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An enzymatic biosensor has been developed for the determination of selenate (SeO 4 2- ), in which selenate reductase (SeR) is chemically attached to a gold disk electrode by lipoic acid N-hydroxysuccinimide ester as linker, allowing the catalytic reduction of the SeO 4 2- to SeO 3 2- . Modification of the gold electrode was characterized by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and electrochemistry. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were performed in different buffers for selenate determination. Under optimum conditions, the calibration curve was linear over the range 7.0-3900.0 μg L -1 with limits of detection and quantification of 4.97 and 15.56 μg L -1 , respectively. The possible interference of the relevant oxyanions SO 4 2- , NO 3 - , NO 2 - , PO 4 3- and AsO 4 3- in the determination of SeO 4 2- was studied. Finally, the proposed biosensor was used to determine SeO 4 2- with recovery between 95.2 and 102.4 % in different real water samples., (© 2022 Wiley-VCH GmbH.)