Beyond the Silica Surface by Direct Silicon‐29 Dynamic Nuclear Polarization

Abstract

Buried truth: High‐field magic angle spinning dynamic nuclear polarization (MAS DNP) enhances the sensitivity of solid‐state NMR spectroscopy, but only for protonated surfaces. Direct 29Si DNP using the biradical TOTAPOL (see picture) circumvents this limitation by producing a 30‐fold enhancement of subsurface 29Si NMR signals in mesoporous silica, a material with applications in photonics, nanotechnology and catalysis.

Dynamic nuclear polarization of spherical nanoparticles

“…2) 29 Si spectra with less intensity due to a less efficient polarization with much lower enhancements compared to Q 4 enhancements ( Table 1). This difference between the direct-excitation and CP 29 Si DNP experiments has been reported previously for a mesoporous silica material, 21 which shows that the surface silicon sites (Q 2-3 ) are best polarized by CP, whereas, the sub-surface sites (Q 4 ) are polarized by directexcitation, which is also the case for the studied nanoparticles. As a result, it can be concluded that there is a preferential DNP polarization phenomenon in both silica based mesoporous and spherical materials, which leads to different DNP enhancements for different 29 Si sites, and as a result, different signal intensities in the spectra.…”

“…56 These values are similar to the ones reported previously. 19 However, B30% larger enhancements were observed for direct-excitation 29 Si DNP experiments recently by using a different microwave (mW) irradiation frequency, under the so-called 21 The power of the DNP enhancements leads to the observation of spectra with high sensitivity in a very short experiment time (B60 or B40 seconds for the 1D CP or direct-excitation experiments, respectively, for experimental parameters see the caption of Fig. 2).…”

“…19 The 29 Si CP spectra show different Q-type (Q n = Si (OSi) n (OH) 4Àn ) silicon sites for both coated and pure silica materials, which are observed at d 29 Si ¼ $90 ppm (see SI 2, ESI † for 1D 13 C and 29 Si spectra of non-functional silicon-oxide nanoparticles). 19,21 The amount of different silicon sites are determined by integrating the specific resonances in spectra recorded via CP (represented below in terms of ratios, %, in each individual spectrum. The differences in those individual ratios between different samples are compared); which are 10/8 (Q 2 ), 57/71 (Q 3 ), and 33/21 (Q 4 ) % for the 25/100 nm nanoparticles coated with arginine, respectively (the ratios and some of the fits are in SI 3, ESI, † as well as for other particles).…”

“…[4][5][6][7] As pointed out recently, this ''renaissance of DNP'' combined with high magnetic fields will further grow in the coming years, enabling demanding applications with better resolution. 8,9 Materials and biological systems such as membrane or microcrystalline proteins, [10][11][12][13] fibrils, 14 ligands bound to receptors, 15 polymers, [16][17][18] surface-bound species, [19][20][21][22][23] and other low concentrated samples were studied beneficially using DNP-NMR. 13,[24][25][26] Cryogenic temperatures are required to slow down nuclear and electron relaxation and to allow an efficient polarization transfer in solid-state DNP NMR experiments.…”

“…Later, Lafon et al demonstrated the characterization of bulk silica sites buried beneath the surfaces in a similar porous material by using direct excitation 29 Si DNP-NMR. 21 Rossini et al showed that the surface sites in small pores, from which the radical is excluded, can be hyperpolarized using DNP NMR methods on functionalized metal-organic frameworks. 22 Furthermore, intrinsic (endogenous) spin labels in polymers enable DNP-NMR without the necessity of supplying exogenous radicals.…”