Due to the presence of various compartments of different chemistry and physical properties, a variety of specific applications are under discussion and exploration currently. Some of these studies have been triggered by theoretical predictions of certain advanced properties in particular of Janus colloids. In several examples in the literature, it is evident that when researchers make clever use of the inherent multicompartment character, the
non-I-14 Introduction centrosymmetric nature and the resulting physical characteristics, new materials with novel properties can be obtained. However, only some of those will be commercialized and make their way to real industrial applications or specialized product.
A first example for an interesting switchable device was shown by Nisisako et al., who made use of the electrical anisotropy of Janus particles filled with white and black pigments in both hemispheres.31 In order to create a switchable display panel, they placed a thin layer of these spheres between two electrodes. Upon switching on an electric field, the particles orient their black sides to the negative electrode and vice versa. The orientation and the color of the display can be flipped simply by reversing the electrical field. With this method, very thin, robust and environmentally stable displays could be created.
Besides, Janus particles can be used as efficient and unique optical probes for biological interactions or rheological measurements in confined space. In recent years, this concept has been brought forward by Behrend and coworkers108-112 who used Janus beads coated on one side with metal. These so-called (magnetically) modulated optical nanoprobes reflect and transmit light or emit fluorescence anisotropically, i.e. depending on their orientation with respect to the observer. Being placed into a specific environment, these particles blink on and off depending on the surrounding conditions. Precisely speaking, the frequency of the flickering can be used to draw detailed conclusions of the microenvironment and the viscoelastic properties, simply because the rotational diffusion of the particles experiences viscous drag.
With this strategy it is possible to create devices ranging from precise nanoviscosimeters to nanothermometers. Further development of these particles has aimed at the incorporation of highly selective receptor sites or magnetic coatings on one side of the particles in order to use them as (bio)chemical nanosensors. Thus the flickering of the particles is not only sensitive to viscous drag, but also to electric and magnetic fields as well as to chemical attraction and biochemical forces.113 For sensor applications, Janus particles could allow an independent biochemical conjugation with the possibility of imaging (microscopy or magnetic resonance tomography) based on dyes or contrast agents located within the other side. Thus, an interference of the two sides is minimized and the sensing functions could be optimized.
Another interesting effect which potentially finds application in nanoscience is the self-propulsion induced by catalytically active Janus beads with a spatially asymmetric distribution of the reaction site.99, 114 Generally, self-motile particles are of interest in nanomedicine as they exhibit an increased diffusion coefficient compared to standard particles. This allows them to screen a larger volume for docking sites within less time and would make drug-delivery vehicles more efficient. Such devices had been recently described by Ryan and coworkers99 They used micron-sized polystyrene particles coated on one side with a thin platinum layer and studied their diffusion by tracking experiments in dependence of the concentration of hydrogen peroxide. The latter served as “fuel” in these experiments as it is catalytically degraded by platinum into two reaction products and leads to an asymmetric distribution of reaction products and an accompanying osmotic potential (see Figure 1 - 5).
Introduction I-15
Figure 1 - 5. Self-propulsion of a Janus particle via the asymmetric distribution of reaction products in case of the catalytic degradation of hydrogen peroxide by platinum into two reaction products.
This nanoscale chemical locomotion leads to enhanced directed diffusion on a short time scale and is randomized for longer time-scales. The overall diffusion coefficient is substantially larger in the presence of fuel. By changing the catalytic centre to an active enzyme, the propulsion mechanism would mimic to some extent a bacterial flagellum.
From an industrial point of view, the surface activity of Janus particles is of fundamental interest.
Due to the corona segregation, amphiphilic Janus particles are expected to strongly adsorb at interfaces. These particles uniquely combine the so-called Pickering effect, known for particles, with the amphiphilicity of classical surfactants. In recent years, several publications have appeared describing theoretically the remarkably high adsorption strength of Janus particles at interfaces.115, 116 For instance, in case of spherical particles, the adsorption energy at a liquid/liquid interface is predicted to be up to three times higher for particles with corona segregation than for particles with a uniform surface. Therefore, Janus particles may be ideally suited as strong future emulsifiers. Following the predictions, Glaser et al.117 recently used pendant drop tensiometry to show that bi-metallic Janus particles indeed lead to a significant reduction of the oil/water interfacial tension as compared to uniform metallic nanoparticles (iron oxide or gold) of similar size (see Figure 1 - 6).
Figure 1 - 6. (a) Schematic representation of bimetallic Janus particles at the hexane/water interface (gold: gold part with surfactant; gray: iron oxide part). (b) Interfacial tension vs time as measured by pendant drop tensiometry (NP: homogeneous nanoparticles; JP: Janus particles).
I-16 Introduction This means that by changing the architecture, stronger and hence better surfactant particles can be created. The application of nano-sized Janus colloids to the stabilization of interfaces will be demonstrated in two chapters of this thesis.