Functionalized nanocomposites with the optimal graphene oxide/Au ratio for amplified immunoassay of E. coli to estimate quality deterioration in dairy product

An amplified electrochemical immunosensor was developed by exploiting poly(diallyldimethylammonium chloride)-functionalized graphene oxide and gold nanoparticles (GO-PDDA@AuNP) as sensing platform. Highlight of this work was to investigate and optimize the ratio of GO-PDDA to AuNP for sensitivity enhancement. The suitable GO-PDDA@AuNP nanocomposites were proven to not only provide an excellent biocompatible microenvironment for the immobilized antibody, but also accelerate electron transfer to enhance electrochemical signal. Moreover, the {_dAb-Au-THi} nanoprobes as biorecognition elements were designed by utilizing the amplification effect of AuNPs to load detection antibody (_dAb) and enormous thionine (THi), in which _dAb was used for specific recognition of E. coli and THi served as electroactive species. Under optimal conditions, experimental results demonstrated that the variations (ΔI) in the peak current linearly depended on the logarithmic value of E. coli concentration from 50 to 5.0×10⁶ cfu mL⁻¹ with the detection limit of 35 cfu mL⁻¹. Recovery experiments were also performed for E. coli assay in dairy product (pure fresh milk, yogurt in shelf-life and expired yogurt), and the recoveries of standard additions were in the range of 89.7–112.6%. The excellent performance of the proposed strategy indicated its promising prospect as a valuable tool to estimate the quality deterioration in dairy product.