Ultrasensitive Electrochemical DNA Assay Based on Counting of Single Magnetic Nanobeads by a Combination of DNA Amplification and Enzyme Amplification

An ultrasensitive electrochemical method for determination of DNA is developed based on counting of single magnetic nanobeads (MNBs) corresponding to single DNA sequences combined with a double amplification (DNA amplification and enzyme amplification). In this method, target DNA (t-DNA) is captured on a streptavidin-coated substrate via biotinylated capture DNA. Then, MNBs functionalized with first-probe DNAs (p1-DNA−MNBs) are conjugated to t-DNA sequences with a ratio of 1:1. Subsequently, the p1-DNA−MNBs are released from the substrate via dehybridization. The released p1-DNA−MNBs are labeled with alkaline phosphatase (AP) using biotinylated second-probe DNAs (p2-DNAs) and streptavidin−AP conjugates. The resultant AP−p2-DNA−p1-DNA−MNBs with enzyme substrate disodium phenyl phosphate (DPP) are continuously introduced through a capillary as the microsampler and microreactor at 40 °C. AP on the AP−p2-DNA−p1-DNA−MNBs converts a huge number of DPP into its product phenol, and phenol zones are produced aroun...