Antioxidants are used as markers of the functional potential of foods, making their accurate quantification essential for assessing bioactivity and possible health benefits. A smart polymer incorporating 2,2-diphenyl-1-picrylhydrazyl (DPPH) motifs (FDPPH) was synthesized via bulk polymerization of N-vinylpyrrolidone, methyl methacrylate, and a diphenylamine-functionalized monomer, followed by solid-phase reactions to introduce the DPPH functionality. FTIR and thermal analysis confirmed successful synthesis. The FDPPH sensor showed a colorimetric response consistent with DPPH in solution, demonstrating its reliability. Twenty-three beet samples were analyzed using both the conventional DPPH assay and the FDPPH method, where polymer discs were immersed and colour changes recorded via smartphone. The hue (H) parameter in the HSV colour-space showed the highest correlation. Calibration using beet samples was more accurate than with Trolox. The optimal response time was 3 h, with a LOD and LOQ of 0.063 and 0.191 mmol Trolox/L, respectively.
