Chemistry Reference
In-Depth Information
Figure 14.8
DNAzyme and AuNP-based colorimetric Pb
2+
detection: (A) Pb
2+
-directed
assembly of DNAzyme-linked AuNPs aligned in a head-to-tail manner; (B) colour of the
AuNPs in the presence of different divalent metal ions. (Reprinted with permission from
J. Am. Chem. Soc
., 2003,
125
, 6642-6643. Copyright 2003, American Chemical Society.)
substrate sequence, leaving a broken linkage between the AuNPs. Consequently, the
dispersed AuNPs displayed a deep red colour due to a shifted absorption to around
522 nm. With increasing level of lead ions, a blue-to-red colour shift was clearly dis-
cernible by the naked eye. The detection of lead ions could also be realized more
accurately by UV/Vis spectroscopy measurements. The ratio of extinction at 522 nm
over 700 nm was used to quantify Pb(II) since it rose with increasing concentration
of lead ions. A detection limit of
∼
100 nM was achieved and the sensor was able to
detect lead extracted from paint.
An important benefi t of the AuNP-based DNAzyme sensor is the easy imple-
mentation of tuneable dynamic ranges. A sensor design that allows simple tuning
of dynamic range is highly desirable to match the requirements of different applica-
tions. For example, the US EPA defi nes the toxic threshold for lead in paint as
∼
2 mM, while that in drinking water is 72 nM. Additionally, regulations on sensitivity
of metal ion detection can change over time. It is often not economical or even
practical to redesign and remake new sensors to meet these regulation changes, thus
giving extra signifi cance to tuneable sensors in practical applications. Taking advan-
tage of the knowledge obtained from biochemical assays and the multiple turnover
property of the DNAzyme, we have demonstrated a general scheme for tuning the
dynamic range of the colorimetric Pb
2+
sensor system.
79
DNAzyme activity was
abolished by changing the G•T wobble pair to a G-C Watson-Crick pair (Figure
14.9 , left).
80
This mutated DNAzyme could still assemble AuNPs similarly to the
native DNAzyme. When a ratio of 5 : 95 was adopted for the active enzyme (17E)
versus inactive enzyme (17Ec), the Pb
2+
-sensitive range shifted to
one order of
magnitude higher Pb
2+
concentrations (Figure 14.9, right). This tuning property is
unique and useful because it allows detection of Pb
2+
over a wide concentration
range without redesigning the sensor.
∼
