Optical magnetic detection of single-neuron action potentials using quantum defects in diamond

At a Glance

Metadata	Details
Publication Date	2016-11-22
Journal	Proceedings of the National Academy of Sciences
Authors	John F. Barry, Matthew Turner, Jennifer M. Schloss, David R. Glenn, Yuyu Song
Institutions	Center for Systems Biology, Center for Astrophysics Harvard & Smithsonian
Citations	573
Analysis	Full AI Review Included

Technical Documentation & Analysis: NV-Diamond Magnetic Sensing for Neuroscience

Executive Summary

This research demonstrates a groundbreaking, noninvasive, label-free method for detecting single-neuron action potentials (APs) using Nitrogen-Vacancy (NV) quantum defects in MPCVD Single Crystal Diamond (SCD).

Core Achievement: Successful magnetic sensing of APs from excised giant axons (squid, marine worm) and, critically, from the exterior of whole, live, opaque organisms over extended periods (24+ hours).
Material Foundation: The technique relies on a custom-fabricated, electronic-grade SCD chip featuring a uniform, high-density NV sensing layer (13 µm thick, ~3x10¹⁷ cm^-3).
Performance Metrics: Achieved a magnetic field sensitivity ($\eta$) of 15 ± 1 pT/√Hz and a temporal resolution of ~32 µs using Continuous-Wave Electron Spin Resonance (CW-ESR) magnetometry.
Vector Sensing Capability: NV-diamond provides full vector magnetometry, enabling the determination of AP propagation direction and conduction velocity asymmetry—a capability superior to scalar measurement techniques.
Scalability & Future Potential: The results pave the way for a ‘quantum diamond microscope’ capable of real-time, 3D magnetic mapping of neuronal networks with projected ~10 nm spatial resolution and ~1 µs temporal resolution.
Key Requirement for Scaling: Future success hinges on optimizing diamond substrates for higher NV density and longer spin-dephasing times (T₂*).

Technical Specifications

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

Parameter	Value	Unit	Context
Diamond Substrate Dimensions	4 x 4 x 500	mm x mm x µm	Electronic Grade SCD
NV Sensing Layer Thickness	13	µm	Uniform, top surface layer
NV Center Density	~3 x 10¹⁷	cm^-3	Achieved via irradiation/annealing
Diamond Isotope Purity	99.99% ¹²C	N/A	Used 25 ppm ¹⁴N precursor
Annealing Temperature	800	°C	In vacuum, 12 hours
Magnetic Field Sensitivity ($\eta$)	15 ± 1	pT/√Hz	Achieved using CW-ESR magnetometry
Temporal Resolution (10%-90% rise time)	~32	µs	Limited by LIA time constant
NV Zero-Field Splitting	2.87	GHz	Triplet ground state
Excitation Laser Wavelength	532	nm	Applied at grazing incidence
Bias Magnetic Field ($B_0$)	~7	Gauss	Applied along two NV axes
AP Magnetic Field (Excised Axon)	4.1 ± 0.2	nT	Peak-to-peak amplitude (N_avg=150)
AP Magnetic Field (Live Worm Exterior)	~1	nT	Peak-to-peak amplitude (N_avg=1650)
Projected Spatial Resolution (Future)	~10	nm	Synapse-scale resolution goal

Key Methodologies

The experiment relied on precise MPCVD diamond engineering and advanced quantum sensing techniques:

Diamond Substrate Fabrication: Electronic grade Single Crystal Diamond (SCD) chips (4 mm x 4 mm x 500 µm) were grown via Chemical Vapor Deposition (CVD). The material was 99.99% ¹²C isotopic purity, crucial for minimizing decoherence.
NV Layer Creation: A high-density NV layer (13 µm thick) was created by irradiating the diamond with 4.6 MeV electrons (1.3 x 10¹⁴ cm^-2s^-1 flux) followed by vacuum annealing at 800 °C.
Sensor Mounting: The diamond chip was mounted on a 330 µm thick Silicon Carbide (SiC) heat spreader. For excised axons, a 2 mm x 25 mm slot in the SiC provided access; for whole organisms, the SiC acted as a spacer.
Optical Excitation: A 532 nm laser was applied at a shallow grazing angle to the diamond top surface, utilizing total internal reflection to excite the NV layer without irradiating the biological specimen.
Microwave (MW) Delivery: MWs were applied via a wire loop (excised axon) or a 25 µm thick copper foil layer (whole worm) located above the diamond sensor.
Magnetometry Protocol: Continuous-Wave Electron Spin Resonance (CW-ESR) Optically Detected Magnetic Resonance (ODMR) was used. MWs were square-wave frequency modulated at $f_{mod}$ = 18 kHz.
Signal Processing: Laser-Induced Fluorescence (LIF) was collected and processed using a Lock-in Amplifier (LIA) with a 30 µs time constant, achieving a photon-shot-noise-limited sensitivity.

6CCVD Solutions & Capabilities

This research highlights the critical need for highly specialized, custom-engineered diamond substrates to advance quantum sensing in neuroscience. 6CCVD is uniquely positioned to supply and collaborate on materials required to replicate and extend this work, particularly in achieving the projected sensitivity and resolution gains.

Applicable Materials for Replication and Extension

Research Requirement	6CCVD Solution & Material Grade	Technical Advantage
High-Purity Substrate	Electronic Grade Single Crystal Diamond (SCD)	Ultra-low strain and high isotopic purity (e.g., < 100 ppb ¹⁴N, > 99.999% ¹²C) essential for maximizing spin coherence times (T₂*), which is critical for achieving the projected 1 µs temporal resolution.
Controlled NV Layer	Custom NV-Engineered SCD	We offer precise control over nitrogen incorporation during CVD growth and subsequent electron irradiation/annealing protocols to achieve the required high NV density (~3x10¹⁷ cm^-3) and uniform 13 µm layer thickness, or optimized thinner layers for enhanced spatial resolution.
Scaling & Field-of-View	Large-Area Polycrystalline Diamond (PCD)	While the paper used SCD, future circuit-scale (~1 cm) field-of-view applications may benefit from our large-area PCD wafers (up to 125 mm diameter) with custom NV implantation for wide-field magnetic imaging.
Integrated Sensing	Boron-Doped Diamond (BDD)	For simultaneous electrophysiology or electrochemical sensing, we supply highly conductive BDD films, which can be integrated adjacent to SCD NV sensors.

Customization Potential for Advanced Quantum Microscopy

The paper emphasizes that future advancements require optimized diamond geometries and integrated components. 6CCVD provides end-to-end fabrication services to meet these needs:

Custom Dimensions and Thickness: The experiment used 4 mm x 4 mm x 500 µm plates. 6CCVD offers custom SCD plates up to 500 µm thick and substrates up to 10 mm thick. We can supply larger, inch-size SCD wafers necessary for scaling the field-of-view beyond the current 4 mm limit.
Surface Quality: The research requires placing biological samples directly on the NV-enriched surface. We guarantee Ra < 1 nm polishing for SCD, ensuring minimal surface defects and optimal contact for biological specimens.
Integrated Metalization: The experiment used external MW delivery structures (wire loops). 6CCVD offers in-house metalization capabilities (Au, Pt, Ti, Cu, Pd, W) for patterning integrated microwave transmission lines directly onto the diamond surface, improving MW efficiency and minimizing perturbation to the sample.
Advanced Fabrication: We provide laser cutting and shaping services to create custom geometries, such as the slots or trenches needed for microfluidic integration or fiber coupling, as required for next-generation quantum diamond microscopes.

Engineering Support

6CCVD’s in-house PhD team specializes in the physics and engineering of MPCVD diamond for quantum applications. We can assist researchers and engineers with material selection, NV creation recipe optimization, and substrate design for similar neuronal magnetic sensing projects, ensuring the material properties (e.g., T₂*, NV concentration, strain) are perfectly matched to the required sensitivity and temporal resolution goals.

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

View Original Abstract

Significance We demonstrate noninvasive detection of action potentials with single-neuron sensitivity, including in whole organisms. Our sensor is composed of quantum defects within a diamond chip, which detect time-varying magnetic fields generated by action potentials. The sensor is biocompatible and can be brought into close proximity to the organism without adverse effect, allowing for long-term observation and superior resolution of neuron magnetic fields. Optical magnetic detection with quantum defects also provides information about action potential propagation that is not easily available with existing methods. The quantum diamond technique requires no labeling or genetic modification, allows submillisecond time resolution, does not bleach, and senses through opaque tissue. With further development, we expect micrometer-scale magnetic imaging of a variety of neuronal phenomena.

Tech Support

Original Source

DOI: https://doi.org/10.1073/pnas.1601513113