Design and Application of Piezoelectric Quartz Crystal-based Immunoassay

Immunoassays are exquisite analytical tests that utilize antibodies. Their use for both qualitative and quantitative analysis has proved to be one of the most productive technological contributions to medicine and fundamental life science research in the twentieth century.1 Traditional immunoassays are typically based on the enzyme, fluorescent, or radioactive element-labeled second antibodies and associate substrates to generate the final signal. Although sensitive and accurate enough, these traditional techniques, however, suffer from drawbacks of complexity in concept and design, timeconsuming procedures, use of potential hazards, expensive materials and the need for skilled personnel. To overcome these limitations, a variety of rapid, simplified immunoassay tests have been developed. For example, dot enzyme immunoassays, dotimmunobinding assays, and various particles agglutination tests have been used for the detection of antibodies against a wide range of antigens. An alternative method to perform the assay of an antigen or antibody is the use of immunosensors, measurement devices that incorporate an immunologically-active sensing element with an electronic transducer. The combination of the physical transducer with the immunoreaction brings about a new generation of immunoassay devices, which are small, selfcontained, specific, cheap, and robust. By means of these devices, assay results can be produced quickly and automatically. Types of immunosensors include potentiometric, optical, amperometric, and piezoelectric (Pz) crystal devices. These transducers convert immunoreaction events into different physical signals and can sense antigen/antibody concentrations by either indirect competitive and displacement reactions or by direct changes in transducer output. The Pz crystal device is an example of the latter type of system. Antigen-antibody affinity reactions can be identified directly by measuring the frequency change which corresponds to a mass change of the sensor surface. This assay concept and procedure are simple and easyto-use; any use of potentially hazardous-labeled materials is eliminated. The most commonly known Pz devices are surface acoustic wave (SAW) device and bulk acoustic wave (BAW) device. In a SAW device, electrodes are on the same side of the crystal. Mass loading on the acoustic path between two sets of electrodes will alter the phase wave velocity and cause a shift in the frequency.2 The extremely high mass sensitivity means that SAW devices have been applied in a number of chemical sensing systems.3 Compared with SAW devices, the latter design, BAW, is more popularly used in biological applications, and thus will be discussed in this paper. Quartz Crystal Microbalance (QCM) is an alternative term frequently used instead of BAW crystal; the term describes the mass sensitive features of the devices. BAW, or QCM, device consists of a quartz crystal wafer sandwiched between two metal electrodes. The electrodes 107 ANALYTICAL SCIENCES FEBRUARY 2000, VOL. 16 2000 © The Japan Society for Analytical Chemistry