Functionalized gold nanorod-based labels for amplified electrochemical immunoassay of E. coli as indicator bacteria relevant to the quality of dairy product

•AuNR@SiO2 was able to load enormous FCA tags for signal amplification.•A sandwich-type electrochemical immunoassay was developed by using {dAb–AuNR–FCA}.•The method shows rapid response, high sensitivity and specificity for E. coli assay.•E. coli in pure fresh milk, yogurt in shelf-life, and expired yogurt was detected.•The recoveries of standard additions were in the range of 95.1–106%.

In this paper, we report an amplified electrochemical immunoassay for Escherichia coli as indicator bacteria relevant to the quality of dairy product using the functionalized gold nanorod-based labels ({dAb–AuNR–FCA}). The {dAb–AuNR–FCA} labels were designed by exploiting silica-functionalized gold nanorods (AuNR@SiO2) as the carriers for immobilization of detection antibody (dAb) and ferrocenecarboxylic acid (FCA), in which dAb was used for recognition of E. coli and FCA tags served as signal-generating molecule. Greatly amplified signal was achieved in the sandwich-type immunoassay when enormous FCA linked to AuNR@SiO2. Compared with the commercially available {dAb–FCA}, the {dAb–AuNR–FCA} labels exhibited a better performance for E. coli assay due to the advantages of AuNR@SiO2 as carriers. Under optimal experimental conditions, it showed a linear relationship between the peak current of FCA and the logarithmic value of E. coli concentration ranging from 1.0×102 to 5.0×104 cfu mL−1 with a detection limit of 60 cfu mL−1 (S/N=3), and the electrochemical detection of E. coli could be achieved in 3 h. Moreover, the proposed strategy was used to determine E. coli in dairy product (pure fresh milk, yogurt in shelf-life, and expired yogurt), and the recoveries of standard additions were in the range of 95.1–106%. This proposed strategy exhibited rapid response, high sensitivity and specificity for E. coli assay in dairy product, and could become a promising technique to estimate the quality of dairy product.

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