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

DNA oligomers can stabilize few-atom Ag clusters in aqueous solution. Some of the resulting AgDNAs are fluorescent with chemical stabilities, excitation and emission wavelengths, and quantum yields that depend strongly on the DNA sequence. 6 This sequence dependence has been exploited for detecting disease-related DNA mutations and for identifying promising biolabels. Rational design of AgDNA fluorophores for such applications will require a better understanding of how cluster nucleobase interactions determine AgDNA properties. Direct coordination of DNA bases with the Ag clusters is evident from the pH dependence of fluorescence, which implicates the N3 sites of thymine and cytosine in cluster binding. However, many fundamental questions pertaining to the sequence dependence remain. For example, how many bases bind the cluster in a given AgDNA? Do bases that are not directly bound to the cluster affect its properties? How does DNA secondary structure influence the stability of AgDNAs? Detailed structural characterization of these emitters awaits improved chemical yields and purification. Here, we explore such issues in the context of a striking feature of AgDNA fluorophores: resonant UV excitation of visible emission via transfer of excitation energy from the DNA bases that stabilize the cluster. This work was motivated by previous studies which found that visible emission from AgDNAs could be excited both by visible light and by UV wavelengths. Since then, despite numerous studies of AgDNA fluorescence, little attention has been paid to UV excitation. The early studies suggested energy transfer from the DNA bases as one possible mechanism; however, excitation spectra for the visible emission were not made to definitively test the role of the bases. Here, we use AgDNAs formed on cytosine and thymine homopolymers and on mixed-base strands to provide strong evidence that the DNA bases provide a general excitation pathway for AgDNA fluorescence.