nanoreactors for nanostructured materials

INTERNATIONAL JOURNAL OF CHEMICAL

REACTOR ENGINEERING

Volume 6 2008 Article A62

Nanoreactors for Nanostructured Materials

Ramdas B. Khomane∗ Bhaskar D. Kulkarni†

∗National Chemical Laboratory, [email protected]†National Chemical Laboratory, [email protected]

ISSN 1542-6580Copyright c©2008 The Berkeley Electronic Press. All rights reserved.

Nanoreactors for Nanostructured Materials

Ramdas B. Khomane and Bhaskar D. Kulkarni

Abstract

Organized systems such as micelles, reverse micelles, vesicles, polyelectrolytecapsules, liquid crystals, etc., formed through a self-assembling process representnanoreactors that can be used for preparing nanostructured materials. Besides afascinating academic subject, these nanoreactors provide a unique way to developa special type of advanced material for a wide variety of applications in electron-ics, photonics, biomedical and other areas. The article examines the formation,functioning, properties and special attributes of these nanoreactors with a view to-wards their engineering analysis, design and possible integration in manufacturingtechnology.

KEYWORDS: nanoreactors, nanostructured materials, organized self-assemblies

1. INTRODUCTION:

Chemical reaction engineering is the science of chemical transformation of input raw-materials into desired products in reaction vessel-called chemical reactor. For a rational design of a reactor due considerations need to be given to understand (i) the effects of concentration, temperature and flow fields etc. on the reactor performance (ii) measure the rate of transformation (as a function of space and time, as the case may be) (iii) to monitor conversion of raw materials, and (iv) formation of desired and other side products, Extensive studies on the type of reactions (series, parallel or a combination thereof), the inter and intra phase heat and mass transport limitations, the presence of exotherms, the mechanism of cooling, the type of flow field- extent and mixing, micromixing, degree of segregated ness etc. on product and product distribution have been reported in the past (Doraiswamy et al., 1984; Carberry et al., 1987; Froment et al., 1990; Fogler, 1981). Enabling technologies such as experimental tools and measuring instruments including on-line sensors help in generating data at required locations and frequency in time which can be analyzed using fundamental models to determine kinetic and transport parameters and along with thermodynamic data provides a rigorous basis to formulate and use such fundamental models for rational reactor design, optimization, control, monitoring, etc. The experimental data and its analysis further drives the search for newer type of contacting devices, generate better catalysts for the transformation process, unravel fundamental transformation mechanisms, and provide for the in-sight for scale-up. The process also yields optimized performance with better quality of product at higher utilization efficiency of material, energy, utilities, lesser load on environment and cost and time for scale-up (Kulkarni, 2003). Chemical reaction engineering has evolved to be a matured area and chemical industries and manufactures have immensely benefited by its applications across the various categories of chemical products ranging from basic chemicals, specialty chemicals, pharmaceuticals, food and biotechnology products, polymers on one hand to new advanced materials with structures and architectures that are responsible for the performance attributes of the products on the other. The research needs to acquire physico-chemical, kinetic and thermodynamic data and to build first principles models to obtain and predict process performance, facilitate reactor design for improved performance, predict and link process conditions to product quality and properties at various length and time scales continue (Klipstein, 2001). The new challenges to reaction engineering emerge from advanced material synthesis and manufacture. Reports of the appearance of new materials replacing the conventionally used materials (wood, glass, metal, fibers etc.) with an improved performance and extended range of applications are clear pointers in

1Khomane and Kulkarni: Nanoreactors for Nanostructured Materials

Published by The Berkeley Electronic Press, 2008

this direction. A number of advanced materials with cost competitiveness and enhanced performance in sectors such as automotive and transportation, energy, electronics and optoelectronics, separations, packaging, coating, biomedical applications, hybrid composites for building-construction, enhanced oil recovery, space modules etc. are poised to bring about economic transformation of the chemical industry. A clear understanding of the fundamental science and especially the chemistry (and biology in some instances) at interfaces together with our ability to model structure-activity-performance relationship and the connected issue of controlling molecular structure during processing conserving its desirable properties is thus the emerging need. Development of non conventional reactors that will allow structured contacting with ease of rapid heating or cooling or use of alternative fields such as electric, magnetic or photo-assisted transformation or self-assembled systems as nanoreactors for nanostructured products are becoming important. In the present work we will take stock of nanoreactors for preparation of such structured materials and examine them from the view point of chemical reaction engineering parameters.

Scientific curiosity about the behavior of matter at smaller length scales, besides generating a significant fundamental understanding, has led to development of a number of new advanced materials with well defined size, structure and form that contribute and decide their unique properties. The materials normally range from nano to micro scale and have greatly impacted various sectors with very many applications in diversified areas. Many different types of nanomaterials synthesized in different types of nanoreactors are summarized in Figure 1.

2 International Journal of Chemical Reactor Engineering Vol. 6 [2008], Article A62

http://www.bepress.com/ijcre/vol6/A62

Figure 1. Classification of nanomaterials and nanoreactors in different areas. Conducting chemical reactions in confined spaces creates an opportunity

to synthesize new materials with unexpected properties. Nature uses relatively simple systems such as enzymes or complex assemblies such as cells to produce different materials of desired properties. The transformation processes are multi-step in nature where multiple components and multi-catalytic species are present or generated at the right location at the right time leading to high yield and selectivity. Coupling of reactions in time and space with intrinsic control on the kinetics of reactions, mass and transport of species, convection and mixing, all together optimizes the performance to obtain final structure with minimum cost, time and waste produced. Construction of an artificial cell while is the ultimate target, the initial attempts to construct covalent or noncovalent reactors and self assemblies of small molecular components have generated results that are sufficiently exciting and continue to fan inspiration in this direction.

In the past few years, research on nanosystems has developed significantly due to their potential applications in biology, electronics, advanced materials and information technology. The nanostructured materials exhibit novel electrical,

Sector Area Materials Nanovescles

Transportation

Energy

Health care

Building/construction

Advanced composite materials

Catalysis

Separations

Electronics

Optics

Medicine biosensing

Metals

Metal oxide / hydroxide

Semiconductor

Porous inorganic structures

Coated colloids, hollow spheres

2-d surface arrays

3-d mesoporous structures

Polymer

Biomaterials

Membranes

Micelles

Reverse micelles

Rod-like micelles

Microemulsions

Vesicles

LB Films

Lipid membranes

Bilayers

Liquid crystals

Microgels

Liposomes

Dendrimers

Polyelectrolytes

Multilayer capsules

Protein & virus cages

Other self-assemble system

Nanomaterials and nanoreactors



optical, magnetic and mechanical properties due to altered electronic structures (Nayak et al., 2005). Synthesis of such materials in self-assembled surfactant templates can provide a means of exercising some control on their shapes, crystallinity and sizes which are so crucial to their properties. Surfactants with hydrophilic polar head and hydrophobic hydrocarbon chain, when dissolved in water can form various aggregates such as micelles, reverse micelles, interconnected cylinders, vesicles, planer or onion like lamellar phases, spherical or cylindrical nanocrystal etc. Extensive studies on the formation and characterization of such self-assembled structures and effects of number of parameters such as nature and type of polar head group, hydrocarbon type and chain length, operating conditions of temperature, concentration of surfactants, extent of dilution, presence of additives, electrolytes etc. on the shape, size, crystallinity, nature and type of self-assembly have been studied. A number of different types of nanoparticles ranging from metals, semiconductors, polymers, inorganic oxides etc. have been prepared in micelles (Samuelson et al., 2001), reverse micelles (Ingert et al., 2001), liquid crystals (Delinger et al., 2001), microemulsion (Hingorani et al., 1995; Manziek et al. 1998), microgels (Antonietti et al., 1997), block copolymers ( Bronstein et al., 1999; Forster et al., 2003), etc. In the rest of the paper, we shall summarily discuss these nanoreacors along with their characteristics, methods of formation, applications to different systems and the rationale for engineering design and analysis.

2. ORGANIZED MEDIA: SOME IMPORTANT FACTS

2.1. Normal Micelle

The literature on normal micelles is extensive and for reasons of brevity we shall only summarize some important facts. Typical normal micelles are depicted in Figure 2.



Figure 2. Normal micelle

• Lyophilic assemblies of colloid size; several hundreds of amphiphilic molecules or ions of surfactants form dispersed phase that is distributed in the bulk of the solvent phase.

• Interphase or stern layer separates the microphase of direct micelles from the dispersion medium. The layer contains polar head groups of surfactant and a shell of retained water molecules. The rigidity, flexibility, water permeability, local ionic strength, electrostatic potential, pH, etc. can be altered and depend on extent and type of electrolyte, nature of organic solvent, cosurfactant, etc.

• The entropic factor in the Gibbs potential (ΔG=ΔH-TΔS, <0) describing the spontaneous micellization contributes more to micelle formation than the enthalpy. This is due to change in the states of the surfactant and water molecules on micellization. The hydrophobic interactions drive the process and eliminates the nonpolar hydrocarbon radicals from water and their association in the nucleus of the micellar systems. Eliminating water contact and restructuring of water depends on concentration of surfactant which decreases with the length of hydrocarbon radicals.

• Micellar systems continuously undergo the formation and breakdown processes. The half life of surfactant micelles range from a millisecond



to second whereas the surfactant molecule in a micelle has half life of 10-5 to 10-7 seconds.

• The packing parameters Ρ( ν/ιа), where a is the surface area of head group and ν and ι are the volume and length of alkyl chains, defines various aggregation geometries possible. For instance 0<P<1/2, the micelle formation is expected whereas for 1/2<P<1, vesicles may form and for P>1 inverted structures are expected. All these geometries have nanometer dimensions and can be used as nanoreactors. The packing size and shape can change from spherical, rod-like to hexagonal type as concentration of surfactant, electrolyte etc. changes.

• Solvation thermodynamics is decisive in the favorable overlap of hydration shells of hydrophobic parts of molecules. A variety of self assembled morphologies can result as a consequence of aggregation in aqueous solution. The aggregation morphologies are decided by the free energy term comprising of contributions from (a) favorable clustering of hydrophobic parts of molecules (b) a tendency to closely pack so as to minimize unfavorable hydrocarbon-water interaction (c) a counter tendency to spread due to electrostatic repulsion between charged head groups, hydration, steric effects etc. The typical aggregation number varies between 40 to 150.

• The presence of hydrophilic surface and hydrophilic core, allows the micelles to dissolve both polar and nonpolar substances and can alter (enhance) the reaction rate due to (a) concentration of reactant at the interface or in the interior, (b) stabilization of transient state of reaction due to favorable interaction with surfactant and (c) the medium effect as reflected in properties such as polarity, microviscosity, charge, etc.

A number of examples illustrating synthesis of different nano particles are included in the Table 1.



Table 1. Some experimental studies on microemulsion for nanoparticles Sr. No

System Special features and applications References

1 An aqueous solution of SDS and reactant cation mixture + methylamine

Normal and reverse micelle microemulsion were used to synthesis single-phase CoCrFeO4 nanoparticles with a controlled size range of 6-16 nm. Using CoCrFeO4 nanoparticles as precursors for the formation of a bulk sample resulted in a material that displayed an unusual susceptibility behavior but not the anomalous temperature-dependent hysteresis trend usually observed in conventional CoCrFeO4 bulk materials.

Vestal et al., 2002

2 Tween 20/clove oil/water Basic acid, an unsatd. triterpene acid isolated from Mimusops elangii, was tested for its antileishmanial properties both in vitro and in vivo. The in vitro antileishmanial activity of basic acid being encouraging, its activity in vivo was evaluated in hamster models of visceral leishmaniasis, both in free form, as well as incorporated in two different delivery systems, viz microemulsions and polylactide nanoparticles.

Lala et al., 2006

3 O/W microemulsion & sub-micron emulsion process & compositions (patent)

A process for the prepn. of an oil in water (O/W) microemulsion or sub-micron emulsion compn. for dermal delivery of at least one pharmaceutically active ingredient.

Larm et al., 2006

4 An aqueous solution of SDS and reactant cation Mixture + NH4OH

Magnesium aluminate, MgAl2O4, spinel nanoparticles were synthesized using normal micelle microemulsion methods.

Vestal et al., 2003

5 SDS/hexane/n-butanol/water Studied the presence in the environment of the Segura et al.,



SDS/hexane/n-pentanol/water SDS/hexane/n-hexanol/water SDS/hexane/3-pentanol/water

polycyclic arom. hydrocarbon benzo[a]pyrene (BaP) in the city of Granada using room-temp. phosphorescence (RTP) applied to microemulsions.

2002

6 Microemulsion and micelle systems containing long polymer chain surfactants and short fatty acid surfactant components for solubilizing drugs

A microemulsion delivery system for water insol. or sparingly water-sol. drugs comprise a long polymer chain surfactant component and a short fatty acid surfactant component, with the amt. of each being selected to provide stable microemulsion or micellar systems.

Dennis et al., 2002

7 SDS/heptane/1-butanol/phosphate buffer SDS/toluene/1-butanol/carbonate buffer

The electrophoretic behavior of five bases and corresponding nucleosides in the oil in water (o/w) microemulsion capillary electrophoresis, microemulsion electrokinetic chromatog. (MEEKC), were examd. in comparison with those in normal capillary zone electrophoresis (CZE).

Furumoto et al., 2001

8 Phase behavior of sodium naphthenates, toluene, and water

In a ternary phase diagram of sodium naphthenates (SN), toluene, and water the following phases can be formed at 25o: normal micellar soln.; inverse micellar soln.; lamellar liq. crystal; microemulsion; and turbid birefringent gel phase.

Horvath-Szabo et al., 2001

9 Tween 80/pentanol/ethyl oleate Tween 80/cyclohexanol/ethyl oleate

Microemulsions are phys. stable oil/water systems that have potential use as delivery systems for many pharmaceuticals which are normally of limited use due to their hydrophobicity, toxicity, or inability to access the site of action.

Al-Adham et al., 2000

10 SDS/aromatic hydrocarbon/propanol

Water-SDS-arom.hydrocarbon-propanol microemulsions are constructed and used as media for I- oxidn. by S2O2-8 ions.

Santhanalakshmi et al., 1995



2.2. Reverse Micelle

Figure 3. Reverse micelle

Reverse micelles provide another example of organized self assemblies of surfactants in solution and are most widely used as reaction media or templates for biomimetic synthesis of various inorganic nanoparticles (Pileni, 1993; Pillai et al., 1995; Kon-no et al., 1997; Adair et al., 1998; Osseo-Asare, 1999; John et al., 2002; Capek, 2004). The biomineralization process in nature uses organized aggregates of bio macromolecules to synthesize nanoparticles with dimensional, morphological and architechtural specificity and excersizing full control over nucleation, growth and the patterns formed. The hydrophilic head and hydrophobic tail of surfactants in apolar solvent self assemble to give reverse micelles where the polar core contains the hydrophilic heads and the apolar shell the hydrophobic chains (see Fig.3). Water can be solubilized in the core forming water-in-oil droplets (5 nm) which eventually become the w/o microemulsion as the water content increases (5 to 100nm). The water to surfactant molar ratio has a decisive influence on the diameter of the reverse micelles. The aggregation number is typically in the range of 20 to 30, lower than in direct-micelles. The shape can be spherical, rod-like or lamellar and depends on the concentration of surfactant, electrolyte, other additives, etc. The droplets undergo continuous collisions and exchange their contents. The anionic sodium bis (2-ethylhexyl) sulfosuccinate (AOT) and nonionic poly oxyethylated surfactants are most commonly used to generate reverse



micelles and have the advantage that no cosurfactant need be used. Also, the micelles formed are nerealy spherical and monodispersed irrespective of the nature of the apolar solvent and the droplet size is independent of the CMC of surfactant. The use of nonionic surfactants also eliminates the ill-effects of counterions. Other ionic or nonionic surfactants can also be used but must be chosen with care to ensure their inertness in the system, interference of the counter ions etc. One of the easiest method of preparing nanoparticles in reverse micelles involves preparing two reverse microemulsions containing reactant A and B and mix them together to produce the product. The possible mechanism of particle formation involves (i) colloiding of two droplets (ii) coalescing to form dimer (iii) exchange the contents (iv) chemical reaction and particle formation and (v) disintegrate into droplets. The collision rate dynamics is usually very fast (1010 dm3/mol/s) at ambient conditions as compared to the rate of interchange of droplet contents (106 to 108 dm3 /mol/s) for AOT (Sagar, 1998). Only a small percentage of droplet collisions therefore result in actual dimer formation and hence the interchange mechanism is often rate controlling step. Also the reaction rate, once the transfer occurs, is usually faster. This has the implications that the bulk nucleation and growth models may not adequately explain the nanoparticles formation. In fact, the nucleation and growth is determined by the intermicellar exchange of reactants (autocatalytic reaction) and/or product (the ripening process). In the former the exchanged reactants undergo reaction at the surface of the particles inside the droplets and deposit and grow on them while in the later the smaller particles from one droplet gets transferred to the other containing larger particle that grows further. The size of the channel containing the two microdroplets and the surfactant film flexibility plays a significant role in the growth process. The surfactant molecule can get adsorbed on the particle surface and stabilize it preventing further growth. The entire surfactant film can participate in the process especially when it is rigid. For flexible surfactant films, the adsorption of surfactant on particles and their transfer across droplets creates an inversion of film curvature which depends on film flexibility and elasticity. The nanoparticles size and size distribution depends on a number of parameters (Pileni, 1997; Lopez-Quitela et al., 2003) which include (1) the droplet size (2) inter droplet exchange (30 reactant concentration (4) the drolet volume fraction (5) critical nucleus (6) the reactant excess ratio (7) film flexibility (8) the growth mechanisms arising out of autocatalysis and ripening. General model that accounts for all such parameters with Monte Carlo simulations give some generic trends for particle size and size distribution; thus it is established that:

• Particle size and polydispersity increases with the increase of reactant concentration

• Particle size increases with the surfactant film flexibility



• Particle size increases with the increase in droplet size • Particle size decreases with the increase in the excess of the reactants

The above generic trends are exhibited by systems that are microemulsuion- controlled systems. In the other case of surfactant-controlled systems, none of the microemulsion based parameters are involved in deciding particle size. In such systems the adsorption behaviour of surfactants or capping agents dictates; in fact the use of microemulsion is irrelevant and similar results can be obtained in direct bulk solutions. Preparation of catalytic particles, where adsorption is strong belong to this type. The presence of cosurfactant can affect the flexibility of surfactant film and the adsorption behavior. The positive entropy change (TΔS) in the Gibbs free energy equation due to adsorption of surfactant decreases with the increase in radius R of the particle. The enthalpy changes on the other hand are nearly constant. A minimum stable critical radius νc (ΔG=0) is therefore predicted. For radius smaller than νc, the particle size is governed by microemulsion controlled behaviour while for the radius greater than νc, the surfactant control behaviour is followed. The shape of nanoparticles synthesized in reverse- micelles would normally be spherical unless some system specific special care is exercised. A number of examples illustrating synthesis of metals, metal oxides and other inorganic particles are included in the Table 2 (see Tables 2.1 to 2.18). Synthesis of non spherical assemblies of nanostructures has also been the focus of attention. Of special interest are the 1-d nanostructures (nano wires, nanotubes, nanorods, etc.) and the self organization of colloidal nanoparticles into 2-d to 3-d assemblies as superlattices exhibiting the combined properties of individual as well as collective particle behaviour. The primary mechanism responsible for the formation of such structures involves either a template-directed growth or the oriented aggregation. In the former, the elongated water droplet generates elongated nuclei that grows into nanostructure with one dimension considerably larger than the template. The nuclei formation can also occur in the interconnecting channels. In the other mechanism, the surfactant encapsulates the primary particles which linearly attach each other due to specific interactions between particles, surfactants and other additives present. The intrinsic crystallographic structure of the solid may also contribute to this mechanism. Numerous examples that subscribe to either one of these mechanisms are reported in the literature. A few assorted examples of nanorods, nanowires and nanotubes are provided in the table 2. The integration of one-dimensional building blocks into ordered superstructures produce hierarchical assembles with complex functional architectures and hence attributes. A few of such examples (nanorods, tubes, cubes, etc.) are also included in Table 2.



Table 2. Some experimental studies on Reverse microemulsion Table 2.1. Some experimental studies on reverse microemulsion for nanoparticles

Sr. No


1 Cetyltrimethylammonium chloride (CTAC), n-hexanol, and cyclohexane

Authors have synthesized and and studied magnetic properties of BaFe12O19 hexaferrite nanoparticles. They report the variation of elec. cond. during the formation and reaction of microemulsion droplets suggesting nonpercolating microemulsion conducting systems.

Xu et al., 2007

2 Water/AOT/isooctane Monodispersed palladium nanoparticles have been synthesized to study structure sensitivity of solvent-free selective hydrogenation of 2-methyl-3-butyn-2-ol .The method for isolation of monodispersed Pd nanoparticles from a reverse microemulsion was developed using hydrocarbon evapn. and methanol-assisted particle purifn. from a surfactant.

Semagina et al., 2007

3 sodiumdodecylsulfate/toluene-pentanol (1:1)/water

The influence of polyampholytes on the phase behavior of microemulsion used as template for the nanoparticle formation has been studied. A series of hydrophobically modified polyampholytes by the copolymn. of the cationic monomer (N,N'-diallyl-N,N'-dimethylammonium chloride) and the anionic monomers maleamic acid or butylmaleamic acid has been synthesized and the influence on the inverse micellar region of the quaternary system has been investigated.

Note et al., 2007

4 Nonionic surfactants, heptane and aq. salt Reverse microemulsions is used, to synthesize BiOX Henle et al.,



solns Nanosized BiOX (X = Cl, Br, I) Particles

(X = Cl, Br, I) nanoparticles. The reverse micelles act as nanoscale templates for the ionic pptn. process.

2007

5 CTAB/hexanol/water Synthesized the nanosized precursors of yttria-stabilized zirconia (YSZ) via a reverse microemulsion.

Tai et al., 2007

6 TX-100 + CTAB / n-butanol + n-hexanol cyclohexane /water

The study demonstrates preparation of single-crystal hydroxyapatite (HAP) nanorods by the reverse microemulsion method with diam. 8-15 nm and length 25-50 nm.

Sun et al., 2006

7 OP/hexanol/cyclohexane/aqueous solution

Methylene blue (MB)-doped silica nanoparticles (NPs) were prepared in a reverse microemulsion and used as a novel matrix for biochem. application.

Xian et al., 2006

8 Triton X-100/ n-hexane/ n-hexanol/water

W/O microemulsion was used to prepare nanoscaled nickel by direct electrodeposition, Ni nano-particles were modified onto the Ni substrate. Further they studied the cathodic process of Ni2+ in the W/O microemulsion .

Zhou et al., 2006

9 n-hexadecil trimethylammonium bromide (CTAB)/ n-butanol/ n-hexanol/water

The magnetite nanopowders were synthesized by a single microemulsion technique in which the aq. phase contains only metal ions (Fe2+ and Fe3+). Further, the microstructure and size control of iron oxide nanoparticles was studied.

Koutzarova et al., 2006

10 OP (polyethylene glycol p-octylphenyl ether, t)/ cyclohexane /n-butanol/water

Nanometer-sized Bi4Ti3O12 particles by chem. reaction of Bi(NO3)3.5H2O, Ti(SO4)2, and NH3 soln. in a reverse microemulsion system has been reported.

Xie et al., 2006

11 (Igepal CO 520, AOT, TOPO)/ (Cyclohexane/cyclopentane/ cycloheptane)/ Water

A method for synthesiszing polyaniline nanoparticles using reverse microemulsion process has been reported.

Jae-Hyun et al., 2006



12 Brij-30/n-heptane Triton X-100/n-hexanol /cyclohexane.

Ultrafine tungsten and tungsten oxide powders with controllable particle size and structure was prepared by a reverse microemulsion-mediated synthesis method.

Xiong et al., 2006

13 Triton X-100/cyclohexane /n-hexanol/water

A synthesis method for nanocryst. lithium silicate by coupling of sol-gel method in reverse microemulsion is reported. The nanoparticle prepd. in the microemulsion shows enhanced CO2 sorption capacity and shorter retention times at higher temp.

Khomane et al., 2006

14 DBSA/iso-Pr alc./isooctane/water The study fabricates the pyrrole-oligomer nanoparticles doped dodecylbenzenesulfonic acid (DBSA) in reverse microemulsion, DBSA plays the roles of both surfactant and dopant.

Han et al., 2006

15 Mixt. of Span80 and Tween80/ theolin/butanol/water

Authors have successfully prepd.Nano-sized monoclinic sulfur particles via the chem. reaction between sodium polysulfide and hydrochloric acid in a reverse microemulsions system.

Guo et al., 2006

16 Coated water soluble nanoparticles comprising an inorganic shell and semiconductor core and their preparation

Core-shell nanoparticles and methods of making these nanoparticles are provided. The nanoparticles may include semiconductor nanocrystals. A shell may encapsulate a nanoparticle core, and may include a non-org. material, esp. silica.

Ying et al., 2006

17 Polyoxyethelyne(5)nonyphenyl ether/ Cyclohexane/ ammonium hydroxide

Using reverse microemulsion techniques combined with templating strategies authors have synthesized four types of nanoparticles and studied the formation of different nanoparticle architectures with tailored silica shell thickness and porosity.

Yi et al., 2006

Table 2.2. Reverse microemulsion as nanoreactor



Sr. No


1 Cetyltrimethylammonium bromide/ n-butanol / cyclohexane

An effective strategy to produce zeolite nanocrystals by using microwave heating in reverse microemulsion.

Chen et al., 2005

2 AOT (2-ethylhexyl sulfosuccinate)/isooctane/water

The preparation method for monodispersed ultrafine indium-tin oxide (ITO) particles in reverse microemulsions is reported. The study further shows that tin mols. are doped into the indium oxide homogeneously rather than forming a discrete tin oxide domain in indium-tin oxide particles formed in microemulsions.

Kim et al., 1999

Table 2.3. Reverse microemulsion for nanorods Sr. No


1 Triton X-100/cyclohexane /n-hexanol /water

Ba0.7Sr0.3TiO3 nanorods in reverse microemulsion at room temp. The tuning of the size of nanorods by changing w value (molar ratio of water to surfactant), aging time and reactant concn is reported .

Chen et al., 2007

2 TX-100 + CTAB/ n-butanol + n-hexanol / cyclohexane/water

A simple system for the synthesis of single-crystal hydroxyapatite (HAP) nanorods by the reverse microemulsion.

Sun et al., 2006

3 SDS/cyclohexane/ n-Amylalcohol

Pure γ-phase CuI nanorods. by reverse microemulsions using low temperature.

Li et al., 2006

4 CTAB/cyclohexane/H2O and NP10/cyclohexane/H2O

Authors have explained the methods to synthesize the BaCO3 nanorods and nanobelts in reverse microemulsion. From various comparison expts.

Li et al., 2006



Authors have shown that several exptl. parameters, such as the concn. of surfactant and reactant, and solvent hydrothermal treatment play important roles in the morphol. control of BaCO3 nanostructures.

Table 2.4. Reverse microemulsion for nanotubes Sr. No


1 AOT /hexane/water

Authors have shown the fabrication method for polypyrrole (PPy) nanotubes with different diams. using cylindrical micelle templates in reverse microemulsions. They further studied electronic response PPy nanotubes with various diameters

Yoon et al., 2006

2 AOT /hexane/water

Authors have described a convenient one-step method to chem. prep. bulk quantities of microns long poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes using reverse microemulsion polymn., the use of potential-time profiling to uncover factors that favor tubular polymer growth, the prepn. of PEDOT-metal and PEDOT-metal oxide composites having tubular morphol., and transport properties and potential device applications of individual PEDOT tubes.

Zhang et al., 2006

3 TOA/H3PO4

Titanium phosphate nanotubes with alternating interlayer spacings have been synthesized in a reverse microemulsion formed in an amine extn. system.

Yin et al., 2004

4 AOT /hexane/water

Polypyrrole (PPy) nanotubes by reverse microemulsion polymn. in an apolar solvent in the

Jang et al., 2003



presence of FeCl3 and sodium bis(2-ethylhexyl)sulfosuccinate have been reported, and factors affecting the formation of PPy nanotubes have also been investigated.

Table 2.5. Reverse microemulsion for nanowires Sr. No


1 SDS/water/heptane/n-hexane

The study reports the synthesis of ZnO nanowires with high-aspect-ratio of up to ca. 600 in a quaternary reverse microemulsion contg. via a hydrothermal method. The role of SDS in the formation of morphologies has been ellucided.

Sun et al., 2006

2 CTAB/BuOH/n-heptane/HAuCl4(aq.) The hydrophobic Au nanostructures with different shapes have been prepared by using BuOH situ redn. in W/O microemulsion through microwave dielec. heating.

Shen et al., 2005

Table 2.6. Reverse microemulsion as a template Sr. No


1 Marlophen NP5 (RO(CH2CH2O)xH, x =5, R = nonylphenyl, M= 440 g/mol, SASOL ) / n-heptane /water

To synthesize BiOX (X = Cl, Br, I) nanoparticles Authors have used Reverse microemulsion. The reverse micelles act as nanoscale templates for the ionic pptn. process.

Henle et al., 2007

2 SDS/toluene-pentanol (1:1)/water Present work focuse on the use of branched poly(ethyleneimine) (PEI) as reducing as well as stabilizing agent for the formation of gold nanoparticles in different media. The reverse microemulsion droplets of the quaternary system

Note et al., 2006



were successfully used for the prepn. of gold nanoparticles

3 SDBS/ dimethylbenzene/ water

Solvothermal synthesis of hollow nanostructure, hollow spindle-like hematite with uniform size and morphol. The formation mechanism as a coordination-assisted dissoln. process occurred in a reverse microemulsion system.

Lu et al., 2006

4 Polyoxyethelyne(5)nonyphenyl ether/ Cyclohexane/ ammonium hydroxide

Using reverse microemulsion techniques combined with templating strategies the work synthesizes four types of nanoparticles and studies the formation of different nanoparticle architectures with tailored silica shell thickness and porosity.

Yi et al., 2006

5 AN/ PPO19–PEO33PPO19 /NMP

Mesoporous polymer materials can be fabricated from micelle/polymer precursors prepared by the micelle template method in reverse microemulsion systems and the pore size could be tuned by varying the type and concentration of surfactant.

Jang et al., 2005

Microemulsion Methods Table 2.7. Reverse microemulsion co-precipitation

Sr. No


1 Triton X-100 /n-heptane/ hexanol and an aqueous solution of cerium and copper nitrates.

Ce-Cu mixed oxide precursors with varing Ce:Cu at. ratio have been prepd. by freeze-drying and microemulsion copptn.

Fuerte et al., 2007

2 Triton X-100/n-heptane/hexanol / aq solution solutions of CuO and CeO2

Nanostructured catalysts based on combinations between oxidised copper and cerium entities prepd. by two different methods (impregnation of ceria and copptn. of the two components within reverse

Gamarra et al., 2007



microemulsions) have been examd. with respect to their catalytic performance for preferential oxidn. of CO in a H2-rich stream (CO-PROX).

3 Sorbitan triesterate/Xylan/ chloroform / cyclohexane /aq solutions of ferrous and ferric salts

This work evaluates an exptl. set-up to coat superparamagnetic particles in order to protect them from gastric dissoln. First, magnetic particles were produced by copptn. of iron salts in alk. medium. Afterwards, an emulsification/crosslinking reaction was carried out in order to produce magnetic polymeric particles.

Silva et al., 2007

4 CTAB/1-butanol/n-heptane/H2O Nanopowder of Y Al garnet (YAG, Y3Al5O12) doped with Nd ions (Nd:YAG) was prepd. in the H2O/cetyltrimethylammonium bromide/1-butanol/n-heptane system. Al, Y, and Nd nitrates were used as starting materials, and NH3 was used as a pptg. agent. Coppt. hydroxide precursors where thermally treated at 900oC to achieve the garnet phase.

Caponetti et al., 2007

5 W/O microemulsion Nanosized SrAl2O4:Eu2+,Dy3+ phosphors with good monodispersity and narrow size distribution were synthesized by the coupling of water-in-oil (W/O) microemulsion with copptn. method. The amt. of surfactant that was used had an important effect on the shape and av. size of the phosphor particles.

Sun et al., 2006

6 CTAB/ 1 hexanol/ aq solution MnCl2, La(NO3)3 and Sr(NO3)2 CTAB/ 1-hexanol/ 1-butanol/ aq solution MnCl2, La(NO3)3 and Sr(NO3)2

Lanthanum-strontium manganites were synthesized using co-pptn. method with a reverse micellar microemulsion. Either oxalic acid, sodium hydroxide or tetramethylammonium hydroxide was used for the pptn. of precursor cations in a form that

Uskokovic et al., 2007



was subsequently calcined under various conditions in order to obtain perovskite manganite phase.

7 Reverse microemulsion mediated co-precipitation

Typical hexagonal plate-like particles with approx. sizes of 80, 180, and 500 nm, were obtained by microemulsion, copptn., and solid-state reaction techniques, resp.

Nedkov et al., 2006

8 Brij-97/cyclohexane/ aq. ferrous and ferric salts solutions

A 1-pot microemulsion method to produce monodisperse and coated small nanoparticles. The nanoparticles are formed by the copptn. reaction of ferrous and ferric salts with two org. bases, cyclohexylamine and oleylamine, into a water-in-oil microemulsion.

Vidal-Vidal et al., 2006

9 CTAB/1-hexanol/water Nanostructured lanthanum-strontium manganites were synthesized using two different co-pptn. approaches, one in bulk soln., and the other in reverse micelles of CTAB/1-hexanol/water microemulsion. In both cases, precursor cations were pptd. by using oxalic acid.


10 SDS /styrene / cetyl alcohol / Water Magnetic polymer-coated microspheres were prepd. by the microemulsion polymn. of styrene (St), methacrylic acid (MAA), acryamide (AM) in the presence of emulsifiers with the size of 1-5 µm. The magnetic material (i.e. Fe3O4) coated with oleic acid used in the prepn. of the microspheres was synthesized in a classical co-pptn. procedure.

Liu et al., 2006

11 Marlipal O13/70/ cyclohexane/ aqueous solutions (salts, ammonia and hydroxide)

In this paper, w/o-microemulsions were employed to produce nanoparticles of the perovskite Ca0.5Sr0.5MnO3, which have a size of approx. 20-50

Lopez-Trosell et al., 2006



nm. The procedure was carried out using sodium hydroxide or ammonia as co-pptn. agent.

12 NP5+NP9 /cyclohexane/ aqueous solution of Pb2+, Zr4+, Ti4+

Microemulsion method has been applied and compared with sol-gel, copptn. and ceramic route with the purpose of encapsulating hematite into zircon crystals used as ceramic pigment. The stabilization of sodium silicozirconate cryst. phases in NaOH copptd. or emulsioned samples avoids the zircon crystn. and then the hematite encapsulation.

Garcia et al., 2003

13 NP5+NP9/ cyclohexane/ aqueous solution Pb(NO3)2–TiO(NO3)2

Ultrafine lead titanate (PbTiO3) powders in tetragonal form have been successfully prepd. via two processing routes, namely, conventional copptn. (CPC) and microemulsion-refined copptn. (MCP). the microemulsion-refined copptn. is the technique that results in the formation of the finer powder of lead titanate than the conventional copptn. Does.

Fang et al., 2002

14 NaAOT/ isooctane / aqueous K3Fe(CN)6

Co(AOT)2 / isooctane / water

Here the authors describe the synthesis of cryst. nanoparticles of three different mol.-based magnetic materials, cobalt hexacyanoferrate, cobalt pentacyanonitrosylferrate, and chromium hexacyanochromate, by copptn. reactions involving mixts. of water-in-oil microemulsions.

Vaucher et al., 2002

15 Brij-97/cyclohexane/ aq solutions of ferrous and ferric salts Igepal CO-520/ n-heptane / aq solutions of ferrous and ferric salts Triton X-100 /cyclohexane / n-hexanol / aq solutions of ferrous and ferric salts.

Three different nonionic surfactants (Triton X-100, Igepal CO-520, and Brij-97) were used for the prepn. of microemulsions, and their effects on the particle size, crystallinity, and the magnetic properties were studied. The iron oxide nanoparticles are formed by the copptn. reaction of ferrous and ferric salts with

Santra et al., 2001



inorg. bases. A strong base, NaOH, and a comparatively mild base, NH4OH, were used in each surfactant to observe whether the basicity has some influence on the crystn. process during particle formation.

16 NP5+NP9/cyclohexane/water Three processing routes have been used to prep. barium titanate powders, namely conventional copptn., single-microemulsion copptn. using di-Et oxalate as the precipitant, and double-microemulsion copptn. using oxalic acid as the precipitant.

Wang et al., 1999

17 NP5+NP9/cyclohexane/water Ultrafine perovskite lead zirconate powders have been prepd. via three types of processing routes: conventional solid reaction, conventional copptns. using either oxalic acid or ammonia soln. as the precipitant, and microemulsion-refined copptns. using either oxalic acid or ammonia soln. as the precipitant.

Fang et al., 1998

18 Poly(oxyethylene)5 nonyl phenol ether + poly(oxyethylene)9 nonyl phenol ether/ cyclohexane/ water

The microemulsion system used consists of cyclohexane as the oil phase, mixed poly(oxyethylene)5 nonyl phenol ether and poly(oxyethylene)9 nonyl phenol ether as the nonionic surfactants, and an aq. soln. contg. cations of lead, zirconium, and titanium as the water phase. Copptn. of the hydroxide precursors was effected through addn. of an aq. ammonia soln. into the microemulsions.

Ee et al., 1998

19 Cyclohexane / n-hexyl alcohol /water solution. OP-10

Cerium(IV) oxide ultrafine particles were prepd. using a reaction within reversed micelles, known as a

Masui et al., 1997



microemulsion method. The ultrafine particles, which were obtained by mixing the water-in-oil microemulsions contg. cerium nitrate soln. with those of ammonium hydroxide, were characterized by high-resoln. electron microscope (HREM) observations.

20 Igipal CA520/n-heptane/water The authors describe a new technique using microemulsions to produce ultrafine precursors of Y Fe garnet. A copptn. of hydroxide or carbonate precursors was made in a W/O microemulsion medium.

Vaqueiro et al., 1997

21 Microemulsion

A method for synthesizing ultrahomogeneous nanoparticles of precursor powder by copptn. in a H2O and oil microemulsion is disclosed.

Shah et al., 1996

22 Alcohol-in-oil microemulsion Ultrafine BaFe12O19 with uniform particle size was synthesized from an alc.-in-oil (nonaq.) microemulsion system where the metal ions were supplied by the surfactant (metal di-2-ethylhexylsulfosuccinate) mols. themselves. A monodisperse, fine-gained ppt. (Ba-Fe oxalate) was ensured by the steric barrier provided by the surfactant monolayer, while the nonaq. environment promoted stoichiometric copptn.

Chhabra et al., 1995

23 AOT/n-heptane/water AOT/n-hexane/water AOT/n-octane/water AOT/n-decane/water CTAB/n-heptane/water

A microemulsion was studied as a reaction medium to control the copptn. of oxalate precursors of the superconducting YBa2Cu3O7

-δ ceramics.

Wang et al., 1995



CTAB/n-hexane/water CTAB/n-octane/water CTAB/n-decane/water

24 Triton N- 10 1/ cyclohexane / hexanol / aq. NH40H. Igepal CO-520 /cyclohexane / aq. NH40H.

water-in-oil microemulsions in which monodisperse silica colloids are produced by the controlled hydrolysis of tetra-Et orthosilicate (TEOS) in water nanodroplets. The resulting pure silica spheres can be grown to 40-80 nm diam. and can be used as seed particles for prodn. of larger silica colloids upon further reactions with TEOS in the microemulsion. CdS quantum dots were incorporated into the silica colloids during the silica sphere synthesis by the simultaneous copptn

Chang et al., 1994

25 NP-5/petroleum ether/water The inverse microemulsion technique has successfully been used to synthesize lanthanum and nickel oxalate particles, which could readily be processed to form a fine LaNiO3 powder.

Gan et al., 1994

26 CTAB/ n-butanol/ octane/ Water Nanoparticles of barium ferrite (BaFe12O19) were synthesized using a novel method called microemulsion processing. In this process, the aq. cores (typically 5-25nm in size) of water-cetyltrimethylammonium bromide-n-butanol-octane microemulsions were used as constrained microreactors for the co-pptn. of precursor carbonates (typically 5-15nm in size).

Pillai et al., 1993

27 Igepal CO-430/ cyclohexane / aqueous solution of metal salts Y:Ba:Cu=1:2:3

A method is presented for prepn. of ultra-homogeneous nanoparticles of precursor oxalate powders by copptn. in the aq. cores of water-in-oil

Kumar et al., 1993



Igepal CO-430 / cyclohexane / aqueous solution of oxalic acid

microemulsions for the prepn. of YBa2Cu3O7-x (123-phase) superconductors.

28 water-in-oil microemulsion The authors describe a new technique for the synthesis of ultrahomogeneous nanoparticles of precursor oxalate powder by copptn. in the aq. core of a water-in-oil microemulsion for the prepn. of Bi-Pb-Sr-Ca-Cu-O (2223) oxide superconductor

Kumar et al., 1993

Table 2.8. Reverse microemulsion and hydrothermal Sr. No


1 Triton X-100 /cyclohexane / 1-hexanol tetrabutyl titanate dissolved in nitric acid (5 mol/L) as the aqueous phase

Nitrogen-doped TiO2 nanocatalysts with a homogeneous anatase structure were successfully synthesized through a microemulsion-hydrothermal method by using some org. compds. such as triethylamine, urea, thiourea, and hydrazine hydrate.

Cong et al., 2007

2 SDS/cyclohexane/n-hexanol/water The zircon-type tetragonal (t-) LaVO4 nanowires were controlled synthesized by a new approach, a microemulsion-mediated hydrothermal method, in which the aq. cores of sodium dodecyl sulfate (SDS)/cyclohexane/n-hexanol/water microemulsion were used as constrained microreactors for a controlled growth of t-LaVO4 nanocrystals under hydrothermal conditions.

Fan et al., 2007

3 CTAB/n-octane/n-butanol/water The single crystal octahedra of tetragonal CdMoO4 were synthesized on large scale via a microemulsion-mediated hydrothermal route at 120ofor 10 h.

Gong et al., 2006



4 CTAB/ water/cyclohexane/n-pentanol Indium hydroxide, In(OH)3, nano-microstructures with two kinds of morphol., nanorod bundles and caddice spherelike agglomerates, were successfully prepd. by the cetyltrimethylammonium bromide (CTAB)/ water/cyclohexane/n-pentanol microemulsion-mediated hydrothermal process.

Yang et al., 2006

5 CTAB/ water/cyclohexane/n-pentanol CaWO4 nanocrystals with av. diams. of 20-30 nm and nanorods with mean lengths of 600-1000 nm were controllably synthesized through a facile microemulsion-mediated hydrothermal procedure.

Sun et al., 2006

6 CTAB/ water/cyclohexane/n-pentanol Octahedral BaWO4 microparticles were prepd. via a simple microemulsion-mediated hydrothermal procedure.

Liu et al., 2005

7 Triton X-100 /cyclohexane / n-hexanol / Tetrabutyl titanate and (NH4)2SO4 were dissolved in the hydrochloric acid as the aqueous phase.

Nanocryst. TiO2 catalysts with different anatase/rutile ratios and high surface area (113-169 m2/g) have been prepd. at low temp. by the microemulsion-mediated hydrothermal method.

Yan et al., 2005

8 CTAB/ toluene/butanol/water The crystal morphol. of silicalite-1 was adjusted through a microemulsion-based hydrothermal synthesis. The surfactant cetyltrimethylammonium bromide (CTAB) with cosurfactant butanol was used to form water-in-oil microemulsions contg. the silicalite-1 synthesis gel. The crystal morphol. of silicalite-1 was adjusted from coffin-shaped to novel rod-shaped and to irregular-shaped nanoparticles by varying the microemulsion compn.

Lin et al., 2005

9 CTAB/ n-hexane/ n-pentanol /water Well-dispersed cryst. SnO2 nanoparticles were prepd. by a novel and simple water-in-oil (w/o)

Chen et al., 2004



microemulsion-assisted hydrothermal process, using low-cost tin chloride as the starting material.

10 CTAB/n-pentanol/n-hexane/H2O Ni sulfide nanocrystals with novel morphologies of two-dimensional nanosheets and 1-dimensional (1D) nanoneedles or nanotubes were successfully achieved in the CTAB/n-pentanol/n-hexane/H2O microemulsion under hydrothermal condition at 130o.

Chen et al., 2004

11 CTAB/cyclohexane/n-pentanol/water A convenient microemulsion-mediated hydrothermal process was employed for the 1st synthesis of BaF2 whiskers with lengths up to 50 µm and aspects ratios ≤1000, each of which is a single crystal with a growth direction of.

Cao et al., 2003

12 PEG 200/isooctane/propanol/water The main steps of the exptl. procedure consisted of the prepn. of a stable reverse water-in-oil microemulsion, dissoln. of Al and Ba precursors (isopropoxides) in the oil phase followed by addn. to the microemulsion, aging, hydrothermal treatment in an autoclave, and recuperation of oxide nanoparticles by rotoevapn.

Balint et al., 2002

13 CTAB/n-octane/n-butanol/water The single crystal octahedra of tetragonal CdMoO4 were synthesized on large scale via a microemulsion-mediated hydrothermal route at 120 degrees C for 10 h.

Gong et al., 2006

14 CTAB/toluene/1-butanol/water The crystal morphology of silicalite-1 was adjusted through a microemulsion-based hydrothermal synthesis. The surfactant cetyltrimethylammonium bromide (CTAB) with cosurfactant butanol was used to form water-in-oil microemulsions containing the

Lin et al., 2005



silicalite-1 synthesis gel. Table 2.9. Reverse microemulsion precipitation

Sr. No


1 Non-ionic emulsifier based on polyethylene glycol hexadecyl ether/light mineral oil/water

The prepn. of titania nanosized spherical powders through a microemulsion pptn. method was investigated. The process is based on the pptn. of particles from a soln. contg. low-cost precursor, previously emulsified in an immiscible liq. The ppts. were obtained dropping an aq. soln. of TiCl4 into a stable microemulsion contg. a light mineral oil and a surfactant.

DeBenedetti et al., 2006

2 Marlipal/cyclohexane/water This study provides the basis for the scale-up of the synthesis route in water-in-oil (w/o)-microemulsion droplets by a detailed exptl. and theor. anal. of the pptn. of calcium carbonate (CaCO3) and barium sulfate (BaSO4).

Niemann et al., 2006

3 CTAB/1-hexanol/water Nanostructured NiZn ferrites were synthesized using two different techniques: first, a pptn. procedure in the reverse micelles of a CTAB/1-hexanol/H2O microemulsion, and second, pptn. in a bulk aq. soln.


Table 2.10. Reverse microemulsion sol-gel Sr. No


1 TX 100/cyclohexane/n-hexanol/water report on the synthesis of nanocryst. lithium silicate by coupling of sol-gel method in reverse microemulsion.

Khomane et al., 2006

2 CTAB/resorcinol/formaldehyde/water Carbon aerogels were successfully fabricated by a Wu et al.,



microemulsion-templated sol-gel polymn. method. When a suitable molar ratio of surfactant to resorcinol (S/R) and an appropriate resorcinol-formaldehyde concn. were selected, the org. gels thus obtained could be dried with little shrinkage by heating at ambient pressure.

2006

3 AOT/hexane or heptane or isooctane/water Size-tunable silica nanotubes were prepd. using a room-temp. reverse microemulsion-mediated self-assembly sol-gel method. Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was used as surfactant for the reverse microemulsion system.

Jang et al., 2004

4 Nonionic surfactants/Al- & Ba-i-propylate in i-octane/water

Novel nano-engineered catalysts for the high-temp. direct oxidn. of methane to synthesis gas have been synthesized via a microemulsion-mediated sol-gel route. Through the addn. of Pt salts, catalysts with large surface areas, a well-controlled morphol. and very homogeneous distributions of highly active Pt nanoparticles in a hexaaluminate matrix are obtained.

Schicks et al., 2003

5 Photoluminescence characteristics of neodymium oxide nanocrystal/titania/ormosil composite sol-gel thin films

Nd (III) oxide nanocrystal/TiO2/organically-modified silane (ormosil) composite thin films were prepd. using a chem. approach consisting of a combination of inverse microemulsion and sol-gel techniques at low temp.

Que et al., 2001

6 AOT/toluene/water Colloidal silica particles are prepd. via a sol gel technique carried out in an inverse microemulsion of water in a toluene soln. of tetraethoxysilane (TEOS), stabilized by either an anionic surfactant AOT or isopropanol.

Espiard et al., 1995



7 SDS/cyclohexane/1-pentanol/water TX 100/cyclohexane/1-pentanol/water

Microemulsion gels and coatings have been obtained by the sol-gel method using titanium(IV) isopropoxide. Three types of fine water dispersions have been used as the base sol: reverse Triton X-100 micelles in cyclohexane; quaternary water-in-oil microemulsions contg. cyclohexane, 1-pentanol, sodium dodecyl sulfate, and water; and dispersions of water in pentanol in the presence of sodium dodecyl sulfate.

Papoutsi et al., 1994

MICROEMULSION STRUCTURE Table 2.11. Nanotubes as nanoreactors

Sr. No


1 Thermal reaction process using CNTs as nanoreactors for Mg3N2 nanowires

Fabrication of high-quality, large-yield, single-crystalline Mg3N2 nanowires

Hu et al., 2006

2 Single-Wall Carbon Nnaohorns (SWNHs) for Gd2O3 nanoparticles

Hollow nanospaces of SWNHs works as a chemical reaction field i.e. Gd-acetate clusters inside SWNHs were transformd into ultrafine Gd2O3 nanoparticles

Miyawaki et al., 2006

3 Carbon nano tubes Nanofluids were obtained by filling carbon nanotubes with toluene using supercritical carbon dioxide. The method claims as a novel route to developing nanoscale chemical reactions using CNTs as nanoreactors.

Wang et al., 2005

4 Nanowires of FexB prepared by boriding Carbon nanotubes as nanoreactors for building iron Han et al.,



Fe nanowires which were encapsulated in carbon nanotubes (CNTs)

nanowires. 2000

Table 2.12. Core Shell Structures

Sr. No


1 polystyrene-b-poly(L-lactide) (PS-PLLA) with PLLA-rich fractions

Authors have invewtigated degradable core-shell cylinder microstructures in chiral diblock copolymers. Microstructures are hydrolyzed to give polymeric nanotubes with potential applications in drug delivery and as nanoreactors.

Ho et al., 2006

Table 2.13. Drug Delivery systems Sr. No


1 poly(2-methyloxazoline)-block-poly(dimethylsiloxane)-block-(2-methyloxazoline) triblock copolymers.

The research is mainly focused on triblock copolymeric nanoreactors as alternative for liposomes as encapsulating carrier for prodrug activating enzymes. The study demonstrated efficient cleavage of three natural substrates and one prodrug, 2-fluoroadenosine, by the nanoreactors.

Ranquin et al., 2005

2 Polymer nanocontainers for drug delivery This review focuses on applications of triblock copolymers in the drug delivery system such as enzyme encapsulation and gene transfection isdiscussed and illustrated using model systems.

Sauer et al., 2004

Table 2.14. Hydrogels Sr. No


1 Hydrogel networks based on N- Authors have prepared hydrogel and synthesized Mohan et



isopropylacrylamide (NIPAM) and sodium acrylate (SA)

highly stable and uniformly distributed silver nanoparticles which can be effectively employed as antibacterial material

al., 2007

Table 2.15. Mesoporous Silicates Sr. No


1 Mesoporous molecular sieve silicate SBA-15

A highly efficient enrichment and subsequent tryptic digestion of proteins in SBA-15 for matrix-assisted laser desorption/ionization mass spectrometry with time-of-flight/time-of-flight analyzer (MALDI-TOF/TOF) peptide mapping. The procedure allows for rapid protein enrichment and digestion inside SBA-15, and has great potential for protein anal.

Zuo et al., 2006

2 Mesoporous high-aluminum aluminosilicate matrices

The study synthesized Ag/AlxSi1-xO2-0.5x nanocomposite materials by using mesoporous high-aluminum aluminosilicate matrices as nanoreactors.

Kolesnik et al., 2004

3 Cationic Surfactant-Templated Mesoporous Silica

The work reviews the phys. chem. of soln. silicate species and surfactants in the synthesis of mesoporous silicas.

Lin et al., 2002

4 Mesoporous MCM-41 materials in aqueous solution

The channels of the mol. sieve are employed as nanoreactors to study free radical attacks on C60 in aq. media and further reveal the proximity effect of free radical reactions in a nano-restricted environment.

Lee et al., 2002

5 Mesoporous silica SBA-15 A method for prep. of highly conductive polypyrrole/poly(Me methacrylate) nanocomposite. Compression-molded nanocompostie exhibited elec.

Lim et al., 2001



cond. of 1.7 S/cm and very highly oriented and uni-directional structures.

Table 2.16. Micro reaction Cages Sr. No


1 Micro Reaction Cages with Tailored Properties

Hollow polyelectrolyte capsules were prepared in micro- and submicrometer size from melamine resin capsules coated by several layers of poly (Na styrenesulfonate) and poly(allylamine-HCl). The modified walls behaved like ion exchange membranes and showed selectivity toward adsorption and permeation of org. ions. The study shows that modified capsules offer many possibilities for novel applications as containers for controlled pptn., as nanoreactors for catalyzed reactions, or as sensors.

Daehne et al., 2001

Table 2.17. Self Assembly

Sr. No


1 Self-Assembled Nanoreactors A review of work on mol. nanoreactors (capsules and boxes, micelles, vesicles), macromol. nanoreactors (polymersomes, polymer micelles), and biomacromol. nanoreactors (protein cages, viruses)

Vriezema et al., 2005

2 Surfactant templated silica mesophases Authors have prepared surfactant templated silica mesophases belong to the class of self-assembled materials that exhibit long range ordered two-dimensional (2D) hexagonal, three-dimensional (3D) hexagonal, or 3D cubic mesostructures when the

Gibaud et al., 2003



compn. of the initial deposited soln. and its aging time have been optimized. Author have further shown that the evapn. rate of the solvent is one of the key parameters that control the final mesostructure and can, under certain conditions, promote the formation of the cubic mesophase.

3 Self-Assembled Nanostructured Materials The work reviews on the "wet" colloid chem. construction of nanosized or nanostructured materials as inspired by biomineralization (the in vivo formation of inorg. crystals and/or amorphous particles in biol. systems) and on hierarchically organized self-assembly (spontaneous stepwise assembly of functional units). With 46 references Author have explained the "wet" colloid chem. prepn. of nanostructured materials in the labs. by the layer-by-layer self-assembly of (1) polyelectrolyte-semiconductor nanoparticle, (2) polyelectrolyte-clay platelets-semiconductor nanoparticle, and (3) polyelectrolyte-graphite oxide (and reduced graphite oxide) ultrathin films.

Fendler et al., 1996

4 PANI/TiO2 nanocomposites in reverse micelles

Spherical polyaniline (PANI)/TiO2 nanoparticles were synthesized in Triton-x100 (OP)/hexamethylene/water reverse micelle nanoreactor. The study shows that reverse micelles provide a template for control of the unique structure and morphol. of the nanocomposites during oxidative polymn. of aniline with ammonium peroxydisulfate in the presence of TiO2 colloidal dispersion. After 60

Sui et al., 2004



days setting time in the reverse micelle, the spherical nanoparticles self-organized into sea urchin-like shape

Table 2.18. Soft colloidal templates Sr. No


1 Soft colloidal templates (reverse micelles) Colloidal solns. were assumed to be very efficient templates for controlling particle size and shape. It is also shown that large no. of groups used reverse micelles to control the size of spherical nanoparticles. This makes it possible to determine the various parameters involved in such processes, and demonstrates that nanoparticles can be considered to be efficient nanoreactors.

Pileni et al., 2003



2.3. Vesicles

Figure 4. Vesicles Surfactants self-assemble in aqueous solution due to presence of dual functionality viz. the hydrophilic and the hydrophobic parts and can acquire a variety of morphologically different structures. The urge to minimize contact with water for the hydrophobic groups is responsible for the aggregation process. A number of different structures are possible ranging from spherical to rod-like to bi-layers. The packing factor, as explained earlier, broadly decides the actual shape and for P >1/2 vesicle formation is predicted. In bi-layers, the hydrophilic parts of the surfactant face the aqueous pool while the hydrophobic parts rearrange themselves in the form of a comb-in-comb locking their teeth to constitute the interior forming planar structures. The opposite open ends of the planar structure can join together creating a closed environment within. These structures as depicted in Figure 4 are called vesicles. In the simplest form they can be spherical and comprised of a single bilayer (unilamellar vesicles) with dimension in the range of radius (R= 4-20nm), also called as small unilamellar vesicles (SUV) or R= 50nm to 10µm, also called as large unilamellar vesicles (LUV). The radius of curvature decides the small or large vesicles. The unilamellar vesicles form the uniform homogeneous solution also called as L4 phase. As the surfactant concentration increase, multilamellar vesicles- containing concentric shells such as in onions are formed. On further increase in concentration the structure transforms into planar bilayers. The various structures may also co-exist in equilibrium in what appears to be a homogeneous solution on macroscopic scale.

Surfactants that can form bilayers in solution are thus essential for vesicle formation. Small head groups with bulky hydrophobic parts can meet the packing requirements of the surfactant in solution and are therefore the possible



candidates. Typically double chain hydrocarbon surfactants such as normal phospholipids, perfluro surfactants or nonionic single chain compounds with small hydrophilic groups have been used. Ionic surfactants mixed with cosurfactants can also meet the packing requirements. The free energy of bending of a bi-layer system is governed by bending elasticity described by the elastic modulus, viz. the mean bending modulus, and the Gaussian modulus. The sponge phase (also called as L3 phase) and the plane lamellar phases are stable for positive and negative values of the Gaussian modules. For large enough negative values a transition from planer lamellar to vesicles is expected.

Preparation of vesicles, in general, involves dispersion of lamellar bilayers in the solvent using external energy input. The simple methods of shaking, stirring, vortex formation are oftentimes enough to form vesicles. In some other situations more intense form of external force such as sonication may become necessary. Alternative methods such as high pressure extrusion of lamellar phases, where the shear tears apart and fragments the sheet have been used. Evaporation of the surfactant solution in volatile solvents leads to thin films which when brought in contact with water can form vesicles. In addition to the use of low molecular weight surfactants, the use of large molecular weight amphiphiles such as block copolymers, graft copolymers or polymerizable amphiphile can also be used to prepare vesicles. The size and size distribution of vesicles is controlled by the molecular weight of the copolymer. Also, the vesicles can be formed in aqueous as well as the oil continuous reverse systems. A number of systems illustrating the use of such aphiphile has now been reported in the literature. In general, the oppositely charged blockes forms the inner part of the vesicle and the two polymers form inner and the outer part of the vesicle. Table 3 shows a few of these assorted examples.

The deformation of bilayer structure when subjected to shear depends on the extent of shear field and rate of shear. For a small field the shear orients the bilayer first and then transformation to MLV occurs as shear increases. The size change is continuous for small change of shear rate while discontinuous changes occur for sudden and large changes of shear rate. In general, an inverse – square root of the shear rate dependency and the size of the MLV is observed. The transformation process to MLV produces a shear thickening effect as the resistance to flow increases; this is followed by shear thinning behavior for further increase in the shear rate and strain. The structural transitions are system specific depending on molecular composition, concentration etc.

Contrary to the general belief that some external force is needed to form the vesicles, a number of reports on spontaneous formation of vesicles are now available (Ninham et al., 1983; Brady et al., 1984). Of course the studies involve the use of small shear that may initiate the spontaneous formation. Anionic surfactants showing strong dependence of vesicle formation on the counterion



present have been reported (Kamenka et al., 1992; Kaler et al. 1989). The classical catanionic systems containing admixture of cationic and anionic surfactants forms vesicles over a wider range. The simple single chain surfactants have been studied in details to dense phase diagrams showing the occurrence of vesicles in some region and their borders or transition to other forms such as micelles etc. The vesicles formation occur through the formation of precipitate which turns to vesicles on heating. Alternatively for excess of anionic or cationic surfactants a unilamellar vesicle can be formed. The nature of surfactant head groups, their chain lengths, counter ion presents etc. will have a decisive influence on the process. The dominating interactions in cationic surfactants are electrostatic in nature and the solution has high ionic strength and the counterions pair to form salt that effectively shield the interactions.

Spontaneous formation of vesicles has also been observed for some nonionic single chain surfactants. The vesicles can be stabilized by addition of cosurfactant, stiff rod-like amphiphiles, surfactants with polymeric hydrophilic head groups, ionic surfactants etc. In addition to the aqueous phase, vesicle formation in organic solvents and their organometallic complexes has also been known. Table 3 illustrates a few assorted examples. The spontaneous formation of vesicles and their transformation to structurally different forms under the influence of change in surfactant type and concentration, counterions present, pH, temperature, shear, ionic strength etc. are system specific as described by its phase behavior. The dynamics of structural formation and transformation are inadequately understood. The experimental evidence suggests that the micellar to vesicle transformation proceeds through the rod-like and/or disc-like aggregates as intermediate structures. The vesicle may gel by a slow ordering process to form glassy structures. Such transformation process can occur at time scales slower or faster and different probing techniques such as conductivity measurements, light scattering, various spectroscopies etc. can reveal their kinetics. The issues of transitions to different structural forms are also related to their thermodynamic stability. At least in some situations and especially for unilamellar vesicles thermodynamic stability seems to be well established. The control of size, size distribution, structure and properties of vesicle is possible through control over their preparation process.



Table 3. Some experimental studies on vesicles

Sr. No


1 Phase behavior and structural studies of one such mixture, sodium dodccylbenzenesulfonate (SDBS) and cetyl trimethylammonium tosylate (CTAT) in H2O.

The SDBS/CTAT mixture has many features that appear to be common to aqueous mixtures of asymmetric cationic and anionic surfactants. Vesicle formation apparently results from the production of an anion-cation surfactant pair which then acts as a double-tailed zwitterionic surfactant. Although unilamellar vesicles have been created by numerous physical and chemical techniques from multilamellar dispersions, all such vesicle systems revert to the equilibrium, multilamellar phase over time.

Kaler et al., 1992

2 Phase behavior and microstructural evolution in mixtures of surfactants with symmetric tail groups: sodium dodecyl sulfate and dodecyltrimethylammonium bromide.

It is known that micelles of anionic surfactants grow upon addition of cationic surfactant. These rodlike micelles are transformed abruptly into vesicles over a very narrow composition range. For this symmetric system, formation of hydrated crystals of 1: 1 anion/cation surfactant dominates the phase behavior. The study develops a theoretical thermodynamic cell model to predict important properties of the mixed micellar solutions such as monomer and micellar composition and counterion binding as well as the equilibria between the crystalline and micellar phases.

Herrington et al., 1993

3 The phase behavior and aggregate morphology of mixtures of the oppositely

Differences in the lengths of the two hydrophobic chains stabilize vesicles relative to other

Yatcilla et al., 1996



charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) are explored

microstructures (e.g., liquid crystalline and precipitate phases), and vesicles form spontaneously over a wide range of compositions in both CTAB-rich and SOSrich solutions. Bilayer properties of the vesicles depend on the ratio of CTAB to SOS, with CTAB-rich bilayers stiffer than SOS-rich ones.

4 Amphiphilic ion pairs, derived from a series of trimethyl-n-alkylammonium bromides and saturated fatty acids, form vesicles upon sonic dispersal in water.

Ion-pair amphiphiles represent a new and unique class of vesicle-forming surfactants that warrant a detailed examination from both a theoretical and practical standpoint. Unlike conventional double-chain surfactants, however, special attention will have to be paid to the nature and concentration of all ionic species present in solution and to the appropriate ion-exchange equilibria.

Fukuda et al., 1990

5 Versatile self-vesiculating amphiphile system based on amino acid surfactanta

Vesicle formation from catanionic micellar mixtures was confirmed by freeze fracture electron microscopy and quasi-elastic light scattering measurements. Molecular reorganization of the N-methylated amino acid headgroups upon vesiculation (E- end 2-configuration equilibrium) was revealed by using sodium N-lauroylsarcosinate as anionic component.

Amb¨uhl et al., 1993

6 Vesicle Formation from Aqueous Solutions of Didodecyldimethylammonium Bromide and Sodium Dodecyl Sulfate Mixtures

Spontaneous vesicle formation in the aqueous mixture of didodecyldimethylammonium bromide (DDAB) and sodium dodecyl sulfate (SDS) has been investigated with differential interference microscopy, transmission electron microscopy, glucose trapping experiments, 5 potential

Kondo et al., 1995



measurements, and surface tension measurements. The micrographs of the DDAB-SDS mixtures confirm the spontaneous formation of polydispersed vesicles including "giant vesicle" with a diameter less 40 pm.

7 Multilamellar vesicles

This study considers the biological systems as nanoreactors and further describes the formation of cadmium sulfide (CdS) particles in the gaps between the layers of the multilamellar vesicles leading to a new pathway in the prepn. of nanometer-scale particles.

Bota, et al., 2007.

8 Block copolymer A review of recent literature concerning amphiphilic block copolymer vesicles, owing to the increasing interest in self-assembled structures from block copolymer materials.

Kita- Tokarczyk et al., 2005

9 9:1 mixture of DPPC and DPPG

This work presents a method that allows the on-demand release and mixing of zepto- to femtoliter vols. of solns. in the interior of vesicular nanoreactors. The reactors comprise a nested system of lipid vesicles, part of which release their cargo in the interior of the others during a thermotropic phase transition.

Bolinger et al., 2004

10 Phospholipid Vesicles A method to embed phospholipid vesicles into polyelectrolyte multilayers built up by the alternate deposition of polyanions and polycations. In this study before deposition, the vesicles are rigidified by polycation adsorption onto their surface to avoid

Michel, et al., 2004



their fusion once deposited on the multilayer surface.11 Enzymes inside lipid vesicles This study focuses on preparation, reactivity and

applications of enzymes in lipid vesicles and reports no. of methods that can be used for the prepn. of enzyme-contg. lipid vesicles (liposomes) which are lipid dispersions that contain water-sol. enzymes in the trapped aq. space. This has been shown by many investigations carried out with a variety of enzymes.

Walde, et al., 2001

12 Self-assembly of regular hollow icosahedra in salt-free catanionic solutions

Salt-free mixts. of anionic and cationic surfactants, such bilayers can self-assemble into hollow aggregates with a regular icosahedral shape. These aggregates are stabilized by the presence of pores located at the vertices of the icosahedra. The resulting structure have a size of about one micrometer and mass of about 1010 daltons, making them larger than any known icosahedral protein assembly or virus capsid.

Dubois et al., 2001

13 Nanostructured Hollow Polymer Spheres Hollow polymer spheres synthesized from a vesicle-directed polymerization can be dried and redispersed in water using a variety of nonionic ethoxylated alcohol surfactants as stabilizers. The final dispersions consist of both polymer shells and surfactant micelles, which remain together in colloidal suspension for at least several months. Small-angle neutron scattering (SANS) is used to measure the polymer shell thickness (63 Å) and core radius (560 Å) of the surfactant-stabilized hollow polymer spheres in the presence of surfactant

McKelve et al., 2002



micelles. 14 Statistical mechanics of closed fluid

membranes The work discusses the statistical mechanics of systems, such as vesicle phases, that consist of many closed, disconnected fluid membranes. It is shown that the internal undulation free energy of a single such membrane always contains a logarithmically scale-dependent ‘‘finite-size’’ contribution, with a universal coefficient, that arises essentially from the absence of undulation modes of wavelength longer than the size of the surface. The study briefly discusses the conceptually related problem of a polydisperse ensemble of fluctuating one-dimensional aggregates, such as rodlike micelles, for which they obtain the experimentally observed scaling of micelle size with concentration. It is concluded by discussing the application of the results to the interpretation of recent experiments on equilibrium vesicle phases.

Morse et al., 1995

15 Thermodynamics of Unilamellar Vesicles The influence of mixing on the curvature free energy of a thermodynamically open, reversibly formed vesicle bilayer is investigated by deriving expressions for the various contributions to the bilayer bending constant kbi as functions of the structure of aggregated surfactants as well as the solution state (electrolyte concentration, average composition in the bilayers, etc).

Bergstr¨om., 2001

16 Stability of spontaneous vesicles Equilibrium unilamellar vesicles are stabilized by one of two distinct mechanisms depending on the

Jung et al., 2001



value of the bending constant. Helfrich undulations ensure that the interbilayer potential is always repulsive when the bending constant, K, is of order kBT. When K ⟩⟩ kBT, unilamellar vesicles are stabilized by the spontaneous curvature that picks out a particular vesicle radius; other radii are disfavored energetically. Adding electrolyte to the sodium perfluorooctanoate/CTAB vesicles leads to vesicles with two bilayers; the attractive interactions

between the bilayers can overcome the cost of small deviations from the spontaneous curvature to form two-layer vesicles, but larger deviations to form three and more layer vesicles are prohibited.

17 Effect of Surfactant Tail-Length Asymmetry on the Formation of Mixed Surfactant Vesicles

The recently developed molecular-thermodynamic theory was used to study the formation of vesicles in mixtures containing cetyltrimethylammonium bromide (CTAB) and sodium alkyl sulfates of various tail lengths. The theory accounts for the essential free-energy contributions to the free energy of vesiculation, gves, with particular emphasis on their relative importance and interplay in the process of vesicle formation. It is shown that mixed surfactant vesicles can be stabilized energetically in highly asymmetric surfactant mixtures, such as those consisting of CTAB and sodium pentyl sulfate (SPS).

Yuet et al., 1996



The complex-vesicular based structures are one amongst the principal structures formed when lipids, proteins and polymers self-assemble in solution. Such closed bilayers are prototype examples and a model system for understanding cell membranes. The possibility to encapsulate active molecules within it makes them interesting for drug delivery and bilayer medicine, gene therapy and variety of other pharmaceutical and cosmetic applications. The surface of a vesicle is more organized than that of a micelle and can be used as a seat for chemical transformation. Likewise interior of a vesicle can be used as nanoreactor for some reactions. The molecular properties of a vesicle interface with water is sensitive to additives, impurities, hydrodynamics and other such factors and can change its activity for chemical transformation. The vesicular catalysis rates are often order of magnitude higher than in micelles and several orders of magnitude higher in comparison to reaction in pure water. The changed microenvironment at the substrate binding site is expected to contribute to such measures. The counterions, besides charge compensation, can also act as a catalyst for some systems. Metal complexes attached to vesicular surfaces provide examples illustrating the role of substrate binding sites and counterions effect. A number of examples such as Cu(II) vesicles for Diels-Alder reaction (Rispens et al., 2001), Cytochrome 450 (Groves et al., 1995) - an enzyme containing Iron (III) protoporphyrin complex (Mansuy, 1994; Feiters, et al., 2000) and active for oxidation, dehydrogenation, oxidative formylation, dehydration reactions, the manganese (III) tetra hexadecyl phenyl porphyrin chloride complex for regio selective epoxidations of steroids, the rhodium (III) formate system for reducing molecular oxygen, Hemocynin system (Klein et al., 2001) for oxidative degradation of ligand, amphiphilic diphosphene ligands along with their rhodium complexes for hydroformylations etc are known. The other set of reactions use the interior of vesicles as a nanoreactors for conducting enzymatic reactions. The catalysts (enzyme) are embedded within the inner compartment while the substrate moves from the bulk solution crossing the membrane barrier. It is imperative that the enzyme and the membrane wall should be sufficiently stable and not degrade, at least over the period of transformation. Also, external factors such as pH, temperature, ionic strength etc. should not destabilize the system. A number of different techniques for preparing the nanoreactor assembly, ranging from simple mixing of various ingradients, to more sophisticated extrusion and dehydration / rehydration methods are in use to create an optimum configuration for reaction environment and better and faster conversion rates and product selectivity and yield. The method of preparation has a decisive influence on the performance since the catalyzed reaction in the reactor critically depends on the diffusion of the reactants and counterdiffusion of products through the lipid bilayer. The characteristic properties of the substrate, product and the surfactant material forming the bilayers also plays important role.



Synthesis of a number of bimolecules ranging in complexity from simple glycogen to complete protein synthesis has been reported in vesicles. The technique of encapsulating necessary enzymes along with other required components holds the key to successive synthesis of complex biomolecules. A few assorted examples are included in table 3. A number of different methods to improve the stability of vesicles are also developed. They include the use of vesicle membrane as a template to organize monomers and then polymerize them improving the stability to mechanical, chemical and attack from digestive enzymes. Other techniques include use of fluorinated lipids, non-ionic sugar lipids and mechanical and thermal treatments.

2.4. Polyelectrolyte Multilayer Capsule (PEMC)

The technique of layer-by-layer (LbL) self assembling of charged polyelectrolytes (PE) on to planer or colloidal structures, allow to create nanovessels with a direct control on their thickness and composition. The interior of polyelectrolyte multilayer capsules (PEMC) so created can be loaded with desired reactants and used for their transformation to products of desired shape, morphologies etc. The outside shell can also be used to anchor different materials such as dyes, metal alkoxides, nanoparticles etc. Small molecules can be easily incorporated into the polyelectrolye film (~Mw 4000) and transformed. The oppositely charged PEs interweave to create void spaces that act as a reactor. A number of nanostructures of metals, metal oxides/hydroxides, semiconductors etc. are prepared using planer polyelectrolyte films. Likewise coated colloids can be used to fabricate hollow spheres and 2-d and 3-d microporous material. The fabrication of polyelectrolyte multilayers involves the simple principle of electrostatic interactions between oppositely charged PEs as the driving force; thus for instance the positive charge on the substrate is reversed to negative charge upon adsorption of polyanions and subsequently becomes vulnerable to attack from polycations. The layer-by-layer deposition can be continued in a controlled manner to achieve desired thickness, composition, permeability etc. The thin films so produced have potential applications as bioactive films, electrochromic membranes, LED, pattering etc. Likewise the colloids can be subjected to LbL deposition to generate hollow capsules of varied thickness and morphologies. It is possible to replace one charged PE component with charged preformed inorganic nanoparticle layer. Areas such as catalysis, drug delivery, separation materials, chromatography, coating, biosensing etc. can immensely benefit from this approach. Nanoparticles are usually embedded in a solid matrix or polymer thin films for them to be usuable in practice. The simple procedure involves mixing of polymers and nanoparticles; however the structures so created can often be inhomogeneous. The other method requires the preparation of the nanoparticles



with appropriate charges and surface modifications in stabilized media and then uses the LbL approach to create orderely and homogeneous films containing nanoparticles. The LbL technique allows control on the film thickness by adjusting deposition cycles; however the control on size of the particle during and after film formation is difficult. The technique of in-situ synthetic approach on the preformed film can become advantageous and a number of particles of metals, metal oxide/hydroxides semiconductors etc. are prepared using this technique. A few assorted examples are listed in Table 4.

The LbL colloidal templating approach can be employed on the preformed polyelectrolyte layers onto colloids to produce uniform core-shell particles and hollow spheres. One can also use the inorganic molecular precursors/PE multilayeres onto colloids or in filtration of preformed nanoparticles into film-coated colloids to conduct suitable nanoreactor chemistry such as reduction, hydrolysis, sol-gel reactions, polymerizations or thermal oxidation etc. Again a few assorted examples are included in the table 6. The generic technique can also be used to obtain tunable morphologies with desired properties and functionalities for 2-d ordered surface arrays and 3-d macroporous materials with controlled pore sizes and periodic structures. Examples illustrating the formation of open or closed pores with controllable pore wall thickness, pore diameter are available in the literature.



Table 4. Some experimental studies on Polyelectrolyte Multilayers Sr. No


1 Polyelectrolyte Capsule Nanoreactors

The study demonstrates result of the synthesis of two types of nanoparticles, these specially designed PEMCs contg. both silver and goethite nanocrystals can be used as antimicrobial capsules, which can move by an external magnetic field. Such a technology has the potential for use in sterilization at the desirable sites.

Choi et al., 2005 (131)

2 Silver nanocomposite multilayer films

Multilayer thin films, prepared through the sequential electrostatic deposition of a pos. charged third-generation poly(amidoamine) dendrimer (PAMAM) and neg. charged poly(styrenesulfonate) (PSS) and poly(acrylic acid) (PAA), were utilized as nanoreactors for the formation of silver nanoparticles.

Liu et al., 2005 (132)

3 Polyelectrolyte multilayer The recent advances in the synthesis and characterization of a variety of nanostructured materials generated by using polyelectrolyte multilayers assembled onto both planar and colloidal substrates as nanoreactors have been discussed.

Shi et al., 2004 (133)

4 Poly(acrylic acid) & poly (allylamine hydro chloride) multilayer films

Polyelectrolyte multilayer nanoreactors for preparing silver nanoparticle composites.

Wang et al., 2002 (134)

5 Hollow Polyelectrolyte Shells

Hollow polyelectrolyte capsules were fabricated by means of stepwise adsorption of polyelectrolytes followed by dissolution of the templating core. The capsule wall thickness was approximately 20 nm. The diameter of the capsules is given by the size of the templates (3.3 μm). These capsules exclude poly(styrenesulfonate) (PSS) from a molecular weight

Sukhorukov et al., 1999



of 4200 upward but are permeable for small ions and 6-carboxyfluorescein (6-CF). By means of adding PSS in acidic form to the bulk solution, a Donnan equilibrium between the bulk and internal solution encapsulated within the capsules was created.

6 Layered Polymeric Multicomposites

Multilayer films of organic compounds on solid surfaces have been studied for more than 60 years because they allow fabrication of multicomposite molecular assemblies of tailored architecture. However, both the Langmuir-Blodgett technique and chemisorption from solution can be used only with certain classes of molecules. An alternative approach-fabrication of multilayers by consecutive adsorption of polyanions and polycations-is far more general and has been extended to other materials such as proteins or colloids.

Decher, 1997

7 Hollow Polymer Shells by Colloid-Templated Assembly of Polyelectrolytes

The study describes a method for constructing hollow polyelectrolyte shells by colloid-templated consecutive polyelectrolyte adsorption followed by decomposition of the templating core. Micron-size polyelectrolyte shells of poly(sodium styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), with thicknesses ranging from a few to tens of nanometers, have been produced.

Donath et al., 1998

8 Hollow Capsule Processing through Colloidal Templating and Self-Assembly

Hollow polymer, inorganic and inorganic - organic composite capsules of submicrometer to micrometer size have been fabricated by using a variety of colloidal particles as templates for the build-up of nanostructured multilayers and subsequently removing the core by chemical or thermal pathways. The figure shows a TEM image of a cross-section of hollow inorganic capsules. The geometry, size, wall

Caruso., 2000



thickness and composition of the hollow capsules can be readily controlled.

9 Nanoengineering of Particle Surfaces

The creation of core-shell particles is attracting a great deal of interest because of the diverse applicability of these colloidal particles; e.g., as building blocks for photonic crystals, in multi-enzyme biocatalysis, and in drug delivery. This review presents the state-of-the-art in strategies for engineering particle surfaces, such as the layer-by-layer deposition process, which allows fine control over shell thickness and composition.

Caruso., 2001

10 Inorganic and hybrid hollow spheres by colloidal templating

Hollow silica and silica-polymer spheres with diameters between 720 and 1000 nanometers were fabricated by consecutively assembling silica nanoparticles and polymer onto colloids and subsequently removing the templated colloid either by calcination or decomposition upon exposure to solvents. The hollow spheres produced are envisioned to have applications in areas ranging from medicine to pharmaceutics to materials science.

Caruso., 1998

11 Physicochemical properties of extremely small colloidal metal and semiconductor particles

Certain topics in the field of research on very small semiconductor particles have been selected in the present article to show that many effects occur in this neglected size dimension which was not anticipated even a few years ago. An increasing number of laboratories are now working in this field.

Henglein., 1989

12 Quantum crystallites and nonlinear optics

A review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity. There are also introductory discussions on physical size regimes, crystallite

Brus., 1991



synthesis, quantum confinement theory, and linear optical properties. The experiments describe CdSe crystallites, exhibiting strong confinement of electrons and holes, and CuCl crystallites, exhibiting weak confinement of the exciton center of mass. In the CdSe system, neither the Stark effect nor the third order nonlinearity is well understood.

13 Semiconductor clusters, Nanocrystals, and quantum dots

Current research into semiconductor clusters is focused on the properties of quantum dots - fragments of semiconductor consisting of hundreds to many thousands of atoms - with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Alivisatos., 1996

14 Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer

Electroluminescent devices have been developed recently that are based on new materials such as porous silicon and semiconducting polymers. By taking advantage of developments in the preparation and characterization of direct- gap semiconductor nanocrystals, and of electroluminescent polymers, we have now constructed a hybrid organic/inorganic electroluminescent device. Light emission arises from the recombination of holes injected into a layer of semiconducting p-paraphenylene vinylene (PPV) with electrons injected into a multilayer film of cadmium selenide nanocrystals.

Colvin et al., 1994

15 Coated colloids with tailored optical properties

Tailored optical properties have been imparted to polystyrene (PS) colloids of various sizes by the layer-by-layer (LbL) assembly of silica-coated gold (Au@SiO2) or silica-coated silver (Ag@SiO2) nanoparticles with different shell

Salgueirino.,2003



thicknesses and an electrostatically bridging polyelectrolyte, poly(diallyldimethylammonium chloride) (PDADMAC). The spectral position of the surface plasmon band of the composite colloid spheres is tailored through (i) the type of silica-coated nanoparticles used, (ii) the thickness of the silica shell, and (iii) the number of nanoparticle layers deposited.

16 Electroluminescence from CdSe quantum-dot/polymer composites

Spectrally clean and fairly narrow electroluminescence (EL) from CdSe quantum dots in thin films containing homogeneous mixture of a stable hole transport polymer (PVK) and an electron transport molecular species (PBD) were obtained. The EL is tunable in the visible by changing the dot diameter. There was apparent size dependence to the efficiency with smaller dots having higher threshold voltages than larger dots. Selecting a lower work function cathode and separating the hole transport and electron transport layers from the emitter layer should improve the turn-on voltage, efficiency, and stability of dot EL devices.

Dabbousi et al., 1995

17 Gold Nanoparticle-Based Sensing of "Spectroscopically Silent" Heavy Metal Ions

A simple colorimetric technique for the detection of small concentrations of aqueous heavy metal ions, including toxic metals such as lead, cadmium, and mercury, is described. Functionalized gold nanoparticles are aggregated in solution in the presence of divalent metal ions by an ion-templated chelation process; this causes an easily measurable change in the absorption spectrum of the particles.

Kim et al., 2001

18 Sensing strategy for lithium ion based on gold nanoparticles

The detection of Li+ is currently in demand for both biomedical and industrial applications. The study reports the functionalization of 4 nm Au particles with a 1,10-phenanthroline ligand that binds selectively to Li+. The ligand

Obare et al., 2002



binds to Li+ by forming a 2:1 ligand-metal complex, causing Au nanoparticles to aggregate. Au nanoparticle aggregation causes a shift in the extinction spectrum with a concomitant color change, providing a useful optical method of detecting Li+ in aqueous solution.

19 multilayered nanostructural films from macromolecular precursors

Sequential adsorption of a cationic polyelectrolyte and individual sheets of the silicate mineral hectorite has allowed controlled, stepwise formation of multilayered films on silicon wafers. Each component adsorbs rapidly by an ion-exchange mechanism, and x-ray diffractometry indicates structural order even in films with thicknesses greater than 0.2 micrometer. The large lateral extent of the silicate sheets (about 25 to 35 nanometers) allows each layer to cover any packing defects in the underlying layer, thus preserving structural order in the growing film.

Kleinfeld et al., 1994

20 Alternate assembly of ordered multilayers of SiO2 and other nanoparticles and polyions

Alternate layer-by-layer assembly of colloidal SiO2 particles with polycations has been investigated by quartz crystal microbalance (QCM), scanning electron microscopy, and atomic force microscopy (AFM). QCM measurement confirmed the high regularity and reproducibility of the assembling process that depends on particle concentration, particle size, and ionic strength. The individual adsorption step was completed within 15 s. The thickness of adsorbed layers increased with increasing SiO2 concentrations at the three particle sizes used (45, 25, and 78 nm in diameter), unlike the case for other polyion assemblies. It also increased with increasing ionic strength of aqueous SiO2 dispersions.

Lvov et al., 1997

21 Molecular self-assembly of Multilayer ultrathin films composed of titanium dioxide Liu et.al.,1997



TiO2/polymer nanocomposite films

nanosized particles and ionic polystyrene molecules have been fabricated on single-crystal silicon, quartz, and glass substrates by a novel molecular self-assembly process. X-ray photoelectron spectroscopy (XPS) indicates that the formed cationic TiO2 particles adsorb only on negatively charged and not on positively charged surfaces. Contact angle measurements demonstrate that the water contact angle oscillates regularly in accordance with which molecules form the outermost layer of the films.

22 Layer-by-Layer Self-Assembly of Alumosilicate-Polyelectrolyte Composites

The morphology and gas permeation properties of montmorillonite-polyelectrolyte self-assembled multilayer systems have been investigated. Without any special pretreatment, a stable film of montmorillonite-polymer composite was assembled on a hydrophobic poly(ethyleneterephthalate) support by means of layer-by-layer deposition. All individual alumosilicate platelets were found to be oriented in parallel to the substrate, while their surface density strongly depended on the nature of the polyelectrolyte (charge and molecular weight). The organic-inorganic films were found to be very flexible and crack-resistant even under a considerable mechanical stress.

Kotov et al., 1998

23 Multilayered Polyelectrolyte Films

Alternating adsorption of polyethyleneimine-metal ion complexes and polyanions results in the formation of multilayered polyelectrolyte films. Postdeposition reduction of the metal ions by heating or exposure to NaBH4 then yields composite films containing metal nanoparticles.

Dai et al., 2002

24 Dendrimer-Silver Complexes and

Silver complexes of poly(amidoamine) (PAMAM) dendrimers as well as different {silver-PAMAM} dendrimer

Balogh et al., 2001



Nanocomposites as Antimicrobial Agents

nanocomposite solutions have been tested in vitro against Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli bacteria, using the standard agar overlay method. Both PAMAM silver salts and nanocomposites displayed considerable antimicrobial activity without the loss of solubility and activity, even in the presence of sulfate or chloride ions.

25 Multilayer Polymer Films Containing Pd Nanoparticles

A simple route to the fabrication of multilayer films containing Pd nanoparticles is described. Following layer-by-layer assembly of PdCl4

2- and polycation, QPVP-Os (a quaternized poly(4-vinylpyridine) complexed with [Os(bpy)2Cl]2+/+), on 4-aminobenzoic acid-modified glassy carbon electrodes, the three-dimensional Pd nanoparticle multilayer films are directly formed on electrode surfaces via electrochemical reduction of PdCl4

2- sandwiched between polymers.

Liu et al., 2001

26 Pt nanoparticles assembled in multilayer films

PtCl62- anions were assembled on a glassy carbon electrode

with [tetrakis(N-methylpyridyl)porphyrinato]cobalt cations through layer-by-layer method, then electrochemically reduced to yield zero valent Pt nanoparticles.

Shen et al., 2002

27 Multilayer Films Containing Pt Nanoparticles on a Glassy Carbon Electrode

A simple route for the preparation of Pt nanoparticles is described. PtCl6

2- and [tetrakis(N-methylpyridyl)porphyrinato]cobalt (CoTMPyP) were assembled on a 4-aminobenzoic acid modified glassy carbon electrode through the layer-by-layer method. The three-dimensional Pt nanoparticle films are directly formed on an electrode surface by electrochemical reduction of PtCl6

2- sandwiched between CoTMPyP layers.

Shen et al., 2003

28 Multilayer Nanoreactors for Multilayer thin films of sequentially adsorbed polyelectrolytes Joly et al., 2000



Metallic and Semiconducting Particles

were utilized as nanoreactors for both metallic (Ag) and semiconductor nanoparticles (PbS). Polyelectrolyte multilayers with a controlled content of free carboxylic acid binding groups were fabricated with weak polyelectrolytes via suitable pH adjustments of the processing solutions. These groups were then used to bind various inorganic ions that were subsequently converted into nanoparticles.



The versatility and simplicity of the LbL approach to build PE multilayers on chosen substrates (planer or colloidal) with tunability afforded through choice of PE composition, solution pH, temperature etc. and availability and possibility to use different molecular precursors have given rise to preparation of number of different nanomaterials with different morphologies and structures. The methodology is intrinsically slow and suffers on account of step-by-step procedure of fabrication. The limitation to remain confined to use of liquid, mostly aqueous phase and charged polymers poses another limitation.

2.5. Liquid Crystals

Figure 5. Liquid crystals

Lyotropic liquid crystals are obtained when molecules with interlinked hydrophobic and hydrophilic blocks are mixed with water. The hydrophobic parts cluster together to minimize water contact whereas the hydrophilic parts dissolve in water. The covalent linkage does not allow microphase separation and appropriate to packing parameters of the two parts and the water to surfactant ratio, liquid crystal with a well defined phase is obtained (see Figure 5). The variation in the amphiphile to water ratio can change the phase from hexagonal to lamellar. Addition of non-polar solvent can increase the hydrophobic contents and eventually leads to obtain an inverse hexagonal liquid-crystalline phase. The liquid crystals have characteristic dimensions in the nanometer range and the confined space in the hydrophobic and/or hydrophilic portions can be used to conduct specific chemical transformation. The size, shape, connectivity and



dimensionality of these reactors is controlled by the nature of the liquid crystalline phase. The 3-d geometry of the aqueous domain for a hexagonal phase becomes 2-d for lamellar phase and 1-d for the inverse phase. The templating effect of the shape of the nano container can decide the shape of the product and correspondingly 3-d spheres, 2-d sheets or 1-d rods can be synthesized (Dellinger et al., 2004). In a typical experiment the liquid crystals containing the substrate to be reduced is shear mixed with another liquid crystal containing the reducing agent. The two interconnect forming the nanomaterial. The transport or diffusion of species from one to another control the rate of formation and is dictated by the geometry. Thus very small particles are formed for a 1-d geometry as compared to 2-d sheet like geometry or 3-d volumetric space. The particle size also depends on the concentration of precursor (substrate) in the liquid crystal and decreases with decrease in concentration due to enhanced liquid crystal order. The amphiphile to water ratio also controls the lamellar spacing in the lamellar phase. The hydrophilic part of the liquid crystal retains the same dimension while the hydrophilic aqueous domain changes the thickness and hence the size of the nanoreactor. The effect is, however, usually not very pronounced to effect particle size. In other words, particle size shows weak dependency on lamellar spacing. Similarly the particle size may show no dependency on the concentration of reducing agent (since it is usually high and any further increase has no effect) or may sometimes increase as the increase in concentration of reducing agent may reduce liquid-crystalline order.



Table 5. Some experimental studies on Swollen Liquid Crystals Sr. No


1 SDS,water, NaCl, pentanol, and cyclohexane, SLCs containing CTAB .

The study reports the prepn. of direct hexagonal liq. crystals, containing oil-swollen cylinders arranged on a triangular lattice in water. SLCs are made by using SDS and CTAB surfactants and their properties have been studied.

Dos et al., 2005

2 Highly Swollen Liquid Crystals using cationic, anionic, and nonionic surfactants

Synthesis and self-assembly of nanomaterials can be controlled by the properties of soft matter. On one hand, dedicated nanoreactors such as reverse microemulsions or miniemulsions can be designed. On the other hand, direct shape control can be provided by the topol. of liq. crystals that confine the reacting medium within a specific geometry.

Surendran et al., 2005

3 PbS Nanocrystallites Synthesized in the Bicontinuous Cubic Phase of a Lipid

The study reports the synthesis and characterization of monodispersed lead sulfide (PbS) particles with diameters in the range 4-14 nm, using bicontinuous cubic phase as a matrix. It was found, through polarized optical microscopy and small-angle X-ray scattering studies, that the viscous isotropic bicontinuous cubic phase continued to exist in the ternary sodium dioctyl sulfosuccinate (AOT)/water/sodium sulfide (Na2S) system as long as the concentration of aqueous Na2S solution was below 0.5 M. The exposure of this ternary mixture to aqueous 0.40 M lead nitrate (Pb(NO3)2) solution led to the formation of PbS nanocrystallites within the mesophase.

Yang et al., 1996

4 Organic Lyotropic Liquid Crystals

Various nanoscale semiconducting superlattices have been generated by direct templating in a lyotropic organic liquid crystal. These include superlattices of CdS, CdSe, and ZnS, templated in a liquid crystal formed by oligoethylene oxide oleyl ether amphiphiles and

Braun et al., 1999



water. The semiconductor growth process copied the symmetry and characteristic dimensions of the original mesophase by avoiding growth of mineral within regularly spaced hydrophobic regions.

5 Semiconducting superlattices templated by molecular assemblies

Organic-inorganic nanostructured composites provide a rich source of new materials for a host of technological applications. For example, the incorporation of organic molecules in an inorganic lattice can toughen an otherwise brittle material, or be used to tailor its electronic properties, and cooperative interactions between organic and inorganic molecules are being used to generate a range of porous materials for separation and catalytic technologies. The study describes the growth of stable semiconductor–organic superlattices based on cadmium sulphide and cadmium selenide.

Braun et al., 1996

6 Liquid-crystal templates for nanostructured metals

Lyotropic liquidcrystalline phases are versatile media for nanostructure design of metals. The reduction of platinum salts dissolved within the aqueous domains of a hexagonal mesophase leads to Pt whose nanostructure is a cast of the liquid-crystalline phase architecture. Hexachloroplatinic acid and ammonium tetrachloroplatinate were used as the Pt sources. the lyotropic liquid-crystalline phases were prepared from octaethyleneglycol monohexadecyl ether, Pt salt and water.

Attard et al; 1997

7 Mesoporous platinum films from lyotropic liquid crystalline phases

The lyotropic liquid crystalline phases of surfactants exhibit a rich polymorphism of structures that have long-range periodicities and whose characteristic repeat distances range from 2 to 15 nm. The electrochem. reduction of platinum salts confined to the aqueous environments of these phases leads to the deposition of platinum films that have a well-defined long-ranged porous nanostructure and high sp. surface areas. These results suggest that the use of liquid crystalline plating solns. could be a versatile way to create

Attard et al; 1997



mesoporous electrodes for batteries, fuel cells, electrochem. capacitors, and sensors.

8 Nanostructured cobalt films from lyotropic liquid crystalline media

Lyotropic liquid crystalline phases formed at high concns. of nonionic surfactants provide a versatile templating medium that can be used to produce nanostructured materials with regular arrays of pores of nanometer dimension and extended periodicity. The authors used this technique to prepare nanostructured cobalt films on gold substrates by electrochem. deposition of cobalt from cobalt acetate dissolved in the aqueous domains of the hexagonal lyotropic liquid crystalline phase of Brij 56 (C16EO10). Low angle x-ray and transmission electron microscope studies show that the resulting cobalt films have a regular nanostructure consisting of a hexagonal array of cylindrical pores.

Bartlett et al; 2001



2.6. Polymer Micelles

The surfactant molecules when dispersed in water is able to create a large variety of self-assembled forms, primarily because it contain two pairs with different and opposite properties. The hydrophobic and hydrophilic parts can be altered (different head groups, hydrocarbon chain, chain length etc.) giving rise to different morphologies. It is easier to visualize replacing the surfactant with other moieties that have such opposite properties. Thus far instance, polymer containing at least two blocks with different properties can aggregate in solvent to create self-assembled structure (see Figure 6). The idea seems interesting especially since polymers can be prepared using different monomers in different compositions, molecular weight and its distribution, functionalities, shape etc. offering a possibility to construct diblocks of desired properties and hence the self-assembled morphologies when dispersed in solvent. The driving force for structures to form is dictated by the microphase separation of the insoluble blocks in the copolymers.

Micelles, micellar rods, vesicles, lamellae, branched worm-like micelles, oriented helices etc. are some of the more common structures formed using diblock copolymers. (Coil-coil) and (rod-coil) diblock copolymers and multi-block copolymers with and without cross-linkable groups have been used. The ratio of the two blocks allow to control the size, polarity, stability of structures such as vesicles. The stability of vesicles formed using block polymers is better as compared to those formed using amphiphiles due to increased stability, thickness and rigidity of the membranes. This, however, also means lower permeability and less fluidic characteristics of the membranes. A number of different polymerosomes giving special functions such as stimuli-responsive vesicles for drug delivery, or incorporating the channel protein in the membrane to allow passive diffusion of small solutes or incorporating ion-carrying ionophores to fascilitate transport of ions, or electron conducting vesicle membranes or catalytically active polymerosomes have been demonstrated. A few assorted examples are included in Table 6.

Figure 6. Polymer micelles



Table 6. Some experimental studies on Block Copolymers Sr. No


1 Polystyrene-block-poly(4-vinylpyridine) (PS-PVP) diblock copolymers

The study investigated changes in the domain period by gold nanoparticles, employing thin films of sym. diblock copolymers. The study reports dependence of the lamellar period on the vol. fraction of nanoparticles by an intermediate case of homogeneous distribution and local segregation of nanoparticles within the PVP lamellae.

Sohn et al., 2002

2 Styrene (PS)-acrylic acid (PAA) diblock copolymer

The study reports the formation of novel self-assembled ultrathin (ca. 20 nm) films comprising in-plane arrays of nanoscale surface cavities. The use of cavitated, metal-loaded PAA domains as arrays of open nanoreactors for inorg. nanocluster synthesis is reported.

Boontongkong et al., 2002

3 Poly(styrene)-co-poly(2-vinylpyridine) (PS-co-P2VP) copolymer

The synthesis of gold nanoparticles from Star-block copolymers and also by using polyelectrolyte complex (PEC) with water-sol. terthiophene deriv. for the redn. of HAuCl4 to gold nanoparticles.

Advincula et al., 2002

4 Amphiphilic block copolymers

Issues related to mediation between incompatible materials, amphiphilic block copolymers vs. surfactants, micelles and nanoreactors, design of porous ceramics, biomimetic mineralization are discussed.

Goltner, 2001

5 Poly(2-vinylpyridine)-block-poly(ethylene oxide)

Interaction of diblock copolymers with noble metal compds. in aq. media and metal nanoparticle formation were studied. The P2VP-b-PEO micelles contg. metal compds. work as "nanoreactors" for metal nanoparticle formation.

Bronstein et al., 1999

6 Polybutadiene-b-poly(ethylene oxide) (PB-b-PEO)

The use of amphiphilic block copolymers (ABCs) as synthetic templates for prepn. of porous silica with metal nanoparticles is reported. The block copolymer is used in a first step as a

Bronstein et.al., 1999



nanoreactor for prepn. of metal nanoparticles inside their micelle core, and then as templates where the micelles are used as porogens.

7 Poly(tert-Bu acrylate-block-Me methacrylate)

star-like amphiphilic block copolymers with 6 and 12-arms with remarkably low polydispersity were prepd and the polymers show a unique response of the mol. geometry to the polarity of the solvent.

Heise et al., 1999

8 Review of metal nanoclusters in block copolymer films

A brief non-comprehensive survey of some recent work on the situ prodn. of metal nanoclusters in polymer films.

Ciebien et al., 1998



Besides vesicles, the normal spherical micelles, micellar rods or hexagonal phase are some of the more common morphologies when block copolymers are dispersed in solvent. The available compartments in these structures can indeed be used as nanoreactors and numerous examples of synthesis of metal nanoparticles are reported in literature. The block polymers also aids in stabilizing the particles. Homopolymers such as PVP (polyvinyl pyrrolidone) can also stabilize metal colloids. The homopolymers-metal particle hydrides behave as micellar systems and number of different methods to form them exists. A number of metal colloids, metal oxides and sulfides have been synthesized in polymer micelles. The polymer-metal hydride itself can be used for reactants transformations. They provide a unique example of heterogenising homogeneous catalyst. The metal colloids form interior core and reactants diffuse from outside through the membrane shell to react inside. The polymer shell is soluble in solvent affording catalyst recovery. Reactions such as catalytic hydrogenation of olefins, metal colloids catalyst for oxidation, C-C coupling reactions etc. have shown immense benefits. The polymer micelles can also be used to encapsulate enzymes for biotransformation that have been shown to remain active.

Similar to polymer micelles, other structures such as covalent systems (dendrimers, star polymers and hyperbranched polymers) can stabilize nanoparticles. The compartment in these systems can be used as nanoreactors. Unlike micelles, they are not self-assembled structures and hence dynamic in nature. A dendrimer is a single molecule with regular branches emanating from the central core in radial directions that adapt to form globular shape. The hyperbranched and star polymers are suitably synthesized structures. A number of examples of their use in catalysis are reported in Table 6.

3. DISCUSSIONS

The type and form of the organized assembly and hence the nanoreactor decides the efficacy of the processes occurring in it. The interaction and the correspondence between the nature of the nanoreactor and the reactants needs to be clearly understood. Description of the various nanoreactors in the earlier section gives out some important hints for the rational choice and engineering analysis and design of such systems. Thus for instance, the choice of type of nanoreactor can be summarized as follow:

• Direct micelles or o/w microemulsion: effective for binding of hydrophobic molecules.

• Reverse micelles or w/o microemulsion: effective for binding of hydrophilic molecules or ions.

• Bile salts: effective for binding aromatic compounds



• Ionenes (cationic + anionic polyelectrolytes): effective for bind both hydrophilic and phobic compounds. Foaming phenomenon absent on mixing, shaking, etc.

Also, on solubilization of reactant, a number of changes occur in the various physico-chemical, spectrochemical, electrochemical and adsorption properties. Typically they include:

• Physical properties: Aggregation, rigidity, adsorption, solubility, etc. • Chemical properties: hydrophobic, hydration, molecular complexation,

conformation protolytic, redox, etc. • Electro-properties: electronic spectrs, electrochemical parameters,

electronic excitation energy transfer, charge transfer. The effective properties such as dielectric constant of the medium can vary over an order of magnitude (80-5) as we move from aqueous to micellar phase over a distance of 1 to 2 nm. Such sharp gradients can be seen in other properties such as microviscosity, polarity and acidity. Use of different types of surfactants can further change these properties. For instance the microviscosity changes by a factor of three for surfactant changing from DDS to TX-100 (Singh et al. 1982). Also change from micelles to vesicular form using dimylristoylphosphatidyl choline changes the aqueous phase viscosity (0.89 cp) to more than 100 cp (Neal et al., 1995). Increase in viscosity lowers the associated vibration energy losses. The fluorescence intensities also likewise undergo order of magnitude variations (Hinze et al., 1984) as compared to aqueous system. The distribution of species upon solubilization into different microphases as also the variations in solubility, rate and dynamics, equilibrium rates etc. change the extent of reaction. The characteristics of inter and intra-molecular excitation-energy transfer, electron transfer, charge distribution on a molecule bring about changes in spectroscopic, electrochemical, electrophoretic conditions affecting the sensitivity and selectivity (Shtykov, 2002; Pramauro et al. 1996). Thus, for instance, we can use

• Charge on the reactant to control its binding efficiency or formation of inclusion-complex.

• Functional groups on surfactant molecules to control catalytic or regioselective ability of self-assembled structure.

• The presence of aromatic rings, unsaturated bonds or presence of chiral atoms in surfactants to control binding.

• A control on aggregation number, microviscosity, charge density, surface potential, hydration etc. can aid in improving the sensitivity and selectivity of reaction and separation processes.

Addition of electrolytes, solvents, co-surfactants helps in establishing and manipulating the control.



The properties of nanoreactors are unique to their nanosize dimensions. Controlling the size of the nanocrystals / nanoparticles in these reactors, which are self assembled structures, is generally a relatively easier task, despite a few discrepancies which may always exist due to peculiar nature of the specific system. Controlling the shape of nanocrystals is however a more intricate and challenging job. The changes in the shape occur due to variations and differences in the growth rates of various crystallographic faces that are influenced in a complex way by a number of factors (Pileni, 2003). The presence of impurities, additives, electrolytes, pH of the medium, nature and type of ions etc. can modify the growth of some crystal faces. The shape is therefore not uniquely decided by the shape of the template. Notwithstanding these complications, once optimized the regularity in the functioning of the nanoreactor can be ascertained. Scale-up of nanoreactors for large scale manufacturing therefore means increasing their number multifold ensuring same properties. The consistency in the quality of material produced thus remains unaffected and no additional parameter optimization is necessary when the scale of production moves from grams to kilograms and more. The small size of the reactor allows for a faster reaction rate and transport rate across the boundaries, leading to savings in time and cost. The yields are quantitative with minimal waste.

In order to understand the supramolecular self assemblies with cavities that can be used as nanoreactors, it is necessary to put them in proper perspective. As we know, even simple atomic/molecular systems self-assemble on length scale measuring in Angstroms. The molecular assemblies can be simulated using ab-initio quantum mechanical and molecular dynamics methods. The large number of structures that are possible can be experimentally probed using instrumental tools such as optical scattering, nmr spectroscopy, neutron scattering etc. The theoretical studies require to properly define the potential interactions and the force field. The simple system involves long range interactions, Vander der Walls dispersion forces and repulsive excluded volume (Leonard-Jone potentials). Such isotropic and centro symmetric potentials can generate a few simple structures. The presence of directional interactions (dipole, hydrogen or covalent-bonding, ligand binding etc.) anisotropic shapes of molecules can generate variety of diverse structures (Glotzer, et al., 2004).

The potential interactions and their types vary considerably as we move from molecules to colloids. They include

• Long range attractive Van der wall’s interaction for particle pairs. • Excluded volume interactions due to adsorbed steric layers. • Repulsive forces between two colloidal particles due to charge distribution

on the surface (especially when electrolyte is present). • Depletion potential (small non adsorbing molecules create attractive forces

between colloids due to entropic, free volume effect). This is especially



important since its range and strength can be independently varied to create assembly.

• Short-range adhesive forces due to changes in steric layer and induced dipole forces.

Like in atomistic/molecular systems only a few limited structures of assembly can be generated using above potential interactions. The presence of noncentro symmetric and anisotropic shape and strength of interactions are needed for a wide choice of assembly structures. The type, strength and range of pair potential interactions indicate thermodynamic limitations on formation of some assembly processes. The nano and colloidal assemblies also suffer kinetic limitations. The crystal grows more slowly with tendencies for defects such as vacancies, wrong stacking, grain boundaries etc. The kinetic pre-exponential factor )( 0K in the nucleation rate per unit volume )]exp([ 0 KTGK Δ−=Ι can be obtained in dimensionless reduced units as )(3/5

0 ϕϕ DK Α= where A is a constant, φ the reduced amorphous liquid volume fraction and )(ϕD the reduced short time self diffusivity generalization of Stokes-Einstein diffusivity. The later can be obtained from Brownian dynamics as mean square displacement of colloidal sphere [ akTtTDt Ot

πμσξ =>Δ<∞→

)(lim 2 ] where a is the sphere

radius. The characteristic time in the nucleation rate )( 2 Da scales as a3 indicating strong kinetic retardation with the particle radius. For the ranges, type and strength of the interaction potentials considered both molecular and colloidal systems can exhibit glass transition as well as gelation phenomenon. The slowing down of dynamics and crystallization rates near glass transition due to caging effect of repulsive excluded volume of surrounding particles can become severe to a level where thermodynamically feasible assemblies can also be blocked. The short range attractive interaction can also slow down the dynamics and hold the system in a nonordered metastable state. Nonequilibrium aggregation and gelation may also occur hampering the assembly process. Simulation methods for self assembly of nanoparticles and colloidal systems use Monte Carlo (MC) simulations (Ethayaraja, et al., 2007) to generate equilibrium phase diagrams by assuming hard sphere, soft sphere, DVLO and other such pair potentials. The MC method operates probabilistically to generate particle configurations with a view to obtain ordered structures in the form of a number of thermodynamic ensembles. The molecular dynamics (MD) and Brownian dynamics (BD) methods (Dudowicz et al., 2004) also use similar pair potentials to deterministically generate the particle configurations. They solve the Newton`s second law equation for force balance where the force on each particle is obtained from the gradient of potential. MD simulations generate constant



energy ensemble particle configurations/trajectories, usually under isothermal and isobaric conditions. The solvent dynamics has to be modeled explicitely that make simulations prohibitive. Methods such as BD include the effect implicitely where the potential of mean force as mediated by the presence of other particles and solvent is used. The equation of motion describing the force balance comprises of three terms viz (i) the conservative force Fc acting on each particle and includes the van der walls, excluded volume, coulumbic interactions, screening effects etc. The effects of particle specifics such as composition, surface modification interactions etc. enter through this term (ii) the drag force, FD, conventionally expressed as FD = 6лaηυ where a is particle radius, η the solvent viscosity and υ the velocity of particle. The hydrodynamic interactions are considered to be absent and (iii) the resultant force FR generated due to Brownian bombardment of solvent molecule as described by the fluctuation-dissipation. Theorem ))'()(()'(),( ttaTktFtF BRR −>=< ηδσπσ with zero mean RF . The dependence of DF and RF on particle size clearly indicates its importance for the assembly process. The characteristic time for a sphere to travel its own radius scales as cube of the radius. For smaller particles, this implies that the dispersion delocalization time is realy short. This has implications for nucleation kinetics, gel and glassy and other phenomenon that mediate the assembly process. In addition to MC, MD and BD simulations, other methods such as discrete particle dynamics (DPD) (Frenkel et al., 2001) use soft particles to represent fluid elements and hard particles to capture packing effects of colloids due to excluded volume. Notwithstanding the method used for simulations, it is important to correctly describe the interactions in these complex systems, which in a broad way, can be assigned as attractive (-philic) or repulsive (-phobic) character. The interaction potential to be used for surfactants, block copolymer, liquid crystals and other forms must be chosen appropriately and may require additional support from ab-initio quantum mechanical (surface morphology) or density functional theories (force). The self assembly of particles into 1-d, 2-d and 3-d arrays is thus a sensitive issue requiring detail understanding of the effects of particle shape, size on potential interactions, Brownian motion, type of solvent, impurities, additives etc. Simulation methods help us build this understanding of the fundamentals which is very useful for new technological applications. Synthesis of nanoparticles and structures in different shapes and form has by now been a standard procedure; however arranging these building blocks into desirable organized structures such as 1-d, 2-d and 3-d assemblies remain as a challenge. A number of question such as what kind of structures should we assemble and for what purpose and how-have been raised in recent years and partial solutions and direction for further research and work are spelled out



(Glotzer et al., 2004). Similarly development of practical methodologies to commercial and technologically important self assembly processes should be addressed. Self assembly as a manufacturing method and its large scale practical implementation also requires the attention. The issue discussed in more details by Tirrell (2005) requires careful considerations to precision synthesis of precursors and building internal guidance into the process of evolution whereby the desired self assembly results. The biological systems posses such in-built mechanisms whereas their artificial analogue have to be provided with sufficient information content that relates to molecular recognition, directionality, tunability of interaction strengths, addressability and programmability of self assembling characters. Nucleic acids have the characteristic ability to store and transcribe genetic information. Incorporating them or their equivalent synthetic analogues into the system can enhance the possibility of specific assembly (base pairing) and eventually the self assembly processing. Tirrell (2005) cite a number of recent studies on self assemblies using nucleic acid bas pairings, templating the synthesis of other materials and conjugating and coupling to peptides, lipids etc.

The variety of synthesis methods described in the earlier part allow us to create structures that can be used as molecular reactors (in various forms such as rings, spheres, tubes, open vessels etc.), mixes, splitters, transport lines (channels, tubes etc.). Many of these and such functionalities are present in biological systems such as cells. Additionally components producing using and dissipating the energy are also present. The cell system is indeed very sophisticated with presence of control elements such as signal carriers and actuators. It is noteworthy to realize that a biological system unit with all its sophistication and complexities is still chemical at its molecular level. The energy devices as well as control elements are thus essentially molecular in nature and can be synthesized as molecular electrical wires, switches, gears, valves, motors, pumps, shuttles etc. We have already seen that by a judicious choice of the hydrophobic interactions, it is possible to direct the self assembly as micelle, lipid bilayers, vesicles etc. Other forms of interactions such as hydrogen bonding, π stacking, metal coordination and electrostatic interactions can also direct the self assembly. Catenanes and Rotaxcenes are good examples of self-assembling of molecular complexes under the influence of electrostatic interactions and orbital overlap forces. The electrostatic forces, under certain conditions can also localize components to create interlocked molecules that can be used as valves, circuits, elevators etc. The idea of integrating all such components into a comprehensive system mimicking the behavior of a cell is very interesting and vigorously pursued. The network of vesicles-nanotubes developed by Shimizu et al (2005) is one such step in this direction. The perspective article by Stephanopolous et al. (2005) vividly describes the process system engineering issues of nanosystems and their integration and should be referred to for details.



4. CONCLUDING REMARKS Organized assemblies, such as the ones described herein, formed through a self-assembling process represent nanoreactors that can be used for preparing nanostructured materials of desired functionalities and attributes. The types and characteristics of these nanoreactors and control of their efficiencies for conducting chemical transformations have been extensively investigated. A number of illustrative examples of formation of nanoparticles and their assemblies in the form of films, spheres, hollow structures, rods, tubes and other two and three-dimensional structures, vividly demonstrate the engineering ability to create the desirable structures and attributes. At the functional level, we understand the fundamental difference between a homogeneous system and a micro-heterogeneous organized media and that local effects can bring about a dramatic change. Thus, our ability to dissolve dissimilar substances, bring them together and concentrate them in microphases, create multi-center multifunctional interactions between components, play with the nature, geometric compatibility, orientations and extent of components, and alterations in the microphase properties such as viscosity, polarity, etc. with large gradients over nanometer distance can all be utilized in a judicious way to create the desired nano architecture. The maneuverability and control at operational level allow us to develop open cellular structure with possibility to tune the material structure at different length scales. Hierarchical porous interconnected structures affords the possibility to introduce transport channels for moving the molecules in and out. High internal surface area with a possibility of local deposition, doping or insertion of active component prove useful for multi step reactions where yield to desired product can be maximized. Computer simulations can provide a very useful lead in elucidating the efforts of various parameters on performance. Self-organized systems are the crude man-made mimics of the natural cell systems. The simple surfactant used for construction of the enclosed confined environments already show a number of features such as compartmentalization, self-assembling etc. of the cell system. Number of studies reporting effects of variation in surfactant type, nature and other attributes improve upon the ability of such systems to some extent and yet are no where near the sophistication seen in natural cell systems. The self-assembly of lipids and detergents into bilayer membrane vesicles with a range of bilayer thicknesses (3-5nm) and vesicular length scales of (100-10,000nm) diameter provided the mimics for cell membrane and brought us a little more close to natural system. The development and use of macro molecules with a ability to impart intelligence for self-assembling in desired architectures kept the march forward with numerous interesting results. Thus for instance macromolecular ampiphiles for vesicular drug delivery systems



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