U. BUFFALO (US) — Chemists have found a way to create tiny, tube-shaped molecular cages to capture and purify nanomaterials.
The “bottle brush” molecules use tiny organic tubes whose interior walls carry a negative charge to selectively encase only positively charged particles.
Because the tubes are made from scratch, the researchers can create traps of different sizes that snare molecular prey of different sizes.
A transmission electron microscopy image of the organic nanotube traps. (Color added through digital enhancement). (Credit: U. Buffalo)
The level of fine tuning possible is remarkable: In the Journal of the American Chemical Society, the researchers report that they were able to craft nanotubes that captured particles 2.8 nanometers in diameter, while leaving particles just 1.5 nanometers larger untouched.
These kinds of cages could ultimately be used to expedite tedious tasks, such as segregating large quantum dots from small quantum dots, or separating proteins by size and charge.
“The shapes and sizes of molecules and nanomaterials dictate their utility for desired applications,” says Javid Rzayev, assistant professor of chemistry at the University at Buffalo who led the research.
“Our molecular cages will allow one to separate particles and molecules with pre-determined dimensions, thus creating uniform building blocks for the fabrication of advanced materials.
“Just like a contractor wants tile squares or bricks to be the same size so they fit well together, scientists are eager to produce nanometer-size particles with the same dimensions, which can go a long way toward creating uniform and well-behaved materials,” he explains
To create the traps, Rzayev and colleagues first constructed a special kind of molecule called a bottle-brush molecule that resemble a round hair brush, with molecular “bristles” protruding all the way around a molecular backbone.
After stitching the bristles together, the researchers hollowed out the center of each bottle-brush molecule, leaving behind a structure shaped like a toilet paper tube.
The carving process employed simple but clever chemistry: When building them, the scientists constructed the heart of each molecule using molecular structures that disintegrate upon coming into contact with water. Around this core, they then attached a layer of negatively charged carboxylic acid groups.
To sculpt the molecule, the scientists then immersed it water, in effect hollowing the core. The resulting structure was the trap—a nanotube whose inner walls were negatively charged due to the presence of the newly exposed carboxylic acid groups.
A series of experiments involving a two-layered chemical cocktail put the tubes’ effectiveness to the test. The cocktail’s bottom layer consisted of a chloroform solution containing the nanotubes, while the top layer consisted of a water-based solution containing positively charged dyes. (As in a tequila sunrise, the thinner, water-based solution floats on top of the denser chloroform solution, with little mixing.)
When the scientists shook the cocktail for five minutes, the nanotubes collided with and trapped the dyes, bringing the dyes into the chloroform solution. (The dyes, on their own, do not dissolve in chloroform.)
In similar experiments, Rzayev and his team were able to use the nanotubes to extract positively charged molecules called dendrimers from an aqueous solution. The nanotubes were crafted so that dendrimers with a diameter of 2.8 nanometers were trapped, while dendrimers that were 4.3 nanometers across were left in solution.
To remove the captured dendrimers from the nanotubes, the researchers simply lowered the pH of the chloroform solution, which shuts down the negative charge inside the traps and allows the captured particles to be released from their cages.
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