Regenerable fluorescent nanosensors for monitoring and recovering metal ions based on photoactivatable monolayer self-assembly and host-guest interactions

Efficient detection, removal, and recovery of heavy metal ions from aqueous environments represents a technologically challenging and ecologically urgent question in the face of increasing metal-related pollution and poisoning across the globe. Although small-molecule and entrapment-based nanoparticle sensors have been extensively explored for metal detection, neither of these extant strategies satisfies the critical needs for high-performance sensors that are inexpensive, efficient, and recyclable. Here we first report the development of a regenerable fluorescent nanosensor system for the selective and sensitive detection of multiple heavy metal ions, based on light-switchable monolayer self-assembly and host-guest interactions. The system exploits photocontrolled inclusion and exclusion responses of an α-cyclodextrin (CD)-containing surface conjugated with photoisomerizable azobenzene as a supramolecular system that undergoes reversible assembly and disassembly. The metal nanosensors can be facilely fabricated and photochemically switched between three chemically distinct entities, each having an excellent capacity for selective detecting specific metal ions (namely, Cu²⁺, Fe³⁺, Hg²⁺) in a chemical system and in assays on actual water samples with interfering contaminants.