Surface-Enhanced NMR Spectroscopy via Solid-to-Solid Relayed Dynamic Nuclear Polarization

Over the past 15 years, dynamic nuclear polarization (DNP) has emerged as a unique and powerful strategy to enhance the solid-state nuclear magnetic resonance (NMR) signals of surface species. This approach, known as DNP surface-enhanced NMR spectroscopy (DNP SENS), has been successfully applied to a broad range of functional materials. DNP SENS relies on the use of exogenous sources of electrons, typically stable organic radicals. A key method for adding these polarizing agents to the target material and obtaining high NMR signal enhancement factors is incipient wetness impregnation (IWI) with a radical-containing solution. However, DNP SENS via IWI is not amenable to all materials due to chemical incompatibilities between the polarizing solution and the surface of interest. This strongly limits the application fields of DNP-SENS. Here, we report the design and synthesis of new porous silica-based solids that incorporate organic biradicals within their bulk of their framework. These polarizing solids can be mixed with other powdered materials, and once impregnated with a pure solvent or solvent mixture, they efficiently transfer the polarization to external surfaces. Numerical simulations provide a framework to describe the 1H-spin diffusion-relayed polarization transfer from the polarizing solid to the target solid. This solid-to-solid relayed DNP approach is demonstrated with pristine and functionalized alumina particles, as well as sensitive catalytic surfaces. Substantial NMR sensitivity gains are reported on surface species, establishing a new strategy for DNP SENS.