## 2 Materials and Methods  
PAA were grown by aluminum foil (99.99%) anodization in mixed phosphoric acid/aluminum hydroxide bath whose composition was 0.4 M H\({}_{3}\)PO\({}_{4}\) added with Al(OH)\({}_{3}\) at different concentration, from 0.04 M to 0.095 M, and 0.13 M. For comparison, the PAA morphology obtained by aluminum anodization in pure 0.4 M H\({}_{3}\)PO\({}_{4}\) aqueous solution was also investigated. Prior to the anodizing, aluminum foils were degressed with acetone and electropolished in a stirred 4:1 mixture of ethanol and 60% perchloric acid for 5 min at 20 V and temperature below 10 \({}^{\circ}\)C, followed by ultrasonic rinsing in a cold ethanol bath for 5 min, and then in cold distilled water for 5 min. Anodization was carried out in a thermally insulated bath at 0 \(\pm\) 1 \({}^{\circ}\)C vigorously stirred by a magnetic needle to maintain uniform temperature on the electrode surface. The low electrolyte temperature was selected in order to form hard and abrasion-resistant anodic layers [4] that are highly desirable in PAA membranes because they are largely handled for any final use. The two-electrode cell for aluminum anodizing was powered at the constant voltage of 160 V for different times, in order to let pass electrical charges from 20 to 160 C cm\({}^{-2}\). Before anodizing, aluminum mass to be anodized was carefully determined by weighing, while the oxidized mass was evaluated by difference between the initial mass, expressed as Al\({}_{2}\)O\({}_{3}\), and PAA mass formed. Since the two values were found to be different, the total consumed aluminum was evaluated and also anodizing efficiency was determined as the ratio of PAA molar mass to oxidized aluminum molar mass (expressed as Al\({}_{2}\)O\({}_{3}\)). The PAA layer detached from the un-oxidized aluminum was weighed after having been again and again washed with distilled water and then dried overnight at 60 \({}^{\circ}\)C. The weight values of both aluminum and PAA were accepted when weighing was stable within \(\pm\)5%. The remnant metal was dissolved chemically in 0.1 M CuCl +20% _w/w_ HCl mixture, while the opening of the pore bottom (alumina barrier film) was achieved by chemical etching in aqueous 1 M NaOH solution. The weight measurements were performed employing a SARTORIUS microbalance (mod. Premium Microbalance ME36S). At acidic pH, more or less hydrated Al\({}^{3+}\) ions are present in solution by dissociation of Al(OH)\({}_{3}\) that is sparingly soluble in aqueous solutions. The true concentration of aluminum ions in solution was evaluated by back-titration with 0.0101 M ZnNO\({}_{3}\bullet\)6H\({}_{2}\)O [79]. A large excess of a standard solution of 0.0101 M EDTA was added to the solution containing Al(OH)\({}_{3}\) whose pH was buffered at 5 by CH\({}_{3}\)COONa/ CH\({}_{3}\)COOH. Xylenol orange was used as indicator. The saturation value of Al(OH)\({}_{3}\) in 0.4 M H\({}_{3}\)PO\({}_{4}\) solution was evaluated by the same procedure and it was found to be 0.138 M, slightly higher than the maximum concentration employed in aluminum anodizing (0.13 M). Characterization of the porous layers was basically conducted by a Field Emission Gun Scanning Electron Microscopy (FEG-SEM). In particular, an FEI QUANTA 200 FEG-ESEM electron microscope was used and both section and plan views were examined. Prior to inserting samples into the microscope chamber, they were gold-sputtered, even if in some cases the environmental option of the microscope was preferred in order to avoid morphological tricks due to the gold film. For the fabrication of nanostructures, in order to make the template electrically conductive, a thin film of Au was sputtered on one side of the membrane. Then membrane was mounted onto copper support by means of a conductive paste and the active surface (about 1 cm\({}^{2}\)) was delimited by means of an insulating lacquer. Electrochemical experiments were performed using a P.A.R. Potentiostat/Galvanostat (mod.