Shell versus Core Dy³⁺ Contributions to NMR Water Relaxation in Sodium Lanthanide Fluoride Core–Shell Nanoparticles. An Investigation Using O-17 and H-1 NMR

Lanthanide nanoparticles (NPs) are useful as paramagnetic contrast agents in MRI, generating contrast through enhanced relaxation of signals from water protons in their vicinity. The overall relaxation depends on the proximity of the water, so that lanthanides at the NP surface would be expected to make a contribution greater than those buried in the core. NPs are usually coated with an organic layer to attain colloidal stability, which will influence the access of water to the lanthanide constituents. Here, we interrogated the relative contributions of NP core versus surface lanthanides to overall water relaxation using a series of core–shell lanthanide fluoride NPs (typical core dimension of 15.5–19.5 nm, and typical shell dimension of 0.5 nm) wherein the core and/or shell regions contained either a contrast-active lanthanide (paramagnetic; dysprosium, Dy 3+ ) or a contrast-inactive lanthanide (diamagnetic; yttrium, Y 3+ ), variously designated Dy–Dy, Dy–Y, Y–Dy, or Y–Y core–shell NPs. The organic coating in each case consisted of a monolayer of oleate (OA) bound to the positively charged nanoparticle surface, intercalated with an outer layer of poly(maleic anhydride- alt -1-octadecene)–polyethylene glycol (PMAO–PEG). Paramagnetic influences were assessed via changes in water 17 O chemical shift and 1 H T 1 relaxivity. 17 O chemical shift changes were minimal, even for Dy–Dy and Y–Dy NPs, consistent with a limited access of water to the NP surface, likely due to occlusion by the OA portion of the organic coating. Nevertheless, water 1 H T 1 relaxivity was enhanced by a factor of 40 for Dy–Dy relative to Y–Y NPs, reflecting the predominant through-space 1 H dipolar relaxation mechanism. Both Y–Dy and Dy–Y core–shell NPs exhibited water 1 H T 1 relaxivity intermediate to that of Dy–Dy and Y–Y core–shell NPs. Modeling of the 1/ r 6 dependence of the dipolar coupling between lanthanide and water indicates that in the presence of the OA/PMAO–PEG organic coating only the outermost layer of NP lanthanides contributes to the relaxation. This suggest that for the Y–Dy and Dy–Y core–shell NPs the as-prepared shells may not be of homogeneous composition and/or thickness.

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