Biomedical Engineering Reference
In-Depth Information
GOD and L-amino acid oxidase have been covalently bonded to chemically
modifi ed graphite electrodes via the cyanuric chloride linkage to yield glucose and
L-phenylalanine sensors, respectively, by Ianniello [5]. These enzymes catalyze the
oxidation of their respective substrates in the presence of O
2
to yield H
2
O
2
as one of
the products. The H
2
O
2
produced during the enzymatic reaction is electrochemically
consumed resulting in a measurable, steady-state current. These electrodes displayed
relatively rapid response, expanded linear response range, and unique catalytic proper-
ties as compared to previously reported amperometric enzyme electrodes. The elec-
trode remained active over a 20-30 day period, provided the proper storage conditions
were maintained.
17.2.1.3 Sol-gel/polymer embedment of protein
So far, the prevalent method to immobilize protein is sol-gel/polymer embedment
of protein. It usually embeds and immobilizes protein in a three-dimensional netlike
structure of macromolecule polymer. This technology has some characteristics such as
mild conditions, multifarious sol-gels, and controllable membrane aperture and fi gure.
It can also retain the bioactivity of protein well when applied to a high concentration
of protein. However, the aggregation process is diffi cult to control.
Fortier [6] found that AQ polymer from Eastman was not deleterious for the activ-
ity of a variety of enzymes such as L-amino acid oxidase, choline oxidase, galactose
oxidase, and GOD. Following mixing of the enzyme with the AQ polymer, the mixture
was cast and dried onto the surface of a platinum electrode. The fi lm was then coated
with a thin layer of Nafi on to avoid dissolution of the AQ polymer fi lm in the aqueous
solution when the electrode was used as a biosensor. These easy-to-make amperomet-
ric biosensors, which were based on the amperometric detection of H
2
O
2
, showed high
catalytic activity.
Tor [7] developed a new method for the preparation of thin, uniform, self-mounted
enzyme membrane, directly coating the surface of glass pH electrodes. The enzyme
was dissolved in a solution containing synthetic prepolymers. The electrode was
dipped in the solution, dried, and drained carefully. The backbone polymer was then
cross-linked under controlled conditions to generate a thin enzyme membrane. The
method was demonstrated and characterized by the determination of acetylcholine by
an acetylcholine esterase electrode, urea by a urease electrode, and penicillin G by a
penicillinase electrode. Linear response in a wide range of substrate concentrations
and high storage and operational stability were recorded for all the enzymes tested.
17.2.1.4 Surfactant embedment of protein
Surfactant has a similar amphoteric structure as lipid, which makes it possible to form
a stable membrane the same as a lipid membrane and can be used to embed proteins.
A surfactant membrane has many characteristics similar to those of a biomembrane,
so that it can retain the bioactivities of proteins well. The process of preparing a sur-
factant/protein-modifi ed electrode is simple and viable. There are usually two methods
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