pH responsive amperometric immunoassay for carcinoma antigen 125 based on hollow polydopamine encapsulating methylene blue

A sandwich-type amperometric immunosensor based on pH responsive sensitivity amplifiable route was elaborately designed for determination of carcinoma antigen 125 (CA 125). The sensitivity of amperometric immunosensor depends on a change of current response (ΔI) attributed to antigen to be tested per unit concentration. Employing zeolitic imidazolate framework (ZIF-8) as the template, the hollow-structured polydopamine framework with high specific surface area can be fabricated and it can enrich large amounts of signal molecules. The pH responsive hollow polydopamine composites which loaded methylene blue (MB) covalently coupled with antibodies (Ab2) were implemented as smart labels and the Au-rGO with high absorption capacity was served as substrate to capture signal molecules. Different from traditional electrochemical immunosensor, HCl led to the destruction of the labels and disassociation of antibody-antigen interaction layer. The impedance of modified interface was dramatically decreased and the distance between signal molecules and the sensing interface was shortened, giving rise to the improvement of electron transfer efficiency and amplification of current response. With the help of pH responsive strategy, ΔI caused by antigen was effectively elevated, leading to the sensitivity amplification of immunosensor. Under optimum conditions, the amperometric immunosensor with good reproducibility, high sensitivity and specificity exhibited an ultralow detection limit of 0.336 μU mL−1 (S/N = 3) and a wide linear range from 0.0001 to 100 U mL−1 for CA 125 detection.