Employing intrinsic fluorone-dye fluorescence in a quenching-based (“on-off”) strategy for diltiazem determination in tablets and capsules

A novel green spectrofluorometric method was developed for the quantification of diltiazem hydrochloride (DLZ), a benzothiazepine-class calcium channel blocker with vasodilatory properties. The assay exploits the rapid fluorescence quenching of Acid Red 87-a fluorone-based dye-upon complexation with DLZ in acidic medium (pH 3.8). This "on-off" mechanism enables selective DLZ detection by measuring the decrease in Acid Red 87 native fluorescence intensity (λ_ex/λ_em = 302.5/545.8 nm). Key parameters (pH, dye concentration, buffer volume) were systematically optimized, yielding a linear response over 50-1100 ng/mL (r² = 0.9991) with a detection limit of 15.5 ng/mL. The method was rigorously validated per ICH Q2(R1) guidelines, confirming precision (RSD < 2%), accuracy (99.76% % recovery), and robustness. It was successfully applied to analyze DLZ in pharmaceutical formulations (tablets/capsules) with no matrix interference, and the statistical comparison (t- and F-tests) showed no significant difference from the reference method. Critically, the procedure uses distilled water as the sole solvent, aligning with green chemistry principles while offering simplicity, cost-efficiency, and high-throughput potential.