Ultra-sensitive fluorene-based fluorescent sensors for Hg²⁺ detection using smartphone-assisted RGB analysis in water...

Mercury is a highly toxic environmental pollutant due to its bioaccumulation, persistence, and severe neurotoxic effects on human health. In aquatic ecosystems, even trace levels of Hg²⁺ can undergo methylation to form methylmercury, a potent neurotoxin that enters the food chain through water and fish consumption. Developing straightforward, accessible, and reliable strategies for mercury detection remains essential. In this work, two fluorene-based Schiff base fluorescent sensors (2 and 3) were synthesized for highly selective detection of Hg²⁺. Upon interaction with Hg²⁺, both sensors showed a remarkable fluorescence enhancement, mainly due to the suppression of photo-induced electron transfer (PET) and the formation of rigid complexes that favor radiative emission. The 1:1 binding stoichiometry and sensing mechanism were confirmed by Job's plot, Benesi-Hildebrand analysis, Nuclear Magnetic Resonance (NMR) titration, Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS), lifetime measurements, and Density Functional Theory (DFT) calculations. Under optimized conditions (λ_ex = 355/345 nm, pH 8.0), the sensors displayed wide linear ranges of 0.50-150.00 μM and 0.30-100.00 μM with low detection limits of 0.15 and 0.08 μM, respectively. Practical applicability was demonstrated through the determination of Hg²⁺ in environmental water and food samples, with recoveries ranging from 96.2 % to 100.6 % and no significant deviation from Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (t_exp < t_critical). Moreover, polystyrene (PS) thin-film kits were developed for on-site analysis. These films, coupled with smartphone-based Red-Green-Blue (RGB) analysis, provided a reusable, robust alternative to paper-based tests. This dual-mode platform offers a practical route for portable and quantitative Hg²⁺ monitoring in environmental and food matrices.