Table of Contents
ISRN Nanotechnology
Volume 2012, Article ID 539601, 8 pages
http://dx.doi.org/10.5402/2012/539601
Review Article

Dissolvable Trimolybdate Nanowires as Ag Carriers for High-Efficiency Antimicrobial Applications

1Key Laboratory for the Physics and Chemistry of Nanodevices and School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China
2Institute of Biophysics, Chinese Academy of Sciences, 15 Datun Road, Chaoyang District, Beijing 100101, China

Received 5 October 2011; Accepted 9 November 2011

Academic Editors: C. A. Charitidis and C. Malagù

Copyright © 2012 Jiongwei Xue et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Elimination of bacteria and other microbes effectively is important to our daily life and a variety of medical applications. Here, we introduce a new kind of trimolybdate nanowires, namely A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O , that carry a large amount of Ag atoms in the lattice and Ag-rich nanoparticles on the surface. These nanowires can eliminate bacteria of E. coli, Staphylococcus aureus, and unknown microbes in raw natural water with high efficiency. For example, they can inactivate more than 98% of E. coli with a nanowire concentration of only 5 ppm in the solution. The excellent sterilization performance is attributed to the combined effects of Ag ions, Mo ions, and Ag-rich nanoparticles of the A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires. These nanowires are not dissolvable in deionized water but can be dissolved by the metabolic materials released from bacteria, making them attractive for many biological applications.

1. Introduction

Silver (Ag), in the form of bulk, sheet, wire, powder, and composite, has a long history of being used as a broad-spectrum antimicrobial material [17]. In particular, with a high surface-to-volume ratio, Ag nanoparticles showed much higher antimicrobial efficiency than Ag microparticles [816]. In some biological applications, it should be rather interesting if Ag composites or nanoparticles could be delivered to target tissues and/or cells with a controllable carrier of nanomaterial. Similar concepts have already been well presented in studies on drug delivery of molecular medicines, characterization, imaging, and targeting with carbon nanotubes, nanowires, and other nanomaterials [1628]. However, in these approaches, side effects caused by the residual nanomaterials inside tissues or cells become a great concern. Some nanomaterials, such as carbon nanotubes, are chemically and biologically inert, and in vivo studies showed that they could accumulate and be permanently trapped in living organs or tissues, causing unexpected multiple toxicity [2933].

Here, we introduce a new kind of trimolybdate nanowires, namely A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O , that carry a large amount of Ag atoms in the lattice and Ag-rich nanoparticles on the surface. Similar to previously reported powders of Ag2MoO4, Ag2Mo2O7, Ag6Mo10O33, Ag2Mo3O10·1.8H2O, and so forth [3437], the A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires show strong antimicrobial properties, for example, they kill E. coli, Staphylococcus aureus, and unknown microbes in raw natural water with high efficiency. The nanowires are not dissolvable in deionized water; however they could be dissolved by the metabolic chemicals released from bacteria, making them attractive for many biological applications.

2. Materials and Methods

The A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires and other nanomaterials were synthesized from a mixed aqueous solution of (NH4)6Mo7O24·4H2O (purity 99.999%) and AgNO3 (purity 99.999%) that were separately dissolved in deionized water. Their mass ratio in the mixture, ranging from 10 : 1 to 1 : 3, had been systematically studied. The pH value of the starting solutions was controlled with drops of diluted HNO3 acid. The mixed solution in an open beaker was immersed into a water bath kept at a set temperature between 20 and 80°C, and a magnetic stirrer was used during the whole reaction process. In minutes yellowish-white precipitates were obtained from the mixed solution, and gradually the whole solution turned into porous, paste-like precipitates with some residual water. After centrifuging and rinsing the precipitates with deionized water for 3–6 cycles, the samples were dried in air and characterized by means of scanning electron microscopy (SEM, Tecnai XL30F), X-ray diffraction (XRD, Rigaku D/max-2400) for morphology, transmission electron microscope (TEM, Tecnai G20, 200 kV) for imaging, in situ energy-dispersive X-ray (EDX) and selected area electron diffraction (SAED), and Cu spectra for surface binding energy of the Ag atoms. Thermogravimetric analysis (TGA, Q600) was performed in the ambient of pure nitrogen gas.

For the cytotoxicity test, 293E cell line was chosen to be the human cells sample. First, the cells sample was cultured with nutrient medium of DMEM, which contained 10% fetal calf serum to reach a concentration of 60–80% in a culture dish with diameter of 10 cm. Next, the nutrient medium was moved away and the cells were washed out with 2 mL PBS. Then trypsogen was added into the sample for 40 seconds. The pseudo pods of the adherent cells retracted and the cells turned into round shapes. Subsequently, 3 mL nutrient medium of DMEM was added into the sample and the solution was blown with a pipette to suspend cells, meanwhile the number of cells was counted with a count plate. After that, about 3 × 1 0 5 cells were taken to each of 12-well plates, and the solution in each well was supplemented to 1 mL with DMEM and stirred until being uniform. At last, the sample was placed at an incubator at 37°C and 5% CO2, adding in nanowires with certain concentrations. The statement of the cell samples was observed closely after different intervals. For the LB plate antibacterial experiments, the E. coli DH5a samples were inverted with PCI-neo-GFP plasmid, and the number of colonies for each trial was counted. The samples were cultured in nutrient Agar (N.A.) at 37°C for 96 hours. For the skin safety experiments, 4 rabbits weighing 2.3–2.5 kg were used. The temperature and humidity of the rabbit hutch were kept at 18–26°C and 40–61%, respectively. Hairs on both sides of the spine of each rabbit were totally sheared away, leaving two pieces of clean skin, each about 3 square centimeters. On one piece of the clean skin, 0.5 mL of 1000 ppm Ag-doped nanowires in deionized water was smeared uniformly, while the other piece was left untreated for contrast.

3. Results and Discussion

The nanowires were synthesized at 30°C from a mixed aqueous solution of (NH4)6Mo7O24·4H2O and AgNO3. Various phases of Ag-rich trimolybdate nanomaterials could be obtained in a controlled way when the pH value of the starting solution was adjusted. Figure 1 shows in the left panel 5 groups of typical SEM micrographs of our samples made under varying pH values and their corresponding XRD spectra in the right panel. From the XRD spectra we distinguished different crystalline phases in the as-synthesized nanomaterials. When the starting pH value was set at 1.0, hexagonal prisms of the NH3(MoO3)3 phase (JCPDS 77-0354) with diameters around one micron were obtained, as shown in Figure 1(a). End product obtained under initial pH of 2.0 consisting of both NH3(MoO3)3 (marked by solid red triangles) and A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O phases is shown in Figure 1(b). Nanowires shown in Figure 1(c) and Figure 1(d) were obtained when the initial pH values were set at 3.0 and 4.0, respectively. Their XRD spectra show the major phase of A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O and peaks of the minority NH3(MoO3)3 phase (marked by solid red triangles). Here, the chemical formula of A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O was obtained by calculations based on both the experimental XRD and TGA data that will be discussed in the following sections.

fig1
Figure 1: Left panel, shows typical SEM micrographs of 5 kinds of samples synthesized at 30°C under varying pH values of 1.0, 2.0, 3.0, 4.0, and 4.3–4.7, respectively. Right panel is their corresponding XRD spectra. The peaks for NH3(MoO3)3 phase are marked by solid red triangles in (b)–(d). Shown in (e), without adding any acid, the mixture solution of (NH4)6Mo7O24·4H2O and AgNO3 naturally maintained its pH value in a narrow range of 4.3–4.7 in the whole reaction process.

Without adding any acid, the mixture solution of (NH4)6Mo7O24·4H2O and AgNO3 naturally maintained its pH value in a narrow range of 4.3–4.7 at 30°C in the whole reaction process. This was attributed to the existence of N H 4 + ions in the solution [38, 39]. Synthesized under this condition, the end products were uniform and long nanowires, as shown in Figure 1(e).

The blue line in Figure 2(a) plots the XRD spectrum of the nanowires shown in Figure 1(e). It reveals a single crystalline phase that matches well to the tetragonal phase of (NH4)2Mo3O10 (JCPDS 79-1905), peaks of which are marked as black lines in Figure 2, with slight offsets. The lattice constants of (NH4)2Mo3O10 are 𝑎 = 1 . 3 1 8 2 n m , 𝑏 = 0 . 7 5 8 9 n m , and 𝑐 = 0 . 9 2 8 6 n m . The lattice constants of our tetragonal nanowire phase are calculated to be 𝑎 = 1 . 3 3 6 n m , 𝑏 = 0 . 7 5 4 n m , and 𝑐 = 0 . 9 2 1 n m , where a becomes 1.35% bigger and 𝑏 and 𝑐 become 0.65% and 0.82% smaller, respectively. As a result, most lattice spacings of our nanowires are larger than their counterparts of (NH4)2Mo3O10 (see Table 1), while the spacings of (212) and (400) are smaller, marked by black triangles in Figure 2(a). Also, the XRD spectrum of our samples matches well to that of NaNH4Mo3O10·H2O (JCPDS 36-0335, not shown here), and again with slight offsets, indicating the similarity among these three kinds of tetragonal lattices. This is the first evidence that in our nanowires Ag atoms have taken the lattice positions.

tab1
Table 1: Experimental XRD data for 𝐴 𝑔 2 𝑥 ( 𝑁 𝐻 4 ) 𝑥 𝑀 𝑜 3 𝑂 1 0 3 𝐻 2 𝑂 nanowires (this work) and (NH4)2Mo3O10 (JCPDS 79-1905). The differences in lattice spacings between these two phases, namely 𝑑 1 and 𝑑 2 , are calculated and listed.
fig2
Figure 2: Experimental XRD data (a) and TGA data (b) of A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires. The two triangles in (a) indicate the peaks having opposite offset direction to that of the rest peaks, as compared to XRD data of (NH4)2Mo3O10 (see the Supplementary figure in the Supplementary Material available online at doi: 10.5402/2012/539601).

The TGA data of the samples also reveal that Ag atoms substitute the (NH4)+ sites in the crystalline lattices. Figure 2(b) shows TGA data of three different batches of the nanowires synthesized under the same conditions shown in Figure 1(e). The three measurement curves match well to each other. The first step of loss in weight, about 7.5%, was measured at around 280–370°C, corresponding to the weight percentage of NH3 and H2O released from the nanowires. The second peak of loss in weight, about 38%, was measured at around 800–1000°C, corresponding to the weight percentage of released Ag2O. Calculated from the TGA data, comparing the similarity of its XRD spectrum to those of NaNH4Mo3O10·H2O and (NH4)2Mo3O10, we determined the chemical formula of the nanowires to be A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O , where 𝑥 0 . 5 .

When the Ag-doped trimolybdate nanowires were used in a high concentration, they could efficiently eliminate the growth of bacteria. Figure 3 shows the results of a culture experiment using Lysogeny broth (LB) plate, a more quantitative test for the antibacterial ability of the A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O against E. coli DH5a. It shows that 2 ppm of nanowires in the sample is the critical value of antibacteria. When the concentration increased to 5 ppm, the number of colonies decreased to 2% of that without nanowires. That is, as much as 98% of the bacteria are inhibited.

539601.fig.003
Figure 3: Results of LB plate culture experiment of the A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires against E. coli DH5a. It shows that 2 ppm is the critical concentration for antibacteria. When the concentration increases to 5 ppm, the number of colonies decreases to 2% of that without nanowires.

We have tested the cytotoxicity of the as-synthesized nanowires to find out the concentration range in which the nanowires are safe to human cells and at the same time have an effective antimicrobial effect [40]. Here, 293E cell line was chosen to be the human cells sample. The results are shown in Figure 6. The blue (dark and light) regions show the concentration range in which nanowires are friendly to human cells but cannot inhibit the growth of microbes effectively. In the red (dark and light) regions the nanowires are antimicrobial but also do harm human cells. The green region is the concentration window in which the nanowires are both friendly to human cells and have an effective antimicrobial effect. The results show that the safety window of concentration depends much on the reaction time duration. Within 4 hours, the ratio of the highest to the lowest concentrations in the safe region is 100, and this ratio decreases to 15 at 24 hours and 2 at 96 hours.

Two major elements in the A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires are expected to have the antibacterial nature, that is, the Ag+ ions and the Mo6+ ions of the composite, both of which have been reported previously [116, 4143]. In this work, we have also observed a large amount of Ag-rich nanoparticles on surface of the nanowires. These Ag-rich nanoparticles may also contribute greatly to the observed antibacterial nature.

Morphology imaging and SAED analyses of the as-synthesized A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires with a TEM showed that the nanowires had a single crystalline structure with a growth direction [ 1 0 1 ] (inset of Figure 4). On the surface of the nanowires, a large amount of randomly distributed dark nanoparticles were observed. The in situ EDX spectra of individual nanowires revealed that, when the electron-beam probe was moved from the center to the edge of the same nanowire, the relative atomic ratio of Ag over Mo was remarkably increased, as typically shown in Figures 4(c) and 4(d), respectively. This suggests that the dark nanoparticles on the nanowires are Ag rich, most probably Ag2O particles, which are formed in the synthesis process. Under the irradiation of an intense electron beam, the dark particles could move slightly on the nanowire surface. The XPS spectra of the nanowires consisted of peaks for Mo, Ag, O, and N. The binding energy of Ag 3d was measured to be 367.97 eV, indicating that Ag atoms occurred in the form of Ag2O. The peak for Mo 3d was measured to be 232.75 eV, showing its chemical status of α-MoO3 (232.6 eV). The atomic ratio of Ag to Mo, indicated by the XPS data, was about 9 : 16.

fig4
Figure 4: (a) A low magnification TEM image of A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires. The inset is a typical SAED of a single nanowire, showing single crystalline phase and the growth direction [ 1 0 1 ] of each nanowire. (b) A close look at a nanowire revealing a large amount of nanoparticles. (c) and (d) EDX spectra of the center region and the edge region of the same nanowire, respectively.

We also performed experiments to filter the natural water taken from the Weiming Lake on the campus of Peking University with nanowire-treated filtering paper. Each layer of the nanowire-treated filtering paper was dipped in a solution of the A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires and dried in air. Repeated experiments showed that, when the natural water was filtered through 4–6 layers of the nanowire-treated filtering paper, 95–99% of the natural microbes in the lake water, although unknown, were eliminated (see the Supplementary figure).

We note that both nanowires of pure (NH4)2Mo3O10 and NaNH4Mo3O10·H2O phases are easily dissolvable in deionized water [39], while the nanowires shown in Figures 1(c), 1(d), and 1(e) are not. However, they can be dissolved in mild acidic solutions. As shown in Figure 5, we have observed clear evidence that these nanowires could be dissolved by the chemicals released from the bacteria during their metabolism processes. Group 1 (G1) images show Staphylococcus aureus bacteria cultured on a base of N.A. at 40°C for 18 hours. Group 2 (G2) images show Staphylococcus aureus bacteria mixed with dry A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires cultured on the same N.A. base at 40°C for 18 hours. For comparison, Group 3 (G3) images show pure A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires on N.A. experimented under the same condition. The Staphylococcus aureus bacteria have been sufficiently eliminated with the presence of the nanowires. We note that, in G2, the nanowires originally seen on the N.A. surface disappeared gradually after 6–18 hours, while, in G3, the nanowires kept their original shapes. This nature should be favorable for certain biological applications, for example, once these nanowire carriers have accomplished their missions, they themselves might be naturally dissolved in the cell or tissue and be removed out of the target regions.

539601.fig.005
Figure 5: Optical images of 3 samples that were spread on the same kind of N.A. base and cultured under the same conditions at 40°C for 18 hours. G1: pure Staphylococcus aureus bacteria. G2: Staphylococcus aureus bacteria mixed with dry A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires. G3: pure A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires. Staphylococcus aureus bacteria had been sufficiently eliminated with the presence of the nanowires. In G2, the nanowires originally seen on the N.A. surface disappeared gradually after 6–18 hours. In G3, the nanowires kept their original shapes.
539601.fig.006
Figure 6: Results of the cytotoxicity test for A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires in 293E human cells. The experiments were performed under varying concentrations (C) in unit of part per million (ppm) of the nanowires used and varying reaction times with the cells (in hours). B.I.: Bacterial infection; n.: normal; L.F.: loss form; C.S.: contents spill; A.: apoptosis; and s: slightly. In the blue (dark and light) regions the nanowires are friendly to human cells but show no obvious antimicrobial effect. In the green (normal) region the nanowires are both friendly to human cells and have an effective antimicrobial effect. In the red (dark and light) regions the nanowires have an effective antimicrobial effect but also attack human cells.

Because the Ag-rich nanoparticles are dispersed on the A g 2 𝑥 ( N H 4 ) 𝑥 M o 3 O 1 0 3 H 2 O nanowires, while the nanowires should be able to wrap on fibers easily, this material may be applied in fabrication of antimicrobial yarns and clothes. We have performed a skin sensitivity test of the nanowires on rabbits for fourteen days. The concentration used in the experiments was 1000 ppm, much higher than the actual concentration that may apply. The skin of rabbit stays in good condition free of erythema or dropsy in the period of experiment, proving the safety of skin with this kind of nanowires.

4. Conclusion

We have presented a novel single crystalline trimolybdate nanowire that carries a large amount of Ag ions in its lattice and Ag-rich nanoparticles on its surface. These nanowires kill E. coli, Staphylococcus aureus, and some unknown microbes in raw natural water with high efficiency. They are not dissolvable in deionized water but could be dissolved in mild acidic solutions or by metabolic chemicals released from bacteria; therefore they may have a great potential for biological applications. For instance, if such nanowires are applied as drug carrier, they could be naturally dissolved and removed out of the target regions after the mission is completed.

Acknowledgments

The authors are grateful to the National Science Foundation of China (Grant no. 11074010 and no. 60925003) and MOST of China (Grant no. 2011CB933002) for financial support. They thank Professor Yuxin Yin for valuable discussions.

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