Prospects for nuclear spin hyperpolarization of molecular samples using nitrogen-vacancy centers in diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-01-21 |
| Journal | Physical review. B./Physical review. B |
| Authors | JeanâPhilippe Tetienne, Liam T. Hall, Alexander J. Healey, Gregory A. L. White, MarcâAntoine Sani |
| Institutions | The University of Melbourne, Centre for Quantum Computation and Communication Technology |
| Citations | 31 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study models the feasibility and requirements for using optically pumped Nitrogen-Vacancy (NV) centers in diamond as a non-invasive platform for nuclear spin hyperpolarization (HP) to enhance Nuclear Magnetic Resonance (NMR) sensitivity.
- Polarization Potential: NV-based HP can theoretically generate average nuclear spin polarizations exceeding 10% over macroscopic sample volumes (greater than or equal to ”L).
- Geometric Requirement: Achieving high polarization requires careful system engineering, specifically high-aspect-ratio micro-structuring of the diamond surface to maximize the contact area between the NV layer and the sample.
- Micro-NMR Enhancement: For micro-NMR experiments (sensing volumes ~10 ”m3), modest signal enhancements (1-2 orders of magnitude) over thermal polarization are achievable using current NV technology.
- Limiting Factor (T2,NV): The finite NV electron spin coherence time (T2,NV) is the critical practical limitation, significantly reducing the cooling rate, especially for low-gamma nuclei (e.g., 13C) which require longer transfer times.
- Liquid Sample Constraint: NV hyperpolarization is generally ineffective for liquid samples due to fast molecular diffusion (Dn > 10-14 m2s-1), necessitating a freeze-thaw cycle for liquid-state NMR applications.
- Nano-NMR Conclusion: For nano-NMR experiments relying on statistical polarization detection, NV hyperpolarization generally provides no net signal-to-noise ratio (SNR) improvement due to the significant temporal overhead of the polarization step.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Electron Spin Polarization Fidelity (FNV) | ~80% | N/A | Typical fidelity achieved by optical pumping. |
| NV Depth (dNV) | 5 nm | N/A | Nominal depth for near-surface HP-NVs. |
| NV Surface Density (ÏNV) (Target) | 1016 m-2 | N/A | Optimum trade-off with T2,NV for high polarization. |
| NV Coherence Time (T2,NV) (Shallow) | 10 - 100 ”s | N/A | Commonly observed range for dense, near-surface NVs. |
| NV Coherence Time (T2,NV) (Optimized) | ~1 ms | N/A | Achievable with optimized diamond surface preparation. |
| 1H Thermal Polarization (Pth) | ~10-5 | N/A | At 3 T and 300 K. |
| 1H Nuclear Spin Density (Ïn) (Frozen Water) | 66 nm-3 | N/A | Equivalent to 1.7 M concentration. |
| 1H Spin-Lattice Relaxation Time (T1,n) | ~1 s | N/A | Typical value in the frozen phase. |
| 13C Spin-Lattice Relaxation Time (T1,n) | ~100 s | N/A | Relevant for low-gamma nuclei. |
| Flip-Flop Time (Ï0) (1H, Flat Surface) | ~30 ”s | N/A | dNV = 5 nm, PulsePol protocol. |
| Flip-Flop Time (Ï0) (13C, Flat Surface) | ~1.2 ms | N/A | dNV = 5 nm, 1 M 13C spins. |
| Molecular Diffusion Limit (Dn) | < 10-14 m2s-1 | N/A | Required for polarization transfer validity (solid/high-viscosity sample). |
| Laser Wavelength | 532 nm | N/A | Used for NV optical pumping/initialization. |
| Laser Intensity (Peak) | ~100 kW/cm2 | N/A | Required for ~1 ”s NV initialization time. |
| Slab Gap Height (hcell) (Target) | ~1 ”m | N/A | Required for 10% 1H polarization at T1,n = 1 s. |
| Slab Aspect Ratio (Target) | ~100 | N/A | Required for achieving hcell ~ 1 ”m in micro-structured devices. |
Key Methodologies
Section titled âKey Methodologiesâ- NV Spin Initialization: The NV electron spin is initialized into the |0> state using continuous optical pumping (532 nm laser). This process requires high laser intensity and includes a dead time (td, typically 2-10 ”s) to account for relaxation and addressing overheads.
- Coherent Polarization Transfer Protocol: The PulsePol protocol is employed for robust, coherent polarization exchange (flip-flop) between the NV electron spin and the external nuclear spins. This requires careful tuning of MW amplitude and pulsing parameters.
- Polarization Build-up Cycle: The NV initialization and transfer protocol (duration Ï) are repeated continuously. The polarization rate u(R) is optimized by setting the cycle time Ï close to the optimum duration Ïopt â 0.74Ï0.
- Geometric Optimization (Slab Architecture): To maximize the average polarization (P) over macroscopic volumes, a slab architecture is proposed, consisting of stacked diamond plates micro-structured with grooves (gap hcell) to maximize the NV-sample contact area (ÏNV).
- Sample State Management: Due to the inefficiency of NV-HP in the presence of fast molecular diffusion, liquid samples must be frozen (at or below the freezing point) during the polarization step, followed by rapid thawing before the NMR measurement (similar to dissolution-DNP).
- Micro-NMR Detection: For micro-NMR applications, shallow HP-NVs polarize the sample, and a separate layer of deep readout NVs (RO-NVs, dRO > 10 ”m) detects the resulting AC magnetic field (BHP) generated by the net hyperpolarized magnetization (MHP).
Commercial Applications
Section titled âCommercial ApplicationsâThe research on NV-based hyperpolarization directly supports advancements in several high-tech fields, particularly those requiring enhanced magnetic resonance sensitivity and miniaturization:
- Enhanced Clinical and Research NMR: Providing a potentially simpler, non-cryogenic alternative to conventional Dynamic Nuclear Polarization (DNP) for boosting the SNR of standard NMR/MRI experiments, especially for low-concentration or low-gamma (13C, 15N) analytes.
- Portable NMR Spectrometers: Enabling the development of highly sensitive, compact, and portable micro-NMR devices by integrating the NV hyperpolarization source directly onto the diamond chip, reducing reliance on large, high-field magnets.
- Nanoscale Materials Characterization: The underlying NV technology is crucial for high-resolution nano-NMR and nano-MRI, allowing for the study of molecular dynamics and surface interactions in nanoscale objects (e.g., lipid bilayers, atomically-thin materials).
- Diamond Quantum Sensor Manufacturing: The stringent requirements for high-density, shallow NV layers with long T2,NV drive innovation in diamond fabrication, including ion implantation, doping engineering, and surface termination techniques.
- Microfluidic Devices: Integration of micro-structured diamond chips (high-aspect-ratio grooves) with microfluidic systems for continuous-flow sample handling and in-situ hyperpolarization.
View Original Abstract
After initial proof-of-principle demonstrations, optically pumped nitrogen-vacancy (NV) centers in diamond have been proposed as a noninvasive platform to achieve hyperpolarization of nuclear spins in molecular samples over macroscopic volumes and enhance the sensitivity in nuclear magnetic resonance (NMR) experiments. In this work we model the process of polarization of external samples by NV centers and theoretically evaluate their performance in a range of scenarios. We find that average nuclear spin polarizations exceeding 10% can in principle be generated over macroscopic sample volumes (â€ÎŒl) with a careful engineering of the systemâs geometry to maximize the diamond-sample contact area. The fabrication requirements and other practical challenges are discussed. We then explore the possibility of exploiting local polarization enhancements in nano/micro-NMR experiments based on NV centers. For micro-NMR we find that modest signal enhancements over thermal polarization (by 1-2 orders of magnitude) can in essence be achieved with existing technology, with larger enhancements achievable via microstructuring of the sample/substrate interface. However, there is generally no benefit for nano-NMR where the detection of statistical polarization provides the largest signal-to-noise ratio. This work will guide future experimental efforts to integrate NV-based hyperpolarization to NMR systems.