Dr Lisa Bushby discusses the new wonder material graphene, its range of uses and considers some of its associated hazards and risks.
What is it?
Graphene is a two-dimensional (one-atom thickness) form of carbon. Initial work on graphene was carried out by two Russian-born scientists at the University of Manchester: Andrei Geim and Konstantin Novosolev, who were awarded the Nobel Prize for Physics for their research.
The material exists as standalone atomic monolayers of carbon arranged in the familiar hexagonal pattern of graphite that resembles chicken wire fencing. The structure is shown in figure 1.
Figure 1: Chemical structure of graphene
In a bid to encourage innovation, the researchers at the University of Manchester did not patent their work on graphene, allowing teams around the world to explore and find uses for the material.
Owing to its special properties, the number of potential uses of graphene is huge, eg it:
absorbs only 2.7% of light, ie is almost entirely transparent
is about 200 times stronger than steel
is the most effective known conductor in the world.
Current uses of graphene are as a replacement material, eg as a replacement for indium tin oxide in display technology (owing to its conductivity, transparency and flexibility properties). Graphene has also been used as a transistor material in place of silicon and semiconductor transistors, and shows great potential to break the frequency limits of established transistor materials in the near future.
Many future potential applications have been suggested. For example:
research has suggested that graphene filters could outperform other techniques of desalination by a significant margin
as graphene has an extremely high surface area to mass ratio, graphene may be used in ultracapacitors
functionalising graphene could enable single-molecule gas detection, or microbial detection and diagnosis devices.
Associated hazards and risks
A large sheet of graphene is considered to be a stable compound and relatively inert. It is therefore, generally not hazardous in normal handling, but good laboratory practices should always be applied.
However, on 18 October 2011, the European Commission adopted the Recommendation on the definition of a nanomaterial. According to this Recommendation, a “nanomaterial” means: “A natural, incidental or manufactured material containing particles in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1nm–100nm.
“In specific cases and where warranted by concerns for the environment, health, safety or competitiveness the number size distribution threshold of 50% may be replaced by a threshold between 1 and 50%.
“By derogation from the above, fullerenes, graphene flakes and single wall carbon nanotubes with one or more external dimensions below 1nm should be considered as nanomaterials.”
And recently, researchers have raised concerns about the health effects of these so-called graphene flakes.
There is precedent: asbestos particles (thin, light, tough) are considerably hazardous when breathed in, and can cause asbestosis and mesothelioma; silicon dust can cause silicosis, and coal dust pneumoconiosis.
Respiratory toxicologists at the University of Edinburgh suggested that tiny flakes of carbon might be transported deep into the lungs in a similar way to asbestos fibres and coal dust. Once lodged, there is no mechanism for their removal or breakdown as the compound is inert.
According to the team at Edinburgh, the problem with graphene flakes is that although they might be labelled as being a few dozen micrometres across on the shipping container from a supplier, the platelets can behave as if they were much smaller and become and remain airborne for some time, acting in the manner of a frisbee. Owing to their aerodymanic properties, they can be inhaled deeper into the lungs than other forms of carbon. The human body can usually filter particles quite effectively, but these particles behave in ways that allow them to slip past the filters and once inside can be too big for white blood cells to engulf.
In vitro tests also showed that these particles can remain on sensitive tissues and can trigger an inflammatory response in lung cells and those found in the pleural space.
In 2011, a team at Brown University in the US published data on graphene’s putative toxicity. It suggested that “biological response will vary across the material family depending on layer number, lateral size, stiffness, hydrophobicity, surface functionalisation, and dose.” The team also suggested that graphene might produce reactive oxygen species in target cells or interfere with membrane lipids because of its extremely high hydrophobic surface area.
In addition, the researchers said that as with asbestos and coal dust, and other smooth, continuous, biopersistent particles that can enter the body, graphene may have the ability to instigate tumour growth.
There is minimal reliable environmental data on graphene regarding its bioaccumulation or mobility in environmental media.
Health and safety
The occupational use of graphene and graphene flakes is regulated under the Control of Substances Hazardous to Health Regulations 2002 (COSHH).
The principle of risk assessment is embedded in COSHH and applies even though all the necessary information on the health and safety aspects of graphene is not available.
Since there is uncertainty about the risks of being exposed to graphene flakes, the regulatory and safe response is to take a precautionary approach.
Examples of a precautionary approach to control measures when working with graphene flakes, include the following.
Using appropriate work processes, systems and engineering controls, and providing suitable equipment and materials to minimise the likelihood of release.
Control exposure at source by carrying out all tasks, including packaging and disposal in a ducted fume cupboard with a high-efficiency particulate air (HEPA) filter, or by using other suitable effective local exhaust ventilation (LEV) fitted with a HEPA filter.
Ensure the LEV achieves and maintains adequate control of exposure at all times and that the system undergoes thorough examination and testing once a year. Employees should be trained in how to check and use the LEV and records should be kept of all the daily, weekly, monthly and annual LEV checks.
Reduce the number of employees handling graphene flakes and minimise the level and duration of exposure and the quantities used.
If possible, keep the material wet or damp to reduce the risk of it becoming airborne.
Provide respiratory protective equipment (RPE). This is for emergencies, and for use only in addition to other control measures. Ensure those employees who use RPE are trained in its use and have had face-fit testing.
Provide personal protective equipment, eg gloves and goggles. Use single use disposable gloves where possible. At least two layers of gloves are recommended to be worn when handling nanomaterials owing to the difficulty in determining diffusion of nanoparticles.
Use wet-wiping wherever practicable for cleaning and avoid the use of vacuum cleaners.
Emergency procedures should be in place to deal with spills, accidents and emergencies.
Employees should be trained in the proper handling of nanomaterials and records should be kept of all training carried out.
There is currently no legal requirement for health surveillance for those working with graphene flakes. However, it is good practice to keep a record of those working with the substance in a similar way to other substances of concern.