5.8.2 Sensing via Catalysts

Catalyst-based detection is realized by immobilized catalyst agents on the surface of the insulator. The target is injected in a well prepared environment which only allows chemical reactions via the catalyst at the interface. The catalytic reaction (e.g. oxidization) yields a release of charge, which can be sensed by the ISFET. For instance, Kharitonov et al. [212] showed a nicotinamide adenine dinucleotide ($N\!AD^{+}$) detector, utilized by a pyrroloquinoline quinine (PQQ) catalyst. The chemical reaction is written as follows:

$$2 N\!AD^{+}+2 H_{2}\!O\autorightleftharpoons{PQQ}{} 2 H^{+}+ 2 N\!ADH + 2 O_{2}\quad.$$ (5.31)

Catalyst spare the need for a selective membrane impeding diffusion of undesired ions to the surface area of the ISFET, due to its immobilization at the surface and restriction to the reaction (5.37). Therefore, such sensors exhibit a high degree of selectivity. However, the predicted and the measured pH level may slightly disagree due to the catalyst molecules introducing auxiliary effects at the insulator surface.