High-quality nanodiamonds offer new possibilities for bioimaging and quantum sensing

Quantum sensing is a rapidly developing field that exploits the 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) with nitrogen vacancy (NV) centers. These centers are created by replacing carbon atoms near lattice vacancies in the diamond structure with nitrogen.

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. These spin state changes can be detected using optical detection magnetic resonance (ODMR), which measures fluorescence changes under microwave irradiation.

In a recent breakthrough, scientists at Japan’s Okayama University have developed a nanodiamond sensor with spin properties comparable to bulk diamond and bright enough for bioimaging. The research, published in ACS Nano on December 16, 2024, was led by Research Professor Masazumi Fujiwara of Okayama University in collaboration with Sumitomo Electric Industries and the National Institute for Quantum and Radiological Science and Technology.

“This is the first demonstration of quantum-grade NDs with very high-quality spins and is a long-awaited breakthrough in the field. These NDs are ideal for quantum biosensing and other advanced applications. It has properties that are highly sought after in Japan,” said Professor Fujiwara. .

Current ND sensors for bioimaging face two main limitations: high concentrations of spin impurities, which perturb the NV spin states, and surface spin noise, which destabilizes the spin states more rapidly. To overcome these challenges, researchers focused on producing high-quality diamonds with very few impurities.

They grew single-crystal diamond enriched with 99.99% 12C carbon atoms and then introduced controlled amounts of nitrogen (30 to 60 ppm) to create approximately 1 ppm of NV centers. Diamonds were ground into NDs and suspended in water.

The average size of the resulting NDs was 277 nanometers and contained 0.6–1.3 ppm of negatively charged NV centers. They exhibit strong fluorescence and achieve photon counting rates of 1500 kHz, making them suitable for bioimaging applications.

These NDs also had improved spin properties compared to commercially available larger NDs. These require 10-20 times less microwave power to achieve 3% ODMR contrast, reduced peak splitting, and significantly longer spin relaxation times (T1 = 0.68 ms, T2 = 3.2 μs). , which was demonstrated to be 6 to 11 times longer than the others. Type Ib ND.

These improvements demonstrate that NDs possess a stable quantum state and can be accurately detected and measured even at low microwave radiation, minimizing the risk of microwave-induced toxicity in cells.

To assess the potential for biological sensing, the researchers introduced NDs into HeLa cells and measured their spin properties using ODMR experiments. The NDs produced narrow and reliable spectra that were bright enough to be clearly visible despite the effects of Brownian motion (random ND movement within the cell).

Additionally, the ND was able to detect small temperature changes. At temperatures around 300 K and 308 K, the ND exhibited distinct oscillation frequencies and a temperature sensitivity of 0.28 K/√Hz, which was better than the bare type Ib ND.

These advanced sensing features enable this sensor to be used in a variety of applications, from biological sensing of cells for early disease detection, to monitoring battery health, to enhancing thermal management and performance of energy-efficient electronic devices. It may be applicable for various purposes.

“These advances have the potential to transform medicine, technology and environmental management, improve quality of life, and provide sustainable solutions to future challenges,” says Professor Fujiwara.