Effect of Treatment Time on Color Strength (*K/S*)

Figure 6 exhibited the effect of treatment time on SeNPs uptake by leather. As depicted, the *K/S* was increased upon increasing time of treatment. The optimum *K/S* value of 18.7 was achieved at 60 min after which, *K/S* started to decrease with increasing time. The decrease in *K/S* as the increase in time from 90 to 120 min may be due to the aggregation of SeNPs as a result of SeNPs accumulation upon the leather surface [48].

**Figure 4.** Effect of *pH* on color strength (*K/S*) of leather/SeNPs.

**Figure 5.** Effect of treatment temperature on color strength (*K/S*) of leather/SeNPs.

**Figure 6.** Effect of treatment time on color strength (*K/S*) of leather/SeNPs.

Effect of SeNPs Concentration on Color Strength (*K/S*)

The color strength of the leather/SeNPs was found to be dependent on SeNPs concentration. Upon increasing the concentration of SeNPs, the *K/S* value was decreased as

shown in Figure 7. The highest *K/S* value for the treated leather was monitored at the lowest concentration. This proved that treating leather with lower concentration of SeNPs (25 mM) was adequate to achieve the optimum *K/S*. The aforementioned behavior could be attributed to the thermal stability of the leather, as it increased at low SeNPs concentration and decreased at high SeNPs concentration. The improvement in thermal stability could be recognized as the crosslinking between SeNPs and collagen of leather [4].

**Figure 7.** Effect of SeNPs concentrations on color strength (*K/S*) of leather/SeNPs.

#### 3.2.4. Exhaustion of SeNPs onto Leather

The UV/Visible spectrum of SeNPs colloidal solution was measured before and after leather treatment to evaluate the SeNPs exhaustion by treated leather as shown in Figure 8.

**Figure 8.** UV/Visible spectrum of SeNPs before and after leather treatment.

The absorbance of SeNPs was measured in the range of 235 to 700 nm. The absorption spectrum of SeNPs showed a sharp absorption band at 290 nm before the exhaustion, while SeNPs spectrum after leather treatment decreased obviously after exhaustion, confirming the deposition of SeNPs on the treated leather.

#### 3.2.5. Physical Properties of Leather/SeNPs

The tensile modulus and elongation at break % values were listed in Table 2. The obtained results indicated that there were no significant differences about the mechanical properties of leathers, mainly between the leather/SeNPs compared to the blank leather. The color fastness properties of leather/SeNPs were evaluated and the results were tabulated in Table 2. Leather/SeNPs revealed excellent fastness results referring to the chemical stability of the SeNPs onto the leather surfaces along with long-term durable interactions between the SeNPs and the leather surface [4].


**Table 2.** Properties of the Leather and Leather/SeNPs under optimum conditions.

Treatment conditions: SeNPs, (25 mM); Time (60) min; Temp. (65) ◦C; *pH* 6. <sup>a</sup> Standard deviation for untreated leather = 0.5, standard deviation for treated leather = 0.5. <sup>b</sup> Standard deviation for untreated leather = 0.5, standard deviation for treated leather = 0.5.

#### 3.2.6. Cytotoxicity of Leather/SeNPs

Leather/SeNPs cytotoxicity was evaluated against healthy human melanocyte cell line (HFB4) using MTT assay. The viability of cells in the presence of leather/SeNPs was 90.67% compared to that of negative control. The average relative cell viability is over 70% [50], indicating the low toxicity of leather/SeNPs toward human skin.

#### 3.2.7. Antibacterial Activity

The antibacterial activity of leather/SeNPs was evaluated against four bacterial strains, including *Bacillus cereus* as Gram-positive bacteria in addition to *Escherichia coli*, *Pseudomonas aeruginosa* and *salmonella typhi* as Gram-negative bacteria. The results revealed that the leather/SeNPs showed outstanding antibacterial activity against the tested strains in comparison with standard drugs as listed in Table 3. Leather treated with 25 mM SeNPs colloidal solution was more effective against *Escherichia coli*, *Pseudomonas aeruginosa* and *Bacillus cereus* than that treated with high concentration (50 mM) due to the difference in average size. SeNPs (25 mM) with small average size exhibited higher specific area and in contact with bacterial cells more than that at the high concentration (50 mM) [51]. There was no obvious variation in inhibition zone diameters for leather/SeNPs before and after five washing cycles that confirms the durability of the leather/SeNPs samples at the tested concentrations. Moreover, the cross-linking between leather and SeNPs as antimicrobial agents protects leather against laundering and mechanical abrasion that makes the leather products more durable [22].

**Table 3.** The inhibition zone (mm) values of leathers/SeNPs treated with different SeNPs concentrations.


The antibacterial effect on bacteria may be due to the release of ions or the formation of reactive oxygen species that lead to DNA damage. Furthermore, the deposited SeNPs can contact bacteria or fungi cells much easier than colloidal form [22].

#### **4. Conclusions**

Leather material was successfully colored and functionalized utilizing selenium nanoparticles (SeNPs) which were synthesized and deposited simultaneously onto the leather surface. The SeNPs decorated the leather surface with shining colors, which can be controlled by adjusting the *pH* at 6, treatment time for 60 min., treatment temperature at 65 ◦C and SeNPs concentration of 25 mM. The results showed that a yellow/brown color was imparted to the leather after the implementation with SeNPs by ultrasound technique. Moreover, the colored leather samples acquired good color fastness to rub, wash, and light. Leathers/SeNPs exhibited excellent antibacterial activities against models of bacteria, including Gram-positive bacteria (*Bacillus cereus*) and Gram-negative bacteria (*Pseudomonas aeruginosa*, *Escherichia coli* and *Salmonella typhi*). The results of coloration, cytotoxicity and antibacterial properties clarified that the SeNPs can be used to impart color and antibacterial properties to leather material. The proposed methodology emphasized the effectiveness and applicability of this simple approach to the footwear industry to color the leather as well as prevent the spread of bacterial infection promoted by humidity, poor breathability and temperature.

**Author Contributions:** T.A.E. conceived the original idea of this study and participated in design and coordination as well as manuscript drafting. S.A.A. participated in the manuscript writing and interpreted the obtained results from colorimetric and mechanical properties study. K.S.-A. prepared the SeNPs and interpreted the obtained results obtained from TEM, SEM analysis, antimicrobial and cytotoxicity tests as well as participation in manuscript writing. K.E.-K. was responsible for the dyeing experiments and leather finishing steps in addition to participation in SeNPs preparation. R.M.A. provided the tanned leather required for this study. All authors have read and agreed to the published version of the manuscript.

**Funding:** This paper is based upon work supported by Science, Technology & Innovation Funding Authority (STDF); Egypt under grant (POST GRADUATE SUPPORT GRANT, CALL-1).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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