Protein adsorption mechanisms at rough surfaces: Serum albumin at a gold substrate

Adsorption kinetics of human serum albumin (HSA) at a gold substrate was studied using the quartz microbalance (QCM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Measurements were performed at pH 3.5 for various bulk suspension concentrations and ionic strengths. The QCM experimental data were compared with the dry coverage of HSA derived from AFM and from the solution of the mass transfer equation acquired using a hybrid random sequential adsorption model. In this way, the water factor and the dynamic hydration function for HSA monolayers were quantitatively evaluated as a function of dry coverage for various ionic strengths. A comparison of these results with previously known for a silica sensor confirmed that the QCM kinetic measurements are sensitive to the roughness of the sensor characterized in terms of the rms parameter. For the more rough gold sensor (rms = 2.5 nm, average surface feature size 6 nm) the QCM mass transfer rate constant was 2.6 times lower than for the silica sensor characterized by rms = 0.86 nm. This gives for the gold sensor the apparent water factor equal to one and zero hydration function. Moreover, the hydration function increased for larger HSA coverage and was dependent on ionic strength in contrast to the silica substrate. This unexpected behavior was interpreted in terms of the buoyancy effect where the HSA molecules adsorbing in cavities existing at rough surfaces replace the stagnant (hydrodynamically bound) water. Hence, these results confirm that the sensor roughness of the size comparable with protein molecule dimensions exerts a decisive influence on their adsorption kinetic derived from QCM measurements.