Renewable carbon from flax shives with silver nanoparticles biosynthesized using Eichhornia crassipes extract for green electrochemical detection of hydroxychloroquine

Hydroxychloroquine (HCQ) is an antimalarial drug that was repurposed during the COVID-19 pandemic. However, due to its limited clinical efficacy and notable side effects, its recommendation was later withdrawn. Despite this, HCQ sales increased by over 800 % in Brazil, and its presence has since been detected in aquatic environments, raising concerns regarding potential risks to both environmental and human health. In response, a novel green electrochemical sensor was developed for the determination of HCQ in water samples. The sensor is based on a glassy carbon electrode modified with hydrochar derived from flax shives and silver nanoparticles biosynthesized using Eichhornia crassipes (water hyacinth) extract as a reducing and stabilizing agent (GC/HC-AgNPs). The sensor was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and cyclic voltammetry (CV). Experimental parameters were optimized, and the device was successfully applied for the detection of HCQ using differential pulse voltammetry (DPV). The sensor exhibited a linear response in the range of 1.0–40.0 µmol L⁻¹, with a limit of detection (LOD) of 0.265 µmol L⁻¹ and a limit of quantification (LOQ) of 0.885 µmol L⁻¹. Excellent repeatability (RSD = 2.27 %) and reproducibility (RSD = 5.75 %) were achieved. Application in tap and lake water samples resulted in recovery values ranging from 89.5 % to 105.0 %, and comparative analysis with UV–vis spectroscopy confirmed the accuracy of the proposed method. The sensor further demonstrated good selectivity even in the presence of relevant interfering species. Green chemistry assessment tools confirmed the sustainable character of the developed method. Overall, the proposed sensor represents a promising, effective, and environmentally friendly platform for HCQ monitoring in water matrices.