Control of calcium carbonate crystallization by using anionic polymethylsiloxanes as templates

Abstract

Sulfonated (SO₃H-PMS) and carboxylated (CO₂H-PMS) polymethylsiloxanes were synthesized and their effects as anionic template modifier on the CaCO₃ crystal morphologies were evaluated. In vitro crystallization assays of CaCO₃ were performed at room temperature by using gas diffusion method at different concentration, pH and time. SEM images of CaCO₃ showed well-defined short calcite piles (ca. 5 μm) and elongated calcite (ca. 20 μm) when SO₃H-PMS was used. When CO₂H-PMS was used, the morphology of CaCO₃ crystals was single-truncated at pH 7–9 and aggregated-modified calcite at pH 10–11. However, at pH 12 the least stable donut-shaped vaterite crystals were formed. EDS and XRD confirmed the presence of Si from anionic PMS templates on the CaCO₃ surfaces and its polymorphism, respectively. Results showed that the selective morphologies of CaCO₃ reflect the electrostatic interaction of anionic groups of functionalized PMS with Ca²⁺ adsorbed on CaCO₃ crystals. Rounded and truncated-modified fluorescent CaCO₃ was also produced by the inclusion of functionalized PMS into the lattice of CaCO₃ matrix. We demonstrated that the anionic PMS offer a good modifier for polymer-controlled crystallization and a convenient approach for understanding the biomineralization field.

Introduction

Biomineralization is the process by which living organisms produce biological composites and exert precise control over minerals they deposit, creating bioceramic materials with uniform particle sizes, novel morphologies, myriad shapes and sizes that are often of high strength and remarkable properties [1], [2]. This process offers an organism more than just structural support and mechanical strength. As nature´s master builder, it is involved in a wide variety of important biological functions such as: protection, motion, cutting and grinding, buoyancy, storage, optical detection, magnetic and gravity sensing. Examples of bioceramic are mollusk and egg-shells, crustacean carapaces, bones, teeth, etc. Therefore, molecular processes involved in this process are of great interest to materials scientists whom seek to manufacture bio-composites and analogous hybrid materials to those formed in nature. Abundant efforts in search of innovative biomimetic processing strategies to produce e.g., inorganic thin films has been performed [3], [4]. The majority of these efforts have focused on exploring the promoting or inhibiting effects of polymeric additives as templates on crystal nucleation and growth [5]. Several approaches can be discerned, according to the nature and structural complexities of the templates employed and have been performed on the in vitro [6], [7], [8], [9] or in vivo [10], [11], [12] assays. Templates used in in vitro refer to those substrates with particular sequence fragments that could mediate special mineral crystals. The presence of biomacromolecules turns the deposition process of calcium carbonate (CaCO₃) from homogeneous nucleation to heterogeneous nucleation, and the reaction condition reaches the kinetic limitation requirements. Thus, polymer molecules adsorbed on the glass substrate can provide the conditions for heterogeneous nucleation of aragonite crystals, which is then under certain condition energetically more favorable than homogeneous nucleation of calcite crystals in solution. These investigations have yielded valuable information on how organic matrix can affect biomineral formation [3], [4], [5], [7].

CaCO₃ is one of the most studied inorganic material, which has facilitated the understanding of biological control of biomineralization [13], [14]. It has found abundant applications in the paper, textile, paint, rubber and adhesive industries. It is well known that anhydrous CaCO₃ have three polymorphs: rhombohedric calcite, orthorhombic aragonite and hexagonal vaterite with decreasing stabilities and increasing solubility limits in aqueous environment at 25 °C. Their solubility constant (Ksp) values are 10^−8.48, 10^−8.34 and 10^−7.91, respectively. CaCO₃ biominerals usually consist in single-crystals intimately associated with organic matrices. Calcite is one of the most common polymorph in nature. Some calcite biominerals, such as sponge spicules [15], [16] and sea urchins [17], are produced as skeletal elements that behave as single-crystal. Therefore, there is a growing interest in CaCO₃ crystallization using polymeric additives as crystal growth modifiers or nucleating agents [18], [19], [20], [21], [22], [23]. Other biomineralized CaCO₃ such as the prismatic layers of many mollusk shells are partly single-crystalline subunits, which are composed of calcite and aragonite [24]. Precipitation of such biominerals take place in confined microenvironments microenvironments by cells under chemical control with specialized function [25]. The research performed of CaCO₃ reveals that crystal growth and morphology are function of a number of experimental variables that include the pH [26], temperature [27], saturation, time [28], crystallization method, polarity of reactants [29], concentration [30], [31] and the nature of additives. Many approaches have been used to synthesize specific forms of CaCO₃ using different templates such as: dynamic liquid–liquid interfaces [32], piles of lipid bilayer stacks [33], functionalized micropatterned surfaces [23], [34], proteins extracted from CaCO₃ rich microorganisms [35], chiral molecules [36], etc.

The crystallization mechanism can be altered by specific interactions with polar functional groups such as: –CO₂H, –PO₃H, –SO₃H [23], [37], [38], [39], [40], [41], [42]. From a thermodynamic point of view, resulting morphology of crystals is an expression of differentiated growth rates in the distinct crystallographic directions. Thus, the obtained crystal morphology minimizes the free enthalpy of the crystal, which is the sum of the products of surface energy and area of all exposed faces (Wulff's rule) [43]. Crystal surface energies can be lowered by the adsorption of additives present in solution modifying the crystalline morphology [44]. The crystallographic planes on which the additives are adsorbed become expressed as crystals faces according to Wulff's rule [45]. Therefore, the use of polymeric templates induces the production of new hybrid materials with specific structure, advanced properties and functions [46]. Studies addressing the biological control over inorganic mineralization have been carried out applying organic substrates as additives. Wei et al. [47] loaded doxorrubicin (DOX) in CaCO₃ hollow spheres to enhance the citotoxicity of the drug by electrostatic interaction, which can be explained by increasing cellular uptake, perinuclear accumulation and nuclear entry. This hybrid nanodevice exhibited good biocompatibility due to CaCO₃ component compared with other synthetic pH platform with great clinical applications [47]. Therefore, polyanionic biomacromolecules have been reported to mediate mineralization by forming aggregates with mineral ions and has been also involved in the temporary stabilization of amorphous precursor phases of crustacean, which occur as a transient phase in CaCO₃ formation [48], [49]. Evidences for the existence of these biomacromolecules have been found within the biominerals as inter-crystalline matrices [50], [51], [52] suggesting that precipitation mediating molecules may partly become incorporated during crystallization.

On the other hand, to our knowledge, few studies concerning the use of poly(organosiloxane)s derivatives as an inorganic templates for controlling CaCO₃ crystals have reported [40], [41], [53]. Polymethylsiloxanes (PMS), usually referred to as silicones, have numerous applications in different fields of chemistry and engineering. PMS also represent the most widely used silicon-containing polymeric systems in different industrial and medical applications [54], [55]. Polydimethylsiloxane (PDMS) and polyhydrogenmethylsiloxane (PHMS) exhibit versatile properties such as thermal stability, hydrophobicity, low surface tension, flexibility, bioactivity, among others [54]. It is known that the current uses of PMS in the medical, personal care, and pharmaceutical industries include: delivery vehicles for active components in personal care products, lubricant in medical devices (e.g., syringes) [56], external applications (intravenous bags and skin patches for transdermal drug delivery), implanted prosthetic devices [57], etc. Widely applied and explored hydrosilylation reaction of polymers have attracted great interest due to the practical outcome and recent development in silicon-based polymers [58], [59]. Hydrosilylation is a general method used for the addition of organic or inorganic silicon hydrides to molecules containing multiple bonds, and many catalytic systems based on platinum complexes such as Speier´s [60], [61], Lamoreaux´s [62] and Karsted´s [63], [64] catalysts. Among them, Speier´s catalyst, which is prepared from hydrated hexachloroplatinic acid (IV) and 2-propanol, is the most effective catalyst for the functionalization of PHMS backbone for obtaining derivatized PMS [65]. The main synthetic route leading to PMS modification is the hydrosilylation reaction of unsaturated compounds, such as allyl derivatives, with polysiloxanes containing Si–H labile bond and by using platinum Speier's catalysts [66], [67].

Considering the above mentioned, PMS polymers could become quite attractive as new templates for in vitro crystallization essays [40], [41], [53], [68] since PMS can be chemically modified with different functional groups as those existing on the surface of organic biomacromolecules, emulating the guide role of these during the in vivo mineralization. Although abundant articles on the production of modified PMS, as medical and non-medical applications exists [69], [70], [71], [72], [73], the available information concerning the effect of PMS as modifier on crystallization is still scarce. This work is motivated by the lack of studies on in vitro inorganic biomineralization by using functionalized PMS as template. We report the preparation of templates based on sulfonated (SO₃H-PMS) and carboxylated (CO₂H-PMS) polymethylsiloxanes through hydrosilylation with dicyclopentadienyl platinum II chloride (Cp₂PtCl₂), a non-commercially available catalyst and to demonstrate their capacity for controlling the morphogenesis and the crystallographic polymorphism of CaCO₃ using a gas diffusion method [74].