NIR Light, Gold Nanoparticles Combine to Inactivate Bacteria

A rapid photothermal technique has been developed that irradiates near-infrared (NIR) light to inactivate bacterial cells, such as E. coli, deposited on surfaces coated with gold nanoparticles. The method could one day help hospitals treat some common infections without using antibiotics, which could help reduce antibiotic resistance.

Scientists from the University of Houston created nanoporous gold disks (NPGDs) in the lab by dissolving gold and reducing it to nanometer-scale particles. Once miniaturized, the particles could be crafted into various shapes including rods, triangles or disks.

Schematic diagram of the thermal run and control experiments. Control 1: A nonirradiated glass substrate was used to test the life expectancy of the bacterial strains inside the covered glass slides. Control 2: A nonirradiated nanoporous gold disk (NPGD) substrate was tested for its toxicity to the bacterial samples. Control 3: One coverslip without NPGD was irradiated to test the effect of irradiation on cell viability. Courtesy of Santos et al./ Optical Materials Express, a publication of The Optical Society (OSA).

The team, led by professor Wei-Chuan Shih, had previously shown in 2013 that gold nanodisks absorbed 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.The disks were riddled with pores, lending the particles a sponge-like look that helps increase their heating efficiency while maintaining their stability, said Shih.

In the new work, the team set out to test the antimicrobial properties of their new nanoparticles when activated by light. They grew bacteria in the lab — E. coli and two types of heat-resistant bacteria that thrive in scorching environments such as the hot springs of Yellowstone National Park: Bacillus subtilis and Exiguobacterium sp. AT1B.

Bacteria cells were placed on the surface of a single-layer coating of NPGDs and irradiated with NIR light. Cell viability tests and scanning electron microscope imaging were used to determine 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 °C nearly instantaneously, delivering thermal shocks into the surrounding array. As a result, all of the bacterial cells were killed within 25 s, the researchers reported. In control trials, neither the gold disks nor light from the laser alone killed nearly as many cells.

E. coli proved most vulnerable to the treatment; all of its cells were dead after 5 s of laser exposure. The other two types of bacteria required the full 25 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. The time needed to achieve similar levels of cell death in other nanoparticle array studies ranged from 1 to 20 min, the researchers said. The robust morphological structure of the substrate is also notable for its ability to withstand the instantaneous thermal shocks.

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. A 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 cleaning.

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

The research was published in Optical Materials Express (doi: 10.1364/ome.6.001217), a publication of the Optical Society (OSA).