**1. Introduction**

Leather is a durable and flexible material made by tanning animal rawhide and skin, most commonly cattle hide [1]. It has unique characteristics, including high tensile strength, elasticity, tear resistance, high porosity, and air/water permeability [2,3]. In addition, collagen is the main component of natural leather and amino acids are the main units of collagen. It contains functional groups such as –NH4 +, –COO<sup>−</sup> as well as OH groups [4]. Collagen of leather can provide ideal conditions such as moisture, temperature, oxygen, and nutrients required for the rapid growth of bacteria and fungi [5].

Leathers have undergone many developments concerning their color and acquired functionalization such as antibacterial, self-cleaning, and UV protection [6,7]. In addition,

**Citation:** Abou Elmaaty, T.; Sayed-Ahmed, K.; Mohamed Ali, R.; El-Khodary, K.; Abdeldayem, S.A. Simultaneous Sonochemical Coloration and Antibacterial Functionalization of Leather with Selenium Nanoparticles (SeNPs). *Polymers* **2022**, *14*, 74. https:// doi.org/10.3390/polym14010074

Academic Editor: Sándor Kéki

Received: 10 November 2021 Accepted: 6 December 2021 Published: 25 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

diverse dyes and dyeing methods have been utilized for sustainable development in the leather industry [8]. The most important of these developments is the use of nanotechnology, which has an eco-friendly effect on the environment [9,10].

Nowadays, coloration and functionalization of leather surfaces with organic nanoparticles have received considerable attention [7,11–16]. The multifunctional features of leather, such as antimicrobial and UV radiation protection, as well as self-cleaning capabilities, have improved leather's adaptability [1,6,9,17–19].

Recently, many studies have reported the use of AgNPs to functionalize leather surfaces due to their antibacterial properties [5,20–23]. In addition, silicon nanoparticles were used with polyurethanes and polyacrylates to produce leather with high water vapor permeability, good mechanical properties as well as good thermal properties [24,25]. Additionally, Ag-TiO2 nanoparticles were deposited on the leather surface to obtain antimicrobial properties and improve photocatalytic activity [26,27]. As well, TiO2-SiO2 nanocomposite was used to enhance the performance such as colorfastness and adhesion [28].

From what was mentioned previously, we found that the multifunctional properties of leathers improved by different kinds of nanoparticles, which were investigated to evaluate their antimicrobial, mechanical, and thermal properties, as well as UV protection and photocatalytic activity. However, the use of NPs as colorants and finishing agents in leather processing has not been sufficiently investigated. Moreover, leather finishing with NPs requires multiple steps, a long reaction time, and the use of more hazardous chemical agents. As a result, there is a great demand for a simple and environmentally safe process for the incorporation of nanoparticles into leather surfaces without the use of hazardous chemical ingredients. One of the most effective nanoparticles in this context is selenium nanoparticles (SeNPs) which have excellent coloring capability [29–32], antimicrobial, antitumor, antioxidant, and antibiofilm properties [33–37]. Moreover, SeNPs exhibit low cytotoxicity [29,38].

In this work we have developed a simple technique to obtain simultaneous coloration and functionalization of leather via the incorporation of SeNPs into the leather under study utilizing an ultrasonic technique. The SeNPs was synthesized through a green process and used as a colorant without any harmful additives, achieving outstanding color fastness and antibacterial properties.

To the best of our knowledge, the utilization of SeNPs for coloration and functionalization of leather has not been reported elsewhere.
