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Controlled hydrothermal synthesis of Ag nanowires and their antimicrobial properties
⁎Corresponding author at: No. 22, Qixiangtai Rd., Heping Dist., Tianjin 300070, China. wangbaiqi@tmu.edu.cn (Baiqi Wang)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Abstract
The therapeutic effect of existing antibacterial agents is weakening as bacterial resistance increases. Therefore, researchers should focus on exploring and developing new antibacterial materials. In this work, Ag nanowires (AgNWs) were synthesized using a hydrothermal method, and the optimal synthesis conditions were determined. The fabricated AgNWs were characterized, and their antibacterial effect was studied. The optimal AgNW synthesis conditions were as follows: reaction temperature of 130 °C, reaction time of 60 min, silver nitrate:trisodium citrate ratio of 1:7.5, and sodium dodecylsulfonate concentration of 1 mM. Under these conditions, the diameter and length of the synthesized AgNWs were approximately 40 ± 5 nm and approximately 10 ± 1.5 μm, respectively. The prepared AgNWs presented good dispersion and a face-centered cubic crystal structure, and nanocrystals preferentially grew along the (1 1 1) crystal plane direction. In addition, AgNWs exhibited a good antibacterial effect against Gram-positive (G+) bacteria, namely Staphylococcus aureus and Enterococcus faecalis, and Gram-negative (G-) bacteria, namely Escherichia coli and Klebsiella pneumoniae. The minimum inhibitory concentrations of AgNWs for the aforementioned bacteria strains were 9.38, 18.75, 4.69, and 1.17 μg/mL, respectively. Therefore, AgNWs were more sensitive against G- bacteria than against G+ bacteria. This work provides a good foundation for the use of AgNW as nanomaterial antibacterial agents.
Keywords
Ag nanowires
Hydrothermal method
Controlled synthesis
Antibacterial
Nanomaterial
1 Introduction
Antibiotics can be credited for the significant increase in life expectancy worldwide in the 20th century. JM Conly and BL Johnston noted that The discovery of penicillin by Fleming in 1928 followed by the discovery and clinical use of sulphonamides in the 1930s heralded the age of modern antibiotherapy. Penicillin came into widespread use during the early 1940s (Conly and Johnston, 2005). However, the extensive use of antibacterial drugs has led to an increase in number of multidrug-resistant bacteria and superbacteria; moreover, bacteria resistance to existing antibiotics and disinfectants has caused great difficulties in treating and preventing infections (Cohen, 1992; Grant et al., 1994; Nikaido, 2009; Boucher et al., 2009). The development of new bactericides and disinfectants to prevent opportunistic pathogens in the environment from developing drug resistance is a current and critical need in the infectious disease prevention and control field. Furthermore, this is a public health problem (Leighow et al., 2020). The increase in number of multidrug resistant bacteria poses great challenges to finding suitable disinfectants. Therefore, medical researchers need to develop new agents to prevent bacteria from developing drug resistance and ensure effective disinfection (Chlemann and Krishna, 2005; Cohen, 1992; Walsh, 2003).
Nanomaterials have exhibited great potential for biosensors, cancer diagnosis, disease treatment, and targeted drug delivery because of their unique physical and chemical properties (Wijnhoven et al., 2009; Zhang et al., 2017; Ahamed et al., 2010; Monopoli et al., 2012; Tetsurou et al., 2020; Nakamura et al., 2019; Chen and Schluesener, 2008; Jain, 2003). In the infectious disease prevention and control field, Ag nanomaterials have attracted increasing attention as new bactericides (Baker et al., 2005; Brandão et al., 2017; Hajipour et al., 2013; Rai et al., 2009). Bezza et al. (2020) Ag nanomaterials present a remarkable antimicrobial effect and broad application prospects in the medical treatment field. Sadanand et al. (2017) coated nanosized Ag on cotton fabric and studied its antimicrobial effect. They determined that Ag nanomaterials presented a good antimicrobial effect on Gram-positive (G+) and Gram-negative (G-) bacteria and proposed that Ag nanomaterials could be widely used for surgical instruments and gowns and dressings for burn patients to prevent bacterial infection. Tetsurou et al. (2020) and Acosta-Torres et al. (2012) added nanosilver to polymethyl methacrylate (PMMA) and determined that nanosilver-containing PMMA-based denture materials presented an antifungal effect. To date, the use of Ag nanoparticles (AgNPs) has been widely studied for applications for infant nipples, antibacterial toothpaste, and antibacterial supplements for burn patients (Dentistry - Dental Technology, 2017; Singh et al., 2015; Sadeghnejad et al., 2014). Therefore, Ag nanomaterials could play important roles in the prevention, control, and treatment of bacterial infections.
Ag nanomaterials present broad-spectrum and long-lasting antibacterial properties and provide long-term disinfection. Ag nanowires (AgNWs) are one of the most important and promising Ag nanomaterials. Nanowires can be defined as one-dimensional structures less than 100 nm in size (Li et al., 2003). Because AgNWs present excellent conductivity, they can be used to fabricate ultrasmall circuits, which are widely used in smart phones, tablets, computers, and other touch screen electronics (Silva et al., 2014). Owing to their nanometer size, AgNWs present excellent light transmittance and flexibility and, therefore, they can be used for thin-film solar cells (Liu and Yu, 2011). In addition, because of their large aspect ratio, AgNWs present advantages when used for conductive and thermally conductive adhesives. Furthermore, owing to their strong antibacterial properties and low toxicity, AgNWs can be widely used for nanosilver-containing products, such as aseptic apparatuses and equipment, medical imaging equipment, functional textiles, antibacterial drugs, and biosensors (Richard and Roger, 2018; Kiran and Lin, 2016; Singh et al., 2013). AgNWs present a relatively large aspect ratio, which results in different surface and adsorption energies on different surfaces (Alhmoud et al., 2016). In addition, some studies have demonstrated that one-dimensional nanomaterials possess strong antibacterial capabilities and excellent antibacterial effects (Kaimlová et al., 2018; Richard and Roger, 2018). However, studies on the synthesis and antibacterial effects of AgNWs have been scarce because of unfavorable factors, such as preparation conditions, product yield, and cost. Moreover, the antibacterial mechanism of AgNWs is not well understood. Therefore, more studies should be performed to prepare AgNWs with good morphology, uniform size, and excellent antibacterial properties and to elucidate the antibacterial mechanism of AgNWs and promote their applications.
To date, several methods have been used to prepare Ag nanomaterials, including the microwave heating (Si et al., 2008), polyol (Wang et al., 2019; Zhao et al., 2010), oxidation–reduction (Zhao et al., 2016), and hydrothermal (Yang et al., 2010) methods. Among these methods, the hydrothermal method has attracted significant attention because of its advantages, such as controllable and mild reaction conditions, uniform particle size distribution of the prepared nanomaterials, and low cost. Therefore, in this study, we synthesized AgNWs using a hydrothermal method. The AgNWs fabricated by controlling the reaction time and temperature and amount of redox agent presented good morphology, uniform size, and excellent physical and chemical properties. In addition, the as-prepared AgNWs presented excellent antibacterial properties against Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, and Klebsiella pneumoniae. The flowchart of this study is presented in Fig. 1. We believe that this study provides significant theoretical guidance and a useful practical foundation for the large-scale applications of AgNWs.
2 Materials and methods
2.1 Chemical reagents
Silver nitrate (AgNO3 solution, 0.1 M), trisodium citrate dihydrate (Na3C6H5O7·2H2O, 99.0% purity), and sodium dodecylsulfonate (SDSN) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., China. AgNPs for blank control experiments were purchased from Dalian Meisia New Materials Co., Ltd., China. All chemical reagents were of analytical grade and were used as received without further purification.
2.2 Synthesis of Ag nanomaterials
First, 1.5 mL of a Na3C6H5O7 solution with a certain concentration was slowly added to a 0.1 M AgNO3 solution. The color of the mixture changed gradually from opalescent to white and colorless. Thereafter, 100 mL of deionized water and 1 mL of a SDSN solution with a certain concentration were added in sequence to the colorless solution at room temperature. Subsequently, the mixture was transferred to a Teflon-lined autoclave and was maintained at 115–145 °C for 60–180 min. After the system was naturally cooled to room temperature, the color of the mixture became yellow-gray. Next, the mixture was washed, centrifuged, and dried to obtain Ag nanomaterials with different morphologies and properties.
2.3 Sample characterization
The crystal structure of the Ag nanomaterial powder was analyzed using a D/Max2500 (Rigaku, Japan) X-ray diffraction (XRD) apparatus with Cu Kα radiation in the 2θ range of 20–80°. The morphology of the samples was analyzed using a HT7700 (Hitachi, Japan) transmission electron microscopy (TEM) instrument. The optical absorption performance of the samples was investigated using a U-3900 (Hitachi, Japan) ultraviolet–visible (UV–Vis) spectrophotometer. E. faecalis (ATCC 29212), S. aureus (ATCC 29213), E. coli (ATCC 25922), and K. pneumoniae (700603) strains were obtained from the Microbiology Laboratory of the Tianjin Stomatological Hospital, China.
2.4 Antimicrobial effect of Ag nanomaterials
Broth culture medium of standard concentration was prepared according to the manufacturer’s instructions, followed by sterilization at 121 °C for 15 min and natural cooling to room temperature. E. faecalis, S. aureus, E. coli, and K. pneumoniae were inoculated onto blood agar plates and incubated for 24 h. An appropriate amount of bacterial strains was collected with an inoculating loop to prepare 0.5 McFarland standard bacterial suspensions with sterile nutrient broth. Mueller–Hinton broth medium (100 μL) was added to each well of a 96-well plate, and a 1200 μg/mL solution of Ag nanomaterial broth culture medium (100 μL) was added to the first wells of rows A, B, C, and D of the well plate. The mixtures were blown at least three times using a pipette to evenly mix the test solution and nutrient broth. Thereafter, 100 μL of the suspension in the first row of wells was transferred to the second row of wells, and 100 μL of broth culture medium was added to the wells. The mixed solutions were blown with a pipette at least three times to evenly mix the test solution and nutrient broth. Thereafter, the concentration of AgNWs was consecutively reduced by half until the eleventh row of wells. Next, 100 μL of the suspension in the eleventh well was removed and discarded. Only bacteria suspensions and broth were added to the twelfth row of wells, which served as the positive control group. E. faecalis, S. aureus, E. coli, and K. pneumoniae suspensions (100 μL) were added to rows A, B, C, and D, respectively. Only broth culture medium was added to the wells in row E, which served as the negative control group. Subsequently, the 96-well plate was placed in a thermostat incubator at 37 °C for 16–24 h, bacterial suspensions were inoculated onto corresponding Petri dishes, and culturing continued for 24 h at 37 °C (Tetsurou et al., 2020). The concentration of AgNWs in the last well where no bacteria grew was recorded as the minimum inhibitory concentration (MIC).
3 Results and discussion
3.1 Optimal AgNW synthesis conditions
3.1.1 Reaction temperature
Reaction temperature significantly affects nanomaterial morphology. Therefore, the effects of reaction temperature on the morphology of Ag nanomaterials was studied. The TEM images of the Ag nanomaterials synthesized at 115, 130, and 145 °C are illustrated in Fig. 2.
Quasi-spherical AgNPs with a diameter of approximately 60 nm were prepared at 115 °C (Fig. 2a), AgNWs with a diameter of approximately 40 ± 5 nm and a length of approximately 10 ± 1.5 μm were fabricated at 130 °C (Fig. 2b), and a mixture of irregular AgNPs and Ag nanorods (AgNRs) was formed at 145 °C (Fig. 2c). Feng et al. also reported that the Ag nanomaterials they synthesized exhibited multiple morphological features when the reaction temperature exceeded 140 °C (Feng et al., 2018). Therefore, when the reaction temperature was increased from 115 to 145 °C, the morphology of the fabricated Ag nanomaterials changed from AgNPs to AgNWs to AgNPs. During the growth process, the sizes of AgNPs was controlled to minimize their surface free energy (Shu-Hong et al., 2005). Based on this mechanism, by changing the reaction temperature, the free energy of the reduced Ag atoms should also change. Higher reaction temperatures would lead to an increase in the diameter of the AgNWs. Therefore, the shape of the Ag nanomaterials changed from AgNPs to AgNWs when the temperature was increased from 115 to 130 °C. However, when the reaction temperature was further increased, the difference in growth rate between different crystal facets of the Ag nanomaterials decreased rapidly. This caused the formation of AgNPs at 145 °C. Considering the changes in morphology of the synthesized Ag nanomaterials, the optimal reaction temperature was 130 °C.
3.1.2 Reaction time
Reaction time, which is an important factor for nanomaterial preparation, can affect the morphology of nanomaterials. In this study, we analyzed the morphology of the Ag nanomaterials prepared at 60, 120, and 180 min while maintaining the other reaction conditions unchanged. The TEM images of the fabricated Ag nanomaterials are presented in Fig. 3.
The morphologies of the Ag nanomaterials fabricate at 130 °C, a SDSN concentration of 1 mM, a AgNO3:Na3C6H5O7 ratio of 1:7.5, and different reaction times were different. The Ag nanomaterial prepared at a reaction time of 60 min consisted of uniform AgNWs approximately 40 nm in diameter and several micrometers in length. When the reaction time was increased to 120 min, the fabricated Ag nanomaterials consisted of a mixture of quasi-nanoparticles approximately 70 nm in diameter and a few AgNRs. When the reaction time was increased to 180 min, the synthesized Ag nanomaterials consisted of spherical AgNPs approximately 90 nm in diameter. Therefore, the optimal reaction time to fabricate AgNWs was 60 min.
3.1.3 AgNO3:Na3C6H5O7 ratio
The amount of Na3C6H5O7 is an important parameter for the reaction system in this study; furthermore, the AgNO3:Na3C6H5O7 ratio can affect the shape of the prepared Ag nanomaterials. The TEM images of the as-prepared Ag nanomaterials at AgNO3:Na3C6H5O7 ratios of 1:1, 1:3.75, and 1:7.5 are illustrated in Fig. 4.
When the AgNO3:Na3C6H5O7 ratio was 1:1, the prepared Ag nanomaterials consisted of spherical AgNPs with a diameter of approximately 9 nm. When the AgNO3:Na3C6H5O7 ratio was 1:3.75, the fabricated Ag nanomaterials consisted of a mixture of torispherical AgNPs with a diameter of approximately 100 nm and AgNRs with a diameter of approximately 30 nm and a length of 500 nm. When the AgNO3:Na3C6H5O7 ratio was 1:7.5, the synthesized Ag nanomaterials consisted of AgNWs. Therefore, the prepared Ag nanomaterials gradually changed from AgNPs to AgNWs with increasing AgNO3:Na3C6H5O7 ratio. In the reaction system, Na3C6H5O7 acted not only as reducing agent, but also as a stabilizing agent (Hu et al., 2004). The AgNO3:Na3C6H5O7 ratio played a critical role in the formation of AgNWs. The concentration of Ag monomer from the reduction of Na3C6H5O7 controlled the growth pattern of Ag nanomaterials and reaction rate. According to the Ostwald ripening theory, when the AgNO3:Na3C6H5O7 ratio was relatively low (1:1 or 1:3.75), the formation of Ag nanospheres was favored. Small AgNPs nanoparticles dissolved in solution and grew into large AgNPs. However, when the AgNO3:Na3C6H5O7 ratio was 1:7.5, the Ag monomer concentration in the reaction mixture increased, and the growth of Ag nanomaterials was diffusion-controlled. As the Ag crystals grew, SDSN was dynamically adsorbed and desorbed on the surface of Ag particles. This was conducive to the formation of AgNWs with good crystallinity and prevented aggregation. The effect of the AgNO3:Na3C6H5O7 ratio on the growth of Ag nanomaterials is illustrated in Fig. 5.
3.1.4 Surfactant concentration
The amount of SDSN used for hydrothermal synthesis can affect the morphology and properties of the as-prepared Ag nanomaterials. The TEM images of the Ag nanomaterials prepared using different SDSN concentrations (0, 1, 1.5, and 2 mM) are presented in Fig. 6.
When no SDSN was added to the reaction system, the prepared Ag nanomaterial consisted of spherical AgNPs with an average diameter of approximately 60 nm (Fig. 6a). At an SDSN concentration of 1 mM, the synthesized Ag nanomaterial consisted of AgNWs with an average diameter and length of approximately 40 ± 5 nm and 10 ± 1.5 μm, respectively (Fig. 6b). When the SDSN concentration was increased 1.5 mM, the prepared Ag nanomaterial consisted of a mixture of AgNPs with an average diameter of approximately 30 nm and AgNRs (Fig. 6c). When the SDSN concentration was increased to 2 mM, the AgNRs disappeared from the reaction mixture, and the prepared Ag nanomaterial consisted of spherical AgNPs with a diameter of approximately 100 nm (Fig. 6d). Therefore, the shape of the synthesized Ag nanomaterials changed from AgNPs to AgNWs with increasing SDSN concentration. However, when the SDSN concentration was further increased, the prepared Ag nanomaterials consisted of spherical AgNPs. This was attributed to SDSN solutions of certain concentrations providing a soft template for Ag nanomaterials and promoting the growth of AgNWs. However, when the SDSN concentration was too high, the excess SDSN molecules capped Ag nanocrystals and prevented their further growth along a crystal plane (Hu et al., 2004).
3.2 Controlled synthesis of AgNWs
The aforementioned results indicated that the optimal reaction conditions consisted of a reaction time of 60 min, reaction temperature of 130 °C, AgNO3:Na3C6H5O7 ratio of 1:7.5, and SDSN concentration of 1 mM. Under these conditions, AgNWs were synthesized, and the TEM images of the fabricated AgNWs are illustrated in Fig. 7. The AgNWs synthesized under optimal reaction conditions exhibited good dispersity. The length and diameter of the AgNWs were approximately 40 ± 5 nm and 10 ± 1.5 μm, respectively. Djadidi et al. studied the toxicity of AgNWs with different lengths (4, 10, and 20 μm) and demonstrated that 10 μm AgNWs were less toxic than AgNWs with other lengths (Djadidi et al., 2019). Moreover, they proposed the “safe-by-design” concept concerning the biosafety of nanomaterials. Therefore, the as-prepared AgNWs should present low toxicity. Furthermore, AgNWs with good morphology and low toxicity should provide a good foundation for subsequent studies.
3.2.1 Crystal structure analysis
The crystal structure of the AgNWs prepared under optimal conditions was analyzed using XRD, and the diffraction pattern is presented in Fig. 8. Four strong diffraction peaks at 2θ = 38.1°, 44.2°, 64.4°, and 77.4° were observed in the XRD pattern of the as-synthesized AgNWs, which were attributed to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystal planes of face-centered cubic phase Ag (JCPDS 04-0783). In addition, the intensities of the aforementioned diffraction peaks decreased as follows: (1 1 1) > (2 0 0) > (2 2 0) > (3 1 1). This revealed that the (1 1 1) crystal direction was the preferred growth direction of AgNWs and was ascribed to Ag crystals growing much faster along the (1 1 1) crystal plane than along the other crystal planes under the optimal reaction conditions in this study (Sun et al., 2003).
3.2.2 UV–Vis absorption capacity of AgNWs
Optical absorption capacity is one of the most important physicochemical properties of AgNWs, and in this study, it was analyzed using UV–Vis spectroscopy. The UV–Vis spectrum of the as-synthesized AgNWs is presented in Fig. 9. The optical features observed in the UV–Vis spectrum correlated well with the TEM images. The peak at approximately 420 nm was attributed to the plasmon resonance of AgNWs (Sun et al., 2002). The shoulder peak at approximately 350 nm was attributed to the plasmon resonance of AgNPs. Moreover, the intensity of the peak at approximately 420 nm was significantly stronger than that of the peak at approximately 350 nm. This indicated that the final product synthesized under optimal reaction conditions was a mixture of AgNWs and AgNPs, but AgNWs were dominant (Yang et al., 2010), which was in good agreement with the aforementioned characterization results.
3.3 Antimicrobial capacity of AgNWs
3.3.1 Selection of target bacteria
The increasing drug resistance of G+ bacteria is concerning because G+ bacteria are responsible for one third of nosocomial infections worldwide (Almanaa et al., 2020). S. aureus is a clinically common pathogenic bacterium that can cause a variety of diseases, such as skin septic infections, pneumonia, pericarditis, septicemia, and sepsis. Currently, the multidrug resistance of S. aureus is a major concern (Andrea et al., 2020). Moreover, the emergence of methicillin-resistant S. aureus (MRSA), an extremely harmful bacterium, has posed significant challenges in clinical anti-infection treatment. MRSA is a common multidrug resistant bacterium in iatrogenic infections that affect intensive care unit and immunodeficient patients (Matteo, 2016). Furthermore, antibacterial drug selection for clinical diagnosis and treatment is challenging (Sadeghnejad et al., 2014). E. faecalis; which is a G+ and hydrogen peroxide-negative coccus, is one of the main bacterium in human and animal intestinal flora. However, the multidrug resistance of E. faecalis is increasing, and the relationship among virulence factors is complex. Moreover, the infection transmission mechanism from animals to humans is complicated. Therefore, research on the development of antibacterial drugs against E. faecalis has become a global research hot spot.
G- E. coli is the main pathogenic bacterium responsible for common digestive tract infections (Huijbers et al., 2020; Wiener et al., 1999). First, E. coli was considered to be a typical intestinal flora bacterium. However, in the mid-19th century, it was demonstrated that E. coli could cause diarrhea, septicemia, and other infections in humans. E. coli is mainly transmitted through the digestive tract, and contaminated food and water are the main transmission routes. Therefore, E. coli is used as an important food and water pollution indicator. Studies have revealed that E. coli is resistant to antimicrobial drugs, and E. coli is used as a monitoring indicator for evaluating the resistance of G- bacteria internationally (Nyirabahizi et al., 2020). K. pneumoniae is highly pathogenic to humans and can cause bloodstream, surgical site, respiratory tract, and digestive tract infections (Aires-de-Sousa et al., 2019; Wang et al., 2018).
The aforementioned types of bacteria are highly resistant to current antibacterial drugs. Therefore, in this study, we evaluated the antimicrobial effect of the synthesized AgNWs on these common nosocomial infection pathogens.
3.3.2 Antimicrobial effect of AgNWs
The double dilution method was used to determine the MIC of AgNWs against the selected bacteria strains and evaluate the antibacterial effect of AgNWs (Andrea et al., 2020). AgNWs suspensions of different concentrations were added to a 96-well plate containing bacterial suspension, and the plate was incubated at 37 °C for 24 h. The growth of S. aureus, E. faecalis, E. coli, and K. pneumoniae was significantly inhibited (Fig. 10). Bacteria growth in each well is summarized in Table 1. S. aureus, E. faecalis, E. coli, and K. pneumoniae stopped growing when the concentration of AgNWs was higher than 9.38, 18.75, 4.69, and 1.17 μg/mL, respectively. Therefore, the MICs of AgNWs against S. aureus, E. faecalis, E. coli, and K. pneumoniae were 9.38, 18.75, 4.69, and 1.17 μg/mL, respectively. To confirm these results, a sterile inoculation loop was used to mix the suspensions, followed by collecting appropriate amounts of mixtures and transferring them to Petri dishes for further culture at 37 °C for 24 h, and the results are presented in Fig. 11. For S. aureus, E. faecalis, and E. coli, the bacteria in the first, second, third, fourth, fifth, sixth, and seventh wells were further inoculated. For K. pneumoniae, the bacteria in the first, fifth, sixth, seventh, eighth, nineth, tenth, and eleventh wells were further inoculated. The results were in good agreement with the data in Table 1. In addition, we determined that the MICs of AgNWs against G- bacteria (E. coli and Klebsiella pneumoniae) were lower than those against G+ bacteria (S. aureus and E. faecalis). Therefore, AgNWs were more sensitive to and presented a stronger antimicrobial effect on G- bacteria. This was attributed to the lipopolysaccharides of G- bacteria presenting more affinity for AgNWs than the cellular wall of G+ bacteria, and lipopolysaccharides trapping and blocking Ag+ cations (Djadidi et al., 2019).
| Bacteria | Well location in 96-well plate | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1st | 2nd | 3rd | 4th | 5th | 6th | 7th | 8th | 9th | 10th | 11th | 12th | |
| AgNW concentration (μg/L) | ||||||||||||
| 150.00 | 75.00 | 37.50 | 18.75 | 9.38 | 4.69 | 2.34 | 1.17 | 0.59 | 0.29 | 0.15 | 0.00 | |
| Staphylococcus aureus | — | — | — | — | — | ＋ | ＋ | ＋ | ＋ | ＋ | ＋ | ＋ |
| Enterococcus faecalis | — | — | — | — | ＋ | ＋ | ＋ | ＋ | ＋ | ＋ | ＋ | ＋ |
| Escherichia coli | — | — | — | — | — | — | ＋ | ＋ | ＋ | ＋ | ＋ | ＋ |
| Klebsiella pneumoniae | — | — | — | — | — | — | — | — | ＋ | ＋ | ＋ | ＋ |
Note: + represents bacteria growth; — represents no bacteria growth.
To further study the antibacterial mechanism of AgNWs, we performed comparative experiments using AgNWs, AgNPs, and AgNO3 solutions with the same concentration, and the results are presented in Table 2. The MICs of AgNO3 against S. aureus, E. faecalis, E. coli and K. pneumoniae were 13.29, 26.57, 6.64, and 0.41 μg/mL, respectively. The MICs of AgNPs against these bacteria strains were greater than 54 μg/mL, and the MICs of AgNWs against the aforementioned bacteria were 9.38, 18.75, 4.69, and 1.17 μg/mL, respectively. Overall, the MICs of AgNWs, AgNPs, and AgNO3 against S. aureus, E. faecalis, and E. coli decreased as follows AgNWs > AgNO3 > AgNPs. These results indicated that the antibacterial effect of AgNWs was higher than those of AgNO3 and AgNPs. The antibacterial effect of the aforementioned Ag species against K. pneumoniae followed a different trend, which was ascribed to the cells of K. pneumoniae lacking flagella. (Regue et al., 2004). To date, the antibacterial effect of Ag nanomaterials was attributed to their ability to release Ag+ ions in solution. AgNPs are isotropic. When proper surface modifiers are used, Ag NPs rarely release Ag+ ions in solution. Conversely, AgNWs are anisotropic, and the activation energies of Ag+ ions on different crystal faces are different. Even if surface modifiers were used to protect the surface of AgNWs, a fraction of Ag+ ions could still be easily released into solution (Yan-Min et al., 2017). Furthermore, the antibacterial effect of AgNWs is stronger than that of AgNPs.
| Bacteria | AgNO3 (μg/mL) | AgNPs (μg/mL) | AgNWs (μg/mL) |
|---|---|---|---|
| Staphylococcus aureus | 13.29 | >54 | 9.38 |
| Enterococcus faecalis | 26.57 | >54 | 18.75 |
| Escherichia coli | 6.64 | >54 | 4.69 |
| Klebsiella pneumoniae | 0.41 | >54 | 1.17 |
In addition, Ag nanomaterials exhibit broad-spectrum antibacterial and bactericidal activities. The schematic diagram of the possible antibacterial mechanism of AgNWs, which consisted of several processes, is presented in Fig. 12. The antibacterial activity of AgNWs mainly depends on the release of Ag+ ions, which present good antimicrobial activity, in solution. Ag+ ions can combine with oxygen or nitrogen electron donors in biomolecules, interact with the sulfhydryl groups of enzymes or proteins, and denature DNA (Kumar and Münstedt, 2005; Feng et al., 2000). Moreover; Ag atoms on the surface of AgNWs can react with oxygen dissolved in water to form silver oxide. Subsequently, silver oxide can release Ag+ ions, which present antibacterial properties. Furthermore, AgNWs can adhere to cell membranes and block the permeability and respiratory function of cells. In particular, AgNWs can strongly interact with S-containing proteins or P-containing biomolecules and promote cell apoptosis (Liu et al., 2013). In addition, AgNWs can react with membrane surfaces and penetrate cells. AgNWs interact with cell membrane receptors and enter cells through endocytosis. Once AgNWs are exposed to the acidic environment of cell lysosomes, they directly induce the production of reactive oxygen species. This can promote cell rupture and trigger apoptosis (Guo et al., 2013; Sondi and Salopek-Sondi, 2004; Pal et al., 2007).
4 Conclusions
The hydrothermal method was used to synthesize AgNWs by controlling the reaction conditions. The AgNWs synthesized under optimal conditions, namely a reaction temperature of 130 °C, reaction time of 60 min, AgNO3:Na3C6H5O7 ratio of 1:7.5, and SDSN concentration of 1 mM were approximately 40 ± 5 nm in diameter and 10 ± 1.5 μm in length. The synthesized AgNWs presented a good dispersibility and face-centered cubic microcrystalline structure and preferentially grew along the (1 1 1) crystal plane. In addition, the fabricated AgNWs exhibited good antibacterial effects against G+ bacteria (S. aureus and E. faecalis) and G- bacteria (E. coli and K. pneumoniae). The MICs of AgNWs against the aforementioned bacteria were 9.38, 18.75, 4.69, and 1.17 μg/mL, respectively. Therefore, AgNWs were more sensitive toward G- bacteria than toward G+ bacteria. This study provides a good theoretical exploration and solid experimental foundation for the research and applications of AgNWs.
Acknowledgements
This work was supported by the Natural Sciences Foundation of Tianjin City of China [grant number 12JCYBJC19100] and the Open Project of the Key Laboratory of Modern Toxicology of the Ministry of Education, Nanjing Medical University [grant number NMUMT201808].
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References
- Cytocompatible antifungal acrylic resin containing silver nanoparticles for denture. Int. J. Nanomed.. 2012;7:4777-4786.
- [CrossRef] [Google Scholar]
- Silver nanoparticle applications and human health. Clin. Chim. Acta. 2010;411(23–24):1841-1848.
- [CrossRef] [Google Scholar]
- Epidemiology of carbapenemase-producing Klebsiella pneumoniae in a Hospital, Portugal. Emerg. Infect. Dis.. 2019;25(9):1632-1638.
- [CrossRef] [Google Scholar]
- Antibacterial properties of silver dendrite decorated silicon nanowires. RSC Adv.. 2016;6(70):65976-65987.
- [CrossRef] [Google Scholar]
- The extreme drug resistance (XDR) Staphylococcus aureus strains among patients: a retrospective study. Saudi J. Biol. Sci.. 2020;27(8):1985-1992.
- [CrossRef] [Google Scholar]
- Characterisation of antibiotic resistance, virulence, clonality and mortality in MRSA and MSSA bloodstream infections at a tertiary-level hospital in Hungary: a 6-year retrospective study. Anna. Clin. Microbiol. Antimicrob.. 2020;19(1):17.
- [CrossRef] [Google Scholar]
- Synthesis and antibacterial properties of silver nanoparticles. J. Nano Nanotechnol.. 2005;5(2):244-249.
- [CrossRef] [Google Scholar]
- Synthesis of biosurfactant stabilized silver nanoparticles, characterization and their potential application for bactericidal purposes. J. Hazard. Mater.. 2020;393:122319
- [CrossRef] [Google Scholar]
- Bad bugs, no drugs: no ESKAPE! An update from the Infectious Diseases Society of America. Clin. Infect. Dis.. 2009;48(1):1-12.
- [CrossRef] [Google Scholar]
- Brandão, S.B., Franco, P.M., Grama, H.M., de ARSC, Augusto, J.F., Danil, G.R., Bittencourt, B.S., 2017. In vitro evaluation of the remineralizing potential and antimicrobial activity of a cariostatic agent with silver nanoparticles. Braz. Dental J. 28(6), 738–743. doi:10.1590/0103-6440201701365.
- Nanosilver: A nanoproduct in medical application. Toxicol. Lett.. 2008;176(1):1-12.
- [CrossRef] [Google Scholar]
- Antimalarial multi-drug resistance in Asia: mechanisms and assessment. Curr. Top. Microbiol. Immunol.. 2005;295:39-53.
- [CrossRef] [Google Scholar]
- Epidemiology of drug resistenc:implications for a post-antimicrobial era. Science. 1992;257(5073):1050-1055.
- [CrossRef] [Google Scholar]
- Epidemiology of drug resistance: implications for a post-antimicrobial era. Science. 1992;257(5073):1050-1055.
- [CrossRef] [Google Scholar]
- Where are all the new antibiotics? The new antibiotic paradox. Can. J. Infect. Dis. Med. Microbiol.. 2005;16(3):159-160.
- [CrossRef] [Google Scholar]
- Dentistry - Dental Technology, 2017. Study Data from Jilin University Provide New Insights into Dental Technology (Antibacterial properties of nano-silver coated PEEK prepared through magnetron sputtering). Biotech Business Week 33(9), E348-E360. doi:10.1016/j.dental.2017.06.014
- Facile synthesis of silver nanoparticles with high antibacterial activity. Materials. 2018;11(12):2498.
- [CrossRef] [Google Scholar]
- A mechanistic study of the antibacterial effect of silver ions on Escherichia coli and Staphylococcus aureus. J. Biomed. Mater. Res.. 2000;52(4):662-668.
- [CrossRef] [Google Scholar]
- Overexpression of Multidrug Resistance-associated Protein (MRP) increases resistance to natural product drugs. Cancer Res.. 1994;54(2):357-361.
- [CrossRef] [Google Scholar]
- Anti-leukemia activity of PVP-coated silver nanoparticles via generation of reactive oxygen species and release of silver ions. Biomaterials. 2013;34(32):7884-7894.
- [CrossRef] [Google Scholar]
- Antibacterial properties of nanoparticles (vol 30, pg 499, 2012) Trends Biotechnol.. 2013;31(1):61-62.
- [CrossRef] [Google Scholar]
- A simple and effective route for the synthesis of crystalline silver nanorods and nanowires. Adv. Funct. Mater.. 2004;14(2):183-189.
- [CrossRef] [Google Scholar]
- Surveillance of antibiotic resistant Escherichia coli in human populations through urban wastewater in ten European countries. Environ. Pollut.. 2020;261:114200
- [CrossRef] [Google Scholar]
- Nanodiagnostics: application of nanotechnology in molecular diagnostics. Expert Rev. Mol. Diagnostics. 2003;3(2):153.
- [CrossRef] [Google Scholar]
- Optimization of silver nanowire formation on laser processed PEN: surface properties and antibacterial effects. Appl. Surf. Sci.. 2018;473:516-526.
- [CrossRef] [Google Scholar]
- Preparation and characterization of bio-based hybrid film containing chitosan and silver nanowires. Carbohydr. Polym.. 2016;137:732-738.
- [CrossRef] [Google Scholar]
- Silver ion release from antimicrobial polyamide/silver composites. Biomaterials. 2005;26(14):2081-2088.
- [CrossRef] [Google Scholar]
- Multi-scale predictions of drug resistance epidemiology identify design principles for rational drug design. Cell Rep.. 2020;30(12):3951.
- [CrossRef] [Google Scholar]
- One-pot green synthesis and antimicrobial activity of exfoliated Ag NP-loaded quaternized chitosan/clay nanocomposites. RSC Adv.. 2013;3(25):9714.
- [CrossRef] [Google Scholar]
- Silver nanowire-based transparent, flexible, and conductive thin film. Nanoscale Res. Lett.. 2011;6(1):75.
- [CrossRef] [Google Scholar]
- Is first-line antimicrobial therapy still adequate to treat MRSA in the ICU? A report from a highly endemic country. Crit. Care (London, England). 2016;20:246.
- [CrossRef] [Google Scholar]
- Biomolecular coronas provide the biological identity of nanosized materials. Nat. Nanotechnol.. 2012;7(12):779-786.
- [CrossRef] [Google Scholar]
- Synthesis and application of silver nanoparticles (Ag NPs) for the prevention of infection in healthcare workers. Int. J. Mol. Sci.. 2019;20(15):3620.
- [CrossRef] [Google Scholar]
- Multidrug resistance in bacteria. Annu. Rev. Biochem.. 2009;78(1):119-146.
- [CrossRef] [Google Scholar]
- Evaluation of Escherichia coli as an indicator for antimicrobial resistance in Salmonella recovered from the same food or animal ceca samples. Food Control. 2020;115:107280
- [CrossRef] [Google Scholar]
- Does the antibacterial activity of silver nanoparticles depend on the shape of the nanoparticle? A study of the gram-negative bacterium Escherichia coli. Appl. Environ. Microbiol.. 2007;73(6):1712-1720.
- [CrossRef] [Google Scholar]
- Silver nanoparticles as a new generation of antimicrobials. Biotechnol. Adv.. 2009;27(1):76-83.
- [CrossRef] [Google Scholar]
- A gene, uge, is essential for Klebsiella pneumoniae virulence. Infect. Immun.. 2004;72(1):54-61.
- [CrossRef] [Google Scholar]
- Marta R: silver nanowires: synthesis, antibacterial activity and biomedical applications. Appl. Sci.. 2018;8(5):673.
- [CrossRef] [Google Scholar]
- Antibacterial cotton fabric with in situ generated silver nanoparticles by one-step hydrothermal method. Int. J. Polym. Anal. Charact.. 2017;22(3):275-279.
- [CrossRef] [Google Scholar]
- Antibacterial nano silver coating on the surface of polyethylene films using corona discharge. Surf. Coat. Technol.. 2014;245:1-8.
- [CrossRef] [Google Scholar]
- Growth of silver nanowires from solutions: a cyclic penta-twinned-crystal growth mechanism. J. Phys. Chem. B. 2005;109(19):9416-9421.
- [CrossRef] [Google Scholar]
- Research on nano-silver colloids prepared by microwave synthesis method and its SERS activity. Acta Photonica Sinica. 2008;37(5):1034-1037.
- [CrossRef] [Google Scholar]
- Short versus long silver nanowires: a comparison of in vivo pulmonary effects post instillation. Part. Fibre Toxicol.. 2014;11(1):52.
- [CrossRef] [Google Scholar]
- Silver nanowires as prospective carriers for drug delivery in cancer treatment: An in vitro biocompatibility study on lung adenocarcinoma cells and fibroblasts. Eur. J. Nanomed.. 2013;5(4):195.
- [CrossRef] [Google Scholar]
- Polyvinyl pyrrolidone/carrageenan blend hydrogels with nanosilver prepared by gamma radiation for use as an antimicrobial wound dressing. J. Biomater. Sci. Polym. Ed.. 2015;26(17):1269-1285.
- [CrossRef] [Google Scholar]
- Silver nanoparticles as antimicrobial agent: a case study on E. coli as a model for Gram-negative bacteria. J. Colloid Interface Sci.. 2004;275(1):177-182.
- [CrossRef] [Google Scholar]
- Crystalline silver nanowires by soft solution processing. Nano Lett.. 2002;2(2):165-168.
- [CrossRef] [Google Scholar]
- Transformation of silver nanospheres into nanobelts and triangular nanoplates through a thermal process. Nano Lett.. 2003;3(5):675-679.
- [CrossRef] [Google Scholar]
- Antibacterial properties of nano-Ag coating on healing abutment: an in vitro and clinical study. Antibiotics (Basel, Switzerland). 2020;9(6):347.
- [CrossRef] [Google Scholar]
- A toxicology-informed, safer by design approach for the fabrication of transparent electrodes based on silver nanowires. Environ. NANO. 2019;6:684-694.
- [CrossRef] [Google Scholar]
- Where will new antibiotics come from? Nat. Rev. Microbiol.. 2003;1(1):65-70.
- [CrossRef] [Google Scholar]
- Wang, H., Li, K.B., Xu, S.C., Li, G.H., 2019. Large-scale solvothermal synthesis of Ag nanocubes with high SERS activity. J. Alloys Compounds 772, 150–156. doi:10.1016/j.jallcom.2018.09.043
- Risk factors for carbapenem-resistant Klebsiella pneumoniae infection and mortality of Klebsiella pneumoniae infection. Chin. Med. J.. 2018;131(001):56-62.
- [CrossRef] [Google Scholar]
- Multiple antibiotic-resistant Klebsiella and Escherichia coli in nursing homes. J. Am. Med. Assoc.. 1999;281(6):517.
- [CrossRef] [Google Scholar]
- Nano-silver – a review of available data and knowledge gaps in human and environmental risk assessment. Nanotoxicology. 2009;3(2):109-138.
- [CrossRef] [Google Scholar]
- One-pot hydrothermal synthesis of silver nanowires via citrate reduction. J. Colloid Interface Sci.. 2010;352(2):285-291.
- [CrossRef] [Google Scholar]
- Surface ligand controls silver ion release of nanosilver and its antibacterial activity against Escherichia coli. Int. J. Nanomed.. 2017;12:3193.
- [CrossRef] [Google Scholar]
- Electrospinning of Ag Nanowires/polyvinyl alcohol hybrid nanofibers for their antibacterial properties. Mater. Sci. Eng. C. 2017;78:706-714.
- [CrossRef] [Google Scholar]
- Large-scale synthesis of size-controllable silver nanoplates and their application in detecting strong oxidants in aqueous solutions. Chem. Eng. J.. 2016;285:690-697.
- [CrossRef] [Google Scholar]
- Size-controlled preparation of silver nanoparticles by a modified polyol method. Colloids Surf., A. 2010;366(1–3):197-202.
- [CrossRef] [Google Scholar]