Effect of Nanoparticle Size on Cysteine‐Gold Surface Interactions

Abstract Gold nanoparticles (AuNPs) are considered for biomedical applications, and their size influences their effectivity and stability in the human body. This study investigates the interactions between citrate‐stabilized AuNPs (5, 10, 15, and 20 nm) and L‐Cysteine (Cys). The interactions were probed by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), cyclic voltammetry (CV), dynamic light scattering (DLS), and X‐ray absorption spectroscopy (XAS). Hydrogenated gold cysteine thiolate molecular ions, gold‐sulfur ions, and Au 3 +/− , as gold atom representatives, were all detected for the different sizes. Smaller intensity ratios of the gold‐cysteine‐related peaks versus the gold reference peaks were observed with increasing AuNP size. CV confirmed stronger interactions of smaller AuNPs with Cys. AuNPs bond strongest to the thiol group, followed by the amino group, while no gold‐carboxyl interactions were probed. The nonspecific properties of the smallest‐sized (5 nm) AuNPs stabilized (less aggregation) by the presence of Cys based on XAS, but all nanoparticle sizes showed more agglomeration in aqueous solution in the presence of Cys based on DLS. The data confirmed that the strength of the binding between AuNPs and Cys is size‐dependent, possibly caused by curvature, surface energy, and/or diffusion processes.