Various electrochemical TERS approaches were developed to probe electrochemical processes at the nanoscale, in real time and over a wide potential window (around 800 mV). EC-TERS was successfully applied to the investigation of the transformation mechanism of different molecular layer architectures, i.e. self-assembled monolayers (SAMs) on gold electrodes and grafted layers via controlled electro-reduction of aryldiazonium salts. A bimolecular electrochemical reaction path, difficult to evidence from the electrochemical response only, is proposed in light of the EC-TERS analysis. The comparison among the in situ reactivity of different layer structures allowed establishing structure-reactivity relationships.
In this communication, we will present the concept of Electrochemical Tip Surface-Enhanced Raman Spectroscopy (EC-Tip-SERS) on a redox- and Raman- active model molecule, the Nile blue, in which a temporal resolution in the order of millisecond can be attained. Then, we will consider the use of Tip-SERS in the characterization of complex rotaxane-based giant assemblies containing porphyrins, and in probing their molecular motion on gold surfaces . Finally, we will demonstrate the use of EC-Tip-SERS in scrutinizing electrocatalytic transient mechanisms, such as those associated to the oxygen reduction reaction using methyl viologen SAMs as electrocatalyts.
The reactivity of electrochemically-active molecular architectures immobilized on electrode surfaces was investigated by electrochemical-TERS, at relatively high potential sweep rate and on broad potential ranges. A complex electrochemical mechanism, involving reaction intermediates and multiple reaction paths, could be resolved on electroactive architectures based on nitrobenzene derivatives. Further EC-TERS investigations on these derivatives assembled as mono- or multilayers on the electrode surface emphasized the influence of the structure of the molecular assemblies on their reactivity. Under specific conditions, azo bonds formation between nitrobenzene derivatives observed by TERS can result from the electrochemical polarization/reaction, and not from photochemical processes.
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