LIEC/UNESPSilver rod-like structures that can only be seen through powerful electron microscopes act against the bacteria Staphylococcus aureus. This particular bacterial strain, known as MRSA, is resistant to the antibiotic methicillin and responsible for hospital infections. “It was merely by chance that we found these silver rods growing spontaneously in a sample of silver tungstate [Ag2WO4],” says Professor Elson Longo of the Universidade Estadual Paulista (Unesp) Chemistry Institute in Araraquara, São Paulo State. “In our analysis of the material, we were studying a new production route for the silver tungstate, which is a semiconductor crystal, or possibly even a sterilant. When we put it under a field emission scanning electronic microscope, we saw the rod. At first we thought the equipment might be faulty, but when we tried it with a transmission microscope, we saw the same phenomenon,” says Longo, who is also the coordinator of the Center for Research and Development of Functional Materials, one of the Research, Innovation and Dissemination Centers (RIDC) of FAPESP.
“This is a new generation of bactericidal materials,” says Longo. Biological tests coordinated by Professor Carlos Eduardo Vergani, of the Unesp Dental School at Araraquara, also showed promising results against fungi. “The fungi are made up of larger molecules and many are resistant to conventional fungicides. We obtained good results in experiments with Candida albicans, and we are testing other species,” says Professor Ana Machado, member of the team coordinated by Vergani. “We further demonstrated that the growth of silver filaments in tungstate enhances the ability of the material to combat the proliferation of MRSA, resulting in a reduction of four times the amount of the substance needed to kill this organism,” says Machado.
This finding proves yet again the use of silver as a potent and natural bactericidal material, one that has been used for this purpose since ancient times. The scientific explanation for this use is related to electric charges or free radicals present in the metal that alter DNA molecules and interact with cell membranes to damage the microorganisms. “But there is nothing in the literature that resembles what we found: a material being formed from within another material,” says Longo. “We experimented with particles—which are electrons present in greater amounts and energy in the transmission microscope—in silver tungstate and only silver filaments were spontaneously generated, like a bean that sprouts after the seed is planted in the ground,” says Longo. “Normally if you have a compound material such as silver chloride, and do something to break it up, you end up with two separate elements: chlorine and silver,” he notes. “We still don’t know exactly what is happening with silver tungstate, and we are doing theoretical calculations to better understand the phenomenon. We know there is both a disorder and an order of silver clusters, which decompose to form metallic silver and move to the surface within the silver tungstate crystal forming threads that consolidate in some parts of the material.” The size of the filaments ranges from micrometers (on the order of 1 millimeter divided by one thousand) in length to nanometers (equivalent to 1 millimeter divided by 1 million) in width.
Transfer to industry
The silver tungstate filaments are not meant for use in medicines, but to be incorporated into metals, plastics or other materials. The silver by itself is currently being used as a bactericide on surgical instruments, and in washing machines, refrigerators and filters, for example. The group coordinated by Longo is also part of the National Institute of Science and Technology for Materials in Nanotechnology, supported by FAPESP and the National Council for Scientific and Technological Development (CNPq). Groups of professors in partnership and led by Longo, formerly at the Federal University of São Carlos (UFSCar), and José Arana Varela of Unesp, the current president of FAPESP, have been working with silver nanoparticles for over a decade. Technological advances using these nanoparticles developed by the two groups were transferred to Nanox (see Pesquisa FAPESP Issue No. 187), a Unesp and UFSCar laboratory start-up company. It produces and sells material made from silver nanoparticles, which add bactericidal and self-sterilizing properties to a number of products such as water purifiers, hair dryers, paints, food packaging, ceramics and surgical instruments.
The new material, presented in the April Scientific Reports issue of the journal Nature, also demonstrates another activity: its use in the decomposition of organic matter in industrial waste and in water from rivers and streams. “We tested the filaments with rhodamine B, a red chemical compound whose color does not easily fade, used in international tests in experiments on water treatment products,” says Longo. “Using the silver rods we were able to degrade rhodamine into water and carbon dioxide [CO2] in 30 minutes in an environment with sunlight, because the filaments are also photoluminescent and react with the dye, helping to degrade it. Existing products for rhodamine decomposition take more than one hour to do this.” Another advantage of this process is that tungstate silver rods are reusable. Now researchers, in addition to having a better understanding of the phenomenon, are also writing patents on the uses of tungstate silver rods.
1. Center for Research and Development of Functional Materials (No. 2013/07296-2); Grant Mechanism Research, Innovation and Dissemination Centers (RIDC); Coordinator Elson Longo/Unesp; Investment R$1,184,793.34 and US$1,060,186.89 per year (FAPESP)
2. National Institute of Science for Materials in Nanotechnology (No. 2008/57872-1); Grant Mechanism Thematic Project Grant/INCT; Coordinator Elson Longo/Unesp; Investment R$838,500.00 and US$772,295.09 (FAPESP).
Longo, E. et al. Direct in situ observation of the electron-driven synthesis of Ag filaments on alpha-Ag2WO4 crystals. Scientific Reports. v. 3, No. 1676. April 2013.