The antimicrobial properties of silver have long been recognized – since ancient times, it has been used for a wide variety of microorganisms. It's advantageous for use in humans because its toxicity to human cells is far lower than its toxicity to bacteria. It can also be used in the soil to kill pesky pathogens before they destroy our crops. However, there are environmental and health concerns about using large amounts of the stuff, as it could cause unknown havoc on the body or to ecosystems.
To overcome these issues, researchers from North Carolina State University, led by Orlin Velev, have created a barrier around silver that still allows us to take advantage of its antibacterial properties, but also biodegrades easily after the silver is used up. This means we wouldn't have to worry about lots of potentially problematic leftovers. The microorganism-fighting nanoparticle is composed of silver infused with a substance from plant cells; all wrapped up in a charged polymer. This polymer has the added bonus of helping the nanoparticle adhere to microbes, so it targets and destroys them more efficiently.
At the moment, the nanoparticle is intended for use in agriculture. However, there are possible other applications, for example to coat and sterilize medical equipment, or for use in more general personal care.
The nanoparticles were tested against a host of different bacterial nasties: E. coli, which can cause food poisoning; Pseudomonas aeruginosa, which causes disease in animals and humans; Ralstonia, a soil-borne pathogen; and Staphylococcus epidermis, which can live on plastics (e.g. catheters) and cause infections in the human body. The nanoparticles were effective bactericides in all these cases. These amazing results have been published in Nature Nanotechnology.
The nanoparticles were made biodegradable by adding a core composed of a biodegradable compound: lignin. Lignin is an organic material produced inside plant cells. It is very rigid and does not rot quickly, making it perfect for supporting structures in plants, such as cell walls. The design allows silver ions to still perform the desired antibacterial action, but with a set lifetime. As the bacteria are wiped out, the silver gets depleted from the nanoparticle and the leftover biocompatible core can degrade safely in the soil. Some active silver will still be left in the soil, however levels are significantly lower than when silver ions are used alone.
There have been concerns in the past about depositing silver in the environment since silver atoms don't biodegrade and continue to act on the environment long after they're deposited. Hopefully, this nanoparticle can solve this environmental issue.