Supramolecules get time shine

Supramolecules get time to shine

1:35 PM, 16th July 2011
Supramolecules get time to shine
Rice University researchers have created a platform to analyze interactions between carbon nanotubes and a wide range of photoluminescent materials. The team is clockwise from top: Professor Matteo Pasquali, Natnael Behabtu, Professor Angel Martí, Avishek Saha and Saunab Ghosh.

HOUSTON, TEXAS: What looks like a spongy ball wrapped in strands of yarn - but a lot smaller - could be key to unlocking better methods for catalysis, artificial photosynthesis or splitting water into hydrogen, according to Rice University chemists who have created a platform to analyze interactions between carbon nanotubes and a wide range of photoluminescent materials.

The microscopic particles assembled in the lab of Angel Martí, an Assistant Professor of Chemistry and Bioengineering, combine single-walled carbon nanotubes with porous silicate materials that can absorb various molecules - in this case, a ruthenium complex.

Martí, Graduate Student and Lead Author Avishek Saha and their colleagues reported their results in the Royal Society of Chemistry journal Chemical Science.

The ability to immobilize individual carbon nanotubes on a solid surface is interesting enough, but combining supramolecular systems with nanomaterials to produce hybrids is unique, they said.

“This can be used as a general platform to study the interaction of not only ruthenium complexes, but most photoactive molecules can be encapsulated within these porous silicates in a very simple way without chemical modification, without anything,” said Martí.

For the experiment the solution suggested by co-author Matteo Pasquali, a Rice professor in chemical and biomolecular engineering and in chemistry, involved dissolving the bundles in chlorosulfonic acid, which protonated, or added protons - and thus a positive charge, to each nanotube.

“We don’t fully understand the mechanism, but the truth is they have a very strong affinity to silicon oxide networks,” said Martí, describing the nanotube-wrapped particles. “Once they’re protonated, they just bind.”

“Protonated nanotubes are cool, but we want to have pristine nanotubes,” added Martí.

Vinylpyrrolidone (VP) did the trick by giving the nanotubes a polymer-like coating while returning them to their pristine states.

The experiment went one critical step further when the researchers introduced ruthenium molecules to the mix. “Basically, we found out that if you put a photoactive species (ruthenium) there and excite it with light, two different processes happen. If it has carbon nanotubes close by, it will transfer an electron to the nanotubes. There’s a charge transfer and we knew that would happen,” Martí said. “What we didn’t expect when we analyzed the spectrum was seeing two different species of ruthenium complexes, one with a very short photoluminescence lifetime and one very long.”

The research leads to some interesting possibilities for materials science, Saha said. “MCM itself has many applications (as a mesoporous sieve in fuel refineries, for instance), and carbon nanotubes are wonderful materials that many people are interested in. We’re just combining these two into a hybrid material that might have the virtues of both.”

Martí said putting charged nanotubes on the surface of a solid also opens the door to use them as catalysts in solar-energy conversion. "You need that driving force, that charge separation, for artificial photosynthesis," he said.

Co-authors of the paper are Rice graduate students Saunab Ghosh and Natnael Behabtu.

The Welch Foundation supported the research.

(C) Rice University News




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