Adsorption Free Energy of Glycine on TiO2 in Water from First Principles Simulations and Electrochemical Impedance Spectroscopy

The adhesion of amino acids and small organic molecules on TiO2 nanoparticles is fundamental for bio-nano functionalization of peptides and proteins. The adsorption free energy is the main physical quantity that regulates the adsorption process. Its evaluation is particularly challenging both experimentally, due to the weak interfacial signal in aqueous environments, and by atomistic simulations, due to the complexity of the physical phenomena occurring at the solid-water interface (polarization and charge transfer effects). We report here an accurate experimental-computational study of hydrated TiO2 nanoparticles interacting with Glycine where we obtain quantitative agreement of the measured adsorption free energy. Ab-initio simulations are performed within the Tight Binding Density Functional Theory in combination with enhanced free energy sampling techniques. The experiments adopt a new and efficient set-up for electrochemical impedance spectroscopy measurements based on screen-printed gold electrodes. The measured adsorption free energy is about -30 kJ/mol (both from experiment and calculation), with preferential interaction of the charged NH3 group which strongly adsorbs on the TiO2 bridging oxygens. The perfect agreement between computation and experiment opens the doors to an extended exploration of the bio-nano interaction for different materials and molecules.