A novel approach to magneto-responsive polymeric gels assisted by iron nanoparticles as nano cross-linkersw Miklo´s Czaun, ab La´szlo´ Hevesi,* b Makoto Takafuji a and Hirotaka Ihara* a Received (in Cambridge, UK) 15th November 2007, Accepted 25th February 2008 First published as an Advance Article on the web DOI: 10.1039/b717721f A new route for the preparation of magneto-responsive poly- meric gels involving iron nanoparticles as nano cross-linkers has been described. Although other stimuli-responsive polymer gels such as tem- perature-, pH- or analyte-sensitive gels have received much attention, 1 preparation of magneto-responsive gels remains a challenging field. Magnetic gels (ferrogels) are a new class of soft polymer materials exhibiting properties controlled by magnetic fields. Due to their shape and size distortion ability in a magnetic field, these magnetic stimuli-responsive polymer networks are of interest for artificial muscles, actuators and micromanipulators. 2 Superparamagnetic gels (e.g. 10 nm sized ferrit nanocrystals in a polymer network) are potential candi- dates as agents for electromagnetic hyperthermia that is a recently developed method for cancer therapy. 3 Furthermore, the magneto-elastic properties of magnetic gels can be applied to the manufacture of sensors, switches, various separations, membranes and drug delivery systems. 4 Magnetic gels which have been fabricated by in situ pre- cipitation of different iron-containing particles so far do not involve chemical bonds between the magnetic particles and the cross-linked polymer matrix. 2–7 The question arises whether the termination of propagating radicals situated on different magnetic particles allows the formation of a polymer network? In other words, whether a covalently cross-linked polymeric gel can be synthesized without using conventional cross-lin- kers? With this problem in mind we attempted to prepare a covalently cross-linked polymer network in which magnetic nanoparticles act as cross-linking points. Iron nanoparticles were chosen as model magnetic particles because their magne- tization is higher than those measured for different iron-oxide species. 8 Generally, there are two approaches to chemically attaching polymer chains to the surface of inorganic particles: (1) the ‘‘grafting to’’ method; 9 end-functionalized polymer chains react with surface groups and (2) the ‘‘grafting from’’ meth- od 10 in which polymer chains grow in situ from initiator molecules that have been pregrafted to the surface of inorganic particles. Among various controlled radical polymerization methods (e.g. NMRP, RAFT, LAP) increasing attention has been placed on atom transfer radical polymerization (ATRP) because of its high tolerance to most functional groups. 11 Herein, we demonstrate the first surface-initiated ATRP from iron nanoparticles as a novel approach to prepare magneto-responsive gels. To fabricate highly dense polymer- grafted iron nanoparticles we divided the process into three steps (Scheme 1): (1) synthesis of nano-sized iron particles (Fe np ), (2) immobilization of ATRP initiator onto Fe np and (3) conducting polymerization from initiator-grafted Fe np . Iron nanoparticles were synthesized according to a modified litera- ture methodw by adding sodium borohydride to an aqueous solution of iron(III) chloride. 12 Radical polymerization initiator [11-(2-bromo-2-methyl)- propionyloxy]undecyltrichlorosilane (1) was synthesized 13 and grafted onto Fe np by the reaction between the surface accessible OH groups on the Fe np and the trichlorosilyl anchoring group of 1. Since initiator 1 was applied in excess to obtain the highest surface coverage as possible, the non- immobilized and physically absorbed initiator molecules had to be removed entirely because ‘‘free’’ initiators may cause polymerization in solution. To avoid this undesirable side reaction initiator-grafted iron nanoparticles (Fe np -1) were repeatedly washed with different organic solvents and water.z The amount of surface-attached initiator was estimated on the basis of thermogravimetric analysis (2.91 nm 2 initiator 1 ).w ATRP processes were carried out using Fe np -1 suspended in a mixture of styrene and toluene in the presence of CuBr and 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA) as cata- lyst precursors giving a magneto-responsive gel.y Both the shape and the length of the Fe np -PS gel could be influenced by a permanent magnet as shown in Fig. 1a. Although the Scheme 1 Synthetic steps for preparation of PS-grafted Fe np . a Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan. E-mail: ihara@kumamoto-u.ac.jp; Fax: +81 96 342 3662; Tel: +81 96 342 3661 b Laboratoire de Chimie des Mate ´riaux Organiques, Faculte ´s Universitaires Notre-Dame de la Paix, 61 Rue de Bruxelles, Namur 5000, Belgium. E-mail: laszlo.hevesi@fundp.ac.be; Fax: +32 81 724 538; Tel: +32 81 724 530 w Electronic supplementary information (ESI) available: Preparation of Fe np and thermogravimetric analysis of Fe np -1. See DOI: 10.1039/ b717721f This journal is  c The Royal Society of Chemistry 2008 Chem. Commun., 2008, 1–3 | 1 COMMUNICATION www.rsc.org/chemcomm | ChemComm