The concept of photoaffinity immobilization of specific binding proteins has been patented and reduced to practice with antibody on glass by this company. The development of additional reagents and reaction procedures is proposed for the affinity coupling (covalent) of receptor molecules (especially antibodies) to the activated surface of passivated (by hydrophilic spacer coating) silicon-based sensors. Additional trifunctional photoreactive reagents will be synthesized and coupled to the sensor surface for the specific covalent binding of monoclonal antibody (using one of the two antibody binding sites), leaving the remaining stabilized antibody binding site available for functional exchange between surface-immobilized analyte and free analyte molecules in the test sample. The specific, one-step purification-covalent immobilization of the antibody through a long, hydrophilic, flexible tether adjacent to the short-spacer immobilized analyte group, is expected to provide cost efficient production of stabilized immunosensors with improved performance (faster response and regeneration, as well as increased sensitivity, particularly where rapid analyses are required). The system proposed will detect toluene and/or related aromatic hydrocarbon derivatives. Demonstrating laboratory feasibility with this immobilized, stabilized immunosensor system with this type of analyte will support the utility of this technology for sensitive, rapid, and selective sensing and monitoring of biologically active agents. The sensor device will reversibly recognize and quantitate analyte content in batch or continuous sampling situations and transduce the signal with electronic recording and transmission of data. Such a device would offer a broad range of applicability for periodic sample analysis and real time, on line monitoring of fluids (air, water) as well as dosimetry measurements in air and physiological fluids.