Development of a Nafion membrane-based colorimetric FRAP sensor for measuring total antioxidant capacity

The ferric-tripyridyltriazine: [Fe(TPTZ)₂]³⁺ complex, the chromogenic FRAP reagent, was electrostatically attached to negatively charged Nafion membrane to manufacture a total antioxidant capacity (TAC) sensor for food. Trolox-equivalent antioxidant capacities for 19 different phenolic and thiol antioxidants were measured directly on the sensor (at 620 nm). The sensor worked well for both phenols and thiols, with high linearity. The method was also applied after reaction with 5,5'-dithiobis(2-nitrobenzoic acid): DTNB, and positive deviations observed in thiol-phenol mixtures were prevented, with quantitative recovery from fruit juices and green tea samples. TAC determination could be performed 10 months after the membrane was prepared, and the color on the sensor remained almost unchanged within 5 days. The three prominent novelties of this work are the first-time design of a colorimetric FRAP sensor, correction of negative error for simple thiols, and elimination of thiols overrating error in FRAP for thiol-phenol mixtures by preliminary reaction with DTNB.