Modulation of the calcium oxalate dihydrate to calcium oxalate monohydrate phase transition with citrate and zinc ions

As one of the most ubiquitous biominerals, calcium oxalate (CaOx) exhibits different hydrate phases. Although the monohydrate (whewellite, COM) is the thermodynamically favorable phase at ambient temperatures, the metastable dihydrate (weddellite, COD) can still stably exist in pathological mineralization but the stabilization mechanisms remain unclear. Herein, the in situ phase transformation of nano-sized COD to COM in the presence of citrate (CA) and zinc ions (Zn2+) as stabilizers was observed by time-resolved atomic force microscopy (AFM) and Raman spectroscopy. By a combination of zeta potential measurement, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), we demonstrate that CA can form a protective layer on the surface of nano-sized COD crystals to inhibit aggregation and delay the phase transformation of COD to COM. Due to the stabilizing/destabilizing effect of CA on the specific faces of COD, atypical-shaped COD crystals form. In addition, Zn2+ can substitute Ca2+ sites within the COD unit cell, shrinking the COD structure by compressing the “zeolite-like” channels to reduce weakly-bonded interchannel H2O molecules. This generates relatively smaller and more stable COD crystals with a typical pyramid shape to further retard COM formation. The obtained results provide new insights into modulating the stability of the metastable CaOx phases for potential control over pathological mineralization.