Surface enhanced resonance Raman spectroscopy (SERRS) is used to observe single molecules. For single molecule experiments, metal colloidal suspensions (usually silver) are made with particles of diameter 90-120nm. Adsorbing analyte molecules tend to cause aggregation of the particles, so very dilute amounts of analyte are used to prevent excessive particle aggregation, and of course to guarantee adsorption of single analyte molecules on every one to two metal particles. The mechanism of the enhancement has not been clearly defined, but it is agreed that there is an enhancement of the local electric field of the particle caused by excitation of the particle’s surface plasmon by the incident light. This effect is maximized when the electric field of the incident light is parallel to the electric transition dipole of the analyte molecule, and when the molecule is bound between two metal particles, since the plasmon fields of each particle overlap and enhance the field experienced by the analyte. A smaller contribution to the enhancement comes from “chemical effect” namely a coupling of the analyte’s electronic states with the broad electronic states of the metal surface, which allow charge transfer either way.

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Single molecule Raman provides more information that single molecule fluorescence in that intrinsic information (bond vibrations) of the analyte can be studied. Deviation of single enzymes and dye molecule’s activity from bulk activity measurements has already been demonstrated, which demonstrates the exciting information that may be gained. Unfortunately there are some drawbacks: analytes studied usually need strong absorption centers, which has limited these studies to dye molecules (Rhodamine) and enzymes with metal sites (heme group in Hb). Studies of enzymes must be done carefully, because adsorption onto metals will change the activity, either decreasing it due to destabilization of the folded structure, or possibly enhancement by electronic coupling to the metal. With continued development, single molecule Raman studies should provide interesting information, and nicely complement other single molecule platforms.
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