Glass-encapsulated ISFET

Sol-gel derived glasses, encapsulating neutral carriers were prepared as ion-sensing membranes in ISFETs or as supported membranes in macroelectrodes. Thromobogenicity of the membranes proved to be much better than that of plasticized poly(vinyl chloride) membranes, which makes them successful candidates in the analysis of biological samples. 

H H van den Vlekkert, N F de Rooij, A van den Berg, and A Gnsel, Multi-ion sensing system based on glass-encapsulated pH-ISFETs and a pseudo-REFET, Sensors and Actuators B, 1 (1990) 395-400... 

ISFETs do not require hydration before use and can be stored indefinitely in the dry state. Despite these many advantages, no ISFET-specific ion electrode appeared on the market until the early 1990s, more than 20 years after its invention. The reason for this delay is that manufacturers were unable to develop the technology to encapsulate the devices to give a product that did not exhibit drift and instability. Several companies now produce ISFETs for the determination of pH, but they are certainly not as routinely used as the glass pH electrode at this time. 

For the purposes of pH measurement, the hydrated silicon oxide dielectric of a MOSFET can be used like a glass membrane. Better response to pH, however, can be achieved by MNSFETs with a silicon-nitride dielectric, which are commonly used for this purpose (but not for in vivo measurement in blood because of the high thrombogenicity of Si3N4). Only the surface of the insulator may be in contact with a liquid sample other parts of the ISFET have to be electrically extremely isolated, e.g., by epoxy encapsulation. 

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