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Properties of Materials Confined in Nano-pores

SURESH CHANDRA, RAJENDRA K. SINGH*, MANISH P. SINGH

Department of Physics, Banaras Hindu University, Varanasi – 221 005, India

E-mail : rksingh_17@rediffmail.com

Abstract

Properties of materials confined in pores or channels having at least one of its dimensions comparable to the molecular size of the confined materials drastically change. This has wide ranging implications in biology, geology, physics, chemistry and various branches of engineering science. This paper gives an overview of the properties and applications of materials in confined geometry. Illustrative examples are given for confined molecular liquids, gases, water, liquid crystals, polymers and ionic liquids. The last one is discussed in relativity greater detail. Confined ionic liquids have resulted in a new group of materials, called as “ionogels”, which are useful in developing electrochemical devices, sensors etc. Keywords : nano pores, mesopores, confinement effect, ionic liquids

1. Introduction

The general perception about the field of nano-science and technology is that it deals with properties of solid state materials having the particle size in the approximate range of 1-100 nm. The two extremums could vary. It has been found that as the particle size is reduced, the surface to volume ratio increases and surface effects come into play. The more interesting aspect of such nano-particles are that quantum effects come into play changing electrical, magnetic, thermal and optical properties. This has led to the development of exotic nano-devices. Most papers in this

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monograph refers to this aspect of nano-sciences. This paper refer to a less explored (and yet having wide ramifications in various disciplines of science & technology) question as to what happens when materials, particularly fluids, are entrapped or confined in the constrained geometry of nanopores or nano-channels having at least one of its dimension comparable to the size of molecules of the confined material. Such a situation is widely prevalent in nature. For example, in living biological system, flow in ion channels, membrane pores, different intracellular environment etc. may be quite different when the sizes are ~ nm. The reactivity in nano-channels or broad channels may be different due to surface interaction effect changing many biological functions including mutation. In geology, many structures like zeolite, clays etc. have mesoporous structures (pore size < 50 nm). Materials entrapped in these pores may show different behaviour. Similarly, ‘micro-fluidics’ in many mechanical and civil engineering structures have important implications. A systemic study of properties of materials confined in nano-pores has only been possible recently because of interesting developments in the area of ‘controlled’ synthesis of such structures. The last decade has seen tremendous progress in this area of research. In the following paragraphs, we first discuss the basics of confining matrix of nano-size followed by a description of properties of typical confined molecular liquids, gases, liquid crystals and polymers. In the end, a new group of materials is introduced obtained by confining ionic liquids in meso-porous matrices. Such materials are called “Ionogels”. Typical applications are also given.

2. Confinement in Nano-pores: Some basics

2.1. Introduction to Confining Nano-porous Matrices

The interaction between the surface wall of the confining matrix and the confined material is predominantly significant when the pore size of the matrix is extremely small (approximately of the

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order of the size of confined molecules). According to the size of pores, the confining matrices can be classified as:

(a) Microporous (pore diameter < 2 nm) (b) Mesoporous (pore diameter between 2-50 nm) (c) Mcaroporous (pore diameter > 50 nm) The material may be confined in 3-D, 2-D and 1-D geometry.

These are schematically shown in Fig. 1. Note that in Fig. 1, the “dimensions” in which the confinement occurs is varying but the effective system always extends in three dimensions.

Fig. 1. Schematic representation of 1-D, 2-D and 3-D confinement

In case of 3-D confinement, the confining pores/ voids are embedded deep into the matrix so that the confined matrial feels the confinement effect from all directions. In 2-D confinement (Fig. 1b), the confinement is in channels with one end open. The channels or pore walls constrain the confining materials from two sides. In the 1-D confinement (Fig. 1c), the confined material form layers stacked between two confining surfaces. The confinement effect is felt only in one of the X- , Y- or Z- directions.

Mesoporous structures (man-made and natural) are more common than what is commonly believed. Some illustrative examples of such structures with nano-pores are shown in Fig. 2. Many geological minerals and clays have built-in nano-pores. The

(a) 3-D

(b) 2-D

(c) 1-D

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famous example is zeolites shown in Fig. 2a. It has nano-channels in which many molecules can be entrapped and such confined molecules1 have been found to have interesting different properties as compared to un-confined molecules in bulk. The presence of nano pores enhance porosity and pore surface area. The latter has been found to provide zeolites its high catalytic activity. Many oxides like SiO2, SnO2, TiO2 etc. particularly synthesised by sol-gel route, are mesoporous. Some commercial materials having ordered mesoporous structures are also available like SBA-15 (Santa Barbara Amorphous-15) MCM-41, (Mobile corporation Material-41) etc. shown in Fig. 2b and Fig. 2c respectively. Carbon nano-tube (CNT) is a recent addition to the list of mesoporous materials. Interest in this material is particularly high because of its conducting nature. Its surface shows significant coulombic interaction with the confined molecules. Properties of materials confined in CNT have been found to be different from those confined in insulating oxide matrices. As stated earlier, mesoporous structures are in abundance in nature. Some of these are also shown in Fig. 2. For example, tree wooden logs have porous structure (Fig. 2d). Some of these pores can carry ‘food’ and water to different portions of the tree. Reef-stone is another well known example of porus material objects. Porous bone, nano-channels, nano-tubules etc. are also examples from biological systems.

(a) (b) (c)

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(d)

(d) Fig. 2. (a) Zeolite beta, (b) SBA-15 (c) MCM-41(d) cross-section of porous wooden log.

2.2. What is the Basic Science behind Confinement ?

A simplistic approach can clearly demonstrate why the properties of materials are likely to behave differently in confinement. Consider a fluid confined in pores/ containers of different sizes. We will have to consider, at least, three relevant factors: confinement size, confinement shape and the extent to which the confined fluid is affected by its interaction with the wall of the confining surface. Molecules belonging to two distinct zones in the confined fluid can be identified as: (i) Surface Layer (molecules near the pore-wall surface) (ii) Inner layer (molecules far away from the pore-wall surface)

The properties of molecules in the surface layer would be affected by substrate wall-fluid interaction. This interaction will slow down the dynamics and so the viscosity in the surface layer is expected to be high. On the contrary, the properties of molecules inside the inner layers would be only controlled by fluid-fluid interaction and there will be insignificant effect of surface-fluid interaction. As a consequence, the viscosity in this layer is expected to be lower. Such a description is reasonably applicable to systems ‘macroscopically confined’ in large diameter capillaries as shown in Fig. 3. However, as the capillary diameter decreases,

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Surface layer

Inner layer

Confining pore

Decreasing diameter from mili-meter to nano-meter

Fig. 3. Schematic representation showing that only surface layer remains as the size of pore approaches to nm scale.

the relative coverage of surface layer zone vis-a-vis inner layer zone decreases. When the capillary pore diameter is ~ nm, the surface layer is quite extensive and the inner layer zone may be negligibly small. Obviously, the properties of fluid will depend on the capillary pore-size. Overall, the situation for molecules in confined geometry is both theoretically and experimentally very complicated. Some experiments have been carried out recently on ionic liquid in contact with charged surfaces and interesting layered structures near the surface have been observed. Mezger et al.2 used X-ray reflectivity technique to determine electron density at different distances from the substrate surface (see Fig. 4a). The measured total electron density is shown in Fig. 4b where a layered structure like variation of electron density is evident.

α

Ionic liquids

Sapphire substrate

(a)

To vacuum pump

Detector

Synchrotron X-ray (λ≈ 0.171 Å)

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(b) Fig. 4.(a) Schematic representation of X ray reflectivity measurement apparatus. See text for description (b) Electron density profile near the substrate surface as derived from X-ray reflectivity studies.

As we go away from substrate, layering slowly disappears as surface-ionic liquid interaction decreases. These results are for ionic liquid [1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate]. The layering has been directly observed by Atkin et al.3 on the same ionic liquid in contact with gold (111) surface using STM-technique. Atkin’s group have done another interesting experiment on many IL/ Substrate interfaces using ATM tips4-6. They measured force needed to pierce the layers near the surface. These results are summarised in Fig. 5. No layering beyond 4 nm from the substrate surface could be seen and clear layering was limited to 4 layers. Such layering has wide range implication in controlling the physical properties of various fluids confined in nano-pores. Typical examples are discussed below.

Ele

ctro

n de

nsity

(x

1010

cm-2

)

Distance from pore wall (Å)

251

Fig. 5. Force applied on AFM tip to pierce various layers of confined ionic liquids formed at different distances from the solid wall surface.

3. Properties of Materials Confined in Nano-pores: Some Examples

The surface interaction with the molecules confined in nano-pores leads to layering or stratification of molecules as discussed earlier. This, in turn, may lead to changes in various physico-chemical properties like: visco-elastic, dielectric, optical, thermal phase transition, electrochemical, catalytic activity etc. In the following sections, some illustrative examples are given.

3.1. Inert and Molecular Gasses

A large number of studies have been reported on inert gases like Helium, Neon, Argon, Krypton, etc. and molecular gases like O2, N2, and H2

7,8. Most studied gases are He, Ne, Ar, O2 and N27,9-

252 12. The freezing and melting transitions in these systems are less complicated because of the spherical symmetry of the constituting atom and the fcc (face centred cubic) structure of the resulting solid. Neon and Helium confined in nano-pores showed pore size dependent depression of melting / freezing temperature on confinement. Knorr’s group7,13 reported similar depression of melting / freezing temperature in Ar. The pore condensate was shown to consist of two to three adsorbed monolayers on the pore wall and the capillary condensate was in the pore centre. Morishige14,15 showed that melting / freezing temperature also depresses in Kr on confinement. Changes in melting / freezing temperatures on confinement have also been reported for molecular gases N2, O2 and H2 7-12,16. Interestingly, it has been found that for O2 and H2

17,18 no freezing occurs when the pore diameters were of the order of 2-3 nm. Contescu et al.19 using quasi-elastic neutron scattering technique have recently reported changes in the dynamics of molecular H2 confined in the pores of different sizes.

3.2. Molecular Liquids

A large number of molecular liquids confined in different nano porous matrices like sol-gel derived silica gelsil/spherosil/ vycor glass/ controlled pore glass (CPG), MCM-41, SBA-15 etc. have been carried out. Depression in Tm and Tg on confinement is a common feature observed in all the studies with only a few exception. Some examples are given in Table 1.

An interesting result observed for many molecular liquids, apart from change in Tm & Tg, is the absence of crystalline phase if the pore diameters are < 5nm. Another notable feature in the studies on some molecular liquids was that the nature of crystalline phase of unconfined and confined systems was different. For example, bulk crystalline phase of cyclohexane is monoclinic but it was found to be cubic for confined system20.

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The pore-size dependence of depression of melting point Tm, is popularly discussed by Gibbs – Thomson equation,

slm m m m

f s

4σ T T - T (d) = T d.ΔH .ρ

where σsl is the surface energy (surface tension) of the solid–liquid interface, Tm is the bulk melting point, Tm (d) is the melting point of crystals confined in pores of diameter d, fΔH is the bulk enthalpy of fusion (per gram of material) and ρs is the density of the solid. This equation predicts that Δ Tm 1/d which has been found to be roughly obeyed (though not always). More complete expressions for Δ Tm are discussed in one of our recent papers21.

Table 1. Example of some organic liquids studied in different confining matrices.

Organic liquid

Confining system

Pore dia. (d)

Effect of confinement

Reference

CCl4 CPG and Vycor glass

4–50 nm

depression in melting point. Δ Tm 1/d

[22]

Nitrobenzene CPG and Vycor glass

4–50 nm

depression in melting point. Δ Tm 1/d

[23]

Benzene MCM-41 and SBA-15,CPG glasses

0.75–1.2 nm

depression in melting point

[24, 25]

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isobutyric acid

MCM-41 SBA-15 CPG glasses

3.3–10 nm

depression in melting point

[26]

Glycerol MCM-41 and SBA-15

3–30 nm

change in glass transition temperature

[27, 28]

3.3. Water

The behaviour of water confined in different constraint geometries, with length scales comparable to that of its diameter, is crucial for understanding many biological, geological and technological processes. This is why a large number of studies have been carried out on “confined” water. Historically, Foote & Saxton29 carried out the first dilatometry experiments on melting and freezing transition of water adsorbed on hydrogels like alumina, ferric oxide and silica which showed that there is no clear freezing at Tm of water. Patrick, and Kemper30 showed depression of freezing point on confinement. Since, then many experiments have been carried out using a wide Variety of techniques like DSC, NMR, Neutron diffraction, X-ray diffraction, Raman and quasi-elastic scattering etc.31-36 The important conclusions are:

(i) Freezing point depresses on confinement. (ii) Ice has hexagonal structure in large pores and cubic phase

in small pores (~ 4nm). (iii) Confinement induces “stratification” in the freezing water

with the formation of distinct non-freezing water layers on and near the pore surface.

(iv) The density of confined water changes on confinement and depends upon the pore diameter of the confining pore36.

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3.4. Liquid Crystals

Liquid crystals (LCs) are interesting materials which, in a particular temperature range, do not have the positional order typical of crystalline solids but have orientational order. So, liquid crystalline phase can be considered intermediate between liquids and solids. On heating, some solids pass through liquid crystalline phase before going to Isotropic (I) melt phase. Absence of orientational order let them show flow properties like liquid but due to the presence of orientational order, the scattering properties are more akin to solids. Depending upon the type of orientational order, there are three types of crystalline phases viz. Nematic (N), Smectic (Sm) and Cholestric. Many nematic and smectic liquid crystals confined in nano-porous silica, zeolites, MCM etc. matrices have been studied. Some important changes in the properties of LCs induced by confinement in nano-pores are summarized below37-40 :

(i) N-I or S-N or I-Sm phase transition temperatures decrease on confinement and are pore size dependent.

(ii) Orientational order parameters also change on confinement. For example, the order parameter of liquid crystal (Merck phase 4) increases with increasing pore diameter.

(iii) The nature of phase transition also changes on confinement in a few cases. For example: octacyanobiphenyl (8CB) changes its phase from nematic to smectic A (SmA). Kralji et al.40 report that first order I-Sm phase transition in dodecylcyanobiphenyl (12CB) changes to second order upon confinement. For pentylcyanobiphenyl (5CB), the usual S-N-I phase transition behaviour gets modified on confinement. I–N phase transition is observed but S-N transition is absent in confined 5CB.

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3.5. Polymers

Polymers are long chain molecules consisting of a large number of monomers. Since the polymer chain is often very large, the confinement effect can be felt even in relatively large pores as compared to effective confining pore for molecular liquids etc.41-48. An unconfined polymer may look like Fig. 6a. When it is confined between two layers separated by ‘d’ which is approximately equal to the steric diameter (2R) of a single polymeric chain, the structure gets compacted and ordered (Fig. 6b). However, when d is very small, as shown in Fig. 6c, then single chain may get opened up. Schematically single polymer chains in micro-porous channels of MCM are shown in Fig. 6 d. Such confinement effect will change Tm, Tg, viscoelastic and mechanical properties of polymers.

Fig. 6. Schemetic representation of confinement of polymer in slit type pores (a, b and c) and (d) in hexagonal type pores.

2R ≈d

d

(b)

(a)

(c)

(d) d

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3.6. Confinement of Biomolecules

This field is in its infancy and deserves more attention. Nonetheless, many studies have been reported on ion transport in channels, enzymes, E-coli cells, proteins, DNA etc.49-54 For example, enzymes entrapped in sol-gel matrices have been found to be more stable, possibly because the rigidity of the cage inhibits denaturing unfolding motions. There may be situations where denaturation may be forced. As an example, the possible denaturation of double stranded DNA confined in nano-pores is schematically shown in Fig. 7. The implication of this will be that many of the properties of DNA will change in confinement. Preliminary studies on thermal properties of DNA confined in nano porous silica matrix have been carried out in our laboratory using differential scanning calorimetry (DSC) technique. The results are shown in Fig. 8 from where it can be seen that DNA-unfolding (see Fig. 7) has resulted in significant changes in the melting point and the phase transition behaviour of confined DNA. The multiple endothermic peaks show the denaturation of double stranded DNA. DSC themogram of confined DNA shows the a broad endothermic peak which engulfs peak observed at 88ºC, 127ºC of pure DNA while endothermic peak observed at 107ºC & 110ºC of pure DNA are shifted to lower temperature at 102ºC & 103ºC respectively55.

3.7. Ionic liquids in Confined Matrices: “Ionogels” as a New Class of Materials for Device Applications

Ionic liquid is a new class of materials used for development of innovative ionic devices.56 Ionic liquids, in, general, have weakly co-ordinating bonds and do not need a solvent for dissociating into free cations and anions (earlier known ionic salts like NaCl, KCl, KBr etc. needed high dielectric constant solvent like water). Due to availability of a large number of mobile cations and anions, ionic liquids (ILs) possess high ionic conductivity, which is necessary for many ionic devices.

258

(a) (b)

Fig. 7. Schematic representation of (a) native coiled DNA (b) linearised DNA after confinement.

-20 0 20 40 60 80 100 120 140-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Pure DNA Confined DNA

Temperature (OC)

Hea

t Flo

w (a

.u.)

-6.0

-4.5

-3.0

-1.5

0.0

Heat Flow

(a.u.)

Fig. 8. DSC thermogram of an unconfined Salmon DNA (2000 base pair) and DNA confined in a porous silica gel matrix (blue solid line) [(Singh et al. to be published)].

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Apart from high ionic conductivity, ILs have a low vapour pressure, high thermal stability, high chemical stability, a wide liquidus range and good capability of dissolving organic / inorganic materials. These properties are recently being exploited to develop an effective ‘green chemistry’ approach for chemical synthesis processes57. However, the use of ILs in ionic device leads to problems of packaging and leakage etc. A novel approach has been developed to confine ionic liquid in various nano porous matrices like silica, titania, zeolites etc. The first attempt in confining ion conducting liquid electrolyte in sol-gel derived SiO2 was possibly by Chandra’s Table 2. Some selected “ionogels” obtained by confining ILs in nano-porous oxide matrices.

Porous matrix Ionic Liquids Reference

SiO2 [EMIM][TFSI] [58]

SiO2 [BMIM][TFSI] [59]

SiO2 [BMIM][BF4] [60]

SiO2 [BMIM][PF6] [61]

SiO2 [EMIM][BF4] [62]

SiO2 [BMIM][OcSo4] [21]

TiO2 [BMIM][TFSI] [63]

TiO2 [EMIM][EtSO4] [64]

SnO2 [BMIM][PF6], [BMIM] [BF4], [BMIM] [Br]

[65]

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group66 who confined aqueous electrolytes obtained by dissolving potassium-di-hydrogen phosphate (KDP), ammonium-di-hydrogen phosphate (ADP) etc. However, its conductivity decreased with time as the solvent water evaporated. Next step in this direction was to use ionic liquids (as electrolytes) which are known to be stable because of its extremely low vapour pressure. Such materials were termed as “ionogels”58. The last decade has seen the development of many ionogels obtained by confining different ILs in a wide variety of nano-porous matrices. Some selected examples are given in table 2.

Ionogels are generally classified on the basis of nature of confining matrices as given below:

(a) Organic ionogels (b) Inorganic ionogels (c) Hybrid (organic and inorganic) ionogels The organic ionogels obtained by gelation of IL by low

molecular weight gellators or organic polymers. Inorganic ionogels have been obtained by using either a conducting porous matrix like carbon nano tube68,69 or an insulating matrix like SiO2, TiO2 or SnO2 matrix. Most studies have been carried out on nano-porous silica matrix obtained by sol-gel process. Fig. 9 Shows various steps involved in the synthesis of ionogels61. The precursor is tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS) which on hydrolysis and condensation gives the SiO2 matrix:

[BMIM]: 1-butyl-3-methyl imidazolium [EMIM]: 1-ethyl-3-methyl imidazolium [TFSI]: bis-(trifluoromethylsulfonyl) imide [Br]: bromide [BF4]: tetrafluroborate [PF6]: hexaflurophasphate [EtSO4]: ethyl sulfate [OcSo4]: octyl sulfate

261

3 4 2 2 3Si(OCH ) +2H O SiO + 4CH OH

2 5 4 2 2 2 5Si(OC H ) +2H O SiO + 4C H OH

IL, which is also added in the sol-gel reaction vessel, gets entrapped in the SiO2 matrix.

Fig. 9. Schematic representation of different steps involved in the preparation of IL confined porous silica gel matrix.

VISCOUS TRANSPARENT GEL

SILICA POROUS GEL

Heated at 40 °C

Left at room temperature to gellify

TEOS + C2H5OH +IL + H2O +HCl

TRANSPARENT SOLUTION

Stirring for 4–5 hours

Add HCl + water drop by drop

TEOS + C2H5OH + IL Stirred two hours at 40 °C

Ionic liquid TEOS + C2H5OH

VACUUM DRYING AT 60 °C

FINAL IL-CONFINED SILICA GEL SAMPLES FOR MEASUREMENT

Mix

262

Many studies on the effect of confinement on the properties of ILs have been reported in the last few years. The more important general conclusions are:

(i) Thermal phase transition behaviour has been found to change on confinement21,59,61,62. Singh et al.21 have shown that change in Tm, Tc, and Tg depend upon the size of the anion. Some examples are given in Table 3. Changes in Tg are generally very small. Table 3. Change in melting point ( Δ Tm) of various ILs on confinement in nano-porous silica matrix

(ii) Thermal stability changes on confinement69,70. Singh et al.69 have explained this on the basis of a new phenomenological model (hinged spring model).

(iii) Vibrational properties (IR/Raman spectra) have been reported to be different for confined ILs. Since the end group of IL cations and anions gets affected due to interaction with the pore wall of the confining matrix61,71-73.

(iv) Fluorescence/ Luminescence spectra also have been found to change27,61.

(v) Dynamics of confined IL have been studied using NMR, neutron scattering and relaxation studies. Neouze et al.59,60 have shown that dynamics of confined IL is intermediate between liquid and solid. We have recently shown in our laboratory74 that

Ionogel Length of anion of con-fined IL (Å)

Δ Tm

(°C) Reference

SiO2:[BMIM][OcSO4] ~ 12.7 52 [21]

SiO2:[BMIM][TFSI] ~ 7.3 20 [59]

SiO2:[BMIM][PF6] ~ 3.3 2 [61]

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dielectric relaxation peak shifts upon confinement in nano-porous matrix. A typical result is shown in Fig. 10

Fig. 10. Dielectric relaxation of IL ([BMIM][PF6]) confined in porous silica matrix. Note that apart from shift in peak of IL on confinement a new peak appear in confined IL.

Many technological applications of ILs confined in nano-porous matrices or polymers are recently emerging75,76. Some important applications are:

(i) As electrolyte for lithium batteries and fuel cells. (ii) In Dye sensitized solar cells (DSSC) [for use of ILs in

DSSC reference may be made to the review article of Chandra77. (iii) Ionogel based sensors/Biosensors: Many ILs have been

shown to have good solubility and compatibility with enzyme, proteins and other bio-molecules. So, many bio-sensors are being tried for detecting hydrogen peroxide, glucose dopamine etc.

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(iv) Drug delivery: This application is in its infancy but appears promising. ILs have high solubility for many Active Pharmaceutical Ingredients’ (APIs) and so these can be immobilized in the ionogels.

(vi) Optical application: Solubilization of many luminescent lanthanide, Samarium and terbium salts is possible in ILs which can be subsequently confined in nano-porous matrices to obtain luminescent ionogels75,76. Ahmad & Deepa79 have used ILs for application in electrochromic devices.

Summary

An overview is given for an emerging field of nano-science based on materials obtained by confining a wide variety of materials in nano-porous matrices. It is highlighted that the properties of materials confined in nano-pores are different than those in bulk. The technological and scientific implications of this are discussed in brief.

Acknowledgements

One of us (SC) is thankful to National Academy of Sciences, India for the award of position of Platinum Jubilee Senior Scientist; RKS is grateful to the UGC, New Delhi, India for providing financial assistance for carrying out this work, MPS is thankful to CSIR, New Delhi, India for award of Senior Research Fellowship (SRF).

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