Structural development of mercaptophenol self-assembled monolayers and the overlying mineral phase during templated CaCO3 crystallization from a transient amorphous film

Formation of biomineral structures is increasingly attributed to directed growth of a mineral phase from an amorphous precursor on an organic matrix. While many in vitro studies have used calcite formation on organothiol self-assembled monolayers (SAMs) as a model system to investigate this process, they have generally focused on the stability of amorphous calcium carbonate (ACC) or maximizing control over the order of the final mineral phase. Little is known about the early stages of mineral formation, particularly the structural evolution of the SAM and mineral. Here we use near-edge X-ray absorption spectroscopy (NEXAFS), photoemission spectroscopy (PES), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to address this gap in knowledge by examining the changes in order and bonding of mercaptophenol (MP) SAMs on Au(111) during the initial stages of mineral formation as well as the mechanism of ACC to calcite transformation during template-directed crystallization. We demonstrate that formation of ACC on the MP SAMs brings about a profound change in the morphology of the monolayers: although the as-prepared MP SAMs are composed of monomers with well-defined orientations, precipitation of the amorphous mineral phase results in substantial structural disorder within the monolayers. Significantly, a preferential face of nucleation is observed for crystallization of calcite from ACC on the SAM surfaces despite this static disorder.     

Growth of polar crystal surfaces on ionized organic substrates

Calcite crystals nucleate on the (01.2) face on a diverse range of organic substrates, including self-assembled monolayers, hydrogen-bonded ribbons, and polymer rafts. The (01.2) face of calcite is a polar surface. Therefore macroscopic crystal growth can only occur if the dipole moment is quenched. We demonstrate that the dipole moment can be quenched for a given polar direction by adsorption onto an organic substrate with arbitrary charge density. The density of ions in the outer calcium plane must be modified, by introducing rows of vacancies, to fulfill the condition of zero net dipole moment. Interfacial energies are calculated for interfaces between the polar (01.2) and (00.1) faces of calcite and stearic acid monolayers with a range of densities. It was found that, contrary to the experimental evidence, the (00.1) face has lower interfacial energy than the (01.2) face with monolayers with equivalent densities. We give an explanation for this discrepancy based on kinetic models.     

Effect of magnesium ions on oriented growth of calcite on carboxylic acid functionalized self-assembled monolayer

The combined effect of templating and solution additives on calcite crystallization was studied. Self-assembled monolayers of mercaptoundecanoic acid supported on silver, as templates, induced the uniform, oriented nucleation of calcite from the (012) plane. The presence of Mg2+ in the crystallizing solution affected the crystal growth dramatically, due to the selective Mg binding to the calcite planes roughly parallel to the c-axis. Highly homogeneous arrays of oriented crystals with characteristic sizes, shapes, and morphology, depending on the relative concentration of Mg and Ca ions, were synthesized.     

Thermodynamics of epitaxial calcite nucleation on self-assembled monolayers

Classical heterogeneous nucleation theory is used to describe the epitaxial nucleation of calcite on self-assembled monolayers (SAMs). Both spherical and faceted clusters are considered. The use of faceted clusters reveals a useful relation between the shape of very small crystals and the ratio of the heterogeneous and homogeneous nucleation barriers. The experimental approach of this paper concerns the measurement of the threshold driving forces for both homogeneous and heterogeneous nucleation of calcite. This is accomplished by preparing solutions with well-defined driving forces and by measuring the resulting types of nucleation that are observed after a fixed experimental time. The results of the experiments and the theoretical shape analysis are compared, and it is shown that in the experiments no homogeneous nucleation of calcite occurs for driving forces up to at least Deltamu/k(B)T approximately equal to 6.0. A calculation of the critical cluster size for heterogeneous nucleation results in a range of 2-28 growth units and faceted critical clusters from 3-28 growth units, depending on the value of the surface free energy of calcite. These sizes are 50-100 times smaller than the crystalline domain sizes of SAMs and therefore small enough to explain the promoting effect of the substrate.     

Effect of alkyl chain parity on the face-selective crystal growth of a drug polymorph

Carboxy terminated alkanethiol self-assembled monolayers promote the face-selective nucleation of the P-monoclinic polymorph of carbamazepine; the type of face nucleated depends on the parity of the alkyl chain.     

Templating Effect of Amion Monolayers on the Formation of Anionic Clay Sheets at an Air-Water Interface

Monolayers have been reported to play a template role in the formation of minerals at interfaces. For example, the oriemted nucleation of calcite is controlled by self-assembled monolayers (SAMs) of to-terminated alkanethiols ((CH₂)_nX)^[1].This presentation shows the templating effect of a Langnuir monolayer of stearic acid (SA) during the preparation a thin film of anionic clay, The work is an extension of our previous works on the formation of a hybrid film of a clay and a metal complex by the Langmuir-Blodgett (LB) method^[2].     

Molecular recognition controls the organization of mixed self-organized bis-urea-based mineralization templates for CaCO(3)

To investigate the role and importance of nondirectional electrostatic interactions in mineralization, we explored the use of Langmuir monolayers in which the charge density can be tuned using supramolecular interactions. It is demonstrated that, in mixed Langmuir monolayers of bis-ureido surfactants containing oligo(ethylene oxide) and ammonium head groups associated with matching or nonmatching spacers between the two urea groups, the organization is controlled by molecular recognition. These different organizations of the molecules lead to different nucleation behavior in the mineralization of calcium carbonate. The formation of modified calcite and vaterite crystals was induced selectively by different phases of mixed monolayers, and they were characterized by SEM, TEM, and SAED. To understand the influence of the mixed Langmuir monolayers on the crystallization process, we studied the mixtures by means of (pi-A) isotherms and Brewster angle microscopy observations. Infrared reflection-absorption spectroscopy experiments were also performed on Langmuir-Schaefer films. From these results, we conclude that the local organization of the two systems discussed here gives rise to differences in both charge density and flexibility that together determine not only polymorph selection and the nucleation face but also the morphology of the resulting crystals.     

Hydrogels coupled with self-assembled monolayers: an in vitro matrix to study calcite biomineralization

This paper describes the control of the nucleation and growth of calcite crystals by a matrix composed of an agarose hydrogel on top of a carboxylate-terminated self-assembled monolayer (SAM). The design of this matrix is based upon examples from biomineralization in which hydrogels are coupled with functionalized, organic surfaces to control, simultaneously, crystal morphology and orientation. In the synthetic system, calcite crystals nucleate from the (012) plane (the same plane that is observed in solution growth). The aspect ratio (length/width) of the crystals decreases from 2.1 +/- 0.22 in solution to 1.2 +/- 0.04 in a 3 w/v % agarose gel. One possible explanation for the change in morphology is the incorporation of gel fibers inside of the crystals during the growth process. Etching of the gel-grown crystals with deionized water reveals an interpenetrating network of gel fibers and crystalline material. This work begins to provide insight into why organisms use hydrogels to control the growth of crystals.     

Growth and analysis of octadecylsiloxane monolayers on Al2O3 (0001)

On solvent-cleaned and piranha-etched single-crystal Al2O3(0001) surfaces, uniform, robust, self-assembled monolayers of octadecylsiloxane (ODS) are formed by 48 h exposure to a solution containing octadecyltrichlorosilane (OTS) in an anhydrous atmosphere. X-ray photoelectron spectroscopy, atomic force microscopy, ellipsometry, and water contact angle measurements confirm the presence of a uniform, complete monolayer. Reducing the exposure time or omitting the piranha-etch leads to much less uniform coverage. The ODS monolayers are stable when stored in ambient atmospheres for month-long periods. Thermal desorption in a vacuum environment (10(-9) Torr) shows the ODS monolayer is thermally stable up to at least 420 K. When heated in 200 mTorr of flowing forming gas (N2-10% H2) for 1 h at 520 K, slow loss of ODS was indicated. A schematic model is proposed which involves island nucleation by covalent bonding of OTS to surface hydroxyl groups followed by growth through the addition of mobile ODS species.     

A quasi-time-resolved CryoTEM study of the nucleation of CaCO3 under langmuir monolayers

Calcium carbonate biomineralization uses complex assemblies of macromolecules that control the nucleation, growth, and positioning of the mineral with great detail. To investigate the mechanisms involved in these processes, for many years Langmuir monolayers have been used as model systems. Here, we descibe the use of cryogenic transmission electron microscopy in combination with selected area electron diffraction as a quasi-time-resolved technique to study the very early stages of this process. In this way, we assess the evolution of morphology, polymorphic type, and crystallographic orientation of the calcium carbonate formed. For this, we used a self-assembled Langmuir monolayer of a valine-based bisureido surfactant (1) spread on a CaCl2-containing subphase and deposited on a holey carbon TEM grid. In a controlled environment, the grid is exposed to an atmosphere containing NH3 and CO2 (the (NH4)2CO3 diffusion method) for precisely determined periods of time (reaction times 30-1800 s) before it was plunged into melting ethane. This procedure allows us to observe amorphous calcium carbonate (ACC) particles growing from a few tens of nanometers to hundreds of nanometers and then crystallizing to form [00.1] oriented vaterite. The vaterite in turn transforms to yield [10.0] oriented calcite. We also performed the reaction in the absence of monolayer or in the presence of a nondirective monolayer of surfactant containing an oligo(ethylene oxide) 2 head group. Both experiments also showed the formation of a transient amorphous phase followed by a direct conversion into randomly oriented calcite crystals. These results imply the specific though temporary stabilization of the (00.1) vaterite by the monolayer. However, experiments performed at higher CaCl2 concentrations show the direct conversion of ACC into [10.0] oriented calcite. Moreover, prolonged exposure to the electron beam shows that this transformation can take place as a topotactic process. The formation of the (100) calcite as final product under different conditions shows that the surfactant is very effective in directing the formation of this crystal plane. In addition, we present evidence that more than one type of ACC is involved in the processes described.     

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.     

Determining the nucleation rate from the dimer growth probability

A new method is proposed for the determination of the stationary one-component nucleation rate J with the help of data for the growth probability P2 of a dimer which is the smallest cluster of the nucleating phase. The method is based on an exact formula relating J and P2, and is readily applicable to computer simulations of nucleation. Using the method, the dependence of J on the supersaturation s is determined by kinetic Monte Carlo simulations of two-dimensional (2D) nucleation of monolayers on the (100) face of Kossel crystal. The change of J over nearly 11 orders of magnitude is followed and it is found that the classical nucleation theory overestimates the simulation J values by an s-dependent factor. The 2D nucleus size evaluated via the nucleation theorem is described satisfactorily by the classical Gibbs-Thomson equation and its corrected version accounting for the spinodal limit of 2D nucleation.     

Kinetics of octadecyltrimethylammonium bromide self-assembled monolayer growth at mica from an aqueous solution

We have studied the growth kinetics of self-assembled monolayers (SAMs) ofoctadecyltrimethylammonium bromide (C18TAB) on mica below the critical micelle concentration at 22, 30, 40, and 50 degrees C. A combination of atomic force microscopy, contact angle goniometry, and transmission infrared spectroscopy was used to follow the growth processes to determine the rates involved in the growth of a C18TAB SAM on mica. The growth of a SAM consisted of four distinct processes: deposition of adsorbate molecules, growth of a disordered 2D liquid phase, nucleation of islands of an ordered 2D solid phase, and subsequent growth of the solid phase. The rates of these various processes are determined, and the activation energies for several processes were calculated including those for the adsorption onto a bare substrate (20 kJ/mol), adsorption into the saturated liquid phase (100 kJ/mol), and nucleation of islands (0.3 kJ/mol). Despite the small activation barrier to island nucleation, the nucleation rate is qualitatively slow, suggesting that entropic effects dominate the nucleation rate.     

Synthesis of ferrocenethiols containing oligo(phenylenevinylene) bridges and their characterization on gold electrodes

Ferrocene-terminated oligo(phenylenevinylene) (OPV) methyl thiols have been prepared by orthogonal coupling of phenylene monomers. Ethoxy substituents on the phenyl rings improve the solubility of OPV, enabling the synthesis of longer oligomers. Self-assembled monolayers containing a mixture of a ferrocene OPV methyl thiol and a diluent alkanethiol were deposited on gold. A cyclic voltammetric study of monolayers containing oligomers of the same length with and without ethoxy solubilizing groups reveals that both solubilized and unsolubilized oligomers form well-packed self-assembled monolayers. Changing the position of the solubilizing groups on an oligomer chain does not preclude packing of the oligomer in the monolayer. Conventional chronoamperometry, which can be used to measure rate constants up to approximately 10(4) s(-1), is too slow to measure the electron-transfer rate through these oligomers over distances up to 35 A. OPV bridges are expected to be highly conjugated unlike oligo(phenyleneethynylene) bridges, which may be only partially conjugated because of rotation of the phenyl rings about the ethynylene bonds. Because of its high conjugation, OPV may prove useful as a molecular wire.     

Directed crystallisation of zinc oxide on patterned surfaces

Patterned self-assembled monolayers of functionalised alkane thiols were prepared on gold substrates, using UV-photolithography. Two alkane thiols, 11-mercaptoundecanoic acid (MUA) and a fluorinated decane thiol (FDT, CF3(CF2)7CH2CH2SH) were used to fabricate chemically structured surfaces which served as templates for zinc oxide (ZnO) crystallisation. When these patterns, containing high (MUA) and low (FDT) surface energy regions were exposed to a 10 mM zinc nitrate crystallising solution, nucleation occurred selectively on the low energy regions. After 90 min, hexagonal prisms had grown upright on these areas. The crystal growth is uniform with a crystal length of about 1 mum and a diameter between 50 and 100 nm. We attribute the selective growth to a combination of crystallographic frustration of the zinc ions on the high energy regions and an accumulation of hydroxide ions on the low energy regions.     

Patterned growth of large oriented organic semiconductor single crystals on self-assembled monolayer templates

This work demonstrates a method for inducing site-specific nucleation and subsequent growth of large oriented organic semiconductor single crystals using micropatterned self-assembled monolayers (SAMs). We demonstrate growth of oriented, patterned, and large organic semiconductor single crystals for potential use in organic electronic devices. The control over multiple parameters in a single system has not yet been reported. The ability to control various aspects of crystal growth in one system provides a powerful technique for the bottom-up fabrication of organic single-crystal semiconductor devices.     

Crystallization of CaCO3 in the presence of sulfate and additives: experimental and molecular dynamics simulation studies

The effects of sulfate and BHTPMP (Bis (hexamethylene) triaminepentakis (methylene phosphonic acid)) on the crystallization rate, phase composition and morphology of calcium carbonate have been studied. It was observed that sulfate reduces the nucleation rate and favors the formation of aragonite form in the calcium carbonate precipitate. Moreover, in the presence of sulfate the rhombohedral morphology of the calcite crystals is modified, and during the formation of calcite, the development of {104} faces are more significantly prohibited than {110} faces. In the presence of sulfate together with BHTPMP, the crystallization process is inhibited and the modified morphology and the dominant calcite form are observed in the solid. The results from molecular dynamics simulations show the more strong combination of sulfate with calcite surface, in particular the {104} face, in comparison with the aragonite surface. The strong interaction of BHTPMP with sulfate and the aragonite surface favors the formation of the dominant calcite phase in the precipitate.     

Controlled immobilization of membrane proteins to surfaces for fourier transform infrared investigations

We show that it is possible to immobilize membrane proteins uniformly and reversibly as self-assembled (sub)monolayers on nitrilotriacetic acid-covered sensor surfaces via hexahistidine sequences present either in the protein or in lipid membranes. Fourier transform infrared spectra of such self-assembled (sub)monolayers deliver important structural information of the membrane proteins and are suited to screen the function of cellular receptors.     

Zinc oxide growth morphology on self-assembled monolayer modified silver surfaces

Using organic molecules to direct inorganic crystal growth has opened up new avenues for controlled synthesis on surfaces. Combined with soft lithography to form patterned templates, self-assembled monolayers (SAMs) have been shown to be a powerful approach for the assembly of inorganic nanostructures. In this work, we show that the surface free energy of SAM-modified silver, which depends on end groups and deposition method of SAMs, has a dramatic effect on the nucleation and growth of crystalline ZnO, a technologically important material, from supersaturated solutions. For SAMs with inert methyl end groups, ZnO nucleation is inhibited. For SAMs with chemically active (carboxylic or thiol) end groups, the ZnO morphology is found to be three-dimensional nanorods on low-surface-energy surfaces and two-dimensional thin films on high-energy surfaces.     

Early stages of crystallization of calcium carbonate revealed in picoliter droplets

In this work, we studied the heterogeneous nucleation and growth of CaCO(3) within regular arrays of picoliter droplets created on patterned self-assembled monolayers (SAMs). The SAMs provide well-defined substrates that offer control over CaCO(3) nucleation, and we used these impurity-free droplet arrays to study crystal growth in spatially and chemically controlled, finite-reservoir environments. The results demonstrate a number of remarkable features of precipitation within these confined volumes. CaCO(3) crystallization proceeds significantly more slowly in the droplets than in the bulk, allowing the mechanism of crystallization, which progresses via amorphous calcium carbonate, to be easily observed. In addition, the precipitation reaction terminates at an earlier stage than in the bulk solution, revealing intermediate growth forms. Confinement can therefore be used as a straightforward method for studying the mechanisms of crystallization on a substrate without the requirement for specialized analytical techniques. The results are also of significance to biomineralization processes, where crystallization typically occurs in confinement and in association with organic matrices, and it is envisaged that the method is applicable to many crystallizing systems.