Biomedical Engineering Reference
In-Depth Information
Another attractive affi nity interaction that has achieved wide acceptance for binding biological
species to surfaces is avidin-biotin linkage, which presents a variety of specifi c advantages over
other immobilization techniques. For example, the binding strength of biotin to streptavidin is
nearly equal to that for a covalent bond (
K
D
=
10
-
15
M) [247]. This interaction is highly
resistant to a wide range of organic solvents, pH range variations, and high temperatures, and
it can be used to link any biomolecules possessing avidin or biotin labels. In addition, the bio-
logical activity of the biomolecule being immobilized by this interaction can also be preserved
effi ciently.
Biotins can be cooperated into the conducting polymers by oxidizing monomers labeled with
biotin [246,248]. Subsequent avidins link to the functionalied polymers by the avidin-biotin affi nity.
A variety of biotinylated biomolecules have been fabricated, for example, GOD [248,249], nucleic
acids [250], peptides, and bacteria [251].
1
×
14.2.2.2 Nanoconducting Polymer
The conducting polymers that are discussed in Section 14.2.2.1 function as a matrix of biomolecules
in the form of bulk membrane. However, recently, nanoscaled forms of conducting polymers and
their applications in the design of biosensors have attracted much interest because they have better
properties compared with the bulk conducting polymers, such as larger surface area, better conduc-
tivity, and higher reaction ability. Several types of biosensors based on the nanoscaled conducting
polymers have been reported.
Zhou et al. [252] fabricated an amperometric glucose biosensor based on platinum micropar-
ticles dispersed in nanofi brous PANI. The nanofi brous PANI fi lm had been synthesized by pulse
galvanostatic method. The single nanofi brous PANI has a diameter of 70-100 nm and a length of
about 10 µm, and the nanofi brous PANI fi lm exhibits a three-dimensional (3-D) structure with a
large number of microgaps and micropores existing between the fi bers. This structure results in a
large specifi c surface area and good ionic and electronic conductivities of the fi lm, which are benefi -
cial for platinum dispersion and enzyme incorporation. So, it can be concluded that the nanofi brous
PA N I fi lm can be used as a better supporting material for catalyst and enzyme.
There is another amusing approach for the immobilization of enzymes based on the nanocon-
ducting polymers. Parthasarathy et al. [253,254] developed a template-based synthetic method
to yield hollow PPy microcapsules of uniform diameter and length, and then they fi lled these
microcapsules with high concentration of enzymes. This method is an ideal approach because
it satisfi es all the criteria that an ideal immobilization method requires, such as employing mild
chemical conditions, allowing for large quantities of enzymes to be immobilized, providing a
large surface area for enzyme-substrate contact with a small total volume, minimizing barriers
to mass transport of substrate and product, and providing a chemically and mechanically robust
system.
Since Martin et al. [255] explored the “template-synthesis” method, nano- or microtubes of
many kinds of materials have been synthesized. It is feasible and facile to synthesize these micro-
containers with various conducting polymers. Recently, Park et al. [256] reported a hollow microcyl-
inder structure comprised of the conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT);
it worked as a container retaining water-soluble electroactive materials and played the role of a cur-
rent collector for the electrochemical reaction of the species retained inside. The current collector
exhibited a fi ne current response due to the redox reaction of K
3
Fe(CN)
6
, and the results show that
guest materials can be stored in this capped cylinder structure without the problems associated with
the use of organic solvents during the manufacturing processes.
As we have discussed above, the encapsulation method offers a number of important advantages
such as general fabrication, chemically benign environment, and a large surface area for enzyme-
substrate contact, so this biomolecule immobilization concept should fi nd extensive applications in
biosensors.
