Copper(ii) sulfate, cobalt(ii) chloride hexahydrate, gold(iii) chloride solution, chloroplatinic acid, cerium(iii) hydrocarbonate, iron(iii) chloride, dopamine, ascorbic acid, Nafion (5% solution in 90% ethanol), 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), dipotassium hydrogen phosphate, potassium dihydrogen phosphate, glutaraldehyde (25%), cetyltrimethylammonium bromide (CTAB), and all other reagents and solvents used in this work were purchased from Sigma-Aldrich.