Table 1.

Synthetic conditions for the materials prepared during the optimization of the amount of urea adsorbed in the NPs. In all cases, solution A contained Na₃(Citrate) (0.20 M) and Ca(NO₃)₂ (0.20 M); solution B contained K₂HPO₄ (0.12 M), Na₂CO₃ (0.10 M) and KNO₃ (0.20 M). The concentration of urea is calculated as amount of urea added per total volume of the reaction. The total nitrogen content (wt%) from elemental analysis is provided for fully amorphous samples.

Sample	Urea added (M)	Total N wt%
U5-NACP	0.01	2.40
U10-NACP	0.02	4.24
U15-NACP	0.03	6.31
U20-NACP	0.04	8.13
U28.6-NACP	0.07	Crystalline urea
U44.4-NACP	0.13	Crystalline urea
U61.5-NACP	0.27	Crystalline urea