(217cy) Development of Nanoparticle-Stacked Membranes Fabricated By Using Functionalized Silver Nanoparticles

Introduction

  A water-treatment system using a membrane has attracted attentions for their energy- and space-saving potential. For this system, membranes with high rejection performance for solute and high flux are required. However, conventional polymeric membranes, fabricated by phase separation method, have a limitation to improve membrane performances. ¹⁾An active separation layer of polymeric membrane is thick, and precise control of pore sizes is sometimes difficult. In order to accomplish high membrane performance, composite membrane which has the separation layer on a supporting membrane has attracted attention.

  Recently, Penn et al. developed ferritin nanoparticles (NPs)-stacked membrane with a very thin, 60-nm active separation layer. ²⁾Their membrane had much higher flux compared than conventional polymeric membranes. NPs-stacked membranes have a great potential to improve membrane performance.

  On the other hand, layer-by-layer (LbL) method is one of the techniques of fabricating thin multilayer on substrate. Previously, the thin film was fabricated by LbL method using polyelectrolytes which have different charge. ³⁾

  In this study, we developed a nanoparticles-stacked ultrafiltration membrane using polyelectrolyte-stabilized silver (Ag) NPs as a model of inorganic NPs. Inorganic NPs have attracted attention as building blocks to construct advanced materials.⁴⁾ Inorganic nanoparticles were prepared by reducing metal ion in water. In that case, stabilizing agents are needed by controlling the aggregation of NPs in water. Moreover, high functionalization of NPs surface such as cationization and anionization is possible by introducing various stabilizing agents. ^{5, 6)}Uniform pore size membrane was fabricated from rigidity of NPs and durability such as chemical resistance is improved.

Experimental

  Oppositely charged Ag NPs were prepared using polyelectrolytes as stabilizing agents and stacked onto a support membrane by an LbL method. Cationic or anionic Ag NPs were prepared by a seed-growth method. ⁷⁾ Ag⁺ ions were reduced by NaBH₄and stabilized with polyethyleneimine (PEI ; Mw = 1,800) or poly(sodium 4-styrenesulfonate) (PSS ; Mw = 200,000). The Ag NPs were deposited onto an anodic aluminum oxide supporting membrane (Anodisc 25, pore size 0.02 mm) by the LbL method. Membrane performances were evaluated with a cross-flow membrane filtration system, and membrane structures were characterized with a field emission scanning electron microscope (FE-SEM).

Result

  From transmission electron microscope images of prepared NPs, the average diameters of cationic PEI-Ag NPs and anionic PSS-Ag NPs are ca. 13.1 nm and 8.8 nm, respectively. FE-SEM images showed that the oppositely charged NPs were uniformly stacked onto the supporting membranes and formed a very thin layer. The membrane repeated the NP depositing 30 times had 9.5 [m³/(m²･day･atom)] of water permeability and could cut off 500 kDa of dextran. On the other hand, the membrane repeated the NP depositing below 20 times hardly showed membrane performances. These results indicate that the Ag NPs stacked membranes fabricated by the presented method are applicable to membranes for solute rejection.

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