Decoding Soft Metals Multifunctional Fluorescent Probes for Selective Ag+ and Hg2+ Sensing

Ag + and Hg 2+ are soft, thiophilic cations of high environmental and biomedical relevance, yet their closely related coordination preferences make differential sensing nontrivial. This review surveys multifunctional fluorescent probes that discriminate or simultaneously report Ag + /Hg 2+ , organized by fluorophore scaffold and transduction mechanism. Across eight scaffold classes, 1,8-naphthalimide, rhodamine/rhodol/xanthene, coumarin, BODIPY, aggregation-induced emission (AIE)-active (e.g., tetraphenylethene), polycyclic aromatics (anthracene/pyrene/naphthalene), heteroaromatics and benzo-fused heterocycles (pyridine, thiophene, benzimidazole/oxadiazole/benzothiadiazole, naphthyridine, triarylamine), and long-wavelength specialty dyes (squaraine, anthraquinone, phosphinine), we map recognition motifs (S/S, Se-enriched chelators, N,O-donor arrays, triazoles, crown/azacrown, thio/semicarbazide, thioketal) to signal transduction pathways, including intramolecular charge transfer (ICT) and photoinduced electron transfer (PET) modulation, chelation-enhanced fluorescence (CHEF)/ chelation-enhanced quenching (CHEQ), excited state intramolecular proton transfer (ESIPT), twisted intramolecular charge transfer (TICT), AIE, and reaction-gated conversions (e.g., rhodamine spirocyclic opening, desulfurization, thioketal cleavage). We compare response formats (turn-on/turn-off, ratiometric, lifetime/anisotropy), media compatibility (buffered aqueous solutions, paper/hydrogel strips, complex water matrices), and biological use cases (live-cell imaging), and benchmark figures of merit (dynamic range, limits of detection, binding stoichiometry/affinity, kinetics, photostability, and selectivity panels against Cu 2+ , Pb 2+ , Fe 3+ , alkali/alkaline-earth cations, and biologically relevant anions/thiols). Case studies highlight orthogonal readouts that enable Ag + /Hg 2+ discrimination, such as distinct emission maxima, colorimetric shifts, reversible masking with chelators, as well as aggregation- or reaction-driven amplification suitable for portable formats. Persistent challenges include the demand for water-soluble, low-background probes that operate with minimal organic cosolvent, sustained performance in complex matrices, and red/near-infrared (NIR) emission for deep-tissue imaging. We conclude with design principles that emphasize atom-economical synthesis, biocompatible scaffolds, orthogonal binding sites, and dual-channel transduction as a route to robust, field-deployable, and bioimaging-ready Ag + /Hg 2+ sensors.