NiCoO@CeO Nanoboxes for Ultrasensitive Electrochemical Immunosensing Based on the Oxygen Evolution Reaction in a Neutral Medium: Application for Interleukin-6 Detection.

The development of highly active electrocatalytic labels is important for constructing sensitive electrochemical immunosensors. Great progress has been made in developing non-noble-metal nanocatalysts toward the oxygen evolution reaction (OER) in the past decade, but non-noble-metal OER nanocatalysts have not been explored as electrocatalytic labels for immunosensing. Herein, we report NiCoO@CeO nanoboxes (NBs) as novel electrocatalytic labels for ultrasensitive immunosensing based on the excellent OER activity of NiCoO@CeO NBs in a neutral solution. The synthesis of NiCoO@CeO NBs involves Ni exchange and heat treatment of ZIF-67 nanocubes to produce NiCoO NBs, followed by the growth of CeO nanoparticles on the surface of NiCoO NBs. The NiCoO@CeO NBs offer superior OER activity to NiCoO NBs because of the synergetic effect between NiCoO NBs and CeO nanoparticles. The formation of ester-like bridging between CeO and the carboxylic groups of antibody enables direct immobilization of the antibody on the NiCoO@CeO surface. A sandwich-type electrochemical immunosensor using NiCoO@CeO NBs as electrocatalytic labels features a broad linear range for interleukin-6 detection from 2.5 × 10 to 10 ng mL, with a low detection limit of 7 fg mL. Our work lays the foundation for developing electrochemical immunosensors and aptasensors based on non-noble-metal OER electrocatalysts.