Self-assembly

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.

[10] 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.

[11] 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.

[6] 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,[14] and shapes.

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.

These nanoscale building blocks can in turn be synthesized through conventional chemical routes or by other self-assembly strategies such as directional entropic forces.

[6] In either the enthalpic or entropic case, self-assembly proceeds through the formation and breaking of bonds,[22] possibly with non-traditional forms of mediation.

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.

[25] P. Chen et al. demonstrated a microscale self-assembly method using the air-liquid interface established by Faraday wave as a template.

[26] Yasuga et al. demonstrated how fluid interfacial energy drives the emergence of three-dimensional periodic structures in micropillar scaffolds.

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.

[30] 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.

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.

[31] 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.

[32] By choosing precursors with suitable physicochemical properties, it is possible to exert a fine control on the formation processes that produce complex structures.

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.

[34] The most common examples of self-assembly at the macroscopic scale can be seen at interfaces between gases and liquids, where molecules can be confined at the nanoscale in the vertical direction and spread over long distances laterally.

[35][36] Another remarkable example of macroscopic self-assembly is the formation of thin quasicrystals at an air-liquid interface, which can be built up not only by inorganic, but also by organic molecular units.

[37][38] Furthermore, it was reported that Fmoc protected L-DOPA amino acid (Fmoc-DOPA)[39][40] can present a minimal supramolecular polymer model, displaying a spontaneous structural transition from meta-stable spheres to fibrillar assemblies to gel-like material and finally to single crystals.

[43] Since the 1950s, scientists have built self-assembly systems exhibiting centimeter-sized components ranging from passive mechanical parts to mobile robots.

Self-assembly of lipids (a), proteins (b), and (c) SDS - cyclodextrin complexes. SDS is a surfactant with a hydrocarbon tail (yellow) and a SO 4 head (blue and red), while cyclodextrin is a saccharide ring (green C and red O atoms).
Transmission electron microscopy image of an iron oxide nanoparticle . Regularly arranged dots within the dashed border are columns of Fe atoms. Left inset is the corresponding electron diffraction pattern. Scale bar: 10 nm. [ 1 ]
Iron oxide nanoparticles can be dispersed in an organic solvent ( toluene ). Upon its evaporation, they may self-assemble (left and right panels) into micron-sized mesocrystals (center) or multilayers (right). Each dot in the left image is a traditional "atomic" crystal shown in the image above. Scale bars: 100 nm (left), 25 μm (center), 50 nm (right). [ 1 ]
AFM imaging of self-assembly of 2-aminoterephthalic acid molecules on (104)-oriented calcite . [ 3 ]
The DNA structure at left ( schematic shown) will self-assemble into the structure visualized by atomic force microscopy at right.