Synthesis and characterization of new SiO 2 –CaO hollow nanospheres by sol–gel method: Bioactivity of the new system G.S. Pappas a , P. Liatsi b , I.A. Kartsonakis b , I. Danilidis b , G. Kordas a,b, * a Department of Materials Science, University of Patras, GR 26 500, Greece b Institute of Materials Science, NCSR Demokritos, Agia Paraskevi Attikis, GR 153 10, Greece Available online 19 November 2007 Abstract A new binary system of SiO 2 –CaO nanospheres has been successfully prepared by sol–gel method in W/O emulsion. Tetraethyl ortho- silicate (TEOS) and calcium nitrate tetrahydrate were used as silica and calcia source, respectively, and an ammonia solution was used as catalyst. The choice of the components of the oil and water phase, which stabilize the emulsion, is the key factor contributing to the formation of spherical particles. The surfactants, Span 80 and Tween 20, were used to control the shell thickness, the surface form, the size and form of the pores. The resulting nanospheres with diameter from 110 to 180 nm were obtained after calcination at 600 °C. Calcium bid fair to conduce in narrow size distribution. The in vitro bioactivity study was carried out by soaking in a simulated body fluid (SBF) at 37 °C. Formation of crystalline phase was observed on the surface of nanospheres after soaking in SBF. The prop- erties of all samples were determined by FT-IR, SEM and XRD techniques. Ó 2007 Elsevier B.V. All rights reserved. PACS: 91.67.Gj Keywords: Bioglass; Biomaterials; STEM/TEM; Nanoparticles 1. Introduction The hollow structure of nanospheres provides significant properties such as low density, large specific surface area, stability and surface penetrability. Encapsulation of vari- ous molecules with bioactivity is also a property that raises both scientific and industrial interest. In the last decade several works [1–6] have been focused on the preparation of nanospheres. Silica nanobottles [1], silica nanospheres with magnetic core and charged surface [2], fluorescent sil- ica nanospheres [3], polypyrrole-magnetite-silica particles [4], CdS–SiO 2 core-shell particles [5] and silica nanospheres encapsulating enzyme [6] are some remarkable results in this area. Significant progress has also been observed in sol–gel bioglasses, where complex systems have been devel- oped so as to improve bioglass properties [7–9]. In the last years there has been an attempt to decrease patients’ convalescence time and enforce bioglass functionality, by the encapsulation of bio-molecules in the sol–gel silica networks [10]. These characteristics promote bioglasses for clinical use. The system of SiO 2 –CaO nanospheres has never been studied. The purpose of this paper is to investigate the synthesis route of the binary system, the effect of Ca ++ presence and the bioactivity after immersing in SBF. 2. Experimental procedure This study is based on a published procedure from Park et al. [11], to synthesize nanospheres with hollow structure. A modification of the components and their compositions so as to achieve the addition of Ca ions in silica network and a smaller diameter of the spheres was necessary. 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.007 * Corresponding author. Address: Institute of Materials Science, NCSR Demokritos, Agia Paraskevi Attikis, GR 153 10, Greece. Tel.: +30 2106503301; fax: +30 2106547690. E-mail address: gkordas@ims.demokritos.gr (G. Kordas). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 354 (2008) 755–760