Biomedical Engineering Reference
In-Depth Information
thickness of DI and G6PDH layers were estimated to be 5.8 and 5.0 nm, and these estimates
are close to the enzyme diameters (DI: 6.4 nm and G6PDH: 6.3 nm), which were calculated
from molecular weight by assuming the enzymes as being rigid, spherical molecules [15].
Lvov et al. [4] reported that the thickness of most of the protein layers was from 22 to 57 nm.
This suggests that most of the enzymes are assembled into monolayers. However, typically,
immobilization of enzymes is very complex, as pointed out by Lvov, and in some cases, the
aggregation of enzymes during the LBL procedure resulted in a multilayer conformation in
one adsorption step [4]. In our work, QCM data showed that monolayer enzyme adsorption
occurred, if the interface was considered to be a plane after immobilization [19].
8.3.2
Atomic Force Microscopy
To identify the immobilization by LBL and clarify the surface smoothness after alternate
adsorption, the interface was viewed by AFM, and observed images are shown in Figure 8.3.
Figure 8.3A is the image of a bare gold electrode surface, and Figure 8.3B is the image of the
surface after alternate adsorption in PEI and DI solution. A relatively smooth surface is
found on a bare gold electrode, and after one bilayer immobilization, it has been changed to
a rough surface, which confirmed that the LBL absorption steps resulted in enzyme
aggregation.
8.4
Application of Layer-by-Layer Adsorption for Biosensors
Organized multilayer architectures containing proteins or other biomolecules have
attracted great interest because of their broad range of applications in biotechnology, espe-
cially in bioanalytical areas such as biosensors [31].
Enzyme immobilization on electrode surface is one of the most important factors in the
design and construction of biosensors because the immobilization procedure should not
(A)
(B)
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FIGURE 8.3
Atomic force microscopy (AFM) images of bare Au electrode (A) and after one bilayer adsorption of PEI/DI on
Au electrode first assembled by 3-MPA. (From Zheng, H., Okada, H., Nojima, S., Suye, S., Hori, T. (2004) Layer-
by-layer assembly of enzymes and polymerized mediator on electrode surface by electrostatic adsorption.
Sci. Technol. Adv. Mater. 5:371-376.)
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