Turn-On Fluoresence Sensor for Hg²⁺ in Food Based on FRET between Aptamers-Functionalized Upconversion Nanoparticles and Gold Nanoparticles

In this study, a turn-on nanosensor for detecting Hg²⁺ was developed based on the fluorescence resonance energy transfer (FRET) between long-strand aptamers-functionalized upconversion nanoparticles (UCNPs) and short-strand aptamers-functionalized gold nanoparticles (GNPs). In the absence of Hg²⁺, FRET between UCNPs and GNPs occurred because of the specific matching between two aptamers, resulting in the fluorescence quenching of UCNPs. In the presence of Hg²⁺, long-stranded aptamers fold back into a hairpin structure due to the stable binding interactions between Hg²⁺ and thymine, leading to the release of GNPs from UCNPs, resulting in the quenched fluorescence restoration. Under the optimized conditions, the nanosensor achieved a linear detection range of 0.2–20 μM and a low detection limit (LOD) of 60 nM. Meanwhile, it showed good selectivity and has been applied to detecting Hg²⁺ in tap water and milk samples with good precision.