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arXiv:2006.03910 · PRX Quantum

Micron-Scale NV-NMR Spectroscopy with Signal Amplification by Reversible Exchange

Nithya Arunkumar, Dominik Bucher, Matthew J. Turner, Patrick M. TomHon, David R. Glenn, Sören Lehmkuhl, Mikhail D. Lukin, Hongkun Park, Matthew Scott Rosen, Thomas Theis, Ronald Lee Walsworth

Published Jan 11, 2021• 50 citations
arXivHTMLPDF

Abstract

Optically probed nitrogen-vacancy (NV)) quantum defects in diamond can detect nuclear magnetic resonance (NMR) signals with high-spectral resolution from micron-scale sample volumes of about 10 pl.However, a key challenge for NV-NMR spectroscopy is detecting samples at millimolar concentrations.Here we demonstrate an increase in NV-NMR proton concentration sensitivity by hyperpolarizing sample proton spins to about 0.5% through signal amplification by reversible exchange (SABRE), enabling micron-scale NMR spectroscopy of small-molecule sample concentrations as low as 1 mM in picoliter volumes.The SABRE-enhanced NV-NMR technique may enable detection and chemical analysis of low-concentration molecules and their dynamics in complex micron-scale systems such as single cells.

DOI
10.1103/prxquantum.2.010305

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Optically probed nitrogen-vacancy (NV)) quantum defects in diamond can detect nuclear magnetic resonance (NMR) signals with high-spectral resolution from micron-scale sample volumes of about 10 pl.However, a key challenge for NV-NMR spectroscopy is detecting samples at millimolar concentrations.Here we demonstrate an increase in NV-NMR proton concentration sensitivity by hyperpolarizing sample proton spins to about 0.5% through signal amplification by reversible exchange (SABRE), enabling micron-scale NMR spectroscopy of small-molecule sample concentrations as low as 1 mM in picoliter volumes.The SABRE-enhanced NV-NMR technique may enable detection and chemical analysis of low-concentration molecules and their dynamics in complex micron-scale systems such as single cells.
Nuclear magnetic resonance (NMR) spectroscopy provides unparalleled access to molecular structure and dynamics but is traditionally limited by weak signal strength, requiring large sample volumes and high magnetic fields. Here, we demonstrate nanoscale deuterium (2H) NMR spectroscopy using nitrogen vacancy (NV) centers in diamond, reproducing the characteristic quadrupolar powder line shapes that are present in the conventional bulk NMR spectra. By detecting statistical spin fluctuations from nanometer scale detection volumes, our approach delivers a sensitivity enhancement of six to eight orders of magnitude over inductive detection while operating at magnetic fields two orders of magnitude lower than those used in conventional NMR. Temperature dependent measurements of a deuterated polymer and molecular solid reveal distinct motional averaging and phase transitions with nanoscale sensitivity. Powder-like NV detected 2H NMR establishes a powerful tool for probing molecular dynamics on the nanoscale and, in the ultimate limit, at the single molecule level - capabilities beyond those of most existing spectroscopic techniques.
Nitrogen vacancy centers in diamond (NVs) are point defects well suited to quantum sensing of magnetic fields. NV spin states can be initialized and read out optically with high spatial resolution, enabling detection of pL-scale volumes of nuclear spins. Using Coherently Averaged Synchronized Readout (CASR), standard NMR spectroscopy techniques such as free decay, spin echo, and PGSE have been performed with Hz-scale spectral resolution using NV-diamond sensors. However, using NV detection for conventional high-resolution NMR at multi-tesla fields is challenging due to the need to manipulate NV electronic spins at resonance frequencies 100 GHz or higher. Low-field J-coupling spectroscopy provides an alternative path to chemical resolution in a regime more easily accessible to NVs. Here, we demonstrate homonuclear and heteronuclear J-coupling spectroscopy using an NV-NMR spectrometer based on permanent magnets operating at $\sim$890 kHz proton Larmor frequency (bias magnetic field $\sim21mT).ThisfieldstrengthenableschemicalcharacterizationbasedonJ−couplingusingsequencessuchasSynchronizedEcho(SyncE)andSpin−EchoDoubleResonance(SEDOR),whichdonotrequirethestrongchemicalshiftdispersionandpristinefieldhomogeneityofhigh−fieldNMR.ThefrequencyrangeiseasilydetectedbyNVsusingstandardXYdynamicaldecouplingsequencesandiswell−suitedforapplyingOverhauserDNP,whichcanboostprotonsignalbytwoordersofmagnitude.WeacquireinitialNV−NMRmeasurementsofT21 mT). This field strength enables chemical characterization based on J-coupling using sequences such as Synchronized Echo (SyncE) and Spin-Echo Double Resonance (SEDOR), which do not require the strong chemical shift dispersion and pristine field homogeneity of high-field NMR. The frequency range is easily detected by NVs using standard XY dynamical decoupling sequences and is well-suited for applying Overhauser DNP, which can boost proton signal by two orders of magnitude. We acquire initial NV-NMR measurements of T21mT).ThisfieldstrengthenableschemicalcharacterizationbasedonJ−couplingusingsequencessuchasSynchronizedEcho(SyncE)andSpin−EchoDoubleResonance(SEDOR),whichdonotrequirethestrongchemicalshiftdispersionandpristinefieldhomogeneityofhigh−fieldNMR.ThefrequencyrangeiseasilydetectedbyNVsusingstandardXYdynamicaldecouplingsequencesandiswell−suitedforapplyingOverhauserDNP,whichcanboostprotonsignalbytwoordersofmagnitude.WeacquireinitialNV−NMRmeasurementsofT_1,T, T,T_2,andOverhauserDNPinglycerolbeforeadvancingtoSEDORspectroscopyofmethanol−, and Overhauser DNP in glycerol before advancing to SEDOR spectroscopy of methanol-,andOverhauserDNPinglycerolbeforeadvancingtoSEDORspectroscopyofmethanol−^{13}Candamethanol−C and a methanol-Candamethanol−^{13}C/acetonitrile−2−C/acetonitrile-2-C/acetonitrile−2−^{13}$C mixture at sample volumes $\sim$50 pL, in which we demonstrate measurement of 1JCH^1J_\text{CH}1JCH​ couplings and resolution between species. We also acquire a homonuclear J-coupling spectrum of a natural-abundance propylene carbonate sample.
Advances in sensing devices that utilize nitrogen-vacancy (NV) center ensembles in diamond are driving progress in microscale nuclear magnetic resonance spectroscopy. Utilizing quantum sensing techniques in the high-field regime significantly boosts sensitivity by increasing thermal polarization and improves spectral quality via enhanced energy shifts. Compatible with the latter, a straightforward manner to further raise sensor sensitivity is to increase NV concentration, although this intensifies detrimental dipole-dipole interactions among NVs. In this Letter, we present a method for detecting NMR signals in high-field scenarios while effectively suppressing dipole-dipole couplings in the NV ensemble. Thus, this approach enhances sensitivity by combining highly doped diamond substrates and elevated magnetic fields.
Nitrogen vacancy (NV) color centers in diamond are a leading modality for both superresolution optical imaging and nanoscale magnetic field sensing. In this work, we address the key challenge of performing optical magnetic imaging and spectroscopy selectively on multiple NV centers that are located within a diffraction-limited field-of-view. We use spin-RESOLFT microscopy to enable precision nanoscale mapping of magnetic field patterns with resolution down to ~20 nm, while employing a low power optical depletion beam. Moreover, we use a shallow NV to demonstrate the detection of proton nuclear magnetic resonance (NMR) signals exterior to the diamond, with 50 nm lateral imaging resolution and without degrading the proton NMR linewidth.
Nuclear magnetic resonance (NMR) spectroscopy is a powerful probe of molecular structure and dynamics but typically requires large spin ensembles. Here, we demonstrate nanoscale NMR detection of fluorinated DNA using individual near-surface nitrogen-vacancy (NV) centers in diamond nanopillar waveguides under ambient conditions. We covalently tether 19^{19}19F-labeled oligonucleotides to mixed nitrogen- and oxygen-terminated surfaces, localizing target spins within the nanoscale sensing volume. Probabilistic spin simulations suggest that the measured signals are consistent with those expected from a few molecules and depend strongly on NV-molecule separation, configuration, and local surface heterogeneity. These results establish a framework for NMR of surface-tethered biomolecules and identify key challenges for single-molecule structural elucidation.
Quantum sensing using nitrogen-vacancy (NV) centers in diamond has emerged as a powerful platform for detecting ultra-low magnetic fields under ambient conditions. Owing to their long spin coherence times, optical addressability, and compatibility with aqueous environments, it has found widespread applications in biosensing and bio-imaging. This review presents the fundamental principles and recent advances in NV-based quantum magnetometry for biosensing applications, with a particular focus on measurements in aqueous medium and at cellular and molecular length scales. We discuss the underlying spin physics of NV centers and highlight two primary detection modalities: optically detected magnetic resonance (ODMR) and T1 relaxometry-based sensing and how these approaches aids in the detection of both static magnetic fields and dynamic magnetic noise arising from biological processes. The review explores key application areas, including nanoscale nuclear magnetic resonance (NMR), monitoring of neural activity, detection of abnormal or rogue cells using NV-based platforms etc. In addition, strategies for enhancing sensitivity, such as surface functionalization of nanodiamonds, femtosecond (fs) laser-written photonic structures, and integration with microfluidic and lab-on-chip systems have also been discussed in depth. We also address the critical challenges, including surface-induced decoherence, charge-state instability, and signal-to-noise limitations in biofluids associated with NV-based biosensing applications. Finally, we outline future prospects, highlighting how NV-based magnetic biosensing provides a promising pathway for translating quantum sensing technologies into practical biomedical applications.