Cerium Oxide Nanoparticles with Entrapped Gadolinium for High T 1 Relaxivity and ROS-Scavenging Purposes

Abstract

Gadolinium chelates are employed worldwide today as clinical contrast agents for magnetic resonance imaging. Until now, the commonly used linear contrast agents based on the rare-earth element gadolinium have been considered safe and well-tolerated. Recently, concerns regarding this type of contrast agent have been reported, which is why there is an urgent need to develop the next generation of stable contrast agents with enhanced spin-lattice relaxation, as measured by improved T ₁ relaxivity at lower doses. Here, we show that by the integration of gadolinium ions in cerium oxide nanoparticles, a stable crystalline 5 nm sized nanoparticulate system with a homogeneous gadolinium ion distribution is obtained. These cerium oxide nanoparticles with entrapped gadolinium deliver strong T ₁ relaxivity per gadolinium ion (T ₁ relaxivity, r ₁ = 12.0 mM^-1 s^-1) with the potential to act as scavengers of reactive oxygen species (ROS). The presence of Ce³⁺ sites and oxygen vacancies at the surface plays a critical role in providing the antioxidant properties. The characterization of radial distribution of Ce³⁺ and Ce⁴⁺ oxidation states indicated a higher concentration of Ce³⁺ at the nanoparticle surfaces. Additionally, we investigated the ROS-scavenging capabilities of pure gadolinium-containing cerium oxide nanoparticles by bioluminescent imaging in vivo, where inhibitory effects on ROS activity are shown.

Figure 1

(a) X-ray diffraction patterns of CeO_x, CeO_x:Gd5%, CeO_x:Gd9%, CeO_x:Gd14%, and CeO_x:Gd19%. The calculated grain sizes as determined by the Scherrer equation are given. (b) Raman spectra for CeO_2, Gd₂O_3, CeO_x, CeO_x:Gd5%, CeO_x:Gd9%, CeO_x:Gd14%, and CeO_x:Gd19%. The Raman spectra are normalized and vertically shifted using a small offset for each spectrum to facilitate line shape comparison. There is an increasing shift of the main Raman peak to lower wavenumbers related to the integration of Gd.

Figure 2

(a–c) High-resolution HAADF-STEM images of the CeO_x, CeO_x:Gd9%, and CeO_x:Gd19% particles, respectively. (d) Vertically shifted core-loss EEL spectra (top) for the same samples, with emphasis on baseline aligned the O-K (bottom left) and vertically shifted Ce-M (bottom right) edges.

Figure 3

Inverse of relaxation times (a) 1/T₁ and (b) 1/T₂ are shown as a function of the concentration of gadolinium for the prepared Gd-CeNPs. The slope of the fitted linear equations denoted as the r₁ (a) and r₂ (b) relaxivities are given within the brackets.

Figure 4

Absorbance spectra and inserted images of CeO_x:Gd9% untreated and treated with 10 μM H₂O₂ on (a) day 0, (b) day 10, and (c) on day 10, another 10 μM H₂O₂ was added to the sample. (d) Plotted red shifts CeO_x:Gd0–19% ([Ce] = 10 μg mL^–1) treated with increasing concentration of H₂O₂. (e) Hydrodynamic diameter of CeO_x treated with H₂O₂ (number-weighted distributions). The photos in panels (a)–(c) courtesy of the first author, P. Eriksson. Copyright 2020.

Figure 5

XPS spectra of (a) CeO_x:Gd9% scan 1, (b) CeO_x:Gd9% treated with 1 μM H₂O₂ scan 1, (c) CeO_x:Gd9% scan 8, and (d) CeO_x:Gd9% treated with 1 μM H₂O₂ scan 1.

Figure 6

T₁-weighted MR images of CeO_x:Gd9% nanoparticles as a function of Gd concentration: (a) schematic image side view of measured tubes, (b) top view, and (c) measured MR signal (T₁-weighted) from the cylindrical volume of 3 mm height and 5 mm diameter.

Figure 7

Antioxidant behavior of CeO_x and CeO_x:Gd5% nanoparticles in immunocompetent mice. (a) Schematic of subcutaneous injection on the dorsal side of a mouse showing positions of different particle formulations including a mixture of CeO_x with PS (hexagonal), a mixture of CeO_x:Gd5% with PS (circle), and PS microparticles only (square) as a control. (b) Bioluminescent image of the ROS signal from a representative mouse on day 3, following subcutaneous injection of particles. (c) Quantification of bioluminescent signals from injection sites over a 5 day period. Error bars are standard of the mean (n = 12 injections on 6 mice with 2 replicates per mouse). *, ^**, ^***, ^**** denote p ≤ 0.05, 0.01, 0.001, and 0.0001, respectively. “ns” denotes nonsignificance, which has p > 0.05.