Antibacterial silver ions and nanoparticles in electrospun fibers

Silver nanoparticles is well-known for its antibacterial property and electrospun nanofiber membrane containing silver nanoparticles have been shown to be effective in inhibiting bacteria growth [Yuan et al 2010]. This has attracted the application of silver nanoparticles loaded electrospun fibers in many areas such as food packaging, filters, wound dressing and implants.

Overview of electrospinning process of AgNPs doped PVA nanofiber mats under different prepared conditions. Note: The orange arrow in the microscope image indicates the alignment direction of AgNPs along the as-prepared electrospun fibers in the PVA-Ag sample. [Lin et al 2014]

There are many strategies used to incorporate silver ions and nanoparticles into electrospun fibers. The most direct method would be through blending into the electrospinning solution [Yuan et al 2010]. An alternative method is to incorporate silver nitrate into the solution followed by photo-reduction using UV irradiation [Son et al 2006, Rujitanaroj et al 2010], thermal reduction at 80 °C [Xu et al 2006] or aging [Rujitanaroj et al 2008]. UV irradiation of silver nitrate incorporated nanofibers showed that the silver nanoparticles were distributed close to the surface of the nanofibers [Son et al 2006, Rujitanaroj et al 2010]. However, thermal reduction of silver nitrate showed uniform distribution of silver nanoparticles throughout the cross-section of the nanofibers [Xu et al 2006]. Lok et al [2007] showed that for silver nanoparticles to be effective in inhibiting bacterial growth, it must first be converted into its ionic form. A study of the release of Ag+ ions in nanoparticles uniformly distributed in nanofibers showed an initial burst release followed by sustained release [Xu et al 2006]. For UV reduced silver nanoparticles on the surface of the nanofibers, an initial burst release was also observed. Compared with unconverted Ag+ ions in the nanofiber, the initial release was greater in the UV reduced silver nanoparticles [Rujitanaroj et al 2010]. This could be due to migration of Ag+ form the core to the surface during the formation of the silver nanoparticles. Using poly(ether amide) as the carrier electrospun fiber material, Liang et al (2014) was able to incorporate only 0.15% AgNO₃ into the material to achieve inhibition rate of >99.9% against E. coli and S. aureus after UV reduction of AgNO₃ in Ag nanoparticles. With 0.05% AgNO₃, the inhibition rate was 88% and 76% for E. coli and S. aureus respectively. Similarly, Nuge et al (2017) showed that Ag nanoparticles loaded electrospun gelatin fibers were less effective against gram positive S. aureus compared to gram negative Escherichia coli. The difference in efficacy has been attributed to the presence of a thick peptidoglycan layer below the cell membrane of gram positive bacteria that protects it from external stresses and possibly reduced the penetration of silver nanoparticle. Thermal decomposition study of the composite material showed that with increasing Ag content, the decomposition temperature was reduced. Therefore, lower amount of AgNO₃ also helps to maintain the stability of the material. However, given the low concentration of Ag nanoparticles, the effectiveness of the antibacterial property of membrane over time needs to be determined. It is possible that leaching of Ag nanoparticles may render the membrane ineffective after the initial release. In terms of antibacterial and cytotoxicity, a study by Lin et al (2014) suggests that silver nanoparticles (AgNPs) formed by UV irradiation is preferred over heat treatment and AgNO₃ with electrospun poly(vinyl alcohol) (PVA) as carrier. Reduction of AgNO₃ in electrospun PVA to AgNPs using heat showed greater diameter dispersion. However, in UV irradiation, the AgNPs diameter was much smaller with narrower distribution. Smaller AgNPs are known to exhibit stronger antibacterial characteristic due to larger surface area for Ag ion release. All the PVA-AgNPs fibrous mat was shown to be effective in inhibiting S. aureus and E. coli. As expected, PVA-Ag@UV electrospun fibers showed the greatest biocidal effect against S. aureus, as no bacterial colonies was detect after only 30 minutes of incubation while small colonies were still found in the PVA-Ag@heat and PVA-Ag samples. Interestingly, the mat showed almost no cytotoxicity with PVA-AgNPs towards PA317 cell line. However, with PVA-AgNO₃, there is a AgNO₃ concentration dependent cytotoxicity.

In most studies, Ag nanoparticles (AgNPs) are reported to be well dispersed in electrospun fibers. However, in some cases, the nanoparticles may require additional assistance to ensure uniform dispersion within the fiber matrix. Zhu et al (2022) used ultrasonic-assisted electrospinning for uniform dispersion of starch-capped Ag nanoparticles in polyvinylpyrrolidone (PVP) fibers. In this setup, the mixture of AgNPs and PVP solution was passed through a tube that coiled round an ultrasonic generator before ending with a metal nozzle. As the solution dispersion flow passes the ultrasonic generator, the ultrasonic vibration breaks up any AgNPs aggregates. The solution dispersion then travels a short distance to the nozzle where a high voltage is applied for electrospinning. Comparing the dispersion of the nanoparticles between electrospinning with and without ultrasonic-assistance, it is apparent in 0.6 wt% concentration of AgNPs formed long strips of aggregated nanoparticles within the fibers without ultrasonic-assistance. With ultrasonic assistance, the AgNPs are evenly dispersed in the electrospun fiber. Hence this setup presents a relatively simple method of dispersing nanoparticles in the solution just before electrospinning.

Apparatus scheme of fabricating AgNPs-PVP nanofiber by ultrasonic-assisted electrospinning [Zhu et al 2022].

TEM images of 0.6 wt.% AgNPs loading on 10 wt.% PVP electrospun fibers without Ultrasonic-Assisted (left) and with Ultrasonic-Assisted (right) [Zhu et al 2022].

Electrospun fibers loaded with silver nanoparticles have been investigated for use in healthcare and other biomedical applications. Jin et al (2018) investigated the potential use electrospun chitosan (CS) membrane incorporated with Ag-CaP as bone guided regeneration membrane. The release of Ag ions will help to prevent infection at the surgical site. Bone marrow stromal cells were used to test biocompatibility of the membrane. Cell viability were compared on electrospun chitosan (CS), CaP/CS and Ag-CaP/CS membrane. CaP/CS membrane showed the highest cell proliferation and Ag-CaP/CS membrane showed lower cell proliferation indicating some inhibition of the cells. However, electrospun pure CS membrane showed the lowest cell proliferation although CS is known to be biocompatible. The use of Ag nanoparticles in implants require more studies as electrospun Ag nanoparticles loaded polyvinyl alcohol (PVA) showed hemolysis percentage under 9 while a percentage under 5 is more suitable for biomedical applications [Preethi et al 2019]. The same membrane also showed moderate cytotoxicity towards human lung cancer cell line A549 although it does not show cytotoxicity towards human normal keratinocytes cell line. The cytotoxicity towards cancer cells may be due to the use of PST001 isolated from T. Indica seed kernal for reduction of AgNO₃ to Ag nanoparticles. PST001 has been shown to be toxic towards cancer cells and its residual presence in the nanofibers may account for the moderate toxicity towards cancer cells [Preethi et al 2019].

Ag loaded electrospun membrane with its antibacterial property has been tested in food packaging material to delay spoilage or bacteria contamination. Chaudhary et al (2014) used an electrospun polyacrylonitrile-silver composite filter media to cover a nutrient media in room condition and passes ambient air through the filter media. When compared to the negative control which is without the protective filter media, the nutrient media protected by the nanofibrous filter remains free of bacteria growth after two months while the unprotected nutrient media show microorganism growth. Taking advantage of the good dispersion of Ag nanoparticles in electrospun fibers, Castro-Mayorga et al (2017) coated a commercial polyhydroxyalkanoate (PHA) substrate with electrospun PHA loaded with Ag nanoparticles to introduce antibacterial properties to the material. To fuse the electrospun fibers with the base substrate, hot compression was used. With compression under heat, the electrospun fibers completely fused with the base substrate such that no fibers were observed following the heat treatment. The nanoparticles were being transferred to the combined substrate and its distributions indicates where the fibers once were. The multilayered substrate retains good transparency and are active against Salmonella enterica (gram negative) but did not show any antibacterial effect against Listeria monocytogenes (gram positive).

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