Developments in Antimicrobial Composite Nanofibers for Bacterial Filtration
Fast industrialization in various parts of the world and continuous population growth have led to extensive environmental pollution. Water pollution is caused by the release of chemicals and biological contaminants from industrial outlets into waterbodies, which leads to a shortage of clean, drinkable water. To mitigate the problem of contaminated water, various efforts are going on worldwide. Conventional water filtration media are not sufficient to remove chemical and biological pollutants. Similarly, air quality is falling day by day due to dust and smoke pollution from construction, vehicles, industries, and bacterial contamination from hospitals and laboratories. Microbial contamination in air can affect human health as due to the smaller size, it can be directly inhaled by the human body, resulting in airborne diseases. In this chapter, several
Nanotechnology in Textiles: Advances and Developments in Polymer Nanocomposites Edited by Mangala Joshi
Copyright © 2020 Jenny Stanford Publishing Pte. Ltd.
ISBN 978-981-4800-81-5 (Hardcover), 978-1-003-05581-5 (eBook) www.jennystanford.com types of metal particles (embedded or decorated) as well as mono- and bimetallic nanoparticles-polymer composite nanofibers for the efficient removal of organic dyes, and antibacterial membrane for air and water filtration are reported.
In the 20th century fast industrialization in various parts of the world and continuous population growth led to extensive environmental pollution. Water pollution is caused by the release of chemicals and biological contaminants in water bodies, which leads to a shortage of drinkable clean water. It is estimated that more than 50% of the countries will face a water crisis by 2025 . The contamination of water is due to different types of pollutants, such as organic dyes, bacterial pathogens, heavy metal ions, pesticides, and pharmaceutical waste. Among these, synthetic dyes (such as Congo red, methylene blue, and methylene orange) and bacterial pathogens (Escherichia coli, Staphylococcus aureus, etc.) are some of the most common and highly hazardous pollutants . Therefore, a large population currently suffers from disease and death caused by waterborne pathogens, particularly in developing countries. The disease and death caused by waterborne pathogens in developing countries are receiving increasing attention as a primary public health concern . According to a World Health Organization report, more than 700 million people, particularly in developing and rural areas, lack adequate safe drinking water, which results in 2 million deaths from diarrhea annually .
Bacterial air contamination also affects human health as microbes easily pass through the respiratory tract and cause lung infection. Such contamination occurs more in a hospital environment. Also, an increase in the use of biological weapons in wars by various countries has made the environment dangerous to health. For the betterment of human life, these pollutants have to be eliminated through bacterial filtration of both air and water. To resolve this issue, various efforts are going on to develop energy- efficient air and water treatment technology. It has become the need of the hour to control various waterborne diseases because of the limited clean water available for a growing population.
Conventionally, various materials (such as activated carbon) and various methods (sediment deposition, adsorption, and many more) are utilized to remove chemical and biological pollutants. Although some of them are efficient and convenient, they have some drawbacks.
Metal ions and metal compounds have been extensively investigated in various applications, like antimicrobial filters, wound dressing materials, water disinfection, sensors, chemical and gas filtration, protective clothing, and air filtration. Metal oxides as antimicrobial agents have the advantage of improved safety and stability as compared to organic antimicrobial agents . To develop new and innovative materials, various nanomaterials are explored for such applications due to their unique properties, such as small dimensions and large surface-to-volume ratios compared to their bulk counterparts . For this reason, nanotechnology has attracted a great deal of attention from the scientific community. Metallic nanoparticles are considered the most promising as they contain remarkable antibacterial properties due to their large surface-to- volume ratio, which is of utmost interest to researchers due to the growing microbial resistance against metal ions and antibiotics and the development of resistant strains . Since most of the biological processes take place at the nanoscale level, a combined application of nanotechnology and biology can effectively solve biomedical problems. Therefore, metal or metal oxide nanoparticles are effective as antimicrobial agents. The toxicity of different metal particles to microorganisms is in the following order: Ag > Hg > Cu > Cd > Cr > Pb > Co > Au > Zn > Fe > Mn > Mo > Sn . Silver particles have the most efficient antimicrobial activity, whereas tin particles have the least antimicrobial activity.
On the other hand due to the high surface-to-volume ratio, nanoparticles tend to agglomerate, which leads to a decrease in the effective surface area, and further use in the powder form may cause their corrosion and a decrease in their antimicrobial efficiency. Also, separation and recycling of utilized nanoparticles is a very difficult task. Therefore, additional steps are required to separate and recycle the utilized nanoparticles, leading to extra operational cost. To overcome these problems and retain or even promote the catalytic stability of nanoparticles, an effective technique is to encapsulate them in polymeric nanofibers to construct an organic- inorganic composite where the polymer component not only serves as a support but also reduces the agglomeration, controls the size and distribution of nanoparticles, and protects them from corrosion .
Nanofibers and Composite Nanofibers
Polymeric or ceramic nanofibrous materials have attracted a huge amount of interest during the last few decades, mainly due to their high surface area, high porosity, small pore size, and compatibility with functionalizing additives. Thus, they are promising for various applications, including filtration, membranes, medical applications, sensors, catalysts, Li-ion batteries, and enzyme carriers . There are different techniques for the synthesis of polymeric nanofibers, such as template assisted , phase separation , solvent evaporation , self-assembly , doctor blading , drawing processing , and spinning . Spinning (from the word "spun”) is the process of making fibers from a given polymer. Such polymeric fiber processing depends upon thermal stability, conductivity, solubility of the polymer in the solvent, etc. Depending on the polymer properties and types of fibers needed, spinning techniques are classified into wet spinning, dry spinning, melt spinning, and electrospinning. Apart from laboratory scale spinning techniques, there are various production methods several companies use to produce nanofibers on an industrial scale. The most commonly used methods are melt blowing, centrifugal spinning, island-in-the-sea splitting, and needleless electrospinning [17-19]. Among the different spinning techniques, electrospinning is a useful one-step and straightforward process for the fabrication of nanofibers.
It is more advantageous than other techniques to draw the polymer nanofibers, because of the relatively low startup cost, ease of fiber deposition onto different substrates, a high surface- to-volume ratio nanofiber fabrication possibility, possibility to spin a wide variety of polymer nanofibers, ease of nanofiber functionalization, and mass production capability. Electrospinning was first discussed in a patent by Cooley and coworkers in 1902 as an apparatus to synthesize polymeric fibers with the application of voltage to a polymer solution. The basis for their research comes from the earlier studies on the effect of electrostatic force on water droplet in the 17th century and excitation of dielectric liquids under that field in the 18th centuiy. But the fundamental idea was studied in a series of patents by Anton Formhals from 1934 to 1944. He described the experimental setup for the fabrication of fibers using an electrostatic field. Figure 12.1 shows the schematic of general electrospinning equipment, and it consists of a DC high-voltage source, a syringe pump, a cylindrical metallic collector, a metallic or plastic syringe with a steel tip for loading the polymeric solution, etc. There are two types of parameters affecting  nanofiber formation via electrospinning, (i) intrinsic parameters (molecular weight, concentration, surface tension, viscosity, and conductivity) and (ii) processing or control parameters (applied voltage, flow rate, tip to collector distance, temperature, and humidity).
Polyacrylonitrile (PAN), an important polymer material, has been used to produce a variety of synthetic fibers used in a number of engineering applications. PAN-based nanofibers produced by electrospinning have attracted wide attention due to thermal stability, mechanical properties, chemical resistance, and abundant functional cyanogroups (-CN) on their macromolecular chains. The web or nonwoven nanofiber sheet is (Figs. 12.2a and 12.2b, SEM images) used in filtration application as a selective layer for removing smaller particles. Compared to the microfiltration (MF), ultrafiltration (UF), and reverse osmosis (RO) membranes, which have porosities from 5% to 35%, the nanofiber membranes could easily be made into MF and UF membranes with porosities in the range of 80% to 90% . In addition, the interconnectivity of the pores in the nanofiber membrane eliminates the inefficiency of the dead-end pores that are commonly formed in conventional membranes. These characteristics significantly increase flux and reduce transmembrane pressure drop.
Generally, water filters of nanofibrous membranes operate in dark and damp conditions and, therefore, are constantly subject to
Figure 12.1 Schematic of general electrospinning equipment.
attacks from environmental microorganisms. The microorganisms (such as bacteria) are readily captured by the filters and grow rapidly due to the conducive environment, resulting in the formation of biofilms. Consequently, the buildup of microorganisms on the filter surfaces deteriorate the quality of purified water. Additionally, it has unfavorable effects on the flow of water. Moreover, contaminated filters with biofilms are difficult to clean; usually, high pressure is required during the operation. This, in turn,
Figure 12.2 PAN nanofibers (a) membrane sheet and (b) SEM of the nanofiber.
increases the cost. To overcome this problem, an antimicrobial layer needs to be added to the surface of the nanofibrous membrane. Metal or metal oxide nanoparticles are thought to kill bacteria by disrupting their metabolic processes. But it is difficult to disperse nanoparticles uniformly over the nanofiber membrane. Therefore, polymer/inorganic composite nanofiber materials containing metal nanoparticles have attracted a great deal of attention because of their unique optical, electrical, and catalytic properties. The properties of these nanocomposites are strongly dependent on the size, content, dispersity, and structure of the metal nanoparticles that are incorporated within the polymer matrix. The size- and shape-dependent properties of nanoparticles provide a challenge to synthetic chemists when obtaining highly functional advanced nanomaterials is important. Since past several years, nanosized metallic particles impregnated with polymer matrix have been synthesized successfully using various synthetic techniques. The polymer can act as a stabilizer and prevent the agglomeration of nanoparticles. The different types of polymers, such as cellulose acetate, polyvinyl alcohol (PVA), PAN, polyvinylpyrrolidione (PVP), polyvinyl chloride, and polyurethane (PU), have been generally used alone or in combination as stabilizers as well as matrices for in situ formation of polymer-metal nanoparticle composite nanofibers.