Visual chiral recognition of mandelic acid enantiomers with l-tartaric acid-capped gold nanoparticles as colorimetric probes

We report a method for visual differentiation between the l - and d -forms of mandelic acid (MA). It is based on the chirality of l -tartaric acid-capped gold nanoparticles ( l -TA-capped AuNPs, ca. 13   nm in diameter) that can be used as chiral selector for l - and d -forms of MA. On addition of l -MA to a solution of the l -TA-capped AuNPs, a color change from red to blue can be seen, while no color change is found on addition of d -MA. The chiral assay can be used to determine the enantiomeric excess of l -MA in the range from −100% to 100%. The effect can be detected with bare eyes and quantified by spectrophotometry. Notably, this method does not require the complicated chiral modification. The method excels by its sensitivity, low cost, good availability of materials, and its simplicity.