From Simplified Pretreatment to Sensitive Analysis: An Immobilization-Free Cathodic Photoelectrochemical Assay for Sulfonylurea Herbicide Residue and Its Accumulation

Sensitive and selective detection of agricultural chemicals in complex matrices necessitates streamlined workflows without performance loss. By combining the inherent high sensitivity of photoelectrochemical (PEC) analysis with the high selectivity of enzymatic recognition, an immobilization-free PEC assay for the sulfonylurea herbicide chlorsulfuron (Chs) was developed, with purification steps omitted and analytical procedure simplified. Specifically, BiOI nanoflakes (NFs)/NiO heterojunction was fabricated as a sensing platform, generating strong and stable cathodic photocurrent in the presence of dissolved O2. The catalytic reaction of acetolactate synthase (ALS) consumed O2, quenching the photocurrent response. Conversely, upon addition of the model target Chs, a representative sulfonylurea herbicide and effective inhibitor of ALS, the signal was recovered. By monitoring the photocurrent change before and after the enzymatic reaction, Chs was quantified over a linear range of 0.25–100 μg/L, with a limit of detection (LOD) of 0.16 μg/L. The assay avoids laborious probe-immobilization steps by directly adding the enzyme into the electrolyte, significantly simplifying the detection protocol. Furthermore, by omitting traditional purification procedures, the overall workflow was further optimized. The method was successfully applied to detect Chs residues in wheat powder and to evaluate its accumulation in wheat seedlings, with accuracy and reliability validated by high-performance liquid chromatography (HPLC). This work achieves highly sensitive and immobilization-free PEC analysis of Chs and offers a feasible approach for sulfonylurea herbicide detection in selected agricultural samples, with streamlined pretreatment process and enhanced analytical efficiency.