Controlling covalent functionalization of graphene oxide membranes to improve enantioseparation performances

At present, the separation of racemates is still a great challenge due to their similarity in chemical structures and physicochemical properties. Here, we proposed a new strategy to improve the enantioseparation performances of graphene oxide (GO) based membranes via controlling the degree of functionalization (DF) of GO by the aid of an epoxide ring-opening reaction with a carboxyl-terminated ionic liquid (IL-COOH) as a spacer, followed by an amidation reaction of the l-glutamic acid (GO-IL-Glu) as chiral selectors. Results show that: i) these membranes are superior in the enantioselectivity and 1–3 orders of magnitude higher in the flux than traditional chiral separation membranes; ii) compared with l-glutamic acid modified GO membranes and their complex membranes with polypeptides in our previous work, they afford a ca. 40–80% increase in enantioselectivities and an order of magnitude increase in fluxes. Herein, IL-COOH groups can not only serve as a spacer, which is propitious to expanding the interlayer spacing of GO-IL-Glu nanosheets, thereby achieving high throughput of enantiomers, but also function as an active site to improve the grafting amount of chiral selectors (l-Glu), helpful for enhancing enantioselectivities. In particular, the flux is not compromised by the improved enantioselectivity in such membranes; on the contrary, it has an order of magnitude increase. Our findings suggest that the strategy used in this work, i.e., combining more chiral selectors with a wider interlayer spacing, could provide new opportunities for simultaneously facilitating high-flux and high-selectivity and a potential application in a great many enantio- and bio-separations.