All chemicals used were of analytical grade and used as purchased without further treatment. Boric acid, iron(iii) nitride, sodium oxalate, potassium bromate, and hydrogen peroxide were obtained from CARL ROTH GmbH (Karlsruhe, Germany). Ammonia at 25% was purchased from Merck (Darmstadt, Germany). Isopropyl alcohol was purchased from Fluka AG in Sigma Aldrich (Steinheim, Germany). Ascorbic acid was purchased from Scharlau Chemicals (Barcelona, Spain) and folic acid was purchased from UNI-CHEM (China) chemicals.