Ultrasensitive colorimetric aptasensor for Hg2+ detection using Exo-III assisted target recycling amplification and unmodified AuNPs as indicators

Facile and ultrasensitive detection of Hg2+ in water environment remains challenging. Exonuclease III (Exo-III)-assisted target recycling is one of the most popular amplification strategies. Although the magnesium (II) ions are widely acting as cofactors of Exo-III, we recognized that Mg2+ cofactors would strongly disturb the charge distribution on citrate-stablized gold nanoparticles (in the general sense, unmodified AuNPs) surface, thus generate false positive colorimetric signals. To address this issue, we first put forward the view that the cobalt (II) ions can function as the Exo-III cofactor and successfully construct a novel label-free colorimetric aptasensor for facile and ultrasensitive detection of Hg2+ using Hg2+-triggered Exo-III-assisted signal amplification and unmodified AuNPs as indicators. A hairpin-looped DNA probe was rationally designed with thymine-rich recognition termini and specifically recognized trace Hg2+ by a stable T–Hg2+–T structure. A blue-to-red color change of AuNPs with the addition of Hg2+ provided the quantitative detection of Hg2+ with a limit of detection of 0.2 nM and a linear working range from 0.5 nM to 5.0 nM. The whole testing time for one assay was approximately 40 min. Real water samples, even containing Hg2+ at 1 nM, could be determined by the aptasensor with recovery rates from 97% to 103%.