Tuning the Iron-Oxygen Bond Length in α-Fe₂O₃ for Efficient Electrochemical Sensing

Transition-metal oxides are widely used as sensing interfaces, yet the lack of reliable descriptors limits the rational design of high-performance electrochemical sensors. Here we establish a direct correlation among the Fe-O bond length, adsorption energy, and electrochemical sensing activity by combining density functional theory with experimental validation. We show that shortening the Fe-O bond enhances Fe-O covalency, weakens Pb²⁺ adsorption, and improves sensitivity, thereby identifying the Fe-O bond length as a reliable activity descriptor. Guided by this principle, α-Fe₂O₃ with the shortest Fe-O bond length was optimized to yield a miniaturized electrochemical sensor with high sensitivity (26.3 μA μM^-1) and an ultralow detection limit (1.14 nM). The sensor enables on-site Pb²⁺ detection in real water and food samples with a performance consistent with ICP-MS analysis. These findings introduce the Fe-O bond length as a generalizable descriptor for oxide-based sensing interfaces and provide a framework for the rational design of advanced electrochemical sensing platforms.