Waste-derived Au@Pd/activated biochar electrode for sensitive detection of 17α-ethinylestradiol in water

17α-Ethinylestradiol (EE2), a synthetic estrogen widely used in pharmaceuticals, is a potent endocrine disruptor capable of causing adverse effects even at trace concentrations. To enable its sensitive determination, we developed a carbon paste electrode modified with activated biochar derived from green coconut mesocarp and functionalized with Au@Pd nanoparticles. The activated biochar presented a porous, high-surface area framework, while the bimetallic nanoparticles enhanced conductivity and electrocatalytic activity. The resulting sensor exhibited excellent linearity in the range of 0.05-5.0 μM (R² = 0.9993), achieving a detection limit of 2.53 nM and a quantification limit of 8.43 nM, nearly three orders of magnitude lower than those obtained by a reference chromatographic method (LOD: 5.14 μM, LOQ: 17.14 μM). Intra-day (RSD = 5.77 %) and inter-day (RSD = 8.14 %) reproducibility met accepted analytical criteria. Selectivity studies revealed significant interference only from 17β-estradiol and ascorbic acid, yielding relative errors of +165.38 % and +72.63 %, respectively. By contrast, urea, NaCl, and caffeine showed negligible effects (-10.38 % to +6.03 %), demonstrating the robustness of the sensor in biological and saline matrices. The applicability of the method was confirmed by quantifying EE2 in a commercial contraceptive (declared: 6.746 μM; found: 6.706 μM; recovery: 99.41 %) and in real and simulated matrices, including groundwater, tap water, synthetic urine, saline solution, and artificial breast milk. These results highlight the potential of this biochar-based nanocomposite electrode as a low-cost, sustainable, and efficient platform for decentralized monitoring of endocrine-disrupting contaminants.