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

Abstract

International audienceDynamic nuclear polarization (DNP) (29)Si 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 (29)Si solid-state NMR spectra are monitored in order to measure the DNP enhancement factors (epsilon(Si) (CP)) as well as the overall sensitivity enhancement (Sigma(Si) (CP)) available from the combination of DNP and CPMG acquisition. Here, Sigma(Si) (CP) (epsilon(Si) (CP)) ((sic)(Si)) root kappa, where theta(Si) is a factor which quantifies reduction of the NMR signal by paramagnetic effects (quenching) and 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 Sigma(Si) (CP) is always substantially lower than the measured value of epsilon(Si) CP due to paramagnetic effects which reduce the II of the (29)Si 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 (29)Si 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

Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves

“…Conditions that produce the largest ratio of signals with and without microwaves may not be the same as conditions that maximize NMR sensitivity. Optimizing the NMR sensitivity with DNP has been discussed in several articles, 18 but in this article, we have presented an additional mechanism that affects the NMR signal not discussed previously.…”

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…Recently, an improvement in surface selectivity more spectacular than that observed with CP was shown using dynamic nuclear polarisation (DNP), where a 100-fold enhancement of the surface 29 Si signals was observed for a mesoporous silica. 148 This technique involves contacting the sample with a small amount of radical-containing solution, which is not chemically innocuous, and it is too early to tell what impact it will have on the field. 17 O NMR data are much less frequent than 29 Si NMR because the natural abundance of the target isotope is lower and makes it difficult to record spectra without isotopic enrichment.…”

Analysis of sensitivity enhancement by dynamic nuclear polarization in solid-state NMR: a case study of functionalized mesoporous materials

“…T 1r H relaxation of protons, and the performance of 13 C DNP-CP-MAS NMR in proline was studied by Lange et al 30 Rossini et al 29 quantified sensitivity enhancements of 29 Si DNP-CP-MAS NMR of passivated hybrid mesoporous silica. In addition to quenching and T 1 H relaxation, they studied the effect of TOTAPOL on T Here, we set out to systematically assess the contributions of various experimental factors to the global sensitivity enhancement in DNP-CP-MAS NMR of both 13 C and 29 Si nuclei.…”

“…One of the main strategies for increasing the sensitivity is to begin by exciting high-g spins and transferring their polarization to the observed low-g nuclei. This approach is commonly used in 1 H -X cross-polarization (CP), as a means of improving the sensitivity of hetero-nuclei (X = 13 C, 29 Si, 15 N, etc.) in solids.…”

“…[23][24][25][26] Several studies have addressed the quantification of the sensitivity enhancement in these experiments. 21,[26][27][28][29][30] The DNP enhancement factor is typically determined by comparing the spectra measured with mw irradiation ''on'' and ''off'', under the same conditions of static field B 0 and sample temperature T, using sufficiently long recycle delays so that relaxation effects can be neglected. In a study of DNP-enhanced 13 C CP-MAS NMR of a solvent-free peptide with covalently attached TOTAPOL 28 a more general enhancement factor was described which also accounts for effects of the radicals on the spin-lattice relaxation times T 1 H of the protons and on paramagnetic broadening ('quenching') of the signals.…”