Aptamers are exciting smart molecular probes for specific recognition of disease biomarkers. A number of strategies have been developed to convert target-aptamer binding into physically detectable signals. Biosensors that employ aptamers as a recognition element are called aptasensors. DNA and RNA aptamers can be modified chemically to undergo analyte-dependent conformational changes. The aptamer-target reaction is independent of both the type of detection system and the kind of transducer employed. Aptasensors can be easily multiplexed to detect a variety of aptamer-target reactions simultaneously. So far, a variety of techniques in combination with a wide range of functional nanomaterials have been used for the design of aptasensors to further improve the sensitivity and detection limit of target determination.
Fig.1 Schematic illustration of a biosensor. (Walter, 2012)
After the aptamers bind to their targets, the transducer translates the binding event into a detectable and measurable signal. Generally, there are seven groups methods which are usually used in biosensors.
This classification is not strictly independent. Different technology can be used in combination. Among them, the following four groups are most wlidely used.
Optical aptasensors are aptamers with fluorescence, luminophore, enzyme, and NP reporters or aptamers with a label-free detection system. Colorimetry is the simplest sensing approach in the development of biosensors and involves monitoring the color changes of a solution by the naked eye or by UV-vis spectroscopy. Fluorescence is the most commonly used technique for visualizing optical aptasensors due to its remarkable properties including high efficiency, high sensitivity, and simple operation.
Electrochemical detection is very popular in sensing because of its facile fabrication and integration of electrochemical cells with lab-on-chip devices. Electroactive labels such as ferrocene, ferricyanide ([Fe- (CN)6]4− or [Fe(CN)6]3−), ruthenium(III) hexaamine ([Ru- (NH3)6]3+), MB, bis-anthraquinone-modified propanediol, and cadmium sulfide (CdS) NPs are typically used as the reporters in aptasensors.
Fig.2 Schematic illustration of electrochemical aptasensor in the signal on (left) and signal off (right) system. (Liu, 2020)
Mass-based aptasensors monitor the minor changes of mass for signal transduction. There are two types of mass-based aptasensors: acoustics-based biosensors and aptamer-based surface plasmon resonance (SPR) biosensors. This is a label-free technique for sensitive detection.
In micromechanical-based aptasensors, the cantilevers serve as a mini platform of a diving board, and the length of bent cantilevers is measured as signal transduction. Surface stress is produced on the cantilevers once the target molecules bind to the immobilized aptamers on one side of the cantilevers.
The levels of various biomarkers in human body fluids reveals the biological status or process in different conditions and may provide useful information for clinical diagnoses. Aptasensors have already focused on sensing and quantifying disease biomarkers. Besides, due to their high affinity and specificity, aptamers have become promising tools for in vivo molecular imaging. In vivo molecular imaging in situ allows direct study of cellular and molecular events under normal physiological conditions.
Altogether, the development of aptamers for quantitative and qualitative clinical applications has been successful. Nanotechnology-based detection platforms will be widely used to perform highly selective, specific, cost-effective, and quick multiplex detection of targets in the future.
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