Quantum sensing is an emerging field that leverages the unique quantum properties of particles, such as superposition, entanglement, and spin, to detect subtle changes in physical, chemical, or biological systems. A particularly promising class of quantum sensors is nanodiamonds (NDs) equipped with nitrogen-vacancy (NV) centers, which offer high sensitivity to various environmental factors, including magnetic fields, electric fields, and temperature. These NV centers, created by replacing a carbon atom with nitrogen near a lattice vacancy in a diamond structure, emit photons that preserve stable spin information. By using optically detected magnetic resonance (ODMR), researchers can detect changes in these spin states, making NDs ideal for applications in quantum biosensing.
In a groundbreaking study published on December 16, 2024, in ACS Nano, scientists from Okayama University in Japan have developed a new class of nanodiamond sensors that are not only bright enough for bioimaging but also exhibit spin properties comparable to bulk diamonds. The study, led by Research Professor Masazumi Fujiwara from Okayama University, in collaboration with Sumitomo Electric Company and the National Institutes for Quantum Science and Technology, marks a significant advancement in the field of quantum sensing. “This is the first demonstration of quantum-grade NDs with exceptionally high-quality spins, a long-awaited breakthrough in the field,” says Prof. Fujiwara. “These NDs possess properties that have been highly sought after for quantum biosensing and other advanced applications.”
Overcoming Key Challenges in Nanodiamond Sensing
Nanodiamond sensors have faced significant challenges in bioimaging and biosensing due to two main limitations: high concentrations of spin impurities that disrupt NV spin states, and surface spin noise that destabilizes these spin states more rapidly. To address these issues, the researchers focused on producing high-quality diamonds with minimal impurities. They grew single-crystal diamonds enriched with 99.99% 12C carbon atoms and introduced a controlled amount of nitrogen (30–60 parts per million) to create NV centers with about 1 part per million. These diamonds were then crushed into nanodiamonds and suspended in water.
The result was a set of nanodiamonds with a mean size of 277 nanometers and a concentration of 0.6–1.3 parts per million of negatively charged NV centers. These nanodiamonds exhibited strong fluorescence, achieving a photon count rate of 1500 kHz, making them highly suitable for bioimaging applications.
Superior Spin Properties and Enhanced Sensing Capabilities
The newly developed nanodiamonds also showed remarkable improvements in their spin properties when compared to commercially available larger nanodiamonds. These advances include requiring 10 to 20 times less microwave power to achieve a 3% ODMR contrast, reduced peak splitting, and significantly longer spin relaxation times (T1 = 0.68 ms, T2 = 3.2 µs). These spin relaxation times were 6 to 11 times longer than those of type-Ib NDs, indicating much more stable quantum states. Such stability enables more accurate and reliable measurements with low microwave radiation, reducing the risk of microwave-induced toxicity in biological cells.
To test the bioimaging potential of the new nanodiamonds, the researchers introduced them into HeLa cells (a commonly used cell line) and performed ODMR experiments. The results showed that the nanodiamonds were bright enough for clear imaging and produced narrow, consistent spectra, despite some interference from Brownian motion—the random movement of the nanodiamonds within the cells. These results confirm that the new NDs are highly suitable for biological sensing applications.
Temperature Sensing and Potential Applications
In addition to bioimaging, the nanodiamonds demonstrated superior temperature sensitivity. When exposed to temperatures around 300 K and 308 K, the NDs exhibited distinct oscillation frequencies, providing a temperature sensitivity of 0.28 K/√Hz—an improvement over bare type-Ib NDs. This enhanced sensitivity makes the NDs ideal for detecting small temperature fluctuations, a valuable capability in a range of applications from biological monitoring to energy-efficient electronics.
With these advanced sensing capabilities, the new nanodiamonds hold great promise for a variety of applications. These include early disease detection through biological sensing of cells, monitoring the health of batteries, and improving thermal management in electronic devices. “These advancements have the potential to transform health care, technology, and environmental management, improving quality of life and providing sustainable solutions for future challenges,” says Prof. Fujiwara.
The breakthrough in nanodiamond-based quantum sensing represents a significant step forward in the development of highly sensitive, versatile sensors for a broad range of fields, from medicine to environmental monitoring, offering exciting prospects for both research and real-world applications.
By Impact Lab
