Magnetic Silica Nanotubes: Synthesis, Drug Release, and Feasibility for Magnetic Hyperthermia Xuecheng Chen,* ,† Rü diger Klingeler, ‡ Matthias Kath, ‡ Ahmed A. El Gendy, ‡,§ Krzysztof Cendrowski, † Ryszard J. Kalenczuk, † and Ewa Borowiak-Palen † † Institute of Chemical and Environment Engineering, West Pomeranian University of Technology, ul. Pulaskiego 10, 70-322, Szczecin, Poland ‡ Kirchhoff Institute for Physics, Im Neuenheimer Feld 227, D-69120 Heidelberg, Germany ABSTRACT: A new kind of silica nanotube with incorporated γ-Fe 2 O 3 nanoparticles has been successfully prepared through sol-gel processes. Hematite particles supported on carbon nanotubes served as templates for the fabrication of the magnetic silica nanotubes. The obtained nanostructures consisting of magnetic Fe 2 O 3 nanoparticles protected by a silica shell were fully characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), N 2 sorption and desorption, and magnetization studies. The hollow inner space and the magnetic functionalization render the material promising for applications in biology and medicine. This is underlined by studies in alternating magnetic fields which show a significant heating effect, i.e., the feasibility for applications in hyperthermia therapies. In addition, the material exhibits enhanced drug-loading capacity which is demonstrated by loading with rhodamine B molecules as drugs and corresponding release experiments. The results show that magnetic silica nanotubes can be straightforwardly synthesized and have a great potential as a multifunctional drug carrier system. KEYWORDS: silica tube, magnetic, carbon nanotube, heating effect, hyperthermia therapies, drug delivery ■ INTRODUCTION Silica-based nanomaterials attract much attention because of their nontoxic nature, tunable particle size and specific surface area, abundant Si-OH bonds on the particle surface, chemical/ thermal stability, high drug loading capability, and sustained drug release from the supports. 1 Being a tubular nanomaterial, silica nanotubes exhibit empty inner space which can be filled by functional loads. In addition, the surface is hydrophilic and biocompatible so that the material can be applied in bioseparation, biocatalysis, biosensoring, and as drug/gene delivery carriers. 2 Hitherto, silica nanotubes have been typically fabricated in various templates, 3 using a surfactant-mediated sol-gel method 4 and a thermal oxidation-etching approach to convert the silicon nanowires into silica nanotubes. 5 Though silica nanotubes with different thickness and dimensions have been fabricated by changing the reaction parameters, the control and design of the wall structure remain contentious but desirable. 6 In addition to the mere properties of pristine silica tubes, adding magnetic functionality will be beneficial for applications in biology and medicine. To be specific, magnetic nanomaterials may be visualized by means of magnetic imaging modalities and/or serve as local probes for external magnetic fields. Hence, drug targeting by means of external magnetic gradient fields or local heating, i.e., magnetically induced hyperthermia, by means of alternating magnetic fields may be envisaged. 7,8 Thus, the combination of silica materials with magnetic particles is of great fundamental interest for, e.g., so- called “nanomedical” applications or liquid separation because of their high surface area and magnetic separability. 9 In general, magnetic nanoparticles have been widely used in different fields, including advanced technological areas and biology. This field of biomagnetics is rapidly growing and consists of a broad range of current applications, including labeling and sorting of cells, 10 cell separation, 11 separation of biochemical products, 12 biosensing, 13,14 and studies of cellular function, 15 as well as a variety of other potential medical and therapeutic applications. 16 For biomedical applications, iron oxides are commonly used, i.e., maghemite (γ-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ), due to enhanced biocompatibility with respect to other ferromagnetic materials. Nevertheless, the reactivity of iron oxide nanoparticles has been shown to greatly increase as their dimensions are reduced, and particles with relatively small size may undergo rapid biodegradation when they are directly exposed to biological environments. 17 Therefore, a suitable coating is essential to overcome such limitations, and encapsulation with silica endows the nano- magnets with several beneficial properties for their use in biomedical applications. Main parameters in this respect are compatibility in biological systems, 18 functionality, high colloidal stability under different conditions, the ability to modulate the magnetic properties with heating, and hydro- phlicity. 19 Previous studies on composite nanomaterials consisting of iron oxide cores and silica shells indeed show promising magnetic properties, low cytotoxicity, chemically Received: March 15, 2012 Accepted: April 9, 2012 Published: April 9, 2012 Research Article www.acsami.org © 2012 American Chemical Society 2303 dx.doi.org/10.1021/am300469r | ACS Appl. Mater. Interfaces 2012, 4, 2303-2309