Biomedical Engineering Reference
In-Depth Information
chemical composition. It is not a comprehensive coverage of these topics, but rather the most impor-
tant advances relevant to novel developments in biosensor over the recent years are highlighted.
14.2.3.1
One-Dimensional Nanomaterials in Biosensor
One-dimensional nanostructures, such as CNTs, metal or metal oxide nanotubes, and conducting-
polymer nanowires, are particularly attractive for biosensor application. Because of their high
surface-to-volume ratio, high aspect ratio, and novel electron transport properties, their electronic
conductance is hypersensitive to surface perturbations (such as those associated with the binding of
macromolecules). Such 1-D materials thus offer the prospect of rapid (real-time) and sensitive label-
free bioelectronic detection and massive redundancy in nanosensor arrays. The extreme smallness
of these nanomaterials would allow the packing of a huge number of sensing elements onto a small
footprint of an array device.
Since the applications of CNTs (Section 14.2.1), and conducting-polymer nanowires (Section
14.2.2.2) in biosensors have been reviewed in the previous sections, in this part, we mainly intro-
duce several biosensor applications of metal and metal oxide 1-D nanostructures that have been
reported recently.
It is known that gold electrodes have been increasingly used in designing electrochemical bio-
sensors. However, enzyme immobilization on fl at gold surfaces often suffers from low amounts
of biomolecules and poor electrical contact to the transducer. Recently, Demoustier-Champagne
and Delvaux reported on the fabrication of ensembles of gold nanotubes aligned parallel to each
other and presenting uniform size and shape by electroless plating of gold into the pores of nanopo-
rous polycarbonate track-etched membranes [279]. Then arrays of gold nanotubes were functional-
ized through the immobilization of
enzymes using SA monolayers as anchor layers [280,281]. The
resulting biosensors showed excellent properties, such as low cost, ease of fabrication, remarkable
sensitivity, good reproducibility, and repeatability. Concerning the high sensitivity, there were two
explanations [280]. First, it came from the higher roughness of the electroless Au surface of the
electrode compared with fl at gold fi lm and from the tubular nature of the electrode, both leading
to an increased surface area of the electrode for the same geometric area. Second, the amount of
enzyme molecules keeping their activity is higher when they are immobilized within a porous sys-
tem than on a fl at surface, because the magnitude of analyte response is not only proportional to the
enzyme loading but also to the enzyme activity on the electrode surface, and the Au nanoelectrode
has better biocompatibility.
A disadvantage of metal nanomaterials that limits their applications in biosensors is the direct
adsorption of redox proteins on unmodifi ed metal surfaces, which usually leads to a dramatic
change in the protein structures and signifi cant loss of their bioactivity [282,283]. But for the metal
oxides, the situation will change, because some metal oxides, such as zinc oxide semiconductors,
have favorable biocompatibility.
Zinc oxide (ZnO) and its 1-D nanostructures have recently been investigated intensively due to
their potential applications in optoelectronics [284,285]. The 1-D ZnO nanostructures have unique
advantages including high specifi c surface area, nontoxicity, chemical stability, electrochemical
activity, and high electron communication features, and more particularly, ZnO has a high IEP
[286], which makes it suitable for the adsorption of low-IEP proteins or enzymes, and thus they may
have great potential in the applications of biosensors.
Zhang et al. [287] reported a reagentless UA biosensor based on uricase immobilized on ZnO
nanorods. The monodispersed ZnO nanorods were prepared by thermal evaporation approach
[288] and dispersed in solution for the surface modifi cation of GCE; the immobilization of uri-
case was done by casting 5 µL uricase on the ZnO membrane. This sensor showed high thermal
stability and electrocatalytic activity to the oxidation of UA without the presence of an electron
mediator. Glucose biosensor based on GOD immobilized on ZnO nanorod array has also been
reported [289].
