Hydrogel ISFET

In a rather unnecessary attempt to define the interface between the ion-selective membrane and the dielectric, an internal hydrogel layer saturated with, for example, a buffer solution, has been interposed between these two layers. It mirrors the miniaturization of conventional symmetrical membrane ion sensors. The volume of the hydrogel layer is very small. This approach creates more problems than it solves. What is the major problem  

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On-wafer membrane deposition and patterning is an important aspect of the fabrication of planar, silicon based (bio)chemical sensors. Three examples are presented in this paper amperometric glucose and free chlorine sensors and a potentiometric ISRET based calcium sensitive device. For the membrane modified ISFET, photolithographic definition of both inner hydrogel-type membrane (polyHEMA) and outer siloxane-based ion sensitive membrane, of total thickness of 80 pm, has been performed. An identical approach has been used for the polyHEMA deposition on the free chlorine sensor. On the other hand, the enzymatic membrane deposition for a glucose electrode has been performed by either a lift-off technique or by an on-chip casting. 

Most ISEs are based on purely physicochemical and non-catalytic recognition elements solid membranes with fixed ionic sites (e.g. the glass pH electrode), ion-exchange polymer membranes or plasticised hydrogel membranes incorporating ionophores [9], Silicon oxide or metal oxides act as the recognition element in pH-ISFETs, gas-sensitive FETs, solid-state electrolyte, solid-state semiconductor and many conductometric gas sensors. 

Berg veld P and Reinhoudt D N 1990 Modification of ISFETs by covalent anchoring of poly(hydroxyethyl) methacrylate hydrogel. Introduction of a thermodynamically defined semiconductor-sensing membrane interface Anal. Chim. Acta 230 59-65... 

The CHEMFET is a special modification of the ion sensitive field effect transistor (ISFET) in which the gate oxide is covered with an ion selective membrane [2]. The ion selectivity is introduced by incorporation of ion-selective receptor molecules in a hydrophobic polymer membrane. Between the sensing membrane and the gate oxide of the ISFET a buffered hydrogel is introduced which serves as inner electrolyte solution and eliminates pH sensitivity and CO2 interference (Figure 1) [3],... 

Actual potentiometric biosensors are commonly designed on the basis of ISFETs, i.e. they are ENFETs. Inclusion in hydrogels is no longer done. Covalent bonding, preferably at carbon surfaces, is preferred. Alternatively, embedding in polymer layers is performed, today preferably in polymers generated elec-trochemically. Also widespread are P VC coatings with specific softeners (as solvents for active substances) as well as layers of silicon rubber and polyurethane. 

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