High-throughput nitrogen-vacancy center imaging for nanodiamond photophysical characterization and pH nanosensing

At a Glance

Metadata	Details
Publication Date	2020-01-01
Journal	Nanoscale
Authors	Maabur Sow, Horst Steuer, Sanmi Adekanye, Laia Ginés, Soumen Mandal
Institutions	Cardiff University, University of Oxford
Citations	32
Analysis	Full AI Review Included

High-Throughput NV Center Characterization and pH Nanosensing in Nanodiamonds: A 6CCVD Analysis

This technical documentation analyzes the findings of the paper “High-throughput nitrogen-vacancy center imaging for nanodiamond photophysical characterization and pH nanosensing” (Nanoscale, 2020) and aligns the core requirements of advanced NV center physics with the high-specification diamond materials and engineering services offered by 6CCVD.

Executive Summary

This study details a sensitive, high-throughput method for characterizing Nitrogen-Vacancy (NV) centers in nanodiamonds (NDs) using two-color wide-field epifluorescence imaging.

Novel Methodology: Utilized fluorescence intensity ratios (Red/Green, R/G ratio) in conjunction with high-throughput single-particle detection to characterize hundreds of NV centers simultaneously.
Fundamental Physics: Directly observed the charge conversion dynamics between the photoactive charge states of the NV center ($\text{NV}^-$ and $\text{NV}^0$) within individual NDs (as small as 10 nm).
Charge State Metrics: Established specific R/G ratio benchmarks for the $\text{NV}^-$ state ($\approx 0.9$) and the $\text{NV}^0$ state ($\approx 0.6$), enabling rapid charge state identification.
Lifetime Quantification: Successfully employed Hidden Markov Modeling (HMM) analysis to calculate distinct dwell-times (lifetimes) for the $\text{NV}^-$ state ($\approx 38$ s) and the $\text{NV}^0$ state ($\approx 3$ s, short component).
Nanosensing Application: Demonstrated reversible pH nanosensing capability using 10 nm doped NDs, where pH variations (4 to 10) directly and reversibly control the dominant NV charge state, paving the way for non-photobleachable biological probes.
Material Challenge: Research highlights that NV charge stability is highly sensitive to ND size, surface chemistry, and internal crystal strain, underscoring the necessity of high-purity, low-strain precursor diamond materials.

Technical Specifications

The following table extracts key quantitative parameters and results achieved in the study:

Parameter	Value	Unit	Context
Excitation Wavelength	532	nm	CW Laser
Excitation Power Density (Imaging)	7.8	$\text{kW cm}^{ -2}$	High-throughput wide-field imaging
Excitation Power Density (Dynamics)	3.4	$\text{kW cm}^{ -2}$	Long dynamic time-traces
ND Particle Size Range Studied	5 to 200	nm	Undoped and doped nanodiamonds
Red Channel Spectral Window	655 to 750	nm	Primarily $\text{NV}^-$ Zero-Phonon Line (ZPL) emission
Green Channel Spectral Window	550 to 620	nm	Primarily $\text{NV}^0$ Zero-Phonon Line (ZPL) emission
$\text{NV}^-$ R/G Ratio (Single Emitter)	0.9	(Unitless)	Corresponds to high red emission
$\text{NV}^0$ R/G Ratio (Single Emitter)	0.6	(Unitless)	Corresponds to high green emission
$\text{NV}^-$ Lifetime ($\tau$)	$\approx 38$	s	Calculated from HMM dwell-time distribution
$\text{NV}^0$ Lifetime ($\tau_{1}$) (Short)	$\approx 3$	s	90% population fitted by double-exponential decay
pH Sensing Accuracy	$\approx \pm 0.4$	pH unit	Estimated accuracy range (pH 4 to 10)
Critical ND Size for pH Sensing	10	nm	Demonstrated reversible pH sensing capability

Key Methodologies

The core of the successful characterization relies on a high-throughput, two-color optical setup coupled with advanced image analysis software (Gapviewer) and modeling (HMM).

Diamond Preparation: Nanodiamonds (NDs) of various sizes (5 nm to 200 nm, doped and undoped) were immobilized at a very low density on a glass microscope slide via spin-coating.
Wide-Field Epifluorescence Imaging: A simplified wide-field microscope setup was used, employing a 532 nm laser for continuous wave (CW) excitation.
Two-Color Spectral Splitting: The NV emission spectrum was split into two distinct channels using dichroics and long-pass filters: a Green channel (550-620 nm, dominated by $\text{NV}^0$) and a Red channel (655-750 nm, dominated by $\text{NV}^-$).
High-Throughput Localization: Software was used to localize up to 500 fluorescent spots (NDs) simultaneously in the field of view. Photon count for each spot was determined for both the Red and Green channels per camera exposure (100 ms).
Charge State Quantification: The Red/Green (R/G) fluorescence intensity ratio was calculated for each detected single NV center to identify the dominant charge state ($\text{NV}^-$ or $\text{NV}^0$) and track dynamic charge transitions.
Dynamic Analysis: Long time-traces (up to 60 minutes) were collected for dynamically switching NDs. Hidden Markov Modeling (HMM) was applied to the R/G ratio time-traces to fit state transitions and calculate the empirical lifetimes ($\tau$) of the $\text{NV}^-$ and $\text{NV}^0$ states.
pH Manipulation: Immobilized 10 nm doped NDs were subjected to solutions with varying pH (4 to 12.8), and the reversible shift in the R/G ratio distribution mode was monitored, demonstrating direct pH nanosensing.

6CCVD Solutions & Capabilities

This research confirms the crucial relationship between diamond quality (especially purity, nitrogen content, and low strain) and the stability and utility of the NV center for quantum and biosensing applications. 6CCVD provides the high-specification single crystal diamond (SCD) and polycrystalline diamond (PCD) required to advance this field, both as primary sensing components and as high-purity precursors for optimal nanodiamond synthesis.

Applicable Materials

The paper emphasizes that high-purity, small NDs with controlled impurities and little crystal strain are essential for high-sensitivity nanosensing. 6CCVD offers materials that directly address these quality requirements:

6CCVD Material Recommendation	Specification & Application Focus
High-Purity Single Crystal Diamond (SCD)	Ideal precursor for producing the highest quality, low-strain nanodiamonds required for robust single-NV quantum sensing. 6CCVD supplies material up to 500 $\text{µm}$ thick with $\text{Ra} < 1 \text{nm}$ polishing.
Nitrogen-Doped SCD (Custom Doping)	Critical for precisely replicating or controlling the NV concentration (e.g., the 1NV/ND samples used in the study). We offer controlled introduction of nitrogen during MPCVD growth.
Optical Grade Polycrystalline Diamond (PCD)	For applications requiring larger scale, high-volume production, 6CCVD provides inch-size PCD wafers (up to 125 $\text{mm}$) necessary for large arrays of sensors or microfluidic platforms referenced in the paper.
Boron-Doped Diamond (BDD) Plates	While the study used NV centers, the charge-dependent sensing mechanism is similar to BDD’s robust electrochemical properties. BDD plates can be engineered for direct integration into microfluidic or in vivo sensing platforms requiring highly stable electrochemical probes.

Customization Potential

The success of next-generation NV-based sensors relies on material integration and precise structuring, capabilities where 6CCVD excels:

Optimized Dimensions and Thickness: We can provide SCD wafers in custom thicknesses (0.1 $\text{µm}$ to 500 $\text{µm}$) and larger PCD plates (up to 125 $\text{mm}$ diameter), suitable for integrating optical components (like the dichroics and mirrors referenced in the experimental setup) or creating microfluidic device platforms.
Advanced Polishing: Surface charge dynamics are crucial for NV stability and pH sensing. 6CCVD guarantees ultra-low roughness polishing ($\text{Ra} < 1 \text{nm}$ for SCD) to minimize surface traps and defects, directly enhancing charge stability compared to the heterogeneous NDs studied.
Integrated Metalization Services: For device integration (e.g., creating electrodes or contacts for electrical potential control mentioned in related NV studies), 6CCVD provides in-house metalization using Au, Pt, Pd, Ti, W, and Cu layers, directly supporting NV manipulation experiments.

Engineering Support

The challenges of achieving stable, bright NV centers—especially in small particles where surface effects dominate—demand expertise in diamond growth and post-processing.

In-House PhD Team Consultation: 6CCVD’s dedicated PhD engineering team specializes in diamond material science and NV quantum physics. We offer expert consultation on optimizing Nitrogen Concentration and NV Depth in precursor SCD material to maximize quantum yield and minimize surface-induced charge instability for novel NV-based sensing projects (such as pH monitoring or temperature sensing).
Global Logistics: We ensure reliable, global delivery of highly sensitive materials through DDU (default) or DDP (upon request) shipping arrangements.

Call to Action: For custom specifications or material consultation tailored to high-performance NV center research, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

A method to observe individual fluorescent crystal defects in nanodiamonds is reported and opens new nanosensing avenues ( e.g. pH nanosensing).

Tech Support

Original Source

DOI: https://doi.org/10.1039/d0nr05931e