Development of sustainable N,S carbon quantum dots turn-off fluorescence method for rivastigmine determination using...

Rivastigmine, a dual cholinesterase inhibitor used for dementia treatment, poses significant environmental concerns due to its persistence in aquatic systems and potential ecotoxicological effects on non-target organisms, necessitating sensitive analytical methods for environmental monitoring. Herein, a novel, sensitive, and environmentally sustainable spectrofluorimetric method was developed for rivastigmine determination using nitrogen and sulfur co-doped carbon quantum dots (N,S CQDs) as fluorescence probes. The N,S CQDs were synthesized via a rapid microwave-assisted carbonization method and subsequently characterized using different techniques to elucidate their morphological, structural, and optical properties. The fluorescence quenching mechanism was then systematically investigated through Stern-Volmer kinetic analysis at three temperatures (298-313 K) revealing inverse temperature dependence of quenching constants Ksv (5.57 × 10⁵ to 3.86 × 10⁵ M^-1), thereby confirming static quenching through ground-state complex formation. Additionally, thermodynamic analysis revealed exothermic binding (ΔH = -21.06 kJ/mol) with spontaneous complex formation. Subsequently, critical analytical parameters including pH, N,S CQDs volume, and reaction time were systematically optimized using Box-Behnken experimental design. Notably, response surface methodology revealed significant factors such as pH and N,S CQDs volume as well as synergistic interactions between these factors, which yielded a predictive mathematical model (R² = 0.9759) with optimized conditions: pH 6.0, N,S CQDs volume 0.9 mL, and reaction time 3 min. The method was validated according to the ICH guidelines demonstrating high analytical performance with linear range 0.05-1.5 μg/mL (r² = 0.9996), low detection limit (0.0165 μg/mL), high accuracy (98.92 ± 1.207 %), and good precision (RSD <2 %). The method was then successfully applied to pharmaceutical formulations and environmental water samples with satisfactory recovery rates (95.43-103.26 %). Comprehensive greenness and blueness assessments using AGREE and BAGI tools revealed superior environmental performance (0.77) compared to conventional HPLC-UV methods (0.63) while maintaining acceptable practical applicability (77.5). The developed method offers a viable green alternative for rivastigmine determination in pharmaceutical and environmental analysis.