Reaction of silver diamine fluoride with hydroxyapatite and protein

Silver diammine fluoride (SDF) is used as an anticaries agent; however, its mode of action is uncertain, whether chemical, physical, mechanical or antibacterial. As a preliminary study, the effect of SDF on hydroxyapatite (HAp) and gelatin (as a chemically-representative protein) was examined.Methods2.5 mg HAp powder specimens and 0.5 mL 10% gelatin (Riedel-de Haën) (initially as a sol at ∼37 °C), were mixed with 0.5 mL of 38% SDF (J. Morita), 4% NaF (Sigma) or 40% AgNO3 (Sigma) and tumbled in 1.5 mL polypropylene tubes (Sarstedt) for 48 h at ∼23 °C, in two series: exposed to laboratory lighting, and kept dark at all times. The HAp specimens were separated by centrifugation and decanting, then these and one set of gelatin specimens were dried at 60 °C in situ; a second parallel set of gelatin specimens were dried at ∼23 °C. Each was washed with 1 mL deionized water for 1 min, 3 times. Treated materials were observed, before and after washing, with scanning and transmission electron microscopy (SEM, TEM); energy-dispersive X-ray analysis (EDX), and electron diffraction (ED).ResultsSDF appeared to produce globular particles of CaF2 on the surface of the HAp, but these disappeared on washing, whilst with AgNO3 yellow cubic crystals of Ag3PO4 formed which were not dissolved on washing, but which darkened, converting gradually to metallic silver, on exposure to light. NaF had no effect on gelatin, whilst with SDF and AgNO3, particles of silver were produced which were resistant to washing.ConclusionsBoth principal components of tooth tissue react with SDF; the solubility of the putative CaF2 formed weakens the case for it exerting a caries-protective effect. The importance of the persistent silver needs further study.