The synthesis of ZrHEDP was carried out as follows. At first, nano-zirconia precursor powder (Zr-precursor) was prepared by a method reported earlier by our group. One gram of the synthesized Zr-precursor powder was dispersed in 25 ml of 1 M HNO3 (Solution A). Next, the required amounts of 1-hydroxy-1,1-ethylidene diphosphonic acid (HEDP, H2PO3-C(OH)CH3-PO3H2) (Fig. ) (Strem Chemicals, 95%) were dissolved in 12.5g of distilled water, followed by heating at 50 °C for 30 minutes (Solution B). Next, both heated solutions (Solution A and B) were mixed at 50 °C and kept stirring for 4 hours, then heated at 70 °C for 4.5 hours in a sealed vessel. Finally, the obtained precipitate was dried at 90 °C, followed by washing with water until neutral pH was attained. HEDP/Zr-precursor ratio (weight) was varied (0.65, 0.90, 1.10, and 1.30) to synthesize ZrHEDPs functionalized with the different amounts of phosphonic acid group, i.e., the several average distances between acid moieties. We labeled ZrHEDP samples by their mole ratio of phosphorous per zirconium (P/Zr).