Researchers at the University of Houston have developed a new technique for killing bacteria in 5 to 25 seconds using highly porous gold nanodisks and light, according to a study published today in Optical Materials Express. The method could one day help hospitals treat some common infections without using antibiotics, which could help reduce the risk of spreading antibiotics resistance.

Gold nanoparticles are used because they absorb light strongly, converting the photons quickly into heat and reaching temperatures hot enough to destroy various types of nearby cells — including cancer and bacterial cells. Scientists create gold nanoparticles in the lab by dissolving gold, reducing the metal into smaller and smaller disconnected pieces until the size must be measured in nanometers. Once miniaturized, the particles can be crafted into various shapes.

In 2013, corresponding author Wei-Chuan Shih, a professor in the electrical and computer engineering department, and his colleagues created a new type of gold nanoparticle in the form of discs riddled with pores, lending the particles a sponge-like look that helps increase their heating efficiency while maintaining their stability, said Shih.

Zapping with light too

In the new work, the researchers set out to test the antimicrobial properties of their new nanoparticles when activated by light. They grew bacteria in the lab including E. coli and two types of heat-resistant bacteria that thrive in even the most scorching environments such as the hot springs of Yellowstone National Park.

Then, they placed the bacteria cells on surface of a single-layer coating of the tiny disks and shone near infrared light from a laser on them. Afterward, they used cell viability tests and SEM imaging to see what percentage of cells survived the procedure.

Using a thermal imaging camera, the research team showed that the surface temperature of the particles reached temperatures up to 180 degrees Celsius nearly instantaneously, “delivering thermal shocks” into the surrounding array — killing all of the bacterial cells within 25 seconds, the researchers report.

E. coli proved most vulnerable to the treatment; all of its cells were dead after only five seconds of laser exposure. The other two types of bacteria required the full 25 seconds, but that’s still much quicker than traditional sterilization methods such as boiling water or using dry-heat ovens, which can take minutes to an hour to work, said Shih. And it’s “considerably shorter” than what other nanoparticle arrays have demonstrated in recent studies, the researchers write. The time needed to achieve similar levels of cell death in those studies ranges from 1 to 20 minutes.

In control trials, the researchers found that neither the gold disks nor light from the laser alone killed nearly as many cells.

Hospital use

The technique has important potential biomedical applications, said Shih. Currently, the researchers are investigating using the particles as a simple coating for catheters to help reduce the number of urinary tract infections in hospitals.

“Any sort of light activated procedure would be much easier to implement at the bedside of a patient,” instead of removing and potentially replacing the catheter every time it needs to be cleaned, he said.

Another potential application they’re exploring is integrating the nanoparticles with filter membranes in small water filters, he said, to help improve water quality.

Abstract of Photothermal inactivation of heat-resistant bacteria on nanoporous gold disk arrays

A rapid photothermal bacterial inactivation technique has been developed by irradiating near-infrared (NIR) light onto bacterial cells (Escherichia coli,Bacillus subtilis, Exiguobacterium sp. AT1B) deposited on surfaces coated with a dense, random array of nanoporous gold disks (NPGDs). With the use of cell viability tests and SEM imaging results, the complete inactivation of the pathogenic and heat-resistant bacterial model strains is confirmed within ~25 s of irradiation of the NPGD substrate. In addition to irradiation control experiments to prove the efficacy of the bacterial inactivation, thermographic imaging showed an immediate averaged temperature rise above 200 °C within the irradiation spot of the NPGD substrate. The light-gated photothermal effects on the NPGD substrate offers potential applications for antimicrobial and nanotherapeutic devices due to strong light absorption in the tissue optical window, i.e., the NIR wavelengths, and robust morphological structure that can withstand high instantaneous thermal shocks.