Production of antibacterial nano silver PET nonwoven fabric
The antibacterial nano silver nano-woven polyethylene terephthalate (PET) fabric has been prepared by a three-step process. First, the fabrics are pretreated by depositing a thin film of organosilicon using an atmospheric pressure plasma system, then silver nanoparticles (AgNPs) are introduced into the fabric by a dry dipping and final dipping process. The same nanoparticles are covered in a second 10-50 nm organosilicon layer, which acts as a separator. Various surface characterization techniques such as SEM and XPS have been performed to study the morphology and chemical composition of silver nanofabrics. Based on these techniques, a uniform immobilization of AgNPs in the PET substrate was observed. The antibacterial activity of the treated fabrics was also examined using P. aeruginosa ,S. aureus and C. albicans . It shows that the thickness of the barrier layer has a strong influence on the bacteria reduction of the fabric. The strength and stability of AgNPs on fabrics were also studied during washing. By doing so, it is confirmed that the barrier layer can effectively prevent the release of AgNPs and the thickness of the silicon barrier is an important parameter to control the release of silver ions.
Nonwoven polyethylene terephthalate (PET) has been used in many applications due to its outstanding properties such as excellent mechanical strength and good chemical stability. However, in recent years, a lot of attention has been paid to achieve more multi-functional performance of PET fabric, especially in medical and sanitary field.
For this purpose, PET fabrics treated with antibacterial agents have been extensively studied and the results show their ability to prevent the growth of pathogenic microorganisms, such as bacteria, fungi, algae, etc. . Various antibacterial agents (silver, copper, metal salts, quaternary ammonium compounds, polyhexamethylene biguanide, triclosan biopolymer chitosan, N-halamine, etc.) on textiles. 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12
Especially when the Covid 19 epidemic spread around the world, taking the lives of millions of people, the production of masks from antibacterial non-woven fabrics to control the epidemic received more and more attention.
Among the antibacterial chemicals mentioned above, silver has been widely used due to its broad antibacterial activity and low toxicity to mammalian cells. The release of silver ions is thought to be the main reason for its antibacterial properties. Ionized silver is highly active, as it binds to tissue proteins and brings about structural changes to bacterial cell walls and nuclear membranes leading to deformation and cell death. Due to their small size, silver nanoparticles (AgNPs) have emerged as a new generation of antibacterial agents with unique properties for diverse medical applications. By immobilizing AgNPs on the substrate material can completely avoid the release of silver nanoparticles in practical applications and only controlled release of silver ions can be achieved. This immobilization method has been intensively studied because it can block the potentially dangerous effects of silver nanoparticles while still having beneficial antibacterial properties.
In this study, PET nonwovens with antibacterial properties will be produced by firmly immobilizing silver nanoparticles through a double layer of plasma-deposited organic film. This is carried out in a three-step process as shown in Figure 1 . At first, an organosilicon thin film is deposited on the fabric surface by a plasma jet deposition system. This first layer, called the “reserve layer” through the paper, is used to immobilize the silver and control the adhesion of the silver nanoparticles to the PET fiber. The fabric was then dipped in a nanosilver suspension to incorporate the silver, and the color of the fabric changed from white to gray ( Figure 2). In the final step of the process, a second organosilicon film is deposited using a plasma jet system.
To better understand the possibilities of the proposed process, the chemical composition and morphology of the nano-silver-added fabrics will be characterized by X-ray photoelectron spectroscopy (XPS) and electron microscopy. scanning element (SEM). The antibacterial activity of the samples will also be tested against three common pathogenic microorganisms, Pseudomonas aeruginosa ( P. aeruginosa ), Staphylococcus aureus ( S. aureus ) and Candida albicans (C. albicans ). The antibacterial effect of fabrics treated in static and mechanical load experiments demonstrated by washing will be analyzed and discussed in detail.
Results and discussion on the production method of nano silver antibacterial fabric
Plasma polymerization on non-woven PET fabric
Plasma polymerization is capable of creating ultrathin, polymer-like layers with a defined, regular structure on the top surface of flat materials. However, in this work, the nonwoven porous material is exposed to plasma polymerization. Therefore, it is very important to study the penetration efficiency of plasma polymerization through PET nonwovens after deposition of the first organosilicon layer. For this purpose, plasma polymerized PET samples were examined with XPS on two sides: the upper side in direct contact with the plasma and the lower side in close contact with the sample holder.
The atomic composition of the untreated and plasma-treated PET nonwovens was determined using XPS and is shown in Table 1 . Raw fabric is composed of 73.4% carbon and 26.6% oxygen. For the samples after plasma polymerization, the top and bottom surfaces all exhibit the same composition: carbon, oxygen and silicon. It is noteworthy that the concentrations of the elements on the two sides are almost identical. Thus, plasma penetrates clearly into the fabric’s structure and plasma deposition can be achieved in the mass of the nonwoven as well as on the underside of the fabric. This can be explained by the efficient transfer of active plasmas by the vertical gas flow along the plasma jet.
Figure 3 shows the C1s and Si2p spectra of the raw fabric and the top/bottom side of the plasma-treated PET nonwovens. These high-resolution XPS spectra can be used to analyze the chemical states of the elements present on the surface. As shown in figure 3(a), the cross section of the top C1 shows the difference between the raw fabric and the treated fabric on the one hand, and the presence of completely different bonds on the surface on the other hand. plasma polymerization. The C1s spectra of the original PET fabric can be decomposed into three components: one at 285.0 ± 0.1 eV due to CC or CH binding, one at 286.5 ± 0.1 eV due to CC or CH binding. CO and one component at 289.1 ± 0.1 eV due to OC = O binding. In contrast, the C1s spectra of the plasma-treated samples have a completely different structure: these surfaces no longer contain OC=O bonds at 289.1 eV but contain a small peak at 283.1 eV indicating a presence of Si-C bonds. Therefore, these C1s spectra indicated that the fabric surface was covered by an organosilicon layer due to plasma polymerization. Figure 3 (b), the raw fabric did not contain any Si2p peaks. However, this peak appeared for plasma polymerized samples again suggesting successful deposition of organosilicon film on PET fabric. The Si2p spectrum in figure 3 (b) can be separated into 4 components: one at 105.eV ± 0.1 eV due to (CH3 ) 3 SiO, one at 102.2 ± 0.1 eV due to (CH 3 ) 2 SiO 2 , a component at 102.8 ± 0.1 eV due to CH 3 SiO 3 , and a component at 103.4 ± 0.1 eV due to SiO 4 . 25Starting from pure TMDSO one can expect to observe only a peak at 102.2 eV but due to the small addition of oxygen in the discharge gas, a mixture of different silicon bonds is present in the plasma deposited films. . It should also be noted that the C1s and Si2p spectra of the top and bottom surfaces of plasma-treated fabrics have almost identical profiles confirming the uniform chemical composition of the residue throughout the PET fabric.
Chemistry and morphology of materials treated at different process steps
The chemical composition of the samples in different experimental steps was analyzed by XPS and the results are presented in Table 2 . In the dipping and drying step, silver was introduced into the fabric with a silver concentration of 1.9% on the surface. The decrease in oxygen and silicon concentrations after incorporation of AgNPs is consistent with our previously published study 25. Silver concentration on the surface decreased to 1.1% and 0.5% after single-layer deposition. separated by 10 nm and 50 nm respectively. In our opinion, this effect is due to the suppression of the silver signal by the barrier surface. It is well known that XPS can be used to study the chemistry of the top 10 nm of the surface. As a result, one can expect no silver concentration to be detected when a 10 nm or 50 nm separator is deposited on top of AgNPs. However, in Table 2, a small concentration of silver remained on the samples with the upper barrier layers. This effect may be due to the presence of cracks, voids or thin coverings on the nanosilver integration points. While the reason for the formation of cracks and voids is not entirely clear, it may be the result of thermal stress induced during plasma deposition because a strong temperature gradient exists between the gaseous and liquid phases. cold background. The presence of those imperfections in the shielding can be observed indirectly by the variation of the Si signal in the XPS measurements. During the dipping and drying step, the silicon concentration decreased from 21.6% to 15.2% due to the incorporation of nano silver. With the deposition of a 10 nm barrier layer, the Si concentration recovered to 19.9%. This value is lower than that of the original organosilicon film (21.6%), which indicates that the surface of AgNP is not completely covered by the 10 nm organosilicon film and some imperfections are present in the barrier. thin. Increasing the barrier thickness to 50 nm increases the silicon percentage by 21.1%, which is close to the original value due to the reduction of those defects. Thus, defects in the 10 nm barrier layer can be clearly detected by XPS and can be significantly reduced by increasing the barrier layer thickness to 50 nm. Correspondingly, barrier thickness will be an important parameter in controlling coating performance and silver release through imperfections. 6%) showed that the surface of the silver nanosheets is not completely covered by the 10 nm organosilicon film and some imperfect regions are present in the thin barrier layer. Increasing the barrier thickness to 50 nm increases the silicon percentage by 21.1%, which is close to the original value due to the reduction of those defects. Thus, defects in the 10 nm barrier layer can be clearly detected by XPS and can be significantly reduced by increasing the barrier layer thickness to 50 nm. Correspondingly, barrier thickness will be an important parameter in controlling coating performance and silver release through imperfections. 6%) showed that the surface of the silver nanosheets is not completely covered by the 10 nm organosilicon film and some imperfect regions are present in the thin barrier layer. Increasing the barrier thickness to 50 nm increases the silicon percentage by 21.1%, which is close to the original value due to the reduction of those defects. Thus, defects in the 10 nm barrier layer can be clearly detected by XPS and can be significantly reduced by increasing the barrier layer thickness to 50 nm. Correspondingly, barrier thickness will be an important parameter in controlling coating performance and silver release through imperfections.
Analysis of silver nanocomposite on PET fibers was performed with SEM. Figure 4 shows the surface morphology of the samples at different fabrication steps: (a) raw PET fabric, (b) deposition of the reserve layer; (c) incorporation of AgNPs; (d) barrier deposition (50 nm). SEM data with two different magnifications, i.e. ×1000 and × 10000, are presented on the left and right, respectively. The raw PET fabric consists of fibers with an average diameter of about 10 μm with a smooth surface as shown in Figure 4 (a). After being treated with the first plasma deposition procedure ( Figure 4 (b)), the surface of the PET fibers was completely covered by a film. Figure 4(c) is performed after the dipping and drying step. Magnification of the surface shows successful incorporation of silver particles on fibers with a diameter of several tens of nanometers. A strong change of surface morphology is observed after barrier layer deposition. The surface of the PET fibers shown in Figure 4 (d) is smoother, which indicates the coverage of particles incorporated by a new film layer.
Antibacterial effect of treated PET material
To test the antimicrobial efficacy of PET fabrics, the following test organisms were used: Pseudomonas aeruginosa ( P. aeruginosa ) ATCC 9027, Staphylococcus aureus ( S. aureus ) Mu50 and Candida albicans ( C. albicans ) SC5314, corresponding to a gram-positive bacteria, gram-negative bacteria and fungi respectively. Infection P. aeruginosa is a serious problem in patients hospitalized for cancer, cystic fibrosis, and burns; with a high mortality rate. 26 P. aeruginosa naturally resistant to many antibiotics. S. aureus is a pathogenic microorganism that causes many diseases such as toxic shock syndrome, superficial skin lesions, deep-seated infections and is a leading cause of nosocomial (hospital) infections of surgical wounds. 26 Moreover, it is resistant to a large number of antibacterial substances. 27 C. albicans is the most common cause of opportunistic fungal infections and it can cause infections ranging from superficial skin infections to life-threatening systemic infections. 28 As all three microorganisms are considered as common potential pathogens for infections, the above three microorganisms were selected to evaluate the antibacterial effect in this study.
The percentage reduction in organisms R indicating the biological yield due to exposure to the sample was determined by the following formula:
R(%)= [(B-A)/B]*100
where A is the CFU per milliliter for media with the substrate treated after incubation and B is the CFU per milliliter of the medium with the control sample after incubation. The control samples in this section correspond to those with deposition of the first film layer.
The antibacterial properties of the samples were tested against P. aeruginosa , S. aureus and C. albicans . The original sample with a 70 nm organosilicon membrane with no antibacterial activity was used as a control. The antibacterial ability of 10 nm and 50 nm barrier-free treated PET fabrics is shown in Figure 5 . All samples with nanosilver exhibited antibacterial activity against the three microorganisms, which clearly indicated that microbial growth in the medium was affected by the presence of AgNPs. All treated PET fabrics showed higher efficacy against S. aureus and against P. aeruginosa , which is consistent with results on commercially available silver-containing dresses. 29 Samples with AgNPs but no separator showed the highest reductions of over 90% S. aureus and C. albicans and 80% P. aeruginosa. The presence of a barrier layer reduces the antibacterial efficiency to almost 50% in the case of a 50 nm barrier layer. Such a strong effect of the barrier layer may be related to how AgNPs exert antimicrobial effects.
At present, the exact mechanism of silver’s antibacterial activity remains unclear. The release of silver ions is thought to be the main contributor to this effect. Ionic silver has a strong affinity for electron donor groups in biomolecules containing sulfur, oxygen, or nitrogen. Therefore, it can bind with thiol (-SH) groups in enzymes and inactivate them and destroy cell membranes. 30 DNA replication can also be inactivated by interaction with silver ions, as suggested by Thiel and Jung. 31 , 32 It is well known that silver nanoparticles can be oxidized after exposure to ambient water, at the material-liquid interface or after the polymeric substrate absorbs water. This leads to the generation of silver ions, which diffuse into the liquid medium through the barrier, as shown in Figure 6 .
For samples without a barrier, the nanosilver on the material is sufficiently exposed to the environment. Therefore, they can rapidly release silver ions into the medium and exhibit the strongest antibacterial activity against microorganisms. When a diaphragm is deposited, the direct contact between the silver nanoparticle and the medium is hindered and the release of silver ions from the nano silver is reduced. In this case, the release of Ag + can only occur through small cracks and voids in the barrier layer as shown in the previous study. 33 Moreover, the thickness of the barrier layer significantly affects the antibacterial effect, which is in contrast to the work of L. Polux et al. 34In their study, a heptylamine (HA) matrix was used to load AgNPs and it was shown that thickness had no significant effect on the release of Ag ions due to HA’s nanomorphological structure, allowing the Ag ions diffuse through it. freely. In the plasma deposition experiments performed in our work, increasing the thickness of the barrier layers resulted in a decrease in the number of cracks and pores in the organosilicon matrix, which could be a way to control the antibacterial effect and to get the long-lasting antibacterial effect. user products.
In addition to controlling the antimicrobial effect through deposition of the coating, it is important for applications to avoid any release of nanosilver during the life of the textile and/or under mechanical stress. occurs during dressing and washing. Therefore, the efficiency of PET samples produced under mechanical stress will be examined in detail in the following section.
Antibacterial effect under mechanical pressure
As mentioned, direct contact of AgNPs with cells should be completely avoided due to their potential toxicity. The cytotoxic effects of silver nanoparticles are mediated by the induction of oxidative stress, which leads to a wide range of physiological and cellular events including stress, inflammation, DNA damage and apoptosis. To have those toxic effects, silver nanoparticles must penetrate membranes and react with organelles, such as mitochondria. According to our approach, silver nanoparticles immobilized inside the coating inhibit cell-nanoparticle reactions and further intracellular effects can be avoided. Therefore, we expect that the cytotoxicity of silver nanoparticles in nanocomposite films will be reduced. In this section, the durability of AgNP incorporated in the material was evaluated through sample wash test after 1, 3, Wash cycles 5 and 10, each cycle for 40 min in 200 ml of deionized water. All samples were dried after washing in vacuum prior to analysis. Figure 7 shows silver concentrations on the top surface of three different samples after multiple wash cycles. The concentrations shown in Figure 7 are the mean of the 9 different sites per sample. For samples without a barrier, the silver concentration fluctuates dramatically with the wash cycle. The interesting fact is that the concentration shows a steady increase of more than 3% in the first 3 washing cycles. Subsequent washings resulted in a reduction of silver concentration to approximately 1.3% and 0.4% after 5 and 10 wash cycles, respectively.
The effect at the beginning of the wash can be explained by loosening the fibers in the nonwoven structure and rapidly desorption of the physically absorbed particles as they migrate from the bulk of the material to the surface. Correspondingly, the increase in the Ag signal in the XPS analysis for the non-shielded material is attributed to the release of free nano silver. The reduction in silver concentration in the following wash cycles is due to the loss of absorption of the nanoparticles on the fiber surface and dispersion in water. In the unscreened samples, silver exhibited uneven release kinetics at the first 5 wash cycles. This confirms the ability to desorb and separate AgNPs from this fabric. For samples with 10 nm and 50 nm barrier layers, the silver concentration on the surface showed almost negligible fluctuations for all washing cycles. This conclusion clearly demonstrates our notion of using barrier layers to prevent any release of AgNPs from the matrix.
The antibacterial activity of nanosilver-treated fabrics in the absence and with an insulator was evaluated against P. aeruginosa , S. aureus and C. albicans after a series of washing cycles. The test results presented in Figure 8 showing a general antibacterial decrease trend are in full agreement with the XPS measurement of silver concentration. For the samples without barrier (orange bar in Figure 8 ), a 100% reduction for the three microorganisms was achieved after the first wash cycle. According to the XPS results in Figure 8, this is due to the higher amount of AgNPs on the surface of the samples. This loose physical adsorption of AgNPs can even induce the penetration of AgNPs into bacterial suspensions and lead to very high antibacterial efficacy. However, this effect should always be eliminated in any application. Furthermore, after 5 washes, the antibacterial activity decreased significantly due to the loss of many nano silver. For the samples with the barrier layer (10 nm is the blue bar and 50 nm is the pink bar), the antibacterial activity is maintained at a constant level and is consistent with the XPS results, which confirms the stability. determination of silver nanowire bonding and the positive effect of a barrier layer.
It is expected that samples with comparable silver concentrations would have similar antibacterial activity. However, this is not always the case as observed for samples after the test wash. According to XPS analysis ( Figure 7 ), there was 0.4% and 0.5% Ag on the samples without barrier and samples with 50 nm barrier after 10 wash cycles respectively. However, as shown in Figure 8, the bactericidal capacity for the three microorganisms of the unscreened sample (orange bar) was higher than that of the 50 nm barrier sample (pink bar). In our opinion, this phenomenon can be explained by the effect of the barrier layer preventing the oxidation of silver nano in liquid medium and the release of silver ions. For the sample without the separator after 10 wash cycles, all AgNPs on the surface were in direct contact with the medium and were affected by oxidation during the washing process leading to the release of many silver ions. While for the sample with a 50 nm barrier after 10 wash cycles, the release channels were limited to cracks and voids in the barrier. Thus, even with comparable silver concentrations on the surface for those two samples, the bacteriostasis was lower for the sample with the 50 nm barrier layer. In summary, the barrier layer thickness should be less than 50 nm if a high level of antimicrobial efficacy is required for the samples.
In summary, antibacterial silver nano-PET fabric is prepared through a three-step process based on atmospheric pressure plasma deposition process. It has been revealed that plasma can penetrate through the non-woven fabric structure and can achieve uniform double-sided deposition. Nano silver is embedded between two layers of organosilicon film: a room layer (1 st layer) and a barrier layer (2 nd layer). The change of the barrier layer thickness is proposed as a new precise method to control the release of silver ions and the antimicrobial activity of the substrate. SEM and XPS results show that AgNPs can be uniformly distributed in PET materials. Antibiotic tests against P. aeruginosa , S. aureus and C. albicans showed that samples with a 10 nm barrier were more active than those with a 50 nm barrier. The bond strength of silver nanoparticles in the substrate as an important factor was also investigated through the washing process. Silver concentrations in the unscreened samples showed significant fluctuations after several wash cycles. This is explained by desorption of AgNPs that are physically absorbed from the nonwoven and migrate to the environment during washing. However, for the shielded samples, effective fixation of silver in the substrate was confirmed with stability of the antibacterial effect even after 10 washing cycles.
Production method of nano silver antibacterial fabric
Plasma beam deposition system
The plasma beam consists of a pin and a grid electrode placed in a quartz tube as described elsewhere. 35 Emission spectroscopy-based investigations revealed that nitrogen plasma contains many excited states of nitrogen molecules (N 2 (A 3 Σ, B 3 Π, C 3 Π)) and ionized nitrogen species ( N 2 + (B 2 Σ u , X 2 Σ g )). A plasma jet generated at N 2 (7000 sccm) with the mixing of O 2 (20 sccm) and tetramethyldisiloxane (TMDSO, Sigma Aldrich) as organosilicon precursor was used for deposition. 33, 36 The PET background is placed 10mm from the nozzle, where the gas temperature is reduced to 310 K, which is very important for PET nonwovens. The plasma head is mounted on a robotic arm (Stepcraft 300, Germany) to achieve large-scale treatment. Scanning uniformity for large area deposition was confirmed using silicon wafers as the substrate.
Preparation of nano silver PET fabric
Nano silver nonwoven PET fabric is prepared by a three-step process. Non-woven PET fabric (DuPont, Spain) was cut into 3 cm x 3 cm size before silver fixation. At first, an organosilicon thin film is deposited on the surface of fabrics using a plasma deposition system. This 70 nm layer was used as a reserve layer for silver immobilization and to control the adhesion of silver nanoparticles to PET fibers. The thickness of the protective and barrier layers in this work was determined in a set of independent experiments on silicon wafers as it was described in our previous work. 36
In the next steps, samples with the upper polymerized plasma layer were immersed in a suspension of nano silver in ethanol and raised to dry. Silver nanoparticles (SSNANO, USA) of size 20 nm with 99.95% purity (trace metal basis) were used throughout the experiments as purchased. The color of the fabric changed from white to gray after incorporating AgNPs. In the final step of the process, a second organosilicon film is deposited using a plasma jet system. This second layer is used as a barrier to prevent the release of AgNPs. Two different film thicknesses (10 and 50 nm) will be tested in this study.
Surface properties of nano silver PET fabric
The surface chemistry of PET fabrics was determined by X-ray photoelectron spectroscopy (XPS) on a Versaprobe II system (Physical Electronics (PHI), USA) equipped with a monochromatic X-ray source AlAl K α ( hν = 1486.6 eV) operated at 23.3 W. The pressure in the analysis chamber was maintained below 10 −6 Pa during the analysis and the photoelectrons detected by the hemispherical analyzer were placed at an angle of 45° to the normal to the sample surface. The survey scans and individual high-resolution scans (C1s, O1s and Si2p) were recorded with transfer energies of 117.4 eV and 23.5 eV, respectively. Particles present on PET surface were identified from XPS survey scans, performed on 3 different analytical areas (200 μm x 600 μm) per sample. The resulting elements were quantified using Multipak software using the Shirley platform and applying relative sensitivity factors provided by the manufacturer of the XPS device. Multipak software was also applied to curve fit the high resolution C1s peaks after the hydrocarbon composition of the C1s spectrum (285.0 eV) was used for energy scale calibration. In the next step,
SEM analysis was performed to investigate the morphology of nano silver in PET samples. Samples were Au coated with JFC-1300 Automatic Fine Coating (JEOL, Belgium) to avoid charge effect. After gold coating, PET fabric was studied using InTouch Scope JSM-6010 SEM (JEOL, Belgium).
Antibacterial efficacy of nano silver PET fabrics
Before testing, all samples were sterilized by exposure to ultraviolet light for 30 min. The microorganisms were cultured on Tryptic soybean agar (TSA) (Oxoid, Drongen, Belgium) ( P. aeruginosa and S. aureus ) or Sabouraud agar (Sab) (BD, Franklin Lakes, NJ) ( C. albicans ) under aerobic conditions at 37 °C. Using sterile forceps, samples were placed into the wells of a 24-well microtiter plate and then 1 ml of cell suspension, containing approx. 10 4 colony-forming units (CFU)/ml was added. Plates were incubated for 24 h at 37 °C. After incubation, samples were transferred to 10 ml of 0.9% NaCl (w/v) and subjected to three cycles of 30 s vortex mixing and 30 s sonication. Ten-fold serial dilutions were performed in 0.9% (w/v) NaCl and the number of CFUs was determined by plate counting. To this end, one ml of each diluent was plated on TSA or Sab and the plates were incubated at 37 °C for 48 h.
