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Advancements in Quantum Sensing with Nanodiamonds
Quantum sensing is making significant strides by exploiting the unique quantum states of particles, such as superposition and entanglement, to monitor changes across various physical, chemical, and biological systems. Among the promising developments in this field are quantum nanosensors in the form of nanodiamonds (NDs) that feature nitrogen-vacancy (NV) centers. The creation of NV centers involves substituting a carbon atom with nitrogen close to a vacancy in the diamond lattice. When stimulated with light, these centers emit photons that preserve stable spin information, making them highly sensitive to external factors like magnetic and electric fields, as well as temperature variations.
Detection of alterations in these spin states is achievable through a technique known as optically detected magnetic resonance (ODMR), which assesses fluorescence variations influenced by microwave radiation. NDs containing NV centers are inherently biocompatible, enabling them to be tailored for interactions with specific biological molecules. This adaptability makes them invaluable for applications in biological sensing. Nonetheless, nanodiamonds employed in bioimaging often display inferior spin qualities compared to their bulk diamond counterparts, consequently diminishing sensitivity and precision during measurements.
In a noteworthy development, researchers from Okayama University, Japan, have designed nanodiamond sensors that exhibit sufficient brightness for bioimaging, achieving spin characteristics that rival bulk diamonds. This pivotal study, published in ACS Nano on December 16, 2024, was spearheaded by Research Professor Masazumi Fujiwara, in partnership with Sumitomo Electric Company and the National Institutes for Quantum Science and Technology.
“This marks the inaugural demonstration of quantum-grade NDs featuring exceptionally high-quality spins, representing a long-anticipated milestone in this domain. These NDs integrate properties that are crucial for quantum biosensing and various advanced uses,” Professor Fujiwara remarked.
The current challenges faced by ND sensors in bioimaging stem largely from two factors: the presence of high spin impurity concentrations that interfere with NV spin states and surface spin noise that can lead to faster destabilization of these states. To address these challenges, the team concentrated on cultivating diamonds of superior quality with minimal impurities. They grew single-crystal diamonds enriched with 99.99% 12C carbon atoms and introduced a calculated amount of nitrogen (30-60 parts per million) to foster an NV center concentration of approximately 1 part per million. The diamonds were subsequently reduced to NDs and dispersed in water.
The resultant NDs, with an average diameter of 277 nanometers, contained between 0.6 and 1.3 parts per million of negatively charged NV centers. They exhibited strong fluorescence, achieving a photon count rate of 1500 kHz, proving their suitability for bioimaging applications. Moreover, these nanodiamonds displayed enhanced spin properties compared to commercially available, larger counterparts. They needed 10-20 times less microwave power to attain a 3% ODMR contrast, presented reduced peak splitting, and demonstrated substantially longer spin relaxation times (T1 = 0.68 ms, T2 = 3.2 µs), outpacing type-Ib NDs by 6 to 11 times. Such enhancements suggest that these NDs possess stable quantum states, allowing for precise detection and measurement using minimal microwave radiation, which reduces the risk of microwave-induced toxicity to cells.
To assess their potential in biological sensing, the researchers introduced the NDs to HeLa cells and conducted ODMR experiments to gauge their spin properties. The NDs proved sufficiently luminous for clear observation and produced narrow, reliable spectral outputs, even though Brownian motion (random movement of NDs within cells) had some impact. Additionally, the NDs demonstrated the capability to detect minor temperature fluctuations, with distinct oscillation frequencies observed at temperatures of approximately 300 K and 308 K. They exhibited a temperature sensitivity of 0.28 K/√Hz, surpassing the performance of bare type-Ib NDs.
These advanced sensing capabilities open up numerous application possibilities, ranging from biological sensing for early disease identification to the monitoring of battery performance and the enhancement of thermal management in energy-efficient electronic devices. “The advancements we have achieved could revolutionize sectors including healthcare, technology, and environmental stewardship, thereby enhancing the quality of life and delivering sustainable solutions for future challenges,” concluded Professor Fujiwara.
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