Sol gel combustion derived monticellite bioceramic powders for apatite formation ability evaluation

Sol-gel combustion method was employed for the synthesis of monticellite (CaMgSiO4) ceramic powders using various fuels. Citric acid, succinic acid, tartaric acid, sucrose, urea, glycine, and L-alanine were used as fuels. Influence of calcination temperature on the phase evolution was investigated. Prepared monticellite powders were characterized using powder-XRD, FT-IR, and DLS techniques. XRD pattern reveals that the L-alanine is a suitable fuel among all the fuels studied and confirms the formation of pure monticellite. FT-IR spectra confirm the presence of characteristic functional groups associated with monticellite. DLS measurements show the particle size of the monticellite powders. Finally, Apatite formation ability studies were carried out by immersing the monticellite and monticellite-polymer (chitosan/chitin) composites in the SBF solution. Pure monticellite shows higher bioactivity than the composites and its surface analysis (SEM and EDS) reveals the deposition of spherical hydroxyapatite particles.


Introduction
Calcium magnesium silicates have potential biomedical, wear resistance, thermo-mechanical, ceramic coating and luminescent applications in the field of material science. Calcium magnesium silicate CaO-MgO-SiO 2 ternary system on crystallization exist in different phases such as monticellite (CaMgSiO 4 ), diopside  [1][2][3][4][5]. Among all, monticellite gains more research attention due to the salient features of the crystal structure. It is a naturally occurring mineral form during metamorphism of siliceous dolostones and named after Italian mineralogist Teodoro Monticelli. The first crystal structure reported by Bragg and Brown (1926) and subsequently refined by Brown (1970). Monticellite has olivine-type crystal structure exist in orthorhombic crystal system with the space group Pmnb [6][7][8].
Considering ) is close to that of Ti-6Al-4V alloy (10.03×10 −6°C−1 ). Monticellite induces the formation of bone-like apatite in physiological solution as well as promotes the osteoblasts cell growth and proliferation [9].
Various synthetic methods used in the past for the preparation of ceramic materials [10][11][12][13]. Especially, monticellite prepared by the solid-state method, sol-gel method and floating zone method, which require either higher calcination temperatures or special reduced atmospheric conditions [9,14,15]. Hence, it is worth attempting to prepare monticellite by a sol-gel combustion method, which doesn't require special atmosphere or high calcination temperatures [16,17]. As the material and its composites not explored, it is interesting to investigate the bioactivity properties in in-vitro conditions, which may be helpful in identifying monticellite as a biomedical material.

Sol-gel combustion synthesis
Sol-gel combustion method [16,17] employed for the synthesis of monticellite in pure phase. As shown in the Later, TEOS added slowly to the reaction mixture and then nitric acid carefully added. The reaction was continued until the formation of a gel with heating at 70°C. Obtained gel was dried in hot air oven at 70°C. Dried gel samples decomposed in a preheated furnace at 400°C for 30 min. After decomposition, the precursor obtained found to be in different colors for different fuels. Thus obtained precursor samples calcined at 1300°C /5 h to obtain monticellite. For phase evolution study, L-alanine assisted precursor compound calcined at different temperatures (600°C, 900°C & 1300°C/5 h).

Apatite formation ability
All the calcined monticellite powders (prepared by using various fuels) made as compact pellets of 13 mm diameter and 3 mm thickness by using hydraulic pellet press. Similarly, ceramic-polymer composite pellets were also prepared by taking different ceramic to polymer proportions (100, 90:10 and 80:20). L-alanine derived pure monticellite, (chitin and chitosan) biopolymers used to prepare the composites.
2X SBF solution was prepared as per Kokubo procedure [18,19]. In brief, all the necessary reagents (NaCl, NaHCO 3 , KCl, K 2 HPO 4 .3H 2 O, MgCl 2 .6H 2 O, HCl, CaCl 2 , Na 2 SO 4 , (CH 2 OH) 3 CNH 2 ) were dissolved in 500 ml of double distilled water one by one at 37°C with continuous stirring. After the stepwise dissolution of the reagents, double distilled water again added to increase the total volume of SBF solution to 1 L. Finally, the pH adjusted to 7.4 using hydrochloric acid solution. The resultant solution filtered and stored in the refrigerator.
Apatite formation ability of monticellite and the composites were investigated by immersing in 2X SBF solution for 10 days and incubated at 37°C. After 10 days the pellets were removed from SBF, washed with deionized water and dried at room temperature. The dried pellets were analyzed by using XRD, SEM and EDS techniques to detect hydroxyapatite formation on the immersed pellet surface.

Characterization techniques
Bruker D8 advance X-Ray Diffractometer (Germany) was employed for the phase analysis by using Cu Kα, Nifiltered radiation (wavelength=1.540600 Å). Shimadzu IR Affinity-1 CE FTIR Spectrophotometer used to record FT-IR spectra of the samples. DLS particle size distribution curves obtained by using Horiba scientific nanopartica nano particle analyzer SZ-100 by dispersing the powder samples in the ethylene glycol medium. Surface morphology and EDS spectrum of the pellet recorded by using EVO 18 Research (Zeiss India).

Sol-gel combustion synthesis
The addition of metal nitrate solution (calcium nitrate and magnesium nitrate) to fuel solution leads to the formation of metal-fuel complex and TEOS upon acid hydrolysis results in the formation of silanol group along with ethanol. All these complexes help in the formation of polymeric network gel (fuel-Ca-Mg-silanol-ethanolnitric acid-water) through the sequential steps such as hydrolysis, condensation, polymerization, and gelation. Different polymeric network structures will form depending on the type of fuel used [20]. Type and number of functional groups present on the fuel will affect the various parameters (Thermochemistry, Enthalpy, time duration, type of gases (nitrates, carbonates) evolves, the amount of carbon content and color of the precursor compound) of the combustion reaction through the formation of different polymeric network structures. Thus, different fuels take different time durations to complete the combustion reaction and the color of obtained precursors varies. However, calcination of precursor prepared using various fuels at high temperature (1300°C) results in the formation of a white color compound, which may be due to the formation of monticellite.

XRD analysis
The figure 2 shows the XRD pattern of the precursor sample (L-alanine derived) calcined at different temperatures for phase evolution study. Dual phases corresponding to diopside and merwinite was observed after calcination of monticellite precursor at 600°C. The presence of these materials commonly noticed during the preparation of tertiary system silicates (calcium magnesium silicate) due to non-stoichiometric calcium-rich sites in the heated sample [21]. The absence of monticellite phase in the XRD pattern was due to the lack of optimum temperature required to achieve its crystallization. Hence, to initiate the phase formation of monticellite the precursor was further calcined at a higher temperature. After calcination at 900°C, characteristic monticellite peaks emerged as dominant phase. The diopside and merwinite phases were found to be highly unstable in the temperature range of 700 to 900°C, leading to their complete elimination. Moreover, akermanite as minor phase was also observed along with monticellite at 900°C. When the calcination temperature of the sample increased to 1300°C, the akermanite phase was found to be completely removed and single monticellite phasic pure monticellite was obtained. The required temperature for the synthesis of pure monticellite by sol-gel combustion method was optimized at 1300°C. This temperature was found to be very low as compared to existing reports [9,22]. The XRD pattern of the sample calcined at 1300°C was matched with standard JCPDS (Card No: 035-0590) and formation of single phasic monticellite in orthorhombic crystal system with space group Pmnb (62) was confirmed.
The monticellite powders prepared using different fuels were calcined at 1300°C and compared XRD pattern shown in figure 3. It was observed that the XRD patterns were composed of dual phases with the existence of monticellite as well as akermanite peaks. This might be due to the formation of stable akermanite when prepared by citric acid, succinic acid, tartaric acid, sucrose, urea, and glycine as fuels. Hence, it can be suggested that the akermanite phase can be eliminated either by increasing the calcination time or temperature. This finding also indicates that the L-alanine is a suitable fuel for the synthesis of pure monticellite. Table 1 represents the lattice parameters, unit cell volume and crystalline sizes calculated for monticellite samples prepared by using various fuels. The crystallites size was calculated by using Scherer's formula for highest intense (131) plane. It was found to be in the range of 16-44 nm. The unit cell volume was found to be in the range of 339.44723-342.37023 (Å 3 ).

Vibrational spectra
The internal modes of SiO 4 are almost independent of lattice vibrations in monticellite crystal structure and these SiO 4 vibrational levels split due to symmetry. Earlier, Handke and Urban determined the splitting modes of the SiO 4 for orthosilicate structures. FT-IR spectra (figure 4) of the precursor calcined at 1300°C (prepared by using various fuels) shows a sharp band at 826 cm −1 which attributed to A 1 (v 1 ) stretching mode, the bands appeared at 993, 961, 866 cm −1 assigned to the F 2 (v 3 ) stretching mode, sharp bands appeared at 601, 516 cm −1 assigned to F 2 (v 4 ) bending mode and bands appeared at 482,436 cm −1 assigned to E(v 2 ) bending modes. All the observed vibrational frequencies in good agreement with the Handke and Urban reports [23]. No significant vibrational variations observed in the FT-IR spectra of the monticellite prepared by using various fuels, which indicates the formation of monticellite phase.  Figure 5 shows the DLS particle size distribution curves of the monticellite prepared by using various fuels. From the results, it was clear that all the samples in the particle size range of 2162 to 2731 nm. There is no significant variation observed in the particle size distribution of the monticellite prepared by using various fuels.

Apatite formation ability
Apatite formation ability of the monticellite pellets (prepared by using various fuels) was tested by immersing in the 2X SBF solution for 10 days ( figure 6(i)). This was done to analyze the influence of fuels on the apatite formation ability of the monticellite. Urea derived monticellite shows crystalline hydroxyapatite deposition and other fuel derived samples shows the amorphous apatite phase. This observation indicates that monticellite samples have potential to induce apatite formation when exposed to the physiological environment. The rapid exchange of alkaline earth ions from urea derived monticellite sample with H + ions from SBF might lead to the formation of the silica-rich layer at a very faster rate. This process resulted in the rapid consumption of Ca 2+ and P ions from the SBF to initiate hydroxyapatite deposition. Previous studies suggest that the formation of the silica-rich layer at the interface of materials is the major step to induce apatite nucleation [24]. The overall mechanism of apatite deposition is explained elsewhere [25]. The pure monticellite shows improved bioactivity within 10 days as compare to earlier reports. Previous articles reveal that the hydroxyapatite deposition on monticellite surface takes place after immersing in SBF for 15 days [9].    Figure 6(ii) shows the XRD pattern of monticellite (L-alanine derived) and its composite pellets after immersing in 2X SBF. After 10 days of immersion, monticellite peaks became less intense and partially disappeared with the appearance of characteristic hydroxyapatite peaks as a major phase. A comparative apatite deposition behavior of monticellite-chitosan and monticellite-chitin composites were also analyzed. It was found that chitosan favors the deposition of hydroxyapatite whereas chitin was unable to induce hydroxyapatite formation on the surface of composites figure 6(ii). This behavior was found similar to the existing report [26]. This study shows that the content as well as the compositional dependent bioactivity of the samples in a physiological environment. Also, as the chitosan content was decreased in the composites an increase in the rate of apatite deposition was noticed. But the monticellite/chitosan composites did not exhibit better apatite deposition ability than pure monticellite. Hence, this analysis indicates that the bioactivity of ceramics can be influenced by fabricating their composites with biopolymers. Literature survey shows that the monticellite composites are very rarely studied while its composites with biopolymers are still unexplored [27]. Figure 7 shows the SEM and EDS analysis data of the pure monticellite pellet after immersing in 2X SBF solution. From the SEM images, the morphological appearance of monticellite pellet surface observed to cover with spherical shaped hydroxyapatite particles, which evenly spread throughout the surface. The appearance of characteristic peaks corresponding to Ca, Mg, Si, O, and P proves the apatite formation ability of monticellite.

Conclusion
For the first time, monticellite powders were synthesized by employing sol-gel combustion method and studied the effect of fuel on the phase formation. The methodology used in current work assisted in reducing the calcination temperature of monticellite by150°C. Phase evolution study indicates that the formation of the monticellite from diopside, merwinite, and akermanite. XRD pattern confirms the formation of pure monticellite phase when L-alanine used as a fuel. In the case of other fuels, the existence of akermanite as a secondary phase observed, which indicates that L-alanine is a suitable fuel for the synthesis of monticellite phase. Vibrational spectra confirm the presence of monticellite functional groups and DLS data reveals the particle size range from 2162 to 2731 nm. Apatite formation ability studies show the bioactive characteristics of the pure monticellite within 10 days. Previous reports reveal that the hydroxyapatite deposition on monticellite surface takes place after immersing in SBF for 15 days. Thus, monticellite may be a potential candidate material for bone regeneration and tissue engineering applications.