NiCoO₂@CeO₂ Nanoboxes for Ultrasensitive Electrochemical Immunosensing Based on the Oxygen Evolution Reaction in a Neutral Medium: Application for Interleukin-6 Detection

Abstract

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.