19F MAS DNP for Probing Molecules in Nanomolar Concentrations: Direct Polarization as Key for Solid-State NMR Spectra without Solvent and Matrix Signals

The efficiency of dynamic nuclear polarization (DNP) enhanced 19F MAS NMR spectroscopy without 19F-containing solvents and matrices, which transport polarization via 19F–19F spin diffusion, is demonstrated. By preventing solvent and matrix signals respectively masking the corresponding resonances, this enables the detection of fluorinated target molecules in nanomolar amounts. As model compound, 1,3,5-tris(2-fluoro-2-methylpropionylamino)benzene (F-BTA) is investigated in a frozen 1,1,2,2-tetrachloroethane (TCE) solution and incorporated into a matrix of isotactic polypropylene (i-PP). While the polarizing agent is homogeneously dissolved within the frozen solution, for the i-PP/F-BTA blend, it is distributed via the incipient wetness impregnation (IWI) technique. For the frozen solutions with an F-BTA concentration of 187.5 mM an εon/off of 260 was obtained. For F-BTA concentrations of 10 and 2.5 mM the sensitivity trend suggests even higher DNP gains. The substantial enhancements could be achieved by direct polarization transfer over distances up to at least 20 Å, derived from a simple geometric model assuming a homogeneous solution, engaging a large part of the sample volume. Cross-polarization (CP) to 13C nuclei allowed selection of the NMR spectroscopic resonances of the minority species in the i-PP/F-BTA blend suppressing the otherwise dominating resonances of the IWI solvent and the polymer matrix. The possibility of exciting 19F via DNP directly and of transferring the polarization to other heteronuclei within close proximity enables spatial spectral editing to clear up spectra otherwise crowded by matrix and solvent signals. We thus expect direct polarization transfer techniques for DNP enhanced NMR spectroscopy to become more important in the future.