Effect of Calcination Temperature on Structure of Mesoporous Borosilicate Bioglass

Recently, most researchers have been focusing on the development of the mesoporous bioactive glasses for medical application. These materials are capable for bone tissue (soft and hard) regeneration and the delivery of bio-responsive active therapeutic molecules such as drug, proteins, nucleic acids, and peptides. However, the structure properties of the mesoporous bioactive glass are easily been controlled with a small change of calcination temperature during the sample preparation. In this perception, a series of mesoporous borosilicate bioactive glasses (MBBGs) with the composition of 10B2O -70SiO2 -15CaO- 5P2O5 were prepared using the combination of sol-gel and evaporation induced self-assembly (EISA) and characterised. The calcination temperature of the bioactive glass preparation was controlled with varied temperature of 400°C, 500°C, 600°C and 700°C. The amorphous nature of the prepared samples were confirmed using XRD pattern. The EDX and FTIR spectra shows a small amount of carbon trapping inside the sample is increases with the increased of calcination temperature. All MBBGs samples show a ratio of Si-O-NBO and Si-O-Si (sym) bigger than 1. It is established that the structure of the mesoporous borosilicate bioactive glass can be tailored by controlling the calcination temperature.


Introduction
In the late 1960s, Larry Hench et al. developed new materials beside ceramic which is called bioactive glasses [1]. Compared to bioactive ceramic, bioactive glasses form a bond much faster. This is due to their dissolution of random amorphous network which is easier than crystalline ceramic, therefore hydroxycarbonate apatite (HCA) forms earlier on glass [2]. Fabrication of bioactive glass by sol gel with evaporation-induced self-assembly (EISA) technique resulting a mesoporous bioactive glass. Those mesoporous bioactive glasses exhibit a pore dimensions are within 2-50nm, which is resulting a superior bioactive property (formation of a HCA layer within a few hours) compared to bioactive glass made by traditional technique [3,4]. Several studies have been reported that borosilicate glass react quickly and convert more complexly to HCA compared to silicate 45S5 bioactive glass [5,6]. Xiu et. al has proved that mesoporous CaO-B2O3-SiO2 glasses have a higher porosity and exhibit better in vitro bioactivity than conventional sol-gel-derived CaO-B2O3-SiO2 glasses [7].
The mesoporous structure of the bioactive glass can be controlled by several ways. One of them by tuning the thermal condition of the mesoporous bioactive glass during the sample preparation. In 2017, Mariappan and Ranza have been investigated the effect of heat treatment on calcium phosphosilicate bioglass ceramics nanoparticles doped with silver [8]. They reported that no change of size and shape was clarified causes by the heat treatment but it shows better bioactivity and antibacterial properties against E. coli and S. aureus [8]. The studies on the effect of thermal treatment was continued by Vale Recently, Delihta et. al studied on 80SiO2-15CaO-5P2O5 with different composition of Na. They found that the synthesis mesoporous bioactive glasses without additional Na prepared by 700 o C for 5h calcination condition exhibit in amorphous behaviour without any carbon entrapment [11]. However this calcination condition is not suitable when the MBG composition was added with Na. Only few works on introduction of boron in MBG have been reported. Two of them is work by Paul et. al [12] and Wu et. al [13] with MBG composition of 50Si-50B and 75SiO2-15CaO-5P2O5-5B2O3, respectively. Those two MBGs glasses have successfully been prepared with the different calculation condition. This proves that the change of glass composition needs a different calcination temperature. Therefore, the aim of this study is to determine the optimum calcination condition for mesoporous borosilicate bioglass with composition of 10B2O3 -70SiO2-15CaO2-5P2O5. The effect of calcination condition on the mesoporous borosilicate bioglass behaviour can be explained more in term of their relation with other properties.

Experimental
A series of mesoporous borosilicate bioglass with composition of 10B2O3 -70SiO2-15CaO2-5P2O5 with different calcination temperature such as 400 o C, 500 o C, 600 o C and 700 o C, have been prepared by EISA method. The mesoporous copper borosilicate bioglasses is synthesized by sol-gel method (using evaporation-induced self-assembly; EISA) as previous studies [14,15]. 8 g of P123 is dissolved in 120 g of ethanol followed by the addition of tetraethyl orthosilicate (TEOS, Si-precursor), boric acid (H3BO3) calcium nitrate tetrahydrate (Ca(NO3)2.4H2O), copper nitrate (Cu(NO3)2·3H2O, and triethylphosphate (TEP) at room temperature under fume hood. The mixture is vigorously stirred for 7 hours, then transfered into petri dish and kept at 35 o C under fume hood for evaporation-induced selfassembly (EISA) process. After 1 day of EISA process, the gel sample is aged at 60 o C for another day. Then the glass was be separated into a several part and applied in different thermal treatments of calcination process (400 o C, 500 o C, 600 o C and 700 o C). The amorphous nature of mesoporous borosilicate bioglasses (MBBGs) is confirmed by X-ray diffraction pattern from Rigaku SmartLab High Resolution X-Ray Diffractometer equipped with CuKα radiation with wavelength 1.5406 Å in the 2Ɵ range of 10-60 and 0.02 step size. The structural properties of MCBBG have be tested by Fouri Transform Infrared (FTIR). The FTIR spectra have been found by pellet the sample with KBr with the ratio of 1:100 and 4 cm -1 resolution.

Results and discussions
The mesoporous borosilicate bioactive glass (MBBG) samples with composition of 10B2O3 -70SiO2-15CaO2-5P2O5 have been prepared by sol-gel and EISA method with different calcination temperature such as 400 o C, 500 o C, 600 o C and 700 o C. Samples with different calcination condition of 400 o C, 500 o C, 600 o C and 700 o C are naming as S400, S500, S600 and S700, respectively. Figure 1 depicted the X-ray diffraction pattern for S400, S500, S600 and S700 samples. It is apparent that, a common diffusion halo is present in the XRD pattern in the range of 20-35 degree, which confers the amorphous nature [16] and short range structural disorder of the glasses [17]. From Figure 1, it also can be seen that there are absent of sharp peak in the XRD pattern, this verify that the structure of glass does not appearance any crystal.  Figure 1. The X-ray diffraction pattern of S400, S500, S600 and S700 samples with different calcination condition.
In order to confirm the present of element in the MBBG, the Energy Dispersive X-ray (EDX) spectroscopy was used. Elemental Analysis of MBBG determined by EDX Spectroscopy, the result from all samples (S400, S500, S600 and S700) are listed in table 1. The element analysis are in the range of 47.0 to 51.2, 28.1 to 30.7, 11.6 to 8.5,0.8 to 1.7, 1.4 to 2.2, and 5.9 to11.0 for O, Si, Ca, P, B and C element, respectively. Besides O element, Si element also? shows a higher weight percentage compared to other element due to the higher amount of TEOS used during the sample preparation compared to other raw material. From table 1, it is clearly seen that, the increment of calcination temperature reduces the weight percentage of the Si, Ca, P, and B element. This indicates that the change of calcination temperature may affect the present of element inside the glass network. On the other hand, the weight percentage of C element is increased with the increase of calcination temperature. One reason for the increment might be due to the carbon trapping inside the glass structure during the calcination process. Nitrogen (N2) adsorption-desorption isotherms are used to determine the mesoporosity and surface area of samples. Figure 2 shows the Nitrogen adsorption-desorption isotherms of MBBG with three different calcination temperatures. Sample S400, S500, S600 and S700 exhibit typical type IV isotherms and hysteresis loops are type 1, which indicate the presence of hexagonal p6nm mesoporous [5]. From those figure sample S700 shows small N2 absorbed volume (weak step), which attributed to the irregular pore structure [18]. The calculated data of the BET surface area, pore volume and pore size of MBBG samples are listed in table 2. The S600 had the highest surface area and pore volume of 388.6 m 2 g -1 and 0.66cm 3 g -1 , respectively, which higher compared to previous studies [19] [14]. It can also be found that the surface area and pore volume S700 is small compared to S400, S500 and S600. This significant difference of surface area and pore volume may result by difference ordered degree of sample. The S700 with 700 o C calcination temperature might causes a carbon trapping inside the glass network. The pore distribution curve was analysed from the N2 desorption branches using the BJH model ( Figure 3). The BJH pore size distribution curve indicates that, all samples S400, S500, S600 and S700 have a narrow size of pores around 2.0 and 2.9 nm, respectively.    Figure 2 shows the FTIR spectra of all MBBG samples with different calcination temperature. Each of the spectra is deconvoluted in the range of 400-2000 cm -1 wavenumber as shown in Figure 3. The moderate band around 1648 cm -1 is attributed to the stretching vibration of the H-O bond of H2O that was absorbed in the materials or silanol group (Si-O-H). The Gaussian shape are consistent in all sample and had been assigned as listed in table 3. It can be seen that, the area of carbonate band near 1390-1430 cm -1 is increased with the increase of temperature, which might be due to carbon trapping inside the sample [20]. This result is agreement with pervious result in EDX analysis, which assigned the carbon contain is increased with the increment of calcination temperature.
Basically, the unit of silicate tetrahedral is participating and linking with four bridging oxygen to neighbouring silicon atoms. The present of network modifiers (B 3+ and Ca 2+ ) in the glass structure can provoke the destruction of the glass network continuity by breaking the Si-O-Si bond. Commonly, the unit of silicate tetrahedral are donated as Q n , where n refer to the number of bridging respectively possess by four, three, two and one non-bridging oxygen atom [21]. From figure 5 the vibration mode of Si-O-NBO and symmetric stretching mode of Si-O-Si is a characteristic of Q 3 and Q 4 group. According to Ma.J in 2010, the relative ratio between Q 3 and Q 4 group can be used to show the polymerization degree of the synthesized glass [22]. Figure 6, show the intensity ratio of Q 3 and Q 4 for MBBG sample as a function of calcination temperature. The value of ratio is increases from 2.61 to 2.99 and then it slightly decreases to 2.79 when the sample is heat with 700 o C in the calcination process. This is indicated that the sample S600 with 600 o C calcination temperature show a higher intensity ratio of Q 3 and Q 4 . Previous studies by Serra J. et. al have reported that, sample with ratio of Q 3 and Q 4 bigger than 1 have a higher potential as bioactive material [23]. This is due to the efficient ion exchange, dissolution pf silicate and formation of HCA layer on the glass surface. Therefore, compared to the previous studies, all the synthesized glasses have a bigger ratio.