Lyle M. Gordon

Nanoscale chemical tomography of buried organic–inorganic interfaces in the chiton tooth

Biological organisms possess an unparalleled ability to control the structure and properties of mineralized tissues. They are able, for example, to guide the formation of smoothly curving single crystals or tough, lightweight, self-repairing skeletal elements. In many biominerals, an organic matrix interacts with the mineral as it forms, controls its morphology and polymorph, and is occluded during mineralization. The remarkable functional properties of the resulting composites—such as outstanding fracture toughness and wear resistance—can be attributed to buried organic–inorganic interfaces at multiple hierarchical levels. Analysing and controlling such interfaces at the nanometre length scale is critical also in emerging organic electronic and photovoltaic hybrid materials. However, elucidating the structural and chemical complexity of buried organic–inorganic interfaces presents a challenge to state-of-the-art imaging techniques. Here we show that pulsed-laser atom-probe tomography reveals three-dimensional chemical maps of organic fibres with a diameter of 5–10 nm in the surrounding nano-crystalline magnetite (Fe3O4) mineral in the tooth of a marine mollusc, the chiton Chaetopleura apiculata. Remarkably, most fibres co-localize with either sodium or magnesium. Furthermore, clustering of these cations in the fibre indicates a structural level of hierarchy previously undetected. Our results demonstrate that in the chiton tooth, individual organic fibres have different chemical compositions, and therefore probably different functional roles in controlling fibre formation and matrix–mineral interactions. Atom-probe tomography is able to detect this chemical/structural heterogeneity by virtue of its high three-dimensional spatial resolution and sensitivity across the periodic table. We anticipate that the quantitative analysis and visualization of nanometre-scale interfaces by laser-pulsed atom-probe tomography will contribute greatly to our understanding not only of biominerals (such as bone, dentine and enamel), but also of synthetic organic–inorganic composites.

Amorphous intergranular phases control the properties of rodent tooth enamel

Key trace minerals greatly strengthen teeth The outer layers of teeth are made up of nanowires of enamel that are prone to decay. Gordon et al. analyzed the composition of tooth enamel from a variety of rodents at the nanometer scale (see the Perspective by Politi). In regular and pigmented enamel, which contain different trace elements at varying boundary regions, two intergranular phases—magnesium amorphous calcium phosphate or a mixed-phase iron oxide—control the rates of enamel demineralization. This suggests that there may be alternative options to fluoridation for strengthening teeth against decay. Science, this issue p. 746; see also p. 712 Differences in strength and stability of various tooth enamels may be due to trace minerals at boundary regions. [Also see Perspective by Politi] Dental enamel, a hierarchical material composed primarily of hydroxylapatite nanowires, is susceptible to degradation by plaque biofilm–derived acids. The solubility of enamel strongly depends on the presence of Mg2+, F−, and CO32–. However, determining the distribution of these minor ions is challenging. We show—using atom probe tomography, x-ray absorption spectroscopy, and correlative techniques—that in unpigmented rodent enamel, Mg2+ is predominantly present at grain boundaries as an intergranular phase of Mg-substituted amorphous calcium phosphate (Mg-ACP). In the pigmented enamel, a mixture of ferrihydrite and amorphous iron-calcium phosphate replaces the more soluble Mg-ACP, rendering it both harder and more resistant to acid attack. These results demonstrate the presence of enduring amorphous phases with a dramatic influence on the physical and chemical properties of the mature mineralized tissue.

Atom Probe Tomography of Apatites and Bone-Type Mineralized Tissues

Nanocrystalline biological apatites constitute the mineral phase of vertebrate bone and teeth. Beyond their central importance to the mechanical function of our skeleton, their extraordinarily large surface acts as the most important ion exchanger for essential and toxic ions in our body. However, the nanoscale structural and chemical complexity of apatite-based mineralized tissues is a formidable challenge to quantitative imaging. For example, even energy-filtered electron microscopy is not suitable for detection of small quantities of low atomic number elements typical for biological materials. Herein we show that laser-pulsed atom probe tomography, a technique that combines subnanometer spatial resolution with unbiased chemical sensitivity, is uniquely suited to the task. Common apatite end members share a number of features, but can clearly be distinguished by their spectrometric fingerprint. This fingerprint and the formation of molecular ions during field evaporation can be explained based on the chemistry of the apatite channel ion. Using end members for reference, we are able to interpret the spectra of bone and dentin samples, and generate the first three-dimensional reconstruction of 1.2 × 10(7) atoms in a dentin sample. The fibrous nature of the collagenous organic matrix in dentin is clearly recognizable in the reconstruction. Surprisingly, some fibers show selectivity in binding for sodium ions over magnesium ions, implying that an additional, chemical level of hierarchy is necessary to describe dentin structure. Furthermore, segregation of inorganic ions or small organic molecules to homophase interfaces (grain boundaries) is not apparent. This has implications for the platelet model for apatite biominerals.

Mapping residual organics and carbonate at grain boundaries and the amorphous interphase in mouse incisor enamel

Dental enamel has evolved to resist the most grueling conditions of mechanical stress, fatigue, and wear. Adding insult to injury, it is exposed to the frequently corrosive environment of the oral cavity. While its hierarchical structure is unrivaled in its mechanical resilience, heterogeneity in the distribution of magnesium ions and the presence of Mg-substituted amorphous calcium phosphate (Mg-ACP) as an intergranular phase have recently been shown to increase the susceptibility of mouse enamel to acid attack. Herein we investigate the distribution of two important constituents of enamel, residual organic matter and inorganic carbonate. We find that organics, carbonate, and possibly water show distinct distribution patterns in the mouse enamel crystallites, at simple grain boundaries, and in the amorphous interphase at multiple grain boundaries. This has implications for the resistance to acid corrosion, mechanical properties, and the mechanism by which enamel crystals grow during amelogenesis.

Selective Formation of Metastable Ferrihydrite in the Chiton Tooth

Metastable precursors are thought to play a major role in the ability of organisms to create mineralized tissues. Of particular interest are the hard and abrasion-resistant teeth formed by chitons, a class of rock-grazing mollusks. The formation of chiton teeth relies on the precipitation of metastable ferrihydrite (Fh) in an organic scaffold as a precursor to magnetite. In vitro synthesis of Fh under physiological conditions has been challenging. Using a combination of X-ray absorption and electron paramagnetic resonance spectroscopy, we show that, prior to Fh formation in the chiton tooth, iron ions are complexed by the organic matrix. In vitro experiments demonstrate that such complexes facilitate the formation of Fh under physiological conditions. These results indicate that acidic molecules may be integral to controlling Fh formation in the chiton tooth. This biological approach to polymorph selection is not limited to specialized proteins and can be expropriated using simple chemistry.

Towards Atom Probe Tomography of Hybrid Organic-Inorganic Nanoparticles

Surface functionalization of inorganic nanoparticles is widely used to modify material properties for a variety of applications including medical therapies and diagnostics. While inorganic nanoparticles may be characterized via electron microscopy and X-ray scattering, study of hybrid nanomaterials and, in particular, buried interfaces is very challenging. Here we discuss the application of atom probe tomography (APT) to imaging organic-inorganic interfaces of biological nanoparticles, ferritin, as a model system. Ferritin is a structurally well-characterized protein, critical across biology to store some 4500 iron atoms and transport iron as mineralized Fe (III) [1]. This 24-subunit 12 nm protein has a spherical 8 nm core into which Fe (II) enters and is subsequently oxidized.

A contribution to the characterization of the silicate-water interface – Part I: Implication of a new polished sample hydration technique

The analysis of the atomic composition of the interface between tricalcium silicate (C3S), the main compound of Ordinary Portland Cement, and surrounding solution is still a challenging task. At the same time, that knowledge is of profound importance for describing the basic processes during hydration. By means of Scanning Electron Microscopy (SEM) and Atom Probe Tomography (APT) we combine modern techniques in order to shed light on this topic in the present study. The results of these methods are compared with conduction calorimetry as a standard technique to study the hydration kinetics of cement. The tests were carried out on powders as well as on polished C3S samples. Results indicate that the progress of hydration is strongly increased when the C3S is used in the form of polished specimen. First C-S-H phases are detected in the powder 2.2 h after contact with water, on the polished section after 5 min. Besides SEM, the formation of C-S-H phases can be detected by APT, leading to an advantageous atomic resolution compared to EDX analysis. We propose that the use of APT will lead to deeper insights on the hydration progress and on the composition of the sensitive C-S-H phases based on these first results.

Atom Probe of Apatites - from Single Crystals to Interphases in Tooth Enamel

Tooth enamel is the hardest tissue in vertebrates. Optimized to withstand the forces of mastication, it is composed of hydroxylapatite nanowires, thousands of which are bundled into rods that are organized in a three-dimensional weave. The outstanding fracture resistance of enamel and its long fatigue life are the consequence of this hierarchical architecture. Tooth enamel is also the target of the most prevalent infectious disease in humans: dental caries. It is an infectious disease that has extremely high morbidity, with 60-90% of children and nearly 100% of adults worldwide having or having had caries. Caries, simply put, is the destruction of tooth biominerals by dissolution and commonly begins with the demineralization of enamel by acids produced in plaque biofilms. It has long been known that the susceptibility of enamel to acid dissolution is greatly dependent on the presence of magnesium, carbonate, and fluoride ions. A major bottleneck in understanding formation, structural evolution, and degradation of enamel under normal conditions and during tooth decay has been that imaging the distribution of these impurities in enamel has remained a great challenge.

Chemical Imaging of Interfaces and in Interphases in Tooth Enamel

Tooth enamel is the hardest tissue in vertebrates. Optimized to withstand the forces of mastication, it is composed of hydroxylapatite (OHAp) nanowires, thousands of which are bundled into rods that are organized in a three-dimensional weave. During tooth development, a preformed organic matrix is thought to be integral to the biological control over the precipitation of an amorphous precursor phase, its transformation into hydroxylapatite, and the growth of individual OHAp nanowires in enamel. This matrix is degraded during enamel maturation, but a small amount of organics remains in the final biocomposite, where its presence and that of water affect the mechanical properties. Once the tooth has erupted, enamel is affected by caries (tooth decay), a chronic infectious disease that affects nearly 100% of adults worldwide. Caries commonly begins with the demineralization of enamel by acids produced in plaque biofilms. It has long been known that the susceptibility of enamel to dissolution is greatly dependent on the presence of magnesium, carbonate, and fluoride ions. However, mapping the distribution of organic and inorganic ‘trace’ constituents is very challenging due to the complex 3D architecture, the importance of primarily low atomic number (Z) constituents, and the sensitivity of the sample to beam damage.

In Situ Environmental Transmission Electron Microscopy of Ice Nucleation

Heterogeneous ice nucleation on atmospheric aerosols results in cloud formation and represents one of the largest uncertainties in predicting climate. Elucidating the fundamental physicochemical and materials properties that result in highly active ice nuclei requires detailed observation of the ice nucleation process of model substrates in situ. Environmental transmission electron microscopy (TEM) represents a suitable tool for high-resolution analysis of nucleation in situ. Unlike a conventional TEM, gas can be introduced to the sample to simulate the reaction conditions of interest.