Contact Mechanism of the Ag-doped Trimolybdate Nanowire as An Antimicrobial Agent

www.nmletters.org Contact Mechanism of the Ag-doped Trimolybdate Nanowire as An Antimicrobial Agent Yujie Jiang1 , Jian Gang1 , Sheng-Yong Xu1,∗ (Received 31 October 2012; accepted 15 November 2012; published online 30 November 2012) Abstract: Antibacterial Ag-agents are intensively applied as broad spectrum, high-stability, high-efficiency and high-safety inorganic antibacterial agents. We have developed a new kind of antibacterial Ag-agent, namely Ag2−x (NH4 )x Mo3 O10 ·3H2 O nanowires (NWs). Carrying Ag atoms in the lattice and Ag-rich nanoparticles on the surface, the Ag-doped NWs show strong antibacterial effects for a variety of bacteria including E. coli, Staphylococcus aureus, Candida albicans and Aspergillus niger. By performing systematic comparison experiments, we have proven that the main antibacterial effects are neither resulted from the tiny amount of Ag+ ions released from the Ag-doped NWs in aqueous solutions, nor resulted from Ag-rich nanoparticles of fragments of the NWs when they are slowly dissolved in the Martin broth. Instead, the effects are mainly resulted from a contact mechanism, under which, the Ag-doped NWs need to be physically in contact with the bacteria to be eliminated. This is a novel phenomenon observed in the interactions between nanomaterials and live cells, which is worthy of further investigation at the molecular scale. As the Ag-doped NWs are not dissolved in pure water or weak acids, one may find practical antibacterial applications in textile industry and food storage industry for these unique nanomaterials. Keywords: Antibacterial agent; Antibacterial Ag-agent; Silver ions; Silver-rich nanoparticles; Silver-doped trimolybdate nanowire; Bio-safety of nanomaterials; Contact mechanism Citation: Yujie Jiang, Jian Gang and Sheng-Yong Xu, “Contact Mechanism of the Ag-doped Trimolybdate Nanowire as An Antimicrobial Agent”, Nano-Micro Lett. 4 (4), 228-234 (2012). http://dx.doi.org/10.3786/ nml.v4i4.p228-234 Introduction Antibacterial agents are important materials that are intensively applied in hospitals, clinics, biological laboratories, medical and food industries, as well as in military practices and daily lives. There are natural antibacterial agents, organic antibacterial agents and inorganic antibacterial agents. Among inorganic antibacterial agents, the antibacterial Ag-agents that consist of element Ag have been widely investigated, and they are extensively applied in many cases as the first choice, because these Ag-agents usually inhibit low cost, broad spectrum, high stability, high efficiency, and above all, high safety [1-10]. To date two main effective parts have been recognized for the strong antibacterial properties of the Agagents: Ag+ ions, and Ag-rich nanoparticles. For the antibacterial effects of Ag+ ions, two kinds of mechanism have been reported. First, it is resulted from the toxic effects of the heavy element Ag+ released from the agent. Second, it is attributed to the strong oxidization effects of reactive oxygen species (ROS) produced by Ag+ , which consequently causes permanent damages on macromolecules or membrane of the bacterium [11-21]. On the other hand, the exact antibacterial mechanism of Ag-rich nanoparticles is not clear. However, at the molecular level one major difference has been reported: In the cells under test the Ag-rich 1 Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China 1 These authors contributed equally to the paper. *Corresponding author. E-mail: Nano-Micro Lett. 4 (4), 228-234 (2012)/ http://dx.doi.org/10.3786/nml.v4i4.p228-234 Nano-Micro Lett. 4 (4), 228-234 (2012)/ http://dx.doi.org/10.3786/nml.v4i4.p228-234 on a PROFILE SPEC (Leeman) atomic emission spectroscopy system. The culturing of various bacteria was performed following a standard biological procedure. A dozen groups of different experiments were done to figure out the antibacterial mechanism of the Ag-doped trimolybdate NWs. In the first sets of experiments, we examined whether the antimicrobial effect of the Ag-doped NWs was resulted from the Ag+ ions in the solution released from the NWs. To determine the concentration of Ag+ ions in an aqueous solution of Ag-doped NWs, a suspension of 1 wt% Ag-doped NWs was prepared by adding 500 mg NWs into 50 ml de-ionized water, stirred thoroughly and kept for 24 hrs at room temperature. After that, an acrodisc syringe filter with average pore diameter of 0.45 μm (Article No. PN 4614, Pall Co.) was applied to filtrate the suspension. Then 10 ml filtered solution was measured with an ICP atomic emission spectroscopy system. For comparison, we prepared 5 aqueous solutions of AgNO3 with Ag+ ion concentrations of 44.7 ppm, 18.6 ppm, 7.76 ppm, 3.66 ppm and 0.7 ppm, respectively. Then 20 ml of each of the six kinds of solutions were individually mixed with Martin medium powder, at a ratio of 1000 ml to 28.5 g. Each mixed solution was further mixed with 1 ml of yeast suspension (namely Saccharomyces cerevisiae, showing an OD600 value of 0.4), and the final samples were cultured at 37℃ for 24 hr. Next, control experiments, namely Groups A, B and C, were performed for culturing the same yeast sample. For Group A, we prepared mixture of Martin medium powders with 20 ml de-ionized water. For Group B, a mixture of Martin medium powder was prepared with 20 ml filtrate of the suspension of 1 wt% Ag-doped NWs in de-ionized water. For Group C, the Martin medium powder was mixed with a suspension of 100 ppm AgNWs in 20 ml de-ionized water. The ratio of powder to solution for Groups A, B and C was all kept at 28.5 g to 1000 ml. Then 1 ml yeast suspension (showing OD600 of 0.4) was added into each sample of Groups A, B and C. All these samples were cultured at 37℃ for varied time up to 20 hrs, and their OD600 values were measured at different stages of the cultured samples. We have repeated this set of experiments for 3 times. The OD600 method was applied to measure the dissolvability in time of the Ag-doped NWs in the Martin broth. Then 5 sets of samples were prepared to determine the solvent(s) for the Ag-doped NWs. In every 28.5 g of Martin powder, there were 5 g of tryptone, 2 g of yeast extract, 20 g of glucose, 1 g of K2 HPO4 and 0.5 g of MgSO4 . For each set of testing samples, 6 mg Ag-doped NWs suspended in 20 ml de-ionized water was mixed with one of the 5 ingredients of the Martin medium. Their OD600 values were measured at the time when the mixtures were just prepared, and at the time after they were treated in a shaker for 8 hrs. nanoparticles do not cause a dense DNA phenomenon, but it is always observed in Ag+ treated cells [22-31]. We have reported that Ag-doped trimolybdate NWs, with a chemical formula of Ag2−x (NH4 )x Mo3 O10 ·3H2 O, have strong antibacterial effects. These NWs carry a certain amount of Ag atoms in the lattice and Ag-rich nanoparticles on the surface, and they can be synthesized at one-atmosphere fro (...truncated)