*2.1. Reagents*

Potassium ferricyanide (K3Fe(CN)6), iron(III) chloride (FeCl3 × 4H2O), copper(II) sulfate (CuSO4), Cerium(IV) sulfate tetrahydrate Ce(SO4)2 × 4H2O, palladium chloride (PdCl3), cobalt(II) chloride (CoCl2 × 6H2O), zinc(II) sulfate (ZnSO4), manganese(II) chloride (MnCl2 × 4H2O), cadmium(II) chloride (CdCl2), neodymium(III) chloride (NdCl3), 2,2- azinobis (3-ethylbenzothiazoline-6-sulfonate) diammonium salt (ABTS), o-dianisidine, hydrogen peroxide (H2O2, 30%), sodium ethylenediaminetetraacetate (EDTA), sodium

L-lactate, Nafion (5% solution in 90% low-chain aliphatic alcohols) and all other reagents and solvents used in this work were purchased from Sigma-Aldrich (Steinheim, Germany). All reagents were analytical grade and were used without further purification. All solutions used ultra-pure water prepared with the Milli-Q® IQ 7000 Water Purification system (Merck KGaA, Darmstadt, Germany).
