*2.2. Green Synthesis of SeNPs*

SeNPs were prepared via redox reaction as reported by Abou Elmaaty et al. [32] with an improved modification. Sodium hydrogen selenite was used at different concentrations (50 mM and 100 mM) as a precursor for SeNPs. Polyvinylpyrrolidone (PVP) was dissolved in sodium hydrogen selenite solution at a concentration of 12 g/100 mL to maintain the nanoparticles stability. Then, ascorbic acid at varied concentrations of 100 mM and 200 mM was mixed with this mixture at a volume ratio of 1:1 and molar ratio of 2:1 (ascorbic acid: NaHSeO3). The color change from colorless to dark orange indicated the formation of SeNPs.

#### *2.3. Implementation of Selenium Nanoparticles (SeNPs) into Leather*

Before treatment, the leather samples were scoured with nonionic detergent (3%) on a weight of leather at 50 ◦C for 15 min at a liquor ratio of 1:50 to remove impurities. Then, the leather sample was rinsed with distilled water, followed by treatment into a solution of SeNPs at a liquor ratio (LR) of 1:50 by ultrasonic water bath. The treatment was carried out at varied temperatures (at room, 40, 50, 60, 65 and 70 ◦C), different periods (30, 60, 90 and 120 min) as well as different *pH* values (3,4,5,6,7,8) and two concentrations of SeNPs of (25 and 50) mmol/l. Subsequently, the leather sample was removed, rinsed with nonionic detergent (2%) on weight of leather at 40 ◦C for 15 min at a liquor ratio of 1:50, rinsed with distilled water, and dried in an oven at 50 ◦C for 25 min. Finally, the leather samples were placed in a desiccator to remove traces of water [4].

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

SeNPs characteristics included morphology and size were characterized by transmission electron microscope (JEOL, JEM 2100F) at 200 kV. A drop of the NPs colloidal solution was loaded onto a 400-mesh copper grid with an amorphous carbon film, and water was evaporated in air at room temperature.

#### *2.5. Leather Characterization*

#### 2.5.1. SEM and EDX Analysis

The surface morphology of the leather samples, including blank samples and those treated with SeNPs, was observed using a scanning electron microscope (JEOL JSM-6510LB, Tokyo, Japan) with attached energy dispersive X-ray spectrum (EDX) unit.

#### 2.5.2. Raman Spectroscopy Analysis

The types of bonds found in the blank or treated leather were detected using a confocal Raman microscope (Jasco NRS-4500, Tokyo, Japan) at the range of 200–1600 cm<sup>−</sup>1. For both Raman data acquisition and processing, Jasco spectroscopy suite software was used.

#### 2.5.3. Colorimetric Study

The color uptake, expressed as the color strength (*K/S*) of blank leather and leather/SeNPs, was determined using a spectrophotometer (CM3600A; Konica Minolta, Japan). *K/S* values were evaluated at the wavelength of maximum absorption (λmax) of the color's reflectance curve at 360 nm. The total color difference (Δ*E*) was represented in terms of CIE LAB color space data. It was calculated using (Equation (1)),

$$
\Delta E = \sqrt{\left(L\ast\_2 - L\ast\_1\right)^2 + \left(a\ast\_2 - a\ast\_1\right)^2 + \left(b\ast\_2 - b\ast\_1\right)^2} \tag{1}
$$

where, Δ*E* is the total difference between blank leather and leather/SeNPs, *L\** is the lightness from black (0) to white (100), *a\** is the red (+)/green (−) ratio and *b\** is the yellow (+)/blue (−) ratio.

#### 2.5.4. Exhaustion of SeNPs onto Leather

The UV/Vis spectrum of the SeNPs colloidal solutions before and after leather treatment was measured by UV/Vis spectrophotometer (Alpha-1860, Noble, IN, USA) to evaluate the SeNPs exhaustion.

#### 2.5.5. Physical Properties of Leather

The color fastness of blank leather and leather/SeNPs was tested according to the following standard methods. They were determined using the AATCC (61-1996) [40], (8-1996) [41], and (16-2004) [42] tests for washing, rubbing, and light fastness, respectively. The tensile strength tests of blank leather and leather/SeNPs were performed according to ASTM D638-14 [43], using a tensile testing machine (Zwick Z010, Staufenberg, Germany). Additionally, the durability to washing was evaluated according to AATCC 61(2A)-1996 [44] after five washing cycles.
