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3 Organic – Inorganic Hybrids and Sol - Gel Composites

3 Organic – Inorganic Hybrids and Sol - Gel Composites

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Sol-Gel Electrochemistry



231



an attractive pathway to benefit from the inertness, rigidity, porosity, high surface area, processability, and other attractive features of sol-gel silicates, and at

the same time to take advantage of the enormous versatility inherent in organic

chemistry [65–72].

Two scientific developments that took place in the early 1980s signaled the

evolution of hybrid organic–inorganic hybrids. Schmidt and coworkers exploited

the fact that Si-C bonds are stable and do not hydrolyze during sol-gel processing in order to develop organically modified ceramics (Ormocers) and silicates

(Ormosils), using organofunctional silane precursors such as methyltrimethoxysilane, MTMOS [73]. In this class of materials, the organic moiety is covalently

bonded to the siloxane backbone. These materials containing Si-C covalent bonds

are now classified as class I organic–inorganic hybrids. The covalent bonding

between the organofunctional group and the cross-linked siloxane backbone is

very similar and almost indistinguishable from materials made of silylation of

large-surface silica gels. However, the Si-containing monomers can form a maximum of three siloxane bonds, which influence the rigidity and other physical properties of the end product. Silylation products are also classified as class I materials,

because the covalent bond should be broken to release the organofunctional group.

Avnir et al. [74] developed class II organic–inorganic hybrids by the so-called solgel doping process. The first dopants were dye molecules that were introduced into

the silicate during sol-gel polymerization. Avnir et al. [75–78] further showed that

it is possible to encapsulate colorimetric reagents that change color in response to

chemicals in the silicate environment. Doped sol-gel materials and impregnated or

adsorbed organic compounds on sol-gel silicates are now termed class II hybrids

because some of the organic moieties can leach out.

Sol-gel hybrids can be prepared by the following methods:









1.Impregnation: Physical adsorption of an organic material on an inorganic support is simply done by first preparing a neat sol-gel silicate

film and then immersing the film in a concentrated solution of the

organic compounds. Evaporation of the external solvent leaves a silicate impregnated by the organic molecules. The interaction between the

organic and inorganic moieties may be direct (e.g., by hydrogen bonding

to silanols) or indirect (e.g., by adsorption of a cluster of molecules to the

silicate surface).

2. Sol-gel doping: An important variant of the last approach is termed solgel doping. Here the guest molecule, be it an organic or inorganic moiety,

is introduced along with the sol-gel precursors and is entrapped in the solgel matrix by the polymerization of the silicate around it. The intimate

contact between the guest molecule and the polymerizing host provides

ample routes for optimized interaction between the inorganic host and the

guest molecule. In addition, caging of the organic moiety within the silicate porous structure can take place. This caging mechanism is particularly interesting from an electrochemical point of view when the silicate

pores form a morphology of bottlenecks that hinder the mobility of large



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Electroanalytical Chemistry: A Series of Advances



encapsulates. These materials have interesting functionalities while still

allowing the movement of electrochemical reactants and products and

thus rapid interaction of modified electrodes with the environment.

3. Polymerization of Ormosils carrying desirable functionality. This procedure involves the synthesis of hybrid materials from silicon monomers

containing hydrolysable groups that can participate in sol-gel polymerization reactions and also carry desirable functionalities that remain bound

by Si-C bonds to the silicate network after gelation. A combination of several functionalities by mixing different monomers is a viable and useful

possibility. Silicon monomers containing double bonds, epoxides, thiols,

aldehydes, and amines (see several examples in Table 4.2) are frequently

used for Ormosil formation. Carboxylates containing monomers are less

useful because they can participate in intramolecule reactions with the

silanols. Redox-active functionalities such as metallocenes (ferrocene),

methylene blue, and quinones were also proposed.

4.Grafting on preprepared Ormosils or inorganic silicate. In this

technique, the organic moiety is bonded to silicate silanol by a covalent

bond. A variant of the same method is to produce organically modified

silicate containing a pendant functionality (e.g., by sol-gel polymerization of epoxy-, vinyl-, thio-, or aminosilicate) that can be used to anchor

the desirable organic moiety after gel formation.



Impregnated and doped sol-gel hybrids sometimes result in permanent immobilization of the guest molecule with zero observed leaching of the organics. However,

because no covalent bonding is involved, some or even the entire organic modifier is removed from the silicate in most cases, particularly when an appropriate

solvent is used for extraction.

4.3.1.1 Encapsulation of Biomolecules

Biomolecules constitute an important class of organic compounds whose immobilization in silicate and other inorganic matrices has attracted considerable attention. The power of this technique, which was first presented over half a century

ago by Dickey [79,80], was unraveled by a series of publications starting with the

work of Braun et al. [81,82]. The versatility and the synergy between the bioentities

and the silicate hosts are still being unraveled, yielding surprising revelations [13].

Delicate, heat-sensitive biological entities capable of direct recognition of small

ligands or large bioentities (e.g., aptamers, oligonucleotides, and antibodies) and

highly specific biocatalysts (e.g., enzymes and catalytic antibodies) can be encapsulated in sol-gel matrices by sol-gel doping or by linkage to pre-prepared silicate or

Ormosil matrices. Not only can these delicate entities survive the harsh conditions

involved in sol-gel polymerization, but at times they can also exhibit improved performance as compared to the native bioentities. Remarkable changes of the binding affinity, turnover numbers, and redox potential of encapsulated enzymes were

reported by a mere change of the encapsulation environment. Increased stability

under far-from-neutral pH conditions or elevated temperature and the ability to



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Table 4.2

Examples of Some Useful Commercially Available Silicon Monomers for Polymerization of Ormosils

Monomer



Structure



Abbreviation



Usage



OCH3



n-octadecyltrimethoxysilane



H3CO Si



ODTMOS



Surface hydrophobization



MTMOS



Surface hydrophobization



OHMOS



Surface hydrophobization



PhTMOS



Surface hydrophobization



OCH3

OCH3



Methyltrimethoxysilane



H3CO



Si

OCH3

OCH3



Octyltrimethoxysilane



H3CO



Si

OCH3

OCH3



Phenyltrimethoxysilane



H3CO



Si

OCH3

OC2H5



Perfluoroheptylethyltriethoxysilane



C2H5O



Si

OC2H5



F F F

F



F



F



F F

F



F



F



CF3



Surface hydrophobization



F



(continued )



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Table 4.2 (Continued)

Examples of Some Useful Commercially Available Silicon Monomers for Polymerization of Ormosils

Monomer



Structure



Abbreviation



Usage



OCH3



3-Cyanopropyltrimethoxysilane



H3CO



Si



CN



Nonpolar surface hydrophilization



OCH3

OCH3

H3CO



Si



SH



MPTS



Metal coordination, binding nanoparticles



APTS



Cationic. Nanoparticles binder, binding proteins



MEMO



Curing agent for polyaddition



GLYMO



Curing agent for polyaddition



OCH3

OCH3



3-Aminopropyltrimethoxysilane



H3CO



Si



NH2



OCH3

O



OCH3



3-Aethacryloxypropyltrimethoxysilane



H3CO



Si



O



OCH3

OCH3



3-Glycidoxypropyltrimethoxysilane



H3CO



Si



O



O



OCH3



(continued )

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Electroanalytical Chemistry: A Series of Advances



3-Mercaptopropyltrimethoxysilane



Pyrenemethyltrimethoxysilane



H3CO



Si



Fluorescent substituent



OCH3



OCH3



2-Ferrocenyl-N-propylacetamidetrimethoxysilane



H3CO



Si

OCH3



NH



CO

Fe



Redox substituent



Sol-Gel Electrochemistry



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OCH3



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Electroanalytical Chemistry: A Series of Advances



function in organic solvents and in the encapsulated form extremely hydrophobic

conditions were reported. For example, alkaline phosphatase exhibits its optimum

activity at pH 9.5 and remains active at a pH as low as 0.9 [83]. Entraped creatine

kinase retains 50% of its activity ten times longer at 47°C than the free enzyme

at the same temperature [84]. Encapsulated lipases that can function in organic as

well as inorganic solvents are commercially available from Fluka [85].

Bioencapsulation is not limited to biomolecules but can be extended to live

microorganisms as well. Viruses, bacteria, yeasts, algae, and even mammalian

tissues were successfully immobilized within silicate shells and retained their

viability [13]. Two inspiring examples include the encapsulation of bioartificial

organs and bioengineered microorganisms. Langerhans islets were encapsulated

in silicate–alginate composites, which were successfully implanted in diabetic

mice and kept the mice alive for a lengthy period [86]. While the electrochemical application of this approach is rather far-fetched, though it can be envisioned

in the future, electrochemistry is likely to play an increasing role in our second

example, the encapsulation of recombinant microorganisms [87–89]. Figure 4.5

(after work conducted by Permkumar et al. [87]) demonstrates the dynamic

response of a single recombinant E. coli bacterium that was engineered to express

green fluorescent protein (gfp) in response to specific stress conditions. Similar

engineering with the expression of β-galactosidase or glucose oxidase will enable

Stress



(a)

(b)

(c)

(d)



Bacterium

E. coli DNA

Genes

Promoter

Physiological

response



Recombinant Bacterium

(e)

Plasmid

Genes for reporting

proteins

(f )

Promoter

Reporting proteins



Figure 4.5  Left: Scheme of stress-induced response of native and recombinant bacteria. The recombinant bacterium senses the stress and expresses green fluorescence protein

in addition to the physiological defense mechanism of the native bacterium. Right: Single

E. coli cell fluorescent response after induction by 1.2 mmolar mitomycin C (an antibiotic). Confocal microscope images were taken after incubation times of 0, 140, 260, 340,

420, and 480 min with the antibiotic. (From Avnir, D., T. Coradin, O. Lev, and Livage, J.,

2006. J Mater Chem 16:1013–30. Used with permission.)



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electrochemical sensing as well. Needless to say, the expression of insulin in

response to glucose deficiency as well as the expression of green fluorescent protein in response to stress conditions imply that the encapsulated cells were viable,

and that nutrients and analytes could be delivered through the hybrid matrix.



4.3.2 Composites in Sol-Gel Electrochemistry

Composites are materials made up of two or more phases. From the materials

chemistry point of view, the straightforward way to classify composites is based

on the degree of interpenetration between the two phases and the plasticity or

elasticity of the components. Because the degree of interpenetration is not a

binary quantity, and it may span a large range of dimensions of the embedding

and guest domains, one would prefer to use an alternative classification based on

how the composite is formed, which bears also on the degree of interpenetration

and wettability of the silicate by the foreign phase.

4.3.2.1 Organic Polymer–Inorganic Host and

Organic Host–Inorganic Polymers

Five different methods are currently being used to design organic polymer–inorganic component composites, and most of them have already found electrochemical usefulness, though practical applications are slow in coming.

4.3.2.1.1 Encapsulation of Preprepared Organic

Moieties in Sol-Gel Silicates

This method of obtaining inorganic–organic composites is by far the most popular and one of the simplest to implement. All that the experimenter has to do is

carry out conventional sol-gel processing in the presence of a foreign organic

polymer in a compatible cosolvent that will allow dissolution of the precursors as

well as the polymer. The method is generic, and it is compatible with all sol-gel

processing techniques, including the conventional dip coating, spray coating, and

spread coating as well as electrophoretic and electrochemical sol-gel deposition

techniques. Numerous applications were reported, including modified working

electrodes, sensors and biosensors, solid electrolytes, stand-alone membranes for

humidity sensors, and fuel cell electrodes.

The list of polymer dopants that have already been used for electrochemical

applications is quite extensive and includes hydrophilic polymers such as polysaccharides and the positively charged chitosan [90–97] as well as exceedingly hydrophobic neutral dopants (e.g., poly(dimethylsiloxane) and DMDS [98,99]). Several

examples are given in Scheme 4.6. The guest polymer can be tailored to retain water

(if the application is in the area of polyelectrolyte membrane fuel cells (PEMFCs))

or reject water (for gas-phase sensors), depending on the choice of the end application. Likewise, many of the polymer dopants are natural polymers, whereas others

are synthetic and may span a large range of price and complexity, including relatively inexpensive poly(ethylene glycol) and expensive, carefully designed redox

polymers such as poly(neutral red) [100] or osmium bipyridine [101].



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[ (CF2CF2)x CF



(CF2CF2)y ] n



[ (CH2 CH ]



O



OH



CH2

F



n



C



Poly(vinyl alcohol)

CF2 CF2



SO3H



CH3



[ (CH2 CH2) O ]

n

OH



Nafion (acid form)



Poly(ethylene glycol)



NH+



NH+



NH



NH



n

Poly(aniline) (conducting form)

OH

HO



H



O



NH2



OH



OH

O



HO



O

NH2



O



HO



O



O



H



NH2



Chitosan

OH

H



HO



O



OH



O HO



OH

O

OH



O HO



OH

O



O



H



OH



Cellulose



Scheme 4.6  Structure of common polymer dopants mentioned in this chapter.



For most electrochemical applications, the mechanical strength of the composites is quite sufficient, although for certain applications such as fuel cell membranes, the loss of strength may necessitate the addition of rigid support that would

decrease the power per weight output of the fuel cell. Nevertheless, it was noted

that better connectivity between the phases (and smaller domain size) increases

the strength and the Young modulus of the composite. Interpenetrating and segregated phases are not usually differentiated by electrochemical means, and therefore electrochemists do not perceive the degree of interpenetration unless clear

optical or structural changes accompany the phase segregation.

Novak [102] provided important guidelines for the preparation of alloys

and interpenetrated polymers relative to conditions leading to the formation of

segregated phases. These general guidelines may be used also as a qualitative

way to control the domain size of partly interpenetrating polymer composites.



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Conditions that are close to those forming interpenetrated polymers may yield

smaller domains of the guest phase. These include the following:

















1. The cosolvent should be compatible with the precursors as well as with the

hydrolyzed intermediates and oligomers that are formed during polymerization, otherwise phase separation will occur. This is not always simple

because most sol-gel precursors are hydrophilic or become hydrophilic

after the initial hydrolysis step (which replaces the hydrophobic silicon

alkoxide by a more hydrophilic silanol), whereas many of the desirable

polymer dopants are hydrophobic in nature. Hence, successful cosolvation for the first stage of polymerization does not necessarily guarantee

that the inorganic phase will not precipitate at an advanced stage of the

sol-gel process.

2.To enhance polymer interpenetration conditions and to form small

domain sizes, compatibility with the solvent should be maintained even

after water or alcohol is released as a polymerization by-product.

3.Better interpenetration of the two phases may be encountered when they

have high affinity toward each other. For example, this may be the reason for obtaining clear, transparent composite from poly(methyl methacrylate), PMMA, poly(vinyl alcohol) (PVA), and poly(vinyl acetate) and

poly(vinyl alcohol) in silicate and methyl silicate gels [102,103].

4.As an extension of the last criterion, hydrogen bonding between the two

phases enhances the interpenetration and yields small-size domains.

This makes polyols, for example, especially attractive as sol-gel guests.

5.Chemical bonding between the two components may enhance polymer interpenetration (though this class of grafted composites belongs

to our next classification). Trimethylsilane terminated polyols and

poly(ethylene oxide) are especially attractive due to the combination of

labile hydrogen bonds and facile formation of covalent linkage to the

siloxane backbone.



Indeed, polysaccharide-silica composites are frequently used for the formation of

electrochemical films, because there is a large affinity between cellulose and the

silicate oligomers in addition to the compatibility in the nature of solvents that

can be used. The opposite is also true, and silicates (particularly sodium silicate)

are useful additives in the paper industry. Chitosan is quite appealing, at least

from the interpenetration point of view, because in addition to all these benefits,

it is also positively charged and thus undergoes electrostatic interactions with the

deprotonated silanols and can also assist in the attachment of biomolecules that

are often negatively charged.

The example of Ormolytes (organically-modified electrolytes). Sol-gelderived silica/alumina/zirconia/titania are poor conductors, and one of the ways

to increase their ambient temperature ionic conductivity is to add organic components that incorporates ionic salts. When the processing is carried out under

ambient conditions, the organic moiety will be intact. The molecular-level



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TEOS

PEG

H2O



Stir



Mixture



60°C/few min

Add salt/stir



Homogenous

solution

40°C



Cast on

petri-dishes



A drop of conc. HCl



Turbid

Stir/60°C mixture



Evaporation



Film

Vacuum oven, 60°C

for several days

Transparent

electrolyte film



Scheme 4.7  Preparation of hybrid, ionically conducting matrix. (From Judeinstein, P.,

and C. Sanchez, 1996. Hybrid organic–inorganic materials: A land of Multidisciplinarity.

J. Mater. Chem 6:511–525. Used with permission.)



mixing of the organic and inorganic precursors ensures homogeneity at the

microscopic level. The literature contains a large number of studies in which

various combinations of polymeric species with different electrolytes are

reported to yield highly conducting matrices. They are referred to as organiclymodified electrolytes (Ormolytes). The conducting hybrids that are generally

used as electrolyte materials are treated as “biphasic” at the microscopic level

[104–106]. The formation of hybrids may be due to weak interactions based

on van der Waals forces, hydrogen bonding, or due to strong covalent bonding [107]. The electrical conductivity depends on the intimate mixing of the

inorganic and organic phases and also the ionic component present. The basic

recipe reported by Judeinstein is used by many with little modification, depending on the components.

A schematic of the process is given in Scheme 4.7, where the use of TEOS

and poly(ethylene glycol) are shown, and the resultant Ormocer with an ionic

component was reported to have very good ionic conductivity, on the order of

10 −6 –10 −4 S/cm at 25°C. Salts based on lithium, magnesium, calcium, zinc, and

other trivalent metal ions such as europium, dysprosium, and ytterbium have been

used to prepare sol-gel-derived hybrid electrolytes [108–114]. Ravine et al. [115]

used lithium nitrate and lithium perchlorate salts to achieve ambient temperature

ionic conductivities of ≈ 7 × 10 −5 S cm−1 at 25°C using SiO2–PEG gels. The metal

ion would be solvated by the ethylene oxide units present in the matrix. Hybrids of

tetraethoxysilane and tetraethylene glycol were used as matrices for lithium salts



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to achieve high lithium ion conductivity [116,117]. In spite of the conductivities

achieved using hybrid materials, there is still a long way to go before they can be

used in actual devices such as batteries. Other considerations, including a solid

polymer interface (SPE) that generally occurs with a polymer electrolyte–electrode interface, thermal cyclability with retention of ionic conductivity, transport

number, etc., are to be addressed further. Further research is required to optimize the gel properties so that the contact resistance with the electrode surface is

minimized. This is an essential requirement for application in the area of electrochemical energy systems, particularly rechargeable batteries. The use of silicates

alone would not give rise to high conductivities because a plasticizer is required

in the form of an organic component to improve ionic conductivity.

4.3.2.1.2 Grafted Organic Polymer–Silicate Network

Criterion 5 (chemical bonding between the two components) for the enhancement of polymer–silicate interpenetration brings us to the next class of composites in electrochemistry. Although the preparation conditions are similar to the

previous protocol, a covalent linkage between the two phases provides strong

attachment of the pre-prepared polymer to the siloxane backbone, and intimate

mixing of the two components is achieved. Covalent bonding is important when

otherwise leachable, linear polymers are processed, or when interpenetrating

polymer formation is desirable, that is, especially when optical transparency is

preferred. The covalent bond between the two polymers is often created by condensation reactions between the silane monomer and polyols. This is indeed the

case in the formation of PVA–poly(vinyl pyridine) (PVA-g-PVP), poly(ethylene

glycol), and dextran sulfate-silica sol composites [118,119].

Limited interpenetration of the inorganic and organic polymers is usually

achieved by preparation of the silicate sol prior to its mixing with the organic

polymer and other additives. This procedure is especially preferred over the mixing of the organic polymer with the silane monomer in order to minimize contact

of the evolving methanol and the starting solution with delicate bioentities that

are used for the production of biohybrids. Another synthesis procedure involves

linear polymers containing reactive end groups such as alkoxysilane or hydroxyl-terminated linear polymers. For example, hydroxyl-terminated poly(dimethyl

siloxane), [OH [Si(CH3)2 O]n Si(CH3)2 OH] [120], and triethoxysilane-terminated

poly(ethylene oxide) are frequently used for composite preparation, depending on

the desirable hydrophobicity. When the embedded phase is highly hydrophobic,

and this is certainly the case for dimethylsiloxanes, the covalent bonding to the

silicate is an absolute must in order to prevent phase separation during sol-gel

processing.

4.3.2.1.2.1   Interpenetrating Composite Aerogels by Supercritical Solvent

Evacuation  A related procedure was reported for the preparation of highly

porous, low-density composites. This process relies on the selection of a cosolvent that is suitable for the preparation of interpenetrating organic–inorganic

composites, and can also be replaced by carbon dioxide under supercritical



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