A silver-specific DNA-based bio-assay for Ag(I) detection via the aggregation of unmodified gold nanoparticles in aqueous solution coupled with resonance Rayleigh scattering

We report a label-free, silver-specific DNA-based bio-assay for the detection of silver ion (Ag+) in aqueous solution. The bio-assay relies on the structural change of cytosine (C)-rich silver-specific DNA and the status transition of gold nanoparticles (AuNPs). In the absence of Ag+, unfolded silver-specific DNA stabilizes the dispersed AuNPs at high salt concentrations, which induces low resonance Rayleigh scattering (RRS) intensity. However, in the presence of Ag+, silver-specific DNA folds into a hairpin conformation through C-Ag+-C mismatches, and cannot coat on the surface of AuNPs. Thus, the uncoated AuNPs aggregate at high salt concentrations and cause a significant increase in RRS intensity, which reflects the amount of Ag+ in the system. Under the optimized conditions, the RRS intensity at 550 nm increased linearly with the concentration of Ag+ ranging from 0.20 mu M to 1.00 mu M, and the limit of detection for Ag+ was determined as 0.202 mu M. The bio-assay also shows high selectivity against coexisting cations.