404 · BIOMEDICAL SENSORS different protocols for immobilization were studied to determine the most reliable one. The greatest efficiency was obtained through physical adsorption of the enzyme coupled with surface reticulation using a dilute glutaraldehyde cross-linking solution. This sensor was advantageous, as it exhibited short response times (15 s to 30 s) and a response to urea over 1 × 10 -2.5 M to 1 × 10 -1.5 M with a viable response lasting 4 weeks. 8.8. PIEZOELECTRIC DETECTION 8.8.1. GENERAL PRINCIPLES The interaction of antibodies with their corresponding antigens is an attractive basis for develop- ing antibody-based chemical biosensors, that is, immunosensors. Theoretically, if an antibody can be raised against a particular analyte, an immunosensor can be developed to recognize it. Despite the high specificity and affinity of antibodies toward complementary ligand molecules, antibody­antigen interactions lead to only weak electronically measurable changes. However, the remarkable selectivity of antibodies has stimulated research in the field to overcome this problem (Luong & Guilbault, 1991; Muramatsu et al., 1987). The piezoelectric effect in various crystalline substances is a useful property that allows the detec- tion of analyte. Piezoelectric detection involves monitoring the change in mass at a sensor surface, either a piezoelectric crystal or an acoustic wave device. This type of biosensor has been used variously for environmental monitoring (Yokoyama et al., 1995), food microbial testing (Ye, Letcher, & Rand,