ISFET sensitivity

The purpose of the CIM is to attract ions present in the solution the incorporated ions represent an equivalent charge near the SiO2 and it is equivalent to a voltage applied to the gate. In particular cases the CIM can be the SiO2 itself or a thin film of S13N4 or other insulators. As an example, in order to obtain an ISFET sensitive to K+ ions in solution, valynomicine may be utilized as a CIM. 

ISFETs Sensitive to Other Ions. Deposition of an ion-selective membrane on top of the gate insulator opens the way to measurement of ions other than H . Most of the sensitive materials for ISE applications described in Section 28,2.3.1,1 have been used in conjunction with ISFETs, providing sensors covering a wide variety of species. Thus, ISFETs with solid membranes have been described with AgCl-AgBr membranes sensitive to Ag . Cr, and Br [149], for example, or with LaFs membranes sensitive to fluoride [150], Ion-sensitive polymer-matrix membranes (liquid membranes with a polymer matrix) have also been used as sensitive membranes for ISFETs. The first experiments involved mainly PVC membranes [151]. [152], but these membranes show poor adhesion and poor mechanical strength. To improve membrane adhesion, modified PVC was utilized as a matrix material [153], The use of silicones as matrix materials has made it possible to prepare very durable ISFETs with polymer-matrix membranes [154], Other reports describe the use of photopolymerized polymers as matrix materials [155], [156],... 

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... 

Selectivity coefficients values for K - and Na -ISFETs with the optimized ion-sen-sing membranes encapsulating valinomycin and bis(12-crown-4) are summarized in Fig. 9. The selectivity coefficient for with respect to Na in the K -ISFET is 2 x 10 " and that for Na with respect to in the Na -ISFET is 3 x 10. The selectivity coefficient values are similar to those for the ISFETs and ion-selective electrodes with the previous membrane materials containing the same neutral carriers. The high sensitivity and selectivity for the neutral-carrier-type ISFETs based on sol-gel-derived membranes can last for at least 3 weeks. 

Typical profiles for the potential response of Na -ISFETs based on the sol-gel-derived membranes modified chemically by alkoxysilylated 16-crown-5 (7) and bis(12-crown-4) (11), together with anion excluder (9) show that both of the Na -ISFETs have high sensitivity with a Nernstian response to Na activity changes in wide activity ranges of 3 x 10 to 1 M (Fig. 12). The potential response is quite fast in the Na -ISFETs in spite of the covalent bonding of the neutral carriers to the ion-sensing membranes, as exemplified in the membrane system of the bis(12-crown-4) (11). Some mobility of the chemically bonded neutral carriers can probably be maintained in the present sol-gel-derived membranes. The response time ( 9o) is several seconds for both of the Na -... 

FIG. 14 Time-course changes of sensitivity (slope of calibration plots) for Na -ISFETs based on sol-gel-derived membranes incorporating bis(12-crown-4) by covalent bonding using (11) ( ) and by encapsulation of bis[(12-crown-4-)ylmethyl] 2-dodecyl-2-methylmalonate (O)- (From Ref 49.)... 

Fig. 2.16. Ion-sensitive field effect transistor (intersection and symbol), (a) n-p-n transistor, (b) IGFET (MOSFET), (c) ISFET.

Recently, Van der Schoot and Bergveld74 developed an ISFET-based microlitre titrator by combination with a microcoulometric pH actuator. As a result of integrated circuit technology they could integrate ten pH-sensitive ISFETs... 

M.N. Hendji, N. Jaffrezic-Renault, C. Martelet, P. Clechet, A.A. Shulga, V.I. Strikha, L.I. Netchipruk, A.P. Soldatkin, and W.B. Wlodarski, Sensitive detection of pesticides using a differential ISFET-based system with immobilized cholinesterases. Anal. Chim. Acta 281, 3—11 (1993). 

FIGURE 10.3 A schematic diagram of an ISFET pH measurement system. A potential responding to the hydrogen ions concentration in the solution is established on the pH sensitive gate-solution interface of the ISFET. It electrostatically influences the current (4-d) flow between the source and drain, therefore, the current 4 d change is directly related to the pH change in the solution. 

Relevant for our discussion is the genesis of the sensitivity behaviour in a class of devices all generated from the well known MOSFET structure (ISFET and GASFET). In particular, the influence of charges into the gate oxide on the threshold voltage and MOSFET behaviour under shrinking conditions will be discussed. 

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