On the Observation of the Central Dark Line in the Human Tooth Enamel Crystals Through SEM, TEM, STEM, and AFM Microscopes

We examine the image contrasts presented at micrometric and nanometric scales of the scanning electron microscope (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM), and atomic force microscope (AFM) in the tapping mode, looking for the observation of the central feature known as "the central dark line" (CDL) in the structure of human tooth enamel crystals. Since our interest is in inorganic material, mainly at the nanometric scale, the enamel samples for SEM and AFM were polished following a metallographic procedure, using progressively finer silicon carbide papers and etching with orthophosphoric acid. For TEM and STEM, the samples were produced by the focused ion beam (FIB) technique. The CDL images were well recorded using TEM and STEM, but not in SEM and AFM images. SEM images revealed the size and morphology of the enamel crystals, while AFM images revealed spherical and band-like structures. The observation of the CDL using TEM and STEM suggests that it may represent a potential chemical compositional site.