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Mechanisms of fibrinogen adsorption on silica sensors at various pHs: Experiments and theoretical modeling.

Adsorption kinetics of human serum fibrinogen at silica substrates was studied using the optical waveguide lightmode spectroscopy (OWLS) and the quartz crystal microbalance (QCM) techniques. Measurements were performed at pH 3.5, 4 and 7.4 for various ionic strengths. The experimental data were interpreted in terms of a hybrid random sequential adsorption model. This allowed determining the mass transfer rate coefficient for the OWLS cell and maximum coverages at various pHs. The appearance of different, pH-dependent mechanisms of fibrinogen adsorption on silica substrates was confirmed. At pH 3.5 the molecules mostly adsorb in the side-on orientation that produces low maximum coverage of ca 1 mg m-2. At this pH, the kinetics derived from the OWLS measurements agrees with that theoretically predicted using the convective-diffusion theory. In consequence, a comparison of the OWLS and QCM results allows determining the water factor and the dynamic hydration of fibrinogen molecules for the entire range of coverage. At pH 7.4, the OWLS method gives inaccurate kinetic data for low coverage range. However, the maximum coverage that was equal to ca. 4 mg m-2 agrees with the QCM results and with previous literature results. It is postulated that the limited accuracy of the OWLS method for lower coverage stems from a heterogeneous structure of fibrinogen monolayers, which consist of side-on and end-on adsorbed molecules. One can expect that the results acquired in this work allow developing a robust procedure for preparing fibrinogen monolayers of well-controlled coverage and molecule orientation, which can be exploited for efficient immunosensing purposes.