This work compares the spreading and relaxation rates of albumin and fibrinogen, inferred from singlecomponent and competitive adsorption kinetic experiments, on model surfaces of varying hydrophobicity. Kinetics from the single-component studies revealed a constant spreading rate, where the adsorbed protein footprint grew linearly in time for at least 15 min. This spreading rate increased with substrate hydrophobicity (ranging from 0.02 to 0.16 nm 2 /molecule/s for albumin and from 0.04 to 0.26 nm 2 /molecule/s for fibrinogen), resulting in a larger extent of footprint growth and a lower ultimate coverage on hydrophobic surfaces when compared with hydrophilic surfaces at the same adsorption conditions. Competitive adsorption studies were in qualitative agreement with the single-component experiments but were able to probe longer spreading time scales. Although spreading appeared to occur initially at a constant rate in the competitive experiments, after 2 h the spreading rate had slowed dramatically and the spreading process had begun to level off.

We report the kinetic behavior of albumin and fibrinogen adsorption and relaxation from gentle shearing flow and phosphate buffer onto C16 self-assembled monolayers. The adsorption kinetics were generally transport-limited; however, the ultimate coverages depended on the rates at which protein molecules arrived at the surface, suggesting that interfacial relaxations determined the ultimate coverage. Of particular note was a dependence of the ultimate coverage of both proteins on the wall shear rate, in addition to the influence of the bulk solution concentration. Analysis of single protein experiments revealed interfacial protein relaxation rates of 0.12 and 0.15 nm2 molecule-1 s-1 for albumin and fibrinogen, respectively. These rates were constant over the range of experimental conditions and represent the initial relaxation rates after protein adhesion to the surface. The initial protein footprints were consistent with the free solution protein dimensions and, in the case of albumin, grew over a factor of 5 as the protein relaxed. For fibrinogen, relaxations were less extensive, increasing the footprint by a factor of 3. The extents of relaxation and the sizes of the protein footprints during the linear regime of spreading suggest that interfacial denaturing contributes significantly to the relaxation process, in addition to simple reorientations. The albumin relaxation behavior was shown, in addition to its influence on albumin coverage, to affect the coverage of fibrinogen in competitive situations. When the C16 layer was passivated with albumin prior to fibrinogen adsorption, short albumin exposures (still sufficient to cover the C16 surface) were ineffective at preventing fibrinogen adsorption. Prolonged incubation of albumin layers in albumin solution or buffer dramatically reduced subsequent fibrinogen adhesion.

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Using total internal reflectance fluorescence (TIRF), we observe that lysozyme adsorption onto hydrophobic surfaces can exhibit kinetic overshoots at some conditions, while, at lower free solution concentrations or flow rates over the surface, the coverage monotonically approaches its final value. This behavior is explained by an interfacial relaxation from an end-on to a side-on orientation, which occurs by rollover and not by the displacement of end-on adsorbed proteins by side-on adsorbing proteins. Rollover and displacement models are compared with data to prove this point. Ultimately, we quantitatively predict the kinetic traces for a variety of different adsorption histories (free solution concentration, flow rate, interruption of adsorption by flowing solvent) using a rollover model with reversible transport-limited adsorption of end-on oriented lysozyme and a single rollover rate constant.

Using AFM (atomic force microscopy) to probe protein conformation and arrangement, and TIRF (total internal reflectance fluorescence) to monitor kinetics, fibrinogen adsorption on three different silica-based surfaces was studied: the native oxide on silicon, acid-etched microscope slides, and acid-etched polished glass. The three are chemically similar, but the microscope slide is rougher and induces AFM tip instabilities that appear as high spots on the bare surface. Fibrinogen's conformation and transport-limited adsorption kinetics are found to be quantitatively similar on all three surfaces. Further, the number of adsorbed proteins in progressive AFM micrographs quantitatively match the coverages measured by TIRF during early adsorption. Surfaces appear full, via AFM, when adsorbed amounts are about an order of magnitude below their true saturation levels (via TIRF) because, above about 0.26 mg/m(2), individual proteins cannot be discerned. The results demonstrate how the appearance of AFM micrographs can be misleading regarding surface saturation. On all three surfaces, fibrinogen is, at most, slightly aggregated, showing limited, if any, surface mobility. The complexities of the microscope slide's surface landscape minimally impact adsorption.

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This article demonstrates how the adhesion rates of micrometer-scale particles on a planar surface can be manipulated by nanometer-scale features on the latter. Here, approximately 500-nm-diameter spherical silica particles carrying a substantial and relatively uniform negative charge experienced competing attractions and repulsions as they approached and attempted to adhere to a negative planar silica surface carrying flat 11-nm-diameter patches of concentrated positive charge. The average spacing of these patches profoundly influenced the particle adhesion. For dense positive patch spacing on the planar collector, the particle adhesion was rapid, and the fundamental adhesion kinetics were masked by particle transport to the interface. For patch densities corresponding to a planar surface with net zero charge, particle adhesion was still rapid. Adhesion kinetics were observably reduced for patch spacings exceeding 20 nm and become slower with increased patch spacing. Ultimately, above a critical or threshold average patch spacing of 32 nm, no particle adhesion occurred. The presence of the threshold average patch spacing suggests that more than one positive surface patch was needed for particle capture under the particular conditions of this study. Furthermore, at the threshold, the length scales of the patch spacing and of the interactive surface area between the particle and the surface become similar: The concept of adhesion dominated by the matching of length scales is reminiscent of pattern recognition, even though the patch distribution on the collector is random in this work. Indeed, fluctuations play a critical role in these adhesion dynamics, hence the current behavior cannot be predicted by a mean field approach.