β-Cyclodextrin-modified silver nanoparticles as colorimetric probes for the direct visual enantioselective recognition of aromatic α-amino acids

Efficient enantiomer discrimination by a convenient method remains a challenge in biochemical and pharmaceutical fields. Herein we have modified silver nanoparticles (AgNPs) with β-cyclodextrin (β-CD) to obtain a probe for the chiral recognition of aromatic α-amino acids. This assay relies on the distance-dependent optical properties of AgNPs and the difference in inclusion binding strength between the aromatic amino acid enantiomers and the β-CD host. The response of β-CD functionalized-AgNPs to certain chiral aromatic amino acid enantiomers namely d - and l -phenylalanine (Phe), d - and l -tryptophan (Trp), and d - and l -tyrosine (Tyr) in phosphate buffer solutions was studied in detail. In the presence of d -Phe (or l -Trp or l -Tyr), an appreciable yellow-to-red color change of the AgNP solution can be observed, whereas no color change is found in the presence of l -Phe (or d -Trp or d -Tyr). This method can be used to quantitatively determine the percentage of d -Phe in a mixture of d - and l -Phe. The chiral assay described in this work is easily readout with the naked eye or using a cheap UV-vis spectrometer. This method excels by its simplicity, low cost, and good availability of materials.