General Overview of our Research
Our research is basically focussed on the synthesis,
characterization and applications of nanoparticles and assemblies thereof.
Core-shell nanoparticles are our favourite systems, so that we can control
the properties of the colloid by means of careful modification of the dimensions
of the core-shell geometry and of the nature of both the core and the shell.
By chosing an insulating material as the shell, we can build up nanostructures,
both in 2D and 3D, where the spacing between neighboring particles is determined
by the thickness of the shell. Such a lattice control permits us to tailor
the properties of the nanostructures. We search for applications mainly
on magnetic and optical properties.
For references, see the Publications page.
Nanoparticle synthesis: size and shape control
Morphology control in the nanoscale is a hot topic because of the spectacular
effects that small changes in the shape of nanoparticles have on a variety of
physical (optical, magnetic, electronic...) properties of
the material. Colloidal synthesis has proven extremely useful to
prepare a wide variety of nanoparticles with tight control of size and shape.
Still, much of the knowledge in this area is empirical and no general rules can
be provided for a rational nanomaterial design. We are particularly interested
in understanding the mechanisms involved in nanoparticle growth, which determine
the final size and shape. Though eminently fundamental, this research is
required for the design of nanoparticle materials with tailored properties that
can be used for practical applications.
Examples of nanoparticles with various morphologies are shown in our picture gallery.
Plasmonics of metal nanoparticles
Nanoparticles of noble metals (Au, Ag, Cu) display very interesting optical properties due to
so-called surface plasmon resonances,
which involve the collective oscillation of conduction electrons in
resonance with the alternating electric field of incident electromagnetic
radiation, as sketched below.
The frequency of the surface plasmon mainly depends on the nature
(dielectric function) of the
metal, but is largely affected by the size and shape of the nanoparticles,
or by their dielectric environment, among other
parameters. Such resonances result in bright colours, as well as
large enhancements of the electric field around the particles.
One of the main interests of our group is the fine tuning of
the optical response of metal nanoparticles with
tailored composition, size and shape. Characterization of
plasmon modes is carried out both for
Examples of nanoparticles and their colours are shown in our picture gallery.
For more information, see a recent review.
Nanoparticle assemblies
The preparation of thin films is based in the layer-by-layer
assembly method, which is sketched below. The (negatively charged) substrate (1)
is first dipped in a solution of a positively charged polyelectrolyte (2), so that a
first monolayer is deposited. Then
the modified substrate is immersed in a colloid of negatively charged nanoparticles
(3), which attach due to electrostatic interactions. The process can be
repeated in a cyclic fashion to build multilayers.
For AFM pictures of the films, visit our picture gallery.
Applications
of these films can be found in the magnetic storage technology, since the
decoupling between magnetic moments provided by the insulating layer implies
a reduction of the noise to signal ratio.
The same methodology can also be applied to assemble nanoparticles on larger
colloidal templates, which we have applied to assemble metal nanoparticles on
latex spheres and carbon nanotubes. A few examples of TEM images are shown in
our picture gallery as well.
Composite colloids
This area of research focuses on the
synthesis of nanoparticles or larger colloids comprising multiple
functionalities, typically magnetic and optical, for applications either in
nanoparticle directed assembly and orientation, or for biomedical uses, such as
biolabelling and hyperthermia. We follow various approaches, either through
seeded growth or through assembly of the components in the colloidal dispersion.
Assembly typically involves using polyelectrolytes or other types of surface
modification of the larger substrate.
Functionalization of carbon nanotubes
Carbon
nanotubes (CNTs), the paradigm material when talking about Nanotechnology, not
only possess unique structure dependent electronic, mechanical, optical and
magnetic properties, but can also reach very high aspect ratios and surface
areas. For these reasons, CNTs can be considered as ideal templates for the
formation of one-dimensional nanoparticle assemblies. Despite the relative
chemical inertness of CNTs, several strategies have been followed for the
preparation of CNT-nanoparticle composites, either through in-situ nanoparticle
synthesis or the assembly of pre-formed nanoparticles. In either case, surface
modification is required, which can involve the chemical development of
defect-sites and subsequent covalent functionalization or non-covalent
adsorption of macromolecules on the side walls. Non-covalent adsorption is of
particular interest because it enables to functionalize the CNTs while still
preserving their electronic structure, since the sp2-hybridized
carbon structure and conjugation remain unaltered, which on the other hand
provides the CNT surface with an inert and hydrophobic character, thus
complicating the adsorption of hydrophilic macromolecules. We
carry out the deposition of various materials on CNTs, so as to impart them with
optical or magnetic functionalities, forming extremely anisotropic systems with
interesting properties and potential applications in multiple areas.
TEM images of nanoparticle-functionalized CNTs are displayed in our picture gallery.
Evaluation of the optical enhancing properties of nanoparticles and nanoparticle arrays
Surface enhanced spectroscopies (SERS, SEF and SEIRA), with detection limits down to the single molecule regime, are known to be the ultimate analytical tools. This family of techniques is also called plasmon assisted spectroscopies because of the need of metallic nanoparticles to provide the electromagnetic field necessary for optical enhancement. Key aspects of the enhancing activity of nanostructures are related with size, shape, composition and surface chemistry of the nanoparticles. In this research line, we evaluate the suitability of different colloids for SES and direct sensing.
Integration of nanoparticles into complex sensors for environmental pollution monitoring diagnosis and biodetection
Besides direct approaches in the evaluation of components in a given sample, other powerful approaches include indirect detection by taking advantage of the spectroscopic properties of certain molecular systems. The fabrication of hybrid systems based on nanoparticles is taking prominence as a method for the fabrication of complex sensor elements based on recognition events (key-and-lock sensors) or indirect interactions (cross-reactive arrays). In this research line, we investigate new sensing technologies by using SPR and SES with applications in high-throughput screening and real time analysis.
Reactivity in in nanoheterogeneous media
We study the capability of different nanoheterogeneous systems such as micelles, microemulsions, vesicles and metallic nanoparticles to modulate chemical reactions, focusing in their kinetic aspects; catalytic activity and changes in the reaction mechanisms.
