What does "graphene" really look like and why is it not "carbon nanotubes"

During the last few years, there has been a continuous and steady increase in the industrial translation of graphene and various related 2D materials. Many different industries and application areas have moved forward from the evaluation stage of graphene materials to the phase of adoption and industrial development, closer to large-scale product manufacturing. Some examples include fabrics, batteries, metal alloys, concrete and other construction materials, and different types of composites used in automotive and aerospace engineering.

At the same time, recent press releases and some health authority advisories have highlighted the need to guarantee that the degree of potential health risks from using graphene in various marketed products is clear to the regulators and consequently to consumers. In a lot of cases, parallels are drawn to some perceived risks from different types of graphene materials and those identified for specific types of carbon nanotubes. Such parallels are in almost all cases inaccurate, and in many cases outright false.

In this Nanopinion, we aim to offer clarification on some fundamental understanding about “graphene”. In particular, we wish to emphasise that “graphene” as a single type of material does not really exist, nor is it used as such in industrial applications. Moreover, we wish to explain why graphene-related materials are really not the same materials as carbon nanotubes. While both consist of only carbon, just like pencils or diamond stones – both are made of carbon atoms arranged differently in space!

Not just one “graphene”: the zoology of graphene-based materials

Hundreds of companies worldwide are claiming to manufacture “graphene”, applying different production methodologies and processes to obtain a range of graphene materials with significant variations in their physicochemical properties (Kauling et al. 2018). ‘’Graphene’’ is the pristine, defect-free, single atom layer carbon sheet, among a family of graphene-based materials, the most popular of which comprise graphene oxide, reduced graphene oxide, exfoliated graphene flakes, graphene nanoplatelets, chemically functionalised versions of all these, and other two-dimensional (2D) materials.

There has already been an attempt and proposition to describe and categorise graphene and related materials on the basis of their chemical and structural features (Bianco et al. 2013). Moreover, a working framework that is mapping some critical physicochemical features of graphene materials has been proposed (Kostarelos and Novoselov, Science, 2014; Wick et al. 2014) to create a landscape and assist in drawing correlations with some more complex features and behaviours, such as pharmacological properties (Bussy et al. 2015) or degradation (Martin et al. 2019) (Figure 1). In combination, both propositions to use a common and accurate nomenclature to refer to the many different types of graphene materials, and a common physicochemical framework against which to map them in relation to each other, should be adopted to provide clarity and a widely accepted common understanding. Sadly, these attempts and propositions have largely remained ‘academic’, not widely adopted by the non-experts who tend to adopt the simplicity of the elusive “graphene” for their marketing communication.   


 

In 2017, a joint terminology from the International Organization for Standardization and the  International Electrotechnical Commission (ISO/IEC) was officially adopted for graphene and related 2D materials (https://www.iso.org/standard/64741.html), defining 99 relevant terms spanning nomenclature, methods of production, properties and characterisation. Being already used in many commercial products, it is the responsibility of companies and stakeholders to accurately report the type of graphene material and its characteristics (Clifford et al. 2021). In order to provide a precise description of each graphene material type, a committee for standards has also reported an ISO/IEC document specifying the sequence of methods for characterising the structural properties of graphene, bilayer graphene and graphene nanoplatelets from powders and liquid dispersions (https://www.iso.org/standard/70757.html). So, in terms of regulatory standards for definitions and nomenclature some steps in the right direction have been taken.

According to the European Chemical Agency (ECHA), protocols for sample preparation and characterisation data analysis are mandatory (https://echa.europa.eu/home) for registration of graphene materials, and these new ISO/IEC standards are of great help. Registration of graphene and graphene oxide has already been filed with ECHA (https://echa.europa.eu/de/registration-dossier/-/registered-dossier/24678, https://echa.europa.eu/de/registration-dossier/-/registered-dossier/27774) by the Graphene Consortium GmbH and these dossiers will need to include more extensive safety information and data as manufacturers aim to obtain registration for production of more than 1 ton per year.

Why “graphene” is not the same as “carbon nanotubes”

At the same time, there is obscurity created frequently by (most commonly non-scientific) articles, posts and announcements connecting, or in some cases, equalising safety risks from graphene materials with those reported for carbon nanotubes. First of all, there is a good body of solid scientific evidence available that has determined safety concerns and the risk for carcinogenicity to be primarily associated with exposure to long, rigid carbon nanotubes (Kostarelos 2008). Surface modifications (such as oxidation) and shortening of even such high risk carbon nanotubes has been shown to lead to dramatic reduction of adverse reactions (Ali-Boucetta et al. 2013). Secondly, there is no real structural parallel or similarity between the flat sheets that graphene-related materials consist of, with the fibre-shaped cylindrical objects that carbon nanotubes are. Thirdly, an association is commonly made with an overgeneralised “graphene” that, as was discussed above, is problematic and inaccurate. Connections and associations between the two types of carbon-based materials are not scientifically justified and can lead to unnecessary confusion. More worryingly, it can lead to unwillingness to perform and fund systematic scientific evaluation of various types of graphene materials – especially those that are finding their way to mass industrial use and consumer products.      

It should be highlighted that a major difference between the two genres of materials exists at the supply end of the chain. Due to the facile production and industrial upscaling of processes for some types of graphene materials, there are now hundreds of producers around the world, with thousands of different graphene material products marketed as “graphene”. These are being incorporated into hundreds of different products (e.g. fabrics, paints, anti-corrosion coatings, structural alloys, etc.) creating an environment that is particularly challenging for the application of quality control measures and regulatory responsibility and accountability. In contrast, there are only a few manufacturers of carbon nanotubes that can be more easily regulated. The ‘democratisation’ of graphene material production therefore should be addressed by future regulatory bodies overseeing the quality of the exact graphene materials used in different products.  

The Graphene Flagship, through its persistent research efforts during the last decade, has been (and still is) generating a significant body of data on the safety, risks and biological response from interaction and exposure to the most prevalent types of graphene materials, that are thoroughly characterised and standardised (Fadeel et al. 2018). The responsibility of the Graphene Flagship project is to accurately and with scientific rigour determine any safety limitations from exposure to such materials. However, all such materials studied – particularly in the study and elucidation of biological mechanisms – are carefully synthesised and characterised with minimum impurities. These materials may have very little in common with the crudely and cheaply produced commercially available materials, nominally of the same class and category. It will not be surprising if some very different biological response data will be generated using such commercially available low-purity material, however, this responsibility should lie with the manufacturers and suppliers.

Biographies of the authors

Alberto Bianco Alberto Bianco is first class Research Director at the CNRS in Strasbourg, France (https://ibmc.cnrs.fr/en/laboratoire/i2ct-en/equipes/therapeutic-multifunctional-carbon-and-2d-nanomaterials/). He received his PhD from the University of Padova (Italy). He was visiting scientist at the University of Lausanne (Switzerland), the University of Tübingen (Germany), as an Alexander von Humboldt fellow, the University of Padova and Kyoto University (Japan). He is fellow of the European Academy of Science and the Academia Europaea. In 2019 he obtained the CNRS Silver Medal. He is Editor of the journal Carbon. Alberto is Deputy Leader of Work Package on Health and Environment in the Graphene Flagship.

Maurizio Prato Maurizio Prato is Chemistry professor at the University of Trieste, Italy, and Ikerbasque Professor at CIC biomaGUNE in San Sebastián, Spain. He was the recipient of an ERC Advanced Research Grant in 2008 and another one in 2020. He is member of Accademia Nazionale dei Lincei. His research focuses on the synthesis of new functional materials and their safety profile, for applications in medicine and energy fields, in particular in spinal cord repair, splitting of water, reduction of carbon dioxide into useful chemicals. He is Leader of Work Package on Health and Environment in the Graphene Flagship.

Kostas Kostarelos is the Professor and Chair of Nanomedicine at the University of Manchester (www.nanomedicinelab.com) and a Severo Ochoa Distinguished Professor at the Catalan Institute of Nanoscience & Nanotechnology (ICN2) in Barcelona, Spain (www.icn2.cat/nanomedicine). He read Chemistry at the University of Leeds and obtained his Diploma and PhD in Chemical Engineering from Imperial College London. He is a Fellow of the Royal Society of Chemistry (FRSC), the Royal Society of Medicine (FRSM) and the Royal Society of Arts (FRSA). Kostas is Head of Division Health, Medicine & Sensors, and Leader of Work Package on Biomedical Technologies in the Graphene Flagship.

References

Kauling et al., Adv. Mater. 2018, 30, 1803784
Bianco et al., Carbon 2013, 65, 1
Kostarelos and Novoselov, Science 2014, 344, 261
Wick et al., Angew Chem Int Ed. 2014, 53, 7714
Bussy et al., Nanoscale 2015
Martin et al., Chem. Comm. 2019, 55, 5540
Clifford et al., Nat. Rev. Physics 2021, 3, 233
Kostarelos, Nat. Biotech. 2008, 26, 774
Ali-Boucetta et al., Angew. Chem. Int. Ed. 2013, 52, 2274
Fadeel et al., ACS Nano 2018, 12, 10582
https://graphene-flagship.eu/

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