Halide-Dependent Dissolution of Dicalcium Phosphate Dihydrate and Its Modulation by an Organic Ligand
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© 2017 American Chemical Society. In situ atomic force microscopy (AFM) combined with X-ray photoelectron spectroscopy (XPS) and ? potential were used to investigate how citrate (50 µM) modified the nanoscale dissolution of brushite (dicalcium phosphate dihydrate, CaHPO 4 ·2H 2 O) by NaX (X = Cl, Br, or I) at a constant pH of 7.0. Results showed that on the brushite (010) surface, halide ions with large sizes (such as I - ) enhanced adsorption of Na + that cannot be desorbed by water flow, inhibiting the retreat rates of  Cc steps. The introduction of 50 µM citrate resulted in desorption of Na + ions and caused the disappearance of specific salt effects on the dissolution of the brushite (010) face. With further increasing salt concentrations up to 100 mM, pit deepening was initiated, and it could possibly be attributed to hydrated Na + ions at the negatively charged surface that orient the interfacial water so as to increase the entropy of the interfacial system, and hence the dissolution rate. Following the addition of 50 µM citrate to 100 mM salt solutions, deep pits immediately disappeared, suggesting that citrate covers the brushite (010) surface to form a protective film to decrease the number of active sites of dissolution reactions on the brushite surface. The findings improve fundamental understanding of biomineral interfacial dissolution, with clinical implications of bone demineralization and resorption as well as prevention of osteoporosis.
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