A biocompatible technique for magnetic field sensing at (sub)cellular scale using Nitrogen-Vacancy centers

At a Glance

Metadata	Details
Publication Date	2020-10-21
Journal	EPJ Quantum Technology
Authors	Ettore Bernardi, Ekaterina Moreva, Paolo Traina, Giulia Petrini, Sviatoslav Ditalia Tchernij
Institutions	Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Australian Nuclear Science and Technology Organisation
Citations	9
Analysis	Full AI Review Included

Technical Analysis and Documentation: Biocompatible NV Magnetic Sensing

Executive Summary

This research demonstrates a highly sensitive, biocompatible magnetic field sensing technique utilizing Nitrogen-Vacancy (NV) centers in MPCVD diamond, achieving sub-cellular resolution.

Core Achievement: Magnetic sensitivity of $\eta = 68 \pm 3 \text{ nT}/\sqrt{\text{Hz}}$ achieved at $80 \text{ mW}$ optical power, suitable for in-vitro biological measurements.
Material Foundation: The sensor relies on a thin, near-surface NV layer (15 nm thick, 10 nm depth) created in an optical grade CVD Single Crystal Diamond (SCD) substrate.
Sensing Volume: The sensing volume was successfully minimized to $(0.015 \times 10 \times 10) \text{ µm}^{3}$, enabling magnetic field mapping at the cellular scale.
Methodology: Utilized a lock-in based Optically Detected Magnetic Resonance (ODMR) technique, enhanced by simultaneous microwave driving of the three hyperfine resonances, improving the signal slope by a factor of $\approx 1.5$.
Future Requirements: Achieving the necessary sensitivity for single neuron activity (1-10 nT) requires advanced materials, specifically isotopically purified SCD for extended coherence time ($T_{2}^{*}$), and thin-film metalization for thermal management.
6CCVD Value Proposition: 6CCVD specializes in providing the high-purity, isotopically controlled SCD substrates and custom metalization services required to replicate and significantly advance this quantum sensing research.

Technical Specifications

The following hard data points were extracted from the experimental setup and results:

Parameter	Value	Unit	Context
Magnetic Sensitivity ($\eta$)	$68 \pm 3$	nT/√Hz	Achieved at 80 mW optical power
CW Shot-Noise Limit ($\eta_{CW}$)	12.5	nT/√Hz	Theoretical limit based on ODMR spectrum
Sensing Volume	$0.015 \times 10 \times 10$	µm³	Defined by NV layer thickness and laser spot size
NV Layer Thickness	15	nm	Along z-axis
NV Layer Depth	10	nm	Below sample surface
NV Concentration	$\sim 3 \cdot 10^{19}$	cm^-3	Resulting from implantation/annealing
Implantation Dose (N ions)	$1 \times 10^{14}$	cm^-2	10 keV N ions at room temperature
Annealing Temperature	950	°C	Held for 2 hours
Excitation Wavelength	532	nm	Nd:YAG second harmonic
Maximum Optical Power	80	mW	Used for sensitivity measurement
Laser Spot Size (x-y plane)	$10 \times 10$	µm²	Defines sensing area
ODMR Spin Resonance Center	2.87	GHz	Microwave frequency
Hyperfine Splitting ($\Delta_{orth}$)	2.16	MHz	Used for simultaneous driving
Biocompatible Power Estimate	> 10	mW	Conservative estimate for sustained use
Target Sensitivity (Neuronal)	1 - 10	nT	Required for single channel activity

Key Methodologies

The experimental protocol focused on creating a shallow, high-density NV layer and optimizing the ODMR readout for maximum signal slope.

Substrate Preparation:
- Used a $3 \times 3 \times 0.3 \text{ mm}^{3}$ “optical grade” CVD Single Crystal Diamond (SCD) substrate.
- Nominal impurity concentrations: Substitutional Nitrogen (< 1 ppm), Boron (< 0.05 ppm).
NV Center Formation (Implantation):
- 10 keV N ions were implanted at room temperature.
- Implantation dose (fluence) was $1 \times 10^{14} \text{ cm}^{-2}$.
NV Center Activation (Annealing):
- The implanted sample was annealed at $950^{\circ}\text{C}$ for 2 hours.
- This process resulted in a 15 nm thick NV layer localized 10 nm from the surface.
Microwave Control:
- The diamond was mounted on a planar ring antenna designed for the 2.87 GHz spin resonance.
- A commercial microwave generator (Keysight N5172B) was used, internally modulated at $f_{mod} = 5001 \text{ Hz}$.
- Simultaneous hyperfine driving was achieved by mixing the microwave signal with a $\sim 2.16 \text{ MHz}$ sinewave.
Optical Readout (ODMR):
- 532 nm excitation light (up to 80 mW) was focused to a $10 \times 10 \text{ µm}^{2}$ spot using an NA = 0.67 objective.
- Photoluminescence (PL) was spectrally filtered (650 nm long-pass) and detected using a photodiode (96% fraction) coupled to a Lock-In Amplifier (LIA).
- The LIA signal was used to track the ODMR shift, which is linearly proportional to the applied magnetic field.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced diamond materials and processing required to replicate this cellular-scale magnetometry research and achieve the necessary sensitivity improvements outlined by the authors.

Applicable Materials

The paper utilized standard optical grade SCD. To achieve the estimated 300-fold sensitivity improvement required for single-neuron sensing, the following 6CCVD materials are essential:

6CCVD Material Solution	Specification & Benefit	Application Context
Isotopically Purified Single Crystal Diamond (SCD)	Ultra-low ¹³C concentration (< 0.1%). Extends the NV center coherence time ($T_{2}^{*}$), crucial for implementing high-contrast pulsed techniques (Ramsey measurements) and achieving higher sensitivity.	Quantum Sensing, Advanced Magnetometry
Optical Grade SCD Substrates	Low substitutional Nitrogen (< 1 ppm) and Boron (< 0.05 ppm). Available in custom thicknesses from $0.1 \text{ µm}$ up to $500 \text{ µm}$, and substrates up to 10 mm thick.	Replication of current experimental setup
High-Purity Polycrystalline Diamond (PCD)	Available for large-area sensing applications up to 125 mm diameter, with surface roughness (Ra) < 5 nm.	Scalable sensor arrays, large-area biological imaging

Customization Potential

The authors noted that laser heating is a major limitation for biocompatibility, suggesting the use of a thin reflective/heatsink layer (e.g., Aluminum) to protect the sample. 6CCVD provides integrated solutions for this requirement.

Custom Service	Specification	Relevance to Research
Custom Metalization	In-house deposition of thin films including Al, Ti, Pt, Au, Pd, and Cu.	Essential for creating the reflective/heatsink layer (Al) suggested by the authors to mitigate laser heating and increase sustainable optical power.
Precision Polishing	SCD surfaces polished to ultra-low roughness (Ra < 1 nm).	Ensures optimal optical coupling and minimizes scattering losses, critical for high-NA confocal microscopy used in this setup.
Custom Dimensions & Thickness	Plates/wafers up to 125 mm (PCD). SCD thickness control from $0.1 \text{ µm}$ to $500 \text{ µm}$.	Allows researchers to optimize substrate size and thickness for specific microwave antenna geometries and thermal loads.
Laser Cutting & Shaping	Precision laser cutting for custom geometries (e.g., micro-pillars, cantilevers) compatible with complex ODMR setups.	Fabrication of specialized diamond components for integrated quantum sensors.

Engineering Support

6CCVD’s in-house PhD team specializes in material science and quantum defect engineering. We can assist researchers in optimizing material selection for similar NV-based Biomagnetometry projects:

Implantation Recipe Consultation: Guidance on selecting the optimal nitrogen implantation dose and energy to achieve specific NV layer depths (e.g., 10 nm) and concentrations, balancing sensitivity and surface proximity.
Surface Termination: Consultation on surface treatments (e.g., oxygen or hydrogen termination) to maximize NV charge state stability and improve biocompatibility.
Thermal Management Integration: Design assistance for integrating thin-film metalization layers that function as effective heatsinks without compromising optical access.

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

Tech Support

Original Source

DOI: https://doi.org/10.1140/epjqt/s40507-020-00088-2

References

2006 - A new generation of the hts multilayer dc-squid magnetometers and gradiometers