Biomedical Engineering Reference
In-Depth Information
biochip development. This technology is based on the detection of biologically functionalized
micrometer- or nanometer-sized magnetic labels using high-sensitivity microfabricated magnetic
fi eld sensors. Although in its infancy, this technology offers high-sensitivity detection (to the
level of a single molecular interaction, in principle), a stable labeling system, low-magnetic
background, and cheap device components. Magnetic NPs have been widely used for the con-
struction of biological assay systems, such as DNA hybridization [317] and ligand-receptor
reactions [318,319].
Chemla et al. [320] developed a new technique for detecting biological targets using antibodies
labeled with magnetic NPs. This technique uses a highly sensitive superconducting quantum inter-
ference device (SQUID) that only detected the antigen-antibody magnetic NPs. The NPs unlabeled
to the antigen were not detected because they relax rapidly by Brownian rotation after pulses of
magnetic fi elds were applied and do not contribute measurable signal. In this way, the ability to
distinguish between bound and unbound labels enables homogeneous assays to be run without the
need to separate and remove the unbounded particles.
Because of their unique physical (structural, electronic, magnetic, and optical) and chemical
(catalytic) properties, NPs and their biosensor applications can be used in a variety of ways. NPs,
including metals, semiconductors, and magnetic particles, have proved extremely useful in the
preparation of optical sensors, electrochemical biosensors of DNA, enzymes, and proteins. Electro-
chemical biosensors based on nanoparticles have been widely investigated, as summarized in many
elaborate reviews [321-324].
14.2.3.3
Nanoporous Materials in Biosensors
In addition to the nanotubes and NPs, novel materials such as porous silicon (PS) [325], porous
carbon [326], and porous Al
2
O
3
membranes [327] with pore size compatible with the dimension of
the chemical-biological agents have been used for biosensor applications.
PS is a well-characterized and versatile inorganic material produced through a galvanostatic,
chemical, or photochemical etching procedure in the presence of HF [328]. Depending upon the
etching conditions, PS has a very complex, anisotropic, and nanocrystalline architecture of high
surface area. Pore formation occurs in a unidirectional manner from the surface into the bulk,
leading to aligned pores and columnar silicon structures. The PS surface is hydrophobic and the
functionalization of the surface renders it as a biomaterial.
The immobilization of catalyst molecules within the pores involves the reaction of a silane-
coupling agent (3-aminopropyl) triethoxysilane with the PS surface (partially oxidized), followed
by the addition of a catalyst solution, forming the “microreactors.” The porous enzyme microre-
actors show a
350-fold increase in GOD activity over those fabricated with planar silicon sur-
faces [329] with the specifi c activity showing dependence upon the doping of the porous layer, the
anodization current, and the depth of the porous matrix.
Novel porous active carbon has shown to be a good matrix for the construction of highly
stable biosensors [330]. The high conductivity of this carbon material is ideal for the elec-
trochemical signal transduction; meanwhile, its high porosity allows the adsorption of large
molecules, such as polyelectrolyte-enzyme complexes without adverse effects on the activity
of the enzyme. And at the same time, the porous active carbon allows for high enzyme loading
without the need for any chemical treatment of the enzyme. This material has been used with
great success for the construction of highly stable and reproducible glucose and lactate biosen-
sors [330-332].
Porous alumina membrane is well-known as a template for the preparation of nanotubes,
nanofi brils, and nanowires of different materials. It can be also employed in the construction
of biosensors providing an available relatively high surface area for the retention of enzymes
or related bioactive compounds and allowing the development of biosensors that show a good
operational stability [327-334].
≦
