Titania nanoshells with an external diameter of 10-30 nm and a wall thickness of 3-5 nm were prepared by dissolving the silver cores of AgTiO sub(2) nanoparticles in a concentrated solution of ...ammonium hydroxide. The nanoshells were assembled layer-by-layer (LBL), with negatively charged poly(acrylic acid) (PAA) to produce coatings with a network of voids and channels in the interior of the film. The diameter of the channels in the titania shells was comparable to the thickness of the electrical double layer in porous matter (0.3-30 nm). The prepared nanoparticulate films demonstrated strong ion-sieving properties due to the exclusion of some ions from the diffuse region of the electrical double layer. The permeation of ions could be tuned effectively by the pH and ionic strength of a solution between "open" and "closed" states. The ion-separation effect was utilized for the selective determination of one of the most important neurotransmitters, dopamine, on a background of ascorbic acid. Under physiological conditions, the negative charge on the surface of TiO sub(2) facilitated the permeation of positively charged dopamine through the LBL film to the electrode, preventing the access of the negatively charged ascorbic acid. The deposition of the nanoshell/polyelectrolyte film resulted in a significant improvement to the selectivity of dopamine determination. The prepared nanoshell films were also found to be compatible with nervous tissue secreting dopamine. Although the obtained data demonstrated the potential of TiO sub(2) LBL films for implantable biomedical devices for nerve tissue monitoring, the problem of electrode poisoning by the by-products of dopamine reduction has yet to be resolved.
In this chapter, we explore the sensitivity of gold nanorods toward changes in the dielectric constant of the surrounding medium. Experimental data for pure and silica-coated nanorods with varying ...shell thickness are compared to calculations based on the boundary element method (BEM). They indicate that anisotropy and sharp tips make nanoparticles more environmentally sensitive. We also find that sensitivity decreases as silica shell thickness increases, as expected from a dielectric screening effect. Even when coated with thin shells, gold nanorods are found to be excellent candidates for biosensing applications.
A large variety of synthetic procedures have been developed for the preparation of noble metal nanoparticles with various sizes and shapes, and which are stable in a wide range of solvents. The ...optical properties of these nanomaterials are currently quite well understood, since the quality of the current preparation methods allows to obtain a high monodispersity, so that simplified theories can be used.
Since it was not within the scope of this chapter, we have only presented a couple of examples of structures that can be fabricated using noble metal colloids as building units. These examples showed that the coupling between particles in close contact leads to modifications in the optical properties, which ultimately allows to engineer the distribution of nanoparticles and thereby obtain the desired optical properties. Since very little has been done in this field by taking profit of the wealth of properties arising from the newly devised anisotropic metal nanoparticles, we should expect huge developments in this area within the coming years.
The chapter describes the core-shell nanoparticles and assemblies. The chapter illustrates that the knowledge of various forces acting on colloid particles, allow particle stability to be controlled, ...and thermodynamic behavior to be predicted. Depending upon the system requirements, it may be desirable to destroy or to preserve particle stability against coagulation. Control of particle aggregation is of primary importance in adhesives, inks, pharmaceuticals, cosmetics, foodstuffs, and lubricants. There are two ways in which colloid particles are stabilized in aqueous media—namely, electrostatic and steric stabilization. In the former case, the two forces acting upon particles are double-layer repulsion and van der Waals attraction. In the latter case, van der Waals and steric forces are present, though double-layer repulsion may also be present if the steric stabilizer is electrically charged. Steric stabilization by organic molecules is the common means for achieving particle stability in nonaqueous solvents. Finally, the chapter focuses on the forces associated with particle-particle interaction.
