Dopamine hydrochloride

Aniline (C₆H₇N), perchloric acid (HClO₄), tetrachloroauric acid (HAuCl₄), sulfuric acid (H₂SO₄), dopamine hydrochloride (C₈H₁₁NO₂∙HCl), di-sodium hydrogen phosphate (Na₂HPO₄), sodium di-hydrogen phosphate di-hydrate (NaH₂PO₄·2H₂O), sodium chloride (NaCl), potassium chloride (KCl), potassium ferrocyanide (K₄[Fe(CN)₆]), potassium ferricyanide (K₃[Fe(CN)₆]), hexaammineruthenium(II) chloride ([Ru(NH₃)₆]Cl₂), hexaammineruthenium(III) chloride ([Ru(NH₃)₆]Cl₃) and human male serum (type AB) were purchased from Merck (Milan, Italy).
Milli-Q water was used for all solutions. The buffer solution used in this work was 0.1 M phosphate buffer with 0.1 M NaCl pH 7.0 (PBS).

SU-8 2035 and 2075 were obtained from MicroChem, (Newton, MA, USA). Dopamine hydrochloride was purchased from Merck (Darmstadt, Germany). Potassium hexacyanoferrate (III) and potassium hexacyanoferrate (II) trihydrate were purchased from Fisher (Houston, TX, USA). The tris-buffered saline (TBS) was prepared from NaCl, KCl (both obtained from Sigma Aldrich, Deisenhofen, Germany) and Tris base purchased from Fisher. The phosphate buffer solution (PBS, pH 7.4) was obtained from Sigma Aldrich. All the chemicals were of analytical grade and deionized water was used throughout the experiments.

Dopamine hydrochloride (DA, 98%), epinephrine hydrochloride (EP, 98%), norepinephrine hydrochloride (NE, 98%), (L)-phenylalanine methyl ester, (L)-cysteine methyl ester, ethylene glycol dimethacrylate (EDMA), 2-hydroxyethyl methacrylate (HEMA), methanol, and acetic acid were supplied from Merck (Darmstadt, Germany). The chemicals used to prepare buffers in this research were sodium dihydrogen phosphate adjusted with phosphoric acid to obtain a pH of 1.5, acetic acid and sodium acetate for the preparation of acetate buffer to obtain a pH of 5.5, and potassium phosphate monobasic and potassium phosphate dibasic for the preparation of phosphate buffer to achieve a pH of 7.4. Ammonium chloride was adjusted by ammonia to obtain a pH of 8.5, and, finally, carbonic acid and sodium hydroxide adjusted the pH to 10.5, which were obtained from Merck (Darmstadt, Germany). Deionized (DI) water (18.2 MΩ cm) was utilized to make all of the aqueous solutions and treated with pure lab Ultra Analytic (ELGA Lab Water, Runcorn, UK). The pH meter was used to measure the pH, and the ultrasonic water bath was also used in solution preparation. Pencil graphite electrodes (PGEs) were used in pencil tips (Tombow 0.5, HB) obtained from local stationery stores.

Whatman filter papers (N. 42, slow) with a reported mean pore size of 2.5 μm and a thickness of 0.39 mm were chosen as commercial cellulose filter papers. Zinc chloride, silver nitrate solution (0.1 M), sodium hydroxide, calcium hydroxide, ethylene glycol and dopamine hydrochloride were purchased from Merck Chemical Co. Polyethyleneimine (PEI, branched, MW 25,000 Da) was supplied by Sigma Aldrich Chemical Co. Graphene oxide (GO) sheets were bought from Platonic Nanotech Co.

Cobalt (II) acetate tetrahydrate > 98.0%, copper (II) chloride dihydrate 99+%, titanium (IV) n-butoxide 99+%, and nickel (II) nitrate hexahydrate > 98.0% were purchased from Alfa Aesar (Kandel, Germany). Dopamine hydrochloride 100%, zinc (II) acetate dihydrate > 99.0%, iron (III) chloride 97%, potassium permanganate 99%, Nafion^® 5%wt solution, aqueous ammonia solution (30%), and sodium hydroxide 99.99% were purchased from Merck (Darmstadt, Germany). All aqueous solutions were prepared with ultrapure water (Milli-Q water, 18.2 MΩ cm). Platinum, nominally 20%, on carbon black and iridium (IV) oxide powder, 99%, purchased from Alfa Aesar, were used as benchmark catalysts.

Poly (lactic acid) grade Ingeo™ 2003D manufactured by Nature Works^® (Minnetonka, MN) was used as a polymer matrix. Dopamine hydrochloride (98%) was purchased from Merck. 1,1,1-Tris(4-hydro-xyphenyl) ethane (> 98.0% (GC)) was purchased from TCI. Hydrochloric acid (37%) was purchased from Sigma-Aldrich. ZnO (< 5 μm particle size, 99.9%) was purchased from Aladdin.