Smart chiral magnetic nanoparticles for highly efficient enantioseparation of tryptophan enantiomers

Herein, we report a simple and facile strategy to prepare one kind of smart chiral magnetic nanoparticles (Fe3O4@PDA@PNG-CD) with high enantioselectivity via combining mussel-inspired polydopamine (PDA) chemistry with surface-initiated atom transfer radical polymerization for effective enantioseparation of tryptophan enantiomers (dl-Trp). The PDA thin layer plays a pivotal role in fabricating high-density poly(N-isopropylacrylamide-co-glycidyl methacrylate)-β-cyclodextrin (PNG-CD) smart polymer brushes onto the Fe3O4 NPs. The grafted PNG-CD plays a significant role in greatly boosting the enantioselectivity of the Fe3O4@PDA@PNG-CD, which is composed of the poly(N-isopropylacrylamide-co-glycidyl methacrylate) (PNG) copolymer chains with numerous appended β-cyclodextrin (β-CD) units. The β-CD units serve as chiral selectors capable of selectively recognizing and binding l-tryptophan (l-Trp) into their cavities by forming stable host–guest inclusion complexes of β-CD/l-Trp, and the PNIPAM chains act as microenvironmental adjustors for the inclusion constants of β-CD/l-Trp complexes. Operating temperature and initial concentrations of dl-Trp are two important factors that significantly affect the separation efficiency of dl-Trp and the enantioselectivity of the Fe3O4@PDA@PNG-CD. Furthermore, the Fe3O4@PDA@PNG-CD also demonstrates satisfactory recycling and excellent magnetic separability from enantiomeric solution. Such smart chiral magnetic NPs with high enantioselectivity developed in this study show great potentials in direct enantioseparation of various chiral compounds.