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, were used as working electrodes for voltammetric measurements. The thickness of the alumina layers varied between 50 and 1000 nm. For comparison, measurements were also performed with a platinum microdisk 12.5 μm radius (Ptdisk). Several electroactive species either charged or uncharged, dissolved in aqueous, non-aqueous and micellar media were probed by the Pt/Al2O3 and Ptdisk electrode systems. In all cases, sigmoid well-shaped cyclic voltammograms were recoded, typical for microelectrodes working under steady-state. It was established that the Pt/Al2O3 samples behaved as ensembles of recessed nanoelectrodes, wired in parallel, each nanoelectrode sufficiently far apart, such that no overlap of individual radial diffusion profiles occurred. The recessed nanoelectrodes were due to defects or pinholes formed during the alumina deposition. The analysis of the steady-state limiting currents, obtained at the different electrode systems, allowed verifying that no pore size effect was evident for either charged or uncharged species, whose average hydrodynamic radii were lower than 1 nm. Size exclusion effects were instead observed at the thickest alumina film, using ferrocene dissolved in the hydrophobic core of sodium dodecylsulphate micelles, whose hydrodynamic radius is about 2 nm. The Pt/Al2O3 systems were also used as substrates for electroplating. In this way, ensembles of platinum black microelectrodes were prepared and employed for the voltammetric sensing of hydrogen peroxide.