Control of synthesis and optical properties of DNA templated silver nanoclusters by varying DNA length and sequence

We have used a simple method to prepare five fluorescent Ag nanoclusters (NCs) through the NaBH4-mediated reduction of Ag+ ions in the presence of various DNA scaffolds. The emission intensities and wavelengths (536-644 nm) of the as-prepared DNA-Ag NCs were dependent on the sequence and length of the DNA scaffold. Electrospray ionization mass spectrometry of the DNA-Ag NCs revealed that different numbers of Ag atoms (2-6 atoms) were present per DNA scaffold, depending on the number and position of the cytosine bases. Using the oligonucleotide 5'-CCC(TTCC)(2)TT(CCAA)(2)CCC-3' (DNA(TAr2)) as the scaffold, we obtained DNA(TAr2)-Ag NCs exhibiting a quantum yield (Phi(f)) of 61% at 608 nm; these NCs were stable in the presence of the tested thiols, Cl- ions and DNase I. Because of their strong fluorescence and stability, the DNA(TAr2)-Ag NCs were highly selective and sensitive for the detection of Hg2+ ions [linear range: 2.5-50 nM; limit of detection (signal-to-noise ratio = 3): 0.9 nM]. We validated the practicality of this probe through analyses of several water samples spiked with Hg2+ ions (10 nM); the recoveries were 98-118%.