Temperature dependence of ofloxacin fluorescence quenching and complexation by Cu(II)

The coexistence of heavy metals and antibiotics is common in the environment, and their interactions may mutually alter their environmental behaviors and risks. This study investigated ofloxacin (OFL)–Cu(II) interaction using fluorescence quenching experiments. The possible artifacts were excluded and OFL quenching was attributed to static quenching as suggested by the linear Stern–Volmer plot and decreased quenching with increased temperature. The OFL–Cu(II) interaction was quantitatively described using a stoichiometry equation. The calculation suggested that OFL–Cu(II) association was the mixture of 1:1 and 1:2 complexes. The negative ΔG values and the negative ΔH values suggested that the complexation is a spontaneous and exothermic process. Cation-π binding and electrostatic interaction were excluded and the complexation of Cu(II) with OFL ketonic and carboxyl groups was proposed through UV–visible spectrum characterization, pH dependent complexation, and thermodynamic analysis.

► OFL fluorescence intensity is statically quenched by Cu(II).
► OFL–Cu(II) association is the mixture of 1:1 and 1:2 complexes.
► OFL complex with Cu(II) through a spontaneous and exothermic process.
► The ketonic and carboxyl groups of OFL are complexed with Cu(II).