125I‑Radiolabeling, Surface Plasmon Resonance, and Quartz Crystal Microbalance with Dissipation: Three Tools to Compare Protein Adsorption on Surfaces of Different Wettability

The extent of protein adsorption is an important consideration in the biocompatibility of biomaterials. Various experimental methods can be used to determine the quantity of protein adsorbed, but the results usually differ. In the present work, self-assembled monolayers (SAMs) were used to prepare a series of model gold surfaces varying systematically in water wettability, from hydrophilic to hydrophobic. Three commonly used methods, namely, surface plasmon resonance (SPR), quartz crystal microbalance with dissipation (QCM-D), and (125)I-radiolabeling, were employed to quantify fibrinogen (Fg) adsorption on these surfaces. This approach allows a direct comparison of the mass of Fg adsorbed using these three techniques. The results from all three methods showed that protein adsorption increases with increasing surface hydrophobicity. The increase in the mass of Fg adsorbed with increasing surface hydrophobicity in the SPR data was parallel to that from (125)I-radiolabeling, but the absolute values were different and there does not seem to be a "universally congruent" relationship between the two methods for surfaces with varying wettability. For QCM-D, the variation in protein adsorption with wettability was different from that for SPR and radiolabeling. On the more hydrophobic surfaces, QCM-D gave an adsorbed mass much higher than from the two other methods, possibly because QCM-D measures both the adsorbed Fg and its associated water. However, on the more hydrophilic surfaces, the adsorbed mass from QCM-D was slightly greater than that from SPR, and both were smaller than from (125)I-radiolabeling; this was true no matter whether the Sauerbrey equation or the Voigt model was used to convert QCM-D data to adsorbed mass.