Protein adsorption on and swelling of polyelectrolyte brushes: A simultaneous ellipsometry-quartz crystal microbalance study

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

Combined QCM-D/GE as a tool to characterize stimuli-responsive swelling of and protein adsorption on polymer brushes grafted onto 3D-nanostructures

A combined setup of quartz crystal microbalance and generalized ellipsometry can be used to comprehensively investigate complex functional coatings comprising stimuli-responsive polymer brushes and 3D nanostructures in a dynamic, noninvasive in situ measurement. While the quartz crystal microbalance detects the overall change in areal mass, for instance, during a swelling or adsorption process, the generalized ellipsometry data can be evaluated in terms of a layered model to distinguish between processes occurring within the intercolumnar space or on top of the anisotropic nanocolumns. Silicon films with anisotropic nanocolumnar morphology were prepared by the glancing angle deposition technique and further functionalized by grafting of poly-(acrylic acid) or poly-(N-isopropylacrylamide) chains. Investigations of the thermoresponsive swelling of the poly-(N-isopropylacrylamide) brush on the Si nanocolumns proved the successful preparation of a stimuli-responsive coating. Furthermore, the potential of these novel coatings in the field of biotechnology was explored by investigation of the adsorption of the model protein bovine serum albumin. Adsorption, retention, and desorption triggered by a change in the pH value is observed using poly-(acrylic acid) functionalized nanostructures, although generalized ellipsometry data revealed that this process occurs only on top of the nanostructures. Poly-(N-isopropylacrylamide) is found to render the nanostructures non-fouling properties. Includes supplemental materials

Combined QCM-D/GE as a tool to characterize stimuli-responsive swelling of and protein adsorption on polymer brushes grafted onto 3D-nanostructures

A combined setup of quartz crystal microbalance and generalized ellipsometry can be used to comprehensively investigate complex functional coatings comprising stimuli-responsive polymer brushes and 3D nanostructures in a dynamic, noninvasive in situ measurement. While the quartz crystal microbalance detects the overall change in areal mass, for instance, during a swelling or adsorption process, the generalized ellipsometry data can be evaluated in terms of a layered model to distinguish between processes occurring within the intercolumnar space or on top of the anisotropic nanocolumns. Silicon films with anisotropic nanocolumnar morphology were prepared by the glancing angle deposition technique and further functionalized by grafting of poly-(acrylic acid) or poly-(N-isopropylacrylamide) chains. Investigations of the thermoresponsive swelling of the poly-(N-isopropylacrylamide) brush on the Si nanocolumns proved the successful preparation of a stimuli-responsive coating. Furthermore, the potential of these novel coatings in the field of biotechnology was explored by investigation of the adsorption of the model protein bovine serum albumin. Adsorption, retention, and desorption triggered by a change in the pH value is observed using poly-(acrylic acid) functionalized nanostructures, although generalized ellipsometry data revealed that this process occurs only on top of the nanostructures. Poly-(N-isopropylacrylamide) is found to render the nanostructures non-fouling properties. Includes supplemental materials

Real-time characterization of ultra-thin organic layers via simultaneous spectroscopic ellipsometry and piezoelectric nanogravimetry

Analysis techniques are needed to determine the quantity and structure of materials composing an organic layer that is below an optical 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. The author describes 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 an 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). The author presents and discusses applications of the approach to studies of organic surfactant adsorption, self-assembled monolayer chemisorption, and multiple-layer target DNA sensor preparation and performance testing. Advisor: Mathias Schuber

Quantification of adsorption on three-dimensional spatially coherent thin films at the solid-liquid interface

Sculptured thin films comprising highly spatially coherent three-dimensional nanostructures exhibit strongly anisotropic properties. The anisotropic response of the nanostructures is modulated by the uptake of adsorbate due to screening of anisotropic polarization charges. This phenomenon, termed here surface-enhanced anisotropy modulation, is a new detection principle that allows for quantification of adsorbate by generalized ellipsometry. Additionally, the nanostructures have a higher surface area than respective flat surfaces and allow for additional attachment. Applications for surface-enhanced anisotropy are numerous and include contraband detection, diagnostics, in-line monitoring of surface modification, and evaluation of the conformational state of organic materials embedded within porous nanostructure thin films. Ellipsometry and quartz crystal microbalance with dissipation are characterization techniques that have been used to benchmark the attachment of adsorbate onto isotropic flat and rough surfaces. The data analysis approaches of these techniques must consider the anisotropic properties and significant surface roughness of nanostructure thin films. An anisotropic Bruggeman effective medium approximation is applied for optical modeling, and a frequency overtone analysis is developed to consider the mechanical effects of liquid on oscillating nanostructures. In this work, data analysis approaches for generalized ellipsometry and quartz crystal microbalance with dissipation are introduced and developed for anisotropic surfaces at the solid-liquid interface. Water and its deuterated analogue are exposed over a slanted columnar thin film to determine the quantity of liquid that rigidly couples with nanostructures during quartz crystal microbalance measurement. The surface-enhanced anisotropy detection principle is demonstrated by monitoring the adsorption of fibronectin protein onto Ti slanted nanocolumns. The formation of a nanohybrid functional material is described, whereby decanethiol chemisorbs onto and functionalizes Ti slanted nanocolumns coated with Pt by atomic layer deposition

Investigation of Bovine Serum Albumin (BSA) Attachment onto Self-Assembled Monolayers (SAMs) Using Combinatorial Quartz Crystal Microbalance with Dissipation (QCM-D) and Spectroscopic Ellipsometry (SE)

Understanding protein adsorption kinetics to surfaces is of importance for various environmental and biomedical applications. Adsorption of bovine serum albumin to various selfassembled 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

Generalized ellipsometry in-situ quantification of organic adsorbate attachment within slanted columnar thin films

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

BSA arrangement scenarios on various SAM surfaces.

<p>Left: Scenario 1 –Side-on; Right: Scenario 2: End-on.</p