Synthesis and immobilization of silver nanoparticles on aluminosilicate nanotubes and their antibacterial properties
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A novel colloidal method is presented to synthesize silver nanoparticles on aluminosilicate nanotubes. The technique involves decomposition of AgNO3 solution to Ag nanoparticles in the presence of aluminosilicate nanotubes at room temperature without utilizing of reducing agents or any organic additives. Aluminosilicate nanotubes are shown to be capable of providing a unique chemical environment, not only for in situ conversion of Ag+ into Ag0, but also for stabilization and immobilization of Ag nanoparticles. The synthesis strategy described here could be implemented to obtain self-assembled nanoparticles on other single-walled metal oxide nanotubes for unique applications. Finally, we demonstrated that nanotube/nanoparticle hybrid show strong antibacterial activity toward Gram-positive Staphylococcus epidermidis and Gram-negative Escherichia coli.
KeywordsNanotube Metal oxide Metal Nanoparticle Antibacterial
Yet another important class of materials, noble metal particles (NMPs), especially silver nanoparticles (AgNPs), has been extensively investigated for catalysis, textile industry, water disinfection, cosmetics, and food packaging owing to their unique physical and chemical properties (Lee and El-Sayed 2006; Rai et al. 2009; Jiang et al. 2005). However, it is a challenge to synthesize MNPs with high dispersion and small particle size because particles suffer from the drawback of agglomeration, which causes undesirable changes to their properties and restricts their applications (Moritz and Geszke-Moritz 2013). There are many traditional synthetic routes to formation of MNPs including chemical treatment (Sun and Xia 2002; Kumar 2007; Sondi et al. 2003), irradiation (Henglein and Giersig 1999; Yin et al. 2004; Abid et al. 2002; Pol et al. 2005), thermal treatment (Navaladian et al. 2007), and photochemical (Huang et al. 1996), or radiolytic reduction (Krklješ et al. 2007). Many of these methods are time consuming and require the usage of additional chemicals such as sodium borohydride (NaBH4) as reducing agent and polyvinyl pyrrolidine (PVP) as stabilizer or protecting agent, which can affect the surface properties of nanoparticles and diminish the overall performance of the material. Prevention of the impurities resulting from the usage of additional chemicals, minimizing the time and preparation costs, achieving large-scale synthesis with good reproducibility and controllable nanoparticle characteristics remains challenging.
To this end, we apply a fast, facile, and generalizable new synthesis strategy to construct novel nanohybrid architectures comprised of AgNPs and AlSiNTs, without using of reducing/stabilizing agents, or irradiation, by one-step facile and convenient colloidal approach, in which AlSiNTs act as unique templates and carriers to AgNPs. While pristine AlSiNTs and AgNPs are known to have remarkable properties, their hybrids not only present a new field of basic research, but also endow new and enhanced functions with performances far beyond those of the individual counterpart materials, which will lead to the creation of novel nanohybrid systems for practical applications with significantly improved performance. In this paper, we show the potential of AlSiNT/AgNP nanocomposites as an antibacterial agent, and this finding may be useful in the application of single-walled metal oxide nanotube-based hybrids against clinically important Gram-negative and Gram-positive bacteria.
A facile, convenient, generalizable, and scalable aqueous phase-based strategy has been developed for the preparation of AlSiNT/AgNP hybrids at room temperature. It involves hydrothermal synthesis of AlSiNTs followed by in situ generation of AgNPs from silver nitrate (AgNO3) solution in the presence of AlSiNTs, without using of an additional chemical or treatment. All chemicals were purchased from Sigma-Aldrich and used as received without further purification.
Aluminosilicate nanotube synthesis
To synthesize AlSiNTs, 0.05 M TEOS (tetraethoxysilane) was mixed with 0.1 M ASB (aluminum tri-sec-butoxide), and the mixture was introduced dropwise to a mildly acidic solution of 0.05 M HCl. The obtained solution was kept under vigorous stirring for 24 h at 25 °C. The solution was then diluted to 0.02 M Al and the temperature was increased to 95 °C. The reaction lasted for 4 days (96 h). At the end of the reaction, the nanotubes were precipitated by dropwise addition of a 30 wt % ammonia solution. The resulting gel was centrifuged, and the supernatant was discarded. Then, 10 N HCl was added dropwise to redisperse the nanotubes. Finally, the dispersion was dialyzed for 4 days against deionized water using a 15-kDa membrane. Pure nanotube networks are obtained by freeze-drying of the precipitates for 24 h.
Preparation of AlSiNT/AgNP nanohybrids
Electron microscope images were taken on a Tecnai F20 transmission electron microscope (TEM) with an operating voltage of 200 kV. Dark-field scanning transmission electron microscopy (STEM) images were obtained by Helios Nanolab 600 FIB at 30 kV. The samples were prepared using ultrathin carbon TEM grids (Ted Pella) by dropping a small amount of the AlSiNT/AgNP nanohybrid dispersion on the grid, removing the extra solution using a blotting paper and allowing the TEM grid to dry.
Assessing antibacterial activity
The Gram-positive S. epidermidis (ATCC 35984) and Gram-negative E. coli (ATCC 25922) were chosen as test organisms. Both organisms were grown in Trypticase Soy broth at 37 degrees C. Overnight cultures of each organism were diluted to approximately 1000 colony forming units (cfu) per 0.1 mL. 1 mL of the diluted culture was combined with 100 µl of the solution that was tested. Antibacterial activities of three samples were tested against each type of bacteria. Samples tested were 0.01 M AgNO3 solution obtained by mixing AgNO3 with DI water, bare AlSiNT dispersion in DI water obtained by sonication for 3 min, and colloidal AlSiNT/AgNP hybrid solution. A 100 microliter control sample of the diluted bacteria was plated on a trypticase soy agar (TSA) plate to verify the cfu/mL. For each sample, the mixture of bacteria and tested material was centrifuged for 1 min at 15,000 rpm. The cells were resuspended and 100 microliters plated on TSA plates at 0 and 60 min. Plates were incubated 24 h at 37 degrees C and the surviving cfu counted.
Results and discussion
In this work, by utilizing AgNO3 as a silver source, we fabricated AlSiNT/AgNP nanohybrids. The whole process of nanohybrid formation is schematically illustrated in Fig. 2.
An important finding from these studies is that AlSiNTs are not only inorganic supports to AgNPs, but also present a structurally unique nanotube system in their capability to directly synthesize and immobilize AgNPs preventing the agglomeration effects without the need for reducing and stabilizing agents. To the best of our knowledge, the formation and stabilization of AgNPs could only be achieved through the use of certain reducing and stabilizing agents, and even then obtaining small particles as small as 3 nm is quite challenging (Bastús et al. 2014; Pinto et al. 2010; Leo et al. 2013). This is the first synthetic route that allows synthesis of highly stable small-sized AgNPs without any chemicals and simultaneous synthesis of novel nanotube-based nanohybrid architectures.
A novel method has been developed for the synthesis of AlSiNT/AgNP nanohybrids. Small size (~3 nm) and uniform distribution of AgNPs on AlSiNTs were obtained at room temperature without using reducing/stabilizing agents, any prior surface modification, irradiation, or heat treatment. The other advantages of the method are its simplicity, scalability, and repeatability. An important and exciting implication of the developed method is that peculiar nanotube surface behaves both as a reducing agent and a stabilizer. Nanoparticle morphology/stability can be controlled by surface characteristics of nanotubes and simple aqueous phase chemistry (i.e., by pH and temperature). The resultant colloidal suspensions can be readily used to prepare nanohybrid films, powders, and gels. Inherent ability of single-walled metal oxide nanotubes to directly form and immobilize metallic agents, without expensive methods and environmentally/biologically toxic substances that hinder their usage, is expected to excite a new field of fundamental research while presenting a unique combination of material systems, which opens up paths for a wide range of applications in nanotechnology from drug delivery to catalysis. Here, we demonstrated that nanohybrids based on AlSiNTs and MNPs show excellent antibacterial activity toward both Gram-positive and Gram negative bacteria and are potential candidate materials to be used in applications where an antibacterial effect is desired.
This work was supported by Young Investigator Seed Funding through Materials Research Center at Missouri University of Science and Technology.
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