Halloysite Nanotubes Supported Ag and ZnO Nanoparticles with Synergistically Enhanced Antibacterial Activity
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Novel antimicrobial nanocomposite incorporating halloysite nanotubes (HNTs) and silver (Ag) into zinc oxide (ZnO) nanoparticles is prepared by integrating HNTs and decorating Ag nanoparticles. ZnO nanoparticles (ZnO NPs) and Ag nanoparticles (Ag NPs) with a size of about 100 and 8 nm, respectively, are dispersively anchored onto HNTs. The synergistic effects of ZnO NPs, Ag NPs, and HNTs led to the superior antibacterial activity of the Ag-ZnO/HNTs antibacterial nanocomposites. HNTs facilitated the dispersion and stability of ZnO NPs and brought them in close contact with bacteria, while Ag NPs could promote the separation of photogenerated electron-hole pairs and enhanced the antibacterial activity of ZnO NPs. The close contact with cell membrane enabled the nanoparticles to produce the increased concentration of reactive oxygen species and the metal ions to permeate into the cytoplasm, thus induced quick death of bacteria, indicating that Ag-ZnO/HNTs antibacterial nanocomposite is a promising candidate in the antibacterial fields.
KeywordsHalloysite nanotubes Nanocomposites Ag nanoparticles ZnO nanoparticles Antibacterial activity
- Ag NPs
Colony forming units
- E. coli
High-resolution transmission electron microscope
Length to diameter
Polyethylene glycol methyl ether
Scanning electron microscope
Transmission electron microscopy
X-ray photoelectron spectroscopy
- ZnO NPs
Antibacterial materials such as metals [1, 2, 3] and metal oxides  inhibit bacteria growth by oxidative stress with the production of reactive oxygen species. Zinc oxide (ZnO) is one of representative metal oxide semiconductors used as commercially antibacterial materials due to low-cost, abundance, and environmentally friendly feature. Several studies have proposed the antibacterial mechanism of zinc oxide nanoparticles (ZnO NPs) to be damaging the cell membrane and releasing reactive oxygen species [5, 6, 7, 8]. However, the easy aggregation into big cluster of ZnO NPs at nanoscale in the solution will weaken the antibacterial effect . The low-photoinactivation efficiency in visible region also impose a negative influence on their antibacterial activity.
The dispersibility of ZnO NPs in aqua can be improved by surface modification, but the highly expensive surfactant increases the manufacture cost, including polyvinylpyrrolidone (PVP), oleic acid (OA), together with diethanolamine (DEA), polyethylene glycol methyl ether (PGME), poly(methyl methacrylate) (PMMA), and polystyrene (PS). Also, graphite sheet and carbon nanotubes possessed larger specific surface area, which can indeed facilitate the dispersion of nanoparticles, but their easy carbonization at high-temperature, high-cost, and complicated preparation process will limit their large-scale applications, whereas halloysite nanotubes (HNTs) as the support could make up for the above disadvantages to some extent. Natural clay minerals, such as kaolinite [9, 10], halloysite , montmorillonite [2, 12, 13, 14, 15, 16], and palygorskite [17, 18, 19, 20] are widely used in the catalysis, energy storage, and wastewater treatment application by loading the traditional nanomaterials, which means that they can be used as cost-efficient matrix to improve the dispersion of ZnO given to their natural nanostructures, unique ion exchange capacities, superior hydrophily, and excellent mechanical properties. Such features may not only bring ZnO NPs to be closer to the membrane of bacteria to hamper the normal function of bacteria  but also increase the local zinc concentration to inhibit the growth of bacteria . A series of novel metal nanoparticles such as gold , silver , and copper  have strong bactericidal activities for bacteria, fungi, and virus. Using a combination of noble metal and metal oxide antibacterial agent, bacterial growth and survival is believed to be effectively inhibited.
Halloysite (i.e., halloysite nanotubes, HNTs) as a dioctahedral 1:1 nanoclay of the kaolin group, consists of hollow cylinders formed by multiple rolled layers [25, 26, 27, 28, 29]. Halloysite-based nanocomposites have gained specific research attention as a potential material for various biological applications (e.g., antibacterial, enzyme immobilization, and controlled drug delivery) . Such interest can be hugely attributed to their physicochemical properties: tubular structures, high-specific surface area, length-to-diameter (L/D) ratio [31, 32], and hydrophobicity. Ag nanoparticles (Ag NPs), as one of the most commercialized bactericidal materials, exhibit higher toxicity to microorganisms by penetrating through the membrane and inducing cell death [1, 15, 33]. Halloysite facilitates the dispersity and controls the distribution of ZnO NPs and brought them close to Ag NPs within 1–10 nm. In this way, ZnO NPs and Ag NPs could efficiently have contact with bacteria cell membrane and remarkably interrupt the membrane functions. A small amount of loaded Ag NPs can achieve the synergistic antimicrobial effect, which could cause direct damage to the bacterial cell membrane  and dramatically enhance the antibacterial activity of ZnO NPs. In this paper, Ag-ZnO/HNTs antibacterial nanocomposites were prepared by incorporating HNTs and Ag NPs into ZnO NPs. The interfacial characteristics of ZnO NPs, Ag NPs, and HNTs were investigated. A typical bacterium Escherichia coli was used to assess the antibacterial activity of Ag-ZnO/HNTs antibacterial nanocomposites and enhanced antibacterial mechanism was proposed.
Raw halloysite mineral was obtained from Chenxi, Hunan province in China. The visible impurities like the brown and black parts were eliminated through hand-selecting process, the white halloysite mineral was milled in an agate mortar before all of the powders passed a 300 mesh sieve. The powder was immersed in water and magnetically stirred for 2 h, then filtered and washed by ethanol, followed by drying at 60 °C for 2 h, finally for the experiment use. A typical process for the synthesis of ZnO/HNTs nanocomposites is described as follows: 2.4 g HNTs, 3.2 g CO(NH2)2, and 3.2 g Zn(NO3)2∙6H2O were dispersed in 50 mL distilled water, ultrasonic dispersion for 15 min and stirred for 3.5 h at 95 °C, and then calcined at 400 °C for 4 h, labeled as ZnO/HNTs. ZnO/HNTs with different ZnO loading (15, 30, 45, and 60%) were prepared by changing the ZnO:HNTs mass ration. For comparison purpose, pure ZnO was synthesized using the same conditions without adding HNTs. As for the synthesis of Ag-ZnO/HNTs nanocomposites, 2 g ZnO/HNTs, 0.07 g AgNO3, and 0.1 g PVP were dispersed in 40 ml distilled water under ultrasonic dispersion for 15 min. Ten milliliter aqueous solution contained 0.02 g NaBH4 was added dropwise under stirred for 30 min. The products were further washed with ethanol and water for several times, dried under vacuum at room temperature, and labeled as Ag-ZnO/HNTs.
The X-ray diffraction (XRD) measurements were recorded on a DX-2700 X-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm). Data were collected from 2θ range of 5–80° with a scan rate of 0.02°/s and at 40 kV and 40 mA. The morphology and the nanostructure of the samples were observed using a JEOL JSM-6360LV scanning electron microscope (SEM) at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) studies were performed using a JEOL JEM-2100 F operating at 200 kV. The particle size and lattice distance of samples were observed with a high-resolution transmission electron microscope (HRTEM, JEM-3010; JEOL). X-ray photoelectron spectroscopy (XPS) measurements were taken using a spectrometer (ESCALAB 250; Thermo Fisher Scientific).
Gram-negative Escherichia coli (E. coli) was used to test the antibacterial activity of the samples (ZnO, ZnO/HNTs, and Ag-ZnO/HNTs). Luria Bertani (LB) broth and nutrient agar were used as sources for culturing E. coli at 37 °C in aerobiosis on the rotary platform. The bacterial was in series diluted to reach the concentration for plate count method. Ten milligram nanomaterial was resuspended in the test tube contained 10 mL LB liquid, 2 mL E. coli was pipetted into the test tubes and placed in a rotary platform at 37 °C for 4 h. To ensure that any decrease in bacterial number was due to the exposure to the nanomaterial treatment, control group was included in the experiment with the absence of nanomaterial. One hundred microliter samples were transferred onto the LB nutrient agar plates and sprayed evenly on top of the plates using a sterile glass rod. After the bacteria were dried, the petri plates were inverted and incubated at 37 °C for 18–20 h, visible colonies were quantified after incubation.
TEM analysis was performed to observe the effect of Ag-ZnO/HNTs on morphology and surface structure of the bacterial cells. TEM images of samples were accomplished using the following procedures: the cells exposed to Ag-ZnO/HNTs for 4 h were centrifuged and fixed with 2.5% glutaraldehyde overnight at 4 °C, followed by washing with 0.1 M PBS, and then postfixed with 1% osmium tetroxide for 1 h, dehydrated in graded concentrations of ethanol, and embedded in epoxy resin. The resin embedded cells was polymerized at 60 °C overnight. Thick 1~2 μm and thin 90 nm sections were cut using an ultramicrotome (LEICA EM UC7). Grids were stained with uranyl acetate and lead citrate stains. Ultrathin 90 nm sections were examined with TEM transmission electron microscope (HT7700) operated at 80 kV.
Results and Discussion
Ag-ZnO/HNTs antibacterial nanocomposites were prepared by incorporating HNTs and Ag NPs into ZnO NPs. HNTs facilitated the dispersion and stability of ZnO NPs and brought them in close contact with bacteria. Ag NPs promote the separation of photogenerated electron-hole pairs and enhance the antibacterial activity of ZnO NPs. ZnO/HNTs shown evident inhibition on bacteria growth with increased nanocomposite concentration than that on equivalent doses of pure ZnO. Ag-ZnO/HNTs nanocomposites showed the highest antibacterial activity and stability. The outstanding results demonstrated excellent antibacterial properties of Ag-ZnO/HNTs antibacterial nanocomposites.
This work was supported by the National Natural Science Foundation of China (41572036), the National Science Fund for distinguished Young Scholars (51225403), the Hunan Provincial Science and Technology Project (2016RS2004, 2015TP1006), the China Postdoctoral Science Foundation (2015 M582346), the State Key Laboratory of Powder Metallurgy, Central South University (2015-19), and the Postdoctoral Science Foundation of Central South University (155219).
HY conceived the project and wrote the final paper. ZS wrote the initial drafts of the work. ZS and YZ designed the experiments and synthesized and characterized the materials. ZS, YZ, and QY analyzed the data. All authors discussed the results and commented on the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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