Magnetometer with nitrogen-vacancy center in a bulk diamond for detecting magnetic nanoparticles in biomedical applications
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-02-12 |
| Journal | Scientific Reports |
| Authors | Akihiro Kuwahata, Takahiro Kitaizumi, Kota Saichi, Takumi Sato, Ryuji Igarashi |
| Institutions | Tokyo Institute of Technology, Center for Integrated Quantum Science and Technology |
| Citations | 132 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Core Innovation: Development of a novel, compact, optical fiber-based magnetometer utilizing negatively charged nitrogen-vacancy (NV-) centers embedded in bulk diamond.
- Application Focus: Highly sensitive detection of magnetic nanoparticles (MNPs) for biomedical applications, specifically targeting deep tissue sensing (e.g., sentinel lymph node biopsy).
- Sensitivity Achievement: The system demonstrated a minimum detectable AC magnetic field sensitivity of 57.6 nT (at 1.025 kHz, with 1 second averaging time).
- Magnetic Field Control: A specialized coil system was implemented, using an excitation coil to magnetize MNPs with an AC field, and a cancellation coil to suppress the residual excitation field at the NV- center location by approximately 99%.
- Detection Range: Successfully detected micromolar concentrations of MNPs (ResovistŸ) at longitudinal distances up to 9 mm (for 40 ”L sample volume).
- Operational Advantage: The NV- center sensor operates effectively at room temperature and ambient magnetic fields (geomagnetic field), offering a practical advantage over cryogenic SQUID or vacuum-dependent OPAM systems.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Type | Bulk (100) | N/A | Used for NV- center ensemble. |
| Diamond Dimensions | 2 x 2 x 0.5 | mm3 | Physical size of the bulk diamond. |
| NV Fabrication Dose | 1 x 1018 | cm-2 | High-energy electron beam irradiation (4.6 MeV). |
| NV Annealing Temperature | 800 | °C | Thermal annealing time: 1 hour. |
| Excitation Laser Wavelength | 532 | nm | Frequency doubled YAG laser (Green). |
| Incident Laser Power | ~100 | mW | Applied to the bulk diamond. |
| ODMR Center Frequency (f0) | ~2.87 | GHz | Without external magnetic field. |
| ODMR Dip HWHM (w) | ~4.3 | MHz | Half Width at Half Maximum. |
| AC Excitation Frequency | 1.025 | kHz | Frequency used for MNP magnetization and lock-in detection. |
| Minimum Detectable AC Field (Experimental) | 57.6 | nT | At 1.025 kHz, 1 s averaging time (SNR ~1). |
| NV Center Sensitivity (Calculated) | 33.2 | nT | Best sensitivity adjusted for NV- center angle. |
| Excitation Coil Field (at NV location) | ~2600 | ”T | Field generated by excitation coil alone. |
| Residual Magnetic Field (at NV location) | 3.5 | ”T | After cancellation coil application (99% suppressed). |
| MNP Detectable Distance (40 ”L sample) | 9 | mm | Longitudinal distance from probe head. |
| MNP Detectable Distance (5 ”L sample) | 5 | mm | Longitudinal distance from probe head. |
| Probe Head Outer Diameter | 18 | mm | Physical dimension of the compact probe. |
Key Methodologies
Section titled âKey MethodologiesâThe magnetometer relies on Optically Detected Magnetic Resonance (ODMR) combined with lock-in detection and precise magnetic field control:
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NV- Center Fabrication:
- Bulk (100) diamond was subjected to high-energy electron beam irradiation (4.6 MeV) at a dose of 1 x 1018 cm-2.
- Subsequent thermal annealing was performed at 800 °C for 1 hour to mobilize vacancies and form the NV- centers.
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Optical Fiber System Integration:
- A compact, bifurcated optical fiber bundle (2x1 coupler) was used for light management.
- A 532 nm green laser (~100 mW) was delivered through the fiber to excite the NV- centers.
- Red fluorescence (>600 nm) was collected through the same fiber port and filtered before detection by a photodiode (PD).
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Magnetic Resonance Control:
- Microwave (MW) irradiation (~2.87 GHz) was applied via a thin copper film (0.04 mm thick) located beneath the diamond to drive the electron spin resonance (ESR).
- A permanent magnet was positioned to apply a static magnetic field, splitting the four NV orientations into eight dips (Zeeman effect) and allowing selection of a single, highly sensitive NV axis for measurement.
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AC Field Generation and Cancellation:
- An excitation coil generated a strong AC magnetic field (1.025 kHz) to actively magnetize the MNPs.
- A concentric cancellation coil was used to nullify the excitation field at the precise location of the NV- center ensemble, reducing the background field from 2600 ”T down to 3.5 ”T.
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Signal Detection:
- The system utilized lock-in detection (DC sensing principle) tuned to the AC excitation frequency (1.025 kHz).
- The magnetic field generated by the magnetized MNPs caused fluctuations in the red fluorescence intensity, which were measured by the lock-in amplifier to extract the weak MNP signal.
Commercial Applications
Section titled âCommercial ApplicationsâThis technology offers significant potential in fields requiring highly sensitive, non-cryogenic magnetic sensing:
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Biomedical Diagnostics and Imaging:
- Sentinel Lymph Node Biopsy (SLNB): Provides a non-radioactive, highly sensitive alternative to conventional gamma probes for detecting MNP tracers accumulating in lymph nodes.
- Deep Tissue Sensing: Potential for detecting small amounts of MNPs (micromolar concentrations) at depths greater than 10 mm, crucial for non-invasive cancer diagnosis and tracking.
- In Vivo MNP Tracking: Real-time monitoring and quantification of magnetic tracers used in drug delivery or hyperthermia treatments.
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Quantum Sensing and Metrology:
- Room-Temperature Magnetometry: Enables the deployment of highly sensitive magnetometers outside of laboratory environments, eliminating the need for liquid nitrogen or helium required by SQUID or OPAM systems.
- Compact Sensor Development: The optical fiber-based design facilitates the creation of compact, handheld, or integrated magnetic probes for industrial or field applications.
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Material Science and Research:
- Nanoscale Magnetic Characterization: Used for magnetic field imaging and characterization of magnetic materials and phenomena at the millimeter scale and potentially smaller.