Abstract
This study used the submerged arc discharge method (SADM) to produce metal fluid containing nanoparticles and submicron particles, whereby the energy focused by an electric arc was used to disintegrate silver (Ag) metal in deionized water (DW). No additional chemical substances were required throughout the process, which enabled production to be fast and straightforward. This study proposed using colorimetry to define the interrelation between concentration and nano Ag as well as the Ag ions (Ag+) within a nano silver colloid (NSC) of a specific dilution factor, involving ultraviolet-visible (UV-Vis) spectroscopy and the measuring of electrical conductivity. The results showed that Ag+ activity increased under various dilution conditions, displaying an upward trend in activity with an increase in the dilution factor. The absorption values of Ag+ and nanoparticles increased over time, and the Ag+ activity increased by up to a factor of two after its dilution. Therefore, the Ag+ and nanoparticles demonstrated an interdependence between each other.
1 Introduction
Different methods have been employed in the production of nano-sized metallic silver (Ag) particles with different morphologies and sizes, for example, chemical reduction [1], [2], [3], electrochemical [4], photochemical [5], [6], microwave-assisted [7], c-rays [8], hydrothermal [9], wet chemical [10], laser ablation [11], sol-gel [12], sonochemical [5], [13], green [14], [15], pyrolysis [16], and the electric spark discharge system (ESDS) [17], [18], [19]. Most of the mentioned methods are based on chemical and physical reduction. According to the literature, reactions between the particles and the target analysis are generally based on electrostatic interaction [20], [21], [22], coordination chemistry [23], host-guest chemistry [24], or antibody-antigen binding [25]. The aforementioned mechanisms reduce spacing and result in particle aggregation. Consequently, the surface plasmon resonance of the nanoparticles shifts, and a change in the color of the colloidal solution can be observed.
Numerous researchers have employed various methods in their study of nanocomposite metal fluids. From an optical perspective, an increase in the concentration of the nano Ag results in a change in the solution color (Figure 1). The most common qualitative analysis method for measuring nano Ag stability is the full wavelength scan using ultraviolet-visible (UV-Vis) spectroscopy. According to the solution figure in a previous study [26], the 10-nm nano Ag aqueous solution exhibited a bright yellow color, and a specific spectral absorption peak occurred at a wavelength of approximately 393 nm in the UV-Vis spectroscopy (Figure 2).
In this paper, the submerged arc discharge method (SADM) is used to produce a nano silver colloid (NSC) solution without adding any chemical substances. As the produced NSC solution releases Ag ions (Ag+) [27], [28], it was discovered to possess antibacterial [29], [30], [31]/antifungal [32], [33] capability and, thus, is friendly to the environment as well as the human body. As a result, it could be used directly for medical treatments, for example, application to organisms or the creation of a dressing and an antibacterial agent.
2 Experimental process
The Ag (99.99% pure) wires with diameters of 1 mm and 2 mm are used as the anode (1 mm) and cathode (2 mm), respectively, and are submerged in deionized water (DW) (Milli-Q, 18.2 MΩ·cm) or ethanol (95%). The 200-ml DW is loaded, and the positive and negative electrodes are fixed and aligned manually. The discharge parameters are then set before discharge. The discharge is finished after a period of time, and finally, the product is taken out. The product is analyzed by spectroscopy, light scattering apparatus, and scanning electron microscopy (SEM).
2.1 Micro-EDM system setup
The micro-electrical discharge machining (EDM) system comprises hardware circuits, power supply, and chuck. A desktop computer equipped with a motion control adapter card is used as the global core. The arcing rate monitoring system consists of logical decision circuit and software. The number of successful discharge, arcing rate, and energy consumed by electrode are instantly displayed on the computer screen. The process performance is observed by changing the arcing rate; thus, the parameter setting for the optimum process efficiency and quality of the NSC [34], [35] was obtained. From the experimental results and analysis, it is confirmed that this method is fast, simple, and easily adaptable to mass production. The setup used to produce Ag nanoparticles is depicted in Figure 3 [30]. This technique is proved to be economical, environmental friendly, and can produce uncontaminated nanoparticles.
Figure 4 schematically illustrates the discharge principle for producing a nano metal colloid by EDM. Figure 4A shows the SADM system. As shown in the figure, the Ag wire with 1 and 2 diameters served as the anode and cathode, respectively. They were submerged in DW.
In order to trigger an arc discharge (plasma) at the tip of both electrodes submerged in the dielectric fluid, the servo system is properly adjusted such that the distance between the two electrodes during the discharging process is achieved.
The gap between the two electrodes was regulated at approximately 30 μm (Figure 4B). After the arc discharge occurred during the on-time (Ton), the metal particles sputtered onto the electrode surface (Figure 4C). Subsequently, two electrodes became isolated during the off-time (Toff), until the next discharging cycle. In Figure 4D, a stirring bar and a magnetic stirrer were installed at the bottom of a collection tank. During the discharging process, the metal particles were stirred until they were evenly distributed in the dielectric fluid [19], [34].
2.2 Silver nanoparticle characterization
According to the experimental conditions regarding the preparation parameters for Ag, when Ton was lower than Toff, the preparation efficiency for the nano fluid was optimal. Employing m-EDM to prepare the NSC, the related parameter settings is shown in Table 1 and the resulting particle characteristics are shown in Figures 5 and 6. The SEM analysis is shown in Figure 7.
Parameter of m-EDM setting and material and testing condition | |
---|---|
TON:TOFF | 10:10 |
IP | V=100 V, I=4.2 A |
Capacitor | Off |
Diameter of Ag | Anode: 1 mm; cathode: 2 mm |
Discharge time | 10 min |
Dielectric fluid | DW |
Beaker | 200 ml |
Filter paper | Advantech |
Atmosphere | 1 atm |
2.3 Analysis equipment
This study used DW as a dielectric liquid to produce Ag particles. The samples were analyzed by spectrophotometry. The absorbance of the NSC was analyzed by spectrophotometry as the concentration index of nanoparticles. A conductivity meter measured the conductivity of the dielectric fluid, guaranteeing its purity. The conductivity of general DW was lower than 5.00 (μs/cm).
3 Experimental results and discussion
3.1 Experimental procedure
The Beer’s law and optical colorimetry were utilized to measure the concentration. According to literature, colorimetry is simple, fast, and free of large equipment as well as easy to implement as an online test [36], [37], [38]. For a solution with a given molar absorption coefficient, the solution concentration is determined to measure the light absorbance. The purpose is to put the experimental data in the center of a measurable range of all instruments. When the colloidal Ag is diluted, the correlation of Ag+ and nanoparticles in colloid with a given concentration during preparation is defined by electrical conductivity and UV-Vis spectroscopy. Therefore, the preparation process was designed to be 50 min, and the interval of the nano Ag preparation process was 5 min. As the NSC is prepared at 5-min interval, the electrical conductivity was measured, presenting a clear uptrend, as shown in Table 2.
Discharge time | Before testing liquid is diluted | After dilution | Dilution ratio | Value after dilution multiplied by the dilution ratio |
---|---|---|---|---|
5 min (std.) | 2.44 | 2.44 | 1.0 | 2.44 |
10 min | 5.56 | 5.30 | 1.05 | 7.95 |
15 min | 8.99 | 6.33 | 1.42 | 12.66 |
20 min | 15.15 | 7.53 | 2.01 | 18.80 |
25 min | 20.49 | 7.81 | 2.61 | 23.40 |
30 min | 25.00 | 6.75 | 3.85 | 30.00 |
35 min | 28.08 | 5.45 | 5.14 | 29.00 |
40 min | 29.87 | 7.61 | 3.92 | 53.42 |
45 min | 40.76 | 7.07 | 5.76 | 53.00 |
50 min | 44.47 | 7.71 | 5.76 | 54.00 |
3.1.1 The electrical conductivity of DW
The electrical conductivity of DW was measured; a good value is about 1.0 (μs/cm). The DW was put in the test container, and water with a magnet was placed on the revolving table.
The electrical conductivity of DW in the container was measured on the revolving table. If it is equal to a previous measured value (±0.2), then the experiment continues. If the value changes too excessively, then the container should be cleaned with DW for another measurement.
3.1.2 The micro-EDM discharges
In order to prepare the NSC at different concentrations, the micro-EDM discharges 10 times, for 5 min each time, totaling 50 min.
Five minutes later, 20 ml of NSC (5 min) was extracted as the initial value, and the electrical conductivity and absorption spectrum (300~600 nm) of the Ag nanoparticles were measured. The testing liquid was then put in a 20-ml serum bottle.
The micro-EDM discharges for the next 5 min. The total time consumed for preparing the NSC was 10 min.
Then, 2 ml of NSC (10 min) was extracted, and its electrical conductivity was measured. Afterward, the 10-min, 15- to 50-min NSC were diluted with 1 ml of DW each time by an adjustable pipette. Colorimetry stops when it was visually identical with the NSC concentration (color) of 5 min. The dilution factor was recorded, and the electrical conductivity after dilution was measured. Then, the diluted NSC was stored in the serum bottle.
Finally, 2 ml of NSC was extracted as the testing liquid before dilution after 10 min, and then Steps (2) and (3) were repeated. Ten bottles of NSC with approximately identical concentrations were obtained by colorimetry.
3.2 Test result analysis
3.2.1 Comparison of electrical conductivity
According to Figure 8, the curves of electrical conductivity (before dilution of the testing liquid) showed an upward trend over time. As colorimetry was used, the testing liquid for each experiment was compared. When the measured electrical conductivity (after dilution) was close to the initial value (5 min), its time curve was nearly flat. Over the duration of putting Ag in the testing liquid, both the Ag+ concentration and the dilution ratio increase gradually. Therefore, the electrical conductivity dilution ratio increases slightly. Finally, the value after dilution was multiplied by the dilution ratio to obtain the activity of the Ag+. It is observed that the activity rises after dilution.
3.2.2 Testing the liquid dilution ratio
Table 3 shows the testing liquid dilution ratio. It is seen that 2 ml of the solution was extracted from the testing liquid every 5 min (testing liquid is extracted and pre-diluted). As colorimetry diluted the testing liquid to a concentration close to the 5-min (std.) absorbance value, the addition of DW increases gradually during dilution, and the dilution ratio increased over time, as seen in Figure 9.
Discharge time | Testing liquid extracted and pre-diluted (ml) | Diluted with DW (ml) | Dilution ratio |
---|---|---|---|
5 min (std.) | 0 | 0 | 0 |
10 min | 2 | 1 | 0.5 |
15 min | 2 | 2 | 1 |
20 min | 2 | 3 | 1.5 |
25 min | 2 | 4 | 2 |
30 min | 2 | 7 | 2.5 |
35 min | 2 | 9 | 3 |
40 min | 2 | 12 | 4.5 |
45 min | 2 | 13 | 5.5 |
50 min | 2 | 12 | 7 |
3.2.3 Activity multiplication factor
Table 4 shows the activity multiplication factor, in which Ag+ activity after dilution is increased by one to two times with time, as shown in Figure 10.
Discharge time | Activity multiplication factor |
---|---|
5 min (std.) | 1 |
10 min | 1.42 |
15 min | 0.74 |
20 min | 1.24 |
25 min | 1.14 |
30 min | 1.16 |
35 min | 1.07 |
40 min | 1.78 |
45 min | 1.30 |
50 min | 1.21 |
3.2.4 Ag+ and Ag nanoparticle UV-Vis spectroscopy relationship
Table 5 shows the Ag+ and Ag nanoparticle UV-Vis spectroscopy relationship. The absorbance of Ag+ and Ag nanoparticles increased with time; hence, Ag+ and the Ag nanoparticles exhibited the same tendency, as shown in Figure 11.
Discharge time | Ag+ absorbance | Ag nanoparticle absorbance |
---|---|---|
5 min (std.) | 0.185 | 0.284 |
10 min | 0.45 | 0.48 |
15 min | 0.78 | 0.76 |
20 min | 1.07 | 0.925 |
25 min | 1.29 | 1.02 |
30 min | 1.49 | 1.49 |
35 min | 1.8 | 1.65 |
40 min | 2.27 | 2.03 |
45 min | 2.51 | 2.25 |
50 min | 2.87 | 2.45 |
3.3 Results and discussion
Through measurements and comparison of the electrical conductivity before and after dilution of the Ag+ and Ag nanoparticles, it was discovered that a decrease in concentration (i.e., increased dilution) did not lead to a linear decline in the electrical conductivity value. Therefore, a decrease in concentration following dilution increased the particle spacing and led to an increase in Ag+ activity. Subsequently, the Ag+ dilution factor increased with the time of the EDM, resulting in an increase in the NSC concentration. The dilution rate required during colorimetry also increased. The calculation of Ag+ activity showed that the ion activity did not exhibit substantial growth following an increase in the dilution ratio, with the activity increasing only by one- to twofold.
The measurement of the UV-Vis absorption peak revealed that an increase in the number of nano Ag particles was accompanied by an upsurge in Ag+. This meant that the amount of Ag+ increased following an increase in the number of nano Ag particles, which helped maintain the suspension characteristics of the nano Ag particles and prevent them from forming precipitates.
The experiment in this study determined that reverse osmosis water did not experience a change in electrical conductivity after being placed in room temperature and standard pressure for 1 h. The electrical conductivity of Ag+ surged from 1.2 to 50 after undergoing SADM. According to the results of this study, the increase in electrical conductivity was caused by an increase in the compounds of Ag+ during SADM. However, further experiments and investigation are required to determine what types of compounds that resulted in the electrical conductivity were formed.
4 Conclusion
The relationship between the SADM-produced Ag nanoparticles and the Ag+ concentration is summarized as follows:
SADM is not merely a physical phenomenon. Under its electric field, an atom will change the electron cloud distribution and share its electrons with the surrounding molecules. For example, Ag contributes one of its own electrons to share with the other molecules, thus, carrying positive-charged Ag+. Therefore, the NSC produced through SADM generates Ag+.
The results of this experiment demonstrate that nanoparticles produced by Ag, gold, copper, or titanium possess suspension characteristics within aqueous liquid, whereas those produced by other metals lose their suspension characteristics for unknown reasons. However, the experiment shows that for a small number of metals such as Ag, the nanoparticle concentration increased over time and was accompanied by an increase in ion concentration. For other metals, this only resulted in low ion concentrations. Even SADM was continued to operate because the additional nanoparticles produced lost their suspension characteristics. This study further demonstrated that Ag+ change with an increase in ion concentration, which led to the discovery of the relationship between Ag+ and nanoparticles.
According to the experimental results, the activity of Ag+ varies under different dilution conditions. It is shown that an upward trend in activity with an increase in the dilution factor is observed. The absorption values of Ag+ and nanoparticles increased with time; therefore, Ag+ and nanoparticles are interdependent.
About the authors
Kuo-Hsiung Tseng received the MSEE degree from Mon- mouth University, NJ, and received the PhD degree in Science and Technology from Nueva Ecija University, Cabanatuan, R.O.P. Currently, he is a Professor with the Department of Electrical Engineering at National Taipei University of Technology, Taipei, Taiwan, R.O.C. His current research interests include power electronics, Biomedical Science, Nano Science and Nano Technology.
Chih-Ju Chou received his MS degree in electrical engineering National Taiwan Institute of Technology in 1986 and PhD degree in electrical engineering from National Taiwan University in 1994. He joined the faculty of Chun Yuan Christian University in Taiwan from 1992 to 2005. After that, he has been with National Taipei University of Technology in Taiwan, where he is currently a professor of electrical engineering. His research interests include grounding system analysis and design, power system fault analysis and protection, harmonic control and filter design and energy applications and planning.
To-Cheng Liu received his MS degree from the Institute of Mechatronic Engineering, Tung Nan University, Taiwan, in 2010. He is currently a CTCI Smart Engineering Corporation and pursuing his PhD degree in the Department of Electrical Engineering, National Taipei University of Technology, Taipei, Taiwan. His research interests include Smart Home Development, HMI and SCADA Design Configuration & Integration applications and planning, dynamic modeling and simulation, Nanotechnology and Its Applications.
Der-Chi Tien earned a BS in Electronic engineering at National Taipei University of Technology, Taiwan, in 1977 and a MS in the Department of Mechanical Engineering at National Taipei University of Technology, Taiwan, in 2005. He joined Professor Tsung’s Nano group in 2003 and earned his PhD in 2009 from the Department of Mechanical Engineering at the National Taipei University of Technology. He is currently the Assistant Professor of Mechanical Engineering, National Taipei University of Technology, Taiwan, since 2009. His current research interests in Nanatechnology.
Chun-Yung Chang was born in Taipei, Taiwan in 1994. He received his Bachelor’s degree from the Department of Electrical Engineering, Chang Gung University, Taiwan in 2016. Currently, he is a student in National Taipei University of Technology, pursuing his Master’s degree of Electrical Engineering.
Leszek Stobinsk Doctoral degree in chemistry, ruled by the Scientific Council of the Institute of Physical Chemistry of the Polish Academy of Sciences on 16.03.1993. He is currently the post doctor of electrical engineering, Warsaw University of Technology. His current research interests include nanotechnology and graphene material.
Conflict of interest: The authors declare that there is no conflict of interest regarding the publication of this paper.
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