HfO 2 :Eu nanoparticles excited by X-rays and UV-visible radiation used in biological imaging * Jaroslaw Kaszewski a, * , Jaroslaw Olszewski b, c , Julita Rosowska a , Bartlomiej Witkowski a , Lukasz Wachnicki a , Karolina Wenelska d , Ewa Mijowska d , Zdzislaw Gajewski b , Marek Godlewski a , Michal M. Godlewski b, c a Institute of Physics, Polish Academy of Sciences, Al. Lotnik ow 32/46, 02-668 Warsaw, Poland b Veterinary Research Centre, Department of Large Animal Diseases with Clinic, Faculty of Veterinary Medicine, Warsaw University of Life Sciences e SGGW, Nowoursynowska 100, 02-797 Warsaw, Poland c Department of Physiological Sciences, Faculty of Veterinary Medicine, Warsaw University of Life Sciences e SGGW, Nowoursynowska 159, 02-776 Warsaw, Poland d Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology in Szczecin, al. Piast ow 45, 70-311 Szczecin, Poland article info Article history: Received 31 August 2018 Received in revised form 8 April 2019 Accepted 10 April 2019 Available online xxx Keywords: HfO 2 Europium Scintillators Hydrothermal Nanoparticles Rare earths abstract Europium doped hafnium dioxide nanoparticles were crystallized using biologically friendly microwave hydrothermal method and employed in observation of small rodents tissues. Main solvent in synthesis is water, in consequence nanoparticles are found surrounded by thin hydroxide layer. Size of the nano- particles is controlled by temperature of calcination. 1200  C heating causes drop of luminescence in- tensity, explained as damage of high symmetry surface emission centers in the nanocrystals. On the other hand, growth of crystallites increases intensity of X-ray and electron beam excited luminescence. This makes them promising activator in photodynamic therapy. © 2019 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths. 1. Introduction Inorganic nanoparticles attract attention of many biology ori- ented researchers. 1 The applications such as imaging, trafficking, drug supplying very fast are gathering many followers. 2,3 Firstly, semiconductor quantum dots are used for above mentioned pur- poses as they exhibit high luminescence yields faciliating tracking in the biological system. However, their biocompatibility is still under debate as they can, and usually contain, class A toxic ele- ments such as Pb, Cd and Hg. 4 Considering that, biological appli- cation of luminescent nanoparticles turned to rare-earth doped materials. 5 Lanthanide ions embed in biologically inert inorganic matrix offer wide range of properties promising in medical diag- nosis and treatment. Luminescent and magnetic properties are possible to be controlled freely without toxic interaction with examined biological system. 6 Lanthanide ions are characterized by 4f electronic shell, which is screened from the surroundings. 7 This prevents its properties to change depending on the chemical environment. It is important because luminescent properties of the lanthanides origin mostly from the electronic transitions within 4f manifold. 8 This mechanism of emission results in the narrow emission lines, which are very well distinguished from the fluo- rophores often used in biology. As bioinert matrixes oxides are used including: titanium oxide, zirconium oxide, hafnium oxide, yttrium oxide, aluminum oxide and other oxides of d-block and p-block metals. Hafnium oxide is especially interesting, it was used e.g., for examination and treat- ment of dental biofilms in rats. 9 Hafnium oxide is wide band gap dielectric (E g z 5.7 eV, t z 30). It crystallizes in few polymorphic forms including atmospheric pressure stable monoclinic (space group P2 1 /c), tetragonal (P4 2 /nmc) and cubic (Fm3m) phases. After increasing pressure orthorhombic polymorphs appear: I (Pbca) and II (Pnma). 10 High temperature t and c forms may be stabilized to reach metastability at room temperature (RT). It can be achieved by doping HfO 2 with trivalent ions (commonly with yttrium 11 ) or by * Foundation item: Project supported by National Science Centre (2012/05/E/ NZ4/02994 and 2012/06/A/ST7/00398). * Corresponding author. Fax: (þ48) 22116 35 07. E-mail address: kaszewski@ifpan.edu.pl (J. Kaszewski). Contents lists available at ScienceDirect Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths https://doi.org/10.1016/j.jre.2019.04.003 1002-0721/© 2019 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths. Journal of Rare Earths xxx (xxxx) xxx Please cite this article as: Kaszewski J et al., HfO 2 :Eu nanoparticles excited by X-rays and UV-visible radiation used in biological imaging, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.04.003