Correlation of photobleaching, oxidation and metal induced fluorescence quenching of DNA-templated silver nanoclusters

Abstract

Few-atom noble metal nanoclusters have attracted a lot of interest due to their potential applications in biosensor development, imaging and catalysis. DNA-templated silver nanoclusters (AgNCs) are of particular interest as different emission colors can be obtained by changing the DNA sequence. A popular analytical application is fluorescence quenching by Hg²⁺, where d¹⁰–d¹⁰ metallophilic interaction has often been proposed for associating Hg²⁺ with nanoclusters. However, it cannot explain the lack of response to other d¹⁰ ions such as Zn²⁺ and Cd²⁺. In our effort to elucidate the quenching mechanism, we studied a total of eight AgNCs prepared by different hairpin DNA sequences; they showed different sensitivity to Hg²⁺, and DNA with a larger cytosine loop size produced more sensitive AgNCs. In all the cases, samples strongly quenched by Hg²⁺ were also more easily photobleached. Light of shorter wavelengths bleached AgNCs more potently, and photobleached samples can be recovered by NaBH₄. Strong fluorescence quenching was also observed with high redox potential metal ions such as Ag⁺, Au³⁺, Cu²⁺ and Hg²⁺, but not with low redox potential ions. Such metal induced quenching cannot be recovered by NaBH₄. Electronic absorption and mass spectrometry studies offered further insights into the oxidation reaction. Our results correlate many important experimental observations and will fuel the further growth of this field.