Christine A. Orme

Formation of chiral morphologies through selective binding of amino acids to calcite surface steps

Many living organisms contain biominerals and composites with finely tuned properties, reflecting a remarkable level of control over the nucleation, growth and shape of the constituent crystals. Peptides and proteins play an important role in achieving this control. But the general view that organic molecules affect mineralization through stereochemical recognition, where geometrical and chemical constraints dictate their binding to a mineral, seems difficult to reconcile with a mechanistic understanding, where crystallization is controlled by thermodynamic and kinetic factors. Indeed, traditional crystal growth models emphasize the inhibiting effect of so-called ‘modifiers’ on surface-step growth, rather than stereochemical matching to newly expressed crystal facets. Here we report in situ atomic force microscope observations and molecular modelling studies of calcite growth in the presence of chiral amino acids that reconcile these two seemingly divergent views. We find that enantiomer-specific binding of the amino acids to those surface-step edges that offer the best geometric and chemical fit changes the step-edge free energies, which in turn results in macroscopic crystal shape modifications. Our results emphasize that the mechanism underlying crystal modification through organic molecules is best understood by considering both stereochemical recognition and the effects of binding on the interfacial energies of the growing crystal.

Molecular modulation of calcium oxalate crystallization by osteopontin and citrate

Calcium oxalate monohydrate (COM), which plays a functional role in plant physiology, is a source of chronic human disease, forming the major inorganic component of kidney stones. Understanding molecular mechanisms of biological control over COM crystallization is central to development of effective stone disease therapies and can help define general strategies for synthesizing biologically inspired materials. To date, research on COM modification by proteins and small molecules has not resolved the molecular-scale control mechanisms. Moreover, because proteins directing COM inhibition have been identified and sequenced, they provide a basis for general physiochemical investigations of biomineralization. Here, we report molecular-scale views of COM modulation by two urinary constituents, the protein osteopontin and citrate, a common therapeutic agent. Combining force microscopy with molecular modeling, we show that each controls growth habit and kinetics by pinning step motion on different faces through specific interactions in which both size and structure determine the effectiveness. Moreover, the results suggest potential for additive effects of simultaneous action by both modifiers to inhibit the overall growth of the crystal and demonstrate the utility of combining molecular imaging and modeling tools for understanding events underlying aberrant crystallization in disease.

Large scale surface structure formed during GaAs (001) homoepitaxy

Atomic force microscopy studies have been performed on GaAs (001) homoepitaxy films grown by molecular beam epitaxy. Multilayered features are seen to evolve when the growth conditions favor island nucleation. As the epilayer thickness is increased these features grow in all dimensions but the angle of inclination remains approximately constant at 1°. The mounding does not occur on surfaces grown in step flow. We propose that the multilayered features are an unstable growth mode which relies on island nucleation and the presence of a step edge barrier.

Raman Spectroscopy of DNA Packaging in Individual Human Sperm Cells distinguishes Normal from Abnormal Cells

Healthy human males produce sperm cells of which about 25-40% have abnormal head shapes. Increases in the percentage of sperm exhibiting aberrant sperm head morphologies have been correlated with male infertility, and biochemical studies of pooled sperm have suggested that sperm with abnormal shape may contain DNA that has not been properly repackaged by protamine during spermatid development. We have used micro-Raman spectroscopy to obtain Raman spectra from individual human sperm cells and examined how differences in the Raman spectra of sperm chromatin correlate with cell shape. We show that Raman spectra of individual sperm cells contain vibrational marker modes that can be used to assess the efficiency of DNA-packaging for each cell. Raman spectra obtained from sperm cells with normal shape provide evidence that DNA in these sperm is very efficiently packaged. We find, however, that the relative protein content per cell and DNA packaging efficiencies are distributed over a relatively wide range for sperm cells with both normal and abnormal shape. These findings indicate that single cell Raman spectroscopy should be a valuable tool in assessing the quality of sperm cells for in-vitro fertilization.

Macroscopic 3D nanographene with dynamically tunable bulk properties.

Polymer-derived, monolithic three-dimensional nanographene (3D-NG) bulk material with tunable properties is produced by a simple and inexpensive approach. The material is mass-producible, and combines chemical inertness and mechanical strength with a hierarchical porous architecture and a graphene-like surface area. This provides an opportunity to control its electron transport and mechanical properties dynamically by means of electrochemical-induced interfacial electric fields.

Ultralow Density, Monolithic WS2, MoS2, and MoS2/Graphene Aerogels

We describe the synthesis and characterization of monolithic, ultralow density WS2 and MoS2 aerogels, as well as a high surface area MoS2/graphene hybrid aerogel. The monolithic WS2 and MoS2 aerogels are prepared via thermal decomposition of freeze-dried ammonium thio-molybdate (ATM) and ammonium thio-tungstate (ATT) solutions, respectively. The densities of the pure dichalcogenide aerogels represent 0.4% and 0.5% of full density MoS2 and WS2, respectively, and can be tailored by simply changing the initial ATM or ATT concentrations. Similar processing in the presence of the graphene aerogel results in a hybrid structure with MoS2 sheets conformally coating the graphene scaffold. This layered motif produces a ∼50 wt % MoS2 aerogel with BET surface area of ∼700 m(2)/g and an electrical conductivity of 112 S/m. The MoS2/graphene aerogel shows promising results as a hydrogen evolution reaction catalyst with low onset potential (∼100 mV) and high current density (100 mA/cm(2) at 260 mV).

Dynamics of Biomineral Formation at the Near-Molecular Level

3.5. Hillock Geometry (Step Density) 4796 3.6. Impurity Interactions 4796 3.6.1. Effects of Additives on Step Kinetics 4796 3.6.2. Step Pinning Models 4797 3.6.3. Effects of Additives on Shape 4797 4. Pathological Crystallization 4799 5. Calcium Oxalate Monohydrate 4799 5.1. Structure and Growth in Pure Solution 4800 5.2. Metal Ions 4802 5.3. Citrate 4802 5.4. Naturally Occurring Macromolecules 4805 5.5. Synthetic Macromolecules 4805 5.5.1. Synthetic 27-Residue Peptides 4805 5.5.2. Synthetic PolyD, PolyE, and PolyAA 4807 6. Brushite 4808 6.1. Growth and Dissolution of Brushite in Solutions without Impurities 4809

Molecular mechanisms of crystallization impacting calcium phosphate cements

The biomineral calcium hydrogen phosphate dihydrate (CaHPO4·2H2O), known as brushite, is a malleable material that both grows and dissolves faster than most other calcium minerals, including other calcium phosphate phases, calcium carbonates and calcium oxalates. Within the body, this ready formation and dissolution can play a role in certain diseases, such as kidney stone and plaque formation. However, these same properties, along with brushite’s excellent biocompatibility, can be used to great benefit in making resorbable biomedical cements. To optimize cements, additives are commonly used to control crystallization kinetics and phase transformation. This paper describes the use of in situ scanning probe microscopy to investigate the role of several solution parameters and additives in brushite atomic step motion. Surprisingly, this work demonstrates that the activation barrier for phosphate (rather than calcium) incorporation limits growth kinetics and that additives such as magnesium, citrate and bisphosphonates each influence step motion in distinctly different ways. Our findings provide details of how, and where, molecules inhibit or accelerate kinetics. These insights have the potential to aid in designing molecules to target specific steps and to guide synergistic combinations of additives.

Extremely sharp carbon nanocone probes for atomic force microscopy imaging

A simple and reliable catalyst patterning technique combined with electric-field-guided growth is utilized to synthesize a sharp and high-aspect-ratio carbon nanocone probe on a tipless cantilever for atomic force microscopy. A single carbon nanodot produced by an electron-beam-induced deposition serves as a convenient chemical etch mask for catalyst patterning, thus eliminating the need for complicated, resist-based, electron-beam lithography for a nanoprobe fabrication. A gradual, sputtering-induced size reduction and eventual removal of the catalyst particle at the probe tip during electric-field-guided growth creates a sharp probe with a tip radius of only a few nanometers. These fabrication processes are amenable for the wafer-scale synthesis of multiple probes. High resolution imaging of three-dimensional features and deep trenches, and mechanical durability enabling continuous operation for many hours without noticeable image deterioration have been demonstrated.

Influence of Chromium and Molybdenum on the Corrosion of Nickel Based Alloys

Abstract The addition of chromium and molybdenum to nickel creates alloys with exceptional corrosion resistance in a diverse range of environments. This study examines the complementary roles of ch...