Extraction Of Flavonoids From Ginkgo Biloba Leaves By Graphene Oxide-Assisted Ethanol Reflux Method And Its Mechanism

Ginkgo biloba leaves?GBL?contain many biological active compounds such as flavonoids and terpene trilactones,which have antihypertensive,antidiabetic and cardioprotective activities.In this paper,the content determination method of ginkgo flavonoids?GF?was firstly studied,and the determination conditions of NaNO₂-NaOH-Al?NO₃?₃ colorimetry were optimized.The results showed that the method was simple,small error and good precision with RSD of 0.88%and the average recovery rate of104.50%.Then,the effects of extraction process including water reflux extraction?WRE?,ethanol reflux extraction?ERE?,graphene oxide?GO?-assisted water reflux extraction?GWRE?and GO-assisted ethanol reflux extraction?GERE?on the yield of GF were compared.The results showed that the highest yield of GF could be obtained by GERE method,and the parameters of GERE were optimized by Box-Behnken method.The optimum extraction conditions of GERE were as follows:80?C of temperature,73%of ethanol concentration?v/v?,85.23 mg/L of GO concentration.Under these conditions,the yield of total flavonoids by GERE reached 83.79%,about5.60%higher than that by the conventional ethanol reflux extraction.Finally,the extraction kinetics of GERE was also studied.The effects of stirring speed,GO dosage,ethanol concentration,temperature and liquid to solid ratio on the extraction kinetics of GF were discussed.The results showed that So-Macdonald model was suitable to describe the extraction kinetics with high values of the coefficient of determination?R²?0.994?and low values of the residual sum of squares?RSS?0.00022?.The stirring speed had no obvious effect and the extraction process was mainly controlled by internal diffusion.The extraction yield of GF was shown to increase with an increase in the ethanol concentration,temperature,the ratio of liquid to solid and the dosage of GO.However,when the ethanol concentration further increased to 70%,?_e^d1was almost unchanged.When the ratio of liquid to solid was greater than 16 mL/g,k_d1,k_d2,?_e^d1 and?_e^d2 were no further increase for GERE.When GO dosage was 1.5 mg/g,?_e achieving a maximum of 82.63%and subsequently decreased.For GERE,when GO dosage was 1.5 mg/g,the relationship between D'and T was7)9)^?=7)9)???²?-473.85/+2?+27)9)?(-_?8?²,and the apparent activation energy obtained was3.69 kJ/mol.For ERE,7)9)^?=7)9)???²?-443.93/+2?,the apparent activation energy obtained was 3.94 kJ/mol.In addition,thermodynamic results showed that the extraction of GF was a process of spontaneous,endothermic and entropy increase.HPLC results indicated that GO could not decompose flavonoids.SEM,XRD and FT-IR results all showed that GO could disrupt the cellulose structure and reduce the crystallinity.The higher yield of ginkgo flavonoids by GERE can be attributed to the catalytic hydrolysis to the cell wall by GO,which can decrease the apparent activation energy and accelerate the leaching of GF at the fast diffusion stage.