Can a diamond quantum sensor take high-resolution NMR of tiny volumes?
Separating the magnetising step from the diamond-based detection step let researchers record NMR spectra ten times sharper than earlier diamond sensors, enough for two-dimensional NMR on picolitre volumes.
Source
Two-dimensional nuclear magnetic resonance spectroscopy with a microfluidic diamond quantum sensor
Study at a glance
- Design
- Other — Lab experiment: analyte prepolarised in a 1.5 T magnet, flowed to a 13 mT detection region and read out by NV-centre ensembles in a diamond membrane inside a microfluidic chip.
- N
- No sample N; measurements on water, trimethyl phosphate and 1,4-difluorobenzene with several diamond membranes.
- Population
- Liquid analytes (water, TMP, DFB) sensed by NV-centre ensembles in 35-micrometre diamond membranes
- Outcome
- NMR linewidth (spectral resolution), concentration sensitivity, J-coupling splittings and 2D COSY cross peaks
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What they did
Liquids were first magnetised by flowing through a strong permanent-magnet array, then moved to a weak, carefully stabilised field where they sat over a diamond membrane containing nitrogen-vacancy centres. Laser and microwave pulse sequences let the NV centres sense the tiny oscillating magnetic field of precessing proton spins in an effective volume of about 40 picolitres. The team calibrated sensitivity with a known test field, measured water, trimethyl phosphate and difluorobenzene, and ran two versions of 2D correlation spectroscopy.
What they found
The narrowest water line was 0.65 Hz wide, about an order of magnitude sharper than previous diamond NMR, and concentration sensitivity was about 27 M s^1/2. Roughly 80% of the prepolarised magnetisation survived transfer to the detector. Spectra resolved heteronuclear J-coupling splittings of 11.04 Hz in trimethyl phosphate and about 6 Hz in difluorobenzene, and a heteronuclear COSY experiment showed cross peaks demonstrating magnetisation transfer mediated by those couplings.
The limits
What it doesn't show
The low detection field of 13 mT means chemical shifts cannot be resolved, so the method cannot yet identify molecules the way high-field NMR does. Sensitivity is far too low for metabolites at physiological concentrations without long averaging, and it varied by about 50% between experiments. Although the sensed volume is picolitres, several millilitres of liquid are needed to fill the flow apparatus, and the linewidth is still wider than water's natural value.
Key terms
- Nitrogen-vacancy centre
- A defect in diamond whose electron spin can be optically initialised and read out, making it a sensitive magnetic-field sensor.
- Prepolarisation
- Magnetising nuclear spins in a strong field before measuring them in a separate, weaker field.
- J-coupling
- An interaction between nuclear spins transmitted through chemical bonds that splits NMR lines by a fixed number of hertz.
- COSY
- Correlation spectroscopy, a two-dimensional NMR experiment whose cross peaks reveal which spin states exchange magnetisation.
- Concentration sensitivity
- The minimum detectable spin concentration for a given signal-to-noise ratio, normalised to one second of averaging.
- Spectral resolution
- How close two spectral lines can be and still be told apart, set here by the NMR linewidth.
Flashcards
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Quiz yourself
What senses the nuclear magnetic field in this spectrometer?
Common questions
Why not magnetise and detect in the same strong field?
Keeping the detection field low makes it much easier to stabilise to sub-hertz precision and lets NV centres use low microwave frequencies, while the strong magnet still provides polarisation.
Why are chemical shifts not seen?
Chemical shift splittings scale with field strength, and at 13 mT they are too small to resolve; the authors suggest raising the detection field to about 0.25 T.
What is the 40 picolitre figure?
It is the effective detection volume, the region contributing half the signal above the laser spot, not the total amount of liquid needed.
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