Conference-AsiaNANO 2016-Micro-Ikebana by Biomimetic Crystallization of Alkaline Earth Carbonates

Biomineralization is the process of forming hard exoor endoskeltons by biological organisms. The physical properties and the morphology of the composite material that is formed depends on the mode of crystal-growth, which depends on the diffusion of the constituents (metal cations, their anions, and additives). We chose a model system, strontium carbonate / silica, because the precipitation of SrCO3 and SiO2 is pH dependent. A lower pH accelerates SiO2 formation while it slows down SrCO3 crystallization, and vice versa. Thus, the precipitation of the composite can occur in an alternating fashion, regulating the morphology of the carbonate precipitate. The solution of SrCl2 and water glass is poured into a shallow container and a glass slide is floated on top, eliminating the effects of bulk precipitation and convection. The pH is adjusted to a starting value of 12, and a small amount of dimethyl carbonate is added. This compound slowly hydrolyses and gives off carbon dioxide, the carbonate source for the crystal growth. The crystal morphology is clearly dendritic and shows several distinct periods of growth. First, a few crystals grow from a common seed, and after a while, the blocking of surface sites on the carbonate crystals by water glass, leads to the start of a more needle-like growth with a significant increase of branching density. Finally, after seven days, coral-like structures are formed. [DOI: 10.1380/ejssnt.2017.65]


I. INTRODUCTION
Biomineralization is the process of organisms to produce an inorganic material.For example, shellfish, corals and phytoplankton such as coccolithophorids are generally well-known.The complicated structure of the coccolithophorid shell is influenced by the environment, e.g.temperature, pH, and CO 2 concentration.Their crystal shape is radically different from abiogenic bulk crystals [1], in which the unit cells of the crystal lattice arrange in a regular three-dimensional solid body with smooth sides and edges [2].The creation of biomimetic versions of biominerals, that are formed by mimicking some aspect of the conditions under which biominerals form, can be useful for applications that require high-surface area materials, such as catalysts [3] or column materials for separation or purification [4].
One popular approach to control the biomimetic crystal growth is the addition of polymers [5,6].Such additives allow the selective growth along certain crystal planes and also allow for the stabilization of metastable crystal phases.Inorganic additives such as SiO 2 can also be used [7], and we have reported on the fabrication of dendritic, flower-like, and coral-like microcrystals in a SrCl 2 / SiO 2 system [8].In the present paper, we further focus on the control of the crystalline form by changing the conditions of crystal growth.

II. EXPERIMENTAL
Strontium chloride hexahydrate (Wako, 99.0%), sodium silicate solution (water glass; Wako 57.0%, mol ratio SiO 2 /Na 2 O 2 : 2.06), dimethyl carbonate (Wako, 98.0%), and sodium hydroxide (Wako, 97.0%) were purchased commercially and used without further purification.Distilled water was used for the preparation of all required solutions.Glass cove slides (Matsunami NEO micro cover glass, 25 × 50 mm, thickness 0.12-0.17mm) were used as the crystallization substrate.The substrate was intensively cleaned with acetone and 2-propanol prior to use.Crystallization experiments were conducted in disposable petri dishes with a diameter of 8.5 mm.Stock-solutions of strontium chloride hexahydrate (0.1 M) and sodium silicate were used as the strontium carbonate and silica precursor respectively.The sodium silicate stock-solution was prepared by mixing 10 g of commercial water glass with 90 g of water and the addition of 15 µl of NaOH (1 M).In a typical diffusion controlled crystallization experiment an aliquot of the strontium chloride solution (200 µl for the 1 mM experiment, 600 µl for the 3 mM experiment, and 1 ml for the 5 mM experiment) were mixed with water to a total volume of 20 ml, followed by the addition of 200 µl of silica precursor (resulting pH = 12.0).This solution was poured in polystyrene dishes with a surface area of 10 cm 2 .Finally, the glass substrate was gently placed at the water surface, and the crystallization setup was covered with a Büchner funnel to avoid disturbance of the sample due to air movement.After substrate was placed at the air/water interface, 100 µl (1.19 mmol) of dimethyl carbonate were added to the crystallization medium for experiments involving DMC and covered by Parafilm R ⃝ to restrict CO 2 absorption from air.According to http://camblab.info/wp/index.php/parafilmfrequently-asked-questions/,Parafilm has a CO 2 permeability of ≤ 1100 cm 3 /m 2 /24h at 23 • C and 50% relative humidity, so that it is estimated that a maximum of 20 µmol of CO 2 could have diffused into the reaction medium each day.The samples were removed from the water surface after three to seven days, and washed copious times with water and EtOH.The sample was allowed to dry in air for further investigations with scanning electron microscopy.SEM pictures were taken with a JSM-7800F (JEOL, Japan), and element mapping was performed with an attached EDS equipment.

III. RESULTS AND DISCUSSIONS
Figure 1 is a schematic diagram of the experiment.W.Noorduin had immersed the substrate in the solution [7].Crystallization from solution leads to concentration depletion, and thus less density of the solution in the vicinity of the formed crystals, and under normal gravity conditions, vertical convection sets in.Thus unevenly distributes material and leads to heterogeneous crystallization.By floating the substrate on the solution, those effects of gravity and liquid convection on the crystal growth are limited.In previous experiments, we have used atmospheric CO 2 as the carbonate source.CO 2 has to diffuse into the reaction solution, and because the floating substrate covers some of the water surface, the carbonate concentration under the substrate shows a gradient.In most of the experiments described here, we use dimethylcarbonate (DMC) as a carbonate source, since DMC hydrolyses in alkaline solution to release CO 2 homogeneously in the solution.The strontium carbonate and silicon dioxide present in the solution precipitate alternately onto the substrate, and gradually SrCO 3 crystals will grow, as can be seen in the scheme in Figure 2. The pH decreases when reactions (1) and (2) take place, because their net reaction is CO 2 +H 2 O+SrCl 2 → SrCO 3 +2HCl.On the other hand, the pH increases during reaction 3, because protons are used up to transform silicate ions into SiO 2 .Thus, when reaction 1 and 2 occur, reaction (3) accelerates, depleting the solution of SiO 2− 3 ions.Since the SiO 2 precipitate on a SrCO 3 crystal will block most crystallization sites, and thus will reduce further H + production, the SiO 2 production will eventually become slower.The remaining active crystallization sites on the SrCO 3 crystals will then again lead to increased SrCO 3 precipitation, and decrease in pH.In a non-equilibrium state in which reactants can diffuse through the solution, reaction (1) + ( 2) and (3) will thus occur in a spatio-temporal pattern.Only selected areas on a SrCO 3 crystal surface are available for crystal growth, leading to highly branched, dendritic, crystals, as is schematically depicted in Figure 2.
The low magnification scanning electron microscopy (SEM) images in the left columns of Figs.3-5 give the reader a qualitative assessment on crystal density, shape, and size variation.The high magnification images in the center and right columns are representatives of the most commonly observed crystal types.
SEM images in Fig. 3  neous CO 2 concentration.Compared to the usual carbon dioxide source from air, dimethyl carbonate gave more dense structures, as can be seen in Figs. 4 and 5.The 'stems' of the flower-like structures have a larger diameter, and are less branched.Also, the 'flower-heads' are denser and less branched for all concentrations, meaning that the solubility product of SrCO 3 is reached even at lower Sr concentrations, because of the high carbonate concentration.Between three and seven days of crystal growth, the structures become even more dense, and a broccoli-like structure is formed.Thus we conclude that the higher carbonate concentration in the aqueous solution leads to faster crystal grow, before the SiO 2 blocking layer can form.
We did not carry out x-ray crystallography in order to determine the crystal structure of the formed carbonates, because it already has been reported that even large organic additives, such as p-aminobenzioc acid, or N-(2hydroxyethyl)ethylenediamine-N, N, N-triacetic acid did not change the crystal structure, and always the most stable orthorhombic strontionite [8].
As we have previously reported, the crystallization takes place at random nucleation sites on the glass substrate surface [9].In order to evaluate what types of surfaces induce crystallization, we used different types of surfaces: a very smooth, defect free hydrophilic surface of mica-an aluminium silicate-, a very rugged amphiphilic polymer honeycomb surface [10], and glass beads.Since mica is an anionic atomically flat surface with exposed silicate anions, we expected to see a similar crystallization blocking by the addition of water glass to the SrCl 2 solutions.And indeed, mica did not induce crystallization or dendritic flower-like growth.Figure 6 shows that the atomically flat mica surface is free of crystals.Only a few crystals are present at the step edges that were present on http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) the mica surface after peeling.Even though honeycomb surfaces can be used to immobilize inorganic nanoparticles [11,12] and induce a patterned adsorption of metal complexes [13], the highly rugged honeycomb surface is not suitable to induce crystallization either.Figure 6 shows just a few crystals at random positions on the honeycomb surface.Glass beads, on the other hand, act similar to planar glass surfaces and induce crystal seeding and growth (Fig. 7).Compared to flat glass surfaces, it is striking that the density of the SrCO 3 crystals is extremely high.The whole glass surface is covered with crystals to the extent that the glass surface cannot be seen anymore, and this is evidence for a much higher nucleation site density on a comparatively rough spherical glass surface as compared to a smooth, industrially made glass slide.
From the elemental analysis mapping with EDX in Figs. 8 and 9, it was verified that Sr, C and O were contained in the crystals that were grown both on flat substrates as well on glass micro beads, suggesting that they consist of SrCO 3 .

IV. CONCLUSIONS
We found that SrCO 3 dendritic crystals form when a SrCl 2 solution is mixed with water glass and covered with a floating glass, mica or honeycomb substrate.The coprecipitating water glass has a strong influence on crystal growths.Furthermore, the shape and dentritic structure can be controlled by adding dimethyl carbonate as a carbon dioxide source.Such structures may be useful as solid supports for catalysts, or to study the biometic crystal growths of biogenic dendritic inorganic crystals.

FIG. 1 .
FIG. 1. Schematic diagram of experimental setup with the floating substrate.