Biomedical Engineering Reference
In-Depth Information
Polypyrrole
H
N
H
N
H
N
Oxidation
+
+
N
H
N
H
n
N
H
Reduction
FIGURE 1.19
A representation of the oxidized form of Polypyrrole is shown, where the local oxidized species depicted is a pos-
itive-charged 'defect' structure. These structures can be eliminated upon reduction back to the neutral species
using either chemical or electrochemical methods. Reprinted from Marx, K.A., Lim, J.O., Minehan, D., Pande, R.,
Kamath, M, Tripathy, S., Kaplan, D. (1994). Intelligent Materials Properties of DNA and Strategies for It's
Incorporation into Electroactive Polymeric Thin Film Systems.
J. Intelligent Mater. Systems Struct.
5:447-454.
Electrical conductivity is a property that can be taken advantage of in the design of biosen-
sors. Polycationic polymers, such as polypyrrole, should be capable of rearranging their
mobile charge distribution in the presence of a polyanion, to form energetically favorable
electrostatic complexes. With this as an immobilization strategy, in a number of studies we
have examined two conducting polymers, polypyrrole and polythiophene—both polyca-
tions, for their ability to interact with and immobilize DNA—a polyanion.
Using low-resolution SEM, we studied electropolymerized polypyrrole films (~100
m
thick) removed from their synthesis electrodes (59). Two strikingly different surface mor-
phologies were observed for these films. A rough surface was exhibited on the solution
exposed polymerization growth face and a smooth morphology on the Pt electrode proximal
face. The interaction of DNA with these electropolymerized polypyrrole films was then stud-
ied. In Figure 1.20 we show a series of images of linear pBR 322 plasmid DNA bound in an
extended conformation to the surface of an electropolymerized and cationic polypyrrole film
(60). Images such as these were obtained from samples where DNA binding kinetics were
measured. These studies revealed that the mass levels of DNA bound/cm
2
surface area to
polypyrrole and polythiophene films depended directly upon both the film electropolymer-
ization conditions and resulting measured conductivity level, reflecting the positive-charged
defect density (61-64). In the case of polypyrrole binding to DNA, the surface binding level
kinetics were observed to exhibit a
t
0.5
dependence and to depend directly on DNA concen-
tration, as Figure 1.21 shows. This concentration-dependent behavior provided evidence to
support surface binding by a simple diffusion-limited DNA binding model with very low
activation energy (62). That this interaction was largely electrostatic in nature was demon-
strated in our study by competition experiments using other anion and polyanion competi-
tors during DNA binding. Once bound, DNA could be competitively removed from the
polypyrrole surface, albeit at a very slow rate. This slow rate undoubtedly resulted from the
multiple electrostatic bonds formed between both polypyrrole and the DNA polymers.
In a further study, using radiolabeled DNA uptake by electropolymerized polypyrrole
films, we showed that the rough and smooth faces of polypyrrole-immobilized DNA from
solution at very different rates (60). The rate differences were correlated with differences
in the films' smooth and rough face surface areas and the number of voids and channels
penetrating throughout the films' interior. Furthermore, we provided evidence for the
internal migration of DNA through channels in the polypyrrole films. Therefore, the elec-
trostatic interactions between DNA and the mobile cations of conducting polymer matri-
ces serves as a simple and convenient immobilization strategy, placing DNA in close
contact with the conducting polymer chain for use in signal transduction schemes. A num-
ber of studies have investigated the properties of DNA in contact with polypyrrole and
other conducting polymers used in biosensor applications (65).
