Biomimetic Synthesis Of Metal Carbonate Microspheres As Templates For Polyelectrolyte Capsules Formation

In the past decade, the biomimetic synthesis of inorganic particle materials with specific size and morphology has become an important research filed due to in the development of new materials such as advanced materials, catalysis, medicine, electronics, ceramics, pigments, etc.. The strategy of using organic additives to control the nucleation, growth, and alignment of inorganic particles has been universally applied for the controlled synthesis of various inorganic materials with unusual and complex forms.Metal carbonates, especially calcium carbonate (CaCO3), were chosen as one of the standard model systems due to their abundance in nature and also their important industrial application in paints, plastics, rubber, or paper. Biomimetic synthesis of biominerals such as CaCO3 crystals in the presence of organic templates and/or additives has been intensively investigated in recent years as reviewed recently. Langmuir monolayers, ultrathin organic films, self-assembled films, foam lamellae,cross-linked gelatin films, polymer substrates, crystalimprinted polymer surfaces,and polymeric matrixes have been used as effective templates or were employed to direct the controlled growth of CaCO3 crystals. It has been shown that special functional low molecular weight and polymeric additives can influence the crystallization of CaCO3 strongly, including complex liquidlike morphologies or stabilized amorphous CaCO3. Similarly, barium carbonate exists in nature as a thermodynamically most stable crystal modification among the heavy metal carbonates (ACO3, A = Sr, Ba, Pb) even though theorthorhombic phase is metastable in the case of calcium carbonate. BaCO3 has also attracted a lot of recent research due to its close relationship with aragonite, a prevalent and important biomineral, with many important applications in the ceramic and glass industries as well as its use as a precursor for magnetic ferrites and/or ferroelectric materials.Polyelectrolyte microcapsules, produced by stepwise adsorption of oppositely charged polyelectrolytes (PEs), also known as Layer-by-Layer (LbL) adsorption onto the surface of colloidal particles followed by core dissolution, are currently being studied intensively. The obtained hollow capsules keep the original size and shape of the template.This base technology has recently inspired considerable attention in the fields of drug delivery, biosensors, microreactors, and bioseparations. Different colloidal templates have been introduced for the fabrication of such microcapsules, ranging from polymers and metal particles to crystals of proteins, low molecular weight compounds, and biological cells.However, problems remain with the dissolution of materials to produce clean capsules. One of the most important steps in capsules preparation, which may affect finally the multilayers, is the core removal process. Most of the capsule wall properties depend on the fabrication history.Most commercially available templates such as melamine formaldehyde (MF) require harsh conditions such as strong acids or organic solvents to be dissolved, and in some cases template materials cannot be completely removed from the capsules because of interaction with capsule walls. It is believed that the MF oligomers, a product of acidic hydrolysis, lead to an increased osmotic pressure during dissolution and therefore affect the integrity of the capsule wall.The amount of MF bound to the capsule wall is hard to control and the formation of MF/polyion complex is undesirable since it makes capsules unsuitable for biomedical applications. Thus, improved templates are essential for efficient production of high-quality capsules.Inorganic crystals, such as carbonates can be perfect candidates. These inorganic carbonate crystal materials are also advantageous for the formation of polymeric micro/nanocapsules, as they can be synthesized from simple reactions, even those as simple as mixing two chemical solutions. More importantly, they can be dissolved under mild conditions to obtain clean capsules, e.g. pH 2-3 or complexing agents, because the metal ions will not interact with polyelectrolyte walls and form complexes such as MF oligomers. For example, carbonate cores can be dissolved with EDTA solution8b without adding acid, which is an additional advantage of the carbonate crystals when employed for making capsules from pH-sensitive species such as enzymes and nucleic acids. Moreover, biocompatibility of possible remaining core material is required for their use in drug delivery.In this paper, spherical MnCO3, CaCO3, BaCO3 particles were separately prepared by a precipitation reaction of ammonium hydrogen carbonate with manganese sulfate monohydrate, anhydrous calcium chloride and barium chloride dehydrate in the presence of ethanol, sodium poly(styrene sulfonate) (PSS) , PSS/SDS complexes and synthetic macromolecules containing a sulfonate group as crystal modifiers at room temperature. The as-prepared products were characterized with scanning electron microscopy (SEM) and X-ray diffraction (XRD)The quality of the resulting microparticles was found to be stronglydependent on the experimental conditions such as the type of additives used and their concentration, temperature, ratio of reactant concentration and rate of agitation of the reaction mixture probably through their effect on the rate of the nucleation process.Different additives to the reaction mixture were shown to exert a profound effect on the morphology of the carbonate microparticles formed. They played an important role of control on the nucleation, growth and alignment of carbonate particles. However, despite great efforts the problem of controllabl crystallization of carbonate resulting in macroscopic quantities of uniform, homogeneous, and nonaggregated carbonate micro- and nanoparticles remains to be solved. We have succeed in preparing monodisperse MnCO3 sphericalparticles.These microspheres are about 5μm in size and their surface is composed of rhombus-shaped rhodochrosite subunits,resulting in somewhat rough surfaces. Following production of MnCO3 particles, hollow polyelectrolyte microcapsules were fabricated by Layer-by-Layer assembly of common polyelectrolyte pair PSS/PAH on the MnCO3 particles, followed by core removal with EDTA. The particles were coated with nine bilayers of {PSS/ PAH} to allow observation with optical microscopy. Such capsules represent a micron sized freestanding PE film consisting only of the desired species. The texture of the particle surface determines the morphology of the derived capsules as well as the capsule wall thickness.