Reaction of silver diamine [corrected] 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. 2.5mg HAp powder specimens and 0.5mL 10% gelatin (Riedel-de Haën) (initially as a sol at ∼37°C), were mixed with 0.5mL of 38% SDF (J. Morita), 4% NaF (Sigma) or 40% AgNO(3) (Sigma) and tumbled in 1.5mL polypropylene tubes (Sarstedt) for 48h 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 1mL deionized water for 1min, 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). SDF appeared to produce globular particles of CaF(2) on the surface of the HAp, but these disappeared on washing, whilst with AgNO(3) yellow cubic crystals of Ag(3)PO(4) 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 AgNO(3), particles of silver were produced which were resistant to washing. Both principal components of tooth tissue react with SDF; the solubility of the putative CaF(2) formed weakens the case for it exerting a caries-protective effect. The importance of the persistent silver needs further study.