Ferric-o-phenanthroline adsorbed on a Nafion membrane: A novel optical sensor for antioxidant capacity measurement of food extracts

Abstract

With increasing interest in consumption of antioxidant-rich food for fighting oxidative stress-related diseases, practical and low-cost tests for total antioxidant capacity (TAC) assessment are on the rise. We developed a sensitive solid membrane optical sensor for screening the TAC of food extracts and plant materials. The sensor was based on immobilizing a chromogenic oxidant, Fe(III)-o-phenanthroline (Fe(III)-phen), onto a Nafion cation-exchange membrane, and colorimetric measurement of the change in 510 nm-absorbance associated with highly-colored Fe(II)-phen formation upon reaction with antioxidants. The calibration curves with respect to the Fe(III)-phen sensing method of individual antioxidants comprising vitamins C and E, polyphenols and flavonoids were constructed, and their apparent molar absorptivities and linear concentration ranges determined. The limit of detection (LOD) and quantification (LOQ) for Trolox using the sensor were 0.26 and 0.87 mu M, respectively. The trolox equivalent antioxidant capacities (TEAC) of antioxidants found with the sensor were close to those of solution-phase Fe(III)-phen method, indicating that the immobilized reagent retained its reactivity toward antioxidants. This colorimetric sensor was validated through linearity, additivity, precision and recovery, showing its reliability and robustness. The sensor was tolerant to pH variations and turbidity, and used for screening the TAC values of some commercial fruit juices without pre-treatment. The sensor was more sensitive than the solution-phase method because the membrane concentrated the color from a larger volume solution. The sensor may be a good choice of field analytical chemists for rapid, simple and versatile determination of TAC of complex samples on-site (like a pH indicator-strip measuring H+ activity). (C) 2017 Elsevier B.V. All rights reserved.