Sodium nitrite

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 99.99%), 2-methylimidazole (98%) and nitric acid (HNO₃, 65–68%) were purchased from Aladdin. Perchloric acid (HClO₄, 70–72%), sulfuric acid (H₂SO₄, A.R.), hydrochloric acid (HCl, 36–38%), acetic acid (CH₃COOH, 99.5%), methanol (CH₃OH, A.R.), ethanol (C₂H₅OH, A.R.), sodium nitrite (NaNO₂, A.R.) and sodium acetate anhydrous (CH₃COONa, A.R.) were purchased from Beijing Chemical Works. Tetraethyl orthosilicate (TEOS, C₈H₂₀O₄Si, 99%) was obtained from Innochem. Ferrous acetate (Fe(C₂H₃O₂)₂, 95%) was purchased from J&K Chemicals. 1,10-phenanthroline and sodium hydroxide (NaOH, 95%) were obtained from Macklin. Commercial Pt/C (40 wt%) was obtained from BASF. Nafion alcohol (5 wt%, D520) was obtained from Aldrich. Nafion 211 membrane was obtained from DuPont. All chemicals were used without further purification. All aqueous solutions were prepared using deionized (DI) water with a resistivity of 18.2 MΩ.

Sulfanilic acid was purchased from TCI (Shanghai) Development Co., Ltd., (Shanghai, China). Natural graphite flakes were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sodium carbonate, concentrated hydrochloric acid, sodium borohydride, hydrazine (80%, N₂H₄·H₂O), and sodium nitrite were purchased from Beijing Chemical Works. Polyacrylonitrile (PAN, average Mw 150,000) was purchased from HuBei ChuShengWei Chemistry Co., Ltd., (Wuhan, China).

Acetonitrile 99.9%, n-hexane 95% and methanol 99.9% were purchased from Fisher Scientific (Shanghai, China). The fatty acids methyl ester (FAME) reference standard mixture 37 (standard 47885-U) was purchased from Sigma (St. Louis, MO, USA). Sugars standards [glucose, sorbitol and d(+)-trehalose and myo-inositol], phenolic standards (gallic, 4-hydroxybenzoic, 3,4-dihydroxybenzaldehyde, homogentisic, p-coumaric and o-coumaric acids) were purchased from Sigma (St. Louis, MO, USA) or Sigma-Aldrich Chem (shanghai, China). 1,1-Diphenyl-2-picryl-hydrazyl (DPPH), BHT, 3-(2-pyridyl)-5,6-bis (4-phenyl-sulfonic acid)-1,2,4-triazine (ferrozine), rutin and ascorbic acid were purchased from Sigma Chemical Co. BSTFA + TMCS (99:1) was purchased from Supelco. Folin–Ciocalteu’s phenol reagent was purchased from Beijing Solarbio reagent company (Beijing, China). Ferrous chloride, sodium salicylate, ferrous sulphate, hydrogen peroxide, potassium ferricyanide, trichloroacetic acid, disodium phosphate, sodium dihydrogen phosphate, ferric chloride, sodium carbonate, sodium hydroxide, aluminium nitrate, sodium nitrite, petroleum ether (60–90 °C), ethanol and methanol were all purchased from Beijing chemical plant (Beijing, China). All chemicals were of analytical grade. Water was treated in a Milli-Q water purification system (TGI Pure Water Systems, USA).