Fine tuning the pH triggers the enantiorecognition of underivatized amino acids by silver nanoparticles: A novel approach based on the focused use of solution equilibria.

Stereoselective amino acid analysis is still a challenging task. In this work, we report a study on the chiral recognition of d,l-Trp and d,l-His using l-Cys capped silver nanoparticles (AgNPs) and copper(II) ion. The AgNPs have been characterized by TEM, UV-Vis spectra and dynamic light scattering (DLS) measurements and used for chiral discrimination. In the l-Cys capped AgNPs, the alpha-amino and alpha-carboxyl groups of the surface-confined amino acid, besides showing either a negative or a neutral charge as a function of the pH, can coordinate the copper(II) ion, which in turn, binds the l- or d-amino acid present in solution forming diastereoisomeric complexes. The resulting systems have been characterized by UV-Vis spectroscopy, exploiting the zwitterionic nature of the cysteine to obtain enantiodiscrimination by a fine tuning of the pH. The analysis of the UV-Vis data by using a multiwavelength approach allows us to determine the kinetic constants ruling the processes.