Biomedical Engineering Reference
In-Depth Information
Fig. 9.10
Electrically
controlled fluorescent in a
hybrid device
a
b
-
F
F
+
-
DNA
-
+
negative
positive
tilted. As a result, the nonradiative energy transfer from the dye molecule to the
metal modulates the fluorescence intensity F according to the relation
F
/
.L
DNA
sin /
3
;
(9.1)
so that strong fluorescence is observed for the upright orientation and quenching
occurs for the tilted DNA molecule. In (
9.1
), is the number of molecules per
area, L
DNA
is the effective length of DNA, and is the angle between the molecule
and the surface, which takes values of 90
ı
and 25
ı
for the upright and tilted DNA,
the fluorescence decreasing ten times in the latter case compared to the former
situation. Fluorescence modulation by an applied bias can only occur for sufficiently
low packing densities of DNA molecules on the surface, such that these molecules
can gyrate freely when the polarity of the electric field reverses. Fluorescence
modulation can follow the electrical excitation for a large number of cycles only
up to frequencies of 10 kHz, for higher frequencies the DNA molecule adopting an
intermediate position between standing and lying. Although the device can work
with both ssDNA and dsDNA, a higher cutoff frequency is observed in the latter
case due to an increase in stiffness. The device works well in a solution with salt
concentration of less than 10 mM since for higher concentrations, the dissolved ions
in solution screen the electrode charge preventing the development of a high electric
field at the surface of the gold electrodes.
Biological molecules are also used in miniature biofuel cells, as detailed in
Heller
(
2004
), consisting of carbon fibers with a diameter of 7m, each coated
with a distinct bioelectrocatalyst enzyme. Such cells deliver a power of 4:3W
when operated at 0.52 V in a physiological buffer solution at 37
ı
C. Biofuel
cells containing single-walled CNTs wrapped with DNA as bioelectrodes, glucose
oxidase as the anode system, and laccase as cathode could even be implanted in
vivo for power implants, for example (
Lee et al. 2010
). Glucose oxidase binds on
the bioelectrodes by covalent bonds. This device delivers a maximum output power
of 442Wcm
2
at a voltage of 0.46 V maintaining almost the same power level for
5 days. Biofuel cells can become an alternative for the conventional fuel cell since
they can harvest energy from unusual sources, such as effluents of animal processes
or carbohydrates from plants, or from unusual places. For example, microbial fuel
cell can harvest energy from the interface between marine sediments and seawater.
A review on microbial and enzymatic biofuel cells can be found in Calabrese
Calabrese Barton et al.
(
2004
).
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