UV Excitation of DNA Stabilized Ag Cluster Fluorescence via the DNA Bases

We report UV excitation through the DNA bases as a universal pathway to visible emission from DNA stabilized silver cluster fluorophores (AgDNAs): AgDNAs with peak emissions throughout the visible spectrum all exhibit two peaks in fluorescence excitation: a visible peak at a wavelength 50-150 nm shorter than the emission peak and a UV peak between 260 and 270 nin depending on the specifics of the DNA Strand. UV excitation produces the same emission spectrum as visible excitation with 2-4 times greater intensity depending on the specific emitter. The excellent agreement between AgDNA excitation and absorption spectra, the similarity between AgDNA and bare DNA absorption spectra, and the ubiquity of UV excitation for all the AgDNAs studied strongly suggest that UV excitation proceeds via the DNA bases rather than by direct excitation of higher energy excited states of the Ag cluster. We explore the possibility that UV excitation can provide information about the type, number, and orientation of cluster-bound bases. As an efficient and universal means of exciting AgDNA fluorescence, UV excitation should prove useful in detailed studies of AgDNA photophysics and in identification and purification of the most stable AgDNA fluorophores.