Folin–Ciocalteu Reagent-Loaded Acrylamide-Based Hydrogel Sensor for Antioxidant Capacity Measurement with the Molybdenum Green Method

The widely known Folin–Ciocalteu (FC) method of total antioxidant capacity (TAC) measurement of phenolic compounds (including phenolic antioxidants) operates only in the solution phase. In this study, a Folin–Ciocalteu sensor, which functions on a solid surface, was developed for the first time as an antioxidant capacity determination method. First, an acrylamide-based transparent hydrogel (AH) was synthesized, and the Folin–Ciocalteu reagent was loaded into this hydrogel to obtain a yellow sensor material (AH-FC). The structure of AH-FC was characterized by infrared (IR), scanning-transmission electron microscopy/energy dispersion X-ray spectrometry (STEM-EDX), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and Brunauer–Emmett–Teller (BET) surface analysis techniques. AH-FC, whose structure was elucidated, interacted with different phenol- and amine-type antioxidants to obtain a green color, different from the starting yellow color. The reason for the color change is thought to arise from the reduction of the molybdenum atom in the structure of the FC reagent from the (+6) oxidation state to the (+5) oxidation state in acidic medium, known as the “molybdenum green” method. The color change in the AH-FC structure was measured with the help of a smartphone application. In selectivity experiments with different phenolic compounds, as opposed to the classical Folin–Ciocalteu method, AH-FC responds only to antioxidants, making AH-FC even more important. Additionally, in experiments conducted in the presence of different cations, anions, sugars, and amino acids, AH-FC responds to antioxidants with a high recovery value of 94.6 to 104.0%. AH-FC applied to real samples of green tea, thyme, apple juice, and red wine was able to determine caffeic acid (CFA) with quantitative recovery in samples supplemented with a caffeic acid standard. Caffeic acid was determined in green tea extract using both the suggested AH-FC and the reference CUPRAC, FRAP, and DPPH methods, and the assay was validated through statistical t- and F- tests.