Bi-Analyte Surface Enhanced Raman Scattering for unambiguous evidence of   single molecule detection
  A method is proposed to pin down an unambiguous proof for single molecule surface enhanced Raman spectroscopy (SERS). The simultaneous use of two analyte molecules enables a clear confirmation of the single (or few) molecule nature of the signals. This method eliminates most of the uncertainties associated with low dye concentrations in previous experiments. It further shows that single-molecule signals are very common in SERS, both in liquids and on dry substrates. 
E. C. Le Ru, M. Meyer, P. G. Etchegoin