective sensor based on dimethyl 4, 4′(o-phenylene) bis (3-thioallophanate) in PVC matrix, Journal of Molecular Liquids, 174 (2012) 11-16. [12] V.K. Gupta, M.L. Yola, N. Atar, A.O. Solak, L. Uzun, Z. Üstündağ, Electrochemically modified sulfisoxazole nanofilm on glassy carbon for determination of cadmium (II) in water samples, Electrochimica Acta, 105 (2013) 149-156. [13] G. Li, P. Miao, Theoretical Background of Electrochemical Analysis, Electrochemical Analysis of Proteins and Cells, Springer2013, pp. 5-18. [14] C. Kokkinos, A. Economou, I. Raptis, C.E. Efstathiou, T. Speliotis, Novel disposable bismuth-sputtered electrodes for the determination of trace metals by stripping voltammetry, Electrochemistry Communications, 9 (2007) 2795-2800. [15] Y.Q. Tian, N.B. Li, H.Q. Luo, Simultaneous determination of trace zinc (II) and cadmium (II) by differential pulse anodic stripping voltammetry using a MWCNTs–NaDBS modified stannum film electrode, Electroanalysis, 21 (2009) 2584-2589. [16] W.W. Zhu, N.B. Li, H.Q. Luo, Simultaneous determination of chromium (III) and cadmium (II) by differential pulse anodic stripping voltammetry on a stannum film electrode, Talanta, 72 (2007) 1733-1737. [17] B.L. Li, Z.L. Wu, C.H. Xiong, H.Q. Luo, N.B. Li, Anodic stripping voltammetric measurement of trace cadmium at tin-coated carbon paste electrode, Talanta, 88 (2012) 707-710. [18] J.-H. Chen, C. Jang, S. Xiao, M. Ishigami, M.S. Fuhrer, Intrinsic and extrinsic performance limits of graphene devices on SiO2, Nature nanotechnology, 3 (2008) 206-209.
[19] Z.-S. Wu, W. Ren, L. Gao, J. Zhao, Z. Chen, B. Liu, D. Tang, B. Yu, C. Jiang, H.-M. Cheng, Synthesis of graphene sheets with high electrical conductivity and good thermal stability by hydrogen arc discharge exfoliation, Acs Nano, 3 (2009) 411-417. [20] Z. Li, T. Chang, G. Yun, J. Guo, B. Yang, 2D tin dioxide nanoplatelets decorated graphene with enhanced performance supercapacitor, Journal of Alloys and Compounds, 586 (2014) 353359. [21] H.D. Jang, S.K. Kim, H. Chang, J.-W. Choi, J. Huang, Synthesis of graphene based noble metal composites for glucose biosensor, Materials Letters, 106 (2013) 277-280. [22] R.I. Jafri, N. Rajalakshmi, S. Ramaprabhu, Nitrogen doped graphene nanoplatelets as catalyst support for oxygen reduction reaction in proton exchange membrane fuel cell, Journal of Materials Chemistry, 20 (2010) 7114-7117. [23] J. Li, S. Guo, Y. Zhai, E. Wang, High-sensitivity determination of lead and cadmium based on the Nafion-graphene composite film, Analytica chimica acta, 649 (2009) 196-201. [24] Z. Wang, P.M. Lee, E. Liu, Graphene thin film electrodes synthesized by thermally treating co-sputtered nickel–carbon mixed layers for detection of trace lead, cadmium and copper ions in acetate buffer solutions, Thin Solid Films, 544 (2013) 341-347. [25] R.J. Grim, Catalytic Activity of an Intermetallic Compound of Cadmium and Copper in the Vapor-phase Reduction of Nitrobenzene, The Journal of Physical Chemistry, 46 (1942) 464-469. [26] Z. Lu, S. Yang, Q. Yang, S. Luo, C. Liu, Y. Tang, A glassy carbon electrode modified with graphene, gold nanoparticles and chitosan for ultrasensitive determination of lead (II), Microchimica Acta, 180 (2013) 555-562.
[27] P.M. Lee, Z. Wang, X. Liu, Z. Chen, E. Liu, Glassy carbon electrode modified by graphene–gold nanocomposite coating for detection of trace lead ions in acetate buffer solution, Thin Solid Films, 584 (2015) 85-89. [28] P. Sahoo, B. Panigrahy, S. Sahoo, A.K. Satpati, D. Li, D. Bahadur, In situ synthesis and properties of reduced graphene oxide/Bi nanocomposites: As an electroactive material for analysis of heavy metals, Biosensors and Bioelectronics, 43 (2013) 293-296. [29] H.L. Poh, F. Šaněk, A. Ambrosi, G. Zhao, Z. Sofer, M. Pumera, Graphenes prepared by Staudenmaier, Hofmann and Hummers methods with consequent thermal exfoliation exhibit very different electrochemical properties, Nanoscale, 4 (2012) 3515-3522. [30] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud'Homme, I.A. Aksay, R. Car, Raman spectra of graphite oxide and functionalized graphene sheets, Nano letters, 8 (2008) 36-41. [31] C. Zhang, N. Mahmood, H. Yin, F. Liu, Y. Hou, Synthesis of Phosphorus ‐ Doped Graphene and its Multifunctional Applications for Oxygen Reduction Reaction and Lithium Ion Batteries, Advanced Materials, 25 (2013) 4932-4937. [32] M. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L. Cancado, A. Jorio, R. Saito, Studying disorder in graphite-based systems by Raman spectroscopy, Physical Chemistry Chemical Physics, 9 (2007) 1276-1290. [33] P. Lespade, R. Al-Jishi, M. Dresselhaus, Model for Raman scattering from incompletely graphitized carbons, Carbon, 20 (1982) 427-431. [34] H. Wang, Y.H. Hu, Electrolyte-induced precipitation of graphene oxide in its aqueous solution, Journal of colloid and interface science, 391 (2013) 21-27.
[35] D. Pan, L. Zhang, J. Zhuang, T. Yin, W. Qin, A novel tin-bismuth alloy electrode for anodic stripping voltammetric determination of zinc, Microchimica Acta, 177 (2012) 59-66. [36] A. Shrivastava, V.B. Gupta, Methods for the determination of limit of detection and limit of quantitation of the analytical methods, Chronicles of Young Scientists, 2 (2011) 21. [37] D. Zhao, X. Guo, T. Wang, N. Alvarez, V.N. Shanov, W.R. Heineman, Simultaneous Detection of Heavy Metals by Anodic Stripping Voltammetry Using Carbon Nanotube Thread, Electroanalysis, 26 (2014) 488-496. [38] A.M. Beltagi, E.M. Ghoneim, M.M. Ghoneim, Simultaneous determination of cadmium (II), lead (II), copper (II) and mercury (II) by square-wave anodic stripping voltammetry at a montmorillonite-calcium
modified
carbon
paste
electrode,
International
Journal
of
Environmental and Analytical Chemistry, 91 (2011) 17-32. [39] W. Song, L. Zhang, L. Shi, D.-W. Li, Y. Li, Y.-T. Long, Simultaneous determination of cadmium (II), lead (II) and copper (II) by using a screen-printed electrode modified with mercury nano-droplets, Microchimica Acta, 169 (2010) 321-326. [40] L. Xiao, H. Xu, S. Zhou, T. Song, H. Wang, S. Li, W. Gan, Q. Yuan, Simultaneous detection of Cd (II) and Pb (II) by differential pulse anodic stripping voltammetry at a nitrogendoped microporous carbon/Nafion/bismuth-film electrode, Electrochimica Acta, 143 (2014) 143151. [41] E. Tesarova, L. Baldrianova, S.B. Hocevar, I. Svancara, K. Vytras, B. Ogorevc, Anodic stripping voltammetric measurement of trace heavy metals at antimony film carbon paste electrode, Electrochimica Acta, 54 (2009) 1506-1510.
[42] J. Wang, C. Bian, J. Tong, J. Sun, S. Xia, Simultaneous detection of copper, lead and zinc on tin film/gold nanoparticles/gold microelectrode by square wave stripping voltammetry, Electroanalysis, 24 (2012) 1783-1790. [43] D.F. Tibbetts, J. Davis, R.G. Compton, Sonoelectroanalytical detection of lead at a bare copper electrode, Fresenius' journal of analytical chemistry, 368 (2000) 412-414. [44] W.H. Organization, Cadmium in drinking-water: background document for development of WHO guidelines for drinking-water quality, (2004). [45] W.H. Organization, Copper in Drinking Water, Background document for development of WHO Guidelines for Drinking-water Quality, (2004). [46] W.H. Organization, Lead in Drinking Water, Background document for development of WHO Guidelines for Drinking-water Quality, (2011).
Table and figure captions Table 1: ID/IG ratios and D and G band widths of Raman spectra measured from GO/GCS, Sn2+GO/GCS and G-Sn/GCS Table 2: Comparison of the analytical performance of modified electrodes for determination of trace heavy metals Table 3: SWASV results measured from tap water samples using G-Sn/GCS for simultaneous determination of Cd2+, Pb2+ and Cu2+ ions
Fig. 1: Schematic diagram showing preparation procedure of G-Sn/GCS electrode Fig. 2: FESEM micrographs of G-Sn/GCS (Sn2+:GO = 0.1) at two different magnifications Fig. 3: Raman Spectra of GO/GCS, Sn2+-GO/GCS and G-Sn/GCS Fig. 4: Schematic illustration of the electroreduction mechanism of G-Sn nanocomposite Fig. 5: SWASV responses of bare GCS, RGO/GCS and G-Sn/GCS to Cd2+, Pb2+ and Cu2+ in 0.1 M ABS (pH 5) containing Cd2+, Pb2+ and Cu2+ of 50 nM each Fig. 6: SWASV responses of G-Sn/GCSs with Sn2+:GO ratios of (a) 0.3 and (b) 0.1 to Pb2+ in 0.1 M ABS (pH 5) containing 50 nM of Pb2+ Fig. 7: Stripping peak current measured using G-Sn/GCS in 0.1 M ABS containing 50 nM of Cd2+, Pb2+ and Cu2+ each with respect to pH of the solutions Fig. 8: Effect of preconcentration potential on stripping peak current measured using G-Sn/GCS in 0.1 M ABS containing 50 nM of Cd2+, Pb2+ and Cu2+ each Fig. 9: Effect of preconcentration time on stripping peak current measured using G-Sn/GCS in 0.1 M ABS containing 50 nM of Cd2+, Pb2+ and Cu2+ each Fig. 10: (a-c) stripping voltammograms of electroreduced G-Sn/GCSs in individual analyses of (a) Cd2+, (b) Pb2+ and (c) Cu2+ over a concentration range of 10 nM to 100 nM each in 0.1 M ABS of pH 5 at -1 V for 150 s, and (d) their respective calibration curves Fig. 11: (a) Stripping voltammograms of G-Sn/GCSs in simultaneous analysis of Cd2+, Pb2+ and Cu2+ over a concentration range of 10 nM to 100 nM each in 0.1 M ABS of pH 5 at -1 V for 150 s, and (b) their respective calibration curves
Fig. 12: Stripping voltammogram of G-Sn/GCS in simultaneous analysis of Cd2+ and Pb2+ of 150 nM each in 0.1 M ABS of pH 5 at -1 V for 150 s
Table 1
Sample
ID/IG
GO/GCS Sn2+-GO/GCS G-Sn/GCS
1.07 1.14 1.69
D band width (cm-1) 123.0 114.2 86.6
G band width (cm-1) 88.6 75.7 73.8
Table 2
Electrode
Technique
LOD of LOD of LOD of Ref. Cd(II) (nM) Pb(II) (nM) Cu(II) (nM)
RGO/Bi nanocomposite CNT thread Montmorillonite-calcium modified carbon paste electrode Screen printed electrode modified with mercury nano-droplets Nafion/Bi/NMC/GCE Antimony film CPE Sn/GNPs/gold microelectrode Sn CPE Bi/PANI/GCE G-Sn/GCS (this work)
DPASV OSWSV SWASV
Cd(II) (nM) 24.9 1.9 4.8
Pb(II) (nM) 2.65 1.5 1.48
Cu(II) (nM) 409 0.27 11.8
[28] [37] [38]
SWASV
12
23
20
[39]
DPASV SWASV SWSV
13.34 7.12 NA
0.24 0.97 14.48
NA NA 31.47
[40] [41] [42]
SWASV SWASV SWASV
10 1.1 0.63
NA 16.5 0.60
NA NA 0.52
[17] [2]
Table 3
Sample Tap water Cd2+
Target metals Added (nM) Measured (nM) 35 35.6 70 68.9
Recovery (%) 102.2 98.4
Pb2+ Cu2+
35 70 35 70
35.8 70.5 33.4 69.1
101.8 100.1 97.2 98.7
GCS
Sn2+ + GO + HCl
Sn2+ tap
GO
Electroreduction
Drop-cast
Bubbling
Vacuum
GCS
Ag/AgCl
Fig. 1
G-Sn
Pt
[Pb2+]
[Cd2+]
100 nM
100 nM
(a)
(b)
[Cu2+]
[Pb2+] [Cd2+]
100 nM
[Cu2+]
10 nM
(c)
(d)
Fig. 10
[Pb2+]
[Cu2+ 100 nM [Cd2+
(a)
[Pb2+]
[Cu2+ [Cd2+
(b)
Fig. 11
Fig. 12
Fig. 2
Fig. 3
GO
Sn Sn4 Electroreduction RGO
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9
S0013-4686(15)01215-3 http://dx.doi.org/doi:10.1016/j.electacta.2015.05.092 EA 25025
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
7-4-2015 11-5-2015 16-5-2015
Please cite this article as: Pui Mun Lee, Zhong Chen, Lin Li, Erjia Liu, Reduced graphene oxide decorated with tin nanoparticles through electrodeposition for simultaneous determination of trace heavy metals, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.05.092 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Reduced graphene oxide decorated with tin nanoparticles through electrodeposition for simultaneous determination of trace heavy metals
Pui Mun Lee1,2,3,4, Zhong Chen3, Lin Li4, Erjia Liu4†
1
Interdisplinary Graduate School, Nanyang Technological University 50 Nanyang Avenue, Singapore 639798
2
Environmental Chemistry and Materials Group (ECMG), Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University 1 Cleantech Loop, Singapore 637141 3
School of Materials Science and Engineering, Nanyang Technological University 50 Nanyang Avenue, Singapore 639798
4
School of Mechanical and Aerospace Engineering, Nanyang Technological University 50 Nanyang Avenue, Singapore 639798
Highlights
► A simple fabrication method of graphene decorated with tin nanoparticles was developed. ► A mechanism for the formation of graphene-tin nanocomposite was proposed. ► The graphenetin nanocomposite electrode developed was able to detect heavy metals at 10-10 M level concentration. Abstract Reduced graphene oxide (RGO) decorated with tin nanoparticles (SnNPs) was electrodeposited on glassy carbon sheet (GCS), i.e. G-Sn/GCS, with a drop-casting method followed by constant potential electroreduction. Raman spectroscopic analysis on the graphitic structure of the GSn/GCS confirmed a good intercalation of SnNPs in the RGO matrix. Field emission scanning electron microscopic measurements illustrated a uniform distribution of the SnNPs on the RGO sheets. The G-Sn/GCS showed a better electroanalytical performance than the bare GCS and RGO/GCS in the simultaneous determination of divalent cadmium ions (Cd2+), lead ions (Pb2+) and copper ions (Cu2+) using square wave anodic stripping voltammetry. The electroanalytical measurements using the G-Sn/GCS were optimized at -1 V for 150 s in a 0.1 M acetate buffer solution (pH 5). The G-Sn/GCS demonstrated a highly linear behavior in the detection of Cd2+, Pb2+ and Cu2+ in the concentration range of 10 nM to 100 nM with detection limits of 0.63 nM, 0.60 nM and 0.52 nM (S/N=3), respectively. A mechanism for the formation of the G-Sn nanocomposite was proposed in this paper.
Keywords: Graphene-tin nanocomposite; Glassy carbon sheet; Electrodeposition; Trace cadmium, lead and copper; Simultaneous detection
†
Corresponding author:
Tel.: +65-67905504 Fax: +65-67924062 E-mail: [email protected] 1. Introduction Industrial activities, such as mining, construction and metal processing have caused the increasing contamination of natural water by heavy metals. Heavy metals, such as copper (Cu), lead (Pb) and cadmium (Cd) are harmful to human. Hence, detection of these heavy metals at trace level is essential due to their high toxicity to biota. Compared to conventional detection methods, such as atomic absorption spectrometry (AAS), inductive coupled plasma-atomic emission spectrometry (ICP-AES) and inductive coupled plasma-mass spectrometry (ICP-MS), electrochemical methods provide shorter processing time, lower cost and portability [1-3]. Two main electrochemical techniques used in trace metal detection are potentiometry using an ion-selective electrode (ISE) and stripping voltammetry [4]. There are two versions of ISE, i.e., liquid-state electrode and solid-state membrane electrode. A solid-state membrane electrode that is usually constructed with poly(vinyl chloride) (PVC) membrane is comparably easy to implement and stable [5]. Several studies have shown that PVC membrane electrodes that have been modified with ligands can perform satisfactorily in detection of trace mercury ions (Hg2+) [5-8] , lead ions (Pb2+) [3,9] and copper ions (Cu2+) [10,11] in wide ranges of concentrations and pH values within a very short processing time. However, the ISE measurements have performed non-ideally in the solutions with very low analyte concentrations in the presence of interfering ions [4]. Meanwhile, stripping voltammetry can achieve lower
LODs than any other analytical techniques, but it could only perform ideally at specific pH levels [4, 12]. In the past, hanging mercury drop electrodes (HMDEs) have been widely used as working electrodes in electrochemical stripping analysis due to their wide cathodic potential range and renewing ability [13]. However, the toxicity of mercury has restricted the popularity of HMDE. At a later stage, bismuth (Bi) film electrodes have been introduced as a potential replacement of HMDEs because of their good detection ability with lower toxicity [14]. Recently, thin film electrodes have been reported for the usage in the determination of divalent Chromium, Cd and Zinc ions [15,16]. Thin film electrodes are attractive because they are stable, non-toxic and inexpensive. The sensitivity of tin film electrodes is around 10 nM that is fairly low [17]. In the last decade, graphene has become one of the most exciting research subjects because it possesses several extraordinary properties, such as high electrical conductivity, thermal conductivity and chemical inertness [18, 19]. Hence, it has been developed for applications in supercapacitor [20], biosensor [21], fuel cell [22], and electrode for tracing heavy metals [23,24]. Graphene can also be functionalized with metal nanoparticles, such as silver [25], gold [26, 27] and Bi nanoparticles [28] to enhance its sensitivity and stability in detection of trace heavy metals. For example, a reduced graphene oxide (RGO)/Bi nanocomposite electrode can detect as low as 24.9 nM of Cd ions (Cd2+) [28]. In this work, RGO was functionalized with tin nanoparticles (SnNPs) and drop-cast on glassy carbon sheets (GCS) followed by electrochemical reduction to form the G-Sn/GCS electrodes. The G-Sn/GCS electrodes were used for the simultaneous detection of Cd2+, Pb2+ and Cu2+ ions contained in acetate buffer solutions (ABS). The results showed that the G-Sn/GCS
had good stability, high sensitivity and good repeatability in heavy metal tracing. A mechanism for the formation of the G-Sn nanocomposite was proposed to provide a new insight to the reader. 2. Experimental 2.1 Reagents and chemicals All chemicals employed in this work were purchased from Sigma-Aldrich with analytical grade and used without further purification. 0.1 M ABS solutions of different pH were prepared by mixing sodium acetate with glacial acetic acid. A solution containing 0.05 M of tin ions (Sn2+) was prepared by mixing tin (II) chloride dehydrate (SnCl2.2H2O) with 1 M of HCl. Stock solutions containing Cd2+, Pb2+ and Cu2+ of 0.05 M each were prepared by mixing cadmium (II) nitrate, lead (II) nitrate and copper (II) nitrate with deionized (DI) water. A solution containing 0.25 M of sodium chloride (NaCl) was prepared by diluting NaCl in DI water.
2.2 Sample preparation GO was prepared by Hummers’ method [29]. At first, 0.5 mg/ml of the GO was dispersed in DI water, followed by the addition of Sn2+ to make two solutions with the ratios of Sn2+ to GO being 0.1 and 0.3, respectively. The mixture was then bubbled with nitrogen gas for 30 min to disperse and deoxygenate the mixture. The bare GCS samples with a size of 1 cm x 3 cm were polished, followed by washing with acetone, ethanol and DI water for 10 min each sequentially. After that, the GCS samples were covered by a polyimide tape with an only 6 mm diameter area of each sample surface exposed, on which 30 µl of the prepared Sn2+-GO mixture was drop-cast. The GCS samples with
the drop-cast Sn2+-GO mixture were put into a vacuum chamber to dry overnight. Next, the dried Sn2+-GO/GCS samples were electrochemically reduced in the 0.25 M NaCl solution at -1 V for 15 min to form the G-Sn/GCS samples, followed by rinsing with DI water and drying with compressed air. The preparation procedure is schematically shown in Fig. 1. The surface morphology of the samples was measured using field emission scanning electron microscopy (FESEM, JEOL JSM 7600F, JEOL Ltd., Tokyo, Japan) operated with 2 kV. The bonding structure of the samples was analyzed by using Raman spectroscopy (RS, Renishaw 1000, Renishaw, UK) with 633 nm laser light.
2.3 Electrochemical analysis Electrochemical experiments were performed using electrochemical potentiostat (Gamry Interface 1000, Gamry Instruments, USA) with a platinum mesh as the counter electrode, silver/silver chloride (Ag/AgCl) as the reference electrode and the fabricated samples as the working electrode. Square Wave Anodic Stripping Voltammetry (SWASV) measurements were applied to study the performance of the electrodes. The three electrodes were dipped into a 0.1 M ABS (pH 5) containing predetermined concentrations of target Cd2+, Pb2+ and Cu2+ ions. In an SWASV measurement, a preconcentration potential of -1.0 V was applied to the working electrode for 150 s with continuous magnetic stirring. After that, a 30 s quiet time was performed to stabilize the solution. Then, an SWASV scanning was performed from -1 V to 0.2 V. In real sample measurements using the G-Sn/GCS, tap water samples were adjusted to pH 5 using glacial acetic acid and sodium acetate, followed by SWASV measurements. After that, Cd2+,
Pb2+ and Cu2+ of two different concentrations each (i.e. 35 nM and 70 nM) were added into the tap water samples to conduct the recovery tests of the G-Sn/GCS.
3. Results and discussion 3.1
Characterization of electrodes Fig. 2 presents the FESEM micrographs of the G-Sn/GCS, where the SnNPs are
uniformly distributed on the RGO surface with the average particle size of about 50 nm. Furthermore, the wrinkled surface of the RGO increases the total surface area for analytes to react. RS is a powerful tool for characterization of carbon products due to their high Raman intensities of conjugated and double C=C bonds [30]. A spectral range of 1000 to 4000 cm-1 is used to investigate the graphitic structures of the GO/GCS, Sn2+-GO/GCS and G-Sn/GCS. Fig. 3 illustrates the Raman spectra of the GO/GCS, Sn2+-GO/GCS and G-Sn/GCS. The spectra of the Sn2+-GO/GCS and G-Sn/GCS consist of three prominent characteristic peaks that are D band (~1350 cm-1), G band (~ 1582 cm-1) and 2D band (~2700 cm-1). The D band represents structural defects, while the G band indicates sp2 hybridized carbon-based materials. The degree of structural defects could be obtained from the D/G intensity ratio (ID/IG) [31]. Table 1 shows that the ID/IG ratio of the GO slightly increases from about 1.07 to 1.14 after the addition of Sn2+, implying the increased structural defects due to the mildly reduced GO to partially reduced GO (GO’). After electroreduction, ID/IG of the Sn2+-GO/GCS further increases from about 1.14 to 1.69, indicating the further increased structural defects due to the intercalation of the SnNPs in the RGO matrix. Furthermore, the D and G bands are getting narrower from the GO/GCS, Sn2+-
GO/GCS to the G-Sn/GCS, as presented in Table 1, proving the restoration of the graphitic structure [32]. The 2D band is the second-order of the D band, which is very sensitive to the stacking order of the graphene sheets [33]. Usually, single layer graphene and graphite possess a strong 2D peak. However, the presence of the stacking order between graphene planes due to strong interactions between adjacent planes leads to a change in the 2D band from one peak to two peaks [32], as shown in Fig. 3.
3.2 Proposed mechanism When Sn2+ is added into the GO suspension, redox reactions will take place. Sn2+, as a reducing agent, will partially reduce GO to GO’, while Sn2+ will be oxidized to Sn4+, as shown in Eq. 1. As GO’ is mildly reduced, it may still maintain its amphiphilic properties. In addition, HCl, a by-product, will lower the pH and the GO’ dispersion will remain stable at this lowered pH level (Table S1). Hence, GO’ could still disperse uniformly in the solution without agglomeration as shown in Fig. 4. At the same time, Sn4+ are drawn towards GO’ by electrostatic interactions. When electroreduction is performed, GO’ will be fully reduced to RGO. Further, Sn4+ will react with GO’ and be reduced to G-Sn as shown in Eq. 2. SnCl2 + GO+ H2O Sn4+ + GO’
3.3
Sn4+ + GO’ + HCl
G-Sn
Comparison of electrodes
(1) (2)
Fig. 5 exhibits the SWASV responses of Cd2+, Pb2+ and Cu2+ simultaneously on the bare GCS, RGO/GCS and G-Sn/GCS electrodes. The preconditioning process is carried out at -1 V for 150 s in the 0.1 M ABS (pH 5) solution containing 50 nM of Cd2+, Pb2+ and Cu2+ without deaeration. There are very weak peaks measured with the bare GCS and RGO/GCS electrodes, but higher current peaks for the three target metal ions were measured with the G-Sn/GCS. The three peaks corresponding to Cd2+, Pb2+ and Cu2+ can be identified at the potentials of -0.79 V, 0.58 V and -0.09 V, respectively. The enhanced signals from the G-Sn/GCS may be due to its strong absorptive ability towards the three metal ions and the increased total surface area of the electrode. In addition, it may be because of the intercalation between SnNPs and RGO preventing the RGO from agglomeration and hence improving the electric conductivity of the GSn/GCS.
3.4
Optimization of the experimental parameters Fig. 6 shows the effect of Sn2+:GO ratio on the detection of Pb2+. The preconditioning
process is carried out at -1 V for 150 s in the 0.1 M ABS (pH 5) solution containing 50 nM of Pb2+ without deaeration. Curve b, which represents Sn2+:GO ratio of 0.1, shows a distinct and strong peak of Pb2+. However, for curve a, which represents Sn2+:GO ratio of 0.3, a weaker peak current of Pb2+ is observed, which is due to the poor reduction of GO when the Sn2+:GO ratio is high. When the concentration of Sn2+ is increased, the GO is further reduced to the RGO. ID/IG of Sn2+:GO ratio of 0.3 is about 1.396, while ID/IG of Sn2+:GO ratio of 0.1 is about 1.14 (Fig. S1), indicating that a higher concentration of Sn2+ leads to further reduction. As the RGO is hydrophobic, it will not disperse uniformly in the solution. In addition, the by-product HCl
lowers the pH of the solution (Table S1), causing the destabilization of the GO suspension, followed by the aggregation of the GO sheets [34]. Hence, the aggregated GO sheets can be only partially reduced to the RGO, where certain areas of the sample surface remain the brownish GO (Fig. S2). In addition, ID/IG of Sn2+:GO ratio of 0.3 after electroreduction is about 1.47, which is lower than that of Sn2+:GO ratio of 0.1 (Fig. S3), indicating a less effectiveness in electroreduction, which may be due to the aggregated GO sheets. Selection of a proper pH value is important to optimize the performance of the working electrode. Fig. 7 shows the performance of the G-Sn/GCS on the detection of Cd2+, Pb2+ and Cu2+ at different pH levels. From pH 3-5, the SWASV responses on the detection of Cd2+, Pb2+ and Cu2+ are enhanced. It is attributed to hydrogen evolution that reduces the electrode surface reactivity, which, otherwise, would occur at a low pH. On the other hand, when pH is increased to about 6, the SWASV intensity is slightly reduced possibly due to the hydrolysis of metal ions that, otherwise, would happen when pH is increased. Hence, pH 5 is chosen as an optimum pH value. The effect of the preconcentration potential on the detection of trace Cd2+, Pb2+ and Cu2+ is studied in the range of -0.8 V to -1.2 V. As the potential is more negative, more analytes are reduced. Hence, the SWASV current is increasing as shown in Fig. 8. However, after the optimal point, at -1 V, the current intensity drops, which is mainly due to the hydrogen evolution that deteriorates the surface activity of the working electrode. Hence, -1 V is selected as an optimal preconcentration potential. Fig. 9 exhibits the SWASV current responses to Cd2+, Pb2+ and Cu2+ in the range of preconcentration time of 60 s to 240 s. As expected, when the preconcentration time is increased,
the current intensity is enhanced. However, a prolonged preconcentration time tends to cause the surface saturation at a high metal ion concentration. Further, as the preconcentration time is increased up to 180 s, the standard deviation of the current, especially the SWASV response to Pb2+, is higher. Hence, to compromise between high sensitivity, low detection limit and high reproducibility, 150 s is selected as an optimal preconcentration time.
3.5 Electrochemical responses of G-Sn/GCS toward Cd2+, Pb2+ and Cu2+ Fig. 10(a-c) illustrate the SWASV responses measured with the G-Sn/GCS to Cd2+, Pb2+ and Cu2+ individually with the concentrations varying from 10 nM to 100 nM in the 0.1 M ABS (pH 5) at -1 V for 150 s. The peaks obtained are well-defined and show good linear relationships with the concentrations of Cd2+, Pb2+ and Cu2+, respectively (Fig. 10(d)). The linear regression equation for Cd2+ is i(µA) = 289.16[Cd2+](µM) - 0.3279, with the correlation coefficient of 0.9967. The lowest detection limit (LOD) is calculated using the equation C=3Sbl/S, where Sbl is the standard deviation of blank measurements and S refers to the sensitivity of the calibration curves [35. 36]. The LOD of Cd2+ obtained is about 0.63 nM with a relative standard deviation of about 6.1%. The linear regression equations for Pb2+ and Cu2+ are i(µA) = 367.73[Pb2+](µM) – 3.6869 and i(µA) = 145.36[Cu2+](µM) – 0.0752, with the correlation coefficients of 0.9955 and 0.9983, respectively. The LODs of Pb2+ and Cu2+ obtained are about 0.60 nM and 0.52 nM with the relative standard deviations of about 7.4% and 2.5%, respectively. The obtained LODs using the G-Sn/GCS developed in this work are compared with the previously reported results measured using other electrodes modified with carbon nanotubes (CNT), nitrogen doped microporous carbon (NMC), gold nanoparticles (GNPs), RGO, bismuth
(BI) and polyaniline (PANI) with the summary of the comparison illustrated in Table 2. From Table 2, the LODs of Cd2+ and Pb2+ from our work are the lowest though the LOD of Cu2+ is slightly higher than the CNT thread electrode. Hence, the G-Sn/GCS has been proved to be an effective working electrode for heavy metal detection. Fig. 11(a) demonstrates the SWASV responses measured with the G-Sn/GCS for the simultaneous detection of Cd2+, Pb2+ and Cu2+ with the concentrations varying from 10 nM to 100 nM in the 0.1 M ABS (pH 5) at -1 V for 150 s. The stripping peak current shows good linear relationships with respect to the concentrations of Cd2+, Pb2+ and Cu2+ (Fig. 11(b)). The linear regression equations for Cd2+, Pb2+ and Cu2+ are i(μA) = 43.572[Cd2+](μM) - 0.2321, i(μA) = 173[Pb2+](μM) + 2.0638 and i(μA) = 86.217[Cu2+](μM) + 1.5109, with the correlation coefficients of about 0.9967, 0.9967 and 0.9952, respectively. However, compared to the SWASV results from the individual trace metals, the sensitivities of these three simultaneous analytes are decreased significantly. For example, the sensitivity of Pb2+ in the individual analysis is about 367.73 µA/µM that is lowered to about 173 µA/µM in the simultaneous analysis. This is mainly due to the competition of the analytes towards the limited active sites on the electrode. Moreover, there are two current peaks observed at about -0.46 V and -0.05 V, which may be due to the formation of intermetallic compounds such as Pb-Cu [43] and Cd-Cu [25] during the deposition stage, which has suppressed the intensities of the Cd2+, Pb2+ and Cu2+ peaks. Without adding Cu2+ in the solution, the peaks of Cd2+ and Pb2+ are less likely affected, as shown in Fig. 12. Nevertheless, the peak current of Cd2+ remains low, which may be due to its poorer absorptivity on the electrode surface, as compared to Pb2+ and Cu2+. The LODs of Cd2+, Pb2+ and Cu2+ obtained are about 7.56 nM, 6.77 nM and 5.62 nM, with comparably high relative
standard deviations of about 11%, 39% and 16%, respectively. The formation of the intermetallic compounds severely affects the stability and reproducibility of the electrode. Furthermore, in order to investigate the feasibility of the proposed method in real sample application, the proposed electrode was used for the simultaneous detection of the three targeted metals in tap water samples. From the SWASV measurements on the tap water samples using the G-Sn/GCS developed in this work, no stripping peaks of the targeted metals are observed. However, after adding Cd2+, Pb2+ and Cu2+ of two different concentrations each (i.e. 35 nM and 70 nM) into the tap water samples, obvious stripping peaks are observed. The results of the recovery tests summarized in Table 3 indicate that the G-Sn/GCS provides an excellent platform for the simultaneous detection of trace heavy metals in real samples.
Conclusions In this work, G-Sn/GCS electrodes were developed through drop-casting and electroreduction of RGO decorated with SnNPs on GCS for the simultaneous electrochemical detection of Cd2+, Pb2+ and Cu2+. RS analysis illustrated that a good intercalation of the SnNPs in the RGO matrix, with strong interactions between adjacent RGO-SnNPs sheets was achieved. A low Sn2+:GO ratio during fabrication was essential to prevent the aggregation of GO that can possibly disrupt the reduction process of the GO. The modified G-Sn/GCS electrodes presented a highly linear behavior for the voltammetric stripping peak current measurement and a high sensitivity for the detection of Cd2+, Pb2+ and Cu2+ individually and simultaneously. The individual detection showed a good stability with the LODs of Cd2+, Pb2+ and Cu2+ of about 0.63 nM, 0.60 nM and 0.52 nM, respectively. However, the stability of the simultaneous detection
was relatively lower due to the formation of some intermetallic compounds, such as Cd-Cu and Pb-Cu. As a result, the LODs of Cd2+, Pb2+ and Cu2+ were reduced to about 7.56 nM, 6.77 nM and 5.62 nM, respectively, in the simultaneous detection. Nevertheless, the obtained LODs above mentioned were still very well below the values stipulated by the World Health Organization (WHO) [44-46]. In addition, the G-Sn/GCS demonstrated good recoveries of the targeted metals in tap water samples, proving that the G-Sn/GCS developed in this work could be a promising device for the simultaneous electroanalysis of trace heavy metals in real samples including environmental and biological samples.
Acknowledgement P.M. Lee was grateful for the PhD scholarship from the Interdisplinary Graduate School (IGS) and the Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University, Singapore. References [1] E.P. Achterberg, C. Braungardt, Stripping voltammetry for the determination of trace metal speciation and in-situ measurements of trace metal distributions in marine waters, Analytica chimica acta, 400 (1999) 381-397. [2] Z. Wang, H. Guo, E. Liu, G. Yang, N. Khun, Bismuth/polyaniline/glassy carbon electrodes prepared with different protocols for stripping voltammetric determination of trace Cd and Pb in solutions having surfactants, Electroanalysis, 22 (2010) 209-215.
[3] A. Jain, V. Gupta, L. Singh, J. Raisoni, A comparative study of Pb 2+ selective sensors based on derivatized tetrapyrazole and calix [4] arene receptors, Electrochimica Acta, 51 (2006) 25472553. [4] R.J. Brown, M.J. Milton, Analytical techniques for trace element analysis: an overview, TrAC Trends in Analytical Chemistry, 24 (2005) 266-274. [5] V.K. Gupta, S. Chandra, H. Lang, A highly selective mercury electrode based on a diamine donor ligand, Talanta, 66 (2005) 575-580. [6] V.K. Gupta, S. Jain, U. Khurana, A PVC‐based pentathia‐15‐crown‐5 membrane potentiometric sensor for mercury (II), Electroanalysis, 9 (1997) 478-480. [7] V.K. Gupta, B. Sethi, R. Sharma, S. Agarwal, A. Bharti, Mercury selective potentiometric sensor based on low rim functionalized thiacalix [4]-arene as a cationic receptor, Journal of Molecular Liquids, 177 (2013) 114-118. [8] V. Gupta, A. Singh, M. Al Khayat, B. Gupta, Neutral carriers based polymeric membrane electrodes for selective determination of mercury (II), Analytica chimica acta, 590 (2007) 81-90. [9] V. Gupta, A. Jain, P. Kumar, PVC-based membranes of N, N′-dibenzyl-1, 4, 10, 13tetraoxa-7, 16-diazacyclooctadecane as Pb (II)-selective sensor, Sensors and Actuators B: Chemical, 120 (2006) 259-265. [10] V.K. Gupta, R. Prasad, A. Kumar, Preparation of ethambutol–copper (II) complex and fabrication of PVC based membrane potentiometric sensor for copper, Talanta, 60 (2003) 149160.
[11] V.K. Gupta, L. Singh, R. Singh, N. Upadhyay, S. Kaur, B. Sethi, A novel copper (II) selective sensor based on dimethyl 4, 4′(o-phenylene) bis (3-thioallophanate) in PVC matrix, Journal of Molecular Liquids, 174 (2012) 11-16. [12] V.K. Gupta, M.L. Yola, N. Atar, A.O. Solak, L. Uzun, Z. Üstündağ, Electrochemically modified sulfisoxazole nanofilm on glassy carbon for determination of cadmium (II) in water samples, Electrochimica Acta, 105 (2013) 149-156. [13] G. Li, P. Miao, Theoretical Background of Electrochemical Analysis, Electrochemical Analysis of Proteins and Cells, Springer2013, pp. 5-18. [14] C. Kokkinos, A. Economou, I. Raptis, C.E. Efstathiou, T. Speliotis, Novel disposable bismuth-sputtered electrodes for the determination of trace metals by stripping voltammetry, Electrochemistry Communications, 9 (2007) 2795-2800. [15] Y.Q. Tian, N.B. Li, H.Q. Luo, Simultaneous determination of trace zinc (II) and cadmium (II) by differential pulse anodic stripping voltammetry using a MWCNTs–NaDBS modified stannum film electrode, Electroanalysis, 21 (2009) 2584-2589. [16] W.W. Zhu, N.B. Li, H.Q. Luo, Simultaneous determination of chromium (III) and cadmium (II) by differential pulse anodic stripping voltammetry on a stannum film electrode, Talanta, 72 (2007) 1733-1737. [17] B.L. Li, Z.L. Wu, C.H. Xiong, H.Q. Luo, N.B. Li, Anodic stripping voltammetric measurement of trace cadmium at tin-coated carbon paste electrode, Talanta, 88 (2012) 707-710. [18] J.-H. Chen, C. Jang, S. Xiao, M. Ishigami, M.S. Fuhrer, Intrinsic and extrinsic performance limits of graphene devices on SiO2, Nature nanotechnology, 3 (2008) 206-209.
[19] Z.-S. Wu, W. Ren, L. Gao, J. Zhao, Z. Chen, B. Liu, D. Tang, B. Yu, C. Jiang, H.-M. Cheng, Synthesis of graphene sheets with high electrical conductivity and good thermal stability by hydrogen arc discharge exfoliation, Acs Nano, 3 (2009) 411-417. [20] Z. Li, T. Chang, G. Yun, J. Guo, B. Yang, 2D tin dioxide nanoplatelets decorated graphene with enhanced performance supercapacitor, Journal of Alloys and Compounds, 586 (2014) 353359. [21] H.D. Jang, S.K. Kim, H. Chang, J.-W. Choi, J. Huang, Synthesis of graphene based noble metal composites for glucose biosensor, Materials Letters, 106 (2013) 277-280. [22] R.I. Jafri, N. Rajalakshmi, S. Ramaprabhu, Nitrogen doped graphene nanoplatelets as catalyst support for oxygen reduction reaction in proton exchange membrane fuel cell, Journal of Materials Chemistry, 20 (2010) 7114-7117. [23] J. Li, S. Guo, Y. Zhai, E. Wang, High-sensitivity determination of lead and cadmium based on the Nafion-graphene composite film, Analytica chimica acta, 649 (2009) 196-201. [24] Z. Wang, P.M. Lee, E. Liu, Graphene thin film electrodes synthesized by thermally treating co-sputtered nickel–carbon mixed layers for detection of trace lead, cadmium and copper ions in acetate buffer solutions, Thin Solid Films, 544 (2013) 341-347. [25] R.J. Grim, Catalytic Activity of an Intermetallic Compound of Cadmium and Copper in the Vapor-phase Reduction of Nitrobenzene, The Journal of Physical Chemistry, 46 (1942) 464-469. [26] Z. Lu, S. Yang, Q. Yang, S. Luo, C. Liu, Y. Tang, A glassy carbon electrode modified with graphene, gold nanoparticles and chitosan for ultrasensitive determination of lead (II), Microchimica Acta, 180 (2013) 555-562.
[27] P.M. Lee, Z. Wang, X. Liu, Z. Chen, E. Liu, Glassy carbon electrode modified by graphene–gold nanocomposite coating for detection of trace lead ions in acetate buffer solution, Thin Solid Films, 584 (2015) 85-89. [28] P. Sahoo, B. Panigrahy, S. Sahoo, A.K. Satpati, D. Li, D. Bahadur, In situ synthesis and properties of reduced graphene oxide/Bi nanocomposites: As an electroactive material for analysis of heavy metals, Biosensors and Bioelectronics, 43 (2013) 293-296. [29] H.L. Poh, F. Šaněk, A. Ambrosi, G. Zhao, Z. Sofer, M. Pumera, Graphenes prepared by Staudenmaier, Hofmann and Hummers methods with consequent thermal exfoliation exhibit very different electrochemical properties, Nanoscale, 4 (2012) 3515-3522. [30] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud'Homme, I.A. Aksay, R. Car, Raman spectra of graphite oxide and functionalized graphene sheets, Nano letters, 8 (2008) 36-41. [31] C. Zhang, N. Mahmood, H. Yin, F. Liu, Y. Hou, Synthesis of Phosphorus ‐ Doped Graphene and its Multifunctional Applications for Oxygen Reduction Reaction and Lithium Ion Batteries, Advanced Materials, 25 (2013) 4932-4937. [32] M. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L. Cancado, A. Jorio, R. Saito, Studying disorder in graphite-based systems by Raman spectroscopy, Physical Chemistry Chemical Physics, 9 (2007) 1276-1290. [33] P. Lespade, R. Al-Jishi, M. Dresselhaus, Model for Raman scattering from incompletely graphitized carbons, Carbon, 20 (1982) 427-431. [34] H. Wang, Y.H. Hu, Electrolyte-induced precipitation of graphene oxide in its aqueous solution, Journal of colloid and interface science, 391 (2013) 21-27.
[35] D. Pan, L. Zhang, J. Zhuang, T. Yin, W. Qin, A novel tin-bismuth alloy electrode for anodic stripping voltammetric determination of zinc, Microchimica Acta, 177 (2012) 59-66. [36] A. Shrivastava, V.B. Gupta, Methods for the determination of limit of detection and limit of quantitation of the analytical methods, Chronicles of Young Scientists, 2 (2011) 21. [37] D. Zhao, X. Guo, T. Wang, N. Alvarez, V.N. Shanov, W.R. Heineman, Simultaneous Detection of Heavy Metals by Anodic Stripping Voltammetry Using Carbon Nanotube Thread, Electroanalysis, 26 (2014) 488-496. [38] A.M. Beltagi, E.M. Ghoneim, M.M. Ghoneim, Simultaneous determination of cadmium (II), lead (II), copper (II) and mercury (II) by square-wave anodic stripping voltammetry at a montmorillonite-calcium
modified
carbon
paste
electrode,
International
Journal
of
Environmental and Analytical Chemistry, 91 (2011) 17-32. [39] W. Song, L. Zhang, L. Shi, D.-W. Li, Y. Li, Y.-T. Long, Simultaneous determination of cadmium (II), lead (II) and copper (II) by using a screen-printed electrode modified with mercury nano-droplets, Microchimica Acta, 169 (2010) 321-326. [40] L. Xiao, H. Xu, S. Zhou, T. Song, H. Wang, S. Li, W. Gan, Q. Yuan, Simultaneous detection of Cd (II) and Pb (II) by differential pulse anodic stripping voltammetry at a nitrogendoped microporous carbon/Nafion/bismuth-film electrode, Electrochimica Acta, 143 (2014) 143151. [41] E. Tesarova, L. Baldrianova, S.B. Hocevar, I. Svancara, K. Vytras, B. Ogorevc, Anodic stripping voltammetric measurement of trace heavy metals at antimony film carbon paste electrode, Electrochimica Acta, 54 (2009) 1506-1510.
[42] J. Wang, C. Bian, J. Tong, J. Sun, S. Xia, Simultaneous detection of copper, lead and zinc on tin film/gold nanoparticles/gold microelectrode by square wave stripping voltammetry, Electroanalysis, 24 (2012) 1783-1790. [43] D.F. Tibbetts, J. Davis, R.G. Compton, Sonoelectroanalytical detection of lead at a bare copper electrode, Fresenius' journal of analytical chemistry, 368 (2000) 412-414. [44] W.H. Organization, Cadmium in drinking-water: background document for development of WHO guidelines for drinking-water quality, (2004). [45] W.H. Organization, Copper in Drinking Water, Background document for development of WHO Guidelines for Drinking-water Quality, (2004). [46] W.H. Organization, Lead in Drinking Water, Background document for development of WHO Guidelines for Drinking-water Quality, (2011).
Table and figure captions Table 1: ID/IG ratios and D and G band widths of Raman spectra measured from GO/GCS, Sn2+GO/GCS and G-Sn/GCS Table 2: Comparison of the analytical performance of modified electrodes for determination of trace heavy metals Table 3: SWASV results measured from tap water samples using G-Sn/GCS for simultaneous determination of Cd2+, Pb2+ and Cu2+ ions
Fig. 1: Schematic diagram showing preparation procedure of G-Sn/GCS electrode Fig. 2: FESEM micrographs of G-Sn/GCS (Sn2+:GO = 0.1) at two different magnifications Fig. 3: Raman Spectra of GO/GCS, Sn2+-GO/GCS and G-Sn/GCS Fig. 4: Schematic illustration of the electroreduction mechanism of G-Sn nanocomposite Fig. 5: SWASV responses of bare GCS, RGO/GCS and G-Sn/GCS to Cd2+, Pb2+ and Cu2+ in 0.1 M ABS (pH 5) containing Cd2+, Pb2+ and Cu2+ of 50 nM each Fig. 6: SWASV responses of G-Sn/GCSs with Sn2+:GO ratios of (a) 0.3 and (b) 0.1 to Pb2+ in 0.1 M ABS (pH 5) containing 50 nM of Pb2+ Fig. 7: Stripping peak current measured using G-Sn/GCS in 0.1 M ABS containing 50 nM of Cd2+, Pb2+ and Cu2+ each with respect to pH of the solutions Fig. 8: Effect of preconcentration potential on stripping peak current measured using G-Sn/GCS in 0.1 M ABS containing 50 nM of Cd2+, Pb2+ and Cu2+ each Fig. 9: Effect of preconcentration time on stripping peak current measured using G-Sn/GCS in 0.1 M ABS containing 50 nM of Cd2+, Pb2+ and Cu2+ each Fig. 10: (a-c) stripping voltammograms of electroreduced G-Sn/GCSs in individual analyses of (a) Cd2+, (b) Pb2+ and (c) Cu2+ over a concentration range of 10 nM to 100 nM each in 0.1 M ABS of pH 5 at -1 V for 150 s, and (d) their respective calibration curves Fig. 11: (a) Stripping voltammograms of G-Sn/GCSs in simultaneous analysis of Cd2+, Pb2+ and Cu2+ over a concentration range of 10 nM to 100 nM each in 0.1 M ABS of pH 5 at -1 V for 150 s, and (b) their respective calibration curves
Fig. 12: Stripping voltammogram of G-Sn/GCS in simultaneous analysis of Cd2+ and Pb2+ of 150 nM each in 0.1 M ABS of pH 5 at -1 V for 150 s
Table 1
Sample
ID/IG
GO/GCS Sn2+-GO/GCS G-Sn/GCS
1.07 1.14 1.69
D band width (cm-1) 123.0 114.2 86.6
G band width (cm-1) 88.6 75.7 73.8
Table 2
Electrode
Technique
LOD of LOD of LOD of Ref. Cd(II) (nM) Pb(II) (nM) Cu(II) (nM)
RGO/Bi nanocomposite CNT thread Montmorillonite-calcium modified carbon paste electrode Screen printed electrode modified with mercury nano-droplets Nafion/Bi/NMC/GCE Antimony film CPE Sn/GNPs/gold microelectrode Sn CPE Bi/PANI/GCE G-Sn/GCS (this work)
DPASV OSWSV SWASV
Cd(II) (nM) 24.9 1.9 4.8
Pb(II) (nM) 2.65 1.5 1.48
Cu(II) (nM) 409 0.27 11.8
[28] [37] [38]
SWASV
12
23
20
[39]
DPASV SWASV SWSV
13.34 7.12 NA
0.24 0.97 14.48
NA NA 31.47
[40] [41] [42]
SWASV SWASV SWASV
10 1.1 0.63
NA 16.5 0.60
NA NA 0.52
[17] [2]
Table 3
Sample Tap water Cd2+
Target metals Added (nM) Measured (nM) 35 35.6 70 68.9
Recovery (%) 102.2 98.4
Pb2+ Cu2+
35 70 35 70
35.8 70.5 33.4 69.1
101.8 100.1 97.2 98.7
GCS
Sn2+ + GO + HCl
Sn2+ tap
GO
Electroreduction
Drop-cast
Bubbling
Vacuum
GCS
Ag/AgCl
Fig. 1
G-Sn
Pt
[Pb2+]
[Cd2+]
100 nM
100 nM
(a)
(b)
[Cu2+]
[Pb2+] [Cd2+]
100 nM
[Cu2+]
10 nM
(c)
(d)
Fig. 10
[Pb2+]
[Cu2+ 100 nM [Cd2+
(a)
[Pb2+]
[Cu2+ [Cd2+
(b)
Fig. 11
Fig. 12
Fig. 2
Fig. 3
GO
Sn Sn4 Electroreduction RGO
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Fig. 9