Modification of membrane properties using ionic liquids investigated with Langmuir monolayers and bilayers in vesicles†

Biocompatible and bioavailable ionic liquids (ILs) are generally used to modify the physicochemical properties of lipid monolayers and bilayers. Understanding the influence of ILs on lipid properties and the molecular level mechanism is crucial for applying ILs in lipid membrane modifications. Here the modification of a lipid membrane composed of 1,2-dioleoyl-sn-glycero-3-phosphoglycerol (DOPG) and cholesterol using alkyl methyl imidazolium-based ILs [C_nmim]Cl with varied alkyl chains (n = 4, 6, 8 and 10) was investigated. Langmuir monolayers were used to evaluate the influence of the ILs on the rigidity and robustness of the membranes. Then, selected compositions were used to prepare small unilamellar vesicles (SUVs) as nanocarriers. A small amphiphilic molecule (4-(4-diethylaminostyry)-1-methyl-pyridinium iodide, D289) was used as a probe to investigate the loading capability and encapsulation efficiency of the SUVs. The permeability of the lipid membrane to small molecules was also evaluated using second harmonic generation spectroscopy analysis. The different effects of the ILs with increasing chain length imposed on the membranes were compared and analyzed.