Engineering of Nanomembranes for Emerging Applications

Membranes play an essential role in nature as well in many industries including water treatment, energy, health and agro-business, where commercial membrane technologies using synthetic polymeric and ceramic membranes have been used for the past 50 years ago. With an annual market valued of about $10 billion and emerging markets for membrane applications in fuel cells, hydrogen production, clean water production, wastewater treatment, air pollution control, catalysis, food processing, drug delivery, and medical devices.

Nanoscience and nanotechnology is recognized as the key strategy to improve conventional and develop new membrane technologies by exploring novel nanomaterials and nano-scale processes. The development of new nanomembranes using advanced nanofabrication approaches has rapidly progressed in recent years, and their application beyond separation processes is extended into new application areas.

Dr Losic and his research group at Ian Wark Research Institute (IWRI), University of South Australia, Adelaide, are working on development of new nanomembranes with particular focus on designing their specific functional properties toward emerging applications, including targeted molecular separations, biosensing, and implantable drug delivery (Fig.1). The approach is directly inspired by nature, for example biosilica membranes in diatoms (single cell algae) where bio-mimetic principles are applied for development of key membrane functions such as the selective molecular transport, energy transport and signalling (sensing).

Figure 1. Nanomembranes for emerging applications: a) molecular separations, b) biosensing and c) drug delivery

To design nanomembranes with desired functions and properties, Dr Losic's group developed a series of fabrication protocols to precisely control their most critical parameters, including pore diameters, pore geometry and surface chemistry. The self-ordering electrochemical process is selected as the nanofabrication approach because it is simple, inexpensive, lithography free and highly flexible to perform structural engineering at the nanoscale.

Typical nanomembrane structures (alumina oxide) routinely fabricated in our lab (Fig. 2) show highly organized, vertically aligned pore channels with controllable structural dimensions, including pore diameters (10-200 nm), inter-pore distances (50 to 400 nm), high pore aspect ratio, pore density (10⁹ - 10¹¹ cm^-2), porosity (10-70 %), membrane thickness (1- 500 µm), and excellent thermal, chemical stability and bio-compatability. The structural features of nanomembranes can be easily controlled and tunned by adjusting conditions (electrolyte, voltage, current, temperature and time) during fabrication.

Figure 2. SEM images of pore structures of nanomembranes (alumina oxide) fabricated by self-ordering electrochemical anodization. A) top surface and b) cross-section

The challenging problem of fabricating nanomembranes with shaped and ratchet pore geometry was solved through development of a unique electrochemical nanofabrication method called cyclic anodization. Therefore design of nanomembranes with complex and hierarchical pore architectures allows us for the first time to use the pore shape as a strategy for molecular separation. A new concept for selective molecule separation using these periodic nano-ratchets is under development to underpin new separation technology (Fig. 3).

Figure 3. Nanomembranes with shaped pore geometries fabricated by cyclic anodization

To advance the properties of fabricated nanomembranes we developed several strategies for their additional structural and chemical functionalization involving modification processes such as metal coating (chemical and electrochemical), carbon nanotube growth, atomic layer deposition, plasma polymerization, and surface functionalization.

Composite nanomembranes, with precisely controlled pore dimeters (down to a few nm), were engineered with gold, nickel, carbon, polymers and nanoparticles. The transport properties and size and chemical selectivity of membranes have been significantly improved to meet demanding requirements for fast and selective molecular separation.

The unique magnetic, ion-exchange, electrocatalytic and optical properties (SERS, interferometric) of these membranes offer excellent potential, with the development of chip based and label-free nanopore biosensors for biomedical diagnostics and implantable drug delivery of particular interest for our group at Ian Wark Research Institute.

References

1. D. Losic, M. A. Cole, B. Dollmann, K. Vasilev, H. J. Griesser, Surface modifications of nanoporous alumina membranes by plasma polymerisation, Nanotechnology, 2008, 19, 245704
2. D. Losic, S. Simovic, Self-ordered nanopore and nanotube platforms for drug delivery applications, Expert Opinion in Drug Delivery, 2009, DOI: 10.1517/17425240903300857
3. L. Velleman, G. Triani, P. J. Evans, J. G. Shapter, D. Losic, Structural and chemical modification of porous anodic alumina membranes, 2009, Microporous and Mesoporous Materials, 2009, 126, 87-94
4. L. Velleman, J.G. Shapter, D. Losic, Gold nanotube membranes functionalised with fluorinated thiols for selective molecular transport, Journal of Membrane Science, 2009, 328,121-126.
5. D. Losic, M. Lillo, D. Losic Jnr., Porous alumina with shaped pore geometries and complex pore architectures fabricated by cyclic anodization, Small, 2009, 5, 1392-1397
6. D. Losic, D. Losic Jnr, Preparation of Porous Anodic Alumina with Periodically Perforated Pores, Langmuir, 2009, 25, 5426-5431
7. K. Krishna, D. Losic, A simple approach for synthesis of TiO2 nanotubes with through-hole morphology, Physica Status Solidi RRL, 2009, 3, No. 5, 139-141
8. K. Vasilev, Z. Poh, K. Kant, J. Chan, A, Michelmore, D. Losic, Tailoring the surface functionalities of titania nanotube arrays, Biomaterials 2009, doi:10.1016/j.biomaterials.2009.09.074.
9. A. M. Md Jani, E. J. Anglin, S. J.P. McInnes, D. Losic, J. G. Shapter, N. H. Voelcker, Fabrication of nanoporous anodic alumina membranes with layered surface chemistry, Chemical Communications 2009, 3062-3064
10. M. Lillo, D. Losic, Ion-beam pore opening of porous anodic alumina: the formation of single nanopore and nanopore arrays, Materials Letters, 2009, 63, 457-460.
11. M. Lillo, D. Losic, Pore opening detection for controlled dissolution of barrier oxide layer and fabrication of nanoporous alumina with through-hole morphology, Journal of Membrane Science, 2009, 327, 11-17.