Nucleation and Growth of Calcium Phosphate from Physiological Solutions onto Self-Assembled Templates by a Solution-Formed Nucleus Mechanism

Synthetic amorphous calcium phosphates (ACPs): preparation, structure, properties, and biomedical applications

Amorphous calcium phosphates (ACPs) represent a metastable amorphous state of other calcium orthophosphates (abbreviated as CaPO₄) possessing variable compositional but rather identical glass-like physical properties, in which there are neither translational nor orientational long-range orders of the atomic positions. In nature, ACPs of a biological origin are found in the calcified tissues of mammals, some parts of primitive organisms, as well as in the mammalian milk. Manmade ACPs can be synthesized in a laboratory by various methods including wet-chemical precipitation, in which they are the first solid phases, precipitated after a rapid mixing of aqueous solutions containing dissolved ions of Ca²⁺ and PO₄^3- in sufficient amounts. Due to the amorphous nature, all types of synthetic ACPs appear to be thermodynamically unstable and, unless stored in dry conditions or doped by stabilizers, they tend to transform spontaneously to crystalline CaPO₄, mainly to ones with an apatitic structure. This intrinsic metastability of the ACPs is of a great biological relevance. In particular, the initiating role that metastable ACPs play in matrix vesicle biomineralization raises their importance from a mere laboratory curiosity to that of a reasonable key intermediate in skeletal calcifications. In addition, synthetic ACPs appear to be very promising biomaterials both for manufacturing artificial bone grafts and for dental applications. In this review, the current knowledge on the occurrence, structural design, chemical composition, preparation, properties, and biomedical applications of the synthetic ACPs have been summarized.

Oligo(l-glutamic acids) in Calcium Phosphate Precipitation: Chain Length Effect

The understanding of calcium phosphate precipitation is of major interest in different fields of science, including medicine, biomaterials, and physical chemistry. The presence of additive biomacromolecules has been known to influence various stages of the precipitation process from nucleation to crystal growth. In the current work, well-defined sequences of short, negatively charged peptides, oligo(l-glutamic acids), were utilized as a model, inspired by contiguous sequences of acidic amino acids in natural biomineralization proteins. The precipitate morphology and phases, the element time profile in solution and in the precipitates, as well as the kinetics during the precipitation process were analyzed to explain the effect of these short peptides on calcium phosphate precipitation. The results show that peptides can delay the phase transformation of an amorphous precursor phase to hydroxyapatite and that there is an optimal chain length for this effect at a given concentration of peptide. This study is the first part of a two-part series and is followed by a subsequent work to reveal the mechanism by which these short peptides influence the calcium phosphate precipitation.

Crystallization Behavior of the Low-Temperature Mineralization Sintering Process for Glass Nanoparticles

Bioactive glasses are promising materials for various applications, such as bone grafts and implants. The development of sintering techniques for bioactive glasses is one of the most important ways to expand the application to biomaterials. In this paper, we demonstrate the low-temperature mineralization sintering process (LMSP) of glass nanoparticles and their crystallization behavior. LMSP is a novel process employed to densify glass nanoparticles at an extremely low temperature of 120 °C. For this new approach, the hydrothermal condition, mineralization, and the nanosize effect are integrated into LMSP. To induce mineralization in LMSP, bioactive glass nanoparticles (BGNPs, 55SiO2-40CaO-5P2O5, mol%), prepared by the sol-gel process, were mixed with a small amount of simulated body fluid (SBF) solution. As a result, 93% dense BGNPs were realized under a temperature of 120 °C and a uniaxial pressure of 300 MPa. Due to the effect of mineralization, crystalline hydroxyapatite (HAp) was clearly formed at the boundaries of BGNPs, filling particles and interstitials. As a result, the relative density was remarkably close to that of the BGNPs conventionally sintered at 1050 °C. Additionally, the Vickers hardness value of LMSP samples varied from 2.10 ± 0.12 GPa to 4.28 ± 0.11 GPa, and was higher than that of the BGNPs conventionally sintered at 850 °C (2.02 ± 0.11 GPa). These results suggest that, in addition to LMSP being an efficient densification method for obtaining bulk bioactive glasses at a significantly lower temperature level, this process has great potential for tissue engineering applications, such as scaffolds and implants.

A mathematical model for persistent post-CSD vasoconstriction

Cortical spreading depression (CSD) is the propagation of a relatively slow wave in cortical brain tissue that is linked to a number of pathological conditions such as stroke and migraine. Most of the existing literature investigates the dynamics of short term phenomena such as the depolarization and repolarization of membrane potentials or large ion shifts. Here, we focus on the clinically-relevant hour-long state of neurovascular malfunction in the wake of CSDs. This dysfunctional state involves widespread vasoconstriction and a general disruption of neurovascular coupling. We demonstrate, using a mathematical model, that dissolution of calcium that has aggregated within the mitochondria of vascular smooth muscle cells can drive an hour-long disruption. We model the rate of calcium clearance as well as the dynamical implications on overall blood flow. Based on reaction stoichiometry, we quantify a possible impact of calcium phosphate dissolution on the maintenance of F0F1-ATP synthase activity.

Exceptional Fossil Conservation through Phosphatization

This paper addresses the taphonomic processes responsible for fossil preservation in calcium phosphate, or phosphatization. Aside from silicification and rarer examples of carbonaceous compression, phosphatization is the only taphonomic mode claimed to preserve putative subcellular structures. Because this fossilization window can record such valuable information, a comprehensive understanding of its patterns of occurrence and the geochemical processes involved in the replication of soft tissues are critical endeavors. Fossil phosphatization was most abundant during the latest Neoproterozoic through the early Paleozoic, coinciding with the decline of non-pelletal phosphorite deposits. Its temporal abundance during this timeframe makes it a particularly valuable window for the study of early animal evolution. Several occurrences of phosphatization from the Ediacaran through the Permian Period, including Doushantuo-type preservation of embryo-like fossils and acritarchs, phosphatized gut tracts within Burgess Shale-type carbonaceous compressions, Orsten-type preservation of meiofaunas, and other cases from the later Paleozoic are reviewed. In addition, a comprehensive description of the geochemical controls of calcium phosphate precipitation from seawater is provided, with a focus on the rates of phosphate nucleation and growth, favorable nucleation substrates, and properties of substrate tissue and pore-fluid chemistry. It is hoped that the paleontological and geochemical summaries provided here offer a practical and valuable guide to the Neoproterozoic–Paleozoic phosphatization window.

Recombinant DNA technology and click chemistry: a powerful combination for generating a hybrid elastin-like-statherin hydrogel to control calcium phosphate mineralization

Understanding the mechanisms responsible for generating different phases and morphologies of calcium phosphate by elastin-like recombinamers is supreme for bioengineering of advanced multifunctional materials. The generation of such multifunctional hybrid materials depends on the properties of their counterparts and the way in which they are assembled. The success of this assembly depends on the different approaches used, such as recombinant DNA technology and click chemistry. In the present work, an elastin-like recombinamer bearing lysine amino acids distributed along the recombinamer chain has been cross-linked via Huisgen [2 + 3] cycloaddition. The recombinamer contains the SNA15 peptide domains inspired by salivary statherin, a peptide epitope known to specifically bind to and nucleate calcium phosphate. The benefit of using click chemistry is that the hybrid elastin-like-statherin recombinamers cross-link without losing their fibrillar structure. Mineralization of the resulting hybrid elastin-like-statherin recombinamer hydrogels with calcium phosphate is described. Thus, two different hydroxyapatite morphologies (cauliflower- and plate-like) have been formed. Overall, this study shows that crosslinking elastin-like recombinamers leads to interesting matrix materials for the generation of calcium phosphate composites with potential applications as biomaterials.

Biomaterials-Potential nucleation agents in blood and possible implications

Blood, simulated body fluids, and many cell culture media are supersaturated solutions with respect to several calcium phosphates. Therefore biomaterials can act as nucleation agents and evoke heterogeneous nucleation of salts on the surface of immersed biomaterials. Depending on the field of application, this can be either beneficial or disadvantageous. Although nucleation from supersaturated solutions is an old and well-known scientific phenomenon it is not standard to test new developed materials with surface analytical methods for their ability to initiate nucleation in vitro. Therefore, this communication aims to review the mineralization effect and to emphasize the possible negative implications, especially to functionalized bone implants. Surface coatings with proteins, growth factors, and, etc., can become ineffective due to deposition of a dense calcium phosphate layer. In the case of drug loaded implants, drug release might be inhibited.

Formation of calcium phosphate nanostructures under the influence of self-assembling hybrid elastin-like-statherin recombinamers

The self-assembly properties of elastin-like-statherin recombinamers have great influence on calcium phosphate mineralization.

Mesoporous Hydroxyapatite as Olanzapine Carrier Provides a Long-Acting Effect in Antidepression Treatment

An antidepressant carrier was designed to maintain over 2 weeks of constant medication release. The carrier was injected into muscle, where cellular activity was employed to achieve the goal of constant release. Mesoporous hydroxyapatite (mesoHAP) was synthesized into an adequate size by a coprecipitation method; it then went through a series of hydrophobic surface modifications for olanzapine (OLZ) loading by physical absorption to produce mesoHAP-OLZ. Because of its hydrophobic nature, OLZ was not effectively released from mesoHAP-OLZ in an aqueous environment. However, once engulfed by macrophages, the lysosome/endosome hybrid ruptured due to alterations in osmotic pressure, resulting in the release of OLZ into the cytoplasm. OLZ was then exocytosed to the extracellular space due to a high calcium ion (Ca(2+)) concentration and finally reached the blood circulation. Our findings provide a useful treatment strategy to achieve long-term drug release with a single intramuscular (IM) injection, helping to solve the problem of nonadherent medication intake that often occurs in antidepressant therapy.

The use of physiological solutions or media in calcium phosphate synthesis and processing

This review examined the literature to spot uses, if any, of physiological solutions/media for the in situ synthesis of calcium phosphates (CaP) under processing conditions (i.e. temperature, pH, concentration of inorganic ions present in media) mimicking those prevalent in the human hard tissue environments. There happens to be a variety of aqueous solutions or media developed for different purposes; sometimes they have been named as physiological saline, isotonic solution, cell culture solution, metastable CaP solution, supersaturated calcification solution, simulated body fluid or even dialysate solution (for dialysis patients). Most of the time such solutions were not used as the aqueous medium to perform the biomimetic synthesis of calcium phosphates, and their use was usually limited to the in vitro testing of synthetic biomaterials. This review illustrates that only a limited number of research studies used physiological solutions or media such as Earle's balanced salt solution, Bachra et al. solutions or Tris-buffered simulated body fluid solution containing 27mM HCO3(-) for synthesizing CaP, and these studies have consistently reported the formation of X-ray-amorphous CaP nanopowders instead of Ap-CaP or stoichiometric hydroxyapatite (HA, Ca10(PO4)6(OH)2) at 37°C and pH 7.4. By relying on the published articles, this review highlights the significance of the use of aqueous solutions containing 0.8-1.5 mMMg(2+), 22-27mM HCO3(-), 142-145mM Na(+), 5-5.8mM K(+), 103-133mM Cl(-), 1.8-3.75mM Ca(2+), and 0.8-1.67mM HPO4(2-), which essentially mimic the composition and the overall ionic strength of the human extracellular fluid (ECF), in forming the nanospheres of X-ray-amorphous CaP.

Surface-engineered growth of AgIn₅S₈ crystals

The growth of semiconductor crystals and thin films plays an essential role in industry and academic research. Considering the environmental damage caused by energy consumption during their fabrication, a simpler and cheaper method is desired. In fact, preparing semiconductor materials at lower temperatures using solution chemistry has potential in this research field. We found that solution chemistry, the physical and chemical properties of the substrate surface, and the phase diagram of the multicomponent compound semiconductor have a decisive influence on the crystal structure of the material. In this study, we used self-assembled monolayers (SAMs) to modify the silicon/glass substrate surface and effectively control the density of the functional groups and surface energy of the substrates. We first employed various solutions to grow octadecyltrichlorosilane (OTS), 3-mercaptopropyl-trimethoxysilane (MPS), and mixed OTS-MPS SAMs. The surface energy can be adjusted between 24.9 and 50.8 erg/cm(2). Using metal sulfide precursors in appropriate concentrations, AgIn5S8 crystals can be grown on the modified substrates without any post-thermal treatment. We can easily adjust the nucleation in order to vary the density of AgIn5S8 crystals. Our current process can achieve AgIn5S8 crystals of a maximum of 1 μm in diameter and a minimum crystal density of approximately 0.038/μm(2). One proof-of-concept experiment demonstrated that the material prepared from this low temperature process showed positive photocatalytic activity. This method for growing crystals can be applied to the green fabrication of optoelectronic materials.

Hydroxylapatite nanoparticles: fabrication methods and medical applications

Hydroxylapatite (or hydroxyapatite, HAp) exhibits excellent biocompatibility with various kinds of cells and tissues, making it an ideal candidate for tissue engineering, orthopedic and dental applications. Nanosized materials offer improved performances compared with conventional materials due to their large surface-to-volume ratios. This review summarizes existing knowledge and recent progress in fabrication methods of nanosized (or nanostructured) HAp particles, as well as their recent applications in medical and dental fields. In section 1, we provide a brief overview of HAp and nanoparticles. In section 2, fabrication methods of HAp nanoparticles are described based on the particle formation mechanisms. Recent applications of HAp nanoparticles are summarized in section 3. The future perspectives in this active research area are given in section 4.

Biomimetic layer-by-layer templates for calcium phosphate biomineralization

Carboxylated, sulfated and/or phosphorylated surfaces are admitted as potential optimal templates for biomimetic deposition of calcium phosphate (CaP) coatings in view of improving implants' osseointegration. Layer-by-layer films were built up consisting of anionic chondroitin sulfate (ChS), a biological carboxylated and sulfated polysaccharide and cationic poly(l-lysine) (PLL). The films were used as soft matrices to immobilize a model phosphoprotein, phosvitin (PhV). The respective roles of ChS, PLL and PhV terminal layers on the heterogeneous nucleation kinetics and the structure of CaP deposits obtained from supersaturated solutions were inspected. Critical supersaturation ratios and induction times preceding heterogeneous nucleation were precisely determined and interpreted within the framework of classical nucleation theory in order to derive the effective interfacial energies of CaP crystals. It was found that the potency of terminal layers toward CaP nucleation increased in the order: PLL<ChS<PhV. Beyond a supersaturation threshold, PhV-terminated films exerted unique influence on the nucleation kinetics, maintaining the induction time at a constant value owing to conformational change of the PhV molecules upon calcium bridging. Promisingly, all films templated the deposition of thin (a few micrometer thick) uniform coatings of octacalcium phosphate and possibly hydroxyapatite, the two most relevant biological phases of CaP.

The role of nanoscale architecture in supramolecular templating of biomimetic hydroxyapatite mineralization

Understanding and mimicking the hierarchical structure of mineralized tissue is a challenge in the field of biomineralization and is important for the development of scaffolds to guide bone regeneration. Bone is a remarkable tissue with an organic matrix comprised of aligned collagen bundles embedded with nanometer-sized inorganic hydroxyapatite (HAP) crystals that exhibit orientation on the macroscale. Hybrid organic-inorganic structures mimic the composition of mineralized tissue for functional bone scaffolds, but the relationship between morphology of the organic matrix and orientation of mineral is poorly understood. Herein the mineralization of supramolecular peptide amphiphile templates, that are designed to vary in nanoscale morphology by altering the amino acid sequence, is reported. It is found that 1D cylindrical nanostructures direct the growth of oriented HAP crystals, while flatter nanostructures fail to guide the orientation found in biological systems. The geometric constraints associated with the morphology of the nanostructures may effectively control HAP nucleation and growth. Additionally, the mineralization of macroscopically aligned bundles of the nanoscale assemblies to create hierarchically ordered scaffolds is explored. Again, it is found that only aligned gel templates of cylindrical nanostructures lead to hierarchical control over hydroxyapatite orientation across multiple length scales as found in bone.

Comparison of the effects of possible mechanical stimuli on the rate of biochemical reactions

The aim of this work is to address the question of what constitutes a mechanical stimulation of biochemical reactions in general and further to compare the importance of the two possible mechanical stimulations: shear rate and the rate of volume variation. Using linear nonequilibrium thermodynamics, the Curie principle (the relation for coupling phenomena) is retrieved for a phenomenological relation for a scalar flux in an isotropic system. From these phenomenological relations for the rate of chemical reaction, it is established that the only scalar quantity related to the rate of deformation tensor D that cannot be neglected is the rate of volume variation D((1)). This leads us to the conclusion that, although tissues are exposed to all variety of mechanical factors: straining, shear, pressure, and even dynamic electric fields, the volume variation rate D((1)) is the most important mechanical stimulus driving the processes in them.

Carboxymethylation of the fibrillar collagen with respect to formation of hydroxyapatite

Control over crystal growth by acidic matrix macromolecules is an important process in the formation of many mineralized tissues. Highly acidic macromolecules are postulated intermediates in tissue mineralization, because they sequester many calcium ions and occur in high concentrations at mineralizing foci in distantly related organisms. A prerequisite for biomineralization is the ability of cations like calcium to bind to proteins and to result in concert with appropriate anions like phosphates or carbonates in composite materials with bone-like properties. For this mineralization process the proteins have to be modified with respect to acidification. In this study we modified the protein collagen by carboxymethylation using glucuronic acid. Our experiments showed unambigously, that N(epsilon)-carboxymethyllysine is the major product of the in vitro nonenzymatic glycation reaction between glucuronic acid and collagen. We hypothesized that the function of biomimetically carboxymethylated collagen is to increase the local concentration of corresponding ions so that a critical nucleus of ions can be formed, leading to the formation of the mineral. Thus, the self-organization of HAP nanocrystals on and within collagen fibrils was intensified by carboxymethylation.

Microporous nanofibrous fibrin-based scaffolds for bone tissue engineering

The fibrotic response of the body to synthetic polymers limits their success in tissue engineering and other applications. Though porous polymers have demonstrated improved healing, difficulty in controlling their pore sizes and pore interconnections has clouded the understanding of this phenomenon. In this study, a novel method to fabricate natural polymer/calcium phosphate composite scaffolds with tightly controllable pore size, pore interconnection, and calcium phosphate deposition was developed. Microporous, nanofibrous fibrin scaffolds were fabricated using sphere-templating methods. Composite scaffolds were created by solution deposition of calcium phosphate on fibrin surfaces or by direct incorporation of nanocrystalline hydroxyapatite (nHA). The SEM results showed that fibrin scaffolds exhibited a highly porous and interconnected structure. Osteoblast-like cells, obtained from murine calvaria, attached, spread and showed a polygonal morphology on the surface of the biomaterial. Multiple cell layers and fibrillar matrix deposition were observed. Moreover, cells seeded on mineralized fibrin scaffolds exhibited significantly higher alkaline phosphatase activity as well as osteoblast marker gene expression compared to fibrin scaffolds and nHA incorporated fibrin scaffolds (0.25 and 0.5g). All types of scaffolds were degraded both in vitro and in vivo. Furthermore, these scaffolds promoted bone formation in a mouse calvarial defect model and the bone formation was enhanced by addition of rhBMP-2.

Mid-infrared reflectivity of experimental atheromas

We report that the pathologic components present within the atheromatous plaques of ApoE knock-out mice can reflect significant amounts of mid-infrared (mid-IR) light. Furthermore, the reflected light spectra contained the unique signatures of a variety of biologic features including those found in unstable or "vulnerable" plaque. This discovery may represent a unique opportunity to develop a new intravascular diagnostic modality that can detect and characterize sites of atherosclerosis.

The nucleation and growth of calcium phosphate by amelogenin

The nucleation processes involved in calcium phosphate formation in tooth enamel are not well understood but are believed to involve proteins in the extracellular matrix. The ability of one enamel protein, amelogenin, to promote the nucleation and growth of calcium phosphate was studied in an in vitro system involving metastable supersaturated solutions. It was found that recombinant amelogenin (rM179 and rp(H)M180) promoted the nucleation of calcium phosphate compared to solutions without protein. The amount of calcium phosphate increased with increasing supersaturation of the solutions and increasing protein concentrations up to 6.5 μg/mL. At higher protein concentrations, the amount of calcium phosphate decreased. The kinetics of nucleation was studied in situ and in real time using a quartz crystal microbalance (QCM) and showed that the protein reduced the induction time for nucleation compared to solutions without protein. This work shows a nucleation role for amelogenin in vitro which may be promoted by the association of amelogenin into nanosphere templates, exposing charged functionality at the surface.

Bismuth Sulfide Thin Films with Low Resistivity on Self-Assembled Monolayers

Using self-assembled monolayers (SAMs), highly crystalline bismuth sulfide thin films with low electrical resistivity have been prepared from aqueous solution at low temperature (40-70 degrees C). The nucleation and growth process of Bi2S3 thin films was investigated in detail by XPS, AES, SEM, XRD, SAED, and HRTEM. Solution conditions have marked effects on the microstructure, growth rate, and mechanism of Bi2S3 films. Increased solution temperature resulted in a higher growth rate and a shorter induction time due to a higher supersaturation degree. In the solution of pH 1.12, homogeneous nucleation and the attachment process dominated the formation of Bi2S3 films. In contrast, at pH 0.47 Bi2S3 thin films were formed via heterogeneous nucleation and growth. The c-axial orientation of bismuthinite films was enhanced with the increase of reaction time. By controlling the solution supersaturation and reaction duration, highly crystalline Bi2S3 films composed of closely packed and coalescent crystallites could be realized, whose dark electrical resistivity could reach as low as 0.014 Omega cm without any post-treatment.

Matrix macromolecules in hard tissues control the nucleation and hierarchical assembly of hydroxyapatite

Biogenic minerals found in teeth and bones are synthesized by precise cell-mediated mechanisms. They have superior mechanical properties due to their complex architecture. Control over biomineral properties can be accomplished by regulation of particle size, shape, crystal orientation, and polymorphic structure. In many organisms, biogenic minerals are assembled using a transient amorphous mineral phase. Here we report that organic constituents of bones and teeth, namely type I collagen and dentin matrix protein 1 (DMP1), are effective crystal modulators. They control nucleation of calcium phosphate polymorphs and the assembly of hierarchically ordered crystalline composite material. Both full-length recombinant DMP1 and post-translationally modified native DMP1 were able to nucleate hydroxyapatite (HAP) in the presence of type I collagen. However, the N-terminal domain of DMP1 (amino acid residues 1-334) inhibited HAP formation and stabilized the amorphous phase that was formed. During the nucleation and growth process, the initially formed metastable amorphous calcium phosphate phase transformed into thermodynamically stable crystalline hydroxyapatite in a precisely controlled manner. The organic matrix-mediated controlled transformation of amorphous calcium phosphate into crystalline HAP was confirmed by x-ray diffraction, selected area electron diffraction pattern, Raman spectroscopy, and elemental analysis. The mechanical properties of the protein-mediated HAP crystals were also determined as they reflect the material structure. Such understanding of biomolecule controls on biomineralization promises new insights into the controlled synthesis of crystalline structures.

Topography and surface energy dependent calcium phosphate formation on Sol-Gel derived TiO2 coatings

Heterogeneous nucleation and growth of calcium phosphate (CaP) on sol-gel derived TiO(2) coatings was investigated in terms of surface topography and surface energy. The topography of the coatings was derived from AFM measurements, while the surface energy was determined with contact angle measurements. The degree of precipitation was examined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The precipitation of CaP was found to be dependent on both topography and surface energy. A high roughness value when combining the RMS roughness parameter S(q) with the number of local maxima per unit area parameter S(ds) enhances CaP formation. The hydrophilicity of the coating was also found to be of importance for CaP formation. We suggest that the water contact angle, which is a direct measure of the hydrophilicity of the surface, may be used to evaluate the surface energy dependent precipitation kinetics rather than using the often applied Lewis base parameter.

One step synthesis of silver nanorods by autoreduction of aqueous silver ions with hydroxyapatite: An inorganic–inorganic hybrid nanocomposite

In this report, a novel method for the synthesis of silver nanoparticles on the surface of hydroxyapatite is described. Hydroxyapatite crystals are synthesized from acid mineralized solution, a byproduct of bone glue industries, by a simple chemical precipitation method, which results in the formation of hydroxyapatite nanocrystals. The reduction of silver ions occurs by the electron transfer from the hydroxyl groups on the surface of hydroxyapatite. This results in the formation of silver nanorods and needle shaped nanoparticles that are bound on the surface of hydroxyapatite, and the observed silver nanocrystals show anisotropic structure. Thus, hydroxyapatite crystals can be used as a new class of inorganic scaffolds for the synthesis of nanomaterials with implications in designing inorganic-inorganic hybrid nanocomposites for different applications.

Oriented growth of TS-1 zeolite ultrathin films on poly(ethylene oxide) monolayer templates

Self-assembled monolayers of poly(ethylene oxide) (PEOM) were used as templates to direct the nucleation and subsequent oriented growth of TS-1 zeolite thin films. The resulting TS-1 zeolite thin films exhibited completely oriented crystalline domains with the 010 crystallographic direction parallel to the surface normal with a little deformation in the in-plane direction. Remarkably, the correlation length of the film is as extremely large as 1700 A (1200 A) in the out-of-plane (in-plane) direction. PEOM act as two-dimensional media for the nucleation and growth of highly oriented TS-1 zeolite thin films. This study demonstrates that the highly oriented TS-1 thin films can be achieved by using the inner parts of PEOM as templates.

Synthesis of hydroxyapatite crystals using amino acid-capped gold nanoparticles as a scaffold

Inorganic composites are of special interest for biomedical applications such as in dental and bone implants wherein the ability to modulate the morphology and size of the inorganic crystals is important. One interesting possibility to control the size of inorganic crystals is to grow them on nanoparticles. We report here the use of surface-modified gold nanoparticles as templates for the growth of hydroxyapatite crystals. Crystal growth is promoted by a monolayer of aspartic acid bound to the surface of the gold nanoparticles; the carboxylate ions in aspartic acid are excellent binging sites for Ca(2+) ions. Isothermal titration calorimetry studies of Ca(2+) ion binding with aspartic acid-capped gold nanoparticles indicates that the process is entropically driven and that screening of the negative charge by the metal ions leads to their aggregation. The aggregates of gold nanoparticles are believed to be responsible for assembly of the platelike hydroxyapatite crystals into quasi-spherical superstructures. Control experiments using uncapped gold nanoparticles and pure aspartic acid indicate that the amino acid bound to the nanogold surface plays a key role in inducing and directing hydroxyapatite crystal growth.