Mesoscale self-assembly is the self-assembly of objects ranging in size from several nanometers to a few hundreds of microns through non-covalent forces like capillary, magnetic, electrostatic, and light forces. This involves the extension of ideas emerging from molecular self-assembly to larger objects. The self-assembly of objects in meso-scale can be used for the fabrication of complex systems which has potential application in microelectronics, optics, displays and microelectromechanical systems.
Regular arrays of topologically complex, meso-scale objects can be prepared by self-assembly in fluids, with the structure of the arrays determined by shapes of the assembling objects, the wettability of their surfaces and the lateral capillary forces at the interfaces. Self-assembly results from minimization of the interfacial free energy of the liquid-liquid interface. The capillary interactions between objects can be viewed as a type of 'bond' that is analogous to chemical bonds that act between atoms and molecules.
Examples of MESA include
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Nanocrystals of CdSe assembled into 3D array by changing solvent polarity
The crystallization of polystyrene spheres assembled on a glass plate or Hg surface using capillary or electrostatic forces (micron-scale self-assembly); and
The formation of ordered structures from polyhedral objects of polydimethylsiloxane using lateral capillary forces (millimeter-scale self-assembly).
Assembly by capillary forces relies upon particle-particle interaction and interaction between particles and their enviornment. In two dimensions, capillary forces result from interaction between the meniscus or interface along the faces of the objects.
From the aspect of holding the components in contact, there are certain similarities between bonding due to capillary forces and covalent atomic bonds. For example, both types of forces are reversible up to certain levels of energy and both forces can be described in terms of overlap of some component in space, the menisci for capillary and wavefunction in the case of covalent bonds. The capillary bonds can be considered as analogous to the chemical bonds leading to molecular self-assembly.
In terms of physical principles, interactions by capillary forces are different from those of covalent bonds between atoms or molecules; a capillary bond results due to high surface tension at the interface between fluids and is due to increased order or decreased entropy of water near the interface. Whereas the covalent bonds between atoms or molecules are primarily due to electrostatic forces. Capillary forces of interaction can be described by classical mechanics while covalent bonds need invoking of quantum mechanics.
Coercing colloids are 3D photonic crystals of colloidal particles with periodic dielectric lattices (periodic optical nanostructures designed to influence the motions of photons) an optical analog of electronic semiconductor (periodicity of potential in semiconductor crystal affecting the motion of electrons). These colloidal crystals and films are prepared by electrophoresis, sedimentation or evaporation induced self-assembly.
These photonic crystals active in visible and near-IR wavelengths would possess exciting optical properties and find applications in signal processing, switching and sensing of light, optical computing and telecommunications. Three-dimensional colloidal photonic crystal can be utilized as optical pulse control devices for ultra-short laser pulses (femtosecond) and high power laser application.
Bioinspired colloidal photonic crystals also find application in templating, catalysis, chromatographic separations and color-based sensors for monitoring changes in environment.
Pixelated photonic crystal films capable of producing red, green, blue and white can be used directly in liquid-crystal display (LCD) devices as replacements for the color filter and backlight units
Several research groups have fabricated optical circuits out of colloidal particles and demonstrated their great potential.
The supramolecular structures are self-assembled structures consisting of two or more molecules, stabilised, guided and governed byÂ intermolecular interactions rather than by traditional covalent bonds. The intermolecular interactions in the supramolecules could be Electrostatic, Hydrogen bonding, ï°-ï° stacking interactions, Van der Waals forces and hydrophobic effects. Simplest example is a host-guest complex. The host is generally a large organic molecule with a cavity and the guest is a small molecule or ion. The host must have spaced-out binding sites with correct size, shape and electronic character to complement and interact with those of the guest like a lock and key.
The stable host-guest complexes are obtained with hosts pre-organised for ideal binding of the guest. Some of the common host molecules are porphyrins, cyclodextrins, metallocrowns, crown ethers, zeolites, cryptophanes etc., Host-guest complexes are observed in Intercalation compounds, inclusion compounds, clatharates, molecular tweezers and biological enzymes.
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Porphyrins Crown ethers Cyclodextrin - Host molecules
At the supramolecular level, recognition and self-assembling of molecules are guided by 'complementarity'. For example,
i) dissimilar portions of functional groups interact with one another;
ii) an electropositive hydrogen bond donor approaching an electronegative acceptor (DÎ´âˆ’ ï‚¾ HÎ´+Â· ïƒ-ïƒ-ïƒ-ïƒ-AÎ´âˆ’);
iii) electrostatic cation-anion interaction in salts and metal complexes (M+Xâˆ’);
iv) protrusion in one part of the molecule ï¬ts into hollows of another portion (hydrophobic interactions), and so on.
Recognition of Complementary shape and bonding feature of interacting molecules give rise to supermolecule and periodic arrangement of supermolecules in a crystal lattice.
By specifically choosing the functional molecular building blocks, weak and selective noncovalent linkages can guide the formation of highly directed crystal packing with molecular-level nanoarchitectural feature control.
Self-assembly and self-organization leads to the formation of organized, discrete aggregates of various intriguing structures like boxes, squares, helicates, catenanes, capsules, grids and many others. Ordered entities of even higher dimensions is obtained by self-organizing these discrete aggregates into an extended lattice and ensembles such as molecular crystals, liquid crystals, micelles, phase-separated polymers, and colloids.
For example, coordination compounds of Co, Cu and Zn with bi or tridentate pyrazolyl and pyridyl ligands showed a C-H Â· Â· Â· Cl-M interaction leading to inorganic supramolecular structures.
Supramolecular crystal lattice self- assembly opens up new avenues for novel nanostructured materials with unique functional properties and for miniaturization and nanofabrication technologies.Â Self-organized supramolecular organic nanostructures have potential applications in molecular electronics, photonics and as precursors for nanoporous catalysts.
Supramolecular metal-ligand assembly finds application as host for catalytic reactions in organic chemistry where the restricted space of the supramolecular structure forces reacting substrates to adopt specific conformations and hence enhance the selectivity as well as rate of the reactions up to a 1000times.
Supramolecular architectures are used to obtain efficient near-IR photoluminescence and electroluminescence using a three-dimensional Ï€ conjugation. Their photoluminescence properties can be tuned for different applications like polarized emission and optically pumped lasers.
Self-assembled monolayers (SAMs) are highly ordered supramolecular arrays of molecules chemisorbed on metal substrates and the design of their molecular structure is crucial to confer specific functions to the resulting surface. SAMs are of technological interest as they can be incorporated in devices like sensors and organic thin-film transistors to modify the metal/organic semiconductor interface properties and hence performance of devices.
Â Nanocrystals and clusters
Nanocrystals are crystals with at least one dimension between 1 and 100 nm.
Nanoclusters are groups of atoms or molecules with an intermediate state of matter between molecules and solids. The diameter of nano clusters range from sub-nanometer to about 10nanometres. Nanoclusters consist up to a few hundreds of atoms, while larger aggregates containing 103Â or more atoms are often called nanoparticles. The properties of nanoparticles gradually approach those of bulk materials. Nanoclusters, have properties and structures which are very sensitive to their composition and size (to the count of every atom) which can lead to new and interesting properties not realised in the corresponding bulk material.Â Traditional materials in the micrometer size particles replaced by metal and semiconductor clusters seem to hold the promise to future leading to miniaturization of devices and eventually a big jump in the world of novel technologies.
Many techniques are developed to produce clusters for use in different applications like thin film manufacture for advanced electronic or optical devices, production of nanoporous structures, and fabrication of thin membranes of nanoporous materials.
Nanoclusters of technologically important inorganic bulk materials e.g SiO2, ZnO, CdS were found to be photoinitiators while their corresponing bulk materials did not exhibit this property; ZnS clusters and its aggregates act as effective photocatalysts in reduction of organic compounds.
The nanoclusters of cerium oxide (CeO2-x) materials have been found to possess a significant concentration of Ce3+Â and oxygen vacancies resulting in excellent poisoning resistance against H2O and CO2Â and substantial reduction in the temperature of selective SO2Â reduction by CO.
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"Nanocrystalline materials" areÂ nano scale particles (dimension less than a micron) with aÂ crystallineÂ structure (single crystal or polycrystalline).
Crystalline nanoparticles are of interest for many reasons; reduction in the size of crystals has dramatic effects on the properties of bulk materials since more number of atoms in solid is found on grain boundaries where they behave differently from those that are not. The nanocrystalline materials composed of crystallites in the 1-10 nm size range possess very high surface area to volume ratios due to the fine grain size.
Nanocrystalline metals exhibit like increased strength, hardness, improved magnetic properties. Semiconductor nanocrystals in <10Â nm size range are often referred to asÂ quantum dots. Ceramic nanocrystalline materials show increase in ductility, improved toughness, reduced brittleness and increased wear resistance.
Crystalline nanoparticles used in a solar panel is claimed to have 12% efficiency (while conventional organic solar panel has 9% efficiency) and is more flexible and cheaper than many other panels.
Crystalline nanoparticles ofÂ zeoliteÂ are reportedly used as filter to convert crude oil to diesel fuel by a method cheaper than the conventional way.
Nanocrystalline and nanoporous Si showed promise due to its electroluminescence, photoluminescence and refractive index changes that could be controlled.
The size control of nanoscale catalyst particles has been achieved in preparation of highly dispersed metal colloids and metal clusters fixed on to substrates by Electro-chemical reduction of metal salts.
Materials with higher hydrogen storage per unit volume and weight and considerably increased rate of hydrogen adsorption, like mixtures of nanoscale particles of Mg and Mg2Ni with high surface area have been produced by mechanical means.
Fullerenes are the third allotropic form of carbon material (after graphite and diamond). Its discovery won a Nobel prize in chemistry in 1996. Fullerenes are closed-cage molecules made entirely of carbon in the form of a hollow sphere, ellipsoid or tube. It is represented as Cn, and contains 12 pentagons and a variable number of hexagons. Individual molecules of fullerenes are about 1 nm in diameter. Fullerenes were named after Richard Buckminster Fuller, an architect who designed theÂ geodesicÂ domes which resemble spherical fullerenes in appearance.
C60 was first produced by physicists W. Krätschmer and D.R. Huffman by creating an arc between two graphite rods in a helium atmosphere and extracting the carbon condensate formed, using an organic solvent. The fullerenes are soluble in common solvents such as carbon disulphide, benzene, toluene or chloroform.
Spherical structure of Fullerene C60 with Carbon at each vertex.
Types of Fullerene:
Since their discovery, many structural variations and modifications of Fullerene have been prepared and reported. Some of them are:
Buckyball clusters: Fullerenes having less than 300 carbon atoms are commonly known as "buckyballs". Fullerene with smallest number of carbon atoms known is C20. This is found in nature in soot of coal. The most common and abundant fullerene, C60 is called buckminsterfullerene.
Nanotubes: Nanotubes are hollow cylindrical tubes of carbon which are a few nanometers in diameter and upto several microns in length, with single or multiple walls.
Megatubes: These are tubes with larger diameter than nanotubes and their walls are made of varying thickness.
Polyfullerenes: Polymers of fullerenes are made of C60 and C70 balls linked by covalent bonds in different ways like directly linked to each other, cross-linked, linked through a polymer backbone, through heteroatoms and so on.
Endohedral Fullerenes: Fullerene derivatives with various atoms enclosed inside. For example, when metal atoms are enclosed they are called metallo-fullerenes
Exohedral fullerenes and Heterofullerenes: Exohedral fullerenes are fullerene derivatives formed by addition or redox reactions on the surface of the fullerene. Heterofullerenes are heteroanalogues of C60 or higher fullerenes with one or moe carbon atoms of the cage replaced with trivalent heteroatoms like nitrogen or boron e.g., C59N, (C59N)2.
Nano-onions: Â structures consisting of carbon spheres of increasing diameters layered on top of each other.Â Due to their layered design these were called nano-onions.Â Â These nano-onions have superior lubrication properties compared to other conventional lubricants.
Fullerene rings: Some researchers have succeeded in making a thirteen-membered ring hole in fullerene through which small molecules can pass through or get included.
Characteristics of Fullerenes:
An important characteristic of CÂ 60Â molecule is its high symmetry with 120 symmetrical operations which map the molecule onto itself. Each carbon atom bonds with three other adjacent atoms using sp2 hybridization.
Chemically fullerenes are stable; breaking the balls requires temperatures over 1,0000C. However, fullerenes are not unreactive;Â as they possess pi- electrons which are free to localise or delocalise in different chemical situations, reactions such as addition reactions and redox reactions, are possible on their surface. This leads to covalent exohedral adducts and salts. Fullerenes are insoluble in water, sparingly soluble in many other solvents and more soluble in toluene and carbon disulphide. Â Fullerenes themselves are non- toxic but some of the derivatives of fullerenes could be harmful to the health.
Applications of Fullerenes:
In Organic photovoltaics, fullerenes are used in solar cells as preferred n-type materials for increasing device efficiency.
In polymer electronics, fullerenes based on C60, C70 and C84 are used in Organic Field Effect Transistors (OEFTS) with improved performance.
Fullerenes hold great promise as antioxidants as they react readily with free radicals, thus preventing cell damage due to oxidation. Pharmaceutical companies are exploring the use of fullerenes in controlling Alzeimer's and other neurological diseases.
Fullerenes are used as additives in polymers and other composites to improve their physical and performance properties.
Fullerenes and their derivatives appear to have potential antiviral activity bearing great implications in treatment of HIV.
Fullerenes and its modified forms are capable of targeted and controlled delivery of drugs and genes into cells.
Endofullerenes with their protected cage-structure are capable of being applied in MRI, X-ray imaging and radiopharmaceutical diagnostic applications.
Dendrimers (organic nanoparticles)
Dendrimers are a new class of polymeric materials. A dendrimer consists of molecular chains branching out from a common center, and there is no entanglement between individual dendrimer molecules. These highly branched macromolecules are monodisperse (all of almost uniform size and mass) unlike linear polymers which are random in sizes and mass. The term 'dendrimer' originates from 'dendron' meaning 'tree' in Greek.Â The size and molecular mass of dendrimers can be controlled during synthesis.
Dendrimer molecules have their diameters of the order of a few tens of nanometers. In solution, dendrimers form a tightly packed ball unlike linear polymers and hence have significantly lower viscosity. With an increase in the molecular mass of dendrimers, their intrinsic viscosity increases, goes through a maximum at the fourth generation and then begins to decline. The presence of many chain-ends makes them highly soluble / miscible and highly reactive. Dendrimers terminating in hydrophilic groups are soluble in polar solvents, while dendrimers having hydrophobic end groups are soluble in nonpolar solvents.
The globular shape and the presence of internal cavities in Dendrimers impart them some unique properties. An important one is the possibility to encapsulate guest molecules in its interior. The shape of the guest and the architecture of the box and its cavities determine the number of guest molecules that can be entrapped.
Representation of a fourth generation dendrimer
Dendrimers are classified on the basis of generation; generation refers to the number of repeated branching cycles that are performed during its synthesis. For example if a dendrimer is made by the branching reactions performed onto the core molecule three times, the resulting dendrimer is considered a third generation dendrimer. The number of terminal groups increases in geometric progression with the number of generation. Each successive generation results in a dendrimer roughly twice the molecular weight of the previous generation. Higher generation dendrimers have more exposed functional groups on the surface, which can be used to customize the dendrimer for a given application.
The structure of these materials has a great impact on their physical and chemical properties. The properties of dendrimers are dictated by theÂ functional groupsÂ on theÂ molecular surface. As a result of their unique behaviour dendrimers are suitable for a wide range of biomedical and industrial applications. Currently more than fifty families of dendrimers, each with tailored surface, interior and core and unique properties, are known for various applications. Many potential applications of dendrimers are based on their unparalleled molecular uniformity, multifunctional surface and presence of internal cavities.
Dendrimers have been applied in in vitro diagnostics in cardiac testing.
Dendrimers are tested in preclinical studies as contrast agents for magnetic resonance(MRI).
Attempts are being made to use dendrimers in the targeted delivery of drugs and other therapeutic agents ; drug molecules being loaded both in the interior of the dendrimers as well as attached to the surface groups.
Dendrimers can act as carriers, called vectors, in gene therapy. Vectors transfer genes through the cell membrane into the nucleus.
Apart from biomedical applications, dendrimers are also used to improve many industrial processes. The combination of high surface area and high solubility makes dendrimers as promising nanoscale catalysts, for both homogeneous and heterogeneous phases. Dendrimers have nanoscopic cavities which act like microenvironment for molecular reactions and reactor sites for catalysis. Both the core as well as the surface can act as two possible catalytic sites. Attempts are made to use dendrimers in enhancing reaction rate and reaction selectivity.
Dendrimers with tailored solubility properties can find use in environment friendly industrial processes. Â Amphiphilic dendrimer are useful in forming interfacial liquid membranes for stabilizing aqueous-organic emulsion; leading to its application to extract chemical compounds between two phases. This arrangement holds promise for the development of organic chemistry in aqueous medium
Scientists have also studied dendrimers for use inÂ sensorÂ technologies. Studied systems includeÂ protonÂ orÂ pHÂ sensors. Dendrimers are also being investigated for use asÂ blood substitutes. Their steric bulk surrounding aÂ heme-mimetic centre significantly slows degradation compared to free heme,[
Carbon Nanotubes - Characteristics and Applications
Carbon nanotubesÂ (CNTs) areÂ a fibrous allotrope of carbonÂ having a hollow cylindricalÂ structure and belong to the fullerene family. The diameter of a nanotube is of the order of a nanometer, but their length is up to several microns. The walls of the long, hollow cylindrical structure are formed by one-atom-thick sheets of carbon, calledÂ graphene. These sheets are rolled at specific angles; the rolling angle and the radius of the cylinder decide the properties of these nanotubes
. File:Carbon nanotube chiral povray.PNG
Carbon Nanotubes are classified asÂ one -atom thick single-walled nanotubes (SWNTs) andÂ multi-walled nanotubesÂ (MWNTs) made of concentric tubes. Individual nanotubes naturally align themselves into "ropes" and are held together byÂ van der Waals forces.
As in graphite, the bonds of carbonÂ in nanotubes are made ofÂ sp2Â hybridised orbitals. These bonds are stronger than theÂ sp3 bondsÂ found inÂ diamond which accounts for the unique strength of the nanotubes. Carbon nanotubes have the highest tensile strength and elastic modulus of all known materials. However, due to their hollow structure and high aspect ratio, they tend to undergoÂ bucklingÂ when placed under compressive, torsional, or bending stress
The electrical properties of a nanotube are strongly dependant on its structure due to the unique symmetry and electronic structure of graphene. The electrons propagate only along the tube's axis due to their nanoscale cross-section and hence are referred to as one-dimensional conductors.
The nanotubes are very goodÂ thermal conductorsÂ along the tube, but good insulators lateral to the tube axis. Measurements show that at room-temperature, the thermal conductivity of a SWNT along its axis is about an order greater than that of copper, a metal well known for its goodÂ thermal conductivity.
Inspite of their great desirable properties, carbon nanotubes can pose a serious risk to human health since the available data suggests that chronic exposure can produce inflammation,Â microscopic nodules,Â fibrosis, and toxicological changes in the lungs.
The superior strength and greater flexibility and the other unique properties of CNT's at nano-scale dimenions make them potentially useful in a wide array of fields.
SWCNTs are 100 times stronger than steel at one-sixth its weight. This makes CNT's to be potentially employed as additives in composites for improved strength at fraction of their weight and their in products ranging from bullet-proof clothes and sports gear to combat jackets and space lifts.
In the field of electronics, nano-tube based transistors, logic devices and integrated memory circuits with significant reduction in size are being built. Highly-ordered carbon nanotube arrays can be used for data storage, displays, sensors & smaller computing devices. As SWCNTs can be clear and transparent, they can be used for developing transparent, electrically conductive robust films as a replacement for currently used Indium-Tin-Oxide in touchscreens and flexible displays.
Depending on structural characteristics, carbon nanotubes could be conducting or semi-conducting. CNT's and nano tube based materials are recently used in fabricating wires with specific conductivity exceeding copper and aluminium. CNTs also show promising applications in solar cells due to their strong UV/Vis-NIR absorption characteristics, as electrodes in building paper batteries, as hydrogen storage materials and Ultracapacitors.
They are used as Actuators for the conversion of electrical energy to mechanical energy and vice versa which can find potential use in robotics, optical fiber switches and displays, prosthetic devices, etc.
Due to their similarity in physical dimensions to those of biologically active macromolecules such as proteins, and DNA, carbon nanotubes find increasing utility in biologically inspired design and engineering of materials.
In the field of medicine, ultra-short SWNTs have been used as in vivo nano-scale capsules. In cancer research, SWNTs inserted around cancerous cells and excited with radio waves results in killing the cancerous cells. sensors, drug delivery, enzyme immobilization and DNA transfection.
NanocompositesÂ Â Applications
A nanocomposite is a matrix reinforced by added nanoparticles. NanocompositeÂ is a multiphase solid material with one of the phases having dimensions <100nanometersÂ (nm).Â The reinforcing material can be particles, sheets or fibres (e.g. carbon nanotubes). Due to the exceptionally high 'surface area to volume' ratio of the reinforcing phase and (or) its highÂ aspect ratio, the area of the interface between the matrix and reinforcement phase(s) in nanocomposites is typically an order of magnitude greater than for conventional composite materials.
The properties of the nanocomposite materials like electrical, thermal, optical, mechanical, electrochemical and catalytic, significantly differ from that of the component materials. The size of the nanoparticle affects the property of the nanocomposites; nanoparticles with <5Â nm influence the Â catalyticÂ activity, <20Â nm magnetic, <50Â nm optical, <100Â nm for mechanical properties.
Nanocomposites are found in nature, (in theÂ mollusc shell and bone) and also have been in use since early days in history although the term "nanocomposites" was not commonly used. This translates to observable effect on the macroscale properties of the composite with even a relatively small amount of nanoscale reinforcement.
Nano-composites as with conventional composites could be one of the following types:
Ceramic-matrix nanocomposites (main part of the volume is aÂ ceramic, i.e. oxides, nitrides, borides, silicides etc.. with mostly aÂ metalÂ as the second component)
Metal-matrix nanocomposites (where metals form the matrix in which carbon nanotubes or other nanoparticles are dispersed)
Polymer-matrix nanocomposites (where aÂ polymerÂ or copolymer is the matrix with nanoparticles or nanofillers is dispersed in the polymer matrix.)
Nano-composites provide a platform for a broad range of applications both in material science as well as in life science.
In life science
Tissue engineering i.e replacement ofÂ tissuesÂ (of skin, bone, cartilage, blood vessels) damaged by sickness or accidents or other artificial means.
Delivery of drugs in general and in tumor therapy by protecting the drugs from destruction in blood stream and controlled rate of delivery. Nanotubes prepared with a responsive polymer attached to the tube opening allow the control of access to and release from the tube
Biosensor applications since core shell fibers of nano particles with fluid cores and solid shells can be used to entrap biological objects such as proteins, viruses or bacteria in conditions which do not affect their functions.
Owing to the possibility of building ordered arrays of nanoparticles in the polymer matrix, future application exist in the manufacture of nanocomposite circuit boards, in neural networks applications, optoelectronics andÂ optical computing and magneto-optical storage media manufacturing.
In Material Science:
Production of batteries with greater power output.Â Example: Anodes for lithium ion batteries made of composite formed with silicon - carbon nanoparticlesÂ make closer contact with the lithium electrolyte, which allows faster charging or discharging of power.
Production of structural components with a high strength-to-weight ratio.Â Example: an epoxy containing carbon nanotubes can be used to produceÂ strong but lightweight and hence longer windmill blades or aircraft components which will increase the amount of electricity generated by each windmill.
Making lightweight sensors with nanocomposites.Â A polymer-nanotubeÂ nanocomposite conducts electricity depending upon the spacing of the nanotubes.
Using nanocomposites to make flexible batteries.Â
due to improved mechanical property such as tensile strength, modulus, heat distortion temperature of nanocomposites they find numerous potential aplplications In automotive and general industries including mirror housings on various vehicle types, door handles, engine covers, impellers and blades for vacuum cleaners, power tool housings, mower hoods and covers for portable electronic equipment such as mobile phones, pagers etc.
The gas barrier property and barrier to oxygen in particular, of nanoclay incorporated materials is expected to considerably enhance the shelf life of many types of food and find application in food packaging industry.
Significant reductions in flammability and transmission of solvents through polymers incorporated with nanoclay fillers can result in the use of these materials as both fuel tank and fuel line components for cars.
Â Biological nanomaterials: Applications
Biological systems are inherently nano in scale. Biological nanomaterials have their properties refined by evolution and hence possess a high level of optimisation compared to many synthetic materials. They provide models which can assist us in providing insight into the behaviour of nanomaterials in general and lead us to approaches like biomimicking. There are many nanophase materials in biological systems; for example living systems produce mineral material of the bone with particle size in the nanometer scale. These biological nanomaterials could even be used as direct source of novel materials or modified through in vivo procedures and in vitro manipulations. Some examples of biological nanomaterials are i) ferritins and related iron-storage proteins ii) nanoparticles found in magnetotactic bacteria.
Ferritin and its derivatives provide valuable model systems for probing the magnetic properties of nanoscale materials and also to investigate iron metabolism in living organisms.
A certain group of bacteria known as magnetotactic bacteria use earth's magnetic lines to orient themselves and move in the direction of nutritional or chemical gradients. This suggests some interesting biomimicking possibilities with respect to magnetic sensor and transducer systems.
The field of biological nanomaterials is still in its initial stages and many exciting possibilities exist in the future.
The formation of inorganic materials with complex form is a widespread biological phenomenon (biomineralization). Among the most spectacular examples of biomineralization is the production by diatoms (a group of eukaryotic microalgae) of intricately nanopatterned to micropatterned cell walls made of silica (SiO2). Understanding the molecular mechanisms of diatom silica biomineralization is not only a fundamental biological problem, but also of great interest in materials engineering, as the biological self-assembly of three-dimensional (3D) inorganic nanomaterials has no man-made analog. Recently, insight into the molecular mechanism of diatom silica formation has been obtained by structural and functional analysis of biomolecules that are involved in this process. Furthermore, the rapid development of diatom molecular genetics has provided new tools for investigating the silica formation in diatoms and for manipulating silica biogenesis. This has opened the door for the production, through genetic engineering, of unique 3D nanomaterials with designed structures and functionalities.
A nanowire is a wire with diameter of the order of a nanometer (10âˆ’9 meters) and an unconstrained longitudinal size. At these magnitudes since quantum mechanical effects are important, nanowires are also known as "quantum wires".
Nanowires could be metallic (Ni, Pt, Au), semiconducting (InP, Si, GaN, etc.), and insulating (SiO2,TiO2). Molecular nanowires are made of repeating organic or inorganic molecular units.
The wire diameter, wire surface condition, crystal structure and its quality, chemical composition, crystallographic orientation along the wire axis etc influence the physical and chemical properties of Nanowires.
Owing to their high density of electronic state, diameter-dependent band gap, enhanced surface scattering of electrons and phonons, increased excitation binding energy, high surface area to volume ratio and large aspect ratio, nanowires of metals and semiconductor exhibit unique electrical, magnetic, optical, thermoelectric and chemical properties compared to their bulk parent counterparts. This in turn holds lot of promises for applications in the fields of electronics, optics, magnetic medium, thermoelectronic, sensor devices etc
The most potential application of nanowires is in the magnetic information storage medium. Studies show that periodic arrays of magnetic nanowire possess the capability of storing 1012 bits of information per square inch of area.
Nanowires exhibit interesting thermoelectric properties. Metal nanowires exhibit many fold increase in Seebeck coefficient and thermopower which make them very attractive for thermoelectric cooling system and energy conversion devices.
Nanowires find potential use in electronic applications such as junctions with good rectifying characteristics (GaAs and GaP). Junction diodes, memory cells and switches, transistors, FETs , LEDs and inverter etc have already been fabricated using nanowire junctions.
Chemical and biological sensors made of nanowires as sensing probe exhibit enhanced sensitivity and faster responsivity compared to conventional sensors since they need less electrical power to work. The development of nanowire based pH sensor and Pb nanowire based hydrogen gas sensor have been reported so far.
Â In the field of medicine, researchers are using nanowires to coat titanium implants which reduce the risk of implant failure. TheÂ muscleÂ tissue which sometimes doesn't adhere well to titanium, when coated with nanowires, can anchor itself to the implant.
Future prospects of nanotechnology
Witnessing an exponential progress, nanotechnology is today among the fastest growing areas of science and technology.
Nanotechnology can be expected to accomplish the following in the near future:
Create integrated circuits using three-dimensional carbon nanotubes contributing to the growth of computer power.
Design solar panels with greater efficiency using nanocrystalline materials
Fabricate lighter and stronger Military equipment using nanomaterial composites
Revolutionise display technologies allowing brighter images, lighter weight, less power consumption and wider viewing angles using nanostructured polymers.
Contribute to more efficient, energy saving chemical manufacturing processes with reduced waste by-products using nanostructured catalysts.
Save humanity from ill health with reformulated pharmaceutical products using nanosized particle, improved targeted drug-delivery, their administration and absorption, create tissue compatible implants, scaffolds for tissue regeneration and even build artificial organs.
Protect the environment from degradation in ways more than one; design nanostructured membranes to capture carbon dioxide in the exhaust, nanoparticles for effective cleaning up of organic solvents, nanoclusters of silver for reducing polluting byproducts, nanowires making alternate energy sources cost effective
Â Make safe drinking water available to all at reasonable cost with water purification bottles, using filters only a few nanometres in width.
Produce smart materials with nanotechnology surfaces which are highly resistant to bacteria, dirt and scratches and fabrics that are highly resistant to liquid and stain-proof.
Nanotechnology is expected to continue the ever increasing miniaturization of semiconductor processing and memory devices.
In short, Nanotechnology has the potential to revolutionize every industry that touches every aspect of our life from health care to construction to digital to space exploration..
The future of Nanotechnology, being an unchartered territory with vast unpredictable possibilities, rests in the hands of the scientists that are able and responsible to take it into the next realm. However, as with every other form of energy, nanotechnology is a powerfully potential force which can save or dissolve everything. Walking a careful line with human well-being as the only goal will be a tricky but worthwhile accomplishment!!