QCM-D publications review
Monday, December 08, 2008
Here is a round up of some of the key publications related to QCM-D over the past six months. In this round up we see a focus on research into protein adsorption, illustrating QCM-D’s value as a research tool.
Bovine Serum Albumin Adsorption on Nano-rough Platinum Surfaces Studied by QCM-D
A Dolatshahi-Pirouza, K Rechendorffa, M.B. Hovgaarda, M Foss, J Chevalliera and F Besenbacher.
Journal: Colloids and Surfaces B: Biointerfaces, Volume 66, Issue 1, 1 October 2008, Pages 53-59.
QCM-D was used to follow the adsorption of bovine serum albumin (BSA) on platinum surfaces with a root-mean-square roughness ranging from 1.49 nm to 4.62 nm. Two different BSA concentrations, 50 μg/ml and 1 mg/ml, were used, and the adsorption studies were complemented by monitoring the antibody interaction with the adsorbed BSA layer.
The researchers found that surface nano-roughness significantly influenced the adsorption process with the surface mass density of the adsorbed BSA layer being enhanced in a non-trivial way with the surface roughness. By a close study of the energy dissipation vs. frequency shift plot it was additionally observed that the BSA adsorption on the roughest surface is subject to several distinct adsorption phases. This indicates structural changes facilitated by the nano-rough surface morphology during the adsorption process, which was particularly noticeable at low (50 μg/ml) BSA concentration. The results, say the researchers, confirm that the nano-rough surface morphology has a significant influence on both the BSA mass uptake and the functionality of the resulting protein layer.
pH-Dependent Immobilization of Proteins on Surfaces Functionalized by Plasma-Enhanced Chemical Vapor Deposition of Poly(acrylic acid)-and Poly(ethylene oxide)-like Films
S Belegrinou, I Mannelli, P Lisboa, F Bretagnol, A Valsesia, G Ceccone, P Colpo, H Rauscher and F Rossi.
Journal: Langmuir, 24 (14), 7251–7261, 2008
Another article that has its focus on protein adsorption where QCM-D is helping with the study of the interaction of the proteins bovine serum albumin (BSA), lysozyme (Lys), lactoferrin (Lf), and fibronectin (Fn) with surfaces of protein-resistant poly(ethylene oxide) (PEO) and protein-adsorbing poly(acrylic acid) (PAA) fabricated by plasma-enhanced chemical vapor deposition.
The aim of the research was to develop an adsorption scenario for each of these proteins, which takes into account electrostatic protein-surface and protein-protein interaction, as well as the pH-dependent properties of the proteins, such as shape and exposure of specific domains.
The researchers focussed on several parameters which are crucial for protein adsorption, i.e., the isoelectric point (pI) of the proteins, the pH of the solution, and the charge density of the sorbent surfaces, with the ξ-potential as a measure for the latter.
The measurements showed adsorption stages characterized by different segments in the plots of the dissipation vs frequency change. It was found that PEO remains protein-repellent for BSA, Lys, and Lf at pH 4-8.5, while weak adsorption of Fn was observed. On PAA, different stages of protein adsorption processes could be distinguished under most experimental conditions. BSA, Lys, Lf, and Fn generally exhibit a rapid initial adsorption phase on PAA, often followed by slower processes. The evaluation of the adsorption kinetics also reveals different adsorption stages, whereas the number of these stages does not always correspond to the structurally different phases as revealed by the D-f plots.
Fibronectin Adsorption on Tantalum: The Influence on Nano-roughness.
M Hovgaard, K Rechendorff, J Chevallier, M Foss and F Besenbacher.
Journal: J. Phys. Chem. B, 112 (28), 8241–8249, 2008
In a different look at nano-roughness, the interest lies in the complex mechanisms of protein adsorption at the solid−liquid interface. This is because of its importance to protein purification, biocompatibility of medical implants, biosensing, and biofouling, for example.
The protein adsorption process depends crucially on both the nanoscale chemistry and topography of the interface. So the researchers looked at the adsorption of the cell-binding protein fibronectin on flat and nanometer scale rough tantalum oxide surfaces using ellipsometry and QCM-D.
On the flat tantalum oxide surfaces, the researchers noted interfacial protein spreading causes an increase in the rigidity and a decrease in the thickness of the adsorbed fibronectin layer with decreasing bulk protein concentration. For the tantalum oxide surfaces with well-controlled, stochastic nanometer scale roughness, similar concentration effects are observed for the rigidity of the fibronectin layer and saturated fibronectin uptake.
The researchers say that the nano-rough tantalum oxide surfaces promote additional protein conformational changes, an effect especially apparent from the QCM-D signals, interpreted as an additional stiffening of the formed fibronectin layers.
Quartz Crystal Microbalance Study of Protein Adsorption Kinetics on Poly(2-Hydroxyethyl Methacrylate)
JH Teichroeb, JA Forrest, LW Jones, J Chan and K Dalton.
Journal: J Colloid Interface Sci. 2008 Sep 1;325(1):157-64
The world of medical implants looms large in this study. Implants may lead to biomacromolecules coming into contact with artificial biomaterials with potentially serious consequences. Hoping to find out more, these researchers looked at the interaction of one of the most widely used biomaterials, polyHEMA, with lysozyme, bovine serum albumin (BSA), and lactoferrin using QCM-D. The concentration dependence of adsorption was measured for the proteins individually as well as for lysozyme-BSA, and lysozyme-lactoferrin combinations.
Applying an extension of Voinovas viscoelastic model to n layers the researchers created thickness-time graphs for adsorption. For each of lactoferrin and lysozyme, two distinctly different timescales of adsorption could be differentiated, apparently having different mechanisms of adsorption. Negative dissipation shifts were measured for low concentrations of lysozyme, trending to positive dissipation at higher concentrations. This suggested that lysozyme was adsorbed initially into the matrix, stiffening the hydrogel, and later onto the surface of polyHEMA. Additionally, trials with commercial no-rub cleaning solutions indicated little added effectiveness over buffer solutions. Mixtures of proteins showed behaviour which differed in some cases from the simple combination of single protein adsorption experiments.
Dispersant Adsorption and Viscoelasticity of Alumina Suspensions Measured by Quartz Crystal Microbalance with Dissipation Monitoring and In Situ Dynamic Rheology.
L Palmqvist and K Holmberg.
Journal: Langmuir. 2008 Sep 16;24(18):9989-96
This study is an example of research with an industrial angle. Related to ceramics, the researchers looked at the adsorption behavior and water content of adsorbed layers of four dispersants for aqueous ceramic processing on alumina surfaces. The dispersants were a poly(acrylic acid), a lignosulfonate, and two hydrophilic comb copolymers with nonionic polyoxyethylene chains of different molecular weights.
As rheology is important in this application a Voigt model was applied to analyze the viscoelastic behavior of the adsorbed dispersant layers. QCM-D results were compared with viscoelastic properties determined by in situ dynamic rheology measurements of highly concentrated alumina suspensions during slip casting.
The QCM-D results indicated the performance of the dispersants. It showed that both the poly(acrylic acid) and the lignosulfonate adsorbed in low amounts and in a flat conformation, which generated thin, highly rigid layers less than 1 nm thick. The water content of these layers was found to be around 30% for the lignosulfonate and 35% for the poly(acrylic acid). High casting rate and strength in terms of storage modulus were observed in the final consolidate of the suspensions with the two polyelectrolytes.
Very different behavior was observed for the copolymers. The high molecular weight comb copolymer adsorbed in a less elastic layer with a thickness of about 6 nm, which is enough to provide steric stabilization. The viscous behavior of this layer was attributed to high water content, which was calculated to be around 90%. Such a water-rich layer gives a lubrication effect, which allows for reorientation of particles during the consolidation process, resulting in a high final strength of the ceramic material. During consolidation, the suspension showed a slow casting rate, most likely due to rearrangement facilitated by the lubricating layer. The short-chain comb copolymer adsorbed in a 1.5 nm thick, rigid layer and gave low final strength to the consolidated suspension. In this case, the researchers say that it is likely that the poor consolidation behavior is caused by flocculation due to insufficient stabilization of the dispersion.
