Selective Separation of Structurally Similar Alkaloids by Graphene Oxide Membranes

Matrine (MT) and oxymatrine (OMT) are alkaloids coexisting in Sophora flavescens, and they have similar structure but different clinical applications. Graphene oxide (GO) membranes with different oxidation degrees were used to explore the separation mechanism of MT and OMT. When the ratios of O-C=O and C=O groups remained basically unchanged, the higher C-OH content and the lower C-O-C content on the GO nanosheets provided higher O/C ratios, more negative Zeta potentials, stronger hydrophilicity, rougher surface, and larger interlayer spacing of the GO membranes. With the increases in the O/C ratio, the rejection rates for MT and OMT exhibited opposite trends, and it was considered that a competitive relationship between MT and OMT on the membrane converted from hydrophilic interaction to electrostatic interaction. Within the separation channels limited by the GO nanosheets, the tertiary amine in the alkaloid catalyzed C-O-C group ring opening and formed C-OH or O-C=O groups. As the C-OH content of the original GO membranes increased, the stability during alkaloid separation was improved. The GO-5 membrane showed stable separation ability within 72 h, and the separation factor of MT and OMT almost doubled during the secondary membrane process. This study provides new insight into the application of membrane technology for the separation of similar molecules of pharmacodynamic substances in traditional Chinese medicine.