In addition, in order to load more nano-silver onto the carrier, nano-silver with different concentrations was prepared. Figure 3 shows the images and UV visible spectra of nano-silver of different concentrations. As shown in the figure, the maximum absorption peak of the nano-silver with a smaller concentration of 0.001 and 0.01 mg/mL is about 400 nm, which indicates that the nano-silver has smaller particle size. With the increase in concentration, the collision between silver nanoparticles intensifies, larger silver particles are formed, verified by the darker color of the suspension, which is also observed from other research [ 17 21 ]. When it is increased to 1 mg/mL, the color of the suspension becomes gray black. It can be seen in the UV spectra that the maximum absorption peak shifts to the right with the increase in concentration, reaching about 450 nm at 1 mg/mL. Moreover, with the increase in concentration, the width of each absorption peak also increases, indicating that the particle size distribution increases and high concentration of nano-silver leads to wider particle size distribution. Although the high concentration of nano-silver suspension can bring convenience to the preparation of antibacterial agents, the larger particle size and distribution are unfavorable to the antibacterial properties. Uniform particle size distribution is beneficial to the antibacterial activity of nano-silver. The absorption peak distribution of 0.05 mg/L nano-silver solution is narrow, the particle size distribution is relatively small, and the maximum absorption peak is about 405 nm, indicating a small particle size of about 10–20 nm, which grantees a high loading and also a high antibacterial activity of silver nanoparticles.
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For nano-silver reduced by sodium citrate, sodium citrate acts as both a reducing agent and surface-treatment agent. As shown in Figure 1 , the liquid color is light when sodium citrate is used for reduction, which indicates that the content of nano-silver generated is low. This is mainly due to the weak reduction ability of sodium citrate, so that a large part of silver ions is not reduced. However, sodium citrate has an obvious effect on the growth of crystal nucleus, so it can cooperate with NaBHto control the particle size of nano-silver [ 17 ]. In the first stage of the reduction reaction, silver ions are preferentially reduced to form smaller particles. After the formation of nano-silver clusters, the pH of the solution is changed to alkaline. At this time, under the effect of sodium citrate, silver ions are slowly reduced to elemental silver on the surface of nano-silver, promoting the growth of crystal nuclei.
As shown in Figure 1 , the state of the nano-silver solution obtained by PVP and LA is similar. According to the electrification of different surface treatment agents, the surface of nano-silver modified by PVP is electrically neutral, and the surface modified by LA is with carboxyl groups. Since the number of hydroxyl groups on the surface of montmorillonite is less than that of LTA zeolite, it may not be able to firmly bond to the LA terminal carboxyl group, while the long-chain polymer PVP is more easily bound to the montmorillonite microstructure. Therefore, PVP-capped silver nanoparticles are used for montmorillonite carrier, while LTA zeolite selects LA-capped silver nanoparticles.
Under the reduction action of NaBH, Agis rapidly reduced to Ag, and these fine Agquickly merge into crystal nuclei. This process usually occurs within 200 ms and the solution color is light [ 17 ]. Then, these nuclei collide and merge continuously to obtain larger nuclei, and nano-silver is obtained through continuous merging and growth. The addition of surface treatment agents inhibits the further growth of the crystals. One end of these surface agents bonds with the nano-silver, and the other end is charged or has repulsive groups, which makes the nano-silver particles repel each other and maintain a good dispersion. With the extension of time, due to the high surface energy of fine particles, they gradually dissolve during the ripening process and promote the gradual growth of large particles, which is a process called Ostwald ripening. When the color of the solution no longer changes, the nano-silver solution after ripening is obtained.
Compared with montmorillonite, zeolite carrier is easier to release silver ions. Montmorillonite has more pores and the exchanged ions need to diffuse through the pores, so the instant antibacterial property is slightly poor. In addition, the exchangeable ions, silver, copper and zinc, are in the interlayer domain of montmorillonite and have strong affinity to the carrier, which are difficult to desorb [ 26 ]. Therefore, in spite of the small particle size of silver nanoparticles, the instant antibacterial property of AgNPs(a)-M is not as good as AgNPs(a)-Z.
Although the silver nanoparticles formed by AgNPs(b)-M in the interlayer domain are smaller in size, usually smaller than 2 nm, the silver nanoparticles are firmly bound in the interlayer domain, which makes oxidation more difficult [ 27 ]. Compared with the antibacterial agent AgNPs(a)-M, the added nano-silver is mainly attached to the pores of montmorillonite rather than entering the interlayer domain. The pre-made nano-silver does not occupy the interlayer domain space, which is mainly for the ion exchange sites of silver ions. Therefore, the antibacterial property of AgNPs(a)-M is better than that of AgNPs(b)-M.
The diameter of inhibition zone determined by vernier caliper is shown in Table 2 . It can be seen that all the agents exhibit similar inhibition zone diameter with slight difference. The antibacterial effect of in situ silver nanoparticles is generally weaker than that of ex situ silver nanoparticles. AgNPs(b)-Z shows the weakest antibacterial ability. During the in situ synthesis of AgNP, silver ions were first exchanged in the pores of the zeolite particles. When they were reduced, many of the generated silver particles occupied the internal space of the zeolite, overflowed the pores and covered the surface of the zeolite particles, seriously affecting the subsequent ion exchange process. Therefore, the amount of silver ions that can be exchanged by the antibacterial agent is small, which cannot form a synergistic antibacterial effect of silver nanoparticles and silver ions, and the instant antibacterial property is relatively poor. AgNPs(b)-M performs slightly better than AgNPs(b)-Z, which is due to the different structures of montmorillonite and zeolite. The in situ formation of silver nanoparticles from montmorillonite mainly occurs in the interlayer domain. Due to the limitation of the interlayer domain, the size of silver nanoparticles generated is smaller, which enhances the antibacterial effect of silver nanoparticles. In addition, the space of interlayer domain becomes larger after the formation of nano-silver, which can be increased from 1.45 nm to 1.54 nm [ 13 26 ]. The increased interlayer domain cannot only accommodate a large amount of nano-silver, but also provide more space and sites for subsequent ion exchange.
The results of the inhibition ring test of the four antibacterial agents against Escherichia coli are shown in Figure 4 . It can be seen from the figure that the control group without antibacterial agent does not show any antibacterial area, while all the antibacterial agents show antibacterial areas, indicating that the antibacterial agent has antibacterial activity.
The antibacterial agent with montmorillonite as carrier using ex situ generation of silver nanoparticles has more advantages than zeolite. This is because the montmorillonite has a large number of macropores, and the average pore diameter is 6.25 nm, which is much larger than that of LTA zeolite of 0.39 nm. Therefore, there is sufficient space to accommodate pre-made nano-silver, which can be oxidized and ionized to produce silver ions to provide antibacterial activity. In the antibacterial inhibition zone test, the agent with the montmorillonite carrier is weaker than that with zeolite because of the ion internal diffusion. However, the large amount of oxidation ionization from nano-silver makes up for this defect. At the same time, the larger concentration gradient accelerates the diffusion efficiency.
AgNPs(b)-Z and AgNPs(b)-M with in situ nano-silver have weaker antibacterial properties. The reason is consistent with the explanation of the activity of antibacterial agents. During the preparation process, silver ions were first reduced, and the obtained silver particles occupied the surface of the zeolite or filled the pores of montmorillonite, which hindered the subsequent ion exchange process and therefore decreased immediate antibacterial activity.
Figure 5 and Table S1 show the test results of the durability of antibacterial coatings. The antibacterial activity of the four antibacterial agents after multiple washing and wiping were tested. All the tests were conducted three times and the relative standard deviation was all within 5%, indicating a good repeatability and reliability. Similar to the instant antibacterial property, AgNPs(a)-Z and AgNPs(a)-M produced by the heterotopic reduction of silver nanoparticles also show higher durability, which can withstand over 20 washing cycles (1200 times). The decline of the antibacterial rate of both coatings is relatively gentle, indicating that the release of antibacterial active substances is relatively stable. As analyzed for their instant activity, AgNPs(a)-Z are more inclined to the action of ionic silver, and AgNPs(a)-M is dominated by the synergistic action of ionic silver and nano-silver. Both situations lead to similar durability results. The antimicrobial durability is much better than that of coatings with silver ions, whose antibacterial rate was reduced to lower than 99% after 12 washing cycles [ 13 ]. The results verify the reservoir function of silver nanoparticles. Furthermore, AgNPs(a)-M and AgNPs(a)-Z show slightly higher activity when compared with commercial nano-silver agents after 20 washing cycles [ 16 ].The antibacterial agent AgNPs(b)-Z has the lowest antibacterial property, with the reduction rate decreasing to lower than 99% (R < 2) at the 15th cycle. Since α-cage volume of LTA zeolite is very small, about 700Å3, the obtained silver particles from reduction leave the zeolite interior to the surface under the pressure of the narrow space and adhere to the zeolite surface through van der Waals force, forming a nano-silver layer, which hinders the subsequent silver ion exchange process, resulting in insufficient silver ions of antibacterial agent. During the washing process, silver ions are gradually consumed, and the synergistic effect of nano-silver–silver ions is gradually lost, so the durability is weakened. In addition, the generated nano-silver is weakly bonded with carrier surface by van der Waals force, and the nano-silver is likely to fall off from the zeolite and loses antibacterial ability.
Compared with AgNPs(b)-Z, AgNPs(b)-M with montmorillonite exhibits better durability, since it is a better AgNP carrier that has larger pores where silver nanoparticles can be settled. The nano-silver generated in the early stage mainly exists in the interlayer domain, which is both the storage space of nano-silver and the exchange site of silver ions. With the encapsulation of hydrophilic materials, the synergistic effect of nano-silver–silver ions is easier to function. However, some literature shows that during the ion exchange process, some silver ions will also replace the nano-silver, extruding the nano-silver out of the interlayer domain and occupying the pores, causing the pores to be blocked and the ion exchange process is thus limited [ 28 29 ]. Therefore, its durability is worse than that with ex situ silver nanoparticles.
The nanoparticle loaded powder can be mixed in paints and applied to hospitals walls , kitchens and toilets to give them antimicrobial coating
A team of Indian researchers have developed a new method that helps prepare silver nanoparticle loaded antibacterial powder, which could potentially be used to clean water and help in waste management.
Developed by researchers at the Mumbai-based Bhabha Atomic Research Centre (BARC), the new technique avoids use of harsh chemicals during synthesis and is environmental friendly.
The nanoparticle loaded powder could also be mixed in paints and applied to surfaces where chances of infection are high, such as walls in hospitals, kitchens and toilets to give them antimicrobial coating, the researchers said.
For more information, please visit Nano Silver for Antibacterial Plastics On Sale.
To synthesise silver nanoparticles, the team deployed the spray-drying technique. In this process, gum arabic — an eco-friendly biopolymer obtained from the Acacia tree — was used. Gum arabic helps in chemical reduction and attachment of silver nanoparticles to the silica substrate.
The study has been published in the journal Applied Nanoscience.
To study the distinctive features of the composite material, the team used advanced techniques such as X-ray diffraction, high-resolution transmission electron microscopy, field emission scanning electron microscopy and Fourier transform infrared spectroscopy.
They tested the antimicrobial activity of the composite on a gram-negative bacterium, Escherichia coli and a gram-positive bacterium, Staphylococcus aureus. The composite harbouring silver nanoparticles could kill the bacteria at very low doses. The composite was also used as a catalyst and found effective, the researchers said.
Filtering silver nanoparticles from waste water has been difficult due to its smaller size.
Some paint companies claim that their products are antimicrobial, but scientists say usually in such types of paints silver nanoparticles are directly added to the acrylic paints, mixed and applied. But studies have shown that around 30 per cent of silver nanoparticles from such paints get released in the environment within a year.
In the synthesised material, however, silver nanoparticles measuring around five nanometers were strongly attached to much larger silica substrate. As a result, the silver-containing particles could be easily removed by filtration.
For example, after being used with a detergent in a washing machine, the particles could be easily separated from the waste water by using a filter membrane at the outlet hose. Moreover, the silica nanoparticles could be replaced with magnetic nanoparticles and thus could be easily separated by using an external magnet after application, the researchers noted.
“The synthesis procedure is green and novel. We took the advantage of evaporation-induced self-assembly of silica nanoparticles and biopolymer during spray-drying,” researchers observed in their study.
“Thus, by just drying a droplet, we were able to achieve surface-functionalised substrate onto which metal nanoparticles could be attached. And the use of spray-drying has the added advantage of scalable production at low cost," they added. (India Science Wire)
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