Resumen de Synthesis and characterization of hybrid nanostructured materials
The aim of the work presented in this thesis comprises the synthesis of new hybrid nano-structured materials, as well as the study of their optical, catalytic or magnetic properties. The interest on these materials stems from their unique nanosize-dependent properties, different from those observed in bulk materials. Such properties have rendered those structures as ideal objects for a considerable number of potential applications. This could be the case of integrating various dissimilar nanomaterials in a single device for the simultaneous sensing of multiple physical phenomena. Thus, various hybrid nano-materials with optical, magnetic and/or catalytic properties have been developed in this work. Two different templates, carbon nanotubes (CNT) and polystyrene spherical colloids (PS), have been used as supports for the fabrication of hybrid nanostructures. In this task it was of great importance the synthesis of metallic nanoparticles and the layer by layer self-assembly technique. Such technique, based on the electrostatic interactions between different components, allowed us to integrate various components in a single nano-material. So, we can divide this work in two main parts: The first part covers the synthesis of hybrid nanostructured materials based on CNTs that were used as templates because of their excellent intrinsic structure-dependent properties. These nanotubes were use as templates for the deposition of metallic platinum and silver nanoparticles. In the first case we have doped CNTs with different types of platinum nanoparticles, spherical and dendritic, which allowed as to identify the high catalytic activity of the dendritic shape particles by using an electron transfer reaction as probe. In the case of the CNTs coated with silver nanoparticles, we intend to develop a stable substrate for SERS-based applications in order to provide higher enhancement factors of the Raman signal. In this chapter we used this material to determine if a bio-analyte, in this case a derivate of cocaine, is or is not present in a sample and how much is present. First we use this substrate to perform the label-free indirect ultra-detection of the bio- analyte in question through an antibody assembled onto the surface of the substrate. The comparison between this indirect detection and a direct detection using the same substrate revealed that, whereas the latter offers a higher detection limit than the specific indirect method, label-free indirect detection offers quantitative results at physiological levels, so that not only drug abuse can be demonstrated, but their consumption can also be quantified with a portable, non-invasive and fast technique. The synthesis of new hybrid nanostructured materials, using spherical particles as support, has been explored in the second part of this work. Monodisperse Colloidal particles of polystyrene were chosen as templates because of their commercial availability in a wide range of sizes, and also because they can be easily removed by means of dissolution with organic solvents or thermal decomposition. In this regard, the deposition of metal nanoparticles onto spherical polystyrene colloids is a useful strategy for the fabrication of hybrid materials with unique properties, which are dependent on the nature of the nanoparticles deposited. Thus, in this work platinum and gold nanoparticles have been deposited onto the surface of polystyrene colloids. In a first example, the catalytic activity of platinum nanocatalyst previously deposited onto polystyrene beads has been exploited for the controlled deposition of a magnetic material onto them, thereby tuning the magnetic properties of the nanostructure. The fact that these polystyrene colloids can be easily dissolved in organic solvents makes them very good candidates for obtaining hollow capsules. It is for that reason that polystyrene templates were coated with a homogeneous and porous silica shell, which allows removing the polystyrene while preserving the hollow structure. Moreover, porous nanocapsules may have different properties and applications attending to the nature of a material encapsulated in their inner space. As an example, the encapsulation of single crystal dendritic platinum nanoparticles renders highly active nanocapsules that were used for the confined deposition of a magnetic material. In a different work, gold nanoparticles encapsulated in silica capsules were shown as attractive optically active substrates for surface enhanced Raman scattering (SERS) -based applications, allowing the fabrication of SERS-encoded submicrometer particles with shape and size unifromity for use in antigen biosensing. Finally, the deposition of CNTs onto the surface of polystyrene beads was attempted as strategy to increase the complexity and functionality of the hybrid material that resemble natural magnificent sea anemones. Thus, the nanotubes play two main roles, helping to reinforce the hollow structure as well as permitting to increase and tune their surface area. Additionally, the incorporation of magnetic nanoparticles into these structures provides them with an extra magnetic functionality, which allows this material to be manipulated by an external magnetic field. In this thesis the chapters are structured by published papers, and this structure allows us to develop in each chapter the protocol for the synthesis of the material and application. The application of hybrid nanostructured material is directly related to material properties.
