Researchers have uncovered the reason behind the fluorescence of carbon nanotubes when they attach to specific molecules. These nanotubes are seen as promising candidates for biosensors, potentially aiding in blood sugar level checks or COVID-19 testing, for example. Their fluorescence increases with the concentration of the target analyte. A collaborative team from Ruhr University Bochum in Germany and the University of Texas at El Paso in the USA utilized terahertz spectroscopy to reveal the underlying mechanism of this light emission. They discovered that the presence of water is crucial for fluorescence, with their findings published online in the journal Nature Communications on August 8, 2024.
At Ruhr University, the research led by Professor Martina Havenith and Professor Sebastian Kruss was conducted as part of the Cluster of Excellence known as “Ruhr Explores Solvation,” or RESOLV. The PhD candidates Sanjana Nalige and Phillip Galonska played key roles in this study.
Carbon nanotubes as biosensors
Single-walled carbon nanotubes have been identified as effective components for biosensors in prior research. Their surfaces can be chemically modified with biopolymers or DNA segments, enabling specific interactions with target molecules. Once these molecules bind, the nanotubes alter their emission in the near-infrared spectrum, which can penetrate deep into biological tissues. This property allows for the detection of specific neurotransmitters, which are chemicals that transmit messages in the brain. Although some of these sensors are already operational, a clear understanding of their functional mechanisms has been lacking.
Water is crucial for fluorescence
Given that many significant biological reactions occur in aqueous environments, the researchers examined the carbon nanotubes in an aqueous solution. Through terahertz spectroscopy, they observed the energy dynamics between the nanotubes and water. The findings revealed the importance of the hydration shell—this refers to the layer of water molecules that encases the nanotubes. When a nanotube is excited, its internal energy can resonate with the vibrations of these surrounding water molecules. For sensors that become more luminous with the presence of the analyte, they release less energy into the water. Conversely, those that lessen in brightness tend to transfer more energy to the surrounding water.
“Terahertz spectroscopy enables us to directly measure phenomena that we only speculated about before,” explains Sebastian Kruss. “These findings provide a foundational principle for designing better-performing biosensors for new applications in both research and medicine.”
Martina Havenith, the spokesperson for the RESOLV Cluster of Excellence, adds: “In this interdisciplinary investigation, our focus shifted from the carbon nanotube itself to the surrounding solvent, water, allowing us to identify a previously unknown link between the changes in the water surrounding the nanotube and its function as a biosensor. This embodies the essence of what RESOLV represents.”
Funding
The research received financial support from the German Research Society (EXC 2033 — 390677874, GRK2376-331085229), VolkswagenStiftung, and the National Science Foundation (CBET-2106587).
