Observation of an organic-inorganic lattice match during biomimetic growth of (001)-oriented calcite crystals under floating sulfate monolayers

Assembly of the Intraskeletal Coral Organic Matrix during Calcium Carbonate Formation

Scleractinia coral skeleton formation occurs by a heterogeneous process of nucleation and growth of aragonite in which intraskeletal soluble organic matrix molecules, usually referred to as SOM, play a key role. Several studies have demonstrated that they influence the shape and polymorphic precipitation of calcium carbonate. However, the structural aspects that occur during the growth of aragonite have received less attention. In this research, we study the deposition of calcium carbonate on a model substrate, silicon, in the presence of SOM extracted from the skeleton of two coral species representative of different living habitats and colonization strategies, which we previously characterized. The study is performed mainly by grazing incidence X-ray diffraction with the support of Raman spectroscopy and electron and optical microscopies. The results show that SOM macromolecules once adsorbed on the substrate self-assembled in a layered structure and induced the oriented growth of calcite, inhibiting the formation of vaterite. Differently, when SOM macromolecules were dispersed in solution, they induced the deposition of amorphous calcium carbonate (ACC), still preserving a layered structure. The entity of these effects was species-dependent, in agreement with previous studies. In conclusion, we observed that in the setup required by the experimental procedure, the SOM from corals appears to present a 2D lamellar structure. This structure is preserved when the SOM interacts with ACC but is lost when the interaction occurs with calcite. This knowledge not only is completely new for coral biomineralization but also has strong relevance in the study of biomineralization on other organisms.

Evolution from Classical to Non-classical Aggregation-Based Crystal Growth of Calcite by Organic Additive Control

Aggregation-based crystal growth is distinct from the classical understanding of solution crystallization. In this study, we reveal that N-stearoyl-l-glutamic acid (C18-Glu, an amphiphile that mimics a biomineralization-relevant biomolecule) can switch calcite crystallization from a classical ion-by-ion growth to a non-classical particle-by-particle pathway, which combines the classical and non-classical crystallization in one system. This growth mechanism change is controlled by the concentration ratio of [C18-Glu]/[Ca(2+)] in solution. The high [C18-Glu]/[Ca(2+)] can stabilize precursor nanoparticles to provide building blocks for aggregation-based crystallization, in which the interaction between C18-Glu and the nanoprecursor phase rather than that of C18-Glu on calcite steps is highlighted. Our finding emphasizes the enrollment of organic additives on metastable nano building blocks, which provides an alternative understanding about organic control in inorganic crystallization.

Surface Tension Drives the Orientation of Crystals at the Air-Water Interface

The fabrication of oriented crystalline thin films is essential for a range of applications ranging from semiconductors to optical components, sensors, and catalysis. Here we show by depositing micrometric crystal particles on a liquid interface from an aerosol phase that the surface tension of the liquid alone can drive the crystallographic orientation of initially randomly oriented particles. The X-ray diffraction patterns of the particles at the interface are identical to those of a monocrystalline sample cleaved along the {104} (CaCO3) or {111} (CaF2) face. We show how this orientation effect can be used to produce thin coatings of oriented crystals on a solid substrate. These results also have important implications for our understanding of heterogeneous crystal growth beneath amphiphile monolayers and for 2D self-assembly processes at the air-liquid interface.

Morphological, chemical and kinetic characterisation of zein protein-induced biomimetic calcium phosphate films

A zein protein layer was used to mineralize thin films of calcium phosphate at the air–solution interface producing an iridescent mineral film with novel nano-morphology.

Calcium carbonate crystal growth beneath Langmuir monolayers of acidic β-hairpin peptides

Four amphiphilic peptides with designed hairpin structure were synthesized and their monolayers were employed as model systems to study biologically inspired calcium carbonate crystallization. Langmuir monolayers of hairpin peptides were investigated by surface pressure area isotherms, surface potential isotherms, Brewster angle microscopy (BAM), atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy. A β-hairpin conformation was found for all peptides at the air-water interface although their packing arrangements seem to be different. Crystallization of calcium carbonate under these peptide monolayers was investigated at different surface pressures and growth times both by in situ optical microscopy, BAM and ex situ investigations such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). An amorphous calcium carbonate precursor was found at the initial crystallization stage. The crystallization process occurred in three stages. It starts from the nucleation of amorphous particles being a kinetically controlled process. Crystal nuclei subsequently aggregate to large particles and vaterite crystals start to form inside the amorphous layer, with the monolayer fluidity exerting an important role. The third process includes the re-crystallization of vaterite to calcite, which is thermodynamically controlled by monolayer structural factors including the monolayer flexibility and packing arrangement of the polar headgroups. Thus, the kinetic factors, monolayer fluidity and flexibility as well as structure factors govern the crystal morphology and polymorph distribution simultaneously and synergistically.

Structure and kinetics of fatty acid Langmuir monolayers on zinc salt solutions

The adsorption of zinc cations under behenic acid Langmuir monolayers was investigated by means of isotherm measurements, grazing incidence X-ray diffraction and Brewster angle microscopy. The structure of the films was characterized as a function of Zn(2+) concentration, for three different counterions (chloride, iodide, bromide) and at two subphase pHs (5.5 and 7.5). At pH 5.5 and in the studied concentration range, Zn(2+) adsorption leads to a condensation of the fatty acid monolayer with the same phase transitions as over pure water. In contrast, at higher pH the organic X-phase is evidenced immediately above a concentration threshold without any ion organization. Even though Cu(2+) and Zn(2+)cations induce both the fatty acid X-phase, the kinetics of its formation appears strongly different. Indeed, as for Mg(2+) and Cd(2+), the intermediate new I-structure is evidenced in the course of Zn(2+) adsorption although superstructures are observed only for Mg(2+) and Cd(2+). However, for Zn(2+), the I-phase evolves to the final state through a new structure called X' and a continuous X'-X transition. Finally, any effect of the counterion is evidenced neither during the kinetic process nor in the final state.

Assembly of amorphous clusters under floating monolayers: a comparison of in situ and ex situ techniques

We report synchrotron X-ray scattering studies of biomimetic crystallization of hydroxyapatite (the primary constituent of bone), using monolayers of fatty acid molecules floating on simulated body fluid (SBF) as well as aqueous solutions of calcium phosphate. A ∼10 Å thick film of amorphous material is observed to form immediately at the molecular monolayer, consistent with the proposed formation of "Posner clusters". This layer becomes denser but not significantly thicker as the subphase concentration and the temperature approach physiological conditions. The amorphous films do not crystallize within 24 h, in contrast to prior reports of more rapid crystallization using electron microscopy on ex situ samples. However, crystallization occurs almost immediately after our films are transferred onto solid substrates. These results illustrate the importance of in situ measurements for model biomineralization experiments.

Charge, stereochemistry, or epitaxy? Toward controlled biomimetic nucleation at mixed monolayer templates

Floating monolayer mixtures of cationic dioctadecyldimethyldiammonium bromide and anionic lipids were used as variable templates for the biomimetic nucleation of calcium carbonate and studied using grazing incidence X-ray diffraction. Varying the ratio of constituents changes the monolayer charge, structure, and molecular tilt. The nucleating surface of calcite also changes as the mixture is varied, and at an 80:20 ratio the (012) face is seen under a floating monolayer template for the first time. Our results indicate that the average template lattice is the major controlling factor in the oriented nucleation of CaCO(3). This is in contrast to the current view that the orientation is controlled by the stereochemical matching of the terminal functional group and molecular tilt with respect to the carbonate groups in the crystal.

Ion-specific induced charges at aqueous soft interfaces

Ionic specificity effects, i.e., ions of the same valence leading to different macroscopic effects, are studied by considering a Langmuir monolayer of arachidic acid over a solution containing either Fe(3+) or La(3+). We systematically vary pH levels as a way to control the interfacial surface charge and characterize the system by surface-sensitive x-ray scattering and spectroscopic techniques. We show that the critical surface pressure at the tilted (L2) to untilted (LS) transition is ionic specific and varies with pH. While the maximum density of surface bound La(3+) per head group of arachidic acid is ∼0.3, the amount necessary to neutralize the surface charge, for Fe(3+) it is nearly 0.6 and it is accompanied with a significant accumulation of the coions Cl(-) as revealed by surface x-ray spectroscopy. We account for the experimental observations by a statistical mechanical model including ion specificity.