Assembly of two-dimensional (2D) molecular arrays on surfaces produces a wide range of architectural motifs exhibiting unique properties, but little attention has been given to the mechanism by which ...they nucleate. Using peptides selected for their binding affinity to molybdenum disulfide, we investigated nucleation of 2D arrays by molecularly resolved in situ atomic force microscopy and compared our results to molecular dynamics simulations. The arrays assembled one row at a time, and the nuclei were ordered from the earliest stages and formed without a free energy barrier or a critical size. The results verify long-standing but unproven predictions of classical nucleation theory in one dimension while revealing key interactions underlying 2D assembly.
Long residence times of soil organic matter have been attributed to reactive mineral surface sites that sorb organic species and cause inaccessibility due to physical isolation and chemical ...stabilization at the organic-mineral interface. Instrumentation for probing this interface is limited. As a result, much of the micron- and molecular-scale knowledge about organic-mineral interactions remains largely qualitative. Here we report the use of force spectroscopy to directly measure the binding between organic ligands with known chemical functionalities and soil minerals in aqueous environments. By systematically studying the role of organic functional group chemistry with model minerals, we demonstrate that chemistry of both the organic ligand and mineral contribute to values of binding free energy and that changes in pH and ionic strength produce significant differences in binding energies. These direct measurements of molecular binding provide mechanistic insights into organo-mineral interactions, which could potentially inform land-carbon models that explicitly include mineral-bound C pools.Most molecular scale knowledge on soil organo-mineral interactions remains qualitative due to instrument limitations. Here, the authors use force spectroscopy to directly measure free binding energy between organic ligands and minerals and find that both chemistry and environmental conditions affect binding.
The characteristic shapes, structures and properties of biominerals arise from their interplay with a macromolecular matrix. The developing mineral interacts with acidic macromolecules, which are ...either dissolved in the crystallization medium or associated with insoluble matrix polymers, that affect growth habits and phase selection or completely inhibit precipitation in solution. Yet little is known about the role of matrix-immobilized acidic macromolecules in directing mineralization. Here, by using in situ liquid-phase electron microscopy to visualize the nucleation and growth of CaCO3 in a matrix of polystyrene sulphonate (PSS), we show that the binding of calcium ions to form Ca-PSS globules is a key step in the formation of metastable amorphous calcium carbonate (ACC), an important precursor phase in many biomineralization systems. Our findings demonstrate that ion binding can play a significant role in directing nucleation, independently of any control over the free-energy barrier to nucleation.
The interplay between crystal and solvent structure, interparticle forces and ensemble particle response dynamics governs the process of crystallization by oriented attachment (OA), yet a ...quantitative understanding is lacking. Using ZnO as a model system, we combine in situ TEM observations of single particle and ensemble assembly dynamics with simulations of interparticle forces and responses to relate experimentally derived interparticle potentials to the underlying interactions. We show that OA is driven by forces and torques due to a combination of electrostatic ion-solvent correlations and dipolar interactions that act at separations well beyond 5 nm. Importantly, coalignment is achieved before particles reach separations at which strong attractions drive the final jump to contact. The observed barrier to attachment is negligible, while dissipative factors in the quasi-2D confinement of the TEM fluid cell lead to abnormal diffusivities with timescales for rotation much less than for translation, thus enabling OA to dominate.
Mechanisms of nucleation from electrolyte solutions have been debated for more than a century. Recent discoveries of amorphous precursors and evidence for cluster aggregation and liquid-liquid ...separation contradict common assumptions of classical nucleation theory. Using in situ transmission electron microscopy (TEM) to explore calcium carbonate (CaCO₃) nucleation in a cell that enables reagent mixing, we demonstrate that multiple nucleation pathways are simultaneously operative, including formation both directly from solution and indirectly through transformation of amorphous and crystalline precursors. However, an amorphous-to-calcite transformation is not observed. The behavior of amorphous calcium carbonate upon dissolution suggests that it encompasses a spectrum of structures, including liquids and solids. These observations of competing direct and indirect pathways are consistent with classical predictions, whereas the behavior of amorphous particles hints at an underlying commonality among recently proposed precursor-based mechanisms.
Recent experimental observations of the onset of calcium carbonate (CaCO3) mineralization suggest the emergence of a population of clusters that are stable rather than unstable as predicted by ...classical nucleation theory. This study uses molecular dynamics simulations to probe the structure, dynamics, and energetics of hydrated CaCO3 clusters and lattice gas simulations to explore the behavior of cluster populations before nucleation. Our results predict formation of a dense liquid phase through liquid-liquid separation within the concentration range in which clusters are observed. Coalescence and solidification of nanoscale droplets results in formation of a solid phase, the structure of which is consistent with amorphous CaCO3. The presence of a liquid-liquid binodal enables a diverse set of experimental observations to be reconciled within the context of established phase-separation mechanisms.
Two-step nucleation pathways in which disordered, amorphous, or dense liquid states precede the appearance of crystalline phases have been reported for a wide range of materials, but the dynamics of ...such pathways are poorly understood. Moreover, whether these pathways are general features of crystallizing systems or a consequence of system-specific structural details that select for direct versus two-step processes is unknown. Using atomic force microscopy to directly observe crystallization of sequence-defined polymers, we show that crystallization pathways are indeed sequence dependent. When a short hydrophobic region is added to a sequence that directly forms crystalline particles, crystallization instead follows a two-step pathway that begins with the creation of disordered clusters of 10-20 molecules and is characterized by highly non-linear crystallization kinetics in which clusters transform into ordered structures that then enter the growth phase. The results shed new light on non-classical crystallization mechanisms and have implications for the design of self-assembling polymer systems.
A classical view on nonclassical nucleation Smeets, Paul J. M.; Finney, Aaron R.; Habraken, Wouter J. E. M. ...
Proceedings of the National Academy of Sciences - PNAS,
09/2017, Letnik:
114, Številka:
38
Journal Article
Recenzirano
Odprti dostop
Understanding and controlling nucleation is important for many crystallization applications. Calcium carbonate (CaCO₃) is often used as a model system to investigate nucleation mechanisms. Despite ...its great importance in geology, biology, and many industrial applications, CaCO₃ nucleation is still a topic of intense discussion, with new pathways for its growth from ions in solution proposed in recent years. These new pathways include the so-called nonclassical nucleation mechanism via the assembly of thermodynamically stable prenucleation clusters, as well as the formation of a dense liquid precursor phase via liquid–liquid phase separation. Here, we present results from a combined experimental and computational investigation on the precipitation of CaCO₃ in dilute aqueous solutions. We propose that a dense liquid phase (containing 4–7 H₂O per CaCO₃ unit) forms in supersaturated solutions through the association of ions and ion pairs without significant participation of larger ion clusters. This liquid acts as the precursor for the formation of solid CaCO₃ in the form of vaterite, which grows via a net transfer of ions from solution according to z Ca2+ + z CO₃2− → z CaCO₃. The results show that all steps in this process can be explained according to classical concepts of crystal nucleation and growth, and that long-standing physical concepts of nucleation can describe multistep, multiphase growth mechanisms.
The ability of proteins and other macromolecules to interact with inorganic surfaces is essential to biological function. The proteins involved in these interactions are highly charged and often rich ...in carboxylic acid side chains
, but the structures of most protein-inorganic interfaces are unknown. We explored the possibility of systematically designing structured protein-mineral interfaces, guided by the example of ice-binding proteins, which present arrays of threonine residues (matched to the ice lattice) that order clathrate waters into an ice-like structure
. Here we design proteins displaying arrays of up to 54 carboxylate residues geometrically matched to the potassium ion (K
) sublattice on muscovite mica (001). At low K
concentration, individual molecules bind independently to mica in the designed orientations, whereas at high K
concentration, the designs form two-dimensional liquid-crystal phases, which accentuate the inherent structural bias in the muscovite lattice to produce protein arrays ordered over tens of millimetres. Incorporation of designed protein-protein interactions preserving the match between the proteins and the K
lattice led to extended self-assembled structures on mica: designed end-to-end interactions produced micrometre-long single-protein-diameter wires and a designed trimeric interface yielded extensive honeycomb arrays. The nearest-neighbour distances in these hexagonal arrays could be set digitally between 7.5 and 15.9 nanometres with 2.1-nanometre selectivity by changing the number of repeat units in the monomer. These results demonstrate that protein-inorganic lattice interactions can be systematically programmed and set the stage for designing protein-inorganic hybrid materials.
Microscopic mechanisms operating at the mineral–aqueous interface control rates of growth and dissolution, isotope fractionation and trace element partitioning during crystal growth. Despite the ...importance of characterizing surface kinetic controls on isotopic partitioning, no self-consistent microscopic theory has yet been presented which can simultaneously model both mineral growth rate and isotopic composition. Using a kinetic theory for AB or di-ionic crystal growth, we derive a model to predict precipitation rate and isotope fractionation as a function of growth solution oversaturation and solution stoichiometry and apply the theory to calcium isotope fractionation during calcite precipitation.
Our model assimilates the current understanding of surface controlled isotope fractionation with kinetic theories of ion-by-ion mineral growth to predict isotopic partitioning during the growth of ionic crystals. This approach accounts for the effect of solution composition on microscopic mineral surface structure and composition, providing numerous testable hypotheses for growth of sparingly soluble AB crystals such as calcite, namely:
(1)Both oversaturation and solution stoichiometry control growth rate and partitioning of isotopes during precipitation;(2)for growth driven primarily by step propagation, distinct expressions describe dislocation- and 2D nucleation-driven growth rates, while the expression for isotope fractionation is the same for both mechanisms;(3)mineral precipitation occurring via the formation of an amorphous precursor will generate isotope effects that are not compatible with ion-by-ion growth theory and may therefore be excluded from comparison; and,(4)the absolute kinetic limit of isotope fractionation may not be accessible at high oversaturation due to the formation of amorphous precursors.
Using calcite as a model system, we derive expressions for growth rate and isotopic fractionation as a function of oversaturation and Ca2+:CO32− in solution. Increasing oversaturation increases mineral growth rate and drives isotope partitioning towards the kinetic limit, while increasing the concentration of Ca2+ relative to CO32− at a given oversaturation tends to drive crystal growth towards isotopic equilibrium. These competing effects attenuate the magnitude of isotope fractionations observable in terrestrial environments.
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