Self-assembly is a term used to describe processes in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction.
Self-assembly can be classified as either static or dynamic. In
static self-assembly, the ordered state forms as a system approaches
equilibriumIn thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. Classical thermodynamics deals with dynamic equilibrium states...
, reducing its free energy.
Self-assembly is a term used to describe processes in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction.
Self-assembly can be classified as either static or dynamic. In
static self-assembly, the ordered state forms as a system approaches
equilibriumIn thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. Classical thermodynamics deals with dynamic equilibrium states...
, reducing its free energy. However in
dynamic self-assembly, patterns of pre-existing components organized by specific local interactions are not commonly described as "self-assembled" by scientists in the associated disciplines. These structures are better described as "self-organized".
Self-assembly in chemistry and materials science
Self-assembly (SA) in the classic sense can be defined as
the spontaneous and reversibleReversibility can refer to:* Reversible dynamics, a mathematical dynamical system, or physical laws of motion, for which time-reversed dynamics are well defined* Reversible diffusion, an example of a reversible stochastic process...
organization of molecular units into ordered structures by non-covalent interactions. The first property of a self-assembled system that this definition suggests is the spontaneity of the self-assembly process: the interactions responsible for the formation of the self-assembled system act on a strictly local level—in other words,
the nanostructureA nanostructure is an object of intermediate size between molecular and microscopic structures.In describing nanostructures it is necessary to differentiate between the number of dimensions on the nanoscale. Nanotextured surfaces have one dimension on the nanoscale, i.e., only the thickness of the...
builds itself.
Distinctive features
At this point, one may argue that any chemical reaction driving atoms and molecules to assemble into larger structures, such as
precipitationPrecipitation is the formation of a solid in a solution during a chemical reaction. When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate...
, could fall into the category of SA. However, there are at least three distinctive features that make SA a distinct concept.
Order
First, the self-assembled structure must have a higher
orderOrder in a crystal lattice is the arrangement of some property with respect to atomic positions. It arises in charge ordering, spin ordering, magnetic ordering, and compositional ordering. It is a thermodynamic entropy concept often displayed by a second order phase transition...
than the isolated components, be it a shape or a particular task that the self-assembled entity may perform. This is generally not true in
chemical reactionA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. They are studied by chemists under a field of science called chemistry. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, often coming about...
s, where an ordered state may proceed towards a disordered state depending on thermodynamic parameters.
Interactions
The second important aspect of SA is the key role of weak interactions (e.g.
Van der WaalsIn physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the attractive or repulsive force between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules...
,
capillaryCapillary action, capillarity, capillary motion, or wicking refers to two phenomena:1 The movement of liquids in thin tubes.
2 The flow of liquids through porous media, such as the flow of water through soil....
, ,
hydrogen bondA hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, like nitrogen, oxygen or fluorine . The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s) with respect to more "traditional" covalent,
ionicAn ionic bond is a type of chemical bond that involves a metal and a nonmetal ion through electrostatic attraction. In short, it is a bond formed by the attraction between two oppositely charged ions....
or
metallic bondMetallic bonding is the electromagnetic interaction between delocalized electrons, called conduction electrons and gathered in an "electron sea", and the metallic nuclei within metals...
s. Although typically less energetic of a factor 10, these weak interactions play an important role in materials synthesis. It can be instructive to note how weak interactions hold a prominent place in materials, but especially in biological systems, although they are often considered marginally with respect to "strong" (i.e. covalent, etc.) interactions. For instance, they determine the physical properties of liquids, the
solubilitySolubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a liquid solvent to form a homogeneous solution. The solubility of a substance strongly depends on the used solvent as well as on temperature and pressure...
of solids, the organization of molecules in biological membranes.
Building blocks
The third distinctive feature of SA is that the building blocks are not only atoms and molecules, but span a wide range of nano- and mesoscopic structures, with different chemical compositions, shapes and functionalities. These nanoscale building blocks (NBBs) can in turn be synthesised through conventional chemical routes or by other SA strategies.
Examples
Important examples of SA in materials science include the formation of molecular
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography...
s,
colloidA colloid is a type of chemical mixture in which one substance is dispersed evenly throughout another.The particles of the dispersed substance are only suspended in the mixture, unlike in a solution, in which they are completely dissolved...
s,
lipid bilayerA lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus and...
s, phase-separated polymers, and
self-assembled monolayerA self assembled monolayer is an organized layer of amphiphilic molecules in which one end of the molecule, the “head group” shows a special affinity for a substrate...
s. The folding of polypeptide chains into proteins and the folding of nucleic acids into their functional forms are examples of self-assembled biological structures.
Properties
Therefore, we can say that SA extends the scope of chemistry aiming at
synthesisingIn chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
products with order and functionality properties, extending chemical bonds to weak interactions and encompassing the self-assembly of NBBs on all length scales. In covalent synthesis and polymerisation, the scientist links atoms together in any desired conformation, which does not necessarily have to be the energetically most favoured position; self-assembling molecules, on the other hand, adopt a structure at the thermodynamic minimum, finding the best combination of interactions between subunits but not forming covalent bonds between them. In self-assembling structures, the scientist must predict this minimum, not merely place the atoms in the location desired.
Another characteristic common to nearly all self-assembled systems is their
thermodynamic stability. For SA to take place without intervention of external forces, the process must lead to a lower
Gibbs free energyIn thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system...
, thus self-assembled structures are thermodynamically more stable than the single, unassembled components. A direct consequence is the general tendency of self-assembled structures to be relatively free of defects. An example is the formation of two-dimensional
superlatticeSuperlattice is a periodic structure of repeating quantum wells that sets up a new set of selection rules which affects the conditions for charges to flow through the structure. This nanostructure consists of two different semiconductor materials, which are deposited alternately on each other to...
s composed of an orderly arrangement of micrometre-sized polymethylmethacrylate (PMMA) spheres, starting from a solution containing the microspheres, in which the solvent is allowed to evaporate slowly in suitable conditions. In this case, the driving force is capillary interaction, which originates from the deformation of the surface of a liquid caused by the presence of floating or submerged particles.
These two properties—weak interactions and thermodynamic stability—can be recalled to rationalise another property often found in self-assembled systems: the
sensitivity to perturbations exerted by the external environment. These are small fluctuations that alter thermodynamic variables that might lead to marked changes in the structure and even compromise it, either during or after SA. The weak nature of interactions is responsible for the flexibility of the architecture and allows for rearrangements of the structure in the direction determined by thermodynamics. If fluctuations bring the thermodynamic variables back to the starting condition, the structure is likely to go back to its initial configuration. This leads us to identify one more property of SA, which is generally not observed in materials synthesised by other techniques:
reversibility.
From what we have written so far, it should be evident that SA is a process which is easily influenced by external parameters: if this can make synthesis more problematic due to the many free parameters that require control, on the other hand it has the exciting advantage that a large variety of shapes and functions on many length scales can be obtained.
Generally speaking, the fundamental condition needed for NBBs to self-assemble into an ordered structure is the simultaneous presence of long-range repulsive and short-range attractive forces. Figure [fig_sa_scheme] exemplifies SA occurring from building blocks made up of two different units (A and B) which are covalently linked (short-range attraction) and repel each other by long-range interactions (e.g. because A is hydrophobic and B is hydrophilic). Since the energy of the system will unfavour configurations where A is close to B, and still no macrophase separation is possible due to A—B covalent bonds, the system will adopt a configuration where the contact area between A and B is minimised. This results in a periodic ordered structure (in figure [fig_sa_scheme] this is exemplified by a lamellar structure). For a list of repulsive-attractive competing forces which can give rise to SA phenomena .
By choosing
precursorIn chemistry, a precursor is a compound that participates in the chemical reaction that produces another compound. In biochemistry, the term "precursor" is used more specifically to refer to a chemical compound preceding another in a metabolic pathway....
s with suitable physicochemical properties, it is possible to exert a fine control on the formation processes that produce complex structures. Clearly, the most important tool when it comes to designing a synthesis strategy for a material, is the knowledge of the chemistry of the building units.
Consistent concepts of self-organization and self-assembly
Self-organizationSelf-organization is a process of attraction and repulsion in which the internal organization of a system, normally an open system, increases in complexity without being guided or managed by an outside source...
and self-assembly are regularly used interchangeably. As complex system science becomes more popular though, there is a higher need to clearly distinguish the differences between the two mechanisms to understand their significance in physical and biological systems. Both processes explain how collective order is developed from “dynamic small-scale interactions” according to an article in a November/December 2008 issue of Complexity.
http://www3.interscience.wiley.com/journal/121358448/abstract?CRETRY=1&SRETRY=0 Self-organization is a nonequilibrium process where self-assembly is a spontaneous process that leads toward equilibrium. Self-assembly requires components to remain essentially unchanged throughout the process. Besides the thermodynamic difference between the two, there is also a difference in formation. The first difference is what “encodes the global order of the whole” in self-assembly where as in self-organization these initial encodings are not necessary. Another slight contrast refers to the minimum number of units needed to make an order. Self-organization appears to have a minimum number of units where as self-assembly does not. These terms are becoming more necessary as more is learned about natural selection. Eventually, these patterns may form one theory regarding the pattern formation in nature.
See also
- Self-organization
Self-organization is a process of attraction and repulsion in which the internal organization of a system, normally an open system, increases in complexity without being guided or managed by an outside source...
- Nanotechnology
Nanotechnology, shortened to "nanotech", is the study of the control of matter on an atomic and molecular scale. Generally nanotechnology deals with structures of the size 100 nanometers or smaller, and involves developing materials or devices within that size.Nanotechnology is very diverse,...
- Langmuir–Blodgett film
- Autopoiesis
Autopoiesis literally means "auto -creation" , and expresses a fundamental dialectic between structure and function.-Meaning:...
- Molecular self-assembly
Molecular self-assembly is the process by which molecules adopt a defined arrangement without guidance or management from an outside source. There are two types of self-assembly, intramolecular self-assembly and intermolecular self-assembly...
External links and further reading
- Book chapter "Challenges and breakthroughs in recent research on self-assembly", Sci. Technol. Adv. Mater. 9 (2007) 014109 (96 pages) (free download)
- Kuniaki Nagayama, Freeview Video 'Self-Assembly: Nature's Way To Do It, A Royal Institution Lecture by the Vega Science Trust.
- Paper Molecular Self-Assembly
- Paper Beyond molecules: Self-assembly of mesoscopic and macroscopic components
- Whitesides, G. M. & Grzybowski, B. (2002) Science 295, 2418-2421.
- Rothemund PWK, Papadakis N, Winfree E (2004) Algorithmic Self-Assembly of DNA Sierpinski Triangles. PLoS Biol 2(12)
- Wiki: C2 Self Assembly from a computer programming perspective.
- Pelesko, J.A., (2007) Self Assembly: The Science of Things That Put Themselves Together, Chapman & Hall/CRC Press.
- A brief page on DNA self-assembly DNA Self Assembly for Nanotechnology
- A brief page on self-assembly at the University of Delaware Self Assembly
- Mohammadzadegan R, Sheikhi MH (2007) DNA Nano-Gears Molecular Simulation 33(13); 1071-1081.
- Structure and Dynamics of Organic Nanostructures
- Metal organic coordination networks of oligopyridines and Cu on graphite