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.
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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].
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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]
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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
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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
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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.
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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
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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.
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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].
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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).
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Figure 1.14: A Concept of the crystallization process.
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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
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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
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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
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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.
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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].
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Figure 1.16: Schematic representation of crystal morphologies.
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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
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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].
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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
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(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.
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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
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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.
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48
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