Silver Secret Weapon

Silver Secret Weapon
July 31, 2015 

No chemical element has been as useful in combating mythical creatures as silver. A silver cane strikes down the Wolf Man, Abraham Lincoln gathers a trainload of silver to combat Confederate vampires and when parasitic zombie vampires attack Manhattan, the ascetic old pawnbroker knows to dust off his cache of silver swords and bullets.

But silver is also effective in killing microbes and, just like its little brother copper, doctors and scientists use it extensively as an antibacterial agent. And also like copper, silver can be toxic to both humans and the environment if it is left to linger around, leaving scientists with the conundrum of how to deliver just the right amount of silver to just the right place in order to kill bacteria but not harm people or the environment.

According to a team of biomedical engineers from NC State University, the weapon for this type of job sounds almost like something you find in a monster movie: an elaborately carved wooden bullet, inlaid and veined with silver and coated in a special substance that sticks to the monsters.

The scientific way to say that is “silver-infused lignin nanoparticles” and Orlin Velev, professor of biomolecular engineering at NC State, says these nanoparticles represent an environmentally friendly way to harness the antimicrobial powers of silver.

The nanoparticles themselves, which are about the size of bacterial cells, are made of lignin. Lignin is a cheap and naturally abundant polymer best known for giving wood its “woodiness.” Within the folds of the lignin polymer, Velev and his team were able to insert silver ions.

The whole nanoparticle is coated in an electrolyte polymer that allows it to stick to the outside of target bacteria, and while it is stuck there, the silver ions are drawn out of the nanoparticle and into the bacteria.

The researchers tested the nanoparticles on E. coli, and several other bacteria that cause everything from hospital infections to crop diseases. In each case the silver ions killed off the bacteria without leaving leftover silver in the nanoparticles to cause trouble anywhere else.

All that remained of the nanoparticles was the lignin frame, and while biomass energy scientists may have a hard time breaking lignin into usable pieces, decomposers in the soil are very efficient at breaking it down and animals can either digest it or flush it away.

Additionally, Velev says the electrolyte layer can be specifically tuned to attack any species of bacteria, which would make the nanoparticles useful as a microbial pesticide to protect crops.

The researchers are currently working to scale up the production process to make these nanoparticles on an industrially useful scale. The paper describing these nanoparticles was published in the journal Nature Nanotechnology.

— Daniel Lane

Daniel Lane covers science, engineering, medicine and the environment in North Carolina.