NiCoO₂@CeO₂ Nanoboxes for Ultrasensitive Electrochemical Immunosensing Based on the Oxygen Evolution Reaction in a...

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 × 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.