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

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 ({ d Ab–AuNR–FCA}). The { d Ab–AuNR–FCA} labels were designed by exploiting silica-functionalized gold nanorods (AuNR@SiO 2 ) as the carriers for immobilization of detection antibody ( d Ab) and ferrocenecarboxylic acid (FCA), in which d Ab 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@SiO 2 . Compared with the commercially available { d Ab–FCA}, the { d Ab–AuNR–FCA} labels exhibited a better performance for E. coli assay due to the advantages of AuNR@SiO 2 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 1.0×10 2 to 5.0×10 4   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.