Electron microscopes, which have a long history of success primarily with metal, ceramic and biological structures, can be used to determine the shape and size of polymeric nanoparticles and nanocapsules. They can also image chains of molecules, for example, protruding from the surfaces of the particles. The polymer scientists in Mainz use three transmission electron microscopes (TEM), which illuminate the sample with an electron beam and thus also provide information about the internal structure of the nanoparticles, for example about the thickness of capsule walls. They also use two scanning electron microscopes (SEM) in which the electron beam scans the surface of the sample and produces a topographical image.
Integrated X-ray fluorescence and electron energy loss spectrometers also provide insight into the distribution of elements in the nanoparticles. It is difficult to study polymer spheres in the electron beam, however, because it destroys the polymers more or less quickly. Ingo Lieberwirth’s microscopy team is one of the few in the world that specializes in the microscopy of soft matter. This requires throttling the beam of electrons down to an extremely low intensity. They also fix the polymers so that they can withstand the electron beam for a longer time.
However, two disadvantages remain: Electron microscopes provide images of only a few particles. Moreover, the particles must be dried or frozen in their dispersion. In order to observe the particles in their natural medium, the researchers in Mainz must use complementary methods (see box on p. 64). The microscopy group is thus working on using high-resolution laser scanning microscopy (STED) for polymers.
Wall paint, printer ink, drugs or contrast media – in practice, nanoparticles are usually used in dispersion form. Their size and shape in the liquid medium can be determined with different methods of light scattering, for example. The scientists in Anja Kroeger-Brinkmann’s analytical group are specialists in this field. The methods are based on illuminating a dispersion of the nanoparticles with a laser beam. The nanoparticles then scatter the light. The intensity and direction of the scattered light depends on the shape and size of the particles.
Dynamic light scattering is one method that allows reliable statements to be made about this. The researchers analyze the fluctuations of the scattered light, which resemble a TV picture with poor reception. This is a function of the Brownian motion of the particles, which in turn is a function of the size. When the researchers analyze the scattered light deflected toward large scattering angles, this technique can also tell them something about the inner structure of the nanoparticles. To this end, they are now working on developing the method further. If a dispersion contains mixtures with different particle sizes, the particles must first be separated or sorted before reliable statements can be made about their size distribution.
One of the methods the researchers use is flow-field-flow fractionation: they pump a dispersion of the particle mixture through a flow channel. A second current, the cross-flow, meets the channel current at a perpendicular angle. This current forces the larger particles more strongly toward the channel wall than it does the smaller ones. Under the influence of this force field and the counteracting diffusion, the different sizes of particles separate. The particles sorted in this way can then be analyzed with light scattering methods.
A mixture of oil and water in which droplets of one liquid float in the other in a fine dispersion. Unlike conventional emulsions, ultrasound tears the droplets apart and makes them a fairly uniform size on the nanoscale. Moreover, the miniemulsion is stabilized by the osmotic reagent: a substance that dissolves almost exclusively in the droplets and prevents larger droplets from forming at the expense of the smaller ones.
Includes all synthetic materials. They consist of chain-like or reticular molecules comprised of building blocks of a monomer, or sometimes different monomers. They are distinguished by their starting materials and their chain type.
A substance whose molecules have a water-soluble and an oil-soluble end.