Magnetometer with nitrogen-vacancy center in a bulk diamond for detecting magnetic nanoparticles in biomedical applications

At a Glance

Metadata	Details
Publication Date	2020-02-12
Journal	Scientific Reports
Authors	Akihiro Kuwahata, Takahiro Kitaizumi, Kota Saichi, Takumi Sato, Ryuji Igarashi
Institutions	The University of Tokyo, Center for Integrated Quantum Science and Technology
Citations	132
Analysis	Full AI Review Included

Technical Analysis of NV-Center Magnetometer for Biomedical Applications

Executive Summary

The analyzed paper details the development and characterization of a novel, compact, optical fiber-based magnetometer utilizing ensemble Nitrogen-Vacancy (NV^-) centers in bulk Single Crystal Diamond (SCD). This technology is engineered for the highly sensitive detection of Magnetic Nanoparticles (MNPs), offering a promising, non-radioactive alternative for biomedical applications like sentinel lymph node biopsy.

Core Achievement: Demonstrated a functional NV- center magnetometer system achieving sensitive detection of AC magnetic fields generated by magnetized MNPs.
Sensitivity Benchmark: Measured minimum detectable AC magnetic field was approximately 57.6 nT (at 1.025 kHz, 1 s averaging time), with a corrected NV- center sensitivity of 33.2 nT.
Biomedical Performance: Successfully detected micromolar MNP concentrations (1120 µg) at a longitudinal distance of up to 9 mm from the probe head.
System Integration: Utilized a compact, fiber-optic platform integrated with a precise excitation/cancellation coil system to effectively magnetize MNPs while suppressing residual magnetic fields at the NV sensor location (achieving 99% cancellation).
Material Basis: The sensor uses a (100) bulk SCD plate prepared via high-energy electron irradiation (4.6 MeV, 1 x 10¹⁸ cm^-2 dose) and 800 °C annealing.
Future Pathway: Performance improvements hinge on maximizing red luminescence extraction efficiency (photon count I₀), which is predicted to reduce the shot-noise limited sensitivity to ~9 nT.

Technical Specifications

Hard data extracted from the study detailing the material and performance metrics of the NV- center magnetometer.

Parameter	Value	Unit	Context
Diamond Material	Bulk SCD	(100)	Dimensions: 2 x 2 x 0.5 cm³
NV Creation Dose	1 x 10¹⁸	cm^-2	Electron irradiation (4.6 MeV)
Annealing Temperature	800	°C	Annealing time: 1 hour
Excitation Wavelength	532	nm	Green laser (250 mW)
Detection Wavelength	> 600	nm	Red fluorescence detection
ODMR Center Frequency (f₀)	~2.87	GHz	Zero magnetic field (S=1, ³A₂ ground state)
Dip Half Width (HWHM, w)	~4.3	MHz	ODMR spectral linewidth
Minimum Detectable AC Field (B_min)	57.6	nT	Experimental sensitivity at 1.025 kHz
NV Center Sensitivity (Calculated)	33.2	nT	Corrected for NV- center orientation angle (109.5°)
Theoretical Sensitivity (Shot-Noise)	~9	nT	Requires maximization of photon count (I₀)
Residual Field at NV Center	~1	µT	After 99% cancellation of 2600 µT excitation field
MNP Detection Distance (Max)	9	mm	For 40 µL (1120 µg) of MNPs (Resovist^®)
Gyromagnetic Ratio (γ_NV)	28.024	GHz/T	Used for Zeeman shift calculation (Δf = γ_NVB₀)

Key Methodologies

The experimental design focuses on creating a high-density NV ensemble and integrating it into a compact, fiber-coupled, magnetically controlled system optimized for AC magnetic sensing.

Diamond Preparation: A (100) SCD plate was subjected to high-energy electron beam irradiation (4.6 MeV, 1 x 10¹⁸ cm^-2 dose) to introduce vacancies, followed by thermal annealing at 800 °C for 1 hour to mobilize vacancies and form NV centers.
Optical Fiber Integration: A compact fiber-optic platform was developed, coupling a 532 nm green laser for excitation and collecting >600 nm red fluorescence using a 2 x 1 fiber coupler and optical adhesives.
Static Field Isolation: A Neodymium permanent magnet (NdFeB, 25 mm diameter) was positioned near the diamond to provide a constant magnetic bias (0.7-4.4 mT). This Zeeman split the ODMR dips, allowing for the selective isolation and sensing of a single NV axis orientation.
Microwave (MW) Delivery: MW radiation (50 dB) was delivered via a 0.04 mm thin copper film patterned beneath the bulk diamond surface to generate the Electron Spin Resonance (ESR) needed for Optically Detected Magnetic Resonance (ODMR).
AC Field Generation and Nulling: A solenoid coil system, comprising an excitation coil and a cancellation coil (588 turns), was used. The cancellation coil was critical for eliminating approximately 99% of the strong excitation magnetic field (up to 2600 µT) at the NV center location, minimizing interference during MNP detection.
Lock-in Detection: The system operated using lock-in detection at an AC frequency of 1.025 kHz. This technique enhanced the Signal-to-Noise Ratio (SNR) by up to 140 times compared to DC sensing, enabling the high-sensitivity detection of the alternating magnetic fields generated by the magnetized MNPs.

6CCVD Solutions & Capabilities

This research validates the use of high-quality bulk Single Crystal Diamond (SCD) as a critical component in next-generation, compact quantum sensing magnetometers for high-sensitivity biomedical applications. 6CCVD is uniquely positioned to supply and engineer the SCD materials necessary to replicate, optimize, and scale this technology.

Requirement/Challenge (Paper)	6CCVD Solution & Value Proposition
Material Foundation	High-Purity Single Crystal Diamond (SCD) Wafers
The experiment required a uniform (100) bulk diamond (2 x 2 x 0.5 cm³).	6CCVD provides low-strain, high-homogeneity Optical Grade SCD plates in various orientations, including the critical (100) orientation, with custom dimensions up to 125 mm diameter (PCD).
NV Center Optimization	Controlled Nitrogen Doping and Post-Processing Support
High NV density was achieved via high-dose irradiation and 800 °C annealing.	We supply SCD materials with tailored nitrogen concentrations optimized for NV formation density. Our expert engineering team assists customers in determining precise parameters for electron irradiation and high-temperature annealing (up to 1200 °C) to maximize T₂* coherence and signal intensity (I₀).
Optical Signal Enhancement	Ultra-Precision Polishing (Ra < 1 nm)
Future improvements require increased red luminescence extraction efficiency (I₀). Optical coupling used adhesive bonding.	6CCVD guarantees ultra-smooth surfaces (Ra < 1 nm for SCD) far superior to standard commercial materials. This low roughness is essential for minimizing scattering losses and maximizing photon extraction efficiency, directly pushing sensitivity towards the theoretical 9 nT shot-noise limit.
Probe Integration & MW Control	Custom Thin-Film Metalization & Micro-Machining
MW delivery was achieved via a 0.04 mm copper thin film applied near the diamond.	We offer internal metalization capabilities (Au, Pt, Pd, Ti, Cu, W) applied via high-precision PVD. This allows for the integration of complex, high-efficiency on-chip microwave transmission lines, optimizing the ESR setup required for the lock-in detection method. We also provide precision laser cutting services for custom component geometries.
Scalability and Volume	MPCVD Production Expertise & Global Supply Chain
This prototype demonstrates feasibility for clinical application requiring mass production and reliability.	As MPCVD diamond specialists, 6CCVD ensures high lot-to-lot material consistency and the capacity for volume production required for scaling compact biomedical devices. We offer reliable, global shipping (DDU default, DDP available) to ensure research and commercial timelines are met.

6CCVD provides the specialized high-purity, optically optimized diamond material essential for the next generation of NV quantum magnetometers.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41598-020-59064-6