Anal. Chem. 2007, 79, 500-507
Reusable Platinum Nanoparticle Modified Boron Doped Diamond Microelectrodes for Oxidative Determination of Arsenite Sabahudin Hrapovic, Yali Liu, and John H. T. Luong*
Biotechnology Research Institute, National Research Council Canada, Montreal, Quebec, Canada H4P 2R2
Boron doped diamond (BDD) macro- and microelectrodes were modified by electrodeposition of platinum nanoparticles using a multipotential step electrodeposition technique and used for the oxidative determination of arsenite, As(III). The formation of Pt nanoparticles was evident from cyclic voltammetry measurement, whereas AFM and SEM revealed the size and size distribution of deposited Pt nanoparticles. Raman spectroscopy illustrated a correlation between the typical BDD signature and the number of platinum deposition cycles. Linear sweep voltammetry performed with the modified BDD microelectrode outperformed its macrocounterpart and resulted in very low detecting currents with enhanced signal-to-noise ratios. With linearity up to 100 ppb and a detection limit of 0.5 ppb, the electrochemical system was applicable for processing tap and river water samples. Over 150 repetitive runs could be performed, and electrochemical etching of platinum allowed the reuse of the BDD microelectrode. The presence of copper and chloride ions, the two most severe interferents at levels commonly found in groundwater, did not interfere with the assay. Soluble and acutely toxic inorganic arsenic of geological origin is found in groundwater used as drinkable water in several parts of the world, but mostly in Bangladesh, India, and China.1a An estimated Bangladeshi population of 65 million is exposed to arsenic poisoning through drinking water1b since groundwater arsenic levels in some locations can reach 2 mg/L.1b Besides the drinking of contaminated groundwater, the people in such countries use this source of contaminated water for crop irrigation. Thus, arsenic compounds find their way into soils used for rice (Oryza sativa) cultivation through polluted irrigation water. The World Health Organization (WHO) provisional guideline value for drinkable water is 0.01 mg/L (10 ppb)1d whereas the U.S. Environmental Protection Agency (EPA) is considering a new * To whom correspondence should be addressed. E-mail:
[email protected]. (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) Arsenic and Arsenic Compounds, Environmental Health Criteria 224 (WHO): http://www.inchem.org/documents/ehc/ehc/ehc224.htm. (c) Tondel, M.; Rahman, M.; Magnuson, A.; Chowhury, I. A.; Faruquee, M. H.; Ahmad, S. A. Environ. Health Perspect. 1999, 107, 727. (d) Abedin, M. J.; Feldmann, J.; Meharg, A. A. Plant Physiol. 2002, 128, 1120. (e) battelle.org/environment/pdfs/arsenic-detection.pdf (Superior Arsenic Detection Capabilities).
500 Analytical Chemistry, Vol. 79, No. 2, January 15, 2007
standard ranging from 2 to 20 ppb.1e The uptake of large dosages of inorganic arsenic leads to gastrointestinal symptoms, disturbances of cardiovascular and nervous system functions, and eventually fatality. Long-term exposure to arsenic via drinking water is causally related to increased risks of cancer in the skin, lungs, bladder, and kidney, as well as other skin diseases such as hyperkeratosis and pigmentation changes. Occupational exposure to arsenic, primarily by inhalation, is causally associated with lung cancer. Coal-fired power generation plants, burning vegetation, and volcanism lead to arsenic contaminations of air, water, and soil. Other sources of contamination include the manufacture and use of arsenical pesticides and wood preservatives. Under reducing conditions, the dominant form As(III) or arsenite exists as arsenous acids (HAsO32-, H2AsO3-, and H3AsO3). Arsenate, As(V), is the stable form in oxygenated environments and exists as arsenic acids (H2AsO4-, HAsO42-, and AsO43-). Elemental arsenic is not water-soluble, but its salts exhibit wide ranges of solubility, depending on pH and the ionic environment. As(III) is more soluble, more mobile, and more toxic than As(V), and the conversion of As(III) into As(V) easily occurs in oxidative environments. Excellent reviews of methods for arsenic analysis have been presented, particularly the use of LCMS-MS for the arsenic speciation with a detection limit of 2 pg for tetramethylarsonium.2a Laboratory-based analytical methods such as atomic absorption spectrometry, inductively coupled plasma (ICP), and ICP/mass spectrometry require expensive instrumentation and high operating costs.2b Low-cost methods are polarography techniques,3 cathodic stripping voltammetry,4 and anodic stripping voltammetry (ASV)5 with the latter being most popular due to its low detection capabilities and simple operations. Among different electrode materials, glassy carbon (GC) electrodes have been modified by gold nanoparticles to detect As(III) as low as 10 ppt using ASV (2) (a) Corr, J. J.; Larsen, E. H. J. Anal. At. Spectrom. 1996, 11, 1215. (b) Story, W. C.; Caruso, J. A.; Heitkemper, D. T.; Perkins, L. J. Chromatogr. Sci. 1992, 30, 427. (3) Kaye, S. Am. J. Clin. Pathol. 1944, 14, 83. (4) (a) Sadana, R. S. Anal. Chem. 1983, 55, 304. (b) Li, H.; Smart, R. B. Anal. Chim. Acta 1996, 325, 25. (c) Greulach, U.; Henze, G. Anal. Chim. Acta 1995, 306, 217. (5) (a) Kopanica, M.; Novotny, L. Anal. Chim. Acta 1998, 368, 211. (b) Forsberg, G.; O’Laughlin, J. W.; Megargle, R. G.; Koirtyohann, S. R. Anal. Chem. 1975, 47, 1586. (c) Sun, Y.-C.; Mierzwa, J.; Yang, M.-H. Talanta 1997, 44, 1379. (d) Davis, P. H.; Dulude, G. R.; Griffin, R. M.; Matson, W. R.; Zink, E. W. Anal. Chem. 1978, 50, 137. (e) Simm, A. O.; Banks, C. E.; Compton, R. G. Anal. Chem. 2004, 76, 5051. 10.1021/ac061528a CCC: $37.00 Published 2007 Am. Chem. Soc. Published on Web 11/16/2006
(linear sweep or square wave).6 However, this technique suffers two major setbacks: lengthy analysis time (15-20 min for the deposition of arsenite) and severe interference caused by copper, another common metal found in natural deposits and groundwater. Indeed, this interference cannot easily be circumvented since there are several metals and ion species, commonly found in the groundwater, that can be codeposited and stripped off under this operating condition. In 1974, the U.S. Congress passed the Safe Drinking Water Act that requires EPA to establish safe levels of chemicals in drinking water which do or may cause health problems.7 These nonenforceable levels, based solely on possible health risks and exposure, are called maximum contaminant level goals (MCLG). The MCLG for copper has been set at 1.3 ppm because EPA believes this level of protection would unlikely cause any of the potential health problems. In this paper, boron doped diamond (BDD) macro- and microelectrodes have been modified by platinum nanoparticles for the oxidative determination of As(III) at levels below 1 ppb. In addition to its applicability for processing river and tap water, the electrochemical system was not affected by Cu2+ and Cl-. The use of BDD electrodes to overcome severe interferences caused by chloride and copper was motivated here since BDD films exhibit a wide working potential window for solvent-electrolyte electrolysis in conventional aqueous media; i.e., a large overpotential exists for the evolution of chlorine,8 oxygen, and hydrogen.9 BDD electrodes also exhibit voltammetric background currents and double-layer capacitances up to 1 order of magnitude lower than for glassy carbon, implying better detection sensitivity.8-10 EXPERIMENTAL SECTION Materials. Dihydrogen hexachloroplatinate (99.9%) was a product of Alfa Aesar (Ward Hill, MA). The arsenic atomic spectroscopy standard solution (Fluka) in nitric acid (1000 µg/ mg, prepared with As2O3, NaOH, and HNO3) and hydrogen tetrachloroaurate(III) trihydrate (HAuCl4) were purchased from Sigma-Aldrich (St Louis, MO). Nitric acid, hydrochloric acid (
