Quantum sensing is a rapidly developing field that uses quantum states of particles, such as superposition, entanglement, and spin states, to detect changes in physical, chemical, or biological systems. A promising type of quantum nanosensor is nanodiamonds (NDs) equipped with nitrogen vacancy (NV) centers. These centers are created by replacing a carbon atom with nitrogen near a lattice vacancy in a diamond structure. When excited by light, NV centers emit photons that maintain stable spin information and are sensitive to external influences such as magnetic fields, electric fields, and temperature.
Changes in these spin states can be detected by optically detected magnetic resonance (ODMR), which measures fluorescence changes under microwave radiation. NDs with NV centers are biocompatible and can be designed to interact with specific biological molecules, making them valuable tools for biological detection. However, NDs used for bioimaging generally exhibit lower spin quality compared to bulk diamonds, resulting in reduced sensitivity and precision in measurements.
In a recent advance, scientists at Okayama University in Japan developed nanodiamond sensors bright enough for bioimaging, with spin properties comparable to those of bulk diamonds. The study, published in ACS Nano on December 16, 2024, was led by research professor Masazumi Fujiwara of Okayama University, in collaboration with Sumitomo Electric Company and the National Institutes of Quantum Science and Technology.
“This is the first demonstration of quantum-grade NDs with exceptionally high quality spins, a long-awaited advance in the field. These NDs possess properties that have been highly sought after for quantum biosensing and other advanced applications,” says Professor Fujiwara. .
Current ND sensors for bioimaging face two main limitations: high concentrations of spin impurities, which disrupt NV spin states, and surface spin noise, which destabilizes spin states more rapidly. To overcome these challenges, researchers focused on producing high-quality diamonds with very few impurities. They grew monocrystalline diamonds enriched with 99.99% 12C carbon atoms and then introduced a controlled amount of nitrogen (30-60 parts per million) to create an NV center with approximately 1 part per million. The diamonds were crushed to ND and suspended in water.
The resulting NDs had an average size of 277 nanometers and contained between 0.6 and 1.3 parts per million negatively charged NV centers. They showed strong fluorescence and reached a photon counting rate of 1,500 kHz, making them suitable for bioimaging applications. These NDs also showed improved spin properties compared to larger commercially available NDs. They required 10 to 20 times less microwave power to achieve 3% ODMR contrast, reduced peak splitting, and demonstrated significantly longer 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 improvements indicate that NDs possess stable quantum states, which can be accurately detected and measured with low microwave radiation, minimizing the risk of microwave-induced toxicity in cells.
To evaluate its potential for biological sensing, the researchers introduced ND into HeLa cells and measured spin properties using ODMR experiments. The NDs were bright enough for clear visibility and produced narrow, reliable spectra despite some impact from Brownian motion (random movement of NDs within cells). Furthermore, the NDs were able to detect small changes in temperature. At temperatures around 300 K and 308 K, the NDs exhibited different oscillation frequencies, demonstrating a temperature sensitivity of 0.28 K/√Hz, higher than bare type Ib NDs.
With these advanced sensing capabilities, the sensor has potential for diverse applications, from cell biological sensing for early disease detection to battery health monitoring and improving thermal management and performance of energy-efficient electronic devices. . “These advances have the potential to transform healthcare, technology and environmental management, improving quality of life and providing sustainable solutions to future challenges,” says Professor Fujiwara.