Assessing a Spectroelectrochemical Sensors Performance for Detecting [Ru(bpy) 3 ] 2 + in Natural and Treated Water Eme A. Abu, a Samuel A. Bryan, b Carl J. Seliskar, a William R. Heineman* a a Department of Chemistry, University of Cincinnati, 301 Clifton Court, Cincinnati, OH 45221-0172, USA b Pacific Northwest National Laboratory, Richland, WA 99352, USA *e-mail: william.heineman@uc.edu Received: March 19, 2012; & Accepted: May 10, 2012 Abstract A spectroelectrochemical sensor that combines three modes of selectivity in a single device was evaluated in natural and treated water samples using tris-(2,2’-bipyridyl) ruthenium(II) dichloride hexahydrate, [Ru(bpy) 3 ] 2 + , as a model analyte. The sensor was an optically transparent indium tin oxide (ITO) electrode coated with a thin film of partial- ly sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SSEBS). As the potential of the ITO electrode was cycled from + 0.7 to + 1.3 V, the analyte changed from the colored [Ru(bpy) 3 ] 2 + complex to col- orless [Ru(bpy) 3 ] 3 + complex and the change in absorbance at 450 nm was used as the optical signal for quantifica- tion. Calibration curves were obtained for [Ru(bpy) 3 ] 2 + in natural well water, river water and treated tap water with detection limits of 108, 139 and 264 nM, respectively. A standard addition method was developed to determine an unknown spike addition concentration of [Ru(bpy) 3 ] 2 + in well water. The spectroelectrochemical sensor deter- mined the concentration of [Ru(bpy) 3 ] 2 + spiked into a sample of Hanford well water to be 0.39 Æ 0.03 mM versus the actual concentration of 0.40 mM. Keywords: Spectroelectrochemical sensors, SSEBS, ITO electrode DOI: 10.1002/elan.201200143 1 Introduction Spectroelectrochemistry based on UV-Vis, infrared, reso- nance Raman, electron spin resonance spectroscopy com- bined with standard electrochemical techniques are ex- tremely useful for characterizing compounds [1]. These techniques primarily investigate spectral and redox prop- erties and mechanisms of electrode reactions including measuring both homogenous and heterogeneous rate con- stants. They also provide information for studies of elec- trode surfaces. Using spectroelectrochemical sensors for specific analytes is a relatively new application. Spectroe- lectrochemistry is advantageous because of its enhanced selectivity for detecting an analyte in the presence of direct interferences [2]. These sensors derive their selec- tivity from combining three modes of selectivity in one device [3], namely selective partitioning, reversible elec- trochemistry and optical measurement. Scheme 1 shows a cross-sectional view of the sensor with a dication as a representative analyte. The first mode is the use of a charge selective film on the ITO electrode surface. In this study, the cation ex- changer, partially sulfonated polystyrene-block-poly(ethy- lene-ran-butylene)-block-polystyrene (SSEBS) [4] was used to preconcentrate [Ru(bpy) 3 ] 2 + into its ion-exchange sites by electrostatic attraction. The electrostatic attrac- tion increased the [Ru(bpy) 3 ] 2 + concentration at the elec- trode surface when compared to the bulk solution, lead- ing to increased optical and electrochemical signals and lower detection limits. The other two modes of selectivity are reversible electrochemistry and spectroscopy. Electro- chemical conversion is used to change the analyte from one redox form to another which constitutes the second mode of selectivity, and is accompanied by a spectral change of the analyte. Light at a specific wavelength that the analyte absorbs is directed at the electrode surface via attenuated total reflectance spectroscopy and an evan- escent wave is produced and comprises the third mode of selectivity. The evanescent wave monitors the amount of analyte that has been oxidized or reduced in the film next to the electrode surface by either absorption or fluores- cence. [Ru(bpy) 3 ] 2 + has been used extensively in the develop- ment of these spectroelectrochemical sensors because its charge enables selectively partitioning into an ionomer film on ITO by ion exchange, it both absorbs with a large molar absorptivity (e mol = 14 600 M À1 cm À1 at 452 nm) and fluoresces in the visible spectral range, and it has reversi- ble electrochemistry that changes these spectral proper- ties [5]. These characteristics also made it a good model analyte in this study. The excellent selectivity of the spectroelectrochemical sensor was demonstrated by sensing ferrocyanide in a complex sample of nuclear waste [2c]. Sodium ferrocya- nide was added to the waste supernate within the Han- ford nuclear waste tanks in combination with nickel sul- Electroanalysis 2012, 24, No. &,1–7  2012 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ Full Paper