pH-dependent adsorption of α-amino acids, lysine, glutamic acid, serine and glycine, on TiO nanoparticle surfaces.

TiO nanoparticles (NPs) are widely used in different applications, and potential exposure to these NPs raises concerns about their impact on human health. In contact with biological fluids, proteins adsorb onto NPs to create a protein corona. Protein adsorption is highly dependent on the affinity between exterior amino acid residues and the NP surface. Thus, studying amino acids adsorption onto NPs can provide insight into protein corona formation. Herein, the pH-dependent adsorption of α-amino acids onto TiO NPs in buffered solutions is described. Methods include attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy to analyze molecular interactions and dynamic light scattering (DLS) to measure changes in size and zeta-potential upon adsorption. Depending on the predominant speciation and TiO NP surface charge, adsorption involves a combination of carboxylate and amine group interactions. Gly and Lys reveal a similar trend of higher adsorption with increasing pH. In contrast, Glu adsorption decreases with increasing pH. Ser adsorption onto TiO NPs surfaces is the highest around pH. These differences are attributed to the different speciation of the functional groups within the amino acids and the TiO surface charge at each pH. Under our experimental conditions, multiple surface species coexist at different pH values. Protonated surface species are present for all amino acids at pH 2. At pH 9, Lys and Glu adsorbate spectra have new peaks at 1740 cm and 1744 cm, respectively. This is a possible result of surface-induced deprotonation of the amine group and proton transfer to the carboxylate. Analyzing the pH-dependent adsorption of amino acids can provide a better understanding of biomolecule-surface interactions in in vivo and different biological milieu.