Unravelling Optical Properties and Self-Assembly Behavior of Ciprofloxacin in Ionic Liquid Environments: Probing the Role of Cationic Residues and Counter Anions

Abstract

This study investigates the interaction of ciprofloxacin (CIP) with three different ionic liquids (ILs) with distinct cationic residues (pyrrolidinium, IL1 vs imidazolium, IL2, with bis (trifluoromethylsulfonyl)imide as counter anion and counter anions (bis (trifluoromethylsulfonyl)imide, IL2 vs hexafluorophosphate, IL3 with imidazolium as cationic residue) in the aqueous environment. A series of spectroscopic studies have been performed to elucidate the role of ILs in optical properties as well as aggregation behavior of CIP. The fluorescence quenching experiment indicated that IL with pyrrolidinium residue showed stronger binding with CIP, while bis (trifluoromethylsulfonyl)imide would be the preferred choice of anion. These quenching effects might be attributed to complex formation mediated by charge-pair and cation−π interactions, along with hydrogen bonding. The Stern−Volmer analysis confirmed a static quenching mechanism, with binding constants (Kb) reflecting the stronger affinity of IL1 due to hydrophobic butyl group and flexible pyrrolidinium cation, resulting formation of larger aggregates. In contrast, the imidazolium residue in IL2 facilitated π−π and hydrogen-bond interactions, disrupting CIP aggregation and resulting in smaller clusters. The polarizable nature of bis(trifluoromethylsulfonyl)imide along with hydrogen bond accepting nature enable stronger binding of ILs to CIP compared to hexafluorophosphate containing ILs. Further studies indicated that pH 6 is optimum for CIP-IL interactions, where CIP remained in zwitterionic form. Increased temperature and ionic strength diminished quenching efficiency, consistent with the reduced stability of CIP-IL complexes under such conditions.