Encapsulation, ISFET

Encapsulation. The larger scale of the device makes encapsulation for laboratory testing much simpler, and the processes involved are more easily carried out than the corresponding procedure required to encapsulate ISFET devices. 

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. 

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. 

ISFETs based on the sol-gel-derived membranes of (7) and (11), being almost comparable to that for similar ISFETs based on sol-gel-derived membranes simply encapsulating bis[(12-crown-4-)ylmethyl]2-dodecyl-2-methylmalonate [27]. 

FIG. 15 Potential response of anion ISFETs based on sol-gel-derived membranes encapsulating trioctylmethylammonium chloride ( ) and modified chemically by alkoxysilylated quaternary ammonium salt (12) (O) to CC-activity changes. (From Ref 50.)... 

Fig. 5.37 An ion-selective field-effect transistor (ISFET). 1, drain 2, source 3, substrate 4, insulator 5, metal lead 6, reference electrode 7, solution 8, membrane 9, encapsulant [22b].

ISFETs do not require hydration before use and can be stored indefinitely in the dry state. Despite these many advantages, no ISFET-specific ion electrode appeared on the market until the early 1990s, more than 20 years after its invention. The reason for this delay is that manufacturers were unable to develop the technology to encapsulate the devices to give a product that did not exhibit drift and instability. Several companies now produce ISFETs for the determination of pH, but they are certainly not as routinely used as the glass pH electrode at this time. 

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