Chemistry

Introduction to Molecular Self-Assembly
Molecular self-assembly is an evolving methodology that aims at producing desired structures of simple molecules and supramolecular specimen by fabricating them using nanotechnology. The desired structures can be formed due to their property of shape-complement. In this method the molecules adopt a distinct arrangement without regulation or management from an outside source.
Self-assembly is defined as the autonomous organization of components into patterns or structures without human intervention. (12 Whitesides  Grzybowski 2002, p.2).

Self-assembling process is widespread throughout nature and technology ranging from the non-covalent association of organic molecules in solution to the growth of semiconductor quantum dots on solid substrates.
Challenges Faced
The technology had a difficult time to begin with as most of it was only theoretical and later on computer simulated. The practical demonstration was not easily possible and was not funded by many universities adding to the woes of the researchers. Self-Assembly has been a challenging area due to the very fact that it is not even possible to mimic the process that occur in the biological process that are happening around. Though they seem to be elementary reactions, the how-tos remain a puzzle.  (13 Whitesides n.d.).

Self assembly has gained wide spread interest due to several reasons
Humans are fascinated by the appearance of order from disorder.
Living cells have the ability to self-assemble.
Self-assembly is an important process of nanotechnology for making ensembles of nanostructures.
Self-assembly can also be applied in the fields of manufacturing and robotics
Advantages of self-Assembly
Synthetic chemistry which has evolved quite highs is used in the most critical part of fabrication using nano technology.
Existing biological examples which have developed complex functional structures provide an unending set of real time data.
The final systems developed which are intended to be defect-free and self healing using this method can directly inherit the existing biological structures.

Types of Self-Assembly
There are different classifications for Self assembly. One way of classifying is intramolecular self-assembly and intermolecular self-assembly. The term molecular self-assembly often refers to intermolecular self-assembly, whereas the intramolecular analog is commonly known as folding. The protein folding theory tries to predict the tertiary and quaternary structure with the primarys information. (4 Jelercic 2009, p.4) Protein folding comes under intramolecular self-assembling molecules which assemble from the random coil conformation into a definite stable structure like secondary and tertiary structure. The formation of a micelle by surfactant molecules in solution comes under intermolecular self-assembly which form supramolecular assemblies like quaternary structure.

The other classification includes two kinds of self assembly  static and dynamic.
Static self-assembly involves systems that are at global or local equilibrium and do not dissipate energy. Static self-assembly involves systems currently at global or local equilibrium and which are not exothermic in nature. (12 Whitesides  Grzybowski 2002, p.2).

As the name refers, static self-assembly required external energy to be provided for the reaction to happen. This can be proving thermal, electrical or mere stirring. The formation of the crystal structure of ribosome, Lipid bilayer formation, base pairing and protein folding are classic examples of static self-assembly.

In Dynamic self-assembly, various interactions amongst the components which are responsible for the formation of structures or patterns occur if the system is exothermic. (12 Whitesides  Grzybowski 2002, p.2).

The patterns resulting in oscillating chemical reactions in time or space domain due to either chemical reaction or diffusion can be brought under the aegis of Dynamic self-assembly. Actin filaments, histones, chromatin and protein aggregates in signaling pathways come under dynamic self-assembly.

Figure  Pictorial representation of various self assembly methods (7 Ozin, et al., 2009)
Principles of Molecular Self-Assembly
For Self assembly molecular programming, supramolecular chemist needs to create nano- or macro scale structures which are composed of certain smaller molecules with defined control over its size, pattern and resolution or periodicity.
Principles of Self-assembly include
Components - which are a group of molecules or segments of a macromolecule that interact with one another which may be the same or different.

Interactions  The method in which equilibrium is attained amongst molecules during self-assembly through attraction and repulsion. It is needed to understand that self-organization is triggered by thermodynamic process and is a non-equilibrium process and the self-assembly is a process tending towards stable equilibrium. (2 Halley  Winkler 2010).

Reversibility (or Adjustability)  refers to the ability of components to generate ordered structure during self-assembly. For the same to happen, the covalent association must be either reversible or must allow re-ordering to a pattern upon completion of forming the required pattern.

Environment  refers to the interface or media which facilitates self assembly. This is generally a solution or an interface media which allows the motion of components for adjustability. The choice of the environment is very important as it affects the entire process of self-assembly and can show in results.

Mass Transport and Agitation  The molecules are energized to be in a state of agitation. This allows mass transport for the purpose of aligning to patterns in the process of self-assembly.
Molecular self-assembly is a bottom-up methodology where the preferred structure is achieved in shape with functional groups exploring nanometer-scale devices and nanostructure materials. This methodology offers advantages such as the three-dimensional assembly, possibility of inexpensive mass fabrication and achievement of atomic feature sizes. In future molecular nanotechnology, molecular self-assembly can be used to make microchips. Challenging molecular topologies like Borromean rings have been constructed using molecular self-assembly.

Research in the self-assembly domain has been going on in multiple dimensions. This area is considered more of a science than of a technology. As there are many aspects which are hidden in this area, efforts currently are on to study the fundamental attributes of non-covalent interactions in the process of molecular building blocks creation. As referred by the term self assembly, it is narrowed down to the blocks which during the process of creation instantaneously assemble in to predefined structures and patterns. The structures thus formed are meant for functional devices and materials. This process is completely reliable and versatile. Its rugged performance has been proven with the existence of various synthetic compounds. The high reliability of these compounds is due to the intrinsic error correction in the thermodynamic process during the assembly. Once this research reaches further heights, one can expect self-assembly to be used in other areas of research, production, technology and even day-to-day activities of a common man.

The formation of nanostructures is a success in the fields of nanotechnology using molecular self assembly methods. Self-assembly of two- and three-dimensional structures of molecules and nano-clusters is being inspired by various mechanisms such as LangmuirBlodgett films, electrostatic interactions, surface forces, chemical self-assembly, hydrophobic  hydrophilic interactions, and bimolecular-mediated self-assembly techniques.

The LB films are constructed by transferring mono-layers which float on water surface on to a solid substrate. These are expected to have a wide range of applications in electronic and bio-electronic devices. (5 Langmuir-blodgett films, n.d.)

Figure-2 Schematic illustration of layer-by-layer molecular self-assembly procedure for a nano-structured thin film (3 Huie 2003).

Combinatorial tools, known as soft lithography, such as microcontact printing and dip-pen nanolithography are also available which are used in combination with self-assembly methods to up-size nano-structured assembly patterns into micrometer scale level. (3 Huie 2003).

Future of Mankind  structured self-assembly with controlled and intelligent nanotechnology
One dimensional nano-structures are being experimented in various research labs and various new products are emanating from this study. This has also helped to understand certain patterns which were not identifiable earlier. A recent work devised was a 3D macroscopic sacs and membranes through interactions between large and small molecular structures. Further this was used to study piezoelectricity in non-centrosymmetric clusters and significant increase in conductivity in hexabenzocoronene nanotubes and Dendron-rod-coil ribbons. A extended study on modular structure termed the Dendron-rod-coil (DRC) to study each segment of this supra molecular structure. (8 Palmer  Stupp n.d).

News was on high when the computer giant IBM produced the first ever application of self-assembly technique in chip manufacturing. This has paved way for a huge area of improvement in the processor speeds and miniaturizing the electronic components.

Ongoing research in the self-assembly nanotechnology area have grown manifold in the recent years. The projects and research in this area are now directed in finding out the hidden treasure which exists around us in all forms various to facilitate better life for future generations.  
Laboratory research is on going for developing nano-particles of semiconductors into twisted ribbon structures using light as agitation source. The geometrical structure that resulted are in micro scale are in researched for further possible usage.

Recently revealed research has demonstrated that control in spatial distribution of nano particles is possible with energy sources such as light and heat without any chemical presence.
The achievement through this research was to obtain linear array structures of varied lengths in the nano scale. This research opens various ways to generate nano particles which can be used for the storage of solar energy which is now practically limited.

The self assembly was recently photographed using X-Ray crystallography and this became a source of more information than from the simulation which was done using computers. Given here is a wheel structure developed during one such experiment.

(10 Scientists photograph nano-particle self-assembly, 2010)
Post this capture, the scope of growth of nanotechnology base self-assembly is seen in the electronics  mainly semi conductors, medicine and drugs, creation of nano-sensors and nano functional equipment in the near future.

The devising of Nano-particle one-dimensional photonic crystals has opened the think tanks for developing improved devices for optical and optoelectronic equipments. This has popped up a new interdisciplinary field called Molecular Optoelectronics which uses the principles of the molecular bistability, conformational and positional switching. (14 Wild et al., n.d.). Another upcoming prospective area is in the field where this technology is inherently present  thats in the Deoxyribonucleic Acid (DNA). Understanding DNA itself was a challenge a few years back but now scientists are targeting to use the technology to store intelligence in the DNA and the formation of the double helix from two single strands using self-assembly. (1 Biomolecular self-assembly, 2005).

Figure These two hinged DNA arms (a) act as selfassembling tweezers when a third fuel strand is added (b) and pulls the two arms together (c). (9 Quellette n.d.). Flexible five digit monochrome display for smart cards has been developed using Fluid Self-Assembly in which large ICs are deposited on plastic substrates. This has found wide range applications in wireless, smartcard and e-technology business. (9 Quellette n.d.). A new array of lasers has been triggered with the advent of Dye-Anchored mesoporous metal-oxide electro-chemiluminiscent device. This laser has the ability of having a broad emission of the dye in a wide range of wavelengths in continuous or discrete patterns. The advantage over solid state laser or the gas laser is the selection of medium which is a solution in the case of self assembled technique.

Earlier, generation of nanocarbons required harsh chemical reactions to open up terminal ends. But now with self assembly it is possible to create the same with intrinsically open ends. (11 Shimizu n.d.). Miniaturized gas ionized sensors using nano carbons have marked the advent of this technology in the aerospace and nuclear engineering as well. These new nano tubes are compact, battery-powered and a quite safe. The sharp tips of nanotubes create high electric fields with low voltages utilization. This lowers the breakdown voltages manifold in comparison to traditional electrodes. (6 Modi, et al., 2003)

Figure Nanotubes generated using self-assembly. (11 Shimizu n.d.).
Optically encrypted and packed silver release in single dimensional photonic crystals is the latest creation of self-assembly using nanotechnology. These aid both in fundamental research and advanced labs and are picking up significant momentum in the commercial market as well.
Nanowires are the next big thing in production although its not seen to a normal eye. Recent chalked out strategy was to create ultrathin nanowires of Sb2S3 and Bi2S3. These wired have exceptional internal necklace structure, flexibility and colloidal stability. (7 Ozin, et al., 2009).

Another study funded by the department of energy has succeeded in creating nutraceuticals which is pharmacy combined with nutrients. With this study, a new set of capsules with nano sized particles are expected to be rolled out.

The development of supramolecular chemistry and nanochemistry experienced great growth in past couple of decades and has come up as important cross-disciplinary field. Nanoscience is developed from physics and materials science with the perception that at the nanoscale, physical properties of matter would undergo transitions and was mostly foreseen for inorganic substances.
Supramolecular chemistry is directed to the concept that chemists could control non-covalent bonds in all structures with the same accuracy achieved by synthetic organic chemistry and the principles of this demonstrated the possibility to craft the size, shape, and internal structure of nanoscale objects. As any other advancement in science, effective and intelligent usage is the need of the hour. These techniques have immense advantages most of which is still unknown. Scientists and researchers should focus their ability and knowledge in this domain for a constructive purpose enabling a cleaner and greener earth.