Membrane-Targetable Probes for Hg²⁺ Detection in Live Cells and Paper-Based Devices

Two novel fluorogenic probes (HGMem-2 and −3) for detecting Hg²⁺ were designed, synthesized and characterized. Probes based on a commercially available fluorophore (fluorescein) with high brightness were equipped with/without a photolabile group (2-nitrobenzyl group), a membrane-anchoring unit (cholesterol) and a Hg²⁺-reactive moiety. Taking advantage of the hydrophobicity of the cholesterol group in the probe, a paper-based device spotted with HGMem-2 was developed as a Hg²⁺ paper indicators, and showed feasibility in complex cell lysate. Only upon UV photolysis, HGMem-3 displayed more than 1900-fold fluorescence enhancement with the addition of Hg²⁺. Furthermore, they exhibited pronounced Hg²⁺-selective fluorogenic signalling behavior over other 45 analytes. Importantly, HGMem-3 was utilized to specifically visualize Hg²⁺ near cell membrane in live HepG2 cells with high spatiotemporal resolution, avoiding the influence of reactive species outside cells successfully. Hence, our newly developed membrane-anchored Hg²⁺ sensors provide useful tools for versatile applications in biological and environmental research.