*2.3. Preparation of Silica Nanoparticles (SiO2 NPs)*

SiO2 NPs were prepared by adding 6 g sodium metasilicate to (394 mL) distilled water in a flask. The temperature was adjusted to 55 ◦C under constant stirring at a 300 rpm speed. CTAB (4 g) was added with stirring until complete dissolution for 15 min. The diluted HCl (200 mL) was added dropwise to the solution in two steps. In the first step, HCl was added dropwise to the mixture until the pH reached 9–9.5. With no further addition of HCl, the solution was agitated for another 10 min. More HCl was added until the pH reached 3–3.5 in the second step. Then, 10 mL of 10% NaCl aqueous solution was added to the reaction mixture at the acquired pH and agitated for an additional 20 min. The precipitated wet-gel silica was aged at 50 ◦C for 24 h, and the produced wet-gel silica was centrifuged and rinsed with distilled water until a negative response for chloride ions was observed, as measured with a 0.1 M AgNO3 solution. The gel was dried at 40 ◦C via a microwave heating process. The total drying time was 70 min for each sample, applied seven times in an intermittent interval of 10 min. The dried gel was calcined at 650 ◦C for 3 h in a muffle furnace to obtain the white powder of nano SiO2 [31,32]. The obtained micrographs showed that the SiO2 NPs size was in the range of 10–40 nm, whereas their shape was spherical (Figure 1). Moreover, most SiO2 NPs had a uniform size, and no aggregation was observed in scanning electron microscopy (SEM) analysis (JEOL JSM-6510LB with field emission gun, Tokyo, Japan). On the other hand, IR spectra were obtained for KBr pellets on a JASCO 410 spectrometer (Jasco, Tokyo, Japan) with only selected absorptions recorded in the range of 2000–200 cm−1. Figure 2 shows that the peaks at 1020–1110 and 807 cm−<sup>1</sup> are attributed to Si-O-Si a symmetric and symmetric stretching style, respectively [33].

**Figure 1.** SEM micrographs of the amorphous SiO2 NPs at different bar scales (**a**) 1 μm, and (**b**) 0.5 μm.

**Figure 2.** FTIR spectra of the amorphous silica prepared with CTAB.

Dynamic Light Scattering (DLS) and Zeta Potential Analysis

Zeta potential and dynamic light scattering (DLS) were measured using a Malvern Zetasize Nano-zs90. As shown in Figure 3, DLS analysis confirmed the preparation of SiO2 particles in nano scale at approximately 90 nm. The obtained size from the DLS analysis is more than that obtained from SEM micrographs, which is caused by the formation of hydrogen bonds between SiO2 NPs because of water addition [34]. Furthermore, the zeta potential value of the colloidal SiO2 NPs solution was −19.4 mV with a single peak.

**Figure 3.** Dynamic light scattering (DLS) (**a**) and zeta potential (**b**) of the obtained SiO2 NPs.
