Research ArticleAPPLIED PHYSICS

Two-dimensional nuclear magnetic resonance spectroscopy with a microfluidic diamond quantum sensor

See allHide authors and affiliations

Science Advances  26 Jul 2019:
Vol. 5, no. 7, eaaw7895
DOI: 10.1126/sciadv.aaw7895
  • Fig. 1 Microfluidic prepolarization NMR setup.

    (A) Comparison of statistical and thermal polarization of protons in water as a function of detection volume. The room temperature water proton density is ρ = 6.7 × 1028 m−3. (B) Prepolarization concept. Analyte is prepolarized by flowing it through a permanent magnet (1.5-T Halbach array). It is subsequently shuttled to a microfluidic chip housed in a stabilized, lower magnetic field (B0 = 13 mT, Helmholtz coils) where it is detected by NV NMR. (C) Detection setup. 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. A set of eight gradient compensation coils is used to eliminate first- and second-order magnetic field gradients along the field direction. The field is stabilized temporally using a coil-based NMR magnetometer in combination with low-inductance feedback coils wound around the main Helmholtz coils. (D) Microfluidic chip setup. The chip is constructed using glass and adhesives (section SV). Two fluidic lines pass to the detection region, one consisting of water (for NMR coil magnetometer) and the other with analyte (for NV NMR). 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. The NMR coil magnetometer consists of a 3-mm-diameter coil wound around a ∼10-μl water volume. The RF excitation loop and NMR coil magnetometer were placed orthogonal to one another to minimize cross-talk. Copper microwave (MW) lines, printed on the interior of the glass chip, provide spin control over NV electron spins. (E) NV NMR geometry. 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. Laser illumination (532 nm) bounces off the printed microwave line, and fluorescence (650 to 800 nm) is detected. The effective analyte detection volume is ∼40 pL (section SXII).

  • Fig. 2 Characterization of prepolarized NV NMR.

    (A) The synchronized readout pulse sequence. It consists of a train of XY8-N pulses that perform successive phase measurements of the ac magnetic field produced by precessing nuclei. The measured fluorescence reflects an aliased version of the nuclear ac field projection. The entire sequence is repeated every 2.5 to 4.25 s (1.25 s for flow and the remainder for detection). (B) NV NMR spectra (absolute value of Fourier transform) of water (red) and an applied 2.5-nT amplitude test field (blue) for an effective acquisition time of 5.2 s (average of 60 traces; total measurement time, 150 s). The NMR signal amplitude obtained from the processed photodetector signal is recorded in μV. The conversion to magnetic field amplitude (in nT) is derived from the calibrated test field (see section SX). Inset: The SD of the noise floor reveals aBmin = 45 pT. From these data, we infer a minimum detectable concentration of 27 M s1/2 (SNR = 3). Incorporating all experimental dead time, the concentration sensitivity is ∼45 M s1/2 (section SXI). (C) A high-resolution NV NMR spectrum of water (imaginary part of Fourier transform) reveals a full width at half maximum (FWHM) linewidth of 0.65 ± 0.05 Hz. Data were obtained by averaging 60 traces, each 3 s long.

  • Fig. 3 1D NMR.

    Time-domain (left) and frequency-domain (right) NV NMR signals for (A) water, (B) trimethyl phosphate (TMP), and (C) 1,4-difluorobenzene (DFB). Signals were averaged over ∼103 traces for a total acquisition of ∼1 hour. A ∼1-kHz-bandwidth bandpass filter is applied to the time-domain data for better visualization. The frequency-domain spectra show the imaginary component of the Fourier transform. Each spectrum is fit with Gaussian functions (black lines). For TMP, we constrain the widths of both lines to be equal with a 1:1 amplitude ratio and find JHP = 11.04 ± 0.06 Hz. For DFB, we constrain the widths of all three lines to be equal with a 1:2:1 amplitude ratio and find JHF¯=6.09±0.05 Hz.

  • Fig. 4 2D COSY NMR of DFB.

    (A) Homonuclear COSY pulse sequence, (B) simulated spectrum, and (C) experimental NV NMR spectrum of DFB. (D) A modified heteronuclear COSY sequence reveals off-diagonal peaks in both (E) simulation and (F) experiment. Color scales correspond to the normalized absolute value of the 2D Fourier transform. Vertical axes (f1fref) correspond to the frequencies of the t1 dimension, and horizontal axes (f2fref) correspond to the frequencies of the t2 dimension. In (C), 14 values of t1 in 0.021-s increments up to 0.294 s were used. Total acquisition time was 22 hours. In (F), 16 values of t1 in 0.021-s increments up to 0.336 s were used. Total acquisition time was 25 hours. In both cases, the t2 acquisition spanned from 0 to 1.25 s. All simulations were performed using the SPINACH package (41). Simulation and experimental data use the same windowing functions (see section SXV).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/7/eaaw7895/DC1

    Section SI. NV NMR detection apparatus

    Section SII. Magnetic field gradient compensation

    Section SIII. Gradients due to magnetic susceptibility mismatch of sensor components

    Section SIV. NMR coil magnetometer feedback system

    Section SV. Microfluidic chip fabrication

    Section SVI. Sample preparation

    Section SVII. Microfluidic flow and switch timing

    Section SVIII. Adiabaticity considerations

    Section SIX. Optimization of flow rates

    Section SX. Magnetic field calibration

    Section SXI. Concentration sensitivity

    Section SXII. NMR field amplitudes and effective sensing volume

    Section SXIII. Analytical calculation for heteronuclear COSY

    Section SXIV. 2D homonuclear COSY of TMP

    Section SXV. SPINACH simulations and windowing functions for 2D NMR

    Fig. S1. Magnetostatic modeling of a diamond immersed in water.

    Fig. S2. Histogram of fitted central frequencies obtained from the NMR coil magnetometer for a typical measurement.

    Fig. S3. NMR signal strength dependence on flow rate and RF pulse length.

    Fig. S4. Saturation curve of the NV NMR.

    Fig. S5. NV NMR spectrum of water.

    Fig. S6. Nuclear ac magnetic field projection amplitude (integrated across the sensor volume) as a function of water volume.

    Fig. S7. Experimental homonuclear COSY spectrum of TMP.

    Table S1. Values of the different J-couplings in a DFB molecule used in the simulation.

  • Supplementary Materials

    This PDF file includes:

    • Section SI. NV NMR detection apparatus
    • Section SII. Magnetic field gradient compensation
    • Section SIII. Gradients due to magnetic susceptibility mismatch of sensor components
    • Section SIV. NMR coil magnetometer feedback system
    • Section SV. Microfluidic chip fabrication
    • Section SVI. Sample preparation
    • Section SVII. Microfluidic flow and switch timing
    • Section SVIII. Adiabaticity considerations
    • Section SIX. Optimization of flow rates
    • Section SX. Magnetic field calibration
    • Section SXI. Concentration sensitivity
    • Section SXII. NMR field amplitudes and effective sensing volume
    • Section SXIII. Analytical calculation for heteronuclear COSY
    • Section SXIV. 2D homonuclear COSY of TMP
    • Section SXV. SPINACH simulations and windowing functions for 2D NMR
    • Fig. S1. Magnetostatic modeling of a diamond immersed in water.
    • Fig. S2. Histogram of fitted central frequencies obtained from the NMR coil magnetometer for a typical measurement.
    • Fig. S3. NMR signal strength dependence on flow rate and RF pulse length.
    • Fig. S4. Saturation curve of the NV NMR.
    • Fig. S5. NV NMR spectrum of water.
    • Fig. S6. Nuclear ac magnetic field projection amplitude (integrated across the sensor volume) as a function of water volume.
    • Fig. S7. Experimental homonuclear COSY spectrum of TMP.
    • Table S1. Values of the different J-couplings in a DFB molecule used in the simulation.

    Download PDF

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article