Histidine Adsorption on TiO₂ Nanoparticles: An Integrated Spectroscopic, Thermodynamic, and Molecular-Based Approach toward Understanding Nano–Bio Interactions

Abstract

Nanoparticles in biological media form dynamic entities as a result of competitive adsorption of proteins on nanoparticle surfaces called protein coronas. The protein affinity toward nanoparticle surfaces potentially depends on the constituent amino acid side chains which are on the protein exterior and thus exposed to the solution and available for interaction. Therefore, studying the adsorption of individual amino acids on nanoparticle surfaces can provide valuable insights into the overall evolution of nanoparticles in solution and the protein corona that forms. In the current study, the surface adsorption of l-histidine on TiO₂ nanoparticles with a diameter of 5 nm at pH 7.4 (physiological pH) is studied from both macroscopic and molecular perspectives. Quantitative adsorption measurements of l-histidine on 5 nm TiO₂ particles yield maximum adsorption coverage of 6.2 ± 0.3 × 10¹³ molecules cm^–2 at 293 K and pH 7.4. These quantitative adsorption measurements also yield values for the equilibrium constant and free energy of adsorption of <i>K</i> = 4.3 ± 0.5 × 10² L mol^–1 and Δ<i>G</i> = −14.8 ± 0.3 kJ mol^–1, respectively. Detailed analysis of the adsorption between histidine and 5 nm TiO₂ nanoparticle surfaces with attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy indicates both the imidazole side chain and the amine group interacting with the nanoparticle surface and the adsorption to be reversible. The adsorption results in no change in surface charge and therefore does not change nanoparticle–nanoparticle interactions and thus aggregation behavior of these 5 nm TiO₂ nanoparticles in aqueous solution