Frozen Acrylamide Gels as Dynamic Nuclear Polarization Matrices

Abstract

Abstract:We show that aqueous acrylamide gels can be used to provide dynamic nuclear polarization (DNP) NMR signal enhancements of around 200 at 9.4 T and 100 K. The enhancements are shown to increase with cross linker concentration and low concentrations of the AMUPol biradical. We show that this DNP matrix can be used in situations where conventional incipient wetness methods fail, such as to obtain DNP surface enhanced NMR spectra from inorganic nanoparticles. In particular, we obtain 113 Cd spectra from CdTe-COOH NPs in minutes. The spectra clearly indicate a highly-disordered cadmium rich surface.Dynamic nuclear polarization (DNP) is a rapidly expanding method that can provide an increase in solid-state NMR signal intensity by 2 orders of magnitude, [1] thereby enabling atomiclevel characterization of systems that were previously completely inaccessible. [1a, 1c, 2] DNP works by transferring polarization from unpaired electrons to nearby nuclei. This is enabled in diamagnetic samples by doping with a stable radical as a polarization source. Additionally, a medium is required to transfer polarization by spin diffusion from the source to the nuclei of interest and to distribute homogeneously the polarizing agents. This typically leads to formulations of frozen solutions of organic nitroxide based biradicals (TEKPol, [3] AMUPol, [4] TOTAPOL [5] …) in glass forming mixtures which can either be aqueous water/glycerol or water/DMSO, or a range of organic solvents from 1,1,2,2-tetrachloroethane [6] to ortho-terphenyl. [7] Substrates are either directly dissolved in the solution, [8] or for materials samples impregnated with the polarizing solution.[9] Formation of a glass has proven to be an essential requirement to avoid separation or precipitation effects upon freezing leading to poor DNP performance. [3,10] However, there is still today essentially only one water based formulation. The most popular DNP matrix is glycerol-d8/D2O/H2O in a ratio of (6/3/1 v/v) which has been empirically optimized to give the best enhancements (typically around 200 at 9.4 T and 100 K), and which is often referred to as "DNP Juice." Even the organic solvents, which have a broad range of properties and are compatible with many substrates, encounter problems in systems prone to aggregation, with nanoparticles being a prime example that have not so far been amenable to study in any ordinary solvents. [11] The importance of these limitations is demonstrated if we consider the work that has been done to find alternatives. De Paëpe and others have used so-called matrix-free DNP methods to characterize liposomes [12] and small proteins [13] or membrane proteins. [14] As well as in the case of in-vivo magnetic resonance imaging of silicon nanoparticles their surface composed of defectbound electrons do not require additional matrix. [15] To prevent aggregation of colloidal solutions of nanoparticles (NPs) at cryogenic temperatures, they have been dispersed in mesoporous silicas, [11] , whereas incipient wetness impregnation of NPs might...

Elucidating the Location of Cd2+ in Post-synthetically Treated InP Quantum Dots Using Dynamic Nuclear Polarization 31P and 113Cd Solid-State NMR Spectroscopy

“…The structural model put forwards on the basis of the sold-state NMR experiments is consistent with previous characterization of Cd-InP QDs by cadmium EXAFS which demonstrated Cd-O and Cd-P coordination 10. More generally this study highlights the utility of DNP SENS for the characterization of QDs and other nanomaterials [19][20][21][22][23][24][25][26][27][28][29][46][47][48][49][50][51][52].…”

“…The sensitivity of solid-state NMR experiments on QD and other nanomaterials can be improved by using DNP surface enhanced NMR spectroscopy (SENS). [19][20][21][22][23][24][25][26][27][28][29] To prepare QD samples for a DNP SENS experiment a colloidal solution of QD is dispersed on a support material, either mesoporous silica 24 or hexagonal boron nitride (h-BN) 27 and then a PA solution is added. Alternatively, to further increase NMR sensitivity the QD can be precipitated from solution, physically mixed with h-BN, then impregnated with a minimal volume of PA solution.…”

Revealing the Surface Structure of CdSe Nanocrystals by Dynamic Nuclear Polarization-Enhanced 77Se and 113Cd Solid-State NMR Spectroscopy

“…60,61 Currently, MAS DNP experiments on semiconductor NCs are performed by using inert support material, such as mesoporous silica, gels, or hexagonal boron nitride (h-BN), to disperse the NCs or minimize the size of the NC aggregates and maintain the proximity between the NC and the DNP polarizing agents after freezing the sample. 56,57,59,62,63 In this way, DNP enables many solid-state NMR experiments on semiconductor NCs such as HETCOR experiments with challenging natural isotopic abundance combinations ( 13 C-111 Cd, 13 C- 29 Si, and 31 P-113 Cd), 113 Cd anisotropic chemical shift -isotropic chemical shift correlations ( 113 Cd-113 Cd), and through-bond and through-space homonuclear double-quantum singlequantum (DQ-SQ) correlation experiments ( 29 Si, 31 P or 113 Cd). 56,57,59,62 Notably, DNPenhanced surface-selective 111 Cd and 113 Cd SSNMR experiments have been performed on carboxylate capped CdS or CdSe NCs.…”