Hydroxyapatite composites with multiwalled carbon nanotubes

Abstract

The structure of composites based on hydroxyapatite filled with multiwalled carbon nanotubes was investigated using differential scanning calorimetry analysis and X-ray diffraction spectroscopy. Hydroxyapatite was prepared by the wet synthesis in the presence of pristine and oxidised carbon nanotubes. An influence of nature of functional groups of the nanotubes boundary layer on a crystallinity of the inorganic part was evaluated using differential scanning calorimetry analysis and X-ray diffraction spectroscopy. Morphology of composites obtained was studied by nitrogen adsorption/desorption technique and scanning electron microscopy. The hydroxyapatite/nanotubes interface was characterised using electrochemical methods. The parameters of the electrical double layer: A density of surface charge versus pH, zeta potential versus pH were described. The study has shown that the synthesis of hydroxyapatite/nanotube composites by formation of inorganic part in the presence of carbon filler significantly affect the microstructure, phase composition, crystallinity, hydroxyl content, chemical composition of the mineral part, as well as thermal properties and electrokinetic properties of composite.

Introduction

Last decade intense studies oriented to synthesis of a bio-hydroxyapatite, other inorganic chemical compounds and composites with desired biological, physical and mechanical properties have been carried out (Wiśniewska et al., 2015a). Hydroxyapatite (HAP) and its composites due to their absorption ability and biocompatibility are of interests for application in medicine. Physical and chemical properties as well as soft indestructible interactions with human tissue make HAP attractive as the object of the in vivo and in vitro research. Carbon multiwalled nanotubes have inspired wide interests in current nanoscience and nanotechnology (Baughman et al., 2002). An increasing interest in CNTs applications is caused by their excellent mechanical properties that allow to produce materials with improved exploitation characteristics (He et al., 2007; Nowicki et al., 2015, Wong et al., 1997). Introducing of carbon nanotubes into ceramic matrices should promote preparation of composites characterised by high stiffness and improved mechanical properties compared to the single-phase ceramic material (Guo et al., 2011; Lupo et al., 2004). The most important tasks in technology of ceramic/nanotubes composites with high performers are achieving the homogeneous mixing between nanotubes and ceramic powders, preserving perfect structure of nanotubes during preparation of composites, and providing strong interfacial between nanotubes and matrix to ensure load translation. However, the HAP/nanotube composites fabricated by the traditional mechanical procedure do not comply with demanded properties. At present the HAP matrix reinforced with nanotubes has not made a breakthrough in the nanotechnology but is undoubtedly a new material but poorly characterised (Chen et al., 2007; Dean-Mo, 1996). In the literature, investigations devoted to synthesis of the nanotubes/HAP composites obtained in an aqueous solution and description of phenomena occurring are scarce. Are scarce in this paper, HAP/nanotube composites were obtained by formation of inorganic part in the presence of carbon filler. The HAP/nanotube composite powders were characterised using differential scanning calorimetry (DSC) method, scanning electron microscopy, nitrogen adsorption and desorption, potentiometric titration and zeta potential measurements.

Materials and methods

Synthesis of HAPs

The HAP/nanotubes composites were prepared by precipitation of Ca(OH)₂ and H₃PO4 in the presence of nanotubes. The two types of nanotubes, pristine and oxidised, obtained from the Chuiko Institute of Surface Chemistry Kalush (Ukraine), were used in synthesis.

In the reaction the following reagents were used

$$10\text{Ca}(\text{OH}{)}_2+6{\text{H}}_3{\text{PO}}_4\to {\text{Ca}}_{10}({\text{PO}}_4{)}_6(\text{OH}{)}_2+{18\text{H}}_2\text{O}$$

The H₃PO₄ solution was dropped into the Ca(OH)₂ suspension placed in the flask for 15 min. While dropping the reaction mixture was stirred vigorously and then dried for 24 h. The synthesis of HAP was conducted on several matrices using the above described method. Then the sediment was washed with redistilled water till the constant value of redistilled water conductivity was achieved. The obtained composites were washed many times to clean them from impurities.

Methods of characterisation

Thermal properties of the materials were investigated in nitrogen atmosphere in the temperature range from 20 to 500℃ using a Perkin Elmer DSC instrument. The samples of ∼11 mg in mass were closed in the standard aluminium pans. The cooling and heating rates were fixed to 10℃/min for typical measurements. After the first heating scan and before second heating one, the samples remained at 500℃ for 2 min (isothermally).

The HAP/nanotube composites were investigated using nitrogen adsorption–desorption (ASAP2405-Accelerated Surface Area and Porosimetry, Micromeritics Instruments, Co.).

Selected electrokinetic properties of nanotubes are presented in the article (Skwarek et al., 2016). Surface charge measurements were performed simultaneously in the suspension of the same solid content, to keep the identical conditions of the experiments in a thermostated Teflon vessel at 25℃. To eliminate the influence of CO₂ all potentiometric measurements were performed under nitrogen atmosphere. pH values were measured using a set of glass REF 451 and calomel pHG201-8 electrodes with the radiometer assembly. Surface charge density was calculated from the difference of the amounts of added acid or base to obtain the same pH value of suspension as for the background electrolyte. The zeta potential and particle size as well as distribution of HAP dispersions were determined by electrophoresis with Zetasizer 3000 by Malvern. The κa was greater than 100, so to calculate the zeta potential the Smoluchowski’s equation was used. The measurements were performed at 100 ppm of the composites which were subjected to ultrasonication of the suspension. Also high-resolution scanning electro-ionic microscope Quanta 3D FEG produced by FEI was used for characterisation.

Results and discussion

Thermal analysis

Unfilled HAP contains a large amount of humidity. Removal of water from the sample is displayed by the endotherm on the DSC curve with a maximum of 105℃ (Figure 1). The broad peak near 177℃ was due to liberation of chemically bonded water (of crystallisation of HAP). On the curve of second heating these were no effects detected that pointed on all contaminants were eliminated in the temperature range under study. Synthesis of HAP in the presence of pristine nanotubes caused the dramatic change of material structure. A new narrow reflex on the DSC curve at 194℃ can be to melting of crystalline assigned structures. The endothermic modes at 438 and 470℃ can be to structure relaxation and filler assigned decomposition. For pure HAP peaks on the curve for the second heating, scans were not observed which points to smaller stability of crystallites. In contrast to pristine nanotubes, the oxidised MWCNTox caused the composite structure amorphisation. After adsorbed water elimination at 95℃ on the DSC curve there were two steps at 408 and 438℃ to the relaxation process in different local structures. Thus, oxidation of the assigned nanotubes surface promotes formation of ‘HAP/MWCNT’ composite with less crystallinity that can be caused by a very small size of crystals obtained or due to the formation of a metastable phase. For this material, the filler decomposition shifts to highest temperatures.

Morphology and structural study

The SEM images (Figures 2 and 3) show well dispersed nanotubes in the: HAP/nanotubes structure and HAP/nanotubesOX. Structure systems HAP/nanotubesOX for the latter is better.

Figure 1.

DSC curves of composite HAP/MWCNTs and HAP/MWCNTox for first and second heating runs. DSC: differential scanning calorimetry; HAP: hydroxyapatite; MWCNT: multiwalled carbon nanotube.

Figure 2.

SEM microscopy HAP/nanotubes. HAP: hydroxyapatite; SEM: scanning electron microscope.

Figure 3.

SEM microscopy HAP/nanotubesOX. HAP: hydroxyapatite; SEM: scanning electron microscope.

As follows from the results of nitrogen adsorption/desorption analysis (Table 1), the pore sizes in the HAP/MWCNT and HAP/MWCNTox structures diminish compared to pure HAP which results from nanotubes getting into the nanotube pore structure (Table 1). The pores in the structure of HAP and its composites are mainly mesopores. The pore sizes in the HAP/MWCNT and HAP/MWCNTox structures diminish compared to pure HAP which results from the crystals’ morphology change. It was found that HAP formed in the presence of the nanotubes which have smaller average pore radius. For composite with pristine nanotubes, a decrease in mesopores size (in the range at d < 50 nm) accompanied with decrease in pores volume and specific surface area compared with pure hydroxyapatite (Table 1). It can be caused by increase in contribution of micropores that point on forming of large HAP crystals. In contrast, for the composite HAP/MWCNTox with the pore size decreasing the pore volume is close to the pristine HAP, while the specific surface area increases. A significant increase in specific surface compared with both HAP/MWCNT and the pristine HAP can be result of the structure fluffing with increasing of surface accessible to adsorption. But, mesopore volume remains the same as for the HAP. So, the presence of oxygen-containing groups on the surface of the carbon nanotubes probably contributed to the formation of smaller crystals with a developed surface and located at a distance from each other more than mesoscale or surface area increase can be due to change in a chemical structure of the obtained material and hence an adsorption ability.

Table 1.

Structural characteristics of hydroxyapatite (HAP) and its composites with MWCNTs.

Samples	HAP	HAP/MWCNT	HAP/MWCNTox
Surface area from BET (m2/g)	79	62	107
Surface area from Langmuir isotherm (m2/g)	116	91	157
Total pore volume from adsorption 1.7 nm < d < 300 nm by BJH method (cm3/g)	0.58	0.32	0.57
Total pore volume from desorption 1.7 nm < d < 300 nm by BJH method (cm3/g)	0.59	0.33	0.57
Average pores radius from BET method (nm)	29.65	20.86	21.20
Average pore radius from adsorption, BJH method (nm)	31.72	21.20	21.12
Average pore radius from desorption, BJH method (nm)	29.63	18.04	18.88
pHpzc	7.2	6.05	6.67

BET: Brunauer-Emmett-Teller; BJH: Barrett-Joyner-Halenda; MWCNT: multiwalled carbon nanotube; XRD: X-ray diffraction.

It is quite interesting that in the case of HAP/MWCNTs composites the specific surface area decreases but for HAP/MWCNTox it increases which corresponds to the SEM photos where for the last one an increased surface area and fewer aggregates are observed.

According to FTIR spectral analysis (Figure 4) a composition of pure HAP and composite HAP/MWCNTs, HAP/MWCNTox, these spectra do not differ significantly.

Figure 4.

Absorption spectra of pure HAP (1) and composites with pristine (2) and oxidised carbon nanotubes (3). HAP: hydroxyapatite.

The phase composition was also confirmed by the FTIR analysis (Figure 4). There were bands at 1091, 1054, 601, and 563 cm⁻¹, which were assigned as the stretching and bending motion of phosphate in the HAP whiskers. The bands at 3581 and 630 cm⁻¹ corresponded to the stretching mode of hydroxyl group of HAP. This further demonstrated the analytical result of X-ray diffraction (XRD). Meanwhile there were also bands at 874 and 1401–1458 cm⁻¹, which were assigned to the acidic phosphate group ( ${\text{HPO}}_4^{2-}$ ) and the carbonate group ( ${\text{CO}}_3^{2-}$ ) in the HAP whiskers, respectively. The results presented in Figure 5 confirm fine crystalline structure for HAP. Figure 5 shows changes in the structure at presence of nanotubes and appearance of new forms for the HAP in dependence on nanotubes surface nature. Sedimentation of HAP in the presence of oxidised nanotubes, that have in surface layer oxygen-containing (mostly hydroxyl and less concentration of the carboxyl and carbonyl) groups, results in formation of the pure HAP (Table 2) with well-defined crystalline according XRD patterns that not reflex on DSC (Figure 1).

Figure 5.

XRD pattern of composite HAP/MWCNTox. HAP: hydroxyapatite; MWCNT: multiwalled carbon nanotube; XRD: X-ray diffraction.

Table 2.

Structural characteristics of hydroxyapatite and composites HAP/MWCNT and HAP/MWCNTox from XRD analysis.

Parameters	HAP			HAP/ MWCNT			HAP/ MWCNTox
Composition	Weight fraction/%
Brushite	–			24.9			–
Calcium hydrogen phosphate	–			15			–
Hydroxyapatite	100			60			100
Peaks position, 2θ (degree)	25.9	31.83	39.74	20.94	25.9	31.83	25.9	31.86
Crystallites size (Scherrer equation) D (nm)	19.69	6.80	3.99	30.32	25.12	5.53	26.02	6.78
Distance between planes, d (nm)	0.205	0.374	0.090	0.089	0.205	0.374	0.205	0.349

HAP: hydroxyapatite; MWCNT: multiwalled carbon nanotube; XRD: X-ray diffraction.

Colloidal stability of the systems

In the literature there are publications about the stability of other colloidal systems (Huynh et al., 2012; Puziy et al., 2007, Wiśniewska et al., 2015b). In order to prepare the HAP/nanotubes composite, it is necessary to study the interaction mechanism between HAP and nanotubes because this has great influence on the integrative mechanical properties of the composites. Since the as-grown HAP and nanotube particles are all in a relative high surface energy state due to their small size and relative high surface area, it can be speculated that, in addition to the electrostatic or Van der Waals’s forces, surface adsorption force between the nanotubes and HAP would play an important role in the compounding, which is favourable for the improvement of mechanical properties of composites. According to the Stern theory the electrical double layer consists essentially of two basic electrical layers. The first (inner) layer is a layer of charged ions, bound to the surface by electrostatic forces. The second (outer) layer is formed by ions that offset the charge of the inner layer. Its structure is determined by a balance between adsorption and electrostatic forces, on one hand, and by thermal motion of ions, on the other hand. Formation of surface charge on contact phase results in ordering of charges called the electrical double layer. Electrokinetic potential, as a part of the overall thermodynamic potential, is one of the basic characteristics of the solid–liquid interface, and one of the forms, reflecting the electrochemical state of the mineral surface. Because of the zeta potential and surface charge are the properties of one and the same surface, it is natural that are strongly interdependent. The form of matter arrangement called the double electrical layer plays a significant role in the problems connected with electrokinetic stability of colloidal systems. This can be understood as the colloidal system in which solid particles dispersed in aqueous solutions do not tend to form aggregates. Thus, they are distributed evenly in the whole solution volume and electrostatic repulsion forces among dispersed particles. The physicochemical quantities which among others determine stability are surface charge density and pHpzc as well as zeta potential and pH_IEP. Dependence of zeta potential on pH for the HAP/NaCl, HAP/MWCNTs/NaCl, and HAP/MWCNTox/NaCl is presented in Figures 6 and 7(a) to (c). In all figures the zeta potential value is smaller than 0 in the whole studied range of pH and decreases with the increasing pH. As follows from Figure 7(a) to (c) for the same concentration of the basic electrolyte, there are assumed different values depending on the studied system: the highest for pure HAP/NaCl and the smallest for HAP/MWCNTox/NaCl. The dissociation of surface carboxyl groups of the MWCNTs is expected to result in the negative surface charge of the nanotubes. Because carbonyl and hydroxide groups are not likely to dissociate at the experimental condition and their surface densities are relatively small, both groups are not expected to contribute significantly to the surface charge of MWCNTs.

Figure 6.

Dependence of zeta potential on pH for the HAP/NaCl system. HAP: hydroxyapatite.

Figure 7.

Dependence of zeta potential on pH for the electrolyte concentration (a) 0.001 mol/dm³ NaCl, (b) for the 0.01 mol/dm³ NaCl, and (c) for the 0.1 mol/dm³ NaCl.

Such shift of electrokinetic potential is probably due to oxidation of surface nanotube particles. In all figures, in the whole studied range pH potential is not smaller than −30 mV, thus all systems are not electrokinetically stable and tend to form agglomerates. Small values of zeta potential of the studied systems cause suspension particles to attract more than to repel. The values of zeta potential for the HAP/nanotubesOX/electrolyte systems for the studied concentrations exhibit the highest absolute values which may indicate that the systems of this type are more stable than HAP/electrolyte or HAP/nanotubes/electrolyte. The difference in electrokinetic potentials for the studied systems at the same concentration of electrolyte broadens possibility of their practical application. For the studied systems the following pHiep values were obtained: for HAP/NaCl equal<4, HAP/MWCNTs/NaCl<4, and HAP/MWCNTox/NaCl <4. For the studied systems using the potentiometric titration and determining dependences of surface charge density on pH (Table 1), the following pHpzc values were obtained: 7.2 for HAP/NaCl, 6.05 for HAP/MWCNTs/NaCl, and 6.67 for HAP/MWCNTox/NaCl. The difference between pHpzc and pH_IEP probably results from heterogeneous surface of the analysed materials, as can be also seen in the SEM photos, as well as from the differences in pore diameters, slid plane overlap in the pores of individual adsorbents, and as a result of blockage during electrophoretic measurements.

Conclusions

The results of investigations on composites of HAP with nanotubes presented in this paper can be summarised as follows: the samples of HAP, HAP/MWCNTs, and HAP/MWCNTox composites were prepared using the wet method; oxidation of the nanotubes surface promotes increase in the specific surface area of the ‘HAP/MWCNT’ composite, but with less crystallinity that can be caused by a very small size of obtained crystals or due to the formation of a metastable phase. For this material, the filler decomposition shifts to the highest temperatures; crystallographic structure determination of the obtained composites by XRD has revealed the presence of HAP phase. The values of electrokinetic potential of the studied systems depend on pH and electrolyte concentration. With the increasing pH value, the zeta potential decreases. The differences between the electrokinetic potentials of individual systems in the same electrolyte are great. The studied systems: HAP/NaCl, HAP/MWCNT/NaCl, and HAP/MWCNTox/NaCl are not electrokinetically stable. Therefore, nanotube reinforced HAP composite is a promising material for high-load-bearing metal implants targeted drug delivery system and other biomedical fields. As follows from the comparison of HAP and composite samples studies by the nitrogen adsorption and desorption method and scanning electron microscopy, the composites have different properties from those of HAP itself.