Self-assembly

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STM image of self-assembled molecular chains of the organic semiconductor quinacridone on graphite .

The term self-assembly (engl. Self-assembly ) is not yet defined in a uniform and is therefore defined in very different in the various contexts. This is accompanied by the additional unclear delimitation from the term self-organization , so that the use of both terms is often interchangeable and unspecific. For the following definition and classification, definitions of terms are taken from the scientific literature and partially expanded so that a clear and unambiguous delimitation is made possible.

definition

The term self-assembly refers to processes of structure and pattern formation that take place autonomously, i.e. without external influences such as B. directing through human intervention. Compared to terms such as formation , training or templating (German: for example matrix-supported, matrix-guided arrangement ), in which only processes are combined that have an external effect on a system and thus shape certain forms or patterns via external information from the environment, the term self-assembly differentiated by the following criteria:

  • The process starts with existing, separate components (e.g. molecules ).
  • The result is a product of the information that the individual components themselves contain (e.g. charge , mass , shape, surface properties) and determine their mutual interactions.

In the English language, a distinction is made between the two processes “self-assembly” and “self-organization”, i.e. “self-assembly” and “self-organization”. While self-organization describes the formation of non-equilibrium structures (no thermodynamic minimum structures), self-assembly describes the formation of thermodynamic minimum structures. In the literature, however, this definition is not always clearly adhered to, so that - although the definition is clear - both terms are used strongly mixed up. At the same time, a combination of both mechanisms often occurs, especially in biological systems. Examples of self-organization are actin polymerisation to actin filaments (ATP is necessary), which keeps structure formation in an imbalance. Self-assembly can be observed, for example, during crystallization, in which ions are assembled to form the thermodynamic minimal structure crystal.

Classification

After this conceptual definition, a classification into different types of self-assembly can also be made (based on, whereby the definition does not differentiate between ordered and complex systems, so that the definition has been supplemented by the aspect of symmetry ):

  • Static self-assembly : symmetrical structures that assume a global or local thermodynamic state of equilibrium.
  • Dynamic self-assembly : Ordered structures in systems with a dissipative structure , ie the symmetrical structure can only be maintained by absorbing energy.
  • Matrix-assisted self-assembly : The structure is not exclusively determined by the interactions between the components, but also by regular patterns in the system's environment (e.g. structure of a crystal surface ).

Of great importance for a technological use of self-assembly is the possibility to influence their interaction and thus the result of self-assembly through the selection or design of the components. With regard to nanotechnologies , atoms , molecules or nanoparticles can be considered as components , with the possibility of chemical design in particular in the case of molecules providing a large scope for variability in interactions. When molecules appear as components, the term molecular self-assembly is occasionally used in publications (e.g. in) without distinguishing between molecular and supramolecular associations. In order to take this difference into account, the following differentiation can be made:

  • Molecular self-assembly : Molecules combine to form a covalent structure, although there is a certain degree of reversibility of the connections when the aggregate is formed. These are z. B. to macropolycyclic structures such as porphyrins and phthalocyanines .
  • Supramolecular self-assembly : molecules associate spontaneously through non-covalent interactions (e.g. hydrogen bonds , van der Waals interactions ) to form reversible systems. These include B. organic films or cell membranes .

Differentiation from the term self-organization

Since the term self-assembly is relatively narrowly limited with these definitions , there is also a possibility of delimiting it from the term self-organization . This makes sense if autonomous structure-forming processes are considered under the aspect of complexity: Processes that lead to a development of complexity can be separated from those without a development of complexity and thus be conceptually assigned differently.

In order to make complexity measurable as a fundamentally measurable variable so that a clear statement about its change is possible, it can be defined as a measure of how much information is at least required about the state of a process in order to be able to predict its future behavior (so-called . statistical complexity).

Based on this definition, a process can be regarded as self-organizing if its statistical complexity increases over time.

Pure self-assembly can thus be clearly separated from self-organization, taking into account all of the above-mentioned features, due to the lack of complexity training and development: In systems created through self-assembly, the information about one of the elements allows the prediction of the behavior and position of all the other elements at the same time it is a matter of high-order associations that form symmetries . In contrast, broken symmetries, which are not yet chaotic, allow complexity: the information about one part of the structure alone is not sufficient to predict the properties and behavior of other parts. Complexity as a characteristic of self-organization forms and develops between order and disorder - in the border area to chaos ( edge of chaos ,) z. B. arises when systems move away from thermodynamic equilibrium and instabilities arise.

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  1. ^ A b Jean-Marie Lehn: Supramolecular Chemistry: Concepts and Perspectives. VCH, Weinheim, Germany, 1995, ISBN 978-3-527-29311-7 .
  2. ^ A b c Cosma R. Shalizi, Kristina L. Shalizi: Quantifying self-organization in cyclic cellular automata . In: Proceedings of SPIE . tape 5114 , 2003, ISSN  1996-756X , p. 108-117 , doi : 10.1117 / 12.485805 .
  3. ^ A b c G. M. Whitesides, B. Grzybowski: Self-Assembly at All Scales . In: Science 295, 2002, p. 2418, doi: 10.1126 / science.1070821 .
  4. ↑ For an extensive discussion of the distinction between the two terms, see: Halley, JD and Winkler, DA: Consistent Concepts of Self-organization and Self-assembly . In: Complexity . 14, 2008, p. 10. doi : 10.1002 / cplx.20235 .
  5. ↑ CR Shalizi, JP Crutchfeld: Computational Mechanics: Pattern and Prediction, Structure and Simplicity . In: Journal of Statistical Physics 104, 2002, p. 817, arxiv : cond-mat / 9907176v2 .
  6. ^ SA Kauffman: The Origins of Order: Self-Organization and Selection in Evolution . Oxford University Press, New York, 1993, ISBN 978-0-19-505811-6 .