Microscopic-scale magnetic recording of brain neuronal electrical activity using a diamond quantum sensor

At a Glance

Metadata	Details
Publication Date	2023-07-31
Journal	Scientific Reports
Authors	Nikolaj Winther Hansen, James L. Webb, Luca Troise, Christoffer Olsson, Leo Tomasevic
Institutions	Sorbonne Université, Laboratoire des Sciences des Procédés et des Matériaux
Citations	14
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond Quantum Sensing for Microscopic Neural Recording

Executive Summary

This documentation analyzes the successful demonstration of microscopic-scale biomagnetic recording of neuronal electrical activity using Nitrogen-Vacancy (NV) quantum sensors embedded in Chemical Vapor Deposition (CVD) diamond.

Core Achievement: First proof-of-principle recording of action potential propagation in fragile mammalian brain tissue (mouse corpus callosum) using remote, passive biomagnetic sensing.
Material Basis: The sensor utilizes a 2 x 2 x 0.5 mm³ [100] oriented Single Crystal Diamond (SCD) substrate with a 20 µm ¹⁴N-doped CVD overgrowth layer, optimized for high-density NV ensemble creation.
Performance Metrics: Achieved a high magnetic field sensitivity of 50 pT/√Hz with a sensing bandwidth of 10 kHz, sufficient to detect weak ionic currents (nano- to femto-Tesla range) associated with compound action potentials (cAP).
Methodological Advantage: The technique is non-invasive, highly biocompatible, and remote (sample separated by ~60 µm), avoiding the damaging direct interaction, phototoxicity, and signal distortion inherent in traditional electrical or optical recording methods (e.g., patch clamp, voltage dyes).
Application Potential: Demonstrated in situ pharmacology using tetrodotoxin (TTX), confirming the sensor’s capability for detailed microscopic study of pathologies affecting neuronal electrical activity, relevant for neurodegenerative disease models.
6CCVD Relevance: This research validates the critical role of high-quality, custom-engineered MPCVD SCD materials for next-generation quantum sensing in biological and medical applications.

Technical Specifications

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

Parameter	Value	Unit	Context
Diamond Substrate Orientation	[100]	N/A	Electronic-grade SCD crystal
Diamond Dimensions	2 x 2 x 0.5	mm³	Overall sensor size
CVD Overgrowth Thickness	~20	µm	N-doped layer for NV creation
Nitrogen Doping (¹⁴N)	~5	ppm	Concentration in the CVD layer
Annealing Temperature	800	°C	Post-irradiation N to NV conversion
Proton Irradiation Energy	2.8	MeV	Used to create vacancies
NV Sensing Volume	300 x 100 x 20	µm³	Volume addressed by pump laser
Magnetic Field Sensitivity	50	pT/√Hz	Achieved sensitivity
Sensing Bandwidth (f_-3dB)	10	kHz	Measurement rolloff frequency
ODMR Linewidth	1	MHz	For a single NV axis
ODMR Contrast	1.5	%	For a single NV axis
Pump Laser Wavelength	532	nm	Green laser light
Pump Laser Power	1.2 - 1.4	W	Linearly polarized, single mode
Static Bias Field	1.5	mT	Applied parallel to diamond [110] direction
Sample-Sensor Distance	~60	µm	Separation between brain slice and diamond surface
ACSF Bath Temperature	25	°C	Stable temperature maintained during recording

Key Methodologies

The microscopic biomagnetic recording relied on precise material engineering and a specialized Optically Detected Magnetic Resonance (ODMR) setup:

Material Growth: A 0.5 mm thick [100] oriented Single Crystal Diamond (SCD) substrate was overgrown via MPCVD with a 20 µm layer, precisely doped with ~5 ppm ¹⁴N to control the density of potential NV centers.
NV Center Formation: N to NV conversion was achieved by 2.8 MeV proton irradiation near the top surface of the CVD layer, followed by high-temperature annealing at 800 °C in an inert atmosphere.
Sensor Integration: The diamond was mounted in an aluminum nitride heatsink. The top surface was covered with 16 µm aluminum foil (acting as a reflector and heatsink) and a 50 µm Kapton tape layer for electrical insulation from the biological sample.
Optical Pumping: The NV centers were optically pumped using 1.2-1.4 W of 532 nm green laser light coupled into the diamond at Brewster’s angle (67°), ensuring the laser light was entirely contained within the diamond and did not interact with the tissue.
Magnetic Resonance: A continuous-wave ODMR scheme was employed, utilizing three-frequency microwave driving (2.7-3 GHz) modulated at 33.3 kHz for lock-in detection, transducing magnetic field shifts into changes in red fluorescence intensity.
Biological Preparation: 400 µm thick mouse brain slices (corpus callosum) were maintained in vitro in a chilled, carbogenated ACSF bath at a stable 25 °C, positioned approximately 60 µm above the NV sensing volume.
Signal Verification: Biomagnetic signals were recorded passively and remotely, and simultaneously verified using an invasive AgCl coated silver electrode to measure the local field potential (LFP).

6CCVD Solutions & Capabilities

This research highlights the critical need for highly customized, high-quality diamond materials for advanced quantum sensing in bio-magnetometry. 6CCVD is uniquely positioned to supply and engineer the required components.

Applicable Materials

To replicate or extend this research, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Required for the base substrate (2 x 2 x 0.5 mm³) to ensure low strain and high optical transparency for efficient 532 nm laser pumping.
Custom ¹⁴N Doped SCD Overgrowth: We offer precise control over nitrogen doping (e.g., 5 ppm ¹⁴N) during MPCVD growth to optimize the NV ensemble density, directly impacting the achievable magnetic sensitivity (50 pT/√Hz).
Isotopically Pure Diamond (Optional Extension): For future experiments requiring enhanced coherence times (T₂), 6CCVD can supply isotopically purified SCD (low ¹³C content), enabling higher spatial resolution and single-shot readout capabilities.

Customization Potential

The success of this experiment relies on precise material dimensions, surface quality, and integration features, all of which are core 6CCVD capabilities:

Research Requirement	6CCVD Customization Capability	Sales Advantage
Custom Dimensions (2 x 2 x 0.5 mm³)	Precision Laser Cutting: We provide custom plates and wafers in exact dimensions required for integration into specialized ODMR setups, including plates up to 125 mm (PCD).	Eliminates the need for post-processing by the researcher, ensuring dimensional accuracy and rapid deployment.
NV Layer Creation & Activation	Integrated Material Recipe: 6CCVD can supply the SCD substrate with the specified 20 µm ¹⁴N-doped CVD overgrowth layer, ready for the customer’s irradiation and annealing protocol, or we can assist in optimizing the growth recipe for specific NV densities.	Guarantees material consistency and optimizes the starting material for maximum quantum sensor performance.
Surface Quality (Optical Pumping)	Ultra-Polishing Services: Our SCD polishing capability achieves surface roughness Ra < 1 nm, critical for minimizing laser scattering, maximizing 532 nm pump efficiency, and reducing heat generation near the fragile biological sample.	Ensures optimal optical coupling and minimizes experimental artifacts caused by thermal instability.
Integrated Metalization (Al Reflector/Heatsink)	Internal Metalization Services: We offer custom deposition of thin films (Au, Pt, Pd, Ti, W, Cu) directly onto the diamond surface. This can replace the external aluminum foil, providing a more robust, integrated reflective layer and heatsink for improved thermal management.	Enhances device reliability and simplifies the assembly of complex quantum-biological interfaces.

Engineering Support

6CCVD’s in-house PhD team specializes in optimizing CVD diamond growth parameters for quantum applications. We offer consultation services to assist researchers in:

Material Selection: Choosing the optimal diamond grade (SCD vs. PCD) and crystal orientation ([100] vs. [111]) based on the specific magnetic field direction and sensing requirements.
NV Optimization: Fine-tuning doping levels and layer thicknesses to maximize the NV ensemble signal-to-noise ratio (SNR) for similar microscopic biomagnetic sensing projects.
Integration Support: Advising on thermal management and metalization schemes necessary for high-power laser operation (1.2-1.4 W) in proximity to living tissue.

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

View Original Abstract

Abstract Quantum sensors using solid state qubits have demonstrated outstanding sensitivity, beyond that possible using classical devices. In particular, those based on colour centres in diamond have demonstrated high sensitivity to magnetic field through exploiting the field-dependent emission of fluorescence under coherent control using microwaves. Given the highly biocompatible nature of diamond, sensing from biological samples is a key interdisciplinary application. In particular, the microscopic-scale study of living systems can be possible through recording of temperature and biomagnetic field. In this work, we use such a quantum sensor to demonstrate such microscopic-scale recording of electrical activity from neurons in fragile living brain tissue. By recording weak magnetic field induced by ionic currents in mouse corpus callosum axons, we accurately recover signals from neuronal action potential propagation while demonstrating in situ pharmacology. Our sensor allows recording of the electrical activity in neural circuits, disruption of which can shed light on the mechanisms of disease emergence. Unlike existing techniques for recording activity, which can require potentially damaging direct interaction, our sensing is entirely passive and remote from the sample. Our results open a promising new avenue for the microscopic recording of neuronal signals, offering the eventual prospect of microscopic imaging of electrical activity in the living mammalian brain.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41598-023-39539-y