“This work reports the first experimental demonstration of NMR spectroscopy with full chemical specificity at the scale of a single biological cell, which has been a major scientific goal for the last 50 years,” says Ronald Walsworth of Harvard University, who led this research effort. “We use a new measurement scheme employing quantum sensors in diamond to realize a 100X improvement in NMR spectral resolution under ambient conditions for a sample volume comparable to that of a single cell – about 10 trillionths of a litre.”

The quantum sensors used by Walsworth and colleagues are nitrogen vacancy (NV) colour centres in diamond. These defects occur when two neighbouring carbon atoms in diamond are replaced by a nitrogen atom and an empty lattice site.

NV centres act like tiny quantum magnets that are isolated from their surroundings and can be manipulated using laser pulses. They are ideal as biological probes because they are non-toxic, photostable and can easily be inserted into or placed adjacent to living cells and tissues. NV centres are capable of detecting the very weak magnetic fields from a single cell, molecule, or organism, as the intensity of the light they emit changes with the local magnetic field. They can thus be used as highly sensitive magnetic probes that can monitor local spin changes in a material over microscopic distances.

Broad NMR spectral lines

“Over the last few years, we and other research groups have been able to apply NV sensors to NMR of nanometre and micrometre volumes,” explains Walsworth. “But until now the measurement techniques produced broad NMR spectral lines (typically greater than 100 Hz), due to both the short spin state lifetime of the NV centre (around 3 ms) and the fluctuating statistical spin polarization of the sample. This spectral resolution is too coarse to resolve molecular structure fingerprints important in chemistry, structural biology and materials research.”

Micro-NMR spectral resolution reaches 1Hz

Walworth’s team says that it has now overcome these problems by using an ensemble of NV centres combined with thermal spin polarization of the sample and a narrowband synchronized readout measurement that can sense NMR signals for as long as 103 seconds. The new technique produces an NMR spectral resolution of about 1Hz in the sample volume of a typical cell (about 10 trillionths of a litre), which allows observation of the key spectral features needed for chemical analysis.

According to the researchers, with further improvements in sensitivity, it might even allow for NMR spectroscopy of small molecules and proteins at the single-cell level.

Potential applications

Dirk Englund of the Massachusetts Institute of Technology, who was not involved in this work says that the new study is an "amazing advance" in the field of quantum sensing. "It takes magnetic field sensing to a new extreme that now allows for resolution of chemical shift spectra at the micron-scale, which matches the lengths of interest in cells. Just a few years ago, this still seemed far off and progress has been tremendous."

Potential applications include NMR studies of single-cell metabolomics and NMR fingerprinting of protein expression in tumour cells, for example, say Walsworth and colleagues. “It may even help in the development of new drugs through the study of very small, hard-to-manufacture samples.”

The research is detailed in Nature doi:10.1038/nature25781.