Lysozyme-stabilized Ag nanoclusters: synthesis of different compositions and fluorescent responses to sulfide ions with distinct modes

In this study, three red-emissive Ag nanoclusters capped with denatured lysozyme (dLys-AgNCs 1-3) were synthesized from different reactant molar ratios of AgNO3 and lysozyme. dLys-AgNCs 1 contained only Ag(0), whereas dLys-AgNCs 2 and 3 contained both Ag(0) and Ag(I). The maximum emission wavelengths of dLys-AgNCs 1-3 were all near 640 nm when excited at 490 nm, but had different photoluminescence intensities. The photoluminescence properties of the dLys-AgNCs in the presence of common inorganic anions were studied. dLys-AgNCs 1-3 had a distinct photoluminescence response to S2-. The photoluminescence of dLys-AgNCs 1 was quenched by S2-, because S2- destroyed the nanostructure of dLys-AgNCs 1 by forming Ag2S. The photoluminescence of dLys-AgNCs 3 was enhanced by S-2, because S-2 bound to Ag(I) in dLys-AgNCs 3 and altered the ligand-to-metal-metal charge transfer. For dLys-AgNCs 2, the photoluminescence was enhanced with a small amount of S2- and quenched when more S2- was added. This was explained by S2- binding to Ag(I) at low concentrations and then to Ag(0) at high concentrations. Other anions had no obvious effects, except that I- quenched the photoluminescence of dLys-AgNCs 1-3. The response mechanisms were investigated by XPS, MS, zeta potential, DLS, and fluorescence lifetime measurements. As-prepared dLys-gNCs 1 and 3 were used as fluorescent probes for S2- detection in tap water and lake water with satisfactory results.