Metal-Enhanced Hg2+-Responsive Fluorescent Nanoprobes: From Morphological Design to Application to Natural Waters

Metal-enhanced fluorescence (MEF) is a powerful tool in the design of sensitive chemical sensors by improving brightness and photostability of target-responsive fluorophores. Compounding these advantages with the modest hardware requirements of fluorescence sensing compared to that of centralized elemental analysis instruments, thus expanding the use of MEF to the detection of low-level inorganic pollutants, is a compelling aspiration. Among the latter, monitoring mercury in the environment, where some of its species disseminate through the food chain and, in time, to humans, has elicited a broad research eflort toward the development of Hg2+-responsive fluorescent sensors. Herein, a Hg2+-sensitive MEF-enabled probe was conceived by grafting a Hg2+-responsive fluorescein derivative to concentric Ag@SiO2 NPs, where the metallic core enhances fluorescence emission of molecular probes embedded in a surrounding silica shell. Time-resolved fluorescence measurements showed that the fluorophore's excited-state lifetime decreases from 3.9 ns in a solid, coreless silica sphere to 0.4 ns in the core-shell nanoprobe, granting the dye a better resistance to photobleaching. The Ag-core system showed a sizable improvement in the limit of detection at 2 nM (0.4 ppb) compared to 50 nM (10 ppb) in silica-only colloids, and its eflectiveness for natural water analysis was demonstrated. Overall, the reported nanoarchitecture hints at the potential of MEF for heavy metal detection by fluorescence detection.

Automated summary

Metal-enhanced fluorescence is a powerful tool that improves the brightness and photostability of target-responsive fluorophores, making it useful in the design of sensitive chemical sensors. Expanding the use of metal-enhanced fluorescence to the detection of low-level inorganic pollutants, such as monitoring mercury in the environment, is an important goal. This study demonstrates the potential of metal-enhanced fluorescence for heavy metal detection through fluorescence sensing, by developing a Hg2+-sensitive nanoprobe.