Surface-enhanced Raman spectroscopy (SERS) was utilized for the quantitative analysis of double-stranded (ds) DNA amplified by a polymerase chain reaction (PCR). 4′,6-Diamidino-2-phenylindole dihydrochloride (DAPI), which intercalates into ds-DNA but does not form a complex with single-stranded (ss) DNA, was added to a DNA solution after amplification by PCR. When the solution was mixed, including ds-DNA–DAPI complexes and free DAPI with silver colloid sol, only free DAPI was adsorbed on the colloid surface. The dye on the colloid gave very intense SERS signals with excitation at 514.5 nm, whereas DAPI engaging in the intercalation with ds-DNA did not show any SERS signal. The SERS spectrum of DAPI on the colloid showed a strong band at 1610 cm-1 due to the C═N stretching mode, and a linear relationship was observed between the peak intensity of the C═N stretching band and the concentration of free DAPI. Therefore one can determine the concentration of free DAPI by the SERS measurement. The more ds-DNA there is in the solution, the less free DAPI there is. Thus it is possible to quantitatively analyze the ds-DNA amplified by PCR indirectly by using SERS. The correlation coefficient between the peak intensity of the C═N stretching band and the concentration of ds-DNA amplified by PCR was calculated to be 0.988 for a concentration range from 0.1 to 1.3 mg/ml.
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