Biomedical Engineering Reference
In-Depth Information
well as the sensitive monitoring of the metal ions released upon acid dissolution
of the QD labels, low detection limits of 30 and 50 nM were obtained for ATP and
cocaine, respectively, in these assays. The high specificity to target analytes and
promising applicability to complex sample matrix made the proposed assay proto-
col an attractive route for screening of small molecules in clinical diagnosis.
5.2 QDs for Photoelectrochemical Analysis
Rapid, specific detection of nucleic acid sequences has attracted significant atten-
tion due to possible applications in fields ranging from pathogen detection to the
diagnosis of genetic diseases. Among various detection techniques, photoelec-
trochemical detection has attracted significant interest. Firstly, this method is
very sensitive with low background signals due to the different forms of energy
for excitation (light) and detection (current) [
45
-
47
]. Secondly, compared to
optical detection methods such as fluorescence, chemiluminescence (CL), and
ECL, which use complex and expensive optical imaging devices, the instrument
of photoelectrochemistry is much simpler and of low cost [
48
]. QDs, with their
unique fluorescence properties and photoelectrochemical functions, are photoac-
tive materials for the development of nucleic acid sensor systems. A competitive
DNA hybridization assay based on the photoelectrochemistry of the semiconduc-
tor quantum dot-single stranded DNA conjugates (QD-ssDNA) was developed by
Deniz et al. [
49
]. A sensing surface is constructed by a self-assembled monolayer
(SAM) formation on ITO surface, activation of surface for immobilization of the
amine-modified ssDNA (probe), and then immobilization of probe on activated
surface. After obtaining sensing surface, competitive DNA assay was performed
on the probe-immobilized surface and the target concentration in the sample was
determined based on photocurrent measurements. As seen in Fig.
5.5
, a current
change, photocurrent, was observed in anodic direction when the light source
was turned on, and the system immediately turned to its initial state after the light
source was turned off. Upon the competition between QD-ssDNA and single-
stranded target DNA, the photocurrent response decreased with the increase in the
target DNA concentration. A linear relationship between the photocurrent and the
target DNA concentration was obtained (
R
2
=
0.991), and limit of detection was
found to be as 2.2
μ
M target ssDNA. The selectivity of system toward the target
DNA was also demonstrated using noncomplementary sample.
For better sensitivity, the amplified detection of DNA was accomplished by
developing a novel architecture of double-stranded DNA-cross-linked CdS nano-
particle arrays on electrode supports and the structurally controlled generation
of photocurrents upon irradiation of these arrays [
50
]. The electrostatic binding
of [Ru(NH
3
)
6
]
3
+
to the dsDNA units provides tunneling routes for the conduc-
tion-band electrons and thus results in enhanced photocurrents. CdS nanopar-
ticles (2.6
±
0.4 nm) were functionalized with thiolated oligonucleotide 1 or 2.
These two oligonucleotides are complementary to the 5′- and 3′-ends of the target
