“The antimicrobial activity we observed can be attributed to the ultrasmall size of the gold nanoparticles, which allows them to better interact with bacteria,” explain the researchers. “The interaction between these particles and bacteria could produce metabolic imbalances in the bacterial cells, leading to an increase in intracellaur reactive oxygen species (ROS) being produced, which prove fatal to them.”

Gold nanoparticles with a size comparable to the Fermi wavelength of electrons (around 1 nm) are also known as nanoclusters. They contain discrete electronic states, unlike larger nanoparticles, and characteristic structures. This means that they have molecular-like properties, such as quantized charging, HOMO-LUMO transitions, molecular magnetism, molecular chirality and strong luminescence.

One good example of their very different properties is that they can efficiently catalyse reactions, such as the selective oxidation of styrene by dioxygen. In contrast, gold nanoparticles larger than 2 nm are completely inactive for this reaction.

A wide range of bacteria destroyed

A team led by Jianping Xie has now found that gold nanoclusters can destroy a wide range of bacteria thanks to their ultrasmall size and thus their high surface-to-volume ratio. In their experiments, the researchers tested both the Gram-positive and Gram-negative types of S. aureus, E. coli, S. epidermidis, B. subtitles and P. aeruginosa.

“When bacteria internalize gold nanoclusters, the active surfaces of the particles can induce a metabolic imbalance in the bacterial cells, which leads to an increased production of ROS that then irreversibly damages the bacteria, killing them as a result,” says Xie. In contrast, nanoparticles bigger than 2 nm were not found to be antimicrobial.

No apparent toxicity

“Gold nanoclusters could help in the fight against antibiotic-resistant superbugs,” he tells nanotechweb.org. “What is more, when tested in normal human cells (colon and blood vessel cells), the nanoclusters did not show any apparent toxicity. This biocompatibility could also be another advantage for transferring this technology to the clinic.”

The team, reporting its work in ACS Nano DOI: 10.1021/acsnano.7b02035, says that it is now trying to further improve the antimicrobial activity of the nanoclusters by engineering their physicochemical properties. “We are also trying to prove their efficacy in a mouse study,” says Xie.