Development of a colorimetric assay for quantification of favipiravir in human serum using ferrihydrite

We developed a colorimetric analytical method for favipiravir (FPV), a promising treatment for COVID-19. FPV forms yellow complexes with ferrihydrite (Fh) by a ligand substitution reaction with the iron (III) hydroxyl surface groups in Fh. Fh-coated microbeads were packed into a capillary tube with an inner diameter of 1 mm. FPV-spiked serum after pretreatment was drawn into the capillary packed with Fh-coated microbeads. The intensities of the yellow-color complexes on the microbeads were transformed into red-green-blue (RGB) pixels using Image-J software 1.8, and the difference between green and blue was used as the analytical signal. The signal increased with increasing FPV concentration. The proposed method showed good linearity (R² = 0.9907) over a concentration range of 25-200 μg/mL. The RSD (n = 3) and the LOD (3σ) values were estimated to be 2.8-15.4% and 16.91 μg/mL, respectively. The proposed method enables us to quantify FPV rapidly and easily without the need for expensive equipment.