Sol-gel-derived membranes incorporating crown ether neutral carriers by covalent bonding have been designed for durable, nontoxic neutral carrier-type ion sensors by sol-gel processing using tetraethoxysilane, diethoxydimethylsilane, and their corresponding alkoxysilylated neutral carriers. A tetraphenylborate anion was also bound chemically to the neutral carrier-type sol-gel-derived membranes for suppressing anion interference and membrane impedance. Ion-sensitive field effect transistors (ISFETs) based on the sol-gel-derived membranes modified chemically by crown ether derivatives, such as 16-crown-5 and bis(12-crown-4) derivatives, showed high performance, high sensitivity in wide cation activity ranges, short response time, and high electrode durability. Specifically, high ion selectivities of practical use in biological systems were attained in the sodium ISFETs based on the chemically modified bis(12-crown-4), which has been successfully applied to sodium assay in blood sera.

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Sol-gel-derived membranes encapsulating neutral carriers, such as valinomycin and a bis(crown ether) derivative, were fabricated as novel ion-sensing membranes for neutral carrier-type ion-sensitive field-effect transistors (ISFETs). The neutral carrier-type sol-gel-derived membranes obtained with an initial diethoxydimethylsilane/tetraethoxysilane ratio of 3 gave excellent results for membrane processibility and sensitivity, response time, and selectivity in the resulting ion sensors. The sol-gel-derived ion-sensing membranes containing neutral carriers are superior in the thrombogenic property to their corresponding plasticized poly(vinyl chloride) membranes. The potassium and sodium ISFETs thus obtained were successfully applied to the ion assay in blood sera.

Neutral carriers, such as 16-crown-5 and calix[4]arene tetraester, were immobilized on silicone-rubber membranes by covalent bonding. Sodium ion-selective electrodes based on silicone-rubber membranes modified chemically by the calixarene ionophore together with an anion excluder exhibited high performance in terms of sensitivity, selectivity and response time.