Novel Smart Polymer-Brush-Modified Magnetic Graphene Oxide for Highly Efficient Chiral Recognition and Enantioseparation of Tryptophan Enantiomers

Multifunctional graphene oxide nanocomposites simultaneously possessing high enantioselectivity, excellent thermosensitivity, and magnetism demonstrate great application potentials in direct enantioseparation. We herein report one novel smart graphene oxide nanocomposite (MGO@PNG-CD) with high enantioselectivity, excellent thermosensitivity, and magnetism for highly efficient chiral identification and enantioseparation of tryptophan enantiomers. The MGO@PNG-CD is composed of graphene oxide nanosheets with immobilized superparamagnetic Fe3O4 nanoparticles and grafted PNG-CD smart polymer brushes. The PNG-CD is made up of poly(N-isopropylacrylamide-co-glycidyl methacrylate) (PNG) chains with numerous appended β-cyclodextrin (β-CD) units, which play a significant role in effective chiral discrimination and resolution of DL-tryptophan (DL-Trp). The β-CD units serve as chiral selectors capable of selectively recognizing and binding L-tryptophan (L-Trp) into their cavities to form stable host–guest inclusion complexes of β-CD/L-Trp. The PNIPAM chains in PNG act as a microenvironmental adjustor for the inclusion constants of β-CD/L-Trp complexes. The resulted MGO@PNG-CD demonstrates high thermosensitive enantioselectivity toward L-Trp over D-Trp based on the chiral discrimination ability of β-CD toward L-Trp and the thermosensitive volume phase transition of PNIPAM chains. Operating temperature and initial concentrations of DL-Trp are two significant factors affecting the separation efficiency of DL-Trp enantiomers. Moreover, the MGO@PNG-CD also displays satisfactory recycling and convenient magnetic separability from enantiomeric solution. Such a multifunctional graphene oxide nanocomposite developed in this study can serve as a high-performance nanoselector for highly efficient chiral recognition and enantioseparation of various chiral compounds.