Single-molecule surface-enhanced Raman spectroscopy with nanowatt excitation.

We demonstrate the possibility of single molecule (SM) detection via surface-enhanced Raman spectroscopy (SERS) in two seemingly challenging and unexpected cases: first with ultra-low excitation powers of the order of nanowatts and second in as-synthesized and not deliberately-aggregated silver colloid solution. The experiments are carried out using the bi-analyte method on a methylated form of Rhodamine 6G and one of its isotopologues excited at 514 nm close to the electronic resonance. This study spectacularly highlights the fact that SM-SERS detection is much more common and easier to achieve than typically thought, in particular in the case of resonance Raman excitation. As a result, SM-SERS detection in such cases should not be viewed as an indication of good SERS substrate performance as sometimes implicitly assumed.