Biomedical Engineering Reference
In-Depth Information
QDs (process 1) would result in the electron transfer from the valence band (VB)
to the conduction band (CB) (process 2), thus yielding electron-hole pairs. As
soon as the charge separation occurs, the electron-hole pairs would be destined
for recombination (process 5 or 6) or the charge transfer (such as process 3 and 4).
The emission of the CdS QDs was used to excite the surface plasmon resonance
(SPR) of the proximal Au NPs (process 7), which would create local electric fields
that in turn modulate the exciton states in the CdS QDs by enhancing the radiative
decay rate (process 8). Along with the SPR effect, Au NPs would also introduce an
additional nonradiative decay route for electron-hole recombination in CdS QDs
(process 5) with exciton energy transfer (EET) from the CdS QDs to the Au NPs
(process 9). Processes 5 and 6 are cooperative, and their overall effect contends
with the electron transfer of process 4, so the concern here is their overall effect
on the excitonic response of the CdS QDs, and hence, on the final photocurrent
intensities, that could facilely be monitored by electrical signal. The photocurrent
decrease was proportional to the DNA concentration logarithmically with the lin-
ear range from 5.0
×
10
−
15
M to 5.0
×
10
−
12
M (
R
2
=
0.9858) and detection limit
of 2.0
×
10
−
15
M. Comparing with gold, silver has a dielectric function,
ε
Ag
(
ω
),
and a stronger plasmon resonance. Plasmon band of Ag NPs fully overlaps with
the absorption band of utilized CdS QDs Ag-NPs-based assemblies can demon-
strate enhanced properties suitable for optical and sensor applications [
58
]. Due
to their natural absorption overlap, the exciton of the QDs and the plasmon of Ag
NPs could be induced simultaneously. The EPI resonant nature enabled manipu-
lating photoresponse of the QDs via tuning interparticle distances (Fig.
5.10
).
Specifically, the photocurrent of the QDs could be greatly attenuated and even be
completely damped by the generated EPI. The photocurrent decrease was propor-
tional to the concentration of labeled target DNA in logarithmic scale with the lin-
ear range from 2.0
×
10
−
15
to 2.0
×
10
−
11
M.
In order to produce photocurrents, photoexcitation of light source is usually
required. However, the appendant light source makes the instrument compli-
cated. In addition, the exciting wavelengths of various photoelectroactive materi-
als are different. Hence, monochromator is needed to bring appropriate exciting
light, which makes the volume of the instrument bigger and departures from the
portable trend for biosensor. Consequently, a strategy for substitution of physical
light source is highly deserved [
59
]. Chemiluminescence is defined as a process in
which excited molecules or atoms generated from chemical reactions release the
excess of energy in light form. Different CL systems can bring emission light of
various wavelengths. At the same time, reaction conditions such as type of fluores-
cence reagent and reaction solution can also affect the emission wavelength. Thus,
by adjusting the conditions of CL reaction, various photoelectrochemically active
species can be excited theoretically, which can realize the photoelectrochemi-
cal detection free from physical light source. The photoelectrochemical analysis
of a DNA analyte without an external irradiation of the QD-modified electrode is
depicted in Fig.
5.11
a. The CdS QDs linked to the electrode were functionalized
with BSA units, and the thiolated nucleic acid probe, 3, that is complementary
with the 5′-end of the analyte, 4, was tethered to the BSA layer. The measurement
