Supercritical Fluid-extracted Citrus aurantium L. var. amara Engl. Essential Oil Nanoemulsion: Preparation, Characterization, and Its Sleep-promoting Effect.

To overcome the defects of Citrus aurantium L. var. amara Engl. essential oil (CAEO), such as high volatility and poor stability, supercritical fluid-extracted CAEO nanoemulsion (SFE-CAEO-NE) was prepared by the microemulsification method. Emulsifiers comprising Tween 80, polyoxyethylenated castor oil (EL-40), and 1,2-hexanediol, and an oil phase containing SFE-CAEO were used for microemulsification. We examined the physicochemical properties of SFE-CAEO-NE and steam distillation-extracted CAEO nanoemulsion (SDE-CAEO-NE), which were prepared using different concentrations of the emulsifiers. The mean particle size and zeta potential were 21.52 nm and -9.82 mV, respectively, for SFE-CAEO-NE, and 30.58 nm and -6.28 mV, respectively, for SDE-CAEO-NE, at an emulsifier concentration of 15% (w/w). SFE-CAEO-NE displayed better physicochemical properties compared with SDE-CAEO-NE. Moreover, its physicochemical properties were generally stable at different temperatures (-20-60℃), pH (3-8), and ionic strengths (0-400 mM). No obvious variations in particle size, zeta potential, and Ke were observed after storing this nanoemulsion for 30 days at 4℃, 25℃, and 40℃, suggesting that it had good stability. The sleep-promoting effect of SFE-CAEO-NE was evaluated using a mouse model of insomnia. The results of behavioral tests indicated that SFE-CAEO-NE ameliorated insomnia-like behavior. Moreover, SFE-CAEO- NE administration increased the serum concentrations of neurotransmitters such as 5-hydroxytryptamine and γ-aminobutyric acid, and decreased that of noradrenaline in mice. It also exerted a reparative effect on the function of damaged neurons. Overall, SFE-CAEO-NE displayed a good sleep-promoting effect.