What is observed in single molecule SERS, and why?

Proposed single molecule surface-enhanced Raman scattering (SM-SERS) mechanisms are discussed and problems in SM-SERS of biological molecules are pointed out. It is unlikely that the single molecule signals can be explained exclusively by electromagnetic (EM) 'hot spots' in fractal structures or in narrow gaps. Care must be taken to recognize and avoid the signal of sp(2)-carbon contamination, the origins of which are chemical impurities and/or photophysical reactions of the chosen adsorbate. As already pointed out by KaII and co-workers, relatively large globular proteins fit only in wider gaps, and therefore the EM enhancement is several orders smaller than often reported enhancements in SM-SERS. SM-SERS of haemoglobin probably shows signals from the Fe-porphyrin constituent among other unknown signals. It cannot be excluded that the haemoglobin is denatured and the haem group is situated in a smaller gap. Copyright (C) 2002 John Wiley Sons, Ltd.