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. Roll graphene into a cylinder and you get carbon nanotubes (CNTs), the key to many emerging technologies.

Now, in a study reported in Chemical Communications, researchers at Japan's Kyushu University learned to control the fluorescence of CNTs, potentially allowing 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 fluorescence depends on the tubes' structure, such as the angle at which they are rolled. Fluorescent CNTs have been studied for use in LED lighting and medical imaging.

The Kyushu team aimed 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."

Although fluorescence is often associated with yellow materials, the fluorescence of these CNTs is infrared, which is 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. This pushed the 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.

Another was bonded to an extra group of atoms (a substituent). Because of the hexagonal shape, the two bonded carbons could be adjacent (denoted "o"), or separated by one carbon ("m"), or by two ("p"). Most studies use the "p" arrangement, where the substituent points away from the CNT, but the Kyushu team compared all three.

The "o" pattern produced very different fluorescence from "m" and "p" - instead of one infrared wavelength, the CNTs now emitted two. This resulted from distortion of the tubes by the substituents, which were squeezed against the tube walls.

Meanwhile, for the "m" and "p" arrangements, the energies depended on which elements were in the substituent. For example, NO2 produced bigger gaps between orbitals than bromine. This was no surprise, as NO2 is better at attracting electrons, creating an electric field (dipole). However, the size of the effect differed between "m" and "p".

"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."

additional report
The electronic origins of fluorescence in carbon nanotubes
Technological progress is often driven by materials science. High-tech devices require "smart" materials that combine a range of properties. An impressive current example is carbon nanotubes (CNTs) - single sheets of carbon atoms rolled into a cylinder.

These ultrathin tubes have enormous mechanical strength and electrical conductivity. They also emit infrared fluorescent light, rendering them detectable. This makes them exciting materials for future bio-imaging technology.

The remarkable fluorescence of CNTs was quickly noticed, but its mechanism has proven surprisingly elusive. The frequency of infrared light emitted by CNTs is shifted when organic molecules are attached to the outside of the tubes. This provides a way to "tune" the fluorescence depending on the required purpose. However, the origin of the frequency shift is hard to investigate, because only a few molecules are actually placed on the tubes. Standard methods therefore struggle to pinpoint them - facing a needle-in-a-haystack type of task.

Now, a trio of researchers at Japan's Kyushu University has made progress in understanding these frequency shifts at the atomic level. In a study published in Nanoscale, they report using the technique of spectro-electrochemistry - applying an electrical potential ("electro") to a fluorescent material, and measuring the resulting emission of light ("spectro").

The use of electricity reveals the electron energy levels in the CNTs - i.e., the orbitals around atoms. This is crucial, because fluorescence consists of "excited" electrons moving from one orbital to another, then releasing energy in the form of light.

"The frequency of fluorescence from CNTs depends on the gaps between electron energy levels," study lead authors explain. "These gaps in turn depend on which elements are bonded to the exterior of the nanotubes. For example, we found that molecules containing bromine pushed the energy levels closer together compared to molecules with hydrogen at the same position."

This narrows the gap - mostly by raising the highest occupied orbital, bringing it closer to the empty orbitals above it - and results in fluorescence with a lower frequency.

The measured changes in electronic states were consistent with the fluorescent shifts. This confirmed that the electron energy levels were the key to frequency tuning, allowing the researchers to rule out an alternative explanation based on the stability of excited electrons.

It seems that the effect is mainly caused by the electric field, or dipole, that is generated when molecules are bonded to the CNTs. This field, in turn, depends on the ability of those molecules to pull electrons away from the carbon in the nanotubes.

"Fluorescent CNTs could play a huge role in biomedicine," the authors say. "Our study method, based on electrochemistry, will allow researchers to understand fluorescent materials in full electronic detail. In the near future, this will open the way to fine-tuning of CNTs, in terms of both optical frequency and brightness, by carefully directed chemical decoration."

The article, "Substituent effects on the redox states of locally functionalized single-walled carbon nanotubes revealed by in situ photoluminescence spectroelectrochemistry," was published in Nanoscale at DOI:10.1039/c7nr05480g.

Research paper

The article, "Near infrared photoluminescence modulation by defect site design using aryl isomers in locally functionalized single-walled carbon nanotubes," was published in Chemical Communications at DOI:10.1039/c7cc06663e.

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Related Links
Kyushu University, I2CNER
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