Graphene quantum dots (GQDs) are flakes dimensionally confined both in-plane and out-of-plane, meaning their size is restricted in all directions: hence the name 'dot'. They are usually smaller than five nanometres, and have potential for many biomedical 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.
For instance, because GQDs can easily enter cells, they make a good platform for drug delivery. In addition, their fluorescent properties mean they can be used to create images of tissues and organs inside the body, leading to applications in medical diagnostics. GQDs also release heat when irradiated with near-infra-red light, meaning that, when injected into a tumor and irradiated, the heat released can selectively destroy the cancerous tissue.
These applications are promising, but if we want to use GQDs for medicinal purposes, we need to know 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 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.
Enzymes are a type of protein that act as a biological catalyst, speeding up reactions in the body. They are also involved in processes that keep our body clean of contaminants. "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 are 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 probe the mechanism at play. 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. Bianco continues: "We believe that this discovery will highlight the potential of graphene quantum dots for biomedical applications. They offer great opportunities in nanomedicine."
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."
Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "The Graphene Flagship recognised, from the beginning, the great potential of graphene and layered materials for biomedical applications. This work confirms yet again the biocompatibility of graphene particles and, even more so, the fact that they can be biodegraded in the human body. This is an excellent result that will push forward this important field of application."