Biomimetic fluorapatite films for conservation of historic calcareous stones

Abstract

Fluorapatite protective films were prepared on marble substrates using a biomimetic method. By mimicking the mineralization mechanism of enamel, phosphorus and fluorine were introduced on the surface of the marble substrate. In the presence of a biological template, namely collagen, an integrated fluorapatite film was produced and the marble substrate was entirely covered. The prepared fluorapatite films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray (EDX) spectroscopy. The performances of the fluorapatite films were evaluated by color changes, capillary water absorption, and acid resistance tests. The results revealed that the fluorapatite films had good compatibility with the marble substrate; the physical properties such as color and capillary water adsorption of the marble substrates were unchanged. The fluorapatite films also had good acid resistance and were stable even in heavy acid rain. biomimetic, fluorapatite film, conservation, calcareous stonesCitation:

Hydroxyapatite and Other Calcium Phosphates for the Conservation of Cultural Heritage: A Review

“…As described in detail in Section 2.8 , incorporation of fluorine ions in the apatite crystal was investigated in Reference [ 54 ], with the aim of forming fluorapatite (instead of HAP).…”

“…Following a similar approach, in another study [ 54 ] fluorapatite (Ca 10 (PO 4 ) 6 F 2 ) was formed by reacting marble with a solution containing 0.1 g/L of Type 1 collagen + 5 g/L of ATP + 0.6 g/L of NH 4 F [ 54 ]. The resulting layer of fluorapatite (about ~60 μm thick) was reportedly coral-like, without any crack or detachment [ 54 ].…”

“…Following a similar approach, in another study [ 54 ] fluorapatite (Ca 10 (PO 4 ) 6 F 2 ) was formed by reacting marble with a solution containing 0.1 g/L of Type 1 collagen + 5 g/L of ATP + 0.6 g/L of NH 4 F [ 54 ]. The resulting layer of fluorapatite (about ~60 μm thick) was reportedly coral-like, without any crack or detachment [ 54 ]. The role of collagen was found to be essential because, when the same concentrations of TAP and NH 4 F were used to treat marble in the absence of collagen, a cracked and partly detached fluorapatite layer was obtained [ 54 ].…”

“…The resulting layer of fluorapatite (about ~60 μm thick) was reportedly coral-like, without any crack or detachment [ 54 ]. The role of collagen was found to be essential because, when the same concentrations of TAP and NH 4 F were used to treat marble in the absence of collagen, a cracked and partly detached fluorapatite layer was obtained [ 54 ]. The role of collagen is to promote the direct formation of fluorapatite on the marble surface, by acting as a template.…”

“…The role of collagen is to promote the direct formation of fluorapatite on the marble surface, by acting as a template. According to the mechanism proposed in the cited paper, in the presence of collagen calcium ions dissolved from the substrate adsorb at active sites (such as carboxyl groups) on the matrix and react with phosphate and fluoride ions, thus leading to fluorapatite formation [ 54 ]. As discussed in Section 3.1 , the fluorapatite film is reportedly able to protect marble from dissolution in acid but no direct experimental test was carried out in the Reference [ 54 ] to check the actual protective effectiveness.…”