Cysteine Surface Engineering of Green-Synthesized Gold Nanoparticles for Enhanced Antimicrobial and Antifungal Activity

Green synthesis of gold nanoparticles (AuNPs) provides a significantly eco-friendly and low-impact counterpart to conventional chemical methods. In the present study, we synthesized gold nanoparticles using Schinus molle (P-AuNPs) aqueous extract as a reducing and stabilizing agent. The obtained nanoparticles were then stabilized by another biocompatible agent, the chiral amino acids L-cysteine (L-Cys-AuNPs) and D-cysteine (D-Cys-AuNPs), to estimate the potential of the surface modification for enhancing AuNPs surface chemistry and antimicrobial action. The synthesized gold nanoparticles were confirmed by UV-Vis spectroscopy, FTIR, XRD, and circular dichroism to validate their formation, crystalline structure, surface properties, and chirality. Physicochemical characterization confirmed the formation of crystalline AuNPs with size and morphology modulated by chiral functionalization. TEM and DLS analyses showed that L-cysteine-functionalized AuNPs were smaller and more uniform, while FTIR and circular dichroism spectroscopy confirmed surface binding and the induction of optical activity, respectively. L-Cys-AuNPs exhibited the highest antimicrobial efficacy against a broad spectrum of microorganisms, including Escherichia coli, Salmonella enterica, Listeria monocytogenes, Staphylococcus aureus, Staphylococcus epidermidis, and, notably, Candida albicans. L-Cys-AuNPs showed the lowest MIC and MBC values, highlighting the synergistic effect of chirality on biological performance. These findings suggest that L-cysteine surface engineering significantly enhances the therapeutic potential of AuNPs, particularly in combating drug-resistant fungal pathogens such as C. albicans. This research paves the way for the development of next-generation antimicrobial agents, reinforcing the relevance of green nanotechnology in the field of materials science and nanotechnology.