|Illustration of a sealed and empty single-walled carbon nanotube (top) and a nanotube filled with water.|
(Courtesy: Xuedan Ma et al/Phys. Rev. Lett.)
The optical properties of single-walled carbon nanotubes (SWCNTs) change when the tiny structures are filled with water. That is the conclusion of scientists in Belgium and the US, who attribute the change to a "quasi-phase transition" that occurs in the water – although the exact nature of the transition is unknown. The research points to a new technique for studying confined water molecules – which is crucial to various branches of science, but it is surprisingly difficult to do. The study could lead to better ways of delivering drugs in the body and even boost our understanding of quantum mechanics.
SWCNTs are hollow hair-like structures with walls that are one atom thick. Normally they are closed at both ends, but sometimes the ends can be open and Sofie Cambré of the University of Antwerp in Belgium and colleagues have previously shown that open SWCNTs rapidly fill with other molecules and hold on to them. Why this occurs is not well understood, but Cambré says "it seems the energy of a molecule inside a SWCNTs is much lower than when you have them separated."
SWCNTs are naturally fluorescent, and the colour of the fluorescent light shifts if the nanotube is filled. "You need dedicated equipment to really see these very small shifts", Cambré says. However, by measuring the shifts, researchers can potentially gain useful insights into the behaviour of confined molecules.
Quasi-phase transition spotted in water-filled carbon nanotubes, Tim Wogan