Scheme 1.

Design strategy of the catalytic and molecular beacon (CAMB). (a) Schematic of the molecular beacon (MB) design. (b) Schematic of the catalytic beacon design. (c) Schematic of the CAMB. The DNAzyme substrate strand is incorporated in the loop of the MB, which binds to the enzyme strand to form a complex structure. Addition of metal ions cleaves the substrate and cuts the molecular beacon into two pieces, resulting in stem dehybridization and enhanced fluorescence signal. In the presence of excess MB substrate, one DNAzyme can catalyze the cleavage of multiple MB substrates and achieve an amplified fluorescence signal.