Biomedical Engineering Reference
In-Depth Information
+
(A)
Fluorescence
Aptamer
Ligand
beacon
+
+
(B)
Ligand
beacon
Aptamer-
target
complex
Target
No
fluorescence
FIGURE 20.8
Functioning of a ligand beacon. (A) In the absence of the target analyte, a fluorescent signal is produced. (B) In
the presence of the target analyte, no fluorescent signal is produced.
This format is also applicable to a multiplex system, in which a number of different ana-
lytes are detected simultaneously with the use of ligand beacons conjugated to fluo-
rophores emitting at different wavelengths (52).
This type of assay once again clearly shows the potential of aptamers for analytical and
diagnostic procedures, which cannot be fulfilled by antibodies.
20.3.4.5 Capillary Electrophoresis
Capillary electrophoresis (CE) is a separation technique based on the same principle as con-
ventional electrophoresis. It offers the advantages of speed, small sample volumes, suit-
ability for automation, sensitivity, and the possibility of performing multiplex assays (52).
CE is now being adapted for immunoassays, in which antibody-antigen complexes are sep-
arated on a fluidic stream under an applied electric field (52). However, there are many
practical difficulties associated with this method, such as poor separation between anti-
body-target complexes and free, labeled, antibodies when the targets are small and
uncharged (52).
Recently, German et al. (65) adapted the technique to use a fluorescently labeled DNA
aptamer for the detection and quantification of human IgE. The technique was able to sepa-
rate free aptamer from the aptamer-IgE complex in the presence and in the absence of
serum, indicating that the aptamer-target interaction was not affected by the complex
medium.
20.3.4.6 Molecular Switches
A very relevant example, as described by Jayasena (52), of where aptamers can be used
as molecular switches is in PCR. To avoid nonspecific amplification, which is often due
to nonspecific binding of primers at suboptimum temperatures and to the ambient
