1. Target-induced Structure Switching mode (TISS)
Binding of the targets to the immobilised aptamer causes the aptamers to alter configuration from a flexible to rigid tertiary structure such as a G-quadruplex. Such conformational switches change the relative positions of signal moieties, leading to initiation of a signal. Examples of biosensors using TISS include Apatmer Beacons.
A reported example features methylene blue incorporation into the aptamer stem-loop structure (figure 1). Upon target binding, methylene blue is separated from the surface resulting in a change in signal.
3. Target-induced Dissociation Mode (TID)
In TID mode, the complementary sequences of aptamers, instead of aptamers themselves are employed as anchors to localise the aptamers. After incubation with targets, the formed target-aptamer complexes will be liberated into the solution, which leads to the changes of detectable signals. TID strategy can be further classified into signal-off mode, signal-on mode, and label-free mode, as shown in figure 3.
Conjugation of aptamers on various nano-materials has led to highly sensitive and selective aptasensors. Examples of non-materials used for biosensors include metallic nanoparticles, quantum dots, silica nanotubes, and carbon nanotubes, with interest in aptamer incorporation increasing in popularity.
We are currently collaborating with several parties to develop aptamer-based biosensors for the reliable detection of disease associated biomarkers or field-based contaminants/ pathogens in a decentralised setting. We see the successful integration of aptamers as a key step toward solving the increasing need for highly sensitive, rapid and economical diagnostic devices.
Aptamers are compatible with a variety of commercially available biosensor platforms e.g. Surface Plasmon Resonance (SPR), BioLayer Interferometry (BLI), electrochemical sensors etc.
Standard functional groups (biotin, thiol, amine etc.) are readily incorporated into aptamers during their synthesis. This allows straightforward immobilisation of the aptamer onto the appropriate sensor; converting a generic sensor platform into a target specific biosensor.
The design strategies of most biosensors have similar elements. These strategies can be divided into four modes:
2. Sandwich like mode
An aptamer is immobilised onto a surface as a capture probe and a second aptamer is modified with a label as a signal read-out probe. The target can be captured on the surface of electrodes by binding to the capture aptamer and then forming a sandwich complex with the second aptamer. During this process, sandwiched binding complexes bring catalytic labels to the surfaces of electrodes to generate colorimetric, voltametric, impedimetric, or gravimetric signals for detection.
A reported example was an electrochemical biosensor for Platelet-derived groeth factor (PDGF) detection. The ‘sandwich’ structure is fabricated based on the fact that PDGF has two aptamer-binding sites, which makes it possible for one PDGF molecule to connect with two aptamers simultaneously. Gold nanoparticles (Au-NPs) amplify the signal of the electrochemical probe [Ru(NH3)5Cl](2+) to obtain a very low detection limit for the analysis of PDGF in serum samples (figure 2).
4. Competitive Replacement mode
In a competitive assay, one component (either the aptamer or the analyte) is immobilised on the sensor’s surface. In the case of immobilised aptamers, a label target is bound to the aptamer. Competitive replacement of the labelled target by an unlabelled target originating from the sample results in a decrease of the signal. In this mode, signal modified target molecules need to be designed and synthesised in advance.
A reported example is the development of an ultra-sensitive electrochemical biosensor for the detection of thrombin and lysosome are co-immobilised onto the gold substrate, followed with the binding of cadmium sulphide (CdS) – labelled thrombin and lead sulphide (PdS) – labelled lysosome. As the sample containing detection targets was added, the label-proteins could be displaced by targets, which could be characterised by through electrochemical stripping detection (figure 4).