Cation Doping for Enhanced <i>p</i> -Nitrophenol Sensing: Tuning Valence States and Oxygen Vacancies in Co ₃ O ₄ Hollow Nanocubes

The development of sensitive and efficient sensors for monitoring organic pollutants in water is of critical environmental importance. Transition metal oxides, particularly spinel Co₃O₄, present a promising platform, yet their performance hinges on their electronic structure. This study successfully synthesized Zn-, Fe-, Mn-, and Ni-doped spinel Co₃O₄ hollow nanocubes using a zeolitic imidazolate framework-8 (ZIF-8) template for the electrochemical detection of <i>p</i>-nitrophenol (PNP). Material characterization confirmed that doping effectively tuned the Co²⁺/Co³⁺ ratio and oxygen vacancy (OVs) concentration. Among the materials, Zn-Co₃O₄ exhibited the highest Co²⁺/Co³⁺ ratio and OVs content, which corresponded to its superior electrochemical performance for PNP detection, including high sensitivity (0.3461 μA μM^-1) and low detection limit (5.24 nM). The enhanced performance is attributed to the elevated Co²⁺/Co³⁺ ratio boosting redox activity and the abundant OVs acting as prime adsorption sites for PNP molecules. The sensor also demonstrated excellent selectivity, stability, and applicability in real water samples. This work confirms that cation doping is an effective strategy for enhancing the sensing performance by regulating valence states and defect engineering.