J.R. Haak

Chemically modified field-effect transistors; a sodium ion selective sensor based on calix[4]arene receptor molecules

The development of an ion-sensitive field-effect transistor for sodium ions is described. Cahx[4]arene derivatives incorporated in a poly(vinyl chloride)-based membrane provide the selectivity. A poly(2-hydroxyethyl methacrylate) interlayer between the silicon dioxide gate and the sensing membrane is necessary to obtain a Na+-sensitive ISFET with Nernstian behaviour. The potentiometric selectivity coefficients (log Kij pot) for Na+ over K+ and Li+ are ?1.9 and ?2.5,

Detection of charged proteins by means of impedance measurements

The electrical resistance of a membrane containing charged proteins is a function of both concentration and mobility of the electrolyte ions present in the membrane, and thus it depends on the Donnan effect. We were able to determine the resistance of such a membrane deposited on an ISFET, by means of a simple a.c. impedance measurement. As an example a membrane containing cross-linked lysozyme was used. We found the membrane resistance to be inversely proportional to the fixed charge density, in good agreement with the theoretical description based on the Donnan theory.

Multi-ion sensing device for horticultural application based upon chemical modification and special packaging of ISFETs

A multi-ion sensing device, based on ISFETs, is described for horticultural applications. The advantages of a new packaging technique based on back-side contacted ISFETs are outlined. The basic pH-sensitive ISFETs can be modified to, e.g., potassium-sensitive devices by using neutral carrier-based membranes and an intermediate hydrogel. Both membrane and hydrogel are chemically anchored to the ISFET surface and the ionophore is chemically bonded to the membrane. It is shown that these chemical modifications of the membrane system lead to an increased lifetime for the sensor.

Molecular materials for the transduction of chemical information by CHEMFETs

This paper describes a strategy to overcome the problems associated with the use of MEMFETs as selective chemical sensors. The problems arising from the thermodynamically ill-defined insulator-membrane interface have been eliminated by a new sensor design, viz., the introduction of an intermediate hydrogel layer. Experiments to assess the beneficial effect of such a hydrogel are described. Furthermore, new membrane materials have been designed, synthesized, and characterized. Hence, photopolymerizable polysiloxanes are used which allow both the covalent attachment of electroactive components and the covalent bonding of the sensing membrane to the hydrogel. It is shown that polysiloxanes are versatile materials for the sensing membrane of an ISFET-based chemical sensor. The described technology results in a robust and stable sensor.