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6CCVD Research

Pulse sequence design for high field NMR with NV centers in dipolarly coupled samples

At a Glance

Section titled ā€œAt a Glanceā€
MetadataDetails
Publication Date2025-08-22
JournalScientific Reports
AuthorsCarlos Munuera-Javaloy, Ander Tobalina, J. Casanova
InstitutionsUniversity of the Basque Country
Citations1
AnalysisFull AI Review Included

Executive Summary

Section titled ā€œExecutive Summaryā€

This research introduces a novel quantum sensing protocol designed to achieve high-resolution Nuclear Magnetic Resonance (NMR) spectroscopy in solid-state, dipolarly coupled samples using Nitrogen Vacancy (NV) ensembles in diamond.

  • Core Value Proposition: Overcomes the primary limitation of microscale NV-NMRā€”the strong homonuclear dipole-dipole couplings prevalent in solid-state materialsā€”by integrating advanced decoupling techniques.
  • High-Field Operation: The protocol is optimized for high external magnetic fields (e.g., 2.1 T), leveraging increased thermal polarization and enhanced chemical shift separation for a stronger signal.
  • Dual-Channel Control: Utilizes synchronized Radio Frequency (RF) and Microwave (MW) radiation. The RF channel applies the robust LG4 sequence for decoupling, while the MW channel uses a tailored pulse train for optimal signal detection.
  • Effective Decoupling: The RF field effectively suppresses dipolar interactions (simulated up to 17 kHz), enabling the extraction of subtle chemical shifts. Cleaner spectra are achieved when the RF Rabi frequency exceeds the dipolar coupling strength by an order of magnitude.
  • Coherence Limit: The method shifts the spectral precision limit from the short NV coherence time (T2) to the significantly longer intrinsic coherence time of the nuclear sample (T2* ~ 0.2 s).
  • Demonstrated Performance: Simulations successfully identified two distinct chemical shifts (327 Hz and 106 Hz) in solid ethanol, proving the ability to resolve relevant energy shifts in complex solid-state systems.

Technical Specifications

Section titled ā€œTechnical Specificationsā€
ParameterValueUnitContext
Sensor TypeNV EnsemblesN/ADiamond-based quantum magnetometer
External Magnetic Field (B0)2.1TSimulation environment (high field)
Sample Temperature (T)300KThermal polarization state
Max Dipolar Coupling (Ethanol)17kHzHomonuclear coupling strength in solid C2H6O
RF Rabi Frequency (Ī©RF)(2Ļ€) x 100 to 200kHzTested decoupling field strength
MW Rabi Frequency (Ī©MW)20MHzNV sensor control frequency
Decoupling SequenceLG4 (Lee-Goldburg 4-block)N/AUsed for homonuclear decoupling
Optimal Decoupling Ratio> 10N/AĪ©RF must be greater than 10x the dipolar coupling strength for clean spectra
Nuclear Sample Coherence (T2*)0.2sLimiting factor for spectral precision
Target Chemical Shift 1327HzCorresponds to 3.66 ppm at 2.1 T
Target Chemical Shift 2106HzCorresponds to 1.19 ppm at 2.1 T
Total Acquisition Time0.5sDuration of simulated experiment

Key Methodologies

Section titled ā€œKey Methodologiesā€

The protocol relies on the synchronized delivery of RF and MW radiation, interspersed with NV measurements, to achieve heterodyne detection of the nuclear energy shifts.

  1. Initial State Preparation: A triggering RF pulse is applied to the nuclear spin sample, setting the initial state (thermal polarization) oriented along the axis perpendicular to the LG4 rotation axes (A and B).
  2. Homonuclear Decoupling (RF): The sample is subjected to a continuous, off-resonant RF field implementing the LG4 sequence (A, A, B, B blocks). This sequence minimizes the effect of strong dipole-dipole interactions, enabling the identification of weaker chemical shifts (Ī“i).
  3. Signal Generation: The LG4 sequence induces a collective nuclear spin rotation. The target energy shifts (Ī“i) slightly alter the spin state, causing the sampleā€™s longitudinal magnetization (ML) to oscillate at a slow, tunable rate (typically tens of KHz). This oscillation generates the magnetic signal s(t).
  4. Optimized Detection (MW): A tailored Microwave (MW) pulse sequence, based on a modified CPMG block, is applied to the NV ensemble. This sequence includes two Ļ€ pulses specifically timed (t1, t2) to maximize the NV phase accumulation, which is proportional to the projection of ML onto the major axis of the elliptic path.
  5. Measurement Cycle: During the time corresponding to the B RF field delivery, a Ļ€/2 pulse converts the accumulated NV phase into a population difference, which is measured. The sensor is then reinitialized, and the cycle repeats.
  6. Spectral Analysis: The sequence of measurements yields data that evolves as a sum of sinusoidal components with frequencies Ī“*. Fourier transformation extracts these frequencies, which are then analytically mapped back to the target chemical shifts (Ī“i) of the molecule.

Commercial Applications

Section titled ā€œCommercial Applicationsā€

This technology extends high-resolution NMR capabilities to microscale solid-state systems, opening applications in fields previously constrained by conventional techniques.

  • Pharmaceutical Research: Characterization of Active Pharmaceutical Ingredients (APIs) and their complex interactions with excipients in solid-state drug formulations, crucial for stability and efficacy studies.
  • Advanced Materials Science: Analysis of local structure and dynamics in solid materials, including polymers, ceramics, and thin films, especially at surfaces or interfaces where sample volume is highly limited.
  • Energy Storage Technology: Characterization of local structure in solid materials used in next-generation batteries (e.g., solid-state electrolytes) and fuel cells, aiding in material optimization.
  • Quantum Sensing and Metrology: Advancement of NV-based magnetometers, enabling operation at high magnetic fields and achieving spectral resolution limited by the sampleā€™s intrinsic T2* rather than sensor limitations.
  • Microscale Chemical Analysis: High-resolution NMR spectroscopy of picoliter volume samples, enabling structural analysis in microfluidic devices or highly constrained environments.
View Original Abstract

Abstract Diamond-based quantum sensors have enabled high-resolution NMR spectroscopy at the microscale in scenarios where fast molecular motion averages out dipolar interactions among target nuclei. However, in samples with low-diffusion, ubiquitous dipolar couplings challenge the extraction of relevant spectroscopic information. In this work we present a protocol that enables the scanning of nuclear spins in dipolarly-coupled samples at high magnetic fields with a sensor based on nitrogen vacancy (NV) ensembles. Our protocol is based on the synchronized delivery of radio frequency (RF) and microwave (MW) radiation to eliminate couplings among nuclei in the scanned sample and to efficiently extract target energy-shifts from the sampleā€™s magnetization dynamics. In addition, the method is designed to operate at high magnetic fields leading to a larger sample thermal polarization, thus to an increased NMR signal. The precision of our method is ultimately limited by the coherence time of the sample, allowing for accurate identification of relevant energy shifts in solid-state systems.

Tech Support

Section titled ā€œTech Supportā€

Original Source

Section titled ā€œOriginal Sourceā€

References

Section titled ā€œReferencesā€
  1. 2008 - Spin Dynamics: Basics of Nuclear Magnetic Resonance

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