Biomedical Engineering Reference
In-Depth Information
Filtration of
GOD (12.5
1% chitosan
(protective layer)
M) +
0.1% chitosan
µ
Al
2
O
3
/GOD/Chit
membrane
Al
2
O
3
membrane
GOD
Al
2
O
3
Chitosan network
Cross-sectional view of the
Al
2
O
3
/GOD/Chit membrane
FIGURE 14.9
Immobilization of GOD in nanoporous alumina membranes. (From Darder, M., Aranda, P.,
Hernández-Vélez, M., Manova, E., and Ruiz-Hitzky, E.,
Thin Solid Films
, 495, 321, 2006. With permission.)
Darder et al. [335] (Figure 14.9) reported a glucose biosensor employing porous alumina mem-
brane as a matrix to immobilize GOD. In their work, nanoporous alumina membranes of different
dimensions were made by electrochemical oxidation of aluminum in an acidic solution; these mem-
branes were used to encapsulate GOD by procuring an external coverage with a thin layer of the
biopolymer chitosan, which avoids enzyme leaching. The enzyme-modifi ed membranes were then
attached to the surface of a Pt electrode for the biosensor construction. The infl uence of membrane
dimensions on the biosensor response was also studied.
14.2.4 F
UNCTIONALIZED
M
ONOLAYERS
Almost any surface can be modifi ed by functionalized monolayers, which possess the desired
specifi c electrical, optical, or chemical property. An ideal monolayer is described as perfectly
aligned and closely packed alkane chains attached to a smooth surface. There are two kinds of
functionalized monolayers, namely, LB membrane and SA monolayers. Such molecular assemblies
have been demonstrated to be adapted for biosensor applications for the immobilization of biomol-
ecules in a biomimetic environment.
14.2.4.1 Langmuir-Blodgett Membranes
The LB method is a powerful technique for transferring the Langmuir fi lms at the air-water interface
to a solid substrate. The Langmuir fi lm is based on the particular properties of organic amphipha-
thic molecules such as lipids, phospholipids, or glycolipids. These organic molecules are composed
of two distinct molecular regions: a hydrophilic (''water-loving'') head group and a hydrophobic
(''water-hating'') tail group, which orient themselves at an air-water interface between the gas-
eous and the liquid phases, minimizing their free energy and forming an insoluble monolayer. For
biological components, the tail groups are most often composed of one or two long alkyl chains.
The biosensors based on LB technology use an active fi lm consisting of monolayers of an amphi-
philic molecule in which the biomolecule is incorporated, and then the monolayer is transferred on
a solid substrate to form one, two, or more mixed layers. The major interest in this technique is to
create an organization of two components at the molecular scale; the very small thickness of such
structures leads to biosensors with short response times.
The functionalization of LB fi lms, conferring biospecifi city, can be obtained by association (or
insertion) of proteins, such as enzymes [336], antibodies [337], or specifi c ligands, presenting specifi c
recognition properties. Insertion of enzyme molecules in LB fi lms has been reported extensively
