Chemistry Reference
In-Depth Information
Figure 14.9
(Left) Secondary structure of the 8-17 DNAzyme for lead ion detection; (Right)
dynamic ranges of the colorimetric sensors with different enzyme compositions. (Reprinted
with permission from
J. Am. Chem. Soc
., 2003,
125
, 6642-6643. Copyright 2003, American
Chemical Society.)
In the fi rst generation colorimetric sensor for lead, an additional heating and
cooling step was needed before the DNA-functionalized AuNPs could be effectively
crosslinked by the extended substrate strand. In fact, detection of lead ions was also
conducted after this annealing step, where absence of lead produced a blue product
and lead ions gave a red colour. This problem most likely arose from the alignment
of the AuNPs. Based on the design, the substrate hybridized at both ends with two
AuNPs tagged with the same cDNA sequence. This means these two AuNPs had to
take a head-to-tail alignment, which induced a signifi cant steric effect to prevent
the crosslinking. To solve this problem, a tail-to-tail design was adopted and aggre-
gation of AuNP was observed without the annealing step.
81
Additionally, the size of
the AuNP was changed from 13 nm to 42 nm, which greatly reduced the time needed
to see a colour change to 5 min.
81
Later, a true ' turn - on ' colorimetric sensor was
developed in which addition of lead ions could directly break up preaggregated
AuNPs to produce a distinctive blue-to-red colour shift.
82,83
To accelerate Pb
2+
-
induced disassembly, small pieces of DNA (called invasive DNA) complementary
to the cleaved substrate strands were employed to facilitate AuNP release. Under
optimized conditions, a blue-to-red colour shift was observed in 5 min at room tem-
perature in the presence of Pb
2+
.
82
Recently, we further optimized the disassembly
conditions by designing asymmetric substrate binding arms so that disassembly
occurred right after cleavage without the use of invasive DNAs.
83,84
14.5.3 Other DNA zyme - Based Metal Ion Sensors
The DNA backbone of the DNAzymes allows convenient modifi cations to the
enzyme to tag a signalling moiety, facilitate immobilization of the enzyme or add
additional functionality. Therefore, a variety of other detection strategies can be
developed for metal ion sensing based on DNAzymes. In one such attempt, Plaxco
and coworkers immobilized the lead-dependent 8-17 DNAzyme onto a gold elec-
trode through the 5
′
-end of the enzyme strand and attached a methylene-blue (MB)
- end.
85
The DNAzyme remained rigid when hybridized with the
substrate strand, separating the MB group from the surface and giving a low rate
of electron transfer. In the presence of Pb
2+
, the substrate was cleaved and released,
group to the 3
′
