Magnetochemistry 2022, 8, 22. https://doi.org/10.3390/magnetochemistry8020022 www.mdpi.com/journal/magnetochemistry Article Structural and Optical Characterization of Silica Nanospheres Embedded with Monodisperse CeO2‐Eu 3+ Nanocrystals Corina Secu 1 , Cristina Bartha 1 , Elena Matei 1 , Cristian Radu 1,2 and Mihail Secu 1, * 1 National Institute of Materials Physics, Atomistilor 405A, 077125 Magurele, Romania; cesecu@infim.ro (C.S.); cristina.bartha@infim.ro (C.B.); elena.matei@infim.ro (E.M.); cristian.radu@infim.ro (C.R.) 2 Faculty of Physics, University of Bucharest, Atomistilor 405, 077125 Magurele, Romania * Correspondence: msecu@infim.ro Abstract: Luminescent nanocrystals embedded into silica microspheres were shown to be useful for silica labeling for biological applications, ensuring mechanical and chemical stability, nontoxicity, biocompatibility and optical properties. We used sol–gel technology to prepare silica nanospheres embedded with fluorescent and magnetic Eu 3+ (1 mol%)‐doped CeO2 nanocrystals. The X‐ray dif‐ fraction pattern analysis and transmission electron microscopy investigations showed CeO2:Eu 3+ (1 mol%) nanocrystals of about 9 nm size and Ce 3+ ions substitution by the Eu 3+ ions; the nanocrystals dispersed inside the nanosized silica spheres of about 400 nm diameters. The photoluminescence spectra recorded under UV‐light excitation showed Eu 3+ ions luminescence peaks ( 5 D0‐ 7 FJ, J = 0–4) accompanied by a weaker 425 nm luminescence due to the silica matrix; the quantum yield was 0.14. The weak hysteresis loop and magnetization curves recorded up to 20,000 Oe showed domi‐ nantly paramagnetic behavior associated with the silica matrix; a slight opening of the hysteresis loop to a very small magnetic field (about 0.005 Oe) was due to the presence of the two rare earth ions. The photonic crystal properties of SiO2‐CeO2:Eu 3+ (1 mol%) silica nanospheres deposited as films on quartz plates were revealed by the two weak attenuation peaks at 420 and 500 nm and were associated with the reflection from different planes. The SiO2‐CeO2:Eu 3+ (1 mol%) nanospheres are attractive potential candidates for photonics‐related applications or for multifunctional bio‐labels by combining the luminescence and magnetic properties of the nanocrystals. Keywords: CeO2; europium; silica spheres; nanocrystals; electron microscopy; luminescence 1. Introduction Rare‐earth‐doped nanomaterials are widely studied due to characteristics related to the presence of unpaired electrons in the inner 4f sub‐shell: fluorescence, downconver‐ sion, upconversion, persistent phosphorescene and also magnetic properties, with appli‐ cations spanning from laser optoelectronics, optical amplifiers, lasers, up‐converters to biosensing, biology carrier applications and multifunctional imaging ([1,2] and references therein). In order to maintain their functionalities for practical applications, it is necessary to isolate them with a thin SiO2 layer around the nanocrystalline core and that results in a core–shell composite. The coating of the nanocrystals with a thin SiO2 shell provides sev‐ eral advantages such as: protection against oxidation and agglomeration, increase in the mechanical stability, and enabling dispersion in various aqueous solvents. Furthermore, the surface of silica particles can be chemically modified to link bioconjugates. As the silica can be easily made controllable in spherical morphology from nano‐to micrometer size [3], a novel approach is to embed the nano‐phosphors in a transparent inert silica matrix [4–7]. Fluorescent microspheres can be produced by the incorporation of luminescent nanocrystals into the bigger silica microspheres and have been demonstrated to be useful for silica labeling in a variety of biological or phosphor related applications [4–6]. What‐ Citation: Secu, C.; Bartha, C.; Matei, E.; Radu, C.; Secu, M. Structural and Optical Characterization of Silica Nanospheres Embedded with Monodisperse CeO2‐Eu 3+ Nanocrystals. Magnetochemistry 2022, 8, 22. https://doi.org/10.3390/ magnetochemistry8020022 Academic Editor: Fabrice Pointillart Received: 14 January 2022 Accepted: 1 February 2022 Published: 4 February 2022 Publisher’s Note: MDPI stays neu‐ tral with regard to jurisdictional claims in published maps and institu‐ tional affiliations. Copyright: © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/).