Metal-enhanced PicoGreen fluorescence: application for double-stranded DNA quantification.

PicoGreen (PG) is a fluorescent probe for both double-stranded DNA (dsDNA) detection and quantification based on its ability to form a luminescent complex with dsDNA as compared with the free dye in solution. To expand the sensitivity of PG detection, we have studied the spectral properties of PG, both free and in complex with DNA in solution, when the fluorophore is in proximity to silver nanoparticles. We show that for a broad range of PG concentrations (20 pM-3.5 microM), it does not form dimers/oligomers and it exists in a monomeric state. On binding to DNA in the absence of silver, PG fluorescence increases approximately 1100-fold. Deposition of PG/DNA complex onto silver island films (SiFs) increases fluorescence approximately 7-fold due to the metal-enhanced fluorescence (MEF) effect, yielding fluorescence enhancement of 7700-fold as compared with the free dye on glass. In contrast to PG in complex with DNA, the free dye on SiFs demonstrates a decrease in brightness approximately 5-fold. Therefore, the total enhancement of PG on binding to DNA on silver reaches a value of approximately 38,000 as compared with free PG on SiFs. Consequently, the metal-enhanced detection of PG fluorescence is likely to find important utility for amplified dsDNA quantification.