Hydroxyapatite (HAp) has been synthesized by a hydrothermal treatment in the presence of a Gemini cationic surfactant. This process is a new strategy of synthesis and mainly consists of two parts, i.e., an ordinary hydrothermal treatment and a liquid-solid-solution reaction (LSS strategy). Crystalline HAp nanorods or nanogranules with length of 50-180 nm and width of 30-40 nm were produced by ordinary hydrothermal treatment. By contrary, HAp spheres with a 3D architecture were fabricated with Gemini cationic surfactant by LSS strategy. For Gemini cationic surfactant concentration of 0.05%, spherical HAp particles with an average diameter of 1.7
Hydrothermal method, a common way to prepare HAp, involves chemical reactions occurring in an aqueous solution at high temperature and pressure. The hydrothermal method could be considered as a chemical precipitation method in which the aging process is carried on inside an autoclave or pressure vessel [
It was difficult to control the shape, size, and size distribution of nanoparticles prepared by the hydrothermal method. To overcome this drawback, the use of organic modifiers or surfactants was proposed. Jiang et al. proposed a soft way for the preparation of HAp of different shapes by using a combination of Na2EDTA and citric acid (CA) at
The ordinary hydrothermal method has many weaknesses, for instance, uneven heating and difficulty in obtaining controlled dispersion of HAp. Carrying out the synthesis in an aqueous phase leads to a solid with small particles, which tends to agglomerate into larger aggregates. Hence, their application is limited. To address this issue, we propose a new type of liquid-solid-solution (LSS) assisted hydrothermal method to synthesize monodispersed HAp nanoparticles.
In 2005, Wang et al. [
Based on the LSS strategy, Wang et al. [
Gemini surfactants are a new type of surfactants, in which two traditional surfactant molecules are connected by a spacer group at the hydrophilic head groups or in close proximity to the head groups. They have advantages over the other monomeric surfactants due to their unique self-assembling ability. We have reported a biomimetic fabrication of HAp with a hollow spherical three-dimensional architecture with polyoxyethylene chain-containing quaternary ammonium salt cationic Gemini surfactant [C12C2C12 (EO)] as the template [
For reasons dissolved above, in this work, we prepared HAp by the LSS assisted hydrothermal strategy and using a new kind of hyperbranched Gemini quaternary ammonium (PCD) as the template. PCD is a macromolecular cationic surfactant, which has 4 ionic head groups (Figure
Structure of PCD.
In a typical experiment, an appropriate amount of PCD was dissolved in 35 mL 0.17 mol/L (NH4)3PO4 solution and magnetically stirred to form solution A. Solution B was made of 35 mL 0.28 mol/L Ca(NO3)2. Then, solution A was added to solution B under magnetic stirring, and the pH was adjusted to 10.50 with ammonium hydroxide. When the process was finished, stirring was continued for 0.5 h. The reaction product was transferred to a 100 mL autoclave at 180°C for 10 h. The samples were recovered by centrifugation and washed three times with cyclohexane and absolute ethanol, respectively. Finally, the powders were dried at 50°C for 1-2 days.
3.2 g sodium linoleate was dissolved in 8 mL oleic acid under stirring for 2 h. Then, 32 mL C2H5OH was added dropwise to the mixed solution under stirring for 15 min to form a suspension. Afterwards, 15 mL 0.28 mol/L Ca(NO3)2 solution was added to the obtained suspension under stirring. A stable yellow suspension resulted after 1 h under stirring. As a result, a three-phase system made of a liquid phase (C2H5OH/oleic acid), a solid phase (sodium oleate), and a solution phase (Ca(NO3)2) was formed. After the exchange between Ca2+ and Na+, calcium dioleate was formed. In the next step, an appropriate amount of PCD was added to 15 mL 0.17 mol/L (NH4)3PO4 solution under magnetic stirring for 30 min; then, (NH4)3PO4 solution was added to the resulting solution under stirring, and white flocs were formed. The stirring was continued for 5 min. Then, the product was transferred to a 100 mL autoclave at 180°C for 10 h. After cooling to room temperature, the product was centrifuged and washed three times with cyclohexane and absolute ethanol, respectively. Finally, the powders were dried at 50°C for 1-2 days.
The phase of obtained samples was analyzed by X-ray powder diffraction (XRD, Philips X’Pert MPD, Philips Company, Netherlands). The diffraction peak at
The chemical composition of powders was tested by Fourier transform infrared spectroscopy (FT-IR, Nicolet Avatar 360, Thermo-Scientific Ltd., USA). The morphology of particles was studied using a Field Emission Scanning Electron Microscope (FESEM, JEOL 7500F, JEOL Company, Japan). The conductivities of solutions with different PCD concentrations were measured by a conductivity meter (DDS-307A). Based on the conductivity data, a graph was prepared to obtain the cmc value at 37°C, based on the critical turning point.
The XRD patterns of HAp in different situations are shown in Figure
XRD patterns of obtained samples at different conditions. (a) The absence of PCD; (b) 0.005%; (c) 0.05%.
FTIR spectra of samples obtained at different conditions. (a) The absence of PCD; (b) 0.005%; (c) 0.05%.
Figures
SEM images of obtained samples with different PCD concentrations at 180°C for 10 hours: (a) the absence of PCD; (b) 0.003%; (c) 0.005%; (d) 0.01%.
The crystal structure of HAp belongs to the hexagonal system, every unit cell having 10 Ca2+, 6 PO43-, and 2 OH-. When the sample is prepared by the classical hydrothermal treatment, two types of Ca2+-based complexes are formed. One is Ca-P6O24, which grows along the
As shown in Figure
Preparation of rod-like HAp nanoparticles using hydrothermal treatment in the presence of PCD as a Gemini cationic template.
To further study the growth process of HAp crystals, an experiment was designed to prove the interaction between PCD and PO43-, OH-, and Ca2+, as shown in Table
Electrical conductivity of PCD solution and (NH4)3PO4-PCD solution (25°C).
Electrical conductivity (ms·cm-1) | C(PCD)/(wt) | ||
---|---|---|---|
0.003% | 0.01% | 0.03% | |
0.05 | 0.07 | 0.08 | |
10.30 | 10.26 | 10.20 | |
10.38 | 10.40 | 10.41 | |
0.08 | 0.14 | 0.21 |
When the temperature was constant, several factors, such as the number of ions, ionic charge, and mobility ratio, affected the conductivity of the solution. From the results listed in Table
As can be seen from Figure
XRD pattern (a) and FTIR spectra (b) of samples obtained with different PCD concentrations at 180°C for 10 hours: (A) the absence of PCD; (B) 0.05%; (C) 0.5%.
The FTIR spectra depicted in Figure
Figure
SEM images of obtained samples with different concentrations of PCD at 180°C for 10 h: (a, b) the absence of PCD; (c, d) 0.01%; (e, f) 0.05%; (g, h) 0.5%.
LSS strategy to HAp with Gemini surfactant as template.
As shown in Figure
In summary, we prepared rod or sphere HAp particles by the LSS assisted hydrothermal method. The control of shape and size of the HAp particles was achieved by the LSS strategy. More specifically, the morphology, size, and structural organization of HAp particles were well controlled by changing the concentration of the Gemini cationic surfactant during the LSS synthesis. The results revealed the formation of particles of 50-180 nm in length and 30-40 nm in width. Moreover, the reorganization of one-dimensional HAp nanoparticles from one-dimensional into three-dimensional architecture was promoted by this innovative strategy. Therefore, at a concentration of PCD of 0.05%, spherical HAp nanoparticles with narrow size distribution and average size of 1.7
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there is no conflict of interest regarding the publication of this paper.
The authors gratefully acknowledge the financial support of the Natural Science Foundation of Fujian Province (Nos. 2016J01595 and 2019J01546), the Project of innovation platform for Fuzhou Health and Family Planning Commission (Nos. 2017-S-wp1 and 2018-S-wp3), the Key Clinical Specialty Discipline Construction Program of Fujian Province, the Major Science and Technology Projects of Fuzhou (2017), and the Project of Fuzhou Health and Family Planning Youth Scientific (2018-S-wq10).