Hydroxyapatites: Key Structural Questions and Answers from Dynamic Nuclear Polarization

Abstract

We demonstrate that NMR/DNP (Dynamic Nuclear Polarization) allows an unprecedented description of carbonate substituted hydroxyapatite (CHAp). Key structural questions related to order/disorder and clustering of carbonates are tackled using distance sensitive DNP experiments using C-C recoupling. Such experiments are easily implemented due to unprecedented DNP gain (orders of magnitude). DNP is efficiently mediated by quasi one-dimensional spin diffusion through the hydroxyl columns present in the CHAp structure (thought of as "highways" for spin diffusion). For spherical nanoparticles and ϕ < 100 nm, it is numerically shown that spin diffusion allows their study as a whole. Most importantly, we demonstrate also that the DNP study at 100 K leads to data which are comparable to data obtained at room temperature (in terms of spin dynamics and line shape resolution). Finally, all 2D DNP experiments can be interpreted in terms of domains exhibiting well identified types of substitution: local order and carbonate clustering are clearly favored.

Recent developments in MAS DNP-NMR of materials

“…For instance, 1 H spin diffusion has been employed in microcrystalline solids, including pharmaceuticals, to transport DNP-enhanced polarization from the surface into the interior of the crystals [65,101,103,104]. 1 H spin diffusion transport has also been applied to non-porous inorganic solids, such as magnesium and calcium hydroxide [105], hydroxyapatite [106,107], ammonia borane [108] or proton conductors [109]. It has also been employed to study porous materials, including mesoporous silica [102,110], Metal-Organic Frameworks (MOFs) [111][112][113] and zeolites [114][115][116][117], for which the PAs are too large to diffuse into the pores.…”

“…DNP has been applied to characterize the surface of nanoparticles used for catalysis [124,184,192,238,268], biomaterials [106,107], cements [269], polymer fillers [85,210] and optoelectronics devices [158,159]. The investigated nanoparticles included functionalized silica [193,227,238,270], alumina [80,124,184,196,225,231,[271][272][273], silica alumina [192,233,234,274], ceria [195,275], sulfated zirconia [276], calcium silicate hydrates [269], phosphates [106,107,210], partially oxidized Sn nanoparticles [151] and crystalline semiconductors, such as CdSe, CdS or InP, in the form of nanoparticles (also called quantum dots) [158,159]. DNP has also been applied to investigate calcium silicate hydrates [269] and crystalline semiconductors, such as CdSe and CdS, in the form of nanosheets (also called nanoplatelets) (see Figure 24) [159].…”

Structural description of surfaces and interfaces in biominerals by DNP SENS

“…Bulk hydroxyl ions in the (radical free) bone mineral particles (thickness  4 nm) here are expected to be polarized by relayed DNP by proton spin diffusion. 22,44,45 Extracting the 31 P slices from the 2D experiment (Fig. 3c) allows the comparison of the corresponding phosphates resonances in the three detected environments (amorphous surface, bulk lattice, CO3 2--containing bulk lattice) and highlights their differences in terms of structural environment: (i) HPO4 2in the amorphous layer displays a broad and slightly asymmetric line shape characteristic of their disordered environment (( 31 P) = 3 ppm; FWHM = 5.3 ppm); (ii) PO4 3in the apatitic bulk are slightly upfield shifted (( 31 P) = 2.9 ppm) and exhibit thinner line width (FWHM= 2.7 ppm); (iii) finally, PO4 3close to B-type CO3 2exhibit a broader line width (LW= 3.3 ppm) due to a local disorder induced by the ionic substitution.…”

“…20 In parallel, synthetic hydroxyapatite nanoparticles were studied by DNP enhanced 43 Ca and 13 C NMR experiments, providing new insights into the Ca 2+ surface species 21 and the structural organization of bulk CO3 2substituted ions. 22 In this communication we investigate the contribution of DNP SENS to the comprehension of surfaces and organo-mineral interfaces in natural biominerals through the study of two key examples belonging to the calcium phosphate and calcium carbonate families: bone apatite and aragonite from nacreous shells (Fig. 1).…”

Large-scale ab initio simulations of MAS DNP enhancements using a Monte Carlo optimization strategy

“…The role of spin diffusion is particularly important when microcrystalline or amorphous bulk solids are studied, as the radicals are unable to penetrate the sample and directly polarize the nuclei in its core. 60,61 Since there is an obvious interest in applying DNP to the studies of glasses, semiconductor nanoparticles, 62 narrow-pore materials (such as metal-organic frameworks, MOFs), [63][64][65][66][67] pharmaceuticals, 11,[68][69][70] biological materials, [71][72][73] and other non-penetrable solids, 60 which often have very long relaxation times that allow for the storage of hyperpolarization, 74 it is important to address the impact that fast-MAS will have on this field.…”