Solution nuclear magnetic resonance spectroscopy on a nanostructured diamond chip

Abstract

Sensors using nitrogen-vacancy centers in diamond are a promising tool for small-volume nuclear magnetic resonance (NMR) spectroscopy, but the limited sensitivity remains a challenge. Here we show nearly two orders of magnitude improvement in concentration sensitivity over previous nitrogen-vacancy and picoliter NMR studies. We demonstrate NMR spectroscopy of picoliter-volume solutions using a nanostructured diamond chip with dense, high-aspect-ratio nanogratings, enhancing the surface area by 15 times. The nanograting sidewalls are doped with nitrogen-vacancies located a few nanometers from the diamond surface to detect the NMR spectrum of roughly 1 pl of fluid lying within adjacent nanograting grooves. We perform ¹H and ¹⁹F nuclear magnetic resonance spectroscopy at room temperature in magnetic fields below 50 mT. Using a solution of CsF in glycerol, we determine that 4 ± 2 × 10^{12 19}F spins in a 1 pl volume can be detected with a signal-to-noise ratio of 3 in 1 s of integration.Nitrogen vacancy (NV) centres in diamond can be used for NMR spectroscopy, but increased sensitivity is needed to avoid long measurement times. Kehayias et al. present a nanostructured diamond grating with a high density of NV centres, enabling NMR spectroscopy of picoliter-volume solutions.

Fig. 1

Picoliter NMR. a Overview of ambient-temperature NMR techniques for small volumes. Points represent experimental values for minimum-detectable nuclear spin concentration in 1 s with SNR = 3 for different techniques: microslot, microcoil, cryogenic probes, atomic vapor magnetometers, giant magnetoresistance (GMR) sensors, anistropic magnetoresistance (AMR) sensors, single NV centers, and NV-doped nanogratings (this work). The solid red line is the projected sensitivity for diamond nanogratings (Eq. (1)), exhibiting volume^−1/2 scaling (see Supplementary Note 1). Solid blue lines indicate constant numbers of spins. b Epifluorescence diamond NMR set-up. c The sensor region consists of dense, high aspect-ratio diamond nanogratings fabricated via interferometric lithography and doped with NV centers. d Experimental geometry. The analyte’s precessing nuclear statistical polarization produces an oscillating magnetic field, which is sensed by adjacent near-surface NV centers

Fig. 2

Diamond nanogratings. a Schematic of large-area nanofabrication process. b Scanning electron micrograph of 400 nm pitch diamond nanogratings. Focused ion beam etching prior to imaging enabled visualization of the nanogratings’ cross-section. Scale bar is 1 μm. c Confocal microscopy images reveal that fluorescence from dye-stained water originates from areas inside the nanograting grooves, confirming wetting. Dashed lines represent the estimated diamond–water boundary. Scale bar is 500 nm. d Comparison of T₂, measured with the XY8-22 protocol, and e fluorescence intensity between flat and nanograting chips implanted at similar conditions. T₂ can surpass ~100 μs with sufficient decoupling π-pulses (see Supplementary Note 2). Error bars in d represent standard error of the mean

Fig. 3

Nanoscale NMR. a Sensing protocols: optical pulses are used to pump and probe NV spin state via the spin-dependent fluorescence; microwave multipulse sequences are applied between optical pump and probe pulses. Red and blue color indicates different microwave phases, which are shifted relative to each other by 90°. NV centers are resonantly tuned to detect a particular nuclear species by setting 4τ = τ_L, where 2τ is the separation between π-pulses and τ_L is the nuclear precession period. In order to reject common-mode noise the sequences are repeated with the phase of the last π/2-pulse shifted by 180°. The resulting signals are then subtracted and normalized to give the measurement results. b Time-domain NMR signal for ¹⁹F nuclei in Fomblin^® oil taken using XY8-13 correlation sequence. c ¹⁹F frequency-domain NMR signal at B₀ = 47.1 mT obtained by Fourier transform of the data in b. d Measured ¹H and ¹⁹F gyromagnetic ratios at different B₀ values. Dashed lines are literature values. Error bars represent standard error of the mean

Fig. 4

NV NMR sensitivity characterization. a ¹⁹F NMR signal from Fomblin^® oil at B₀ = 47.1 mT for flat and nanograting sensors implanted at 20 keV using XY8-13 correlation sequence. b NMR signal-to-noise ratio as a function of averaging time. Fits to the function SNR = $\alpha \sqrt{t_{\mathrm{avg}}}$ give good agreement for both sensors (solid lines). The coefficient α was 2.4 times larger for the nanograting chip. Error bars represent standard error of the mean

Fig. 5

Solution NMR. a, b NMR spectra of 20% by weight CsF solution in glycerol at 47.2 and 40.5 mT. c NMR spectrum of pure glycerol. All spectra were measured with an XY8-10 correlation spectroscopy pulse sequence tuned to the ¹⁹F precession frequency. Each spectrum was fit to a single Lorentzian function with central frequency set to the anticipated ¹⁹F Larmor frequency. The linewidth was fixed to that of the ¹H linewidth in glycerol obtained under similar conditions. The only free fit parameters were peak amplitude and offset. We assume that each spectrum contains at most one NMR line (from ¹⁹F) and other fluctuations are due to noise. Error bars represent standard deviation of off-resonant points

Fig. 6

Diffusion-limited NMR. a Diffusion of analyte molecules through the sensing volume. b Example of temporal decay of the correlation signal for immersion oil. The decay is exponential with a characteristic correlation time τ_C. Dashed line represents the decay envelope. c Correlation time τ_C as a function of nitrogen implantation energy for immersion oil and glycerol. Error bars represent fit uncertainty. Solid lines are fits to the one-sided diffusion model discussed in the text