Nanoparticles, Viruses and Copolymers
T. P. Russell
Polymer Science and Engineering
Department, University of Massachusetts,
Amherst, MA 01003
Synthetic nanoparticles, like CdSe have been shown to assemble at the interface between two immiscible fluids, like water and an inorganic solvent. Grazing incidence x-ray scattering measurements have shown that a monolayer of particles assemble at the interface where the nanoparticles pack in a liquid-like manner, are highly mobile at the interface and undergo exchange with nanoparticles dispersed in the organic phase. These unusual characteristics arise from the small size of the particles and the small energetic gain in placing the particles at the interface. It has been a challenge, however, to quantify the dynamics of the particles at the interface, the exchange dynamics of the particles, and the dynamics of the particles during assembly. While fluorescence microscopy measurements can provide limited, qualitative information, it has been far from sufficient to understand the assemblies on a quantitative level. The brilliance, small beam size, coherence and timing structure open the possibility of probing such nanoparticle assemblies on flat interfaces, on single liquid droplets stabilized by the nanoparticles, on droplets emerging from the tip of a micro-pipette and at the interface between two droplets in contact with each other. The latter we have shown to exhibit a Coulomb blockade effect, yet the assembly of the rearrangement of the particles at the interfaces to produce this effect is beyond our capabilities at present. In addition, size dependent exchange of the nanoparticles occurs at the interface and the difference in the size of the particles assembled at the interface gives rise to a two dimension phase separation process at the interface. The characteristics of the ERL will enable the characterization and quantification of these mixed assemblies. A unique aspect of assemblies at liquid interfaces is the ability to perform chemistries on the nanoparticles or ligands attached to the nanoparticles at the interface. For example, the assemblies can be stabilized by cross-linking the ligands. Yet, chemical reactions, in general, result in a volume change or, in the case of these assemblies, surface-area coverage of the interface by the nanoparticles. For encapsulated droplets, this change in surface area is critical to understand. Imagine being able to perform in situ studies on the nanoparticles assembled at the interface. Studies on synthetic particles have recently been extended to biological particles, namely virus particles, where size and shape can be used to influence the assemblies. However, the electron density contrast to characterize these assemblies is much less than that for the synthetic particles and, as such, the brilliance of the beam on the interfacial assemblies is of extreme importance. With current sources it has been a challenge to obtain even rudimentary information on these assemblies.
Block copolymers are emerging as a versatile platform for the generation of platforms and scaffolds for the fabrication of nanostructured materials. Thin films of block copolymers, ranging in thickness from several tens of nanometers to microns, can be prepared on flat surfaces where the orientation of the domains can be controlled by interfacial interactions or external fields. In general, films are characterized by scanning probe microscopy techniques after the structures are generated and, in addition, on surface information is obtained via this route. With the exception of electron tomography, a time consuming art in the case of polymers, only scattering methods provide a chance of examining the internal structure of the films and any hierarchical ordering that occurs in the film. A highly collimated, high brilliance x-ray beam is required to provide suitable scattering for characterization. In addition, kinetic information on the orientation of the domains during processing is important in designing processes that minimize the time required to generate the copolymer templates. In cases where long-range order is required, being able to characterize the growth of grains formed from close-packing of the copolymer domains can only be done, at present, by stopped-time experiments. Enabling the in situ characterization of the grain growth is essential in optimizing processing condition. In addition, phase selective chemistries have been developed where metals and oxides can be selectively grown in one domain. In situ characterization of the chemistry and the uniformity of the chemical reactions within the domains is only a dream at present, though important for the fabrication of hybrid nanostructured materials. The proposed ERL will enable of these.
