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Open Access Nano Express

Engineering optical properties of gold-coated nanoporous anodic alumina for biosensing

Laura P Hernández-Eguía, Josep Ferré-Borrull*, Gerard Macias, Josep Pallarès and Lluís F Marsal

Author Affiliations

Department of Electronic, Electric and Automatics Engineering, Universitat Rovira i Virgili, Avinguda dels Països Catalans 26, Tarragona 43007, Spain

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Nanoscale Research Letters 2014, 9:414  doi:10.1186/1556-276X-9-414

Published: 21 August 2014

Abstract

The effect in the Fabry-Pérot optical interferences of nanoporous anodic alumina films coated with gold is studied as a function of the porosity and of the gold thickness by means of reflectance spectroscopy. Samples with porosities between 14 and 70% and gold thicknesses (10 and 20 nm) were considered. The sputtering of gold on the nanoporous anodic alumina (NAA) films results in an increase of the fringe intensity of the oscillations in the spectra resulting from Fabry-Pérot interferences in the porous layer, with a reduction in the maximum reflectance in the UV-visible region. For the thicker gold layer, sharp valleys appear in the near-infrared (IR) range that can be useful for accurate spectral shift measurements in optical biosensing. A theoretical model for the optical behavior has also been proposed. The model shows a very good agreement with the experimental measurements, what makes it useful for design and optimization of devices based on this material. This material capability is enormous for using it as an accurate and sensitive optical sensor, since gold owns a well-known surface chemistry with certain molecules, most of them biomolecules.

Keywords:
Nanoporous anodic alumina; Gold coating; Thin-film; Reflectance spectroscopy; Pore widening; Effective medium approximation; Modeling; Fabry-Pérot interferences