The analysis of single molecules instead of a molecular ensemble provides a more detailed insight into molecular states and environment. Surface- and also tip-enhanced (resonance) Raman spectroscopy are becoming an interesting alternative to, in this respect, commonly employed fluorescence spectroscopy because of the much richer spectral information that is obtained from unmodified, label-free species. However, sometimes it is difficult to distinguish between the target molecule and contamination or decomposition of the sample, and unfortunately, it is not always ensured that the observed Raman features actually stem from the investigated species. The important question that arises in enhanced Raman studies addressed here is how to distinguish between the target molecule and the carbonaceous species.

The creation of a highly enhanced electromagnetic (EM) field underneath a scanning tunneling microscope (STM) tip enables Raman spectroscopic studies of organic submonolayer adsorbates at atomically smooth single crystalline surfaces. To study the sensitivity of this technique, tip-enhanced resonance Raman (TERR) spectra of the dye malachite green isothiocyanate on Au(111) in combination with the corresponding STM images of the probed surface region were analyzed. The detection limit for unambiguous identification of the dye and semiquantitative determination of the surface coverage reaches < or =0.7 pmol/cm(2), or approximately five molecules present in the enhanced-field region, which is confirmed by STM images. Because of well-defined adsorption sites at atomically smooth Au(111) surfaces, no variation in band positions or relative band intensities was observed at the single- or few-molecule detection level when employing TERR spectroscopy.

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We have studied the dependence of tip-enhanced Raman (TER) spectra composed of discrete Raman lines and an intense Lorentzian-shaped background on varying interparticle distance. For increasing tip-sample separation, we observe a remarkable blueshift of the background maximum that is ascribed to an energy shift of the localized surface plasmon resonance. In addition, we report on a steep background and Raman band intensity decrease within 5–10 nm tip retraction that highlights the underlying near-field enhancement mechanism of TER spectroscopy

We show that label-free detection of picomole amounts of the four nucleobases, respectively, adsorbed at atomically smooth Au(111) is straightforward with tip-enhanced Raman (TER) spectroscopy. In addition to TER spectra that allow identification of the corresponding DNA base, topographic images of the nanometer-sized sample can be obtained. Only 130 adenine molecules present in the enhanced field region in the tip−sample gap are sufficient to record the adenine fingerprint, that is, the Raman band of the characteristic ring-breathing mode at 734 cm-1.