Ultrasensitive electrochemical biosensor for detection of circulating tumor cells based on a highly efficient enzymatic cascade reaction

There has been great interest in the enzymatic cascade amplification strategy for the electrochemical detection of circulating tumor cells (CTCs). In this work, we designed a highly efficient enzymatic cascade reaction based on a multiwalled carbon nanotubes–chitosan (MWCNTs–CS) composite for detection of CTCs. A high electrochemical effective surface area was obtained for a MWCNTs–CS-modified glassy carbon electrode (GCE) for loading glucose oxidase (GOD), as well as a high loading rate and high electrical activity of the enzyme. As a ‘power source’, the MWCNTs–CS composites provided a strong driving power for horseradish peroxidase (HRP) on the surface of polystyrene (PS) microspheres, which acted as probes for capturing CTCs and allowed the reaction to proceed with further facilitation of electron transfer. Aptamer, CTCs, and PS microspheres with HRP and anti-epithelial cell adhesion molecule (anti-EpCAM) antibody were assembled on the MWCNTs–CS/GCE to allow for the modulation of enzyme distance at the micrometer level, and thus ultra-long-range signal transmission was made possible. An ultrasensitive response to CTCs was obtained via this proposed sensing strategy, with a linear range from 10 cell mL−1 to 6 × 106 cell mL−1 and a detection limit of 3 cell mL−1. Moreover, this electrochemical sensor possessed the capability to detect CTCs in serum samples with satisfactory accuracy, which indicated great potential for early diagnosis and clinical analysis of cancer.