Deposition of Polyelectrolyte Multilayer Film on a Nanoporous Alumina Membrane for Stable Label-Free Optical Biosensing

The stabilization of inorganic nanostructures is a key challenge in developing label-free optical biosensors using inorganic nanoporous films. In the present study, a polyelectrolyte multilayer film (PMF) composed of poly(acrylic acid) and poly(allylamine hydrochloride) was deposited on porous anodic alumina (PAA) film (pore diameter = 40 nm) by the layer-by-layer technique. The characteristics of the PMF as a protective coating layer against dissolution of the PAA film were examined by means of in situ optical waveguide spectroscopy (OWS), ex situ scanning electron microscope/energy-dispersive spectroscopy (SEM/EDS), and infrared reflection–adsorption spectroscopy (IR-RAS). The results obtained by OWS and SEM/EDS indicated the formation of PMF at both the alumina pore surface and the PAA film surfaces. Processing the PMF-deposited PAA (PMF–PAA) film at 180 °C to promote thermal cross-linking allowed PMF to act as a strong protective coating layer in aqueous buffer solutions (pH 2.5–8.7) but also in common alumina matrix etchant. Residual amino groups in the cross-linked PMF could also be used to conjugate single-probe DNA at the PMF–PAA film for OWS-based DNA–DNA hybridization assay. From the results obtained in this study, it was concluded that the deposition of PMF followed by thermal cross-linking is potentially useful not only as a protective coating and but also for immobilizing biorecognition elements in the PAA film.