Lund, A., et al., Direct dynamic nuclear polarization targeting catalytically active (27)Al sites. Phys Chem Chem Phys, 2015. 17(38): p. 25449-54.
Here we present a systematic study of direct (27)Al Dynamic Nuclear Polarization (DNP) as induced by three different mono-radical probes with side groups of varying charge states. By employing 4-amino TEMPO that adsorbs to negatively charged surface sites of Al-SBA-15, we achieve a (27)Al signal enhancement factor of approximately 13 compared to a signal enhancement factor of approximately 3-4 from mono-radicals that do not adsorb as strongly to the surfaces of Al-SBA-15, here 4-carboxy- and 4-hydroxy-TEMPO. By performing Electron Spin Echo Envelope Modulation (ESEEM) experiments and continuous wave (cw) Electron Paramagnetic Resonance (EPR) lineshape analysis using various nitroxide probes imbibed in Al-SBA-15, we find that direct (27)Al DNP enhancements achieved with different spin probes can be attributed to proximity and local concentration of the spin probes to aluminum on the surface of mesoporous alumina-silica.