Electronic structure engineering of NiFe layered double hydroxide via first-row transition-metal doping for efficient electrochemical oxidation of sulfamethazine in food samples

Abstract

Layered double hydroxides (LDHs) are promising electrocatalysts due to their excellent electrochemical performance. However, NiFe-LDH suffers from poor electrical conductivity and a tendency to stack and aggregate, limiting its practical applications. To overcome these drawbacks, we engineered a series of first-row transition-metal-doped LDHs, NiFeM-LDH (M = Ti, V, Cr, Mn, Co, Cu, and Zn), for comparative electrochemical detection of sulfamethazine (SMZ). Among them, NiFeCu-LDH-modified glassy carbon electrodes (NiFeCu-LDH/GCE) exhibit superior electrochemical oxidation performance toward SMZ. This enhancement arises from the partially filled e_g orbital of Cu²⁺ (t_2g⁶e_g³), which induces a Jahn–Teller distortion, promoting strong electronic coupling between Cu and Ni/Fe in the layered hydroxide. The resulting structural distortion enhances electron delocalization and accelerates the Ni²⁺ to Ni³⁺ conversion, increasing the number of electroactive sites. The Jahn–Teller effect of Cu²⁺ (d⁹) induces axial distortion of CuO₆ octahedra, perturbing the local metal-oxygen environment and enhancing Ni 3d-O 2p orbital overlap. Additionally, the formation of Ni–O–Cu and Fe–O–Cu linkages improves electrical conductivity and facilitates faster charge transfer. As a result, NiFeCu-LDH/GCE achieves a low detection limit of 11.23 nM and a high sensitivity of 0.0869 µA µM⁻¹ cm⁻², with stable performance over one month. Real-sample analysis in food (chicken, beef, milk, cheese, and honey) and environmental water demonstrates excellent recovery, highlighting its practical applicability. These results confirm that NiFeCu-LDH is a highly efficient electrocatalyst for sensitive and reliable SMZ detection in food and environmental monitoring.