"The carbon nanotube in this case is used as an inert nanoscopic reaction vessel representing the world's smallest test tube with a zeptolitre [10-21 litre] volume," Andrei Khlobystov of the University of Nottingham and David Britz of the University of Oxford told nanotechweb.org.

The nanotubes had an inner diameter of about 1.2 nm and were 2 micrometres long. According to the researchers, the Guinness Book of World Records has verified that the tubes formed the world's smallest reaction vessels.

To carry out the reaction, the scientists mixed C60O with opened single-walled nanotubes with diameters of 13.6 or 14.9 Angstroms. They filled the nanotubes with C60O from supercritical carbon dioxide at 50°C over a period of six days. The molecules entered the nanotubes to form C60O@SWNT "peapod" structures.

"The most important challenge was to insert molecules bearing reactive chemical groups into nanotubes and to make sure that the molecules do not react with each other until encapsulated," said Britz and Khlobystov. "To implement this we developed a technique allowing insertion of molecules into nanotubes at low temperature using supercritical fluids. In this way, we can load reactive molecules into nanotubes and then initiate chemical reaction inside the nanotubes by increasing the temperature."

The team heated the C60O@SWNT peapods to 260°C for three days. As a result, the C60O molecules polymerized head-to-tail.

"Generally speaking, the selectivity and yield of organic reactions carried out inside nanotubes can be expected to be better and higher than in solution because of the isolated environment inside the nanotube," added the scientists. "The nanotube can be thought of as a shield around the molecules, lowering the activation barrier for some chemical reactions and/or favouring the formation of one isomer out of many possible in solution."

So far, the team has only demonstrated one type of reaction in the nanotubes. "More studies are needed to suggest practical applications for this process," they said. "One of the most important challenges ahead is the removal of reaction products from the nanotubes."

The plan is also to investigate how different molecular interactions occur within confined spaces, and how the diameter and chirality of the nanotubes affect the reaction products.

The researchers reported their results in Chemical Communications.