Understanding protein adsorption kinetics to surfaces is of importance for various environmental and biomedical applications. Adsorption of bovine serum albumin to various self-assembled monolayer surfaces including neutral and charged hydrophilic and hydrophobic surfaces was investigated using in-situ combinatorial quartz crystal microbalance with dissipation and spectroscopic ellipsometry. Adsorption of bovine serum albumin varied as a function of surface properties, bovine serum albumin concentration and pH value. Charged surfaces exhibited a greater quantity of bovine serum albumin adsorption, a larger bovine serum albumin layer thickness, and increased density of bovine serum albumin protein compared to neutral surfaces at neutral pH value. The quantity of adsorbed bovine serum albumin protein increased with increasing bovine serum albumin concentration. After equilibrium sorption was reached at pH 7.0, desorption of bovine serum albumin occurred when pH was lowered to 2.0, which is below the isoelectric point of bovine serum albumin. Our data provide further evidence that combinatorial quartz crystal microbalance with dissipation and spectroscopic ellipsometry is a sensitive analytical tool to evaluate attachment and detachment of adsorbed proteins in systems with environmental implications.

With a coupled spectroscopic ellipsometry-quartz crystal microbalance with dissipation (QCM-D) experimental setup, quantitative information can be obtained about the amount of buffer components (water molecules and ions) coupled to a poly(acrylic acid) (PAA) brush surface in swelling and protein adsorption processes. PAA Guiselin brushes with more than one anchoring point per single polymer chain were prepared. For the swollen brushes a high amount of buffer was found to be coupled to the brush-solution interface in addition to the content of buffer inside the brush layer. Upon adsorption of bovine serum albumin the further incorporation of buffer molecules into the protein-brush layer was monitored at overall electrostatic attractive conditions [below the protein isolectric poimt (IEP)] and electrostatic repulsive conditions (above the protein IEP), and the shear viscosity of the combined polymer-protein layer was evaluated from QCM-D data. For adsorption at the "wrong side" of the IEP an incorporation of excess buffer molecules was observed, indicating an adjustment of charges in the combined polymer-protein layer. Desorption of protein at pH 7.6 led to a very high stretching of the polymer-protein layer with additional incorporation of high amounts of buffer, reflecting the increase of negative charges on the protein molecules at this elevated pH.

Analysis techniques are needed to determine the quantity and structure of materials composing an organic layer that is below an ultra-thin film limit and in a liquid environment. Neither optical nor acoustical techniques can independently distinguish between thickness and porosity of ultra-thin films due to parameter correlation. A combined optical and acoustical approach yields sufficient information to determine both thickness and porosity. We describe application of the combinatorial approach to measure single or multiple organic layers when the total layer thickness is small compared to the wavelength of the probing light. The instrumental setup allows for simultaneous in situ spectroscopic ellipsometry and quartz crystal microbalance dynamic measurements, and it is combined with a multiple-inlet fluid control system for different liquid solutions to be introduced during experiments. A virtual separation approach is implemented into our analysis scheme, differentiated by whether or not the organic adsorbate and liquid ambient densities are equal. The analysis scheme requires that the film be assumed transparent and rigid (non-viscoelastic). We present and discuss applications of our approach to studies of organic surfactant adsorption, self-assembled monolayer chemisorption, and multiple-layer target DNA sensor preparation and performance testing.

Generalized ellipsometry in-situ quantification of organic adsorbate attachment within slanted columnar thin films" (2012 Abstract:We apply generalized ellipsometry, well-known to be sensitive to the optical properties of anisotropic materials, to determine the amount of fibronectin protein that adsorbs onto a Ti slanted columnar thin film from solution. We find that the anisotropic optical properties of the thin film change upon organic adsorption. An optical model for ellipsometry data analysis incorporates an anisotropic Bruggeman effective medium approximation. We find that differences in experimental data from before and after fibronectin adsorption can be solely attributable to the uptake of fibronectin within the slanted columnar thin film. Simultaneous, in-situ generalized ellipsometry and quartz crystal microbalance measurements show excellent agreement on the amount and rate of fibronectin adsorption. Quantitative characterization of organic materials within three-dimensional, optically anisotropic slanted columnar thin films could permit their use in optical sensor applications.

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