Surface NMR Using Quantum Sensors in Diamond

Abstract

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.

Advances in nano- and microscale NMR spectroscopy using diamond quantum sensors

“…A recent study has expanded the aluminum oxide platform for surface NMR demonstrated by Liu et al 77 , including additional binding moieties such as biotin or azides by multiple follow-up functionalization of the Al2O3 support 100 . This approach enables the efficient coupling of biomolecules such as proteins or DNA to the aluminum oxide deposited on the diamond's surface and expands the prospect of surface NV-NMR for biomolecules or biochemistry 101 .…”

“…Nevertheless, nanoscale NV-NMR employing shallow NV ensembles has shown its ability to measure NMR signals originating from surfaces and interfaces under chemical-relevant conditions expanding the toolbox of modern chemical analytics with widespread applications in catalysis, electrochemistry, and material research 77 .…”

“…On these small length scales, the number of nuclear spins contributing to the NMR signal is not determined by the Boltzmann nuclear polarization (BNP) -but by random spin fluctuations (statistical nuclear polarization 72 . Thus, the diffusion of molecules determines the linewidth achievable in nanoscale NMR, currently restricting it to high viscosity/low diffusion constant samples, such as oils 67,[73][74][75] , polymers 67,76 , or solids 73,77 . For the latter, anisotropic interactions such as dipolar couplings or chemical shift anisotropy induce spectral line broadening.…”

“…Interfacial and surface properties are vital for numerous material functions and applications in catalysis, energy conversion, or bioanalytics, especially under chemical-relevant conditions 77 .…”

“…Even then, chemical reaction kinetics and molecular dynamics are ultimately challenging to measure under ambient conditions. Liu et al 77 recently demonstrated the application of NV-NMR in combination with atomic layer deposition (ALD) to study self-assembled monolayers (SAM) under ambient conditions (see Figure 5a). ALD is a technique that enables the surface deposition of thin films ranging from pure metals, common polymers (utilizing molecular layer deposition, MLD), or metal oxides 92 .…”