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.