Effects of Mn2+ on oligonucleotide-gold nanoparticle hybrids for colorimetric sensing of Hg2+: improving colorimetric sensitivity and accelerating color change.

In this paper, we present a simple and rapid colorimetric assay--using the polythymine oligonucleotide T(33), citrate-capped gold nanoparticles (AuNPs), and phosphate-buffer saline (PBS) in the presence of Mn(2+)--for the highly selective and sensitive detection of Hg(2+) in an aqueous solution. Citrate-capped AuNPs adsorbed on randomly coiled T(33) were dispersed well in PBS because of strong electrostatic repulsion between DNA molecules. In the presence of Hg(2+), the formation of Hg(2+)-T(33) complexes enabled the removal of T(33) molecules from the NP surface, resulting in salt-induced NP aggregation. However, the T(33)-capped AuNPs (T(33)-AuNPs) were dispersed in PBS solution after the addition of 1.0 microM Hg(2+), indicating that T(33)-AuNPs had poor colorimetric sensitivity toward Hg(2+). We uncovered that the addition of Mn(2+) to a solution containing 0.75 nM T(33)-AuNPs and 0.2x PBS resulted in an acceleration of the analysis time (within 5 min) and a 100-fold sensitivity improvement for the detection of Hg(2+). As a result, the present approach enables the analysis of Hg(2+) with a minimum detectable concentration that corresponds to 10 nM. This is probably attributed to that Mn(2+) binds strongly to the phosphate backbone of DNA, thereby accelerating Hg(2+)-induced aggregation of the T(33)-AuNPs. Because Mn(2+) can stabilize the folded structure of the Hg(2+)-T(33) complex, Hg(2+) facilitates the removal of T(33) from the NP surface in the presence of Mn(2+). This probe was successfully applied to the determination of Hg(2+) in pond water.