Preparation of core–shell nanospheres of silica–silver: SiO 2 @Ag J.C. Flores a , V. Torres a , M. Popa a,b , D. Crespo a , J.M. Calderón-Moreno a,b, * a Department of Applied Physics, EPSC and Center for Research in Nanoengineering, Universitat Politècnica de Catalunya, Av. Canal Olimpic s/n, Castelldefels, 08860 Barcelona, Spain b Institute of Physical Chemistry, Academia Romana, Spl. Independentei 202, Bucharest, Romania article info Article history: Received 9 November 2007 Received in revised form 19 August 2008 Available online 22 October 2008 PACS: 82.70.Dd 78.67.Bf 81.07.Bc 81.07.-b 36.40.Mr Keywords: Colloids Nanocrystals Nanoparticles Absorption Silica abstract We prepared SiO 2 @Ag core–shell nanospheres: silver nanoparticles (4 ± 2 nm in diameter) coated silica nanospheres (50 ± 10 nm in diameter). The preparation route is a modification of the Stöber method, and involves the preparation of homogeneous silica spheres at room temperature, combined with the deposition of silver nanoparticles from Ag + in solution, by using water/ethanol mixtures, tetraethyl- orthosilicate as Si source and silver nitrate as Ag source in a single-pot wet chemical route without an added coupling agent or surface modification, which leads to the formation of core@shell homogeneous nanospheres. We present the preparation and characterization of the SiO 2 @Ag core–shell nanospheres and also of bare silica spheres in the absence of silver, and propose a reaction mechanism for the forma- tion of the core–shell structure. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Noble metals nanoparticles (NPs) have applications in many sci- entific and technological fields due to their attractive physical– chemical properties. The characteristic surface-plasmon [1] reso- nance of silver nanoparticles has been used prominently in diverse areas of biological [2,3] and bio-medical science, such as in molec- ular labeling [2], SERS, and nonlinear optics, for the fabrication of biosensors, labeling of cells and biomolecules, therapies against cancer and HIV-1 [2], and also for the fast detection of DNA macro- molecules and antibodies due to the change of the plasmon reso- nance. The plasmon peak wavelength depends on the size, form [4,5] and inter-particle coupling. On the other hand, core–shell nanoparticles represent one of the most interesting areas of mate- rial science because of their unique combined and tailored proper- ties for several applications. Silica is a good candidate to prepare core–shell nanoparticles. Liz-Marzan, Mulvaney and co-workers have prepared metal–silica core–shell particles by the use of a si- lane coupling agent to provide the metal surface with silanol an- chor groups [6]. They have shown that silica shells not only enhance the colloidal and chemical stability, but also control the distance between core particles within assemblies through shell thickness [7]. Kobayashi et al. have reported more recently silica coating of silver nanoparticles via a modified Stöber [8] method using dimethylamine [9]. Other groups have prepared silica-coated silver nanowires [10] simply through hydrolysis and condensation of tetraethyl-orthosilicate (TEOS) in ethanol. The direct coating of PVP-stabilized metal nanoparticles using TEOS has also demon- strated [11]. When cores are covered, it is possible to change their conductivity and optical properties [12]. Silica NPs emerge partic- ularly like a suitable matrix due to their surface functionality, thus allowing bio-conjugation to bio-activate molecules for objectives such as monitoring and marking. Recently, silver–silica core–shells have been obtained via a wet chemical modified Stöber route by using N-[3-(trimethoxysi- lyl)dropyl]ethylene as a coupling agent [13]. Gedanken and coworkers have readily synthesized different metallic nanoparti- cles and core–shells with encapsulated metal without using cou- pling agents, by employing intense ultrasound irradiation [14– 17]. Silica–metal core–shell structures with gold and silver nano- particle shells on silica and titania have also been obtained using a power ultrasound sonochemical process [18–21]. We have previ- ously used poly(vinylpyrrolidone) as both a reducing agent and a 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.09.014 * Corresponding author. Address: Institute of Physical Chemistry, Academia Romana, Spl. Independentei 202, Bucharest, Romania. E-mail addresses: jose.calderon@upc.edu, calderon@fa.upc.edu (J.M. Calderón- Moreno). Journal of Non-Crystalline Solids 354 (2008) 5435–5439 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol