Electrochemical immunosensor for Bacillus anthracis PA toxin using a polypyrrole-gold nanoparticle/multiwall carbon nanotube sensing platform and cadmium sulphide nanocrystal signal tag†

Abstract

A sandwich electrochemical immunosensor was proposed for the sensitive detection of Bacillus anthracis protective antigen (B. anthracis PA) toxin based on cadmium sulphide nanocrystals (CdS NCs) and polypyrrole-gold nanoparticle-modified multiwalled carbon nanotubes (PPy-AuNPs/MWCNTs). Herein, PPy-AuNPs/MWCNTs were used as a biocompatible and conducting matrix for immobilization of rabbit anti-PA antibody [RαPA antibody, i.e. capturing antibody (Ab1)] and to facilitate excellent electrical conductivity. PPy-AuNPs/MWCNTs were synthesized through a one-step chemical reaction of pyrrole and Au³⁺ on the surface of MWCNTs. CdS NCs were employed as a label for covalent binding of monoclonal mice anti-PA antibody [MαPA antibody, i.e. secondary antibody (Ab2)]. When B. anthracis PA toxin was present in an analytical sample, the sandwich immunoassay was formed, and an electrochemical potentiometric stripping analysis (PSA) signal was generated. The PSA response produced during the detection was caused by Cd²⁺ ions released from CdS NCs upon acid dissolution. The synergistic action of PPy, AuNPs and MWCNTs was responsible for the electrochemical signal amplification in the immunosensor. Under optimal conditions, the developed immunosensor showed a linear range from 100 pg mL⁻¹ to 10 ng mL⁻¹ (R² = 0.992) with a detection limit of 10 pg mL⁻¹ (S/N = 3) for B. anthracis PA toxin. The developed electrochemical immunosensor also exhibited excellent stability, reproducibility and good specificity. The immunosensor was applied for the analysis of human serum spiked with B. anthracis PA toxin with satisfactory results.