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.

A wireless and electrodeless high-frequency quartz crystal microbalance (QCM) was developed for monitoring biochemical reactions in real time without using any labeling procedures. An analytical vibrational calculation for a multilayered plate showed that the QCM sensitivity significantly deteriorates when metallic electrodes are present on the crystal surfaces, indicating the importance of an electrodeless QCM biosensor. The 30-mm-thick electrodeless QCM immunosensor was excited, and its mechanical vibration was detected contactlessly using an antenna located outside the QCM cells. The homebuilt QCM system was used to detect human immunoglobulin G (hIgG) using staphylococcal protein A (SPA) immobilized on the crystal surface and to demonstrate the higher sensitivity of the electrodeless QCM than that of the conventional QCM. The 30-mm-thick QCM, with a 54 MHz fundamental frequency, successfully monitored the hIgG-SPA binding reaction for a hIgG concentration of 100 pg/mL.

This study presents a fundamental concept of piezomagnetic biochemical sensor driven in a wireless-electrodeless manner. A stepped cylindrical rod of nickel is used as the oscillator, which traps the vibrational energy of axially-polarized surface-shear waves in the central part, where the diameter is slightly larger. A meander-line coil surrounding the oscillator with an air gap can cause and detect the resonant vibrations of the surface-shear waves via the piezomagnetic effect. The resonant frequency of the trapped-mode resonance is continuously measured to detect human immunoglobulin G (IgG). It decreased by 0.08% when a solution containing IgG was injected into the glass cell where the oscillator was placed alone. This oscillator is useful for fundamental studies of various biochemical reactions in a closed system in different environmental gases and different pressures.

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