Metal Oxide Nanoparticle–Polymer Composite Nanofibers

The emergence of antibiotic-resistant pathogens has become a serious health issue, and thus, numerous studies have been reported to improve the current antimicrobial therapies. It is known that over 70% of bacterial infections are resistant to one or more of the antibiotics that are generally used to eradicate the infections. The development of new and effective antimicrobial agents seems to be of paramount importance. The antimicrobial activity of metals has been known and applied for centuries. Different metal and metal oxide nanoparticles, like ZnO, ТЮ2, Fe203, CaO, MgO, and CuO, are used industrially for several applications, including cosmetics, paints, plastics, and textiles. These metal oxide nanoparticles also exhibit antimicrobial properties against pathogenic bacteria. To use metal oxide nanoparticles for water purification, it is necessary to choose a suitable and stable "carrier.”

ZnO is recognized as a safe material by the Food and Drug Administration [22]. Even though the mechanism of action is still only vaguely understood, ZnO is used as an antibacterial agent in food packaging [23], restorative dental materials [24], wound dressings [25], and tissue engineering applications [26]. To use ZnO nanoparticles as antibacterial materials, they need to be loaded into polymers to make membranes. The incorporation of nanoparticles in the polymer matrix and electrospinning into composite nanofibers is one of the ways to utilize metal oxide nanoparticles. A wide range of natural and synthetic polymers can be used as a matrix.

Augustine et al. [27] reported the effect of ZnO nanoparticles on polycaprolactone (PCL)/ZnO composite nanofiber membranes' antibacterial properties. PCL is biocompatible and biodegradable. It has immense potential as a biomaterial for various biomedical applications, including tissue engineering scaffolds, wound dressings, and hemostats [28-30]. Electrospinning of PCL has been reported in many studies over the past few years. The effect of ZnO concentration ranging from 0.1 to 6 wt% on antibacterial activity of PCL electrospun nanofibers has been studied by Augustine et al.

SEM of electrospun neat polycaprolactone membrane (a), fiber diameter distribution (b), and the pore space distribution (c). Reprinted from Ref. [27], Copyright (2011), with permission from Elsevier

Figure 12.3 SEM of electrospun neat polycaprolactone membrane (a), fiber diameter distribution (b), and the pore space distribution (c). Reprinted from Ref. [27], Copyright (2011), with permission from Elsevier.

[27], in which ZnO particles of ~60 nm are mixed with 15 wt% of PCL solution. The PCL-ZnO nanocomposite fiber membranes are almost uniform in fiber diameter and highly porous. It is reported that ZnO nanoparticles of various concentrations, from smaller 0.1-0.9 wt% to a higher range of 1-6 wt% significantly influence the fiber morphology, provided the other experimental parameters remained constant.

Figure 12.3 shows SEM micrographs of a neat PCL nanofiber membrane in which the individual fibers are smooth and the average nanofiber diameter is 2500 nm. After the incorporation of nanoparticles in PCL-ZnO nanocomposite fibers, their diameters vary significantly. Up to 1 wt% of ZnO, the nanofiber diameter decreases continuously and the smallest nanofibers, of average diameter 1340 nm, are obtained at 0.4 wt% ZnO concentration.

Figure 12.4 shows the SEM image of 0.5 wt% of ZnO nanoparticle-embedded PCL/ZnO nanofibers, in which fibers of a maximum diameter between 750 nm and 1500 nm can be observed in the membrane. After that, up to a particular limit, the fiber diameter seems to increase. It is well known that the overall tension in fibers depends on the self-repulsion of the excess charges on the jet. The addition of ZnO nanoparticles results in the accumulation

Scanning electron micrograph of an electrospun polycapro- lactone membrane wit

Figure 12.4 Scanning electron micrograph of an electrospun polycapro- lactone membrane with 0.5 wt% of ZnO nanoparticles (a), fiber diameter distribution (b), and the pore space distribution (c). Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Journal of Polymer Research, Ref. [27], Copyright (2014).

of a higher charge density on the surface of the ejected jet during the process of electrospinning, and the overall electric charge carried by the electrospinning jet significantly increases [31]. As the charge carried by the jet increases, higher elongation forces that could overcome the self-repulsion are brought down to the jet under the electrical field. Thus, as the charge density increases, the diameter of the final fibers becomes substantially smaller and the diameter distribution of fibers becomes narrower [32]. At higher concentrations of the filler, the viscosity of the solution tends to increase, which leads to the apparent increase in the fiber diameter [16,33,34].

This reveals that after 0.5 wt% of ZnO concentration, the fiber diameter continuously increases and at 1, 2, 3, 5, and 6 wt% of ZnO, the average fiber diameters observed as 2680, 2630, 2300, 2608, and 2700 nm, respectively. As the ZnO nanoparticle content in the fibers increases, the surface of the fibers becomes rougher due to the agglomeration of ZnO nanoparticles [Fig. 12.5).

Further, Augustine et al. [27] investigated the interaction between ZnO nanoparticles and PCL in PCL-ZnO nanocomposite fibers

Scanning electron micrograph of an electrospun polycapro- lactone membrane with 4 wt% of ZnO nanoparticles

Figure 12.5 Scanning electron micrograph of an electrospun polycapro- lactone membrane with 4 wt% of ZnO nanoparticles (a), the fiber diameter distribution (b), and the pore space distribution (c). Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Natur e, Journal of Polymer Research, Ref. [27], Copyright (2014).

by Fourier-transform infrared (FTIR) investigation and reported that the decrease in the carbonyl stretching vibrations peak in PCL- ZnO nanofibers spectra reveals the interaction between carbonyl groups of PCL with ZnO nanoparticles. The decrease in the intensity of the C=0 group with an increase in the concentration of ZnO nanoparticles reveals the interaction of ZnO nanofillers with PCL polymer chains [27]. The interaction may be physical interaction like van der Waals force, where oxygen of the carbonyl group can bound physically with ZnO, which probably weakens the strength of the ester bond present in the PCL. The antibacterial activity of the PCL/ZnO fiber mats is assessed by observing their activity (using the disk diffusion method) against both gram-negative (F. coli) and gram-positive (5. aureus) bacteria. The activity of neat PCL membranes against these bacteria is used as a control. It is reported that the membrane has good antimicrobial activity against both F. coli and S. aureus. The neat PCL membranes and PCL/ZnO fiber mats with nanoparticle content less than 5wt% do not show any activity against the bacteria. The PCL membrane containing 5 wt% of ZnO nanoparticles demonstrated statistically considerable antibacterial activity with an inhibitory zone diameter of 8.76 ± 1.2 (P = 0.0278) and 9.98 ± 0.6 (P = 0.0082) against £. coli and S. aureus, respectively. The PCL membrane containing 6 wt% of ZnO nanoparticles confirms an increase in the inhibitory zone diameter of 9.81 ± 0.8 (P = 0.0095) and 10.22 ± 1.3 (P = 0.0067) against E. coli and S. aureus, respectively [27]. The antimicrobial activity is apparent only with the 5 and 6 wt% of ZnO nanoparticles. At lower concentrations of ZnO, the nanoparticles are trapped inside the polymer matrix and, thus, those in direct contact with bacterial cells are very few in numbers. The antibacterial activity of ZnO nanoparticles is present only if the nanoparticles are in direct contact with the bacterial cell wall. Further, in the disk diffusion method, the antibacterial agent should be able to diffuse into the agar medium to show antibacterial activity. At higher concentrations of ZnO nanoparticles, the interaction between the polymer matrix and the filler will be apparently low due to higher filler-filler interactions. Thus, there will be more freedom for the entrapped nanoparticles to diffuse into the agar medium and maintain a minimum concentration of ZnO to effectively inhibit bacterial growth. The antibacterial activity of the PCL/ZnO membranes is higher against S. aureus than against E. coli. Reddy et al. have reported similar results for ZnO nanoparticles [35]. Such an observation can be explained in terms of the difference in the cell wall structure of these bacteria. The outer cell membrane of gramnegative bacteria contains lipopolysaccharide in its outer leaflet and phospholipids in the inner leaflet. But gram-positive bacteria lack such a lipopolysaccharide layer. ATMACA et al. [37] proposed that the higher susceptibility of gram-positive bacteria against ZnO nanoparticles could be related to differences in cell wall structure, metabolism, cell physiology, or the degree of contact points. Many researchers have reported that the antibacterial activity of zinc oxide could be due to damage to the membrane of bacterial cells by hydrogen peroxide or the affinity between zinc oxide nanoparticles and bacterial surfaces [37].

Like ZnO, elemental copper and its compounds are also recognized as antimicrobial materials by the US Environmental Protection Agency [38]. Copper oxide, due to its unique biological, chemical, and physical properties; antimicrobial activities; as well as low cost of preparation, is of great interest to the research community [39]. Moreover, in order to make possible the use of CuO particles for bacterial filtration, it is necessaiy to choose a suitable and stable "carrier." One way to solve this problem is incorporation of particles into the polymer matrix. Ungur et al. [40] chose the PU polymer matrix for the incorporation of CuO due to its excellent elastomeric and mechanical properties, tensile strength, durability, and water insolubility [41-43]. Two types of CuO particle size effects on antibacterial filters for water purification are reported. PU solutions are modified by the incorporation of CuO microparticles (700 nm to 1 mm) and nanoparticles (50 nm) in order to compare the influence of the dimensional characteristics of the modifier on the properties of composite filters. CuO micro- and nanoparticles of concentrations 5, 7, 9.5, and 12 wt% are introduced directly into the pre-electrospinning PU solutions. In order to produce filtration materials at an industrial scale, nanofiber filters need to be produced by a commercially viable technique and implemental technology. The composite PU nanofiber membranes are produced by using the industrial nanospider technique [40]. The antimicrobial activity of pristine and composite nanofibers is evaluated against gram-negative E. coli and gram-positive Staphylococcus gallinarum bacterial strains according to the Cornell test (ASTM E2149). The antimicrobial efficiency for micro- and nanoparticle-modified PU fibers after 24 h of contact between the bacterial solutions and samples is depicted in Table 12.1. The antibacterial activity grew with an increase in CuO concentrations for particles of both sizes. There is no particular difference between the antibacterial properties of the samples with micro- and nanoparticles against the E. coli strain. This reveals that all of the produced composite layers with a content of CuO particles in the concentration range from 7% to 12% demonstrated excellent activity against the gramnegative strain. The samples with micro- and nanoparticles against S. gallinarum are slightly different.

Nanofibers with microparticles show higher activity against a gram-positive strain, but the negative distinction is evident only for the nanofibrous substrates with 5% of CuO nanoparticles (Table 12.1).

Table 12.1 Antibacterial efficiency against two bacterial strains (contact time between bacterial solutions and modified samples 24 h)

Efficiency (%) E. coli

Efficiency (%) S. gallinarum

Sample

цт; nm

ЦЛ1; nm

PU + 5% CuO

97; 96.8

98.8; 62.7

PU + 7% CuO

99.7; 99.8

100; 98.2

PU + 9.5% CuO

100; 100

100; 98.8

PU +12% CuO

100; 100

100; 99.6

Source: [40]

Figure 12.6 shows antibacterial test results with S. gallinarum for samples with 5% and 12% of micro- and nanoparticles of CuO (contact time 24 h).

The number of grown bacterial colonies for the inoculum (reference test without the sample) and for the nonmodified fibers is similar. It is reported that 5% of CuO nanoparticles are not sufficient to impart PU nanofibers with good antimicrobial properties against S. gallinarum whereas 12% of CuO micro- and nanoparticles demonstrated the same effect. This is because nanoparticles form large aggregates in the polymer solution and in the structure of the fibers. This leads to a loss of the unique properties caused by the nanoscale characteristics of the particles.

Ungur et al. [40] evaluated the practical application of nanofiber membranes under real conditions for bacterial air filtration. The bacterial filtration efficiency of pristine and modified PU nanofibers was tested using the antimicrobial filtration tester device (Fig. 12.7). The purpose of the test was to verify the extent to which the filter is able to prevent the penetration of aerosolized inoculum with bacteria into the purifying area. The measurement was done by simulating the passage of the aerosolized contaminated inoculum through the tested sample. The presence of bacteria that were injected into the testing apparatus and those passing through the filter media was analyzed. Petri dishes with agars were placed at the end of the apparatus to determine the number of bacteria in the device.

It is reported that the bacterial filtration test corresponds to the values of the surface density for all prepared samples. The surface

Images of agar plates show the results of antibacterial tests against S. gaUinarum (contact time 24 h). Reprinted by permission from Springer Nature Customer Service Centre GmbH

Figure 12.6 Images of agar plates show the results of antibacterial tests against S. gaUinarum (contact time 24 h). Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Journal of Polymer Research, Ref. [27], Copyright (2014).

density of the nanofibers modified by microparticles is found to be higher in comparison to layers containing nanoparticles [40].

These results lead to the assumption that it is sufficient to use nanofibers with a high surface density for bacterial filtration and that antibacterial modification of the nanolayers is not desired. Ungur et al. [40] reported that results of the "smear test” confirmed the antibacterial activity of all of the modified nanofibers in eliminating captured bacteria after the bacterial filtration test. The samples with 9.5% and 12% of microparticles demonstrated complete elimination of trapped bacteria. The micromodified nanofibrous

Scheme of the antimicrobial filtration tester device. Reproduced from Ref. [40], Copyright (2017), with permission of the Royal Society of Chemistry

Figure 12.7 Scheme of the antimicrobial filtration tester device. Reproduced from Ref. [40], Copyright (2017), with permission of the Royal Society of Chemistry.

layers are able to capture more bacterial units due to their higher surface density. Homogeneous distribution of microparticles, without the formation of large aggregates in the structure of nanofibers, enables efficient elimination of the captured bacteria. It reveals that microparticles of CuO are more efficient additives for the antibacterial modification of PU nanofiber filters from economical and technological points of view than nanoparticles. However, it may also be possible that CuO microparticles were present more on the surface of nanofibers (because they cannot go inside due to their large size) than nanoparticles and thus show more activity toward bacterial filtration.

 
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