Researchers have created advanced nanodiamond sensors featuring nitrogen-vacancy (NV) centers, which possess remarkable brightness and spin characteristics for applications in quantum sensing and bioimaging. These new nanodiamonds exceed the performance of existing commercial options, needing 20 times less energy and preserving quantum states for 11 times longer durations. Their heightened sensitivity to magnetic fields and temperature offers precision for various applications, including disease screening, battery evaluation, and thermal regulation in electronics. This marks a notable leap forward in utilizing nanotechnology for quantum sensing, particularly in biological and industrial fields.
Quantum sensing is an emerging area of study that employs the quantum properties of particles—like superposition, entanglement, and spin states—to identify alterations in physical, chemical, or biological environments. A notable kind of quantum nanosensor consists of nanodiamonds (NDs) embedded with nitrogen-vacancy (NV) centers. These centers are formed by substituting a carbon atom with nitrogen adjacent to a lattice vacancy within the diamond structure. When illuminated, the NV centers emit photons that keep stable spin information and react sensitively to outside factors, such as magnetic and electric fields, as well as temperature.
Changes in these spin states can be monitored via optically detected magnetic resonance (ODMR), which captures fluorescence variations under microwave radiation. NDs with NV centers are biocompatible and can be tailored to interact with certain biological molecules, rendering them excellent instruments for biological detection. However, NDs utilized for bioimaging traditionally demonstrate inferior spin quality compared to bulk diamonds, leading to lower sensitivity and precision in measurements.
A recent breakthrough by researchers from Okayama University in Japan led to the development of nanodiamond sensors that are sufficiently bright for bioimaging, featuring spin properties similar to bulk diamonds. This study, which appeared in ACS Nano on December 16, 2024, was spearheaded by Research Professor Masazumi Fujiwara from Okayama University, in partnership with Sumitomo Electric Company and the National Institutes for Quantum Science and Technology.
“This marks the first demonstration of quantum-grade NDs having exceptionally high-quality spins, a long-awaited advancement in this domain. These NDs possess attributes that are greatly coveted for quantum biosensing and other sophisticated applications,” Professor Fujiwara stated.
Current ND sensors used in bioimaging face two critical challenges: high levels of spin impurities that interfere with NV spin states and surface spin noise that leads to faster destabilization of these spin states. To address these issues, the team concentrated on fabricating high-quality diamonds with minimal impurities. They cultivated single-crystal diamonds enriched with 99.99% 12C carbon atoms and subsequently added a controlled amount of nitrogen (30-60 parts per million) to generate an NV center at about 1 part per million. The diamonds were then crushed into NDs and dispersed in water.
The resulting NDs measured an average size of 277 nanometers and contained 0.6-1.3 parts per million of negatively charged NV centers. They exhibited robust fluorescence, achieving a photon counting rate of 1500 kHz, making them apt for bioimaging applications. Additionally, these NDs exhibited improved spin characteristics compared to larger commercially available NDs. They required 10-20 times less microwave power to reach a 3% ODMR contrast, showed diminished peak splitting, and experienced significantly extended spin relaxation times (T1 = 0.68 ms, T2 = 3.2 µs), which were 6 to 11 times longer than those of type-Ib NDs. These enhancements signify that the NDs have stable quantum states which can be reliably detected and measured using low microwave radiation, thus reducing the likelihood of microwave-induced toxicity in cells.
To assess their capabilities in biological sensing, the researchers introduced NDs into HeLa cells and evaluated the spin characteristics through ODMR experiments. The NDs were sufficiently bright for clear observation and produced narrow, consistent spectra despite some effects of Brownian motion (random ND movement in cells). Moreover, the NDs were adept at identifying minute temperature changes. At temperatures around 300 K and 308 K, the NDs displayed distinct oscillation frequencies, demonstrating a temperature sensitivity of 0.28 K/√Hz, superior to that of bare type-Ib NDs.
With these advanced sensing abilities, the sensor offers a variety of potential applications ranging from biological cell sensing for early disease detection to monitoring battery conditions and enhancing thermal management for energy-efficient electronic devices. “These advancements hold the potential to greatly influence healthcare, technology, and environmental oversight, thereby improving life quality and presenting sustainable solutions to future challenges,” Professor Fujiwara concluded.
