Aptamer-based aggregation assay for mercury(II) using gold nanoparticles and fluorescent CdTe quantum dots

Abstract

We have developed an ultrasensitive homogeneous fluorometric assay for Hg(II) ion. It is based on the different affinities of gold nanoparticles (AuNPs) for unfolded and folded aptamers. AuNPs are capable of recognizing conformational changes of aptamers under conditions of high ionic strength. A highly Hg(II)-specific T-rich aptamer was selected and adsorbed on the surface of AuNPs. This prevents the salt-induced aggregation of AuNPs. The red AuNP/aptamer system reduces the green fluorescence of cysteamine-capped CdTe quantum dots (CA-CdTe QDs). However, in the presence of Hg(II), the aptamer specifically reacts with Hg(II) to form a “T-Hg(II)-T” structure, and the AuNPs therefore form blue aggregates because they are not protected by the aptamer under conditions of high ionic strength. The blue aggregates cannot quench the fluorescence of the CA-CdTe QDs. Therefore, on addition of Hg(II), the fluorescence of the CdTe QDs is recovered. Fluorescence intensity is linearly proportional to the concentration of Hg(II) in the 50 pM to 1.0 nM concentration range, with a 2.5 pM detection limit. This ultrasensitive method in our eyes holds a large potential for environmental monitoring Hg(II) ion, and of other ions if respective aptamers are available.