DOI: 10.1002/elan.201400539 Electrochemical Investigation of Metal Oxide Conducting Electrodes for Direct Detection of Sulfide Md. Abdul Aziz,* [a] Manzar Sohail,* [a] Munetaka Oyama, [b] and Wael Mahfoz [a] 1 Introduction The sensing of sulfide has gained substantial importance due to its extensive presence in systems ranging from oil and gas reserves to environmental and biological systems. Sulfide ions are released into waterways and the environ- ment by industrial effluents, as well as by anaerobic me- tabolism of bacteria through degradation of sulfur con- taining proteins and other types of organic compounds [1]. Free sulfides in natural water systems and wastewater serve as an important index of pollution due to the risk they pose to many aquatic organisms. Even at micro- molar levels, sulfide can lead to loss of consciousness, per- manent brain damage, and even death. When present in drinking water, hydrogen sulfide can give the water an undesirable taste and odor. In the oil industry, hydrogen sulfide contaminates the geothermal brine encountered when drilling oil wells [2, 3]; promotes the corrosion of the drilling equipment, pipelines, and vessels; forms sul- fide scales; and poses problems in regard to maintaining the quality of the produced oil. Various methods, including spectroscopic, titrimetric, colorimetric, chromatographic and electrochemical tech- niques, have been used for sulfide determination [1, 3, 4]. However, most of these analytical methods are ill suited to deployment in the field due to factors such as massive instrumentation, high cost, long sample preparation and conditioning times, and the need for standards. Neverthe- less, electrochemical techniques hold significant promise as the basis for a simple, direct, one-pot, inexpensive and on-site analytic method for sulfide determination. A vari- ety of electrochemical methods such as potentiometric, coulometric, amperometic and stripping voltammetry techniques have been developed over the last three de- cades [1, 3, 5–9]. All of these techniques require tedious electrode surface modification processes and an indirect reaction at the electrode surface to achieve selectivity [1, 3, 5–9]. Direct sulfide determination at the surfaces of many electrodes, including metallic electrodes (platinum or gold), is hampered by the corrosiveness and high affin- ity toward metallic surfaces of different sulfide species [2, 10, 11]. Moreover, the direct electrochemical oxidation of sulfide is highly irreversible with a high overpotential at bare electrodes [12, 13]. Additionally, the oxidation products of sulfide adsorb onto the electrode surfaces, leading to fouling and passivation of the electrode surfa- ces and thus to poor reproducibility and unstable electro- chemical signals [2, 7]. Overall, the most pressing need at present is to attain adequate selectivity and stability rather than sensitivity because the performance of most current techniques is negatively impacted by other mem- bers of the sulfur family [1]. In a recent study of the electrochemical determination of sulfide at various commercially available carbon elec- trodes, Lawrence et al. found that edge-plane pyrolytic graphite (EPPG) shows a good electrocatalytic response for direct determination of sulfide ions at different pHs [14]. They found that the catalytic activity of sulfide de- Abstract : This study examined the performance of four conducting metal oxide electrodes for the direct electro- chemical analysis of sulfide; the electrode materials stud- ied were indium tin oxide (ITO), fluorine doped tin oxide (FTO), aluminum doped zinc oxide (AZO) and gallium doped zinc oxide (GZO). Cyclic voltammetry (CV) re- sults obtained using the ITO, AZO, GZO and FTO elec- trodes showed direct electrooxidation peak potential of sulfide at 381, 507, 400, and 850 mV vs. Ag/AgCl, respec- tively; however, the less positive oxidation potential and high catalytic current response of the ITO electrode made it the electrode of choice for the direct oxidation of sulfide. The effects of different electrolytes and buffer solutions on the CV responses were also evaluated. A linear concentration range up to 350 mM and a detection limit of 8.0 mM were achieved. CV response was highly reproducible, remaining unaffected even after 50 meas- urements. The sensor was found to have good selectivity, with no interference from sulfite, sulfate or chloride ions. The present findings demonstrate that the bare ITO elec- trode can be used as the basis of an inexpensive, sensitive, selective and robust sulfide sensor. Keywords: Sulfide · Indium tin oxide · Electrochemical sensor · Stability · Selectivity [a] M. A. Aziz, M. Sohail, W. Mahfoz Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals Dhahran 31261, Saudi Arabia tel: + 966-13-860-3744, fax: + 966-13-860-7264 *e-mail: maziz@kfupm.edu.sa [b] M. Oyama Department of Material Chemistry, Graduate School of Engineering, Kyoto University Nishikyo-ku, Kyoto 615-8520, Japan www.electroanalysis.wiley-vch.de  2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2015, 27, 1 – 9 &1& These are not the final page numbers! ÞÞ Full Paper