Retarded transport properties of graphene oxide based chiral separation membranes modified with dipeptide

The separation of racemates is still a great challenge, since enantiomers commonly exhibit identical physicochemical properties in achiral environments, but differ in biological responses in chiral surroundings. Recently, graphene oxide (GO) based enantionseparation membranes had been proven to follow the facilitated or retarded transport mechanism, dependent on the choice of chiral selectors. Herein, a dipeptide was selected to act as the chiral selector for separating racemates, since it has a longer molecular length than single amino acids and two chiral centers as well, thus more potentially penetrating into the interlayer space in GO membranes. Results show that dipeptide-modified GO membranes exhibit a maximum separation factor of 1.85 with a 1–3 orders of magnitude improved flux in comparison to conventional chiral separation membranes, particularly a reversal of the transport mechanism from facilitated to retarded with respect to GO membranes previously reported. This is a likely consequence of the L-isomeric dipeptide that possesses a much stronger affinity toward the D-isomeric chiral analyte than the L-isomeric one due to the occurrence of extra nonstereoselective interaction between probes and the opposite GO substrate, so that such the dipeptide modified GO membrane exhibits a retarded transport mechanism, thereby offering a promise to achieve the continuous operation. These findings demonstrate that the extra nonstereoselective interaction plays a key role in the chiral recognition, especially in the confined space, and provides new opportunities for controlling the conversion from facilitated to retarded transport mechanisms and designing the novel enantioseparation membranes, aiming at the large-scale production of diversely pure enantiomers.