Production of the antibacterial PVP nanofibers containing silver nanoparticles via electrospinning method

Nanotechnology and nanoscience provide an opportunity to synthesis, characterisation, manufacturing and utilization of materials which have nanostructures. Nanomaterials are referred as the materials which have at least one external dimension in the nanoscale or internal or surface structure in the nanoscale. Nanomaterials are generally classified according to their number of dimensions excluding nanoscale dimension. In this context, zero-dimensional (0-D) nanomaterials like nanoparticles have 3 external dimension in the nanoscale, one- dimensional (1-D) nanomaterials like nanofibers, nanotubes have 2 external dimension in the nanoscale, two-dimensional (2-D) nanomaterials like thin films, nanocoatings have 1 external dimension in the nanoscale. In the case of three-dimensional (3-D) materials like nanocomposites, they have no external dimension in the nanoscale but contain 0-D, 1-D or 2-D nanomaterials inside their matrix. Owing to increased surface area and quantum effect, nanomaterials differ from atomic-molecular or bulk materials in the same compositon with regards to physical and chemical properties. As the size of the material decreases, surface to volume ratio increases, and hence surface atom fraction is increases since then surface atoms have more mobility, entropy and internal energy than interior atoms, chemical reactivity increases, melting and evaporation temperatures decrease, etc. Also, quantum effect cause to change nanomaterials properties such as magnetic and optical after reaching to the critical size. Nanomaterial production methods are based on top-down and bottom-up techniques, where top-down approach builds nanomaterials from the bulk materials, on the contrary bottom-up appoach builds nanomaterials from atoms or molecules. Polymeric nanofibers have generally diameters smaller than 1 μm or 0,5 μm with novel properties due to large specific surface area, small pore size, high porosity, and improved mechanical structures. To produce these nanofibers variety of methods like dwaring, template synthesis, phase separation, self-assembly and electrospinning can be used. Among these methods, electrospinning gets attention having advantages of long and continuous nanofiber production ability, cost effectiveness, easy set-up, allowing diameter control and variety of materials usage. Electrospinning technique enable to easly and rapidly fabricate generally non-woven nanofibrous mat with diameters from micrometer to nanometer size. An electrospinning system consist of three main components; namely, power supply for high voltage generation, capillary tube with spinneret for flow of polymer solution or melt, and collector. In this techique, feeding of polymer solution to spinneret lead to pendant drop on the capillary tip. High voltage is applied to this droplet to obtain electrostatic forces which draws it as liquid jet to the collector. This liquid jet flows in the electrical field extends and dries through solvent evaporation, and finally solid nanofibers formed on the collector. Morphology and diameters of electrospun nanaofibers are effected from many parameters such as; solution parameters (concentration, molecular weight, viscosity, surface tension, etc.), process parameters (voltage, spinning distance and flow rate), and ambient parameters (humidity and temperature). By establishing of the optimum parameters nanofibers with desired properties can be obtained. Electrospun polymeric nanofibers find potentially usage in diverse fields such as medical, filtration, sensors, textile and various industrial applications. Among these application areas nanofibers are widely used in medical field. Silver ions and compounds have been used hygiene and threatment purposes from antiquity. While usage of silver ions and compound was decrease after discovery of antibiotics and other disinfectants, growing antimicrobial resistance to this materials over time, make researchers to search new antimicrobial agent named as nano-antimicrobials. Thus, metallic silver is gained striking attention again form as silver nanoparticles. Silver nanoparticles have broad-spectrum antimicrobial effect to many type of bacteria, fungi, and virus despite low toxicity, moreover, they exhibit biocidal effect via multiple mechanism, and release Ag+ ions slowly, this properties make them strong antimicrobial agent. Silver nanoparticles also have remarkable physico-chemical properties such as high thermal and electrical conductivity, chemical stability, catalytic activity. Silver nanoparticles are synthesized with many techniques being classified as physical, chemical and biological methods. Chemical methods are based on reduction of silver ions in the medium of water or organic/inorganic solvent system are easy, cost effective, and high yield and consist of three components: silver precursor, reducing agent and stabilizing agents. Chemical methods include chemical reduction, electrochemical, microemulsion and photochemical synthesis. Chemical reduction method is the most used method for the synthesis of nanoparticles with different size and shape. As an alternative to the classical heating method, microwave synthesis method is a pratical method which makes nanoparticle production in a short time and provide homogeneous heating. Since polymeric nanofibers are mostly used in medical areas, it is important to obtain nanofibrous mat with antimicrobial properties. For this purpose, antimicrobial nanofibers can be obtained with adding silver nanoparticles to the nanofiber matrix. The aim of this study is to fabricate antibacterial poyvinlypyrrolidone (PVP) nanofibers with silver nanoparticles (AgNP) via electrospinning method for biomedical application. PVP is known as biocompatible, hydrophilic, soluble in water and various solvent also, extensively used effective stabilizing agent for silver nanoparticle synthesis. AgNP were produced via chemical methods by solving AgNO3 salt in ethanol solution containing low molecular weight PVP (L-PVP) as stabilizing agent. At the beginning AgNO3 was added to the solution having PVP/ethanol ratio of 1% by considering PVP/AgNO3 weight ratio (R) as 10 and 50. To reduce the Ag+ ions, solution is placed into the microwave oven and threated at 180 and 600 W for 30 and 60 s. To investigate the effect of polymer concentration on the properties of AgNP produced different L-PVP/ethanol ratios (5%, 7,5% and 10%) are studied. For the solutions which have the L-PVP/ethanol ratios of 5%, 7,5% and 10%, colourless solutions were easily changed yellow to brown colours by adding of AgNO3 salt which indicates the AgNP formation in the solution. Because of this observation microwave treatment was not applied for these solutions. AgNP obtained in different solutions were characterised by colour change, DLS and UV analysis. From the colour changes observed for the solutions having L-PVP/ethanol ratios of 1%, it was concluded that microwave threatment at 180 W for 30 and 60 s was not sufficient to reduce the Ag+ ions. DLS analysis results showed that the change in the value of R ratio did not effect the nanoparticle size. However, increase in the L-PVP/ethanol ratio cause to decrease in the nanoparticle size to a certain point. After that point, further increase in L-PVP/ethanol ratio, did not change the nanoparticle size. The UV spectrum of the AgNP solution with L-PVP/ethanol ratio of 7,5%, contains an absorption peak at 414 nm which proofs AgNP formation clearly. In this study, electrospinning was applied to the AgNP solutions firstly but they could not be converted to nanofibers because of low molecular weight of the polymer. Because of this reason high molecular weight PVP (H-PVP) was used to obtain required solution viscosity for electrospinning process. Electrospinning solutions were prepared by mixing H-PVP/ethanol solutions with the same volume of L-PVP solutions and then electrospinning was applied to these mixed solutions. The thermal treatment at different temperatures was then applied to nanofibers in order to prevent solubility in water and other solvents. SEM images were used to investigate the morphology and size of the nanofibers. The thermal treatment efficiency was determined by observing the solubility of nanofibers in deionized water, XRD analysis of AgNP containing nanofibers was also conducted before and after thermal treatment. Finally, antimicrobial properties of nanofibers against gram-negative and gram-positive bacteria and a fungus were tested with zone of inhibition method. From SEM images, it was observed that increase in polymer concentration and H-PVP ratio in the electrospinning solution leaded to increase in nanofiber diameters. Moreover, smooth and long nanofibers were obtained from electrospinning solution which prepared by mixing L-PVP/ethanol and H-PVP/ethanol solutions having the same ratios. Existence of AgNP in the electrospinning solution resulted a decrease in the nanofiber diameter due to increasing conductivity of electrospininnig solution. Application of the thermal treatment to nanofibers at 150, 175 and 200°C showed that, 200°C provided the most effective crosslinking of the polymer. XRD patterns of the nanofibers gave (200), (220) and (222) crystal faces of AgNP. Antimicrobial test results showed that while PVP nanofibers without AgNP allowed microbial growing on their surface; PVP nanofibers with AgNP formed a small zone of inhibition against E.coli and S.aureus. However, no antifungal activity was observed.