Green spectrofluorimetric method using nitrogen–phosphorus carbon quantum dots and Box–Behnken optimized salting-out assisted extraction for valbenazine determination in environmental samples

A novel, sensitive, and sustainable spectrofluorimetric method was developed for valbenazine determination in spiked environmental water samples by integrating nitrogen and phosphorus co-doped carbon quantum dots (N,P-CQDs) as fluorescent nanoprobes with salting-out assisted liquid–liquid extraction (SALLE). The N,P-CQDs synthesized via microwave-assisted pyrolysis exhibited quasi-spherical morphology, high quantum yield, and strong blue–green fluorescence (excitation/emission: 355/441 nm, Stokes shift: 86 nm). Valbenazine induced fluorescence quenching consistent with a static mechanism involving ground-state complex formation, as supported by temperature-dependent Stern–Volmer analysis and thermodynamic studies which revealed spontaneous exothermic binding. Critical analytical parameters including pH, N,P-CQDs concentration, buffer volume, and reaction time were systematically optimized to maximize fluorescence quenching efficiency. Besides, SALLE conditions including pH, acetonitrile volume, salt concentration, and centrifugation time were optimized through four-factor three-level Box–Behnken design, yielding a predictive model with high statistical validity (R 2 = 0.9106, adjusted R 2 = 0.8777) for efficient extraction from complex environmental matrices. Under optimized conditions, the method exhibited high linearity (0.02–4.0 µg/mL, R 2 = 0.9998), sensitivity (LOD: 6.2 ng/mL), high accuracy (99.65% recovery), and precision (RSD < 2%). Successful application to river and tap water samples yielded recoveries of 94–103% with RSD below 5%. Comprehensive green analytical chemistry assessment using AGREE (0.64), CaFRI (82/100), BAGI (72.5/100), and WAC RGB 12 model (87.7% whiteness) indicated satisfactory environmental sustainability, low carbon footprint, and high practical applicability, with reduced organic solvent consumption and shorter analysis time relative to conventional LC–MS/MS and HPLC methods reported for structurally related pharmaceuticals. The method offers a green, cost-effective proof-of-concept analytical platform with demonstrated feasibility for potential future monitoring of emerging pharmaceutical contaminants in aquatic ecosystems.