1. Introduction
Antibacterial metal and alloy implants have been widely used in order to reduce incidences of medical-implant-associated infections (IAIs) [1]. It has been reported that IAIs not only have become a public health problem but also affect the economy [2]. Metal and alloy implants possess many good mechanical properties, such as high strength, good wear resistance, and biocompatibility, thus resulting in extensive research into the development of antibacterial metal and alloy implants. Notwithstanding the excellent characteristics of the materials, it is critical to understand the interaction between implants and bacteria to ensure that effective antibacterial metal and alloy implants are developed. The interplay between the bacteria and implant can be categorized into four steps: (1) the initial adsorption of the bacteria onto the surface of the implants, (2) the bacterial colonization needs to occur on the implant surface, (3) the biofilm then forms and matures, and (4) the bacteria start to proliferate [1]. Apparently, the incidences of IAIs can be considerably reduced if metal and alloy implants prevent initial bacterial adhesion on the implant surface.
According to the literature information, metals such as copper (Cu) and silver (Ag) have been used as antibacterial materials for centuries due to their toxicity to microorganisms [3,4,5]. Although the antibacterial mechanisms of each metal might be different, the overall mechanisms include the following four possibilities [3]. (1) Metallic elements can cause protein dysfunction in bacteria. However, many proteins require metal ions to maintain their structure or to participate in key cellular reactions. Therefore, protein function abnormalities will occur if metal ions of the same valence electron are used to replace metals in protein. For instance, Lead (II) has been reported to replace the catalytic zinc (II), and nickel (II) has been shown to replace the zinc (II). This helps maintain the original protein structure, though it results in the dysfunction of protein activities [6,7]. (2) Genotoxicity can be the result of reactive oxygen species (ROS) and antioxidant depletion, which are generated by metal ions. Many studies have demonstrated that ferrous iron and ferrous copper can increase the production of intracellular ROS; for example, iron metabolism can damage deoxyribonucleic acid, resulting in genotoxicity in Escherichia coli (E. coli) [8]. (3) Bacterial membrane function can be impaired. Bacterial cell membranes contain negatively charged polymers and can be destroyed by using positively charged metal ions, such as silver or aluminum, to cause the loss of membrane potential [9]. (4) Metal elements can interfere with nutrient uptake of bacteria by triggering a starvation response. In this case, Chromium (IV) has been shown to inhibit the uptake of sulfate and the growth of yeast [1].
The level of toxicity of metals to microorganisms indicates the extent of damage(s) they can cause to human cells. Therefore, metallic alloys are more suitable because they achieve antibacterial effects while improving the mechanical properties of bulk metals. The alloying elements are primarily copper and silver due to their strong antibacterial effect on a broad range of microorganisms (bacteria, fungi, and viral pathogens) [1,3,10]. Rare earth elements are also a good choice for an antibacterial agent because low traces of rare earth elements can show good antibacterial activity. For example, it has been reported that only 0.5, 1.5, or 3 wt% Cerium (Ce)-doped 316L stainless steels demonstrated a strong antibacterial effect [11]. Ultimate tensile strength was considerably improved, and the creep rate was reduced when Ce was alloyed with biodegradable zinc, suggesting that alloys containing rare earth metals can be potentially used in biomedical applications [12].
There are four methods for preparing alloys. The first one is powder metallurgy, a method of processing metal or alloy powder into the desired shape by sintering. Porous alloys and high-performance composites can be prepared by powder metallurgy with improved properties achieved depending on the materials used during processing [13,14]. The second method is the melting method. In this approach, the metals or alloys are melted and formed into the desired shapes through cooling and solidifying. Melting must be conducted in a vacuum or inert gas environment to avoid contamination and to obtain high-purity alloys [15]. The third method is deposition, which is mainly used to create a thin layer of alloy. This method includes physical or chemical vapor deposition, electrophoretic deposition, ion beam deposition, etc. [16]. There are numerous advantages of deposition methods, depending on the method employed. For example, the electrophoretic deposition allows easy adjustments to the composition, thickness, and morphology of the alloys [17]. Lastly, the fourth method is mechanical alloying, where alloys are prepared by mechanically forcing metal powders to mix. Mechanical alloy methods include ball milling, cryogenic milling, high-energy ball milling, etc., which are used to prepare high-strength and high-toughness alloys [18]. Although different alloy preparation methods might require different verification tests, some characterization tests are similar. Thus, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and field-emission scanning electron microscopy are often applied to surface characteristics of alloys. The laser particle analyzer, computerized potentiostat, Vicker’s hardness tester, nano-indenter, and 3-point bending test are commonly used to measure the physical-chemical properties of alloys [19,20,21].
During the manufacturing process of alloy, intermetallic compounds (IMCs) can be produced by melting and casting alloys [22]. IMCs have specific geometric and electronic structures that are different from the random atomic arrangement in the solid solution of alloys [23]. Also, IMCs exhibit excellent catalytic properties, including increased activity, selectivity, and stability, resulting in significant progress in catalytic research [24]. For example, Cu atoms were removed from the ordered and disordered AlCu3 IMC by leaching to produce nanoporous alloys. The result showed that ordered AuCu3 IMC displayed more regular pores, resulting in higher catalytic properties for CO oxidation [25]. Research has shown that the crystallization states of metal, such as crystallinity, as well as the crystal plane, defects, and electronic structures, affect catalyst performance [26]. Because metals such as Cu, Ag, Ce, gold (Au), and iron (Fe) alone have been used as catalysts or antibacterial agents, exploring these alloys as antibacterial materials is imperative.
This study explores Cu48Ag48Ce4 alloys for their inhibiting activity on the growth of E. coli (Gram-negative) and Staphylococcus aureus (S. aureus, Gram-positive). These two bacteria are the most common pathogens for medical-implant-associated infection [27]. Different crystallization states of Cu48Ag48Ce4 alloys were generated in order to investigate the potential relationship between the surface microstructure of alloys and their antibacterial activity. Characterization techniques such as X-ray diffraction analysis were applied before and after the alloys were cultured with bacteria to analyze the changes in surface crystalline structures. The surface morphology of Cu48Ag48Ce4 alloys was observed using a scanning electron microscope, and the quantity of the residual metals was analyzed in order to understand the potential antibacterial mechanism of Cu-Ag-Ce alloys.
2. Materials and Methods
2.1. Preparation of Alloy Ribbons
The alloy Cu48Ag48Ce4, also known as CuAgCe4, was prepared by arc melting in a furnace (Yamato Scientific Co., Ltd., Tokyo, Japan) under an argon atmosphere. The raw materials used include copper (Cu, purity 99.99%, Kojundo Kagaku Co., Ltd., Saitama, Japan), silver (Ag, purity 99.99%, Furuuchi Kagaku Co., Tokyo, Japan), and Cerium (Ce, purity 99.9%, Furuuchi Kagaku Co.). The piece of each alloy was then placed into a quartz tube of 10 mmφ, and the sample was melted by a high-frequency induction heating coil. Subsequently, the liquid was ejected from the quartz capillary tube by argon and impinged onto the outer surface of a rotating copper roll (i.e., a single-roller melt spinning method) at 4000 revolutions per minute. As a result, the amorphous CuAgCe4 alloys were prepared in the shape of ribbons, and the crystallization of amorphous CuAgCe4 ribbons was performed at either 250 °C or 450 °C for three hours. For all the alloy ribbons, it was observed that one side was lighter than the other.
2.2. Observation of Surface Morphology
The CuAgCe4 alloy ribbon was cut into smaller sizes of about 0.25 × 0.25 cm and fixed to the metal support directly without any treatment. It was imaged using an S-3000H scanning electron microscope (SEM, Hitachi, Ibaraki, Japan) at 15 kV under low-vacuum conditions. The SEM images were taken at different magnifications, such as 500×, 1000×, and 2000×, respectively, for both sides of the ribbons. Additionally, the elemental distribution on the ribbons was analyzed using an energy dispersive spectrometer (EDS) (Oxford Instruments, High Wycombe, UK) at a set resolution of 1.0 nm at 15 kV for the scanning electron images.
2.3. X-ray Diffraction Analysis
The crystallinity degree of the alloy ribbons before and after being cultured with bacteria was analyzed using an X-ray diffractometer (XRD; Empyreen, Malvern Panalytical, Malvern, UK). For investigation, a piece of the ribbon was recorded via an XRD using Cu-Ka irradiation generated at 45 kV and 40 mA, and two theta (2θ) values were obtained with scanning diffraction angles of 20° to 90° in steps of 3.3°.
2.4. Antibacterial Activity Testing
Two bacterial strains were used in this study: E. coli (American-type culture collection, ATCC 8739) and S. aureus (ATCC 6538). The immersion method was employed to ascertain whether the alloys could inhibit the growth of E. coli. Accordingly, we mixed 0.3 g of amorphous, semi-crystallization, crystallization at 250 °C or 450 °C CuAgCe4 ribbons with 5 mL of E. coli cultured liquid when the concentration of E. coli was 1 × 105 CFU (colony-forming unit). The mixture was incubated at 37 °C for 24 h (hrs), and E. coli-cultured liquid was considered as control. The next day, 1 mL of the mixture was transferred into 9 mL of Tryptone Soya Broth, and dilutions of 10, 102, and 103 were made. Each diluted solution was mixed vigorously and sprayed onto two plates of Tryptone Soya Agar. The plates were incubated at 37 °C for 24 h, and the number of bacterial colonies was then counted (plate count). The antibacterial activity (log reduction) was calculated using the following formula in Equation (1) [28]:
(1)
Alternatively, the direct contact method was used to determine whether alloys could inhibit the growth of S. aureus. Then, the alloy ribbons were cut and attached into a 1 cm × 1 cm square, and we added 200 μl of S. aureus-cultured liquid with a concentration of 1 × 106 CFU to the surface of a ribbon square and incubated at 37 °C for 24 h. The following day, the ribbons were rinsed with 1800 μl of distilled water, and all the liquid was collected into an Eppendorf tube. A series of dilutions were made with Tryptone Soya Broth, including 10, 102, and 103 dilutions, and each dilution was sprayed onto three Tryptone Soya Agar plates. The plates were then incubated at 37 °C for 24 h, and the bacterial colonies were counted the next day for the antibacterial rate calculation.
2.5. Residual Metal Analysis
After the CuAgCe4 alloy ribbons were cultured with E. coli-cultured liquid for 24 h, 0.1 g of the resulting mixture was tested for the residual metal amounts [29]. Aqua regia was prepared by mixing nitric acid and hydrochloric acid in a ratio of 1:3. Then, 0.1 g of the mixture was added to 40 mL of aqua regia, and the solution was filtered after six hours. Afterward, the concentrations of the residual metals were analyzed via inductively coupled plasma optical emission spectrometry (ICP-OES; Optima 8000, PerkinElmer, Shelton, CA, USA).
3. Results and Discussion
3.1. Characterization of CuAgCe4 Alloy
After the preparation of the amorphous or crystalized CuAgCe4 alloy, the surface morphology was examined. Both sides of the ribbons were observed, and the examination revealed rougher on the darker sides compared to the lighter sides of the ribbons (Figure 1). As shown in Figure 1, dark spots were observed on both sides when CuAgCe4 alloy ribbons were crystallized at 450 °C, and the elements on the surfaces were analyzed using EDS.
Table 1 shows that the Cu, Ag, and Ce elements were detected through the EDS analysis. Notably, the presence of oxygen suggests the ribbons could be oxidized. Thus, the crystalline structures of different alloys were consequently analyzed. The elements of dark spots seen on CuAgCe4 ribbons when crystallized at 450 °C were also identified as Cu, Ag, or Ce. However, the detailed surface morphology could not be observed with EDS; thus, XRD was introduced to investigate the surface crystalline structures.
Figure 2a–d presents the XRD spectral images. The images show a wide range of peaks between 40° and 48° observed in the amorphous and semi-crystallization CuAgCe4 ribbons (Figure 2a,b). Characteristics or sharper peaks could be seen when the CuAgCe4 alloy ribbons are crystallized at 250 °C and 450 °C, suggesting the crystalline structure results showed that the metallic lattice became ordered. When CuAgCe4 was crystallized at 250 °C, rough peaks at 38° (Ag and Ce), 44° (Cu), 45° (Ag), 65° (Ag), and 78° (Ag) were observed (Figure 2c). Peak characteristics of Ag, Cu, and Ce were more obvious when CuAgCe4 was crystallized at 450 °C, and peaks at 50° and 74° represented Cu were observed (Figure 2d). Although the surface microstructures were not distinguishable among different samples by using SEM, XRD displayed better surface crystalline structures. In addition, oxygen content was not detected via XRD analysis, suggesting that the oxygen element detected via EDS might not be accurate. Indeed, it has been shown that the first eight elements might not be detected or with very low sensitivity by EDS [30,31]. XRD would be analyzed after CuAgCe4 ribbons were cultured with bacteria in the subsequent experiments.
3.2. Examining Different Crystalline Structures of CuAgCe4 and Their Inhibition on E. coli Growth
The surface crystalline structures of different CuAgCe4 ribbon samples showed some variations. Hence, we tested their ability to inhibit E. coli growth. Table 2 shows that the antibacterial activity of all the CuAgCe4 samples against E. coli was larger than the 3-log reduction.
Figure 3 presents the XRD images of CuAgCe4 ribbons before and after their culture with E. coli. The crystalline structures of the CuAgCe4 samples after being cultured with bacteria were also investigated and compared with the XRD result before culturing. According to the legend in the plot, the black and red lines represent the spectral trend before and after culture with E. coli. In Figure 3a,b, Ag, Cu, and Cu2O peaks could be observed after culturing even though only wide ranges of peaks were observed before culturing. New Cu2O peaks were observed after crystalized CuAgCe4 ribbons were cultured with E. coli when crystallized at 250 °C and 450 °C. The blue dots around the peaks in Figure 3c,d represent the characteristic peaks of Cu2O, and the feature was only observed after the ribbons were cultured with E. coli, suggesting that Cu elements were oxidized.
Further, the concentrations of residual Cu and Ce were analyzed after CuAgCe4 ribbons were cultured with E. coli using the ICP-OES method. The plots in Figure 4 present the concentrations of Cu and Ce after the amorphous and different crystallized types of CuAgCe4 ribbons were cultured with E. coli for 24 h. From the plot, it could be deduced that 0.55 parts per million (ppm) of Cu and 1.949 ppm of Ce were released from amorphous CuAgCe4 ribbons to the bacterial solution, respectively (Figure 4). In contrast, the concentrations of Cu and Ce ions smaller than 0.2 ppm were released from CuAgCe4 ribbons when the crystallization state was semi-crystallization and crystallization at 250 °C and 450 °C. Evidently, the Cu or Ce ion concentration released in amorphous CuAgCe4 samples is higher than the concentrations in the other alloys with different crystalline structures.
From the examination, it was observed that the metallic lattice structure of the CuAgCe4 alloy was altered after it was cultured with E. coli, irrespective of its initial crystalline structures. After the culture with E. coli, peaks around 38°, 65°, and 78° were noted in all the examined samples. The new peak around 38° could be attributed to Ag or Ce cubic, while 65° and 78° represented Ag. An amorphous lattice means electrons are prevented from moving around freely, resulting in lower conductivity and more electronegativity [32]; therefore, positively charged Cu, Ag, or Ce could not interact with the bacterial membrane, resulting in a loss of membrane potential. However, the surface of all CuAgCe4 alloys might become more positively charged since the peaks of Cu, Ag, and Ce were observed. This could result in the disruption of the bacterial membrane and the death of E. coli.
Another potential antibacterial mechanism of CuAgCe4 alloy displayed by the immersion method might be the release of positively charged metal ions. Therefore, the residual Cu and/or Ce ions in the solution were analyzed after the alloys were cultured with bacteria. Apparently, the concentrations of residual Cu and Ce ions were high in the amorphous alloy sample, as depicted in Figure 4. It has been shown that 1 ppm of released Cu2+ ions could be sufficient to inhibit the growth of E. coli and S. aureus [1]. Although the residual Cu ion concentration was around 0.55 ppm, the original release of Cu ion might be more than 1 ppm. In addition, the residual Ce ion concentration was higher than 1.9 ppm, which might be too toxic for bacterial growth. Previous research has demonstrated that Ce-containing stainless steels could be an antibacterial agent; however, the Ce release concentration required further investigation [11]. The current result showed that 0.044 ppm of Ce ion might be sufficient to cause an inhibiting effect against E. coli growth, suggesting that Ce ions might be more effective than Cu ions as an antibacterial agent. The potential antibacterial mechanism of metal ions release could then be the genotoxicity of bacteria caused by ROS and antioxidant depletion. This possibility could also be explained by the observation of Cu2O peaks around 37° and 42° in all samples and around 61° in alloys when crystallized at 250 °C and 450 °C (Figure 3c,d). Clearly, it has been shown that Cu2O could inhibit bacterial growth and possibly destroy/kill the bacteria without producing ROS supplying Cu+ into the bacterial solution [33]. So, it is evident that the combination of metal ion release and crystalline Cu2O may enhance the production of ROS and the generation of Cu ions to facilitate the death of E. coli. Whether ROS production or antioxidant depletion actually occurs in CuAgCe4 alloys when using the immersion method requires measuring the generation of hydrogen peroxide and byproducts.
Since Ag and Cu have been shown to be good antibacterial agents, it becomes imperative to validate the need to investigate the antibacterial activity of the CuAgCe4 alloy. Cerium in compounds can have valence II, III, and IV, making Ce act as a good catalyst for CO oxidation, water-gas shift reaction, and HC conversion [34]. One advantage of using Ce as an antibacterial agent was that Ce ions could be released with different positive charges (2+, 3+, or 4+), increasing the chances of causing the genotoxicity of bacteria. Another advantage is the rising concern for Ag or Cu tolerance in bacteria [35,36], thereby confirming Ce could support/maintain the antibacterial activity of Cu-Ag alloys. In addition, crystallized CuAgCe4 showed a reduction in the concentrations of Cu and Ce ions released (Figure 4). The gradual release of metal ions could be beneficial to maintaining the long-term antibacterial activity of the implants and avoiding the cytotoxic effect on human cells due to large amounts of metal ion release. Apparently, further investigation will be required to ascertain the biocompatibility of CuAgCe4 alloy as well as its long-term antibacterial activity.
3.3. The Crystalline Structures of CuAgCe4 and Their Corresponding Inhibiting Growth Against S. aureus
The amorphous and different types of crystallized CuAgCe4 alloy were all cultured with S. aureus to investigate whether CuAgCe4 could also inhibit the growth of S. aureus. For this study, we employed the direct contact method. Because Cu, Ag, and Ce could be detected on both sides of the ribbons, as presented in Table 1, the lighter sides of the ribbons were chosen to be in contact with the bacteria for consistency.
Figure 5 shows that the crystalline structures remained unaffected after the CuAgCe4 ribbons were cultured with S. aureus despite the crystallization types. This shows that regardless of the crystallization types, the antibacterial activity of the CuAgCe4 alloys against S. aureus was larger than the 2-log reduction, as Table 3 depicts.
The result showed that all the CuAgCe4 samples could inhibit the growth of S. aureus regardless of the ribbon’s crystallization state (Table 3). As the crystalline structures remained unchanged after the alloys’ contact with the bacterial solution (Figure 5), characteristic peaks and/or significant spikes of Cu, Ag, and Ce cubic were observed in all the samples investigated. It has been shown that Enterococcus hirae could be destroyed/killed by Cu ions. It could be deduced that the number of dead bacteria significantly increased when the copper surface was in direct contact with the bacteria [37]. In contrast, Pseudomonas aeruginosa could be sufficiently and quickly eliminated without contact with copper surfaces when using Cu ions [33]. This phenomenon is referred to as a copper ion burst-releasing model, suggesting that bacteria are rapidly destroyed when in contact with copper surfaces [38]. Although there exist uncertainties around the impact of bacteria on exposure to Cu surfaces, the findings from this study suggest that S. aureus could be terminated by direct contact with CuAgCe4 alloys. Therefore, it is important to examine the live/dead stains. If dead bacteria cells are stained on the surface of CuAgCe4 alloys while Cu ions are also detected, it can be suggested that both the release of Cu ions and direct contact with the alloy surface are required for destroying S. aureus. Direct contact with silver nanoparticles has been shown to kill bacteria, but the majority of research still focuses on eliminating bacterial growth via the release of silver ions [39,40]. The released amounts of Cu, Ag, and Ce ions should be further investigated. It is recommended that the study be conducted at different times/points in order to understand how rapidly the metal ions can be released and possibly the rate at which they kill the bacteria.
Following the variations on the crystalline structures when CuAgCe4 ribbons were cultured with bacteria using immersion and direct contact methods. It could be suggested that the alloy ribbons encountered a large amount of liquid, which resulted in changes in crystalline structures via oxidation. Apparently, the current results provide different applications for CuAgCe4 ribbons. It could also be assumed that the crystalline structures are not critical if CuAgCe4 ribbons are used on the surface of the implant. Since implants require specific mechanical properties, it would be appropriate to apply CuAgCe4 ribbons as antibacterial agents in implant applications. However, the crystalline structures of CuAgCe4 ribbons will be critical when they are applied on the body surface, such as wound dressing, since the bacteria will have direct contact with the ribbon. Accordingly, Wang et al. [41] reported that bacterial cell membranes were damaged through direct contact with exposure to wound dressing materials. Although the current results have demonstrated that amorphous, semi-amorphous, and crystallized CuAgCe4 alloy ribbons could inhibit the growth of S. aureus, it is yet unknown whether CuAgCe4 ribbons with different crystalline structures will present different antibacterial activity against other kinds of bacteria.
4. Conclusions
The current study investigated the antibacterial activity of CuAgCe4 alloys against E. coli and S. aureus. The results showed that the crystalline structures of CuAgCe4 alloy were altered after being cultured with E. coli using the immersion method, regardless of the initial crystalline states of the alloys. The traces of residual Cu and Ce ions depict the antibacterial activity of CuAgCe4 alloy that could be attributed to the release of metal ions. For the direct contact method, the crystalline structures of CuAgCe4 alloy remained unchanged after the culture with S. aureus and could possibly inhibit bacterial growth. However, some of the results suggest that the crystalline structures of CuAgCe4 alloy do not align with the antibacterial activity. It is evident that the findings from this investigation have provided useful information that CuAgCe4 alloys may be applied as antibacterial implants in the future.
Conceptualization, H.-W.F.; methodology, C.-Y.S. and S.K.; validation, Y.-C.T., Q.-P.C., S.-H.W. and Y.-X.L.; formal analysis, Y.-C.T., Q.-P.C., S.-H.W. and Y.-X.L.; investigation, C.-Y.S. and S.K.; data curation, C.-Y.S. and Y.-X.L.; writing—original draft preparation, C.-Y.S.; writing—review and editing, S.K. and H.-W.F.; visualization, C.-Y.S. and S.K.; supervision, H.-W.F.; funding acquisition, S.K. and H.-W.F. All authors have read and agreed to the published version of the manuscript.
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.
In memory of An-Pang Tsai, whose guidance and inspiration greatly impacted this work.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 1. SEM micrographs representing the dark and light sides of amorphous and various crystallized CuAgCe4 ribbons. The scale bar shows 50 μm with 1000× magnification.
Enlarge this image.Figure 2. XRD results of (a) amorphous, (b) semi-crystallization, and crystallization of CuAgCe4 alloy ribbons at (c) 250 °C and (d) 450 °C.
Enlarge this image.Figure 3. XRD results of CuAgCe4 ribbons before and after culture with E. coli. (a) amorphous, (b) semi-crystallization, and crystallization at (c) 250 °C, and (d) 450 °C.
Enlarge this image.Figure 5. XRD results for CuAgCe4 alloy ribbons before and after culture with S. aureus (a) amorphous, (b) semi-crystallization, and crystallization at (c) 250 °C and (d) 450 °C.
The elemental distributions in percentage (%) on each surface of CuAgCe4 alloy ribbons.
| Types of Crystallization | Amorphous | Semi- | Crystallization at 250 °C | Crystallization at 450 °C | |||||
|---|---|---|---|---|---|---|---|---|---|
| Dark or Light Side | Dark | Light | Dark | Light | Dark | Light | Dark | Light | |
| Element | |||||||||
| Cu | 34.2 | 28.7 | 44.3 | 42.0 | 40.6 | 28.9 | 27.8 | 36.6 | |
| Ag | 30.2 | 25.8 | 36.7 | 39.5 | 35.5 | 27.3 | 36.9 | 30.1 | |
| Ce | 2.0 | 1.8 | 2.7 | 3.1 | 2.1 | 0.9 | 2.1 | 1.9 | |
| O | 33.5 | 43.8 | 16.4 | 15.4 | 21.8 | 42.9 | 33.2 | 31.4 | |
The antibacterial activity of CuAgCe4 alloy with different types of crystallization against E. coli.
| Type of Crystallization | Concentrations of E. coli | Concentrations of E. coli | Antibacterial Activity |
|---|---|---|---|
| Amorphous | 5.0 × 105 | 6.67 × 101 | 3-log reduction |
| Semi-crystallization | 5.0 × 105 | 2.5 × 101 | 4-log reduction |
| Crystallization at 250 °C | 5.0 × 105 | 2.0 × 102 | 3-log reduction |
| Crystallization at 450 °C | 5.0 × 105 | 2.0 × 101 | 4-log reduction |
The antibacterial activity of CuAgCe4 with different types of crystallization against S. aureus.
| Type of Crystallization | Concentrations of S. aureus | Concentrations of S. aureus | Antibacterial Activity |
|---|---|---|---|
| Amorphous | 4.10 × 109 | 6.70 × 106 | 2-log reduction |
| Semi-crystallization | 5.03 × 109 | 2.67 × 105 | 4-log reduction |
| Crystallization at 250 °C | 3.27 × 109 | 6.70 × 106 | 2-log reduction |
| Crystallization at 450 °C | 5.67 × 108 | 2.33 × 106 | 2-log reduction |
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1 Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec. 3, Zhongxiao E. Rd., Taipei 10608, Taiwan;
2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan;
3 Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec. 3, Zhongxiao E. Rd., Taipei 10608, Taiwan;
4 Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, 1, Sec. 3, Zhongxiao E. Rd., Taipei 10608, Taiwan;