Box-Behnken optimized green analytical method for mirtazapine detection using N,P CQDs with quantum mechanical...

A novel spectrofluorimetric method for sensitive mirtazapine determination was developed based on fluorescence quenching of nitrogen and phosphorus co-doped carbon quantum dots (N,P CQDs). The N,P CQDs were synthesized via microwave-assisted hydrothermal treatment and comprehensively characterized using dynamic light scattering, transmission electron microscopy, FTIR spectroscopy, and fluorescence spectroscopy, revealing uniform nanoparticles (3.57 ± 1.94 nm) with high quantum yieldand optimal excitation/emission wavelengths at 360/435 nm. Subsequently, mechanistic investigations through UV-visible spectroscopy, temperature-dependent fluorescence studies, and quantum mechanical calculations confirmed static quenching via ground-state complex formation. Stern-Volmer analysis revealed high association constants (Ka = 4.20 × 10⁵ M^-1 at 298 K) with negative temperature dependence, while thermodynamic studies indicated spontaneous binding (ΔG = -32.09 kJ/mol) driven by favorable enthalpy and entropy contributions. Quantum mechanical calculations demonstrated specific interactions through hydrogen bonding and electrostatic interactions between phosphonic acid functionalities and mirtazapine's protonated tertiary amine. Following mechanistic elucidation, Box-Behnken experimental design optimized critical parameters (pH 5.5, N,P CQDs concentration 26 μg/mL, reaction time 4 min) achieving maximum quenching efficiency. The validated method demonstrated excellent analytical performance with linear range 0.05-3.0 μg/mL (r² = 0.9998), detection limit 0.016 μg/mL, quantification limit 0.049 μg/mL, accuracy 98.78 ± 1.393%, and precision RSD values below 2%. Furthermore, statistical comparison with reference HPLC method confirmed equivalent performance (t-test p = 0.844, F-test p = 0.759). Recovery studies in pharmaceutical formulations and environmental water samples (river and tap water) yielded 96.27-104.46% recoveries with RSD < 4%, demonstrating broad method applicability. Finally, green chemistry assessment using AGREE (0.72), BAGI (77.5), and RGB12 (89.0% whiteness) tools confirmed superior sustainability compared to conventional methods. The developed method offers a cost-effective, environmentally friendly alternative for routine mirtazapine analysis in pharmaceutical quality control and environmental monitoring, addressing the growing need for sustainable analytical approaches while maintaining excellent analytical performance.