Nanorice and Nanospears from Polymer Nanospheres

Nanometer-sized functional particles are attractive for optical, electrical, magnetic, and biological applications. Recently, in addition to size, the shape of a nanoparticle was reported to be crucial, for example, to how it interacts with light, as presented by Halas and co-workers. They found that riceshaped nanoparticles made of gold and iron oxide are the most sensitive surface plasmon resonance (SPR) nanosensors yet devised. They hope to get a clearer picture of proteins and unmapped features on the surfaces of cells by attaching them to scanning probe microscopes. Although there are tremendous potential advantages of using anisotropic nanoparticles like nanorice instead of conventional spherical nanoparticles, the development of controlling such shape on the nanoscale is in its early stages. Some controlled growth of the end shape of nanorods with specific equipment at specific conditions has been reported for inorganic or metallic nanoparticles. Even though polymer nanorods and nanotubes have commonly been produced since the fabrication was introduced by Wehrspohn and co-workers, terminal contour control of polymer nanoparticles like nanorice or nanospears has remained a challenging task. If functional polymer nanoparticles can be shaped in the desired forms on the nanoscale, they can easily be functionalized to have much enhanced multifunctional properties using them as templates or substrates. In the fabrication of nanorods and nanotubes, template-assisted fabrication is gaining widespread interest because of its simplicity. Such novel nanostructures are expected to provide new functions in optoelectronic and biological applications that can not be attained with conventional spherical nanoparticles. Researchers have used various kinds of membranes such as polycarbonates and anodized alumina membranes as templates for the fabrication of nanotubes and nanorods. However, no report of polymer nanorods has shown control of both aspect ratio and terminal contour. Additionally, no polymer nanospheres have been incorporated into the production of nanorods. Mostly, monomers, polymer melts, or solutions are introduced into the nanopores for the production of nanorods and nanotubes. Other template-assisted techniques use the step-edge method and other methods involving template molecules in solution. Other nanorod-production techniques that do not require templates include the electrospining of nanofibers and also the use of biomolecules and self-assembly processes. The techniques that use membranes as templates include electrodeposition, layer-by-layer deposition, and methods using commercially available metal-plating solutions. Zheng et al. have fabricated copolymer nanotubes and nanowires by having polymerizing copolymers inside the pores of alumina membranes. Anisotropic metallic nanoparticles like conical nanotubes rather than cylindrical nanotubes and nanorods have also been fabricated. Such anisotropic nanoparticles were fabricated using either a complex chemical vapor deposition setup or a tapered pore that was coated with gold. However, in this paper we report, for the first time, the fabrication of tapered nanorods of varying aspect ratio and controlled terminal contour (e.g., from nanorice to nanospears) using a simple method. First, our method exploits the injection of a controlled amount and size of nanospheres into the cylindrical nanopores of an alumina substrate. Second, the resulting nanostructures are controlled during the nonuniform heating of the polymer nanospheres by capillary forces, and wetting (or dewetting) onto the cylindrical alumina walls. To achieve faster production and also obtain subtle variation in the shape, size, and aspect ratio of nanoparticles, we created a fabrication method using polymer nanospheres and anodized alumina membrane templates that controls the structure of the resulting nanoparticles. Our system for fabricating novel nanorice and nanospears is easy to set up and perform on a laboratory bench top. Nanoscale fabrication was made possible without the use of an elaborate setup involving vacuum chambers or expensive equipment that are normally required for lithography-based fabrication. The cost of running the experiment was very low and the fabrication of different types of nanostructures was easily performed by changing key parameters, such as the particle size, template pore size, duration of ultrasonication, and temperature or heating time. In this work, we used commercially available alumina membranes (Whatman anodized alumina membrane) as templates and polystyrene (PS) nanospheres (neutral, carboxylated, or sulfated PS) for the fabrication of controlled aspect ratio and terminal-contour-controlled nanoparticles (i.e., nanorice and nanospears). The pores of these membranes were filled with C O M M U N IC A IO N S