Colorimetric chiral discrimination and determination of enantiometric excess of D/L-tryptophan using silver nanoparticles

We report on a method for visual differentiation between the D- and L-forms of tryptophan (Trp). It is based on the inherent chirality of citrate-capped silver nanoparticles (AgNPs; ca. 15 nm in diameter) that can be used as chiral selector for D- and L-forms of Trp. On addition of D-Trp to a solution of the AgNPs, a color change from yellow to red can be seen, while no color change is found on addition of L-Trp. The chiral assay can be used to determine the enantiometric excess of D-Trp in the range from −50 % to 100 %. The effect can be detected with bare eyes and quantified by spectrophotometry. Notably, this method does not require any labeling or chiral modification. The method excels by its sensitivity, low cost, good availability of materials, and its simplicity. The silver nanoparticles can discriminate the absolute configuration of Trp and determine the enantiomeric excess of Trp