Biomedical Engineering Reference
In-Depth Information
14.2.2.3 NonconductingPolymer
Nonconducting polymers, which possess impressive advantages such as excellent permselectivity
and high reproducibility, have also been emerging in biosensors as support matrices for the
immobilization of biomolecules [257]. The property of the nonconducting polymers that are
mainly discussed here is the permselectivity. As far as the permselectivity is concerned, the
membrane synthesized by electropolymerization at about neutral pH showed excellent per-
formance. The fi lm growth is self-limited by its insulating and self-sealing character, and the
resulting fi lms are very thin (ca. 10-100 nm), so that the substrates and products can diffuse
rapidly to and from the enzyme, and this diffusion will further increase the overall concentra-
tion of enzymes and may increase the current response. It has also shown that these fi lms are
continuous, hydrophobic, and “defect free,” and they are able to reject interferents and hence can
improve selectivity [258,259].
Work in this area involved enzyme entrapment in thin fi lms that is electrochemically polym-
erized from phenol, phenylenediamine, or other monomers. Bartlett and Caruana [260] reported
that the electropolymerization of phenol at a platinum electrode surface gave a thin-layer fi lm with
a thickness of 38 nm, and glucose biosensor was fabricated by repetitive potential cycling in an
aqueous solution of phenol and GOD. Nakabayashi et al. [259] used poly(3-aminophenol) to immo-
bilize ferrocene and HRP on a carbon-paste electrode for the development of H
2
O
2
biosensor, and
this biosensor is not easily infl uenced by oxidizable species such as L-AA and UA. Yang et al.
[261] employed poly(
m
-phenylenediamine) (PMPD) for the fabrication of a needle-type glucose
sensor with high selectivity. Since the permeability of PMPD fi lm to interferents is lower than
that of poly(
o
-phenylenediamine) and poly(
p
-phenylenediamine) fi lms [262], the PMPD fi lm is a
promising material for fabricating glucose sensors.
As we have mentioned in Sections 14.2.2.1 and 14.2.2.2, PPy is one of the most extensively
used conducting polymers in designing biosensors because of its high conductivity and stability
as well as simplicity and fl exibility of the immobilization procedure. Recently, a nonconducting
PPy based on the overoxidation of pyrrole has emerged as a novel material for biosensor construc-
tion. Overoxidation of PPy (PPyox) appears to be attracted by the positive potentials in water- and
oxygen-containing environment, and in this case it is moving toward partial destruction of poly-
meric backbone and generation of oxygen-containing (carboxyl, carbonyl, and hydroxyl) groups.
The PPyox is a nonconducting, permselective polymer membrane with excellent interferential rejec-
tion properties and is often used as a discrimination membrane that signifi cantly increases selectiv-
ity of electrochemical biosensors [263,264]. Many biosensors, such as glucose and cholesterol, have
been investigated based on the enzymes immobilized on a PPyox-modifi ed electrode [265-267].
14.2.3 N
ANOMATERIALS
Nanotechnology has recently become one of the most exciting forefront fi elds in analytical chem-
istry. Nanotechnology is defi ned as the creation of functional materials, devices, and systems
through control of matter at the 1-100 nm scale. Owing to their small size, such nanomateri-
als have unique optical [268,269], electrical [270,271], magnetic [272,273], and catalytic [274]
properties that differ from those of the bulk materials. Such properties, together with the diver-
sity in composition (inorganic or organic and metals or semiconductors), the diversity of shapes
(particles, rods, wires, tubes, cubes, tetrapods, or triangles), and the readiness for surface func-
tionalization (physical, chemical, or biological), have enabled the fabrication of various functional
nanoscale devices including biosensors [275-277].
Many interesting applications of nanomaterials in biosensors often incorporate biological
components in the sensor transducer elements or separation phase or extraction materials [278].
T her efor e, most l it er at u r e r ev iewe d her ei n fo cuses on na nom at er ia ls conjugat e d w it h biolog ica l com-
ponents. The review in this section divides nanomaterials into three sections, 1-D nanostructures,
NPs, and nanoporous materials according to their shapes, followed by different types based on
