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

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.