Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning C-13 and Si-29 NMR Enhanced by Dynamic Nuclear Polarization
We show that dynamic nuclear polarization (DNP) can be used to enhance NMR signals of C-13 and Si-29 nuclei located in mesoporous organic/inorganic hybrid materials, at several hundreds of nanometers from stable radicals (TOTAPOL) trapped in the surrounding frozen disordered water. The approach is demonstrated using mesoporous silica nanoparticles (MSN), functionalized with 3-(N-phenylureido)propyl (PUP) groups, filled with the surfactant cetyltrimethylammonium bromide (CTAB). The DNP-enhanced proton magnetization is transported into the mesopores via H-1-H-1 spin diffusion and transferred to rare spins by cross-polarization, yielding signal enhancements epsilon(on/off) of around 8. When the CTAB molecules are extracted, so that the radicals can enter the mesopores, the enhancements increase to epsilon(on/off) approximate to 30 for both nuclei. A quantitative analysis of the signal enhancements in MSN with and without surfactant is based on a one-dimensional proton spin diffusion model. The effect of solvent deuteration is also investigated.