Diethylene glycol

HPLC-grade water, methanol, and acetonitrile were purchased from Fisher Scientific (Pittsburgh, PA, USA). Diethylene glycol was obtained from Junsei Chemical Co., Ltd. (Tokyo, Japan). Formic acid, pyridine, methoxyamine hydrochloride, N-methyl-N-(trimethylsilyl) trifluoroacetamide (MSTFA), potassium persulfate, 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), 6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox), sodium acetate, acetic acid, 2,4,6-Tris (2-pyridyl)-s-triazine (TPTZ), hydrochloric acid (HCl), iron (III) chloride hexahydrate, sodium carbonate, Folin–Ciocalteu’s phenol, sodium hydroxide, gallic acid, and naringin were obtained from Sigma-Aldrich (St. Louis, MO, USA).

Nickel oxide nanoparticles/reduced graphene oxide (NiO NPs/rGO) was synthesized using a microwave-assisted method. First, 10 mg of rGO (Angstron Materials, Dayton, OH, USA) and 50 mL of diethylene glycol (DEG, Junsei, Tokyo, Japan, 99%) were mixed in a 50 mL glass vial with ultrasonication for 3 h. Subsequently, 1 mL of 50 mM nickel (II) chloride hexahydrate (NiCl₂∙6H₂O, Junsei, 97%) and 1 mL of 0.5 M sodium hydroxide (NaOH, Junsei, 97%) were added into the solution, followed by sonication for 1 h. The solution was transferred to a 250 mL round flask and heated in a microwave oven with 700 W for 2 min. After cooling to room temperature, the solution was transferred to a 50 mL conical tube and centrifuged at 8000 rpm for 50 min. Then, the supernatant liquid was decanted, and the residual catalyst was washed with acetone (Daejung, Siheung, Korea, 99.9%) several times. Finally, the samples were dried in a vacuum oven at 60 °C overnight. As a control sample, nickel oxide nanoparticles/graphene oxide (NiO NPs/GO) were also synthesized by the same process.