*3.1. Surface Morphology and Elemental Composition of the Nanomaterials*

Figure 1 shows the SEM images of (a) silver, (b) tin oxide, and (c,d) silver tin oxide nanomaterials grown at 800 °C with growth time of 6 h. The micrographs revealed the presence of nanoparticles, wires, and cotton-like structures grown in random directions.

Table 1 exhibits the atomic and elemental composition of the SnO2-Ag composite nanomaterials at 1:4 ratio, 2:3 ratio, 3:2 ratio and 4:1 ratio. The resulting elemental and atomic composition confirmed the presence of Ag, Sn, and O in the composite nanomaterials.

**Figure 1.** SEM images of nanomaterials (**a**) silver; (**b**) tin oxide; and (**c**,**d**) silver tin oxide.

The two controls used in the study are shown in Figure 2a,b. Figure 2a shows an agar plate without any bacterial colony. Instead of adding bacterial solution to the sterile petri dish, only distilled water was poured into it. On the other hand, Figure 2b shows an agar plate with several bacterial colonies seen as tiny white spots.


**Table 1.** Energy-dispersive X-ray spectroscopy (EDX) analysis of SnO2-Ag nanomaterials at 1:4 mixture, 2:3 mixture, 3:2 mixture, and 4:1 mixture.

It can be seen in Figure 3 that there are fewer CFUs on the agar plate with Ag nanomaterials compared to the agar plate with the bulk Ag powder. Likewise, there are fewer CFUs on the agar plate with the SnO2 nanomaterials than on the agar plate with bulk SnO2 powder. This is consistent with the finding of Espulgar and Santos [12] that the antimicrobial property of bulk material was not only carried over but was enhanced by its nanomaterial counterpart.

Figure 4 shows that there are fewer CFUs for the agar plate with the 1:4 ratio of SnO2-Ag composite nanomaterials as compared to the 2:3, 3:2, and 4:1 ratio of SnO2-Ag composite nanomaterials.

**Figure 2.** Agar plates used as control for comparison: (**a**) Agar plate without bacteria; (**b**) Agar plate with bacteria.

**Figure 3.** Comparison on colony forming units (CFU) between (**a**,**c**) powder and (**b**,**d**) nanomaterial on (**a**,**b**) Ag and (**c**,**d**) SnO2.

**Figure 4.** Agar plates containing mixtures of bacterial solution and nanomaterials of different ratio: (**a**) 1:4; (**b**) 2:3; (**c**) 3:2; (**d**) 4:1.

Table 2 summarizes the result of the antibacterial test where the number of CFU after 24 h of incubation was shown. It can be seen that Ag is more toxic to *E*. *coli* than SnO2 and that the nanomaterials are more toxic than their bulk form. Also, it can be observed that as the percentage of Silver increases over tin oxide, the CFU number decreases. This is consistent with the observation that Ag is more toxic to *E*. *coli* than SnO2. Moreover, results reveal that the 1:4 ratio of tin oxide and silver composite nanomaterials exhibits the greatest antimicrobial effect among the other ratios and material composition. Such a finding is consistent with previous reports [13,14] that the combination of Ag and a metal oxide may lead to an increase in bactericidal effect.


**Table 2.** Colony forming units (CFU) *vs*. material composition.

The mechanisms of the bactericidal effect of silver and silver nanoparticles (NPs) were discussed in different studies according to literature [15]. A study proposed that silver NPs can be attached to the surface of the cell membrane disturbing the permeability and respiration functions of the cell [4]. Smaller silver NPs having large surface area that are available for interaction would be more bactericidal than the larger silver NPs [16]. Moreover, it is possible that silver NPs will not only interact with the surface of membrane but can also penetrate inside the bacteria [17].
