High purity water obtained from a Millipore Rios 5TM reverse
osmosis and a Millipore Milli-Q Academic™ system (resistivity
18.2 MO cm) was used throughout to prepare all solutions.
Suprapur™ nitric acid (Merck, Darmstadt, Germany) was always
used in all steps of developed procedure. A solution of tungsten
for carry out the permanent modification, was prepared dissolving
0.1794 g of Na2WO42H2O (Merck) to a volume of 100 mL.