July 15, 2020

Chemistry researchers at UNT have developed a test to more easily identify toxic silver ions, which can be harmful to humans and the environment at high concentrations.

Silver nanoparticles (AgNPs) are well known for their antimicrobial properties and can be found in a number of products such as bedding, toothpaste and toys. But, over time, silver ions can leach from AgNP products into the surrounding environment.

“Right now, scientists are studying how AgNPs move from a product into the environment,” research assistant professor Sreekar Marpu says. “In one instance, researchers looked into various types of food packing containers doped with AgNPs. In the case of the containers containing acidic foods, there was a measurable movement of silver from the packaging to the food product in as little as 10 days. Where, in the form of ions, the silver is toxic.”

The ability to differentiate between the presence of silver nanoparticles and silver ions is important in both determining the toxicity of a substance and also discovering the time it takes silver ions to leach from nanoparticles in various products, knowledge that could have a huge impact on the environment.

While there are tests to determine if there is silver in a substance, there is no quick and easy way to determine if it is the ion or nanoparticle form, and sensitivity below the order of part-per-million (sub-ppm) has been lacking.

The patent-pending test created by Marpu and Professor Mohammad A. Omary, both in UNT’s College of Science, will be able to tell the difference at sub-ppm levels.

“Certain gold (I)-based macrocyclic molecular systems can interact with silver ions (Ag+) and can be used as sensors,” Marpu says. “We use a phosphorescent gold(I) complex that not only attracts silver ions selectively versus nanoparticles but also only changes its emission color based on which is present (bright green with Agand faint red with AgNPs)."

When exposed to ultraviolet light, the silver ion-attracting gold (I) complex will emit red color on its own or when in the presence of silver NPs. Over time, as the silver NPs break down and silver ions are released, the red color will shift toward bright green. To better understand the chemical processes involved, Marpu set up a month-long experiment to observe the change.

“We monitored the leaching of silver ions from nanoparticles over a period of 35 days and watched the color change from red to green as the ratios changed,” Marpu says. “I believe this is the only ratiometric luminescence-based silver sensor to successfully differentiate between the ions and the nanoparticles.”

Marpu and his co-authors Omary and Erin Benton, a former UNT Ph.D. student, published their initial findings used to develop the new test, "Ratiometric Phosphorescent Silver Sensor: Detection and Quantification of Free Silver Ions within Silver Nanoparticles" in the March 22, 2019, American Chemical Society’s Applied Materials and Interfaces journal. They were assisted in their research by UNT Ph.D. student N.A.K. Rajitha Perera.