structure at left (
schematic shown) will self-assemble into the structure visualized by
atomic force microscopy at right. Self-assembly in the classic sense can be defined as
the spontaneous and reversible 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 nanostructure builds itself. Although self-assembly typically occurs between weakly-interacting species, this organization may be transferred into strongly-bound
covalent systems. An example for this may be observed in the self-assembly of
polyoxometalates. Evidence suggests that such molecules assemble via a dense-phase type
mechanism whereby small oxometalate ions first
assemble non-covalently in solution, followed by a
condensation reaction that covalently binds the assembled units. This process can be aided by the introduction of templating agents to control the formed species. In such a way, highly organized covalent molecules may be formed in a specific manner. Self-assembled nano-structure is an object that appears as a result of ordering and aggregation of individual nano-scale objects guided by some
physical principle. A particularly counter-intuitive example of a physical principle that can drive self-assembly is
entropy maximization. Though entropy is conventionally
associated with disorder, under suitable conditions Another important class of self-assembly is field-directed assembly. An example of this is the phenomenon of electrostatic trapping. In this case an
electric field is applied between two metallic nano-electrodes. The particles present in the environment are polarized by the applied electric field. Because of dipole interaction with the electric field gradient the particles are attracted to the gap between the electrodes. Generalizations of this type approach involving different types of fields, e.g., using magnetic fields, using capillary interactions for particles trapped at interfaces, elastic interactions for particles suspended in liquid crystals have also been reported. Regardless of the mechanism driving self-assembly, people take self-assembly approaches to materials synthesis to avoid the problem of having to construct materials one building block at a time. Avoiding one-at-a-time approaches is important because the amount of time required to place building blocks into a target structure is prohibitively difficult for structures that have macroscopic size. Once materials of macroscopic size can be self-assembled, those materials can find use in many applications. For example, nano-structures such as nano-vacuum gaps are used for storing energy and nuclear energy conversion. Self-assembled
tunable materials are promising candidates for large surface area electrodes in
batteries and organic photovoltaic cells, as well as for microfluidic sensors and filters.
Distinctive features At this point, one may argue that any chemical reaction driving atoms and molecules to assemble into larger structures, such as
precipitation, could fall into the category of self-assembly. However, there are at least three distinctive features that make self-assembly a distinct concept.
Order First, the self-assembled structure must have a higher
order 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 reactions, where an ordered state may proceed towards a disordered state depending on thermodynamic parameters.
Interactions The second important aspect of self-assembly is the predominant role of weak interactions (e.g.
Van der Waals,
capillary,
\pi-\pi,
hydrogen bonds, or
entropic forces) compared to more "traditional" covalent,
ionic, or
metallic bonds. These weak interactions are important in materials synthesis for two reasons. First, weak interactions take a prominent place in materials, especially in biological systems. For instance, they determine the physical properties of liquids, the
solubility of solids, and the organization of molecules in biological membranes. Second, in addition to the strength of the interactions, interactions with varying degrees of specificity can control self-assembly. Self-assembly that is mediated by DNA pairing interactions constitutes the interactions of the highest specificity that have been used to drive self-assembly. At the other extreme, the least specific interactions are possibly those provided by
emergent forces that arise from entropy maximization.
Building blocks The third distinctive feature of self-assembly 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, functionalities, and shapes. Research into possible three-dimensional shapes of self-assembling micrites examines
Platonic solids (regular polyhedral). The term 'micrite' was created by
DARPA to refer to sub-millimeter sized
microrobots, whose self-organizing abilities may be compared with those of
slime mold. Recent examples of novel building blocks include
polyhedra and
patchy particles. dimer, discs, rods, molecules, as well as multimers. These nanoscale building blocks can in turn be synthesized through conventional chemical routes or by other self-assembly strategies such as
directional entropic forces. More recently, inverse design approaches have appeared where it is possible to fix a target self-assembled behavior, and determine an appropriate building block that will realize that behavior. possibly with non-traditional forms of mediation. The kinetics of the self-assembly process is usually related to
diffusion, for which the absorption/adsorption rate often follows a
Langmuir adsorption model which in the diffusion controlled concentration (relatively diluted solution) can be estimated by the
Fick's laws of diffusion. The desorption rate is determined by the bond strength of the surface molecules/atoms with a thermal
activation energy barrier. The growth rate is the competition between these two processes.
Examples Important examples of self-assembly in materials science include the formation of molecular
crystals,
colloids,
lipid bilayers, phase-separated polymers, and
self-assembled monolayers. The folding of polypeptide chains into proteins and the folding of nucleic acids into their functional forms are examples of self-assembled biological structures. Recently, the three-dimensional macroporous structure was prepared via self-assembly of diphenylalanine derivative under cryoconditions, the obtained material can find the application in the field of regenerative medicine or drug delivery system. P. Chen et al. demonstrated a microscale self-assembly method using the air-liquid interface established by
Faraday wave as a template. This self-assembly method can be used for generation of diverse sets of symmetrical and periodic patterns from microscale materials such as
hydrogels, cells, and cell spheroids. Yasuga et al. demonstrated how fluid interfacial energy drives the emergence of three-dimensional periodic structures in micropillar scaffolds. Myllymäki et al. demonstrated the formation of micelles, that undergo a change in morphology to fibers and eventually to spheres, all controlled by solvent change.
Properties Self-assembly extends the scope of chemistry aiming at
synthesizing products with order and functionality properties, extending chemical bonds to weak interactions and encompassing the self-assembly of nanoscale building blocks at all length scales. In covalent synthesis and polymerization, 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 self-assembly to take place without intervention of external forces, the process must lead to a lower
Gibbs free energy, 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
superlattices 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 self-assembly. 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 self-assembly, which is generally not observed in materials synthesized by other techniques:
reversibility. Self-assembly is a process which is easily influenced by external parameters. This feature can make synthesis rather complex because of the need to control many free parameters. Yet self-assembly has the advantage that a large variety of shapes and functions on many length scales can be obtained. The fundamental condition needed for nanoscale building blocks to self-assemble into an ordered structure is the simultaneous presence of long-range repulsive and short-range attractive forces. By choosing
precursors 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. For example, it was demonstrated that it was possible to use
diblock copolymers with different block reactivities in order to selectively embed
maghemite nanoparticles and generate periodic materials with potential use as
waveguides. In 2008 it was proposed that every self-assembly process presents a co-assembly, which makes the former term a misnomer. This thesis is built on the concept of mutual ordering of the self-assembling system and its environment. == At the macroscopic scale ==