Anal. Chem. 2006, 78, 762-769
Electrochemical Determination of Arsenite Using a Gold Nanoparticle Modified Glassy Carbon Electrode and Flow Analysis Ehsan Majid, Sabahudin Hrapovic, Yali Liu, Keith B. Male, and John H. T. Luong*
Biotechnology Research Institute, National Research Council Canada, Montreal, Quebec, Canada H4P 2R2
A flow analysis electrochemical system has been developed, characterized, and optimized for the determination of arsenite (As(III)). Sensitivity was significantly improved by the electrochemical deposition of gold nanoparticles on a dual glassy carbon electrode, which was inserted into a cross-flow thin-layer electrochemical cell. The electrochemical system was linear up to 15 ppb with a detection limit of 0.25 ppb. Gold deposition was evident from cyclic voltammetry measurements, whereas atomic force microscopy and scanning electron microscopy revealed the size and distribution of deposited gold nanoparticles. The size and density of the nanoparticles were related to the gold salt concentration, deposition time, and potential as well as the electrode position. The response to arsenite was directly related to the frequency, increment, and amplitude of the square wave voltammetry as well as the deposition time and potential. Estimated reproducibility was (1.1% at 95% confidence interval for 40 repeated analyses of 8 ppb arsenite during continuous analysis. The reproducibility was far superior if the electrochemical reduction of arsenite was performed in nitric acid instead of hydrochloric or sulfuric acid. The electrochemical system was applicable for analysis of spiked arsenic in mineral water containing a significant amount of various ion elements. Arsenic is a poisonous chemical that is widely distributed in nature and occurs in the form of inorganic or organic compounds, ranked as twentieth in abundance among the elements in the earth’s crust. Arsenic can exist in four valency states: -3, 0, +3, and +5. Under reducing conditions, arsenite, As(III), is the dominant form; arsenate, As(V), is generally the stable form in oxygenated environments. Elemental arsenic is not soluble in water. Arsenic salts exhibit a wide range of solubilities depending on pH and the ionic environment. Inorganic compounds consist of water-soluble arsenite (As(III)), the most toxic form, and arsenate (As(V)), the less toxic form, and such pollutants have been associated with many health problems such as skin lesions, keratosis (skin hardening), lung cancer, and bladder cancer.1a-c Organic arsenic species, abundant in seafood, are very much less harmful to health and are readily eliminated by the body. The release of arsenic in the environment occurs in a variety of ways * To whom correspondence should be addressed. E-mail: John.Luong@ cnrc-nrc.gc.ca.
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through industrial effluents, pesticides, wood preservative agents, combustion of fossil fuels, and mining activity.1 Indeed, arsenical insecticides have been used in agricultures for centuries and particularly lead arsenate was used in Australia, New Zealand, Canada, and the United States.1c,2 Arsenic contamination has been reported from many parts of the world including the United States, UK, Canada, and Australia; however, in terms of severity of the problem, Bangladesh tops the list, followed by India, and China.1a In these countries, arsenic has been released in the groundwater by oxidation of the arsenopyrites/pyrites (arsenic is present in more than 200 mineral species, the most common of which is arsenopyrite) from the subsoil or oxyhydroxide reduction. It has been estimated that about one-third of the atmospheric flux of arsenic is of natural origin. Volcanic action is the most important natural source of arsenic, followed by low-temperature volatilization. Inorganic arsenic of geological origin is found in groundwater used as drinking water in several parts of the world, for example, Bangladesh.1c Besides the drinking of contaminated groundwater, the people in such countries use this water for crop irrigation. Therefore, arsenic compounds find their way into soils used for rice (Oryza sativa) cultivation through polluted irrigation water and through historic contamination with arsenic-based pesticides.3 Arsenic contamination poses a particular challenge, as this pollutant can enter plants through their phosphate transporter4 and its contamination is invisible and has no taste or smell. In some areas in Bangladesh, groundwater arsenic concentrations can reach 2 mg/L (2 ppm),1c,5 whereas the World Health Organization (WHO) provisional guideline value for drinkable water is only 0.01 mg/L (10 ppb).3 An estimated Bangladesh population of 65 million is exposed to the threat of arsenic poisoning through drinking water,1a and surprisingly, at least 32 million Americans consume water containing more than 2 ppb (1) (a) Rashid, M. H.; Mridha, A. K. Arsenic contamination in groundwater in Bangladesh. 24th WEDC Conference, Sanitation and Water for All, Islamabad, Pakistan, 1998; pp 162-165. (b) Abernathy, C.; Calderson, R. L.; Chappel, W. R. Arsenic exposure and health effects; Elsevier Science Ltd.: London, 1999. (c) Arsenic and Arsenic Compounds; Enviromental Health Criteria 224 (WHO); http://www.inchem.org/documents/ehc/ehc/ ehc224.htm. (2) Peryea, F. J.; Kammereck, R. Water Air Soil Pollut. 1997, 93, 243-254. (3) Abedin, M. J.; Feldmann, J.; Meharg, A. A. Plant Physiol. 2002, 128, 11201128. (4) Lee, R. B. Ann. Bot. 1982, 50, 429-449. (5) Tondel, M.; Rahman, M.; Magnuson, A.; Chowhury, I. A.; Faruquee, M. H.; Ahmad, S. A. Environ. Health Perspect. 1999, 107, 727-729. 10.1021/ac0513562 CCC: $33.50 Published 2006 Am. Chem. Soc. Published on Web 12/22/2005
arsenic.6 The U.S. Environmental Protection Agency (EPA) is now considering a new standard in the range of 2-20 ppb.6 The presence of arsenic in food and water above a certain level presents a serious threat to public health, and it is of utmost importance to develop analytical methods for total arsenic and arsenic speciation in water and food. Excellent reviews of methods for analysis of arsenic have been presented,7a-d particularly the use of LC-MS-MS for speciation of arsenic with a detection limit of 2 pg for tetramethylarsonium.7e Laboratory-based analytical methods such as atomic absorption spectrometry, inductively coupled plasma (ICP), and ICP/mass spectrometry have been developed for sensitive analysis of arsenic compounds.8 Such methods, however, require expensive instrumentation and cost up to U.S. $8-10/sample.9a The main advantages of ICPMS over ICP-AES (Auger electron spectroscopy) are lower detection limits (subnanogram to subpicogram) with a wide linear range and isotope analysis capability of high precision. The detection limits of ICP-AES are typically in the range of submicrograms to subnanograms. However, ICPMS is more susceptible to isobaric interferences arising from the plasma. For example, hydrochloric acid and perchloric acid are not desirable for sample preparation, because the chloride ions generated in the plasma combine with the argon gas to form argon chloride. This has the same mass as arsenic (75), which could lead to error if not corrected. Field test kits are much less expensive, U.S. $0.5/sample, but they are less reliable for samples with 100 ppb or less.9a Indeed, ∼33% of the measurements would have to be considered false negative, particularly in the range below 70 ppb.9b In brief, such test kits involve the reduction of arsenite and arsenate by zinc to give arsine gas, which is then used to produce a stain on mercuric bromide paper1b or silver diphenyl dithiocarbamate.9a This procedure has been known as the classical Gutzeit test and can be used to detect arsenic in body fluids.11 Zinc can be replaced by sodium borohydride, and the use of a calculator-style device to measure the stain developed photometrically rather than by eye provides a detection limit down to 5 ppb in laboratory environments.11 Nevertheless, the test requires several hundred milliliters of the sample to be acidified by strong acid, and the production of toxic arsine gas might present some significant drawbacks for this procedure. Electrochemical detection techniques may provide an attractive alternative due to low-cost instrumentation and ease of portability. (6) Superior Arsenic Detection Capabilities. battelle.org/environment/pdfs/ arsenic-detection.pdf. (7) (a) Talmi, Y.; Feldman, C. The determination of Traces of Arsenic: A review. In Arsenical Pesticides; ACS Symposium Series 7;Woolson, E. A., Ed.; American Chemical Society: Washington, DC, 1995. (b) Hung, D. Q.; Nekrassova, O. P.; Compton, R. G. Talanta 2004, 64, 269-277. (c) Kumaresan, M.; Riyazuddin, P. Curr. Sci. 2001, 80, 837-846. (d) Cavicchioli, A.; La-Scalea, M. A.; Gutz, I. G. R. Electroanalysis 2004, 16, 697711. (e) Corr, J. J.; Larsen, E. H. J. Anal. At. Spectrom. 1996, 11, 12151224. (8) (a) Story, W. C.; Caruso, J. A.; Heitkemper, D. T.; Perkins, L. J. Chromatogr. Sci. 1992, 30, 427-432. (b) Shum, S. C. K.; Neddersen, R.; Houk, R. S. Analyst 1992, 117, 577-582. (9) (a) Jones, E. M. Arsenic 2000: An Overview of the Arsenic Issue in Bangladesh; WaterAid Bangladesh: Dhaka, Bangladesh, 2000. (b) Rahman, M.; Mukherjee, D.; Sengupta, M. K.; Chowdhury, U. K.; Lodh, D.; Chanda, C. R.; Roy, S.; Selim, M.; Quamrussaman, Q.; Milton, A. H.; Shahidullah, S. M.; Rahman, M. T.; Chakraborti, D. Environ. Sci. Technol. 2002, 36, 53855394. (10) Vogel, A. E. Special tests for small amounts of arsenic. In A Textbook of Macro and Semi-Micro Qualitative Inorganic Analysis, 4th ed.; Vogel, A. E., Ed.; Longmans: London, 1954; pp 242-247. (11) Kaye, S. Am. J. Clin. Pathol. 1944, 14, 83-85.
Methods include polarography techniques,11 cathodic stripping voltammetry12 and anodic stripping voltammetry (ASV),13 the latter being most popular due to its low detection capabilities and simpler operations. Arsenic is first deposited on a working electrode and then stripped. Different electrode materials including, platinum, mercury, boron-doped diamond, modified glassy carbon (GC), and the very popular gold have been used in combination with a variety of acids such as HCl, H2SO4, HClO4, HNO3, etc., as the supporting electrolyte for arsenic determination since the electrochemical reduction of arsenic must be performed in an acidic milieu.14a-g In general, a gold electrode affords better sensitivity than a mercury electrode, and it has been assumed that As(V) is electrochemically inactive.14g The above methods report limits of detection as low as 10 parts per trillion (ppt) up to several parts per billion (ppb). For example, GC electrodes (3 mm) can be modified by gold nanoparticle (11 nm) deposition (potential stepped from +1.055 to -0.045 V) and arsenic is detected as low as 10 ppt by ASV (linear sweep or square wave) in a standard three-electrode system (static cell).14d However, the lowest contamination level of As(III) in commercially available HCl and HNO3 is just below 10 ppt, and this could pose an analytical challenge for any attempt to detect arsenic around this level. In addition, the reproducibility of the arsenite signal and the reusability of the electrode surfaces for many of these detection systems have not been clearly addressed. For example, anodic stripping voltammetry of As(III) with gold ultramicroelectrode arrays does not display adequate signal reproducibility and reusability of the electrode.14a Determination of arsenic with a detection limit of 10 µg/L by hydride generation gas diffusion flow injection analysis with electrochemical detection was reported.15a Flow injection analysis has also been designed to monitor arsenic using a Prussian blue modified screen-printed electrode with a detection limit of 2 ppb.15b This paper presents a highly reproducible ASV technique with a reusable gold nanoparticle-modified GC electrode within a flow cell to detect trace levels of arsenite at concentrations lower than 1 ppb, which is well below the standard of 10 ppb set by the WHO.3 Such a system could be adapted for continuous monitoring online for arsenic contamination within the environment. We have focused on trivalent inorganic forms (As3+) since they are highly toxic, about 60-100 times more poisonous than their pentavalent salts or organoarsenic compounds.16 In addition, up to 10% of total (12) (a) Sadana, R. S. Anal. Chem. 1983, 55, 304-307. (b) Li, H.; Smart, R. B. Anal. Chim. Acta 1996, 325, 25-32. (c) Greulach, U.; Henze, G. Anal. Chim. Acta 1995, 306, 217-223. (13) (a) Kopanica, M.; Novotny, L. Anal. Chim. Acta 1998, 368, 211-218. (b) Forsberg, G.; O’Laughlin, J. W.; Megargle, R. G.; Koirtyohann, S. R. Anal. Chem. 1975, 47, 1586-1592. (c) Sun, Y.-C.; Mierzwa, J.; Yang, M.-H. Talanta 1997, 44, 1379-1387. (d) Davis, P. H.; Dulude, G. R.; Griffin, R. M.; Matson, W. R.; Zink, E. W. Anal. Chem. 1978, 50, 137-143. (e) Simm, A. O.; Banks, C. E.; Compton, R. G. Anal. Chem. 2004, 76, 5051-5055. (14) (a) Feeney, R.; Kounaves, S. P. Anal. Chem. 2000, 72, 2222-2228. (b) Hignett, G.; Wadhawan, J. D.; Lawrence, N. S.; Hung, D. Q.; Prado, C.; Marken, F.; Compton, R. G. Electroanalysis 2004, 16, 897-903. (c) Salimi, A.; Hyde, M. E.; Banks, C. E.; Compton, R. G. Analyst 2004, 129, 9-14. (d) Dai, X.; Nekrassova, O.; Hyde, M. E.; Compton, R. G. Anal. Chem. 2004, 76, 5924-5929. (e) Banks, C. E.; Simm, A. O. Bowler, R.; Dawes, K.; Compton, R. G. Anal. Chem. 2005, 77, 1928-1930. (f) Holak, W. Anal. Chem. 1980, 52, 2189-2192. (g) Hua, C.; Jagner, D.; Renman, L. Anal. Chim. Acta 1987, 201, 263-268. (15) (a) Farrell, J. R.; Iles, P. J.; Yuan, Y. J. Anal. Chim. Acta 1996, 334, 193197. (b) Zen, J.-M.; Chen, P.-Y.; Kumar, A. S. Anal. Chem. 2003, 75, 60176022.
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arsenic has been found to exist as As3+ in uncontaminated surface and deep ocean waters.16a Applicability of the electrochemical system for analysis of spiked arsenite in mineral water was demonstrated. EXPERIMENTAL SECTION Materials. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4) and arsenic atomic spectroscopy standard solution (Fluka) in nitric acid (1000 mg/kg, prepared with As2O3, NaOH, and HNO3) were purchased from Sigma-Aldrich (St. Louis, MO). Nitric acid, hydrochloric acid (
