Monitoring spin coherence of single nitrogen-vacancy centers in nanodiamonds during pH changes in aqueous buffer solutions

At a Glance

Metadata	Details
Publication Date	2019-01-01
Journal	RSC Advances
Authors	Masazumi Fujiwara, Ryuta Tsukahara, Yoshihiko Sera, Hiroshi Yukawa, Yoshinobu Baba
Institutions	Kwansei Gakuin University, Graduate School USA
Citations	32
Analysis	Full AI Review Included

Technical Analysis and Documentation for 6CCVD

Executive Summary

This study, analyzing the spin coherence stability of single nitrogen-vacancy (NV) centers in nanodiamonds within aqueous buffers, highlights critical challenges in nanoscale quantum sensing. 6CCVD’s MPCVD diamond solutions directly address these limitations by providing superior material platforms for high-precision biological sensing.

Core Finding: Single NV centers in 25-nm nanodiamonds exhibit significant spin coherence time (T₂) and resonance frequency ($\omega_{0}$) fluctuations (up to ±12% and ±0.2 MHz, respectively) during continuous pH changes (4 to 11).
Limiting Factor: The fluctuations are primarily attributed to surface chemistry effects—specifically photoionization and charge-state instability caused by the proximity of the NV centers to the non-uniform nanodiamond surface in an aqueous environment.
Application Relevance: These fluctuations are comparable to the signal changes expected for physiological nanoscale thermometry (e.g., a $\pm$0.3 MHz shift corresponding to $\pm$4 °C change). Achieving reliable bio-sensing requires vastly improved material stability.
6CCVD Solution: Utilizing 6CCVD’s high-purity, Optical Grade Single Crystal Diamond (SCD) films or substrates provides a flat, highly controlled surface termination, minimizing surface-induced decoherence and charge-state noise inherent to nanodiamonds.
Engineering Requirement: The research underscores the need for highly stable substrates and surfaces (e.g., surface-oxidized bulk diamond) for robust NV quantum sensing systems in physiological media.

Technical Specifications

Data extracted from the experimental results regarding stability and measurement parameters.

Parameter	Value	Unit	Context
Nanodiamond Size (Median)	25	nm	Commercially available nanodiamonds
pH Range Investigated	4 to 11	-	Continuous change in aqueous buffer solutions
T₂ Stability Fluctuation	±12	%	Fluctuation of mean spin coherence time
$\omega_{0}$ Stability Fluctuation	±0.2	MHz	Fluctuation of mean resonance frequency
T₂ (Phosphate Buffer, Mean)	1224	ns	Single NV center stability at constant pH 6.1 (19 h)
T₂ (Air, Mean)	662	ns	Single NV center stability in air (24 h)
High Optical Excitation Intensity	90	kW·cm^-2	Used for image scanning; linked to destabilization/photoionization
Low Optical Excitation Intensity	5.4	kW·cm^-2	Used for stability testing
ODMR Frequency Temperature Dependence	-74	kHz·K^-1	Theoretical sensitivity for NV-nanothermometry
Target Temperature Sensitivity (Biological)	±4	°C	Corresponds to $\pm$0.3 MHz $\omega_{0}$ shift
Buffer Flow Rate	80	µL·min^-1	Continuous flow rate used during optical excitation

Key Methodologies

The following parameters and techniques were central to characterizing the NV center stability.

Sample Preparation: Nanodiamonds (25 nm median size) were purified, spin-coated, and immobilized on a coverslip. A 25-µm-thin copper wire antenna was integrated onto the coverslip surface for microwave delivery.
Perfusion Chamber Setup: An acrylic chamber with inlet/outlet tubes was glued onto the coverslip, allowing continuous flow of buffer solutions to maintain chemical stability and prevent photothermal aggregation.
Buffer Solutions: Two types of mixed buffer solutions were used for stepwise pH variation:
- pH 4-7: Citric acid (0.1 M) and Na₂HPO₄ (0.2 M).
- pH 7-11: Na₂CO₃ (0.1 M) and HCl (0.5 M).
Optical & Microwave Setup (ODMR): A home-built confocal fluorescence microscope was used with a 532 nm continuous-wave laser excitation. Microwaves were generated (Rohde & Schwarz SMB100A) and amplified by 45 dB, feeding into the Cu linear antenna.
Spin Measurements: Pulsed ODMR sequences were employed to determine spin properties:
- Rabi sequence: Determined the pulse durations ($\pi$ and $\pi$/2).
- Spin Echo sequence: Determined the spin coherence time (T₂).

6CCVD Solutions & Capabilities

This research validates the critical need for advanced diamond materials with highly controlled surfaces to realize the potential of NV quantum sensing in physiological environments. Nanodiamond limitations (surface inhomogeneity, photoionization, and aggregation in high ionic strength buffers) necessitate a shift toward high-quality CVD diamond films.

6CCVD provides the specialized material solutions required to overcome the stability constraints identified in this paper.

Applicable Materials for NV Quantum Sensing

To replicate or extend this research with enhanced stability and precision, 6CCVD strongly recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Required for achieving maximum T₂ times and $\omega_{0}$ stability. The inherent low defect density minimizes spectral diffusion and spin decoherence noise, providing a stable platform superior to nanodiamonds.
- Recommendation: High-purity, nitrogen-doped (to create NV centers) SCD substrates or thin films (down to 0.1 µm thickness) grown via MPCVD.
High-Purity Polycrystalline Diamond (PCD): For applications requiring large-area coverage or integration into microfluidic arrays. 6CCVD can supply PCD plates up to 125 mm diameter, offering scalable sensing platforms.

Customization Potential for Enhanced Stability

The paper highlights that charge-state instability and photoionization are directly linked to surface termination. 6CCVD’s advanced engineering capabilities offer precise control over the material properties and integration features essential for reliable quantum sensors:

Requirement from Research	6CCVD Customization Service	Technical Benefit
Controlled Surface Termination	High-precision Polishing: SCD: Ra < 1 nm; PCD: Ra < 5 nm.	Essential for subsequent controlled oxidation/functionalization needed to stabilize NV charge state (NV^- vs. NV⁰).
Integrated Microwave Delivery	Custom Metalization: Deposition of Au, Pt, Ti, Cu layers.	Allows researchers to integrate planar microwave antennas directly onto the diamond surface, optimizing coupling efficiency and excitation uniformity.
Substrate Thickness Control	Custom Thickness: SCD films from 0.1 µm to 500 µm.	Permits the fabrication of thin quantum layers for near-surface sensing while maintaining the robust physical properties of the bulk CVD substrate.
Large-Area Sensing Arrays	Custom Dimensions: PCD wafers up to 125 mm diameter.	Supports scaling from single-NV studies to parallelized sensing architectures for real-time biological monitoring.

Engineering Support

The successful implementation of NV-diamond sensors relies heavily on specialized material selection and preparation (e.g., surface oxidation and functionalization for biocompatibility).

6CCVD’s in-house PhD material science team specializes in customizing diamond properties—including NV creation, isotopic purification, and surface engineering—to meet stringent quantum application demands. We can assist researchers in selecting the optimal substrate purity and thickness for biological projects involving nanoscale thermometry and quantum decoherence spectroscopy by mitigating surface-induced charge instability.

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

View Original Abstract

We report on the sensing stability of quantum nanosensors in aqueous buffer solutions for the two detection schemes of quantum decoherence spectroscopy and nanoscale thermometry.

Tech Support

Original Source

DOI: https://doi.org/10.1039/c9ra02282a