A colorimetric/fluorescent dual-mode sensor for ultra-sensitive detection of Hg²

A highly sensitive colorimetric/fluorescent dual-mode sensor based on hybridization chain reaction (HCR) combining multifunctional Au NPs is presented for the detection of Hg²⁺ in aqueous solution. In Hg²⁺ absent solution, the surface of Au NPs was covered by hairpin auxiliary DNAs and a single strand DNA (ssDNA), which prevented Au NPs from salt-induced aggregation. At the same time, the fluorescence intensity of the dye-labeled hairpin probes was significantly quenched by Au NPs. In the presence of Hg²⁺, T-Hg²⁺-T coordination chemistry between helper DNA and the hairpin probes was induced which triggered the formation of extended double-stranded DNA (dsDNA) polymers via HCR. The formed dsDNA polymers were stiffer which couldn't attach to Au NPs, resulting in a red-to-blue color change along with salt-induced aggregation of Au NPs for colorimetric sensing. Meanwhile, the fluorescence of dye-labeled DNA turns on. Due to the HCR amplification effect, a highly sensitive detection of Hg²⁺ was achieved with detection limit of 0.1nM. Colorimetry is suitable for the analysis in salt solution with concentration lower than 100mM. It serves as an intuitive method that Hg²⁺ down to 1.0nM could be identified by naked eyes. For high-concentration salt solutions such as industrial effluent, fluorescent sensing acts as a better choice. Both the color and fluorescence changes of the proposed sensor exhibited high selectivity against other metal ions. Lake water was collected and analyzed using the dual-mode sensor, the results confirmed the practicability of the proposed approach.