Citronella Oil Nanoemulsion: Formulation, Characterization, Antibacterial Activity, and Cytotoxicity

Abstract: The excessive and unreasonable use of synthetic bactericides in the agricultural field has caused many serious problems, including toxic effects on human health and environmental pollution. Therefore, searching for low toxicity, highly efficient, and no-residue natural bactericides is urgently needed. Plant essential oil has become an emerging and hot topic in the agricultural field because of its excellent bactericidal activity, good biocompatibility, and abundant sources. Citronella oil is a natural plant essential oil with insect repellent, insecticidal, and antibacterial activities, which mainly includes citronellal, geraniol, and citronellol. At present, the major of research on citronella oil focuses on the repellency and control of sanitary pests, but there are relatively few reports on the control of agricultural pathogenic bacteria. In addition, the hydrophobicity and volatility of citronella oil lead to its low bioavailability and hinder its full biological activity. Therefore, constructing a delivery system for improving the hydrophobicity and reducing the volatility of citronella oil is urgently needed. Nanoemulsions have the advantages of fine and uniform droplets, better physical stability, efficient permeation ability, and enhanced bioavailability. Therefore, nanoemulsions are important drug delivery systems for hydrophobic pesticides. In this study, the influences of emulsifier type (hydrophilic-lipophilic balance (HLB)), dosage, and emulsifying time on the formation and stability of citronella oil nanoemulsions were investigated by observing the appearances and microstructures of samples and measuring droplet size, thereby the optimized formula of the citronella oil nanoemulsions was determined. Furthermore, the bioactivity and biosafety of citronella oil nanoemulsions were also investigated. The results showed that nanoemulsions using castor oil polyoxyethylene ethers EL-40 (hydrophilic-lipophilic balance=13.5) as an emulsifier had the best performance, and the stability of nanoemulsions improved as the emulsifier dosage increased from 3% to 7% (w, mass fraction). In addition, the nanoemulsion prepared through high speed shearing for 3 min was the most stable. The optimal formula for citronella oil nanoemulsions was determined to contain 5% (w) citronella oil, 6% (w) emulsifier (EL-40), and 89% (w) deionized water, upon high speed shearing for 3 min. Then, the inhibitory effect of citronella oil nanoemulsions against the growth of Pantoea ananatis was studied. The concentration for 50% of maximal effect (EC50)of citronella oil nanoemulsions against Pantoea ananatis was 74.85 mg·L−1. The cell viability of L02 cells treated with the citronella oil nanoemulsions (below 100 mg·L−1) was above 83% after 24 h, and the apoptosis rate was 6.93%, indicating that the citronella oil nanoemulsions had low cytotoxicity. This research facilitated the design and fabrication of stable, efficient, and safe agricultural nanoemulsions, and it provides a practical solution for using plant essential oils as agricultural bactericides.