🔬 materials science

Materials and spin characteristics of amino-terminated nanodiamonds embedded with nitrogen-vacancy color centers

This study systematically characterizes nitrogen-vacancy centers in amino-terminated fluorescent nanodiamonds, revealing that while zero-field splitting parameters evolve with size, wet-chemical Hofmann amino termination uniquely enables a high, laser-power-independent negative charge state in 140-nm particles by degrading surface paramagnetic defects.

Original authors: Nikoletta Jegenyes, Vladimir Verkhovlyuk, Szabolcs Czene, Attila Csáki, Olga Krafcsik, Zsolt Czigány, David Beke, Adam Gali

Published 2026-08-17

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Nikoletta Jegenyes, Vladimir Verkhovlyuk, Szabolcs Czene, Attila Csáki, Olga Krafcsik, Zsolt Czigány, David Beke, Adam Gali

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where tiny specks of diamond, smaller than a grain of sand, can act as super-sensitive spies. These aren't just any diamonds; they are "fluorescent nanodiamonds" (FNDs) that glow when you shine a light on them. Inside these tiny gems live special defects called "Nitrogen-Vacancy" (NV) centers. Think of an NV center as a tiny, trapped electron with a magnetic personality, or a "spin," that can be read like a switch. Scientists love these because they can sense invisible things around them—like magnetic fields, electric fields, or even temperature changes—right inside a living cell or a chemical reaction.

However, for these diamond spies to work well, they need to be clean and stable. If the surface of the diamond is messy or covered in the wrong chemicals, the spy's "spin" gets confused, and it might stop glowing or change its message. The surface is like the diamond's skin; if the skin is irritated, the brain (the NV center) can't think straight. The big question scientists have been asking is: How do we coat these tiny diamonds with the right "skin" to make them perfect for biology, without messing up their superpowers? Specifically, can we attach amino groups (chemical tags that love to stick to proteins and DNA) without ruining the diamond's ability to sense the world?

This paper takes a deep dive into that exact problem. The researchers started with a batch of nanodiamonds of different sizes, ranging from tiny 10-nanometer specks to larger 140-nanometer chunks. They treated these diamonds in three different ways: leaving them as they came from the factory (with oxygen on the surface), washing them to make them hydroxyl-terminated (covered in -OH groups), and using a clever wet-chemical trick called "Hofmann degradation" to coat them with amino groups (-NH2). They wanted to see how these different "skins" affected the diamonds' size, their chemical makeup, and most importantly, how well the NV centers inside could hold their charge and sense their environment.

The team found two major things that change how these diamond spies behave. First, they discovered that the size of the diamond matters a lot for its internal "symmetry." They measured two specific numbers, called D and E, which describe how the NV center's energy levels are split. They found that the "E" number, which acts like a measure of how much the diamond's shape is being squashed or twisted by its surroundings, gets smaller and smaller as the diamonds get bigger. It drops from about 8 MHz in the smallest diamonds down to about 5 MHz in the largest ones. This suggests that in the tiniest diamonds, the surface is so close that it physically strains the diamond lattice, but in the bigger ones, the NV center is far enough away that this physical strain disappears. However, the "D" number, which is the main energy gap, only changes in the very smallest diamonds (10 and 30 nm). For everything bigger than that, it stays steady, just like a perfect, bulk diamond. This helps scientists separate the noise caused by physical squeezing from the noise caused by electric fields.

The second, and perhaps more exciting, finding was about the amino coating. Usually, when you try to stick amino groups onto a surface, you might accidentally create new defects that mess up the diamond's performance. But here, the researchers found that the Hofmann degradation method actually cleaned up the surface. In the largest diamonds (140 nm), this amino coating resulted in a remarkably high and stable amount of NV centers staying in their useful, negative charge state (about 80% of them). This was true even when they changed the brightness of the laser shining on them. Why? Because the process didn't just add amino groups; it seems to have degraded or removed some of the "bad" magnetic defects on the surface that usually steal electrons and turn the NV centers off. It's like the chemical treatment didn't just put a new coat of paint on the wall; it also scrubbed away the mold that was making the room unhealthy.

However, this magic only worked for the big diamonds. In the smaller ones, the NV centers are too close to the surface, and the "bad" defects are still too close to be fully neutralized by the new coating, so the high stability didn't appear. The paper rules out the idea that the amino groups themselves directly changed the electron affinity to fix the problem; instead, it suggests the fix came from removing the surface defects that were causing the trouble in the first place.

In short, this study shows that if you want to use these diamond spies for biology, you should pick the larger ones (around 140 nm) and give them a gentle amino coating using this specific wet-chemical method. This approach keeps the diamond's internal "brain" calm and stable while giving it the perfect "hands" to grab onto biological molecules, making it a much better tool for sensing the microscopic world.

Technical Summary: Materials and Spin Characteristics of Nanodiamonds Partially Covered by Amino Groups and Embedded with Nitrogen-Vacancy Color Centers

Problem Statement
Fluorescent nanodiamonds (FNDs) containing nitrogen-vacancy (NV) centers are promising nanoscale quantum sensors for detecting electric and magnetic fields, temperature, and other physicochemical quantities in biological and chemical environments. However, the performance of these sensors is critically dependent on surface chemistry. Because NV centers in nanodiamonds often reside near the surface, their charge state stability (specifically the ratio of the negatively charged NV(−) to the neutral NV(0)) and spin relaxation times are highly sensitive to surface termination. While amino-functionalized surfaces are desirable for bioconjugation, the methods used to introduce them (often plasma-based) can degrade spin properties or create new surface defects. Furthermore, a unified understanding of how particle size and specific surface termination (hydroxyl vs. amino) jointly influence the zero-field splitting (ZFS) parameters and charge state stability of NV ensembles has been lacking.

Methodology
The study utilized commercially available FNDs derived from high-pressure, high-temperature (HPHT) diamonds, ranging in size from 10 to 140 nm. The NV centers were generated via electron irradiation and annealing. The researchers prepared three distinct surface conditions for each size fraction:

As-received/Washed: Oxygen-terminated surfaces, with a subset washed to remove sulfur-related residues.
Hydroxyl-terminated (−OH): Synthesized via LiAlH₄ reduction.
Amino-terminated (−NH₂): Synthesized using a wet-chemical Hofmann degradation mechanism, which selectively converts oxygen-containing surface groups (e.g., −COOH) to amino groups without graphitization or aggregation.

The samples were characterized using a comprehensive suite of techniques:

Structural/Morphological: Scanning Electron Microscopy (SEM) and High-Resolution Transmission Electron Microscopy (HRTEM).
Chemical Surface Analysis: Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), and Zeta potential measurements.
Optical and Spin Properties: Photoluminescence (PL) spectroscopy to determine the NV(−) fraction ( $f_{NV(-)}$ ), Electron Spin Resonance (ESR) under laser illumination (photo-ESR) to characterize paramagnetic defects, and Continuous Wave (cw) Optically Detected Magnetic Resonance (ODMR) to measure the ZFS parameters ( $D$ and $E$ ).

Key Contributions and Results

1. Size Dependence of Zero-Field Splitting Parameters
The study systematically resolved the ZFS parameters ( $D$ and $E$ ) of the NV(−) ensemble across the 10–140 nm size range for all surface terminations.

The $E$ Parameter: The symmetry-breaking parameter $E$ was found to decrease monotonically from approximately 8 MHz to 5 MHz as particle size increased, regardless of surface termination.
The $D$ Parameter: The axial parameter $D$ remained close to the bulk diamond value (2.87 GHz) for particles larger than 30 nm. However, in the smallest particles (≤30 nm), $D$ shifted significantly (down to ~2.867 GHz).
Interpretation: By correlating these findings with theoretical frameworks, the authors disentangle two contributions to the spin levels:
- Static Strain: Dominates in particles ≤30 nm, causing the shift in $D$ and contributing to $E$ .
- Fluctuating Electric Fields: Dominates in larger particles (>30 nm), contributing to $E$ but not shifting $D$ . The magnitude of this electric field noise scales with the distance between the NV center and surface defects (parasitic defects), which increases with particle size.
ESR Correlation: ESR data confirmed that the density of broad $S=1/2$ surface dangling bond signals decreases with increasing size and with amino termination, mirroring the trend observed in the $E$ parameter.

2. Impact of Hofmann Amino Termination on Charge State Stability
The study investigated the effect of the Hofmann degradation method on the NV(−) charge state fraction ( $f_{NV(-)}$ ).

General Trends: For most samples, $f_{NV(-)}$ increased with particle size and reached a plateau. The plateau occurred at smaller sizes for hydroxyl-terminated and washed samples compared to as-received samples.
The 140-nm Anomaly: A remarkable result was observed in 140-nm particles with amino termination. These samples exhibited a high and laser-power-independent $f_{NV(-)}$ of approximately 0.8. In contrast, 140-nm particles with other terminations showed values around 0.6.
Mechanism: Photo-ESR analysis revealed that the Hofmann degradation process did not introduce new paramagnetic defects (as often seen with plasma treatments). Instead, it led to a net degradation of surface paramagnetic defects (specifically, a reduction in the relative weight of the broad $S=1/2$ surface signal and a corresponding increase in the relative weight of the P1 center).
Conclusion on Mechanism: The suppression of surface acceptor-like defects (dangling bonds) prevents the photo-ionization-induced ejection of holes that would otherwise convert NV(−) to NV(0). This effect is size-dependent; in smaller particles, the NV centers are too close to the surface for this passivation to be fully effective, and the high, power-independent plateau is not achieved.

Significance
The paper claims two principal advances. First, it provides a unified picture linking particle size and surface chemistry to the ZFS parameters, successfully separating static strain effects (dominant in small particles) from fluctuating electric field effects (dominant in larger particles). Second, it demonstrates that the wet-chemical Hofmann degradation route is a non-invasive method for amino termination that preserves and, in the case of large (140 nm) particles, enhances the NV(−) charge state stability. This is achieved not by altering the electron affinity of the surface groups directly, but by chemically reducing the density of surface paramagnetic defects.

The authors conclude that Hofmann-based amino termination offers a viable surface for biological functionalization while maintaining the quantum sensing capabilities of NV centers, particularly in larger nanodiamonds where the NV centers are sufficiently distant from the surface to benefit from the reduced defect density.

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