Biomedical Engineering Reference
In-Depth Information
The PPy polymer with low oxidation potential is easily formed from mild aqueous buffer
solutions in which biomolecules are stable, so it has been widely used for the immobilization of
enzymes, antibodies, and nucleic acids [209-211]. The polythiophene monomer is insoluble in
water and requires higher potentials than PPy [212]. PANI polymers are formed from similar con-
ditions as of PPy, but the formation of the most highly conducting form requires the presence of
acid, which limits its use in biosensors because of the denaturization of biomolecules. Fortunately,
the attachment of simple functional groups, such as sulfonates, to the monomer overcomes these
problems [213].
There are several strategies for the immobilization of biomolecules using conducting polymers,
namely, entrapment of biomolecules within electropolymerized fi lms, electrosynthesis of biomolecules-
functionalized monomers, and the attachment of the biomolecules to the underlying fi lms.
14.2.2.1.1 Entrapment of Biomolecules within Electropolymerized Film
This one-step method is the most straightforward strategy of immobilization. When an appropri-
ate potential is applied to an electrode soaked in an aqueous solution containing monomer and
biomolecules, a polymer that incorporates biomolecules homogenously during its growth process is
formed. It should be noted that such immobilization occurs under mild conditions without chemical
reaction that could alter the activity of the biomolecule. The entrapment of enzymes in conducting
polymers provides a facile means for ensuring proximity between the active site of the enzyme and
the conducting surface of the electrode. This reagentless electrochemical approach is easily appli-
cable to a wide variety of biological macromolecules.
This conventional electrochemical method of biomolecule entrapment was mainly focused on
the immobilization of enzymes. GOD has been successfully entrapped in PPy fi lms [214], PANI
fi lms [215], and in special copolymers [216]. Biosensors of other enzymes, such as peroxidase [217],
COD [218], lactate oxidase [219], uricase [220], tyrosinase [212], and multienzyme that require
sequential immobilization [221] or coimmobilization [222], have also been studied by the one-step
strategy of immobilization. Although the immobilization of enzymes was widely studied by this
method, few examples have been devoted to the immobilization of other biomolecule species such
as antibodies and cells [223,224].
Theoretical models associated with the electrochemical entrapment of enzymes have also been
studied along with the roles of polymer layer thickness, enzyme loading, and spatial location on the
functioning of the biosensor [225-227].
14.2.2.1.2 Electrosynthesis of Biomolecules-Functionalized Monomers
For this procedure, monomers are fi rst functionalized with biomolecules and then the combined
monomers are electrosynthesized into copolymers. The fi rst report was the covalent attachment
of GOD to the PPy, terminal amine groups of the enzyme were chemically modifi ed with pyrrole
moieties, and the pyrrole-modifi ed GOD was copolymerized with pyrrole to produce conducting
polymer fi lms containing covalently immobilized GOD. The resulting enzyme electrodes showed
increased activity and thermal stability compared with
PPy-immobilized GOD in which the enzyme
was physically entrapped as
a counteranion [228].
As an extension of the earlier work, a few reports of interaction of DNA with conducting poly-
mers are available [229-231]. Livache et al. [232] used chemically modifi ed DNA with a pyrrole
moiety and proceeded to the polymerization of pyrrole to fabricate a DNA-based biosensor with a
reliable DNA-PPy grafting. Korri-Youssoufi et al. [233] developed a synthetic route toward function-
alized conjugated polymers and a copolymer of 3-acetic acid pyrrole and 3-
N
-hydroxyphthalimide
(NHP) pyrrole with an aminooligonucleotide. The resulting modifi ed copolymer was used for DNA
recognition. Specifi c hybridization of the grafted oligonucleotide (ODN) with its complementary
ODN target in solution induces a signifi cant modifi cation in the electrochemical response of PPy
and enables a sensitive electrical reading of the recognition process.
