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)2TT(CCAA)2CCC-3′ (DNATAr2) as the scaffold, we obtained DNATAr2–Ag NCs exhibiting a quantum yield (Φf) 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 DNATAr2–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%.