Efficient Enhancement of Electrochemiluminescence from Cadmium Sulfide Quantum Dots by Glucose Oxidase Mimicking Gold Nanoparticles for Highly Sensitive Assay of Methyltransferase Activity

Herein, an original electrochemiluminescence (ECL) method for the detection of DNA methyltransferase (MTase) activity is presented based on the efficient enhanced ECL of CdS quantum dots (QDs) through catalytic generation of coreactant and energy transfer by glucose oxidase mimicking gold nanoparticles (Au NPs). Briefly, a double-stranded DNA (dsDNA) containing the symmetric sequence of 5'-CCGG-3' was bonded to the CdS QDs modified glassy carbon electrode (GCE). After that, the electrode was incubated with M.SssI CpG MTase which catalyzed the methylation of the specific CpG dinucleotides. Subsequently, the electrode was treated with a restriction endonuclease HpaII which could recognize and cut off the 5'-CCGG-3' sequence. Once the CpG site in the 5'-CCGG-3' was methylated, the recognition function of HpaII was blocked, and it could not cut off the ds-DNA. Later, Au NPs were combined-with the end of the ds-DNA section which was not cut off and has -SH groups. Therefore, the higher M.SssI MTase activity could lead to more Au NPs immobilized on ds-DNA. Au NPs could not only catalyze the oxidation of glucose with cosubstrate oxygen, producing gluconate and hydrogen peroxide (H2O2) which served as the ECL coreactant of CdS QDs, but also enhanced CdS QDs ECL via energy transfer (ET). Thus, the methylation event corresponding to the MTase activity could be monitored and amplified by this method. Finally, a logarithmic linear correlation between the ECL intensity of CdS QDs and the activity of M.SssI MTase that ranged from 1.0 to 120 U mL(-1) with the detection limit of 0.05 U mL(-1) was obtained.