Synthesis and Characterization of Antibacterial Viscose Nonwoven Fabric by the Cooperative Action of Se Nanoparticles and Amino Hyperbranched Polymer

This research provides a new method for preparing nanoparticle-coated cellulose fabrics, which has broad application prospects in the functional ber industry. In this work amino-terminated hyperbranched poly (HBP)-capped Selenium nanoparticles (Se NPs) were synthesized for coating viscose nonwoven fabric (VNF) via impregnation method to produce a controllable and uniform Se NPs coating on the viscose ber surface. The prepared Se NPs and the treated VNF were characterized by the transmission electron microscope (TEM), x-ray diraction (XRD), x-ray photoelectron spectroscopy (XPS), eld emission scanning electron microcopy (FE-SEM), and antibacterial measurement. The results indicate that the Se NPs were spherical shaped with an average size of 10 nm. FESEM, XRD, and XPS characterizations demonstrated that Se NPs can adsorbed and distributed uniformly on the ber surface. Se NPs-coated VNF showed above 99.9% bacterial reduction of Staphylococcus aureus and Escherichia coli while the Se element content on VNF was about 292 mg/kg.


Introduction
As a regenerated cellulose ber, viscose is made from a cellulose solution, such as cotton linters, Given the excellent hygroscopicity, softness, degradability, and good air permeability, VNF has an important role in medical care applications. However, because of its large surface area and ability to retain moisture in the fabric grid, VNF also provides an excellent environment for microbial reproduction, limiting its application. With the growing concern about public health, antimicrobial cellulose products Se is an essential trace element for human body, which has the characteristics of enhancing body immunity, high biological activity, and biocompatibility, and can highly acceptable by human cells Sadalage et al. 2020;Tang et al. 2020 However, very few research concerns on the synthesis of Se NPs on the bers surface were noticed. For example, Se NPs were also prone to aggregation into large clusters in aqueous solution, leading to a lower bioactivity and bioavailability. To solve this problem, the researchers found that polysaccharides and hyperbranched polymer can modify the interface of Se NPs, control the growth, and stabilize the solution of Se NPs. For another, Se NPs lack of binding force with the ber, during the nishing process, some binders, such as polyurethane resin polyacrylic acid and citric acid were required to x the Se NPS on the bers (Tang et al. 2020;Wadhwani et al. 2016). Therefore, the softness, air permeability, and hygroscopicity of the treated ber may affected by these chemicals.
In our previous study (Zhang et al. 2013), amino hyperbranched polymer (HBP) was successfully designed and applied to the control synthesis of metallic NPs. The prepared metallic NPs can completely be adsorbed on cellulose bers depending on the strong molecular interactions between the amino HBP and the hydroxyl on the cellulose ber surfaces. In this study, Se NPs coating was formed by the circular introduction of HBP to the VNF surface through a simple cyclic impregnation method. The physicalchemical properties of the VNF were evaluated by Fourier transform infrared spectroscopy (FTIR), Field scanning electron microscopy (FE-SEM), x-ray photoelectron spectroscopy (XPS), and x-ray diffraction (XRD). The antibacterial activity of the Se NPs-coated VNF against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were measured quantitatively.

Synthesis of Se NPs
Scorbic acid solution (Vc) with concentration of 1 mM was dropwise in the Na 2 SeO 3 and HBP solutions, where the mixture was stirred with a magnetic stirrer for three hours. The concentration of Na 2 SeO 3 in the reaction system was 0.05 mM and 0.1 mM, the molar ratio of Na 2 SeO 3 and Vc was maintained at 1:3, and the concentration of HBP is 2g/L. F; hence prepared by centrifugation at 8000 rpm for ten minutes to produce precipitation.

Preparation of Se NPs-Coated VNF
To prepare Se NPs aqueous solution, 0.05 mM, 0.1 mM, and 0.15 mM Na 2 SeO 3 were added into the 2 g/L HBP solution. Vc with concentration of 1 mM was added dropwise in the mixing solution with the molar ratio of Na 2 SeO 3 and Vc at 1:3. Afterward, 1 g VNF was dropped into the mixing solution with bath ratio 1:50 in a constant temperature water bath at 30 ℃, 50 ℃, and 90 ℃ for two hours. The Se NPs-coated VNF was thermally pretreated at 80 ℃ for ve minutes and then at 150 ℃ for three minutes.

Characterization of Se NPs and Se NPs-coated VNF
The morphology and lattice characteristics of Se NPs were characterized by transmission electron microscopy (TEM) (2010, JEOL, Tokyo, Japan). The crystalline phases of Se NPs and Se NPs-coated VNF were analyzed by XRD (Philips, Amsterdam, The Netherlands) via a CuKα x-ray source at a voltage and current of 40 kV and 30 mA, respectively. FTIR spectroscopy (NICOLET 6700), XRD (Philips, Amsterdam, Netherlands), and XPS were used to analyze the viscose nonwoven fabric before and after modi cation. The morphology and elements of viscose nonwovens were studied by FE-SEM (Scios DualBeam, Czechia) and energy dispersive spectrometer (EDS) (Carl Zeiss, EVO15, Oberkochen. The antibacterial test uses E.coli (AATCC 1555) and S. aureus (AATCC 547) as microbial models. According to GB/T20944.3-2008, the antibacterial activity of VNF loaded with Se NPs against E.coli and S. aureus was determined by the shaking method. The HBP was used as a stabilizer to solve the problem of easy agglomeration of Se NPs mainly because HBP contains many cavities inside, which is a typical molecular cage structure, and the reduced selenium atoms enter the molecular cage. As more selenium atoms are reduced in the reaction and when larger nanocrystals are formed, the growth of Se NPs particles are restricted by the molecular cage, thereby forming stable growth and nonagglomerated Se NPs. Given the protection of HBP, the prepared Se NPs has good stability and not aggregated for two weeks.

Results And Discussion
To further explore the morphologies, TEM was used to depict the shape and size of the Se NPs. The TEM images of the prepared Se NPs are shown in Fig. 1. The results showed that Se NPs were well-dispersed spherical particles in the solution ( Fig. 1 (a)), and the diameter of Se NPs was about 20 nm. The crystalline phase of the prepared Se NPs was characterized by the powder XRD method, as shown in Fig.   1  As Se NPs have large surface energy, dispersing and combining it with VNF is di cult. To provide the Se NPs-coated VNF and impart the VNF with antibacterial properties, the VNF was treated with the aqueous solution of HBP/Se NPs by the impregnation method at 60℃ and the concertation of Se element was 0.05 mM, 0.1 mM, and 0.15 mM. As shown in Fig. 3 (a), VNF with clean and smooth surface has an obvious change between the VNF and Se NPs treated VNF, where the Se NPs treated sample of Fig. 3 (bd) became rough, and parts of Se NPs can be found on the surface of treated VNF. When the concentration of Se NPs is 0.05 mM, the number of Se NPs on the ber surface is less. More Se NPs can be found on the surface and the nanoparticles were well-dispersed with increased concentration of Se NPs at 0.1 mM and 0.15 mM. The contents of Se NPs were further characterized by ICP method, as shown in Fig.S1. The amount of attached Se NPs on the surface of increased VNF with increasing Se concentration results in the adsorbing capacity of about 290 mg/g. However, when the concentration of Se is from 0.1mM to 0.15mM, the contents of Se on VNF showed little difference.
In the preparation process of Se NPs-coated VNF, the reaction temperature has also in uenced the loading of Se NPs. Fig. 4 showed the morphology of the treated VNF by 0.1 mM Se NPs at different treating temperature. An obvious change between the Se NPs treated VNF at different temperature was observed, as shown in Fig. 4 (a). Very few nanoparticles can be found at 30 ℃ treatment. With increasing the reaction temperature, more Se NPs can be found. Fig. 4 (b) shows that when VNF is treated by 50 ℃ HBP/Se aqueous solution, the Se NPs on the surface of VNF were mainly spherical structures. The morphology of the Se NPs on the VNF change at the reaction temperatures of 70 ℃ and 90 ℃ (Fig. 4 (c), (d)) because of the adsorption between the amino group and SeO 3 2− , which hinders the mutual contact and collision of the reactant molecules, thereby slowing down the reaction speed. When the temperature is low, the nucleation rate in the solution is greater than the growth rate, which is conducive to obtaining small selenium crystal grains. The viscosity of the solution decreased above 90°C, the ability of HBP to control the growth of Se NPS crystals was weakened, the temperature increase accelerated the growth rate of Se crystals, and the morphology of Se NPs is combination of spherical and rod-shaped. VNF and VNF treated by 0.1mM Se NPs at 60 ℃ were further characterized by the FTIR method, as shown in Fig.   S2. The peak of Se NPs-coated VNF at 1650.79 cm −1 belongs to the acetamide groups, and the characteristic absorption peaks of hydroxyl group peak of Se NPs-coated VNF at 3340cm −1 are larger than the VNF attribute to the amino group of HBP. The change of absorption peaks may indicate an interaction between hydroxyl groups of VNF and surface of Se NPs.
The elemental distributions on VNF were further investigated using the EDS spectra and elemental maps of the treated VNF, as shown in Fig. 5a and Fig.S3. The treated VNF contains C, O, and Se. The observed Se elements can con rm the syneresis of Se on the surface of VNF. The surface scanning images shown in Fig. 5(b-d) show that O and C elements of the cellulose ber as well as the even distribution of Se on the VNF surface are found, which con rmed the possibility of the existence of Se element on the VNF. The surface scanning images are in good agreement with the SEM measurement.
To con rm the Se NPs which were combined with VNF, the XRD patterns of the control and treated VNF were tested, as shown in Fig. 6 XPS analysis was performed to identify the chemical state of Se element in VNF. Fig. 7a shows that VNF bers displayed peaks of O1s and C1s. New Se3d peaks were observed after treated with Se NPs (Fig.  7a), indicating the coating of Se element on the VNF ber. Fig. 7 shows the Se3d spectra. The binding energies of two peaks were at 55.9 and 55.1 eV, corresponding to the Se3d and presents an unambiguous proof that the nanoparticles on the VNF are indeed Se0 (Se NPs).

Conclusion
Se NPs were successfully prepared in Na 2 SO 3 , Vc and HBP mixing solution. The average diameter of the Se NPs was about 20nm. The prepared Se NPs showed good stability and dispersion and not obviously aggregated in two weeks. Vc as a reducing agent and HBP as a stabilizing agent during the synthetic process. The VNF was treated by Se NPs through an impregnation method with different concertation and temperature. All results of FE-SEM, ICP-AES, FTIR, and XPS determinations con rmed that Se NPs have been xed and well dispersed on VNF surface through chemical and electrostatic interactions between the hydroxyl and amino groups. The Se NPs-coated VNF fabrics showed more than 99.9% antibacterial properties against both S. aureus and E. coli at the content of 290 mg/Kg. The mechanical property of VNF and Se NPs treated VNF have not changed very much.

Declarations Declaration of Competing Interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper. Figure 1 (a) TEM images (b) XRD of Se NPs.

Figure 2
Zeta potential of HBP/Se NPs and VNF with different pH value