Schiff base-stabilized electron-deficient copper species on MOFs as durable sensor of nitrate in water

The electrochemical sensor has been considered an efficient and portable platform for the rapid quantification of nitrate ions in industrial wastewater and natural water bodies. Albeit of the high sensitivity to NO₃⁻, the electron-deficient copper sites (Cu^δ+) were unstable under the negative working potentials for detection (Cu^δ+ was readily reduced to Cu⁰). Herein, the Schiff base was grafted on MIL-125 (Ti) via covalent binding to stabilize the Cu^δ+, leading to a composite Cu^δ+/Schiff base/MIL-125 (Ti) material (abbreviated as CuSM) for NO₃⁻ sensing. Moreover, the unique porous structure of CuSM can speed up the diffusion rate of NO₃⁻ from the bulk solution to the surface of the modified electrode, enhancing the sensitivity of NO₃⁻ assay. As a result, the CuSM-based sensor not only exhibited a wide linear range from 1.18 μM to 58.8 mM and a low detection limit (S/N = 3) of 0.253 μM but also possesses high specificity, excellent reproducibility (the RSD of the five inter-batch sensors was 1.138%) and the ability to detect real water samples.