Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy

Abstract

Many of the functions and applications of advanced materials result from their interfacial structures and properties. However, the difficulty in characterizing the surface structure of these materials at an atomic level can often slow their further development. Solid-state NMR can probe surface structure and complement established surface science techniques, but its low sensitivity often limits its application. Many materials have low surface areas and/or low concentrations of active/surface sites. Dynamic nuclear polarization (DNP) is one intriguing method to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude. In a DNP experiment, the large polarization of unpaired electrons is transferred to surrounding nuclei, which provides a maximum theoretical DNP enhancement of ∼658 for (1)H NMR. In this Account, we discuss the application of DNP to enhance surface NMR signals, an approach known as DNP surface enhanced NMR spectroscopy (DNP SENS). Enabling DNP for these systems requires bringing an exogeneous radical solution into contact with surfaces without diluting the sample. We proposed the incipient wetness impregnation technique (IWI), a well-known method in materials science, to impregnate porous and particulate materials with just enough radical containing solution to fill the porous volume. IWI offers several advantages: it is extremely simple, provides a uniform wetting of the surface, and does not increase the sample volume or substantially reduce the concentration of the sample. This Account describes the basic principles behind DNP SENS through results obtained for mesoporous and nanoparticulate samples impregnated with radical solutions. We also discuss the quantification of the overall sensitivity enhancements obtained with DNP SENS and compare that with ordinary room temperature NMR spectroscopy. We then review the development of radicals and solvents that give the best possible enhancements today. With the best polarizing mixtures, DNP SENS enhances sensitivity by a factor of up to 100, which decreases acquisition time by five orders of magnitude. Such enhancement enables the detailed and expedient atomic level characterization of the surfaces of complex materials at natural isotopic abundance and opens new avenues for NMR. To illustrate these improvements, we describe the successful application of DNP SENS to characterize hybrid materials, organometallic surface species, and metal-organic frameworks.

Molecular Level Characterization of the Structure and Interactions in Peptide‐Functionalized Metal–Organic Frameworks

“…To avoid important CO2 adsorption in the MOF as observed in the study of Yang et al, 27 side channels that are not experimentally accessible were blocked using rare gas atoms. The Monte-Carlo simulations were done in the Grand Canonical ensemble 28 at 300 K with CO2 fugacity of 1,3,5,10,13,15,20,23,25,30,33,35,40,43, and 45 bar. For each simulation at a given CO2 fugacity, an equilibration run of one million steps took place, followed by five million steps production run.…”

“…In a second step, the natural abundance 15 N NMR spectra of grafted In-MIL-68-NH-Pro and In-MIL-68-NH-Gly-Pro were collected by DNP SENS. The low detection limit provided by the DNP NMR technique 40 makes it well suited to study host-guest type interactions in solids, 41 including post-synthetically functionalized MOFs. 42 DFT-calculated 15 N chemical shifts of selected MIL-68-NH-Pro and MIL-68-NH-Gly-Pro models were compared with 15 N NMR signals obtained from the DNP SENS experiments.…”

Ultra-Low Temperature Nuclear Magnetic Resonance

“…This hyperpolarization of nuclei, known as dynamic nuclear polarization (DNP), has only recently started to be routinely exploited [3][4][5][6][7]. This is due to the technological breakthroughs resulting from the intra-MIT collaboration and perseverance of Griffin, Temkin, and coworkers [8,9], as well as the subsequent commercialization of the necessary hardware [10].…”

“…This is due to the technological breakthroughs resulting from the intra-MIT collaboration and perseverance of Griffin, Temkin, and coworkers [8,9], as well as the subsequent commercialization of the necessary hardware [10]. Nowadays, high-resolution spectra, recorded under magic angle sample spinning (MAS) conditions, can be acquired in a fraction of the previously required experimental time for a large variety of systems ranging from biomolecules [4,5] to functional materials [6,7]. The boundaries of the technique still strive to be pushed further.…”

Time domain DNP with the NOVEL sequence

“…12 Initially, the primary application of DNP was preparation of polarized targets for neutron scattering experiments; 13 however, over the past decade, DNP has been used extensively to enhance the inherently low sensitivity of nuclear magnetic resonance (NMR) signals. [14][15][16][17][18][19] Enhancements on the order of 10 2 − 10 3 were made possible via the solid effect using narrow-line radicals 20-23 and cross effect using biradicals 24-27 as polarizing agents and high frequency gyrotrons as microwave sources. [28][29][30] The latter operate in the 140-560 GHz regime and enable DNP to be performed at magnet field strengths used in contemporary NMR experiments (5-20 T).…”