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A new study has illuminated the reasons behind the fluorescence of carbon nanotubes when they interact with specific molecules. These nanotubes are emerging as valuable biosensors, which could play a pivotal role in applications such as monitoring blood glucose levels or facilitating COVID-19 tests. The fluorescence emitted by the nanotubes increases in intensity with higher concentrations of the analyte. Researchers from Ruhr University Bochum in Germany and the University of Texas at El Paso in the United States utilized terahertz spectroscopy to investigate the light emission mechanism. Their findings, published online in the journal Nature Communications on August 8, 2024, underscore the importance of the aqueous environment in influencing fluorescence.
At Ruhr University, Professors Martina Havenith and Sebastian Kruss spearheaded the collaborative research, which was part of the Cluster of Excellence titled “Ruhr Explores Solvation” (RESOLV). Key contributions were made by PhD students Sanjana Nalige and Phillip Galonska during the study.
Carbon nanotubes as biosensors
Single-walled carbon nanotubes are recognized as potent building blocks for the development of biosensors, supported by earlier research. Their surfaces can be customized with biopolymers or DNA fragments, enabling them to selectively interact with particular target molecules. This interaction alters their near-infrared emission, allowing for deeper tissue penetration to detect specific neurotransmitters, which serve as signaling molecules in the brain. While these sensors are currently in practice, their operational mechanisms remain largely misunderstood.
Water is decisive for fluorescence
Recognizing that most biological processes occur in aqueous environments, the research team evaluated the carbon nanotubes within a water solution. Using terahertz spectroscopy, they explored how energy transfers occur between carbon nanotubes and water. A crucial aspect of their findings is the hydration shell, comprising the water molecules encasing the nanotubes. When excited, a carbon nanotube can share its internal energy with the vibrations of this hydration shell. The study revealed that sensors displaying increased brightness in response to the analyte are less inclined to transfer energy to the surrounding water, whereas those that grow dimmer do so by transferring more energy into their aqueous environment.
“Terahertz spectroscopy enables us to obtain direct measurements of phenomena we previously only theorized about,” noted Sebastian Kruss. “These findings present a foundational design principle for creating more effective biosensors tailored for various research and medical applications.”
Martina Havenith, the spokesperson for the RESOLV initiative, emphasized, “This interdisciplinary exploration shifted the focus from the carbon nanotube itself to its surrounding solvent, water. We discovered a novel correlation between the changes occurring in the water around the nanotube and its function as a biosensor, aligning perfectly with RESOLV’s objectives.”
Funding
The research received support from the German Research Society (EXC 2033 — 390677874, GRK2376-331085229), VolkswagenStiftung, and the National Science Foundation (CBET-2106587).
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