2023 Volume 64 Issue 9 Pages 2163-2167
Silver nanoparticles (Ag NPs) were quick synthesized by electrochemical method combined with heat treatment using silver electrodes and trisodiumcitrate (TSC) as reducing agent. This method is highly productive, with simple equipment, which is easily deployed on an industrial scale. The size of the Ag NPs is controlled through the amperage and the concentration of TSC. The effect of the synthesis’ conditions on optical properties of Ag NPs was surveyed. The Ag NPs were used to fabricate the Ag NPs-coated nonwoven fabrics. The bactericidal testing showed that Ag NPs-coated nonwoven fabrics were able to kill over 80% of E. coli and 96.9% of B. subtilis bacteria. These results can be considered as a new choice to prevent the spread of diseases caused by bacteria and viruses, thereby contributing to the protection of human health.
Fig. 2 UV-Vis absorption spectra of Ag NPs fabricated with: different current intensities (A), different concentrations of TSC (B), different electrochemical time (D), and TSC molecular structure (C).
Currently, there are many diseases caused by bacteria and viruses, which have the ability to spread rapidly within communities, causing severe harm to human health. These diseases can even lead to fatalities, like the Covid-19 pandemic. There are many methods to prevent the spread of Covid-19, as well as the other bacteria and viruses, in the case of infection, the antibacterial products should be given special attention thanks to their considerable benefits. Hence, the researches on the application of biocompatible materials to prevent the spread of disease are continuously on the frontline and are considered as one of most important tasks in all countries.
Silver nanoparticles (Ag NPs) are known to have outstanding antibacterial properties and have been used in medicine, cosmetics and consumer goods.1–4) In the last three years, publications have shown that Ag NPs effectively inhibit the growth of Covid-19,3,5–7) and various antibacterial products based on Ag NPs, such as masks, sanitizers, and mouthwash, have been developed as new choices to help preventing the spread of Covid-19 and to protect the human health. Ag NPs can be fabricated by mean of various methods.8) Chemical methods are efficient ways to produce materials with high yield in a short amount of time. These methods involve the use of simple equipment and straightforward procedures, making them easy to employ but they often rely on precursors, such as AgNO3, NaBH4 that have toxic effects on both the environment and living organisms.9–11) Physical methods, such as the laser ablation, can result in clean products without involvement of harmful chemicals but they have a relatively low yield and require complex specialized equipments.12–14) Combined physical and chemical methods such as the sonoelectrochemical methods, are also used to fabricate Ag NPs thanks to several advantages such as using the silver electrodes to replace the silver salts, which not only helps to reduce a product cost but also removes unwanted chemicals from the products.15,16) However, the use of ultrasonic waves requires specialized equipment, which makes it difficult to deploy on large scales. The electrochemical methods have also been developed, but the synthesis time is too long, and the Ag NPs concentration is low and the effect of the preparation conditions on the product quality has not been studied in detail.17)
This paper introduces a new method for a quick synthesis of Ag NPs by combining the electrochemical method with the chemical reduction method assisted heat treatment to maximize the advantages of each one. While the electrochemical process provides excess silver ions, the reduction reduces them to produce the Ag NPs in solution. The effect of the initial conditions on optical properties of Ag NPs is investigated in details. Then, the Ag NPs are dispersed onto the nonwoven fabric to create the Ag NPs-coated nonwoven fabrics. The crystal structure, morphology and optical properties of Ag NPs and their antibacterial efficiency are also presented in details.
The Ag NPs were fabricated by combining electrochemical method with heat treatment. The electrodes were made of silver with dimensions of 4 × 20 × 200 mm. The distance between the electrodes was 20 mm. The electrolyte solution containing 2 g of trisodium citrate dissolved in 1000 ml of distilled water and stored in a 1500 ml bottle. The amperage was adjustable from 15 to 150 mA with a fixed time of 15 minutes. During the electrochemical process, the solution temperature was set at 90°C.
2.2 Fabrication of Ag NPs-coated nonwoven fabricsThe nonwoven fabric was purchased from HaiCaTex Company, Hanoi, Vietnam. The silver nanoparticle solutions were prepared at a concentration of 20 to 60 ppm in a large bath depending on the size of the nonwoven fabric. The nonwoven fabric was then dipped into that baths so that the Ag NPs were absorbed and dispersed into the fabric. After that, the nonwoven fabric was dried at 60°C. The Ag NPs-coated nonwoven fabrics samples are denoted by SNF 0, SNF 20, SNF 40 and SNF 60, corresponding to the concentrations of 0, 20, 40 and 60 ppm of Ag NPs solution, respectively.
2.3 Antibacterial experimentsThe antibacterial fabric was sterilized by placing under UV light. The samples, were then cut into sizes of 2 × 2 cm2 and weighed. Shaking the bacteria for 2 hours at 37°C, then aspirate 1 ml of bacteria into antibacterial fabric and let stand for 15 minutes, next add antibacterial fabric to the falcon tube containing 9 ml 0.9% NaCl solution and vortexed. The cell density of E. coli or B. subtilis was quantified by diluting the sample with sterile 0.9% NaCl solution, plating on LB agar, then leave the dish in the incubator at 37°C overnight for bacteria (if alive) to grow, then count the colony and the collected data is then processed accordingly. The cell density (in CFU ml−1) of each sample was calculated by using the standard formulas.
2.4 Characterization of the synthesized samplesThe structure of Ag NPs was examined by X-ray diffractometer (XRD) D5005, Bruker, using Cu Kα radiation. The morphology of Ag NPs and silver nano nonwoven fabric was observed by a JEM-1200EX TEM instrument working at an accelerating voltage of 80 kV and a field-emission scanning electron microscope (FESEM) Nova NANOSEM 450, FEI, respectively. UV-vis absorption spectra were recorded by a Shimadzu UV 2450 PC spectrometer.
The XRD patterns of the Ag NPs (I = 50 mA, c = 2 g/l) are shown in Fig. 1(A). The figure shows the position of diffraction peaks at 2θ values of 38.11°, 44.29°, 64.39° and 77.50° corresponding to the (111), (200), (220) and (311) diffraction planes of face-centered cubic structure Ag, respectively. The lattice constant determined from these patterns is a = 4.081 Å which is in a good agreement with the standard diffraction patterns of the cubic metallic silver. The average size of Ag NPs is estimated at 22 ± 2 nm by using the Scherrer formula.18)
XRD patterns (A), the SAED image (B), the high resolution TEM (C) and the TEM images (D) of Ag NPs (I = 50 mA, c = 2 g/l).
Figure 1(B) shows the patterns of the Selected Area Electron Diffraction (SAED) of Ag NPs, and Fig. 1(C) shows the plane-view projection of Ag NPs obtained by using the high-resolution (HR) TEM. As observed, the spacing between (111) planes in a face centered cubic Ag is 2.38 Å, which is in agreement with previous researches.19,20) The TEM image of Ag NPs (I = 50 mA, c = 2 g/l) shown in Fig. 1(D), indicating that the Ag NPs are uniform and spherical with a diameter range of 40–50 nm.
3.2 Effect of preparation conditions on the size and optical properties of Ag NPsThe size and optical properties of prepared Ag NPs are influenced by various factors including current intensity, concentration of TSC solution, electrolyte temperature, as well as area and distance between the operating electrodes. For this study, the surface area and the distance between the electrodes were kept fixed, and the synthesis temperature was maintained at 90°C. The other parameters were changed one after another, starting from current intensity, TSC concentration and electrochemical time.
Figure 2(A) shows the UV-Vis absorption spectra of Ag NPs obtained with different current intensities (while the other factors were fixed). As the current intensity increases, the surface plasmon resonance absorption peak of Ag NPs tends to shift towards the long wavelength. According to Mie theory,21,22) the shifts of absorption peaks towards the long wavelength indicate that the presence of larger nanoparticles, and inversely the shifts towards the short wavelength correspond to smaller ones. The increase in size of the Ag NPs can be explained due to the fact that the greater amount of Ag ions is released into the solution per unit time as the current intensity increases.
UV-Vis absorption spectra of Ag NPs fabricated with: different current intensities (A), different concentrations of TSC (B), different electrochemical time (D), and TSC molecular structure (C).
Figure 2(B) shows the UV-Vis absorption spectra of Ag NPs fabricated by using different TSC concentrations. The results indicate that the absorption peaks are shifted towards the shorter wavelength as TSC concentration increases. This phenomenon can be explained as follows: when TSC is mixed with water as a solvent, the TSC molecules dissociate into ions according to the equations (see the structure of TSC given in Fig. 2(C)):
| \begin{equation} \text{Na$_{3}$C$_{6}$H$_{5}$O$_{7}$} \to \text{3Na$^{+}$} + \text{C$_{6}$H$_{5}$O$_{7}{}^{3-}$} \end{equation} | (1) |
| \begin{equation} \text{C$_{6}$H$_{5}$O$_{7}{}^{3-}$} + \text{H$_{2}$O} \leftrightarrow \text{HC$_{6}$H$_{5}$O$_{7}{}^{2-}$} + \text{OH$^{-}$} \end{equation} | (2) |
To evaluate the effect of electrochemical time on the size and concentration of Ag NPs in the solution, the samples were fabricated under the same experiment conditions (I = 15 mA, c = 2 g/l) at different intervals of 2, 5 and 10 minutes, a 2 ml sample was aspirated, and the UV-Vis absorption spectra were examined (Fig. 2(D)). The spectra show that, increasing the electrochemical time caused a marked increase in the intensity of the absorption peak, while the peak position stays the same. According to the Lambert-Beer law,23) the absorption intensity of the UV-Vis spectrum is proportional to the particles concentration in the solution, thus the increase of the absorption peak intensity indicates the larger concentration of Ag NPs. However, the constant peak position indicates that the size of Ag NPs remains the same, irrespective of electrochemical time. This means that the electrochemical time has a significant impact on the concentration of Ag NPs, but not on their size.
3.3 The antibacterial ability of Ag NPs-coated nonwoven fabricsThe FESEM image of normal nonwoven fabrics and Ag NPs-coated nonwoven fabrics are depicted in Fig. 3(A) and Fig. 3(B), respectively. It can be observed that the Ag NPs are homogeneously attached to the fabric fibers. The antibacterial potential of Ag NPs-coated nonwoven fabrics was evaluated against E. coli and B. subtilis. The results were presented in Fig. 4 and Table 1. As shown, the normal nonwovens fabric did not exhibit any antimicrobial action, while the Ag NPs-coated nonwoven fabrics samples displayed remarkable antibacterial activity with over 80% of E. coli and 96.9% of B. subtilis have been killed.
FESEM image of normal nonwoven fabric (A) and Ag NPs-coated nonwoven fabrics (B).
Photographs of E. coli (top row) and B. subtilis (bottom row) colonies on LB agar plates incubated with different Ag NPs-coated nonwoven fabrics samples.
There are some publications about the bactericidal ability of the Ag NPs-coated nonwoven fabrics. Xiaolong Deng et al. showed that E. coli reduction rates of the Ag NPs-coated nonwoven fabrics from 16.96 to 95.99% when Ag NPs ratio increases from 0.1 to 7% (w/w), respectively.24) Guangyu Zhang et al. showed that the bacterial reduction rates of the E. coli of the Ag NPs-coated nonwoven fabrics can increase from 44 to 99% when the Ag NPs ratio increases from 0.25 to 0.75% (w/w).25) The antibacterial activity of Ag NPs depends on the conditions, size, shape and surfactants, as well as strains and concentration of bacteria, and testing methods.26–28) It is worth noting that the concentration of bacteria in our test was very high (107 CFU/ml), whereas the test typically uses only 102–106 CFU/ml.24,25)
Ag NPs have been successfully fabricated by a new combined method of electrochemical synthesis and heat treatment. The lattice constant of Ag NPS determined from XRD pattern, is a = 4.081 Å. The size range of fabricated Ag NPs depends on technological conditions. The surface plasmon resonance absorption peak of Ag NPs shifts towards the long wavelength when the current intensity increases and towards the short wavelength when TSC concentration increases. The Ag NPs were applied to make the Ag NPs-coated nonwoven fabrics and the results of testing showed that Ag NPs-coated nonwoven fabrics samples was able to kill over 80% E. coli and 96.9% B. subtilis.
