Magnetically controlled immunosensor for highly sensitive detection of carcinoembryonic antigen based on an efficient “turn-on” cyanine fluorophore

Early cancer diagnosis is critically important for early intervention and it can significantly enhance the treatment efficacy and the chance of cure. Herein, for the first time, the cyanine fluorophore namely, SLSO₃, was synthesized and found to possess a high binding affinity towards a typical biomarker, carcinoembryonic antigen (CEA) in which it served as an excellent turn-on labelling fluorophore for the detection of CEA. On this basis, the target CEA antigen is captured by a primary antibody conjugated on magnetic silica coated iron oxide nanoparticles and then followed by secondary antibody-loaded SiO₂ nanoparticles, which provide more binding sites for the labelling dyes resulting in 6-fold increase in fluorescence and thus heightened sensitivity. The immunosensor is capable of differentiating target analyte from other cancer-associated proteins and has been applied to quantify CEA in human serum sample and the result was consistent to that obtained from a commercial ELISA kit. This simple and direct detection assay has achieved a low limit of detection in the pg mL⁻¹ regime with small sample consumption (only 10 μL) and without any sample pre-treatment and purification.