QCM-D with focus on variations in oscillation amplitude
Abstract: The QCM-D (quartz crystal microbalance with dissipation monitoring) technique is an increasingly popular tool in studies of molecular interactions at interfaces, e.g. binding/unbinding reactions at surfaces, studies of thin film properties, and conformational changes. The technique is based on real time monitoring of changes in resonance frequency, df, and in energy dissipation, dD (Q-factor); parameters allowing for extraction of information about coupled mass- and viscoelastic properties of the system under study. In this work, we have extended the original parameter space by adding an actuator ability via variations of the oscillation amplitude of the crystal. By adding a second signal generator, simultaneous driving of the quartz crystal at two harmonics was realized; one harmonic for probing of changes in f and D, and one for continuous driving of the crystal at a variable amplitude. Studies of the possibility to influence various binding reactions and spontaneous bilayer formation kinetics showed that both processes are, under certain conditions, influenced by elevated amplitudes. Bilayer formation is the process by which intact vesicles adsorb on the sensor surface, and where, at a critical surface coverage, they rupture and form a supported lipid bilayer (SLB). It is concluded that there exists a critical oscillation amplitude, Ac, above which bond formation between colloidal entities and the surface is inhibited, but below which the binding is unaffected. It was shown that Ac is proportional to the inverse of the radius of the binding entities. This means in turn that the smallest size that can be influenced is determined by the maximum attainable oscillation amplitude. Furthermore, the maximum unit size that can be influenced is determined by the limited penetration depth of the liquid shear wave. In addition, investigations on the effects of elevated amplitudes on SLB formation showed that the kinetics is influenced, but that the end result always is a complete bilayer. Also this observation was shown to originate from the existence of Ac. The Gaussian amplitude distribution over the sensor surface, together with the existence of Ac, generates a radially distributed surface concentration of adsorbed vesicles, being high at the periphery and low at the center of the crystal. The critical vesicle coverage required for spontaneous SLB formation will thus first be reached in peripheral areas, after which the process progresses towards the center of the crystal. This is in contrast to the normal (zero amplitude) scenario, where the SLB formation starts patch wise simultaneously over the entire crystal. Consequently, the two cases display different kinetics. In summary, the perspective of the developed setup is promising, and it is believed to have great potential in a variety of combined sensor/transducer applications and amplitude induced selection/distribution experiments
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