Synthesis of nanosized calcium hydroxyapatite particles by the catalytic decomposition of urea with urease
- PDF / 305,505 Bytes
- 6 Pages / 585 x 783 pts Page_size
- 87 Downloads / 214 Views
Nanosized calcium hydroxyapatite particles were synthesized by the catalytic decomposition of urea with urease. The influence of the synthesis parameters, i.e., temperature and ratio of the reagents/urease concentrations, on the size and morphology of the hydroxyapatite particles were investigated. As reagents for the synthesis of hydroxyapatite, various amounts of CaCl2, NaH2PO4, urea, and urease were used. The urease-catalyzed decomposition of urea leads to the homogeneous precipitation of hydroxyapatite because of the release of OH− ions during the hydrolysis of urea. In the same process, CO32− ions are also released and incorporated into the crystal structure of the hydroxyapatite. In the temperature range from 30–40 °C, it was found that low crystalline carbonate hydroxyapatite was formed in all cases. It was found that decreasing the ratio of the concentrations of the reagents to the urease concentration leads to decrease in the particle size to below 100 nm and the morphology changes from leafy-like to spherical.
I. INTRODUCTION
During the past few decades, special attention has been paid to hydroxyapatite (HAP) materials because of their physicochemical and sorption properties, thermal stability, and biocompatibility.1 Because of these properties, HAP is used in chromatography, drug delivery, heavy metal immobilization, soil remediation, and for the repair and reconstruction of diseased and damaged parts of the skeleton.1–4 The above-mentioned properties and applications of calcium HAP materials are strictly linked to the synthesis and processing of the HAP powder. To obtain HAP powders with enhanced properties, different kinds of synthesis methods have been used, such as solid-state reactions,5 the sol-gel process,6 hydrothermal procedures,7,8 precipitation,9 etc. Calcium HAP can be synthesized by precipitation in the reaction of Ca(OH)2 and H3PO4 or by a change of the pH value of the salt solution with bases such as NH3 or (NH4)2CO3. In this case, the resulting very fast local pH increase results in a nonhomogeneous precipitation of the apatite phase. To achieve a homogenous precipitation of apatite powder with a defined particle size distribution and morphology, the hydrothermal decomposition of urea can be used.10–12 The hydrolysis of urea can also be a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0170 1156
J. Mater. Res., Vol. 22, No. 5, May 2007
performed by catalytic decomposition using the enzyme urease. The advantage of this method is that the presence of urease enhances the rate of urea decomposition at low temperatures and the enzyme is not consumed in the reaction.13 In this case, the mechanism by the hydrolysis reaction of urea is as follows: CO(NH2)2 + H2O + urease → NH3 + CO(NH2)OH
, (1)
CO(NH2)OH + H2O → NH3 + H2CO3 ,
(2)
H2CO3 → H+ + HCO3−
(3)
HCO3− → H+ + CO32− CO3
2−
+ H2O → HCO
− 3
, ,
(4) −
+ OH
.
(5)
In these reactions, hydroxyl ions are uniformly released throughout the whole volume of the solution, enabling a uniform i
Data Loading...
