Electrochemical Surface Plasmon Resonance: Basic Formalism and Experimental Validation Shaopeng Wang, † Xinping Huang, †,‡ Xiaonan Shan, † Kyle J. Foley, † and Nongjian Tao* ,† Center for Bioelectronics and Biosensors, Biodesign Institute, Arizona State University, 1001 South McAllister Avenue, P.O. Box 875801, Tempe, Arizona 85287-5801, and Department of Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China A quantitative formalism of electrochemical surface plas- mon resonance (EC-SPR) was developed for studying electrochemical reactions. The EC-SPR signal from the reactions was found to be a convolution function of electrochemical current, and therefore, EC-SPR is a powerful tool that can provide information similar to the conventional current-based electrochemical techniques. As an example, potential-sweep EC-SPR was analyzed in details and was found to provide a new way to measure convolution voltammetry without the need of numerical integration. In addition to the benefits provided by the conventional convolution voltammetry, the EC-SPR has several unique advantages, including (1) spatial resolution that is particularly attractive for studying heterogeneous reactions, (2) optical properties of the reactions species that may assist identification of reaction mechanisms, and (3) high surface sensitivity for studying surface binding of the reaction species. Experiments and numerical simulations were carried out for a model system, hexaam- mineruthenium(III) chloride. The simultaneously mea- sured electrochemical current and SPR response con- firmed the relationship between the two quantities, and the numerical simulations were in excellent agreement with the measurements. Surface plasmon resonance (SPR) technique is highly sensitive to various processes taking place on a metal film. It has emerged as a powerful label-free method to study molecular binding processes taking place on a surface. Another important but less explored area of applications is electrochemical SPR (EC-SPR) for studying local electrochemical reactions on electrode surfaces. 1 One of the first SPR studies of electrochemical reactions was based on detecting local surface potential. 2 Other approaches include detection of surface-bound redox species, 3-9 redox- induced conformational changes in surface-bound proteins, 7 potential-controlled DNA melting, electrochemical poly- merization, 10,11 and anodic stripping and detection of metal ions. 4 These studies are mainly focused on adsorption/desorption processes or changes in the adsorbed species. SPR is also sensitive to the charge density in the metal film, 12 which has been employed recently by us to develop a surface impedance imaging tech- nique. 13 In addition to applications based on the SPR dependence on the intrinsic dielectric properties of the metal films and molecular adsorption on the metal films, SPR measures local refractive index in the bulk solution near the metal film. The latter is largely responsible for the observed electrochemical reaction of Fe(CN) 6 3-/4- reported in literature. 3 Despite of the promising applications of SPR in electrochem- istry, a basic and quantitative formalism has not yet been developed for EC-SPR, which is the focus of the present work. We establish a quantitative relationship between the EC-SPR signal and the current measured by conventional electrochemical methods. As an example, we apply the formalism to the most widely used electrochemical method, potential-sweep measure- ments. Furthermore, by considering diffusion-controlled reversible redox reactions, we obtain explicit expressions of the EC-SPR signal in terms of important electrochemical parameters as a function of potential and time. Finally, we carry out simultaneous measurements of electrochemical current and EC-SPR of the redox reaction of hexaammineruthenium(III) chloride and show excellent agreement between the theory and experiments. THEORY SPR measures (1) molecular binding onto a metal film, (2) bulk refractive index changes near the metal film, and (3) dielectric property changes of the metal film. The first property is best known and widely applied to affinity studies of biomol- ecules. We will focus here the second and third properties, which are, as we will show below, directly related to faradic current and double layer charging current, respectively. * To whom correspondence should be addressed. E-mail: njtao@asu.edu. Fax: (480) 965-9457. † Arizona State University. ‡ Lanzhou University. 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