Field Effect Transistor Sensitive to Penicillin
Steve Caras and
Deparfment of Bioengineering, University of Utah, Salt Lake City, Utah
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
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
Papariello et al. (2) and independ-
ently Nilsson et al.
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
Penicilloic acid is a strong acid which releases protons and
depresses the pH at the surface of the pH electrode. Nilsson
and Enfors et al.
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
recently Danielsson et al.
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-
different organic substrates by immobilizing a suitable
enzyme layer over the surface of an ISFET gate.
The primary purpose of this work has been
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
and their performance
characteristics could serve as a standard for our devices.
Materials. Unless stated otherwise,
solutions were prepared
from analytical reagents and deionized
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 188.8.131.52,
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