Confined Nano‐NMR Spectroscopy Using NV Centers

Abstract

Nano nuclear magnetic resonance (nano‐NMR) spectroscopy with nitrogen‐vacancy (NV) centers holds the potential to provide high‐resolution spectra of minute samples. This is likely to have important implications for chemistry, medicine, and pharmaceutical engineering. One of the main hurdles facing the technology is that diffusion of unpolarized liquid samples broadens the spectral lines thus limiting resolution. Experiments in the field are therefore impeded by the efforts involved in achieving high polarization of the sample which is a challenging endeavor. Here, a scenario where the liquid is confined to a small volume is examined. It is shown that the confinement “counteracts” the effect of diffusion, thus overcoming a major obstacle to the resolving abilities of the NV‐NMR spectrometer.

Advances in nano- and microscale NMR spectroscopy using diamond quantum sensors

“…Furthermore, confinement of liquid samples for example in nanofluidics may restrict diffusion potentially enabling high spectral resolution. 72,102 Yet, recent results indicate that the diffusion can lead to sharp-peaked spectral lines which could enable improved resolution. 103,104 Another promising way to gain molecular information is the detection of quadrupolar nuclei (such as 2 H, 11 B, 14 N, .…”

“…Molecular diffusion is a substantial issue for liquid nanoscale NMR; water molecules, for example, diffuse through the detection volume of a single NV center in a few nanoseconds, effectively limiting the sensing time and, accordingly, broadens the linewidth (for water a few MHz to GHz). 72 Thus, the diffusion of molecules determines the linewidth achievable in nanoscale NMR, currently restricting it to high viscosity/low diffusion constant samples, such as oils, 67,[73][74][75] polymers, 67,76 or solids. 73,77 For solids, anisotropic interactions such as dipolar couplings or chemical shift anisotropy induce spectral line broadening.…”

“…However, unlike established DNP, there is no need for added paramagnetic radicals or cryogenic temperatures. Especially NV center-doped nanodiamonds as a polarization source for classical NMR demonstrate promising results 137 and have been realized using small form factor setups at room temperature, 138 which could facilitate the 42,79 Novel pulse sequences enabling detection at higher magnetic fields 145 Nanofluidics sample confinement for diffusion restriction 72 Magic angle spinning 78 High magnetic fields (e.g., 3 T) in combination with advanced quantum sensing schemes 74…”

Advances in nano- and microscale NMR spectroscopy using diamond quantum sensors

“…Magic angle spinning known from classical NMR might solve the latter with significant engineering challenges. Furthermore, confinement of liquid samples for example in nanofluidics may restrict diffusion potentially enabling high spectral resolution 72,102 . Yet, recent results indicate that the diffusion can lead to sharp-peaked spectral lines which could enable improved resolution 103,104 .…”

“…In a typical nanoscale NV-NMR experiment where the NV center is 2 -10 nm below the diamond's surface, the NV, therefore, detects couple of hundreds to thousands of spins (1 nm 3 = ~60 protons in water). On these small length scales, the number of nuclear spins contributing to the NMR signal is not determined by the Boltzmann nuclear polarization (BNP) -but by random spin fluctuations (statistical nuclear polarization 72 . Thus, the diffusion of molecules determines the linewidth achievable in nanoscale NMR, currently restricting it to high viscosity/low diffusion constant samples, such as oils 67,[73][74][75] , polymers 67,76 , or solids 73,77 .…”