Caffeic

Standard solutions were prepared for the fifteen investigated phenolic compounds and flavonoids, namely: kaempferol, luteolin, phenylacetic acid (all Alfa Aesar, Ward Hill, MA, USA), apigenin, chrysin, quercetin, p-coumaric acid, naringenin (all Sigma-Aldrich, Merck, Darmstadt, Germany), ferulic, syringic, vanillic, caffeic, ellagic, gallic and benzoic acids (Acros Organics, Geel, Belgium). The solutions were prepared by dissolving the standards in HPLC grade methanol (ultragradient grade; Carlo Erba, Milan, Italy) to produce stock solutions of 100 mg/L, which were then used to prepare 50, 40, 30, 20 and 10 mg/L solutions for the standard plots. In addition, a mixture containing 30 mg/L of each phenolic compound in methanol was also prepared. Two mixtures of p-coumaric and ferulic acid were prepared, one with equal concentrations of 50 mg/L each and the other with 30 mg/L p-coumaric and 70 mg/L ferulic acid. Another similar set of mixtures was also prepared using vanillic and caffeic acids. We measured the absorbance of the samples of 100 mg/L of each phenolic compound and flavonoid with a UV-Vis spectrophotometer (UV-2600; Shimadzu, Tokyo, Japan) at wavelengths between 180 and 480 nm to find the optimum wavelength for the HPLC-DAD measurements.

Vanillic (4-hydroxy-3-methoxybenzoic) acid (+98.5%), caffeic (3,4 dihydroxycinnamic) acid (98%), p-Coumaric acid (ACROS Organics, Thermo Fisher Scientific, Fair Lawn, NJ, USA) (98%), gallic (3,4,5-trihydroxybenzoic) acid (99%) (Xilong Chemical Industry Incorporated Co., Ltd, Guangdong, China) and tyrosol ((2-(4-hydroxyphenyl) ethanol) (98%) (Aldrich, German) were used to create an OMW synthetic solution.
CNTs (NC7000, Nanocyl company, Sambreville, Belgium), polyethersulfone (PES) (MW = 58,000 g/moL with polymerization degree 259, Goodfellow Company, Huntingdon, London), polyvinylpyrrolidone (PVP) (MW = 58,000 g/moL, ACROS Organics, Thermo Fisher Scientific, Fair Lawn, NJ, USA), 70% N-methyl-2-pyrrolidinone (NMP) (SupraSolv, Darmstadt, Germany), Folin-Ciocalteu phenol reagent (Scharlau, Barcelona, Spain) and sodium carbonate anhydrous (Na₂CO₃) (Fischer, Shanghai, China) were also used in this study.