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Microbiology

Fighting infections with graphene

Nanomaterial enables low doses of light to eliminate bacteria

The bacteria Staphylococcus aureus and Escherichia coli (images A and B) and the same microorganisms covered by graphene oxide (C and D)

Romero, M.P. et al. 

When administered together with visible light, graphene, a hexagonal film of carbon atoms, showed the potential to kill bacteria populations and treat skin infections. A team of researchers from the Optics and Photonics Research Center (CEPOF) at the São Carlos Institute of Physics of the University of São Paulo (IFSC-USP) and the Center for Advanced Research in Graphene, Nanomaterials, and Nanotechnologies (MACKGRAPHE) at Mackenzie Presbyterian University studied the effects of using graphene oxide films illuminated by LEDs to combat two common species of bacteria: Staphylococcus aureus and Escherichia coli, which were grown in a laboratory solution. The graphene films cover the microorganisms and enhance the effect of light therapy.

“We completely eliminated the bacteria within 20 minutes of irradiation,” says Ecuadorian physicist María Paulina Romero, one of the authors of the paper describing the results of the experiments, published in the scientific journal Frontiers in Microbiology in January. Now a professor at the National Polytechnic School of Quito, Ecuador, Romero did a postdoctoral fellowship at IFSC between 2017 and 2019.

Without the graphene oxide coating, the red LED lights used in the experiment, with a wavelength of 630 nanometers, were able to illuminate and heat the bacteria, but not to kill them. The study used graphene oxide films of 2,000 nanometers and 340 nanometers in size. The two bacteria responded similarly to both sizes. The most visible difference was that due to its morphology, Staphylococcus aureus required higher concentrations of the nanomaterial to be completely covered by the graphene oxide films than Escherichia coli.

The lethal dose of light combined with graphene oxide was also tested in vitro on fibroblasts—a type of skin cell—obtained from newborn babies. There was little to no toxicity, depending on the variant of the material used. The graphene oxide films were synthesized at MACKGRAPHE and the antimicrobial tests were conducted at CEPOF, one of the Research, Innovation, and Dissemination Centers (RIDC) funded by FAPESP. “For biological applications, we use strict protocols to produce graphene oxide. The purification stage is extremely important,” explains MACKGRAPHE chemist Cecília Silva, another author of the paper. “Impurities in the synthesis process, such as metal ions and acids, can affect cell viability.” By associating oxygen and hydrogen atoms with the hexagonal carbon fiber, graphene oxide retains almost all the properties of pure graphene, such as extreme lightness, hardness, and flexibility. But it is cheaper and easier to produce.

Based on previous research by groups in other countries, scientists at USP and Mackenzie knew that irradiating graphene with red light increases its temperature and releases energy. They decided to find out if these thermal and photodynamic properties could be useful to fight bacterial infections. In the experiments, they found that by adding graphene oxide films to solutions containing the two infectious agents, the material completely covered the bacteria and enhanced the antimicrobial effects of the light. When LEDs (Light Emitting Diodes) were targeted at bacteria coated by the nanomaterial, oxygen molecules (O2) were released from carbon oxide and the local temperature rose from 55 ºC to 60 ºC. “Irradiation excites the oxygen and makes it toxic to bacteria,” explains Romero. “We used an economical arrangement of red LEDs, which allowed us to radiate an area of at least 12 square centimeters.”

According to physiopathologist Natalia Inada from IFSC, the results indicate that the new approach could be used to disinfect large areas of skin that do not respond well to intravenously administered antibiotics. “Graphene also has the potential to cause fewer side effects and microbial resistance issues,” says Inada, who is also a coauthor of the study. CEPOF coordinator Vanderlei Bagnato, a physicist who also coauthored the scientific article, notes that graphene has potential applications in various fields. “The life sciences and pharmacology sector is one of the most promising,” he says. The next step will be to test graphene-enhanced phototherapy in the treatment of skin tumors.

Projects
1. CEPOF – Optics and Photonics Research Center (nº 13/07276-1); Grant Mechanism Research, Innovation, and Dissemination Centers (RIDC); Principal investigator Vanderlei Salvador Bagnato (USP); Investment R$44,106,793.11
2. Graphene: Photonics and optoelectronics. UPM-NUS collaboration (nº 12/50259-8); Grant Mechanism Research Grant; Program Spec; Principal Investigator Antonio Helio de Castro Neto (Mackenzie Presbyterian University); Investment R$14,956,394.43.

Scientific article
ROMERO, M. P. et al. Graphene oxide mediated broad-spectrum antibacterial based on bimodal action of photodynamic and photothermal effects. Frontiers in Microbiology. Jan. 15, 2020.

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