Paper-based colorimetric immunosensor for visual detection of carcinoembryonic antigen based on the high peroxidase-like catalytic performance of ZnFe2O4–multiwalled carbon nanotubes

Abstract

A new paper-based colorimetric immunosensor for the detection of carcinoembryonic antigen (CEA) was developed based on the intrinsic peroxidase activity of ZnFe₂O₄–multiwalled carbon nanotubes (ZnFe₂O₄@MWNTs). The immunosensor platform was prepared by depositing chitosan and porous gold onto filter paper and entrapping the primary antibodies (Ab₁) onto the layers. Secondary antibodies (Ab₂) were assembled on the surface of the functionalized ZnFe₂O₄@MWNTs. The immunosensor response was quantified as a color change resulting from ZnFe₂O₄@MWNTs catalyzing the oxidation of 3,3′,5,5′-tetramethylbenzidine in the presence of H₂O₂. The catalytic performance of ZnFe₂O₄@MWNTs was higher than ZnFe₂O₄ due to the high electrical conductance of MWNTs, moreover, the electron communications between ZnFe₂O₄@MWNTs and substrates are electrically “wired”. Detection was achieved by measuring the color change when the concentrations of CEA were different. The color change can be quantified with the naked eye but a digitalized picture can also be used to provide more sensitive comparison to a calibrated color scheme. This method was simple for CEA detection with a linear range from 0.005 to 30 ng mL⁻¹ and a detection limit of 2.6 pg mL⁻¹. Such an equipment-free immunoassay has great potential in resource-limited environments.