Hierarchical Effect behind the Supramolecular Chirality of Silver(I)-Cysteine Coordination Polymers

Cysteine is a sulfur-containing amino acid that easily coordinates to soft metal ions and grafts to noble metal surfaces. Recently, chiroptical activity of Ag+/cysteine coordination polymers has been widely studied, while, on the other hand, the appearance of a plasmon-enhanced circular dichroic signal (PECD) at the plasmonic spectral region (lambda > 400 nm) has been observed for AgNPs capped with chiral sulfur-containing amino acids. These two events are both potentially exploited for sensing applications. However, the presence of Ag+ ions in AgNP colloidal solution deals with the competition of cysteine grafting at the metal NP surface and/or metal ion coordination. Herein we demonstrate that the chiroptical activity observed by adding cysteine to AgNP colloids prepared by pulsed laser ablation in liquids (PLAL) is mainly related to the formation of CD-active Ag+/cysteine supramolecular polymers. The strict correlation between supramolecular chirality and hierarchical effects, driven by different chemical environments experienced by cysteine when different titration modalities are used, is pivotal to validate cysteine as a fast and reliable probe to characterize the surface oxidation of AgNPs prepared by pulsed laser ablation in liquids by varying the laser wavelengths.