Nanoscale NMR Spectroscopy Using Nanodiamond Quantum Sensors

While conventional nuclear magnetic resonance (NMR) spectroscopy is widely used for chemical analysis of macroscopic sample volumes, quantum NMR sensors based on nitrogen-vacancy (N-V) centers in bulk diamond have brought NMR spectroscopy to the nanoscale, demonstrating single-nucleus sensitivity and chemical structure determination. However, the detection volume for existing nanoscale N-V NMR devices extends only nanometers above a bulk diamond chip, which prevents measurements in thicker structures, such as cells. Here, we introduce N-V NMR devices based on nanodiamonds and use them to detect and distinguish multiple nuclear species in a sample volume of about ${20}^{3}\phantom{\rule{0.1em}{0ex}}\mathrm{n}{\mathrm{m}}^{3}$. We describe how analyte concentrations can be measured in situ using nuclear spins on the nanodiamond surface for calibration. The nanodiamond host provides a small size (\ensuremath{\sim}30 nm diameter), low toxicity, and opportunities for surface functionalization, which makes these devices promising for intracellular applications. These results mark an important step toward feasible and reliable nanoscale NMR measurements inside living cells.