Magnetically Guided Titania Nanotubes for Site-Selective Photocatalysis and Drug Release

The nanoscale encapsulation of ferromagnetic structures has received a great deal of attention because of the exciting possibilities to use these materials in various applications that range from novel electromagnetic to biomedical devices. For example, nanoscale magnetic entities could be transported and concentrated at pretargeted locations or organs within the human body bymeans of an external magnetic field in order to exert a specific function with high local and temporal precision. Therefore, functionalized magnetic nanodots, nanowires, or nanotubes have a high potential for in vivo applications such as magnetic resonance imaging or siteselective drug delivery systems, if the magnetic property is combined with an appropriate drug loading and release mechanism. TiO2 nanotubes are a highly promising encapsulating material for a magnetic core as a high degree of biocompatibility can be combined with a broad range of other functionalities. Since the pioneering work of Fujishima and Honda in 1971, it has been established that TiO2 is a highly active photocatalyst; this is based on the ability of TiO2 to produce electron–hole pairs upon light irradiation and thereby create highly reactive radical species. This property of TiO2 has been intensively explored in the form of photoelectrodes for the decomposition of various organic pollutants in water and air, and it has been used in self-cleaning, disinfecting, and anticancer materials. The photocatalytic ability of TiO2 can be enhanced by using nanosized TiO2 materials because of their large specific surface area. Herein, we describe a simple way of embedding magnetic properties into TiO2 nanotubes and demonstrate their different site-selective photocatalytic applications. Not only can these tubes be used as a magnetically guided photocatalyst for the decomposition of organic matter but also the photocatalytic mechanism can be exploited to release an active species (a model drug). Among the various synthetic routes used to prepare TiO2 nanotubes, [26–28] anodization approaches have gained significant attention as they lead to highly ordered nanotubular arrangements. During the past few years, our research group has contributed several generations of anodically grown self-organized TiO2 nanotube layers by anodization of Ti in aqueous and organic electrolytes. In our approach, we use nanotube layers (Figure 1a) that were produced in ethylene glycol/NH4F electrolytes [36–38] (see Section S1 and Figure S1 in the Supporting Information). These TiO2 nanotubes were filled with magnetic nanoparticles by sucking a droplet of ferrofluid placed on the top of the nanotube layer using a permanent magnet (see the Supporting Information). Figure 1b shows topand side-view SEM images of the nanotubes that are loaded with the magnetic nanoparticles. It is clear from these images that the majority of the inside tube walls were coated relatively uniformly with the magnetic particles leaving a hollow space inside the tubes. In contrast to other established but time-consuming and