Signal amplification based on tannic acid-assisted cyclic conversion of Fe(III)/Fe(II) for ultrasensitive electrochemical immunoassay of CA 12-5

A signal amplification protocol based on tannic acid (TA)-assisted cyclic conversion of Fe(III)/Fe(II) was designed for highly sensitive immunoassay of carbohydrate antigen 12-5 (CA12-5). Initially, graphene oxide-methylene blue (GO-MB) and ionic liquid functionalized reduced graphene oxide-Au (IL-rGO-Au) composite were sequentially covered on the surface of electrode as stable signal substrate. The labeling antibody/Au-reduced graphene oxide/zeolitic imidazolate framework-metal polyphenol network (Ab2/Au-rGO/ZIF-MPN) as novel probe was prepared by firstly coating TA on ZIF-8 to form ZIF-TA, then adsorbing a certain amount of Fe(III) to form intense metal polyphenol network (MPN) as the coating of probe on account of combination of TA and Fe(III), finally adsorbing Au-rGO composite on ZIF-MPN for immobilization of Ab2. In the existence of target CA12-5, a sandwiched electrochemical immunosensor was successfully constructed via specific recognition events between antigen and antibody. With acidic H2O2 stimulation, the coating of probe was cracked into TA and Fe(III), then TA reduced Fe(III) into Fe(II) with significant catalytic capacity, which reacted with H2O2 to activate Fenton reaction and meanwhile produced Fe(III) and hydroxyl radicals (·OH). Subsequently, the gained Fe(III) was reduced to Fe(II) again by TA, forming the cyclic conversion of Fe(III)/Fe(II) to produce adequate ·OH for degradation of MB. The tactic averted the employment of vulnerable enzymes and noble metal materials, meanwhile achieving conspicuous assay performance with a linear range from 0.00005–500 U mL−1 and ultralow detection limit of 2.3 × 10−3 mU mL−1 under optimal conditions.