Biomedical Engineering Reference
In-Depth Information
◄
Fig. 5.9 a
Schematic diagram of the process to fabricate the energy transfer-controlled PEC
system.
b
Schematic mechanism of the operating PEC system. Process 1, photoexcitation of the
CdS QDs; 2, photon absorption and electron transfer from the valence band (
VB
) to the con-
duction band (
CB
); 3, hole neutralization by electron donor; 4, electron ejection to the electrode
for photocurrent generation; 5, nonradiative electron-hole recombination; 6, radiative elec-
tron-hole recombination; 7, spontaneous emission originating from radiative decay; 8, plasmon
enhancement on radiative decay; and 9, exciton energy transfer (
EET
) from CdS QDs to Au NPs.
c
Photocurrent intensity in 0.10 M PBS containing 0.08 M ascorbic acid of (
a
) CdS/ITO elec-
trode modified with 20 mL, 1 mM capture DNA and blocked by MEA, and after hybridization
with (
b
) Au NP-labeled target DNA, (
c
) bare target DNA, and (
d
) SiO
2
NP-labeled target DNA.
d
Effect of different concentrations of target DNA on the differential photocurrent responses.
Inset
the corresponding calibration plot (
Δ
I
=
I
0
−
I
,
I
0
and
I
are the photocurrents of the capture
DNA/CdS/ITO electrode prior to and after hybridization). The working potential was 0.0 V, and
the excitation wavelength was 420 nm. Reproduced with permission from Ref. [
57
]. Copyright
2011, Royal Socienty of Chemistry
complementary to a nucleic acid (3) that is being released by the machine. Upon
the hybridization of (2) with the region (I) of (1) and the subsequent addition of
polymerase, the dNTPs mixture, and the nicking enzyme Nb. BbvCI, polymeriza-
tion of the complementary strand to (1) is initiated. The replication of domain II
yields, however, the key for the machine operation since the generated strand in
the duplex of region II includes the nicking site for the enzyme. Thus, while the
initial replication of the template proceeds, the nicking process establishes a new
polymerization site. The subsequent polymerization involves the displacement of
the originally replicated strand (3). Thus, the replication, nicking, and displace-
ment of (3) proceed autonomously, and the displaced product (3) may be viewed
as a “waste product.” The “waste product” (3) is then used as the bridging unit
for the assembly of the CdS nanoparticles. CdS nanoparticles were modified with
the thiolated nucleic acid (4) that is complementary to the 5-end of (3) Fig.
5.8
b.
The Au electrode was functionalized with the thiolated nucleic acid, (5), that is
complementary to the 3-end of (3). The initiation of the machine operation in the
presence of the (5)-modified electrode results in the hybridization of (3) with the
electrode and the subsequent hybridization of the (4)-modified CdS nanoparticles
with the modified electrode, Fig.
5.8
b. Thus, the photoelectrochemical response of
the machine may act as a transduction signal for the time-dependent assembly of
the CdS nanoparticle aggregates.
The above works in this field are exclusively based on the change in direct
electron transfer process between the photoactive materials and the ambient envi-
ronment prior to and after the biorecognition events. Chen et al. present the first
exploitation of energy transfer between CdS QDs and Au NPs in a PEC system
to search for an advanced energy transfer-based PEC bioassay protocol [
57
]. As
shown in Fig.
5.9
a, the new energy transfer-based PEC detection format involved
the modification of an indium tin oxide (ITO) electrode with CdS QDs, followed
by the integration of Au NPs into the system for sensitive DNA detection, on the
basis of the interparticle energy transfer between CdS QDs and Au NPs bridged
by the biorecognition of DNA. As shown in Fig.
5.9
b, photoexcitation of the CdS
