Universal Dependence of Nuclear Spin Relaxation on the Concentration of Paramagnetic Centers in Nano- and Microdiamonds

At a Glance

Metadata	Details
Publication Date	2022-08-21
Journal	Materials
Authors	A. M. Panich
Institutions	Ben-Gurion University of the Negev
Citations	5
Analysis	Full AI Review Included

Executive Summary

This research establishes a universal relationship between nuclear spin relaxation and the concentration of paramagnetic centers (PCs) across various diamond materials, crucial for engineering applications in quantum technology and biomedicine.

Universal Law Established: A universal dependence governing nuclear spin-lattice (T₁) and spin-spin (T₂) relaxation times was confirmed, valid for nanodiamonds (NDs) in suspensions, gels, and solid powder states.
Linear Rate Dependence: The nuclear spin relaxation rates (R = 1/T) exhibit a direct linear dependence on the concentration of paramagnetic centers (PCs), consistent with fundamental spin relaxation theory.
Hyperbolic Time Dependence: Conversely, the nuclear spin relaxation times (T₁ and T₂) demonstrate a hyperbolic dependence on the concentration of PCs.
Materials Tested: The study analyzed purified detonation nanodiamonds (DNDs), DNDs surface-grafted with high-relaxivity ions (Gd³⁺, Cu²⁺), and milled high-pressure high-temperature (HPHT) nanodiamonds (SYP series).
Defect Sources: PCs included intrinsic defects (P1 centers, dangling bonds), grafted metal ions, and surface defects induced by mechanical milling.
Application Relevance: These findings are critical for optimizing nanodiamonds for use as high-performance contrast agents in Magnetic Resonance Imaging (MRI) and for applications requiring precise spin control, such as quantum computing and spintronics.

Technical Specifications

Parameter	Value	Unit	Context
DND Primary Particle Size	4.5-5	nm	Determined by DLS, TEM, and AFM
NMR External Magnetic Field (B₀)	8.0	T	Used for ¹H and ¹³C powder measurements
¹H Resonance Frequency	340.52	MHz	Corresponding to B₀ = 8.0 T
¹³C Resonance Frequency	85.62	MHz	Corresponding to B₀ = 8.0 T
Suspension NMR Temperature	310.1 (37)	K (°C)	Aqueous suspension measurements
Powder NMR Temperature	295 (Room)	K	Powder sample measurements
Paramagnetic Defect Density (Purified DND)	(4 to 7) x 10¹⁹	spin/g	Total defect density measured via EPR
Paramagnetic Defect Density (Milled SYP ND)	6.7 x 10¹⁸ to 3.3 x 10¹⁹	spin/g	Varies inversely with particle size (smallest fraction has highest density)
Gd(III) Unpaired Electron Spin (S)	7/2	N/A	Contributes significantly to relaxivity
Gd(III) Magnetic Moment	7.9	µ_B (Bohr Magneton)	High magnetic moment enhances relaxation
Stretched Exponential Parameter (a)	0.5 < a < 1	N/A	Used to fit ¹³C T₁ recovery due to paramagnetic defects

Key Methodologies

Nanodiamond Synthesis and Purification: Detonation nanodiamond (DND) particles were produced and subjected to rigorous purification and de-agglomeration processes to ensure narrow size distribution (4.5-5 nm average).
Surface Functionalization (Grafting): Aqueous suspensions of DNDs were mixed with solutions of copper acetate (Cu(CH₃CO₂)₂) or gadolinium nitrate (Gd(NO₃)₃·6H₂O). Dissociated metal cations (Cu²⁺ or Gd³⁺) underwent ion exchange with surface carboxyl groups, chemically bonding the paramagnetic ions to the nanoparticle surface.
Milled Diamond Production: Submicron diamond powders (SYP series) were manufactured by milling initial HPHT microdiamond crystallites (100 µm average size) to yield fractions ranging from 18 nm to 386 nm.
Defect Concentration Measurement: Electron Paramagnetic Resonance (EPR) spectroscopy was utilized to quantify the concentration of intrinsic (P1 centers, dangling bonds) and milling-induced paramagnetic defects in all powder samples.
NMR T₁ Measurement: Spin-lattice relaxation times (T₁) were measured using an Inversion Recovery pulse sequence (for ¹H suspensions) or a saturation comb pulse sequence (for ¹³C powders). Magnetization recovery was fitted using a stretched exponential function.
NMR T₂ Measurement: Spin-spin relaxation times (T₂) were measured using the Carr-Purcell-Meiboom-Gill (CPMG) sequence (for ¹H suspensions) or the Hahn echo method (for ¹³C powders).

Commercial Applications

The established universal spin relaxation law and the characterized materials are highly relevant for several advanced technological fields:

Biomedical Imaging (MRI):
- Contrast Agents: Gd-grafted DND suspensions exhibit high relaxivity, making them promising candidates for novel, potentially safer MRI contrast agents.
- MRI Phantoms: Nanodiamond suspensions can serve as standardized phantoms for calibrating T₁ and T₂ relaxation times in clinical MRI systems.
Quantum Technologies:
- Quantum Computing: Nanodiamonds containing specific paramagnetic defects (like Nitrogen-Vacancy centers, NV) are foundational materials for solid-state quantum processors.
- Spintronics: Utilizing the controlled spin properties of surface-grafted ions and intrinsic defects for developing spin-based electronic devices.
Nanophotonics: Applications leveraging the optical properties of color centers within the nanodiamond structure.
Materials Engineering: The quantitative understanding of how defects and surface modifications affect spin dynamics allows for precise material optimization for specific applications in suspensions, gels, and solid matrices.

View Original Abstract

An analysis of our data on 1H and 13C spin-lattice and spin-spin relaxation times and rates in aqueous suspensions of purified nanodiamonds produced by detonation technique (DNDs), DNDs with grafted paramagnetic ions, and micro- and nanodiamonds produced by milling bulk high-temperature high-pressure diamonds is presented. It has been established that in all the studied materials, the relaxation rates depend linearly on the concentration of diamond particles in suspensions, the concentration of grafted paramagnetic ions, and surface paramagnetic defects produced by milling, while the relaxation times exhibit a hyperbolic dependence on the concentration of paramagnetic centers. This is a universal law that is valid for suspensions, gels, and solids. The results obtained will expand the understanding of the properties of nano- and microdiamonds and will be useful for their application in quantum computing, spintronics, nanophotonics, and biomedicine.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma15165774

References

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