A sliver of silver

When the ancient Greeks cooked their meals in silver pots, or Cleopatra sipped her wine from a silver chalice, they weren’t just showing off their wealth.

They were taking advantage of the germ-fighting capabilities of silver.

“The antibacterial properties of silver have been known for a long time,” Ismat Shah, professor of materials science and engineering, says. “In fact, the expression ‘born with a silver spoon in the mouth’ has a dual meaning. It means the person is wealthy and also that he’s eating hygienically.”

Today, substances containing silver are used to fight bacteria in burn patients and in drinking water, for example. Most states in the United States require drops of a silver nitrate or antibiotic solution to be put into babies’ eyes at birth to prevent infection. But, Shah says, the idea of using silver to coat everyday objects that could benefit from its germ-fighting properties is too expensive to be practical.

“Due to its high cost, it is not economically feasible to use large quantities of silver for obtaining antibacterial surfaces,” Shah and fellow UD researchers write in a recent article in the Journal of Nanoscience and Nanotechnology. “In [our] research, we have demonstrated the efficiency of silver nanoparticles for inhibiting the growth of bacterial colonies.”

The article, “Synthesis and Antibacterial Properties of Silver Nanoparticles,” was co-written with Darrin J. Pochan, assistant professor of materials science and engineering, and Colin Baker, Anshu Pradhan, Abdullah Celan and Lisa Pakstis, all current or former research assistants in the materials science or physics and astronomy departments.

Nanoparticles are extremely small—a nanometer is one-billionth of a meter—and nanotechnology is a type of engineering that designs tiny electronic materials and other devices that are constructed at the molecular level. As a result of their small size, nanoparticles have a relatively large surface area in comparison to their volume. This larger surface area, Shah says, enables silver nanoparticles to interact more easily with other substances and therefore increases their antibacterial efficiency.

“When things get that small, you see properties you don’t see in the bulk materials,” he says.

Shah says silver nanoparticles might be useful in such applications as dressings for wounds, children’s toys and baby pacifiers, all of which would resist bacteria growth without having to be treated with chemical disinfectants. The widespread use of those chemicals has resulted in bacteria that are resistant to antibiotics, but Shah says research to date has found that bacteria “fail to develop an immunity to silver.”

Another advantage of utilizing the natural antimicrobial properties of silver is that the substance has proved highly toxic to such troublesome bacteria as Escherichia coli (E. coli) and Staphylococcus aureas, Shah says.

Shah’s research is not related to the use of colloidal silver, in which silver particles are suspended in a liquid that can be ingested and that sometimes is marketed as a dietary supplement or a cure for an assortment of serious illnesses. The federal Food and Drug Administration has banned over-the-counter colloidal silver products as ineffective and unsafe.

Once the UD researchers decided to try to use silver nanoparticles, they faced two problems: how to make them in sufficient quantities and how to disperse them into fabrics or other materials. “We think we’ve solved both of these problems,” Shah says.

The researchers use a manufacturing process that evaporates silver wire with a laser, then cools the particles so that they condense and can be collected on a filter. The process, Shah says, allows them to make nanoparticles “in kilogram quantities.”

To be useful, the particles must be dispersed fairly evenly over a material, and the research team has developed a method to accomplish that by building on research involving fibers that is conducted by John Rabolt, professor and chairperson of the Department of Materials Science and Engineering. Shah says the nanoparticles are mixed with fibers, which then can be made into materials in a cost-effective process.

“Since you have a large surface area, one gram of silver nanoparticles can cover hundreds of square meters,” he says. “As a result, this material would add very little to the cost of an item.”

Shah, who has a joint appointment in the College of Arts and Sciences’ Department of Physics and Astronomy, earned his bachelor’s degree in metallurgical engineering from the University of Karachi in Pakistan. He earned his doctorate in materials science and engineering at the University of Illinois at Urbana-Champaign, where he also worked as a postdoctoral fellow.

In 1987, he began work as a senior research scientist for the DuPont Co. and also as an adjunct professor at UD in physics and astronomy.

In 1999, he joined UD as a professor and a member of the College of Engineering’s Center for Catalytic Science and Technology.

Pochan has been a member of the UD faculty since 1999 and previously was a National Research Council postdoctoral associate at the National Institute of Standards and Technology. He earned his bachelor’s degree in chemistry at the University of Wisconsin at Madison and his master’s and doctoral degrees in polymer science and engineering at the University of Massachusetts at Amherst.
Earlier this year, Pochan was awarded the National Science Foundation’s prestigious Faculty Early Career Development Award. The five-year, $440,000 award will support his research on the use of biopolymers to construct advanced materials.

—Ann Manser, AS ’73, CHEP ’73