Dual synergistic modulation of photo-induced electron transfer processes between molecules and gold nanopillars for ultrasensitive plasmon-enhanced Raman scattering

This work presents a synergistic approach to boost plasmon- or surface-enhanced Raman scattering (SERS) by combining molecular and electrical modulators that fine-tune the electronic structure of metal–molecule interfaces, especially the charge transfer (CT) states, allowing molecular resonances. Paraquat (PQ2+) was interfaced with nanopillar SERS substrates whose surface excess of charge was modulated by intercalating anionic Au complexes (AuCl4−, Au(CN)2−) as well as by applying external electric potentials. Such concurrent dual modulation tuned the energy of the CT states of the substrate–anion–PQ2+ triads in resonance with the excitation laser, resulting in a large enhancement of the PQ2+ SERS bands. The results point to a novel coherent through-bond CT contribution of SERS, analogous to the superexchange mechanism for electron transfer in donor–bridge–acceptor systems. The large amplification enables high sensitivity for detecting PQ2+ and ultimately enables the on-site detection of PQ2+ in unprocessed real samples (coffee drink). This study accounts for new physicochemical variables affecting electron transfer processes in nanostructured metal–molecule interfaces and provides a path for further exploring chemical strategies for greater Raman enhancement and for developing ultrasensitive Raman platforms.