Ultrasensitive detection of carcinoembryonic antigen by a simple label-free immunosensor

A simple label-free immunosensor was developed to detect carcinoembryonic antigen (CEA) ultrasensitively. This immunosensor was based on the modulation of the surface bound redox species thionine to increase current of the immunosensor. Glassy carbon electrode surface was first drop-coated with a mixture of thionine and chitosan, and then covered with nafion membrane containing SiO2 nanoparticles. After corroded by NaOH, silver nanoparticles were deposited to facilitate electron transfer process. After binding with polyethyleneimine (PEI), glutaraldehyde was used to cross-link PEI and anti-CEA. Binding of CEA to the surface bound epitope resulted in attenuation of the electrochemical current. Under optimal conditions, the label-free immunosensor had a linear range of 1 fg mL−1 to 100 pg mL−1 with a detection limit of 1 fg mL−1 (S/N = 3). The immunosensor was simple and ultrasensitive, and had high selectivity, a broad linear range and low detection limit. The present method could be of significance to be widely applied to other clinical research of important biological molecules.