Methods and assay for the portable colorimetric detection of an antioxidant in a food sample. The method includes the steps of providing a colorimetric reagent, the reagent including a plurality of ceria nanoparticles immobilized to a support, contacting the colorimetric reagent with the food sample, and detecting an optical property of the colorimetric reagent, where a change in the optical property of the colorimetric reagent is associated with the presence of antioxidant in the food sample. The change in the optical property of the colorimetric reagent is dependent upon the concentration of the antioxidant in the food sample.