Amino Acid Adsorption on Stainless Steel and Titanium Surfaces under Physiological Conditions: A Spectroscopic Analysis

• Adsorption of amino acids on stainless steel and titanium was studied using SERS and SEIRA. • Cysteine forms S-S bonds upon adsorption. • Leucine shows dominant hydrophobic interactions, with higher affinity for titanium. • Threonine adsorption involves hydroxyl (-OH) interactions, stronger on titanium. • Tryptophan adsorption is influenced by π-electron interactions of the indole ring. The adsorption behavior of biomolecules on metallic surfaces is critical for applications in biomaterials, biosensing, and corrosion inhibition. This study investigates the adsorption mechanisms of cysteine (Cys), leucine (Leu), threonine (Thr), and tryptophan (Trp) on stainless steel (SS) and titanium (Ti) surfaces using Raman, Fourier-transform infrared (FT-IR), surface-enhanced Raman spectroscopy (SERS), and surface-enhanced infrared absorption spectroscopy (SEIRA). The results indicate distinct adsorption behaviors for each amino acid, influenced by their functional groups and the physicochemical properties of the metal surfaces. Cys undergoes partial dimerization upon adsorption, forming cystine, with stronger interactions observed on Ti. Leu exhibits predominantly hydrophobic interactions through its alkyl side chain, showing a greater affinity for Ti. Thr adsorption is primarily influenced by hydroxyl (-OH) interactions, which are more pronounced on Ti due to hydrogen bonding with the TiO 2 layer. Trp interacts with both metal surfaces through π-electron interactions of the indole ring, adopting a tilted orientation, while NH 3 + and COO⁻ groups contribute to surface binding. The adsorption processes on both surfaces are generally similar; however, analysis of the relative intensities of characteristic bands in the SERS and SEIRA spectra of the adsorbed amino acids indicates stronger molecular adsorption on the Ti surface compared to SS. This enhanced adsorption is attributed to the formation of a stable TiO 2 layer on the Ti surface, which promotes stronger electrostatic interactions and hydrogen bonding. These findings provide insights into amino acid–metal surface interactions and contribute to a more comprehensive understanding of biomaterial surface modification and protein adsorption mechanisms in physiological environments.