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Fabrication of Double-Alumina Membrane Filters with Controlled Pore Diameters by Direct Dissolution of Al through Nanopores in Anodic Porous Alumina
Summary
Researchers fabricated a double-layered alumina membrane filter with independently controllable pore sizes in each layer — allowing particles of a specific size range to be selectively captured between the two membranes — and demonstrated the concept by size-sorting polystyrene particles and identifying them by Raman spectroscopy directly through the filter. The approach offers a new tool for analyzing nanoplastics and fine atmospheric particles by size class without needing to transfer samples. Improved size-selective capture and analysis methods are essential for tracking the smallest, most biologically concerning plastic particles in environmental samples.
Filters composed of two membrane filters with different pore diameters can be used to selectively capture particles of specific sizes from solutions or the gas phase. Such double-membrane filters are useful in environmental analysis and particle size classification. In this study, a double-membrane filter made of alumina was fabricated by anodization of an Al substrate. We found that a double-membrane filter can be fabricated by forming anodic porous alumina on both sides of an Al sheet and directly dissolving residual Al from the sample’s nanopores. The interpore distance and pore diameter of the two alumina membranes constituting the obtained filter can be independently controlled. Furthermore, because the thickness of the alumina membranes in the filters can be controlled by varying the anodization time, a filter composed of two thin membranes with high permeability was obtained. When an aqueous solution containing three types of particles with different sizes was passed through the obtained double-membrane filter, it was demonstrated that the particles could be size-selectively captured between the two filters. After size-selectively capturing polystyrene particles on a double-membrane filter, Raman spectroscopy of the samples was performed through the filter, resulting in the detection of a Raman spectrum attributable to polystyrene. From these results, the double-membrane filter obtained by this method is expected to be applicable to the size-selective analysis of marine nanoplastics and atmospheric particulate matter.