Electrochemical immunoassay platform for high sensitivity protein detection based on redox-modified carbon nanotube labels

Abstract

We report a highly sensitive immunoassay protocol based on the use of redox-modified multi-walled carbon nanotubes (MWNTs) as electrochemical labels. The MWNTs were coated with methylene blue (MB) at an optically-determined loading of 3.41 × 10⁻³ mol g⁻¹, and were then attached to secondary antibodies (Ab₂) by adsorption. As a model analyte mouse IgG was collected by primary antibody (Ab₁)-coated magnetic beads. Following binding of the MB-MWNT-Ab₂ conjugates, IgG could be measured by MB reduction. Using differential pulse voltammetry for quantification, IgG was calibrated with a dynamic range of 0.1 pg mL⁻¹ to 100 pg mL⁻¹. Given the different possible Ab₁-MB-MWNT-Ab₂ orientations on the magnetic beads, it was likely that not all the MB communicated with the electrode. A greater quantity of MB could be accessed by using the Fe(CN)₆^3−/4−redox couple as a solution phase mediator. This enabled us to lower the dynamic range down to 5 fg mL⁻¹ to 100 fg mL⁻¹.