Single Molecule Detection with an Aptamer-Integrated Nanopore - Nanopores: Sensing and Fundamental Biological Interactions - page 69

high affinity (7.4 nM) for A ricin ￿ricin [ 39 ]. By determining the interval between the
first and second block, Dt 12 with the ricin concentra-

tion (Fig. 3.8c ), we obtained the calibration curve:

Dt 12 ¼ t 2 t 1 , and correlating

Dt 12 decreased from 35 to
2.6 min as the ricin concentration increased from 2.8 nM to 2.8

m M.

3.4.6 Advantages and Perspectives

The aptamer-encoded nanopore demonstrates a series of advantages over other
synthetic nanopore systems. The first is the selective detection. The aptamer-
encoded nanopore can distinguish between transient current blockades caused by
non-specific molecules passing through the nanopore and much longer blocks
resulting from the target binding. The digital signal produced by individual target
molecules (stepwise blocks, Figs. 3.7 and 3.8 ) distinguishes them from the analog
background signal. This is particularly useful in real-time detections, when the
background current dynamically drifts or fluctuates with the environment, one is
still able to identify discrete single molecule events, thus greatly enhancing the
signal/noise ratio. The same important characters also include simple, rapid and
label-free detection with high sensitivity.

3.5 Conclusions

The sophisticated aptamers can be integrated with the sensitive nanopore single
molecule detectors, to explore life science problems that have great implications in
biomedicine, biotechnology and biodefense. These problems range from dynamic
structures of aptamers such as folding/unfolding processes triggered by the environ-
mental factors, to the utilization of aptamers for ultrasensitive bio-detection, such as
clinical biomarkers and bioterrorist materials (because of their importance to homeland
security). The potential of aptamer-encoded nanopore sensors is broad. They can be
applied to medical diagnoses, quantifying biomarkers in blood samples, and used for
environmental monitoring, detecting pollution or contaminating materials in water.
It depends on the aptamer development. If the aptamer is ready, one can use it for just
about anything. Using aptamers is also advantageous because they are more durable
than most protein receptors, resisting most denaturing and degrading conditions,
including immobilization; yet, they are simpler to synthesize, modify, and immobilize
using low-cost methods. The affinity and specificity of aptamer-target interactions can
also be fine-tuned through rational design or molecular evolution. Having already
created a nanopore detector for IgE and ricin, we now aims to devise a nanopore sensor
that will use an aptamer to detect anthrax spores - well known as a bioterrorist threat.
Scientists already have developed anthrax aptamers [ 14 ], paving the way for a
nanopore sensor.