Harnessing zinc(ii) chelation for fluorescence determination of besifloxacin in ophthalmic and biological matrices†

Besifloxacin (BFN), a fluoroquinolone antibiotic, exhibits weak native fluorescence intensity. A sensitive and rapid spectrofluorimetric approach was developed to enhance the intrinsic fluorescence of BFN through complexation with zinc(II) as an electron acceptor in phosphate buffer (pH 6.0). The resulting BFN–Zn²⁺ complex demonstrated high fluorescence intensity, enabling highly sensitive detection of BFN. Among various tested metal ions, Zn²⁺ was effective in enhancing the native fluorescence of BFN due to its favorable ionic radius, flexible coordination number, and strong affinity for oxygen and nitrogen donor atoms present in BFN. Under optimum conditions, a significant increase in the intensity of BFN fluorescence was observed at 440 nm following excitation at 274 nm. The quantification and detection limits were calculated to be 6.08 and 2.0 ng mL⁻¹, respectively, and the relationship between emission intensity and BFN concentration was linear from 10 to 150 ng mL⁻¹. Notably, the binding ratio between BFN and Zn²⁺ ions was found to be 2 : 1, respectively, and this ratio was confirmed by the Job's plot method. The method was validated according to ICH guidelines. The proposed method was successfully applied to quantify the cited drug in its pure state, ophthalmic preparations, and spiked aqueous humor with recovery values ranging from 96.29% to 98.21% in aqueous humor samples.