4 4 bipyridine

All chemicals were of analytical grade and were used without further treatment. Phosphate buffer solution (PBS, 0.1 M) was prepared by mixing NaH₂PO₄ and Na₂HPO₄, obtained from Macklin. CuSO₄·5H₂O (99%), NaCl, NaBr, NaI, 4,4’-bipyridine, and ethanol were bought from Macklin. Hydrogen peroxide (H₂O₂, 30 wt%) was purchased from Beijing Chemicals (Beijing, China) and stored in a 4 °C refrigerator. Ultrapure water was used to prepare all aqueous solutions throughout a Millipore system (18.2  MΩ; Millipore Co., USA).

4,4-bipyridine, iodomethane, 3-bromo-1-propanamine hydrobromide, bromoacetic acid, carboxymethylcellulose sodium salt (CMC, 1200 cps), Polyethyleneimine (PEI, M_n = 10 K), Hyaluronic acid (HA, M_n = 100 K), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC·HCl), N-hydroxysulfosuccinimide sodium salt (sulfo-NHS), MES (2-morpholinoethanesulfonic acid) buffer (0.2 M, pH = 5.5), 1-adamantanamine hydrochloride (AdH), ferric chloride (FeCl₃), potassium thiocyanate (KSCN), cobaltous chloride (CoCl₂·6H₂O), 2-methylimidazole (2-MI), Zinc Nitrate (Zn(NO₃)₂·6H₂O), horseradish peroxidase (HRP), glucose oxidase (GOD) and o-phenylenediamine (OPDA) were purchased from Macklin. l-lactic acid (LLA), 4-Phenylazophenol, 2-Hydroxynaphthalene, 1-Pyrenol, Poly-l lactic acid (PLLA, M_n = 3.0 K), and Stannous octoate were purchased from J&K. The above reagents were used as received unless otherwise noted. All anhydrous solvents (toluene, dichloromethane (DCM), acetonitrile (MeCN), tetrahydrofuran (THF), N, N-dimethylformamide (DMF), and ethanol) were purchased commercially and used without further purification.