Transport and functional behaviour of poly(ethylene glycol)-modified nanoporous alumina membranes

The development of hybrid organic–inorganic membranes with a low propensity for protein adsorption and highly uniform nanometre size pores is described. Poly(ethylene glycol) (PEG) monolayers were grafted to nanoporous alumina membranes using covalent silane and physical adsorption poly(ethyleneimine) (PEI) immobilization chemistries. X-ray photoelectron spectroscopy (XPS) and electron microscopy were used to investigate the chemical and physical surface properties of the membranes. The adsorption behaviour of a serum albumin on the membranes was characterized with fluorescence spectroscopy and it was determined that the PEG coating reduced nonspecific protein adsorption to a level too small to be measured. The gas and liquid permeabilities of membranes were measured to determine if the surface chemistries changed the functional behaviour of the membranes. Surprisingly, the silane chemistry produced little change in the permeabilities while polymer adsorption resulted in a total loss of water permeability. The diffusion of ovalbumin through the membranes was also measured and compared with a theoretical value. Diffusion of ovalbumin through the silane-PEG-modified membranes was found to be 50% slower than the unmodified membranes, which suggests that the pores are coated with a dense film of PEG. These results suggest that hybrid organic–inorganic membranes can provide significantly improved functional behaviour over existing organic or inorganic membranes. Membrane filtration technologies are important for many industrial activities. Due mostly to cost considerations, polymeric membranes are by far the most extensively used type of membranes [1, 2]. However, there has been growing interest in inorganic nanoporous membranes due to their high pore density, narrow pore distribution, and high mechanical strength. Inorganic membranes have become widely used for ultrafiltration and nanofiltration in biotechnology applications, such as separation, bioreactors, tissue culture, and supports for 3 These two authors have contributed equally to this work. 4 Author to whom any correspondence should be addressed. analytical devices [3, 4]. Among these inorganic membranes, anodized alumina membranes have found wide acceptance in the pharmaceutical, food, and electronics industries [5]. Alumina membranes made by anodic oxidation are reported to contain parallel circular pores with a very tight pore size distribution [6]. The pore diameter depends on the applied voltage and can be varied between 20 and 200 nm. The thickness of such membranes is a function of the anodization time and varies between a few tens of nanometres and hundreds of microns. The transport properties of nanoporous membranes have been intensively studied [5, 7–10]. The well defined pore 0957-4484/05/081335+06$30.00 © 2005 IOP Publishing Ltd Printed in the UK 1335