Protein corona of SiO2 nanoparticles with grafted thermoresponsive copolymers: Calorimetric insights on factors affecting entropy vs. enthalpy-driven associations

The mechanism of protein adsorption on various surfaces of tailored functionality is often accessed by isothermal titration calorimetry (ITC), the powerful tool yielding the full set of thermodynamic parameters in one label-free experiment. In this work, the non-ionic polymer brushes with thermo-switchable hydrophilic-hydrophobic balance, based on diethylene glycol methacrylate (DEGMA) and 4-vinyl pyridine (4VP) copolymers, grafted to SiO2 nanoparticles, were titrated with individual blood proteins, their mixture, and diluted human plasma. The concentration of adsorption sites was calculated from the sizes of protein molecules, assuming the formation of a monolayer. For the titrations with the protein mixture and diluted plasma, the concentration of titrant was an estimate, limiting the accuracy of thermodynamic parameters. The particles with grafted polymer brushes showed negative xi-potentials similar to the blood proteins and their interactions occurred against the Coulomb forces. Two cases of exothermic hydrophobic and endothermic polar interactions were distinguished. The strength of adsorption, expressed with the affinity constant Ka, was dependent on brush composition and temperature. The adsorption of proteins from blood plasma was always exothermic and, therefore, the dominating hydrophobic interactions were assumed.