Direct chemical mapping of electrochemically generated spatial composition gradients on thin gold films with surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) was employed to map the composition of electrochemically generated gradients in aromatic thiols on Au electrodes. Two commonly used substrates, electrochemically roughened (ORC) and metal film over nanosphere (MFON) electrodes, were tested for this purpose. Only the MFON substrates displayed the requisite electrical continuity and signal-enhancement properties needed for vibrational spectroscopic mapping of electrochemical gradients. One-component gradients and counterpropagating two-component gradients were formed from naphthalenethiol (NPT), chlorobenzenemethanethiol (ClBMT) and acetamidothiophenol (ACT) and were characterized by Raman scattering on Au MFON substrates. The surface coverage observed approximated the coverage predicted by the combination of a linear potential gradient with local Nernstian behavior of the reductive desorption reaction. For two-component gradients of NPT/ClBMT and NPT/ACT it was possible to perform measurements of surface composition as a function of spatial position (potential), thereby achieving chemical mapping of the surface with the spatial resolution of the focused laser. Because the desorption kinetics for aromatic thiols are relatively slow, surface coverages could also be controlled by changing the length of time the electrochemical potential gradient was applied.