Herein, we have shown the interaction of an antibiotic drug ciprofloxacin (CIP) with three surface-active ionic liquids (ILs), having the same cation and different anions, namely, 1-decyl-3-methylimidazoliumtetrafluoroborate [C₁₀mim][BF₄], 1-decyl-3-methylimidazolium bromide [C₁₀mim][Br], and 1-decyl-3-methylimidazolium chloride [C₁₀mim][Cl]. This study has been performed by exploiting various spectroscopic techniques such as steady-state fluorescence, time-resolved fluorescence, and UV-visible spectroscopy. The fluorescence emission study of CIP with ILs was performed at various concentrations of all the three ILs. The emission spectra of CIP decreased in the presence of ILs, suggesting complex formation between CIP-IL. The effect of different concentrations of ILs on the emission spectra of CIP was exploited in terms of quenching and binding parameters. Further, fluorescence emission study was validated by the time-resolved fluorescence technique by measuring the average lifetime (τ_avg) of CIP in the presence of all the three ILs. The τ_avg value of the drug changed with the addition of ILs, which suggests complex formation between the drug and ILs. This complex formation was also confirmed by UV-visible spectroscopy results of CIP with all the three ILs. Further, for evaluating the thermodynamic parameters of the CIP-IL interactions, isothermal titration calorimetry (ITC) was performed. The ITC experiment yielded the thermodynamic parameters, ΔH (change in the enthalpy of association), ΔG (Gibbs free energy change), ΔS (entropy change), and binding constant (K_a). The binding parameters driven by ITC revealed that CIP-IL interactions are spontaneous in nature and enthalpy-driven, involving hydrophobic forces. Further, the classical density functional theory (DFT) calculations were performed, which provided deep insight for CIP-IL complex formation.

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