Biomedical Engineering Reference
In-Depth Information
(A)
(B)
Polymer
+
+
+
+
−
−
−
−
−
−
+
+
+
+
−
+
PANI/HSO
4
−
+
+
+
PANI/PAA
Gold pads
20
−
60 nm
A
1
µ
m
FIGURE 4.8
(A) PANI and poly(acrylic acid) (PAA) nanojunction sensor. (B) SEM image of PANI-PAA films deposited
on gold pads with 20-60 nm gaps [41]. (From Forzani, E. S., Zhang, H., Nagahara L. A., Ishamshah, A., Tsui, R.,
Tao, N. J. (2004). A conducting polymer nanojuntion sensor for glucose detection.
Nano Lett
. 4:1785-1788.
With permission.)
the polymer. This process offers attractive possibilities to develop materials that show behav-
iors resulting from the combination of properties due to CPs and those due to the functional
groups. The functionalization of CPs can be carried out in three ways: before, during, and
after the polymerization process. The fourth procedure is an entrapment technique where the
target material is immobilized during electrochemical polymerization processes.
The first technique involves covalently linking a specific group to the starting monomer
and subsequently preparing the functionalized polymer [42]. For example, the hydrogen
attached to the nitrogen on the pyrrole molecule can easily be substituted with a specific
group. This method can be adopted only if the specific group is stable during the poly-
merization. A good application of this technique was demonstrated by Cosnier who syn-
thesized conducting PPY films functionalized by biotin groups [43,44]. The strategy
involved the conversion of pyrrole to the
N
-biotinylated pyrrole (Figure 4.9) followed by
its electropolymerization and the subsequent functionalization of the resulting conducting
PPY film by avidin and, finally, its coupling using biotinylated biomolecules. This method
combines the advantages of a spatially localized deposition with those of the avidin-biotin
coupling scheme, in particular the accessibility of the immobilized biomolecule.
Other pyrrole derivatives are also possible but cationic derivatives of pyrrole N-substituted
with alkylammonia groups are interesting because they can be used in the immobilization
of enzymes. The derivatives are amphiphilic molecules as they contain both polar and
nonpolar zones that generate stable electroactive cationic films. Electrochemical polymer-
ization of these surfactants can be carried out in aqueous media without added electrolyte,
which aids the incorporation of biomolecules with a negative charge during formation of
the polymer. The monomer and biomolecule are immobilized together by adsorption on
the electrode surface before the electropolymerization step. This method has permitted the
immobilization of different enzymes [43,45,46] with an increased enzyme loading com-
pared with the entrapment technique.
The disadvantage of N-substituted pyrroles is the slower rate of polymerization and a
marked drop in the conductivity of the PPY matrix due to nonplanarity of the PPY chain
induced by the substituents. An alternative to the N-substitution is derivatization of the
pyrrole ring in the 3 or the 4 positions, which normally requires lower anodic potentials
for its oxidation and has higher electronic conductivities as demonstrated by Garnier and
coworkers [47].
In the second route, some of the specific anions can be electrostatically incorporated
simultaneously during the electropolymerization. In this way, the functionalization is
