Tannic acid

The lipid used for making lipid/polymer membrane was tetradodecylammonium bromide (TDAB), and the preparation solvent was tetrahydrofuran (THF), both of which were purchased from Sigma-Aldrich, Inc. (St. Louis, MO, USA). Dioctyl phenyl-phosphonate (DOPP) and polyvinyl chloride (PVC) were purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). The caffeine sample was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). The modifiers studied were 2,6–DHBA, 3,4-dihydroxybenzoic acid (3,4–DHBA), 3,5–dihydroxybenzoic acid (3,5–DHBA), 2–hydroxybenzoic acid (2–HBA), 2,5–dihydroxybenzoic acid (2,5–DHBA), and 2,4,6–trihydroxy benzoic acid (2,4,6–THBA). All modifiers were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Tannic acid and potassium chloride (KCl) were used to make the reference solution, and these were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Figure 1 shows the molecular formulae of TDAB, DOPP, and caffeine.

The lipid/polymer membrane comprises tetradodecylammonium bromide (TDAB) as the lipid, dioctyl phenyl-phosphonate (DOPP) as the plasticizer, and polyvinyl chloride (PVC) as the supporting material. Tetrahydrofuran (THF) was used as the organic solvent. TDAB and THF were purchased from Sigma-Aldrich (St. Louis, MO, USA). DOPP was acquired from Dojindo Molecular Technologies (Kumamoto, Japan), while PVC was obtained from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan).
Four types of modifiers were utilized, including terephthalic acid (TPA), 2-hydroxyterephthalic acid (2-HTA), 2,6-dihydroxyterephthalic acid (2,6-DHTA), and 2,6-DHBA. 2-HTA and TPA were procured from Tokyo Chemical Industry (Tokyo, Japan). 2,6-DHTA was sourced from BLDpharm (Shanghai, China), and 2,6-DHBA was acquired from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan).
MSG, potassium chloride (KCl), tartaric acid, quinine hydrochloride, tannic acid, and sucrose were purchased from Kanto Chemical Co. (Tokyo, Japan). MSA was purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). Figure 1 shows the structural formula of TDAB, PAEE, TPA, 2-HTA, 2,6-DHTA, 2,6-DHBA, MSG, and MSA.