A low-cost electrochemical screening platform for danazol in anti-doping analysis

Doping in sports, particularly using danazol (DZ), a steroid prohibited by the World Anti-Doping Agency (WADA), can lead to serious health risks due to its unregulated use. Current detection methods are costly and performed in specialized laboratories. In this work, we developed a low-cost electrochemical screening platform based on the direct electrochemical reduction of graphene oxide (GO) onto a glassy carbon electrode (GCE), without the use of chemical reductants or multi-step functionalization. The GCE/ErGO electrode was characterized by cyclic voltammetry (CV) using the redox probe [Fe(CN)₆]^3-/4- and by double-layer capacitance measurements in Britton-Robinson buffer (pH 3.0), confirming the improved performance of the modified surface. The net current observed in the presence of DZ increases almost three times when ErGO modifies the electrode surface. Based on the response obtained for DZ detection using Square-Wave Adsorptive Stripping Voltammetry (SWAdSV), chemical and electrochemical parameters were optimized. The optimal conditions were Britton-Robinson buffer at pH 3.0, an accumulation potential (E_acc) of -0.4 V, and an accumulation time (t_acc) of 300 s. Under these conditions, the calibration curve yielded limits of detection and quantification of 7.0 and 20.0 µg L^-1, respectively. Spiked water, synthetic urine and synthetic saliva samples were analyzed using the standard addition method, achieving satisfactory recoveries. The sensor maintained a measurable DZ signal in the presence of cortisol, urea, and nandrolone, with no anti-fouling or carryover observed. In mechanistic terms, ¹H-NMR spectra of electrolyzed DZ supported hydroxylation at the allylic position, delivering 6-hydroxy-4,5-dihydro-DZ as the main oxidation product.