Chapter-1 1.1 Introduction - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/8196/7/07... ·...

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Chapter-1

1.1 Introduction

Biomimetics and Bionics are artificial concepts. The term biomimetics is a

synthesis meaning - bios (life) and mimesis (to imitate, to mimic). Biomimetics turns

to “natures patent bureau” and takes as its role model such organism based

achievements. Biomimetics is the field of scientific endeavour, which attempts to

design systems and synthesize materials through biomimicry. Bio meaning life and

mimesis meaning imitation are derived from Greek. Perceptions regarding the scope

of biomimetics appear to vary very widely depending upon the specialized discipline

of the investigator. Japanese electronic companies are supporting biomimetic research

with a view to learning the way biological system’s process information. Recent

interest of Japanese in biomimetic research seems to arise from the health and welfare

problems of an ageing society leading to studies on the development of human

supportive robots [1]. Biomedical engineers consider biomimetics as a means of

conducting tissue engineering and trace the origins of biomimetics to ancient times

when Mayan, Roman and Chinese civilizations had learnt to use dental implants made

of natural materials. Material scientists view biomimetics as a tool for learning to

synthesize materials under ambient conditions and with least pollution to the

environment. Chemists have always wondered at the ease with which ammonia is

produced in biological nitrogen fixation, methanol produced in biological oxidation of

methane and oxygen generated in photosynthesis [2].They hope to learn the synthesis

of polymers that can perform the roles of enzymes in such processes. Biologists study

biomimetics not only for an understanding of the biological processes but also to trace

the evolution of various classes of organisms. Biochemists have interest in the field

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due to the complexities associated with the interaction of biopolymers with ions of

metals leading to the mineralization in living organisms. Even geologists have an

interest in biomimetics because of biomineralization: the formation of extra- or intra-

cellular inorganic compounds through the mediation of the living organism. Engineers

attempt to explore the relationship between structure and function in natural systems

with a view to achieve analogous synthetic design and manufacture. On the whole, the

field of biomimetics addresses more than one issue. Those engaged in this field of

research activity try to mimic natural methods of the manufacture of chemicals in

order to create new ones, learn new principles from phenomena observed in nature

(flight of birds and insects, swimming of fish and aquatic animals), reproduce

mechanisms found in nature and copy the principles of synthesizing materials under

ambient conditions and with easily available raw materials. Even though biomimetics

literally means to mimic biology, Vincent[3] has argued that it is far more difficult to

exactly mimic a living system for engineering purposes than to understand the

underlying ideas and principles for designing systems of use. He argues that the

farther we are from the natural the more powerful will be the concepts developed and

greater the chances of transferring the technology from nature to engineering. It has

also been suggested that blindly copying nature may not be advisable.

1.2 Biomineralization

Nature employs more than 60 inorganic materials together with organic matter

to create myriads of organisms. In the bones of vertebrates, the most common mineral

phase is carbonate hapatite and the organic part is collagen fibril. Members of phylum

Echinodermata (Fig.1.1) have calcite that contains magnesium and is distributed in

protein matrix. The shells of molluscs (snails, slugs, clams, oysters, cuttlefish, squid


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etc.) contain aragonite (Fig.1.2). The composition of the crystals produced is also

tailored to suit specific requirements. Sometimes, it varies along the length of even a

single crystal. The calcite prisms found in the tooth of a sea urchin provide the best

example. The magnesium carbonate concentration in the calcite varies from about 4-

5% at one end to about 13% at the other. The chemical composition is also specific to

the taxa [9]. There has been a debate if the biogenic inorganic phases are identical to

the synthetic crystals. Evidence now available suggests that the biogenic crystals are

different in the sense that they have considerable intercalation of acidic

macromolecules derived from the proteins [10].

Amongst different plants, silica is more commonly observed. It is also found

in some marine organisms such as radiolaria, sponges and the marine plants; diatoms

[4]. Other inorganic phases found are mostly of calcium oxalate, gypsum, barite and a

few iron oxides. Even nanocrystals of metals are found in living organisms in their

native state. Klauss et al [5] have found that a silver-resistant microorganism called

Pseudomonas stutzeriAG 259 can accumulate silver in the form of 200 nm sized

crystals embedded in its organic matrix. The weight of silver accumulated could be as

high as 25% of the biomass. The silver crystals form in the space between the outer

membrane and the plasma membrane and have been shown to be in mostly elemental

form.

Some evidence was also provided for the crystallization of silver sulphide and

another as yet undetermined silver-bearing structure. Other examples include the

formation of tellurium in Escherichia coli K12, enzymatic reduction of technetium

and the production of selenium by several organisms [6]. Magnetosomes, the

magnetite crystals produced by magnetotactic bacteria (proteobacteria) in the

intracellular space, best illustrate the functionality of minerals produced through


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biomineralization. These crystals have size specificity (with most of them falling

within 35 to 120 nm along the long axis) and distinctive morphology. The size

enables uniform magnetization of the crystallite with a single domain being operative.

The particles are arranged such that their crystallographic axes of easy magnetization

align themselves. These characteristics are ideally suited for magnetotaxisin the

earth’s magnetic field [7]. Another very recent finding is the detection of 100 m

crystals of ilmenite in the combs of the hornet vespaorientalis by Stokcrooset al [8].

These magnetically polydomain crystals are said to act like spirit levels and aid the

hornets to assess the symmetry and balance of the cells they are building. Whether

these crystals are collected by the hornets or synthesized by them is unknown.

However, the presence of the titanium and iron in the body of the hornet cannot rule

out the possibility of synthesis in situ.

The composition of the crystals produced is also tailored to suit specific

requirements. Sometimes, it varies along the length of even a single crystal. The

calcite prisms found in the tooth of a sea urchin provide the best example. The

magnesium carbonate concentration in the calcite varies from about 4-5% at one end

to about 13% at the other. The chemical composition is also specific to the taxa [9].

There has been a debate if the biogenic inorganic phases are identical to the synthetic

crystals. Evidence now available suggests that the biogenic crystals are different in

the sense that they have considerable intercalation of acidic macromolecules derived

from the proteins [10].


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Figure 1.1 Phylum Echinoderamata – Echinoidea species.


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Figure1.2 Shells of molluscs: a) & b) CaCO3 Mollusc shells c) snail d) squids e) Pearl oyster

Biomineralization occurs under ambient and mild conditions.

Biomineralization could be extra-cellular or within the cell (or organism). The

production of magnetite by iron reducing bacterium Geobactermetallireducens (GS-

15) is an example of biologically induced mineralization. The bacterium produce

achieves the production of magnetite by coupling the oxidation of organic matter to

the reduction of ferric ion [11]. In extra-cellular crystallization, the cell or organism

may affect the solute concentrations in the medium and provide sites for nucleation


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and growth. The secretion of biopolymers like collagen and chitin into the

crystallization domain enables the construction of large-scale structures such as bone

and shell. The growth of such large structures through extra-cellular nucleation

requires control of both the biopolymer and crystal deposition at large distances by

the cells. Usually growth occurs by the movement of a mineralization front. Cells on

this front control the nucleation process while growth takes place in the regions

behind the front [12]. Biomineralisation within the organisms has attracted the attention

of both biologists and material scientists in view of its importance both in

understanding species diversity and evolution as also the mechanisms of formation

and functional characteristics of the mineral produced. The size, shape, composition

and crystallographic orientation of the mineral produced are controlled very closely.

Formations of calcite in coccoliths, magnetite particles in A. magnetotacticum, etc. are

examples. Biomimetic approaches based on an understanding of the biomineralisation

process are aimed at synthesizing nanoparticles, polymer mineral composites and

templated crystals [13-15]. Studies over the last few decades show that there are several

distinct characteristics of biomineralisation.

(a) Biomineralisation occurs in well-constructed compartments or

microenvironments.

(b) These compartments have the ability to promote the nucleation and growth

of crystals of the required inorganic material at chosen sites while effectively

preventing the formation of other crystals.

(c) The crystal size and shape are well defined and show little diversions.

(d) Formation of the macroscopic structure is through the packaging of many

such units.


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The structures that arise are highly organized from molecular (1–100Å) to

macro-scales through nanometric (10–100nm) and mesoscopic (1–100m) domains.

These are hierarchical in nature and meet the functional requirements. The substrate

on which mineralization occurs is composed of proteins, glycoproteins, lipids or

polysaccharides. In some cases, it may just provide a passive support. Often, the

substrate provides both stereo chemical basis and physiosorption for nucleation and

growth of the mineral [16]. The biological macromolecules of the substrate also collect

and transport the material required for mineralization. The organic matrix also

controls the shape and orientation of the resultant crystal [17]. Some of the matrix

material gets occluded into the crystal and can determine fracture behaviour of the

crystal [18]. A number of studies have shown that some of the proteins have the ability

to even determine the structure of the crystal. For example, specific proteins are

responsible for the formation of amorphous calcite [19], silica and calcium phosphate

[20]. When proteins extracted from mollusk shells, which are either made up of calcite

or aragonite, are added to the system in which crystallization of either of these phases

is taking place they change the phase being crystallized. Proteins extracted from

aragonite bearing shells help nucleate aragonite while those extracted from calcite

containing shells nucleate calcite [21]. In abalone, the outer portion of the shell

contains calcite while the inner portion contains aragonite. Recent atomic force

microscopic studies have shown that the transition from calcite to aragonite during

crystallization from a solution can be induced by the addition of soluble proteins

extracted from the abalone shell nacre without the need for a pre-formed matrix [22].

Many of the proteins have both soluble and insoluble parts. In the mollusc

shell, for example, two types of proteins are found on the basis of its solubility in

ethylene diamine tetra acetic acid (EDTA). Proteins rich in aspartic acid and glutamic


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acid with some covalently bound sulphated polysaccharide content and proteins with

serine and a large fraction of sulphated polysaccharides are soluble portion of the

organic matrix. Proteins rich in glycine, alanine, phenylanaline and tyrosine are

insoluble in EDTA. It is believed that the insoluble part provides nucleation sites.

Such parts of the protein contain a high concentration of carboxylate groups such as

glutamic and aspartic acid residues, which enable interaction between, organic chain

and mineral ions like Ca2+.Weiner has suggested that amino acid sequences of the

type Asp-X-Asp (where the ‘X’ stands for a neutral residue) are good binding sites for

calcium [23]. The soluble part is generally highly anionic and their role is to restrict

nucleation to specific sites by inhibiting nucleation in the bulk. There are also a

number of examples where the polymer from the protein gets entrapped in the

growing crystal thereby controlling the shape of the crystal [24].

1.2.1 BIOMINERALS

Biominerals are the bridges between the organic living and the nonliving

mineral world. Living organisms form these crystalline minerals. Biominerals are

materials such as bones, teeth etc. that perform important functions, and provide

robust support and defence (mollusk shell, skeleton) to the individual. They are also

used as gravity sensors, for metal storage and for detoxification. Biominerals are

remarkably examples of Nature’s ability to produce bioorganic-inorganic composites

with distinct geometric shapes that can be classified as either amorphous,

polycrystalline or single crystal in structure. Hence, today’s biologists, chemists,

physical chemists, and engineers are reunited under the same umbrella to synthesize

materials identical in properties with those naturally produced. They consider Nature

as a model and an educator trying to understand and imitate it. Thus, a new area in

science appeared, called biomimetics. The term itself is derived from bios, meaning


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life, and mimesis, meaning to imitate. This new science represents the study and

imitation of Nature’s methods, designs, and processes.

Biominerals are classified into three broad categories based on their

morphological design i.e., amorphous minerals, polycrystalline and crystalline

minerals. Amorphous minerals are the material of choice of many of the most

morphologically exotic biominerals, such as the silicaceous diatoms and radiolaria.

Amorphous material can be readily moulded to give desired product shape.

Polycrystalline biomaterials also exhibit a wide range of morphologies, and it is again

small crystalline building blocks that can be organized to give complex forms. Final

category of biominerals, the ‘Single crystal’ defined by regular, planar faces, where

the external form is a reflection of the internal symmetry of the crystal lattice.

1. Biominerals are formed by all living organisms, starting from bacteria to

higher plants and animals. These minerals are formed in the matrix of bio-

macromolecules like proteins, polysaccharides and lipids. Most biominerals

are organized hierarchically and are ordered over many length scales starting

from nano- to micro-scale.

2. They often have remarkable physical characteristics. Kinetic control on the

nucleation and crystal growth of biominerals morphology is important for

functional use. Biomineralization is the process of mineralization in living system.

These materials are deposited in the intra- or extra-cellular matrix. These

processes are intimately connected to cellular metabolic processes. Thus,

biomineralization as a field of scientific study falls within several branches of

basic sciences.


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3. Intimate association of organic and inorganic parts is the hallmark of

biomineralization. [25, 26]. In many cases, the integration is at the super-structural

level, where mineral particles and biopolymers are organized to give composites

of unusual strength and toughness. In organisms, several different interconnecting

levels regulate the physico-chemical properties of minerals. There is strong inter-

relationship between biomineralization and biomimetic material chemistry.

4. There are fundamentally two different modes of biomineralization. Organic matrix

mediated & biologically induced mineralizations are the two processes which lead

to a variety of naturally occurring biominerals. Biologically induced

mineralization is less rigorously controlled than organic matrix mediated process.

The biologically induced mineralization creates conditions suitable for the

chemical precipitation of extra-cellular mineral phases. The organic matrix

mediated biomineralization in which inorganic particles are grown within or on

some organic matrix. Large extent of biomineralisation is controlled by organic

macromolecules that govern many properties of the biominerals such as crystal

size, texture, crystallographic orientation and some chemical properties of the

biogenic crystals. The major constituent of naturally producing biogenic material

is CaCO3 which is present in most of the exoskeletons in the form of calcite &

aragonite. Recently, past year’s microscopic skeletal structures of CaCO3 using

micelles, reversed micelles, micro emulsions and liquid crystals as templates have

been synthesized. For the last two decades, extensive studies have been done on

the biomineralization of calcium minerals. Table 1.1 and 1.2 show the

biomineralisation concepts and organic boundaries in biomineralisation.


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Table 1.1: Biomineralization concepts and related biomimetic approaches in

Inorganic materials.

Process Concept Synthetic strategy

Boundary-Organized

biomineralisation

Supramolecular

preorganisation Confinement

Organic matrix mediated

biomineralisation

Interfacial molecular

recognition Template- directed

Morphogenesis Biomineral

tectonics

Vectorial regulation

Multilevel processing

Morphosynthesis crystal

tectonics

Table 1.2: Organic boundaries in biomineralisation and their biomimetic

counterparts in spatially confined material synthesis.

Biomineralisation Biomimetic synthesis

Phospholipid vesicles Synthetic vesicles

Ferritin Artificial ferritins

Cellular assemblies Bacterial threads

Macromolecular frameworks Polymer sponges

5. From these studies it has now been established that biominerals grow on organic

templates containing oxygen and nitrogen donor atoms. These structures are

stabilized through ion–ion; hydrophobic–hydrophilic interactions etc., which in

turn control the structure and morphology of the crystals. Metals like Ca2+ and

Ba2+ prefer oxygen donor ligands owing to hard acid–hard base interactions. A


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study of in vivo CaCO3biominerals of mollusk shell has revealed the fact that Ca2+

binds to polyanionic sites [27] of proteins, which change the CaCO3 from the

calcite to the aragonite phase. These proteins contain special kind of sequence that

mainly contains L-aspartic acid and L-glutamic acid residues.

6. Depending on the amino acids present at the nucleation centre biominerals grow

with different morphologies that perform various physiological roles. Synthetic

biominerals of calcium, [28].Barium [29] and iron [30] have been prepared.

7. Biominerals such as calcium carbonate, calcium phosphate, amorphous silica and

iron oxide, are deposited as functional materials in a wide range of organisms.

Many of these biominerals have remarkable levels of complexity. The diversity of

biominerals became evident by the publications on biominerals that emerged in

the late 1980’s to 1990’s [31-41], which has continued with a few more recent

additions to this nice collection [42-46]. Some of the biologically formed minerals

[47-50] are presented in table 1.3.


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Table 1.3: Examples of the Diversity of biominerals.

Biogenic minerals

Formula Organism Biological location

Biological function

Calcium carbonates (calcite, vaterite, aragonite, amorphous)

CaCO3, (Mg,Ca)CO3, CaCO•nH2O

Many marine organisms, Aves, Plants, Animals

Shell, test, eye lens, crab cuticle, eggshells, leaves, Human inner ear

Exoskeleton, optical, mechanical strength, protection, gravity receptor, Ca store

Calcium phospates (hydroxyapatite, dahllite, octacalcium phosphate)

Ca10(PO4)6(OH)2, Ca5(PO4,CO3)3(OH) Ca8H2(PO4)6,

Vertebrates, Mammals, Fish, Bivalves

Bone, teeth, scales, gizzard plates, gills, mitochondria

Endoskeleton, ion store, protection, precursor

Calcium oxylates (whewellite, wheddellite)

CaC2O4•H2O, CaC2O4•2H2O

Plants, fungi, mammals

Leaves, hyphae, renal stones

Protection /deterrent, Ca storage/ removal, pathological

Iron oxides (magnetite, lepidocrocite, ferrihydrite)

Fe3O4, α-FeOOH, γ-FeOOH, 5Fe2O3•9H2O

Bacteria, chitons, tuna/ salmon, mammals

Intracellular, teeth, head, filaments, ferritin protein

Magnetotaxis, magnetic orientation, mechanical strength, iron storage

sulfates (gypsum, celestite, barite)

CaSO4•2H2O, SrSO4, BaSO4

Jellyfish, Acantharia, loxodes, chara

Statoconia, Statoliths

gravity receptor, skeleton gravity device/receptor

Halides (flourite, hieratite)

CaF2 Mollusc, Crustacean

Gizzard plate, statocyst

Crushing gravity perception

Sulfides (pyrite, sphalerite, wurtzite, galena, greigite)

FeS2, ZnS, PbS, Fe3S4

Thiopneutes

Cell wall Sulfate reduction/ion removal

Silicon oxides (silica)

SiO2•nH2O Diatoms, radiolaria, plants

Cell wall, cellular leaves

Exoskeleton, skeleton protection


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Calcified biominerals

The calcium carbonate (CaCO3) biominerals of invertebrates, such as mollusc

shells, sea urchin spines and coccoliths of unicellar algae, have been particularly well

studied due to their accessibility and high degree of crystallographic control (Fig.1.3).

Figure 1.3: a) Calcium carbonate mollusc shell b) calcite spines of the sea

urchin c) Ascidian dogbone spicule composed of a core of amorphous

calcium carbonate d) & e) Calcite coccoliths f) Trumpet shaped calcite in

Discospaeratubifera.


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The calcium phosphate biominerals of the vertebrates (Fig.1.4), such as bones

and teeth, have also been extensively investigated because of their remarkable

structure and mechanical properties. Seventy percent of bone is made up of the

inorganic mineral hydroxyl apatite. Carbonated-calcium deficient hydroxyl apatite is

the main mineral component of dental enamel and dentin. Hydroxyl apatite crystals

are also found in gritty material of Pineal gland of human brain known as corpora

arenacea or 'brain sand'. Calcium oxalate biominerals can be found in plants, but the

majority of studies have focused on the pathological form of calcium oxalate that is

found in kidney stones, which often occurs in conjunction with calcium phosphate

deposits [38-44].

Figure 1.4: a) & b) “Fibrous” biominerals found in the enamel of rat teeth

polycrystalline “rods” or “prisms” of hydroxyapatite (Reproduced from 83

and 86).

Non calcified biominerals

There are a variety of non-calcific biominerals as well, such as the bio silica

found in diatoms (Fig.1.5), sponge spicules, and some plants. The biogenic silica is

synthesized intracellularly by the polymerization of silicic acid monomers. This

material is then extruded to the cell exterior and added to the wall. Diatom cell walls


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are also called frustules or tests, and their two valves typically overlap one over the

other like the two halves of a Petri dish.

Figure 1.5: SEM images of structure of a diatom cell wall consist of

biosilica a) Biddulphia reticulata. b) Diploneis species c) Eupodiscus

radiatus d) Melosira varians (source: http://dx.doi.org/10.1371

/journal.pbio.0020306)

Iron oxides (magnetite) are found in chiton and limpet teeth, as well as

magnetotactic bacteria (Fig.1.6). Magnetite is a ferromagnetic mineral with chemical

formula Fe3O4, one of several iron oxides and a member of the spinal group. The

chemical name (IUPAC) is iron (II, III) oxide and the common chemical name

ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3,

which is one part wurtzite (FeO) and one part hematite (Fe2O3). This refers to the


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different oxidation states of the iron in one structure, not a solid solution. The most

unusual biomineral, copper hydroxide found in the teeth like jaws of blood worm [51].

Figure 1.6: a) & b) Magnetosome chains (nanoparticles of magnetite) in

Magnetotactic bacteria.

1.3 Physicochemical principles in biomineralisation

The common assumption in the biomineralisation field is that the delicate

biomineral morphologies are fabricated under the total control of specific

biomolecules. Hence, biomineralisation is genetically controlled process, which

transforms the genetically engineered organic scaffolds into hard matter. Some of the

biomineralisation mechanisms are more based on physicochemical principles like

nucleation inhibition rather than specific biopolymer structures and functions.

Although a protein may exhibit a complex structure, its actual function may be simple

like for example serving as a polyelectrolyte in a biomineralization process. It

therefore makes sense to investigate how far physicochemical principles play a role in


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biomineralization. An excellent example for this is the pattern formation in diatoms,

which can be explained by a phase separation model of amphiphilic polyamines [52].

Physicochemical principles can be transferred to biomimetic mineralization

experiments for the generation of advanced materials as summarized for the

polyamine [53]. It will be interesting to reveal further physicochemical mechanisms in

bio- and biomimetic mineralization like the minimization of interface energies by the

creation of an amorphous surface layer and growth inhibition by foreign additives/

impurities [54, 55]. These principles will lead to a deeper mechanistic understanding of

biomineralization processes and thus extend the tool box of biomimetic mineralization

by a transfer of biomineralization principles to the synthetic materials world.

Physicochemical principles also play a role in the formation of mesostructures

based on inorganic/organic assembly. Self assembled periodic silicates (known as

MCM-41) were found in 1992 as the first demonstration of a new strategy for

materials synthesis by liquid crystal templating [56]. Using lyotropic liquid crystalline

surfactants and phase separated block copolymers such as PEO based block

copolymers as the template to direct the assembly of an inorganic framework, a large

amount of materials composed of oxides, sulfides, phosphates, and other inorganic

compounds have been synthesized[57-60].

1.4 Morphosynthesis

Morphosynthesis strategies have been developed for the synthesis of

organized inorganic based micro and nanostructures. The synthesis of new

morphological forms on several length scales from nanometer to macroscopic scale is

currently of much interest. Macroscopic to nano scale hierarchical structures can be

autonomously built up from assemblies of smaller units formed, during the process of


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morphogenesis. Many imaginative routes are developed and reported for construction

of biomimetic materials with unusual morphological form (morphosynthesis).

1.4.1 Morphosynthesis of biomimetic form

The morphogenesis of biominerals with complex time dependent form is

dependent on the vectorial regulation of growth within shaped biological

compartments, such as vesicles and macromolecular frameworks. One major

challenge in biomineral- inspired materials chemistry is the synthetic reproduction of

analogous structures, using an approach that we shall refer to as morphosynthesis.

Here are some examples of how inorganic materials with complex morphologies can

be produced. Two strategies, based on physical and chemical patterning, are

illustrated in table 1.4. This study is inspiring the development of morphosynthetic

routes to inorganic materials with complex form.


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Table 1.4: Morphosynthesis of Inorganic minerals.

Strategy Approach Product System Materials

Physical

patterning

Supramolecular

templates

Mineral tubes

and helices

Lipid tubules Cu, Ni,

Al2O3, SiO2

Inorganic

sponges

Copolymer gels

Bacterial threads

Fe3O4, TiO2

SiO2,

Zeolites

Reaction field Hollow shells Emulsion

droplets

CaCO3, SiO2

Replication Cellular thin

films

Microemulsion

foams

CaCO3,

MnOOH,

FeOOH

Porous hollow

shells

Foams+latex

beads

CaCO3,

FeOOH

Chemical

patterning

Reaction field

instability

Microskeletal

frame works

Bicontinuousmic

roemulsions

Ca(OH)2(PO

4)6, SiO2

Twisted

/coiled

filaments

Reverse

microemulsions

BaSO4,

BaCrO4,

CaSO4

Nested

Filaments

Block copolymer

micelles

Calcium

phosphates


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1.4.2 Strategies for crystallization control

Crystallization control in biomimetic mineralization relies on nucleation and

crystal growth control by a thermodynamic or kinetic method with the aim to control

crystal properties such as size, shape, orientation, aggregation and texture.

1.5 Chemical control of biomineralisation

1.5.1 Nucleation

In most cases, biomineralisation is associated with the formation of a new

crystalline phase via nucleation followed by growth [61]. Nucleation plays an

important role as the first step of biomineralization, in building up a complex

structure. However, the nucleation process is hard to study since the nuclei are usually

unstable because of their high surface energy and grow soon after their formation.

There are only a few studies on this key process up to now. The classical theory of

nucleation considers the spontaneous formation of spherical molecular clusters with

size-dependent free energies that continue to grow only when larger than the threshold

value of the critical radius[62, 63]. The model predicts the size of the critical nucleus

and associated activation energy and nucleation rate, as well as their dependence on

super saturation, but is severely limited by a number of assumptions and

simplifications. In particular, the structure and composition of the nucleus are

considered to be the same as the bulk crystalline phase and the formation of

subcritical supramolecular clusters or complexes [64] and multi-step assembly of large

unit-cell structures are not addressed.


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The thermodynamic driving force for inorganic precipitation, super saturation,

is often offset by the kinetic constraints of nucleation. There are two types of

nucleation processes:

a) Homogenous nucleation that occurs due to the spontaneous formation of

nuclei in the bulk of the supersaturated solution.

b) Heterogeneous nucleation that occurs due to the formation of nuclei on the

surfaces of a substrate present in the aqueous medium.

The free energy of formation of a nucleus, ΔGN is given by the difference

between the surface (interfacial) and bulk energies:

ΔGN = ΔGI – ΔGB

The interfacial energy is always positive and dependent on the surface area,

whereas the bulk energy is negative and a function of volume. For the classical case

of a spherical nucleus.

ΔGI = 4 π r2 σ

Where σ is the interfacial free energy per unit surface area, and

ΔGB = 4 π r3 ΔGV /3VM

Where ΔGV represents the free energy per mole associated with the solid-

liquid phase change, and VM is the molar volume. The combination of the two

functions ΔGI and ΔGB, which equates to ΔGN, goes through a free energy maximum

(ΔGN *) at a critical cluster size (r*), as shown in the following Fig. 1.7.

Figure 1.7: Nucleation graph.

G GI

* GN

GB


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Heterogeneous nucleation occurs, at lower super saturation levels than those

required for homogeneous nucleation because the nuclei are stabilized by attachment

to the foreign surface, particularly if there is chemicals and structural

complementarity. The activation energy and rate of nucleation is determined by the

interfacial energy of the critical nucleus and the level of super saturation. These

factors can be biologically controlled in biomineralisation through the evolutionary

design of organic matrices and the membrane regulation of ion concentration

gradients.

1.5.2 Crystal Growth

When the super saturation level falls to the equilibrium, then the growth of the

crystal nuclei occurs. Crystal growth is dependent on the level of supersaturating and

occurs through surface-controlled processes. The rate of growth [65], JG, is given by

JG = k (SA) x

where k is the rate constant and SA is the absolute super saturation raised to the

power x. In principle, there are three models involved in the crystal growth and these

depend on the super saturation level. At moderate super saturation (x = 1), the crystals

grow by the classical layer-by-layer mode furnished by Stranskii [66] and Kossel [67].

This mechanism involves a surface of the crystal having active sites, called steps and

kinks. The kink sites have higher binding energy than the steps because they have

three “faces” in contact with the crystal surface (Fig.1.7). The kink sites are the most

favorable positions for the incorporation of the ions into the solid phase. This model

includes the following consecutive steps:


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1. Bulk diffusion of ions from solution to the crystal surface (stage A).

2. Surface adsorption and dehydration of ions on the crystal terraces (B).

3. Two-dimensional diffusion across the surface to the steps (C).

4. One dimensional diffusion along the step to the kink site (D).

5. Incorporation into the kink site (E).

At higher super saturation (x > 2), the growth process is governed generally by

the two dimensional growth mechanism (Fig 1.7). This model consists in multiple two

dimensional surface nuclei formed on the crystal surfaces that spread by further

incorporation of ions into the kink site.

At lower super saturation (x = 2), the predominant growth mechanism can be

described bytheScrew-dislocation model proposed by Frank [68].The growth is induced

by crystals with lattice defects, which are sites for further crystal growth (Fig 1.8).

Figure 1.8: (A) Layer-by-layer mechanism of crystal growth. (The scheme is

partly based on that in [119]) (B) Two-dimensional mechanism. (Reproduced

from [119]) (C) Screw dislocation mechanism.


26

1.5.3 Crystal growth

Thermodynamic crystal growth

The shape of inorganic crystals is normally related to the intrinsic unit cell

structure and the crystal shape is often the outside embodiment of the unit-cell

replication and amplification. According to the thermodynamic view point, the crystal

morphology depends on the differences of the crystal faces in surface energy and

external growth environment [69]. The equilibrium morphology of a crystal is defined

by the minimum energy resulting from the sum of the products of the surface area of

all exposed faces and the relative surface energies (Wulff rule). The fast growing

faces have high surface energies and they will vanish in the final morphology and vice

versa. Although this purely thermodynamic treatment cannot always predict the

crystal morphology, as crystallization and the resulting morphology often also rely on

kinetic effects as well as on defect structures, it is the basis for the explanation of

additive mediated crystal morphology changes in biomimetic mineralization. The

surface energy of a crystal face can be lowered by the adsorption of an additive.

Therefore, the shape of crystals can be affected by various factors, i.e., inorganic ions

or organic additives. The anisotropic growth of the particles can be explained by the

specific adsorption of ions or organic additives to particular faces, therefore inhibiting

the growth of these faces by lowering their surface energy. Only the additives that can

adsorb on special crystal faces can change the surface energies of the different crystal

faces and change the crystallization process and the final morphologies.

Small molecular additives such as small inorganic ions, small molecular

organic species, and solvents have been found to exert strong influences on the shape

of inorganic crystals when they selectively adsorb on special faces of the crystals [70].


27

However, primary nanocrystal building blocks are usually not temporarily stable

enough for further structure development by the action of small molecular additives

through electrostatic stabilization, since the Debye length is usually in the range of the

attractive van der Waals forces [71]. At the same time stabilization by polyelectrolytes

is very effective for electrostatic stabilization such as in the iron oxyhydroxidesystem

[72], which often induces non-selective adsorption at all faces. Block copolymers with

a polyelectrolyte block-short enough for selective adsorption but long enough for

sufficient interaction with the crystal surface-and a stabilizing block for steric

stabilization are more effective additives in the crystallization process. Closer to real

biosystems, insoluble and soluble biomacromolecules are another kind of effective

additives in crystallization control. De Oliveira and Laursen have shown an excellent

example of using protein secondary structures to control the orientation of chemical

functionality so that the protein can bind to a targeted crystal face [73].

Otherwise, templating is another strategy of biomimetic mineralization.

Biomineralization occurs within specific subunit compartments or

microenvironments, which implies stimulation of crystal production at certain

functional sites and inhibition or prevention of the process at all other sites. In

bioinspired biomineralization, there are hard templates which control the external

environment of crystals and soft templates which work as kinetically stable templates

such as micro emulsions, micelles, and other aggregates formed in solution and

normally have special interactions with minerals or crystals. Soft-template controlled

crystallization is an important part of biomimetic mineralization by organized soft

structures, as can be seen from the amount of publications on this topic [74-77]


28

Kinetic crystal growth

In general, kinetic crystallization control is based predominantly on the

modification of the activation-energy barriers of nucleation, growth, and phase

transformation (Fig. 2). In such cases, crystallization often proceeds by a sequential

process involving structural and compositional modifications of amorphous

precursors and crystalline intermediates through a kinetic effective pathway, rather

than a single-step thermodynamic pathway [78, 79]. How far these phase

transformations proceed along a series of increasingly stable intermediates depends on

the solubility of the minerals and on the free energies of activation of their inter-

conversions, all of which are strongly influenced by additives. The corresponding

changes in composition and structure usually occur by dissolution–recrystallization

processes closely associated with the surface and/or the interior of preformed

particles. Kinetically driven crystallization often involves an initial amorphous phase

that may be non- stoichiometric, hydrated, and susceptible to rapid phase

transformation. Amorphous calcium carbonate (ACC) for instance is highly soluble

and rapidly transforms to calcite, vaterite, or aragonite unless kinetically stabilized. In

biomineralization, this stabilization is achieved by ions such as Mg2+ and PO43-, or by

enclosing the amorphous phase in an impermeable sheath of organic macromolecules,

such as polysaccharides [80] or mixtures of polysaccharides and proteins rich in

glutamic acid, threonine, and serine [81, 82].

Kinetic control of crystallization can be achieved by modifying the

interactions of nuclei and developing crystals with solid surfaces and soluble

molecules [83]. Such processes influence the structure and composition of the nuclei,

particle size, texture, habit and aggregation, and stability of intermediate phases. In

biomineralization, for example, structured organic surfaces are considered to play a


29

key role in organic matrix-mediated deposition by lowering the activation energy of

nucleation of specific crystal faces and polymorphs through interfacial recognition.

Soluble macromolecules and organic anions, as well as inorganic ions can also have

an important kinetic effect on crystallization, particularly with regard to polymorph

selectivity and habit modification. These interactions can be highly specific, for

example, proteins extracted either from calcite- or aragonite-containing layers of the

abalone shell (Fig.1.9) induce the crystallization of calcite or aragonite, respectively,

when added to supersaturated solutions of calcium hydrogen carbonate in the

laboratory [84, 85]. Although there is much experimental evidence to support these

molecular-based mechanisms, the studies usually concern macroscopic crystals with

well-established faces and edges, so information about the early stages of growth at

the colloidal level is not generally available.

Figure 1.9: Abalone shell


30

1.6 General pathways of the Crystallization process

As shown in Figure , whether a system follows a one step route to the final

mineral phase (pathway A) or proceeds via sequential precipitation (pathway B),

depends on the activation energy barriers of nucleation (N), growth (g) and

transformation (i).

The most important factor in controlling the crystallization pathway is the

structure of the critical nucleus. When the nucleus involves strong interaction between

the ions, then the pathway A should be considered. This is in agreement with

macroscopic thermodynamics [86], stating that the phase that is formed first is the one

having the lowest free energy. When the nucleus involves weak interactions between

the ions, the amorphous phase is precipitated first, followed by a polymorphic series,

consistent with Ostwald* step rule [87] and pathway B in Figure 1.10.

Figure 1.10: Pathways to crystallization and polymorph selectivity.

A) Direct. B) Sequential.


31

The changes in composition are mediated by phase transformations. The

important sequence of multiphase pathway is proposing a central role for organic

polymeric matrices for controlling the structure of biominerals by primary nucleation

(Fig. 1.11).

Amorphous CaCO3 → Vaterite → Aragonite → Calcite

↑

Accelerators

Figure 1.11: Regulation of polymorph selectivity.

1.7 Mineralization of metal carbonates

The carbonate crystals implicated are calcium, strontium and barium

carbonate. The most familiar biomineral found in biological systems is CaCO3 in a

number of different forms. In nature, SrCO3, and BaCO3 are less abundant and occur

only in the aragonite phase, namely strontianite and witherite. Earlier studies reported

also a high-temperature, cubic and rhombohedral polymorphs of Strontium and

Barium carbonates that are reminiscent of calcite. However, little is known about the

high temperature polymorphs of Strontium and Barium carbonates because they are

not quenchable.

1.7.1 Calcium Carbonate Mineralization

The Ions Involved: Calcium Ion, Ca2+

When life originated on earth, calcium (the name derived from the Latin word

calx meaning limestone) was abundant in the igneous rocks [88], present in the earth’s

hot crust, and was unavailable for use by living matter [89]. As the earth cooled,

various chemical and biological reactions appearedand, thus, calcium became the


32

chemical basis of many compounds essential for life. The biogeochemistry of calcium

[90] is shown in (Figure1.12).

Figure 1.12: Biogeochemistry of calcium. The precipitation of calcium

carbonate and phosphate are the major inorganic constituents of skeletal

structures.

Calcium is an earth-alkaline element with the atomic number 20 and a radius

of 0.99 Å. The story of calcium began in 1808, when Humphry Davy isolated this

element from alkaline earth [91]. Later on, Sydney Ringer first demonstrated the

biologicalsignificance of calcium; for example: its role in egg fertilization [92] and

development of tissues [93] (bones, teeth and shells). Further, calcium has been found

to be involved in the conduction of nerve impulse to muscle[94], in the plant growth

[109] and to maintain the cytoskeleton architecture of all cells [95]. Calcium forms part

also of biogeochemical compounds that include carbonates (calcite, aragonite and

vaterite), sulphates (gypsum), phosphates (apatite) and silicates.


33

The wide-range of the role of calcium lies in the chemistry of this element [96]

(molecular structure, irregular geometry, valence state, binding strength, ionization

potential and kinetic parameters in biological reactions). Its chemical proprieties are

similar with barium and strontium which have been found to be able to substitute the

requirement of Ca2+ ions, for example in regulating enzyme activity [97, 98]

Carbonate Ion, CO32-

The carbonate ion is a polyatomic anion consisting of one central carbon atom

surrounded by three identical oxygen atoms in a trigonal planar arrangement with a

O−C−O bond angle of 120°. It is formed by dissolving carbon dioxide in water.

According to Henry’s law, carbon dioxide (CO2 (g)) dissolves in water and

further reacts with water forming carbonic acid [99], H2CO3. Carbonic acid is an

instable intermediary of the reaction first isolated by Loerting et al [100]. We note that

only a certain amount of the dissolved CO2 (aq) exists as H2CO3.

CO2(g) ↔ CO2 (aq)

CO2(aq) + H2O ↔ H2CO3

In aqueous solutions, carbonic acid is in equilibrium with hydrated carbon

dioxide [101], H2CO3 (conventionally, both are treated together as they were one

substance), and dissociates in two steps:

H2CO3*+ H2O ↔ H3O + HCO3

- pK1 (25 C) = 6.35

HCO3-+ H2O ↔ H3O + CO3

2- pK2 (25 C) = 10.33

The relative concentrations of H2CO3 and the deprotonated forms, HCO3-

(bicarbonate) and CO32− (carbonate), depend on the pH (Fig. 1.13) [102].


34

Figure 1.13: Distributions of the carbonate species in relation to the pH of the

solution. H2CO3represents the sum of dissolved CO2&H2CO3, and

predominates at low pH range. HCO3- is the most abundant species at

intermediate pH values; CO32−dominates at high pH.

1.7.2 Classical Picture of Crystal Formation

It is observed that the formation of crystals proceeds in two steps either

consecutive or simultaneous steps, i.e., formation of nuclei (nucleation) and crystal

growth [103]. These two steps give up the classical picture of crystallization (Fig.1.14).


35

Figure 1.14: A Concept of the crystallization process.


36

1.7.3 Crystal nucleation & Growth

Calcium carbonate crystal formation

The calcium carbonate forms when the positively charged calcium ion attaches

to the negatively charged oxygen atoms of the carbonate ion. The onset of the CaCO3

crystals in solution is determined by a critical factor called the solubility product

(Ksp), which indicates the level of super saturation of a solution. When the solubility

product is less than the activity product (Kap) of a solution then the precipitation

occurs until Ksp = Kap (Fig. 1.13).

Ca2+ + CO32- CaCO3 Ksp = [Ca2+] [CO3

2-]

Figure 1.15: Schematically shows a general precipitation mechanism proposed

by Nielsen [118].

1.7.4 Polymorphs of Calcium Carbonate

Polymorphism

Polymorph selectivity can be controlled chemically by kinetic effects

involving additives or through transformation processes which proceed along a series

of structures with decreasing solubility and increasing thermodynamic stability. The

structure of the critical nucleus is an important factor in controlling the crystallization

pathway.

Proteins extracted from calcite contains prismatic layers or the aragonite

nacreous layers of sea shells induce the crystallization of calcite or aragonite,

respectively, when added to supersaturated solutions of calcium bicarbonate in the


37

laboratory. Both minerals are made of CaCO3 but clearly the selection of different

unit-cell structures indicates fundamental changes in the crystallization process.

From the perspective of chemical control, the selection of a particular

polymorph can arise by kinetic effects that influence the nucleation and growth

pathways. Polymorph selectivity arises from a transformation process which starts

with an amorphous mineral and proceeds through a series of structures with the

similar composition but increasing thermodynamic stability.

Polymorphism is the ability of a crystalline system to exist in more than one

crystal structure, and it has been recognized for centuries. Polymorphism has great

technological importance due to the dependence of material behavior such as hardness

or optical properties on the solid-state structure. Polymorphism is important in many

fields like agrochemicals, pigments, dyestuffs, foods, and explosives and is especially

relevant for pharmaceutical compounds because dissolution rates depend on the

polymorph and patents are filed for a particular polymorph [104]. Calcium carbonate is

one of the most widely studied biominerals, because of both its high abundance and

rich polymorphism, and it has been widely used as a model mineral in biomimetic

experiments, leading to the understanding of the mechanisms of biogenic control over

mineral polymorph, orientation, and morphology.

Calcium carbonate (CaCO3) is one of the most abundant minerals in nature,

exists mainly as three anhydrate polymorphs and three hydrated polymorphs(Fig.

1.16), They are amorphous calcium carbonate (ACC), calcium carbonate

hexahydrate(CaCO3 6H2O) and calcium carbonate monohydrate(CaCO3 H2O) and

vaterite, aragonite and calcite, respectively, with the increasing order of

thermodynamic stability[105]. Calcite and aragonite, which have similar


38

thermodynamic stabilities under standard conditions, are both common in biological

and geological samples (Table1.5). Vaterite is meta-stable with respect to calcite and

aragonite, and is extremely rare in nature, being detected as a minor component of

only a few biomineralised structures, and not in geological samples. The application

of calcium carbonate particles to industry is mainly determined by the polymorphs of

CaCO3. Calcium carbonate has three crystal polymorphs: rhombic calcite, needlelike

aragonite and spherical vaterite. Calcite is usually the dominant polymorph at a high

solution pH, low temperature and the most stable phase at room temperature under

atmospheric conditions. While vaterite and aragonite are mostly produced at a low

solution pH, high temperature, they transform to stable calcite. Synthetic conditions of

calcite, aragonite and vaterite are different. Vaterite, which is the most unstable

crystal morphology, is obtained by controlling the CaCO3 solution composition.

Calcite is abundant in nature and well utilized in industry due to regular crystal size

and smooth surface.

Industrial application of CaCO3 is strictly related to its properties such as

chemical purity, specific surface area, particle size and morphology. For instance the

calcite particles are extensively used in metallurgical and cement industries for their

excellent brightness, corrosion resistance and thermo- and chemical-stability; the

aragonite particles are used as fillers of biomedical materials and novel composites [6].

Spherical hollow, cubic and linear CaCO3 particles are used as fillers in paper-,

plastic-and rubber-making [107]. Therefore, controllable synthesis and investigations

on the properties of CaCO3 in the presence of additives such as polysaccharides [108–

113], poly- peptide [114,115], proteins [116-119] and surfactants [120–122] have attracted much

interest .Mukkamala etal[123] showed, the influence of PABA and Nicotinic acid on

CaCO3 crystallization. It was found that PABA and Nicotinic acid inhibits growth of


39

CaCO3 by adsorption on the newly formed crystal surfaces and edges, which results in

flower shaped vaterite and hexagonal rods are identified.

Eq. (1) reaction is a gas-liquid one, its contact method affects the granularity

and particle shape of CaCO3 [124, 125]. Also, this reaction has a high degree of

flexibility and does not produce soluble byproduct.

Eq. (2) is mainly used for large scale production. The gas-liquid reaction has

little effect on CaCO3 morphology because it makes water as byproduct. Since gas

reactant flows over the liquidsurface,

Eq. (3) is difficult to realize in an industrial process.

Ca(OH)2(aq)+CO2(aq, gas)→CaCO3(solid)+H2O(byproduct)-------(1)

CaO(aq)+CO2(gas)→CaCO3(solid) --------------------------(2)

CaCl2 (aq) +Na2CO3 (aq) →CaCO3 (sol) +2NaCl (byproduct) ------- (3)

On the other hand, the liquid-liquid reaction for CaCO3 crystallization makes

both CaCO3crystals and NaCl byproduct as Eq. (3) [126-130]. Nucleation and growth of

crystal are affected by NaCl because the byproduct has high ionic strength. However,

this reaction is frequently used for CaCO3 crystallization since the reaction easily

controls saturation concentration in solution and produces various morphologies of

CaCO3 crystalline. It is also easy to make biominerals from CaCO3 crystallization

reaction by a liquid-liquid reaction with organic materials. Previous reports showed

that the polymorph of calcium carbonate is dependent on the operating conditions of

crystallization, such as super saturation [131], solution composition [132], pH [133],

temperature [134], and presence of additives [135-142]. The effect of additives,

concentration and pH has been studied on the crystallization of calcium carbonate.


40

Biomineralization produced biominerals with good mechanical property by

adding amino acid to CaCO3 [143,144].

Double hydrophilic block copolymers were reported recently in the literature

but only under the aspect of micellization upon increase of hydrophobicity of one

block with pH or temperaturechanges. Examples are poly(methy1 vinyl ether)- block-

poly(viny1 alcohol) [145], poly(2-vinylpyridine)-blockpoly( ethylene oxide) [146],

poly(methy1 vinyl ether)-blockpoly( methyltriethy1ene glycol vinyl ether) [147], or

poly(2-dimethylaminoethylmethacrylate)-block-poly(2-iethylaminoethylmethacrylate)

[148].

Henderson et al[149]. indicated that the presence of the carboxylic acids

inhibited crystal growth and calcium carbonate solubility increased. No effect was

observed with respect to enhanced calcium carbonate solubility as a function of

carboxylic acid chain length. In Tong’s paper, the amino acid kinds on the interface of

organic template and calcium carbonate have been searched to be rich in aspartic acid

and glutamic acid. Thus, it can be assumed that the carboxyl groups of aspartic acid

and glutamic acid provide abundant Ca2+ ions bonding sites [150]. Calcium carbonate

crystals were precipitated in glycine-containing aqueous solutions using two methods:

CO32− dropping method or diffusion method. By modulating the dropping velocity

and glycine concentration, they can control the morphologies and proportion of the

acquired vaterite particles [151].


41

Figure 1.16: Schematic representation of crystal morphologies.


42

Control of mineral morphologies of Calcium carbonate

The high degree of control over form and function observed in biominerals is

a source of wonder and inspiration to biologists and material scientists. Biological

systems control the growth and development of mineral morphologies to generate

biominerals with complex shapes and structures. On the basis of these ideas, a rapidly

developing research field has evolved that is generally termed bioinspired or

biomimetic materials chemistry [152,153].

Bioinspired morpho synthesis provides an important and environmental

friendly route to generate materials with controlled morphologies. These tools include

the application of templates and confined reaction environments; spatially controlled

mineral deposition; the presence of soluble additives as crystallization modifiers,

inhibitors, or nucleation agents. During the past decade, exploration and application of

these bio-inspired strategies have enabled complex materials with specific sizes,

shapes, orientation, compositions, and structural organizations to be fabricated [154,155].

Additional interest in these strategies also comes from their potential application to

the bottom-up synthesis of advanced materials, where biominerals can be used as a

source of inspiration for the design and fabrication of future materials.

Mineralization of calcium carbonatein nature [156] occurs in the presence of

organic molecules, which have significant effects on the polymorph and habit of the

crystals produced. Many studies that were carried out on the mechanisms involved in

biomineralization processes and several new biologically inspired synthetic routes

were designed to control the formation of the mineral phase. It has been shown that

polymorphism, morphology, and structural properties of CaCO3 can be controlled by


43

the use of specific additives, macromolecules, small organic molecules, and inorganic

ions.

The possible role of the macromolecules occluded within calcium carbonate

biominerals in controlling mineral morphologies has been studied in some detail,

showing that interaction of these soluble molecules with growing calcite crystals can

result in specific morphological changes, characterized by the display of new, well

defined faces [157-160]. This behavior has been interpreted using two main approaches:

adsorption of additives to particular crystal faces [161], and adsorption to step edges

and terraces [162, 163]. In the former mechanism, macromolecules are preferentially

absorbed onto crystal faces where a structural match exists between the functional

groups on the macromolecules and the atomic arrangement on the crystal planes,

causing the additive to interact with and stabilize these faces. Following adsorption to

specific crystal faces, the macromolecules become overgrown and are occluded

within the crystals, as demonstrated by calcite crystals grown in the presence of

fluorescence labeled sea urchin macromolecules [164]. Crystallization in the presence

of polyelectrolyte without Mg2+ resulted in the formation of aggregates of spherical

crystal morphology with diameter 20 μm[165].

Low molecular weight organic additives such as carboxylic acids can be highly

selective of the crystal face [166]. Carboxylic acids are a natural choice for CaCO3

because of the similarity of the carboxyl groups on the additive and carbonate groups

in the mineral. Carboxylates and the corresponding acids can, be therefore, selectively

adsorbed onto CaCO3 faces, causing a change in morphology. A good example is that

of α-ω-dicarboxylic acids, which adsorb to the faces of calcite if the carboxyl groups

are ionized. This inhibits the growth of these faces, leading to the formation of

elongated calcite crystals [167].


44

1.8 Objective

The major objective of the present work involves the development of novel

strategies to prepare three biomimetic systems, namely calcium carbonate (CaCO3)

Barium carbonate (BaCO3) and strontium carbonate (SrCO3) that are widespread

minerals throughout nature, occurring as the main mineral constituents of

sedimentary rocks and as inorganic components in the skeletons and tissues of many

organisms. The influence of various ligands on metal ions for the nucleation of

CaCO3, BaCO3 & SrCO3 microstructures has been studied in detail. Other critical

factors that influence the crystal growth are pH and metal/ligand mole ratio in the

synthesis process.

Carbonates are some of the most abundant minerals in nature and, as such,

have been of considerable interest in geo- and biosciences, as well as in materials

research. Recently, carbonates have been commonly used in many industrial

applications, and there is need for development of carbonate materials of novel

morphological, physical, and chemical properties by simple synthetic procedures in

a cost-effective manner. The control of the crystal formation and development of

different morphologies and physicochemical properties of carbonate precipitates has

been developed to a remarkable degree by using organics during the precipitation

process [168-174].

One of the most abundant biominerals, calcium carbonate (CaCO3), has

attracted much interest due to its applications in paint, plastic, rubber or paper. Its

morphologies and crystal phases including calcite, aragonite, and vaterite have been

focused on and affected by organic templates or additives. Surfactants are used

mostly as template to control the synthesis of crystals with various morphologies,

polymorphs and phases due to their influences on one or several crystallization steps


45

(nucleation, crystal growth, aggregation) [175, 176]. In aqueous solution, surfactants

with different hydrophilic groups resulted in various morphologies and crystal

phases of CaCO3 [177]. Studies have demonstrated that a promising approach is to use

organic additives and/or templates to control nucleation, growth, and alignment in

the synthesis of inorganic materials [178-181].This strategy has led to the formation of

a variety of shapes of CaCO3 in recent years [182-186].It is noteworthy that this process

is strongly coupled with the rearrangement of organic molecules adsorbed on the

surface of CaCO3 that play an important role in the growth of CaCO3 crystals [187-

189].

Barium carbonate (BaCO3) as a common mineral has some important

applications in industry for producing barium salts, pigment, optical glass, ceramic,

electric condensers, and barium ferrite [190].It is also used as a precursor for

producing superconductor and ceramic materials [191]. Previous studies have shown

that the specific surface area, size, morphology, and so on, can affect the

performance of BaCO3[192]. Therefore, controlled synthesis of BaCO3 crystals has

attracted much interest. Different organic additives or templates have been

intensively used for controlled growth of carbonate minerals [193], such as Langmuir

films [194], self assembled monolayers [195], various soluble additives like synthetic

peptides [196], dendrimers [197], and common polymers [198], various kinds of BaCO3

crystals with morphologies, such as candy-like, needle-like, or olivary-like, have

been prepared by using a double-jet feed semibatch technique or adding different

crystal growth modifiers [199].Sondi et al. [200] obtained spherical and rod structures

of BaCO3 crystals using urease enzyme-catalyzed reaction.


46

Figure 1.17: Structure of Barite

Strontium carbonate (SrCO3), which is widely used as starting material for

preparing a variety of strontium oxide compounds [201, 202].has two traditional main

applications: as an additive in the production of glass for color television tubes and

as a constituent of ferrite magnets [203, 204]. Various kinds of SrCO3 crystals with

morphologies such as spheroidal, needle-like, rod like or ribbon like have been

prepared with the aid of urease enzyme-catalyzed reaction[205], on centered

rectangular self-assembled monolayer substrates [206], within thermally evaporated

sodium bis-2-ethylhexylsulfosuccinate thin films [207],using the Fungus Fusarium

oxysporum [208], or in a simple cationic micro emulsion system under solvothermal

conditions [209].

It is assumed, we assume that the morphologies of metal carbonate crystals

are affected by the metal/ligand ratios and pH. The analytical aims of the present

study are thrrefore study the morphological change of metal carbonates with high

resolution scanning electron microscope (HR-SEM), X-ray diffraction spectrometer


47

(XRD) and Fourier transform infrared spectroscope (FT-IR), as well as to discuss

the mechanism. HR-SEM was used to analyze morphology and crystal size. XRD

was used to measure peak intensities and the presence of metal carbonate

polymorph. Two kinds of crystals were confirmed by FT-IR spectrum. Crystal

morphology change with reaction time was identified with measured peak areas of

XRD pattern and FT-IR data. The work would provide good insight into the metal

carbonate crystallization in the presence of biomacromolecules.


48

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