| The application of plant-based repellents with environment-friendly manufacturing method is a new approach in mosquito bit prevention for today and in the near future. Citronella oil (Cymbopogon nardus (L.) Rendle, Family Poaceae) is perceived by consumers as a safe substance as compared to the gold-standard synthetic repellent DEET (N,N-diethyl-3-methylbenzamide). However, it is most likely that many volatile constituents in citronella oil tend to be unstable under normal storage condition and its mosquito repellent activity is limited, often 1-2 h of exposure. Microencapsulation of the citronella oil using spray drying method is expected to prolong mosquito protection time and increase stability of the volatile substances. Objective: To determine the optimal conditions of the spray drying method for citronella oil microencapsulation using statistic method and to characterize the physicochemical properties of the resulting microcapsules for mosquito repellent patch application. Materials and Methods: A 23-factorial, 6-axial, 6-center central composite design was applied to optimize the spray drying conditions of the citronella oil microcapsules.
The microencapsulated oil produced under the optimal processing conditions were then characterized for its physicochemical properties (i.e., Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), light microscopy, scanning electron microscopy, thermal gravimetric analysis (TGA), gas chromatograph-mass spectrometry (GC-MS), and physicochemical stability) and predicted the interaction between the key volatile constituents and binding pocket of Aedes aegypti olfactory protein 3K1E using the in-silico molecular docking technique. Results: The optimal microencapsulation conditions were gum Arabic, citronella oil, water, and Inlet air temperature of 14 %w/w, 3.64 %w/w, 82.36 %w/w, and 150 oC, respectively. The microencapsulation did not change the chemical structure of gum Arabic and citronella oil as determined by FTIR. GC-MS revealed that the volatile substances encapsulated in microcapsules did not show significant difference from pure oil. Long lasting evaporation of citronella oil by microcapsules was confirmed by TGA compared to that of pure essential oil and the odor perception level was found to be unchanged up to 90 days when kept the formulation at 25oC. The key volatile constituents, citronellal and geraniol, could insert their molecules into the binding pocket of 3K1E with interactive H-bonding and Van der Waals forces. Furthermore, the citronella oil microcapsules could be successfully embedded onto the mosquito repellent patch. Conclusion: The environment-friendly spray drying method should be considered as a candidate for encapsulating the citronella oil in microcapsules and provided long-lasting effect of the odor.
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