Carbon is not just the most important element for life, it also has fascinating properties of its own. Graphene – a pure carbon sheet just one atom thick – is one of the strongest materials known; roll graphene into a cylinder and you get carbon nanotubes (CNTs), the key to many emerging technologies.
Now, in a paper published in Chemical Communications, researchers at Kyushu University in Japan report learning how to control the fluorescence of CNTs, potentially leading to new applications.
CNTs are naturally fluorescent – when placed under light, they respond by releasing light of their own, a process called photoluminescence. The wavelength (color) of this fluorescence depends on the tubes' structure, including the angle at which they are rolled up. So far, fluorescent CNTs have been studied for use in LED lighting and medical imaging.
The Kyushu team wanted to gain finer control over the emission wavelength. "Fluorescence occurs when electrons use energy from light to jump into higher orbitals around atoms," the lead authors explain. "They sink back to a lower orbital, then release excess energy in the form of light. The wavelength of emitted light differs from the input light, depending on the energy of the emitting orbital." CNTs naturally fluoresce at infrared wavelengths, which are invisible to the eye but can be detected by sensors.
The researchers used chemistry to tether organic molecules – hexagons of carbon atoms – onto the CNTs, which pushed their orbitals up or down, thus tuning the fluorescence. One of the six atoms in each hexagon was bonded to a CNT, anchoring the molecule to the tube, while another was bonded to an extra group of atoms, termed the substituent. Because of the molecule’s hexagonal shape, the two bonded carbons could be adjacent to each other (denoted ‘o’), separated by a single carbon atom (‘m’), or by two carbon atoms (‘p’). Most studies use the ‘p’ arrangement, where the substituent points away from the CNT, but the Kyushu team compared all three.
They found that the ‘o’ arrangement produced very different fluorescence from the ‘m’ and ‘p’ arrangements. Instead of one infrared wavelength, the CNTs now emitted two, as result of the substituents distorting the tubes as they were squeezed against the tube walls. For the ‘m’ and ‘p’ arrangements, the energies depended on which elements were in the substituent. For example, nitrogen dioxide (NO2) produced bigger gaps between the orbitals than bromine. This was no surprise, as NO2 is better at attracting electrons and thus creating an electric field (dipole). However, the size of the effect differed between the ‘m’ and ‘p’ arrangements.
"The variation in orbital energies with different substituents gives us fine control of the emission wavelength of CNTs over a broad range," the authors say. "The most important outcome is to understand how dipoles influence fluorescence, so we can rationally design CNTs with the very precise wavelengths needed by biomedical devices. This could be very important for the development of bioimaging in the near future."
This story is adapted from material from Kyushu University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.