Melamine-promoted formation of bright and stable DNA-silver nanoclusters and their antimicrobial properties

A new method has been developed for the preparation of brightly fluorescent and stable DNA-silver nanoclusters (DNA-AgNCs). The approach takes advantage of specific interactions occurring between melamine and thymine residues in a DNA template. These interactions cause the formation of a melamine-DNA-AgNC complex (Mel-DNA-AgNCs), in which a change in the environment of the DNA template causes binding of additional Ag⁺ and an enhancement in the fluorescence efficiency and stability. The effects of the nature of the template DNA, DNA : Ag⁺ : NaBH₄ ratio, pH and temperature were systematically assessed in order to maximize the melamine-promoted fluorescence enhancement. The results show that the Mel-DNA-AgNCs, generated under the optimal conditions, exhibit a ca. 3-fold larger fluorescence efficiency and long-term stability (70 d) in contrast to those of DNA-AgNCs in the absence of melamine. Importantly, the bright and stable Mel-DNA-AgNCs exhibit antimicrobial activities against Gram-positive and Gram-negative bacteria that are superior to those of DNA-AgNCs alone. To the best of our knowledge, this is the first report describing the synthesis of DNA-AgNCs that have improved fluorescence efficiencies and that function as effective antimicrobial agents.