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

•Calculated pH of AgNPs solutions was found in agreement with the experimental one.•The AgNPs exhibit enantiodiscrimination properties towards underivatized amino acids.•The role played by pH was investigated by using thermodynamic stability constants.

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 α-amino and α-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.

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