Metal Nanoparticle–Polymer Composite Nanofibers

Since ancient times various metals, such as silver, mercury, copper, gold, titanium, zinc, cobalt, iron, magnesium, tin, molybdenum, lead, and chromium, have been used as antimicrobial agents. Each metal has its own special properties, potencies, and spectra of activity.

Among the different metals, since ancient times, Ag has been known to be an effective reagent for killing a broad range of microorganisms. Ag nanoparticles are the most popular inorganic nanoparticles widely used and recognized as broad-spectrum biocidal agents effective against bacteria, fungi, and viruses but nontoxic to human cells [44]. Ag nanoparticles possess high electrical conductivity, the highest antibacterial activity, and biocompatibility and release harmful toxic disinfection byproducts infrequently [45, 46]. The antimicrobial property of Ag additives is beneficial in various injection molded plastic products, textiles, and coating- based [47]. Ag nanoparticles also possess a range of biomedical applications [48]. It has been revealed that Ag nanoparticles show a high antimicrobial activity comparable to their ionic form [49]. The kind of materials used for preparing the nanoparticles as well as the particle size are two important parameters that affect the resultant antimicrobial effectiveness [50, 51]. Generally, nanoparticles have different properties compared to the same material with larger particles, which is because the surface-volume ratio of nanoparticles increases considerably with a decrease in the particle size [52]. Indeed, in nanometer dimensions, the fraction of the surface molecule noticeably increases, which in turn improves some properties of the particles, for example, heat treatment, mass transfer, dissolution rate, and catalytic activity [53, 54]. The bactericidal activity of nanoparticles depends on the size, stability, and concentration in the growth medium. While growing in a culture media amended with nanoparticles, the bacterial population growth can be inhibited by specific nanoparticle interactions [55]. It is demonstrated that Ag nanoparticles are potential antimicrobial agents against drug- resistant bacteria [56]. The antibacterial action of Ag nanoparticles results from damage of the bacterial outer membrane [57]. Some researchers assume that Ag nanoparticles can induce pits and gaps in the bacterial membrane and then fragment the cell [58, 59]. It is also assumed that Ag ions interact with disulfide or sulfhydryl groups of enzymes, which leads to the disruption of metabolic processes, causing cell death [47]. The fundamental mechanism by which silver kills bacteria is by disrupting their metabolic processes [60].

In general, bacterial cell size is in the micrometer range, while the outer cellular membranes have pores in the nanometer range. Since nanoparticles can be smaller in size than bacterial pores, they have the unique ability to cross the cell membrane. Due to a high surface- to-volume ratio, Ag nanoparticles tend to agglomerate with each other, which leads to a decrease in the effective surface area, and use in the powder form may cause their corrosion and decreased microbial efficiency. To overcome the problems of nanoparticles, an effective technique is to encapsulate them in polymeric nanofibers to construct an inorganic composite in which the polymer component not only serves as a supporter but also restrains the agglomeration, controls the size and distribution of nanoparticles, and protects them from corrosion.

Electrospun polymer nanofibers loaded with silver nanoparticles are prepared using different methods, such as the silver mirror reaction (SMR) method; the silver nanoparticles are uniformly dispersed on the surface of nanofibers [61]. The in-situ inclusion of silver nanoparticles is carried out by electrospinning a mixed solution of AgN03 and polymer, followed by the chemical reduction method to reduce Ag+ to Ag° [62-65]. In the ion exchange method, silver nanoparticles are in the form of Ag2C03, which responds to visible light due to the narrow bandgap [2.27 eV), resulting in enhanced bactericidal activity. However, due to selfphotocorrosion, Ag2C03 is not stable. So nanoparticle-encapsulated polymer composite nanofibers need to be created [66]. Atmospheric plasma treatment and high temperature treatment are also used to prepare electrospun nanofibers loaded with silver nanoparticles [67, 68].

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