Page 411 - Organic Electronics in Sensors and Biotechnology

388 Chapter Ten

ways to explore the electrochemical and electrocatalytic properties of
carbon nanotube films. Polymer particulates/blocks can be electro-
phoretically moved under the action of an electric field. Hence this
provides a broader selection of processing routes for composite mate-
rial systems for both miniaturized and complex configurations using
EPD.
The electrophoretic patterning of polyaniline from its colloidal
suspension has proved to be a scalable technique: it can be applied to
macroscopic substrates as well as microfabricated device features.
Electrophoresis of charged polymer colloids on patterned electrode
surfaces allows us to form colloidal monolayers and to grow colloidal
crystals with different lattice structures, where the patterns of the
electrode substrates determine the structures of the colloidal crystals
grown on them. Achieving highly accurate growth and control of
packing symmetry in colloidal crystals is of paramount importance
for photonic applications. There is particular interest in this issue as
the propagation of light in photonic crystals is highly dependent on
their lattice structures. Colloidal monolayers fabricated by this
method can be further used for the preparation of three-dimensional
colloidal crystals with low packing symmetry, e.g., by means of a
layer-by-layer deposition of oppositely charged colloidal particles.
Furthermore, the monolayers can have additional applications, e.g.,
to study local reactions that take place in hollow spheres (nanocap-
sules) that are used as delivery vehicles for the controlled release of
substances such as drugs, cosmetics, dyes, or inks, or for protection of
sensitive agents such as enzymes and proteins. Electrophoretic depo-
sition results in the formation of uniform, dense, pinhole-free nano-
structured films of conducting polymers. These nanostructured poly-
meric films can be used as an effective matrix in light-emitting
devices, photovoltaic cells, organic photocells, biosensors, etc.
In biosensor applications, nanostructured polymer matrices have
been found to provide increased surface area for high enzyme loading.
Furthermore, the high surface free energy of a nanostructure strength-
ens binding and stabilizes the desired enzyme. These unique proper-
ties of nanostructured conducting polymers offer excellent prospects
for interfacing biological recognition events with electronic signal
transduction and for designing new bioelectronic devices. In compari-
son with the traditional approaches such as the spin-coating method,
the electrophoretic deposition requires less solubility and thus is appli-
cable to a wider range of materials and composites. Electrophoretic
deposition can be used for on-chip manipulation and assembly of col-
loidal particles. The structures assembled include cells, conductive
microwires from metallic nanoparticles, and switchable two-
dimensional crystals from polymer microspheres. The electric field-
driven assembly of cells and nanoparticles can be used to create new
types of biosensors, microbioassays, and bioelectronic circuits. This