Advanced post-treatment strategy for quantum-grade fluorescent nanodiamonds

At a Glance

Metadata	Details
Publication Date	2025-09-25
Journal	Frontiers in Quantum Science and Technology
Authors	Masfer Alkahtani, Yahya Alzahrani, Ayla Hazrathosseini, Abdulmalik M. Alessa, Maabur Sow
Analysis	Full AI Review Included

Advanced Post-Treatment Strategy for Quantum-Grade Fluorescent Nanodiamonds (FNDs)

Analysis of Alkahtani et al., Frontiers in Quantum Science and Technology (2025)

This document analyzes the reported multi-step purification protocol (KNO₃-Acid-Alkaline, or KAA) for fluorescent nanodiamonds (FNDs) and connects the material requirements to the advanced MPCVD diamond solutions offered by 6CCVD.

Executive Summary

The research successfully developed a scalable, multi-step surface treatment protocol (KAA) to produce high-quality, quantum-grade FNDs, overcoming limitations associated with surface defects and ionic contamination in conventional nanodiamonds.

Performance Enhancement: The KAA protocol resulted in a near two-fold improvement in the spin-lattice relaxation time (T₁) of NV centers, increasing the average T₁ from ~1045 µs (untreated) to ~2045 µs.
Spin Readout Fidelity: Optically Detected Magnetic Resonance (ODMR) contrast was dramatically enhanced from ~3% (untreated) to a reproducibly high ~11.5%, confirming stable NV charge environments.
Colloidal Stability: The treatment achieved excellent long-term colloidal stability, yielding monodisperse particles with a narrow hydrodynamic size of 100 nm and a strong negative zeta potential of -30 mV.
Morphological Refinement: The process effectively removed sp² carbon and residual graphitic shells, transforming irregular, aggregated particles into well-faceted, individually dispersed nanodiamonds with smooth surfaces.
Scalability: The integrated chemical, morphological, and spin-performance improvements establish a robust and scalable route for producing FNDs suitable for high-fidelity quantum sensing and biophotonic applications.

Technical Specifications

Parameter	Value	Unit	Context
Final T₁ Relaxation Time (Average)	2045	µs	KAA-FNDs (Quantum Sensing Figure of Merit)
ODMR Contrast (Average)	11.5	%	KAA-FNDs (Spin Readout Fidelity)
Zeta Potential	-30	mV	KAA-FNDs (Excellent Colloidal Stability)
Hydrodynamic Diameter	100	nm	KAA-FNDs (Monodisperse Distribution)
Photostability	Fully Restored	N/A	Elimination of charge-trapping impurities
KNO₃ Etching Temperature	580	°C	Step 1: Oxidative etching
Acid Oxidation Temperature	75	°C	Step 2.1: H₂SO₄/HNO₃ (9:1 v/v)
Alkaline Wash Temperature	90	°C	Step 2.2: 0.1 M NaOH
Material Recovery Yield	96	%	High efficiency of the KAA protocol

Key Methodologies

The KAA protocol is a multi-step surface engineering strategy designed to eliminate graphitic residues, neutralize surface charges, and remove ionic contaminants.

Molten KNO₃ Oxidative Etching (Step 1):
- FNDs mixed with potassium nitrate (KNO₃) and heated at 580 °C for 10 minutes under ambient atmosphere.
- Purpose: Enables morphological reshaping (from irregular to truncated/faceted) and partial oxidation, removing surface-bound sp² carbon.
Post-Acid Cleaning (Step 2.1):
- KNO₃-treated FNDs immersed in concentrated H₂SO₄/HNO₃ (9:1 v/v) and stirred continuously at 75 °C for 72 hours.
- Purpose: Aggressively removes residual graphite, oxidizes surface carbon, and grafts hydrophilic functional groups (-COOH, -OH).
Alkaline Cleaning (Step 2.2):
- Acid-treated FNDs resuspended in 0.1 M NaOH solution and heated at 90 °C for 2 hours.
- Purpose: Neutralizes residual acids, converts -COOH groups to the soluble -COO¯ form, and dissolves remaining metal ions (including potassium).
Final Acid Cleaning (Step 2.3):
- Nanodiamonds treated with 0.1 M HCl at 90 °C for 2 hours.
- Purpose: Removes residual alkali/transition metal contaminants and reprotonates carboxylates, achieving a final neutral suspension pH (6.5).

6CCVD Solutions & Capabilities

The research demonstrates that achieving high-performance quantum sensors requires starting with high-purity diamond material and applying precise surface engineering. While the paper focuses on nanodiamonds, 6CCVD specializes in high-quality MPCVD Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) substrates, which are the foundation for next-generation quantum devices and high-coherence NV center fabrication.

Applicable Materials for Quantum Sensing Replication and Extension

To replicate or extend this research into deterministic quantum devices, 6CCVD recommends materials that offer superior intrinsic purity and structural control compared to the HPHT FNDs used in the study:

Research Requirement	6CCVD Material Solution	Technical Advantage
Highest Spin Coherence (T₁ > 2045 µs)	Optical Grade Single Crystal Diamond (SCD)	MPCVD SCD offers ultra-low nitrogen content and minimal lattice defects, providing the quietest electronic environment for creating deterministic, high-coherence NV centers via ion implantation.
Surface Functionalization (KAA protocol)	Polished SCD/PCD Substrates	We provide SCD polished to Ra < 1nm and inch-size PCD polished to Ra < 5nm. An ultra-smooth starting surface is critical for uniform chemical functionalization and subsequent integration.
Bulk Sensing/Thermal Management	Thick SCD Substrates (up to 10mm)	For bulk quantum sensing or high-power optical applications, thick SCD provides superior thermal conductivity and structural integrity compared to dispersed NDs.
Large-Scale Integration	Large Area MPCVD PCD Wafers (up to 125mm)	For high-throughput manufacturing of quantum devices or biosensors, our large-format PCD wafers offer a scalable platform.

Customization Potential for Advanced Quantum Devices

The KAA protocol focuses on optimizing the diamond surface chemistry. 6CCVD provides the necessary foundational engineering to support advanced surface treatments and device integration:

Custom Dimensions and Thickness: We supply SCD and PCD plates/wafers in custom dimensions up to 125mm, with thicknesses ranging from 0.1µm to 500µm (SCD/PCD films) and substrates up to 10mm.
Metalization Services: The paper mentions the need for on-chip photonic and spintronic device engineering. 6CCVD offers internal, high-precision metalization capabilities (including Au, Pt, Pd, Ti, W, Cu) for creating coplanar waveguides, microwave structures, and electrical contacts essential for high-fidelity ODMR measurements.
Surface Termination Control: We can deliver substrates with specific terminations (e.g., H-terminated or O-terminated) as the optimal starting point for subsequent wet-chemical functionalization protocols like KAA.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD growth parameters and defect engineering. We can assist researchers and engineers in selecting the optimal diamond material (e.g., specific nitrogen concentration or isotopic purity) for similar quantum sensing, bioimaging, or nanophotonics projects, ensuring the material properties maximize the benefits of post-processing techniques like the KAA protocol.

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

View Original Abstract

Fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV − ) centers are promising platforms for quantum sensing and bioimaging, but their performance is often limited by surface defects, residual graphitic carbon, and ionic contamination. Here, we report a multistep surface treatment strategy combining molten potassium nitrate (KNO 3 ) thermal oxidation with sequential acid and alkaline cleaning to produce high-quality, quantum-grade FNDs. Molten KNO 3 etching at 580 °C enables morphological reshaping and partial oxidation, while subsequent H 2 SO 4 /HNO 3 , NaOH, and HCl washes eliminate graphitic residues, neutralize surface charges, and remove metal ions. This protocol yields discrete, colloidally stable FNDs with enhanced photoluminescence, a high ODMR contrast of 11.5%, and extended average spin-lattice relaxation time (T 1 ≈ 2045 µs). Dynamic light scattering and ζ-potential measurements confirm excellent dispersion (∼100 nm, −30 mV). The integration of chemical, morphological, and spin-performance improvements establishes a scalable route for producing FNDs suitable for high-fidelity quantum sensing and biophotonic applications.

Tech Support

Original Source

DOI: https://doi.org/10.3389/frqst.2025.1687810

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