NiCoO2@CeO2 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 NiCoO2@CeO2 nanoboxes (NBs) as novel electrocatalytic labels for ultrasensitive immunosensing based on the excellent OER activity of NiCoO2@CeO2 NBs in a neutral solution. The synthesis of NiCoO2@CeO2 NBs involves Ni2+ exchange and heat treatment of ZIF-67 nanocubes to produce NiCoO2 NBs, followed by the growth of CeO2 nanoparticles on the surface of NiCoO2 NBs. The NiCoO2@CeO2 NBs offer superior OER activity to NiCoO2 NBs because of the synergetic effect between NiCoO2 NBs and CeO2 nanoparticles. The formation of ester-like bridging between CeO2 and the carboxylic groups of antibody enables direct immobilization of the antibody on the NiCoO2@CeO2 surface. A sandwich-type electrochemical immunosensor using NiCoO2@CeO2 NBs as electrocatalytic labels features a broad linear range for interleukin-6 detection from 2.5 x 10(-5) to 10 ng mL(-1), with a low detection limit of 7 fg mL(-1). Our work lays the foundation for developing electrochemical immunosensors and aptasensors based on non-noble-metal OER electrocatalysts.