Recently, single molecule tip-enhanced resonance Raman spectroscopy of dye molecules has been claimed based on the observation of temporal spectroscopic fluctuations.[1,2] However, tip-enhanced Raman spectroscopy (TERS) of molecules not showing any resonance Raman effect is still difficult, due to the extremely small cross section of normal Raman scattering. In this work, a 2-mercaptopyridine sub-monolayer, which does not absorb visible light, was studied. Temporal intensity fluctuations of different vibrational modes were observed. This suggests that the sensitivity of our TERS setup has reached the single molecule level without the additional resonance Raman effect. Thanks to the high sensitivity of TERS, studies on biological samples become possible. We investigated different bio-molecules, including amino acids and nucleic acids deposited on Au substrates. Spectra comparable to the results by surface-enhanced Raman spectroscopy (SERS) were obtained. It provides an unprecedented opportunity to identify, monitor and analyze label free bio-systems with nanometer spatial resolution at ambient conditions.
[1] Zhang, W.; Yeo, B.; Schmid, T.; Zenobi, R. J. Chem. Phys. C 111, 1733, (2007).
[2] Neacsu, C. C.; Dreyer, J.; Behr, N.; Raschke, M. B. Phys. Rev. B 73, (2006).
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