Selective Detection of As(III) - American Chemical Society

Oct 25, 2010 - Mohammad Rezaur Rahman, Takeyoshi Okajima, and Takeo Ohsaka*. Department of Electronic Chemistry, Interdisciplinary Graduate School ...
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Anal. Chem. 2010, 82, 9169–9176

Selective Detection of As(III) at the Au(111)-like Polycrystalline Gold Electrode Mohammad Rezaur Rahman, Takeyoshi Okajima, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259-G1-5 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Selective electrochemical detection of As(III) using a highly sensitive platform based on a Au(111)-like surface is described. The Au(111)-like surface was achieved for the first time by the partial reductive desorption of n-butanethiol (n-BT) from polycrystalline gold (poly-Au), on which a self-assembled monolayer (SAM) of n-BT was formed previously, which allows the selective blockage of the Au(100) and Au(110) surface domains by n-BT while the Au(111) domain remains bare. Square wave anodic stripping voltammetry (SWASV) using the Au(111)-like poly-Au electrode confirms the successful detection of As(III) without any interference from Cu(II). The fabricated electrode is stable and highly sensitive even in the presence of Cu(II), and it shows a linear response for As(III) up to 15 µM. The detection limit (S/N ) 3) toward As(III) is 0.28 ppb, which is far below the guideline value given by World Health Organization (WHO). The electrode was applicable for the analysis of spiked arsenic in tap water containing a significant amount of various other ion elements. The results indicate that the Au(111)-like polyAu electrode could be promising for the electrochemical detection of trace level of As(III) in real samples without any interference from Cu(II). Arsenic (As) is a toxic substance with acute as well as chronic effects. The contamination of groundwater by As, which leads to adverse health effects,1-6 has been reported in 20 countries where arsenic levels in drinking water are above the WHO’s arsenic guideline value of 10 µg L-1 (i.e., 10 ppb).4,6 It has been reported that at least 32 million Americans consume water containing more than 2 ppb of As. The U.S. As drinking water limit of 50 ppb was set to a new standard in the range of 2-20 ppb.7 The problem is most prominently observed in Bangladesh where at least 50 million people are at the risk of As poisoning and, according to the WHO, “the contamination of groundwater by arsenic in * To whom correspondence should be addressed. Phone: +81-45-9245404. Fax: +81-45-9245489. E-mail: [email protected]. (1) Cullen, W. R.; Reimer, K. J. Chem. Rev. 1989, 89, 713–764. (2) Morita, M.; Edmonds, J. S. Pure Appl. Chem. 1992, 64, 575–590. (3) Li, H.; Smart, R. B. Anal. Chim. Acta 1996, 325, 25–32. (4) World Health Organization, May 2001, fact sheet no. 210. (5) Mandal, B. K.; Suzuki, K. T. Talanta 2002, 58, 201–235. (6) Khan, M. M. H.; Kobayashi, K.; Sakauchi, F.; Yamashita, T.; Mori, M.; Hossain, M. K.; Ahmed, M. F.; Hossain, M. D.; Quamruzzaman, Q. Int. J. Environ. Health Res. 2005, 15, 289–301. (7) Majid, E.; Hrapovic, S.; Liu, Y.; Male, K. B.; Luong, J. H. T. Anal. Chem. 2006, 78, 762–769. 10.1021/ac101206j  2010 American Chemical Society Published on Web 10/25/2010

Bangladesh is the largest poisoning of a population in history”.4,8,9 In Bangladesh, As is present naturally in the underlying strata, and the main source of contamination is the erosion and dissolution that occurs in shallow drinking water wells. According to a recent report on the survey of all 64 districts in Bangladesh, the As level in groundwater of 50 districts is over the WHO guideline value.8,10 Among these As-contaminated districts, the As level in 42 districts is above 50 ppb.11 For these reasons, it is important to have a method which is both sensitive and sufficiently simple that it can be readily converted for use in the field. Various methods12-22 including inductively coupled plasma mass spectrometry (ICPMS),16 atomic absorption spectrometry,17 and high performance liquid chromatography with ICPMS18 have been developed for detection of As. Although these methods are successful in detecting As at subpicogram to subnanogram levels, they require expensive instruments, laboratory setup, and high operating cost and cannot be used for routine in-field monitoring of a large number of samples. The low-cost electrochemical methods, particularly stripping voltammetry, have attracted significant interest for their excellent sensitivity and unique ability to detect the trace levels of elements in distinct oxidation states. Due to accurate measurements of metal ions at ppb level with a (8) Smith, A. H.; Lingas, E. O.; Rahman, M. Bull. World Health Org. 2000, 78, 1093–1103. (9) Alam, M. G. M.; Allinson, G.; Stagnitti, F.; Tanaka, A.; Westbrooke, M. Int. J. Environ. Health Res. 2002, 12, 235–253. (10) Rahman, M. M.; Chowdhury, U. K.; Mukherjee, S. C.; Mandal, B. K.; Paul, K.; Lodh, D.; Biswas, B. K.; Chanda, C. R.; Basu, G. K.; Saha, K. C.; Roy, S.; Das, R.; Palit, S. K.; Quamruzzaman, Q.; Chakraborti, D. J. Toxicol. Clin. Toxicol. 2001, 39, 683–700. (11) Chowdhury, U. K.; Biswas, B. K.; Chowdhury, T. R.; Samanta, G.; Mandal, B. K.; Basu, G. C.; Paul, K.; Chanda, C. R.; Lodh, D.; Saha, K. C.; Mukherjee, S. K.; Roy, S.; Kabir, S.; Quamruzzaman, Q.; Chakraborti, D. Environ. Health Perspect. 2000, 108, 393–397. (12) Le, X. C.; Lu, X. F.; Li, X. F. Anal. Chem. 2004, 76, 26A–33A. (13) Cavicchioli, A.; La-Scalea, M. A.; Gutz, I. G. R. Electroanalysis 2004, 16, 697–711. (14) Brainina, K. Z.; Malakhova, N. A.; Stojko, N. Y. Fresenius J. Anal. Chem. 2000, 368, 307–325. (15) Hung, D. Q.; Nekrassova, O.; Compton, R. G. Talanta 2004, 64, 269–277. (16) Feng, Y. L.; Chen, H. Y.; Tian, L. C.; Narasaki, H. Anal. Chim. Acta 1998, 375, 167–175. (17) Anezaki, K.; Nakatsuka, I.; Ohzenki, K. Anal. Sci. 1999, 15, 829–834. (18) Thomas, P.; Snitechi, K. J. Anal. At. Spectrom. 1995, 10, 615–618. (19) Kim, I.-B.; Bunz, U. H. F. J. Am. Chem. Soc. 2006, 128, 2818–2819. (20) Liu, Y.; Chang, X.; Hu, X.; Guo, Y.; Meng, S.; Wang, F. Anal. Chim. Acta 2005, 532, 121–128. (21) (a) Simm, A. O.; Banks, C. E.; Compton, R. G. Electroanalysis 2005, 17, 1727–1733. (b) Jia, Z.; Simm, A. O.; Dai, X.; Compton, R. G. J. Electroanal. Chem. 2006, 587, 247–253. (22) Hignett, G.; Wadhwan, J. D.; Lawrence, N. S.; Hung, D. Q.; Prado, C.; Marken, F.; Compton, R. G. Electroanalysis 2004, 16, 897–903.

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rapid analysis time, stripping voltammetric methods are popular for the analysis of As at conventional electrodes,21-26 e.g., mercury (Hg), platinum (Pt), silver (Ag), and gold (Au) electrodes. In practice, Au and Au nanoparticle-modified electrodes have been found to be superior for the detection of As,21-24,27-33 and the limit of detection (LOD) achieved has been 0.02-0.75 ppb21,24,27,32,33 with anodic stripping voltammetry (ASV),27,32,33 differential pulse ASV,28 and sono-electroanalytical methods.21a Although high sensitivity and low LOD can be achieved for As detection by using Au and Au nanoparticle-modified electrodes, the major problems associated with the available Au-based electrodes are (i) the interference due to other metal ions such as Cu(II) present in the natural water, (ii) high detection potential, and (iii) the interference due to supporting electrolyte anions. Among these, anodic stripping voltammetric detection of As(III) without interference of Cu remains a challenging aspect since the presence of Cu(II) either increases or decreases the stripping current (depending on the concentration)34 which restricts the design of Aubased As sensors for the real sample analysis. The electrochemical behavior of Au electrodes exhibits a strong relationship with their crystallographic orientation. A single crystal Au(111) electrode with a clean, well-defined and wellordered surface can give more definite electrochemical behavior for the As detection and is suitable to study the deposition mechanism.21b The fabrication of a self-assembled monolayer (SAM) of short chain thiols on Au electrodes is a process of great interest to achieve a single crystallographic domain at the polycrystalline gold (poly-Au) electrode. The potential-controlled partial reductive desorption of the SAM formed over the poly-Au electrode enables the design of a SAM at the molecular level at which the SAM molecules can selectively block specific domains of the poly-Au surface while leaving other domains bare (i.e., uncovered).35 In the present study, we introduce a simple and easy method for the fabrication of a Au(111)-like poly-Au electrode which is employed for the first time for successful and selective detection of As(III). The fabrication of a Au(111)-like poly-Au electrode was carried out by the formation of a submonolayer of n-butyl thiol (n-BT) on a poly-Au electrode. At this modified electrode, n-BT selectively blocks the Au(100) and Au(110) domains of the polyAu surface while the Au(111) domain remains free. To best of (23) Simm, A. O.; Banks, C. E.; Compton, R. G. Electroanalysis 2005, 17, 335– 342. (24) Forsberg, G.; O’Laughlin, J. W.; Megargle, R. G. Anal. Chem. 1975, 47, 1586–1592. (25) Sadana, R. S. Anal. Chem. 1983, 55, 304–307. (26) Ivandini, T. A.; Sato, R.; Makide, Y.; Fujishima, A.; Einaga, Y. Anal. Chem. 2006, 78, 6291–6298. (27) Hua, C.; Jagner, D.; Renman, L. Anal. Chim. Acta 1987, 201, 263–268. (28) Kopanica, M.; Novotny, L. Anal. Chim. Acta 1998, 368, 211–218. (29) Hamilton, T. W.; Ellis, J.; Florence, T. M. Anal. Chim. Acta 1980, 119, 225–233. (30) (a) Feeney, R.; Kounaves, S. P. Anal. Chem. 2000, 72, 2222–2228. (b) Feeney, R.; Kounaves, S. P. Talanta 2002, 58, 23–31. (31) Salaun, P.; Planer-Friedrich, B.; van den Berg, C. M. G. Anal. Chim. Acta 2007, 585, 312–322. (32) Dai, X.; Nekrassova, O.; Hyde, M. E.; Compton, R. G. Anal. Chem. 2004, 76, 5924–5929. (33) (a) Hossain, M. M.; Islam, M. M.; Ferdousi, S.; Okajima, T.; Ohsaka, T. Electroanalysis 2008, 20, 2435–2441. (b) Chowdhury, A.-N.; Alam, M. T.; Okajima, T.; Ohsaka, T. J. Electroanal. Chem. 2009, 634, 35–41. (c) Jena, B. K.; Raj, C. R. Anal. Chem. 2008, 80, 4836–4844. (34) Song, Y.; Swain, G. M. Anal. Chim. Acta 2007, 593, 7–12. (35) El-Deab, M. S.; Ohsaka, T. Electrochem. Commun. 2003, 5, 214–219.

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our knowledge, the present strategy for the fabrication of an As sensor by reductive desorption of n-BT from the poly-Au substrate for the selective detection of As has not been reported so far. It would be promising to use such a modified electrode for the voltammetric sensing application as high sensitivity and low detection limit can be achieved easily. This Au(111)-like poly-Au electrode is found to be highly sensitive and stable and can be employed for the selective detection of As(111) well below the guideline value set by WHO. EXPERIMENTAL SECTION Materials. All the reagents (of analytical grade) used in this study were purchased from either Kanto Chemicals Co. Ltd. (Tokyo, Japan) or Wako Pure Chemicals Industries Ltd. (Osaka, Japan) and used without further purification. All the solutions were prepared with Milli-Q (18 MΩ cm) deionized water. A fresh solution of 0.1 M phosphate buffer (PB) [NaH2PO4/H3PO4, pH 1] stock was prepared daily. The real sample analysis was done by using Yokohama tap water from Yokohama Waterworks Bureau, Japan. (Caution! The As sample is highly toxic; proper care must be taken in handling and waste disposal.) Instrumentation. All electrochemical measurements were performed using a two-compartment three-electrode cell with a poly-Au or modified poly-Au working electrode, a spiral Pt wire counter electrode, and Ag|AgCl|KCl (sat.) reference electrode. The counter electrode compartment was separated from the working electrode compartment by a sintered frit. Cyclic voltammograms (CVs) and square wave anodic stripping voltammograms (SWASVs) were recorded using a computer-controlled ALS CHI-760D electrochemical analyzer. Prior to each experiment, either N2 or O2 gas was bubbled directly into the cell for 30 min to obtain either N2 or O2 saturated solution and electrochemical measurements were carried out under either of these two gases according to the requirement. All the measurements were accomplished at room temperature (25 ± 1 °C). The pH of the PB solution was measured using a standard pH meter (IM-55G, TOA Electronics Ltd., Japan). Preparation of the Electrodes. Prior to modification, polyAu electrodes (1.6 mm in diameter) with an exposed surface area of 2.01 × 10-2 cm2 were polished with aqueous slurries of successively finer alumina powder (down to 0.06 µm), sonicated for 10 min in Milli-Q water and then electrochemically pretreated in 0.1 M H2SO4 solution by repeating the potential scan in the range of -0.20 to 1.50 V vs Ag|AgCl|KCl (sat.) at 0.1 V s-1 until CV characteristics for a clean Au electrode was obtained. A roughness factor (r.f.) of 1.7 was estimated for the poly-Au electrode as calculated from the charge consumed during the formation of the surface oxide monolayer.36 Steadystate voltammograms were obtained at the rotating ring-disk electrode (RRDE) using a rotary system from Nikko Keisoku, Japan. For the steady-state voltammetric measurements, a compartment of 200 cm3 was used to eliminate the possible depletion of O2 concentration, and O2 gas was flashed over the cell solution during the measurements. The Au disk (Φ ) 6.0 mm)-Pt ring (Φin ) 7.0 and Φout ) 9.0 mm) RRDE was (36) Trasatti, S.; Petrii, O. A. Pure Appl. Chem. 1991, 63, 711–734.

mechanically polished in the same manner as the poly-Au electrode. After being polished to a mirror surface and cleaned ultrasonically, the Au-disk of the RRDE was electrochemically treated in 10% HCl solution at 1 mA cm-2 for 30 min to remove any Pt contamination. Then, the RRDE was transferred into 0.1 M H2SO4 solution, and hydrogen gas was evolved at both the Pt-ring and Au-disk at 100 mA cm-2 for 1 min to desorb the chloride ions.37 Prior to its use, the Au(111) single-crystal electrode was annealed in a hydrogen flame until the characteristic cyclic voltammogram (CV) (taken after quenching in deionized water) in 0.01 M HClO4 solution containing 0.09 M NaClO4 was obtained.38 The fabrication of the SAM of n-BT over the poly-Au electrodes was carried out by immersing the clean poly-Au electrodes into 1.0 mM ethanolic solution of n-BT for 1 h. The poly-Au electrodes were then washed with ethanol to remove the nonchemisorbed thiol molecules and introduced into the cell containing N2saturated 0.5 M KOH solution to perform the partial reductive desorption in order to prepare a submonolayer coverage of the n-BT. This was done by repeating the potential scan 4 times between -0.35 and -0.90 V vs Ag|AgCl|KCl (sat.) at a scan rate of 0.05 V s-1. This potential window allows the reductive desorption of the weakly bound (chemisorbed) n-BT from the Au(111) component of the poly-Au surface, leaving the polyAu surface with a submonolayer coverage of n-BT. The electrodes covered with full monolayer and submonolayer of n-BT are hereafter denoted as BT/Au and sub-BT/Au electrodes, respectively. For the determination of the surface coverage of n-BT (Γ), the chemisorbed n-BT on the poly-Au electrode was electrochemically oxidized by sweeping the potential between 1.00 and 1.50 V in H2SO4 solution at a scan rate of 0.1 V s-1. RESULTS AND DISCUSSION Generation of the Au(111) Surface at the Poly-Au Electrode via the Partial Reductive Desorption. The cathodic desorption of alkane thiol, sulfide, and disulfide SAMs on Au-single crystal and mercury surfaces by sweeping the potential to sufficiently negative potentials in strong alkaline media has been previously described in many reviews39,40 as the most common method for electrochemical removal of such films. It has been reported that, for short-chain thiol species (e.g., cysteine and mercaptoacetic acid), multiple reductive desorption peaks can be observed for the reductive desorption of the respective thiol from different crystallographic surface domains of the poly-Au electrode.41 Thus it is possible to generate a Au(111) surface on the poly-Au electrode by a partial reductive desorption of the thiol within a specific potential range which allows the removal of thiol molecules specifically from the Au(111) domain. But the SAMs formed by these thiols with bulky end groups would be disorga(37) Maruyama, J.; Inaba, M.; Ogumi, Z. J. Electroanal. Chem. 1998, 458, 175– 182. (38) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1–11. (39) Finklea, H. O. Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; J. Wiley and Sons: Chichester, 2000. (40) Love, J. C.; Stroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1170. (41) El-Deab, M. S.; Arihara, K.; Ohsaka, T. J. Electrochem. Soc. 2004, 151, E213-E218.

Figure 1. CVs obtained at the BT/Au electrode in N2-saturated 0.5 M KOH solution for the reductive desorption of the chemisorbed n-BT SAM. The inset shows the CVs for the reductive desorption of the n-BT formed over (a) Au(111), (b) Au(100), and (c) Au(110) singlecrystalline electrodes in N2-saturated 0.5 M KOH solution. Potential scan rate: 0.05 V s-1.

nized and porous and could work as a ‘molecular brush’.42 On the other hand, thiols with a longer alkane chain would be organized and can block the electrode surface due to the lateral interaction between the long alkane chains.43 But the problem regarding these long alkane chain thiols is associated with the difficulty in their potential-controlled selective removal from the Au(111) facet of the poly-Au during reductive desorption.43-45 In the present study, it has been found that the n-BT gives multiple reductive desorption peaks during the reductive desorption of n-BT SAM from the poly-Au electrode in strong alkaline media, and sufficient lateral interaction between the alkane chains can be expected to make a compact coverage. Figure 1 shows the CVs for the reductive desorption of the n-BT SAM formed over the poly-Au electrode measured in N2-saturated 0.5 M KOH solution at a potential scan rate of 0.05 V s-1. One small cathodic peak along with a broad one was recognized at -0.78 and -1.10 V, respectively. The origin of the multiple reduction peaks could be attributed to the existence of different single-crystalline surface domains comprising the poly-Au electrode surface.46 A noteworthy observation of Figure 1 is the disappearance of the first cathodic peak located at -0.78 V after the second potential cycle in contrast to the cathodic peak at -1.10 V which decreases with potential cycle gradually and exists even after several cycles. This reflects the relatively weak interaction between the thiol molecules and the surface of the poly-Au electrode responsible for the former peak and also a stronger binding interaction between the thiol and the Au surface domains responsible for the latter broad peak. In order to assign the origin of each peak to its corresponding surface domain, the reductive desorption experiments were performed for the n-BT adsorbed at the Au(111), Au(100), and Au(110) single-crystalline electrodes. The inset of (42) Herzog, G.; Arrigan, D. W. M. Anal. Chem. 2003, 75, 319–323. (43) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (44) Campina, J. M.; Martins, A.; Silva, F. Electrochim. Acta 2008, 53, 7681– 7689. (45) Esplandiu, M. J.; Hagenstrom, H.; Kolb, D. M. Langmuir 2001, 17, 828– 838. (46) Zhong, C.-J.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9–13.

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Figure 1 shows a comparative study for the reductive desorption of n-BT formed at the single-crystalline Au electrodes [i.e., Au(111) (curve a), Au(100) (curve b), and Au(110) (curve c)] measured in N2-saturated 0.5 M KOH solution at a potential scan rate of 0.05 V s-1. Sharp cathodic peaks were observed at -0.78, -1.00, and -1.10 V corresponding to the reductive desorption of n-BT from the Au(111), Au(100), and Au(110) single crystal electrodes, respectively. From the comparison of the peak position of the reductive desorption patterns shown in the inset of Figure 1 with that observed for the reductive desorption of n-BT from the poly-Au electrode surface (Figure 1), the peaks for the reductive desorption of n-BT from the Au(100) and Au(110) domains of the poly-Au surface appeared as a broad one similar to that reported for cystamine.42 The inset of Figure 1 also shows that the Au(111) single crystal electrode surface (curve a) exhibits the lowest binding strength with the n-BT, as indicated from its relatively low cathodic reductive desorption potential compared to those of the Au(100) and Au(110) single crystal electrodes (curves b and c, respectively). Therefore, the observed cathodic peaks at -0.78 and -1.10 V (Figure 1) can be attributed to the reductive desorption of n-BT from the Au(111) and [Au(100) + Au(110)] domains of the poly-Au electrode, respectively. Consequently, the potential-controlled partial reductive desorption was utilized to selectively make the Au(111) domain on the poly-Au electrode surface uncoated for the fabrication of the sub-BT/Au electrode via cycling the potential four times between -0.35 and -0.90 V, keeping the other conditions same as Figure 1 (Figure S-1, Supporting Information). Characterization of the Sub-BT/Au Electrode. Determination of Surface Coverage. For the determination of the surface coverage of n-BT (Γ) over the poly-Au electrode, quantitative oxidation of the chemisorbed n-BT (shown by eq 1) at the polyAu electrode was carried out in N2-saturated 0.1 M H2SO4 solution by sweeping the electrode potential over the potential zone of the electrode surface oxidation (Figure S-1, Supporting Information).47 AuSR + 2H2O a Au + RSO2H + 3e- + 3H+ (pH < 4) (1) The amount of charge (Q) consumed for the oxidation of the specifically adsorbed n-BT was determined by subtracting the charge of the poly-Au electrode oxidation from the total oxidation charge (n-BT + poly-Au electrode), and Γ was calculated from eq 2,48,49 Γ ) Q/nAe

(2)

where Γ is the surface coverage of n-BT, n is the number of electrons involved in the oxidation of chemisorbed n-BT and equals 3 according to eq 1, e is the charge of an electron (1.60 × 10-19 C), and A is the geometric area of the poly-Au. Values of Γ for the BT/Au (ΓBT) and sub-BT/Au (Γsub-BT) electrodes were (47) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335–359. (48) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283–289. (49) Miah, M. R.; Ohsaka, T. Anal. Chem. 2006, 78, 1200–1205.

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Figure 2. CVs obtained at (a,b) BT/Au, (c) sub-BT/Au, and (d) polyAu electrodes in (a) N2- and (b, c, d) O2-saturated 0.5 M KOH solutions. Potential scan rate: 0.1 V s-1.

2.02 × 10-9 and 1.43 × 10-9 mol cm-2, respectively. The ratio of the surface coverage of n-BT for the sub-BT/Au to that for the BT/Au is 0.70, which reflects that about 30% of the surface area of the poly-Au was uncovered during the partial reductive desorption of n-BT from the poly-Au surface. Thus the real surface area of the uncovered Au(111) domain can be calculated as 0.0102 cm2 from the determined real surface area of the poly-Au electrode (0.0341 cm2). Oxygen Reduction Reaction (ORR) at the Sub-BT/Au Electrode. The ORR is used in this section as a probing reaction, which can reflect the surface state of the poly-Au electrode after different pretreatment processes (i.e., full SAM, sub-SAM, or bare poly-Au electrodes). This reaction exhibits a strong relationship with the crystallographic orientation of the Au electrodes.50 Figure 2 shows the CVs obtained for the ORR at (a, b) BT/Au, (c) sub-BT/Au, and (d) poly-Au electrodes in (a) N2- and (b, c, d) O2-saturated 0.5 M KOH solutions at a scan rate of 0.1 V s-1. The ORR is completely blocked at the BT/Au electrode (Figure 2b) in the potential range between 0.20 and -0.50 V vs Ag|AgCl|KCl (sat.). It has been reported that the cathodic reduction of O2 is not feasible at SAM/Au electrodes, but it can only proceed if some pinholes are formed within the SAM/Au.35 When the partial reductive desorption of n-BT SAM was performed to uncover the Au(111) domain of poly-Au to fabricate the sub-BT/Au electrode, pores are expected to be formed in the SAM. Figure 2c shows the ORR at the sub-BT/Au electrode where the reduction of O2 can be observed, and it can be concluded that the ORR is taking place at the bare (uncovered) fraction of the poly-Au surface, i.e., at the Au(111) surface domain, but not at the other surface domains of the poly-Au electrode [i.e., Au(100) and Au(110)]. On the reverse scan, the oxidation peak of HO2- (the 2e- reduction product of O2), which is absent at the poly-Au electrode (Figure 2d), can be observed at the subBT/Au electrode, further confirming the removal of the thiol from the Au(111) domain.35 It has been reported that the alkane chains (all in the trans-configuration) of the alkane thiols are tilted slightly on the metal surface by ∼20-30°, resulting in the formation of a densely packed, highly oriented monolayer.51 The tilted alkane chains of the thiol molecules on the Au(100) and Au(110) surface (50) Strbac, S.; Adzic, R. Electrochim. Acta 1996, 41, 2903–2908. (51) Gooding, J. J.; Hibbert, D. B. Trends Anal. Chem. 1999, 18, 525–533.

domains may create a “shade” over the Au(111) domain even after the partial reductive desorption which results in a resistance to the diffusion of O2 to the Au(111) domain. Thus a negative shifting in the ORR was observed at the sub-BT/Au electrode compared to that observed at the poly-Au electrode. In addition, hydrodynamic voltammetry was also employed to confirm the ORR at the uncovered Au(111) domain of the subBT/Au electrode. The hydrodynamic voltammograms were obtained at the modified electrode in O2-saturated 0.5 M KOH solution at electrode rotation rates in the range of 400-2000 rpm and at a potential scan rate of 0.005 V s-1 (inset of Figure S-3, Supporting Information). The curves in the negative and positive current regions represent the disk current (ID) for O2 reduction and the corresponding ring current (IR) for the reoxidation of the product HO2- at various rotation rates, respectively. The limiting current (iL) for the ORR at the Au disk of the RRDE can be presented by eq 3 known as the Levich equation.52-54 iL ) 0.62nFACbD02/3ν-1/6ω1/2

(3)

where n is the number of electrons involved in the reduction of O2, F is the Faraday constant (96,485 C), A is the geometric surface area of the RDE, Cb is the bulk concentration of O2 (1.1 × 10-3 mol dm-3), D0 is the diffusion coefficient of O2 (1.93 × 10-5 cm2 s-1), ν is the kinematic viscosity of the solution (ca. 0.01 cm2 s-1),55 and ω is the angular rotation rate of the electrode expressed as radian per second (ω ) 2πf/60, where f is the rotation in rpm). Thus, the above equation gives a linear dependence of iL on ω1/2 with a zero intercept and a slope proportional to the apparent number of electrons exchanged per O2 molecule in the overall cathodic reaction. The limiting current densities (jL) were plotted as a function of ω1/2 (Figure S-3, Supporting Information). The experimental values nicely fall on a straight line passing through the origin, suggesting that the reaction is controlled by the diffusion of O2 to the electrode surface. The values of jL were also calculated based on eq 3 using the values of n equal to 2 and 4 and other parameters as given above and plotted as a function of ω1/2 (Figure S-3, dotted lines). The slope of the experimental line was 0.19 mA cm-2 (rad s-1)-1/2 which is consistent with that of the theoretical line for n ) 2 (0.20 mA cm-2 (rad s-1)-1/2), suggesting that the ORR at the sub-BT/Au electrode proceeds through a two-electron pathway. The pathway of the ORR can also be confirmed using the Koutecky-Levich (K-L) equation given by eq 4.52-54 1/j ) 1/jk + 1/(Bω1/2)

(4)

where jk is the kinetic current density and equal to nFCk, B is the so-called B-factor and equal to 0.62 nFCbD02/3ν-1/6, k is the rate constant, and the other symbols have their usual meanings. A plot of j-1 vs ω-1/2 should yield a straight line having a slope equal to B-1 and an intercept equal to jk-1. The values of j-1 (52) Chang, C.-C.; Wen, T.-C.; Tien, H.-J. Electrochim. Acta 1997, 42, 557–565. (53) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamental and Applications; J. Wiley and Sons: New York, 2001; Chapts. 3 and 9, and the refs therein. (54) Miah, M. R.; Ohsaka, T. J. Electrochem. Soc. 2007, 154, F186–F190. (55) Tsushima, M.; Tokuda, K.; Ohsaka, T. Anal. Chem. 1994, 66, 4551–4556.

were obtained at various potentials of the polarization curves shown in Figure S-3 inset and were plotted as a function of ω-1/2. Figure S-4 (Supporting Information) shows the thusobtained typical K-L plots. The linearity and parallelism of the lines obviously suggest that the ORR follows first-order kinetics.56,57 The dotted line corresponds to the theoretical K-L plot for n ) 2. The figure clearly shows that the slopes of the experimental lines are very close to that of the theoretical line, suggesting that the ORR at the sub-BT/Au electrode proceeds through an exclusive two-electron pathway. The values of n at -0.40, -0.50, -0.70, and -0.75 V were calculated from the slopes of the corresponding K-L plots as 1.94, 1.93, 1.93, and 1.94, respectively. The value of IR/NID (where N is the collection coefficiency) is also very important in predicting the pathway of the ORR.58,59 The IR/ NID ratio is very close to unity, suggesting that the ORR proceeds exclusively through a two-electron pathway forming H2O2 while this ratio is zero for an exclusive four-electron ORR producing H2O. The IR/NID values were calculated using the hydrodynamic voltammograms (Figure S-3 inset, Supporting Information) obtained at the sub-BT/Au-disk and Pt-ring RRDE in O2-saturated 0.5 M KOH solution and plotted as a function of the electrode potential (Figure S-4 inset). The figure clearly shows that the values of IR/NID are almost close to unity, implying that the ORR at this electrode essentially takes place through a one-step two-electron pathway. From the fact that the reductive desorption of n-BT from the [Au(100) + Au(110)] surface domains was observed at more negative potential compared to that obtained from the Au(111) surface domain and the ORR proceeded through a two-electron pathway at the sub-BT/Au electrode, it can be concluded that the potential-controlled partial reductive desorption of n-BT from the poly-Au electrode selectively allows the Au(111) domain to be uncoated with n-BT, resulting in the sub-BT/Au electrode. Detection of As(III). The electrochemical response of the subBT/Au electrode toward As(III) was first examined with cyclic voltammetry. Figure 3a depicts the typical CV obtained at the subBT/Au electrode for As(III) in 0.1 M PB solution (pH 1). The broad voltammetric peak observed at -0.20 V during the cathodic sweep is ascribed to the reduction of As(III) to As(0), whereas the sharp peak observed at 0.16 V during the reverse anodic sweep corresponds to the reoxidation of As(0) to water-soluble As(III). The potential for the reduction of As(III) to As(0) is much less negative (∼100 mV) than that observed on the Au nanoparticlemodified GC electrode,32 indicating that the reduction process is more favorable at the Au(111) domain. No such peaks were observed in the absence of any As(III) at this electrode (Figure 3c). Figure 3b shows the CV response at the BT/Au electrode in the presence of As(III) under the same conditions as Figure 3a. No voltammetric peak was recognized at this electrode for the As(III)/As(0) redox process, which further confirms a complete coverage of the poly-Au electrode by n-BT after its chemisorption. The stripping voltammetric technique is known to be one of the most sensitive techniques in the electroanalysis of trace metals (56) Gochi-Ponce, Y.; Alonso-Nunez, G.; Alonso-Vante, N. Electrochem. Commun. 2006, 8, 1487–1491. (57) Pattabi, M.; Castellanos, R. H.; Castillo, R.; Ocampo, A. L.; Moreira, J.; Sebastian, P. J.; McClure, J. C.; Mathew, X. Int. J. Hydrogen Energy 2001, 26, 171–174. (58) El-Deab, M. S.; Ohsaka, T. Electrochem. Commun. 2003, 5, 214–219. (59) El-Deab, M. S.; Ohsaka, T. J. Electrochem. Soc. 2006, 153, A1365–A1371.

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Figure 3. CVs obtained at (a) sub-BT/Au and (b) BT/Au electrodes in the (a, b) presence and (c) absence of 20 µM As(III) in 0.1 M PB solution (pH 1). Potential scan rate: 0.1 V s-1.

Figure 4. SWASV responses of 5 µM As(III) obtained at the subBT/Au electrode in 0.1 M of different supporting electrolytes: (a) PB (pH 1), (b) HNO3, and (c) H2SO4.

in different samples.60 The SWASV technique has been used for the detection of trace amount of As(III). Detection of As(III) using SWASV involves two steps: (i) deposition of As(0) at an optimized potential for a particular duration of time, and (ii) anodic stripping of the deposited As(0). The anodic stripping signal has been used to monitor the concentration of As(III) in solution. The deposition potential and the time were optimized to be -0.30 V and 100 s, respectively; 100 s deposition time was used to avoid the saturation response at higher concentration of arsenic. The magnitude of the stripping current depends on the choice of supporting electrolyte (Figure 4). The sub-BT/Au electrode shows significantly higher stripping current when PB solution (pH 1) was used as the supporting electrolyte (Figure 4a). Although Cl- ions can make a complex with As(III) to give an intense stripping peak,30,61 in the present study, no HCl solution was used as supporting electrolyte due to strong adsorption of Cl- ions at the Au(111) domain. Because the peak current for stripping of As(0) is significantly higher in the case of PB solution, we have (60) (a) Kissinger, P. T.; Heineman, W. R., Eds. Laboratory Techniques in Electrochemistry; Marcel Dekker: New York, 1984; p 499. (b) Sinko, I.; Dolezal, J. J. Electroanal. Chem. 1970, 25, 299–306. (c) Lund, W.; Salberg, M. Anal. Chim. Acta 1975, 76, 131–141. (d) Wu, H. P. Anal. Chem. 1994, 66, 3151–3157. (61) Arnold, J. P.; Johnson, R. M. Talanta 1969, 16, 1191–1207.

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Figure 5. SWASV responses at the sub-BT/Au electrode toward As(III) at various concentrations (from bottom to top: 0, 0.12, 0.25, 0.50, 1.00, 1.50, 2.00, 3.00, 4.00, 5.00, 6.00, 8.00, 10.00, 12.50, and 15 µM) in 0.1 M PB solution (pH 1). The inset shows the corresponding calibration curve.

used 0.1 M PB solution as the supporting electrolyte in all the experiments for the detection of As(III). The number of As(0) atoms deposited on the sub-BT/Au electrode was obtained from the charge consumed during the stripping of As(0) from the electrode surface by linear sweep voltammetry at a particular sweep rate (0.05 V s-1). Under the optimized conditions (deposition potential: -0.30 V, deposition time: 100 s, [As(III)]: 5 µM, supporting electrolyte: 0.1 M PB solution of pH 1), the charge obtained by integrating the area under the stripping peak was 3.10 µC (6.32 × 1014 atoms cm-2). The maximum theoretical coverage of As(0) on the sub-BT/ Au electrode can be calculated using the atomic radius of As(0) (1.38 × 10-8 cm)30a and assuming an atomically flat electrode surface and densely packed atomic layer. The charge theoretically calculated for the deposition of monolayer coverage of As(0) on the sub-BT/Au electrode was 8.19 µC, which corresponds to 1.67 × 1015 atoms cm-2. Comparison of the calculated value with the experimentally obtained value implies that only ∼38% of the sub-BT/Au electrode is covered by As(0). Figure 5 shows the SWASV responses of the sub-BT/Au electrode toward As(III) at different concentrations under the optimum conditions. Linear increase in the peak current was observed up to the concentration of 15 µM with a correlation coefficient of 0.999. The sensitivity of the sub-BT/Au electrode was obtained from the slope of the calibration plot (Figure 5 inset) and was 27.27 ± 0.01 µA cm-2 µM-1. The experimental detection limit was 0.28 ppb (S/N ) 3) which is much lower than that obtained at poly-Au electrode (13.7 ppb) using the same method of detection.23 As the detection limit is well below the guideline value of As(III) in drinking water set by WHO, the sub-BT/Au electrode can be successfully used for the real sample analysis. Selectivity. Selective detection of As(III) in the real sample is a challenging task, as the other metal ions commonly present in the groundwater can be coprecipitated and stripped off under the experimental conditions used for the detection of As(III). Among the metal ions, Cu(II) shows the major interference in the ASV detection of As(III). The electrodes which have a high sensitivity

Figure 6. SWASV responses obtained at the sub-BT/Au electrode in 0.1 M PB solution (pH 1) containing 5 µM As(III) in the presence of various concentrations of Cu(II). Each addition increased the concentrations of Cu(II) by 5 µM. Initial concentration of Cu(II) is 0 µM. The dotted line corresponds to the blank response.

Figure 7. SWASV responses obtained at the Au(111) single-crystal electrode in 0.1 M PB solution (pH 1) containing 5 µM As(III) in the presence of various concentrations of Cu(II). Each addition increased the concentrations of Cu(II) by 10 µM. Initial concentration of Cu(II) is 0 µM. The dotted line corresponds to the blank response.

toward As(III) suffer from the interference from Cu(II).23,62 Besides, the peak current for the oxidation of the As(0) to As(III) at the poly-Au electrode was found to vary irregularly with the concentration of the coexisting Cu(II) ion (Figure S-5, Supporting Information). To examine the analytical performance of the subBT/Au electrode toward As(III) in the presence of Cu(II), measurements have been carried out in the presence of Cu(II). Figure 6 shows the SWASV responses obtained at the sub-BT/ Au electrode for 5 µM As(III) in the presence of different concentrations of Cu(II) (5-40 µM). The peak observed at 0.13 V corresponds to the anodic oxidation of the deposited As(0), whereas the peak at 0.28 V is due to the oxidation of Cu(0) to Cu(II). The stripping signal obtained for Cu(II) is 150 mV more positive than that for As(III), and the position and current of the stripping peak of As(III) do not change during the addition of Cu(II). The interference of Cu(II) toward the sensitivity of the subBT/Au electrode for As(III) detection was also studied. SWASV measurements were performed for the detection of various concentrations of As(III) at the sub-BT/Au electrode in the presence of 30 µM Cu(II). The sensitivity of the sub-BT/Au electrode toward As(III) in the presence of Cu(II) was determined as 27.01 ± 0.01 µA cm-2 µM-1 compared with that (27.27 ± 0.01 µA cm-2 µM-1) obtained in the absence of Cu(II). The presence of Cu(II) did not affect the sensitivity of the present electrode toward As(III) and only a 5 mV positive shifting of the As(III) peak was recognized compared with the case of the absence of Cu(II). It has been reported that the interference of Cu(II) during the ASV detection of As(III) at the poly-Au electrode arises from the formation of intermetallic compounds, such as Cu3As2.33,34,63 Interestingly, the sub-BT/Au electrode does not seem to favor the formation of such compounds, and thus it has been successfully used for the detection of As(III) in the presence of Cu(II). In the present study, we also employed a Au(111) singlecrystal electrode for the detection of As(III) in the presence of various amounts of Cu(II) where no interference from Cu(II) was

recognized (Figure 7), confirming the detection of As(III) at the Au(111) surface domain of the present sub-BT/Au electrode. The superior detection of As(III) without any interference from Cu(II) has been also reported by others at the Au(111) facet-enriched Au nanoparticle-modified electrodes.33b,c Stability and Reproducibility. The stability of the sub-BT/ Au electrode was tested by employing the electrode for 15 repetitive stripping voltammetric measurements in 0.1 M PB containing 5 µM of As(III). This electrode was then kept in deionized water and subjected to another 10 measurements after 24 h. No observable change in the stripping peak position and peak height for As(III) was noticed in both sets of experiments. The voltammetric measurements were also performed at the subBT/Au electrode for measuring the current at regular intervals (4-6 h) over a period of 24 h. The magnitude of the stripping current did not change appreciably in all these experiments (Figure S-6A, Supporting Information), showing that the sub-BT/ Au electrode is very stable and is free from deactivation. The longterm storage stability was tested by measuring the electrode response for a period of one week (Figure S-6B, Supporting Information). No significant change in the magnitude of the peak height was noticed within the first 4 days, and only 8% decrease in the peak height was observed after a storage time of 7 days in deionized water at room temperature, demonstrating the longterm storage and operational stability. Analysis of As(III) in Tap Water. Since it is difficult to obtain arsenic-contaminated water in Japan, tap water spiked with As(III) was selected as a quasireal sample. The tap water for drinking, according to Yokohama Waterworks Bureau, is well controlled and it has pH of 7.51 and contains various minerals (in ppm: 0.03 Al, 0.01 Fe, 0.002 Cu, 39 Ca and Mg, 5.0 Cl-, and 0.70 NO3-).64 There is no As, Zn, Cr, and Pb reported. Tap water samples were collected from five railway stations in Yokohama and tagged as YTW1-YTW5. Figure 8 shows the SWASV response of Yokohama, Japan, tap water (S1) spiked with 1, 3, 5, 10, and 15 µM As(III). The inset shows that a linear relationship (R2 ) 0.999) obtained when the current response was plotted against

(62) (a) Dai, X.; Compton, R. G. Electroanalysis 2005, 17, 1325–1330. (b) Song, Y.; Swain, G. M. Anal. Chem. 2007, 79, 2412–2420. (63) Dai, X.; Compton, R. G. Electroanalysis 2005, 17, 1835–1840.

(64) http://www.city.yokohama.jp/me/suidou/os/suidou-suishitsu/suidou/ suishitsu-kekka.html. (Accessed February 12, 2010).

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Figure 8. SWASV responses at the sub-BT/Au electrode toward spiked As(III) in Yokohama tap water with various concentrations of As(III) (from bottom to top: 0, 1.00, 3.00, 5.00, 10.00, and 15.00 µM) in 0.1 M PB solution (pH 1). The inset shows the corresponding calibration curve. Table 1. Determination of Spiked As(III) in Yokohama Tap Water at the Sub-BT/Au Electrode (n ) 3)

CONCLUSIONS A simple and easy method of fabrication of the Au(111)-like poly-Au electrode has been successfully introduced for the selective detection of environmentally toxic As(III) using anodic stripping square wave voltammetry for the first time. The potentialcontrolled partial reductive desorption of the n-BT SAM formed over the poly-Au electrode enabled the selective desorption of some thiol molecules, and thus switching of the electrochemical behavior of the poly-Au electrode to single-crystalline ones could be achieved. This fabricated electrode is highly sensitive and selective to As(III) and can detect As(III) even in the presence of a high concentration of Cu(II) without any interference. The exposed Au(111) surface domain of the present electrode plays a key role in the selective detection of As(III). The analytical application of the present electrode toward the detection of As(III) in tap water samples collected from different places of Yokohama, Japan, has been successfully demonstrated. The Au(111)-like poly-Au electrode prepared by a partial reductive desorption of the n-BT SAM can be used for the detection of trace amounts of As(III) in the real samples even in the presence of naturally occurring Cu(II). Utilization of this electrode for the sensing of other environmentally hazardous materials is underway.

concentrations of As(III) samples

taken (µM)

found (µM)

RSD (%)a

recovery (%)

YTW 1 YTW 2 YTW 3 YTW 4 YTW 5

5.00 3.00 10.00 4.00 5.00

4.92 3.01 10.02 3.99 4.95

0.20 1.32 0.26 0.75 0.85

98.4 100.3 100.2 99.7 99.0

a

Relative standard deviation.

the As(III) concentration, validating the applicability of the subBT/Au electrode for electrochemical measurement of As(III). To further demonstrate the practical use of the present electrode, all the water samples (YTW1-YTW5) were spiked with As(III) at different concentration levels, and the recovery of the As(III) was determined using the SWASV. The results are compiled in Table 1. It can be noted that the trace level of Cl- present in the tap water did not interfere with the detection of As(III), indicating that the proposed method was highly accurate, precise, and reproducible. It can be used for a direct analysis of the relevant real samples.

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ACKNOWLEDGMENT This work was financially supported by Grant-in-Aid for Scientific Research (A) (No. 19206079) to T. Ohsaka from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. SUPPORTING INFORMATION AVAILABLE Data for the fabrication of the sub-BT/Au electrode, the oxidation of n-BT from the BT/Au and sub-BT/Au electrodes, hydrodynamic voltammogram, Levich plot and K-L plot for the ORR, Cu(II) interference at the poly-Au electrode during As(III) detection, and operational and long-term storage stability of the sub-BT/Au electrode. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review May 7, 2010. Accepted October 9, 2010. AC101206J