Electrospun synthesis of silver/poly (vinyl alcohol) nano-bers: investigation antibacterial activity and ame retardant property

Polyvinyl alcohol (PVA) and Ag nanoparticles-embedded polyvinyl alcohol (Ag/PVA) nano-bers have been synthesized successfully via the electro-spinning technique at room temperature. XRD patterns conrmed the presence of Ag nanoparticles in the electro-spinning PVA nano-bers. FE-SEM images displayed that silver nanoparticles with an average particle size of 32 nm are uniformly dispersed in PVA nano-bers. Also, the average diameter of Ag/PVA nano-bers was estimated to be 142 nm. The EDX analysis of Ag/PVA nano-ber determined the aggregation of Ag nanoparticles in the range of polymer nano-bers. The antibacterial property of Ag/PVA nano-ber for inhibition of Escherichia coli (E-coli) growth was approved. Since polyvinyl alcohol is a biocompatible and water-soluble polymer, it can be considered as unique candidate for the preparation of wound dressing nano-bers. It is also found that the nontoxic nanostructures can appropriately enhance both ame retardant and thermal stability properties of the Ag/PVA matrix. These results show that the Ag/PVA nano-bers may nd practical applications as self-reinforced medical devices and tissue-engineering scaffolds.


Introduction
The nanostructured materials with their unique properties can be modi ed and controlled in optical properties and surface functionalization [1]. They have been applied in various elds of application like energy, health, medicine, biotechnology, electronics, environmental food, and agriculture. Transition metal nanoparticles, such as zinc (Zn), iron (Fe), gold (Au), and silver (Ag), have great opportunities in biomedical applications because of the low volume ratio to surface area [2][3]. Among such transition metal elements, Ag is the most attractive because of its low human toxicity, wide spectrum, and strong activity. Recently, the inducement of metal nanoparticles on certain surfaces to form macroscopic 3D structures has attracted a great deal of attention, which can be used in catalysis, cancer treatment, optical sensing, and electronic devices [4]. The electrospun method is one of the simple and effective techniques that under a strong electrostatic eld can extract continuous nano-bers from melts or polymer solutions [5]. This technique has received special attention for healthcare textiles and medicine due to its effectiveness in producing brous structures or nano-bers. By adjusting the manufacturing parameters and polymer solution types, the bers can be produced with various diameters from few nanometres to several micrometres. Each type of nano-ber that prepared from various polymers, like synthetic and natural polymers, has several attractive characteristics like the exibility of the surface, functionality, high porosity, and speci c surface area [6]. The produced nano-bers can be applied as photoelectric devices, components of biosensors, wound dressings, tissue scaffolds, lter materials, biomedical elements, vascular catalysts, and reinforced composite materials [7]. Polyvinyl alcohol (PVA) is a synthetic and nontoxic polymer that is known as a water-soluble polymer. Due to its excellent biocompatibility, chemical resistance, biodegradability, and good ber-forming property, it is widely used in practical applications [8]. Ultra ne Polyvinyl alcohol bers with different potential applications can be prepared by electrospun process. The electrospun of hybrid materials consisting of polymers and metal nanoparticles within the nano brous materials has become one of the most promising and growing technologies [9].
Recently, the utilization of functional nanoparticles in the PVA nano-ber has been widely applied because of their uniquely promising properties. The Ag/PVA nano-bers can show much anti-microbial e ciency than conventional Ag/PVA micro-bers due to their low volume ratio to surface area. Many research papers have investigated the antibacterial properties of Ag nanoparticles [10][11][12]. For medical applications, PVA nano-ber with metal nanostructure is more active than the large-scale modi cation of material [13]. This research focused on the characterization and antibacterial study of PVA/Ag nanobers produced by the electro-spinning technique that is a promising candidate for applications as wound dressings.

Materials and Characterization
All reagents, including PVA, Ag (NO 3 ) 2 , NaBH 4 , and starch were the analytical grade and used without further puri cation. The phase formation of the prepared nano-bers was identi ed using X-ray diffraction (XRD, λ=1.5418Å). Field-emission scanning electron microscopy (FE-SEM) was used to study the morphology of samples. FTIR spectra were taken on an AVATAR 360 Fourier transform infrared spectrometer. The thermal stability of the prepared nanocomposite was studied by thermogravimetric analysis(TGA).
2.2 preparation of Ag nanoparticles 0.10 g of AgNO 3 in distilled water (100 ml) was dissolved. Then, 0.5 g starch as a surfactant was added to the previous solution. About 2 ml of NaBH 4 solution (10 g/lit) was slowly mixed to the above solution, and the solution was stirred for 15 min. After that, the prepared precipitation was collected by centrifugation, washed, and dried at room temperature.

Synthesis of PVA nano-bers
Various concentrations (5-15%) of polyvinyl alcohol and distilled water were prepared as the starting solutions. The solution was placed on a magnetic stirrer and set to 50 °for 5 hours. The solution was well dissolved and ready to spin.
2.4. Synthesis of (90%:10%) Ag/PVA nano-bers Using the ratio of 10% silver nanoparticles synthesized to the polymer solution, add 0.1 g of silver nanoparticles powder to 0.9 g of polyvinyl alcohol solution and place at 50 ° C on the magnetic stirrer for 3 h to obtain a uniformly concentrated solution. Then put in an ultrasonic bath for 20 minutes and then ready to spin.
Results And Discussion Fig.1 displays a schematic diagram of the experimental setup (precipitation procedure) that was used for nanoparticle preparation. Fig.2 shows the schematic of the electro-spinning process for producing nanobers with a high voltage around 22kV, distance between needle and collector 150 mm, and ow rate about 0.5 mlh -1 . To determine the phase structure of the prepared samples, the X-ray diffraction analysis was performed at room temperature, over the 2θ angular range 10-80•. Fig.3 shows the XRD pattern of the electrospun PVA nano-bers. There are two broad hump peaks at 2θ= 20 and 51 indexed as (101) and (200) planes attributed to the semi-crystallinity nature of the electrospun PVA nano-bers. These peaks may be related to the intermolecular hydrogen bonding between PVA molecular chains [14]. The XRD pattern of Ag/PVA nano-bers, is shown in Fig.4. There are extra diffraction peaks at 2θ= 38.1° and 77.4°t hat can be assigned to the (1 1 1) and (3 1 1) planes of FCC structure of Ag element (JCPDS; # 04-0783), respectively. It is seen that for Ag/PVA bers the peak intensity at 2θ = 20 decreased because of the complexation of Ag nanoparticles in PVA nano-bers. noodles shows the formation of an inhomogeneous structure during the electrospun process. It is seen that there are no noticeable nano-bers in this sample. The SEM images of PVA nano-bers that were prepared of 10% concentration are shown in Fig.6. It can be seen the uniform and suitable mono-disperse nano-bers with average diameter size less than 336 nm successfully are prepared. The SEM images of the silver nanoparticles, prepared using starch as a capping and surface-active agent, is illustrated in Fig.7. This gure clearly shows the Ag nanoparticles with spherical shape, and the average particles size of about 32 nm. Fig. 8 displays the SEM images of Ag/PVA composite nano-bers. These images con rm that the nano-bers with an average diameter size of 142 nm are synthesized. The presence of silver nanoparticles in the polymeric ber matrixes is con rmed. Also, for better identi cation, the energy dispersive X-ray (EDX) analysis of PVA-Ag nano-bers was taken, and the result is shown in Fig.9. The EDX spectrum approved the presence of all three elements silver, carbon, and oxygen with K α of carbon and oxygen and also L α and L β of silver, respectively.  Gentamicin disc and control disk were inserted beside synthesized Ag/PVA disc. As we expected, there is no inhibitor zone around the control disc while the diameter around Gentamicin disc treatment is about 4 cm. The diameter of the growth inhibitor zone in the treatment of the desired material is around 2 cm.
Thermal gravimetric analysis (TGA) of the Ag/PVA nanocomposite is depicted in Fig. 13. It is found that with the addition of Ag nanoparticles to the polyvinyl alcohol, the thermal stability of Ag/PVA nano-bers was improved. As the nanostructured materials can act as dams, which slow the production of thermal transport and evaporation during polymer decomposition, the existence of nanoparticles can shift the thermal decomposition towards higher temperatures. Silver effect on the re retardancy of the cellulose acetate and has been examined applying UL-94 test (sample 130 × 13 × 1.6 mm is applied). A re (1.5 cm) is used to sample (time: 10 s) twice. When the specimen is extinguished in less than 10 seconds after re contact classi ed as V-0, particle drips are allowed as long as they are not in amed. A V-1 type is for a sample with re time less than 30 s (drips are like V-0). V-2 classi cation has re time like V-1 while aming drips are permitted. When the total aming time is above 50 s it is not classi ed (NC); nally, horizontal burning with a rate less than 76 mm/min is HB [17].
UL-94 tests for pure PVA is V-2 while Ag/PVA approves V-0 classi cation (Fig 14). Flame retardancy of nanocomposite is because of high surface to volume ratio, which can disperse into the matrix homogeneously, and formation of a char dam during the combustion. This dam decreases the evaporation of organic segments and reduces oxygen and ame, reaching the product.

Conclusions
Ag nanoparticles were prepared by an easily applicable chemical reaction at room temperature. Polyvinyl alcohol nano-bers and their nanocomposites with Ag were prepared by the electro-spinning method. The FE-SEM images showed that silver nanoparticles are uniformly dispersed in PVA nano-bers. The average diameter of Ag/PVA nano-bers was about 142 nm. Silver antibacterial property for inhibition of E-Coli growth was investigated. Also, thermal stability and ame retardant property of the Ag/PVA matrix were studied. Results showed that the Ag/PVA nano-bers may nd practical applications as self-reinforced medical devices, tissue-engineering scaffolds and ame retardant material.

Declarations
We declare that we have no nancial and personal relationships with other people or organizations that could have appeared to in uence the work reported in this paper.