(6a) One-Step Fabrication of Core-Shell Nanofibers from Electrified Coaxial Jets

New materials with fiber-like shapes, core-shell structure and characteristic diameters below one micrometer have recently been proposed for applications such as fiber reinforced films for enhanced mechanical properties [1], mats for sensor applications [2] and drug delivery [3], among others. For core-shell fiber fabrication, most of the preparation techniques make use of self-assembly and template-based procedures. Up to now, these methods have arguably constituted the simplest and most effective way to prepare these nanostructures, despite of the fact that they often present severe problems. For example, the synthesis techniques are complex and involve careful chemical design and preparation since chemical or physical adherence of the external material onto the surface of the template depends on case-specific interactions i.e. a particular approach may not even be adaptable to the synthesis of a chemically similar material. A method to obtain core-shell nanofibers and nanotubes in just one step has been recently reported [4]. It is based on the formation of coaxial nanofibers by the injection of two immiscible?or poorly miscible?liquids through a pair of concentric needles, to which a high electrical voltage is applied. The two liquids flow coaxially under the action of electro-hydro-dynamic (EHD) forces forming a very thin compound jet where the inner material is surrounded by a shell of the other one [5]. If only the shell solidifies around the interior liquid, nanotubes are obtained after spontaneous or forced removal of the inner liquid. On the other hand, if both shell and core solidify coaxial nanofibers are obtained. This novel procedure is not subject to the chemical constraints of those based on self-assembly and solid templates. In addition, no solid template is required since the inner liquid, whether it polymerizes or not, serves as a template. Finally, the method here reported may be applied for the electrospinning of a broad variety of materials, both of organic and inorganic precursors. Examples of fabrication of nanotubes and coaxial nanofibers are presented.