A facile and sustainable method based on deep eutectic solvents toward synthesis of amorphous calcium phosphate nanoparticles: The effect of using various solvents and precursors on physical characteristics

• ACP nanoparticles were synthesized in various DESs based on choline chloride.
• The effect of precursors on the properties of ACP nanoparticles was investigated.
• The size of ACP nanoparticles varied from 24 to 39 nm upon change of DESs.
• ACPs synthesized using various precursors did not show significant differences.
• ACP nanoparticles were successfully synthesized in the recovered DES.

Amorphous calcium phosphate (ACP) nanoparticles were synthesized using a sustainable method based on precipitation of calcium and phosphate precursors in deep eutectic solvents (DESs). To this aim, three types of DESs, i.e., choline chloride–urea, choline chloride–ethylene glycol, and choline chloride–glycerol were prepared by simple heating-mixing method. The DESs were used as synthesis media for synthesis of ACP nanoparticles from Calcium nitrate tetrahydrate/calcium chloride and di-potassium hydrogen orthophosphate/di-ammonium hydrogen orthophosphate precursors. Characterization of the synthesized ACP nanoparticles by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy–selected area electron diffraction, energy dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy confirmed the formation of amorphous nanoparticles with spherical morphology and a high elemental/structural purity. Based on the results, with the change of DES from choline chloride–urea to choline chloride–ethylene glycol the diameter and Ca/P molar ratio of ACP nanoparticles changed from 24 to 39 nm, and 1.15 to 1.01 respectively. No significant changes in particle size of ACP nanoparticles were detected with change of precursors. After synthesis, DESs were recovered and re-used for synthesis of ACP nanoparticles. Analysis results suggested successful synthesis of ACP nanoparticles with high phase/elemental purity in the recovered DESs.