Design of self-assembling materials
- Ivan Coluzza (Center for Cooperative Research in Biomaterials, Spain)
- Christoph Dellago (University of Vienna, Austria)
- Francesco Sciortino (University of Rome I 'La Sapienza', Italy)
- Jure Dobnikar (Institute of Physics, Chinese Academy of Sciences, United Kingdom)
The goal of this workshop is to bring together scientists who study self-assembling in the biological world and those from the material science community, with specific emphasis on soft-matter systems. The meeting will provide an overview of natural systems and novel artificial systems, that are able to reliably self-assemble in predefined structures avoiding amorphous arrested (glassy) structures. It then appears of extreme importance to bring together scientist from the communities of biophysics of bio-molecules, colloidal systems, dynamics of arrested fluids, both with theoretical and experimental background. In all these communities the goal of self-assembling is of great importance. However, the separation between each field makes the contacts among the scientists more difficult. Hence, the opportunity of establishing a common table of discussion about the current understanding of such systems is of paramount importance for the realization of more controllable self-assembling materials that can be designed to have desired macroscopic properties.
Self-assembly is the process by which a substance exclusively driven by non-covalent interactions spontaneously develops into a specific long-lived conformation with a well defined structure . Such structures may be highly inhomogeneous but still strongly differ from random amorphous materials which are typically dynamically arrested . In fact by having an easily accessible ground state, self-assembling materials have the property of forming structures characterized by few defects, and they often have the capacity to adapt to changes in the environment [1,2]. Self assembly is the method that most biological systems use to control the synthesis of complex structures . If properly understood, the fundamental principles used by living organism can be applied to design artificial materials with a novel properties, such as active response to the environment, catalytic properties or three dimensional specific connectivity which could extend the possibilities of modern electronics based on surface lithography.
Bio-polymers such as proteins and DNA will have an important representation in the workshop. In particular proteins as they are fundamental building blocks of living organisms and viruses. Their function is encoded in the same structural elements that composes them. This is a rather unique property that stirs the interest of scientists from very different backgrounds. However, they have reached a high level of complexity (making the control step very difficult), and they are optimised for very specific tasks which may not meet the general needs. While there are many different proteins with different structures and functions, they are all composed of chains of the same 21 fundamental chemical units called amino acids. Although only few sequences will determine a stable ground state, the chemical heterogeneity of the alphabet insures an enormous variety of combinations, each of which folds into complex arrangements of predominantly two types of secondary structures: alpha helices and beta sheets [3-5]. Due to this complexity the system represents a big theoretical challenge. Large attention will also be devoted at the recent development in the fields of self-assembling in colloidal system, in particular of patchy colloids. Colloids are an optimal system as not only their interactions are highly controllable, but their size often allows for tracking and characterization in real time by means of confocal microscopy. Moreover, colloids and in particular patchy colloids have been more and more used as a model system for the biological systems, in particular in studies devoted to the understanding of protein aggregation and protein crystallization. This is the right time for a workshop focused on this topic since chemists and material scientists are starting to gain control on the shapes and on the local properties of the colloidal particles. Hard cubes, tetrahedra, cones, rods as well as composed shapes of nano or microscopic size have made their appearance in the labs, and will hopefully become available in bulk quantities in the near future . Patterning of the surface properties of these particles is adding extra directions to the anisotropy axis space envisioned by Glotzer and Solomon  in their recent review. Patches on the particle surface can be functionalized with specific molecules (including DNA single strands) to create hydrophobic or hydrophilic areas, providing specificity to the particle-particle interaction. In the same way as sterically stabilized colloids have become the ideal experimental model system for investigating the behavior of hard-spheres and simple liquids, the new physico-chemical techniques will soon make available to the community colloidal analogs of several molecular systems.
The overlap between bio-polymers and colloidal science has been been made more evident by the recent development in [8-11] where, with different techniques, chains and membranes of colloidal particles have been synthesized. The workshop will be an occasion for scientists from these two communities to exchange ideas and increase the area of overlap even further.In order to expand the discussion beyond equilibrium properties, we will also provide a discussion table about the dynamic of the self-assembly process. The importance of dynamics becomes rapidly clear when we look at a typical self-assembling phenomena like crystallization. The crystallization rate can be largely influenced by the propensity of the system to form a glass and get trapped in an arrested state. Another example is the protein folding, where the free energy landscape of a designed protein presents mainly a single global minimum that corresponds to the ensemble of configurations near the folded structure. Hence, the dynamics should be straight-forward downhill towards the global minimum and an analysis of the equilibrium properties should fully characterize the system. However, this is not universally true and could strongly depend on the complexity of the target structure, hence especially for the assembly of complexes it is important to measure the rate at which the global minimum is reached.
The workshop that we propose will focus on the following subjects :
1. Natural occurring Self-assembling phenomena with particular attention to the study of protein [12-14] and DNA design. Important results have been obtained in these two fields with the creation of first fully artificial proteins  and complex self-assembled DNA based structures also called DNA “Origami” [16-18].
2. Artificial systems designed with controlled self-assembling properties. Typical examples in this category are patchy colloidal systems that have already demonstrated to posses a rich space of possible ordered structures, both theoretically and experimentally [6-7]. In addition to patchy particles, we will give large attention the bio-polymer/nano-particles hybrid systems, such as the DNA-coated colloidal particles.
3. The dynamics of self-assembling is the third important category that we will include in the workshop. Dynamics is of great interests for the design of self-assembling systems, because it is essential that any target structure is reachable in a time frame compatible with experiments. It is in fact common for such complex systems to get trapped in disordered configuration for very long time . It becomes clear then that the discussion should include also the contributions from scientists experts in the field of arrested matter, with particular attention to glasses.All these subject will be represented by scientists with either a theoretical or experimental background.
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