A study recently conducted by Graphene Flagship partners the University of Strasbourg and CNRS, France, in collaboration with Nanyang Technological University in Singapore, has shown that graphene quantum dots are biodegradable by two enzymes found in the human body.

Graphene quantum dots (GQDs) are tiny flakes usually smaller than five nanometres that have potential for many applications. GQDs are fluorescent, so they can absorb light and then emit it, often at a different wavelength. They are also so small that they can penetrate cells. Together, these properties pave the way to a wide array of applications in bioimaging, biosensing and new therapies - among other potential uses.

Despite great potential for use in biomedical applications, it would first have to be determined if they can biodegrade – otherwise, the build-up of foreign molecules and materials inside the body could eventually cause harmful effects. This is a critical drawback of other types of quantum dot, which may have similar properties, but often contain toxic metals, like lead and cadmium. For this reason, Alberto Bianco, Graphene Flagship Work Package Deputy for Health and Environment at Graphene Flagship partner CNRS, with colleagues at the University of Strasbourg, and within a collaboration with Nanyang Technological University, set out to study the biodegradability of GQDs by exposing them to human enzymes.

"We used two human enzymes, myeloperoxidase and eosinophil peroxidase, to prove that graphene quantum dots are biodegradable," begins Bianco. The team examined the fate of these materials using a combination of spectroscopy, microscopy and fluorescence measurements. They found that the GQDs became less fluorescent over time, indicating that the enzymes were breaking them down.

Bianco explains that this happens because peroxidases are the enzymes in mammals responsible for degrading and eliminating exogenous molecules, microorganisms and materials – all objects that are foreign to the body. "These enzymes take part in a cycle that generates reactive oxygen species, which react with the graphene material and ultimately decomposes it," he adds. These biochemical reactions transform graphene into carbon dioxide as the final product.

The team conducted molecular dynamics computer simulations to study the mechanism. They found that the shapes of the enzymes and GQDs were slightly distorted towards each other, favoring and interaction leading to the breakdown of the GQDs.

Maurizio Prato, Work Package Leader for Health and Environment, comments: "Graphene quantum dots have completely different properties to graphene. They have very small dimensions and have good potential for biological applications, especially in sensing. This work by Bianco and colleagues demonstrates that even these small nanoparticles are not persistent, as they undergo biodegradation by human peroxidases."