Biomedical Engineering Reference
In-Depth Information
electrical parameters, a calibration curve was generated using different
concentrations of target antigen molecule using two controls; bovine serum
albumin (BSA) was added to the immobilized antibody to test for nonspecific
binding to nanobody, and target antigen without immobilized antibody was
used to measure background binding to the sensor surface. From the cali-
bration curve, it is possible to accurately detect antigen down to a limit of
detection of 1 picogram mL 1 indicating that it should be possible to detect low
femtomolar concentrations of target antigen in clinical patient samples.
These studies suggest that label-free detection of specific oligomeric
aggregate species holds great promise for use as sensitive biomarkers for
neurodegenerative disease.
d n 4 t 3 n g | 2
5.2.4 Optical Sensing Platforms
Optical biosensors based on whole cell sensing are gaining widespread use in
cellular research largely as a result of a recent paradigms shift in drug discovery
from the target-directed approach to the system biology centered strategy.
A major technique rapidly growing in interest with respect to the detection of
cellular interactions is surface plasmon response (SPR). This method was
introduced in Chapter 1 and is summarized briefly here. The technology
exploits evanescent waves to characterize molecular or cellular interactions on
the sensor surface. SPR instruments incorporate a prism to direct a wedge of
polarized light, covering a range of incident angles, into a planar glass substrate
covered with an electrically conducting metallic film to excite surface plasmons.
The resulting evanescent wave interacts with, and is absorbed by, the electron
clouds in the metallic layer, generating electric charge density waves
(the surface plasmons), and thus causing a reduction in the intensity of the
reflected light. The resonance angle at which this intensity minimum occurs is a
function of the refractive index of any biological moiety present at the sensor
surface (Figure 5.9(a)). As outlined in Chapter 1, interactions at the sensor
surface cause shifts in the resonance angle, which is depicted again in
Figure 5.9(b).
In applications involving the label-free measurements of neural electrical
activity, the method, unlike electric detection, is artifact-free. Electrical
recording involves stimulation, which generates artifacts in the detecting signal.
Furthermore, voltage-sensitive fluorescent dyes are expensive, toxic, involve
time-consuming labeling procedures and are affected by photobleaching. From
an optical sensing point of view, changes in scattering and birefringence of a
nerve can be correlated with neural activity. When an action potential
propagates through an axon in the nerve, reorientation of molecular dipoles
across the membrane occurs altering the refractive index of the axon
membrane. In addition, action-potential propagation produces an osmolality
difference across the membrane, which in turn leads to cellular swelling, an
increase in the cell volume through the influx of water molecules. These
alterations in the refractive index and microanatomy of the nerve result in
optical scattering changes. It should be noted that the birefringence change
n 3 .
 
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