Proton µ-PIXE mapping, AFM imaging and size statistics of mineral granules in a dental composite

We applied proton microbeam particle-induced X-ray emission (µ-PIXE) for mapping Ca, Zr, Ba and Yb, and atomic force microscopy (AFM) for imaging the surface landscape of a dental composite which releases Ca2+ and F− for the protection of hard dental tissues. Three areas ∼250 × 250 µm2 located ∼0.5–2 mm apart on a smooth surface specimen were mapped with 3.1 MeV protons focused to a ∼3.0 µm spot and at ∼3.9 µm pixel size sampling. The maps evidenced particles with diameters of 3.2–32 µm (Ca), 20–60 µm (Zr), ≤ 4 µm (Ba) and 10–50 µm (Yb). Cross-section area histograms of Ca-rich particles fitted with 2–6 Poisson functions revealed a polydisperse size distribution and substantial differences from an area to another, possibly implying large local variations of Ca2+ released in the hard tissue near a dental filling of a few millimeters in diameter. Such imbalances may lead to low local Ca2+ protection of the dental tissue, favoring the onset of secondary caries. Similarly, AFM images showed high zone-dependent differences in the distributions of grains with apparent diameters of 1–4 µm, plausibly recognized as Ca- and Ba-containing particles. In a simple model based on demineralization data, lateral diffusion of Ca2+ between adjacent domains containing high- and low-area Ca-rich grains is described by exponential concentration gradients. These gradients may generate appreciable electromotive forces, which may enhance electrochemically the local tissue demineralization. Similar effects are to be expected in the protective action of F− ions released from microgranules of YbF3 and of Ba fluoroaluminosilicate glass. Copyright © 2010 John Wiley & Sons, Ltd.