**3.3 SiO2 microspheres**

The purpose of the study is to learn the behavior of bacteria with an inorganic microsphere of appropriate size. As mentioned above, silica is well compatible with cement and is therefore used as a small transport system. The synthesis process is relatively cheap and easily manageable. The desired ball size is about 1 μm. Through the sol-gel method, SiO2 micro-containers were combined into two series of experiments by changing the parameters of the experiment while keeping the remaining constant. In the 1st series of experiments, we increased the amount of TEOS added. In the second series of experiments, the volume of the solvent increased. In both cases, the ratio of H2O/Si, Rw and the change in size of the microspheres was investigated. We used ethanol, water, TEOS, and 30% NH3 as the catalyst. The experimental SiO2 ball synthesis process was done using 150 ml boiling glass in which EtOH and H2O were mixed and left to stir for about 10 minutes. After that, we added NH3 and stirred for 20 minutes. The reaction starts when the TEOS drip is added. Leave the final solution for about 20 hours under the same conditions, then centrifuge and dry. The whole experimental process takes place at ambient temperature. **Table 2** gives the ratios H2O/ Si, Rw for each case. This ratio results from the added water in each experiment. The water comes from the 30% NH3 solution. That is, 0.3 ml H2O is contained in 1 ml of NH3. **Table 3** gives the exact amounts of chemicals used. The resulting SEM micrographs are shown in **Figure 8**. According to the SEM image, the SiO2 microsphere synthesis process shows repeatability by giving spherical particles, with a similar diameter and small size distribution. The dispersion proves that there is a good size distribution.

The characterization of the size of SiO2 microspheres was done with DLS. The study was carried out to find the optimal sample concentration for the correct measurement in DLS. In this case, it was found that the optimal concentration is 10 mg/L. According to the literature [3], a suspension is considered stable (i.e. not cracked) when it has a potential positive of +30 mV and more negative than -30 mV. According to the DLS measurement, the size of the SiO2 microspheres is 782 ± 46 nm, which is also in line with the measurement of SEM. The ζ-potential is −56 ± 0.361, which attests to the stability of the sample composed. In summary, the synthesis of an inorganic microspheres of about 1 μm size was achieved by the sol-gel.


### **Table 2.**

*The ratio Rw resulting by changing the amount of TEOS in the first case of experiments, keeping the remaining parameters constant.*


### **Table 3.**

*The ratio Rw, which results by changing the amount of solvents in the second case of experiments, keeping the remaining parameters constant.*

**Figure 8.** *SEM image of 725 ± 84 nm size SiO2 microspheres.*
