An electrochemical immunoassay for protein based on bio bar code method

An electrochemical bio bar code assay based on DNA modified gold nanoparticles (Au NPs) provides a nonenzymatic method for quantitative detection of protein. By sandwiching an antigen (Ag) between a magnetic bead (MB) modified with antibody (Ab) and an amplifier Au NP bifunctioned with second Ab and bio bar code DNA, the detection method was established. Subsequent electrochemical stripping analysis of the cadmium dissolved from CdS NPs on the congregates provided a means to quantify the concentration of Ag in the analyte. Using AFP detection as a model, we demonstrate a linear dose response in the range from 0.02 to 3.5 ng/mL. The detection limit of this assay for AFP determined is 9.6 pg/mL, 100-fold improvement compared to the best enzyme-linked immunosorbent assay (ELISA) system. In addition, the amount of anti-AFP loaded on one MB, and the numbers of the secondary anti-AFP and bio bar code DNA modified on one Au NP were all determined for the first time. The ratio of MB and anti-AFP was approximately 1/16,700, and the ratios of Au NPs, anti-AFP, bar code DNA probes, and CdS NPs were about 1/16/101/77. It would be valuable for clinical immunoassay due to the high sensitivity and selectivity.