Chemists Create Nanomaterial-Purifying Molecular Traps

Tiny nanotubes created by chemists are able to trap molecules of different sizes, filtering and separating different particles and molecules. Traps resembling tube-shaped hairbrushes have been created by a team of chemists at the University of Buffalo (Buffalo, NY) to capture and purify nanomaterials.

Uniformity of particle size is an important factor when creating advanced materials, so the ability to separate proteins by size and charge, or separating large and small quantum dots, could aid in material developments, and could also lead to advancements in fluid filtration devices. By creating what the scientists call a "bottle-brush" molecule, the tubes can selective encapsulate only positively charged particles, and can differentiate between a particle sizes.

To create the trap, the team stitched together several of the bottle brush molecules, which have molecular bristles sticking out all around the molecular backbone. After the molecules were stitched, the center of each bottle-brush molecule was hollowed out since the structures used for the heart of each molecule disintegrate in water. A layer of negatively charged carboxylic acid groups were then attached around the core, resulting in a nanotube with negatively charged inner walls that can trap positively charged particles.

To test the trap, a two-layered chemical cocktail was created, with a heavier chloroform solution containing the nanotubes on the bottom, and a thinner, water-based solution containing positively charged dyes floating on top. The team shook the mixture, and after five minutes, the dyes, which would otherwise not mix in the chloroform solution, were trapped by the nanotubes, changing the color of the entire solution.

Researchers discovered in another test that certain sizes of positively charged molecules called dendrimers could be extracted from an aqueous solution by the nanotubes. The dendrimers that were 2.8 nanometers in diameter were trapped, and the ones just 1.5 nanometer larger were not. To release the dendrimers from the nanotube traps, the pH of the chloroform solution was lowered, which shut down the negative charge in the traps, allowing the captured particles to escape.

The researchers reported the findings in the Journal of the American Chemical Society, and are continuing to test bottle-brush molecules in nanomembranes for water filtration and also in creating polymers able to reflect visible light in a way similar to butterfly wings.