Central Composite Design Assisted Spectrofluorimetric Determination of Solifenacin via Quenching of Acid Red 51: Application in Pharmaceutical Formulations and Plasma

This study describes the development and validation of a novel photochemical sensing platform based on the fluorescence quenching mechanism between acid red 51 (AR51) and solifenacin succinate (SLF). Upon excitation at 530 nm, AR51 exhibits native fluorescence at 554 nm, which undergoes significant quenching in the presence of SLF through ion pair complex formation. The photophysical interaction was characterized by Stern–Volmer analysis, revealing a quenching constant of 1.44 × 10⁵ mol/L⁻¹, indicative of a static quenching mechanism. Job's plot confirmed a 1:1 stoichiometric ratio between AR51 and SLF. A central composite design optimized the experimental conditions affecting the photochemical interaction, with buffer pH, buffer volume, and AR51 concentration identified as critical parameters. Under optimized conditions (acetate buffer pH 4.2, 1.1 mL buffer volume, 1.1 mL AR51 solution), the method demonstrated linearity in the concentration range of 0.2 to 44 μg/mL with detection and quantification limits of 0.0554 and 0.1678 μg/mL, respectively. The developed sensor was successfully applied for SLF determination in pharmaceutical formulations and human plasma samples with recovery rates of 99.77% and 99.01%, respectively, offering advantages over conventional methods by operating at longer wavelengths, thereby minimizing interference from endogenous fluorophores.