Biomedical Engineering Reference
In-Depth Information
Specific aggregate morphologies of a-syn and Ab are likely to be present in
CSF samples only at very low concentrations (nanomolar or less) and therefore
successful detection of these targets requires a biosensor with very low detection
limits. The first step to determine biosensor sensitivity is to determine suitable
electrical parameters which will enable detection of the bound target antigen
using electrochemical impedance spectroscopy. Since binding of antigen to
target takes place over a high density array of nanopores, the impedance signal
correlates to the average signal obtained over all the pores, ensuring that even if
some pores do not contain immobilized capture agent the measured impedance
will be reproducible within an acceptable margin of error. The dimensions
of the electrical double layer within the nanopores are approximately 50 nm.
An 100mV peak-to-peak pulse is utilized to characterize changes to the
capacitance of the electrical double layer induced by biomolecules binding. The
second parameter that needs to be defined is the sensing frequency. Since
double layer capacitance dominates the impedance spectrum for frequencies
less than 1 kHz, in order to determine the optimum frequency for these studies,
the frequency response is recorded by adding a range of monomeric target
antigen concentrations to immobilized antibody using a frequency range from
50Hz to 1MHz. A frequency of 100Hz gave maximum visible shifts in the
impedance induced by biomolecule binding (Figure 5.8).
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Using the defined
d
n
4
t
3
n
g
|
2
n
3
.
Figure 5.8
Choosing the most discriminating frequency value for maximum sensi-
tivity detection from a range from 50Hz to 1MHz: the inset shows a
zoomed spectrum for various doses at relevant frequencies. Because
100Hz showed maximum separation shifts between the spectra for indi-
vidual doses in the impedance changes induced by biomolecular binding, it
was chosen as the sensing frequency.
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(Reprinted by kind permission of the Royal Society of Chemistry.)
