Multi-layer electron transfer across nanostructured Ag-SAM-Au-SAM junctions probed by surface enhanced Raman spectroscopy

Abstract

We have developed a new layered Au–Ag electrode for studying interfacial electron transfer processes by surface enhanced resonance Raman (SERR) spectroscopy. The device consists of a nanostructured Ag support which is separated from a Au film via a thin self-assembled monolayer (SAM) of amino-terminated mercaptanes (C_y-NH₂, with y = 6, 8, 11). The Au film is biocompatibly coated to allow for binding of redox-active proteins. We have explored the performance of this device for analysing interfacial electron transfer processes by stationary and time-resolved SERR spectroscopy, using the heme protein cytochrome c (Cyt-c) as a benchmark protein. The SERRS intensity of Cyt-c on Ag-(C_y-NH₂)-Au electrodes and Ag electrodes was comparable when the protein was electrostatically attached to the metal coated by a SAM of carboxyl-terminated mercaptanes (C_x-COOH) surface but 25 times higher upon covalent attachment via Cys102 to the bare Au surface. In the case of electrostatic adsorption the protein remained exclusively in its native state. Electron transfer between the protein and the Ag electrode occurred in an almost ideal Nernstian behaviour with a number of transferred electrons close to one (n = 0.8–0.9). Conversely, the covalent attached Cyt c showed two broad redox transitions (n = 0.3) and a partial conversion to a non-native species which remained redox inactive in the potential range from +0.1 to −0.3 V. For the electrostatically immobilised Cyt, apparent electron transfer rates of 0.8 and 49 s⁻¹ were obtained for y = 11 and x = 15 and 10, respectively, indicating a fast long-distance electron transfer through the multilayer with the electron tunneling through the C_x-COOH SAM being the rate limiting step.