Precise pore size tuning and surface modifications of polymeric membranes using the atomic layer deposition technique

We demonstrated that atomic layer deposition (ALD) technique is an effective method in decorating pore walls of polymeric porous membranes (track etched polycarbonate membranes) by coating the membrane surface and pore walls with a uniform and conformal layer of aluminum oxide. Through ALD coating, the membrane obtained a thin, outer shell of oxide, showing improved hydrophilicity and resistance to acids and organic solvents. More importantly, the membrane pore size could be tuned continuously at a sub-angstrom preciseness simply by altering the numbers of ALD cycles, because the average growth rate of the deposited aluminum oxide was 0.8 Å/ALD cycle. Filtration experiments demonstrated that membranes subjected to increasing ALD cycles gave progressively rise in retention to the solute (bovine serum albumin) and drop in flux. Interestingly, the slightly deposited membrane (subjected to 10 ALD cycles) showed greatly enhanced retention at only a slight expense of the loss of flux because of the hydrophilic nature of the deposited aluminum oxide layer. The ALD approach can be extended to modify/functionalize other membranes since ALD is universal in depositing thin films on almost every type of polymeric and inorganic materials.