EDTA Assisted Highly Selective Detection of As3+ on Au Nanoparticle

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EDTA Assisted Highly Selective Detection of As on Au Nanoparticle Modified Glassy Carbon Electrodes: Facile in situ Electrochemical Characterization of Au Nanoparticles Hsiao-Hua Chen, and Jing-Fang Huang Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 24 Nov 2014 Downloaded from http://pubs.acs.org on November 29, 2014

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Analytical Chemistry

EDTA Assisted Highly Selective Detection of As3+ on Au Nanoparticle Modified Glassy Carbon Electrodes: Facile in situ Electrochemical Characterization of Au Nanoparticles Hsiao-Hua Chen and Jing-Fang Huang* Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, R.O.C. ABSTRACT: A facile electrochemical characterization technique of Au nanoparticles (AuNPs) developed by Wang et al.1 was used to track the variation in the particle size and density of AuNPs in situ and to assist in optimizing the conditions of analysis and catalysis. In this method, the combination of total surface area determined by Pb underpotential deposition (UPD) and the amount of Au obtained by anodic stripping of Au in HCl solution was used to evaluate the average diameter of AuNPs and the number of particles on the electrode. The detection of As3+ in aqueous solution by a AuNP modified glassy carbon electrode (Aunano@GCE) using the electrochemical characterization technique was examined. The AuNPs with a uniform shape and size, deposited onto the GCEs using multiple-scan cyclic voltammetry (MSCV), were suitable for the electrochemical evaluation. The calibration curve for the detection of As3+ had a dynamic range of 0.1-15.0 µg L−1 (from 1.30 to 200 nM, y = 0.21x (in µA L µg−1) + 0.01 (R2 = 0.999)) and showed a sensitivity of 0.21 µA L µg−1 (16.15 µA µM−1). Detection limit as low as 0.0025 µg L−1 (32.5 pM) was achieved. The chelating agent ethylenediaminetetraacetate (EDTA) selectively chelated with the interfering metal ions and effectively inhibited the interfering ions from competing with the ion of interest (As3+), in the pre-concentration process. The presence of EDTA effectively eliminated interference from several metal ions, especially Cu2+ and Hg2+. This method was validated by analyzing the As3+ content in real water samples.

Au nanoparticles (AuNPs) have recently attracted considerable scientific attention, since they have excellent sizedependent electrical, optical, and electronic properties. They have been extensively investigated for practical applications in many fields such as catalysis, biosensors, nanodevices, and electroanalysis.2 The characterization of their composition, size, and surface morphology, which determine their catalytic activities and properties in target applications, is an important process in the synthesis of nanosized materials. Typical techniques employed for the characterization of nanoparticles include scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning tunneling microscopy (STM), and X-ray diffraction (XRD).3 These techniques are in general limited to the instantaneous tracking of the as-synthesized nanoparticles, owing to their inefficiency in terms of cost and time. Besides, it is also difficult to represent the average properties of entire samples based on the data obtained from these characterization techniques. Recently, Compton’s group proposed a simple and fast in situ electrochemical method for the characterization of nanoparticles.1 In this technique, the average diameter and the number of AuNPs could be directly evaluated from the total surface area and the amount of AuNPs. This approach has been applied to characterize the “nanosize effects” in electrocatalytic reactions, including the electro-oxidation of Lascorbic acid and nitrite and the electro-reduction of oxygen.4 In the field of electroanalysis, the use of nanoparticle-modified electrodes or nanosized electrodes presents many advantages

including enhanced diffusion of electroactive species, improved selectivity, improved catalytic activity, higher signalto-noise ratio (S/N), and unique optical properties.5 These factors are significant in the development of a facile in situ method for the evaluation of as-prepared electrode materials for electroanalysis, since the sample preparation restrictions for some typical techniques prevent the instantaneous observation of modified electrodes. In this study, the electrochemical characterization technique for AuNPs described above was extended to allow the examination of AuNPs in electroanalysis. Arsenic is a toxic substance and inorganic arsenic contamination in drinking water is a serious worldwide threat to human health. As3+ and arsenate (As5+) are the inorganic forms of As found in groundwater and are more toxic than the organic forms, with As3+ being the most toxic.6 It has been reported that the pollution of groundwater from arsenic has caused adverse health effects in 20 countries. In these countries, the arsenic levels in drinking water are above the provisional guidelines of 10 µg L−1 (10 ppb) for drinkable water prescribed by the World Health Organization (WHO).7 Square-wave anodic stripping voltammetry (SWASV) employed for the detection of trace As3+ with a AuNPs coated glassy carbon electrode (Aunano@GCE) was selected as a model electroanalytical system. The electrochemical method for the detection of trace heavy metals has attracted considerable attention because of its low cost, ease of operation, good sensitivity, and high reproducibility.6b,8 It has been reported that Au and Au-nanoparticle-modified electrodes

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provide a sensitive anodic current response for As detection.9 A major problem in typical electrochemical methods is interference from other metal ions present in natural waters.10 In this study, it was found that the presence of EDTA effectively eliminated interference from several metal ions, particularly Cu2+ and Hg2+, generally considered to be major interferents in the electroanalysis of As3+.

EXPERIMENTAL SECTION Chemicals. As2O3 99.995% (Aldrich), HAuCl4·3H2O 99.99% (Alfa), Cu(NO3)2 99.7% (JT-Baker), Pb(NO3)2 99.7% (JT-Baker), NaOH (JT-Baker), HCl (Aldrich), and H2SO4 (Aldrich) were used without purification. All the solutions were prepared in deionized water (with a specific resistivity of 18.2 MΩ cm). A stock solution with an As3+ concentration of 1000 mg L−1 was prepared in NaOH by dissolving the required quantity of As2O3. The pH was adjusted to 5.0 using concentrated HCl. A 0.1 M phosphate buffer (PB; NaH2PO4/H3PO4, pH 5) containing 0.01 M EDTA stock solution was prepared daily. Electrode Preparation and Modification. The electrochemical experiments were conducted using a CHI 660C potentiostat/galvanostat. A three-electrode electrochemical cell was used for all the electrochemical experiments. A saturated calomel electrode (SCE) and Pt wire were used as the reference and counter electrodes, respectively. Prior to each electrode modification, a glassy carbon electrode (GCE) was successively polished with 1.0, 0.3, and 0.05 µm alumina powder cloths (Buchler), sonicated in deionized water, and dried before use. Next, a Aunano@GCE electrode was obtained by multiple-scan cyclic voltammetry (CV) between 1.5 V and -0.25 V (vs. SCE) in a 0.5 M H2SO4 solution containing 1.0 mM AuCl4- at a scan rate of 100 mV s−1. The Au loading was controlled by controlling the CV sweeping cycles. AuNPs Characterization. The surface morphology of the Aunano@GCEs was observed using the JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM). The as-prepared working electrodes (Aunano@GCEs) were electrochemically cleaned by potential cycling between 0.0 V and 1.5 V vs. SCE, 10 times in an Ar-purged 0.5 M H2SO4. The electrochemical surface area (ECSA) (or microscopic surface area, Am) was estimated from the integrated reduction current of gold oxide and by using a conversion factor of 390 µC cm−2.11 The roughness factor (Rf) and the specific ECSA (s-ECSA: ECSA/Au loading, m2 g−1) were directly calculated from the value of ECSA. Rf is the ratio of the microscopic surface area (Am) and the geometric area (Ag) of the polished GCE.11 The CV of Pb underpotential deposition (Pb UPD) on Aunano@GCEs was carried out in a 0.1 M NaOH solution containing 1.0 mM Pb(NO3)2 at a scan rate of 50 mV s-1 in the potential range of -0.20 V and -0.80 V (vs. SCE).12 The charges corresponding to the anodic stripping of Au deposits from the Aunano@GCEs in 0.1 M HCl were used to determine the Au loading.13 The stripping experiments on the Aunano@GCEs were conducted in 0.1 M HCl, in the range of 0.70 V to 1.30 V (vs. SCE) at a scan rate of 10 mV s-1. Electrochemical Measurements. Aunano@GCE was used as the working electrode for As3+ determination. The as-prepared Aunano@GCE was dipped in a stirred analyte solution containing As3+ and kept at −0.6 V vs. SCE for the time required for pre-concentration (120 s). Quantitative determinations were

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then conducted using square-wave voltammetry (SWV). The potential was scanned in the anodic direction from −0.6 V to 0.7 V (vs. SCE). A 0.1 M PB buffer (pH = 5.0) solution containing 0.01 M EDTA that was purged with Ar for 5 min was used as electrolyte for the SWV experiments. After recording the voltammograms, the electrode was regenerated by treating in a 0.1 M PB buffer solution containing 0.01 M EDTA at 0.6 V vs. SCE for 30 s. The regenerated electrode was then checked in a bare supporting electrolyte for any memorized signals, prior to conducting the next measurement. Groundwater and drinking water were collected from the middle areas of Taiwan and the campus of National Chung-Hsing University in Taiwan, respectively, and stored in pre-cleaned polypropylene bottles after filtration. The standard addition method was used to evaluate the As3+ content in the water samples.

RESULTS AND DISCUSSION Preparation of Aunano@GCEs. In Figure 1A, the multiplescan cyclic voltammetry (MSCV) of Au3+ in a 0.5 M H2SO4 solution containing 1.0 mM AuCl4- recorded on a GCE, is presented. The electrode potential was initially scanned in the negative direction starting from +1.0 V vs. SCE, at a scan rate of 100 mV/s. In the first MSCV cycle, two reduction waves, c1 and c1’, were observed in the forward potential scan from +1.0 to −0.25 V, and one oxidation wave, a1, appeared in the reverse potential scan. The first and second reduction waves, c1 and c1’, are considered to arise from the reduction of AuCl4− to AuCl2− and the further reduction of AuCl2− to Au metal,14 respectively. The oxidation wave a1 observed during the reverse scan indicated the reoxidation of Au electrodeposits. Besides, a current loop was observed in the first cycle, which disappeared during the subsequent potential cycles. This phenomenon indicates that higher overpotential and nucleation processes are needed for the electrodeposition of Au at a GCE.15 The area of the anodic stripping wave a1 was also found to be smaller than its reduction counterpart. These features imply that the Au electrodeposits cannot be completely reoxidized in each reverse scan. After the first cycle, the Au3+ reduction wave shifted to more positive potentials in the subsequent cycles, implying that once Au deposits are formed on the electrode surface, a much lower potential is needed for additional Au electrodeposition. The amount of Au deposits remaining on the electrode are likely to increase with continued potential scanning cycles, owing to the continuous growth of the new pair of redox waves (a2/c2). The anodic peak a2 at 1.2 V occurred on the second cycle and gradually increased with an increase in the number of scanning cycles. This indicates the formation of Au “oxides” (AuOx), resulting from the sorption of OH- ions onto different crystallographic faces, through a very complicated mechanism.11 On the reverse scan, the cathodic peak c2 at 0.89 V corresponds to the subsequent reduction of the previously formed oxides. Besides, the charge corresponding to the anodic stripping of Au deposits from a Aunano@GCE in 0.1 M HCl was also used to determine the Au loading (Figure 1B). Recently, Zhou et al. have demonstrated that the anodic stripping charge is quantitatively related to the Au loading, with 1.9 ± 0.1 electrons transferred per Au atom.13 Linear scan stripping voltammograms acquired on Aunano@GCEs with different Au loadings in 0.1 M HCl aqueous solutions are shown in Figure 1B. These voltammograms were obtained by anodic scanning from 0.5 V to 1.3 V (vs. SCE) and each of these voltammograms exhibit an oxidation

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E (V vs. SCE) Figure 1. (A) MSCV of Au3+ recorded on a GCE in 0.5 M H2SO4 solution containing 1.0 mM AuCl4- at 100 mV s-1. (B) Linear scan stripping voltammograms for Aunano@GCEs with different Au loading in 0.1 M HCl aqueous solutions at 10 mV s-1. Inset is Au loading on Aunano@GCEs evaluated from the anodic stripping charge of Au deposition vs. the number of CV sweeping cycles. (C) SEM image of Aunano@GCE (Au loading of 7.71 µg cm−2) obtained by MSCV. (D) CVs of Aunano@GCEs with different Au loadings, recorded in 0.5 M H2SO4 at 50 mV s-1. Inset is Rf (○) and s-ECSA (●) of Aunano@GCEs vs. the number of CV sweeping cycles in MSCV.

wave resulting from the anodic stripping of Au deposits from Aunano@GCEs. The fact that the stripping charge increases with the number of CV scanning cycles in MSCV and no signal is observed on a bare GCE, indicate the stripping charge is entirely due to the oxidation of Au deposits and the Au loading is controlled by the number of CV scanning cycles. A SEM image of the Aunano@GCEs obtained by MSCV is shown in Figure 1C. The deposited AuNPs appear to be spherical and uniformly distributed, with a size distribution in the range of 20-40 nm. This suggests that the Aunano@GCE could be fabricated easily by MSCV. Aunano@GCE Characterization. The ECSA is an important factor that needs to be determined for potential catalytic and electroanalytic applications. The CVs of Aunano@GCEs with various Au loadings recorded in 0.2 M H2SO4 at a scan rate of 50 mVs-1 in the potential range of 1.5 V to 0.0 V (vs. SCE), are shown in Figure 1D. These voltammograms show current peaks resulting from the formation of surface gold oxides in the anodic scan and subsequent gold oxide reduction in the cathodic scan. The ECSA was evaluated from the reduction charge of the Au oxide.11 The results indicated that the ECSA increased with an increase in the Au loading, owing to the increased reduction charge of the Au oxide. In order to gain an insight into the ECSA of Aunano@GCEs, the Rf and the s-ECSA can also be directly calculated from the ECSA. These two factors are significant in determining the surface roughness and catalytic activity of the electrode materials (inset of

Figure 1D). Besides the high analytical signal for sensitive response, a reduction in the background noise is crucial to effectively improve the analytical performance in sensing applications. Although higher values of Rf and s-ECSA directly enhance the analytical signal during electroanalysis, the high background noise from the capacitive current is a serious hindrance for very rough electrode surfaces. While the Rf of Aunano@GCEs gradually increased with the number of CV sweeping cycles in MSCV, the largest value of Rf was still lower than 1.0. The largest s-ECSA value (12.8 m2 g-1) was quickly reached at about 10 cycles (Au loading of 0.14 µg cm2 ) and then began to decrease with an increase in the number of CV sweeping cycles. These results indicate that the Aunano@GCE not only has a high s-ECSA, but also has a low surface roughness due to the ultra-low Au loadings. SEM was employed for the morphological observation of AuNPs obtained in two solutions containing 0.2 mM and 1.0 mM AuCl4- (Figure S-1 and Table S-1). The results show the deposited AuNPs always appear spherical or hemispherical and are uniformly distributed, regardless of the concentration of AuCl4-. Histograms of particle diameter were constructed by measuring the diameter of 200-1000 particles, in each SEM image of AuNPs obtained from MSCV (Figure 2A and 2B). The average particle size increased as a function of the number of CV sweeping cycles. It is apparent that the particle growth rate is higher in high concentration AuCl4- solution (1.0 mM) compared to that in low concentration AuCl4- solution (0.2 mM). These results demonstrate that MSCV is an appropriate



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Figure 2. Histograms of particle size constructed by counting 200-1000 particles in SEM images of Aunano@GCEs (in Figure S-1), obtained by MSCV in 0.5 M H2SO4 solutions containing (A) 0.2 mM AuCl4- and (B) 1.0 mM AuCl4-. (C) CVs of Pb UPD on a GCE and Aunano@GCEs with various Au loadings, recorded in 0.1 M NaOH solution containing 1.0 mM Pb(NO3)2 at 50 mV s-1. (D) Average diameter and density of AuNPs evaluated from the combination of the total surface area and amount of AuNPs on Aunano@GCEs with various Au loadings obtained from MSCV in two 0.5 M H2SO4 solutions containing 0.2 mM and 1.0 mM of AuCl4-.

method to prepare Aunano@GCEs with uniform size and shape of AuNPs. The combination of the total surface area (S) determined by Pb UPD and the amount of Au (Q) obtained by anodic stripping of Au in HCl solution has been used to evaluate the average size of AuNPs and the number of particles on the electrode.1 The UPD phenomenon generally involves monolayer or sub-monolayer formation of various metals on foreign metal substrates, at potentials that are anodic relative to the potential for the reversible electrodeposition of the bulk metal.16 Assuming that the AuNPs on the electrode are uniformly distributed and are hemispherical or spherical in shape, the surface area and the amount of Au can be easily calculated from equations (1) and (2), respectively1 = π × (1)

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The number of particles, N, can then be obtained from the average radius. This is a facile in situ approach for the determination of two significant properties, namely particle size and density, which are useful evaluation in the applications of

nanoparticles. However, the uniform shape and distribution of the nanoparticles could significantly affect the accuracy and precision of evaluation of particle size and density. The SEM images shown in Figure S-2 indicate that the uniform morphology of AuNPs obtained from MSCV could render it suitable for electrochemical evaluation. Pb UPD on Au in alkaline solutions have been proved to be suitable probes for the estimation of the surface area and the surface structure of AuNPs.12 Typical CVs of Pb UPD on a bare GCE and those of Aunano@GCEs with various Au loadings obtained from MSCV recorded in a 0.1 M NaOH solution containing 1.0 mM Pb(NO3)2, are shown in Figure 2C. The voltammograms exhibit well-known redox peaks between -0.10 V and -0.8 V, which correspond to Pb UPD/stripping on different crystallographic facets of AuNPs at different potentials. In comparison, no signal was observed on a bare GCE under the same experimental conditions. Figure 2C shows that the charge corresponding to Pb deposition/stripping clearly increases with an increase in the Au loading. The surface area of AuNPs can be determined from the oxidative charge of the anodic stripping peaks. The peak at a potential of -0.56 V (vs. SCE) is associated with the Au(111) domain and the Pb UPD charge is 380 µC cm-2. Au(110) and Au(100) crystal planes contribute to a broader stripping peak at -0.35 V (vs. SCE). Further, the charges associated with Pb UPD on the Au(110) and Au(100) crystal planes are 330 µC cm-2 and 340 µC cm-2, respectively.12-13,17 The average diameter and density of AuNPs have been calculated per the above method for Aunano@GCEs with various Au loadings obtained from MSCV


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increase in the number of CV sweeping cycles. The enlargement and aggregation of AuNPs also cause a decrease in the particle density and s-ECSA of the AuNPs. However, the larger size of AuNPs could significantly roughen the electrode surface. These results demonstrate that the electrochemical characterization of AuNPs provides a facile procedure to rapidly track the properties of AuNPs, which would assist in optimizing the conditions for analytical and catalytic applications.

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Detection of As3+. The usefulness of the optimized Aunano@GCE for the detection of As3+ in aqueous solutions was tested next. The electrochemical response of bare GCE and a Aunano@GCE (with a Au loading of 7.71 µg cm−2 obtained from MSCV) towards As3+ was examined using CVs recorded in a 0.1 M PB buffer solution containing 4.0 mg L−1 of As3+ (pH 5.0) (Figure 3A). It is clear that As3+ was detected with high sensitivity and selectivity, only on the Aunano@GCE and no response was detected on the GCE. The broad cathodic peak observed at −0.37 V (vs. SCE) is assigned to the reduction of As3+ to As, whereas the sharp symmetric anodic stripping peak observed at −0.11 V (vs. SCE) during the reverse positive sweep corresponds to the reoxidation of As to watersoluble As3+. The detection sensitivity was further improved by using the more sensitive SWASV technique (Figure 3B). The detection of As3+ using SWASV involves two steps,

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in two 0.5 M H2SO4 solutions containing 0.2 mM and 1.0 mM AuCl4- (Figure 2D and Table S-1). The average diameter of AuNPs increased with the number of CV sweeping cycles. The average diameter and the growth rate of AuNPs in solutions with high AuCl4- concentrations (1.0 mM) are higher than those in solutions with low AuCl4- concentrations (0.2 mM). The average diameter of AuNPs (40-250 nm) prepared from 1.0 mM AuCl4- solution is also larger than that prepared from 0.2 mM AuCl4- solution (17-50 nm). Irrespective of the concentration of AuCl4-, the electrochemically determined particle diameters and growth rates are consistent with the estimates from the SEM images (Figure S-2). An increase in the particle density as a function of the number of CV sweeping cycles can be noticed up to 10 CV sweeping cycles, beyond which the particle density sharply decreases up to 20 cycles and gradually and continually decreases up to 80 cycles. Interestingly, the particle diameter and density trends as a function of the number of CV sweeping cycles, are very close to those of Rf and s-ECSA of Aunano@GCEs, shown in Figure 1D. Since the particle size is small, the AuNPs are expected to show higher s-ECSA. Then, the AuNPs gradually aggregate with other particles on the electrode surface and grow with an

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E (V vs. SCE) Figure 3. (A) CVs of As3+ recorded on a GCE and a Aunano@GCE (Au loading of 7.71 µg cm−2 obtained from MSCV) in a 0.1 M PB buffer solution containing 4.0 mg L−1 of As3+ at 50 mV s-1 (pH 5.0). (B) Typical SWASVs for the detection of As3+ on a GCE and Aunano@GCEs with various Au loadings obtained from MSCV in 0.1 M PB aqueous solution containing 0.01 M EDTA and 100.0 µg L−1 As3+ with 120 s of pre-concentration time. Edep is −0.6 V. Inset: The SWASV peak current, ip, for solution containing 100.0 µg L−1 of As3+, background signal (capacitive current), and the S/N ratio vs. number of CV sweeping cycles in the MSCV.

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E ( V vs. SCE) Figure 4. SWASVs obtained at a Aunano@GCE (Au loading of 7.71 µg cm−2 from the MSCV) in 0.1 M PB solution containing various concentrations of As3+ for 120 s of pre-concentration time, in the presence of 10.0 µg L−1 interfering metal ions. Edep is −0.6 V. (A) Cu2+ and (B) Hg2+, (Upper SWASVs) without EDTA, (Lower SWASVs) with 0.01 M EDTA. The pH in solution was 5.



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Figure 5. (A) Typical SWASVs for the detection of As3+ on a Aunano@GCE (Au loading of 7.71 µg cm−2 from the MSCV) in 0.1 M PB aqueous solution with 0.01 M EDTA containing 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 4.0, 6.0, 8.0, 12.0, and 15.0 µg L−1 of As3+ for 120 s of preconcentration time. Edep is −0.6 V. Inset: calibration curve of the SWASV peak current vs. the concentration of As3+, from 0.1-15.0 µg L−1. Calibration curves showing SWASV peak current versus the concentration of As3+, from 0.1-15.0 µg L−1, recorded in the presence of (B) 10.0 µg L−1, (C) 50.0 µg L−1, and (D) 100.0 µg L−1 of interfering metal ions (Cu2+ and Hg2+).

namely the cathodic deposition of As at an optimized potential (Edep, −0.6 V) for a duration of 120 s and anodic stripping of the deposited As. The peak currents were compared among bare GCE and Aunano@GCEs with various Au loadings. As shown in Figure 3B, in the case of a solution containing 100.0 µg L−1 of As3+, a SWASV peak was selectively observed with Aunano@GCEs and the peak current increased as a function of Au loading. However, it was observed that the increase in Au loading not only enhances the SWASV peak current corresponding to As3+, but also gradually causes an increase in the background noise (capacitive current). The SWASV peak current corresponding to As3+ sharply increased with an increase in the number of CV sweeping cycles from 5 to 40 and then gradually leveled off (inset of Figure 3B). The size and density of AuNPs are two major factors that may cause these results. In Figure 2D, the diameter of AuNPs obtained from a 1.0 mM AuCl4- solution ranges from 38 nm to 118 nm. Although smaller nanoparticle sizes should exhibit larger s-ECSA and activities in catalytic applications, 50-120 nm sized AuNPs have been demonstrated to be more stable and exhibit superior performance in the detection of As3+.5 A higher concentration of AuCl4- was selected for the preparation of Aunano@GCEs. Besides, the density of AuNPs also decreased after 40 CV sweeping cycles, as shown in Figure 2D. A reduction in the density of the AuNPs could further limit the increase in the SWASV peak current for the detection of As3+. The continual increase of background noise may be directly caused by the increase in Rf of the Aunano@GCEs with an increase in the Au loading. The optimum S/N ratio shown in Figure 3B was ob-

served on the Aunano@GCE obtained with 40 CV sweeping cycles. Elimination of Interferences. Cu2+ and Hg2+ are generally considered to be major interfering metal ions in the typical voltammetric determination of As3+. Severe interference caused by these metal ions is clearly observed in Figure 4. The anodic stripping peak corresponding to As was completely replaced by the much stronger signal from the oxidation of Cu and Hg, as a result of the formation of a metal alloy or amalgam during the accumulation of As. It has been reported that the complexation constant of EDTA with these interfering metal ions is much larger than that with As3+.18 In our previous studies, EDTA was used to selectively chelate with several interfering metal ions, forming bulky complexes or bulky anions that were excluded from the Nafion modified layer.9b,19 Interestingly, in the absence of a modified layer in this study, the voltammograms obtained after the addition of 0.01 M EDTA show the obvious reappearance of the peak at −0.08 V corresponding to the anodic oxidation of As. The signals from the interfering metal ions were almost entirely inhibited by the addition of EDTA. Further, the addition of EDTA effectively increased the selectivity in the detection of As3+, even in the absence of the modified layer. In order to further understand the possible mechanism of EDTA-assisted selective As3+ detection by Aunano@GCEs, CVs of Cu2+, Hg2+, and As3+ were recorded in a 0.1 M PB buffer solution containing 4.0 mg L−1 of individual metal ions (pH 5.0) (Figure S-2). After the gradual addition of EDTA into these solutions, redox peak potentials for the cathodic deposition and anodic stripping of Cu2+


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and Hg2+ obviously shifted in the negative direction and the peak currents also gradually decreased. This implies that the formation of metal-ion complexes increased the reductive overpotential of interfering metal ions and the mass transport of bulky metal complexes was also slower than that of free metal ions.11 Interesting, the As3+ redox behavior was not significantly changed after the addition of EDTA. These results also suggest that the selective chelation of the interfering metal ions by EDTA could inhibit the interfering metal ions from competing with the ion of interest (As3+) in the accumulation process. Although more acidic electrolyte solutions (pH = 1) were preferred for As3+ detection in many previous reports,6d,9a considering the importance of selectivity, EDTA was used to reduce interference from other metal ions. In highly acidic solutions, a hexaprotic ion, designated as H6Y2+, forms the major EDTA species and its chelating ability with metal ions is inferior.20 Calculation of the EDTA speciation as a function of pH showed that the major EDTA species formed, namely H2Y2−, HY3−, and Y4−, chelate more strongly with metal ions in aqueous solutions at pH values greater than 4.0.9b To obtain good sensitivity and selectivity in the detection of As3+, a solution pH of 5 was selected as the optimum condition. Calibration data were obtained under the optimized experimental conditions mentioned above. Figure 5 shows typical SWASV voltammograms for the Aunano@GCE in solutions with different As3+ concentrations. In all the cases, a stripping response was observed at a potential in the vicinity of −0.08 V vs. SCE and a calibration graph was constructed based on the observed peak currents (inset of Figure 5A). The graph shows a dynamic range from 0.1-15.0 µg L−1 (from 1.3-200 nM). The dynamic range of the calibration curve, y = 0.21x (in µA L µg−1) + 0.01 (R2 = 0.999), showed a linear behavior with a slope (sensitivity) of 0.21 µA L µg−1 (16.15 µA µM−1). The detection limit, which is equal to three times the standard deviation of the background, was 0.0025 µg L−1 (32.5 pM). This detection limit is comparable to or better than the detection limits offered by previous reports, which are shown in Table S-2.21 To examine the effect of EDTA addition on the detection of As3+, calibration curves of As3+ constructed in solutions containing 10 µg L−1, 50 µg L−1, and 100 µg L−1 of interfering metal ions (Cu2+ and Hg2+), are shown in Figure 5B-5D. In the presence of EDTA, the detection of As3+ in the presence and absence of the interfering metal ions is the same. This strongly demonstrates that EDTA effectively inhibits these major interfering metal ions during the detection of As3+. The interference of various ions in the detection of As3+ was examined. The use of Aunano@GCE with EDTA in the solution successfully eliminated such interference. For a solution containing 10.0 µg L−1 of As3+, the results showed that an over 50fold excess concentration of Cu2+, Hg2+, Zn2+, Bi3+, Ni2+, Sn2+, Fe3+, and Co2+ can be tolerated (Table S-3). Real Sample Analysis. The detection of As3+ in real water samples was evaluated using Aunano@GCE. Ground waters from the middle part of Taiwan and drinking water from Taichung, Taiwan, were studied. The results are summarized in Table S-4 and indicate good accuracy for all the samples. The As3+ contents in these samples were calculated by factoring in the dilution factors (10-fold) used for each sample during preparation.

CONCLUSIONS

An electrochemical technique for the characterization of the “nano-sized” effects of AuNPs was developed for electroanalytical applications. The method offers a simple and economical process for the optimization of electrode materials compared to the state-of-the art method for the morphological characterization of AuNPs by image observation. The detection of As3+ in aqueous solutions by a Aunano@GCE using the electrochemical characterization technique was examined. The analytical performance and S/N ratio for As3+ sensing were optimized by considering Au loading, effective s-ECSA, and Rf. EDTA was used as a chelating agent to successfully mitigate interference from metal ions that are not of interest. EDTA was able to effectively inhibit the interference of competing ions in the pre-concentration process, owing to its excellent selective chelating ability. Under the optimal conditions, a high sensitivity of 0.21 µA L µg−1 (16.15 µA µM−1) and a low detection limit of 0.0025 µg L−1 (32.5 pM) were obtained using SWASV. This method was also applied for the detection of As3+ in real water samples.

ASSOCIATED CONTENT Supporting Information Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of the Republic of China, Taiwan.

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EDTA Assisted Highly Selective Detection of As3+ on Au Nanoparticle

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