Monitoring the electrochemistry of single molecules by surface-enhanced Raman spectroscopy.

Coherent control of chemical species in complex systems is always subject to intrinsic inhomogeneities from the environment. For example, slight chemical modifications can decisively affect transport properties of molecules on surfaces. Hence, single-molecule (SM) studies are the best solution to avoid these problems and to study diverse phenomena in biology, physics, and chemistry. Along these lines, monitoring SM redox processes has always been a "holy grail" in electrochemistry. To date, claims of SM electrochemistry by spectroscopy have come only from fluorescence quenching of polymers and redox-fluorescent molecules. In unconnected developments, the potential of the bianalyte surface-enhanced Raman scattering (SERS) method as a technique with SM sensitivity has been demonstrated. Raman spectroscopy has the potential to explore SM detection of any molecule, independent of its chemical nature. We provide definitive proof of SM events following redox cycles using SERS. The superior sensitivity and spectral richness of SERS makes it general enough to study, in principle, SM electron transfer of any (label-free) molecule.