Surface Modification of Titanium with a Gold Nanoparticle Monolayer: AFM-IR Insights into Tryptophan and Cysteine Adsorption

Titanium (Ti) is widely used in biomedical implants due to its high strength, corrosion resistance, and excellent biocompatibility. However, surface functionalization is essential to enhancing its interaction with biological environments. In this study, monolayers of citrate-stabilized, negatively charged gold nanoparticles (TCAuNPs(−)) were fabricated on Ti substrates precoated with poly(allylamine hydrochloride) (PAH) to facilitate electrostatically driven adsorption. The adsorption behavior of two amino acids, cysteine (Cys) and tryptophan (Trp) on TCAuNPs(−) deposited on the modified Ti surface, was investigated using atomic force microscopy combined with surface-enhanced infrared absorption (AFM-SEIRA) spectroscopy, enabling nanoscale chemical analysis of biomolecular interactions on the modified surface. The AFM-SEIRA spectra revealed characteristic bands associated with COO– and NH3+ groups for both amino acids. In addition, Trp exhibited prominent signals from the aromatic indole ring, suggesting that these molecular fragments were adsorbed onto the TCAuNPs(−)/Ti surface. AFM-SEIRA results indicate that the strongest amino acid signals were localized at the nanoparticle apexes. The results provide insight into the early stage molecular interactions at the bionano interface and offer a framework for designing new functionalized implant surfaces.