Impedance characterization of a piezoelectric immunosensor. Part I: antibody coating and buffer solution.

This study investigated the impedance characteristics of a 5 MHz quartz crystal resonator oscillating in a thickness-shear mode for utilization as a biosensor. An impedance analyzer measured the impedance of the quartz crystal, which determined all mechanical properties of the oscillating quartz and its immediate environment. In this study, the impedance behavior of the oscillating crystal was characterized in air, in potassium phosphate buffer solution, and with immobilization of antibodies using protein-A. The potassium phosphate buffer behaved like a Newtonian liquid. The series resonance frequency shifted down about 900 Hz on contact with the buffer. An immobilized-antibody layer on the quartz surface behaved like a rigid mass when immersed in the buffer solution. The impedance curve following immobilization of antibodies was shifted down in frequency by about 200 Hz compared with its value when the bare crystal was immersed in the buffer solution.