QCM-D technique

It appears that in most cases the techniques are comparable in terms of resulting sensitivity. Whereas the SPR has the advantage in terms of real sensitivity, the fact the QCM technique also measures entrapped water amplifies the gravimetric response and may render it sensitivity comparable in macromolecular binding experiments [44, 45]. An added advantage of QCM over SPR is the availability of the QCM-D technique which is a measurement of the dissipation energy. A film that is viscoelastic or soft will not fully couple with the quartz crystal s oscillation... 

One unique capability of the QCM-D technique is its successful extraction of quantitative information about a film s viscoelasticity. Figure 8.9 shows the variation of viscosity and shear elastic moduli of a pectin layer during pectin adsorption on the BSA surface from the Voigt model. It is noted that the shear elastic moduli is much... 

In summary, the QCM-D technique has successfully demonstrated the adsorption of pectin on the BSA surface as well as determined the viscoelastic properties of the pectin layer. As pectin concentrations increase, the adsorbed mass of pectin estimated from the Voigt model show higher values than those estimated from the Sau-erbrey equation because the former takes into account the hydrated layer. But the similar increase of thickness of pectin suggests that the pectin chains form a multilayer structure. In agreement with our previous rheology results, the main elastic character of the pectin layer in terms of Q-tool software tells us the network structure of the pectin layer on the BSA surface. In summary, QCM-D cannot only help to better understand the polysaccharide/protein interactions at the interface, but also to gain information of the nanoscale structure of polysaccharide multilayers on protein surface. 

The practical application of this measurement principle is the QCM-D technique (quartz crystal microbalance with dissipation monitoring), patented by Q-Sense [55]. The QCM-D technique extracts frequency, /, and dissipation, D = Rs/ coLs), or the respective changes A/ and AD (see Chap. 12 in this volume). 

The QCM-D technique allows measurement of these parameters several times per second and the performance of measurements on quartz crystals either in vacuum, gaseous, or liquid environment. Additionally it is possible to switch between fundamental frequency and overtones [56]. 

The QCM has added valuable information about the mechanism of vesicle fusion on a surface. For instance, Kasemo and coworkers have unraveled the formation of planar lipid bilayers on Si02 and glassy surfaces by means of the QCM with dissipation (QCM-D) technique in conjunction with SPR, atomic force microscopy (AFM), and computer simulations [5-12]. They found that the process of bilayer formation occurs in three successive steps (1) in the first stage, vesicles attach to the surface via inter molecular interactions (2) at a critical surface coverage, the vesicles start to rupture, fuse on the surface, and thus form bilayer islands coexisting with vesicles and uncovered substrate (3) eventually, a coherent bilayer is formed covering the entire surface. 

Hybrid Devices and Novel Resonance Techniques. Combinations of the QCM with other techniques and novel variations of the standard QCM resonance technique have recently provided interesting advances in the study of different systems. One such approach is the relatively new QCM-D technique, a discontinuous resonance decay approach for obtaining energy dissipation behavior of film surfaces [198]. We believe that this convenient device for measuring the viscosity, elasticity, and thickness of films will continue to expand in popularity for film characterization. 

The QCM-D Technique for Probing Biomacromolecular Recognition Reactions 429 The energy dissipation is a dimensionless quantity defined as ... 

We beheve that this approach of handhng membrane proteins on solid supports constitutes a promising route for analytical strategies aimed at analysis of membrane proteins and consequently also for the QCM-D technique in the development of patterned surfaces for array-based analysis of biorecognition events. 

Kasemo s laboratory at Chalmers University of Technology and Goteborg University was the source of the QCM-D technique [10], now embodied in instrumentation offered by Q-Sense AB. They define the dissipation, A as the inverse of the quality factor Q of the quartz crystal resonator [47] (see equation (4)). In the Q-Sense instrumentation, the driving RF power to the oscillator, causing it to respond at resonant frequency f, is switched off and the exponentially damped sinusoidal wave decays with a time constant r, where D -. .More than 100... 

QCM-D technique Kasemo and co-workers have developed an interesting technique that measures the resonance frequency f and the dissipation factor D, which is the inverse of the Q-factor, of the oscillation simultaneously by a ring down method. The quartz plate is excited every second with a frequency generator followed by switching off the source and recording the free decay of the quartz oscillation. The dissipation factor and resonance frequency are obtained from each cycle by a curve fit of an exponentially damped harmonic oscillator function (Figure 2C). 

Adsorption studies performed with the QCM-D technique showed multilayer or aggregate formation of asphaltenes at the silica surface. Asphaltenes were irreversibly adsorbed as a rigid film. Visual inspection of the resin-coated silica particles clearly showed that the initially hydrophilic silica (Aerosil 200 and Aerosil 7200) adsorbed considerably more resins than the hydrophobic silica (Aerosil 972). By analyzing the toluene solutions for remaining asphaltenes with near-infrared (NIR) spectroscopy, they could conclude that the initially hydrophilic particles had adsorbed considerably more asphaltenes than the hydrophobic particles. All the products exhibited less adsorption of resins than of asphaltenes and the influence of their adsorption was dramatic, especially on the wettability of the hydrophilic silica. Very hydrophilic particles like the unmodified 200 or very hydrophobic particles like the 972 preferred the water and oil phase, respectively. In contrast, the Aerosil 7200 seemed to be equally partitioned between the two phases, indicating intermediate wetting properties. Generally, the stabilization efficiency was enhanced by adsorption of crude oil components onto very hydrophilic or very hydrophobic silica. 

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