Preparation of cinnamaldehyde nanoemulsions: Formula optimization, antifungal activity, leaf adhesion, and safety assessment

Plant essential oils have become an increasingly popular topic in agriculture due to their outstanding fungicidal activity, good biocompatibility and abundance of sources. Cinnamaldehyde is found in cinnamon essential oil and has good antifungal activity, but cinnamaldehyde also has negative characteristics such as high volatility, poor water solubility and easy oxidative degradation, which seriously hinder the potential of its antimicrobial properties and its application in agriculture. In this study, the effects of the surfactant ratio, concentration and emulsification method on the stability of cinnamaldehyde nanoemulsions were systematically investigated by observing the sample appearance, mean droplet size and differential scanning calorimetry (DSC) analysis. The results showed that the optimal formula of cinnamaldehyde nanoemulsion was a surfactant (R5:848) ratio of 1:1 and a concentration of 7%, and the emulsification method was E1: adding dropwise aqueous phase (deionized water) to the oil phase (cinnamaldehyde + surfactant). Cinnamaldehyde nanoemulsions showed good antifungal activity against rice sheath blight (Rhizoctonia solani), wheat sheath blight (Rhizoctonia cerealis) and wheat take-all (Gaeumannomyces graminis). The nanoemulsions also had excellent wetting properties on rice and wheat leaves, higher biosafety for nontarget environmental organisms such as zebrafish and earthworms, low cytotoxicity to human BASE-2B cells, and no inhibition of germination and growth of wheat and rice. The results of this research provide important ideas for the design of efficient, safe and environmentally friendly plant-derived fungicides.