Electrochemical Detection of Arsenic(III) - ACS Publications

Feb 3, 2013 - Here, we report a disposable platform completely free from noble ... electrochemical performance than commonly used noble metals. ..... ...
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Electrochemical Detection of Arsenic(III) Completely Free from Noble Metal: Fe3O4 Microspheres-Room Temperature Ionic Liquid Composite Showing Better Performance than Gold Chao Gao,†,‡,§ Xin-Yao Yu,†,§ Shi-Quan Xiong,† Jin-Huai Liu,† and Xing-Jiu Huang*,†,‡ †

Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, PR China ‡ Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China S Supporting Information *

ABSTRACT: In recent decades, electrochemical detection of arsenic(III) has been undergoing revolutionary developments with higher sensitivity and lower detection limit. Despite great success, electrochemical detection of As(III) still depends heavily on noble metals (predominantly Au) in a strong acid condition, thus increasing the cost and hampering the widespread application. Here, we report a disposable platform completely free from noble metals for electrochemical detection of As(III) in drinking water under nearly neutral condition by square wave anodic stripping voltammetry. By combining the high adsorptivity of Fe3O4 microspheres toward As(III) and the advantages of room temperature ionic liquid (RTIL), the Fe3O4-RTIL composite modified screen-printed carbon electrode (SPCE) showed even better electrochemical performance than commonly used noble metals. Several ionic liquids with different viscosities and surface tensions were found to have a different effect on the voltammetric behavior toward As(III). Under the optimized conditions, the Fe3O4-RTIL composites offered direct detection of As(III) within the desirable range (10 ppb) in drinking water as specified by the World Health Organization (WHO), with a detection limit (3σ method) of 8 × 10−4 ppb. The obtained sensitivity was 4.91 μA ppb−1, which is the highest as far as we know. In addition, a possible mechanism for As(III) preconcentration based on adsorption has been proposed and supported by designed experiments. Finally, this platform was successfully applied to analyzing a real sample collected from Inner Mongolia, China.

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number of samples. On the contrary, the electrochemical methods are potentially the most promising techniques available for use in the field, because they are cheap and can provide very accurate measurements with a rapid analysis time. Among all of the electrochemical methods, stripping voltammetry analysis is the most popular due to it’s excellent sensitivity and unique ability to detect the trace levels of elements in distinct oxidation states.5,6,11−15 As reported in the earlier studies, electrochemical analysis of As(III) has been achieved using the hanging mercury drop electrode16,17 and the mercury film electrode.18 However, due to the potential toxicity of mercury together with operational limitations, portable sensors utilizing mercury electrodes were subsequently replaced by other solid metal substrates, such as platinum (Pt),19,20 silver (Ag),21 and gold (Au).22 Among all the solid metal substrates considered to date, gold was found to be the superior substrate for the working electrode. In addition to solid gold substrate electrodes, various kinds of gold-based

he contamination of arsenic in water is a serious worldwide threat to human health, which has become a challenge for the scientists and the analytical chemists.1 Arsenic can occur in the environment in several oxidation states (−3, 0, +3, and +5), but in natural waters, arsenic is mostly found in inorganic form as oxyanions of trivalent arsenite[As(III)] or pentavalent arsenate[As(V)].2 As(III) is reported to be 25−60 times more toxic than As(V).3,4 Such pollutants in drinking water have been associated with many health problems such as skin lesions, keratosis (skin hardening), lung cancer, and bladder cancer.5 It has been reported in 20 countries that the arsenic levels in drinking water are above the World Health Organization (WHO)’s arsenic guideline value of 10 μg L−1 (i.e., 10 ppb).6 Thus, it is rather important to have an accurate, fast, and sensitive method to detect and monitor this environmental pollutant in drinking water. A variety of accurate analytical methods can be used for arsenic detection in water,7 such as hydride generation atomic fluorescence spectrometry,8 atomic absorption spectroscopy,9 and inductively coupled plasma mass spectrometry.10 However, these methods require expensive instruments, high operating cost, and well trained technicians to conduct the measurements, which are not suitable for routine in-field monitoring of a large © 2013 American Chemical Society

Received: October 18, 2012 Accepted: February 3, 2013 Published: February 3, 2013 2673

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electrodes, such as gold film,23 gold ultramicroelectrode array,11 gold nanoelectrode ensemble,14 and gold nanoparticle modified electrodes,5,6,13,24 have been conducted to improve analytical performance. Although these methods are promising, goldbased electrodes still face some obstacles to commercialization. First, gold is an expensive resource, which is not cost-effective for commercial products. Second, the popular gold-based electrodes need to be operated in strongly acidic media (such as HCl, H2SO4, HClO4, HNO3, etc.), which could cause the problems of producing toxic arsine gas, generating interference from H2 evolution and causing unsafety for transport. Third, surface fouling is a common problem which needs to be resolved.11,25−27 Furthermore, the electrochemical behavior of gold-based electrodes exhibits a strong relationship with their crystallographic orientation, thus causing complexity in fabricating the platform.15 In an effort to overcome these limitations, we try to find an efficient system to fundamentally get rid of the dependence on noble metal, which could also avoid the problem of surface fouling. Previously, Cox and Kulesza reported a thin film of mixed-valent ruthenium(III, II) cyanide modified glassy carbon electrode for detection of As(III) in 0.5 M NaCl (pH = 2).28 Zen and Kumar reported a prussian blue modified screenprinted electrode for flow injection analysis of As(III) in 0.1 M KCl/HCl solution (pH = 4).29 After that, Compton’s group reported an iridium oxide modified boron doped diamond electrode for amperometic detection of As(III) in phosphate buffer solution (pH = 4.30).30 Recently, cobalt oxide nanoparticle modified glassy carbon electrode has been used for detection of As(III) in phosphate buffer solution (pH = 7).31 Despite that these works have made big progress on nonprecious metal systems for electrochemical detection of As(III), seeking a platform completely free from noble metal that could have similar or even better analytical performance compared with gold is still a big challenge. Ferroferric oxide (Fe3O4) is a low-cost, environmentally friendly, and easy-prepared material, which has been widely used in electrochemical sensing.32−35 Recently, monodispersed Fe3O4 nanocrystals were exploited to remove arsenic from water by strong adsorption.36 On the basis of our previous results37−40 that electrochemical behavior was closely bound up with adsorption ability and a previous report41 that the redox couple of Fe(III)/Fe(II) may have some relevance for the reduction of As(III), we surmised that a breakthrough could be obtained using Fe3O4 to detect As(III). However, considering to further enhance the sensitivity, we next seek to find a mediator which can provide the necessary conduction pathways for electrons on the electrode surface to amplify current signal. Recently, room temperature ionic liquids (RTILs) have been widely used in electrochemical sensors and have shown excellent electrochemical behaviors due to their advantages, such as high viscosity, good intrinsic conductivity, low volatility, wide electrochemical windows, and high chemical and thermal stability.42−47 Thus, we believe that RTILs would be a good choice. In the present work, for the first time, we try to use Fe3O4RTIL composite modified screen-printed carbon electrode (SPCE) for electrochemical detection of As(III). Screenprinted electrode is a cost-effective disposable device, which can effectively avoid surface fouling. The main motivation of the present work was to use Fe3O4-RTIL composites to replace precious gold. Particular efforts were devoted to optimize the main influencing factors of analytical performance, such as the

type of RTIL, supporting electrolytes, pH value, deposition potential, and deposition time. Encouragingly, this nonprecious metal system allowed fast, sensitive, and selective detection of As(III) in nonstrong acid solution (pH = 5) by square wave anodic stripping voltammograms (SWASV), which showed even better electrochemical performance than commonly used gold. The possible mechanism of preconcentration based on adsorption was further clarified and verified. Finally, this composite was successfully applied in a real water sample.

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EXPERIMENTAL SECTION Chemical Reagents. 1-Butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfon-yl)imide ([C4dmim][NTf2]), N-propyl diethylmethylammonium bis(trifluoromethy-lsolfonyl)imide ([N 2 ,1 , 1 ,3 ][NTf 2 ]), and 1-butyl-3-methylimidazolium trifluorotris(pe-ntafluoroethyl)phosphate ([C4mim][FAP]) were supplied by Merck KGaA (Darmstadt, Germany). 1Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4 mim][NTf2]) was prepared by standard literature procedures.48,49 The success of the above procedures in purifying the ionic liquids was sensitively assessed by means of voltammetric measurements on blank solvents. Impurity free nitrogen was purchased from Nanjing Special Gases Factory Co., Ltd. for electrochemical experiments as described below. All experiments were carried out at a temperature of 295 ± 3K. All other reagents were commercially available from Sinopharm Chemical Reagent Co., Ltd. (China) with analytical grade and were used without further purification. Acetate buffer solutions of 0.1 M for different pH were prepared by mixing stock solutions of 0.1 M NaAc and HAc. Phosphate buffer solutions (PBS) of 0.1 M were prepared by mixing stock solutions of 0.1 M H3PO4, KH2PO4, K2HPO4, and NaOH. NH4Cl-NH3·H2O (0.1 M) solution was prepared by mixing stock solutions of 0.1 M NH4Cl and NH3·H2O in different proportion. The water (18.2 MΩ cm) used to prepare all solutions was purified with the NANOpureDiamond UV water system. Preparation of Fe3O4 Microspheres. Fe3O4 microspheres were synthesized according to a previous report.50 In brief, 1.25 mmol of FeCl3·6H2O (0.675 g) was dissolved in 18 mL of ethylene glycol to form a clear solution. Then, NaAc (1.8 g) and polyethylene glycol (1.0 g) were added into the above solution. After vigorous stirring for 30 min, the mixture was transferred into a Teflon-lined stainless steel autoclave (20 mL capacity), heated at 200 °C in an electric oven for 6 h, and then cooled to room temperature naturally. The resulting black product was centrifuged and washed with deionized water and absolute alcohol for several times and finally dried at 60 °C under vacuum for 24 h. Fabrication of Fe3O4-RTIL Composite Modified Electrode. Ultrasonic agitation (for 5 min) was used to disperse the four kinds of RTILs ([C4dmim][NTf2], [C4mim][FAP], [C4mim][NTf2], and [N2113][NTf2]) into absolute alcohol in a volume ratio of 1:200, respectively. Then, a certain amount of the synthesized Fe3O4 microspheres were added into the above RTILs solutions and ultrasonicated for 10 min to give a suspension (3.0 mg mL−1). For fabricating the modified electrode, an aliquot of 10 μL of the suspension was pipetted onto the surface of the carbon working electrode in SPCE, respectively. The solvent was evaporated under room temperature to obtain the Fe3O4-RTIL composite film modified SPCEs. For comparison, single Fe3O4 microsphere modified SPCE was prepared using the same process. 2674

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Electrochemical Detection of As(III). Square wave anodic stripping voltammetry (SWASV) was used for the detection of As(III) under optimized conditions. As(0) was deposited at the potential of −0.5 V for 120 s by the reduction of As(III) in 0.1 M NaAc-HAc (pH 5.0). The anodic stripping (reoxidation of As(0) to As(III)) of electrodeposited As(0) was performed in the potential range of −0.3 to 0.4 V at the following parameters: frequency, 15 Hz; amplitude, 25 mV; increment potential, 4 mV; vs Ag. After each measurement, the modified electrode was regenerated in fresh stirred supporting electrolyte by electrolysis at +0.5 V for 140 s to remove the previous residual As(0) from the electrode surface. Apparatus. Electrochemical measurements were performed with SPCEs (ref 110, DropSens, Edificio Severo Ochoa, Spain). The electrochemical cell consists of a three-electrode arrangement with a carbon (4 mm-diameter) serving as the working electrode, with a carbon used as the counter electrode. A silver pseudo reference electrode completed the circuit. All measurements were performed with a CHI 660D computer-controlled potentiostat (ChenHua Instruments Co., Shanghai, China). The scanning electronmicroscopy (SEM) images were taken using a field-emission scanning electron microscope (FESEM, Quanta 200 FEG, FEI Company, USA). X-ray diffraction (XRD) patterns of the samples were obtained with a Philips X’Pert Pro X-ray diffractometer with Cu Kα radiation (1.5418 Å). X-ray photoelectron spectroscopy (XPS) analyses of the samples were conducted on a VG ESCALAB MKII spectrometer using an Mg Kα X-ray source (1253.6 eV, 120 W) at a constant analyzer.

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RESULTS AND DISCUSSION Morphologic and Structure Characterization of Fe3O4 Microspheres. Figure 1 shows the representative scanning electron microscopy (SEM) image of as-synthesized Fe3O4 microspheres at different magnifications. With a detailed observation (Figure 1a, upper right), it demonstrates that the surface of Fe3O4 microspheres is rough and rugged. This shape and structure can significantly increase the effective surface area and thus increase available adsorption and/or deposition sites, which helps to enhance the electrochemical response and decrease the detection limit. The crystalline structures of Fe3O4 microsphere were further characterized by a powder X-ray diffraction (XRD) pattern (Figure 1a, lower left), and all the diffraction peaks observed can be indexed to the pattern for Fe3O4 (JCPDS No. 65-3107). The particle size distribution of Fe3O4 microspheres was estimated on the basis of dynamic light scattering technique. As shown in Figure 1b, with an overall view of the scanning electron microscopy (SEM) image of as-synthesized Fe3O4 microspheres, the diameters of the generated microspheres are mainly distributed in the range of 400−520 nm (inset in Figure 1b). Electrochemical Characterization of Fe3O4-RTIL Composite Modified SPCE. Potassium ferricyanide was selected as a probe to evaluate the performance of the proposed electrodes. The typical cyclic voltammograms of the bare SPCE, Fe3O4 modified SPCE, and Fe3O4-[C4dmim][NTf2] composite modified SPCE were compared in neutral solution of 5 mM Fe(CN)63−/4− containing 0.1 M KCl (Figure S1a, Supporting Information). As compared with the bare SPCE, the anodic and cathodic peaks increase at the Fe3O4 modified electrode, which could be attributed to the conductivity of Fe3O4. For the Fe3O4-[C4dmim][NTf2] composite modified SPCE, the peak current further increases. The improved electrochemical

Figure 1. SEM image of Fe 3 O 4 microspheres at different magnifications. Insets in panel a are XRD pattern (lower left) of the Fe3O4 microspheres and the close-up of the surface of Fe3O4 microspheres (upper right). Inset in panel b is the particle size distribution of Fe3O4 microspheres.

response resulted from the introduction of good conductive RTIL, which could provide the necessary conduction pathways on the electrode surface. The capability of electron transfer on these electrodes was also investigated by electrochemical impedance spectroscopy (EIS), which is a valuable method to monitor the impedance changes of the electrode surface after being modified. The obtained result further indicates that the high conductive RTIL promotes the electron transfer process at the modified electrode surface (Figure S1b, Supporting Information). In addition, according to the Randles-Sevcik equation, ip = (2.69 × 105)n3/2ACD1/2v1/2, the electrochemical active electrode surface of bare, Fe3O4, and Fe3O4-[C4dmim][NTf2] modified SPCE are calculated to be 0.0766, 0.0833, and 0.136 cm−2, respectively (Figure S2, Supporting Information). This result indicates that the introduction of good conductive RTIL could greatly enhance the electrochemical active electrode surface. Optimization of RTILs. To further understand the effect of RTIL on detetion of As(III), several other ionic liquids ([C 4 mim][FAP], [N 2113 ][NTf 2 ], [C 4 mim][NTf 2 ], and [C4dmim][NTf2]) were chosen to fabricate Fe3O4-RTIL 2675

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chemical investigations. In order to get the maximum sensitivity for trace arsenic detection with Fe3O4-[C4dmim][NTf2] composite modified SPCE, the voltammetric parameters (supporting electrolytes, deposition potential, deposition time, and pH value) were optimized in solution containing 10 ppb As(III) (see Supporting Information for details, Figure S4). Electrochemical Detection of As(III). Under the optimal experimental conditions, Fe3O4-[C4dmim][NTf2] composite modified SPCE was successfully applied to the detection of the target As(III) by SWASV. To clarify the individual roles of Fe3O4 and [C4dmim][NTf2] for detection of As(III), bare SPCE and single Fe3O4 microsphere modified SPCE were also applied to the analysis of As(III), and their analytical performances were compared. For bare SPCE (Figure 3a), in a concentration range of 24 to 120 ppb, As(III) is detected with the sensitivity of 0.08 μA ppb−1 and the correlation coefficient of 0.973. The theoretical limit of detection (LOD) was calculated to be 0.015 ppb (3σ

composite modified SPCE. The analytical performance toward As(III) was systematically compared, as shown in Figure 2.

Figure 2. Comparison of (a) sensitivity and (b) LOD (3σ method) for SWASV detection of As(III) at Fe3O4-[C4mim][FAP], Fe3O4[N2113][NTf2], Fe3O4-[C4mim][NTf2], and Fe3O4-[C4dmim][NTf2] composite modified SPCE, respectively.

Their corresponding SWASV responses and linear calibration plot can be found in Supporting Information, Figure S3. Among these composite modified SPCEs, Fe3O4-[C4mim][FAP] and Fe3O4-[C4mim][NTf2] have the same cations but different anions. By comparing their analytical performance toward As(III), they are found to have the similar sensitivity and limit of detection (LOD). This result indicates that the type of anions do not have obvious influence on detection of As(III). However, for Fe3O4-[N2113][NTf2], Fe3O4-[C4mim][NTf2], and Fe3O4-[C4dmim][NTf2], they have the same anions but different cations. Interestingly, they are found to have obvious difference in sensitivity and LOD, which showed that the type of cations can have obvious influence on detection of As(III). We believe that this phenomenon is due to different RTILs having different viscosities, surface tension, and conductivity, which could have an effect on the voltammetric behavior.51 However, more studies are necessary to clarify this. Among various Fe3O4-RTIL composite modified SPCEs, Fe3O4-[C4dmim][NTf2] shows the best performance. Thus, Fe3O4-[C4dmim][NTf2] was selected for subsequent electro-

Figure 3. Typical SWASV response of (a) bare SPCE, (b) Fe3O4 microsphere modified SPCE, and (c) Fe3O4-[C4dmim][NTf2] composite modified SPCE for analysis of As(III) in different concentration ranges. Insets in panels a, b, and c are corresponding linear calibration plots of peak current against As(III) concentrations, respectively. Supporting electrolyte: 0.1 M acetate buffer (pH 5.0). Deposition potential, −0.5 V; deposition time, 120 s; amplitude, 25 mV; increment potential, 4 mV; frequency, 15 Hz. The dotted line refers to the baseline. 2676

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Table 1. Comparison of Performance for Electrochemical Detection of As(III) at Gold-Based and Nonprecious Metal Systemsa electrodes

electrolyte

linearity range (ppb)

sensitivity (μA ppb−1)

LOD (ppb)

refs.

Au-UMEA Au NPs/GCE Au NPs/GCE sonically assisted gold microdisk electrode Au NPs/GCE Au-coated diamond thin-film electrode gold nanoelectrode ensembles Au(111)-like polycrystalline gold electrode gold−carbon composite electrode MWCNTs/gold electrode IrOx/BDD PBSPE MWCNT/Aro/GCE CoOx/GCE Fe3O4-RTIL/SPCE

2 M HCl 1 M HCl 3 M HCl 0.1 M HNO3 1 M HNO3 1 M HCl 1 M HCl PBS (pH = 1) 0.1 M HNO3 acetic buffer (pH = 4) PBS (pH = 4.3) KCl/HCl (pH =4) PBS (pH = 7) PBS (pH = 7) acetate buffer (pH = 5.0)

0−500 0−7.5 0−87 7.5−75 0.5−15 0.01−40 0.1−3 0−1125 1.5−16.5

0.044 0.24 4.27 0.363 0.024 0.0097 3.14 0.3636 0.133 0.236 0.056 3.87 × 10−4 0.00143 0.00148 4.91

0.013 0.0096 1.8 0.2775 0.25 0.005 0.02 0.28 0.375

11 6 24 12 5 13 14 15 52 53 30 29 54 31 this work

1.5−3750 3.75−22500 0−500 15−300 1−10

0.15 1.875 1 0.825 0.0008

a Au-UMEA: gold ultramicroelectrode array; Au NPs: gold nanoparticles; GCE: glassy carbon electrode; PBS: phosphate buffer solution; IrOx: iridium oxide; BDD: boron doped diamond; mvRuCN: mixed-valent ruthenium (III, II) cyanide; PBSPE: prussian blue-modified screen-printed electrode; MWCNTs: multiwalled carbon nanotubes; Aro: arsenite oxidase; CoOx: cobalt oxide.

Possible Preconcentration Mechanism Based on Adsorption. In stripping analysis, the efficient preconcentration of the target analyte onto a certain substrate is significant. Thus, the voltammetric signal, such as As(III), was controlled by how well the electrode materials can adsorb As(III), which are subsequently accumulated on the electrode surface. To confirm this preconcentration mechanism, we have proposed, X-ray photoelectron spectroscopy (XPS) was performed to clarify the relationship between the stripping peak current and the amount of adsorbed As(III) under several different conditions: different supporting electrolytes, different pH value, and different electrodes, respectively. The sample was prepared by dipping the electrodes into the solution containing 750 ppb As(III) for 2 min with magnetic stirring and then washing to remove the unadsorbed As(III). (Considering the resolution of XPS, a high concentration of As(III) is chosen for the experiments.) Figure 4a shows the typical XPS spectra of As3d adsorbed on Fe3O4-[C4dmim][NTf2] composite modified SPCE at pH 5.0 in 0.1 M NH4Cl-NH3·H2O, PBS, and NaAc-HAc, respectively. As seen, the intensity of the peak at 44.5 eV corresponding to As3d is the strongest in NaAc-HAc, which indicates that the amount of adsorbed As(III) on the electrode surface is the most in NaAc-HAc. That is why we got the strongest current signal in NaAc-HAc. Similarly, in Figure 4b, it is found that the amount of As(III) adsorbed on the surface of Fe3O4[C4dmim][NTf2] composite modified SPCE in 0.1 M NaAcHAc is the most at pH 5.0. This result confirms our hypothesis that the pH value greater/less than the pHPZC of Fe3O4 may influence the specific adsorption toward arsenite[As(III)]. The lower amount of As(III) adsorbed at low pH 3.0 also confirms the supposition that weak peak current obtained at low pH 3.0 is probably due to this oxide being less stable at low pH, and thus, less sorption will occur. Such connection can also be found for different electrodes. As shown in Figure 4c, compared with bare SPCE, the Fe3O4 modified SPCE has more As(III) adsorbed on its surface, confirming the role of Fe3O4 for accumulating the As(III), and consequently enhances the sensitivity. By introducing RTIL, the intensity of As3d peak is further enhanced at the Fe3O4-[C4dmim][NTf2] composite modified SPCE. This result suggests that the improved

method), and the lowest detectable concentration actually measured is 24 ppb. For Fe3O4 modified SPCE (Figure 3b), in a relatively low concentration range of 16−56 ppb, the obtained sensitivity is 1.09 μA ppb−1, which is about 14-fold of that for bare SPCE. A higher correlation coefficient of 0.984 and a lower theoretical LOD of 0.005 ppb (3σ method) were also obtained compared with bare SPCE. The lowest detectable concentration actually measured is 16 ppb. The greatly enhanced sensitivity is ascribed to the high adsorptivity of Fe3O4 microspheres toward As(III), which can accumulate more As(III) on the electrode surface. While for Fe3O4[C4dmim][NTf2] modified SPCE (Figure 3c), the stripping peak current is proportional to the concentration of As(III) from 1 to 10 ppb, which is below the guideline value (10 ppb) given by the World Health Organization (WHO). After introducing [C4dmim][NTf2], the obtained sensitivity is about 5-fold of that for Fe3O4 modified SPCE, with even higher correlation coefficient of 0.997 and even lower theoretical LOD of 0.0008 ppb (3σ method). The lowest detectable concentration actually measured is 1 ppb. These results indicate that the presence of RTIL can provide the necessary conduction pathways for electrons, which plays an important role in accelerating the electron transfer on the electrode surface. According to the theory of Compton’s group,55,56 the peak potential shifts with increasing As concentrations are the consequence of the overlap of diffusion layers produced from the stripping of the As(0) to As(III) in solution. In our work, we think the presence of the ionic liquids have an influence on the overlap of diffusion layers. When the diffusion layers start to overlap, the rate of stripping is restricted, thus resulting in a higher peak potential. On the other hand, there are more electrodeposited As(0) on the electrode with increasing As(III) concentrations, therefore requiring a longer sweep for the stripping of the As(0) to As(III). It is worthy to note that, due to the mobility of RTIL (even though it is viscous), the RTIL cannot be attached well on the surface of SPCE; thus, the RTIL modified SPCE was not included in the present work. In contrast to gold-based and nonprecious metal systems, the obtained sensitivity (4.91 μA ppb−1) in this work is the highest and the corresponding LOD is the lowest (Table 1). 2677

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coprecipitated and stripped off under the experimental conditions used for the detection of As(III). Among various metal ions, Cu(II) shows the major interference in the detection of As(III). There are lots of electrodes, which have a high sensitivity toward As(III), affected from the interference from Cu(II).11−13 Thus, Cu(II) was usually chosen for interference studies. Figure 5 shows the SWASV responses

Figure 5. SWASV response of the Fe3O4-[C4dmim][NTf2] composite modified SPCE at 0, 10, 20, 30, 40, 50, 60, and 70 ppb As(III) in the presence of 192 ppb Cu(II) in 0.1 M NaAc-HAc solution (pH 5.0), showing the interference of Cu(II) on the anodic peak currents of As(III). Inset is the corresponding linear calibration plot of peak current against As(III) concentrations. SWASV conditions are identical to Figure 3. The dotted line refers to the baseline.

obtained at the Fe3O4-[C4dmim][NTf2] modified SPCE for different concentrations of As(III) in the presence of Cu(II) (192 ppb). The stripping signal obtained for Cu(II) at about −0.12 V is almost kept the same, while the stripping peaks for As(III) gradually increase with a position shift. Comparing Figure 5 with Figure 3c, in the absence of Cu(II), the sensitivity obtained was 4.91 μA ppb−1. However, in the presence of Cu(II), the sensitivity obtained (0.35 μA ppb−1) was only 1/14 of the former. The decrease in sensitivity can be attributed to competition for deposition sites at the electrode surface by the interfering Cu(II) and also the formation of intermetallic compounds, such as Cu3As2. In addition, when the concentration of As(III) is fixed, the loss of arsenic signal as the concentration of Cu(II) changed is provided in Figure S5, Supporting Information. Another phenomenon to mention is that the relative position of the peak for As(III) and Cu(II) in this work is different from that observed at gold-based electrode, and we still do not know the particular reason. Some other common heavy metal ions were tested to evaluate the selectivity of Fe3O4-[C4dmim][NTf2] composite modified SPCE, and we found that Hg(II), Cu(II), Pb(II), and Cd(II) could also be detected (Figure S6, Supporting Information). Figure 6 shows the comparison of sensitivity for individual analysis of Hg(II), Cu(II), Pb(II), Cd(II), and As(III) at Fe3O4-[C4dmim][NTf2] composite modified SPCE. The sensitivity obtained for As(III) is nearly 10, 621, 153, and 377 times of that for Hg(II), Cu(II), Pb(II), and Cd(II), respectively. Therefore, this system has a good selectivity toward As(III), which showed promise for application in the real sample. Real Sample Analysis. For the purpose of practical application of the present electrode, a test on a real water

Figure 4. Typical XPS spectra of As(III) adsorbed on Fe3O4[C4dmim][NTf2] composite modified SPCE under (a) different supporting electrolytes (pH = 5.0) and (b) different pH values in 0.1 M NaAc-HAc solution. (c) Typical XPS spectra of As(III) adsorbed on bare SPCE, Fe3O4 microsphere modified SPCE, and Fe3O4[C4dmim][NTf2] composite modified SPCE in 0.1 M NaAc-HAc solution (pH = 5.0).

sensitivity after combining RTIL is not only because of the good conductivity of RTIL but also because that RTIL might have a positive effect on the adsorption of As(III). This hypothesis may well explain the phenomenon, as we have mentioned above, that there is a difference between different RTILs in affecting analysis of As(III). The explanation is that different RTILs could have different adsorptivity toward As(III). However, to clarify this effect, more studies are necessary. Interferences and Selectivity. Sensitive detection of As(III) in the real sample without interference is a challenging task, as the other metal ions commonly present can be 2678

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CONCLUSIONS In the present study, sensitive detection of As(III) completely free from noble metal was achieved by Fe3O4-RTIL composite modified SPCE. This platform permits fast, selective, and direct detection of As(III) within the desirable range (10 ppb) in drinking water as specified by the WHO. Furthermore, considerable simplicity and economy of electrode preparation as compared to other electrochemical methods for arsenic detection was offered by this platform. As the Fe3O4-RTIL composite provided a specific interface for arsenic to accumulate and exchange electrons, the obtained sensitivity and LOD was even better than that of commonly used goldbased electrodes, which represents an important advancement in electrochemical detection of As(III). Different RTILs were also found to have an effect on the voltammetric behavior toward As(III). The proposed preconcentration mechanism was verified and thus established the new bridge between adsorption and electrochemical behavior. Finally, this disposable platform has been successfully employed for detection of As(III) on a real sample collected from Inner Mongolia.

Figure 6. Comparison of sensitivity for individual detection of Hg(II), Cu(II), Pb(II), Cd(II), and As(III) at Fe3O4-[C4dmim][NTf2] composite modified SPCE.

sample has been performed. The real sample was collected from groundwater in Xing Wang Zhuang Village, Togtoh County, Hohhot City, Inner Mongolia Autonomous Region, China. The real sample was diluted with 0.1 M NaAc-HAc buffer solution (pH 5.0) in a ratio of 1:99, and no further sample treatment was done. Standard additions of As(III) were performed in the diluted sample. The SWASV response and the corresponding calibration plots are shown in Figure 7. The

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ASSOCIATED CONTENT

S Supporting Information *

Cyclic voltammograms and Nyquist diagram of electrochemical impedance spectra for bare, Fe3O4, and Fe3O4-RTIL composite modified SPCEs (Figure S1); scan rate study at bare, Fe3O4, and Fe3O4-[C4dmim][NTf2] modified SPCE (Figure S2); typical SWASV response of various Fe3O4-RTIL composite modified SPCEs for analysis of As(III) (Figure S3); optimization of experimental conditions (Figure S4); interference study (Figure S5); typical SWASV response of the Fe3O4[C4dmim][NTf2] composite modified SPCE for individual detection of Hg(II), Cu(II), Pb(II), and Cd(II) (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-551-5591142. Fax: +86-551-5592420. Author Contributions

Figure 7. Typical SWASV responses of standard additions of As(III) into a real water sample diluted with 0.1 M NaAc-HAc solution (pH 5.0) in a ratio of 1:99. SWASV conditions are identical to Figure 3.

§

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Scientific P r o g r a m - N a n o s c i e n c e a nd N a no t e c hn o l o g y ( N o . 2011CB933700), National Natural Science Foundation of China (No. 21103198, 21073197, and 90923033), and the China Postdoctoral Science Foundation (No. 20110490386 and 2011M501073). X.J.H. acknowledges the One Hundred Person Project of the Chinese Academy of Sciences, China, for financial support.

concentration of As(III) in the real sample was calculated to be 73.5 ppb. The total arsenic content of the real sample was determined using inductively coupled plasma-mass spectroscopy (ICPMS) for comparison, and the measured concentration is 102.6 ppb. Compared with the value obtained by our proposed method, the higher value obtained by ICPMS is due to the real sample possibly having As(V), which was not detected by our proposed method. In order to evaluate the validity of the proposed method for the detection, recovery studies were carried out on real samples to which known amount of As(III) was added. The recovery obtained is varied from 95.7% to 103.6%. These results reveal that the proposed composite has important practical application potential.

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REFERENCES

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