Highly porous and monodisperse magnetic silica beads prepared by a green templating method† S ebastien Abramson, * a Cl  ementine Meiller, a Patricia Beaunier, b Vincent Dupuis, a Laurent Perrigaud, a Agn es B ee a and Val  erie Cabuil a Received 11th January 2010, Accepted 8th March 2010 First published as an Advance Article on the web 26th April 2010 DOI: 10.1039/c000525h We describe the preparation of magnetic silica nanocomposite millimetric beads using alginate as a green biopolymer template. The simple and soft method which is used here is particularly suitable since the alginate template allows a multiscale control of the structure of the material, both its morphological characteristics at the millimetric scale and its porosity at the nanometric level. These nanocomposites are characterised by a high monodispersity, a perfect spherical shape, a very large and multiscale porosity with pore diameters ranging from 2 nm to more than 50 nm, a homogeneous dispersion of the magnetic nanoparticles in the silica matrix and a high magnetic susceptibility which increases linearly with the volume fraction of the nanoparticles. These highly porous materials which can be used as magnetic adsorbents in water treatment, showed a good sorption capacity for methylene blue, chosen as a model dye. Introduction Biopolymers produced from renewable resources are increas- ingly used for the synthesis of hybrid inorganic/organic or purely inorganic materials since they are non-toxic, biodegradable and inexpensive, and thus comply with the requirements of so called ‘‘green chemistry’’. 1 Moreover they can induce an original organisation of the material, frequently at a multiscale level, in the same way as in natural processes occurring in nature, where natural biocomposites such as pearls, shells or bones, with a high degree of hierarchical organisation are synthesised by living organisms. 2 Natural polysaccharides such as alginate, chitosan or carra- geenan can be easily shaped as nanometric, 3,4 micrometric, 5,6 or millimetric 7–9 hydrophilic gel beads. This property has been exploited by several groups to synthesise highly monodispersed biocomposite beads where the biopolymer is not only responsible for the structuring of the material, but also induces interesting properties such as biocompatibility, biodegradability, adsorbing or catalytic capacity. Thus silica/alginate, 10–14 clay/alginate, 15 silica/chitosan, 16,17 silica/carrageenan, 18 titania/alginate, 19 calcium phosphate/alginate, 20 calcium carbonate/alginate, 21 or copper hydroxide/alginate 22 beads were recently prepared. These materials have great potential as heterogeneous catalysts, 17,19 as adsorbents, 21 or as matrix for encapsulation of living cells, 10,13,14 enzymes, 11,12,14 other proteins 14,20 or drugs 18,20,22 for biomedical or biotechnological applications. However, relatively few works have been devoted to the use of the biopolymer simply as a sacrificial template in order to prepare purely inorganic mon- odispersed beads, although this method presents several advan- tages. First, the syntheses are generally easy-to-perform, safe, and can be modulated to various inorganic phases. Secondly, since the elimination of the polysaccharide generally maintains the size and monodispersity of the beads in parallel with the apparition of a large porosity, highly structured materials are obtained, which may be used for example in cosmetics, 23 or in catalysis. 24 Inorganic beads such as boehmite, 23 silica, 16,24–26 alumina, 27 zinc or cadmium sulfide, 28 cobalt, copper or nickel oxide, 29 or cobalt 30 have been thus produced. All these materials are characterised by a single inorganic phase and the synthesis of composite inorganic beads has not been yet described. Magnetic nanocomposite silicas are usually made of magnetic nanoparticles such as cobalt, nickel, iron, ferrite, maghemite or magnetite dispersed in a silica matrix. They combine the main characteristics of silica—high chemical and thermal stability, optical transparency, low conductivity, high porosity, easy chemical derivatisation, good adsorbing properties—with magnetic properties provided by the nanoparticles. Conse- quently, they have been studied as catalysts, 31,32 adsorbents for water treatment, 33,34 or for analysis of biomolecules, 35,36 sensors, 37 magneto-optical 38 or electronic 39 devices, or drug delivery matrices. 40,41 These materials were also prepared to study the effect of confinement on the shape, size, mono- dispersity, thermal stability, aggregation state, and optical and magnetic properties of the nanoparticles. 42–45 Magnetic silicas have thus tremendous interest from both fundamental and applied points of view. In opposition to previous works that report the incorporation of magnetic nanoparticles in alginate, 46–48 chitosan, 49 or hybrid silica/alginate 50,51 beads, we describe here the preparation of purely inorganic nanocomposite millimetric beads made of magnetic nanoparticles and silica, using an alginate network as a Laboratoire de Physicochimie des Electrolytes, Collo€ ıdes et Sciences Analytiques - (PECSA-UMR 7195 UPMC-CNRS-ESPCI) UPMC – 4, place Jussieu, 75 252 Paris Cedex 5, France. E-mail: sebastien. abramson@upmc.fr; Tel: +33 1 44273174 b Laboratoire de R eactivit e de Surface (UMR 7197 UPMC-CNRS) UPMC – 4, place Jussieu, 75 252 Paris Cedex 5, France † Electronic supplementary information (ESI) available: Spectra and images of beads. See DOI: 10.1039/c000525h 4916 | J. Mater. Chem., 2010, 20, 4916–4924 This journal is ª The Royal Society of Chemistry 2010 PAPER www.rsc.org/materials | Journal of Materials Chemistry