Toward monitoring electrochemical reactions with dual-wavelength SERS: Characterization of rhodamine 6G (R6G) neutral radical species and covalent tethering of R6G to silver nanoparticles

The combination of electrochemistry (EC) and single molecule surface-enhanced Raman spectroscopy (SMSERS) has recently proven to be a sensitive method to investigate electron transfer (ET) reactions at the single molecule level. SMSERS can both detect single redox-active molecules and potentially monitor both the oxidized (O) and reduced (R) forms of a one-electron ET reaction in a single experiment. Herein, we report progress toward complete monitoring of single ET reactions with EC-SMSERS. We first obtained the solution phase resonance Raman (RR) spectrum of the Rhodamine 6G (R6G) neutral radical (R) with thin-layer resonance Raman spectroelectrochemistry (EC-RRS). The experimental spectrum was then correlated with the spectrum calculated by density functional theory (DFT). We then describe our approach to address the problem of adsorbate (R) loss caused either by desorption or reaction of the neutral radical with trace water or oxygen during the EC-SMSERS experiment. We have investigated a covalent cross-linking reaction which tethers R6G to SERS-active substrates (Ag nanoparticles). Covalently tethered R6G is subsequently characterized by surface cyclic voltammetry (CV) and SERS. Lastly, an optimized cross-linking reaction is devised which enabled the first direct detection of the one-electron reduced form of R6G with SERS. Our findings demonstrate that SERS can simultaneously monitor both O and R of a one-electron ET reaction.