Ultrasensitive colorimetric and fluorometric detection of Hg(II) based on the use of gold nanoparticles and a catalytic hairpin assembly

An ultrasensitive method for colorimetric and fluorometric determination of Hg(II) is described. It involves a dual signal detection strategy based on the use of gold nanoparticles (AuNPs) and of a catalytic hairpin self-assembly. Three fluorophore-labeled metastable hairpin DNA probes were adsorbed on AuNPs to quench the fluorescence of the labels. The addition of Hg(II) causes catalytic self-assembly of the hairpins due to formation of T-Hg(II)-T base pairs with the aid of helper DNA to form a branched junction. The branched junction is a dsDNA that is released from the surface of the AuNPs. As a result, fluorescence recovers. Its intensity, best measured at excitation/emission wavelengths of 492/517 nm, increases linearly in the 0.2 to 100 nM of Hg(II) concentration range. The limit of detection is 0.1 nM. This is comparable to, or better than in other amplification based methods. The detection scheme offers both colorimetric (visual) and fluorescent read-out.