Platinum/Alumina thin films prepared by r.f. magnetron sputtering as platforms in voltammetric sensing

Thin alumina films coated platinum substrates (Pt/Al2O3), prepared by r.f. magnetron sputtering, wereused as working electrodes for voltammetric measurements. The thickness of the alumina layers var-ied between 50 and 1000 nm. For comparison, measurements were also performed with a platinummicrodisk 12.5 µm radius (Ptdisk). Several electroactive species either charged or uncharged, dissolved inaqueous, non-aqueous and micellar media were probed by the Pt/Al2O3and Ptdiskelectrode systems. Inall cases, sigmoid well-shaped cyclic voltammograms were recoded, typical for microelectrodes work-ing under steady-state. It was established that the Pt/Al2O3samples behaved as ensembles of recessednanoelectrodes, wired in parallel, each nanoelectrode sufficiently far apart, such that no overlap of indi-vidual radial diffusion profiles occurred. The recessed nanoelectrodes were due to defects or pinholesformed during the alumina deposition. The analysis of the steady-state limiting currents, obtained atthe different electrode systems, allowed verifying that no pore size effect was evident for either chargedor uncharged species, whose average hydrodynamic radii were lower than 1 nm. Size exclusion effectswere instead observed at the thickest alumina film, using ferrocene dissolved in the hydrophobic coreof sodium dodecylsulphate micelles, whose hydrodynamic radius is about 2 nm. The Pt/Al2O3systemswere also used as substrates for electroplating. In this way, ensembles of platinum black microelectrodeswere prepared and employed for the voltammetric sensing of hydrogen peroxide.