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Octacalcium phosphate microscopic superstructure self-assembly and

evolution by dual-mediating combination

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DOI: 10.1039/b823210p

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Octacalcium phosphate microscopic superstructure self-assembly and

evolution by dual-mediating combination†
Zhongru Gou,*a Xianyan Yang,a Xin Gao,a Xinli Zhang,b Kang Ting,b Benjamin M. Wuc and Changyou Gaoa
Received 23rd December 2008, Accepted 23rd March 2009
First published as an Advance Article on the web 7th April 2009
DOI: 10.1039/b823210p

The morphologies of biomimetic mineralized octacalcium phosphate (OCP) are extremely versatile.
Numerous studies have addressed the gel phases, surfactants, polyelectrolytes and thin layers as
convenient media to produce OCP nanostructures with various morphologies. However, hardly any
studies have to date discussed the concept of controlling the structural evolution of these
nanostructures by organic molecule mediation. This paper aims at opening the pathway to such OCP
assemblies with levels of morphological expression and evolution from an aqueous solution consisting
of highly diluted acidic macromolecule and amphiphilic small molecule as co-mediators. This approach
differs in that the usual organic molecules are used to shape the single OCP morphology, and
specifically establish an evolutionary relationship between the lamellar clusters and spherical
aggregates. We present a possible model that gives qualitative description for the morphology of OCP
grown in a variety of molar ratios between acidic macromolecule and small molecule surfactant. We
believe that extension of the present approach to other biominerals and inorganic materials will prepare
a large variety of highly flexible biomimetic nanostructures with elaborate superstructures across
nanoscale to macroscopic dimensions.

Introduction OCP is another fascinating mineral as an intermediate phase in

the biological apatite in hard tissue.7 The attractive biocompat-
Control of architecture and pattern of inorganic materials with ible, bioresorbable and potential osteoinductive properties of
nano- to microscopic dimensions has rapidly developed into OCP-derived materials have led to their use in particle, coating or
a promising field in materials chemistry. Organisms can usually scaffold form in bone implant, tissue engineering, and protein
control polymorph, shape, and hierarchical assembly of a wide delivery system, whereas the initial biological performances of
range of biominerals. The self-assembly of inorganic fluorapatite these materials are significantly influenced by their morphological
(FAP) and carbonated apatite (CAP) nanocrystals into needle- complexity.8 In most cases, the lamellar or ribbon-like OCP
or plate-like microstructures,2 is a critical step toward high- crystals were readily synthesized where templates or organic
performance composites with properties well-adapted to their substrates such as small molecules, polysaccharides, or gelatin
specific functions in the body.3 This is a result of well-defined gel were typically employed.9 Sometimes it was possible to grow
structures that are formed by self-assembly processes assisted by aggregated OCP with spherical (shell) architectures in the pres-
polysaccharides or proteins matrices.4 Over the past decade, an ence of polymers or fluorides.10 Moreover, OCP–amelogenin
impressive investigation regarding the controllable morphogen- interactions in mimicking enamel mineralization have been
esis led to the FAP development from the hexagonal prismatic exploited by Iijima and Moradian-Oldak.11 Their experimental
crystal to the dumbbell shape and finally closed to the spheres in models elucidate that amelogenin exerts a strong influence on
a gelatin gel system.5 Such FAP superstructures were also ribbon to rod transformation and suppresses OCP growth via
confirmed to be achieved on the surface of an enamel slice in selective interactions with specific crystal faces. However, the
gelatin.6 well-defined self-assembly of OCP, the process by which nano-
crystal clusters evolve and hierarchical architectures exquisitely
self-organize, hitherto remains to be unraveled.
a
Bio-Nanomaterials and Regeneration Medicine Research Division, Here we present what we think is the first endeavor for the
Zhejiang California International Nanosystems Institute, Zhejiang
University, Hangzhou, 310029, ChinaHuajiachi Campus. E-mail:
tailorable OCP morphogenesis via a biomimetic dual-mediating
zhgou@zju.edu.cn; zhgou@gmail.com; Fax: +86 571 8697 1539; Tel: approach. Under appropriate conditions, controlling additives
+86 571 8697 1782 of such macromolecules (i.e., polyaspartic acid; PAsp) and
b
Dental and Craniofacial Research Institute, School of Dentistry, surfactants (i.e., cetyltrimethylammonium bromide; CTAB)
University of California, Los Angeles, 90095, CA, USA
c results in the elaboration of regular intergrowth of short plates
Department of Bioengineering, School of Engineering, University of
California, Los Angeles, 90095, CA, USA and long ribbons from nano- to micrometer length scale. The
† Electronic supplementary information (ESI) available: Growth formation of these infrequent architecture entities represents
patterns of OCP nanocrystal individuals along the core in the aqueous a new class of biological analogues, though simple biomimetic
solution by PASP/CTAB dual-mediating combination (Fig. S1); FTIR crystallization and growth. Such morphological diversity and
spectra of OCP samples synthesized from a dual-mediating approach in
the aqueous solution with different Rvalues, in contrast with the pure complexity with continuous architecture evolution have been
OCP and CTAB (Fig. S2). See DOI: 10.1039/b823210p carefully characterized. In order to refer to the relative charge

This journal is ª The Royal Society of Chemistry 2009 CrystEngComm, 2009, 11, 1585–1590 | 1585
amounts, we therefore define a stoichiometric molar ratio (R) changing the CTAB and PAsp concentrations under the same
between the positive charges (amino groups) from CTAB and the temperature and initial pH conditions. The so-called pure OCP
negative charges (carboxylic groups) from PAsp using the R plates were also prepared in the absence of both PAsp and CTAB
value. When R ¼ 0, there was no CTAB in the medium; when R additives while the other reaction conditions remained the same.
¼ 1, the PAsp molecules were exactly neutralized by oppositely
charged CTAB; and when R>1, there was excess CTAB. Characterization

The dried OCP specimens were determined by X-ray diffraction

Materials and methods
(Rigaku D/max-rA) with Cu Ka radiation at a scanning rate of
Materials 0.01 min1 and Fourier transform infrared (Nicolet) for the
phase composition. The morphology and chemical composition
High purity grade Ca(CH3COO)2$H2O, Na2HPO4$12H2O, and of the specimens were determined by transmission electron
NaH2PO4$2H2O (BBI) have been used without further treat- microscopy (JEOL JEM-2010) connected with energy-dispersive
ment. Poly-L-aspartic acid (PAsp) (sodium salt, molecular X-ray analysis (INCA EDAX, element>B) operating at 200 kV.
weight (Mw) 5.5 kDa) was used without further treatment. The scanning electric microscopy images were taken on a JEOL
Cetyltrimethylammonium bromide (CTAB, >99.0%) was JEM-6700F microscope. Samples were deposited onto quartz
recrystallized three times from a heat water medium. The water slides. Thermograwimeteric analysis (TGA) was performed using
used in all experiments was prepared in a thee-stage Millipore a TG/DTA6200, with a heating rate of 10  C min1 in air.
Mill-Q Plus 185 purification system and had a resistivity of 18.2 Dynamic light scattering (DLS) technique for measuring the
MU.cm1. micelle size of PAsp/CTAB mixtures with different R values was
performed in sodium phosphate aqueous solutions. All samples
Methods were washed with DDW and dried to remove the physisorbed
organic molecules prior to analysis.
In a typical procedure, for instance of experiment No. 6 (R ¼ 20)
as mentioned in Table 1, a concentrated solution of Ca(CH3-
COO)2 (100 mM) together with CTAB (1200 mM, variable in Results and discussion
different experiments) in water were added dropwise to a stoi-
The synthesis details and conditions for the OCP assemblies can
chiometric amounts of pH-adjusted sodium phosphate solution
be seen from Table 1. An overview of various precisely tuned
(pH 5.0) containing Na2HPO4/NaH2PO4 (25 mM with respect to
macrostructures and architecture evolution from closed spheres
phosphate) and PASP (1.00 mM, variable in the different
to dispersed ribbon assemblies are shown in Fig. 1. At a glance,
experiments. The reaction in the 1000 mL thee-neck round-
the aggregates mineralized and integrated on a macroscopic scale
bottom flask was undertaken at 60  C in a water bath with
(200 mm) and their outward appearance and evolution was
mechanical stirring (450 RPM). The suspensions were stirred for
attained though the correct association of thousands of OCP
1–3 h (ageing time) and the precipitates were then left from the
crystals. Firstly, only PAsp addition with a concentration of 2.0
mother solution by filtering though a 0.45 mm membrane. The
mM yielded close spheres (Fig. 1a; No.1). However, at lower
precipitates were washed with high purity water 3 times followed
PAsp concentration of 1.75 mM, addition of minimal amounts of
by immersing in anhydrous ethanol (100 mL) at 37  C for 1 h,
surfactant initiated destabilization of the spherical morphology
and afterwards filtered and dried in a vacuum stove at 60  C
toward loose structures. The OCP aggregates displayed a hemi-
overnight. To understand the structure evolution, more OCP
spherical morphology and their thin plate individuals could be
aggregates were synthesized at different R values by only
discernible (Fig. 1b; No. 2). Furthermore, the plate individuals
overgrew in length up to 100 mm with a significant increase of R
Table 1 Synthesis details and conditions for the preparation of hierar- (R ¼ 125, Fig. 1i; No. 9).
chically ordered OCP superstructure More interestingly, a range of spatially hierarchical architec-
tures with annular arrangements analogous to a staminate array
Superstructure
analogue were varied from a combination of short and long plates when the
(relative crystal mediators co-existed in the reactant solution (Fig. 1c–h; No. 3–8).
Entry No. CCTAB/mM CPAsp a/mM Time/h length b)/% Carrying out the reaction time for 1.5 h resulted in a remarkable
No.1 0 2.00 (40) 1.0 Watermelon (18.2)
selective growth inhibition, involving distinct growth steps at the
No.2 50 1.75 (35) 1.0 Halfball (23.5) nano- and microscopic scales, depending on the relative
No.3 100 1.50 (30) 1.5 Konjak (28.1) concentrations of CTAB and PAsp (by changing the R value, 3.3–
No.4 200 1.25 (25) 1.5 Mushoom (36.4) 66). These results primarily suggest that PAsp is the structure-
No.5 250 1.00 (20) 1.5 Sunflower (47.8)
No.6 300 0.75 (15) 1.5 Sunflower (55.2) directing agent while CTAB serves as a co-surfactant that
No.7 350 0.50 (10) 1.5 Orchid (68.7) strongly facilitates crystallite anisotropic growth. Macroscopi-
No.8 400 0.30 (6) 1.5 Dandelion (81.5) cally, the self-organized architectures resemble the naturally
No.9 500 0.20 (4) 1.0 Dandelion (94.7)
occurring forms, such as konjak, mushoom, sunflower and
No.10 600 0.20 (4) 1.0 Dandelion (100)
dandelion. These flower-like superstructures remained stable,
a
The corresponding carboxylic group concentration of PAsp was independently of their ageing time in the aqueous medium (data
estimated in the brackets. b The intensity ratio of the strongest peak
with OCP at 4.788 /2q relative to that of No.10.
not shown). Indeed, this morphogenesis and evolution exceeds
the form developments of already known various

1586 | CrystEngComm, 2009, 11, 1585–1590 This journal is ª The Royal Society of Chemistry 2009
Fig. 1 Overview (SEM images) of the OCP morphological expression and evolution synthesized in an aqueous solution with different PAsp and CTAB
concentrations. (a)–(i) corresponds to No.1–No.9 and the insets show the detailed hierarchical structure.

morphologically controlled materials, such as spheres, dumb- Now it becomes evident that such self-assembly and evolution
bells, and bundles.10,12 In particular, there were less finely tuned behavior is hard to reconcile with the classical concept of crystal
structures if PAsp kept constant with the increase of CTAB growth. It is possible that the long-chain PAsp plays multiple
concentrations (Fig. 1c1–c3; relative to Fig. 1c–e), but in total roles in the construction of OCP assemblies, acting as biological
agreement with the data reported early.10c Thus we were able to macromolecules do, to exquisitely modulate the neighboring
conclude that the complex OCP architectures obtained from crystallite surface (Fig. 2c). The periodicity of PAsp 6.7–6.9 Å
PAsp/CTAB dual-mediating pathway undergo a transformation would preferentially fit the c-direction of (100) face of OCP (6.87
from the short plate mesostructures to the long ribbon structures. Å). Thus, weak adsorption of PAsp on the (100) face of primary
This superstructure’s sensitive response of OCP to CTAB-addi- OCP plates reduces the surface charge and inhibits crystal
tion suggests an interesting phenomenon: the flower-like archi- growth, but this interaction of polypeptides is possibly tempo-
tecture is a transition point of the spheres to the ribbon structures. rary and kinetic.13 On the other hand, CTAB possesses a hydro-
In contrast to the OCP spheres grown slowly in gelatin gel,10a the carbon tail and a positively charged head group that is able either
OCP architectures in this study do not only reflect the surface to partly neutralize the oppositely charged carboxylic groups or
morphology change, but also represent a variety of highly to interact with the phosphate ions of the crystal lattice, with the
ordered self-organizations as the patterns in nature. consequence that the rate of plate individual growth is enhanced
The thee representative architectures in more detail in the due to anionic macromolecule neutralization (Fig. 2f). Thus, the
absence and presence of CTAB, and their oversimplified sche- formation of ribbons in PAsp/CTAB mixtures, as a function of
matic representation are shown in Fig. 2. Microscopically, the increasing CTAB concentrations, was caused by hydrophobic
scanning electron microscopy (SEM) images clearly show that repulsion of CTAB bilayers at the (010) surface (Fig. 2i).
the partially broken sphere was characterized by discontinuous Fig. 3 shows typical X-ray diffraction (XRD) patterns of OCP
core/shell assembly with inner flannelette architecture and aggregates grown in aqueous solution at zero (R ¼ 0; No.1),
a nearly parallel arrangement of plate individual in the shell area moderate (R ¼ 8; No.4), and maximum (R ¼ 150; No.10) CTAB
(Fig. 2a and b). The average plate size in the shell of the sphere concentrations in the present study conditions. It can be seen that
was found to be only approximately 3–5 mm, which was in with increasing R value, the diffraction pattern evolves from
agreement with previous experimental results.10b Notably, OCP poorly- toward highly-crystalline OCP, which indicates a signifi-
nanocrystals were arranged in a cactus-like structure and the cant increase of the relative intensities of the (100) reflection. This
aggregate became looser with respect to the packing arrangement variation might essentially be in accordance with the increasing
of the plate individuals with the addition of a suitable amount of percentage of crystal length relative to the ribbon individual at
CTAB (Fig. 2d and e), implying that some nanocrystals the maximal R value (Table 1). Additionally, Fourier-transform
encountered growth suppression whereas the others required infrared (FTIR) spectra indicate that these assemblies are
growth enhancement. Further increase of CTAB concentration comprised of OCP, but PAsp and CTAB are scarcely detected
shows completely loose overgrown ribbon assemblies as shown (Fig. S2, ESI),† indicating the interaction between organics and
in Fig. 2g and h. However, no significant variations of OCP crystals is very weak.
morphology, as well as the overlong ribbon architectures, have Further characterization using a transmission electron
been appreciated at larger CTAB concentration (600 mM, No.10; micrograph (TEM) and selected area electron diffraction
data not shown). The artificial damaged ribbon assemblies yiel- (SAED) in Fig. 4a and b supports the individual growth char-
ded small aggregates (inset of Fig. 2h), indicative of the forma- acteristics on samples from two critical CTAB concentrations in
tion of ribbons from the same seed nuclei. This growth behavior the present study conditions. On the one hand, high R value (R ¼
can also be confirmed by optical microscopy observation 150; No.10) yields ribbon individuals elongated along the c-axis.
(Fig. S1, ESI).† SAED analysis identifies a well-resolved diffraction pattern with

This journal is ª The Royal Society of Chemistry 2009 CrystEngComm, 2009, 11, 1585–1590 | 1587
Fig. 2 SEM images of the OCP units and crystallites and its schematic illustration at nanoscales. As the CTAB concentration increased, the controlled
assembly of the units made up the various morphologies and epitaxial growth. (a)–(c): No.1; (d)–(f): No.4; (g)–(i) No.10.

differential thermal analysis (TG-DTA). Fig. 4c shows the data

of the spherical and ribbon species. The weight loss can be
assigned to thee steps: two processes between 50 and 250  C,
corresponding to loss of adsorbed water and to removal of lattice
water from OCP, but the combustion process for PAsp
combustion relative to CTAB (from 250 to 550  C) is barely
clarified due to its limited amount.
At the present time we assign the underlying mechanisms to
precisely tailored OCP nanocrystals to spatially selective growth.
Indeed, besides the formation of CTAB micelles above its
concentration theshold in our synthesis, the PAsp/CTAB
complex micelles may also be observed in sodium phosphate
solution, even though the starting concentrations of PAsp
additive are rather low (<2.0 mM). To validate our hypothesis
that the PAsp/CTAB complex micelles were also responsible for
the initial aggregation according to a previous report,14 we
Fig. 3 XRD patterns of OCP from different R. Insets are SEM images
carried out several additional experiments to testify such nano-
of the samples with different outer morphology.
scale micelle by DLS (dynamic light scattering) at very low
concentrations of PAsp and CTAB (Fig. 5). Empirically, another
circular fine dots, which is representative of single crystal; factor that must be taken into account is the ionic strength of
whereas on the other hand the individuals derived from the full multivalent electrolytes (i.e., phosphate) or surfactant for micelle
damaged sphere (R ¼ 0; No.1) display randomly oriented platelet formation.15 An increase in surfactant and calcium ions may
clusters and the d (hkl) values in the SAED diagram corre- neutralize partially the micellar surface charge and decrease the
sponded to those of OCP. The content of water and organic destabilizing repulsion of the likely charged head groups.16 This
components were both obtained by thermogravimetry and results in a drop of the critical micelle concentration and an

1588 | CrystEngComm, 2009, 11, 1585–1590 This journal is ª The Royal Society of Chemistry 2009
 Fig. 5 DLS analysis of PAsp/CTAB mixture in sodium phosphate
Fig. 4 TEM-SAED images (a, b) and TG-DTA analysis (c) of OCP aqueous solutions with different R values (No. 2–9).
plate and ribbon individuals synthesized in aqueous solution with
different R values. (a) The prolonged ribbon individual synthesized with
high R value of 600/4 (No.10); (b) the short plate individual synthesized and may ultimately help to extend the knowledge to the assess-
with low R value of 0/40 (No.1). The weight loss up to about 250  C is due ment of cell responses to their superstructure messages.
to adsorbed and crystal-bound water, where two thermal processes are
displayed: the first one which is reversible, takes place at temperature
lower than 75  C and has been ascribed to reversible removal of one water
Acknowledgements
molecule from the OCP structure; the second one, at about 140  C, The authors would like to acknowledge the financial support
corresponds to the removal of two water molecules per OCP molecule.
from STDZP (2008C21058) and ZCNI (J30802).

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