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1 Supporting nline Material for Imaging Local Electrochemical Current via Surface Plasmon Resonance Xiaonan Shan, Urmez Patel, Shaopeng Wang, Rodrigo Iglesias, Nongjian Tao* *To whom correspondence should be addressed. Published 1 March 1, Science 37, 1363 (1) DI: 1.116/science This PDF file includes: Materials and Methods SM Text Figs. S1 and S References ther Supporting nline Material for this manuscript includes the following: Movie S1

2 Materials and methods 1. Measuring electrochemical current from SPR signals SPR measures electrochemical reaction-induced changes in the bulk refractive index near the electrode. The SPR response (e.g., resonance angle, θ(t)) can be described in terms of the reactant and product concentrations, C and C R, and given by θ SPR (t) = B [α C (z,t) + α R C R (z,t)]e z / l d(z /l) (S1) where α and α R are the changes in the local refractive indices per unit concentration for the oxidized and reduced molecules, respectively. The constant, B, in Eq. S1 measures the sensitivity of the SPR angle to a change in the bulk index of refraction, which can be calibrated for a given SPR setup and reaction species. The exponential term in the integral is the decay of the evanescent field from the metal into the solution phase, where the decay length, l, is on the order of nm. Eq. 1 can be simplified if the time scale of measurement is slower than the diffuse time of the reaction species over a distance of l ~ nm. In this case, Eq. 1 is replaced by θ t) B[ α C ( z, t) α C ( z, t) ] (S) ( = + SPR z= R R z= where C (x,y,z,t) z= and C R (x,y,z,t) z= are the concentrations of the oxidized and reduced molecules near the electrode. For most ions and molecules in dilute solutions, with diffusion coefficient in the range of 1-9 to 1-11 m /s ( m /s for Ru(NH 3 ) 6 3+ )(1), the diffusion time given by l /(D) is less than 1 ms, so Eq. holds well for most electrochemical measurements. According to Eq. S, SPR directly measures the local concentrations of oxidized and reduced species on the electrode surface. In contrast, conventional electrochemical methods measure current density vs. potential or time, which is related to C and C R according to()

3 C i(t) = nfd z = nfd C R z= R z z= (S3) where n is number of electrons transferred per reaction, F is Faraday constant, and D and D R are the diffusion coefficients of the reaction species. Let us consider a redox reaction, + e R where and R are oxidized and reduced species. The diffusion equation for is C t = D C t (S4) where Co (z,t) and D are the concentration and diffusion coefficient of. Note that only the diffusion in z direction is considered, which is a good approximation as long as the lateral length scale of the image is far greater than the depth (~ nm) of the evanescent field in z direction. Performing Laplace transform on Eq. 4, we have ~ C [ ( s / D ) z] 1/ 1 ( z, s) = s C + A( s) exp (S5) where A(s) is a function to be determined from boundary conditions at the electrode surface. To relate the concentrations to current density, we perform Laplace transform on Eq. S3 and combine it with Eq. S5, which leads to ~ 1 1/ 1 1/ (, s) s C ( nfd ) s = C ~ I ( s) (S6) A similar relation can be obtained for the reduced species, given by ~ R R R Combining Eqs. S6, S7 and S, we have C 1 1/ 1 1/ ~ (, s) = s C ( nfd ) s I ( s) (S7) 3

4 i( t) = bnfl 1 ~ { s ( )} 1/ Δθ s which allows us to calculate current from the SPR signal. SPR, (1). Calibration To obtain b, SPR angular shifts per unit concentration for the oxidized and reduced forms of the ruthenium complex were determined using an independent SPR system (BI- from Biosensing Instruments, First, the ruthenium complex in the oxidized state at each concentration was injected into the flow cell, and the SPR response was determined and used to calculate Bα. Second, in order to determine Bα R, we converted the ruthenium complex to the reduced state by applying a negative electrode potential value (-.3 V). Bα R and Bα were found to be.5 mdeg/mm and 5 mdeg/mm, respectively. From b = [B(α R D 1/ R α D 1/ )] 1, and assume D R = D = 5.3*1-1 m /s, we have b =-9.1x1-6 m mm Deg -1 S -1/. Conventional electrochemical detection of TNT on fingerprint Figs. 4B and C in the main text show electrochemical current images and local cyclic voltammograms of a fingerprint covered with TNT particulates. During the measurement, conventional voltammetry was also recorded with an AutoLab potentiostat simultaneously. The voltammogram shown in Fig. S1 does not reveal obvious reduction peaks that can be associated with TNT reduction. 4

5 Fig.S1 Conventional voltammogram of TNT on fingerprint. The potential sweep rate:.5 V/sec. Electrolyte:.5 M KCl. Detection of trace TNT particulates in the presence of interference particulates We mixed candle wax and TNT particulates onto the fingerprint, and obtained the local voltammetry of the surface by imaging the local electrochemical current while cycling the potential. The regions of the wax particulates show little contrast changes with the potential (marked by black arrows). while the regions containing TNT particulates show large and characteristic changes (Fig.S). 5

6 Fig. S. Detection of TNT particulates in the presence of wax particles. (A) SPR image of TNT + wax particulates (marked by a blue circle) on fingerprint. (B-C) Electrochemical current image at different potential (-.5V and -.7V). References 1. A. J. Bard, L. R. Faulkner, Electrochemical methods : fundamentals and applications. (Wiley, New York, ed. nd, 1), pp. 813, Table C.4... A. J. Bard, L. R. Faulkner, Electrochemical methods : fundamentals and applications. (Wiley, New York, ed. nd, 1), pp.833. Supporting nline Material Video Clip Files Movie S1 Electrochemical current density video of a finger print at different potentials recorded during continuous cycling of the electrode potential between -.1V to -.34V at a rate of.1v/s. The electrolyte is.5m phosphate buffer containing 1mM Ru(NH 3 ) The Figs.1 C-F in the manuscript are the snapshots from this video. 6