Detecting and identifying a single DNA base is of basic scientific and practical interest. For example, one approach in rapid DNA sequencing is based upon spectroscopic single molecule detection. To detect and identify single bases by fluorescence, they must be labeled by fluorescent dye molecules to achieve large enough fluorescence quantum yields and distinguishable spectral properties. Recently, we demonstrated a method for detecting and identifying a single DNA base which does not require any labeling because it is based on the intrinsic surface-enhanced Raman scattering (SERS) of the base. From the obtained vibrational pumping, effective surface enhanced Raman cross sections of adenine and of its nucleotide can be derived to be on the order of 10 -16 cm 2/molecule when adenine is attached to small colloidal silver clusters in aqueous solution. Such an enormous cross section for a base or a nucleotide suggests SERS as an exciting new tool for DNA sequencing and for biomedical research in general. SERS can provide a spectral fingerprint of a base with a cross section comparable to effective fluorescence cross sections of common laser dyes.
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