Electron-nuclear double resonance (ENDOR) is a fundamental technique in electron paramagnetic resonance (EPR) spectroscopy that directly detects hyperfine transitions of nuclei coupled to a paramagnetic centre. Despite its wide use, spinsensitivity and restricted spectral resolution in powder samples pose limitations of this technique in modern application fields of EPR. In this contribution, we examine the performance of an ENDOR pulse sequence that utilises a preparation scheme different from conventional Davies ENDOR. The scheme is based on electron-nuclear cross-polarisation (eNCP), which requires concomitant microwave (MW) and radio-frequency (RF) irradiation satisfying specific matching conditions between the MW and RF offsets and the hyperfine coupling. Changes in nuclear polarisation generated during eNCP can be detected via a conventional ENDOR read-out sequence consisting of an RF π -pulse followed by EPR-spin echo detection. Using 1 H-BDPA as a standard sample, we first examine the CP matching conditions by monitoring the depolarisation of the electron spin magnetisation. Subsequently, so-called CP-edited ENDOR spectra for different matching conditions are reported and analysed based on the provided theoretical description of the time evolution of the spin density matrix during the experiment. The results demonstrate that CP-edited ENDOR provides additional information with respect to the sign of the hyperfine couplings. Furthermore, the sequence is less sensitive to nuclear saturation effects encountered in conventional ENDOR.

NMR is a noninvasive, molecular-level spectroscopic technique widely used for chemical characterization. However, it lacks the sensitivity to probe the small number of spins at surfaces and interfaces. Here, we use nitrogen vacancy (NV) centers in diamond as quantum sensors to optically detect NMR signals from chemically modified thin films. To demonstrate the method’s capabilities, aluminum oxide layers, common supports in catalysis and materials science, are prepared by atomic layer deposition and are subsequently functionalized by phosphonate chemistry to form self-assembled monolayers. The surface NV-NMR technique detects spatially resolved NMR signals from the monolayer, indicates chemical binding, and quantifies molecular coverage. In addition, it can monitor in real time the formation kinetics at the solid–liquid interface. With our approach, we show that NV quantum sensors are a surface-sensitive NMR tool with femtomole sensitivity for in situ analysis in catalysis, materials, and biological research.

Characterization of the molecular properties of surfaces under ambient or chemically reactive conditions is a fundamental scientific challenge. Nuclear magnetic resonance (NMR) spectroscopy would be the ideal technique, however it lacks the sen-sitivity to probe the small number of spins at interfaces. Here we use nitrogen vacancy (NV) centers in diamond as quantum sensors to optically detect NMR signals from chemically modified thin films. Aluminum oxide (Al2O3) layers, common supports in catalysis and materials science, are prepared by atomic layer deposition and are subsequently functionalized by phosphonate chemistry to form self-assembled monolayers (SAMs). The surface NV-NMR technique detects NMR signals from the monolayer, indicates chemical binding, and quantifies molecular coverage. In addition, it can monitor in real-time the formation kinetics at the solid-liquid interface. This work demonstrates the capability of NV quantum sensors as a sur-face-sensitive (femtomole) NMR tool for in-situ analysis in catalysis, materials and biological research.

No abstract