title: Determination of total antioxidant capacity of humic acids using CUPRAC, Folin-Ciocalteu, noble metal nanoparticle- and solid-liquid extraction-based methods.
abstract: Total antioxidant capacity (TAC) of humic acid (HA) samples was determined using CUPRAC (CUPric Reducing Antioxidant Capacity), FC (Folin-Ciocalteu), QUENCHER-CUPRAC, QUENCHER-FC, Ag-NP (Silver nanoparticle)‒ and Au-NP (Gold nanoparticle)‒based methods. Conventional FC and modified FC (MFC) methods were applied to solid samples. Because of decreased solubility of Folin-Ciocalteu's phenol reagent in organic solvents, solvent effect on TAC measurement was investigated using QUENCHER-CUPRAC assay by using ethanol:distilled water and dimethyl sulfoxide:distilled water with varying ratios. To see the combined effect of solubilization (leaching) and TAC measurement of humic acids simultaneously, QUENCHER experiments were performed at 25°C and 50°C; QUENCHER-CUPRAC and QUENCHER-FC methods agreed well and had similar precision in F-statistics. Although the Gibbs free energy change (ΔG°) of the oxidation of HA dihydroxy phenols with the test reagents were negative, the ΔG° was positive only for the reaction of CUPRAC reagent with isolated monohydric phenols, showing CUPRAC selectivity toward polyphenolic antioxidants. This is the first work on the antioxidant capacity measurement of HA having a sparingly soluble matrix where enhanced solubilization of bound phenolics is achieved with coupled oxidation by TAC reagents.
authors.affiliation: Yalova University, Faculty of Engineering, Department of Chemical and Process Engineering, 77100, Yalova, Turkey.
authors.forename: Şeyda
authors.initials: Ş
authors.lastname: Karadirek
authors.affiliation: Yalova University, Faculty of Engineering, Department of Chemical and Process Engineering, 77100, Yalova, Turkey.
authors.forename: Nergis
authors.initials: N
authors.lastname: Kanmaz
authors.affiliation: Yalova University, Faculty of Engineering, Department of Chemical and Process Engineering, 77100, Yalova, Turkey.
authors.forename: Zeynep
authors.initials: Z
authors.lastname: Balta
authors.affiliation: Yalova University, Faculty of Engineering, Department of Chemical and Process Engineering, 77100, Yalova, Turkey.
authors.forename: Pelin
authors.initials: P
authors.lastname: Demirçivi
authors.affiliation: Istanbul University, Faculty of Engineering, Department of Chemistry, Avcilar 34320, Istanbul, Turkey.
authors.forename: Ayşem
authors.initials: A
authors.lastname: Üzer
authors.affiliation: Yalova University, Faculty of Engineering, Department of Chemical and Process Engineering, 77100, Yalova, Turkey. Electronic address: hizalyucesoy@gmail.com.
authors.forename: Jülide
authors.initials: J
authors.lastname: Hızal
authors.affiliation: Istanbul University, Faculty of Engineering, Department of Chemistry, Avcilar 34320, Istanbul, Turkey; Turkish Academy of Sciences (TUBA), Turkey.
authors.forename: Reşat
authors.initials: R
authors.lastname: Apak
mesh_terms: D000975:Antioxidants; D006812:Humic Substances; D059625:Liquid-Liquid Extraction; D053768:Metal Nanoparticles; D010636:Phenols; D010936:Plant Extracts; D052616:Solid Phase Extraction
chemical_list: D000975:Antioxidants; D006812:Humic Substances; D010636:Phenols; D010936:Plant Extracts; C011471:folin
keywords: Antioxidant capacity; CUPRAC method; Folin–Ciocalteu method; Humic acids; QUENCHER method; Silver and gold nanoparticles