Functionalization of Nanoporous Materials Using Atomic Layer Deposition

Nanoporous alumina membranes provide several advantages over polymeric membranes for use in selective transport of biological materials. A process involving anodization, stripping of the oxide, and re-anodization is used to fabricate these materials. In nanoporous alumina membranes, nearly cylindrical pores are perpendicularly oriented to the surface of the material; the pores are arranged in a close-packed hexagonal cell structure.

Unlike polymeric membranes, nanoporous alumina membranes can be processed with straight pores, uniform pore sizes, high pore densities, small pore sizes (10-200 nm), and low membrane thicknesses (5 µm). Unlike nanoporous silicon, nanoporous alumina does not simulate calcium phosphate deposition and is stable in physiologic solutions.

Professor Narayan and colleagues at the Joint Department of Biomedical Engineering, University of North Carolina and North Carolina State University, Argonne National Laboratory, Savannah River National Laboratory, and North Dakota State University have recently performed several studies to examine the use of atomic layer deposition for modifying the surfaces of nanoporous alumina membranes. This technique may be used to decrease pore size while maintaining a narrow pore distribution.

In addition, atomic layer deposition may be used to impart biologically-relevant properties, including antifouling and antimicrobial properties, to the surfaces of nanoporous alumina membranes. In atomic layer deposition, self-terminating gas-solid reactions enable growth of a thin film on the surface of a nanoporous alumina membrane in a layer-by-layer fashion. Individual gas-solid reactions are separated by purge steps, which involve saturation with an inert gas. All of the surfaces of the coated material receive a conformal coating of identical thickness since the surfaces are saturated during each reaction. Due to these unique attributes, atomic layer deposition may be used for depositing conformal coatings with precise thicknesses onto nanoporous alumina membranes.

In one recent study, photocatalytic titanium oxide coatings were deposited on nanoporous alumina membranes by means of atomic layer deposition (Figure 1). Ellipsometry of Si (100) witness samples revealed that the atomic layer deposition growth rate for titanium oxide was 0.86-1.0 Å/cycle. Raman spectroscopy and powder X-ray diffraction of titanium oxide-coated nanoporous alumina membranes demonstrated features corresponding to the anatase phase of titanium oxide.

Figure 1. Plan-view scanning electron micrograph of a nanoporous alumina membrane after deposition of a conformal titanium oxide coating by means of atomic layer deposition.

Two pathogenic bacteria, Staphylococcus aureus and Escherichia coli, were used to examine the microbial proliferation on titanium oxide-coated and uncoated nanoporous alumina membranes. 20 nm pore size titanium oxide-coated nanoporous alumina membranes exposed to ultraviolet light demonstrated lower rates of Staphylococcus aureus and Escherichia coli attachment than uncoated membranes; these results were attributed to photocatalytic activity associated with ultraviolet light-titanium oxide interaction.

In another recent study, nanoporous alumina membranes were initially coated with platinum using atomic layer deposition and subsequently coated with 1-mercaptoundec-11-yl hex(ethylene glycol) using a self-assembly process. We evaluated adsorption of platelets, proteins, and other blood components of human platelet rich plasma to PEGylated, platinum-coated nanoporous alumina membranes. The pores of the PEGylated, platinum-coated nanoporous alumina membrane largely remained free of protein fouling; on the other hand, the pores of the platinum-coated nanoporous alumina membrane and the uncoated nanoporous alumina exhibited significant pore fouling and protein aggregation.

The results of our work suggest that atomic layer deposition may be used to impart biologically-relevant properties to nanoporous alumina membranes as well as other nanostructured biomaterials. Atomic layer deposition-modified nanoporous alumina membranes have numerous potential medical applications, including use in drug delivery and biosensing.

References

1. Adiga SP, Curtiss LA, Elam JW, Pellin MJ, Shih CC, Shih CM, Lin SJ, Su YY, Gittard SA, Zhang J, Narayan RJ, Nanoporous materials for biomedical devices, JOM, 60: 26-32, 2008
2. Narayan RJ, Monteiro-Riviere NA, Brigmon RL, Pellin MJ, Elam JW, Atomic layer deposition of TiO₂ thin films on nanoporous alumina templates: Medical applications, JOM 61: 12-16, 2009
3. Adiga SP, Jin C, Curtiss LA, Monteiro-Riviere NA, Narayan RJ, Nanoporous membranes for medical and biological applications, WIREs Nanomedicine and Nanobiotechnology, 1: 568-591, 2009
4. Narayan RJ, Adiga SP, Pellin MJ, Curtiss LA, Hryn AJ, Stafslien S, Chisholm B, Shih CC, Shih CM, Lin SJ, Su YY, Jin C, Zhang J, Monteiro-Riviere NA, Elam JW, Atomic layer deposition-based functionalization of materials for medical and environmental health applications, Philosophical Transactions of the Royal Society A, 2010, in press