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Ultra-thin films riddled with nanoscale pores would be useful in applications ranging from water purification to drug delivery, especially if the pores could change size in response to their environment.
Researchers from the King Abdullah University of Science and Technology (KAUST), Saudi Arabia, and DESY, Germany's high-energy physics laboratory in Hamburg, have demonstrated a relatively simple technique to manufacture such films, with pores responsive to changes in pH.
Suzana Nunes and colleagues dissolved a polystyrene polymer in a solution also containing ions of a transition metal, such as copper. Immersion in water produced nanoscale polymer micelles — spherical polymer that arrange together to encase the hydrophobic tails away from water. The copper ions then bond to link these micelles together forming a film, with the gaps between the micelles becoming pores. The sizes of these pores depend on the pH of the environment.
Testing the films to filter a dilute solution containing organic molecules with widely varying molecular weight and size, the researchers found that the flux through the film changed by a factor of a hundred as the pH varied from 6 to 4. Smaller pores thereby blocked larger molecules.
Various microscopy techniques showed that the pore size is uniform across the film, which has a very high overall porosity.
"The highest flux we measured is ten times larger than possible with commercial membranes with similar pore size," says Nunes. "These films have exceptional porosity."