Fabrication of mechanically robust, large area, polycrystalline nanotubular/porous TiO₂ membranes

Abstract

This work describes the fabrication of self-standing, mechanically robust, polycrystalline (anatase) TiO₂ nanotube array membranes of uniform pore size distribution. The membranes are made as flat sheets with thicknesses ranging from approximately 4.4 μm to 1 mm. To evaluate biofiltration applications, we have used glucose, phenol-red, bovine serum albumin (BSA), and immunoglobulin (IgG) as model molecules for studying the diffusion characteristics through the nanotube array membranes. It is found that the natural log of the molecule diffusion coefficients scale linearly with molecular weight. To date, we have fabricated membranes up to 12.5 cm², a size limited only by the processing equipment.

Introduction

The majority of membranes currently used for separation of sub-micron particles in biochemical applications are of the asymmetric or anisotropic variety. They are either made from polymers such as polysulfone, polyacrylonitrile, polyamides, etc. or ceramics such as alumina (Millipore isopore or Whatman filters). There are several incompatibilities associated with such membranes: (a) sensitization to sterilization techniques; (b) wide pore size distribution; (c) adhesion of various proteins and biomolecules to membranes thus leading to biofouling and (d) leaching of contaminants from polymers [1]. Most common polymeric membranes are made through a solvent-casting process, which results in a pore size distribution with variations as large as 30%. The use of ion-track etching to form membranes (Millipore Isopore) produces a much tighter pore size distribution (±10%). However, these membranes have low porosities (<10⁹ pores/cm²), limited pore sizes, and the pores are randomly distributed across the surface. Porous alumina (e.g. Whatman filters) has also been used to achieve uniform pore size. Although, these membranes have higher pore density (>10¹⁰ pores/cm²), only certain pore sizes (typically greater than 40 nm) can be achieved, and the pore configurations and arrangements are difficult to control. Thus, development of well-controlled, stable and uniform membranes capable of complete separation of viruses, proteins or peptides is an important consideration for biofiltration application. To that end, we have developed TiO₂ nanotubular/porous membranes with well-controlled and uniform pores capable of separating nano-scale particles.

There are several techniques of fabricating more robust metal–oxide nanoporous membranes. For example, porous titania membranes can be fabricated via nanoparticle sintering; however these membranes do not possess uniform pore size and shape. Crystalline mesoporous membranes with well-defined pore size and shape can be fabricated via surfactant templating or similar routes, but are generally not strong enough for normal handling. Ultrathin nanoporous titania membranes can be fabricated by preparing a self-supporting polymer–titania composite film on a nanoporous alumina disk, and then removing the polymer using chemical methods or plasma treatment [2]. However, these membranes are also difficult to handle and the polymer removal disturbs the pore size and shape. Further, nanoporous alumina membranes, which can be easily fabricated and are mechanically robust, have relatively poor biocompatibility thus limiting their application to biofiltration [3], [4], [5]. Nanoporous alumina has been used as a template for membrane construction, with the exterior surface coating material deposited onto the alumina membrane [6]; practical application of these membranes to biofiltration requires a perfect coating without pinholes such that the alumina is not exposed to the ambient.

In this work, we have developed fabrication strategies for successful development of flat and large-area TiO₂ (titania) nanotube array membranes of uniform nano-scale pore size. Titania is a unique semiconductor ceramic that possesses excellent biocompatibility and high photocatalytic activity. The so-called ‘nanotube array’ consists of parallel-oriented, vertically aligned nanotubes, with each nanotube having an open top end and closed bottom end (barrier layer) like that of a common laboratory test tube. The initial nanotube array fabrication route reported in 2001 by Gong et al. [7] yielded nanotube lengths up to approximately 0.4 μm [7], [8], [9]. The authors then made the first report of other electrolyte compositions that could yield nanotube lengths up to about 6 μm [10]. In 2006, Grimes and co-workers reported the first use of polar organic solvents such as dimethyl sulfoxide (DMSO), ethylene glycol, formamide, and N-methylformamide to achieve nanotube array lengths of several hundred microns [11], [12]. Recently we reported the fabrication of nanotube arrays up to 1005 μm in length [13] and application of the resulting membranes to bio-diffusion. Our research group has also developed techniques to precisely control the structural characteristics of the nanotube array films, including individual nanotube dimensions such as pore size, wall thickness, length, tube-to-tube connectivity [14], and crystallinity [15], [16]. Proper selection of the electrolyte composition, anodization voltage and duration has enabled variation of pore size from 10 nm [17] to 242 nm [18], outer diameters from 48 nm [12] to 256 nm [11], wall thicknesses from 5 nm [17] to 34 nm [19], [20] and tube-to-tube spacing from several tens of nanometers to effectively nothing [15]; thus enabling us to fabricate membranes with porosities ranging from 60 to 70%. We propose to use these titania nanotubular membranes for protein separation applications. Of particular interest is that TiO₂ is an active photocatalytic material, giving rise to the possibility of self-cleaning, bioactive filters when used in combination UV light.

Glucose (MW: 180 Da), and Immunoglobulin G (IgG; MW: 150 kDa) were chosen as model molecules to study the diffusive transport through TiO₂ nanoporous membranes. The molecular weight of glucose is similar to that of the metabolic wastes (MW < 12 kDa) associated with kidney failure (e.g. urea MW: 60 Da, uric acid MW: 168 Da, etc.) that are targeted for removal by dialysis and haemofiltration, and hence was used as model molecule for diffusion studies. Further, IgG is a serum antibody secreted by b-cells and plasma cells that accounts for 75% of serum antibodies. Unlike the other types of immunoglobulins, IgG is the smallest class of immunoglobulin and circulates in the body as a monomer. It aids the immune system in protecting the body against potentially harmful antigens such as bacteria, viruses, and cancer cells by binding to the antigens and eliminating them through neutralization, opsonization, and complement activation. It also plays a role in transplant rejection since they are designed to recognize any foreign particles that might be harmful. Furthermore, the specificity of IgG makes them ideal candidates for targeted therapeutic applications. Currently therapeutic antibodies are used to treat a variety of illnesses such as cancer, autoimmune disease, and cardiovascular disease. Controlled diffusion of IgG through membranes is important for successful immunoisolation type devices; hence it was used as model molecule to investigate diffusive properties of nanotubular membranes. We also used phenol red (MW: 354.4 Da) and bovine serum albumin (MW: 66 kDa) as model molecules for diffusion studies since their molecular weights are between glucose and IgG.