IONIC SELF-ASSEMBLY AND HIERARCHIES OF POLYMERIC ... · systems, as lightweight structural materials that reduce fuel consumption, especially in transportation, and as more efficient

88 A. Puzari and J.P. Borah

© 20&3 Advanced Study Center Co. Ltd.

Rev. Adv. Mater. Sci. 34 (2013) 88-106

Corresponding author: Amrit Puzari, e-mail: [email protected]

IONIC SELF-ASSEMBLY AND HIERARCHIES OFPOLYMERIC STRUCTURES GENERATING NANOSCALE

ARCHITECTURE: OPPORTUNITIES AHEAD FROMINDUSTRIAL PERSPECTIVE

Amrit Puzari1 and Jyoti Prasad Borah2

1Department of Chemistry, National Institute of Technology Nagaland, Chumukedima, Dimapur,P.O.Chumukedima, Pin: 797 103, India

2Department of Physics, National Institute of Technology Nagaland, Chumukedima, Dimapur, P.O.Chumukedima, Pin: 797 103, India

Received: November 08, 2012

Abstract. Ionic self-assembly and hierarchies of polymeric structures for fabrication of nanoscalearchitectures has been reviewed along with discussions on future perspective. The process ofself-assembly has attained greater scientific interests because of its capability to control fabricationof macromolecular objects with required morphologies and size, for novel scientific utility. Thescope and opportunity of using these nanoscale materials for various applications such aselectronics, photonics, biomedicinal application, clean energy, organic solar cells, catalysis,light energy conversion, defense, semiconductor device, etc. have been discussed from the viewpoint of industrial perspectives. Emphasis has been made to bring to front the relevance of theprocess of self-assembly in addressing various issues confronted by human society. Developmentattained in various sectors have been discussed under different heading and sub-headings.Finally a brief concluding remark has been provided describing the future expectations from thisfield of research and comments on the frontiers to which the future scope of this interesting filedof work might expand.

1. INTRODUCTION

Fundamental advances attained in the field ofnanoscience along with the development of newermethodologies for obtaining multifunctionalmacromolecular structures with nanoscaledimension by harnessing of self-assembly, hasintroduced a newer dimension in the field ofmacromolecular science. Research activities in thefield continues to expand with the goal of meetingthe global challenges confronted by the society invarious fronts such as the vast need of sustainableenergy, clean air, water, preservation of food stocks,need of the alternatives for petroleum basedpolymeric materials etc. In addition to enabling

technologies, scientists have realized the futurepotential of the research in the field ofmacromolecular chemistry involving non-directional(non covalent) weak interactions like hydrogenbonding, Van-der Wall’s, - interaction etc. andgenerating aggregated species of requireddimension through ionic self-assembly. Developmentof new polymeric materials might be an importantstep forward in this respect. In fact polymericmaterials can be projected as the most importantclass of materials today since the structural featuresof such macromolecular framework can be tuned toobtained materials with desired properties to meetthe growing technological needs. Stepwise covalent

89Ionic self-assembly and hierarchies of polymeric structures generating nanoscale architecture...

synthesis of large molecules has long been prac-ticed and these processes are time consuming andlaborious apart from other disadvantages such aslow yield of desired products.

1.1. Functional nanomaterials

Functional self-assembled materials are now ofgreater interests for applications in electronics,photonics, light-energy conversion and catalysis.Interesting nanostructures can be generated by ionicself-assembly of two oppositely charged species[1,2]. The electrostatic force existing between thoseoppositely charged building blocks leads to theaggregated macromolecular species and therebygenerating important nanostructures. Porphyrinsand other tetrapyrroles are attractive building blocksfor obtaining such functional nanostructures [3].

Aggregates from some synthetic porphyrins pos-sess interesting optical electronic properties[4] andthese aggregates sometimes exists in the form ofuseful nanostructures like nanorods, nanofibres, etc.[5]. Incorporation of functional group will help tuningthe properties of such assemblies to obtain materi-als with desired behavior. Rigid organic linkers maybe used to control the size and shape of such mac-romolecular framework while the use of flexible linkermight lead to pseudo rigid assemblies and withundesired polymeric products.[6] similarlyfunctionalized liquid crystalline assemblies repre-sents another class of self-organized soft materialshaving novel utility. [7]The availability of new instruments able to “see”

and “touch” at this scale, has encouraged scien-tists to discover more and more products with novelutility. Apart from scanning tunneling microscopewith expanded capability of investigation and scan-ning probe microscope several similar techniqueshave evolved from these instruments to observe dif-ferent properties at nanometer scale. Older instru-ments such as electron microscopy of course con-tinued to reveal its relevance and now can image inthe nanometer range. Currently, there are a largenumber of complementary instruments that helpscientists in the nano realm.

2. ROLE OF POLYMER SCIENCE INMEETING THE CHALLENGES OFSOCIETAL NEED

2.1. Importance of polymer science forsocietal need

Polymer science has greater relevance in fulfillingmany of the societal needs and its importancecontinues to grow significantly and paralleled with

the development attained in the field of research inpolymer science. Polymer science has the potentialto address the huge technological challengesconfronted by the society in various fields includingdefense and healthcare and have found utility invarious aspects [8]. Indeed polymers have the abilityto serve in many different capacities from majorstructural components to high value addedingredients on the scale of grams. That includescarbon fiber reinforced thermo sets, different fiberreinforced plastics, doped polymers and thecomposite materials such as Bisphenyl-Polymer/Carbon-Fiber-Reinforced Composite [9]. With thesuccessful synthesis of various architecturalcomponents of polymeric materials and with theincreasing ease of understanding, the relationshipbetween polymeric architecture and materialproperties have tremendously increased the scopeof using polymeric material for societal need.Scientists are now trying to replace the traditionalmaterials with polymeric ones to meet some diversesocial needs and therefore more exciting results arewaiting in future days. In the field of energy, polymersexpected to serve at least three broad functions: ascomponents in new energy generating and storagesystems, as lightweight structural materials thatreduce fuel consumption, especially intransportation, and as more efficient platforms forseparation technologies. Development of lightweightmaterial will significantly help to reduce mass inplanes, trains and automobiles. Thus, the idea ofnext generation polymer membrane [10] are beingintroduced for finding out ways for industrial scaleseparation of liquids and gases.

The progress attained in the field of syntheticchemistry has now rendered it possible to synthesissmall molecules to almost any structure.Subsequently these methods are now being usedto prepare a wide variety of commercial polymers.Scientists have now developed methods toassemble single molecules to larger supramolecularassemblies consisting of many molecules arrangedin a well defined manner. For example self-assemblyof bis(imidaole)-annulated terphenyls disc shapedmolecules to columnar supramolecular order [11].These approaches utilize the concepts of molecularself-assembly and /or supramolecular chemistry toautomatically arrange themselves into some usefulconformation.

2.2. Role of polymerizednanomaterials

Fullerites are the highly incompressible nanotubeforms of which polymerized single-walled nanotubes


(P-SWNT) are the ones those are comparable todiamond in terms of hardness [12]. Research workscarried out so far has revealed that single and multi-walled nanotubes can produce materials withtoughness un-matched in the man-made and naturalworlds [13,14]. James D. Iversion and Brad C.Edwards have also revealed that possible crosslinking of carbon nanotubes prior to incorporation toa polymer matrix might lead to a super high strengthpolymer composite material which may have tensilestrength of the order of 20 million psi and couldpotentially revolutionize many aspects of engineeringdesign where low weight and high strength isrequired [15]. They are used as bulk nanotubes andthe strength of such materials are sufficient for manyapplications. Bulk carbon nanotubes have alreadybeen used as composite fibers in polymers toimprove the mechanical, thermal and electricalproperties of the bulk product. Carbon nanotubes incombination with tiny carbon bulkyballs calledfullerenes, forms solar cells. The bulkyball in suchcompounds can grab electrons when excited withsunlight and the nanotubes like copper wire makesthe electron flow to continue [16].

Combination of conjugated polymer withnanomaterials of the type for example goldnanoparticles can lead to kind of molecules whichcan potentially increase the sensitivity in opticalbiosensors because of the amplification [17] of thequenching via rapid electron transfer and alsobecause of electrostatic attraction between thepolymer and nanoparticle. Water soluble conjugatedpolymers [18] are thus receiving importance ascomponents in high-performance fluorescencesensor applications [19]. The utility of theseluminescent polyelectrolytes in biosensors is favoredby their high absorption coefficients in combinationwith high fluorescent quantum yields. Goldnanoparticles are considered as efficient quenchersof conjugated polymers fluorescence.

2.3. Role of greener polymericmaterials

Use of greener polymeric materials such asbiodegradable polymer as a replacement for theconventional ones are now become inevitable fromthe view point of environmental safety and also itrequired that the current and future macromolecularmaterials are made greener in terms of theirsynthesis and processing too. Polymers have findsignificant use in our society such as in numerousbio-medicinal applications including drug deliverysystems, artificial replacement for hip, heart and

prostheses and in tissue engineering. Polymericmaterials can be projected as the most vitalcomponent for diverse societal use. In principlepolymers enables the production of low cost andportable devices which are suitable candidates aseffective sensors. Polymeric materials have alreadyencompassing a vast area starting automobileindustry, defense application, sensor application,organic electronics etc. Polymeric materials are themost important class of materials today which havethe potential to confront the global challenges facedby the society in various aspects like demand forclean energy, sustainable energy source, the needto supplement, reuse and replace the petroleumbased polymeric materials, technologicaladvancement required in the fields like health,medicine, security, defense, etc.

3. SELF ASSEMBLY ANDHIERARCHICAL STRUCTURE ASFUNCTIONAL OBJECTS

3.1. Process of self-assembly

Stepwise covalent synthesis of large molecules areoften time consuming and a tedious job. Thereforescientists are now concentrating on the concept ofself-assembly which is based on the interaction thatoccur between molecules. The term self-assemblyrefers to those processes in which disorderedsystems of pre-existing components are convertedinto an organized structure by using some specificlocal interactions among the components, withoutusing external direction. Concerted action of weakand non covalent interaction generates hierarchicalstructures through the process of self assembly [20]The local interaction may corresponds to someelectrostatic attraction between positive and negativecharged particles, hydrogen bonds, dispersionforces like weak dipole-dipole interaction that arepresent in and between all molecules or hydrophobiceffects i.e. non polar bonding in an aqueous solution.When the constituent components are molecules,the process is termed molecular self-assembly. Self-assembly can be a static one or a dynamic one. Inthe first case the ordered state forms as a systemapproaches equilibrium, reducing its free energy. Inthe later case specific local interactions bringingabout the organization of the pre-existingcomponents. These structures are better describedas “self-organized”.

Thus over the past two decades the concept ofself-assembly has greatly been pursued forconstruction of macromolecular architecture with


interesting physical properties those are showingcompatibility for using in molecular electronics.Different self-assembled processes have beendeveloped which can generate supramolecularnanostructure with internal order and threedimensional patterns using phase separation ofmacromolecules. The process can also helpgenerating supramolecular assemblies that are twodimensional, non-centrosymmetric with a largevariety of architecture. Even more complex structurewith hierarchical order can be generated byintegrating the process of self-assembly with internalforces or external fields. The materials may beobtained in the form of gels, films, liquid crystal,nanoparticles and the like.

3.2. Self-assembly of syntheticpolymers

Self assembly of synthetic polymers tend to rely ondispersive forces. Manipulation of secondaryinteractions in addition to other kinetic andthermodynamic factors may even lead to dynamicand responsive complex polymeric structures. Blockand heterocopolymers and other hybrid functionalmaterials and analogous supramolecularorganizations are included in such category.Complex polymer systems from di- and tri-blockcopolymers [21] to hierarchical self assembledstructures can be tuned to produce spherical, rodlike or layered structures. For example One suchrod-rod type self assembled nanostructure obtainedfrom diblock or triblock molecules ofoligo(ethersulfone)-b-oligoetherketone (OES-OEK)(Fig. 1) has been reported and synthesized [22].The observed nanoscale architecture was reportedto be promising for application towards nano- ormicroelectronics and templating throughmanipulation of the block length and composition.

The enhanced possibility of generating diversenanoparticle systems enhances the possibility ofobtaining novel functional polymeric material havingpotential application in the field of drug delivery [23],photonics [24], catalytic [25, 31] and energyapplication [26]. Among various processes

Fig. 1. Di- and triblock rod-rod molecules (a) OES-b-OEK (b) OEK-OES-OEK.

developed for the generation of complex polymericstructures ionic self-assembly is one among themost prominent ones. Variety of architecturalfeatures can be developed by exploiting the chemicalcomposition differences within block polymersystems and tuning the secondary interactions.

3.2.1. Self-assembled block co-polymers (BCPs)

Block co-polymers (BCPs) are projected to haveextensive technological implications for present andfuture technological developments. Blockcopolymers when deposited on a variety ofsubstrates for obtaining thin films, using solutionprocessing technique, the dissimilar blocks canseparate into distinct domains with controlleddimensions and functionalities. The organization ofthese domains are controlled by factors likeinteraction of the BCP with the substrate, filmthickness, etc. [27,28]. Most studies on BCPs areconcentrated on coil-coil block copolymer whereeach block is a fully flexible polymer chain. The self-assembled domain shapes for such BCPs can betuned by adjusting the relative volume fraction ofeach block, degree of polymerization, etc. [27]. Thisfeature project BCPs as a potential candidate forgenerating variety of materials using self-assembledBCPs with various pattern. Templating of magneticmaterials[29], nanoparticles[30-33] and continuousthin films[34], by tuning the patterning of diblockcopolymers, are thus reported in literature. Self-assembled hierarchical structural patterns of blockco-polymers are having promising technologicalapplications and are being utilized in the fields likemicroelectronic manufacturing, photovoltaic cellsetc. of which the former is already approachingcommercialization [35]. BCPs are amenable tosolution based processing methods and thisproperty is exploited in working out the aboveapplications. While the use of solar cells forphotovoltaic applications are turned out to be anexpensive one, polymer based photovoltaic presentsa more scalable and inexpensive alternative [36].BCPs are thus inexplicably can be used for


Fig. 2. Ideal morphology of an OPV cell. Blue: acceptor domains Red: donor domains. The width of eachdomain is compatible to the exciton diffusion length (Color online).

generating Organic Photovoltaic Cells (OPV). BCPprovides a promising way to generate a structureas shown in Fig. 2. When processed in films thedonor and acceptor block forms separate domainleading to structures approaching idealized vision[35].

Despite the drawbacks associated in thoseprocesses, the worthy point to be mentioned in thisregards is their lower cost of production. The self-assembled architecture of BCPs is also found usefulin the field of optical lithography [37]. The hierarchicalstructures generated by the process of self-assembly in BCPs can thus lead to promisingnanoscopic functional materials with long rangeorder and robust etch resistance and also cangenerate diverse pattern which might havetremendous technological im]lications for today’sand tomorrow’s technological world.

3.3 Biomolecular self-assembly

Another interesting aspect to be highlighted in thisrespect is biomolecular self-assembly. Biomolecularmaterials can undergo self-assembly [38], a processin which a complex hierarchical structure isestablished without external intervention. One of thebest examples of biomolecular self-assembly is thelong protein molecules which fold themselves intocomplicated three-dimensional structures. Researchon self-assembly of bio-molecular materials is anexciting new discipline lying at the intersection ofmolecular biology, the physical sciences, andmaterials engineering. Bio-molecular materials arethose whose molecular-level properties areabstracted from biology i.e. they are structured andprocessed in a way that is characteristics of

biological materials, but they are not necessarily ofbiological origin. Research on bio-molecular self-assembly can help us obtaining newer functionalmaterials with greater technical importance. Themost interesting point is that if the principle of bio-molecular self-assembly can be extended to modernmaterial synthesis. Directly mimicking biologicalsystems or processes might help to achieve suchgoal or otherwise study of how nature accomplish atask or how it creates a structure with unusualproperties and then applying similar techniques ina completely different context or using completelydifferent material could help attaining suchobjectives. Study of laminated structure of clan shellsand application of analogous principle to design metalceramic composite twice strong as othercomposites constitutes an example of this kind [39].Highly ordered and tougher materials compared toits natural analogue could be generated from suchapproach. So successful application of the findingsof the phenomena of biomolecular self-assemblytowards obtaining hierarchical structures and hencefunctional materials will have significant impact onmaterials and process.

3.4. Hierarchical structures forgeneration of functional materials

Hierarchical structures using charges provide neweropportunity to meet the challenges of generatinghighly ordered structure. Controlled multilayerassembly methods combine different polymerbackbone in nanoscale blend. The morphology ofsuch structures can be controlled using inter-diffusion of charged species during assembly. Theenhanced capability attained with the development


of science in controlling the chain length distribu-tion of polymeric structures, branching and archi-tecture, stereoregularity and precisely located func-tional groups equally enhances the possibility ofobtaining intelligent functional materials which mightsuccessfully address the global challenges facedby the human society in many important fields. Inthis respect hierarchical structure using chargesprovides us the newer opportunities to generatecontrolled multilayered assembled structure withnanoscale dimension. Of course, in such cases thepossibility of undesired aggregation or complexationof the charged species can lead to some other sortof complexities. In such cases use of sophisticatedchemistry can help attaining control over suchphenomena to obtain molecular architecture withdesired functional behavior.

The concept of molecular assembler has lead tothe development of molecular nanotechnology, alsoknown as molecular manufacturing, where nanoscaleobjects can be produced with desired morphology.This has clear distinction from the conventionaltechnologies used to manufacture nanomaterialssuch as carbon nanotubes and nanoparticles.Nanoscale materials can also be used for bulkapplications; most present commercial applicationsof nanotechnology are of this f lavor. DNAnanotechnology utilizes the specificity of Watson-Crick base pairing to construct well-definedstructures out of DNA and other nucleic acids. Init’s true essence the conce]t of molecular self-assembly seeks to utilize the concept ofsupramolecular chemistry, and molecularrecognition in particular, to force single moleculecomponents to automatically arrange themselvesinto some useful conformation. It has also beenanticipated that molecular self-assembly techniquessuch as those employing di-block copolymers [40]can be used for fabrication of nanowires, which arebeing used in semiconductor fabrication such asdeep ultraviolet lithography [41]. Newer techniquessuch as Dual Polarisation Interferometry [42] areenabling scientists to measure quantitatively themolecular interaction that take place at the nano-scale.

Multifunctional nanosystems possess severalfeatures within a single construct which renders itpossible for such device to target tumour or otherdisease tissue, facilitate in vivo imaging, and delivera therapeutic agent [43]. Such type of nanosystemobviously creates new hope for the cancer patientsand therefore tumour targeted nanosystems arecurrently gaining more importance in the field ofresearch. Following fabrication, the surface of these

nanostructures are functionalized for passive or ac-tive targeted delivery to the tumors. The phenom-ena of self-assembly allows integration of compo-nents with varied properties into a usefulnanostructure. Thus self-assembly of macromolecu-lar building blocks with specific functionalities inaqueous media can yield a large variety ofnanoparticle systems which can be screened fur-ther for its efficiency towards delivering therapeuticagents to targeted tumors. Another approach wassuggested by the same school of workers wherethey emphasized on synthesizing a library of largenumber of polymeric materials using different mono-mers and then screening the individual polymers fortheir potentiality towards nanoparticle fabrication.Design of such type of nanoscale objects withfavourable properties and diverse application poten-tial specifically in the field of cancer treatment willdefinitely be a commendable job.

4. IONIC SELF ASSEMBLY AS A MEANFOR FABRICATION OFNANOSCALE OBJECTS

4.1. Fabrication of patternednanoparticles from di- andtriblock co-polymer

Fabrication of nanoscale objects, especially pat-terned nanoparticles are generally difficult to pre-pare by conventional fabrication techniques. Com-plex polymer systems from di- and tri-block co-poly-mers to hierarchical self-assembled structures canbe used for fabrication such patterned nanoparticleshaving tubular, spherical, rod like or layered struc-ture. The difference in chemical compositions of di-an triblock copolymers facilitates the generation ofhighly specific and spatially localized region duringcolloidal assembly. The enhanced possibility of gen-erating diverse nanostructures increases the poten-tial utility of such fabricated structures for applica-tions like drug delivery system, in photonic, energyapplications, catalysis, etc. [44]. For fabrication ofsuch architectures, self-assembly has been usedas a guide in producing nanostructured materialswhere the selection of the interactions that will guidethe assembly and the choice of processing tech-niques will finally guide us to the observednanostructure. Precise fabrication of two and threedimensional materials with nanometric control anddevelopment of a environmentally benign process-ing techniques always remains as a challenge be-fore the scientific community which possibly canbe addressed using the concept of self-assembly.


Even self-assembly can generate three dimensionalstructures but with limited set of symmetries.

4.2. Ionic self assembled multilayer(ISAM) technique fornanoarchitecture

Ionic self assembled multilayer (ISAM) provides aprecisely controlled way of producing nm thick thinfilm on any charged species [46-49]. ISAMtechnique is largely independent of the nature, sizeand topology of the substrate and various substratesincluding glass, silicon, metal, organic/inorganic ormetal nanoparticles, etc. can be used. Thus ISAMbased gold films has significant importance for futuretechnological advancement. Composite gold filmconsisting of gold nanoparticles and polyelectrolytewas fabricated through ISAM technique to nmthickness and was deposited on an end-faces ofmultimode optical fibers to construct LocalizedSurface Plasmon Resonance (LSPR) [50]. SurfacePlasmon resonance (SPR) is the phenomena ofcollective oscillation of valence electrons in a solidstimulated by incident light. SPR in nanometer sizedstructures is called localized surface Plasmonresonance (LSPR). The ISAM deposition processalso known as layer by layer (LBL) technique isshown in Fig. 3 [48,51-54]. The LSPR spectrumobtained in this case can be tuned through ISAMtechnique and it was demonstrated that the ISAMadsorbed on optical fibers potentially provides asimple, fast and cost effective way of obtainingbiosensors.

LSPR is sensitive to change in the localdielectric environment and hence studiedextensively for use in optical spectroscopy andbiosensor application [55].

Fig. 3. ISAM film deposition process of first two layers.

4.3. Ionic self-assembly for porphyrinbased nanodevice and polymericstructures

Porphyrins can easily assemble into a variety ofsupramolecular architectures. The fact that porphyrincan accommodate a metal ion in their centre throughcoordination to four nitrogen atoms, renders sucharchitectures potentially useful for generation ofmaterials with particular electronic properties. Forexample ionic co-assembly of sulfonate andpyridinium porphyrins results in a binary dendriticarchitectures that resemble four-leaf clovers [56].Depending on the metal occupying the central cavityof the porphyrin ring, the prophyrin cores are eitherelectron donor [with Zn(II)] or electron acceptor[withSn(IV)]. Porphyrin nanotubes obtained through ionicself assembly of two oppositely charged porphyrinsnamely anionic meso-tetra(4-sulfonatophenyl)porphyrin dihydrochloride (TPPS4) and cationicMeso-tetra(4-pyridyl) porphyrin (T4MPyP)showsappreciable structural stability due to theelectrostatic force between the porphyrin blocks.Porphyrin nanotubes are capable of reducing metalcomplexes and deposit gold selectively on the outersurface of the tubes. The colloidal solution of goldnanoparticles lead to gold strip inside nanotube.Investigations carried out on these porphyrinnanotubes and nanotubes functionalized by goldreveals the possibility of obtaining a new kind ofnanodevice for photovoltaics, nanophotonics andsensing [57].

The concept of ionic self-assembly has also beenextended to design and generate polymericstructures for obtaining redox-active thermotropicliquid crystalline materials. Such a series of materialwere constructed from the complexation of a seriesof polyferrocenylsilane (PFS) polyelectrolytes withseveral oppositely charged surfactants (Fig. 2) [58].


An ordered mesostructure was obtained for suchmaterials due to strong coulombic attractionsbetween the starting building blocks. The techniquealso allowed incorporation of photoactiveazobenzene containing surfactant for obtainingordered mesostructures with potentialoptoelectronic applications.

5. SELF-ASSEMBLY IN DENDRITICARCHITECTURE ANDFABRICATION OF NANOSCALEOBJECT

5.1. Importance of dendrimer asnanoscopic building blocks

Dendrimers are the most interesting nanoscalearchitectures [59]. Dendrons and dendrimers areorganized polymeric structures having repeatingbranched building blocks and their structures aremonodisperse. Unlike dendrons, dendrimers arestructures in which a number of dendrons areattached to a single core unit. The ease of chemicalsynthesis and structural control attained with suchpolymeric architectures along with the ease offunctionalization of the branched and peripheralgroups projects them as potential candidate fornanotechnological applications. Dendrimers anddendrons are therefore can also be considered as afamily of nanoscopic building blocks [46].Amphiphilic dendrimers containing an extended rigidblock represents a class of self assembling systems

which are increasingly used for the construction ofsupramolecular architecture with well defined shape.The phenomena of self-assembly of dendriticmolecules can help fabrication of nanoscale materialswith novel utility. Bulk nanoscale self-assembly oforganic matter is promising for using as scaffoldsfor photonic materials and other intelligentcomponents of molecular electronics. Dendrons anddendrimers are particularly versatile in generatingsuch periodic nanostructures. Self-assembly ofnanosized dendritic supramolecules around atemplate system can generate a system having thecapability of controlled encapsulation and releaseof active ingredients. Non covalent intermoleculardendron-dendron interaction can even lead tohierarchical assembly of nanostructured materials.For such systems the molecular scale informationare being expressed in macromolecular scale andhence finds application in material science. Multiplesurface groups available in such architecture providemultiple interactions with large surfaces analogousto those found in biological systems. The possibilityof multivalent interaction between dendron surfacesand biological molecules opens up the scope of usingsuch architectures for medicinal therapies.Nowadays a broad range of dendrimers are availableand some of them are even available commercially,and have found to be promising towards importantchemical processes and also as drug or genedelivery devices, as carriers for catalytically activesite in flow reactors and also as chiral auxiliaries forasymmetric synthesis.

Fig. 4. Self-assembled dendritic architecture.


5.2. Hierarchical self-assembly indendritic architecture

Zimmerman et. al. in 1996 reported the first exampleof non covalent interaction between dendronmolecules that assembled the individual dendronsinto a supramolecular dendritic architecture [60].They reported self-assembly in a Dendron which wasresulted due to the hydrogen bond mediatedcarboxylic acid dimerization to form hexamericrosette. They demonstrated how the dendriticbuilding blocks can self-assemble in a controlledmanner and how the dendritic branches exerteddirect control over the self-assembly process. Theyused an organic template to direct the dendritic self-assembly process via anthyridine-amidiniuminteractions (Fig. 4) [61]. They assembled asupramolecular dendrimer with a mass of over10,000. The template cation choosed for the purposeis active against Pneumocystis carinii pneumonia.This example demonstrates the way howsupramolecular dendrimer can be assembled byusing individual dendrons around active cores.Research groups of Reinhoudt and Frechet alsoreported supramolecular dendrimers based onhydrogen bonded hexameric rosettes [62].

Research group of D.K. Smith have advocated aunique microenvironment for the template speciesjust like what was being observed for covalentdendritic encaptulation [63].Investigation carried outby this group on the interaction between a polybasichydrophilic dye and a Dendron with an acidic groupat the focal point indicated that the hydrophilic dyescould be solubilized in apolar sovents in which theyotherwise have no solubility and with increasingDendron generation the solubility of the dye alsoincreases [64] it was proposed that the acid baseinteraction generated a complex in which thehydrophilic dye was encapsulated within a noncovalently generated dendritic microenvironment.Optical properties of the dye were also found to bemodified by the process.

Works carried out by the research group of D.K.Smith also demonstrated the ability ofsupramolecular chemistry to assemble nanoscalearchitecture using simple biocompatible buildingblocks and reversibility of non covalent assemblymethods [65]. Hierarchical self-assembly of dendriticmolecules produces even more interestingarchitectures such as supramolecular gels [66].Dendrimer molecules are of particular interest forthe formation of gels and simple Dendron typearchitectures. They have a much greaterpredisposition for gelation than fully formed spherical

dendrimers. The control of different structural pa-rameters those assist the hierarchical self-assembly of dendritic molecular building blockshelps obtaining gels with a variety of properties. Thisfact other way indicates the power of this kind ofsystem in the ‘bottom-u]’ fabrication of controllednanostructures. The factors such as acid-baseinteraction, dendrimer generation, spacer chain,solvent, gelator surface group, molar ratio ofcomponents etc. play the significant role forfabrication of such architecture.

5.2.1. Electrostatic self-assembly inPAMAM dendrimers

Rotello et. al. employed PAMAM dendrimers ofdifferent generation to assemble gold nanoparticles.In this case direct control of inter particle separationwas achieved through the choise of dendrimergeneration. For this purpose gold nanoparticles werefunctionalized with carboxylic acid group. Salt-bridgeformation between the dendrimer amino groups andthe NP peripheral carboxylic acid groups led toelectrostatic self-assembly between the dendrimerand NP components resulting in well-controlledaggregates (Fig. 5) [67]. These dendrimer mediatedgold nanoparticle assemblies open the door tocreate tailored magnetic nanoparticle structures [68]

Thus dendritic architectures are interestingbuilding blocks for exploiting for the formation ofnanostructured systems. By effectively tuning thenoncovalent interaction, it is possible to obtainedself-assembled nanosized supramoleculardendrimers and such systems are potentially usefulfor controlled encapsulation and release processes.Non-covalent interactions between the branches ofadjacent dendritic systems can give rise tohierarchical self-assembly of nanostructuredmaterials having prospects for application inimportant fields.

5.3. DNA binding systems fromdendrimer self-assembly

It is noteworthy that binding of DNA using dendriticsystem is of potential therapeutic relevance sincethat is helpful for delivery of therapeutic DNA intothe cells, the so called gene therapy [69]. Gene-therapy may provide some way for the treatment ofthe diseases like sickle cell ammonia, cancer, cysticfibrosis, etc. The use of high generationpoly(amidoamine) (PAMAM) dendrimers to bind andtransfect DNA has already been indicated by thegroups of Tomalia and Szoka [70]. Later on wide


Fig. 5. Schematic representation of electrostatic self-assembly of carboxylic acid terminated gold nanoparticleand PAMAM dendrimer.

range of DNA binding system was developed [71].In all the cases interaction between protonatedamines and polyanionic DNA for the formation ofdendrimer-DNA complex was of prime importance.Diederich et. al. thus reported about a small dendroncapable of self assembling and binding to DNA (Fig.6) and which again employed protonated aminesurface group to bind to the anionic DNA [72].Additionally it has hydrophobic tails to encouragethe process of self-assembly.

So this type of dendritic molecules with tailoredsurfaces can be of greater interest in biological ‘softnanotechnology’ with ]otential a]]lication inmedicinal chemistry. New type of therapeutics canpossibly be generated through precise control of theinteraction with biological target as well as thestructural control of the dendritic moieties. Furtherthere is scope for utilizing these very high affinity ofDNA binders for the encapsulation and protectionof genetic materials.

6. SELF ASSEMBLED LIQUIDCRYSTALLINE NANOSTRUCTUREFOR ORGANIC ELECTRONICS

6.1. Liquid crystalline nanostructureas soft materials

Soft materials are getting enormous importance asfunctional materials because of their dynamic nature;

despite the fact that they are not as highly durableas hard materials. Introduction of orders to softmaterials induces newer dynamic functions. Liquidcrystals are ordered molecular entities consistingof self organized molecules and can be used aspotential functional material in various fields.Appropriate functionalization of liquid crystallinematerials will allow design of variety of new self-organized functional materials having application invarious industrial perspectives. Liquid crystallinematerials can exists in various nanostructuredphases, such as layered, micellar, bicontinuous,columnar, cubic etc. which are formed bysegregation of immiscible parts in molecules on thenanometer scale [73]. These soft materials aregaining importance in various fields of advancedtechnologies [73(a), 74] and have the potentialapplicability as dynamic functional materials forinformation and mass transport, sensing, catalysis,stimuli responsive as well as electro-optical displays.Discotic liquid crystals are promising alternativesfor use in electronics because they can self-organizeinto highly ordered superstructures via -interactions[75]. In these structures molecules assemble intocolumnar structures which form a percolationpathway for electrons and holes [76]. High chargecarrier mobility along the one-dimensional stackshave been measured with values of up to 1.1cm2 V-1s-1 for hexabenocoronene (HBC) liquid crys-


Fig. 6. Dendron reported by Diederich et. al. that binds through self-assembly with DNA.

tal in the crystalline state [77].Design of newer mol-ecules and self assembled structures from nano tomacro scale is important to obtain dynamic func-tional molecules [73(a),78].Functional behavior ofthe molecules developed depends on the control ofnano segregation at the molecular scale in the ther-modynamically stable single phase which is com-pletely different from macrophase separation form-ing thermodynamically different phases.

6.2. Self-assembled nanoscalearchitectures from liquidcrystalline compounds

Self assembled nanoscale architecture from liquidcrystalline compounds can be obtained by usingspecific molecular interactions like ionic bond,charge transfer interactions and hydrogen bonds [79-84]. The highly ordered arrangement of liquidcrystalline assemblies improves the ionicconductance in such material. Ionic liquids arepotential candidates for application inelectrochemical devices and also in catalysis. Selfassembly and phase segregation of an ionic liquidcrystal (Fig. 7) into columnar liquid crystallinestructures was reported that possess inner ion-conducting paths [85].

These columnar phase are stable over a widetemperature range including room temperature.Anisotropic measurements of ionic conductivitiesreveals higher ionic conductivities as well as higheranisotropy for these materials and these constitutethe first example of 1D ion conduction in organicmaterials [86].

Self-assembly of liquid crystalline molecules canlead to a variety of phase-segregated structures in Fig. 7. The columnar ionic liquid crystals.

soft states and control of their structural organiza-tions to nanoscale dimension yields new complexnanostructures and induces new functions likecharge conduction [87]. Nano segregation is a keyfor obtaining nano structures with desired dimen-sion. Molecular assemblies in liquid crystalline ioncomplex polymers show interesting thermal prop-erties and characteristic orientational behavior, whereionic group plays significant rule in forming thesmectic layer and enhancing thermal stability [88].For discotic liquid crystalline polymers induction ofchirality to columnar achiral donor polymers throughdoping with chiral acceptors, produces importantassembled structures [89]. Similarly molecular self-assembly in supramolecular helical dendrimers canproduce complex electronic materials [90]. Electroactive films can be generated through self assem-bly of polymerizable ionic amphiphiles (A) and thewater soluble precursor (B) of conducting polymersas represented in Fig. 8, into columnar structures[91]. Nano segregation in such type of assembliesimproves the photo-physical properties. Formationof columnar liquid crystalline structures having in-ner ion-conducting paths was reported where ionicliquid crystals possessing ionic imidazolium salts


and hydrophobic alkyl moieties, were employed [85].The phases obtained are stable over a wide rangeof temperatures including room temperature. Byapplying mechanical shearing between gold elec-trodes it is possible to observe higher ionic conduc-tivity as well as anisotropy in ionic conduction fromsuch materials. Liquid crystalline structures can alsobe applied to obtain photonic materials. Thomas andco-workers have prepared temperature-tunable pho-tonic materials from liquid crystalline materials hav-ing a band gap in the visible region [92]. The phe-nomena of self-assembly along with appropriate nanosegregation in liquid crystalline materials might yieldimportant functional material which can be conceivedas having proper relevance from industrial perspec-tives.

6.2.1. Liquid-crystal compositematerials

Liquid-crystal composite materials, a new type ofself organized soft solid, are also known and one ofthis category of materials are being constructed fromliquid crystals and self assembled solid materialsmade of low molecular weight organic compounds[93] or nanoparticles [94]. This type of soft materialshave improved electro-optical properties. Forexample liquid crystal triphenylene derivatives whichcan function as anisotropic photoconductors getimproved its photo conductivities through physicalgelation [95]. It is to be noted that liquid crystallinephysical gels are composite materials formed fromliquid crystalline material and self assembled fibersfrom low molecular weight organic compounds.

6.3. Self-assembly in liquid crystallinedendrimers

Liquid crystalline dendrimers are also anotherpotential class of materials which are capable ofself-assembling into nanoscale dimension togenerate newer functional molecules [96]. Fullerenecontaining liquid crystalline dendrimers has inter-

Fig. 8. Polymerizable amphiphiles(A) and precursor of conducting polymer (B).

esting physical properties and therefore can encour-age enthusiastic studies in the field of supramo-lecular chemistry and materials science throughdesign of newer self organized structures contain-ing the fullerene unit. Especially this process of self-assembly using supramolecular interactions mighthelp generation of nanoscale architecture from func-tional groups which are otherwise not well adapted.Control of dendritic generation will help tuning theproperties of such supramolecular architecture andto compensate the negative effect of bulk C

60 unit,

incorporation of enough mesogenic subunit in thedendritic addend is essential. Fulleropyrollidines, asshown in Fig. 9, are representative examples of suchtype of soft materials. Fulleropyrrolidines are im-portant class of C

60 derivatives which are even ad-

vantageous than the methanofullerenes as they canproduce a stable reduced species having potentialtowards development of fullerene based redox mo-lecular switches. The functionalization of C

60 with

liquid crystalline addends constitutes an appropri-ate method for the elaboration of fullerene contain-ing thermotropic liquid crystals. Appropriate selec-tion of the dendritic addends can provide a mean toprepare tailor made fullerodendrons through propercontrol of the dendrimer generation. Attachment ofsuitable substituent to the N-atom of thefulleropyrrolidine can generate nano architecture thatmay be suitable for photovoltaic applications orsupramolecular switches [97].

The capability of dendritic liquid crystalline com-pounds to self assemble to well defined andnanosized architecture have continually increasestheir importance in material science and serve asan interesting and fascinating motif in nanoscience,nanotechnology and other interdisciplinary field.Thus ionic derivatives of linear carboxylic acid withpoly (amido amine) (PAMAM) or poly(propyleneimine) (PPI) liquid crystalline dendrimers aremodified to obtain birefringent glasses at roomtemperature and viscous smectic A phase at highertemperature [98]. These dendrimers possess ter-


Fig. 9. Representative examples of Fulleropyrrolidines (Generation 3; G-3).

Fig. 10. Structural representation of polymer PBTTT.

minal flexible alkyl carboxylic acids separated by aregion of high density of the ionic ammonium car-boxylates and these LC dendrimers may find inter-esting application as anisotropic ionic conductivematerial [99]. According to Percec and his group,when electron acceptor or donor organic groups areplaced at the focal point of suitable dendrons, thesupramolecular organization in such casesproduced liquid crystalline dendrimer with highcharge carrier mobility. The materials produced couldhave interesting electronic and opto-electronicapplications [100]. Metallocenyl dendrimersbecause of their electrochemical properties haveattracted much attention in the field of molecularelectronics.

6.4. Self-assembled liquid crystallinepolymers as functional materials

Liquid crystalline materials are apparently been usedin electronic and photonic devices such astransistors, photovoltaics, OLEDs and Lasers [101].Organic photovoltaics and OLEDs requireoptimization of both charge transport and opticalproperties. OLEDs are widely available in mobilephones, and niche lighting markets are opening forwhite-light panels using organicelectroluminescence. The phenomena of self-assembly of liquid crystalline phases has lead tothe development of liquid crystalline semiconducting,thiophene polymers and copolymers many of whichcontain thienothiophene groups (Fig. 10) [102-107].One example of polymer of this kind is the polymer


poly2,5-bis(3-alkylthiophene-2-yl) thieno [3,2-b]thiophene (PBTTT) which is shown in figure-10 [101]This polymer shows exceptionally high field effectmobility up to1 cm2 V-1S-1.

Such polymers exhibit a mesophase above roomtemperature associated with lower side chaindensity then the prototype semiconducting polymerpoly3-hexylthiophene (P3HT), which allows interdigitations of the non conducting side chains. Onannealing to the mesophase the backbone caneasily move to produce a more organized structureand hence removing defects which ultimatelyimproves the semiconducting behavior. Upon coolingthe side chain crystallizes to maintain a higher levelof order and layer structure and that increases thecapability of the molecule for using in OFET.Improvement in the orderings of both aliphatic partand polymeric backbone in such type of orderedmolecular architecture also allows very high valueof charge carrier mobility in polymer based OFETs.

7. IONICALLY SELF-ASSEMBLEDHYBRID NANOSTRUCTURES FORINDUSTRIAL APPLICATION

Hybrid materials represent systems comprising ofmixtures of organic and inorganic material likepolymers and other materials, such as organic andinorganic additives are finding increasinglyimportance due to their superior properties.Hierarchical structures using charges provide new

Fig. 11. Diagram showing experimental setup for monitoring the deposition of PDDA/PS119 multilayer ontooptical fibers and illustration of assembled multilayer at the end of optical fiber.

opportunities to design novel molecular functionalmaterials with higher order. Controlled multilayerassembly methods allow us to combine differentpolymer backbones in nanoscale blend to obtain abroad range of inorganic/organic hybrid materialswith controlled compositions. Manipulation ofmorphology of such systems can be readilyperformed through inter diffusion of charged speciesduring assembly and also by controlling otheradditional interactions like hydrogen bonding. Suchtype of molecular design helps attaining the keytechnological goals like obtaining eff icientphotovoltaics, improved polymeric semiconductors,improved organic solar cells, obtaining superiorphotonic and electronic materials etc. Ionic selfassembly or more generally speaking self-assemblyprocess can act as a guiding principle for design ofsuch macromolecular species with higher degreeof order and superior functional behavior. A largevariety of hybrid materials are obtained[108-109]which include organic groups (alkyl, aryl..), chelatingsystems (cyclames, crown ether, porphyrins) andpolymers. All these systems are covalently bondedto the inorganic matrix [110].

Such ionically self assembled hybrid materialsnot only represent a creative alternative to designnew materials and compounds for academicresearch, but their improved or unusual features alsomake them important for innovative industrialapplications. Evidently these new generations ofhybrid materials, born from the very fruitful activities


in this research field, will open up possibilities for alarge area of applications including optics,electronics, ionics, mechanics, energy,environment, biology, medicine, as membranes andseparation devices, functional smart coatings, fueland solar cells, catalysts, sensors, etc.

A recent review highlights the current developmentof hybrid materials based on semiconductornanocrystals integrated into polymer matrices fordirect light conversion [111]. Semiconductingnanocrystals are smart materials with unique size-dependent optical properties and therefore arepromising for application in photovoltaics, LightEmitting Diodes (LED) and other optoelectronicapplications, memory devices, thermoelectricapplications etc. Colloidal semiconductornanocomposites can form various hybrid materialswhen integrated into different host materials includingpolymer. By integrating these materials into polymermatrices one can obtain newer materials with tailoredproperties. For example first hybrid nanocompositepolymer Light Emitting Diode was reported by Colvinet al. [112] in 1994 where they integrated cadmiumselenium nanocrystals with semiconductingpolymer. Since then significant progress have beenattained in this field.

8. IONIC SELF-ASSEMBLY FORFABRICATION OF OPTICALCOMPONENTS AND COMPOSITEMATERIALS

Electronic Humidity sensor has been extensivelyused as a well-established tool for many years.However, in some circumstances, the measurementof humidity using fiber optical sensor is desirableand additionally optical sensor can offer manyadvantages over conventional electronic sensor [113].Deposition of ionically self-assembled polyelectrolytethin film on optical fiber was demonstrated by HaiHu et. al. where it was observed that the behavior ofpolyelectrolyte thin film vary in different humidityenvironment [Fig. 11] [114]. Multilayer thin films ofalternately adsorbed of polyelectrolyte PDDA ( polydiallyldimethylammonium chloride) and PS-119(polymeric dye poly s-119) were formed on opticalfibers through the ionic self-assembly technique.Intrinsic Fabry-Perot cavities were fabricated bystepwise assembling the polyelectrolytes onto theends of optical fibers for the purpose of fiber opticaldevice and sensor development. Ionically self-assembled polyelectrolyte multilayer thin films, inwhich there are hydrophilic side groups with strong

affinity towards water molecules, represents a cat-egory of humidity-sensitive functional materials.

The remarkable electrical and mechanicalproperties of carbon nanotubes make them usefulfor a variety of applications such as chemical andbiological sensors, interconnects, electrodes,supercapacitors, and fuel cells. Nanotubes havealso been used as fillers for polymer composites;however, conventional techniques to preparenanotube based composites require variousfunctionalization schemes in order to obtain a goodinterface between the polymer and the nanotubesurface. Sandeep Razdan et al. present a processfor synthesizing ionically self-assembledpolyelectrolyte-complex-based carbon nanotubefibers using a simple noncovalent stabilization ofcarbon nanotube aqueous dispersions where nosurface functionalizations of the nanotubes werenecessary [115]. The polyelectrolyte-carbonnanotube composite fibers have mechanical,electrical and chemical properties which make thema choice of materials in applications such asbiosensors, chemical electrodes or flexibleelectronics. The fibres showed reasonable strengthand conductivity as high as 45 S/cm for single-walledcarbon nanotubes and 80-90 S/cm for multiwalledcarbon nanotubes, due to the presence of aninterconnected network of carbon nanotubesembedded inside the fibres.

There is a sustained interest toward thedevelopment of high-quality and robust opticalcomponents that would enable the further integrationof photonic systems. Photonic crystals offer apromising platform for the manipulation of lightcompared to traditional methods relying on totalinternal reflection [116]. Development of a fabricationof platform that enables the use of ionically self-assembled monolayers (ISAM) films in photonicdevices has also been reported [117]. Morespecifically, the use of Nanoimprint lithography (NIL)to pattern the NLO-active polyanion, Poly{1-[4-(3-carboxy-4-hydroxyphenylao)benenesulfonamido]-1,2-ethanediyl, sodium salt}(PCBS) wasdemonstrated and was used in conjunction with theNLO-inactive polycation, Poly(allylaminehydrochloride) (PAH) for the synthesis of the (ISAM)films. Photoelectron and infrared spectroscopyconfirmed that the chemical composition of the filmswas not affected by the heating cycle required byNIL. In addition, measurement of the SecondHarmonic Generation (SHG) also confirmed that theNIL process did not affect the alignment of thechromophores responsible for the nonlinear


properties of the material. These observations con-firm that these types of materials are suitable to bepatterned by nanoimprinting, a key feature thatwould enable the cost-effective manufacturing ofnanophotonic devices leveraging these materials.

9. CONCLUSION

There are lot more expectations from this field ofresearch. Scientists are of the view that research inthe field of nanotechnology will enable the worldcommunity to have faster computers, cheapproduction of goods and medical breakthrough.Nanotechnology is expected to appear in productssuch as tennis rackets, self cleaning cars,cosmetics, food, paints, etc. Scientists are alsoclaiming the possibility of obtaining fastercomputers, cheap production of goods, and medicalbreakthroughs by exploiting the newer opportunitiesassociated in this field. Self-assembly provides auseful and effective strategy for organizingnanostructure into more ordered structures.Formation of more complex aggregates andstructured aggregates might lead to intelligentmaterials with significant functional characteristicsthose are different from simple nanoparticles. Theprocess of ionic self-assembly and hierarchies mayrevolutionize in industrial sector encompassingalmost all aspects with small wonder and endlessfrontiers. This might prove as one of the major driversof economic growth for future development ofmankind.

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