Optimization of a Diamond Nitrogen Vacancy Centre Magnetometer for Sensing of Biological Signals

At a Glance

Metadata	Details
Publication Date	2020-10-19
Journal	Frontiers in Physics
Authors	James L. Webb, Luca Troise, Nikolaj Winther Hansen, Jocelyn Achard, Ovidiu Brinza
Institutions	Centre National de la Recherche Scientifique, Laboratoire des Sciences des Procédés et des Matériaux
Citations	38
Analysis	Full AI Review Included

Technical Analysis and Documentation: Optimization of a Diamond Nitrogen Vacancy Centre Magnetometer for Sensing of Biological Signals

Executive Summary

This document analyzes the research by Webb et al. (2020) on optimizing NV-ensemble magnetometry for biosensing, highlighting critical material requirements and connecting them directly to 6CCVD’s advanced MPCVD diamond capabilities.

Core Achievement: Demonstrated magnetic field sensitivity of ~100 pT/√Hz in the critical DC/low frequency range (10-500 Hz), essential for detecting biological signals like neural action potentials.
Material Requirement: High-purity, isotopically enriched ¹²C CVD diamond (20 µm layer, 5 ppm ¹⁴N doping) was necessary to achieve the narrow ODMR linewidths (~1 MHz FWHM) required for high sensitivity.
System Optimization: The setup utilizes Brewster’s angle laser coupling and advanced noise rejection techniques (balanced photodetectors, digital subtraction) to overcome laser technical noise and reach near shot-noise limited performance.
Thermal Management: Successful operation at high pump powers (up to 2 W) required robust thermal dissipation strategies, including the use of AlN plates and thin insulating layers (Kapton/Al foil) to maintain biological sample viability (35-37 °C).
Customization Need: The experiment required custom electrodes (Pt/Ir, Ti, W) and precise control over diamond thickness and substrate integration, areas where 6CCVD offers specialized fabrication services.
Future Direction: Sensitivity improvements are targeted through pulsed magnetometry schemes and advances in diamond material quality, including strain management and patterning for enhanced light extraction.

Technical Specifications

The following table extracts key performance metrics and material parameters achieved or targeted in the research:

Parameter	Value	Unit	Context
Magnetic Field Sensitivity (DC/LF)	~100	pT/√Hz	Achieved sensitivity for biological measurements
ODMR Linewidth (FWHM)	~1	MHz	Achieved using ¹²C purified diamond
Target Shot Noise Limit Sensitivity	10-20	pT/√Hz	Estimated theoretical limit for DC/LF fields
CVD Layer Thickness	20	µm	¹²C purified layer used for NV creation
Nitrogen Concentration	5	ppm	Doping level in the CVD-grown layer
Pump Laser Power (Max)	2	W	Used to maximize fluorescence output
Fluorescence Collection (Max)	6.5	mW	Maximum collected output
DC Offset Magnetic Field	~1.6	mT	Applied to maximize ODMR contrast (up to 5.1%)
Readout Time (Ensemble Decay)	200-300	µs	Achievable minimum readout time at high power
Optimal Biological Temperature	35-37	°C	Required for maintaining living tissue viability
Laser Incidence Angle	67.5	°	Brewster’s angle for diamond

Key Methodologies

The experimental success relied on precise material engineering and sophisticated optical/microwave control:

Diamond Material Engineering:
- Growth of a 20 µm ¹²C purified CVD layer doped with 5 ppm ¹⁴N.
- Subsequent irradiation using H⁺ ions (at U. Leipzig) followed by annealing at 800 °C to create high-density NV centers with narrow linewidths.
Optical and Microwave Coupling:
- Laser illumination (532 nm) coupled into the diamond at Brewster’s angle (67.5°) using a tilted mirror to maximize transmission and minimize pump laser leakage into the biological sample.
- Microwave (MW) delivery via a custom-designed broadband antenna on a PCB board, optimized to counter the detuning effects of the adjacent aqueous solution.
Sensing Technique (CW ODMR):
- Continuous Wave (CW) sensing performed using MW frequency modulation (33 kHz) and detection via a lock-in amplifier.
- A three-frequency drive method was used to boost sensitivity.
Noise Mitigation:
- Laser technical noise rejected using a balanced photodetector or digital subtraction of the input laser beam signal.
- Automated ODMR sweeps were implemented to correct for thermal drift and maintain the microwave frequency at the point of maximum field sensitivity.
- Advanced FFT filtering was applied to remove strong 50/60 Hz mains harmonics and phase drift broadening, reducing noise by orders of magnitude.
Biocompatible Setup:
- Inverted microscope geometry with a custom 3D-printed bath chamber for holding biological samples in carbogenated solution (5% CO₂, 95% O₂).
- Thermal isolation achieved using thin layers (20 µm Al foil, 30 µm Kapton tape) between the heated diamond and the sample.
- Custom, non-ferrous electrodes (Pt/Ir, Ti, W) were developed to prevent magnetization and disruption of the DC offset field.

6CCVD Solutions & Capabilities

The research highlights the critical need for highly specialized diamond materials and integrated components to push the boundaries of NV biosensing. 6CCVD is uniquely positioned to supply and customize the required materials for replicating and advancing this work.

Applicable Materials for Quantum Biosensing

To achieve the narrow linewidths and high sensitivity demonstrated, researchers require ultra-pure, low-strain diamond with precise nitrogen control.

Material Requirement (Paper)	6CCVD Solution	Technical Advantage
Isotopically Purified ¹²C Diamond	Optical Grade SCD (¹²C Enriched)	Minimizes ¹³C spin noise, maximizing coherence time (T₂*) and achieving narrow ODMR linewidths (~1 MHz).
Thin Active Layer (20 µm)	Custom SCD Thickness Control	6CCVD supplies SCD layers from 0.1 µm up to 500 µm, allowing precise replication of the 20 µm active layer used for optimal NV ensemble density.
Controlled Nitrogen Doping (5 ppm)	Custom Doping Recipes	We offer precise control over nitrogen incorporation during CVD growth, essential for optimizing NV concentration and maximizing fluorescence contrast.
Thermal Dissipation Substrates	SCD/PCD on Substrates	We can supply SCD or PCD bonded to high-thermal-conductivity materials (e.g., SiC or AlN) for efficient heat sinking, crucial for high-power laser operation (up to 2 W).

Customization Potential & Integrated Components

The paper emphasizes the need for custom geometries, thin interfaces, and non-magnetic electrodes. 6CCVD’s in-house fabrication capabilities directly address these challenges.

Custom Dimensions and Polishing: 6CCVD provides custom plates and wafers up to 125mm (PCD) and offers ultra-smooth polishing (Ra < 1nm for SCD) necessary for minimizing light scattering and maximizing fluorescence collection efficiency.
Integrated Metalization for Electrodes and Antennas: The use of custom non-ferrous electrodes (Pt/Ir, Ti, W) is critical. 6CCVD offers internal metalization services (Au, Pt, Pd, Ti, W, Cu) to deposit these non-magnetic materials directly onto the diamond surface, enabling integrated microwave antennas or stimulating electrodes without disrupting the DC offset field.
Thin Interface Fabrication: The experiment required extremely thin insulating layers (20-30 µm) to minimize the sample-diamond separation. 6CCVD can provide thin SCD wafers (down to 0.1 µm) or custom-thinned substrates to meet stringent proximity requirements (order of µm separation).

Engineering Support

The complexity of NV creation (irradiation and annealing) and system integration (Brewster’s angle optics, noise filtering) requires specialized knowledge.

NV Creation Optimization: 6CCVD’s in-house PhD team provides consultation on optimizing material selection, post-processing (irradiation and annealing protocols), and surface termination to maximize NV yield and stability for Biomagnetometry and Quantum Sensing projects.
Global Supply Chain: We ensure reliable, global shipping (DDU default, DDP available) of sensitive, high-purity diamond materials, supporting international research efforts.

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

View Original Abstract

Sensing of signals from biological processes, such as action potential propagation\nin nerves, are essential for clinical diagnosis and basic understanding of physiology.\nSensing can be performed electrically by placing sensor probes near or inside a\nliving specimen or dissected tissue using well-established electrophysiology techniques.\nHowever, these electrical probe techniques have poor spatial resolution and cannot easily\naccess tissue deep within a living subject, in particular within the brain. An alternative\napproach is to detect the magnetic field induced by the passage of the electrical signal,\ngiving the equivalent readout without direct electrical contact. Such measurements are\nperformed today using bulky and expensive superconducting sensors with poor spatial\nresolution. An alternative is to use nitrogen vacancy (NV) centers in diamond that promise\nbiocompatibilty and high sensitivity without cryogenic cooling. In this work we present\nadvances in biomagnetometry using NV centers, demonstrating magnetic field sensitivity\nof ∼100 pT/√Hz in the DC/low frequency range using a setup designed for biological\nmeasurements. Biocompatibility of the setup with a living sample (mouse brain slice)\nis studied and optimized, and we show work toward sensitivity improvements using a\npulsed magnetometry scheme. In addition to the bulk magnetometry study, systematic\nartifacts in NV-ensemble widefield fluorescence imaging are investigated.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2020.522536

References

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