Single-molecule Raman spectroscopy of a cyanine dye in aqueous silver colloidal solution with the use of surface-enhanced Raman scattering at near-infrared excitation (NIR-SERS) is reported. A characteristic Poisson distribution of SERS signals due to the Brownian motion of single dye molecule-loaded silver particles reflects the probability of finding 0, 1, or 2 1,1 ' -diethyl-2,2 ' cyanine (PIC) molecules in the probed volume during an actual measurement and is evidence that single-molecule detection by SERS has been achieved. Spectra measured in 1 s collection time with 100 mW nonresonant 830 nm excitation provide a clear 'fingerprint' of a single PIC molecule by showing its typical Raman lines between 700 and 1700 cm -1. Single-molecule Raman signals are also detected for the first time at the anti-Stokes side of the excitation laser. Effective Raman cross sections for PIC of 10 -16 cm2 per molecule can be inferred from the ratio between 'pumped' anti-Stokes and Stokes signals.
Katrin Kneipp, Harald Kneipp, Geurt Deinum, Irving Itzkan, Ramachandra R. Dasari, and Michael S. Feld, "Single-Molecule Detection of a Cyanine Dye in Silver Colloidal Solution Using Near-Infrared Surface-Enhanced Raman Scattering," Appl. Spectrosc. 52, 175-178 (1998)