Biomedical Engineering Reference
In-Depth Information
to nanomaterial surfaces reduces protein unfolding, resulting in enhanced stability of the
enzyme attached to the nanoparticle surface. Furthermore, for nanometals or metal oxides,
their unique ability is to promote faster electron transfer between the electrode and the active
site of the desired enzyme. In addition to the promising performance features, the unique
solution behaviors of the nanoparticles also point to an interesting transitional region
between heterogeneous and homogeneous catalysis. Theoretical and experimental studies
demonstrated that particle mobility, which is governed by particle size and solution viscos-
ity, could impact the intrinsic activity of the particle-attached enzymes [12].
As mentioned above, nanoparticles are desirable from several perspectives. However,
their dispersion in reaction solutions and their subsequent recovery for reuse are often
found to be a daunting task. It appears that the use of nanofibers would overcome this
limitation while providing the advantageous features of nanosize materials. The collection
of randomly arrayed nanofibers usually forms a nonwoven mesh (or membrane) with
reusability. Thus enzyme-immobilized nanofibrous membranes have functions of bioca-
talysis and separation simultaneously, which is generally accepted as the fundamental
requirement for the enzymatic membrane-bioreactor. Applications in biosensors and bio-
fuel cells are also allowed for these nanofibrous membranes. Compared with other nano-
structured supports, nanofibrous supports show many advantages for their high porosity
and interconnectivity. Electrospun nanofibers have been proven to be excellent supports
for enzyme immobilization [13]. However, studies of this issue are still limited to a small
number as there are still problems in their large-scale application.
Carbon nanotubes (CNTs), discovered in 1991 by Iijima, represent an important group of
nanoscale materials. CNTs can display metallic, semiconducting, and superconducting
electron transport, possess a hollow core suitable for storing guest molecules, and have the
largest elastic modulus of any known material. Basically, there are two groups of CNTs,
multiwall carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs).
An MWCNT is a central tubule of nanometric diameter surrounded by graphitic layers
separated by ca. 34 nm, and an SWCNT is only the tubule and no graphitic layers. CNT-
based electrochemical transducers offer substantial improvements in the performance of
amperometric enzyme electrodes, immunosensors, and nucleic acid sensing devices. The
greatly enhanced electrochemical reactivity of hydrogen peroxide and NADH at CNT-
modified electrodes makes these nanomaterials extremely attractive for numerous oxi-
dase- and dehydrogenase-based amperometric biosensors [14].
Enzyme encapsulation via the sol-gel approach has been one of the most popular meth-
ods for enzyme immobilization since the first report by Avnir et al. [15]: that enzymes
encapsulated into sol-gel matrices maintained their activities. In a typical synthetic proto-
col, tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) is hydrolyzed into “sol,” and
the addition of enzyme solution into “sol” initiates condensation reaction leading to “gel,”
where enzymes are encapsulated into silicate matrices. In this approach, various pores
and channels are formed in the final silicate matrices, ranging from 0.1 to 500 nm in size.
A careful optimization process is required in order to prevent the leaching of encapsulated
enzymes. Once enzyme leaching is prevented, the sol-gel approach results in a fairly sta-
ble form of enzyme immobilization since the close fit of the enzyme molecule within the
sol-gel pore likely prevents unfolding and denaturation of encapsulated enzymes [10].
However, due to the small pore size, the immobilized enzymes usually show lower activ-
ity than the free enzymes. Furthermore, the nonuniform pore sizes of most silica gel sup-
ports made processes less reproducible.
Each nanomaterial has unique advantages as well as disadvantages. Thus other important
support composites were attracting more attention. Composites are engineered materials
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