Published: July 26, 2011 r2011 American Chemical Society 10484 dx.doi.org/10.1021/la201723a | Langmuir 2011, 27, 10484–10491 ARTICLE pubs.acs.org/Langmuir Synthesis of Thermosensitive Microgels with a Tunable Magnetic Core Marco Laurenti, † Pablo Guardia, ‡ Rafael Contreras-C  aceres, § Jorge P  erez-Juste, || Antonio Fernandez-Barbero, § Enrique Lopez-Cabarcos, † and Jorge Rubio-Retama* ,† † Departamento de Química-Física II, Facultad de Farmacia, Universidad Complutense de Madrid, Spain ‡ Departamento de Física Fundamental e Instituto de Nanociencia y Nanotecnología, Universidad de Barcelona, Spain § Departamento de Física Aplicada, Universidad de Almería, Spain ) Departamento de Química-Física y Unidad Asociada CSIC, Universidad de Vigo, Spain ’ INTRODUCTION Since Pelton and Chivante’s 1 first work, the preparation and characterization of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microgels produced via surfactant-free emulsion po- lymerization (SFEP) have attracted extensive interest due to their potential applications in many fields, such as templates for nanoparticles, 2 contrast agents, 4 sensors, 3 catalyst support, 5,6 or biomedical applications. 79 In many of these works, the thermosensitive microgels are combined with inorganic components such as quantum dots, 10 silver, 11 gold, 12 or magnetic nanoparticles 13 to yield nanostruc- tured and multifunctional hybrid material. The combination between the organic and the inorganic components establishes a symbiotic relation in which the microgels give colloidal stability as well as stimuli responsive features, while the inorganic counter- parts provide quantum properties such as photoluminescence, 14,15 surface plasmon resonance, 16 or magnetism. 17 In some works, the localization of the inorganic material is on the outer part of the microgels. This sort of decoration can be produced either by exploiting charge interactions between the nanoparticles and the microgels 16,18 or by covalent bonds between the microgels and the magnetic material. 19 However, one of the major drawbacks concomitant with this strategy is a reduction of the colloidal stability which leads to the system coagulation. Another approach used to synthesized hybrid magnetic nanoparticles is based on the preparation of magnetic nano- particles in a bulk solution, where microgels have been previously dispersed. This approach usually yields nanoparti- cles homogeneously distributed within the microgels. 20 Nevertheless, this sort of decoration presents two major disad- vantages: First, not all the inorganic nanoparticles can be done in the presence of microgels, limiting this strategy only to syntheses performed under mild conditions. Second, the high specific surface of the nanoparticles can favor interactions between the nanoparticles and the microgel matrix, which normally affect the thermal response of the microgels being possible to block the low critical solubility temperature (LCST). This is the reason why only microgels with a limited amount of inorganic material have been produced using this method. 21,22 To overcome the previous disadvantages, many efforts have been done to produce hierarchical microgels in which the inorganic nanoparticles are located in the core of the system. 2326 However, these methodologies have not been successfully applied to Fe 3 O 4 nanoparticles since its surface inhibits the polymerization reaction. This impediment has been related to their capacity to mediate in redox processes 27 and to transfer an electron to the radical, which subsequently stops the polymerization. To overcome this problem, some authors have incorporated pNIPAM on the surface of magnetic nanoparticles using a layer-by-layer deposition technique. 28 However, this procedure requires many steps when one wants to grow a thick polymer layer around the inorganic core. Other authors cover the surface of the magnetic nanoparticles with SiO 2 to overcome the above problems. 13,23,29 However, Received: May 9, 2011 Revised: June 29, 2011 ABSTRACT: In this work, we describe a new methodology for the preparation of monodisperse and thermosensitive microgels with magnetic core. In order to produce such a material, hydrophobic magnetic Fe 3 O 4 nanoparticles were prepared by two methods: thermal decomposition and coprecipitation. The surface of these nanoparticles was modified by addition of 3-butenoic acid, and after that these nanoparticles were dispersed in water and submitted to free radical polymerization at 70 °C in the presence of N-isopropylacrylamide (NIPAM) and bisacrylamide. The result of this reaction was mono- disperse microgels with a magnetic core. By varying the amount of 3-butenoic acid, it was possible to obtain hybrid microgels with different magnetic core sizes and different architectures.