Quantum Sensing Use Cases
Free Radical Detection by T1-Relaxometry
NV T₁-relaxometry measures time required to repopulate electron spin states from optically polarized spin states (“bright”) back to their thermal equilibrium states (“darker”). The value of T₁ is highly sensitive to the presence of magnetic noise (e.g., paramagnetic species) external to NV. In comparison with conventional methods, T₁-relaxometry is capable to reveal free radical concentration at nano/micromolar range and in real-time, as had been demonstrated in numerous use cases in basic and translational biomedical research. Biofunctionalized nanodiamond sensors can be specifically targeted enabling free radicals detection at the mitochondrial membrane or on the nucleus. Decorating viruses or bacteria with nanodiamonds helped to reveal free radicals load on surfaces of the pathogens and elucidate mechanisms of an immune response in in vitro cellular cultures.
T₁ relaxometry instrumentation specifically tailored for measurement of free radicals in a variety of biomedical use cases is available from a startup company QTSense, the Netherlands: https://qtsense.com/
Products optimized for use in T₁ relaxometry:
- NDNV70nmHi10ml was selected by end users as optimal for detection of free radicals in live cells.
- Multicolor NDNV/NVN120nm1mg is recommended for multiplexed imaging and sensing.
- NDNV10nmMd2ml was used for detection of Hydrogen Peroxide (H₂O₂) by catalyzing production of radical intermediates.
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Magnetic Modulation of NV Fluorescence for Paramagnetic Analyte Detection
The fluorescence intensity of NV centers is sensitive to magnetic fields. This behavior results from the electronic spins of NV centers, which emit more or less photons depending on their spin state projection. Magnetic fields (B-fields) change the spin state projections of NV electronic spins and alter the number of emitted photons. The change in fluorescence of NV centers in the presence of an external magnetic field as compared to zero (or lower) magnetic field is the magnetically modulated fluorescence contrast of the NV center(s) (Fig. 1). Oscillating magnetic fields will therefore result in oscillating NV fluorescence intensity.
The oscillatory response of NV fluorescence is exploitable for use of established phase-sensitive signal processing methods (e.g., lock-in amplification), removing contributions of non-modulated background fluorescence noise. This provides greatly enhanced sensitivity.
Paramagnetic analytes such as Gd³⁺, Fe³⁺, and reactive oxygen species (ROS) generate fluctuating magnetic fields due to their unpaired electron spins. This fluctuating magnetic noise interacts with NV centers and alters their fluorescence contrast. Therefore, NV centers can be used to detect for the presence of paramagnetic analytes, which are important in, for instance, cellular metabolic pathways. NV centers incorporated into biocompatible nanoscale diamond particles can be useful tools for the study of intracellular metabolism or paramagnetic analyte generating chemical reactions. For instance, Fig. 2 shows a fluorescence contrast measurement over time in the presence of an enzyme generating superoxide. When the enzyme is suppressed, no change in signal is observed.
Fig. 1. Schematic of magnetic field induced fluorescence contrast of NV centers. Application of an oscillating magnetic field will cause oscillations of the NV fluorescence signal intensity. Phase-sensitive detection methods can then be used to remove background fluorescence contributions. The fluorescence intensity of the NV in the presence of a magnetic field as compared to the absence of a magnetic field results in a fluorescence contrast. Paramagnetic analytes alter the magnitude of this contrast.
Fig. 2. Xanthine oxidase is a model system which produces reactive oxygen species (ROS). When the enzyme is active, the NV fluorescence contrast shows a response proportional to ROS generation. This type of measurement can be used in highly fluorescence and turbid media as an alterative to communally used ROS sensor DCFH-DA for the measurement of oxidative stress.
Temperature Measurement
Changes in temperature alter the local symmetry of NV centers in diamond. This manifests on the ODMR spectrum as shifts in the center of the spectrum (the zero field splitting parameter, D). The D parameter depends on temperature at a coupling constant of about –80 kHz/K around 300 K. Therefore, if the temperature increases, the center of the ODMR spectrum will shift to lower frequencies, and if the temperature decreases, the center of the ODMR spectrum will shift to higher frequencies (Fig. 3). This temperature dependence can be used to measure temperature fluctuations on the order of 10 mK. Such sensitivity can be exploited for monitoring of intracellular temperature fluctuations.
See the application of NV-containing nanoscale diamond sensors for
imaging of distribution of temperature over a sample:
(i) Wide-Field Magnetic Field and Temperature Imaging in Integrated Circuits ACS Appl. Mater. Interfaces 2020
(ii) Organelle-Specific Thermometry providing Insights into Cellular Metabolic Thermodynamics: J. Am. Chem. Soc. 2025
Fig. 3. Shifting of the center of the NV ODMR spectra in response to changes in temperature. Temperature changes alter the zero field splitting parameter (D) away from 2.87 GHz.
Magnetic Field Measurement
Fig. 4 shows a magnetic field measurement utilizing randomly oriented NV centers in diamond nanoparticles. The magnetic field is measured through its contribution to the splitting in the ODMR spectra due to the Zeeman effect. In the absence of a magnetic field, the zero-field splitting 𝐷 defines the energy difference between the 𝑚ₛ=0 and 𝑚ₛ=±1 states. When an external magnetic field is applied, it further splits these 𝑚ₛ=±1 states through the Zeeman interaction. Due to the random orientation of the NV center axes in a distribution of nanodiamond particles (powder) with respect to the magnetic field, a distribution of splitting values is observed. The particles whose NV quantization axis is aligned with the magnetic field produce the largest splitting. The distance between extrema, indicated by the dots in the graph, is proportional to the magnetic field strength and the constant 2×γNV (56 Hz/mT).
See the application of NV-containing nanoscale diamond sensors for
imaging of distribution of magnetic field over a sample:
(i) Wide-Field Magnetic Field and Temperature Imaging in Integrated Circuits ACS Appl. Mater. Interfaces 2020
Fig. 4. Example showing measurement of an external magnetic field applied to a distribution of nanodiamond particles with NV centers. Experimentally derived values are labeled as 𝐵meas, while the nominally estimated values obtained from a gauss meter are denoted as 𝐵true.