Amplified colorimetric sensor for detecting radon by its daughter lead based on the free-fixed auto-assembly structure of Duplex-hemin/G-quadruplex.

A novel signal amplification strategy based on a Pb2+-dependent DNAzyme is proposed for detecting Pb2+ and radon through Pb2+-induced DNAzyme cleavage and the auto-assembly of a duplex-hemin/G-quadruplex with two loop-stem structures and enzyme-mimicking catalytic activity. First, the Pb2+-specific DNAzyme cleaves a primer sequence, which induces assembly of the hairpin probe Hp1/Hp2 into a double helix structure. Subsequently, a G-quadruplex forms after the insertion of hemin into the free G-rich sequences; this G-quadruplex possesses the catalytic activity of horseradish peroxidase and changes colorless TMB to its deeply colored oxidized state. lead-dependent DNAzymes were constantly sheared by Pb2+, and the free primer strands were continuously assembled into double chains by hybridization with hairpin probes, providing amplification for the detection of lead (II) and radon. Under the optimum conditions, there was a good linear relationship between ΔA and the lead concentration for Pb2+ concentrations ranging from 2.58 to 18 nM, and the detection limit was 0.77 nM. Analysis of actual samples indicated that when the radon concentration was in the range of 5.41 × 103- 1.65 × 105 Bq·h/m3, the radon concentration and the absorbance were linearly correlated with a detection limit of 1.62 × 103 Bq·h/m3. During the process of radon sampling and detection, radiation damage from the radioactive gas radon can be avoided. In this study, the primer dissociated by the DNAzyme was in a free state. Compared with the fixed-state primer chain, this system will be more convenient for the biological analysis of ultratrace metal ions. Therefore, this strategy has good application prospects for biosensors and is expected to become a novel platform for the amplification and detection of metal ion signals.