Changes in Morphology and Ionic Transport Induced by ALD SiO 2 Coating of Nanoporous Alumina Membranes Virginia Romero, † Víctor Vega, ‡ Javier García, ‡ Robert Zierold, § Kornelius Nielsch, § Víctor M. Prida,* ,‡ Blanca Hernando, ‡ and Juana Benavente* ,† † Departamento de Física Aplicada I, Facultad de Ciencias, Universidad de Ma ́ laga, E-29071 Ma ́ laga, Spain ‡ Departamento de Física, Facultad de Ciencias, Universidad de Oviedo, Calvo Sotelo s/n, E-33007 Oviedo, Spain § Institut fü r Angewandte Physik, Universitä t Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany * S Supporting Information ABSTRACT: Nanoporous anodic alumina membranes (NPAMs) were produced by the two-step anodization method in sulphuric, oxalic and phosphoric acidic electrolytes displaying a hexagonally ordered spatial arrangement of pores with well controlled nanopore size distribution and low porosity. Some selected NPAMs were further modified by conformal coating their surface and inner pore walls with a thin layer of SiO 2 by means of atomic layer deposition (ALD), which reduces both the pore radii and porosity but it also seems to affect to the electric fixed charge on the membranes surface. A comparative study about the influence of silica modification of NPAMs surfaces on the ionic transport through the nanoporous membranes has been performed by measuring membrane potentials and electrochemical impedance spectroscopy with NaCl solutions. According to these results, a direct correlation between the membrane effective fixed charge and the NaCl diffusion coefficient can be established. The coating with a SiO 2 thin layer causes a reduction of 75% in the positive effective fixed charge of the NPAMs independently of their pore radii and the increase in counterion transport (cation transport number and diffusion coefficient) even through constrained nanopores, which can be of interest in several applications (microfluidics, drug delivery, nanofilter devices, etc.). Moreover, slight changes in the membrane/solution interface due to the SiO 2 cover layer are also indicated. KEYWORDS: nanoporous alumina membranes, ALD surface coating, membrane potentials, impedance spectroscopy 1. INTRODUCTION Nanoporous anodic alumina membranes (NPAMs) synthesized via electrochemical anodization of aluminum are formed by self-ordered structures with parallel aligned and well-defined pores keeping honeycomb structure geometry. These NPAMs have been widely employed as ordered templates for the synthesis of nanoparticles, nanotubes and nanowires, 1,2 and they are also applied in catalysis, hydrogenation, nano- electronics and optoelectronics devices. 3−5 The excellent chemical and thermal stability of NPAMs have favored their use in separation processes, mainly when heavy metal or corrosive products are involved. 6,7 By using tubular or multichannel geometry, it is possible to overpass their fragility, which is a negative characteristic of planar and thin alumina membranes when compared with polymeric samples. 8 Furthermore, the practically ideal porous structure of NPAMs allows their use as model systems for the study of mass and ions transport trough confined channels depending on both the solute/particle size and the pore effective charge, although this latter parameter might significantly reduce the co-ion transport and significantly increase interfacial effects, e.g., concentration- polarization. 9−11 Particularly, NPAMs are employed in biosensors construction because of their relatively high surface area for the retention of enzymes or bioactive compounds. 12,13 On the one hand, their accurate nanopore diameter and narrow pore size distribution are basic requirements for the precise control of molecular transport in areas such as biosensors or biomedical (drug-delivery) applications; 14,15 but on the other hand, specific features such as surface biocompatibility or hydrophilicity may also be of importance depending on the particular application. 16−18 In order to overpass surface effects on the transport of ions or charged molecules across NPAMs, but also tuning the pore size and chemical selectivity, surface coating by adequate materials is also performed. 19−22 In this context, SiO 2 is also widely used because of its excellent thermal and chemical stability together with its outstanding biocompat- ibility, features that make it interesting for bio-MEMS (or biological microelectromechanical systems) applications in drug-delivery devices. 23−25 Atomic layer deposition (ALD) is one of the most suitable techniques to perform controlled coatings of a wide range of materials over complex three-dimensional structures without Received: October 15, 2012 Accepted: April 10, 2013 Published: April 10, 2013 Research Article www.acsami.org © 2013 American Chemical Society 3556 dx.doi.org/10.1021/am400300r | ACS Appl. Mater. Interfaces 2013, 5, 3556−3564