Mechanochemical synthesis of zinc-doped hydroxyapatite for tunable micronutrient release

Abstract

Mechanochemical synthesis of Zn-laden calcium hydroxyapatite (HAP) nanocrystals was performed from a chemically precipitated monetite/brushite precursor. ZnCO₃ served as the Zn²⁺ source. Powder XRD results showed that up to 15% ZnCO₃ can be incorporated into the HAP lattice during the mechanochemical transformation of the precursors. The resulting spectral properties, as investigated using Raman and infrared spectroscopy, showed stark differences between the mechanochemically prepared sample and the control HAP and ZnCO₃ mixed sample. In particular, a peak at 874 cm⁻¹ was observed in infrared and assigned to the HAP-incorporated carbonate ion C–O (ν₂) vibration as opposed to ZnCO₃ at 834 cm⁻¹ indicating that not only Zn²⁺ but also CO₃²⁻ is incorporated into the HAP lattice. Distinct thermal properties were also observed in the thermal analysis of mechanochemically reacted Zn-HAP with the endothermic peak at 235 °C completely absent as opposed to the mixed control of HAP and ZnCO₃. Electron microscopy analysis showed ∼10 × 40 nm HAP nanocrystals formed with a uniform Zn²⁺ ion distribution within them, especially at low 5% ZnCO₃ loading. Zn²⁺ dissolution experiments in soil-relevant 2% citric acid solution showed a distinct delayed dissolution pattern of mechanochemically obtained Zn-HAP with six-fold slower dissolution than that of mixed HAP and ZnCO₃, a commonly used zinc supplement. This study presents room-temperature synthesis methods of plant nutrient-laden HAP with tunable dissolution kinetics needed for efficient nutrient uptake.