Sulfuric acid h2so4

Multi-wall carbon nanotubes (MWCNTs, 98%, Sigma Aldrich, Seoul, Korea), zinc acetate dihydrate (Zn(OAc)₂∙2H₂O, 98%, Duksan, Seoul, Korea), hydrochloric acid (HCl, 35%, Duksan, Seoul, Korea), Sulfuric acid (H₂SO₄, 98%, Duksan, Seoul, Korea), ethanol (CH₃CH₂OH, 99.9%, Duksan, Seoul, Korea), 8-hydroxyquinoline (99%, Junsei Chemical, Tokyo, Japan), aluminum chloride (AlCl₃, 99%, Fluka, Seoul, Korea), and potassium hydroxide (KOH, 85%, DAEJUNG, Seoul, Korea). All chemicals were used as received without any further purification.

Chitosan-coated iron oxide nanomaterials (γ-Fe₂O₃, average hydrodynamic diameter: 50 nm) were purchased from Chemicell (Berlin, Germany). Potassium dichromate (K₂Cr₂O₇, 99%) was purchased from Sigma Aldrich (Yongin, Korea). Hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH), acetic acid (CH₃CO₂H, 99%), acetone, sulfuric acid (H₂SO₄, 97%), and 1,5-diphenylcarbazide (DPC, C₁₃H₁₄N₄O, 99%) were purchased from Duksan Pure Chemicals Co. Ltd. (Daejeon, Korea). All reagents were of analytical grade and used without further purification. Hydrophilic poly(vinylidene fluoride) membrane (PVDF, pore size = 0.20 μm, diameter = 47 mm) was purchased from Hyundai Micro Co. Ltd. (Daejeon, Korea). The vacuum filtration apparatus was purchased from Phenomenex Inc. (Torrance, CA, USA).

Graphite powder, dopamine (DA) hydrochloride, L-(+)-ascorbic acid (AA), uric acid (UA), citric acid (CA), glucose, potassium hexacyanoferrate (K₃[Fe(CN)₆]), phosphate buffer saline (PBS, pH 7.4, 0.1 M), and hydrogen tetrachloroaurate (III) (HAuCl₄) were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). All the other chemicals used such as sodium hydroxide (NaOH), sodium nitrate (NaNO₃), sodium carbonate (Na₂CO₃), potassium permanganate (KMnO₄), hydrogen peroxide (H₂O₂), and sulfuric acid (H₂SO₄) were obtained from Duksan Pure Chemicals Co. (Ansan, Korea). All the reagents were analytical grade and were used without further purification. Deionized (DI) water was used throughout the work.