Spectroscopic and DFT study of imidazolium based ionic liquids with broad spectrum antibacterial drug levofloxacin

Herein, we have shown the interaction of levofloxacin (LVF) with two imidazolium based ionic liquids (ILs), 1-butly-3-methylimidazolium chloride ([Bmim][Cl]) and 1-decyl-3-methylimidazolium chloride ([Dmim][Cl]) by utilising spectroscopic techniques along with computational approach. Both [Bmim][Cl] and [Dmim][Cl] quenched the fluorescence emission of LVF suggesting complex formation between ILs and the drug. The steady-state and time-resolve fluorescence studies revealed that the quenching of fluorescence emission of LVF in the presence of [Bmim][Cl] and [Dmim][Cl], which signified the non-fluorescent complex formation between LVF and ILs. The complex formation between LVF and ILs were also validated by the UV-visible spectroscopy method. The cyclic voltammetry (CV) results further suggest the strong interaction between LVF and ILs. The estimated binding constant (K_b) and free energy change (ΔG) parameters shows the substantial binding of LVF with both the ILs and spontaneous in nature. The value suggested that LVF have stronger binding with [Dmim][Cl] than [Bmim][Cl]. Further, in order to support the results classical density functional theory (DFT) model was performed. The DFT calculations were utilized to explore the 3D structure and the molecular orbitals (HOMO and LUMO) of ILs, LVF and their complexes using Gaussian 09 software. The aggregate size (D_h) and zeta potential of ILs and IL-drug complexes were determined by dynamic light scattering (DLS) and zeta potential in aqueous medium.