Ultrasensitive electrochemical immunosensors for clinical immunoassay using gold nanoparticle coated multi-walled carbon nanotubes as labels and horseradish peroxidase as an enhancer

A novel electrochemical immunosensor for the ultrasensitive detection of a-fetoprotein (AFP) is described with the use of a gold nanoparticle/(3-mercaptopropyl) trimethoxysilane modified electrode as a desirable platform and gold nanoparticle functionalized multi-walled carbon nanotube (MWCNT) nanocomposite labeled secondary antibodies (Ab(2)) as a signal tag. At first, gold nanoparticles (AuNPs) were attached on the MWCNTs surface using poly(diallyldimethylammoniumchloride) (PDDA) as a linkage reagent to obtain AuNP-MWCNT nanocomposites. Subsequently, horseradish peroxidase (HRP) and thionine (Thi) labeled secondary antibodies (Thi-Ab(2)) were used to bind AuNP-MWCNT nanocomposites with a high load amount and good biological activity. Moreover, the immunosensor can provide amplified signals because of the increased surface area and biocompatibility of AuNP-MWCNT nanocomposites. On the other hand, the AuNP functionalized (3-mercaptopropyl) trimethoxysilanes (MPTS) were used for the biosensor platform to increase the surface area as well as improving the electronic transmission rate to capture a large amount of primary antibodies (Ab(1)). Thus, amplified signals can be obtained by an electrochemical sandwich immunoassay protocol in the presence of H2O2. The proposed sensing strategy provides a wide linear dynamic range from 0.01 to 50 ng mL(-1) with a low detection limit of 3 pg mL(-1) (S/N = 3). Significantly, the developed immunoassay method showed a good stability, selectivity and reproducibility.