Universal Behavior in Enantioselective Adsorption of Amino Acids onto Chiral Terbium Phosphate Nanocrystals

The homochirality of biomolecules motivates the search for chiral surfaces capable of enantioselective recognition, yet systematic links between amino acid functional-group geometry and adsorption on chiral inorganic nanocrystals (NCs) remain limited. Here we investigate the enantioselective adsorption of asparagine, serine, and histidine onto intrinsically chiral Λ- and Δ-TbPO4·H2O NCs, which crystallize in a monoclinic (pseudo-hexagonal) structure. Individual-enantiomer adsorption isotherms and enantiomeric excess (ee) from racemic mixtures were determined using circular dichroism spectroscopy combined with potentiometric titration. Adsorption of single enantiomers followed the Langmuir model at low coverage, with L-enantiomers preferring Λ-NCs and D-enantiomers preferring Δ-NCs; matched chiral pairs showed roughly twofold higher equilibrium constants than mismatched pairs. At relatively high surface coverage, the adsorption behavior deviated from the Langmuir model and fitted the Frumkin model, which contains lateral attraction between adsorbed molecules. Enantioselectivity was highest at low racemate concentrations and declined with increasing total racemate concentrations, mirroring trends previously found for tartaric acid and aspartic acid on the same NCs. Tb3+–Tb3+ spacings on the dominant facets closely matched intramolecular functional-group distances in the amino acids, supporting a three-point chiral recognition mechanism.