Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning ¹³C and ²⁹Si NMR Enhanced by Dynamic Nuclear Polarization

Abstract

We show that dynamic nuclear polarization (DNP) can be used to enhance NMR signals of13C and 29Si 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 1H-1H spin diffusion and transferred to rare spins by cross-polarization, yielding signal enhancements εon/off of around 8. When the CTAB molecules are extracted, so that the radicals can enter the mesopores, the enhancements increase to εon/off ≈ 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. Disciplines Chemistry CommentsReprinted (adapted) with permission from Journal of Physical Chemistry C 117 (2013) ABSTRACT: We show that dynamic nuclear polarization (DNP) can be used to enhance NMR signals of 13 C and 29 Si 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 1 H− 1 H spin diffusion and transferred to rare spins by cross-polarization, yielding signal enhancements ε on/off of around 8. When the CTAB molecules are extracted, so that the radicals can enter the mesopores, the enhancements increase to ε on/off ≈ 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.

Is solid-state NMR enhanced by dynamic nuclear polarization?

“…Amorphous materials such as silicas (SiO 2 ) can exhibit inhomogeneous broadening at RT to such an extent that the DNP conditions make little difference, and have thus already been extensively studied with this technique [29,46,[59][60][61][62]. Furthermore, extremely rigid systems [44] and systems with nuclei of interest with a relatively large quadrupolar coupling, such as 27 Al [36][37][38] and 17 O [6], will also experience little change in resolution upon addition of DNP matrices and polarizing agents and are therefore also highly pertinent for MAS-DNP studies.…”

“…Furthermore, metal(loid) oxide based materials, such as silica and alumina, are academically and industrially valuable systems that can produce broad ssNMR lines at room temperature and may not show further significant broadening for MAS-DNP studies [36,46]. Hence, there has been a recent plenitude of results presented on these highly "MAS-DNP-suitable" types of material systems [37,38,59,62], particularly those based on mesoporous silica [29,32,60,65,75]. The latter will not be visited here in much detail since there is a recent review article that covers this topic [29].…”

Theoretical aspects of Magic Angle Spinning - Dynamic Nuclear Polarization

“…The advent of Dynamic Nuclear Polarization (DNP), using gyrotron microwave (MW) sources, combined with MAS has opened the door to many advanced studies of solids at high magnetic fields [1]. The development and implementation of high field gyrotrons up to the Terahertz range has enabled the construction of high field MAS-DNP instruments, and led to the commercialization of MAS-DNP spectrometers by Bruker Inc. [2] operating up to 18 T. Theses high field DNP instrument developments have triggered a vast interest of the solid-state NMR community in MAS-DNP experiments, both for biological [3][4][5][6][7][8][9][10][11] and material science [12][13][14][15][16][17][18][19][20][21][22] applications. Most of these experiments performed today are using nitroxide based bi-radicals such as TOTAPOL [23,24,2,8,25,26,9], and recently new nitroxide based biradicals have been introduced with varying longitudinal relaxation times, phase memory times (or transverse relaxation time), electron-electron dipolar couplings, and relative gÀtensor orientations such as AMUPOL, bCTbK, or TEKPOL [27,26,28].…”

Sub-THz technology for dynamic nuclear polarization in nuclear magnetic resonance (DNP NMR): transverse confinement of microwave propagation through heterogeneous solid DNP samples

“…Application examples range from the study of biomacromolecules [5-6] to material samples like zeolites or mesoporous silica with small molecules or catalytic compounds adsorbed at the inner surfaces of the mesoporous material [7][8][9].…”