Abstract
The main aim of the presented research was to obtain antibacterial fibers containing nanosilica with immobilized silver nanoparticles. The nanomodifier in an amount of 250 ppm, 500 ppm, 1,000 ppm, and 2,000 ppm were introduced into the cellulose fiber matrix during the cellulose dissolution process. In order to assess the influence of the nanomodifier's amount in the fiber on the antibacterial activity of modified fiber, a quantitative test of the antibacterial activity of the fibers was performed. The basic parameters of modified fibers, such as the mechanical and hygroscopic, were estimated. The size and shape of the nanomodifier in the selected fibers, as well as microanalysis of the polymer matrix, were examined. The investigations were conducted by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Energy Dispersive Spectrometry (EDS). The obtained results allowed the selection of optimal fibers with strong antibacterial properties that can be potentially used for personal protection or medical purposes.
1 Introduction
As fibers are the basic structural element for all types of textiles, their properties are key to obtain the final product with the desired characteristics. The growing demand for textile products in increasingly diverse industries, as well as the increasing demands from modern consumers, has meant that the area of special fibers is intensively developed. More and more high-performance textile products are appearing in the commercial market, which offer consumers new, unique properties, such as antibacterial, insect repellent, antistatic, or high thermal insulation or a particular “moisture management” system. Bioactive fibers are of great interest due to, among others, the need to prevent nosocomial infections in hospitals, which are now recognized as one of the most severe global epidemiological and sanitary-hygienic problems. Private consumers also create the observed development of new technologies for the production of bioactive products. In this case, bioactive fibers (antibacterial, anti-fungal, and deodorizing) are used in sportswear, underwear, towels, etc. [1]. All these create new perspectives for the development of biotextiles, currently recognized as one of the fastest growing sectors of the global textile industry.
For years, nanotechnology has played a considerable role in many fields of science and technology such as material science, mechanics, electronics, optics, medicine, aerospace, plastics, and textiles [2, 3]. The ability to create nanostructures such as nanocoatings, nanofinishing, nanofibers, and nanocomposites allows obtaining intelligent, high-performance textiles for advanced applications. In addition, the use of nanoparticles in conventional textile technologies allows achieving new functionalities such as self-cleaning surfaces, antimicrobial properties, controlled hydrophilicity or hydrophobicity, antistatic properties, UV protection, and protection against fire and others, without an influence on the bulk properties of fibers and fabrics [2, 4].
Silver, especially nanosilver, has unique physical and chemical properties such as, among others, optical, electrical, thermal, and biological [5, 6]. For this reason, it is used in various fields, including pharmaceuticals, cosmetics, water treatment, industrial and household products, textiles, and especially in many fields connected to medicine; this is because of their excellent antibacterial, anti-fungal, anti-viral, and anti-inflammatory properties [7, 8]. The possibilities of silver nanoparticle modification were used in textile technologies in the production of antibacterial textiles due to their potential to reduce infection transmission in medical environments [9]. The functionalization of textile products with the use of silver nanoparticles can be accomplished by depositing silver nanoparticles on the surface of the finished textile product by a finishing treatment, e.g., spraying (using low-temperature plasma), surfacing by pad baths or coating (sol-gel or “layer-by-layer” methods), or by producing silver nanoparticles directly on the surface and inside the fibers [10, 11, 12]. Silver nanoparticles can also be introduced into the spinning solution in the fiber production process [13, 14].
Among the known nanoparticles, recently silica nanoparticles have attracted significant attention due to their possible application in many fields of science and industry. They are used for self-cleaning coatings [15] for high-durable superhydrophobicity and antibacterial activity [16], for improvement in thermal performance of firefighter protective clothing [17, 18], and improved durability and mechanical properties of concrete [19]. For this reason, research on novel methods of nanosilica production has been undertaken, which would allow for a significant improvement in the physicochemical properties of particles, as well as better control over their size, shape, and porosity. The most recent literature indicates the availability of a wide range of nanosilica synthesis techniques [20]. Spherical silica nanoparticles as uniform particles from 5 nm to several microns can be synthesized [21], among others, by using the following methods: the sol-gel process [22, 23], the hydrolysis of tetraethyl orthosilicate (TEOS) in an ethanol medium [24], wet chemical synthesis [25], flame spray pyrolysis [26], the low-temperature vapor-phase hydrolysis of SiCl4 [27], water-in-oil microemulsion [28], and microwave synthesis [29]. The choice of the synthesis method determines the basic parameters of the obtained silica nanoparticles (especially particle-size distribution and shape). For better results, the functionalization of nanosilica is frequently used [4].
Nanosilica is most commonly used in the surface modification of textiles. El-Gabry et al. have treated polyester fabrics with silica nanoparticles and acrylate copolymer was used as a binder. Excellent antibacterial activity and durability were obtained after this treatment [30]. Mini-emulsified butyl acrylate/acrylonitrile copolymer — silica nanoparticles nanocomposites — were used by Ahmed et al. in the treatment of both cotton and polyester fabrics. Both fabrics were printed using printing paste with the prepared nanocomposites. When nanosilica was used, better results were achieved in terms of color, rubbing fastness, washing fastness, perspiration fastness, light fastness, UV protection, and self-cleaning [15]. Modification of silica nanoparticles by the introduction of reactive groups using special silane coupling agents was carried out by Riaz et al. [16]. This modified nanosilica was applied to cotton fabric using the conventional pad-dry–cure method. The investigations demonstrated excellent antibacterial activity and superhydrophobicity, along with high comfort properties and maximum durability for laundering. Naeem et al. used silica-based aerogels as insulation materials in protective clothing, especially for firefighters at higher temperatures [17, 18]. Four combinations of high-performance fabrics (multilayer) were prepared, and transmission of heat through them was determined to evaluate the thermal protective performance in terms of transmitted flux density and percentage transmission factor. The results revealed that treatment of silica-based aerogels creates a significant improvement in the thermal protection of firefighter clothing and can be applied for a high range of protection usages.
The process of producing nanosilica containing silver nanoparticles is noteworthy. Scientists developed this method of synthesis from the Industrial Chemistry Research Institute in Warsaw [31, 32]. First, a reaction mixture, which includes ethyl alcohol, aqueous ammonia, and distilled water in an appropriate amount, was prepared. Then, tetraethoxysilane (TEOS) was added to the reaction mixture, stirred at a constant speed for 2 h. TEOS was used as an alkoxysilane precursor and then was distilled immediately before the use in the preparation of nanoparticles. In the next step, silver nitrate as a precursor of nanoparticles was added to the reaction mixture. The final product is nanopowder of nanosilica containing immobilized silver nanoparticles. The authors showed that due to the very high stability of silver nanoparticles immobilized on the surface of silica nanospheres, their required effective concentration is deficient, not exceeding a few ppm. Based on the presented results, it can be stated that these materials are highly effective biocides against both Escherichia coli and Staphylococcus aureus. Nanosilica with immobilized silver nanoparticles can be applied in the form of polymer composite fillers, especially in medicine, textiles, footwear industry, and household appliances.
In addition, the Department of Man-Made Fibers from the Lodz University of Technology has used nanosilica as a modifier in the process of obtaining fibers. Kulpinski [33] modified the cellulose fibers produced by the N-methylmorpholine N-oxide (NMMO) method. Nanosilica (Ludox SM30) as a modifier was introduced into the spinning solution. Results have shown that obtaining fibers is possible with the introduction of up to 30% of nanosilica. However, the use of a modifier above 5% is associated with a significant deterioration in fiber-conditioned tenacity, while reducing the fibrillation tendency.
Niekraszewicz [34] obtained antibacterial cellulose fibers of the Lyocell type by introducing silver-zirconium phosphate (AlphaSan) into the spinning solution at the stage of cellulose dissolution in NMMO. Together with the antibacterial agent, Ludox SM30 nanosilica was also introduced. The results have shown that an additional introduction of nanosilica has improved the antibacterial properties of the obtained cellulose fibers. Both the bacteriostatic and bactericidal activities of the fibers increased. This effect can be attributed to the addition of nanosilica, which creates stronger bonds with silver, consequently leading to a slower release of silver from the fibers. As a result, it is possible to use fewer inorganic antibacterial agents. This is economically important because silver antibacterial agents are more expensive than nanomodifiers, especially Ludox SM30 nanosilica. The presented research shows that the introduction of modifiers in the form of silver-zirconium zeolite and nanosilica did not cause significant changes in either the physicomechanical indicators of the obtained fibers or their hydrophilic properties.
Smiechowicz et al. [35] conducted research concerning antibacterial nanocomposite Lyocell cellulose fibers modified with silver nanoparticles and nanosilica. Silver nanoparticles were generated by the chemical reduction of silver nitrate (AgNO3) in a 50% aqueous solution of NMMO, which was used as a direct cellulose solvent for the production of Lyocell fibers. Nanosilica (Ludox SM30) was used as the second modifier. The presented research shows that the use of nanosilica in the process of obtaining cellulose fibers can contribute to the elimination of the adverse effects of the presence of silver nanoparticles in the fiber matrix, which was confirmed by in vitro studies using human and mouse cell lines. The introduction of nanosilica into the fiber matrix has eliminated the toxicity of silver nanoparticles on human tissue. Scientists proved that the obtained cellulose fibers modified with silver nanoparticles and nanosilica have excellent antibacterial properties and are safe for human tissues and the environment, which means that they are appropriate for medical applications.
The presented manuscript concerns the production of antibacterial fibers containing nanosilica with immobilized silver nanoparticles. Our earlier research was based on introducing the same type of modifier into the matrix of fibers, however, separately in the form of two types of nanoparticles, both in the form of nanosilica and as silver nanoparticles. Silver nanoparticles were synthesized under various conditions. Our extended involvement in such research shows that the generation of nanoparticles is an overly complicated, multi-faceted process that is still not fully understood. In this manuscript, the introduction of nanoparticle modifier into the fiber matrix with strictly defined parameters was demonstrated. The novelty of this work is the elimination of uncontrolled factors occurring during the process of silver nanoparticle synthesis (e.g., generation of large diameter nanoparticles and formation of their aggregates and agglomerates). Thus, the applied fiber's modification process is more efficient, more controlled, and simpler to perform. From an application point of view, the modification of fibers with nanoparticles of specific parameters could reduce the costs of the production process of antibacterial cellulose fibers modified with nanosilica with immobilized silver nanoparticles.
2 Experimental
2.1 Materials
Cellulose pulp (Rayonier Ltd., Wildlight, Florida, USA) containing 98 wt. (%) of α-cellulose with an average degree of polymerization (DP) of about 1,250 was used.
NMMO as 50% aqueous solution from Huntsman (Holland BV, the Netherlands) was used for the preparation of the spinning dope.
The propyl ester of gallic acid (Tenox PG®) from Aldrich (Gillingham, Dorset, UK) was applied as an antioxidant.
In order to modify the cellulose fibers, a water-alcohol solution of nanosilica with immobilized silver nanoparticles (size of nanoparticles: 13 nm; content of silver nanoparticles: ~69,546 ppm; dry matter content: 5.15%) was used as a nanomodifier. The nanomodifier was made at the Department of Polymer Technology and Processing (Industrial Chemistry Research Institute, Warsaw, Poland) according to the procedure described in patent PL-217617 [10].
2.2 Methods
2.2.1 Preparation of the fibers
The spinning solutions were prepared in an IKAVISK kneader by adding a water-alcohol solution of nanosilica with immobilized silver nanoparticles to the cellulose pulp. The amount of the used modifier reached an amount of silver nanoparticles of 250 ppm, 500 ppm, 1,000 ppm, and 2,000 ppm in cellulose fibers. The dissolution process was carried out in the kneader until a homogeneous spinning dope was obtained. The cellulose concentration in the obtained solutions was 8%. The fibers were spun using the dry-wet method in a laboratory spinning machine, at a speed of 55 m/min, as described in previous works [36].
Abbreviations for the samples used in the present research:
F-000 — unmodified cellulose fibers (without modifier)
F-250 — cellulose fibers modified with nanosilica with immobilized AgNPs (amount of AgNPs in fibers = 250 ppm)
F-500 — cellulose fibers modified with nanosilica with immobilized AgNPs (amount of AgNPs in fibers = 500 ppm)
F-1000 — cellulose fibers modified with nanosilica with immobilized AgNPs (amount of AgNPs in fibers = 1,000 ppm)
F-2000 — cellulose fibers modified with nanosilica with immobilized AgNPs (amount of AgNPs in fibers = 2,000 ppm)
2.2.2 Methods of fiber characteristics determination
SEM analysis of the modified fibers morphology was performed using the TESCAN VEGA3–EasyProbe (TESCAN Brno, s.r.o., Czech Republic) scanning electron microscope equipped with the VEGA TG software (high vacuum secondary electron mode; accelerating voltage 7 kV). Samples were sputtered with Au/Pd (Cressington 108 Auto Sputter Coater, UK) for 120 s, resulting in the formation of a 30 -nm thick Au/Pd layer on the samples.
TEM analysis of the modifier in the fibers was performed via TEM at the Polish Academy of Sciences in Krakow. A transmission electron microscope TENCAI G2 FEG 20 (200 kV) was used.
The Japanese Industrial Standard (JIS L 1902: 1998 “Testing method for antibacterial activity of textiles”) was used for the evaluation of the antibacterial efficiency of the modified fibers. The test was performed using the Gram-negative strain of E. coli (ATCC 11229) and the Gram-positive strain of S. aureus (ATCC 6538).
In order to examine the mechanical parameters of the obtained fibers, fiber linear density was determined according to ISO 1973:1995 (E). Conditioned tenacity and elongation at break were measured according to PN-EN ISO 5079:1999. The measurements were performed using the ZWICK/Z 2.5/TN1S (Ulm, Germany) tensile testing machine with TestXpert v. 7.1 software.
Determination of the moisture absorption of the obtained fibers was carried out at 65% relative humidity at 20°C according to Polish standard PN-71/P-04635 and determination of water retention in accordance with PN-72/P-04800. Samples of fibers were immersed in distilled water containing a surface-active agent (Rokafenol N-8, 0.1%) for 24 h and then centrifuged for 10 min at 220 rad/s.
3 Results and discussion
3.1 Estimation of the antibacterial activity of the fibers
The Japanese Industrial Standard JIS L 1902:1998 was used to determine the antibacterial efficiency of the modified fibers. Fibers F-250, F-500, F-1000, and F-2000 were evaluated. Their antibacterial activity against E. coli, as a representative of Gram-negative bacteria, and S. aureus, as a representative of Gram-positive bacteria, was estimated. The obtained results on the antibacterial effect of the fibers are presented in Tables 1 and 2.
Results of tests on the antibacterial activity of the modified fibers against E. coli
| Sample | Time [h] | Number of bacteria [jtk/pr] | Bacteriostatic effectiveness (S) | Bactericidal effectiveness (L) | Antibacterial activity |
|---|---|---|---|---|---|
| Reference sample | 0 | 1,4 × 105 | - | - | - |
| Reference sample | 24 | 1,4 × 108 | - | - | - |
| F-250 | 24 | <20 | 1,4 | −1,6 | Only bacteriostatic |
| F-500 | 24 | 6,1 × 106 | 6,9 | 3,9 | Strong |
| F-1000 | 24 | <20 | 6,9 | 3,9 | Strong |
| F-2000 | 24 | <20 | 6,9 | 3,9 | Strong |
Results of tests on the antibacterial activity of the modified fibers against S. aureus
| Sample | Time [h] | Number of bacteria [jtk/pr] | Bacteriostatic effectiveness (S) | Bactericidal effectiveness (L) | Antibacterial activity |
|---|---|---|---|---|---|
| Reference sample | 0 | 13,8 × 104 | - | - | - |
| Reference sample | 24 | 3,6 × 106 | - | - | - |
| F-250 | 24 | 2,7 × 103 | 3,2 | 1,2 | Negligible |
| F-500 | 24 | 5,0 × 101 | 6,9 | 4,9 | Strong |
| F-1000 | 24 | <20 | 5,3 | 3,3 | Strong |
| F-2000 | 24 | <20 | 5,3 | 3,3 | Strong |
Due to the potential medical use of the fibers, an essential point in this work was to perform antibacterial testing. An analysis of the results (Table 1) showed that F-250 fibers (250 ppm silver) did not display bactericidal activity against E. coli, but only had negligible bacteriostatic activity. The results in Table 2 show that F-250 fibers display weak bacteriostatic activity and negligible bactericidal activity against S. aureus. This is due to the low content of nanosilver enclosed in the cellulose fiber matrix, which was about 0.56% relative to the amount of α-cellulose. It can, therefore, be concluded that F-250 fibers, containing nanosilica with immobilized silver nanoparticles with a content of 250 ppm, do not display satisfactory antibacterial properties against both Gram-negative and Gram-positive bacteria. Other fibers, i.e., F-500, F-1000, and F-2000, in which the nanosilver content is at the level of 500 ppm, 1000 ppm, and 2000 ppm, respectively, display strong bacteriostatic and bactericidal properties against E. coli and S. aureus bacteria. This means that the introduction of silver nanoparticles immobilized in nanosilica in an amount approximately 500 ppm relative to α-cellulose into the polymer matrix is sufficient to obtain fibers with an excellent antibacterial effect against both Gram-negative and Gram-positive bacteria. Therefore, the F-500 fiber (500 ppm of silver) was selected for microscopic analysis.
3.2 Determining the mechanical properties of cellulose fibers
This research was aimed at assessing the influence of silver nanoparticles immobilized in nanosilica introduced into the fibers on average parameters defining the mechanical properties of the fibers, namely their linear density, conditioned tenacity, and elongation at break.
The conditioned tenacity for fiber modification was in the range of 27.29–31.49 cN/tex, whereas for unmodified fibers it was 29.20 cN/tex. The differences between the conditioned tenacity values for fibers obtained with different modifier contents are small and are within the limits of the measurement error. No significant influence of the amount of introduced modifier on the conditioned tenacity values was observed. No significant influence of the amount of introduced modifier on the elongation at break values was also observed. This was probably because small amounts of modifier that were introduced into the spinning solution during the preparation of individual fibers. The amount of modifier is dependent on the amount of silver in ppm that was assumed to be introduced into the fiber matrix. The theoretically calculated content of the modifier corresponding to the highest concentration of silver nanoparticles in the fiber material, i.e., 2,000 ppm, corresponds to 4.5% of the introduced modifier in the cellulose-NMMO system. In the other modified fibers, the modifier share was proportionally smaller and amounted to 0.58% for the fiber with a concentration of silver nanoparticles at 250 ppm. The significant lack of influence for the amount of nanosilica introduced into the spinning solution on the conditioned tenacity of the fibers obtained as a result of modification is consistent with the previous tests [33]. Studies of the fibers obtained with the use of nanosilica have shown that the introduction of nanosilica in the amount of up to 5% based on α-cellulose content into the spinning solution has no significant impact on the conditioned tenacity of the modified fibers. This is highly beneficial and means that the modification of the fibers with nanosilica with immobilized silver nanoparticles does not cause deterioration of the mechanical properties of the fibers. One can only notice an increase in the linear density of the modified fibers with an increase in the modifier content in the fiber matrix, which is associated with an increasing amount of higher density substances (modifier) compared to the density of the cellulose matrix.
3.3 Determination of moisture absorption and water retention for the obtained fibers
The estimation of moisture absorption was carried out in order to determine the influence of the modifier on the hygroscopic properties of the obtained fibers.
An analysis of the moisture absorption results of fibers (Table 4) demonstrates that the values of the tested parameter for all fibers (both modified and unmodified) are at a very similar level – about 11%. The differences between the values obtained for fibers with different modifier content are less and it can be said that the amount of introduced modifier does not significantly affect the amount of moisture absorption of the modified fibers.
The linear density and mechanical properties of fibers
| Sample | Linear density [dtex] | Tenacity [cN/tex] | Elongation at break [%] |
|---|---|---|---|
| F-000 | 2.980 | 29.20 ± 4,01 | 6.64 ± 0,22 |
| F-250 | 3.024 | 31.49 ± 4,29 | 6.98 ± 0,24 |
| F-500 | 3.112 | 30.86 ± 4,58 | 7.43 ± 0,32 |
| F-1000 | 3.136 | 27.29 ± 4,37 | 6.84 ± 0,26 |
| F-2000 | 3.580 | 29.58 ± 4,99 | 6.26 ± 0,23 |
Results of tests on the moisture absorption and water retention of fibers with modifier
| Sample | Moisture absorption [%] | Water retention [%] |
|---|---|---|
| F-000 | 11.16 | 67.43 |
| F-250 | 11.26 | 72.12 |
| F-500 | 11.21 | 79.55 |
| F-1000 | 11.28 | 79.12 |
| F-2000 | 11.16 | 81.25 |
On the other hand, the results of water retention of fibers presented in Table 4 reveal that as the amount of nanosilica with immobilized silver nanoparticles was introduced into the polymer matrix, the water retention of fibers increases. Water retention of unmodified fibers was about 67.43%, whereas for modified fibers it was between 72.12% (F-250 fibers with 250 ppm nanosilver content) and 81.25% (F-2000 fibers with 2,000 ppm nanosilver content). This effect is probably related to the structure of this type of nanosilica.
3.4 SEM analysis of modified fibers
In order to characterize the nanomodifier introduced into the polymer matrix of fibers, an SEM analysis was conducted. The analysis was performed for the selected fiber marked as F-500 (500 ppm silver). The image obtained from the scanning electron microscope is shown in Figure 1.
SEM image of modifier in the polymer matrix of cellulose fiber.
The presented SEM image is only the basis for further analysis of the distribution of modifier nanoparticles in the fiber using the TEM method. To obtain fibers with antibacterial properties, an important goal was to analyze the distribution of nanoparticles in the whole matrix of cellulose fiber, as well as on its surface. In order to estimate the optimal antibacterial effects of the fibers, it was necessary to confirm the presence of modifier nanoparticles on the surface of cellulose fibers. The presented SEM image confirmed the proper occlusion of nanoparticles in the fiber matrix and indicated the proper dispersion of the modifier.
3.5 TEM analysis of the size and shape of silica nanoparticles with immobilized silver nanoparticles
The analysis of the size and shape of silica nanoparticles with immobilized silver nanoparticles was carried out in order to characterize the nanomodifier, which was introduced into the fibers by becoming enclosed in the polymer matrix. The analysis was performed for the selected F-500 fiber (500 ppm silver) by TEM. Images of the nanoparticles obtained using the TEM method are presented in Figures 2 and 3. The histogram presenting the diameter distribution of the nanoparticles was created based on TEM pictures for the selected fiber (particles were counted in each of the images). The relation between the distribution of particle diameter and their number in the matrix of cellulose fibers was presented in the histogram (Figure 3).
For the size distribution of particles, the average size of the particles (
), the standard deviation of the particles’ diameter (s), and their minimum and maximum diameter (Dmin, Dmax) were estimated.
3.6 Microanalysis of the polymer matrix of fibers using the EDS method
Using EDS, major inorganic elements (silver nanoparticles and nanosilica) in the cellulose fibers (F-2000) were identified. Figure 4 shows a TEM image in (a) the dark field of nanosilica with immobilized AgNPs and (b) the EDS spectrum of the area marked in red in (a). The fiber with the highest amount of nanomodifier equaled to 4.5% (corresponding to 2,000 ppm of silver) in relation to α-cellulose in the fiber that was selected for microanalysis.
TEM images: (a) HREM images of silica nanoparticles with immobilized silver nanoparticles; (b) and (c) silica nanoparticles with immobilized silver nanoparticles and their aggregates in the bright field.
Nanosilica with immobilized silver nanoparticles size distributions in the polymer matrix of fibers F-500.
TEM image of silver nanoparticles in fiber F-2000 in (a) dark field with marked red areas (points 1 and 2) and (b) EDS spectrum analysis in point 1.
Both the EDS spectrum and microscopic images (SEM and TEM analyses) confirm the presence of a nanomodifier in the cellulose fiber matrix. Tests on the chemical composition presented above in the marked areas of the selected fibers confirmed that the particles marked with red in the TEM pictures are silica nanoparticles with immobilized silver nanoparticles. The EDS spectra for fibers F-2000 ppm show clear peaks indicating the presence of silica and silver in the fibers. High-resolution HREM images showed that the silica nanoparticles encapsulated in the fiber matrix with immobilized silver nanoparticles have a spherical shape. The images in the “light field” show that these nanoparticles are relatively evenly distributed in the polymer matrix of the fibers. The images (b2) also show a little amount of nanoparticle aggregates with a diameter of about 80–200 nm. Analyzing the histogram of sizes of silver nanoparticles in fiber shows a very narrow size distribution of nanoparticles in the range of 6–17 nm. It was observed that the majority of particles are sized about 12–14 nm, which is consistent with the sizes mentioned in the datasheet of the modifier.
4 Conclusions
This paper shows that antibacterial cellulose fibers can be obtained by introducing nanosilica with immobilized silver nanoparticles into the fiber matrix.
Tests on the antibacterial activity of modified fibers have confirmed that the fibers display strong antibacterial (both bactericidal and bacteriostatic) properties. The introduction of silver nanoparticles immobilized on nanosilica in the amount of 500 ppm relative to α-cellulose into the polymer matrix is sufficient to obtain fibers with excellent antibacterial effect against both gram-negative and gram-positive bacteria.
The differences between the moisture absorption values obtained for fibers with different modifier content are very less and it can be said that the amount of introduced modifier does not significantly affect the amount of moisture sorption by the modified fibers. The introduction of more significant amounts of modifier into the fibers demonstrates improved water retention (at a level of about 4–14%). This was probably due to the structure of silica, as shown in previous examinations at the Department of Man-Made Fibers, in which colloidal nanosilica was used [35]. No significant influence of the amount of introduced modifier on the conditioned tenacity and elongation at break values was observed. This was probably due to the small amounts of modifier that were introduced into the spinning solution during the preparation of individual fibers. Only an increase in the linear density of the modified fibers was noticed with an increase in the modifier content in the fiber matrix, which is associated with an increasing amount of higher density substances (modifier) compared to the density of the cellulose matrix.
Microscopic analysis confirmed the presence of spherical silica nanoparticles with immobilized silver nanoparticles with a diameter of about 13 nm in the matrix of fibers. The TEM images show an even distribution of the modifier in the fiber matrix with a little amount of nanoparticle aggregates having a diameter of about 80–200 nm. This means that the introduced modifier does not have an increased tendency to form aggregates of nanoparticles, which is associated with the immobilized silver nanoparticles on nanosilica nanoparticles. The immobilization of silver nanoparticles on silica nanoparticles has allowed the elimination of the negative tendency of silver nanoparticles to form aggregates and agglomerates, which results from their metallic structure.
The introduction of nanosilica with strictly defined parameters into the fiber matrix allowed obtaining antibacterial fibers in a more efficient, more controlled, and more straightforward way. The obtained nanocomposite antibacterial cellulose fibers have potential use both for the personal protection of a person and for applications in many fields connected to medicine.
Acknowledgments
Authors thank the Industrial Chemistry Research Institute (currently Łukasiewicz-Industrial Chemistry Institute) for preparation of nanosilica samples.
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