Bio-inspired synthetic approaches: From hierarchical, hybrid supramolecular assemblies to CaCO3-based microspheres

Article abstract of DOI:10.1039/c7dt00914c

A bio-inspired synthetic approach to unprecedented hybrid supramolecular assemblies [Ca(Me₂hda)(H₂O)₃]·H₂O (1) and [Ca(C₁₂hda)(H₂O)₂]·H₂O (2), that are stabilized by iminodiacetate-substituted organic ligands is reported. The results of the single-crystal X-ray analysis of 1 further allowed the use of electron microscopy to verify the supramolecular structure of the fibrous assemblies of 2 that incorporate extended alkyl-substituted ligand derivatives. 2 reveals interesting features that distinguish these soft structures from purely inorganic, brittle materials: meshes of nanobelts transform on solid supports to form homogeneous films covering extended, micro-sized areas. The use of the reported ligand system as a habit modifier for CaCO₃ results in hierarchical calcite aggregates. The structure-influencing effects of the ligands and their supramolecular assemblies promote the formation of calcite disks that tessellate into hollow microspheres that contain distinctive openings.

Full text of DOI:10.1039/c7dt00914c

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Bio-inspired synthetic approaches: from
hierarchical, hybrid supramolecular assemblies to
CaCO3-based microspheres†
Cite this: Dalton Trans., 2017, 46,
6456
a
Bartosz Marzec, a Lei Zhang, a,b Nianyong Zhua,c and Wolfgang Schmitt
*
A bio-inspired synthetic approach to unprecedented hybrid supramolecular assemblies [Ca(Me2hda)
(H2O)3]·H2O (1) and [Ca(C12hda)(H2O)2]·H2O (2), that are stabilized by iminodiacetate-substituted organic
ligands is reported. The results of the single-crystal X-ray analysis of 1 further allowed the use of electron
microscopy to verify the supramolecular structure of the fibrous assemblies of 2 that incorporate
extended alkyl-substituted ligand derivatives. 2 reveals interesting features that distinguish these soft
structures from purely inorganic, brittle materials: meshes of nanobelts transform on solid supports to
form homogeneous films covering extended, micro-sized areas. The use of the reported ligand system as
a habit modifier for CaCO3 results in hierarchical calcite aggregates. The structure-influencing effects of
the ligands and their supramolecular assemblies promote the formation of calcite disks that tessellate into
hollow microspheres that contain distinctive openings.
Received 13th March 2017,
Accepted 20th April 2017
DOI: 10.1039/c7dt00914c
rsc.li/dalton
and charge, establishing sites of nucleation and growth
whereby predominantly donor atoms of alcohol, amine and
Introduction
Minerals of biological origin and their hybrid organic– carboxylate functionalities inhibit crystal faces, determining
inorganic composites often display a complex, hierarchical the growth directions and shapes of the minerals.5–9
organization of structure.1 Sea urchin spines, coccoliths,
Biological, affinity-driven aggregation of pre-organized
bones, teeth and shells are the products of sophisticated molecules that characterise the initial steps of the formation
synthetic methodologies where genetically-controlled and of biominerals gives rise to the formation of hybrid organic–
evolution-refined mineral deposition processes result locally inorganic materials and are comparable to supramolecular
in amorphous, poly- or single-crystalline phases.2 Nature’s self-assembly processes which are also developing into a key
remarkable ability to utilize such poorly processable inorganic process in chemistry and material design.10,11 Self-assembly
materials to create biominerals characterised with impressive processes that give rise to ordered structures generally rely on
mechanical properties provides inspiration and motivation for structural dynamics whereby individual entities either equili-
materials scientists.3 All biological mineral depositions involve brate between aggregated and non-aggregated states, or adjust
self-organized organic macromolecules including proteins and their positions relative to one another once the ordered aggre-
polysaccharides imparting selectivity and specificity to bio- gate has formed.12–15 In such systems the local environment
mineralisation processes.4 The interactions between the generally influences prevailing inter-component forces
organic matrixes and the mineralizing components are highly whereby lateral boundaries and other possible templates
complex and require an appropriate geometry, stereochemistry impart directing effects and reduce possible defects.16
Theoretical calculations reported by Goetz et al. underline that
the amphiphilicity of self-assembling molecules is one crucial
parameter that determines the structure and properties of the
aSchool of Chemistry & CRANN, University of Dublin, Trinity College, Dublin 2,
Ireland. E-mail: schmittw@tcd.ie; Fax: +353-1-6712826; Tel: +353-1-8963495
resulting products whereby small amphiphilic molecules or
short polymers were proved to generate membrane-like assem-
blies in aqueous solutions.17 The low bond energies of the
interatomic forces that drive supramolecular assemblies to the
thermodynamically most stable arrangements can give rise to
reversibility and error checking/correction.18
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002,
P. R. China
cInstitute of Molecular Functional Materials & Department of Chemistry,
Hong Kong Baptist University, Waterloo Road, Hong Kong, China
†Electronic supplementary information (ESI) available: Full experimental pro-
cedures and crystallographic data in CIF format for the new structure. CCDC
793553. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c7dt00914c
Our research activities focus on the usage of bio-inspired,
structurally well-characterised supramolecular coordination
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assemblies that self-organize into hierarchical hybrid nano- chemistry of related Ca2+ species remains underrepresented
materials. Our approach takes advantage of the ability to syn- and less explored.31–34 The here reported study provides
thesize hydrogen-bonded or counter-ion mediated networks insights into the coordination chemistry of these biologically
that are based on a modular set of aromatic iminodiacetic acid important main-group systems. Structural information was
ligands (Fig. 1a). We previously demonstrated using transition obtained using a combination of electron microscopy, single-
metal complexes that the resulting supramolecular architec- crystal and powder X-ray diffraction experiments.
tures are controlled by the substituents of these hda (2,2′-(2-
hydroxybenzylazanediyl)diacetic acid) ligands producing
lamellar structures, hexagonal assemblies or open-framework
Results and discussion
materials.19–21 Many of the structures show resemblance to
Ca2+ binding characteristics and supramolecular organization
biological systems e.g. bi-layer membranes.22 In related experi-
ments we succeeded in producing highly aligned Na2CO3 con-
of the selected class of organic ligands
taining nanofibres that are contained in carbon micro-contain- In order to obtain detailed structural information on the
ers which form upon thermolysis within a single crystal of a binding mode on a molecular level, we reacted the tetradentate
coordination network containing the starting components for dimethyl-substituted hda ligand (Me2hda) with CaCl2 in a
the mineralization in a pre-arranged form;23 some of the inves- MeOH/H2O mixture to synthesize the novel compound
tigated transition metal complexes have the ability to incorpor- [Ca(Me2hda)(H2O)3]·H2O (1), which separated phase-pure
ate carbonate ligands that form upon fixation of atmospheric calcite from the reaction mixture as colourless needle-shaped
CO2.24
crystals. Single crystal X-ray analysis revealed that the species
The present study explores the role of the hda ligand system is composed of mononuclear secondary building units
in the formation of supramolecular Ca-based coordination (Fig. 1b). The Ca2+ ions are surrounded by the six closest
assemblies and its influence as a habit modifier for the bio- donor atoms in a highly unsymmetrical octahedral arrange-
inspired preparation of crystalline CaCO3 materials. We ment. Three water ligands adopt a facial arrangement and the
present a coordination chemistry approach to habit modifi- inner coordination environment is completed by two cis coordi-
cation whereby the amphiphilicity of the organic ligand nating O-donors that originate from deprotonated carboxylic
systems was expected to influence supramolecular aggregation acid functionalities of a Me2hda ligand and a symmetry gener-
in solution, potentially resulting in confinement of the reac- ated O-donor, from a carboxylate function of a neighbouring
tion volume. Thus the present approach relates to biological secondary building unit, to give 1D chains that extend parallel
templates that display various degrees of organization of the to the crystallographic a-axis. The N-donor of the amine func-
structure. Whilst supramolecular coordination assemblies and tion and the O-donor of the protonated phenolic alcohol func-
network structures of transition metal ions that benefit from tion of the Me2hda ligand can be considered as a part of the
the ligand field stabilization are well-established,25–30 the outer coordination environment of the Ca2+ ion to give a very
unsymmetrical 8-coordinated polyhedron. The amphiphilic
character of the complexes promotes the formation of a
layered lamellar supramolecular structure in which hydro-
phobic organic and hydrophilic inorganic layers are separated.
Hydrogen bonds which involve the water ligands, the consti-
tutional water molecules, the phenolic OH function and car-
boxylate O-donors link the chains within the ab-plane to form
neutral layers (Fig. 1c and d). The hydrogen bonds in 1 range
between 2.219(2) Å and 2.933(2) Å, stabilizing neutral layers
with a cross-sectional thickness of ca. 1.7 nm. These layers
interact through dispersion forces with each other and pack in
the c-direction to form a layered lamellar structure. The
layered, amphiphilic nature of 1 combined with the observed
anisotropic growth directions within a neutral layer gives rise
to the formation of 1D or 2D crystal and assembly mor-
phologies. In fact, crystalline microfibres of 1 whose dimen-
sions are characterised by high aspect ratios (millimeter
lengths and diameters of ca. 700 nm) can be grown upon
Fig. 1 (a) Hda-type ligands used in the present study. (b) Structure of slight variation of the preparation conditions.
the mononuclear secondary building unit in 1. (c) Mononuclear building
units are linked through coordination and hydrogen bonds to produce
neutral layers extending in the crystallographic ab-plane. (d) Neutral
layers aggregate through van der Waals forces to produce a layered,
Isolated metal complexes with three facial water molecules
are relatively rarely observed since vacant H2O ligand sites
often promote hydrolytic condensations, dimerization or oligo-
merizations.35 The observed type of complex is generally
lamellar arrangement within the crystal structure of 1; perspective view
in the direction of the crystallographic [001]-axis.
regarded to impose directionality in condensation reactions
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whereby the complex can provide a capping unit that stabilises
portions of condensing mineral structures. The three water
molecules provide distinct binding sites for chelating moieties,
in particular for carbonate and hydrogen carbonate ions. In
this context one of the most remarkable, partially hydrated
main-group binding sites in nature is represented in ribulose-
1,5-bisphosphate (RuBP) carboxylase, whose Mg2+ site initiates
the first step in photosynthetic CO2 fixation providing carba-
mate binding sites.36 Depending on the pH of a given reaction
system, metal-bound aqua ligands undergo deprotonation to
form hydroxo moieties that can act as nucleophiles, e.g. in
the presence of CO2, promoting its conversion to carbonate
and hydrogen carbonate in d10 Zn-containing carbonic
anhydrases.37
Influencing the amphiphilicity of the Ca-complexes – towards
functional, soft, hierarchical nanomaterials
The nature of the ligand substituents in the ortho and para
positions with respect to the phenolic OH-functionality pro-
vides a synthetic parameter to control the amphiphilicity to
exploit the described types of compounds for self-assembly
purposes and to generate hierarchical soft materials.
Increasing the hydrophobicity of the hda ligand and introdu-
cing an n-C12-alkyl chain in the para position (Fig. 1) influence
the morphology of the obtained self-assembled materials. The
species [Ca(C12hda)(H2O)2]·H2O (2) forms when calcium chlor-
ide is reacted with the C12hda ligand under similar conditions
to those that led to the formation of 1. 2 rapidly precipitates
from the solution producing fibrous assemblies with high
Fig. 2 (a–c) SEM images of the assemblies of 2 confirm their fibrous
morphology. (d–g) HR-TEM images reveal that 2 has a layered lamellar
structure, where the separation distance between the interference
fringes is ca. 2.5 nm. The inset shows the electron diffraction image
recorded for a single fibre and confirms that 2 is initially an amorphous
material.
aspect ratios. The constitution of 2 was substantiated by CHN organic ligands. The electron diffraction pattern shows no evi-
analysis, EDX spectroscopy and thermogravimetric analysis dence of further ordering within the compound (inset Fig. 2e).
(see the ESI†). Differences between 1 and 2 in the thermal
The soft, flexible nature distinguishes these hierarchical
degradation pathway in an air atmosphere mainly derive from bio-inspired assemblies of 2 from brittle crystalline materials.
the presence of different quantities of coordinating solvent We noticed that the fibrous structures in the as-prepared solid
molecules and the length of the alkyl moieties of the organic samples are capable of undergoing structural rearrangements
ligands. The degradation of the organic ligands occurs above at room temperature. When the sample is left for two weeks on
ca. 200 °C initially resulting in the formation of CaCO3 then a SiO2 surface, extended film areas (max. observed dimension
forming CaO at further elevated temperatures. A comparison 0.3 mm × 0.5 mm) appear to form (Fig. 3a and b). We noticed
of the IR spectra of 1 and 2 (ESI, Fig. S3†) indicates that that this morphological rearrangement seemed to involve the
signals attributable to the asymmetric and symmetric formation of crossbar or grid motifs whereby single belts are
vibrations of the carboxylate functionalities of the organic crossed in an ordered fashion (Fig. 3c and d). After four weeks
ligands display similar shifts. The relative differences Δν = from the preparation, meshes of entangled belts amalgamate
νa − νs agree with the observed binding mode in 1. Thus, this to produce homogeneous, continuous films that extend over
observation supports the assumption that C12Mehda and the surface of the sample (Fig. 3e and f). Overview SEM images
Me2hda bind to Ca2+ ions in a similar fashion and result in at intermediate stages further reveal distinct areas of film for-
comparable coordination environments. Electron micrographs mation; often almost rectangular, 2D crystalline patches with
(Fig. 2a–c†) confirm that a typical sample contains belt-like micrometre dimensions appear at various places on the sur-
assemblies of sub-millimeter lengths whose thickness and faces. As the time progresses some of these patches further
widths vary between 30 and 50 nm and 80 and 300 nm, develop into larger extended areas.
respectively. HR-TEM analysis (Fig. 2d–f) confirms that the
To further investigate the nature of the observed transform-
layered lamellar supramolecular structure is preserved in the ations, we compared the FT-IR and Raman spectra of 2 as well
assemblies. Neutral layers extend parallel to the longest and as the corresponding powder X-ray diffraction (P-XRD) patterns
shortest belt axes and stack across the width of the belt with a of freshly precipitated samples and samples that were one and
separation distance of ca. 2.5 0.2 nm (Fig. 2g) which is very two weeks old. No significant shifts in the IR or Raman spectra
well in agreement with the extent of the alkyl moieties of the occurred over the examined time period, strongly suggesting
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that are influenced by the chemical environment and the
nature of involved chemical bonds whereby transformation to
the equilibrium state is characterised by coarsening effects,
e.g. a dimensional expansion of a characteristic dominant
structure. The observed morphological change did not occur
in samples that were fully dried or fully dehydrated after the
preparation (by treatment at elevated temperatures or keeping
the samples under vacuum). This observation suggests that
the driving force of the rearrangement is associated with the
evaporation/re-location of the solvent molecules and H-bonds,
which occurs when a sample is left under ambient conditions.
In aqueous environments the surface of the fibrous assemblies
of 2 is composed mainly of water molecules. As a direct conse-
quence of the anisotropic morphology of the belt-like fibres,
neutral layers in 2 arrange perpendicular with respect to the
deposition surface – this arrangement appears to be a require-
ment for the observed coarsening effect to form extended
films. A pictogram showing the HR-TEM imaging and high-
lighting the relationship between the molecular and micro-
scopic structure of 2 is presented in Fig. 4. On the relatively
hydrophilic SiO2 surface, the layers may rearrange whereby
hydrophilic parts of the structure are expected to preferentially
interact with the substrate. Competing hydrogen bonds and
electrostatic interactions that occur between the hydrophilic
parts of the assemblies and van der Waals forces involving the
organic residues seemed responsible for the formation of thin
and smooth films, whereby larger domains seemed to grow at
the expense of smaller domains (similar to Ostwald ripening
effects). Thus, these underlying weak intermolecular forces in
combination with the layered lamellar supramolecular struc-
ture of the material are responsible for fibrous assemblies that
are in contact with each other to amalgamate and rearrange to
form continuous, homogeneous films.
Fig. 3 SEM images recorded for a sample of 2 deposited on a Si sub-
strate after two weeks (a–d) and four weeks (e–f) show the morphologi-
cal transition from a cross-bar type motif to a smooth film. (g) Powder-
X-ray diffraction patterns of assemblies of 2 recorded over a time period
of two weeks; (the sample was deposited on a low background Si stub)
– (blue) fresh sample; (green) one week after preparation; (red) two
weeks after preparation.
The observed phenomenon that promotes the formation of
homogeneous, continuous films that extend over large areas
may also apply to other structurally related materials with
that the composition and molecular structure of the samples advantageous surface or electronic/optical properties, thus pro-
remain largely unchanged during the rearrangement and the viding a future perspective of supramolecular approaches
film formation. The recorded time-dependent P-XRD patterns being exploited for biocompatible coatings or electronic/
revealed that although the freshly precipitated fibres of 2 were optical device fabrication.39 The observed characteristics may
initially amorphous or poorly crystalline, the material also be of interest in the context of self-healing materials.40
rearranged within the first week to give a more ordered crystal- Furthermore, soft fibrous materials with high aspect ratios
line phase. During this investigated time period, one observes that degrade into more stable and safe inorganic products (e.g.
the development of a series of reflections in the lower 2θ range
at 6.31°, 9.58°, 12.88° and 16.15°, which can be attributed to
the stacking distance between the neutral layers in 2 (for n = 2,
3, 4 and 5, respectively, according to the Bragg equation,
Fig. 3g). The lattice separation distance calculated for these 2θ
values equals 2.79 nm and is in agreement with the distance
between the interference fringes observed in HR-TEM images
(Fig. 2g, ∼2.5 nm). Reflections related to the lattice parameter
that corresponds to the thickness of the film appear as rather
weak, broad signals.
The initially deposited entangled fibrous material of 2 may
be regarded as a metastable assembly at a lateral boundary,38
Fig. 4 Scheme highlighting the relationship between the molecular
which is prone to structural rearrangements on time-scales and microscopic structure in 2.
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CaCO3) are desired composite materials for regenerative
medical purposes.41
Use of the ligand system as habit modifiers for CaCO3
The crystallographic study of 1 provided us with a detailed
molecular understanding of the complexing ability of the hda
ligands. The partially ‘open’ Ca2+ environments that contain
terminal H2O sites which can be involved in hydrolytic con-
densations or which can provide strong binding affinity for
additional chelating O-donor groups suggest that this ligand-
type could be an interesting class of habit modifier selectively
inhibiting the growth directions of specific CaCO3 crystal
faces. Moreover, the self-organization tendency to form 2
underlines the capability of the ligand system to generate
highly ordered amphiphilic aggregates that can provide appro-
priate spatial periodicity to produce morphology-directing
matrices or confinement effects. The glycine-type functionality
of the chosen ligand system correlates biomimetically very well
with the low isoelectric points of proteins that are typically
involved in natural CaCO3 mineralization and which contain
primarily aspartate or glutamate residues.42–45 Related pre-
viously reported studies to control and template calcifications
have focused on structured organic surfaces including thin
films and Langmuir monolayers;46,47 other studies use organic
and inorganic additives,48 synthetic polymers49 and co-poly-
mers50 or extracted biopolymers.51
Fig. 5 (a–e) False color HeIM micrographs of microspherical CaCO3
structures that assemble in the presence of C12hda as the templating
agent; (a) overview of a usual sample. (b) Enlarged example of the dis-
cussed morphology. (c–e) Microspherical morphology. (f) TEM image
on the edge of an individual calcite disk highlighting the position of the
selected area aperture that was used to collect the corresponding elec-
tron diffraction pattern (g).
In consecutive experiments we increased the Ca2+/C12hda
mole ratio in aqueous solutions up to 100 : 1 and induced
CaCO3 formation through addition of NaHCO3. The reaction
mixture was constantly agitated and during initial experiments
the temperature of the reaction mixture was kept at 40 °C.
X-ray powder diffraction and IR spectroscopy confirm that have confirmed that the structures are composed of calcium
above a Ca2+/C12hda ratio of 25 : 1, phase-pure calcite products carbonate and powder X-ray diffraction revealed that calcite is
separate from the reaction mixture. The phase identity of these the predominant phase.
crystallites was further confirmed in a TEM electron diffraction
The TEM electron diffraction experiment confirmed that
experiment, whereby the recorded diffraction pattern was the individual disks assembling into the hollow sphere were
indexed as calcite. At lower Ca2+/C12hda ratios the chelating composed of calcite (Fig. 5f and g). We speculate that the
ability of the ligand suppresses CaCO3 precipitation and we amphiphilicity of the ligand or of the resulting Ca2+ complexes
observe the afore-described fibrous assemblies of metal promotes the formation of micellar or vesicular assemblies in
complex 2. Preliminary tests clearly demonstrated that the water which provide morphology-directing matrices for the
ligand system distinctively perturbates the CaCO3 mineraliz- CaCO3 mineralization. This assumption is substantiated by
ation at a Ca2+/C12hda ratio higher than 15 : 1 and below 50 : 1 the observation of spherical assemblies of hda transition metal
and we focused our attention on exploring slight variations of complexes that form under comparable reaction conditions.20
the experimental conditions in this concentration regime. It is
As expected, during the CaCO3 precipitation at a Ca2+/
noteworthy that in a Ca2+/C12hda range between 20 : 1 and C12hda ratio of 25 : 1 the initial pH of 6.9 increased to 8.3
25 : 1 our SEM investigations revealed the formation of spheri- within 24 h and then remained constant. X-ray powder diffrac-
cal structures (Fig. 5a–e). The spheres are created by aggrega- tion experiments confirm that CaCO3 precipitation initially
tion of characteristic calcite discs. Each disk is composed of produces amorphous phases (ACC) that age to yield the co-
small nanosized particulates and has a hollow centre. The existing phases of calcite and vaterite after 1 h; calcite phases
cross-sectional diameters of the disks typically vary between containing sphere-like morphologies were obtained after
ca. 2 and 3 μm and the central openings are characterised by 24 hours (Fig. 6a). We further investigated the formation con-
diameters that range between 400 and 800 nm. The entire ditions of this unexpected morphology and varied the initial
spherical assembly has a diameter of ca. 8–12 μm and is typi- temperature and pH range. While elevating the temperature
cally built of 7–11 spherical disks producing a hollow micro- above 60 °C increased the solubility of the C12hda ligand, it
structure with a wall thickness that does not exceed 1 μm. also resulted in the precipitation of aragonite needles (Fig. 6c),
FT-IR and EDX spectroscopy including elemental mapping which is in agreement with the observations of Beck et al.52
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The formation of the observed spheres as shown in Fig. 5
may be rationalized by taking into account the composition of
the reaction media and considering the initial formation of
ACC. Based on literature examples two possible formation
pathways may apply. One possibility involves the self-trans-
formation of hda-stabilized solid ACC microspheres via
surface-directed Ostwald ripening to form hollow calcite
spheres. Considering this scenario, the observed surface pat-
terning is the result of CO2 outgassing processes, thus, the for-
mation relates to processes that were previously observed by
Mann and co-workers.54 A second possible formation mechan-
ism may involve the aggregation of ACC/hda nanospheres at
the underside of CO2 droplets trapped in the meniscus at the
air/water interface. This approach can explain the distinct
curved shape of the disks. As the CO2 templated bubble starts
to sink, further discs are formed and tessellate the surface.
The latter formation approach is supported by the observation
that individual disks are indeed observed in the reaction
system (see Fig. 6b); these can exist in significant quantities
depending on the reaction temperature and thus are influ-
enced by dynamic processes at the air/water interface.55 The
formation of the observed spheres may further involve the for-
mation of transient vaterite spheres, which is a morphology
often assumed by this particular polymorph.56
Fig. 6 (a) Powder X-ray diffraction patterns recorded for the precipitate
forming in a 16.7 mM Ca2+ reaction crystallisation mixture (C12hda/
−
Ca2+/HCO3 mole ratio 1 : 25 : 50, see ESI, Table S1†); after 5 minutes
(blue), 30 minutes (green), 60 minutes (red) and 24 hours (black). The
highlighted areas show the typical positions of calcite (violet) and vater-
ite (green) reflections. (b) SEM image of individual calcite disks observed
in the reaction mixture. (c) SEM image of needle-shaped aragonite crys-
tals precipitated in the presence of C12hda at temperatures higher than
60 °C. (d) SEM image of calcite spheres obtained upon acidification of
the reaction system (see ESI, Table S1†). (e) SEM image of a rhombohe-
dral calcite crystal containing residual quantities of 2 that formed upon
addition of NaOH (1 M) to the growth mixture (initial pH above 8.0, ESI
Fig. S15 (g & h)†).
Conclusions
In conclusion, we report a bio-inspired synthetic approach that
results in unprecedented hybrid supramolecular assemblies 1
and 2, and CaCO3 materials with micro-spherical mor-
phologies. The crystal structure of 1 provided a detailed under-
standing of the supramolecular structure and binding mode of
the involved donor atoms, providing clues on how hda-type
ligands can be exploited as habit modifiers to template hier-
The described spherical morphologies predominantly form in archical CaCO3 mineral structures. The results of the X-ray
a pH range between 6.9 and 8.3 and between 30 and 40 °C. analysis of 1 further allowed us to use electron microscopy to
Acidification of the reaction mixture, which was achieved verify the supramolecular structure of the fibrous assemblies
through the addition of 0.2 M HCl, resulted either in the dis- of 2 that incorporate extended C12hda ligands. The resulting
solution of the CaCO3 precipitates below pH 6.5, or in the pre- soft, hybrid materials reveal interesting features that dis-
cipitation of morphologically related spheres which are shown tinguish them from purely inorganic, brittle materials: meshes
in Fig. 6d. These form at an initial pH value of 6.5 (ESI, of nanobelts transform into highly ordered crossbar assem-
Table S1†). Increasing the initial pH value above 8.0 through blies and homogeneous films whereby the mononuclear
the addition of NaOH results in the precipitation of ca. 20 µm amphiphiles rearrange to produce extended, micro-sized areas.
large, rhombohedral calcite crystals that contain residual The observed phenomenon may in the future be applicable to
amounts of 2 (Fig. 6e).
self-healing films or regenerative medical purposes, thus pro-
The observed spherical, bio-inspired morphologies of viding a perspective of supramolecular approaches being
CaCO3 reveal some vague resemblance to natural coccospheres applied in materials science. Soft fibrous materials with high
which are produced by several species of marine algae, includ- aspect ratios that slowly degrade into calcium carbonate- or
ing Emiliania huxleyi and Gephyrocapsa oceanica and which phosphate-containing hybrid materials are expected to
represent fascinating CaCO3 biomaterials whose complex enhance epitaxial bone or enamel regeneration in vivo.57 The
structural attributes are represented by the observed inherent use of the reported ligand system as a habit modifier for
chirality that is expressed at several distinct scale levels.53 It CaCO3 results in hierarchical calcite aggregates. The structure-
should be clearly stated that the natural formation of these influencing effects of the ligands and their supramolecular
species is not mechanistically related to the here applied for- assemblies promote the formation of micro-spherical calcite
mation processes of the observed spheres.
disks (most likely in the presence of CO2 bubbles) that tessel-
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Experimental
The hda ligands were prepared by a Mannich reaction between
the appropriate phenol, iminodiacetic acid and aqueous
formaldehyde.58
1: 0.027 g (0.1 mmol) of Me2hda was dissolved in a 1 : 1
(v/v) mixture of water and methanol under ambient conditions.
Addition of 28 μL of triethylamine (0.215 mmol) and 50 μL of
an aqueous CaCl2 solution (1 M) afforded the formation of
crystals of 1 after seven days. Yield: 44%. Elemental analysis
expected for C13CaH21NO8: (without crystallization solvent
molecules): C: 43.45%, H: 5.89%, Ca: 11.15%, N: 3.90; found:
C: 42.92%, H: 5.97%, Ca: 10.81%, N: 3.69%. IR/cm−1: 2923
(m), 2852 (m), 1586 (s), 1410 (m), 1325 (s), 1246 (s), 1221 (s),
1152 (s), 990 (s), 867 (s), 721 (m).
2: 0.042 g (0.10 mmol) of C12hda was dissolved at 40 °C in a
mixture of 3.5 ml of water and 6 mL of MeOH. Addition of
28 μL (0.2 mmol) of triethylamine and of 0.5 mL of an
aqueous CaCl2 solution (0.1 M) caused the instant formation
of a white precipitate of 2. Yield: 84%. Elemental analysis
expected for C24CaH43NO7 (without free solvent molecules): C:
57.92%, H: 8.71%, Ca: 8.05%, N: 2.81%; found: C: 58.27%, H:
8.15%, Ca: 8.10%, N: 2.83%. IR cm−1: 3853 (vw), 3225 (s,
broad), 2918 (s), 2848 (s), 2368 (vw), 1655 (m), 1614 (m), 1582
(s), 1489 (w), 1415 (m), 1322 (w), 1241 (w), 1217 (w), 988 (w),
865 (w), 721 (m).
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CaCO3 precipitation experiments
CaCO3 precipitates form upon addition of CaCl2 to an aqueous
solution of C12hda and NaHCO3 at specific temperatures.
Detailed reaction conditions and analyses of the resulting
CaCO3 materials are provided in the ESI (see Table S1†).
Crystallography
Details of data collection and refinement for 1 are given in the
ESI (Table S2†).Crystallographic data, CCDC 793553.
Acknowledgements
Financial Support from the Science Foundation Ireland (SFI;
06/RFP/CHE174 and 13/IA/1896), the European Research
Council (CoG 2014-647719) and DRHEA (B. M.) is gratefully
acknowledged.
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