Caras80 - A nal. Chem. 1980, 52, 1 935-1937 1935 Field...

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Anal. Chem. 1980, 52, 1935-1937 Field Effect Transistor Sensitive to Penicillin 1935 Steve Caras and Jig Janata" Deparfment of Bioengineering, University of Utah, Salt Lake City, Utah 84 112 A feasibility study of an enzyme-coupled field effect transistor has been done. This device was constructed by depositing a co-cross-llnked penicllllnase-albumin layer over a pH-sen- sitive field effect transistor. The differential mode of mea- surement largely eliminates the temperature sensitivity and the effects of ambient pH variation. The new probe has a llfetime of 2 months and time response T~~ = 25 s. The range and sensitivity are comparable to the conventional penicillin- sensitive macroelectrodes. The small size of the sensitive gate requires only a minute amount of enzyme (-2.5 X lo-' IU) which could prove to be an important factor in construction of other enzymatic sensors utilizing more expensive enzymes. Enzyme electrodes have proved to be useful in biochemical analysis and have been developed and characterized for a variety of substrates (1). Papariello et al. (2) and independ- ently Nilsson et al. (3) developed a penicillin enzyme electrode by immobilizing penicillinase over a pH glass electrode. In these probes penicillinase catalyzes the hydrolysis of penicillin to penicilloic acid according to the reaction R CO N H p,---; ", penicillinase RCONH~" 0 CO2 Ho-l 0 Penicilloic acid is a strong acid which releases protons and depresses the pH at the surface of the pH electrode. Nilsson et al. (4) and Enfors et al. (5) have characterized similar penicillin enzyme electrodes and have found that their sen- sitivity depends on the buffer capacity of the bulk solution. The possibility of an enzymatically coupled ion-sensitive field effect transistor (ENFET) has been postulated (6), and recently Danielsson et al. (7) described a urea-sensitive device on the basis of their gas-sensing FET and called their device an "enzyme transistor". As with conventional ion-sensitive electrodes (ISEs), ion- sensitive field effect transistors (ISFETs) can be made sen- sitive to different organic substrates by immobilizing a suitable enzyme layer over the surface of an ISFET gate. The primary purpose of this work has been demonstrate the feasibility of a directly operating enzymatically coupled field effect transistor. A penicillin-sensitive transistor was chosen for two reasons: first, a pH ISFET is the simplest ISFET because it does not require an additional ion-selective membrane. Second, analogous penicillin-sensitive electrodes have been well characterized (2-51, and their performance characteristics could serve as a standard for our devices. EXPERIMENTAL SECTION Materials. Unless stated otherwise, all solutions were prepared from analytical reagents and deionized 15-MR water. Solutions of bovine serum albumin (5, 10, and 15%) (BSA) were prepared in 0.02 M phosphate buffer, pH 6.8. Glutaraldehyde (2.5% v/v) for cross-linking was prepared by diluting 25% stock solution with water. Penicillinase ((3-lactamase from Bacillus cereus, EC, 350 IU/mg, Sigma, or 66 IU/mg, Calbiochem) was dissolved in 0.02 M phosphate buffer, pH 6.8, to a unit concentration of 16000
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Caras80 - A nal. Chem. 1980, 52, 1 935-1937 1935 Field...

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