Chemistry Reference
In-Depth Information
functions of human body at low levels, over-exposure to these ions can be detri-
mental. For example, iron is a critical component of the transportation system
for oxygen and electrons in the human body, and is tightly associated with many
enzymes. Defi ciency in iron leads to anaemia, which is especially harmful to preg-
nant women.
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On the other hand, it has also been shown that excess 'free iron' in
the human body can produce oxidative stress, and is involved in many pathological
manifestations (arteriosclerosis, diabetes, aging, etc.).
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Therefore, it is clearly benefi -
cial to be able to monitor levels of different metal ions as part of disease diagnoses
or treatment plans. In a related issue, since metal ions can enter the human body
through contact with the environment, for example drinking water, it is essential to
monitor and control metal ion levels in these sources to reduce metal-related health
risks.
Laboratory techniques, such as atomic absorption spectrometry, inductively
coupled plasma mass spectrometry (ICP-MS), anodic stripping voltammetry, kinetic
phosphorescence analysis, X-ray fl uorescence spectrometry and microprobes, have
been routinely used for metal ion analysis with high sensitivity. Many of them can
simultaneously quantify multiple metal ions. However, most of the above techniques
require sophisticated equipment and sample pretreatments, or skilled operators,
making it diffi cult and quite costly for on-site, real-time monitoring. To circumvent
these problems, DNAzymes have been developed as metal sensors because of their
metal - ion - dependent catalytic activities.
DNAzymes have many advantages as metal ion sensors compared to many
conventional metal detection techniques. First, DNAzyme sensors can reduce the
requirement for complex and expensive instruments, and can be built into a portable
package for on-site monitoring. Second, DNAzymes are selective, due to the depend-
ence on specifi c metal ion cofactors. Their catalytic activities can be readily engi-
neered to generate and amplify detectable signals for sensitive analysis. DNA-based
sensors are also environment-friendly by being biocompatible and biodegradal, and
should pose minimum toxic effects on the human body. Compared to antibody- and
RNA-based sensors, DNAzymes are more resistant to hydrolysis and stable for
long-term storage and application. They can even be denatured and renatured for
many cycles without losing much of the activitiy. Lastly and perhaps most impor-
tantly, DNAzymes provide a common platform for developing sensors for new metal
ions of interest or even targets beyond metals. Once the strategy of signal transduc-
tion to convert the catalytic activity to measurable signals is developed, the same
design can be readily applied to another target as soon as a DNAzyme is selected
for the target. This fl exible platform is expected to reduce the cost of developing
sensors for different targets in large quantities, and make these sensors more suita-
ble for parallel and multiple detection of complex samples in an array format.
14.5.1 Fluorescence - Based DNA zyme Sensors Based on Catalytic Beacons
A fl uorescence - based lead sensor was initially constructed based on the ' 8 - 17 '
DNAzyme (Figure 14.6 ).
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Fluorescence was employed because of its proven high
sensitivity. An enzyme DNA strand and a substrate strand were hybridized to form
