Adhesion force evolution of protein on the surfaces with varied hydration extent: Quantitative determination via atomic force microscopy

The adhesion force evolution of protein on surfaces with continuously varied hydrophobicity/hydration layer has not been completely clarified yet, limiting the further development of environmental applications such as membrane anti-biofouling and selective adsorption of the functional surfaces. Herein, chemical force spectroscopy using atomic force microscopy (AFM) was utilized to quantify the evolution of the adhesion forces of protein on hydration surfaces in water, where bovine serum albumin (BSA) was immobilized on an AFM tip as the representative protein. The stiffness, roughness and charge properties of the substrate surfaces were kept constant and the hydrophobicity was the only variant to monitor the role of hydrated water layers in protein adhesion. The adhesion force increased non-monotonically as a function of hydrophobicity of substrate surfaces, which was related to the concentration of humic acid, and independent of pH values and ionic strength. The non-monotonic variation occurred in the range of contact angle at 60-80° due to the mutual restriction between solid-liquid interface energy and solid-solid interface energy. Hydrophobic attraction was the dominant force that drove adhesion of BSA to these model substrate surfaces, but the passivation of hydration layers at the interface could weaken the hydrophobic attraction. In contrast to the measurements in water, the adhesion forces decreased as a function of surface hydrophobicity when measured in air, because capillary forces from condensation water dominated adhesion forces. The passivation of hydration layers of protein was revealed by quantitatively determining the evolution of adhesion forces on the hydration surfaces of varying hydrophobicity, which was ignored by traditional adhesion theory.