Killing Two Birds with One Stone: Coating Ag NPs Embedded Filter

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Functional Inorganic Materials and Devices

Killing Two Birds with One Stone: Coating Ag NPs Embedded Filter Paper with Chitosan for Better and Durable Point-of-Use Water Disinfection Meikun Fan, Lin Gong, Ji Sun, Dongmei Wang, Feng Bi, and Zhengjun Gong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13985 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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ACS Applied Materials & Interfaces

Killing Two Birds with One Stone: Coating Ag NPs Embedded Filter Paper with Chitosan for Better and Durable Point-of-Use Water Disinfection Meikun Fana, b*, Lin Gonga, Ji Suna, Dongmei Wanga, Feng Bia, Zhengjun Gonga* a Faculty

of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu, 610031, China.

b

State-province Joint Engineering Laboratory of Spatial Information Technology of High-Speed Rail Safety, Chengdu, 610031, China.

KEYWORDS: Ag NPs, point-of-use, water disinfection, filter paper, chitosan membrane

ABSTRACT: In this study, porous chitosan (CS) coated Ag NPs embedding filter paper (CAEFP) was fabricated for point-of-use (POU) water disinfection application. Thanks for the presence of CS coating, the tensile strength of the CAEFP in wet condition was found to be 1.8 MPa, 700% increase compared with where there was no CS coating, making it much more durable. In addition, the coating with CS could greatly boost the Ag NPs loading without worrying the excessive releasing of Ag into treated water, thereby significantly improve the bactericidal efficiency but still safe to drink in terms of Ag

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release. Furthermore, by controlling the amount of CS used, the flow rate and bactericidal efficiency of the CAEFP could be manipulated (customized). When the CS content increased from 0.5 to 2.0 wt%, the flow rate of CAEFP would drop from 9.3 to 0.53 L/min/m2, and the bactericidal efficiency against E. coli and B. subtilis could improve from 4 and 3.6 to 4.9 and 4.8 log reduction, respectively. At optimum condition, the total Ag in treated water by CAEFP was below 45 µg/L, only 1/10 of that from Ag NPs loaded filter paper without CS coating, half of the WHO drinking water requirement (< 100 µg/L). Natural surface water samples were used for the demonstration of the bactericidal performance of the CAEFP. Both the total bacterial and E. coli counts met the WHO standard.

1. INTRODUCTION According to the WHO, more than 660 million people do not have access to safe drinking water, and 530 million of them are in rural areas 1. In addition, the spread of preventable water-borne diseases by microbial contaminants has caused many casualties. For example, approximately 2 million people, mostly children under the age of 5, die annually from diarrhea due to the lack of safe drinking water 2. As a result, water disinfection is of great importance for human health 3, and low cost POU drinking water disinfection technologies that fit for resource limited situations are urgently needed 4. Thus, POU water treatment technologies have received extensive attention in scientific communities 5-7.


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Some nanomaterials have shown great potential as water disinfectants 8. Among them, Ag NPs have been proved to have superior bactericidal properties

9-11.

Thus, Ag NPs-

decorated materials have been widely reported as POU water disinfectants in the literature in recent years 12. For instance, ceramics have been used as Ag nanopatch carriers for POU water disinfection applications

13.

The material could be simply placed in a household

bucket to let the silver ions diffuse out and then kill the pathogens in water. The material was reported to have a long working life (154 days) and large water disinfection capacity. However, long processing time (8 h) makes it more suitable for household applications. Ag NPs based POU water disinfectants that focus on the disinfection speed instead of the volume have also been reported. This type of POU disinfectants aims for the situation where clean water is urgently needed for individuals, such as in disaster relief. Typically, the use of filtration for POU water disinfection is a low cost and easy-to-operate approach. Ag NPs loaded filter paper naturally becomes one of the most competitive choices. For example, paper sheets impregnated with Ag NPs have been utilized as POU water disinfectant

14.

It was found that E. coli and Enterococcus faecalis had a 6 and 3 log

reduction in effluents, respectively. In another work, an environmentally benign method for the in situ preparation of Ag NPs in paper using microwave irradiation has been reported

15.

The loading of Ag NPs was achieved by leaving both AgNO3 and glucose-

soaked paper in a microwave for 3 min. The log reductions of 8.1 and 2.3 for E. coli and E. faecalis with the as-prepared filter paper have been reported, respectively.



However, there are still issues that need to be improved for filter paper based POU disinfectants. For example, the mechanical strength of filter paper becomes very low under wet conditions. This makes the filter paper based POU disinfectants very fragile in the disinfection process and potentially a safety concern, since a leak could happen and in turn void the disinfection process. It also leads to poor durability of the disinfectants. On the other hand, though the increase of silver loading can improve the bactericidal efficiency of the filter paper based POU disinfectant 16, it will inevitably lead to the excessive release of total silver content in the treated water, resulting in yet another safety concern 17. Here in this work, CAEFP was fabricated through a simple thermal reduction of Ag on filter paper followed by fast phase separation method to coat CS porous membrane. Due to the modification of chitosan porous membrane, the proposed POU disinfection material has some excellent properties as follows. First, the silver content on filter paper increased significantly so that the bactericidal performance of the material was greatly improved. Second, thanks for the existence of CS membrane, total Ag in treated water met the drinking water standard of WHO even though the Ag loading was very high. Third and the most important, the tensile strength of 700% increase of the material in wet condition was achieved with CS coating, making it much more durable. In addition, the filtration rate of CAEFP could be manipulated by adjusting the content of CS, offering a customizable CAEFP synthesis strategy for the processing of water with various degree of bacterial contamination for the best processing efficiency (balance between processing speed and


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log reduction rate of bacteria). Real surface water samples were processed with satisfactory results. 2. EXPERIMENTAL SECTION 2.1. Materials. Silver nitrate (AgNO3), chitosan (CS), ninhydrin, acetic acid and glucose were purchased from Sigma-Aldrich (Shanghai, China). Filter papers were purchased from Whatman (Cat No. 1001-110, Shanghai, China). Anhydrous ethanol, 95% ethanol, pancreatic peptone, beef extract, yeast extract liquid, sodium chloride (NaCl) and agar powder were bought from Kelong Chemicals Co., Ltd (Chengdu, China). Escherichia coli (E. coli, ATCC 25922) and Bacillus subtilis (B. subtilis, ATCC 6633) were purchased from Wuhan University (Wuhan, China). Ultrapure water (18.2 MΩ·cm) was used all through this work. 2.2. Preparation of Ag NPs Loaded Filter Paper (AFP). In this work, Ag NPs were loaded on paper by in situ thermal reduction using glucose as green reducing agent

18.

Briefly, aqueous solutions of glucose (0 to 1.25 M) and AgNO3 (0 to 0.125 M) at molar ratio of 10:1 (glucose: AgNO3) were prepared. Filter paper was then immersed into the freshly prepared solution for 15 min. After immersing, filter paper was incubated in oven for 1 h at 105 ℃. The color of the filter paper changed from white to yellow brown in the process. In order to remove the excessive glucose remaining on the surface of paper and the Ag NPs that are not tightly attached, the filter paper was rinsed with copious amount of ultrapure water.



2.3. Preparation of CAEFP. Aqueous acetic acid (1 wt%) solution containing CS (0.5 to 2.5 wt%) and polyethylene glycol (PEG, 15 wt%) was prepared. The AFP described above was soaked into the mixture overnight. Then the filter paper was removed and left in the air for 3 h at room temperature. After that, the filter paper was immersed in 1 M NaOH aqueous solution overnight to extract PEG for the forming of the porous CS membrane on the surface of paper. Finally, the filter paper was rinsed with ultrapure water until the pH of the effluent reached 7. 2.4. Measurement of Ag Content on Filter Paper and Total Ag in Treated Water 19. An inductively coupled plasma-mass spectrometer (ICP-MS; NexION 2000, PerkinElmer, USA) was used to analyze the Ag content on paper and total Ag in treated water. In order to analyze the silver content of the filter paper, 100 mg dry paper was put in a beaker containing 5 mL 70% nitric acid and then boiled until the paper was dissolved. After a few minutes of cooling, 5 mL H2O2 was added to the beaker and boiled again. Then, the suspension was filtered with a glass filter and the effluent was diluted with distilled water to 100 mL in a brown bottle. ICP-MS was used to analyze the sample. For the total Ag in the treated water, 10 mL of treated water was collected and digested with 10 mL of 67% HNO3 following the same protocol as for the filter paper prior to ICPMS analysis. All experiments were conducted in triplicate. For quality control, analysis of spiked sample was performed, which was accomplished by adding appropriate amounts of standard silver solution to the two sample matrices prior to HNO3 digestion.


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2.5. CS Content on The Filter Paper. The amount of CS coated on the filter paper was analyzed by the ninhydrin method 20. Briefly, 30 mg dry CS paper pieces were left in a solution containing 0.4 mL of pure water and 0.5 mL ninhydrin reagent. After incubating in a water bath at 100 ℃ for 30 min, the mixture was cooled to room temperature. Then, 2.5 mL 50% (v/v) n-propanol aqueous solution was added to the mixture to stop the reaction between the primary amino group and the ninhydrin reagent. The absorbance of the solution was measured at 570 nm wavelength by a UV-vis spectrophotometer (UV2600, Shimadzu, Japan). The standard curve was created using 2-10 mg solid chitosan in the ninhydrin reagent mixture. 2.6. Bactericidal Tests. The relevant experiments on characterization of bactericidal properties of materials are described in detail below. 2.6.1. Bacterial Culture and Preparation of Bacterial Suspension. The liquid LB medium was used to incubate bacteria. Briefly, 400 mL aqueous solution containing 6 g peptone, 2 g NaCl and 2 g yeast extract was boiled and adjust pH to 7-7.5 with 1 M NaOH. E. coli and B. subtilis, as the representatives of the gram negative and positive bacteria, were inoculated in LB medium and placed in the incubator (E. coli 37 ℃, B. subtilis 30 ℃) for 24 hours. After centrifugation (3000 revolutions per minute, 20 min), the isolated bacteria were then diluted to a suitable multiple by 0.9 % NaCl saline and shake evenly for later use. 2.6.2. Agar Diffusion Method. The solid LB medium was used for this experiment, and the preparation method is the same as the liquid LB medium, though an additional 6 g of



agar powder was added before boiling. When the mixture was cooled to about 40 ℃, it was poured into glass dishes for solidification. 1 mL 108 cfu/mL E. coli and B. subtilis were respectively coated on the culture medium evenly, and then the paper samples tailored with a diameter of about 8 mm were placed on the culture medium, respectively. The Petri dish inoculated with E. coli and B. subtilis were cultured at 37 ℃ and 30 ℃ for 24 h, respectively. All the instruments in this experiment were sterilized at high temperature (120 ℃, 110 kPa, 30 min), and the whole experiment process was completed in a sterile environment. 2.6.3. Plate Count Method 21. The prepared bacterial suspensions (108 cfu/mL) of E. coli and B. subtilis were individually flowed through the prepared filter paper. Then 1 mL of each of the effluent was diluted to a suitable multiple in the test tubes. Onto the solid LB medium described above, 0.1 mL of the diluent was applied evenly. After 15 min, the culture dish was inverted and put into the incubator (E. coli 37 ℃, B. subtilis 30 ℃) for 24 hours. Finally, culture dishes were removed and counted. All the instruments in this experiment were sterilized at high temperature (120 ℃, 30 min), and the whole experiment process was completed in a sterile environment. 2.6.4. Real (Natural) Surface Water Samples for POU Disinfection Tests. As there are numerous types of the bacterial species in the surface water, the best way to investigate the bactericidal performance of the CAEFP is to introduce the real surface (natural) water samples instead of bacteria suspension prepared in the lab. The Multi-tube Fermentation Method and MPN Method were used to count the number of E. coli in the natural water


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samples before and after disinfection 21. The difference was that the agar used here was nutrient agar medium. Briefly, 1000 mL aqueous solution containing 10 g peptone, 5 g NaCl, 3 g beef extract and 15 g agar powder was boiled and adjusted to pH 7-7.5 with 1 M NaOH. All the instruments in this experiment were sterilized at high temperature (120 ℃, 30 min), and the whole experiment process was completed in a sterile environment.

2.7. Measurement of Filtration Rate. The circular CAEFP (effective diameter: 10 cm) was clamped on the top of the water collecting tank without any support (Figure S1). Then 1 L of raw water was gradually poured into the holder. During the filtration process, the depth of the water above the filter paper was kept at 5 cm. The filtration rate was calculated using the volume of water treated divided by the effective area of the filter and the time needed. 2.8. Characterization Instruments. Scanning electron microscopy (SEM; Inspect F, FEI, USA) was used to characterize the internal structure of the material at the high voltage of 20 kV. 0.02 g AFP and CAEFP were dried and coated with gold before analysis with SEM. High power optical microscope (ECLIPSE LV100ND, Nikon, Japan) was used to obtain the optical image of the filter. The tensile strength of the AFP and CAEFP in dry and wet state was measured at room temperature using a tensile testing instrument (HDB609A-S, HAIDA, China). The samples were cut into 10×30 mm and the extension rates were set as 2 mm/min. 3. RESULTS AND DISCUSSION



3.1. Characterization of AFP. The Ag content of the filter paper was controlled by changing the concentration of AgNO3. Figure 1 showed the filter paper made by soaking in different concentrations of AgNO3 solution. With the increase of the concentration of AgNO3 solution, the color of paper gradually changed from pale yellow to dark brown. The interlaced fibers of filter paper under optical microscope were shown in Figure S2. With the increase of the concentration of AgNO3 solution, the Ag NPs modified fiber became darker, which might be the result of the aggregation of Ag NPs and forming of larger grains (see the discussion part in section 3.2).

Figure 1. AFP formed from different concentration of AgNO3 (a-f: The concentrations of AgNO3 used were 0, 0.25, 0.5, 0.75, 0.1, 0.125 M, respectively). 3.2. Optimization of Ag Content on Filter Paper. The Ag content has remarkable influence on the bactericidal properties. In a certain range, the larger the Ag content, the better the bactericidal effect of the filter paper will be. However, larger Ag content can easily lead to more Ag being released into the treated water. Here in this work, we adapt CS coating to address this issue. The Ag contents of the CAEFP at different concentrations


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of AgNO3 were shown in Figure 2. The maximum Ag content in the filter paper reached about 14 mg/g, which was a very large amount in terms of such kind of disinfectants 14. However, even at this high level, the total Ag in treated water was just about 45 µg/L, less than half of the WHO requirement (< 100 µg/L). We suspect that this is because CS coated on the paper has abundant and uniformly distributed amino groups, which provides an excellent adsorption capacity to combine with Ag (and hence prevent it from releasing into treated water) 22-23. It was clear that the presence of Ag NPs significantly enhanced the antibacterial activity compared to pure paper (Figure 3). Figure 3 also showed that the bactericidal effect of filter paper improved gradually with the increase of silver content. When the concentration of AgNO3 was 100 mM, the bactericidal effect reached the best. Surprisingly, when the

concentration of AgNO3 was further increased to 125 mM, the bactericidal effect decreased remarkably. This was also proved by Agar diffusion method (Figure S3). As shown in Figure 4, when AgNO3 was 125 mM, the silver on the filter paper showed flake-alike shape and were much larger. According to the literature, smaller Ag NPs have better bactericidal efficiency24 because they have larger surface area to volume ratio, which promotes frequent Ag NP-bacteria cell contact, and in turn boosts the release of larger amount of Ag+ into the bacteria cells within this close proximity25. We believe this is the reason that the bactericidal efficiency dropped. The result also showed that E. coli was slightly more sensitive than B. subtilis when exposed to the CAEFP. This might be the result of the



structural differences in the cell walls of the two types of bacteria 26-28. In summary, 100 mM AgNO3 was used to load Ag NPs in subsequent experiments.

Figure 2. Ag content of AFP. a-f: The concentrations of AgNO3 used were 0, 0.25, 0.5, 0.75, 0.1, 0.125 M, respectively.

Figure 3. Bactericidal efficiency of the CAEFP formed from different concentrations of AgNO3.


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Figure 4. SEM images of AFP. a-e: AFP formed at 0.25, 0.5, 0.75, 0.1, 0.125 M of the AgNO3. f: Filter paper without any modification. 3.3. Customizing the CAEFP Disinfectant for Different Raw Water Quality. High disinfection rate (processing speed) is a desired property of POU water disinfectant in addition to bactericidal efficiency. Here we show that, by varying the amount of CS coated, the disinfection rate of the CAEFP could be customized according to raw water quality with little compromise on bactericidal efficiency. As shown in Figure 5, with 0.5% of CS coating, the CAEFP showed 4 and 3.6 log reduction to E. coli and B. subtilis, respectively. When the content of CS increased, the bactericidal efficiency improved, reached 4.9 and 4.8 log reduction for E. coli and B. subtilis with 2% of CS coating, respectively. The change in bactericidal efficiency was caused by contact time variation with the Ag NPs. It can also be seen in Figure 5, when the CS content was increased from 0.5 to 2.0 wt%, the flow rate



of CAEFP was decreased from 9.3 to 0.53 L/min/m2. The adjustable processing speed is important. That is, for heavily contaminated raw water with bacteria, one could increase the CS content to 2% and get roughly 1 order of magnitude improvement in bactericidal efficiency. On the other hand, if the bacteria count in raw water was reasonably low, 0.5% of CS coating could be used with ~ 20 times faster disinfection rate (from 0.53 L/min/m2 to 9.3 L/min/m2). In the latter case, with a CAEFP size of 10×10 cm2, in roughly 20 min 2 L of disinfected water could be obtained, which was good for one person for one day.

Figure 5. Bactericidal efficiency and Flow Rate of CAEFP at different CS content. Note the y axis for bactericidal efficiency is on the left, while that for the flow rate is on the right. 3.4. Tensile Strength of CAEFP. The mechanical strength of filter paper is of great importance to its practical application. In other words, larger tensile strength at wet condition means better mechanical property, which implies longer working life and larger processing capability. Under dry condition, the tensile strength of CAEFP is 7.8 MPa,


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about 200% as that of AFP (Figure 6). In addition, as can be seen from Figure 6, the tensile strength of AFP and unmodified filter paper was almost the same, which showed that loaded Ag NPs did not affect the mechanical strength of the filter paper. When in wet conditions (working condition), however, the results varied dramatically. The mechanical strength of the filter paper without CS was reduced to only about 0.2 MPa, just about 11% of the CAEFP under the same conditions. The results showed that the mechanical strength of the disinfectant was significantly improved under the wetting condition after CS modification, which could greatly expand the life time and durability. This is because the chitosan film formed on and between the cellulose fibers in the filter paper (Figure S4). The pulling force applied on the cellulose fibers was distributed by the chitosan film, making the CAEFP be able to withstand a stronger pulling force than the filter paper alone 20.

Figure 6. Tensile strength of the filter paper, the AFP and the CAEFP in dry and wet conditions, respectively.



3.5. Total Ag in Treated Water. With the increase of silver content in filter paper, the bactericidal efficiency of filter paper became stronger. Normally, the total silver content in the treated water increases with increasing Ag loading, which hinders the improvement of the disinfection efficiency. In this work, however, the results showed the total Ag in treated water with CAEFP was only 45 μg/L, less than half of the WHO drinking water requirement (< 100 μg/L), even though the loading of Ag in the filter paper was about 13 mg/g. For comparison, under the same conditions, the total Ag in treated water with AFP was 449 μg/L. It is believed that the abundant amino groups on CS chains prevents Ag NPs and Ag+ from releasing into the processed water 29. 3.6. Application for POU Water Disinfection. Different from the bacteria suspension prepared in the laboratory, the composition of the natural water sample is complex. It is believed that the total organic matter (TOC), inorganic substance, turbidity and other factors of natural water (basic water quality matrix) could have negative impact on the bactericidal effect

30-31.

Based on this, it is very important to carry out POU disinfection

test for real water samples instead of bacterial suspension prepared with distilled water. The natural water samples from the Southwest Jiaotong University campus were used for disinfection experiments, and the basic water quality parameters were list in Table S1. After treatment, as can be seen in Table 1, the total number of bacteria in all the water samples were much lower than the requirement for drinking water of WHO (< 100 cfu/mL). In addition, E. coli could not be found in each 100 mL treated water. In order to further explore the stability of the bactericidal effect of materials, 1 L water was continuously


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filtered through a CAEFP of 10 cm in diameter, and a bacterial count was carried out every 200 mL. The results showed that the total bacteria remained below 8 cfu/mL after being treated and no E. coli was detected, which indicated that CAEFP had good stability and large processing capacity (Figure 7). Table 1. Bacteria counts in natural water samples before and after being treated with CAEFP. Total bacteria a E. coli b Index original water treated water original water treated water (cfu/mL) (cfu/mL) (cfu/mL) (cfu/100 mL) sample 1 320 2 130 Negative sample 2 450 9 160 Negative sample 3 270 1 90 Negative a Using the Plate Count Method. The total number of bacterial colonies in a 1 mL sample was obtained after 48 h incubation at 37 ℃ in nutrient agar under aerobic conditions. b

Using the Multiple Tube Fermentation Method. A group of Gram-negative, aerobic and anaerobic bacteria that can ferment lactose, produce acid and gas, at 37 ℃ within 24 h.

Figure 7. Bactericidal stability test of the CAEFP. Note that the experiment was done by analyzing every 200 mL. The total volume of the disinfected water was 1 L.



4. CONCLUSION In this study, porous CS membrane coated Ag NPs embedding filter paper was prepared for POU water disinfection. The benefits of CS coating are two folds. On the one hand, it greatly improved the durability of the filter paper-based disinfectant at wet conditions (700% increase of tensile strength compared with where there is no CS coating). On the other hand, it prevented the excessive release of total Ag into the disinfected water. Higher Ag NPs loading could be realized for better bactericidal efficiency without worrying the potential safety concern of leaked Ag content in the processed water. At optimum conditions, the total Ag in treated water by CAEFP was below 45 µg/L, just about 1/10 of Ag NPs loaded filter paper without CS, and half of the WHO drinking water standard (< 100 µg/L). The proposed method also offers a customizable protocol for the preparation of filter paper-based disinfectant for the processing of water with different quality. With 0.5 wt% CS coating, the CAEFP still had bactericidal efficiency against E. coli and B. subtilis of 4 and 3.6, respectively. A 10×10 cm2 such CAEFP sheet could provide enough disinfected water for single person for one day in just 20 min. Natural surface water samples were used for bactericidal experiments, and the results showed that the water samples were treated satisfactorily. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.


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Set-up for water disinfection; AFP images under optical microscope; Agar diffusion method of CAEFP; SEM images of CAEFP; Basic water quality matrix of real surface water. AUTHOR INFORMATION Corresponding authors *[email protected] *[email protected] ACKNOWLEDGMENTS The authors are thankful for the financial support from the National Natural Science Foundation of China (no. 21677117), Science and Technology Project of Sichuan Province (no. 2018GZ0400) and the Fundamental Research Funds for the National Universities of China (2682017CX085) for financial support. REFERENCES (1) Loeb, S.; Hofmann, R.; Kim, J. H. Beyond the Pipeline: Assessing the Efficiency Limits of Advanced Technologies for Solar Water Disinfection. Environmental Science & Technology Letters 2016, 3 (3), 73-80, DOI: 10.1021/acs.estlett.6b00023. (2) Hong, X. S.; Wen, J. J.; Xiong, X. H.; Hu, Y. Y. Silver nanowire-carbon fiber cloth nanocomposites synthesized by UV curing adhesive for electrochemical point-of-use water disinfection. Chemosphere 2016, 154, 537-545, DOI: 10.1016/j.chemosphere.2016.04.013. (3) Cheng, P.; Zhou, Q.; Hu, X.; Su, S.; Wang, X.; Jin, M.; Shui, L.; Gao, X.; Guan, Y.; Nözel, R.; Zhou, G.; Zhang, Z.; Liu, J. Transparent Glass with the Growth of Pyramid-Type MoS2 for Highly Efficient Water Disinfection under Visible-Light Irradiation. ACS Applied Materials & Interfaces 2018, 10 (28), 23444-23450, DOI: 10.1021/acsami.8b06656. (4) Song, K.; Mohseni, M.; Taghipour, F. Application of ultraviolet light-emitting diodes (UV-LEDs) for water



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Figure 1. AFP formed from different concentration of Ag NO3 (a-f: The concentrations of Ag NO3 used were 0, 0.25, 0.5, 0.75, 0.1, 0.125 M, respectively). 80x53mm (300 x 300 DPI)


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Figure 2. Ag content of AFP. a-f: The concentrations of silver nitrate used were 0, 0.25, 0.5, 0.75, 0.1, 0.125 M, respectively. 80x56mm (300 x 300 DPI)



Figure 3. Bactericidal efficiency of the CAEFP formed from different concentration of Ag NO3. 80x56mm (300 x 300 DPI)


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Figure 4. SEM images of AFP. a-e: AFP formed at 0.25, 0.5, 0.75, 0.1, 0.125 M of the AgNO3. f: Filter paper without any modification. 140x91mm (300 x 300 DPI)



Figure 5. Bactericidal efficiency and Flow Rate of CAEFP at different CS content. Note the y axis for bactericidal efficiency is on the left, while that for the flow rate is on the right. 85x60mm (300 x 300 DPI)


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Figure 6. Tensile strength of the filter paper, the AFP and the CAEFP in dry and wet conditions, respectively. 80x56mm (300 x 300 DPI)



Figure 7. Bactericidal stability test of the CAEFP. Note that the experiment was done by analyzing every 200 mL. The total volume of the disinfected water was 1 L. 80x56mm (300 x 300 DPI)


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Killing Two Birds with One Stone: Coating Ag NPs Embedded Filter

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