Bringing diamond quantum sensors into application in biology and chemistry
Final Report Abstract
Nuclear magnetic resonance (NMR) spectroscopy is one of major methods for structural analysis in the molecular sciences. However, its intrinsically low sensitivity limits its application for sample-limited experiments or on the micronscale. Recently, fluorescent defects in diamond called nitrogen vacancy (NV) centers have shown to be sensitive atomic-scale detectors for NMR signals of even single protons and proteins. However, the best reported spectral resolution for NV-detected NMR is insufficient to resolve key spectral parameters of molecular structure such as J-couplings or small chemical shifts. This low spectral resolution is caused by (a) limited interrogation time set by the spin state lifetime of the NV-center (b) the correlation time set by sample diffusion though the sensing volume in nanoscale spin noise detecting experiments. In this project, we have overcome these limitations and developed a high spectral resolution NMR method with NV-centers in diamond. We solved limitation (a) by introducing a new NV-readout scheme called “synchronized readout (SR)”. The SR protocol allows us to detect ac magnetic fields with arbitrary frequency resolution. The capability of this approach is shown by detecting ac magnetic fields generated by coils with sub mHz spectral resolution. Diffusion issue (b) was overcome by detecting thermal spin polarization instead of spin noise detection in previous NV-based NMR experiments. For detecting the NMR signal of thermal polarized spins, a highly sensitive (30 pT/Hz^-1/2) NV-magnetometer had been developed. It is based on a diamond chip with a dense surface NV layer of circa 10 µm thickness, total internal reflection geometry of the excitation laser and efficient light collection through a light pipe. This sensor is placed in a homogenous magnetic field of an electromagnet at 880 G. The NMR detection volume is defined by the laser spot size and the NV-layer which is around 10 picoliter in this work. For high spectral resolution spectroscopy, the static magnetic field of the electromagnet had to be stabilized < 1 ppm, which was achieved by a second NV-diamond magnetometer. This sensor in combination with the SR readout method is used to detect the free induction decay from thermally-polarized nuclear spins on the micron scale. Importantly, we obtain proton NMR signal with a spectral resolution of ~ 3 Hz, which is nearly two orders of magnitude narrower than previously demonstrated with NV based techniques. It allows us to resolve chemical shifts and J-couplings of small organic molecules for the first time on a micron length scale. The NMR detection volume of the demonstrated method is ideally matched to the size of single cells, which will allow us to monitor single cell processes with chemical resolution in future. Although focused on high-resolution NMR at the micron-scale, the technique could also be applied to chip-based NMR spectroscopy of microliters with millimeter-sized NV ensemble detectors with superior sensitivity. Moreover, this should enable parallel operation with individual cross-talk free optical readouts. This would open the way for high-throughput analytical NMR spectroscopy for biomedical sciences. The work has been featured in physics world (Diamond sensors boost NMR resolution, May 25, 2017).
Publications
- High Resolution Magnetic Resonance Spectroscopy Using Solid-State Spins
Bucher, D.B., Glenn, D.R., Lee, J., Lukin, M.D. ,Park, H., Walsworth, R.L.
