**3. Results**

#### *3.1. Transmission Electron Microscopy (TEM) Analysis*

The morphology and size of the synthesized SeNPs at different concentrations (25, 50 mM) were characterized using a transmission electron microscope. SeNPs prepared at the concentrations of 25 and 50 mM ranged from 36–77 nm and 41 to 149 nm, indicating the increase in average size with the increase in SeNPs concentration. The obtained TEM micrographs revealed that SeNPs were spherical in shape and well dispersed in the colloidal solution, especially that prepared at the low concentration. Furthermore, no agglomeration or deformation of SeNPs was observed, as displayed in Figure 1a,b. The width of bins in SeNPs histogram was 20 nm, as illustrated in Figure 1c, and these bins were centered at 40, 60, 80, 100, 120 and 140 nm. For example, all particles with diameters between 70 nm and 90 nm were considered together as particles with a diameter of 80 nm. In the case of low concentration (25 mM), SeNPs around 60 nm in size exhibited the highest percentage value of 76.06. On the other hand, the majority of SeNPs at the high concentration of 50 mM were from 70 to 110 nm, which confirmed the increase in SeNPs diameter and polydispersity as the increase in NPs concentration.

**Figure 1.** TEM images of SeNPs synthesized at different concentrations of (**a**) 25 mM and (**b**) 50 mM in addition to (**c**) the size distribution histogram of the prepared SeNPs.

Over and above, many SeNPs at the low concentration had a hollow shape (ringshaped), while SeNPs at the concentration of 50 mM were ordinary solid spherical particles, illustrating that SeNPs specific surface area increased with the decrease in SeNPs concentration due to the reduction in NPs average size and the change in their morphology as shown in Figure 1.

#### *3.2. Leather Characterization before and after Functionalization*

#### 3.2.1. Scanning Electron Microscopy (SEM) Analysis

The micrographs obtained from SEM analysis showed that the surface of untreated leather is typically clear with clean scales and smooth longitudinal collagen fibers as shown in Figure 2a. On the other hand, the SEM micrographs of leather/SeNPs revealed that the leather surface was coated sufficiently with a layer of SeNPs without aggregation and the SeNPs were distributed well on the leather surface as displayed in Figure 2b,c. Moreover, the chemical elements analysis of leather/SeNPs confirmed the presence of SeNPs on the surface after the dyeing process as shown in Figure 2d.

**Figure 2.** SEM micrographs of (**a**) blank leather and (**b**,**c**) leather dyed with SeNPs under the optimum conditions at different scales as well as (**d**) EDX spectrum of the dyed leather.

#### 3.2.2. Raman Analysis

Raman analysis was conducted to confirm the deposition of SeNPs on the dyed leather surface as shown in Figure 3. The comparison between the spectra of leather surface before and after treatment with SeNPs was investigated in the range of 200–1600 cm−1. After treatment two intense peaks at 230 and 295 cm−<sup>1</sup> appeared as a result of SeNPs deposition. Similar results were obtained by Abou Elmaaty et al. [29] who treated the polypropylene fabric with SeNPs and found a peak around 236 cm−<sup>1</sup> after SeNPs treatment, while Lukács et al. [47] found a pronounced peak at 292 cm−<sup>1</sup> as an indicator for the presence of SeNPs that confirms the successful deposition of SeNPs on the treated leather surface.

**Figure 3.** Raman spectra of untreated leather and leather/SeNPs−.
