Abstract
This study uses the submerged arc discharge method (SADM) and the concentrated energy of arc to melt silver metal in deionized water (DW) so as to prepare metal fluid with nanoparticles and submicron particles. The process is free from any chemical agent; it is rapid and simple, and rapid and mass production is available (0.5 L/min). Aside from the silver nanoparticle (Ag0), silver ions (Ag+) exist in the colloidal Ag prepared by the system. In the preparation of colloidal Ag, polyvinyl alcohol (PVA) is used as an additive so that the Ag0/Ag+ concentration, arcing rate, peak, and scanning electron microscopic (SEM) images in the cases with and without PVA can be analyzed. The findings show that the Ag0/Ag+ concentration increases with the addition level of PVA, while the nano-Ag and Ag+ electrode arcing rate rises. The UV-Vis absorption peak increases Ag0 absorbance and shifts as the dispersity increases with PVA addition. Lastly, with PVA addition, the proposed method can prepare smaller and more amounts of Ag0 nanoparticles, distributed uniformly. PVA possesses many distinct features such as cladding, dispersion, and stability.
1. Introduction
The production of nanosized metallic silver particles with different morphologies and sizes using different methods has been reported in previous studies, such as the electric spark discharge system (ESDS) [1–3]. PVA (Mw 89,000–98,000, 99+% hydrolyzed, 341584, 9002-89-5, MDL: MFCD00081922) was the eligible polymer since it stands out for its viscoelastic behavior, hydrophilicity, chemical stability [4] and biocompatibility [5]. PVA contains a large amount of –OH functional groups, which can form chelate composed of metal ions [6]. It is a white powdered resin polymer, with features of viscoelasticity, hydrophilicity, chemical stability, and biocompatibility [7–9]. PVA is used in many medical devices approved by the FDA (Food and Drug Administration, USA) such as contact lenses, membranes, drug delivery systems, and orthopedic devices [10], It is extensively used in biomedical and pharmaceutical applications [11, 12], waste water treatment, and artificial articular cartilages [13]. Initially, we used a biocompatible polymer, PVA, as a reducing agent to convert silver salt (AgNO3) to Ag-NP. Then, we obtained PVA/Ag-NP composite nanofibers via electrospinning [14]. A series of monodispersed Ag-NPs with 25, 35, 45, 60, and 70 nm sizes was reported by using PVP as the surfactant [15]. Also, Ag-NPs can be incorporated within biodegradable poly(lactic acid) [16] or deposited onto modified titanium surfaces [17] as an antibacterial scaffold for tissue engineering and medical applications. Poly(vinyl alcohol) (PVA) is a biodegradable polyester that has been investigated extensively as a biomedical material. Many researchers have utilized electrospinning to fabricate nanofibrous PVA scaffolds for use in wound healing [18–20]. PVA and its nanocomposites have found a wide range of industrial applications such as fiber and textile sizing, coating, adhesives, emulsifiers, and film packaging in food and optical holographic industries [21]. Nevertheless, there were only two articles about PVA/TiO2 nanocomposite prepared by the ultrasonic irradiation method. To the best of our knowledge [22, 23], high molecular materials, such as PVA [24], by stabilizing with polymer matrix, the silver nanoparticles, are surface modified. Hence, they can also act as capping agents. The homogeneous distribution of silver nanoparticles into the polymer matrix will also increase the surface area and make them fit for catalytic applications. There are several methods to fabricate silver nanoparticle polymer composites [25–27]. Due to PVA’s special characteristics, the nano-Ag will distribute uniformly on antibacterial dressings such that the index of the antibacterial dressing becomes more smooth and consistent. The products can be made as a film or gel, or weaved into a fiber.
In recent researches of nanoparticles, it is shown that the color of the Ag nanoparticle, produced by different methods such as presented in literature [28], is getting darker with the increase in concentration, usually from light yellow (since the peak absorption of Ag nanoparticle is 395 nm) to dark brown. In this study, the color of the 10 ppm, 35 ppm, and 60 ppm nanoparticles is light yellow, yellow, and dark brown, respectively, as shown in Figure 1.
2. Research Method and Process
The microelectric discharge machine system comprises the overall mechanism, hardware circuit, power supply, chuck, and so on. The motion control adapter card of the computer is used as the global core. The process performance of parameter setting is observed by changing the arcing rate, thus obtaining the parameter setting for the optimum process efficiency and quality of the nano-Ag colloid. Figure 2 shows the micro-EDM system [29, 30].
2.1. Preparation of Nanosilver by Electrical Spark
This study used SADM to split silver material into nanosized particles by arc discharge. The process was free from chemical agents. Pure solutions, such as pure water, were used as the medium [1].
The silver (99.99% pure) wires with a diameter of 1 mm are used as anode and cathode and are submerged in DW or ethanol. The 200 ml DW is loaded, the positive and negative electrodes are manually fixed and aligned, and then the discharge parameters are set before discharge. The discharge is finished after a period of time, and the product is taken out. The system framework is shown in Figure 3 [3].
2.2. Silver Nanofluid Product Analysis
This study uses DW as a liquid medium to prepare Ag particles. The prepared sample is analyzed by spectrophotometry for the spectral characteristic of the product. The absorbance of nano-Ag colloid is analyzed by spectrophotometry based on the concentration index of nanoparticles. The conductivity of dielectric fluid is measured by a conductivity meter, the purity of dielectric fluid is guaranteed, and the conductivity of general DW is lower than 5.00 μs/cm.
Nanosilver fluid is fabricated. For energy dispersive X-ray spectroscopy (EDX) analysis, a silicon wafer was used as a carrier. Carbon, oxygen, and sodium were typically accompanied with the PVA, and therefore, Ag, C, O, and Na were present. Table 1 and Figure 4 display the EDX analysis results, and SEM analysis is shown in Figures 5(a)–5(d).
(a)
(b)
(c)
(d)
3. Experimental Results and Discussion
3.1. Experimental Procedure
The 2.3 μs/cm Ag+ solution (without PVA) and 20 μs/cm Ag+ solution (with 10 ml PVA) preparation processes are designed, and the nano-Ag is then prepared at intervals of 6 min. The correlation between Ag+ and nano-Ag of the nano-Ag colloid and the concentration in the preparation process are defined by electrical conductivity and spectrophotometry (UV-Vis).
3.1.1. Silver Ions
200 ml DW is mixed with 2 ml, 4 ml, 6 ml, 8 ml, and 10 ml of the PVA, respectively. Ag is driven in 5 min, and the electrical conductivity is measured after the PVA is added to DW. When the PVA addition is 8 ml and 10 ml, the electrical conductivity does not distinctly increase, approaching the saturation concentration, as shown in Table 2.
3.1.2. Silver Ion Solution (with/without PVA)
The 2.3 μs/cm Ag+ solution (DW) and 20 μs/cm Ag+ solution (with 10 ml PVA) preparation processes are designed. Then, the nano-Ag is prepared at intervals of 6 min. The correlation between Ag+ and nano-Ag of the nano-Ag colloid and the concentration in the preparation process are defined by electrical conductivity and spectrophotometry (UV-Vis), as shown in Table 3.
3.2. Test Result Analysis
3.2.1. Vissim and UV-Vis
The laboratory report chart of PVA additive in Ag nanoparticle preparation (Vissim and UV-Vis) is shown in Figures 6–11.
3.2.2. Zeta Potential and Size Distribution
The laboratory report chart of PVA additive in Ag nanoparticle preparation (zeta potential and size distribution) is shown in Figures 12–15.
3.3. Results and Discussion
The experimental discussion about the interactive relationship between Ag+/nano-Ag and PVA is described below.(1)With PVA, when 8 ml (52.7 μs/cm) or 10 ml (59.5 μs/cm) of DW (200 ml) is added, the electric conductivity has no significant change, meaning that it has reached saturation.(2)During the discharge process, since the PVA covers the Ag particle and increases the surface charge of the Ag particle, it is seen that, from Figures 6 and 7, the discharging efficiency rises with the increase in the PVA concentration. The diameter of the Ag particle is 30 nm, and it remains 30 nm after the PVA covers the Ag particle; the overall complex may be larger than 30 nm after cladding. This complex is relatively big, and the distance between particles is relatively long. The overall complex is connected after cladding, the complex distance increases, two complexes cannot be connected, the covered complex appearance is negatively charged, and each covered complex is negatively charged. The repulsive effect is generated during mutual collision, whereby the negative electric field on the covered complex appearance is higher than the original surface electric field, from 30 mv to 40–45 mv; and the negative electric field on the covered complex appearance is strong so that the covered nano-Ag particles are farther from each other in liquid or colloid. The following situation is tenable. In the same electric field, the farther the Ag particle is, the more unlikely the Ag particle is to cause a short-circuit bridge. As long as the short-circuit bridge does not occur in the discharge process, the controller makes a continuous electrode discharge, and thus, a lot of nano-Ag0 and Ag+ are generated so that the electrode consumption per unit time increases.(3)According to Figure 8, the UV-Vis absorbance peak of DW discharge is driven in Ag+ < 190 nm and the peak of absorption is a waveform < 190 nm (near 190 nm). According to Figure 9, the UV-Vis absorption peak of DW + PVA discharge is driven in Ag+ = 194 nm. According to Figures 8 and 9, the UV absorbance peak of DW + PVA shifts right. Carefully inspecting Figures 8 and 9, it is obvious that Ag+ may shift right under the addition of PVA introduced. This is the first breakthrough of the current study.(4)According to Figure 10, in the case of DW-Ag0 (6 min), Ag0 surrounds the AgOH and Ag2O, and the corresponding wavelength of UV-Vis absorbance peak is 394 nm. As shown in Figure 11, when the Ag0 surrounds the Ag+-PVA− compound, the corresponding wavelength of absorption peak in the UV-Vis (300–600) spectrum is 410 nm, and the UV-Vis (190–300) absorption peak increases to at least 1.2. This suggests that the Ag+ concentration rises to 12 ppm, and 1 ppm in general approximates to 1 μm/cm. The total electrical conductivity is 24 μm/cm, meaning at least 12 μm/cm of the concentration results from Ag+ and the rest of 12 μm/cm results from the PVA derivant. Figures 10 and 11 indicate that the DW-Ag0 (6 min) wavelength is 76 nm at absorbance 70%, and the PVA (6 min) wavelength is 94 nm at absorbance 70%. Another new finding is that the particle size is measured by the Zetasizer. It is deduced that the Ag0 complex is combined with PVA, which leads to increase in the equivalent diameter, thus causing the peak of Ag0 shifting from 394 nm to 410 nm. Moreover, at the neck of the UV-Vis absorbance wavelength 70% in Figures 10 and 11, the wavelength of DW-Ag0 (6 min) is 76 nm. The width of the neck of DW + 0.5%/w/w-Ag0 (6 min) UV-Vis wavelength at 70% is 96 nm. This is because the (Ag+-PVA−) complex makes the overall Ag0 UV-Vis spectrum neck width wavelength increase from 76 nm to 96 nm.(5)According to Figures 14 and 15, the particle size and distribution are 50–100 nm in the DW solution without PVA. When (0.05%/w/w) PVA is added, the particle diameter is 25–75 nm.
4. Conclusion
The experimental results about the interactive relationship between Ag+/Nano-Ag and PVA are described below.(1)According to the increment ratio of electrical conductivity and absorption peak in the experiment, the PVA is correlated to the Ag0/Ag+ concentration.(2)As the electrode discharges continuously, a lot of Ag0 and Ag+ are generated, and the electrode consumption per unit time increases, so that the Ag0 and Ag+ arcing rate rises.(3)When Ag+ is combined with PVA (0.05%/w/w), it forms a (Ag+ · PVA−) compound. The molecular weight of this compound is much heavier than (Ag+)2O− and Ag+OH−, and the absorption peak of Ag+ shifts right to 194 nm.(4)The PVA not only helps the formation of Ag+ but also increases the formation of Ag0. After the PVA is added, the concentration of Ag+ increases 5 times (1.2/0.227 = 5.28) and Ag0 increases 3 times (0.899/0.247 = 3.63). Here, Ag0 is covered by PVA, increasing its dispersity and forming the Ag+-PVA− compound. The neck width of Ag0 on the UV-Vis absorbance wavelength at the height of 70% shifts from 76 nm to 96 nm, with its Ag0 peak shifting from 394 nm to 410 nm.(5)According to SEM image in Figures 5(a) and 5(b), the particle agglomeration is severe before the Ag0 is mixed with PVA. The particles have irregular shape, and the particle size and distribution are nonuniform. When 0.5% PVA is added in, the particle size becomes smaller and is distributed over the PVA surface uniformly. Meanwhile, the surface zeta potential of Ag0 increases, and the catalytic activity rises, so that the Ag-PVA complex is easier to disperse.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.