Preparation, physicochemical characterisation and assessment of liquid and vapour phase antimicrobial activity of essential oil loaded lipid nanoparticles

Many essential oils have garnered attention as promising antimicrobials; however, their inherent volatility and susceptibility to degradation limit their effective utilisation. Encapsulation can address these limitations by entrapping the essential oils within protective matrices. In the current study, native Australian essential oils, Tasmanian mountain pepper (Tasmannia lanceolata) and lemon myrtle (Backhousia citriodora) and non-native thyme (Thymus vulgaris) were encapsulated in lipid nanoparticles (LNs) using high-pressure homogenisation. The morphology, size, stability and release properties of the LNs was assessed using cryo-TEM, DLS, turbiscanning and dialysis, respectively. The encapsulation efficiency of the LNs was determined using ultrafiltration and GC-FID analysis. The liquid and vapour phase antimicrobial activity and mode of action were determined using microdilution, inverted petri plate assays, and TEM, respectively. Stable spherical LNs (<200 nm size) with encapsulation efficiencies <88% and polydispersity index <0.2 were formulated. LNs generally showed higher antimicrobial activity than unencapsulated oils except for vapour phase activity which varied amongst the oils. Time-dependent cell membrane damage was confirmed using TEM. Essential oils exhibited a bi-phasic release profile from LNs characterised by initial burst release, followed by sustained release. In summary, encapsulation stabilised, promoted sustained release and enhanced the antimicrobial efficacy of the essential oils.