Most commercial devices use enzymes as the biological recognition element. For a given analyte-recognition element reaction, several transduction schemes may be applicable. However, constraints may be imposed by the intended use. The most common transduction parameters and corresponding device types are summarized in Figure 2. Amperometric devices detect changes in current as constant potential.
Conductimetric devices detect changes in conductivity between two electrodes.
In the intrinsic mode, the incident wave is not directed through the bulk sample, but propagates along a wave guide and interacts with the sample at the surface within the evanescent field. Other surface methods of optical detection of biological recognition are based on modulation of the field excited at the interface between different materials due to incident light. Of these, surface plasmon resonances probably received more attention. In the intrinsic mode, the incident light passes through the sample and interacts directly with the sample. The mass detector is generally based on a quartz crystal whose frequency is predictably affected by minor changes at the surface such as antibody binding to a surface-immobilized antigen.