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ABSTRACT Biosensors are a special class of chemical sensors that take advantage of the high selectivity and sensitivity of biologically active material. We are currently investigating the characteristics of various deposited electrode coatings ( Au and Pt ) on 10 MHz quartz crystals using the sputtering method. We are also investigating the effect of magnetic behavior ( by intermixing Fe and Ni with electrodes ) on the binding nature of antigen with the substrate. A change in mass occurs due to the binding of antigens and antibodies on the surface of the thin film coating. The frequency change as a result of a change in mass makes it possible to use these crystals as biological sensor devices. This paper describes the construction of antibody-based piezoelectric crystals capable of detecting mycobacterial antigens in diluted cultures of attenuated M.tuberculosis. The microstructural features of these crystals have been studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The crystallographic properties have been characterized using the X-ray diffraction (XRD) method. The long term objective of this research is to develop a rapid quantitative method of analysis for the diagnosis of tuberculosis (TB) and other infections caused by mycobacteria, using biosensor technology.

INTRODUCTION Piezoelectric immunosensors are based upon the measurement of small changes in mass. Piezoelectricity may be defined as the generation of electric charge in a substance by a mechanical stress that changes its shape, and a proportional change in the shape of a substance as a voltage is applied (1]. Some crystalline substances, such as quartz, possess this property. Certain crystals show positive and negative charges when they are compressed and these charges are proportional to the pressure on certain portions of the crystal's surface. The resonant frequency of a piezoelectric crystal can be affected by a change in mass at the crystal surface. The modification of the surface of the crystal with organic compounds or enzymes when bound to a particular substrate provides enhanced detection specificity. Piezoelectric immunosensors are a special class of the biosensor family. A biosensor is a device that recognizes an analyte in an appropriate sample and interprets its concentration as an electrical signal via a suitable combination of a biological recognition system and an electrochemical transducer [2, 31. Potential applications for biosensors range from mining and industrial to military and security, including agricultural and the clinical and veterinary biomedical areas. 501

Mat. Res. Soc. Symp. Proc. Vol. 459 0 1997 Materials Research Society

Biosensors consist of a biocomponent that specifically reacts or interacts with the analyte of interest resulting in a detectable chemical or physical change. A biosensor has the following components: a) a receptor, which is responsible for the selectivity of the sensor. Examples are enzymes, antibodies, lipid layers etc.; and b) a detector, which plays