Biomedical Engineering Reference
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Fig. 2.5
(
a
) Scheme of the colorimetric detection of a specific target by activation of an
allosteric DNAzyme followed by rolling-circle amplification, PNA, and DiSC2(5) colorimetry
(Reproduced from Ref. [
74
] by permission of John Wiley & Sons Ltd.). (
b
) Aptamer HCR
system. Upon the binding of ATP to its aptamer, the exposure of sticky end triggers the further
HCR mechanism to form a long nicked double helix [
78
]. (
c
) Amplified analysis in which target
DNA triggered two functional hairpins to form nanowires consisting of multiple Mg
2C
-dependent
DNAzymes (Reprinted with the permission from Ref. [
79
] Copyright 2011 American Chemical
Society). (
d
) Scheme of target DNA triggering two functional hairpin structures to the generation
of nanowires consisting of the HRP-mimicking DNAzymes for the colorimetric signal output
(Reprinted with the permission from Ref. [
80
] Copyright 2012 American Chemical Society)
acid (PNA), which is complementary to RCA products, and an organic dye, which
could bind to a DNA/PNA duplex with changed color (Fig.
2.5
a). By incorporating
the complementary sequence of G-quadruplex DNAzyme into the RCA circular
template, the products of target-induced RCA reaction would have many G-
quadruplex repeating units, which, in the presence of hemin, have peroxidase
activity to catalyze the oxidation of ABTS
2
by H
2
O
2
to generate colorimetric
signal [
75
,
76
]. By immobilizing the RCA products on electrode surface with
the accumulation of electrical indicators on DNA, the amplified electrochemical
detection of the PDGF protein has also been realized with high sensitivity and
selectivity [
77
]. Different from enzymatic RCA, HCR has been first reported by
Pierce's group [
78
] as an enzyme-free signal amplification mechanism. In Fig.
2.5
b,
two stable sets of DNA hairpins coexist in solution until the introduction of initiator
strands triggers a cascade of recognition and hybridization events, yielding nicked
long double helices. By elongating hairpin sequences with DNAzyme sequences,
after the triggered assembly, amplified fluorescence [
79
] or colorimetric detection
[
80
] of various targets can be realized (Fig.
2.5
c-d).
When sensing happens at solid surfaces, a critical challenge is low efficiency
of molecular recognition compared with that in solution, which might be due to
the reduced accessibility of targets to probes on a heterogeneous surface. Yan's
group [
81
] proposed the strategy of molecular recognition process in solution
and signal collection on surface by assembling recognition elements on DNA 2D
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