Electrochemical sensing platform based on tris(2,2′-bipyridyl)cobalt(III) and multiwall carbon nanotubes–Nafion composite for immunoassay of carcinoma antigen-125

A new strategy for constructing a sensitive mediator-type electrochemical immunosensor for the detection of carcinoma antigen-125 (CA125) was developed. In this strategy, mediator tris(2,2′-bipyridyl)cobalt(III) (Co(bpy)₃³⁺) was incoporated into the multiwall carbon nanotubes–Nafion (MWNTs–Nafion) composite film via a simple ion-exchange route. Then, gold colloidal nanoparticles (nano-Au) were attached onto Co(bpy)₃³⁺/MWNTs–Nafion film through electrostatic interaction between negatively charged nano-Au and positively charged Co(bpy)₃³⁺. Finally, CA125 monoclonal antibody (anti-CA125), used as a model antibody, was assembled onto the surface of nano-Au to achieve an immunosensor for the determination of CA125 antigen. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to characterize the assembly process of the modified electrode. The resulting immunosensor showed a high sensitivity, wide dynamic range consisting of two linear parts from 1.0 to 30 U mL⁻¹ and 30 to 150 U mL⁻¹ with a low detection limit of 0.36 U mL⁻¹ at 3 times the background noise. Moreover, it displayed good reproducibility and stability, and would be potentially attractive for clinical immunoassay of CA125. The integration of mediator Co(bpy)₃³⁺ and MWNTs–Nafion composite would offer potential promise for the fabrication of biosensors and biocatalysts.