Radial Flow Assay Using Gold Nanoparticles and Rolling Circle Amplification to Detect Mercuric Ions.

A novel colorimetric assay employing oligonucleotide-conjugated gold nanoparticle (AuNP probes) and rolling circle amplification (RCA) was developed for simple detection of mercuric ions (Hg). The thymine-Hg-thymine (T-Hg-T) coordination chemistry makes our detection system selective for Hg. In the presence of Hg, the thymine 12-mer oligonucleotide is unable to act as a primer for RCA due to the formation of T-Hg-T before the RCA reaction. However, in the absence of Hg, DNA coils as RCA products are generated during the RCA reaction, and is further labeled with AuNP probes. Colorimetric signals that depend on the amount of DNA coil-AuNP probe complexes were generated by drop-drying the reaction solution on nitrocellulose-based paper. As the reaction solution spread radially because of capillary action, the complexes formed a concentric red spot on the paper. The colorimetric signals of the red spots were rapidly measured with a portable spectrophotometer and determined as the Δ value, which indicates the calculated color intensity. Our assay displays great linearity (detection limit: 22.4 nM), precision, and reproducibility, thus demonstrating its utility for Hg quantification in real samples. We suggest that our simple, portable, and cost-effective method could be used for on-site Hg detections.