Self-assembly in chemistry and materials science Self-assembly

1 self-assembly in chemistry , materials science

1.1 distinctive features

1.1.1 order
1.1.2 interactions
1.1.3 building blocks


1.2 examples
1.3 properties

self-assembly in chemistry , materials science

the dna structure @ left (schematic shown) self-assemble structure visualized atomic force microscopy @ right. image strong.

self-assembly (sa) in classic sense can defined spontaneous , reversible organization of molecular units ordered structures non-covalent interactions. first property of self-assembled system definition suggests spontaneity of self-assembly process: interactions responsible formation of self-assembled system act on strictly local level—in other words, nanostructure builds itself.

self-assembled nano-structure object appears result of ordering , aggregation of individual nano-scale objects guided physical principle.

self-assembled nanostructure arises in strong non-equilibrium conditions. famous example of self-assembly phenomenon occurrence of life on earth. plausible hypothesize happens because sun generates strong temperate gradient in environment. general idea has been confirmed in experiment of self-assembly of carbon nanotubes.

another interesting example of self-assembly phenomenon of electrostatic trapping. in case electric field applied between 2 metallic nano-electrodes. particles present in environment polarized applied electric field. due dipole interaction electric field gradient particles attracted gap between electrodes.

fabricating crystal placing atom after atom not realistic. self-assembly of crystals works well. fabricating 3d nano-structure not realistic well. self-assembly of 3d nano-structure becomes easy , inexpensive way fabricate them.

nano-structures such nano-vacuum gaps used storing energy , nuclear energy conversion. self-assembled tunable materials promising candidates large surface area electrodes in batteries , organic photovoltaic cells, microfluidic sensors , filters.

distinctive features

at point, 1 may argue chemical reaction driving atoms , molecules assemble larger structures, such precipitation, fall category of sa. however, there @ least 3 distinctive features make sa distinct concept.

order

first, self-assembled structure must have higher order isolated components, shape or particular task self-assembled entity may perform. not true in chemical reactions, ordered state may proceed towards disordered state depending on thermodynamic parameters.

interactions

the second important aspect of sa key role of slack interactions (e.g. van der waals, capillary,



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−
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, hydrogen bonds) respect more traditional covalent, ionic, or metallic bonds. although typically less energetic factor of 10, these weak interactions play important role in materials synthesis. can instructive note how slack interactions hold prominent place in materials, in biological systems, although considered marginal respect strong (i.e. covalent, etc.) interactions. instance, determine physical properties of liquids, solubility of solids, , organization of molecules in biological membranes.

building blocks

the third distinctive feature of sa building blocks not atoms , molecules, span wide range of nano- , mesoscopic structures, different chemical compositions, shapes , functionalities. research possible three-dimensional shapes of self-assembling micrites examines platonic solids (regular polyhedral). term ‘micrite’ created darpa refer sub-millimeter sized microrobots, self-organizing abilities may compared of slime mold. recent examples of novel building blocks include polyhedra , patchy particles. examples included microparticles complex geometries, such hemispherical, dimer, discs, rods, molecules , multimers. these nanoscale building blocks (nbbs) can in turn synthesised through conventional chemical routes or other sa strategies such directional entropic forces.

examples

important examples of sa in materials science include formation of molecular crystals, colloids, lipid bilayers, phase-separated polymers, , self-assembled monolayers. folding of polypeptide chains proteins , folding of nucleic acids functional forms examples of self-assembled biological structures. recently, three-dimensional macroporous structure prepared via self-assembly of diphenylalanine derivative under cryoconditions, obtained material can find application in field of regenerative medicine or drug delivery system. p. chen et al. demonstrated microscale self-assembly method using air-liquid interface established faraday wave template. self-assembly method can used generation of diverse sets of symmetrical , periodic patterns microscale materials such hydrogels, cells, , cell spheroids.

properties

sa extends scope of chemistry aiming @ synthesising products order , functionality properties, extending chemical bonds weak interactions , encompassing self-assembly of nbbs on length scales. in covalent synthesis , polymerisation, scientist links atoms in desired conformation, not have energetically favoured position; self-assembling molecules, on other hand, adopt structure @ thermodynamic minimum, finding best combination of interactions between subunits not forming covalent bonds between them. in self-assembling structures, scientist must predict minimum, not merely place atoms in location desired.

another characteristic common self-assembled systems thermodynamic stability. sa take place without intervention of external forces, process must lead lower gibbs free energy, self-assembled structures thermodynamically more stable single, unassembled components. direct consequence general tendency of self-assembled structures relatively free of defects. example formation of two-dimensional superlattices composed of orderly arrangement of micrometre-sized polymethylmethacrylate (pmma) spheres, starting solution containing microspheres, in solvent allowed evaporate in suitable conditions. in case, driving force capillary interaction, originates deformation of surface of liquid caused presence of floating or submerged particles.

these 2 properties—weak interactions , thermodynamic stability—can recalled rationalise property found in self-assembled systems: sensitivity perturbations exerted external environment. these small fluctuations alter thermodynamic variables might lead marked changes in structure , compromise it, either during or after sa. weak nature of interactions responsible flexibility of architecture , allows rearrangements of structure in direction determined thermodynamics. if fluctuations bring thermodynamic variables starting condition, structure go initial configuration. leads identify 1 more property of sa, not observed in materials synthesised other techniques: reversibility.

sa process influenced external parameters. can make synthesis more problematic due many free parameters require control. on other hand, self assembly has exciting advantage large variety of shapes , functions on many length scales can obtained.

generally speaking, fundamental condition needed nbbs self-assemble ordered structure simultaneous presence of long-range repulsive , short-range attractive forces.

by choosing precursors suitable physicochemical properties, possible exert fine control on formation processes produce complex structures. clearly, important tool when comes designing synthesis strategy material, knowledge of chemistry of building units. example, demonstrated possible use diblock copolymers different block reactivities in order selectively embed maghemite nanoparticles , generate periodic materials potential use waveguides.

in 2008, advances in colloid , interface science published study in concluded every self-assembly process in reality presents co-assembly, makes former term misnomer of kind. thesis built on concept of mutual ordering of self-assembling system , environment.