Detection is performed at 13 mT using Helmholtz coils, simplifying the task of stabilizing NMR linewidths to sub-hertz levels while enabling the use of high-sensitivity diamond quantum sensing protocols at low microwave frequencies ( 15).

Strong permanent magnets (1.5 T) are used to generate nuclear spin polarization. This is accomplished by spatially separating the polarization and detection phases of the experiment in a microfluidic setup ( 16, 17). Here, we report an order-of-magnitude improvement in spectral resolution, 0.65 ± 0.05 Hz, and realize a concentration sensitivity of ∼27 M s 1/2. The effective analyte detection volume is ∼40 pL (section SXII). Laser illumination (532 nm) bounces off the printed microwave line, and fluorescence (650 to 800 nm) is detected. An NV-doped diamond membrane (1 mm by 1 mm by 0.035 mm) is located on the surface of a microfluidic channel (width: 2 mm, height: between 0.2 mm and 1 mm) in contact with the analyte. Copper microwave (MW) lines, printed on the interior of the glass chip, provide spin control over NV electron spins. The RF excitation loop and NMR coil magnetometer were placed orthogonal to one another to minimize cross-talk.

The NMR coil magnetometer consists of a 3-mm-diameter coil wound around a ∼10-μl water volume. A radio frequency (RF) excitation loop, placed in between the NMR coil magnetometer and the NV NMR sensor, excites nuclear spin coherence in both channels. Two fluidic lines pass to the detection region, one consisting of water (for NMR coil magnetometer) and the other with analyte (for NV NMR). The chip is constructed using glass and adhesives (section SV). The field is stabilized temporally using a coil-based NMR magnetometer in combination with low-inductance feedback coils wound around the main Helmholtz coils. A set of eight gradient compensation coils is used to eliminate first- and second-order magnetic field gradients along the field direction. Prepolarized analyte flows to a microfluidic chip where it is stopped via fluidic switches (not shown), and the NV NMR signal is detected using a custom-built epifluorescence microscope with a numerical aperture (NA) of ∼0.8. It is subsequently shuttled to a microfluidic chip housed in a stabilized, lower magnetic field ( B 0 = 13 mT, Helmholtz coils) where it is detected by NV NMR. Analyte is prepolarized by flowing it through a permanent magnet (1.5-T Halbach array). The room temperature water proton density is ρ = 6.7 × 10 28 m −3. ( A) Comparison of statistical and thermal polarization of protons in water as a function of detection volume.

Diamond dimensions 208