Aptamers are increasingly being used in Biosensors. Biosensors are devices that use biological molecules, especially enzymes or antibodies, or living organisms to detect the presence of an analyte. By modifying an aptamer with an appropriate reporter group, we are able to develop biosensors that measure analytes qualitatively and/or quantitatively.
Modes in which aptamers are utilised in biosensors include:
Target-Induced Structure Switching mode (TISS)
Target-induced structure switching is a characteristic of aptamers that we can take advantage of. In the absence of a target, the selected aptamer will be in a state that can either quench or provide a signal. After a target is introduced to the biosensor, a conformational change is induced in the aptamer with target engagement – target induced structure switch.
One way the structure switch is used is when an aptamer contains a hairpin loop. A reporter molecule can be quenched inside the loop. When the aptamer binds the target, the reporter is exposed resulting in a change in signal. Signal molecules which have been used for TISS include; dyes like SYBR Green and redox reporters such as methylene blue.
For a sandwich mode biosensor to work, the target needs to have at least two binding sites. Aptamers are then able to ‘sandwich’ the target creating an aptamer-target-aptamer complex. The first aptamer is immobilised on the biosensor to capture the target. The second aptamer is a reporter which binds to the target enabling analyte measurement.
A key advantage of the sandwich-like mode is that it can be used to complement existing technologies, for example forming aptamer-target-antibody complexes. As aptamers can use different binding sites to antibodies, they can be used in combination, increasing selectivity and/or sensitivity.
Antibodies have proven to be useful tools for developing detection systems but are restricted to a limited range of targets. Aptamers, however, can be selected against small and large molecule targets, viruses, cells and whole organisms.