Search

We develop the wireless-electrodeless multichannel quartz-crystal microbalance (QCM) biosensor using quartz plates of slightly different thicknesses. Their shear vibrations are simultaneously excited and detected by a pair of antenna wires to perform the noncontacting measurement. Their fundamental resonance frequencies are between 43 and 55 MHz, and vibrations at up to 10 channels are measured in liquids. Owing to high affinity of naked quartz surfaces for proteins, we immobilized various receptor proteins on different quartz plates nonspecifically and detected various antigen-antibody reactions separately. The exponential coefficient of the frequency change, rather than the amount of the frequency decrease, is found to be useful for distinguishing between specific and nonspecific binding reactions.

We develop a highly sensitive quartz crystal microbalance (QCM) biosensor with a fundamental resonance frequency of 170 MHz. A naked AT-cut quartz plate of 9.7 microm thick is set in a sensor cell. Its shear vibration is excited by the line wire, and the vibration signals are detected by the other line wire, achieving the noncontacting measurement of the resonance frequency. The mass sensitivity of the 170 MHz QCM biosensor is 15 pg/(cm2 Hz), which is better than that of a conventional 5 MHz QCM by 3 orders of magnitude. Its high sensitivity is confirmed by detecting human immunoglobulin G (hIgG) via Staphylococcus protein A immobilized nonspecifically on both surfaces of the quartz plate. The detection limit is 0.5 pM. Limitation of the high-frequency QCM measurement is then theoretically discussed with a continuum mechanics model for a plate with point masses connected by elastic springs. The result indicates that a QCM measurement will break down at frequencies one-order-of-magnitude higher than the local resonance frequency at specific binding cites.

This paper proposes a replacement-free and surface-modification-free quartz crystal microbalance (QCM) biosensor. With the use of significant nonspecific adsorption of streptavidin on naked quartz surfaces, target analyte is detected through biotin-tagged receptors on streptavidin. The wireless-electrodeless QCM technique is developed with a 30 microm thick AT-cut quartz plate, whose fundamental resonance frequency is 55 MHz, and the naked quartz surfaces are used for the nonspecific adsorption of streptavidin. Once it is installed in the sensor cell, it can be used semipermanently; it never needs to be replaced. The equilibrium dissociation constant of streptavidin on quartz is determined to be 1.3 x 10(-7) M. The flow rate affected the number of the adsorbed streptavidin on quartz as well as the binding velocity.

In this paper, we developed a 55-MHz wireless-electrodeless quartz crystal microbalance (QCM) and systematically studied the effects of flow rate on the sensitivity to the detection of proteins and on the affinity between biomolecules evaluated by the flow injection system. Brownian motion of proteins in liquid suggests a low probability of meeting, and the convection effect plays an important role in the sensitivity and the affinity in the flow cell injection system. The wireless quartz crystal was isolated in the QCM cell, and flow rates between 50 and 1000 microL/min were used for monitoring binding reactions between human immunoglobulin G and Staphylococcus aureus protein A. The sensitivity was significantly increased as the flow rate increased, while the affinity value remained unchanged. However, the affinity value was affected by the reaction time for a large-concentration analyte, indicating the need of a high-sensitivity biosensor system for accurate evaluation of affinity. The electrode effect on the QCM sensitivity was also theoretically investigated, showing that the electrode significantly deteriorates the QCM sensitivity and makes the Sauerbrey equation invalid.

The binding affinity between human immunoglobulin G (IgG) and protein A was studied by the homebuilt wireless-electrodeless quartz crystal microbalance (QCM). Protein A was immobilized on the electrodeless AT-cut quartz plate of 0.05 mm thick and its fundamental resonance frequency near 34 MHz was measured by a noncontacting manner using a line antenna. The vibrational analysis was performed to ensure higher sensitivity of the electrodeless QCM. A flow-cell system was fabricated to continuously measure the resonance frequency during the injection sequence of the IgG solutions with concentrations of 1-20,000 ng/mL. The exponential frequency changes were recorded to determine the affinity based on the Langmuir kinetics. The equilibrium constant K(A) significantly varied between 6 x 10(6) and 6 x 10(10) M(-1), depending on the IgG concentration, which is attributed to various formations of IgG-protein A complexes.

This paper presents a contactless technique to measure shear bulk wave resonance frequencies of an isolated quartz crystal in a flow cell. The line antenna placed outside the cell generates and detects the resonance frequencies in a wireless-electrodeless manner. It is revealed that this mechanism relies on the quasistatic electric field. A 0.3-mm-thick AT-cut quartz was used, and its overtone resonance frequencies up to 80 MHz were measured in liquids. Exact vibrational analysis was carried out for a triple-layered resonator system consisting of the adsorbed material layer, the electrode film, and the quartz plate. It predicts higher frequency sensitivity to the adsorbed material at higher modes when the electrode layer is removed. The 13th overtone (72-MHz resonance frequency) was used to detect human immunoglobulin G with concentrations between 0.1 and 20 microg/mL captured by protein A immobilized on one side of the crystal. The real-time measurement of the frequency response yielded the equilibrium constant KA=5.21 x 10(7) M(-1).