Colorimetric Bioassay Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 49

the aggregation of the Au NPs. 87 This approach provides an LOD of
500 nM for thrombin.
The detection of Pb 2+ ions is of great importance to human
health and for environmental monitoring. Recently, researchers have
developed several varieties of DNAzyme-based analytical methods
for Pb 2+ detection. DNAzymes are small catalytic DNA molecules
that eficiently cleave target RNA molecules. The catalytic activity
of some DNAzymes is speciic toward divalent metal ions — just
as the catalytic activity of some protein enzymes is dependent on
metal ion cofactors. 97 Taking advantage of the high speciicity of
17E DNAzyme for Pb 2+ , highly sensitive and selective colorimetric
sensors have been developed for Pb 2+ . Lu and coworkers recently
developed a strategy based on 17E DNAzyme, involving Au NPs as
the sensing elements. 98,99 The DNAzyme molecules are modiied
onto the Au NPs and then hybridize with the substrate to form duplex
DNA. The dissociation of DNA strands from the Au NP surfaces
results in Au NP aggregation at a high salt concentration because
the unfolded single-stranded DNA can be adsorbed on the citrate-
protected Au NPs while double-stranded DNA (dsDNA) cannot. Thus,
unfolded ssDNA stabilizes the Au NPs in the presence of a given high
concentration of salt while dsDNA cannot, and the Au NPs aggregate
in the presence of the same concentration of salt. A similar strategy
has been developed for Pb 2+ using unmodiied Au NPs. 100 Initially,
17E DNAzyme is hybridized with its substrate to form the duplex. In
the presence of Pb 2+ , the substrate strand is cleaved, resulting in the
release of fragments that are adsorbed on the Au NP probes, leading
to stable Au NPs in the presence of a given high concentration of
salt (the red solution appears). In the absence of Pb 2+ , however, the
remaining duplex cannot be adsorbed on the Au NP probes, leading
to the formation of Au NP aggregates (see Fig. 3.11). The detection of
Pb 2+ can be realized within 20 min with a detection limit of 500 nM.
Figure 3.11 Strategy for Pb 2+ detection using Au NP colorimetry.