Methionine-Controlled Impediment of Secondary Nucleation Leading to Nonclassical Growth within Self-Assembled Gold Nanoparticles.

The conventional key steps for seed-mediated growth of noble metal nanostructures involve classical and nonclassical nucleation. Furthermore, the surface of the seed catalytically enhances the secondary nucleation involving Au to Au reduction, thus providing in-plane growth of the seed. In contrast to this well-established growth mechanism, herein, we report the unique case of a methionine (Met)-controlled seed-mediated growth reaction, which rather proceeds impeding secondary nucleation in the presence of citrate-stabilized gold nanoparticles (AuNPs). The interaction between the freshly generated Au and the thioether group of Met in the medium restricts the secondary nucleation process of further seed-catalyzed Au reduction to Au. This incomplete conversion of Au, as confirmed by X-ray photoelectron spectroscopy, results in a significant enhancement of the zeta (ζ) potential even at low Met concentrations. Nucleation of generated small-sized particles (nAuNPs) takes place on the parent seed surface followed by their segregation from the seed. The self-assembly process of these nAuNPs arises from the aurophilic interaction among the Au. Furthermore, the time-dependent growth of smaller particles to larger-sized particles through assembly and merging within the same self-assembly validates the nonclassical growth. This strategy has been successfully extended toward the seed-mediated growth reaction of AuNPs in the presence of three bio-inspired decameric peptides having varying numbers of Met residues. The study confirms the nucleation strategy even in the presence of a single Met residue in the peptide and also the self-assembly of nucleated particles with increasing Met residues within the peptide.