ISFETs

Fig. 1. Schematic for chemoreceptor-modified ISFET biosensor for detection of acetylcholine where the source and drain are both n-ty e siHcon.

The explicit mathematical treatment for such stationary-state situations at certain ion-selective membranes was performed by Iljuschenko and Mirkin 106). As the publication is in Russian and in a not widely distributed journal, their work will be cited in the appendix. The authors obtain an equation (s. (34) on page 28) similar to the one developed by Eisenman et al. 6) for glass membranes using the three-segment potential approach. However, the mobilities used in the stationary-state treatment are those which describe the ion migration in an electric field through a diffusion layer at the phase boundary. A diffusion process through the entire membrane with constant ion mobilities does not have to be assumed. The non-Nernstian behavior of extremely thin layers (i.e., ISFET) can therefore also be described, as well as the role of an electron transfer at solid-state membranes. 

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. 

Future trends may include the commercialization of ISE s for other clinically significant ions such as bicarbonate, magnesium and phosphate. Solid contact electrodes and ISFET s may allow for mass production of smaller, less expensive devices. However, a high standard of performance must be achieved before conventional electrodes become obsolete. 

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... 

FIG. 4 Selectivity comparison among silicone-rubber-membrane Na -ISFETs based on calixarene neutral carriers (1), (2), and (5). 

FIG. 7 Potential response of K -ISFETs based on valinomycin-encapsulated sol-gel-derived membranes fabricated with initial DEDMS/TEOS ratios of 1 (A), 2 (B), and 3 (O)- (From Ref. 27.)... 

Sol-gel-derived membranes encapsulating a bis(crown ether) derivative [bis(12-crown-4-ylmethyl) 2-dodecyl-2-methylmalonate] [28] were also fabricated with an initial DEDMS/TEOS ratio of 3 for Na -ISFETs. The Na -ISFETs showed a Nernstian response to Na+ activity changes in the activity range of 1 x 10 " to 1 M and a short response time of 2 s. Employment of a polythiophene interlayer improved the potential instability and the lower detection limit in both of the K - and Na -ISFETs based on the sol-gel-derived membranes with the initial DEDMS/TEOS ratio of 3 (Fig. 8). 

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. 

FIG. 8 Potential response of K+- and Na+-ISFETs based on optimized sol-gel-derived membranes of valinomycin ( ) and bis(12-crown-4) (O) with interlayer of polythiophene [27]. The initial DEDMS/TEOS ratio for sol-gel processing is 3. 

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