Superoxide dismutase

Cupric chloride (CuCl₂), l-ascorbic acid (AA), and sodium hydroxide (NaOH) were purchased from J&K Scientific (Beijing, China). 3,3′,5,5′-Tetramethylbenzidine (TMB) and 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) was bought from Macklin (Shanghai, China). Acetic acid (HAc), sodium acetate (NaAc), and potassium persulfate (K₂S₂O₈) was obtained from Sigma-Aldrich (Shanghai, China). Thiol-polyethylene glycol-OH (SH-PEG-OH) (Mw: 1000 Da) was purchased from Yare Biotech. Inc. (Shanghai, China). Catalase (purified powder from bovine liver), superoxide dismutase (purified powder from bovine erythrocytes), and peroxidase (purified powder from horseradish roots) were from Sangon Biotech (Shanghai, China). Phosphate-buffered saline (PBS, pH 7.4, Na₂HPO₄–NaH₂PO₄, 10 mM) solution was prepared in the laboratory. All chemicals and reagents were of analytical grade and used as received without further purification. Ultrapure water (18.2 MΩ cm⁻¹ at 25 °C) purified by a Milli-Q system was used throughout the experiment.

Chloroauric acid tetrahydrate (HAuCl₄-4H₂O), silver nitrate (AgNO₃), hydrogen peroxide, sulfuric acid and ascorbic acid (AA) were obtained from YangZhou LanTian Chemicals Co. Ltd. (China). 4-aminothiophenol (4-ATP), sodium chloride and ethanol were acquired from Jiangsu Younuo Chemicals Co. Ltd. (China). All of the materials were applied directly without further processing. Meanwhile, ELISA kits, superoxide dismutase and four necked round bottom flasks were all purchased from Sangon Biotech (Shanghai, China). Deionized water (resistivity >18.2 Ω) was used for the preparation of the specimens and throughout all the experiments. All glassware was dipped in aqua regia [HNO₃/HCl = 1:3 (v/v)] for over 24 h and washed with deionized water.