Electrochemical DNAzyme sensor for lead based on amplification of DNA-Au bio-bar codes

An electrochemical DNAzyme sensor for sensitive and selective detection of lead ion (Pb2+) has been developed, taking advantage of catalytic reactions of a DNAzyme upon its binding to Ph2+ and the use of DNA-Au bio-bar codes to achieve signal enhancement. A specific DNAzyme for Ph2+ is immobilized onto an Au electrode surface via a thiol-Au interaction. The DNAzyme hybridizes to a specially designed complementary substrate strand that has an overhang, which in turn hybridizes to the DNA-Au bio-bar code (short oligonucleotides attached to 13 nm gold nanoparticles). A redox mediator, Ru(NH3)(6)(3+), which can bind to the anionic phosphate of DNA through electrostatic interactions, serves as the electrochemical signal transducer. Upon binding of Pb2+ to the DNAzyme, the DNAzyme catalyzes the hydrolytic cleavage of the substrate, resulting in the removal of the substrate strand along with the DNA bio-bar code and the bound Ru(NH3)(6)(3+) from the Au electrode surface. The release of Ru(NH3)(6)(3+) results in lower electrochemical signal of Ru(NH3)(6)(3+) confined on the electrode surface. Differential pulse voltammetry (DPV) signals of Ru(NH3)(6)(3+) provides quantitative measures of the concentrations of Pb2+, with a linear calibration ranging from 5 nM to 0.1 mu M. Because each nanoparticle carries a large number of DNA strands that bind to the signal transducer molecule Ru(NH3)(6)(3+), the use of DNA-Au bio-bar codes enhances the detection sensitivity by five times, enabling the detection of Pb2+ at a very low level (1 nM). The DPV signal response of the DNAzyme sensor is negligible for other divalent metal ions, indicating that the sensor is highly selective for Pb2+ Although this DNAzyme sensor is demonstrated for the detection of Pb2+, it has the potential to serve as a general platform for design sensors for other small molecules and heavy metal ions.