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 picoliters. However, a key challenge for NV-NMR is detecting samples at millimolar concentrations. Here, we demonstrate an improvement in NV-NMR proton concentration sensitivity of about $10^5$ over thermal polarization by hyperpolarizing sample proton spins through signal amplification by reversible exchange (SABRE), enabling micron-scale NMR of small molecule sample concentrations as low as 1 millimolar 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.
Highlights
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
Because of the finite sensitivity of the NV-NMR sensor, its application is restricted to highly concentrated pure samples, which limits its utility for most chemical and biological problems
Bucher et al [10] used dynamic nuclear polarization (DNP) based on the Overhauser mechanism [11], where polarization is transferred to the sample nuclear spins from the electronic spins of dissolved molecular radicals, and obtained a proton number sensitivity enhancement of 2 orders of magnitude for micron-scale coherently averaged synchronized readout (CASR) NV-NMR spectroscopy
Summary
The iridium-based catalyst used in SABRE hyperpolarization is {[IrCl(COD)(IMes)] (COD = 1,5-cyclooctadiene; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol2-ylidene)}. The SABRE catalyst precursor is chemically activated by our supplying hydrogen gas and a substrate (e.g., pyridine or nicotinamide). Hydrogen undergoes oxidative addition onto the iridium, and the COD in the catalyst precursor is hydrogenated to cyclooctane. The COD will no longer interact with the catalyst. The substrate coordinates with the iridium to form the active spin-order transfer catalyst. Enriched parahydrogen gas is used to activate the catalyst for experimental convenience, the spin state of the hydrogen gas is irrelevant during the activation process
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