Cysteine Adsorbed on Gold Foils with Varying Surface Chemistries: A ToF-SIMS Study

Cysteine adsorption on nanostructured gold surfaces is essential for applications in biosensing and nanotechnology, yet studying its interaction with gold nanoparticles is limited by their complex and inaccessible surface chemistries. To address this, we investigated cysteine adsorption on gold foil substrates with controlled surface chemistries, namely as-received, solvent-cleaned, and aqua regia-etched, using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and principal component analysis (PCA). This dimensionality reduction technique effectively distinguished the substrates and cysteine-adsorbed samples by exploring the relationships among ion intensities, revealing contamination patterns and adsorption behavior. Cysteine adsorbed on all three gold surfaces, with its thiol group displacing adventitious sulfur via ligand exchange, and formed the most stable monolayer on etched gold, where additional gold-amino interactions were also observed. These findings highlight the utility of ToF-SIMS and PCA in resolving subtle surface interactions and demonstrate the value of the gold foil substrate as a model system for studying adsorption mechanisms that are otherwise difficult to access.