Fluorescent DNAzyme Biosensors for Metal Ions Based on Catalytic Molecular Beacons

In this chapter, methods for designing metal ion sensors using fluorophore- and quencher-labeled DNAzymes are discussed. In contrast to the classical molecular beacon method based on binding, the methods described here utilize catalytic cleavage to release the fluorophore for detection and quantification, making it possible to take advantage of catalytic turnovers for signal amplification. Unlike classical molecular beacons that detect only nucleic acids, catalytic molecular beacons can be applied to different DNAzymes to detect a broad range of analytes. The methods described are based on the finding that almost all known trans -cleaving DNAzymes share a similar structure comprised of a catalytic DNAzyme core flanked by two substrate recognition arms. Using a typical DNAzyme called the “8-17” DNAzyme as an example, the design of highly sensitive and selective Pb²⁺ sensors is described in detail. The initial design employs a single fluorophore-quencher pair in close proximity, with the fluorophore on the 5′-end of the substrate and the quencher on the 3′-end of the enzyme. Although this sensor is highly sensitive and selective at 4�C, high background fluorescence is observed at higher temperatures. Therefore a new design with an additional quencher attached to the 3′-end of the substrate is employed to suppress background fluorescence. The dual quencher method allows the sensor to perform at ambient temperatures with a high signal-to-noise ratio.