A Label-free Electrochemical Immunosensor for Carcinoembryonic Antigen Based on Graphene and Thionine

A label-free immunosensor was developed to detect the presence of an antigen. This immunosensor was based on the modulation of the electrochemistry of the surface bound redox species thionine (Thi). The model antigen was carcinoembryonic antigen (CEA) and the model epitope was the antibody of CEA (anti-CEA). Glassy carbon electrode surfaces were first drop-coated with a mixture of graphene, Thi and Nafion and air-dried. The electrode surface was then electrodeposited in HAuCl4 solution to form Au nanoparticles (AuNPs). The resulted AuNPs were used to immobilize anti-CEA. Binding of CEA to the surface bound epitope resulted in the attenuation of the Thi electrochemistry. Under optimal conditions, the response of the label-free immunosensor had a liner range from 10 fg/mL to 100 ng/mL with a detection of 3.5 fg/mL (S/N=3).