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 T33, citrate-capped gold nanoparticles (AuNPs), and phosphate-buffer saline (PBS) in the presence of Mn2+ – for the highly selective and sensitive detection of Hg2+ in an aqueous solution. Citrate-capped AuNPs adsorbed on randomly coiled T33 were dispersed well in PBS because of strong electrostatic repulsion between DNA molecules. In the presence of Hg2+, the formation of Hg2+–T33 complexes enabled the removal of T33 molecules from the NP surface, resulting in salt-induced NP aggregation. However, the T33-capped AuNPs (T33-AuNPs) were dispersed in PBS solution after the addition of 1.0 μM Hg2+, indicating that T33-AuNPs had poor colorimetric sensitivity toward Hg2+. We uncovered that the addition of Mn2+ to a solution containing 0.75 nM T33-AuNPs and 0.2× PBS resulted in an acceleration of the analysis time (within 5 min) and a 100-fold sensitivity improvement for the detection of Hg2+. As a result, the present approach enables the analysis of Hg2+ with a minimum detectable concentration that corresponds to 10 nM. This is probably attributed to that Mn2+ binds strongly to the phosphate backbone of DNA, thereby accelerating Hg2+-induced aggregation of the T33-AuNPs. Because Mn2+ can stabilize the folded structure of the Hg2+–T33 complex, Hg2+ facilitates the removal of T33 from the NP surface in the presence of Mn2+. This probe was successfully applied to the determination of Hg2+ in pond water.