Category Archives: 29Si

Dynamic nuclear polarization and ESR hole burning in As doped silicon #DNPNMR

Järvinen, J., D. Zvezdov, J. Ahokas, S. Sheludiakov, L. Lehtonen, S. Vasiliev, L. Vlasenko, Y. Ishikawa, and Y. Fujii. “Dynamic Nuclear Polarization and ESR Hole Burning in As Doped Silicon.” Physical Chemistry Chemical Physics 22, no. 18 (2020): 10227–37.

https://doi.org/10.1039/C9CP06859G

We present an experimental study of the Dynamic Nuclear Polarization (DNP) of 29Si nuclei in silicon crystals of natural abundance doped with As in the temperature range 0.1-1 K and in strong magnetic field of 4.6 T. This ensures very high degree of electron spin polarization, extremely slow nuclear relaxation and optimal conditions for realization of Overhauser and resolved solid effects. We found that the solid effect DNP leads to an appearance of a pattern of holes and peaks in the ESR line, separated by the super-hyperfine interaction between the donor electron and 29Si nuclei closest to the donor. On the contrary, the Overhauser effect DNP mainly affects the remote 29Si nuclei having the weakest interaction with the donor electron. This leads to an appearance of a very narrow ( 3 mG wide) hole in the ESR line. We studied relaxation of the holes after burning, which is caused by the nuclear spin diffusion. Analyzing the dynamics of the hole in the spectrum with a simple one-dimensional diffusion model leads to a value of the diffusion coefficient D = 8(3)10􀀀9 G2/s. Our data indicate that the spin diffusion is not completely prevented even in the frozen core near the donors. The emergence of the narrow hole after the Overhauser DNP may be explained by a partial “softening” of the frozen core caused by decoupling of the donor electron and remote 29Si nuclei.

DNP-NMR of surface hydrogen on silicon microparticles #DNPNMR

Shimon, Daphna, Kipp J. van Schooten, Subhradip Paul, Zaili Peng, Susumu Takahashi, Walter Köckenberger, and Chandrasekhar Ramanathan. “DNP-NMR of Surface Hydrogen on Silicon Microparticles.” Solid State Nuclear Magnetic Resonance 101 (September 2019): 68–75. 

https://doi.org/10.1016/j.ssnmr.2019.04.008

Dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) offers a promising route to studying local atomic environments at the surface of both crystalline and amorphous materials. We take advantage of unpaired electrons due to defects close to the surface of the silicon microparticles to hyperpolarize adjacent 1H nuclei. At 3.3 T and 4.2 K, we observe the presence of two proton peaks, each with a linewidth on the order of 5 kHz. Echo experiments indicate a homogeneous linewidth of $ 150 À 300 Hz for both peaks, indicative of a sparse distribution of protons in both environments. The high frequency peak at 10 ppm lies within the typical chemical shift range for proton NMR, and was found to be relatively stable over repeated measurements. The low frequency peak was found to vary in position between À19 and À37 ppm, well outside the range of typical proton NMR shifts, and indicative of a high-degree of chemical shielding. The low frequency peak was also found to vary significantly in intensity across different experimental runs, suggesting a weakly-bound species. These results suggest that the hydrogen is located in two distinct microscopic environments on the surface of these Si particles.

Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles: a study by conventional and DNP-enhanced 29Si solid-state NMR #DNPNMR

Kobayashi, T., et al., Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles: a study by conventional and DNP-enhanced 29Si solid-state NMR. Phys. Chem. Chem. Phys., 2017. 19(3): p. 1781-1789.

https://www.ncbi.nlm.nih.gov/pubmed/28058422

Solid-state NMR spectroscopy, both conventional and dynamic nuclear polarization (DNP)-enhanced, was employed to study the spatial distribution of organic functional groups attached to the surface of mesoporous silica nanoparticles via co-condensation and grafting. The most revealing information was provided by DNP-enhanced two-dimensional 29Si-29Si correlation measurements, which unambiguously showed that post-synthesis grafting leads to a more homogeneous dispersion of propyl and mercaptopropyl functionalities than co-condensation. During the anhydrous grafting process, the organosilane precursors do not self-condense and are unlikely to bond to the silica surface in close proximity (less than 4 A) due to the limited availability of suitably arranged hydroxyl groups.

Studying the Conformation of a Silaffin-Derived Pentalysine Peptide Embedded in Bioinspired Silica using Solution and Dynamic Nuclear Polarization Magic-Angle Spinning NMR

Geiger, Y., et al., Studying the Conformation of a Silaffin-Derived Pentalysine Peptide Embedded in Bioinspired Silica using Solution and Dynamic Nuclear Polarization Magic-Angle Spinning NMR. J Am Chem Soc, 2016. 138(17): p. 5561-7.

http://www.ncbi.nlm.nih.gov/pubmed/26451953

Smart materials are created in nature at interfaces between biomolecules and solid materials. The ability to probe the structure of functional peptides that engineer biogenic materials at this heterogeneous setting can be facilitated tremendously by use of DNP-enhanced solid-state NMR spectroscopy. This sensitive NMR technique allows simple and quick measurements, often without the need for isotope enrichment. Here, it is used to characterize a pentalysine peptide, derived from a diatom’s silaffin protein. The peptide accelerates the formation of bioinspired silica and gets embedded inside the material as it is formed. Two-dimensional DNP MAS NMR of the silica-bound peptide and solution NMR of the free peptide are used to derive its secondary structure in the two states and to pinpoint some subtle conformational changes that the peptide undergoes in order to adapt to the silica environment. In addition, interactions between abundant lysine residues and silica surface are identified, and proximity of other side chains to silica and to neighboring peptide molecules is discussed.

One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition

Rossini, A.J., et al., One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition. Chem. Sci., 2012. 3(1): p. 108-115.

http://dx.doi.org/10.1039/C1SC00550B

Dynamic nuclear polarization (DNP) 29Si solid-state NMR spectra of a hybrid mesoporous silica material impregnated with aqueous biradical solutions have been acquired with cross-polarization (CP) and cross-polarization Carr-Purcell Meiboom-Gill (CP/CPMG) pulse sequences. The integrated intensities (II) and signal to noise ratios (S/N) of the 29Si solid-state NMR spectra are monitored in order to measure the DNP enhancement factors (?Si[space]CP) as well as the overall sensitivity enhancement ([capital Sigma]Si[space]CP) available from the combination of DNP and CPMG acquisition. Here, , where [small theta]Si is a factor which quantifies reduction of the NMR signal by paramagnetic effects (quenching) and [small kappa] is the square root of the ratio of nuclear longitudinal relaxation times of the dry material and material impregnated with radical solution. It is found that [capital Sigma]Si[space]CP is always substantially lower than the measured value of ?Si[space]CP due to paramagnetic effects which reduce the II of the 29Si CP solid-state NMR spectra at high biradical concentrations. In this system, it is observed that the sample preparation which provides optimal DNP signal enhancement does not provide optimal overall signal enhancement. Notably, optimal signal enhancements are obtained for CPMG acquisition of the 29Si solid-state NMR spectra when lower radical concentrations are employed due to slower transverse relaxation rates. To the best of our knowledge this is the first study which seeks to quantify the overall sensitivity enhancements available from DNP solid-state NMR experiments.

Untangling the Condensation Network of Organosiloxanes on Nanoparticles using 2D (29)Si-(29)Si Solid-State NMR Enhanced by Dynamic Nuclear Polarization

Lee, D., et al., Untangling the Condensation Network of Organosiloxanes on Nanoparticles using 2D (29)Si-(29)Si Solid-State NMR Enhanced by Dynamic Nuclear Polarization. J Am Chem Soc, 2014. 136(39): p. 13781-8.

http://www.ncbi.nlm.nih.gov/pubmed/25153717

Silica (SiO2) nanoparticles (NPs) were functionalized by silanization to produce a surface covered with organosiloxanes. Information about the surface coverage and the nature, if any, of organosiloxane polymerization, whether parallel or perpendicular to the surface, is highly desired. To this extent, two-dimensional homonuclear (29)Si solid-state NMR could be employed. However, owing to the sensitivity limitations associated with the low natural abundance (4.7%) of (29)Si and the difficulty and expense of isotopic labeling here, this technique would usually be deemed impracticable. Nevertheless, we show that recent developments in the field of dynamic nuclear polarization under magic angle spinning (MAS-DNP) could be used to dramatically increase the sensitivity of the NMR experiments, resulting in a timesaving factor of approximately 625 compared to conventional solid-state NMR. This allowed the acquisition of previously infeasible data. Using both through-space and through-bond 2D (29)Si-(29)Si correlation experiments, it is shown that the required reaction conditions favor lateral polymerization and domain growth. Moreover, the natural abundance correlation experiments permitted the estimation of (2)J(Si-O-Si)-couplings (13.8 +/- 1.4 Hz for surface silica) and interatomic distances (3.04 +/- 0.08 A for surface silica) since complications associated with many-spin systems and also sensitivity were avoided. The work detailed herein not only demonstrates the possibility of using MAS-DNP to greatly facilitate the acquisition of 2D (29)Si-(29)Si correlation spectra but also shows that this technique can be used in a routine fashion to characterize surface grafting networks and gain structural constraints, which can be related to a system’s chemical and physical properties.

In vivo magnetic resonance imaging of hyperpolarized silicon particles

Cassidy, M.C., et al., In vivo magnetic resonance imaging of hyperpolarized silicon particles. Nat Nano, 2013. 8(5): p. 363-368.

http://dx.doi.org/10.1038/nnano.2013.65

Silicon-based micro- and nanoparticles have gained popularity in a wide range of biomedical applications due to their biocompatibility and biodegradability in vivo, as well as their flexible surface chemistry, which allows drug loading, functionalization and targeting. Here, we report direct in vivo imaging of hyperpolarized 29Si nuclei in silicon particles by magnetic resonance imaging. Natural physical properties of silicon provide surface electronic states for dynamic nuclear polarization, extremely long depolarization times, insensitivity to the in vivo environment or particle tumbling, and surfaces favourable for functionalization. Potential applications to gastrointestinal, intravascular and tumour perfusion imaging at subpicomolar concentrations are presented. These results demonstrate a new background-free imaging modality applicable to a range of inexpensive, readily available and biocompatible silicon particles.

In vivo magnetic resonance imaging of hyperpolarized silicon particles

Cassidy, M.C., et al., In vivo magnetic resonance imaging of hyperpolarized silicon particles. Nat Nano, 2013. 8(5): p. 363-368.

http://dx.doi.org/10.1038/nnano.2013.65

Silicon-based micro- and nanoparticles have gained popularity in a wide range of biomedical applications due to their biocompatibility and biodegradability in vivo, as well as their flexible surface chemistry, which allows drug loading, functionalization and targeting. Here, we report direct in vivo imaging of hyperpolarized 29Si nuclei in silicon particles by magnetic resonance imaging. Natural physical properties of silicon provide surface electronic states for dynamic nuclear polarization, extremely long depolarization times, insensitivity to the in vivo environment or particle tumbling, and surfaces favourable for functionalization. Potential applications to gastrointestinal, intravascular and tumour perfusion imaging at subpicomolar concentrations are presented. These results demonstrate a new background-free imaging modality applicable to a range of inexpensive, readily available and biocompatible silicon particles.

Molecular-level characterization of the structure and the surface chemistry of periodic mesoporous organosilicates using DNP-surface enhanced NMR spectroscopy

Gruning, W.R., et al., Molecular-level characterization of the structure and the surface chemistry of periodic mesoporous organosilicates using DNP-surface enhanced NMR spectroscopy. Phys Chem Chem Phys, 2013. 15(32): p. 13270-4.

http://www.ncbi.nlm.nih.gov/pubmed/23440402

We present the molecular level characterization of a phenylpyridine-based periodic mesoporous organosilicate and its post-functionalized organometallic derivatives through the fast acquisition of high quality natural isotopic abundance 1D (13)C, (15)N, and (29)Si and 2D (1)H-(13)C and (1)H-(29)Si solid-state NMR spectra enhanced with dynamic nuclear polarization.

Molecular-level characterization of the structure and the surface chemistry of periodic mesoporous organosilicates using DNP-surface enhanced NMR spectroscopy

Gruning, W.R., et al., Molecular-level characterization of the structure and the surface chemistry of periodic mesoporous organosilicates using DNP-surface enhanced NMR spectroscopy. Phys Chem Chem Phys, 2013. 15(32): p. 13270-4.

http://www.ncbi.nlm.nih.gov/pubmed/23440402

We present the molecular level characterization of a phenylpyridine-based periodic mesoporous organosilicate and its post-functionalized organometallic derivatives through the fast acquisition of high quality natural isotopic abundance 1D (13)C, (15)N, and (29)Si and 2D (1)H-(13)C and (1)H-(29)Si solid-state NMR spectra enhanced with dynamic nuclear polarization.

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