- Nano Express
- Open Access
A Convenient Ultraviolet Irradiation Technique for Synthesis of Antibacterial Ag-Pal Nanocomposite
© The Author(s). 2016
- Received: 4 July 2016
- Accepted: 20 September 2016
- Published: 27 September 2016
In the present work, palygorskite (Pal) was initially subjected to an ion-exchange reaction with silver ions (Pal-Ag+). Subsequently, Ag-Pal nanocomposites were assembled by a convenient ultraviolet irradiation technique, using carbon dots (CDs) derived from wool fiber as the reducing agent. The obtained nanocomposites were characterized by powder X-ray diffraction (XRD), ultraviolet-visible (UV-vis) spectroscopy, transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy. The XRD patterns and UV-vis absorption spectra confirmed the formation of the Ag nanoparticles (NPs). Meanwhile, the TEM images showed that the Ag NPs, which exhibited sizes in the range of 3–7 nm, were located on the surface of the Pal nanofiber structures. Furthermore, the antibacterial activity of the nanocomposites was evaluated against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria by applying the disc diffusion method and minimum inhibitory concentration test. Owing to their good antibacterial properties, the Ag-Pal nanocomposites are considered to be a promising bactericide with great potential applications.
- Carbon dots
- Ag NPs
- Antibacterial activities
Among the various materials used for antimicrobial activities, nanostructured materials are considered to be more effective because of their high surface-area-to-volume ratio [1–3]. Additionally, the use of inorganic nanoparticles (NPs) as antimicrobial agents has several benefits, such as improved stability and safety, compared with the use of organic antimicrobial agents [4, 5]. Typical inorganic NPs consist of Ag, Cu, ZnO, TiO2, SnO2, and CuO .
Ag NPs have been reported to be an effective antimicrobial agent; they have been widely used in a broad spectrum of applications including water purification, wound care, and food production [7, 8]. However, free Ag NPs without a supporting matrix might be inadequate for applications where prolonged Ag release is needed (e.g., in medical devices) . On the other hand, the small size of the NPs favors their aggregation, which might cause disadvantageous changes in their chemical, physical, and antibacterial properties . To overcome these issues, hybrid processes combining Ag NPs with other environmentally friendly, inert, and cheap materials are adopted to fully exploit their properties in various applications .
Among the possible matrices, clay minerals are particularly attractive because of their cost effectiveness and widespread availability [12, 13]. Palygorskite (Pal), a member of the clay group, is a hydrated magnesium aluminum silicate with reactive hydroxy groups on the surface . Pal consists of a three-dimensional network of densely packed rods; the single rod crystal is the smallest structure unit with a length in the range of 200–500 nm and a diameter of 10–25 nm . Owing to the special nanostructure, inherent stability, and large specific surface area, considerable effort has been devoted to discover new approaches of using Pal in its native and modified forms [16–18].
Composites of metal NPs supported on clay can be fabricated by using various techniques, such as ion-exchange processes, solvothermal synthesis, chemical reduction methods, and sol-gel processes . However, the development of a simple and green synthetic method with environmentally friendly reagents still represents a great challenge . As an emerging class of carbon nanomaterials, carbon dots (CDs) have drawn much attention due to their outstanding photostability, low environmental risk, high biocompatibility, and interesting electron transfer behavior [21–23]. In recent years, significant research efforts have been dedicated to produce noble metal NPs using CDs as the reducing agent [24, 25], providing new opportunities for the synthesis and applications of metal nanomaterials .
In the present work, wool fiber, a cheap and abundant bioresource, was used to fabricate wool fiber-derived CDs (WCDs) by a hydrothermal method. Then, using the obtained WCDs as a reducing agent, Ag-Pal nanocomposites were fabricated by ultraviolet (UV) radiation method, which is a simple and green process (90 min exposure to a UV lamp). The composites were then characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Finally, the antibacterial activity of the nanocomposites was evaluated using the minimum inhibitory concentration (MIC) test and Kirby-Bauer disk diffusion method. Owing to the excellent antibacterial properties against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria, the Ag-Pal nanocomposite is considered a promising bactericide with great potential applications.
Pal (Jiangsu Autobang Co., Ltd., China) was treated with HCl and H2O2 to remove the impurities and activate the hydroxyl groups on the surface . AgNO3 was obtained from Aladin Ltd. (Shanghai, China). E. coli (ATCC 25,922) and S. aureus (ATCC 25,923) were purchased from China General Microbiological Culture Collection Center (Beijing, China). Deionized (DI) water was used for the experiments.
TEM was performed on a JEOL-2010 TEM (Japan) at 200 kV. FTIR spectra were collected within the 4000–400 cm−1 wavenumber range using a Nicolet 360 FTIR spectrometer with the KBr pellet technique. Nitrogen (N2) adsorption-desorption isotherms were measured by an ASAP 2010 analyzer with N2. Powder XRD (PXRD) patterns were obtained with a Rigaku-Dmax 2400 diffractometer using Cu-Kα radiation in the 2θ range of 5°–70°. UV-visible (UV-vis) spectra were recorded using a Perkin-Elmer Lambda 20 UV-vis spectrometer. Excitation and emission spectra were collected using a Hitachi F-4500 Fluorescence spectrophotometer. Quantitative determinations of Ag were obtained by inductively coupled plasma optical emission spectrometry (ICP-OES; IRIS Advantage ER/S spectrophotometer). The UV-vis absorption spectra were recorded on a Perkin-Elmer Lambda 950 spectrophotometer.
Synthesis of the WCDs
WCDs were prepared by the hydrothermal treatment of wool fibers. In a typical synthesis, 0.4 g of wool fibers was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 180 °C for a period of 12 h. When the reaction was completed, the autoclave was cooled down naturally. The resulting light yellow solution was centrifuged at 16,000 rpm for 30 min to remove the large dots, yielding a light brown aqueous solution of WCDs (yield ~36.5 %).
Preparation of the Ag Ion-Exchanged Pal (Pal-Ag+)
A 25 mg of AgNO3 was dissolved in 25 mL of DI water. Next, 200 mg of Pal was subjected to the ion-exchange process in a AgNO3 solution at 25 °C for 5 h with continuous stirring. At the end of the reaction, Pal-Ag+ was collected by centrifugation at 8000 rpm for 30 min. The product was then dried at 80 °C overnight. It could be shown from the EDX that the concentration of elemental silver varied from 3.0 to 8.7 % for the product (Additional file 1: Figure S1), which indicated the formation of Pal-Ag+.
Fabrication of the Ag-Pal Nanocomposite
Part of Pal-Ag+ (40 mg) was dispersed in ethanol by ultrasonic process; 4 mg of WCDs was added. Then, the mixture was placed in a UV quartz cuvette (1 × 1 × 5 cm) located 3 cm away from a 6 W UV lamp (Vilber Lourmat). After the photoreaction (254 nm, 50 min), the color of the solution changed from light yellow to dark brown, implying the formation of the Ag-Pal nanocomposite (Additional file 2: Figure S2).
E. coli and S. aureus were cultivated in a beef extract peptone medium at 37 °C for 12 h with a shaking incubator. The disk diffusion test was performed according to a reported procedure , while equal amounts of Pal-Ag+ and Ag-Pal nanocomposites (200 ± 10 μg) were loaded into the filter papers. The bacterial suspension (100 μL of 104–105 CFU mL−1) was applied uniformly on the surface of the nutrient agar plates. Then, the disks were placed on the plates and incubated at 35 °C for 24 h. After the incubation period, the zones of inhibition were measured and digital images of the plates were captured. For the minimum inhibitory concentration (MIC) test, the survival of the organisms was observed by visual inspection as recorded .
To evaluate the stability of the Ag-Pal nanocomposite, leaching tests were performed. Typically, 0.2 g of nanocomposite was dispersed in 20 mL of DI water and vigorously shaken in a shaking thermostatic bath (30 °C, 200 rpm) for various time periods. After shaking, the suspensions were centrifuged at 8000 rpm for 20 min. The supernatant was analyzed using ICP-OES to establish the quantity of Ag leached into the water.
Characterization of WCDs
Characterization of the Ag-Pal Nanocomposite
Antibacterial Properties of the Ag-Pal Nanocomposite
Leaching test results of the Ag-Pal nanocomposite
In summary, Ag-Pal nanocomposite was assembled by a rapid and facile UV radiation method (λ = 254 nm), using CDs derived from wool fiber as a reducing agent. The PXRD spectra and TEM observations confirmed the formation of Ag NPs on the clay surfaces. The antibacterial activity tests revealed that the nanocomposite has a good biocidal effect on both Gram-positive (S. aureus) and Gram-negative (E. coli) strains, while the leaching tests showed that the nanocomposite remained stable under vigorous agitation. Thus, the obtained Ag-Pal nanocomposite is considered to be a promising bactericide with great potential applications.
This work was supported by the National Natural Science Foundation of China (No. 41330317), the Science Foundation of Hebei (No. D2014402046), and the Program for One Hundred Innovative Talents in Universities of the Hebei Province (No. BR2-204). The authors are indebted to Jianjun Liu (Hebei University of Engineering, Handan) for the supplement of lyophilizer.
SH and SQ fabricated all the nanostructures and drafted the manuscript. HZ performed the antibacterial property tests on the composite nanostructures. LK and XL planned the whole work and revised the manuscript. CZ and YJD performed the PXRD tests of the composite nanostructures. All authors read and approved the final manuscript.
SH worked as a docent in the College of Science, Hebei University of Engineering. His research interests are nanochemistry, nanotechnology, and coordination chemistry. HZ is a Ph.D student in the College of Life Science, Lanzhou University. Her research interests are nanochemistry and biochemistry. Both LK and XL worked as docents in the College of Science, Hebei University of Engineering, their research interests are nanochemistry and nanotechnology. CZ and YD worked as technicians in the College of Science, Hebei University of Engineering, their research interest is nanotechnology. SJQ works as the Dean of the College of Science, Hebei University of Engineering. His research interests are nanobiotechnology and mineral chemistry.
The authors declare that they have no competing interests.
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