Diverse use of QCM-D demonstrated in latest research
Friday, November 07, 2008
A number of recent studies highlight the diverse application of QCM-D in research. At the school of Botany at the University of Melbourne in Australia biofouling is the topic of interest. Their published work, The Quartz Crystal Microbalance: A New Tool for the Investigation of the Bioadhesion of Diatoms to Surfaces of Different Surface Energies, (Langmuir. 2008 Jun 1; 24(13): 6730-7) uses QCM-D to examine differences in the viscoelastic properties of the extracellular adhesives produced by the marine diatoms Amphora coffeaeformis Cleve and Craspedostauros australis Cox interacting with surfaces of differing wettability; 11-mercaptoundecanoic acid (MUA) that is hydrophilic and 1-undecanethiol (UDT) that is hydrophobic.
Marine diatoms are known to attach more strongly to hydrophobic surfaces, than hydrophilic ones but the compositions of the adhesives they make are little understood. As the researchers point out, “Even less is known about the underlying processes that characterize the interaction between the adhesive and a given surface, including those of differing wettability.”
QCM-D results revealed that while the overall delta f/delta D ratios were slightly different, the trends were the same for both diatom species, with the layer secreted upon UDT to be more viscoelastic and far more consistent over several experiments, compared to that on MUA which was less viscoelastic and demonstrated far more variability between experiments.
The researchers observed that adhesives secreted by fouling diatoms differ significantly in their interaction with surfaces depending on their wettability, as well as illustrating the unique mechanics behind the adhesion of A. coffeaeformis upon hydrophobic surfaces, that may contribute significantly to the cells success in colonizing hydrophobic surfaces. This is certainly a major step forward in understanding fouling mechanisms while demonstrating the power of QCM-D.
Tuning surface wettability
The issue of wettability turns up again in a different field – polymers in research published by Spanish researchers at the New Materials Department, CIDETEC, Centre for Electrochemical Technologies and CIC NANOGUNE Consolider. Markus Döbbelin et al in their paper entitled Tuning Surface Wettability of Poly(3-Suphopropyl Methacrylate ) Brushers by Cationic Surfactant-driven Interactions (Macromolecular Rapid Communications, May 2008, Volume 29 Issue 11, Pages 871 – 875) study the kinetics of the polymerization reaction were followed by means of the quartz microbalance technique with dissipation (QCM-D).
Though this is a relatively straightforward use of QCM-D where frequency and dissipation are followed, it allowed the researchers to track the changes in real-time as polyanionic poly(potassium 3-sulfopropyl methacrylate) (PSPM) brushes were switched from hydrophilic to hydrophobic by exchange of the counter cations.
Poly(potassium 3-sulfopropyl methacrylate) brushes were grown by atom transfer radical polymerization (ATRP) from thiol monolayers of initiating -mercaptoundecyl bromoisobutyrate and mixed monolayers of thiol initiator and 1-undecanothiol (blank thiol) attached to gold surfaces. “The collapse of PSPM brushes in the presence of cationic surfactants like quaternary ammonium salts (tetraethylammonium bromide, hexadecyltrimethylammonium bromide) and imidazolium salts (1-dodecyl-3-methylimidazolium bromide, 1H,1H,2H,2H-perfluoro-1-decyl-3-methylimidazolium bromide) was shown by QCM-D,” say the researchers. In addition, water contact angle measurements proved that the wettability of the surface could be tuned reversibly from hydrophilic values (<30 °) to hydrophobic ones (>85 °).
Multidisciplinary cell research
In research published in the paper, Extracellular Matrix Remodelling During Cell Adhesion Monitored by the Quartz Crystal Microbalance (Biomaterials 2008, 29, 2581-2587), QCM-D is one of the techniques used to study a cell’s ability to remodel adsorbed protein layers on surfaces. This ability is known to be influenced by the nature of the protein layer itself.
The researchers say that remodelling is often required to accomplish cellular adhesion and extracellular matrix formation which forms the basis for cell spreading, increased adhesion and expression of different phenotypes.
QCM-D monitoring and fluorescence microscopy was used to study the adhesion of NIH3T3 (EGFP) fibroblasts to serum protein (albumin or fibronectin) precoated tantalum (Ta) and oxidised polystyrene (PSox) surfaces.
The cells were either untreated or treated with cycloheximide to examine the contribution of endogenous protein production during cell adhesion to the QCM-D response over a period of 2 hours. Following adsorption of albumin onto Ta and PS., there was no difference detected between the response to seeding untreated and cycloheximide treated cells.
“The QCM-D was able to detect differences in the untreated cellular responses to fibronectin versus serum precoated Ta and PSox substrates, while cycloheximide treatment of the cells produced the same QCM-D response for fibronectin and serum precoatings on each of the materials. This confirmed that the process of matrix remodelling by the cells is dependent on the underlying substrate and the preadsorbed proteins and that the QCM-D response is dominated by changes in the underlying protein layer,” note the researchers.
Viscoelastic properties
The study of viscoelastic properties is an area where QCM-D excels and this new research published as Viscoelastic Properties of Adsorbed and Cross-linked Polypeptide and Protein Layers at a Solid–Liquid Interface ( Journal of Colloid and Interface Science, Volume 324, Issues 1-2, August 2008, Pages 55-60) demonstrates the power of the technique.
QCM-D tracked the real-time changes in viscoelasticity of adsorbed poly(L-lysine) (PLL) and adsorbed histone (lysine rich fraction) due to cross-linking by glutaraldehyde and corresponding release of associated water. Total reflection Fourier transform infrared spectroscopy (ATR/FTIR) was also used to confirm QCM-D measurements.
The kinetics of PLL and histone adsorption were measured through changes in mass adsorbed onto a gold-coated sensors from changes in frequency and dissipation by applying the Voigt viscoelastic model. Before cross-linking, the shear viscosity and shear modulus of the adsorbed PLL layer were and 2.5×105 Pa, respectively; after cross-linking, these parameters were increased to 2.5×106 Pa. For the adsorbed histone layer, shear viscosity and shear modulus increased modestly from 1.3×10−3 to and from 1.2×104 to 1.6×104 Pa, respectively.
The adsorbed mass estimated from the Sauerbrey equation and the Voigt viscoelastic model differ appreciably prior to cross-linking whereas after cross-linking they converged. This is because trapped water molecules were released during cross-linking.
The researchers note,” The variation in viscoelastic properties increased substantially after cross-linking presumably due to fluctuation of the randomly cross-linked network structure.” Further the US team believes that an increase in fluctuation of the viscoelastic properties and the loss of imbibed water could be used as a signature of the formation of a cross-linked network and the amount of cross-linking, respectively.
