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Additive controlled crystallizationRui-Qi Songa and Helmut C€olfen*b

DOI: 10.1039/c0ce00419g

Additives play a decisive role in crystallization processes. We survey the ongoing efforts in therealm of additive-controlled crystallization. Emphasis is placed on the commonly used andmost studied types of additives and their influences on prenucleation, stabilization ofamorphous precursors, nucleation, crystallization, chiral resolution, nanocrystal assembly,polymorph, and thus morphologies. We will highlight three types of nonclassical trajectoriesadopted by additive controlled crystallization, including oriented attachment, mesocrystals,and biomorphs.

1. Introduction

Control of crystallization events by addi-

tives is as old as application or research on

crystallization itself in the chemical,

pharmaceutical, and food industries.1 A

large number of scientific and industrial

crystallization processes are controlled by

additives but often in an empirical

manner. Biominerals on the other hand

demonstrate that nature is a real master of

the additive-controlled crystallization.2--5

Biomineralization is generally a highly

additive controlled process. There are two

principal types of additives, which are

used for the control of crystallization

events. The first one, associated with the

so-called insoluble or structural matrix, is

composed of water insoluble molecules

like chitin or collagen. The organisms

utilize organic components to construct

scaffolds for the subsequent crystalliza-

tion reactions. Crystallization reactions

are then controlled by soluble additives—

the so-called soluble or functional matrix.

Combination of these two additive types

in a synergistic way, leads to the exquisite

aDepartment of Materials Science &Engineering, Cornell University, Ithaca, NewYork, 14853-1501, USA. E-mail: [email protected] of Konstanz, Physical Chemistry,Universit€atsstr. 10, D-78457, Konstanz,Germany. E-mail: [email protected]

This journal is ª The Royal Society of Chemistry

control, which is generally found in bio-

mineralization events. There is therefore

much to learn from these controlled

reactions. But biomineralization has

proven to be very difficult to investigate in

terms of the precise crystallization control

mechanisms. There are multiple reasons.

The most important ones are that crys-

tallization needs to be observed in a living

system and that usually different additives

are applied at different times or even

multiple additives at the same time.

Moreover, recent research on bio-

mineralization has led to the development

of alternative concepts beyond the clas-

sical textbook knowledge on crystalliza-

tion. These concepts are oriented

attachment,6--8 mesocrystals,9--13 amor-

phous,14--16 or liquid precursors.17,18 While

already significant evidence was found for

the role of amorphous precursors in bio-

mineralization,16,19 the evidence for mes-

ocrystals is less preponderant,11,20 and

that for oriented attachment is rare.21 For

liquid precursors, so far no direct

evidence could be found in a bio-

mineralization system, although biomi-

metic experiments give convincing

evidence in vitro that this mechanism

could be used by living organisms to build

up biominerals like bone.22

Nevertheless, all the above mentioned

nonclassical crystallization pathways

have already been advantageously used

for crystallization control of synthetic

crystals. They are usually additive-

2011

controlled and allow for enhanced possi-

bilities of crystallization control.

Together with additive controlled clas-

sical crystallization events, which can also

lead to size, shape and polymorph

control, the amount of literature on

additive controlled crystallization is

unmanageable. Fig. 1 roughly represents

the current state of knowledge of the roles

of additives in controlled crystallization.

We will extract the main concepts of

additive controlled crystallization here,

and on top of that discuss the possibilities

of the various existing approaches.

2. Crystallization control byinsoluble additives

Crystallization control by insoluble

additives is a templating approach by

which the insoluble additive serves as

a template to control either the nucleation

or polymorphs of a nucleated crystal. In

addition, they can promote crystallization

by heterogeneous nucleation which has,

lower activation energy barriers than

homogeneous nucleation. A number of

different templating possibilities were re-

ported including Langmuir mono-

layers,23--27 self-assembled monolayers,28--32

latexes,33,34 colloidal crystals,35,36 and

other insoluble scaffolds like sea urchins

spine replicas37 and viruses.38 These

insoluble templates have advantages and

disadvantages with various characteris-

tics.

CrystEngComm, 2011, 13, 1249--1276 | 1249

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Fig. 1 An overview of the roles of soluble and insoluble additives in crystallization control.

2.1. Langmuir monolayers

Langmuir monolayers can be used to

control the polymorph and the nucleation

face of a crystal. Their common advan-

tage is that they can be compressed and

therefore, the distance and packing

between the functional surfactant groups

can be varied.39--41 The possibility of

compression makes Langmuir mono-

layers an ideal candidate to find out the

functional group distances to nucleate

a desired polymorph or crystal face. A

large number of different monolayers and

crystal systems was investigated.42

Indeed, the control of nucleation faces or

polymorphs was found possible. Mann

and co-workers proposed that the lattice

match between polar headgroups of fatty

acids and crystal planes of CaCO3 is

a critical factor in controlling the crystal

orientation.43 They also found that the

1250 | CrystEngComm, 2011, 13, 1249--1276

degree of compression of a stearic acid

monolayer influences the homogeneity of

vaterite nucleation. The stereochemical

and electrostatic matching was proposed

as the possible reason.44

Like the {001} aragonite nucleation in

nacre templated by an organic matrix,

epitaxial crystallization is a paradigm in

biomineralization. Langmuir monolayers

were also used extensively to elucidate the

basic mechanisms of crystal nucleation in

biomineralization, especially those rele-

vant to epitaxial crystallization. However,

the epitaxial match between monolayers

and nucleated crystal faces was found to

be a negligible factor in the study of

CaCO3 crystallization on macrocyclic

monolayers.23,24,26,27 Volkmer and

coworkers found that the charge density

of templating monolayers is much more

important than the epitaxial match. In

their case, changing the functional group

This journ

distances by a variation of the monolayer

compression enables the polymorph

selection of CaCO3 switching from calcite

to aragonite or vaterite.

For CaCO3 crystallization on stearic

acid monolayers, recent findings show

that the templated crystal is formed by

pre-nucleation clusters,45,46 which

undergo aggregation and form amor-

phous CaCO3 rather than an ‘‘ion by ion’’

growth mechanism (Fig. 2).46 This

precursor phase attaches to the mono-

layer. Then a crystal forms inside the

amorphous phase in contact with the

monolayer. Only when the crystal orien-

tation is favourable to the monolayer can

the crystal grow. In other cases it will

disintegrate again. Although Langmuir

monolayers have been successfully used

for the control of crystallization events,

Langmuir monolayers cannot be consid-

ered as rigid templates and the precise

al is ª The Royal Society of Chemistry 2011

Fig. 2 Schematic pathway of the mineralization of an organic matrix. Step 0: formation of pre-

nucleation clusters. Step 1: aggregation of the clusters to form 30 nm ACC nanoparticles. Step 2:

clustering and growth of the ACC particles at the surface of the organic matrix. Step 3: start of the

crystallization; formation of poorly crystalline particles. Step 4: formation of nanocrystalline

domains inside the amorphous particle. Step 5: prevalent growth of the crystalline domain stabilized

by the template. Step 6: formation and growth of oriented single crystals.46 Copyright 2009,

American Association for the Advancement of Science.

mechanism of crystallization control on

Langmuir monolayers is not a simple

epitaxial crystallization. There is

a complex interplay between the growing

crystal and the Langmuir monolayer it-

self.46

2.2. Self-assembled monolayers

(SAMs)

In contrast to Langmuir monolayers, self-

assembled monolayers (SAMs) do not

compress since the surfactant molecules

are fixed on a surface. For SAMs, the

functional group of the surfactant is of

importance as well as the substrate on

which the surfactants are attached. A

number of SAMs were obtained from al-

kanethiols on gold and silver surfaces

immobilized on silicon substrates.32 By

functionalization of the thiols with

different terminal groups, --COO�, --OH,

--SO3�, and --PO3

2� for example, SAMs

can be used for a precise control over the

oriented nucleation of a series of specific

crystal faces of calcite.28--31 The length of

the surfactant alkane chains is also of

importance for crystallization control, as

demonstrated by Han and Aizenberg for

the occurrence of the odd--even effect.47

Another great advantage of SAMs over

Langmuir monolayers is the possibility of

patterning them for example by poly-

dimethylsiloxane (PDMS) stamping. Ai-

zenberg et al. presented a localized

deposition of CaCO3 with predefined

orientation by using this technique.48


They also designed a commensurate

strategy enabling the use of a square array

of posts sandwiched between two

substrates to create a calcite single crystal

thin film from ACC precursors. These

experiments suggest the importance of

porosity in the growth of a single crystal

with large size. Like all large biological

single crystals, large single crystals must

be porous in order to release the

mechanical stress and allow water to be

expelled during the transformation from

ACC to calcite single crystals. The

strategy (Fig. 3A and B) can ensure not

only the stabilization of amorphous

precursors in a well-defined environment

but also nucleation in a controlled

manner.

SAMs can also be used as a model for

the insoluble biomineralization matrix

and have been therefore combined with

soluble molecules49,50 or gels51 to investi-

gate the combined action of these two

types of additives in biomimetic crystal-

lization of CaCO3. Indeed, SAMs control

the crystallization by a flexible interplay

with an amorphous CaCO3 (ACC)

precursor phase rather than acting as an

epitaxial template. As found by De

Yoreo and coworkers, 3- and 4-mercap-

tophenol monolayers are highly orga-

nized before the deposition of ACC.

ACC deposition causes SAMs to lose the

order. However, SAMs can turn to

ordered again by the mutual templating

effect between SAMs and crystallizing

ACC moieties.52

2011

2.3. Rigid solid body templates

Inspired by the 2D single crystal growth,48

rigid bodies were used as templates to

shape the final crystal. Colloidal crystals

or latexes are especially advantageous

since they are easy to obtain. A number of

crystalline inverse opals have been

synthesized via a templating approach

with a colloidal crystal. For example,

polystyrene colloidal crystals can be in-

filtrated with amorphous calcium phos-

phate (ACP)36 or ACC35 which

subsequently crystallizes in the interstices

of the colloidal crystals thus forming

a macroporous crystalline replica of the

colloidal crystal (Fig. 3C). This is specifi-

cally of interest for using inverse opals as

photonic crystals, since the refractive

index of inorganic materials is very vari-

able. Also, latexes can be used as

templates to create size adjustable

porosity in single crystals like ZnO34 or

CaCO3.39,53 It turned out that the latex

surface must be sufficiently compatible

with the crystal to get incorporated

successfully.33 More recently, Qi and

coworkers reported the controlled growth

of ZnO nanopillars by using a zinc foil-

assisted monolayer colloidal crystal

(MCC).54 They demonstrated that in-

verted MCC and connected MCC can

define the growth sites and spaces of ZnO

nanopillar arrays with a weak defect-

related emission at room temperature.

Conversion of biotemplates into

biomimetic minerals is a practical

approach to generate diversified

morphologies based on naturally grown

structures. Sea urchin skeletal elements

provide mechanical support, but they also

play a dominant role in calcite morpho-

genesis and nutrition transportation.55

Work by Meldrum and Ludwigs demon-

strated that an echinoid skeletal plate can

be used for the production of a crystalline

replica.37 Crystallization within the

hydrophobic polymeric replica of a sea

urchin skeletal plate has been shown to

yield a single crystal with a triple periodic

minimal surface (TPMS) structure. A

calcite single crystal replica of the original

spine of several tens of micrometers was

obtained by this method (see Fig. 4).56

Making the polymeric replica surface

hydrophilic or increasing the supersatu-

ration led to the formation of poly-

crystalline replicas due to several

simultaneous nucleation events. Similar

CrystEngComm, 2011, 13, 1249--1276 | 1251

Fig. 3 Schematic diagrams: (A and B) 2D templating growth of calcite single crystal film.48

Copyright 2003, American Association for the Advancement of Science. (C) Fabrication of 3D

ordered macroporous calcite single crystals by using poly(styrene-methyl methacrylate-acrylic acid)

spheres assembled colloidal crystals as templates.35 Copyright 2008, Wiley-VCH.

Fig. 4 Templated single crystal of calcite

precipitated in a sponge like polymer

membrane from 0.02 M reagents.56 Copyright

2002, Wiley.

replicas were fabricated with a number of

different inorganic materials. Some of

these materials are single crystals

including SrSO4, PbSO4, and Cu-

SO4$5H2O.53 These results indicate that

1252 | CrystEngComm, 2011, 13, 1249--1276

almost any 3D shape of a crystal is

accessible by the templating approach,

which is otherwise impossible to create

solely using a soluble additive-based

approach.

This journ

2.4. Viruses and hollow capsules

Viruses or other hollow capsules like

ferritin have been used for the precipita-

tion of a number of crystals inside the

hollow capsules.38,57--59 Viruses or ferritin

are quite rigid and therefore allow

controlled crystallization in the interior.60

Douglas and Young used empty (nucleic

acid-free) cowpea chlorotic mottle virus

(CCMV) capsids for the encapsulated

crystallization of spatially constrained

nanoparticles of polyoxometalate salts.61

The positive charges on the interior

interface direct encapsulation and

promote inorganic crystallization reac-

tions. Since viral capsids of different sizes

and shapes are available, the size and

shape of the crystals are indirectly

controllable, even though the capsid

interior does not act as a mould for the

1 : 1 reproduction. Knez et al. reported

the synthesis of metal nanowires using the

interior cavity of the rod-shaped tobacco

mosaic virus as the constrained environ-

ment.62 These systems are more inter-

esting from the viewpoint of

nanoreactors, which can be used for the

crystallization of various nanoparticles.

For example, through protein design and

genetic engineering, the charge on the

interior surface of the CCMV capsid can

be changed, from positive to negative.

The highly anionic capsid interior inter-

face provides an effective stabilization for

the surface nucleation of transition metal

oxides (Fe2O3, Fe3O4, and Co2O3).63

Fig. 5A--C show the schematic of the

encapsulation of Fe2O3 nanoparticles

within the capsid. The hard--soft interface

of the material can be viewed from the

spatially resolved elemental image in

Fig. 5D. A similar direction is the appli-

cation of vesicles as crystallization


Fig. 5 (A) Cryo-electron micrograph reconstruction of CCMV. (B) Cut-away view of the CCMV

cage showing the hollow interior cavity. (C) Schematic of a ‘‘guest’’ material encapsulated within the

cage. (D) Spatially resolved spectral imaging by high-angle annular dark field scanning transmission

electron microscopy of genetically modified CCMV with Fe2O3 synthesized within the cage [blue, N

(from the protein); yellow, Fe (from the Fe2O3)], indicating the spatial relationship between the hard

inorganic guest material (Fe2O3) and the soft viral protein cage.63 Copyright 2006, American

Association for the Advancement of Science.

Fig. 7 A calcite single crystal with gyroid

morphology after removal of the PS template.69

Copyright 2009, Wiley-VCH.

templates.64 Vesicles have been applied

for the synthesis of magnetite in magne-

tosomes of magnetotactic bacteria.65

However, vesicles are more deformable

than viral or ferritin capsids. Their shape

might change during the mineralization

process.

In addition to the interior interface of

the viral capsid architecture, the exterior

and even the interface between protein

subunits making up the capsid can also be

used as templates for crystallization of

inorganic materials. Belcher and

coworkers reported the 1D assembly of

crystalline nanoparticles using a geneti-

cally modified M13 bacteriophage virus

scaffold.66 The incorporation of nucle-

ating peptides into the virus coat structure

provides a viable template for the

assembly of ZnS, CdS, CoPt, and FePt

particles. The blocking effects of the

nucleating peptides prohibit particles

attached to the virus from fusing.

Removal of the viral template by anneal-

ing promotes the oriented aggregation-

based crystal growth, forming single-

crystal ZnS and CdS nanowires.

Fig. 6 Hydroxyapatite whiskers grown in an

aggregate of hydrophobically modified poly-

ethylenoxide-block-polymethacrylic acid block

copolymer.67 Copyright 1998, Wiley-VCH.

2.5. Block copolymer self-organized

templates

Deformation of an organic self-organized

template upon mineralization can lead to

complex structures if a synergetic struc-

turation of crystallizing and organic

mesophase can be reached, which can lead

to a feedback loop. Such structuration

could be achieved for hydroxyapatite

(HAP) synthesized in the aggregate of

a hydrophobically modified double

hydrophilic block copolymer.67 Delicate

neuron like structures (Fig. 6) formed by

adsorption of the polymethacrylic acid


block onto all HAP faces parallel to the c-

axis. This blocking effect inhibits these

surfaces from further growth and only

allows for c-axis growth of the HAP fila-

ments. On the other hand, the growth of

HAP filaments deforms the polymeric

aggregate, which in turn influences the

growth of HAP crystals in a feedback

loop. This synergetic structure formation

process shows the advantage of feedback

loops in the synthesis of complex non-

equilibrium structures.

It is increasingly obvious that ACC is

extremely effective in penetrating and

molding the formation of single crystal-

line calcium carbonate to templates.

While biotemplates demonstrate a high

effectiveness in templating single crystal

morphologies, this technique is limited

due to the requirement of using the native

biotemplates as the starting materials.

Additionally, the size and structure

geometry of the produced crystals are

rigorously restricted to those of the orig-

inal biotemplates. For example, pore sizes

in all echinoderms are invariably 10--15

2011

mm. As a well-known analogue of bio-

logical self-assembly, block copolymers

(BCPs), on the other hand, self-assemble

into periodic spherical, cylindrical, gy-

roid, or lamellar arrays of nanostructures

over macroscopic distances.68 They

present a commensurate scaffold for

deposition and crystallization of ACC

nanoparticles. Steiner and Meldrum et al.

recently reported calcite single crystals

with gyroid morphology (Fig. 7). They

prepared a film with the gyroid

morphology using a polystyrene-b-poly-

isoprene (PS-b-PI) block copolymer.69 PI

was then selectively removed by exposure

to UV and washing in ethanol. To carry

out crystallization of calcium carbonate,

the copolymer film was immersed in

a methanol/water solution of calcium

chloride and exposed to ammonium

carbonate vapor. After crystallization,

the PS template was removed by heating

at 385 �C under oxygen for 2 hours.

2.6. Gel scaffold grown single crystals

In common with extracellular gel scaf-

folds found in nacreous layers,70,71

mollusk shell prisms72,73 and fish otholith

matrix,74 the porous network structure of

gels places a highly complex restriction on

the crystallization reaction environment,

including material diffusion suppression,

nucleation retardation, and stress allevi-

ation.75 These benefits have motivated

a substantial research in the crystalliza-

tion community to realize the formation

of single crystals. For example, a group of

composites containing calcite single crys-

tals have been grown using gels as scaf-

folds. Qi and coworkers successfully

prepared eight-arm and star-like calcite

single crystals in an agarose gel.76 The

CrystEngComm, 2011, 13, 1249--1276 | 1253

Fig. 8 Tomographic reconstruction of an

agarose network inside of a calcite single

crystal section.78 The binding box of the 3D

reconstructions measures 1453 nm by 975 nm

by 220 nm. Copyright 2010, American Asso-

ciation for the Advancement of Science.

formation of calcite single crystals was

suggested as an evident consequence of

the gel-restricted ion diffusion.

C�arcamo and coworkers grew calcite

and KH2PO4 single crystals in industrial

sodium silicate.77 They found the

morphology of calcite defined by a rhom-

bohedron. The agarose gel-grown calcite

single crystals reported more recently by

Estroff and coworkers are also rhombo-

hedral in shape.78 After a 24-hour-growth

in the bulk gel by the gas-diffusion

method, calcite crystals were separated

from the gel. Electron tomography

reveals a continuous network of agarose

gel, which is randomly incorporated in

calcite single crystals (Fig. 8). The result is

consistent with what Gavira and Garcia-

Ruiz observed for agarose fibers distrib-

uted randomly in protein crystals that

were grown in agarose gels.79 Two types

of commercial agaroses were investigated

by Li and Estroff to investigate the influ-

ence of gel fracture strength on its incor-

poration. The gel incorporation mainly

relies on the gel fracture strength and

reaction rate.80

Fig. 9 Scanning electron microscopy (SEM) images of rac-glutamic acid crystals: (A) crystal

morphology of rac-glutamic acid crystallized from solution; (B) crystal morphology of rac-glutamic

acid crystallized onto a chiral D-cysteine surface (plate like); (C) crystal morphology of rac-glutamic

acid crystallized onto a chiral (�)-L-cysteine surface (rectangular). Scale bar ¼ 200 mm (A), 50 mm

(B), and 20 mm (C).81 Copyright 2009, Royal Society of Chemistry.

2.7. Enantioselective crystallization

on nanostructured chiral surfaces

Nanostructured chiral solid surfaces can

be used for chiral resolution.81 Dressler

and Mastai studied the enantioselective

crystallization of racemic and also

1254 | CrystEngComm, 2011, 13, 1249--1276

conglomerate crystals of amino acids on

chiral self-assembled nanofilms of

cysteine.82,83 They used nanosize chiral

surfaces of cysteine for the chiral recog-

nition and crystal morphology modifica-

tion of glutamic acid. The nanochiral

surface was fabricated by the SAM

assembly technique. X-Ray diffraction

(XRD) demonstrates that enantiomers of

glutamic acid with identical chirality to

that of the cysteine surface grow in an

unchanged manner on the surfaces. For

example, while a preferential growth of

the D-enantiomer along the [020] direction

was evidenced by XRD patterns of D-

glutamic acid crystallized on an L-cysteine

surface, L-glutamic acid does not show

preferential orientation when crystallizing

on the L-cysteine surfaces. Thus, enan-

tiomers with opposite chirality to that of

the chiral surface grow in a particular

direction on the surface. Morphology

effects were also observed for the crys-

tallization of rac-glutamic acid crystals

grown on nanochiral surfaces of cysteine

as shown in Fig. 9.

Based on enantioselective crystalliza-

tion on chiral polymeric microspheres,

Mastai and coworkers presented a new

approach to chiral resolution.84 Chiral

microspheres were prepared from poly(N-

vinyl a-L-phenylalanine) (PV-L-Phe)

using PS microsphere templates by

a single-step swelling process. The crys-

tallization of DL-valine was selected as

a model system to study the chiral

discrimination ability of these chiral

microspheres for chiral racemic crystalli-

zation. XRD and differential scanning

calorimetry indicate the occurrence of an

enantioselective crystallization on the

chiral microspheres, with an enantiomeric

excess of ca. 25%.

This journ

3. Control by soluble additivesbefore crystallization

In addition to insoluble additives which

act as templates, soluble additives can

influence crystallization reactions to

a large extent in terms of morphology,

size, and polymorph of the crystal. In bi-

omineralization processes, this is the so-

called soluble or functional matrix,

mainly consisting of soluble macromole-

cules. Additives usually perform multiple

roles in a crystallization process, which

start with the complexation of ions,

generate the local ion enrichment, and

decrease the supersaturation of solutions.

Since crystallization at least of common

biominerals involves stable prenucleation

clusters,45 these are the next species after

the ions, which can interact with addi-

tives. Subsequently, nucleation can be

influenced by additives with coded

control over crystal size, morphology,

and polymorph. After nucleation, the

crystallization path diversifies into amor-

phous or crystalline species, stable nano-

particles, nanoparticle aggregates or

particle growth, depending on if the

crystallization reaction follows the clas-

sical or the nonclassical crystallization

pathway.85 For example, anisotropic

growth can be gained by either face

selective adsorption of soluble additives

or nanoparticle aggregation with coded

influence of additives over shape control.

Furthermore, additives can stabilize

mesocrystal intermediates against Ost-

wald ripening in nonclassical crystalliza-

tion processes and can lead to mechanical

reinforcement and toughness increase.

This is actually what was observed in

biominerals.86 At least nine different

roles of additives were identified in


Fig. 11 Schematic illustration of how hetero-

geneous nucleation depends on the contact

angle between a flat substrate, nucleus, and the

fluid.91 Copyright 2003, American Chemical

Society.

crystallization reactions.87 Usually several

of them can be found in a crystallization

reaction. However, the quantification of

additive interactions in terms of charac-

teristic numerical parameters, which

would allow to generate property profiles

and predictions about the additive

controlled crystallization process, is still

a matter of ongoing research. We there-

fore only focus on so far known roles of

soluble additives in homogeneous nucle-

ation, heterogeneous nucleation, nucle-

ation promotion, retardation, and

inhibition, pre-nucleation, as well as the

formation of polymer induced liquid

precursors.

Fig. 10 Structural evolution of nuclei modeled by colloidal particles: (A) the initial structure of

nuclei is liquid-like. (b) As the nucleus grows, its core first becomes ordered and the exterior layer

remains liquid-like. (c) The nucleus becomes completely ordered after the size exceeds a critical

value.90 Copyright 2009, Wiley-VCH.

3.1. Classical homogeneous

nucleation

Crystal nucleation in solution has been

described in terms of two distinct steps for

a spherical nucleus. The first step involves

the aggregation of the dissolved molecules

in the supersaturated solution into orga-

nized nuclei, thus developing a surface

that separates them from the growth

environment. The free enthalpy change

associated with the formation of new

surfaces, which is positive, is proportional

to the squared radius r of the nucleus. On

the other hand, the free enthalpy change

arising from the molecular aggregation

within the nucleus is negative and

proportional to the volume of the nucleus

and so to r3. Therefore, the formation of

nuclei is a dynamic process dominated by

these two terms. For a small radius r,

where the positive surface energy term

predominates, the nucleus is unstable and

disintegrates. However, once the nucleus

has grown over a critical size, the bulk

energy term dominates and the total free

enthalpy change begins to become nega-

tive, resulting in the continuous growth of

nuclei.88,89

However, this classical nucleation

model based on the energetic counterplay

between the surface energy and bulk

energy of the nucleus fails quantitatively

and qualitatively in various situations.

For example, the critical nucleus differs

drastically from the eventual nucleation

phase in composition and structure.

Attempts have been made to develop the

nucleation theory by computational

approaches, such as molecular dynamics.

These approaches glean information on

molecular crystalline assembly and thus


provide new guidelines for crystal nucle-

ation studies with a combination of

experimental approaches. Zhang and Liu

reported the first experimental observa-

tion of the structural transition of nucle-

ating clusters at the initial stage. The

precondition of their observation is based

on the assumption that the phase

behavior of colloidal suspensions and that

of atomic and molecular systems is

similar.90 They built a setup with PS

colloidal particle suspension sealed

between two indium tin oxide-coated

conducting glass plates separated by

insulating spacers. The attractive force

between PS colloidal particles can be

enhanced by increasing the amplitude or

decreasing the frequency of the alter-

nating electric field. The results of their

experiments suggested that the initial

structure of the crystal nuclei is supersat-

uration-dependent. At high degrees of

supersaturation, classical nucleation

theory is plausible in describing the

dynamic behavior of nucleation. At low

degrees of supersaturation, the crystal

nuclei tend to nucleate with a metastable

liquid-like structure. Subsequently the

liquid-like structure evolves to the stable

and crystalline-ordered structure

(Fig. 10). Such a gradual route signifi-

cantly facilitates the nucleation dynamics

with a lower nucleation barrier.

3.2. Heterogeneous nucleation

A nucleus undergoes further growth

towards the formation of a crystal above

its critical size. During this process, the

presence of a substrate or template can

exert influence on nucleation. In addition

to the nucleus--liquid interfacial free

energy, the nucleus--substrate and the

2011

liquid--substrate free energies should be

considered.91 Since the surface energy of

a nucleus on a surface is in most cases

lower than that in solution due to the

lower nucleus--liquid interfacial free

energy, heterogeneous nucleation is

usually energetically favored. The inter-

action between these three phases

(Fig. 11) will lead to a stabilized poly-

morph or a specific crystal plane parallel

to the template surface. This is eloquently

expressed in the well known seeding

experiments in crystallization.

In terms of templating nucleation,

a crystalline nucleus was suggested to

evolve from an amorphous phase by

aggregation.10 The suggestion is consis-

tent with Zhang and Liu’s observation

on the evolution of crystalline structures

inside the amorphous phase assembly of

polystyrene spheres in an electric field.92

In the process, templated crystal nucle-

ation occurs via merging and ordering

of a few small crystalline nuclei in

a large amorphous cluster. Also, Som-

merdijk et al. observed aggregation of

prenucleation clusters, before their

attachment to a surface as shown in

Fig. 2.46

CrystEngComm, 2011, 13, 1249--1276 | 1255

Fig. 12 AFM phase images of model substrates during in situ nucleation rate measurements

(captured at pH: 5.0, s: 2.14, and T: 25 �C). Prominent silica particles are highlighted with circles. (a)

COOH-terminated surface with silica nuclei at early and late experimental stages. Surface striations

and submicrometre pits and islands are features of the underlying Au (111) surface. (b) NH3+-

terminated surface displaying no evidence of silica deposition nearly 2 h after nuclei were first

observed on the carboxylated surfaces. (c) NH3+/COO� surface displaying a greater density of silica

nuclei than measured on COOH-terminated surfaces after the same amount of time as in panel (a).93

Copyright 2009, American Chemical Society.

Wallace et al. set out to look for

evidence to address the contribution of

a substrate on the deposition of natural

biosilica.93 In this study, they developed

an atomic force microscopy (AFM)-

based in situ experimental approach to

compare the influence of amine-,

carboxyl-, and hybrid NH3+/COO�-

terminated SAMs on the kinetics of silica

nucleation. They found that carboxyl and

hybrid NH3+/COO� substrates are active

for silica deposition, whereas amine-

terminated surfaces do not promote silica

nucleation (Fig. 12). The rate of silica

nucleation is 18� faster on the hybrid

substrates than on carboxylated surfaces,

even though free energy barriers towards

cluster formation on both surface types

are similar. These findings suggest that

surface nucleation rates are more sensitive

to kinetic parameters than previously

believed and that cooperative interactions

between surfaces terminated with oppo-

site charges play an important role in di-

recting the onset of silica nucleation.

Fig. 13 (a) Slice through an emerging nucleus for a single-particle additive with a low affinity for the

solute and an effective size greater than that of a solute molecule. (b) Slice through the solute

aggregate for the single particle additive with a high affinity for the solute and an effective size

greater than that of a solute molecule. (c) Snapshot of the solute aggregate for a weakly amphiphilic

dimer additive: the additive molecules tend to be oriented parallel to the surface. (d) Slice of the

emerging nucleus for an amphiphilic dimer additive: the additive molecules are mostly oriented

perpendicular to the surface.94 Copyright 2009, Wiley-VCH.

3.3. Nucleation promotion,

retardation, and inhibition by

additives

An appropriate additive can enhance,

retard, or inhibit crystal nucleation, and

therefore assist in the selective crystalli-

zation of a polymorphic form or enable

a desired crystal habit to form. Based on

molecular simulations, Anwar and

coworkers studied the mode of action for

additives that influence crystal nucle-

ation.94 The key factors that determine

1256 | CrystEngComm, 2011, 13, 1249--1276

the ability of an additive to modulate

crystal nucleation are the strength of its

interaction with the solute, its disruptive

ability (which may be based on steric,

entropic or energetic effects), interfacial

properties, and the degree of self-associ-

ation. If additive--additive interactions

are too strong, their interactions with

solute molecules and thus impact on

solute nucleation will minimize. If the

additive has a low affinity for the solute,

This journ

the additive particles are excluded from

the interior of the solute cluster and only

retard nucleation at best. For additives

with a high affinity for the solute, the

solute molecules tend to structure around

them, conflicting with the emerging solute

lattice and hence causing nucleation

inhibition (Fig. 13a and b). Large addi-

tives cause complete inhibition, whereas

small additives are only able to retard

nucleation as they become incorporated

into the solute lattice. In contrast to the

solute-philic additive, the amphiphilic

additive tends to reside at the solute/

solvent interfacial areas. Whereas the

weakly amphiphilic dimers mostly align

parallel to the interface (Fig. 13c),

amphiphilic dimers generally orient

perpendicular to the interface (Fig. 13d).

For the former, both inhibition and

promotion are possible. When the second

particle of the dimer additive is larger

than a solute particle, the inhibition will

be observed. When it is smaller than

a solute particle, a rapid nucleation event

will be observed. For the latter, however,

only promotion or at best retardation can

be observed. Anwar et al.’s finding94 is

helpful for the design of new additives for

the inhibition or promotion of nucleation

in specific systems.


Fig. 14 High-resolution cryo-TEM image of

prenucleation clusters in a fresh 9 mM

Ca(HCO3)2 solution.46 Copyright 2009,

American Association for the Advancement of

Science.

Fig. 16 CaCO3 mineralized in a PHEMA gel

via a PILP precursor shows a bicontinuous

structure very similar to the outer core region

of the original sea urchin spine, with 3D in-

terconnected pores similar to those of the

original sea urchin spine.103 Copyright 2006,

American Chemical Society.

3.4. Prenucleation clusters

As already discussed by Gosner et al. in

1965, clusters might occur preferentially

via ion aggregation to an amorphous

phase. Navrotsky et al. also predicted the

formation of small clusters of ions in the

nucleation of amorphous mineral phases.

Recent advances revealed the formation

of stable prenucleation clusters in ther-

modynamic equilibrium with the ions in

solution for crystallization of CaCO3.45

Sommerdijk et al. demonstrated the size

of CaCO3 prenucleation clusters of about

2 nm in diameter (Fig. 14).46 These clus-

ters further aggregate to form amorphous

and homogeneous nanoparticles that

nucleated in solution, which develop to

larger sizes that allow the nucleation of

crystalline domains when attaching to

a template (see also Fig. 2). Charged

polymeric additives can exert multiple

influences on the fate of prenucleation

clusters, such as delaying the onset of

nucleation to form an amorphous

phase.87 The role of prenucleation clusters

in crystallization events is still largely

unexplored and it is not yet clear, how

general this precursor species is in crys-

tallization reactions.

Fig. 15 Schematic illustration depicting a PILP process. (A) As a critical concentration is reached

during the infusion of the carbonate species, isotropic droplets (2--5 mm in diameter) phase-separate

from the solution and accumulate on the substrate. (B) The droplets coalesce to form a continuous

isotropic film. Some late-forming droplets may be partially solidified, or crystalline, and do not fully

merge with the film. (C) Patches within the isotropic film become birefringent as crystal patches

nucleate and spread across the precursor film. The transformation sometimes progresses in an

incremental fashion, where prominent transition bars form from diffusion-limited exclusion of the

polymeric impurity. The linear bars delineate sectors within single-crystalline calcite tablets but are

concentric within spherulitic films that transform in the radial direction. (D) The tablets continue to

transform as the crystals grow laterally to form a continuous film. The transformed film is about half

a micrometre thick and composed of single-crystalline patches of calcite, or spherulitic patches of

vaterite, which range from tens to hundreds of microns in diameter.18 Copyright 2000, Elsevier

Science B.V.

3.5. Polymer induced liquid

precursor (PILP) formation

Gower and coworkers proposed that the

polymer-induced-liquid-precursor (PILP)

process (Fig. 15) may play a fundamental

role in biomineralization.17,18,95,96 In

a PILP process, the negatively charged

polymeric additive, like poly(aspartic

acid) or poly(acrylic acid), induces

a highly hydrated amorphous phase


separated in the crystallization solution of

mineral systems like CaCO3, while

simultaneously delaying crystal nucle-

ation. The metastable precursor phase

usually coalesces into amorphous films

settling on the substrate. A pure liquid

phase can only be observed when large

quantities of the phase accumulate at the

interface of an air bubble. The observa-

tion of partially coalesced particles at the

micrometre size scale in the systems

suggests that the polymer and associated

hydration water impart some fluidic

character to the amorphous

precursor.17,18,96 This was explained as the

consequence of a kinetically dominated

process, where both the excess water and

polymer become excluded with time as the

carbonate species compete with the poly-

mer for its bound calcium.

Since first discovered for the polymer-

controlled crystallization of CaCO3, the

PILP process has been expanded to the

crystallization of other material systems,

like calcium phosphate, barium

carbonate, and strontium carbonate17 and

also to organic systems like amino acids97

or pigments.98 Due to their liquid nature,

PILPs are especially well suited for

2011

morphogenesis with templates as they can

easily adapt to any shape and can even

enter small cavities by capillary forces.

Depending on how the precursor droplets

are deposited, different nonequilibrium

morphologies can be formed as well, such

as nanofibers,99,100 helices,101 templated,102

and ‘‘molded’’ crystals.103

Amazingly, PILPs can even mineralize

collagen. They are able to enter the

CrystEngComm, 2011, 13, 1249--1276 | 1257

nanometre sized gap zones of collagen

resulting in a structure partly resembling

that of bone.104 The results suggest that

bone mineralization might also proceed

via a PILP precursor stage. Similar to

what is overviewed in Section 1, replicas

of complex structures such as that of a sea

urchin spine were presented by Cheng and

Gower using polyAsp as the additive

(Fig. 16).103 The intricate structure of

a calcite polycrystal demonstrates

remarkable possibilities of using PILP

phases as templates to achieve complex

morphologies.

4. Control by soluble additivesafter crystallization

4.1. Face selective adsorption

For crystal synthesis, a specific goal, both

fundamentally and technologically, is the

control of anisotropic crystal properties

and the synthesis of a crystal with

a specific and defined geometric shape,

which is predictable by modeling

approaches. In 1901, Wulff described the

dependence of the equilibrium

morphology of a crystal on its minimum

surface free energy.105 According to

Wulff’s rule, all crystals have a definite

geometric shape, dominated by faces with

low surface energies and slow growth

rate. The interface energy of a crystal

surface relies on the strength of surface

dangling bonds and on its interaction

with solvent.106 A crystal may form ionic

faces with charges, coordinatively binding

faces, electrically neutral but dipolar

faces, or highly polarizable faces as well as

hydrophobic faces. Hydrophilic or

hydrophobic faces may also form in one

and the same chemical crystal system.11 In

addition to the anisotropy of faces in

Fig. 17 Face selective adsorption of an addi-

tive (illustrated with spheres in blue) on crystal

atoms, molecules, or ions lowering the surface

energy of the red atom by partial saturation of

its dangling bonds (yellow arrows). A bulk

atom has all bonds satisfied (black arrows).

Fig. 18 Predicted morphologies based on

atomistic simulation of calcite surfaces in the

presence of various additives. (Left) {001}

Tabular, stabilized with Li+ and (right) pris-

matic rhomb {1�10}/{104}, stabilized with

HPO42�.117 Copyright 1993, Elsevier.

1258 | CrystEngComm, 2011, 13, 1249--1276

surface energy, their interaction with

solvent and additive can be quite

different. Additives can recognize the

surface bonds of some faces of a crystal

and the adsorption process results in

a partial saturation of the surface bonds.

It is therefore no surprise that the surface

energy of crystal faces can be reduced by

the adsorption of additives (as shown in

Fig. 17). As a result, crystals with defined

shape can be formed in a predictable and

selective way, if the additive adsorption is

face selective.

Although only valid for the limited case

of crystallization under equilibrium

conditions, Wulff’s rule provides guide-

lines for crystal morphogenesis studies

and is thus an incentive to glean infor-

mation on crystal anisotropic growth by

face selective additive adsorption.

Nowadays, the adsorption of dyes on

specific faces of crystals can be monitored

by optical microscopy.107--111 The solvent-

dependent reconstruction of high energy

surfaces can be manifested by scanning

force microscopy.112 The formation of

chiral surface textures with chiral addi-

tives can even be observed with AFM.113

Soluble additives used so far to control

the anisotropic growth of crystals can be

finely divided into simple ionic or low

mass additives, synthetic polymer addi-

tives, synthetic bio-macromolecules or

those extracted from biominerals, and

active adsorbing impurities from reaction

processes. Each type of additives presents

its own particular enchantment.

However, merely as a thermodynamic

equilibrium treatment, Wulff’s rule

cannot explain the experimentally formed

crystal morphologies in many cases due to

kinetic factors.

4.1.1. Ion substitution. Inspired by

the rich variety of CaCO3 morphologies

in nature, many attempts have been made

to look for chances of habit modification

in CaCO3 crystallization by the addition

of simple ionic additives. Carboxylic

acids114,115 as well as inorganic ions116,117

were used to generate habit modification.

Many different types of CaCO3

morphologies were obtained by these

additives. Considerable results were

observed for CaCO3 crystal shape when

additives, such as Mg2+, Li+, and HPO42�,

were used.116,117 Calcite {001} faces are

unstable faces due to their highly charged

surface. Simulations revealed that the

This journ

equilibrium shape of a calcite crystal can

change from rhombohedral to hexagonal

platelet by lowering the surface energy of

calcite {001} faces.118 One way of altering

the surface energy of {001} faces is to

incorporate additives. Li+, for example,

can substitute Ca2+ ions and make

unstable {001} faces become the most

stable ones after incorporation, while all

neutral crystal faces become destabi-

lized.117 The substitution produces an

effective negative charge, which can be

compensated by the addition of Li+ in

interstitial sites or by incorporation into

the crystal lattice.117 Consequently, the

calcite crystal tends to take on the

hexagonal shape with {001} morpholog-

ically dominant (Fig. 18, left). For the

substitution of CO32� by HPO4

2�, the

surface energy decrease favours the {1�10}

faces, which are then expressed in the

crystal morphology, along with the {104}

faces (Fig. 18, right). The morphologies

are in agreement with experimental

results.117

4.1.2. Selective adsorption of poly-

mer additives. Recent progress shows

that double hydrophilic block copolymers

(DHBCs)119,120 are highly effective for

stabilization of specific planes of crystals.

Examples include Au,121 ZnO,122--124

calcium oxalate,119 PbCO3,125 BaCO3,126

CaCO3,127 and BaSO4.128 For a DHBC, its

short sticking block offers the advantages

of face selective adsorption, combined

with particle stabilization due to the

longer stabilizing block. As illustrated in

Fig. 19,129 in contrast to low molar mass

additives and homopolymers, DHBCs

combine the advantages of electrostatic

particle stabilization with those of steric

particle stabilization, due to an optimized

molecular design. The design of DHBC is

actually analogous to the structures of


Fig. 19 Face selective adsorption of ions or low molar mass additives (a), steric particle stabili-

zation by polymers (b) and face selective adsorption and particle stabilization by DHBC (c).129

Copyright 2004, Royal Society of Chemistry.

proteins involved in biomineralization,

such as statherin130 or Asp-rich

proteins,131 in which blocks of acidic

moieties interact with a crystal, and other

blocks provide additional functionality.119

The formation of Au triangular

nanoplates that used poly(ethylene

glycol)-b-poly(1,4,7,10,13,16-hexaazacyc-

looctadecane ethylene imine)

(PEG-b-hexacyclen) as a crystal modifier

is an elegant example for crystal

morphogenesis by face selective adsorp-

tion of a DHBC.121 A typical image in

Fig. 20a shows that the presence of this

polymer can lead to the production of very

thin and thus electron transparent trian-

gles, truncated triangular nanoprisms,

and hexagons of Au. The particles display

high crystallinity as confirmed by the

selected area electron diffraction pattern.

The selective adsorption of the functional

group of the PEG-b-hexacyclen polymer

occurs on the (111) face of Au. Molecular

modeling evidenced a good geometrical

match of the interacting nitrogens in the

Fig. 20 (a) TEM image and electron diffraction p

reduction of 10�4 M HAuCl4 solution in the presenc

of the Au (111) surface and a hexacyclen molecule in

molecule to the hexagonal atom arrangement on Au

Figure drawn to scale.121 Copyright 2004, American


hexacyclen part to the Au hexagons on the

(111) face, which effectively minimizes the

surface energy. The result backs the pref-

erential adsorption of PEG-b-hexacyclen

onto the (111) faces. As shown in Fig. 20b,

the distance of the neighboring --NH2

groups matches the distance between the

neighboring Au atoms within the (111)

face very well supporting the face selective

adsorption of the polymer.

Another remarkable example is the

formation of BaCO3 helices by the ‘‘pro-

grammed’’ self-assembly of elongated

orthorhombic BaCO3 units using

a racemic phosphonated DHBC (poly

(ethylene glycol)-b-[(2-[4-dihydroxy-

phosphoryl]-2-oxabutyl) acrylate ethyl

ester] (PEG-b-DHPOBAEE)).126 The

non-chiral polymer itself did not form

helices in the water solution, even in the

presence of barium ions. However, the

helix formation is a result of the interac-

tion between BaCO3 nanoblocks and the

polymer. Due to the selective adsorption

of the sterically demanding PEG-b-

attern of Au nanoparticles synthesized by self-

e of PEG-b-hexacyclen. (b) Molecular modeling

vacuum, which show an excellent match of this

(111). Yellow: Au; blue: N; gray: C; white: H.

Scientific Publishers.

2011

DHPOBAEE on the BaCO3 {110} faces,

BaCO3 fibers form with a diameter of

a few hundreds of nanometres and lengths

up to millimetres. The coded self-

assembly of helices from brick-like elon-

gated nanocrystals relies on a staggered

arrangement. The arrangement is

controlled by the aggregation direction of

the initial three nanocrystals. Once

a particle approaches an aggregate along

its perpendicular direction, which is pre-

sented with favorable and unfavorable

adsorption sites, a twist occurs along the

aggregate, leading to the formation of

a helical BaCO3 composite (Fig. 21).

4.1.3. Binding effects of biomole-

cules. Peptides and proteins are mono-

disperse in size. Their ability to form

defined secondary structures allows

a specific match between the orientation

of chemical functionality and the surface

structures of distinct crystal faces, thus

producing well-defined changes in crystal

morphologies. An excellent example of

using protein secondary structures to

control the orientation of chemical func-

tionality and thus protein binding to

a targeted crystal face was reported by

DeOliveira and Laursen.109 They skill-

fully designed an a-helical peptide (CBP1)

with an array of aspartyl residues for

binding onto the {1�10} prism faces of

calcite.109 The effect of CBP1 and other

peptides on calcite crystal growth was

investigated by adding the peptide to

rhombohedral seed crystals growing

from a saturated Ca(HCO3)2 solution.

When CBP1 was added to seed crystals

as shown in Fig. 22A, a continued

growth of calcite crystals with elongation

along the [001] direction (c-axis) with

rhombohedral {104} caps (Fig. 22B) was

observed. After washing the crystals with

water and replacing the mother solution

with a fresh saturated Ca(HCO3)2 solu-

tion, a regular rhombohedron shape

formed with a subsequent growth on the

putative prism surfaces (Fig. 22C). At 25�C, CBP1 is only about 40% helical. As

a result, studded crystals were formed

under these conditions by epitaxial

growth perpendicular to each of the six

rhombohedral surfaces (Fig. 22D and E).

After washing these crystals and re-

growing them in a fresh Ca(HCO3)2

solution, repair of the non-rhombohe-

dral surfaces was again observed

(Fig. 22F).

CrystEngComm, 2011, 13, 1249--1276 | 1259

Fig. 21 (Left) Primary nanocrystalline witherite building block in vacuum not representing the

observed face areas in solution but just illustrating the orientation of the relevant faces. (110) ¼green, (111) ¼ blue, (011) ¼ red and (020) ¼ pink. (Right) BaCO3 helical superstructures obtained

with the PEG-b-DHPOBAEE additive.126 Copyright 2005, Nature Publishing Group.

Fig. 22 Left) SEM micrographs showing the effect of CBP1 on the growth of calcite crystals. (A)

Calcite seed crystals showing typical rhombohedral morphology. (B) Elongated calcite crystals

formed from seed crystals in saturated Ca(HCO3)2 containing ca. 0.2 mM CBP1. (C) ‘‘Repair’’ and

re-expression of rhombohedral surfaces when crystals from (B) are allowed to grow in saturated

Ca(HCO3)2 after removal of CBP1 solution. (D and E) Respective earlier and later stages of growth

of calcite crystals from rhombohedral seed crystals at 25 �C in saturated Ca(HCO3)2 containing ca.

0.2 mM CBP1. (F) ‘‘Repair’’ and re-expression of rhombohedral surfaces when crystals from (E) are

allowed to grow in saturated Ca(HCO3)2 after removal of CBP1. (Right) The footprint of two a-

helical peptide (CBP1) molecules binding to the (1�10) prism faces of calcite. The filled circles are Ca2+

ions and open circles are CO32� ions. Large circles are ions in the plane of the surface and small

circles are 1.28 �A behind this plane. The hexagons indicate that peptide carboxylate ions occupy

CO32� sites on the corrugated surface.109 Copyright 1997, American Chemical Society.

Face selective adsorption of impurities

produced during reaction processes

Bulk Pt high-index planes, {210} and

{410} for example, exhibit higher cata-

lytic activity than that of the most

common stable planes, such as {111},

{100}, and {110}, due to a high density of

atomic steps, edges, and kinks, which can

serve as active sites for breaking chemical

bonds.132--135 Synthesis of noble metal

nanocrystals with high-index facets is

a popular pursuit to improve catalytic

activities. However, the preparation of

shape-controlled nanocrystals exhibiting

high-index facets is a challenge due to

1260 | CrystEngComm, 2011, 13, 1249--1276

their high surface energy. To address this

issue, dynamic oxygen adsorp-

tion--desorption mediated by a square-

wave potential is under consideration for

the generation and stabilization of high-

index planes, such as the {730} and {210}

facets. Wang and coworkers recently

presented tetrahexahedral (THH) Pt

nanocrystals prepared by an electro-

chemical method.136 The THH shape

exhibits facets of {730} planes and vicinal

planes such as {210} and {310} (Fig. 23).

It is interesting to explore why the

{730} or the {210} type of facets that

define the THH shape is stable during

This journ

growth. Ascorbic acid is excluded because

THH Pt nanocrystals can still be obtained

in ascorbic acid-free solution. In the

square-wave potential procedure, two

processes repeat periodically at

a frequency of 10 Hz. First, at 1.20 V, the

surface Pt atoms on the nanospheres can

be oxidized and partially dissolved to

form Pt ions. Then, these Pt ions diffuse

to the glass carbon surface and are

reduced to Pt atoms between �0.20 and

�0.10 V. At 1.20 V, the Pt surface is

oxidized and covered by oxygen species

originating from the dissociation of H2O

in solution. For the low-energy planes,

surface atoms have larger coordination

numbers, such as 9 for atoms on the (111)

plane, so oxygen atoms are relatively

difficult to adsorb at those surface sites.

However, for high-index planes, the

coordination numbers of surface atoms

are relatively low, only 6 for stepped

atoms on the {730} plane. The oxygen

atoms preferentially adsorb at these

stepped atoms without replacing them,

and ordered surfaces are preserved. Such

THH Pt nanocrystals show an enhanced

catalytic activity in electro-oxidation of

small organic fuels of formic acid and

ethanol, demonstrating a potential for use

in the traditional applications of Pt group

metal nanoparticles.

4.2. Chiral control

4.2.1. Asymmetric growth of non-

chiral crystals. Any material with a bulk

chiral structure can expose chiral enan-

tioselective surfaces. However, it is

possible to prepare achiral minerals that

will expose chiral surfaces in the presence

of chiral molecules. De Yoreo and Orme

et al. reported the chiral morphogenesis of

calcite through the interaction with chiral

amino acid molecules.113 Another achiral

crystal, CaSO4$xH2O, also displays

asymmetric growth and exposes chiral

crystal planes in the presence of chiral

organic compounds.137 Enantioselective

adsorption of additives onto an achiral

mineral that exposes chiral surfaces is

a possible explanation for the asymmetric

growth of non-chiral minerals.

4.2.2. Controlled crystallization of

chiral molecules. When a chiral molecule

crystallizes from solution, it can form

either racemic crystals, conglomerates of

separate left- or right-handed crystals of


Fig. 23 (A) TEM image of THH Pt nanocrystals recorded along the [001] direction. A careful

measurement of the angles between surfaces indicates that the profiles of the exposed surfaces are

{730} planes (a ¼ 133.6�, b ¼ 137.6�). The inset is a [001] projected model of the THH. (B) Cor-

responding SAED pattern with square symmetry, showing the single-crystal structure of the THH Pt

nanocrystals.136 Copyright 2007, American Association for the Advancement of Science.

Fig. 25 Schematic illustration of 1D self-

construction of nanostructures by oriented

attachment: (a) collision of nanocrystals, (b)

adjustment of orientation, (c) attachment and

fusion.

the pure enantiomers, or a racemic solid

solution in which the two enantiomers

coexist in a disordered manner. Statisti-

cally, only 5--10% of all racemates form

conglomerate crystals.138 Although

HPLC can separate enantiomers, large-

scale chiral separation still relies on crys-

tallization. Based on the stereoselective

adsorption of additives at the surface of

crystals, Addadi and coworkers realized

the kinetic resolution of racemic

conglomerates by the addition of enan-

tiospecific chiral inhibitors that prevent or

delay the growth of one of the enantio-

morphs.139 Zbaida et al. introduced the

use of chiral polymers to chiral resolu-

tion.140 C€olfen and Mastai et al. demon-

strated that the enantiomeric excess of

one enantiomer can be maximized by

using chiral DHBCs in the chiral control

of racemic crystal systems by the kinetic

control of crystallization.141 They found

that appropriate polymers can thermo-

dynamically slow down the formation of

the most stable racemic crystals as well as

the formation of one of the pure enan-

tiomeric crystals. In addition, the pres-

Fig. 24 Crystal morphology of (R,R)- and (S,S)-C

mL�1 PEG-b-PEI-(S)-ascorbic acid; crystal planes

CaT.141 Copyright 2002, Wiley-VCH.


ence of DHBCs results in the formation of

calcium tartrate tetrahydrate (CaT) with

unusual morphologies (Fig. 24).

5. Nonclassical crystallization

5.1. Oriented attachment

Literally, an Oswald ripening process

occurs when crystals coarsen from

a precipitate.142,143 It is a thermodynami-

cally driven spontaneous process.144

Small particles, with a higher collision

frequency, a greater mobility and a higher

surface to volume ratio, have a higher

surface energy than large particles. The

classical model assumes that the initial

formation of many small crystals is fol-

lowed by the growth of a few larger ones

via monomer attachment, at the expense

of the smaller particles. However, this

treatment overlooks a substantial

evidence that coalescence or even oriented

attachment can take effect predominantly

during crystal growth in cases far away

from thermodynamic equi-

librium.6,7,9,12,145--147 In contrast to the

aT formed in the presence of Co-labeled 10 mg

are marked. (left) (R,R)-CaT and (right) (S,S)-

2011

classical model, oriented attachment

provides an energetically favored

pathway to the production of defect-free

single crystals by the crystallographic

fusion of adjacent nanoparticles (Fig. 25).

Penn and Banfield christened the

‘‘oriented attachment’’ concept in 1998

when they coarsened the aggregates of

titania nanocrystallites under hydro-

thermal conditions.6,7 They found that

adjoining nanoparticles can spontane-

ously join with each other by sharing

a common crystallographic orientation

(Fig. 26).6 Their studies drew considerable

attention to oriented attachment both

theoretically and experimentally. Penn

and coworkers treated primary nano-

particles as molecules which were

supposed to form a dimer through

a transient particle complex (P/P) anal-

ogous to outer sphere complexes.148,149

Removal of solvent molecules from the

interface between the primary particles

and reorientation of primary particles are

required to gain the transformation from

a P/P to an oriented aggregate (P--P).

Using Derjaguin--Landau--Verwey--Over-

beek (DLVO) theory, they further ex-

plained the observation that the rate

constant of crystal growth by oriented

attachment slows down with time.

However, the assumption that only

dimers form imposes limitations on this

model. To get a better understanding of

crystal growth by oriented attachment,

different kinetic models have been devel-

oped, as reviewed recently.150,151 For

example, Ribeiro and coworkers pre-

sented a polymerization model.152,153

Their model derived from the classical

model of stepwise polymerization with

primary particles as the monomer and

oriented aggregates as multimers. It

provides the number of primary particles.

Employing DLVO theory, population

CrystEngComm, 2011, 13, 1249--1276 | 1261

Fig. 26 (a) TEM micrograph of hydrothermally coarsened anatase particles forming a chain-like

nanostructure and (b) HRTEM of a part of such an assembly demonstrating the single crystalline

nature.8 Copyright 1999, Elsevier.

Fig. 27 Snapshots of the aggregation of the large symmetric nanocrystals. These figures are taken

(a) at the beginning of the simulation; (b) after 100 ps; (c) after 160 ps; and (d) after 1.0 ns. Oxygen

atoms are shown in red (dark) and titanium atoms are shown in white (light). (e and f) The

magnitude of the interparticle force on each atom when two small, symmetric nanoparticles are in

the initial stages of aggregation. Forces are shown relative to the minimum observed force, which

takes the value of one.157 Copyright 2009, American Chemical Society.

balance models can make predictions

about particle size, size distribution of

secondary particles, and the influence of

suspension conditions on the kinetics

of oriented attachment.154

In cases where nanoparticles are free to

move, such as in solution or where

nanoparticles are coated with abundant

surface-bound water, a merger between

particles can eliminate high energy

surfaces, leading to a substantial reduc-

tion of the overall energy. It is anticipated

that using liquid cell in situ transmission

electron microscopy,155 a new approach

for the observation of nanocrystals

growing from chemical reactions can

enable the characterization of crystal

growth by the oriented attachment

mechanism. Classical molecular

dynamics (MD) simulations contribute

significantly to understanding oriented

attachment. Qin and Fichthorn reported

that oriented attachment can generally

arise through interactions mediated by

the solvation forces between colloidal

nanoparticles when they rotate to

approach one another in a solution via

preferred pathways.156 More recently,

they proposed a ‘‘hinge’’ mechanism

based on the aggregation of anatase

nanocrystals with certain preferred

orientations in a vacuum environment.157

Fig. 27 shows such a trajectory of

symmetric anatase particles. The particle

shape is consistent with the experimental

result.6

The center-of-mass separation is �8.5

nm in the initial configuration (Fig. 27a).

A relative orientation occurs with

a shorter distance between particles after

100 ps (Fig. 27b). A ‘‘hinge’’ forms after

160 ps with a {001} face of one particle

contacting the edge between two {101}

faces of another particle (Fig. 27c).

Aggregation subsequently takes place

after 160 ps with one particle rotating

around the ‘‘hinge’’ and its {101} face

contacting the {001} face of the other

particle (Fig. 27d). As shown in Fig. 27c

1262 | CrystEngComm, 2011, 13, 1249--1276

and d, the interparticle forces arise from

the electrostatic interactions between

two-coordinated O atoms with the highest

density along the edges between two

{101} faces and under-coordinated Ti

atoms on the {001} face. Dipole--dipole

interactions have been inferred to be the

driving force for directed aggregation in

some previous studies. However, Alimo-

hammadi and Fichthorn’s MD result

indicates that aggregation rarely occurs

along the direction of the dipole, even for

the asymmetric anatase particles with the

This journ

permanent dipole moment as large as 250

D.157 It is worth mentioning that the MD

simulation was studied in a vacuum

environment with a timescale of nano-

seconds. Despite the simulated aggregates

resembling some of the experimental

images,6 they are polycrystalline rather

than single crystals with aggregation on

{112} as reported by Penn and Banfield.6

To date, zero, one, two, and even three-

dimensional (0D, 1D, 2D, and 3D)

nanomaterials have been prepared by the

oriented attachment mechanism.12,146,147

All these studies demonstrated that an

additive is an influential but not exclusive

reason for the oriented attachment

growth from nanoparticles to single

crystals. Single crystals, such as ZnO

nanorods,158 MnO multipods,159 TiO2,160

CuO microspheres,161 Cu2O poly-

hedrons,162 and PbWO4 dendrites,163 can

be assembled by the oriented attachment

of nanoparticles under additive-free


Fig. 28 HRTEM images ((a), (b), and (d)) and SAED pattern (c) of gold nanochains synthesized in

the presence of glutamic acid. ‘DL’ and ‘TB’ stand for ‘dislocation’ and ‘twin boundary’, respec-

tively.166 Copyright 2009, Nature Publishing Group.

Fig. 29 Mechanism of in vitro hierarchically organized microstructure formation by self-assembly

of nucleated apatite nanocrystallite--Amel nanosphere mixtures based on experimental evidence

(solid arrows) and theoretical analysis (dotted arrows).167 Copyright 2008, American Chemical

Society.

Fig. 30 Oriented attachment strategies: HRTEM images of CdSe nanowires synthesized by re-

acting cadmium acetate with selenourea in the presence of long-chain amines of different lengths—

DDA stands for dodecylamine, while HDA stands for hexadecylamine.175 Copyright 2006, Amer-

ican Chemical Society.

reaction conditions. Here we would like

to pick up some typical examples to show

why and how additives can influence

crystallization to occur by oriented

attachment of nanoparticles.

5.1.1. Oriented attachment in the

presence of simple organic additives.

TiO2 adjacent nanoparticles can share

a common crystallographic orientation in

the presence of adipic acid, acetic acid,

and glycine, and subsequently fuse at

planar interfacial areas.6,7 In the aniso-

tropic growth of CuO uniform mono-

crystals, formamide was used as the

additive.164 Formamide molecules can

face selectively adsorb on CuO primary

nanocrystals when they take shape and

facilitate the preferential one-dimensional

(1D) assembly along the [001] direction.

Finally, the monocrystalline structure

forms by oriented aggregation in a 3D

manner.

Another choice for controlling the

nanoscale arrangement of nanoparticles

is using charged moieties of polydentate

additives to functionalize nanoparticle

surfaces. For example, Trizma

((HOCH2)3CNH2) was used to function-

alize anatase nanocrystals in benzyl

alcohol.165 Upon redispersion in water,

the nanoparticles organized into pearl

necklace structures. The structures are

composed of nanoparticles assembled

along the [001] direction via oriented

attachment, exhibiting lattice fringes

typical for a single crystal. The aniso-

tropic assembly is a consequence of

the water-promoted desorption of the

organic moieties selectively from the

{001} faces of the crystalline nanosized

building blocks together with the

adsorption of water on these crystal faces.

The use of amino acids as additives can

also trigger oriented attachment. For

example, gold nanospheres can fuse with

each other and form nanochains or

nanowires through dipole--dipole inter-

actions due to the zwitterionic nature of

amino acids. This was demonstrated in

a recent study on the synthesis of gold

nanochains by reducing an AuCl4�

aqueous solution with NaBH4166 in the

presence of glutamic acid and histidine as

stabilizers. TEM measurements evi-

denced the occurrence of oriented

attachment (Fig. 28).

Oriented attachment processes can also

be suppressed by simple organic

This journal is ª The Royal Society of Chemistry 2011 CrystEngComm, 2011, 13, 1249--1276 | 1263

additives, when the additive molecules

prevent contact among the crystal planes

after selective adsorption. This hypothesis

of imperfect oriented attachment was

proven by Penn and Banfield in the

studies of TiO2 nanocrystals with dislo-

cations.7


presence of biomolecules. Nancollas and

colleagues found that amelogenin (Amel)

can accelerate hydroxyapatite (HAP)

nucleation kinetics and decrease the

induction time in a concentration-depen-

dent manner.167 They reported the

formation of nanorod intermediates

assembled by synergistic interactions

between flexible Amel protein assemblies

Fig. 31 Four principal possibilities to explain t

a mesocrystal. (a) Alignment of nanoparticles by

alignment by physical fields or mutual alignment of

mutual alignment by physical fields or the faces. (c

a mineral bridge connecting the two nanopart

constraints. Upon growth of anisotropic nanoparti

will align throughout growth according to the spa

particle in the arrangement of already grown particl

building units are shown to be monodisperse for the

systems. Also, the mutual order is not necessarily


1264 | CrystEngComm, 2011, 13, 1249--1276

and rigid calcium phosphate nano-

crystallites. These intermediate structures

further assembled by oriented attachment

to form the final hierarchically organized

microstructures that are compositionally

and morphologically similar to natural

enamel. The mechanism is shown in

Fig. 29.


presence of surfactants. Oriented

attachment can also occur, when ordinary

surfactants are used as additives to adsorb

on nanocrystal surfaces. BaSO4 fibers

were obtained by a reverse microemulsion

method using sodium bis(2-ethyl-

hexyl)sulfosuccinate (AOT) as an addi-

tive.168 BaSO4 and BaCrO4 fibres with 30

he 3D mutual alignment of nanoparticles to

an oriented organic matrix. (b) Nanoparticle

identical crystal faces. The arrows indicate the

) Epitaxial growth of a nanoparticle employing

icles. (d) Nanoparticle alignment by spatial

cles in a constrained environment, the particles

ce restrictions as indicated by the open drawn

es in the lower image of (d). Please note that the

sake of clarity. This is not always the case in real

that of the shown crystallographic register.13

This journ

nm in diameter and up to hundreds of

nanometres in length can also form in the

presence of polyacrylate or a phospho-

nated DHBC.169

Also, alkyl amine additives can induce

oriented-attachment in cubic lattice

systems.170 These additives remarkably

stabilize the (111) facets of PbSe nano-

crystals. PbSe nanocrystals can therefore

assemble into zigzag nanowires, in which

the shape originates from the amine

ligand stabilized anisotropic nano-

crystals. Nanowires can form in the

presence of oleic acid, trioctylamine, or

oleic acid, which cap the (100) surface of

PbSe nanocrystals.171 PbSe nanorings

were obtained by oriented attachment

along different directions simultaneously.

The importance of this work is presenting

a method to generate dipoles along

various crystallographic directions in

a cubic system. Likewise, nanowires for

other cubic structures like In2Se3 nano-

wires were also reported.172 Further

interesting examples are Au nanowires,173

Pt nanowires,174 and Pd nanowires,174

which have been obtained in the presence

of oleylamine, octadecylamine, and do-

decyltrimethylammonium bromide,

respectively, by the oriented attachment

mechanism.

By reacting cadmium acetate and sele-

nourea in long-chain amines, CdSe

ultrathin nanowires (Fig. 30) formed at

100--180 �C. The diameter of nanowires is

adjustable by changing the reaction

temperature or length of amines.175

Oriented attachment is energetically the

most reliable mechanism for the forma-

tion of nanowires.176 The big advantage is

the small number of defects in the wires

from oriented attachment compared to

wires which are generated by face selective

additive adsorption, which show

branches as defects due to incomplete

additive adsorption.

ZnSe, another crystal with wurtzite

structure, can also form in the shape of

ultrathin nanowires by oriented attach-

ment.177 For CdSe and ZnSe nanowires,

quantum confinement effects were

observed due to the high crystallinity and

small diameter of the wires.

5.2. Mesocrystals

Classical crystallization places a crucial

restriction on the definition of the primary

crystal building units, which are atoms,


Fig. 32 The effect of Pif on calcium carbonate crystallization in vitro. (A) Each fraction separated

by means of gel filtration HPLC was subjected to SDS-PAGE under reducing or nonreducing

conditions. (B) SEM image of a piece of calcium carbonate crystal. Scale bar¼ 50 mm. (C) A cross-

section of the white box in (B) prepared by means of focused ion beam was observed with TEM. The

crystal was formed in the space between the chitin membrane and the glass plate. (D) The electron

diffraction pattern of the grey circle area in (C), indicating a single aragonite crystal and its c-axis

perpendicular to the glass plate. 2ME, 2-mercaptoethanol; C, chitin; G, glass plate.184 Copyright

2009, American Association for the Advancement of Science.

ions or molecules. However, biominerals

with single crystalline behavior but

without the expected cleavage planes, like

sea urchin spines, cannot be explained

with the classical model of crystallization.

Studies of these biominerals suggest

a crystallization process based on nano-

particle building units. A nonclassical

crystallization model was suggested based

on studies of bio- and biomimetic miner-

alization mechanisms. It can be used to

explain the hierarchically complex

morphologies and structures found in bi-

ominerals and in biomimetic mineraliza-

tion experiments.9,10 The primary

building blocks in nonclassical crystalli-

zation mechanisms are nanoparticles. If

they form a mesoscopically structured

crystal with 3D ordered nanoparticle

superstructure, a mesocrystal is gen-

erated.11--13 The term ‘‘mesocrystal’’ is an

abbreviation for ‘‘mesoscopically struc-

tured crystal’’. There is an analogy

between structural levels in a mesocrystal

and those found in biopolymers.11

A mesocrystal can be an intermediate

for a single crystal if the nanoparticle

surfaces are not fully stabilized against

crystallographic fusion by additives. Once

fused with each other, nanocrystals will

crystallographically align to a single

crystal domain finally resulting in a single

crystal with a lot of defects like pores.

Thus, it is difficult to distinguish a meso-

crystal from a single crystal, since both

electron and X-ray scattering patterns of

a mesocrystal behave like a single crystal.

Nevertheless, the transition from a meso-

crystal to a single crystal was experimen-

tally evidenced for FeOOH178 and DL-

alanine.179

In a recent progress report, we

proposed four possibilities to explain the

3D mutual alignment of nanoparticles

(Fig. 31).13 The alignment along an

oriented organic matrix is a strategy

which is used in biomineralization. For

example, in bone the hydroxyapatite

nanoplatelets are aligned in an ordered

collagen matrix (Fig. 31a). Physical fields

like electric or magnetic fields but also

dipole or polarization forces or aniso-

tropic van der Waals forces can lead to

a mutual alignment of the nanocrystals

(Fig. 31b). If the nanocrystals are bridged

by mineral bridges, the mesocrystal is

actually crystallographically connected

(Fig. 31c). This structure was discussed

for biominerals like sea urchin spines


which develops from an amorphous

precursor phase.180 The last possibility is

the growth by spatial constraints in

a confined reaction environment where

a particle, which grows in a gap can only

grow if it aligns (Fig. 31d). The mutual

order in mesocrystals must not necessarily

be the perfect 3D crystallographic align-

ment. A continuous structural transition

from a single crystal, via a mesocrystal, to

a polycrystal without any orientation was

presented depending on the size and

degree of orientation of the building

units.181 In this respect, a mesocrystal can

also be considered as a structural inter-

mediate between a classical single crystal

and a polycrystal when the random

aggregation of nanoparticles occurs.

Mesocrystals have some unique

features, such as high crystallinity, high

porosity and inner connection bridged by

organic components or inorganic nano-

crystals. The possibility of preparing

single crystal-like 3D nanoparticle super-

structures and their easy processability

due to micrometre size but with properties

of the nanocrystal subunits, which are

intrinsically difficult to handle, have

2011

brought mesocrystals into the forefront of

materials science research. To date,

a significant portion of relevant research

is focusing on mesocrystals due to their

potential for enhancing material perfor-

mance or achieving new prospects in

desired applications.13 Our previous

reviews span different types of meso-

crystals found in biominerals and a broad

range of synthetic mesocrystals of

important application value.11,12 There is

clear evidence that, in contrast to the

classical atom/ion/molecule-based crys-

tallization mechanism, nanoparticle

aggregation-based nonclassical crystalli-

zation promises an advantageous

pathway for the generation of ordered

nanostructured composite materials. It

also provides ideas for the design of new

advanced materials.13,182

While the number of mesocrystal

systems is increasing, their formation

mechanisms still remain largely unex-

plored. The mutual aggregation of nano-

particles in mesocrystals is usually driven

by an oriented attachment mechanism of

nanoparticles towards single crystals or

grain growth after grain rotation and

CrystEngComm, 2011, 13, 1249--1276 | 1265

Fig. 33 (a) The association of a phosphate ion to a gelatin triple helix. For clarity only the hydrogen

bonds of the phosphate ion to the protein fiber (dO/H z 1.3 A) are depicted (dashed purple lines).

Though three hydroxyproline--phosphate bonds are formed, only two strands of the triple-helix are

involved. This imbalance causes tensile stress to the right side of the fiber, while the left side is less

affected. As a consequence, the previously straight polypeptide is bent (H white, O red, C gray, N

light blue, P purple). (b) Left: model of a triple-helical polypeptide including a solvent shell (about

5000 water molecules) of 1.2 nm minimum thickness together with one attached (included) calcium

ion. Yellow ribbons represent the backbones of the three peptide strands of the protein fiber. Middle:

the same model without solvent molecules. Right: enlargement of the binding site. Calcium forms

ionic bonds (purple dashed lines) to carbonyl oxygen atoms of the protein (dCa/O z 2.3 A). Further

ionic interactions involve oxygen atoms (highlighted in green) of neighboring water molecules (dCa/

O z 2.7 A) (H white, O red, C gray, N light blue, Ca dark blue).189 Copyright 2006, Wiley-VCH.

Fig. 34 (Top left) Ion-scanning image of a calcite--gelatin composite individual at an early stage of

morphogenesis. (Top right) Overview TEM image of the FIB thin cut showing the structure of the

composite consisting of two different areas (belly and branch). (Bottom left) The belly region is only

poorly crystalline and consists of nanodomains in a mosaic arrangement and of pores. (Bottom

right) The filtered high-resolution TEM image reveals that the branch shows a perfect periodic

pattern.199 Copyright 2009, Wiley-VCH.

coalescence in a polycrystalline mate-

rial.183 In a broad sense, the role of addi-

tives in mesocrystal formation can involve

additive-induced formation of a dipole

field, formation of mineral bridges,

nanoparticle alignment in an organic

matrix, formation of a gel matrix, nano-

particle stabilization, and further effects

of polymer additives. Here, we intend to

discuss the mechanisms closely related to

using additives in a more concise way in

the interest of a better understanding.

5.2.1. Mesocrystal growth by nano-

particle alignment along an organic

matrix (Fig. 31a). A mesocrystal can form

when nanocrystals align along an organic

matrix or compartments in a crystal are

filled with crystalline matter organized by

a structured organic matrix (Fig. 31a).13

Either the nanocrystals could be gener-

ated first and then attach to the organic

matrix and get oriented or they could

alternatively directly nucleate on the

organic matrix, which could serve as

a heterogenous nucleation site.

1266 | CrystEngComm, 2011, 13, 1249--1276 This journ

Nacre mesocrystals, for example, are

characterized by a stacked compartment

structure with a uniformly oriented c-axis

of aragonite crystals in each compart-

ment.184,185 Considerable examples,

indeed, are known for matrix-mediated

nanocrystal growth on a fibrous organic

matrix, chitin for example, as found in

nacre.71 A peptide CAP-1 separated from

a crayfish consists of a chitin binding

motif, a phosphoserin unit promoting the

initial formation of ACC, and an acidic

binding domain for CaCO3 binding.186

The peptide can induce the formation of

a calcite mesocrystal film through nucle-

ation, crystallization, and nanocrystal

alignment on a film composed of parallel

a-chitin microfibrils. Very recently, the

specific binding affinity of an acidic

matrix protein, Pif, to aragonite crystals

was identified.184 The protein was sepa-

rated from the pearl oyster Pinctada fu-

cata. In vitro experimental studies indicate

that Pif can facilitate the formation of

aragonite single crystals with character-

istics close to those observed in the

nacreous layer (Fig. 32).

5.2.2. Mesocrystal formation in

a gel. Mesocrystal formation in a gel

matrix is related to nanoparticle align-

ment along an organic matrix. Kniep and


Fig. 35 (a) The DL-alanine unit cell. C¼ grey, O¼ red, N¼ blue, H¼white. (b) (001) surface cut of

DL-alanine (yellow dashed line) showing that (001) is a charged face. (c) (010) surface cut of DL-

alanine (yellow dashed line) showing that this face is hydrophobic, electrically neutral but dipolar

and polarisable. (d) SEM image of DL-alanine mesocrystals. Very thin, platelet-like crystals, aligned

along their c-axis to a multilayer stack. Scale bar¼ 2 mm. (e) SEM image of a DL-alanine mesocrystal

surface. Scale bar ¼ 3 mm.203 Copyright 2005, Wiley-VCH.

coworkers reported that in gelatin--fluor-

oapatite mesocrystals, hexagonal-pris-

matic subunits stack with their long axes

parallel to the [001] direction of fluo-

roapatite.187,188 The content of gelatin in

the composite is equal to the value iden-

tified in the mature tooth enamel. Based

on molecular dynamics simulations, it

was suggested that ion impregnation pre-

structures the gelatin gels prior to the

mineral composite formation.189 A

straightforward example of the pre-

structuring effect of ion impregnation was

observed in gelatin matrices under

double-diffusion conditions. According

to atomistic simulations, phosphate ions

can form two to three hydrogen bonds

with hydroxyproline side-groups of

gelatin and partially also with amino

groups, resulting in a heavy bending of

the applied (Gly-Pro-Hyp)12 polypeptide

(Fig. 33a). Ionic binding of calcium ions

to the oxygen atoms of the carbonyl

groups of the polypeptide backbone and

the side chains of proline and hydroxy-

proline could only cause a marginal

configurational change of the local struc-

ture of the protein fibers (Fig. 33b). Thus

intermediates obtained in a Liesegang

band next to the calcium reservoir display

a fan-like straight and hard morphology,


whereas a fractal growth with bent and

soft features can be observed for the

intermediates obtained from both the

band next to phosphate source and

the middle band. A mixture of both

archetypes was found in the middle band

when calcium ions were supplied faster

than phosphate ions.

Kniep and Simon found that the

intrinsic pattern of gelatin microfibrils

embedded within the periodic matrix of

the fluorapatite--gelatin nanocomposite

was already present within a young

composite seed with the perfect hexag-

onal-prismatic habit.190 This finding

shows that the observed fractal growth in

gelatin--fluoroapatite is not due to the

splitting influence of the basal planes of

young seeds. A situation with a high level

of complexity is brought about due to

polar gelatin triple helices, which favour

the mesocrystal formation of fluorapatite.

When all dipoles add up, a macroscopic

electric dipole forms, producing an

intrinsic electric field,191 which can orga-

nize further microfibrils along the direc-

tion of the developing electrical field. As

a consequence, even the crystallographic

orientation of the local areas in mature

seeds is also consistent with periodicity of

the nanostructured composite. Brick-

2011

mann and coworkers simulated the

structure formation process in remark-

able agreement to the experimental results

based on the alignment of dipoles.192

Once a mesocrystal is formed, it can be

transformed to the mesocrystal of other

components by topotactic conversion

under favorable conditions.13,193--195 The

conversion opens up an advantageous

way for the production of hierarchically

organized mesocrystal structures by

means of simple chemical reactions. In the

case of hydroxyapatite (HAP), the top-

otactic conversion of dicalcium phos-

phate dihydrate (DCPD) to dicalcium

phosphate (DCP) by dehydration and

further conversion to HAP by rapid DCP

hydrolysis with NaOH solution184 are

successful. The main reason is because

nanocrystals making up the mesocrystal

structure in the educt and product are

similar in their sizes and densities and the

lattice angle of the laminated structure

remains unchanged. The inclusion of

gelatin is important for the topotactic

conversion because a similar HAP struc-

ture cannot be obtained when performing

the same reaction cycle on commercial

DCPD crystals. Gelatin provides a gel

matrix for the formation of gelatin

incorporated DCPD precursor crystals to

inhibit these crystals from growing too

fast. In addition, gelatin promotes the

phase transition of DCPD to DCP by

dehydration at 60 �C. The specific inter-

action of gelatin with the {100} and {001}

DCPD faces also limits DCP from growth

during the phase transition.

In 2003, Grassmann and coworkers

showed that calcite octahedral meso-

crystals can be grown in gels. The additive

they used is a polyacrylamide gel, which is

neutral and cannot chemically interact

with CaCO3.196,197 A growth model based

on hierarchical aggregation of rhombo-

hedral subunits was suggested. Later, they

prepared calcite cubo-octahedral meso-

crystals by modifying the polyacrylamide

gels with charged acrylamidopropanesul-

fonate groups.197 Their results suggest

that the habit change is a common event

for mesocrystal growth in either neutral

or charged polyacrylamide gel media. As

a key factor for morphogenesis, Grass-

mann proposed that the permeability

through a homogeneous gel should

increase with decreasing crosslinker and

polymer content. The explanation was

evidenced by the fact that calcite cleavage

CrystEngComm, 2011, 13, 1249--1276 | 1267

Fig. 36 Mechanism of the final morphology change in calcite crystals due to selective adsorption of

PSS to one (001) face and the resulting generation of an inner dipole moment within the crystal along

the c direction. The primary crystals are asymmetric as they bind the polymer only on one side. These

primary blocks assemble to give flat, pseudo-symmetric mesocrystal structures. When a certain size

is exceeded, not only primary platelets, but also amorphous intermediates are attracted. By

recrystallization of those species, bent crystalline structures without translational order can

develop.204 Copyright 2006, Wiley-VCH.

Fig. 37 SEM image of the calcite mesocrystal

surface obtained in the presence of poly

(styrene-alt-maleic acid).206 Copyright 2008,

Wiley-VCH.

rhombohedra with a pore size of 150 nm

were obtained when the polymer content

of the hydrogel was fixed at 5%. A recent

report198 from Helbig supports this result.

Following their pioneering work on

gelatin--apatite mesocrystals, Kniep and

coworkers investigated the growth of

calcite in gelatin gels.199 They obtained

calcite--gelatin composite pseudo-

dodecahedral particles by a double diffu-

1268 | CrystEngComm, 2011, 13, 1249--1276

sion method in gelatin gel matrices.

Compared to bulk calcite crystals, these

calcite mesocrystal particles possess

a lower density due to the inclusion of

gelatin (1.9--2.6 wt%). Two different kinds

of structures were found in the individual

particles (Fig. 34), one dense structure in

the branch area and one porous structure

in the belly region. The branch area shows

a perfect periodic electron diffraction

This journ

pattern of calcite, whereas the belly region

shows a reduced crystallinity consisting of

nanodomains in a mosaic arrangement.

However, the whole area shows

a common crystallographic orientation,

in agreement with the X-ray scattering

properties of single crystals.

5.2.3. Mesocrystal growth driven by

an additive adsorption-induced dipole

(Fig. 31b). All particles with equal

polarity can, to some extent, attract each

other because of the presence of van der

Waals forces at the surface. In the case of

crystals with intrinsic anisotropy, their

faces can be ionic faces with excessive

charges, electrically neutral but dipolar

faces, highly polar faces or simple

hydrophobic faces. All of them have

a different surface energy: potentially ex-

isting in one and the same chemical crystal

system. Therefore, fluoroapatite in gelatin

matrices187,188 and ZnO,200--202 for

example, can undergo mesocrystal

formation via assembly of primary nano-

particles driven by the intrinsic electric

field. The first demonstration of organic

mesocrystals was obtained using a chiral

double hydrophilic block copolymer

poly(ethylene glycol)-block-poly(ethylene

imine)-S-isobutyric acid (PEG4700-

PEI1200-S-iBAc). It allows the growth of

DL-alanine mesocrystals from a supersat-

urated aqueous solution.203 Demon-

strated in Fig. 35a--c is a unit cell of a

DL-alanine crystal and the anisotropic

structure of its surfaces. Compared to the

charged (001) face, the (010) face of DL-

alanine is hydrophobic, electrically

neutral but dipolar. The mesocrystal

formation by the assembly of nanocrystal

platelets in this case was interpreted by

the adsorption of charged butyric acid


Fig. 38 Schematic illustration of the formation process of (NH4)3PW12O40 mesocrystals.208

Copyright 2006, Springer.

blocks on the high energy (001) face of DL-

alanine.203 The inhibited crystal growth

produces a dipole moment along the c-

axis. Subsequently, nanoplatelets stack

along the direction under dipole--dipole

attraction, leading to the formation of DL-

alanine mesocrystals (Fig. 35d and e).

Alternatively, one can engender aniso-

tropic interaction forces that are inher-

ently not present in the crystal itself

through the introduction of anisotropy by

selective adsorption of an additive.

Selective adsorption of polymeric addi-

tives on nanoparticles can lead to the

exposure of highly charged faces simul-

taneously with opposite charges on the

counter-faces. A good example of an

additive adsorption-induced dipolar

environment can be found in the forma-

tion of calcite mesocrystals involving

selective adsorption of a polyanion.204

The binding of poly(styrenesulfonate)

(PSS) to calcium cations shifts the mech-

anism from traditional ionic growth of

calcite rhombohedra to mesoscale nano-

particle aggregates with a con-

vex--concave doughnut-like morphology

(Fig. 36). Although calcite is not dipolar

due to its triclinic symmetry, selective

adsorption of PSS on the highly polar

(001) face of calcite resulted in the

formation of nanoparticles with dissim-

ilar charges on the opposite (001) faces

and thus the generation of dipolar nano-

particle subunits. These subunits were

then built up into a mesocrystal. This led

to a multi-curved convex--concave struc-

ture with a central hole formed on one

side. The broken symmetry along the c-

axis is a clear evidence of long-range

dipolar interaction. The as-obtained

calcite mesocrystals are highly porous205

and show a rather perfect tensorial

birefringence under crossed polarizers. A

similar mechanism is applicable to the

formation of rounded calcite mesocrystals

with a six-fold symmetry precipitated in

the presence of a double hydrophilic block

copolymer poly(ethylene oxide)-block-

poly(styrenesulfonate) (PEO22-b-PSS49)181

and to the formation of calcite meso-

crystals with self-similar morphologies by

using poly(styrene-alt-maleic acid) as

a stabilizer of calcite {001} faces.206

Calcite mesocrystals obtained in the

presence of poly(styrene-alt-maleic acid)

are characterized by the alignment of

equally shaped anisotropic nano-

crystals.206 Adsorbing on the triangular


charged {001} faces, the anionic copol-

ymer is responsible for the stabilization

and expression of {001} in the

morphology of the primary nano-

crystals.206 Side faces of nanocrystals,

however, are neutral and do not interact

with the polymer. When nanocrystals

approach each other, they will align side

by side, where the unprotected {011}

faces will lead to a mutual orientation and

form a crystallographic lock (Fig. 37).

Even though there are some voids and

pores in this self-similar structure, the

nanocrystal subunits in the mesocrystals

are still crystallographically connected.

5.2.4. Mesocrystal growth without

soluble additive—formed with mineral

bridges (Fig. 31c). Although the nano-

particle building units in a mesocrystal are

already in a crystallographic register as

shown in Fig. 31c, they are initially not

connected in most cases.13 For meso-

crystals found in biominerals, like the sea

urchin spine, Imai and Oaki et al.

proposed a mineral bridge mechanism.180

Mineral bridges are assumed to form

before the next nanoparticle building unit

grows in the crystallographic

register.180,207 The continuous growth of

these nanocrystals at the defective sites of

2011

the additive adsorption layer on the

nanoparticle surface allows the genera-

tion of a mineral bridge under supersat-

uration conditions. If the mineral bridge

formation takes place in an oriented

alignment process of nanoparticles after

nanoparticles come into alignment,

a porous and oriented architecture of

nanocrystals will become fixed by mineral

bridges. This is shown in the following

example.

In the case of (NH4)3PW12O40, nano-

crystallites first formed loose and spher-

ical mesostructures. These mesostructures

then slowly transformed to dodecahedral

mesocrystals with rough surfaces and

single crystal scattering behaviour

(Fig. 38).208 These mesocrystals were

called sponge crystals.

The mutual reorientation of the nano-

crystals in a close proximity can occur

when two equivalent nanocrystal faces

approach each other. The nature of the

short range repulsive forces in the above

example is not yet revealed but they must

be temporarily present. Otherwise, crys-

tallographic fusion of the oriented nano-

crystals would immediately occur.

Simultaneously, the decrease of surface

areas indicates a crystallographic fusion

of primary nanocrystals to some extent in

CrystEngComm, 2011, 13, 1249--1276 | 1269

Fig. 39 (A) Schematic illustration of calcite dodecahedron formation in the presence of PSS-co-

MA: (1) formation of polyelectrolyte stabilized ACC nanoparticles; (2) aggregation of ACC

nanoparticles to form liquid-like aggregates (the dots represent the ACC nanoparticles for clarity

reasons); (3) calcite nanoparticle crystallization with six {104} faces; (4) growth along the six {104}

faces and deformation of the liquid aggregate within the crystalline skeleton; (5 and 6) Ostwald

ripening of {104} and further growth fuses three {104} faces at both sides of the crystal leaving rough

crystallized former P-surfaces in between forming the dodecahedra. (B) Schematic illustration of

calcite pseudo-octahedron formation in the presence of PSS-co-MA: (1 and 2) as described above for

the dodecahedra; (3) crystallization of calcite nanoparticles with one charged high energy face

stabilized by the polyelectrolyte and further growth by oriented attachment of crystalline nano-

particles or ACC; (4) crystalline rods have a dipole field (shown only for one rod for clarity reasons).

Minimization of interaction between dipole fields leads to three perpendicularly oriented rods with

mutual 90� angles; (5) deformation of the liquid aggregate along the crystalline skeleton leads to P-

surface formation while patches of liquid-like ACC aggregates attach to the particle (not shown for

clarity reasons); (6) further disposed patches of liquid-like phase crystallize without fusion to a single

interface leading to the scalloped surface of the mesocrystal.212 Copyright 2009, American Chemical

society.

the mesocrystal formation process. This

indicates fixation of the structure by the

formation of mineral bridges between

adjacent nanoparticles. For a given

system, the occurrence of mineral bridges

depends on the solubility of the mineral,

the amount of included material, and the

binding strength of the interface. Excess

material entrapped between the two

adjacent nanocrystals or incompletely

crystallized planes can locally re-crystal-

lize to form the mineral bridge at the

expense of the surrounding ions. The

exact reason why mineral bridges form

between two crystallographically aligned

crystal faces in proximity of only a few

nanometres still remains unknown.

5.2.5. Mesocrystal growth in the

presence of nanoparticle stabilizers. Of

particular significance for mesocrystal

formation is the temporary stabilization

of nanocrystals by additives. This facili-

tates controlled aggregation and orienta-

tion to a mesocrystal by interaction

forces. Reverse microemulsions can give

rise to the so-called ‘‘correction mode’’ for

nanoparticles to balance colloidal inter-

actions and preserve orientation

mobility.13 The nanoparticles are synthe-

sized after fusion of micelles containing

the reactants and are temporarily stabi-

lized in the organic solvent by the

surfactant against immediate single

crystal formation or uncontrolled aggre-

gation. Hydrophobic interactions

between the surfactant tails occur and can

drive the self-assembly process of the

nanoparticles, minimizing the surface

energy of the surfactant phase. A recent

example for the successful application of

this strategy is the synthesis of CaMoO4

mesocrystals. A n-octane/CTAB/n-

butanol system was used to adjust the

hydrophilic--hydrophobic balance.209 A

similar mechanism was proposed for the

growth of GaPO4 mesocrystals in the

presence of an amphiphilic triblock

copolymer [(EO)106(PO)70(EO)106]--OH

(F127).210

5.2.6. Mesocrystal growth in the

presence of additives with multiple

roles. Polymeric additives offer great

promise for the production and observa-

tion of mesocrystals because they can

adopt more than one of the above

mentioned five roles in a crystallization

process. A polyacrylic acid (PAA) addi-

1270 | CrystEngComm, 2011, 13, 1249--1276

tive was used to prepare Fe3O4 meso-

crystals by high-temperature

hydrolysis.211 The monodisperse Fe3O4

nanoparticles were obtained with sizes

ranging from 30 nm to 180 nm. Each

individual particle is an aggregate of

crystallographically oriented nano-

crystallites. A two-stage growth model, in

which primary nanocrystals nucleate

under supersaturation and then aggregate

into mesocrystals, was suggested for the

growth of the Fe3O4 mesocrystals. In this

case, the adsorption of PAA limits the

This journ

growth of Fe3O4 nanocrystals. Moreover,

the PAA layer stabilizes and makes

nanocrystals highly dispersed in water.

When the reaction rate was increased by

addition of NaOH, the size of the meso-

crystals increases but the primary nano-

crystals do not grow. As a result, the

Fe3O4 mesocrystals show super-

paramagnetic behavior at room temper-

ature although their sizes exceed 30 nm.

In addition, the mesocrystal magnetiza-

tion is much stronger than that of single

Fe3O4 nanodots. The superparamagnetic


Fig. 40 (a) TEM image of vaterite nanofilaments prepared in NaAOT reverse microemulsion at

[H2O]/[NaAOT]/[CaCO3] ¼ 340 : 34 : 1. (b) Single vaterite bundle with two nanofilaments showing

the direction of the crystallographic c-axis determined by electron diffraction pattern in (c).216


properties, high magnetization, and high

water dispersibility of Fe3O4 mesocrystals

indicate that physical properties which

would be impossible for classical crystals

can be obtained by means of mesocrystal

formation.

The formation of NH4TiOF3 meso-

crystals provides a good example of the

multiple role of nonionic polymer addi-

tive Brij 58.194 Once the surfactant

participated in the chemical reaction for

the synthesis of NH4TiOF3 with the

poly(ethylene oxide) moiety as Lewis base

and oxygen donor interacting with metal

centers, a viscous amorphous phase was

formed. Afterwards, aggregation

occurred along with the cooperative

reorganization of organic and inorganic

compounds, producing a larger hybrid

particle. Crystallization took place at the

third step. Brij 58 can act as a matrix for

the mesocrystal formation since it has

a strong affinity to the {001} faces of

NH4TiOF3.

Our group contributed an interesting

strategy to CaCO3 morphogenesis using

poly(styrenesulfonate)-co-(maleic acid)

(PSS-co-MA) as an additive.212 Calcite

mesocrystals with large single crystalline

parts were obtained at low supersatura-

tion showing a pseudo-dodecahedral

shape with curved faces.213 Increasing

supersaturation generated calcite meso-

crystals with single crystalline skeleton,

pseudo-octahedral shape, and scalloped

surfaces.213 Further increase of supersat-

uration induces the formation of poly-

crystalline spheres. Such an easily

attainable methodology overcomes chal-

lenges facing in traditional crystal engi-

neering strategies. It requires no changes

in reaction temperature, chemical

components, or solvent to achieve an

access to nanostructured composite

products with control over both poly-

morphs214 and morphologies.

A unifying nanoparticle aggregation

formation mechanism was proposed to

understand the morphogenesis.212 It is

a synergetic consequence of nonclassical

crystallization and surface area minimi-

zation principles (Fig. 39).212 In the initial

growth stage of all these morphologies,

amorphous CaCO3 nanoparticles were

observed as the early product. From

titration measurements, nucleation is

largely inhibited when PSS-co-MA

instead of PSS is used as the additive in

the crystallization process. The poly-


anionic additive provides multiple

adsorption sites and can therefore

temporarily stabilize the amorphous

precursor nanoparticles. Similar obser-

vations were made when using other

polymer additives in mesocrystal forma-

tion.87 We can consider the primary

amorphous nanoparticles assembled into

nanoparticle aggregates with fluid prop-

erties. The liquid-like intermediate phase

can store primary nanoparticles and

prevent the random aggregation before

they mutually align into the crystallo-

graphic domains. This liquid-like amor-

phous phase is different from PILPs.17,18

PILPs were described as classical molec-

ular liquid precursors without recogniz-

able substructure transforming to

polycrystalline products, whereas meso-

crystals were observed as kinetically

stable products in our case. As a prom-

inent indication for the presence of the

liquid-like intermediate, micrometre sized

nanocrystal aggregates with the rhombo-

hedral minimal primitive (P)-surface

shape were captured.213 PSS-co-MA can

selectively adsorb onto calcite nano-

crystal faces with polarity like the (012)

face giving rise to the mutual alignment of

the nanocrystals. The triclinic hexapod is

2011

a straight consequence of the oriented

alignment. The polymer additive-aided

evolution from amorphous precursors via

liquid aggregates through P-surface

intermediates to mesocrystals is reshaping

our previous knowledge of the roles that

polymer additives can fulfill in meso-

crystal formation.

To achieve an exquisite control over the

synthesis of calcium carbonate crystalline

material, various additives were used as

templates or matrices.48,51,215 Li and

Mann demonstrated that the formation

of vaterite mesocrystals in that case was

a consequence of the balance among the

interactions between surfactant--CaCO3

interfaces, the limited crystal growth in

the microemulsion environment, and the

intrinsic crystal anisotropy of vaterite

(Fig. 40).216 Sodium bis(2-ethylhexyl)

sulfosuccinate (NaAOT) can stabilize the

formation of ACC nanoparticles.

However, the transformation from ACC

nanoparticle aggregates to vaterite or

aragonite depends on the hydration effect

and the electrostatic interaction at the

surfactant--inorganic interfaces.

Zhang and coworkers introduced the

co-addition of two surfactants, Tween 20

and sodium dodecyl sulfate, for the

CrystEngComm, 2011, 13, 1249--1276 | 1271

Fig. 41 (a) Schematic representation of the liquid diffusion experimental setup. SEM image and

TEM images of the two-dimensional superstructures obtained in the presence of [SDS] ¼ 2 g L�1,

[Tween 20]¼ 2 g L�1, and [Ca]¼ 10 mM, at 25 �C for 7 days: (b) SEM image of the fan-like units; (c)

TEM image; (d) HRTEM image (right corner inset is the electron diffraction pattern).217 Copyright

2009, American Chemical Society.

formation of a 2D structure full of fan-

like vaterite single crystals.217 An easily

attainable experimental setup (Fig. 41a)

was designed to control the reactant

diffusion in the liquid system. Tween 20

inserted into the molecules of SDS form-

ing reticular micelles with SDS in the

aqueous solution. The micelles provided

a template for the crystallization of

CaCO3. In addition, SDS polar groups

provided active sites for the occurrence of

nucleation. The adsorption of SDS

molecules onto a certain crystal face led to

the formation of vaterite. All together,

these factors determined the formation of

the network structure. When carbonates

Fig. 42 SrCO3 biomorphs prepared in a silica gel at

6 h (a) and 9 h (b).219 Copyright 2003, Elsevier. (c and

Copyright 2008, American Chemical Society.

1272 | CrystEngComm, 2011, 13, 1249--1276

approached calcium ions, they competed

with the surfactant and release the

calcium ions to form CaCO3, because the

electrostatic interaction between them

was stronger than the interaction between

surfactants and calcium ions. The SEM

image and TEM image of an individual

fan-like building block are shown in

Fig. 41b and c. As shown in Fig. 41d, the

building blocks show a single crystal

diffraction pattern.

6. Formation of biomorphs

Biomorphs are silica--BaCO3, SrCO3 or

CaCO3 composites obtained by copreci-

pH 10.5 containing 0.01 M of carbonate ions in

d) CaCO3 biomorphs prepared in a silica gel.220

This journ

pitation of carbonate salts and silica in

alkaline media in the absence of any

organic additives or templates. Garcia

Ruiz, Hyde and Kunz are leading the way

to the exploration of biomorphs with

complex morphologies resembling those

of biominerals (Fig. 42).218--220 Garcia

Ruiz and Hyde et al. recently proposed

a mechanism that explains the formation

of the complex morphologies described

for biomorphs by an oscillating pH-based

chemical coupling, which induces alter-

native precipitation of silica and

carbonate.221 When barium carbonate

crystallized from silica rich solutions or

from silica gels, it formed polycrystalline

aggregates. These carbonate nanocrystals

were cemented by amorphous silica, thus

forming a self-assembled composite

material with an amorphous phase of

silica intimately intertwined with

a carbonate nanocrystalline phase. As

a consequence, morphologies with non-

crystallographic symmetry are shaped by

‘‘differentiable’’ surfaces of well-defined

and smoothly varying Gaussian curva-

ture. Beyond classical crystallization, the

role of silica as a structure inducing

additive evolves dynamically during the

process, far beyond the role of a simple

static template.

7. Summary and outlook

Additive-controlled crystallization is wit-

nessing a particularly rapid development

and reveals a wealth of important infor-

mation on crystallization mechanisms:

from the templating effect of insoluble

additives to the selective adsorption effect

of soluble additives, from homogeneous

nucleation to heterogeneous nucleation,

from nucleation promotion to inhibition,

from prenucleation clusters to PILPs,

from chiral resolution by chiral molecules

to chiral nanosurface-based enantiose-

lective crystallization, from Oswald

ripening to oriented attachment or from

single crystals to mesocrystals (see also

the overview in Fig. 1). We expounded in

this highlight on what is possible using

various strategies and how to understand

the mechanism of these strategies in order

to launch a point of departure towards

complicated control of crystallization

using additives.

The possibilities in this field are

tremendous and rapidly developing with

increased understanding of the


underlying mechanisms. First and fore-

most, the ongoing research on bio-

minerals is providing more and more

evidence for the synergy of different types

of additves.71,185 Approaches to un-

ravelling the mystery in nature must take

that into account by a combination of

additives222--224 or additives with multiple

functional blocks.225 Another significant

challenge is how to obtain single crystal or

mesocrystal based nanostructured

composite materials with extraordinary

properties.226,227 Here again we can learn

from nature. Studies of biominerals have

produced a great deal of results to help us

to interpret complex phenomena in bio-

mineralization.19,228--233 With nanocrystals

as building blocks, oriented attachment

and mesocrystal formation mechanisms

show great promise at this end. In addi-

tion, the production of synthetic organic

single crystals with defined polymorph

and morphology by additive controlled

crystallization still remains difficult.

However, it is intriguing to note that the

progress of basic research in the inorganic

area also impacts the organic area.234,235

The distinctive properties of mesocrystals

make them particularly appealing for the

creation of advanced organic materials.

In summary, additives will be increas-

ingly important for the control of crys-

tallization reactions and the steadily

increasing understanding of the under-

lying mechanisms enables further prog-

ress in this area. The field is moving

forward from an empirical testing of the

influence of various additives to an

understanding of the role of additives in

the crystallization process. In the fore-

seeable future, it will likely even be

possible to predict the outcome of an

additive controlled crystallization reac-

tion, which nowadays is still a major

challenge for modeling approaches or

even impossible. There is still much to be

explored in this wide field in years to

come.

Acknowledgements

R.Q.S. thanks the NIH (DE018335) for

support of her current research.

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