A team of scientists at the University College London (UCL) have developed a new antibacterial material that could help cut hospital-acquired infections. They used a combination of two dyes with nanoscopic particles of gold, which, they say, is deadly to bacteria when activated by light, including average indoor lighting. And, in a first for this type of substance, it has also shown impressive antibacterial properties in total darkness.

Fig. 1 – Samples of silicone with the various dyes infused (Credit: Sacha Noimark, UCL Chemistry)

Hospital-acquired infections continue to be a major issue, even though medical establishments have stringent cleaning policies, insist on frequent hand washing by staff, and have powerful drugs at their disposal. But it is still difficult to eliminate these infections. The researchers say that one way to attack the problem is by making the hospital environment more hostile to microbes by developing alternative strategies such as antibacterial coatings that make surfaces less accommodating to germs. These surfaces, they explained, are not like antibacterial fluids that wash off. Their goal is to make a surface that is intrinsically deadly to harmful bacteria.

“There are certain dyes that are known to be harmful to bacteria when subjected to bright light,” explained the study’s corresponding author Ivan Parkin (Head of UCL Chemistry). “The light excites electrons in them, promoting the dye molecules to an excited triplet state and ultimately produces highly reactive oxygen radicals that damage bacteria cell walls. Our project tested new combinations of these dyes along with gold nanoparticles, and simplified ways of treating surfaces which could make the technology easier and cheaper to roll out.”

The team tested several different combinations of the dyes crystal violet (already used to treat staph infections), methylene blue, and nanogold, deposited on a surface of silicone, which is widely used as a sealant, a coating, and to build medical apparatus such as tubes, catheters, and gaskets.

A New Approach

While previous efforts to create antimicrobial surfaces have often concentrated on complex ways of bonding dyes to the surface, this study took a simpler approach. The re searchers used an organic solvent to swell the silicone, allowing the methylene blue and gold nanoparticles to diffuse throughout the polymer. They then dipped the silicone into a crystal violet solution to form a thin dye layer at the polymer surface. (See Figure 1)

In their tests, in which infected surfaces were subjected to light levels similar to those measured in hospital buildings, surfaces treated with a combination of crystal violet, methylene blue, and nanogold showed the most potent bactericidal effect ever observed in such a surface, they said. Moreover, they found that the treatment did not significantly change the inherent properties of the silicone, and the coating was not affected by rubbing with alcohol wipes, meaning it can stand up to the repeated cleaning that goes on in hospitals, without wearing off.

“Despite contaminating the surface with far more bacteria than you would ever see in a hospital setting, placed under a normal fluorescent light bulb, the entire sample was dead in three to six hours, depending on the type of bacteria,” says the paper’s lead author, Sacha Noimark. “That was an excellent result, but the bigger surprise was the sample which we left in the dark. That sample too showed significant reductions in bacterial load, albeit over longer timescales of about 3 to 18 hours. The precise mechanism by which this dark-kill works is not yet clear, though.”

This is the first time a light-activated antimicrobial surface has had any kind of effect in the dark. This, along with its unprecedented performance under hospital lighting conditions, and relatively simple and cost-effective manufacture, could be extremely promising for future applications. Their research was published in the journal, Chemical Science, and the team has received a patent on the formulation.