Sensors ISFETs

Biological Membranes Attractive supporting membranes for commercial exploitation of biological material, e.g. immobilization ot membrane bound enzymes in solid state sensors ISFET tvpe structures. 

Membrane-casting Techniques. Uhtil recently, PVC membranes have been exclusively formed by solvent casting techniques but which are not well-suited to the fabrication of ISFET devices. Membrane components in tetrahydrofuran are difficult to manipulate on a micro scale and are prone to absorb atmospheric moisture, thus weakening the adhesion at the sensor-ISFET interface. Che innovation which dispenses with the tetrahydrofuran casting stage is based on an in situ photolysis of the model calciun sensor cocktail admixed with monobutyl methacrylate + benzoyl peroxide + benzoin methyl ether at 340 nm (3). Hie resultant matrix adhered well to the ISFET gate and its potentiate trie response compared favourably with the analogous P and P2-MPMA ISE (Table II). 

The results provided in the 1st PT campaign for pH, conductivity, Ca, K, Na and chloride were obtained by in-house fabricated microelectrodes (Chapter 4.1.4 of this book). The sensor used for pH determination is a pH-ISFET with silicone nitride membrane, the sensors for Na+, K+, Ca2+ and Cl- are ISFETs with ion-selective membranes, while the sensors for conductivity is a 4-bar platinum electrode. All the circuits used for measuring with the sensors (ISFET meter and conductivity meter) were also developed in-house. The results for the ions (Na+, K+, Ca2+ and Cl-) were received being expressed in activity, therefore could not be compared with the other PT results. 

Poghossian A, Berndsen L, Schoning MJ (2003) Chemical sensor as physical sensor ISFET-based flow-velocity, flow-direction and diffusion-coefficient sensor. Sens Act B 95 384-390... 

Campanella, L., C. Colapicchioni, L. Aiello, M. Toma.ssetti, Solid state sensors (ISFETs) for environmental analysis. Sens. Microsyts., Proc. Ital. Conf., 1st, 1996,157—162. 

Such sensors utilizing solid-state electronics have significant advantages. The actual sensing area is very small. Hence, a single miniaturized solid-state chip could contain multiple gates and be used to sense several ions simultaneously. Other advantages include the in-situ impedance transformation and the ability for temperature and noise compensation. While the concept of the ISFET is very... 

The ISFET is an electrochemical sensor based on a modification of the metal oxide semiconductor field effect transistor (MOSFET). The metal gate of the MOSFET is replaced by a reference electrode and the gate insulator is exposed to the analyte solution or is coated with an ion-selective membrane as illustrated in Fig. 

The highly dispersible calix[4]arene neutral carriers can also improve the durability for neutral-carrier-type ion sensors. Time-course changes in both sensitivity (slope for Na+ calibration graph) and selectivity (selectivity coefficient for Na" " with respect to K+) were followed in the Na -ISFETs based on ion-sensing membranes of silicone rubber-(l), plasticized PVC-(l), and plasticized PVC-(2) (Fig. 2). Deterioration proceeded quite quickly in the Na -ISFETs of plasticized PVC-(2) both the Na+ sensitivity and selectivity... 

Appropriate fabrication of sol-gel-derived membranes encapsulating neutral carriers such as valinomycin can afford an excellent type of neutral-carrier-type ion-sensing membranes for ISFETs [27] as already mentioned. The simple encapsulation of neutral carriers in sol-gel-derived membranes, however, has a drawback the encapsulated neutral carriers are still apt to exude from the membranes into aqueous sample solutions, which thereby makes the resulting ion sensors less durable and more toxic. Incorporation of neutral carriers to sol-gel-derived membranes by covalent bonding is desirable. 

Applicability in biological ion assay is an important factor for biocompatible potentio-metric ion sensors. Attempts were made to determine Na" " concentrations in human blood sera by using silicone-rubber membrane Na+-ISFETs based on (5) [Fig. 17(a)] [29]. The found values for Na concentration in undiluted, 10-fold diluted, and 100-fold diluted serum samples are in good agreement with the Na" " calibration plots. Even in the undiluted serum samples, only a slight potential shift was observed from the calibration. This indicates that the calixarene-based silicone-rubber-membrane Na+-ISFETs are reliable on serum Na assay. For comparison with the silicone-rubber membrane, Na -ISFETs with corresponding plasticized-PVC membrane containing (2) or (5) were also tested for the Na assay. The found values of Na" " concentration... 

Apart from the advantage of very rapid response, ISFETs can be easily constructed as real microprobes, although their construction requires considerable skill. At present the ISFET as a pH sensor is less selective than the... 

Iida T., Kawabe T., Noguchi F., Mitamura T., Nagata K., Tomita K., ISFET-type -glutamate sensor using thermophilic glutamine-synthetase from a thermophile, Nippon-Kagaku-Kaishi 1987 10 1817-1821. 

MJ. Schoning, M.H. Abouzar, S. Ingebrandt, J. Platen, A. Offenhauser, and A. Poghossian, Towards label-free detection of charged macromolecules using field-effect-based structures scaling down from capacitive EIS sensor over ISFET to nano-scale devices, in Mater. Res. Soc. Symp. Proc. 915, 0915-R05-04 (2006). 

J.F. Schenck, Technical difficulties remaining to the application of ISFET devices, in Theory, Design and Biomedical Applications of Solid State Chemical Sensors (P.W. Cheung, ed.), pp. 165—173. CRC Press, Boca Raton (1978). 

Although a few amperometric pH sensors are reported [32], most pH electrodes are potentiometric sensors. Among various potentiometric pH sensors, conventional glass pH electrodes are widely used and the pH value measured using a glass electrode is often considered as a gold standard in the development and calibration of other novel pH sensors in vivo and in vitro [33], Other pH electrodes, such as metal/metal oxide and ISFETs have received more and more attention in recent years due to their robustness, fast response, all-solid format and capability for miniaturization. Potentiometric microelectrodes for pH measurements will be the focus of this chapter. 

---