Physicochemical characteristics and toxicity of nanomaterials

“Physicochemical characteristics” and toxicity of nanomaterials

By the AVICENN team – Last updated June 2022

At the nano scale, we can no longer consider that “it is the dose that makes the poison”. This sentence from the physician and alchemist Paracelsus, the “father” of toxicology, is very often invoked to evaluate the risks related to synthetic chemicals. Scientifically, it is now being questioned, especially in the specific case of nanomaterial toxicity, which is strongly influenced by the physicochemical characteristics of the nanomaterials considered.

Each of the parameters mentioned below affects the toxicity of nanomaterials and each of these is itself subject to variation during the life cycle of nanomaterials. Hence the complexity, for researchers, to evaluate the toxicity of nanomaterials.

– Their chemical composition:

• This is the identity of the substance, for example “silver” or “titanium dioxide”.

• One can extrapolate the knowledge one has about the substance to the macro state, whose known properties (and sometimes toxicity) can be increased tenfold due to surface reactivity.
  
• New properties (or new toxicity) may also appear specifically at the nanoscale; they are much more difficult to predict, and often not enough is known about them.
  
• However, nanomaterials of the same family cannot be considered as a “monosubstance”: within the same family, different substances can present different toxicity and genotoxicity¹Results of the European Nanogenotox program on the genotoxicity of nanomaterials, presented in French at ANSES, during the ‘Restitution du programme national de recherche environnement santé travail’: Substances chimiques et nanoparticules: modèles pour l’étude des expositions et des effets sanitaires: Dossier du participant et Diaporama, November 2013. And “Toxicological assessment of nanomaterials needs to evolve, says European research project,” APM International, November 14, 2013. On the genotoxicity of nanomaterials, see also: Genotoxicity of Manufactured Nanomaterials: Report of the OECD expert meeting, OECD, December 2014.

– Their dimension (size and size distribution):

Their nanometric size allows nanomaterials to penetrate the cell and cause adverse effects²See for example Size determines how nanoparticles affect biological membranes, Dunning, H., Imperial College London, September 17, 2020 (press release) and Size dependency of gold nanoparticles interacting with model membranes, Contini, C et al, Nature Communications Chemistry, 130, 2020. Their size is not the only factor and the items below are also decisive.

– Their shape (or morphology):

There is a great diversity of nanoparticle shapes: nanotubes, nanowires, nanosheets, nanocubes, etc. It seems that the tubular, fibrous or multifaceted structures present a greater toxicity than the smooth structures (such as spheres), in connection with the surface reactivity³See for example: The influence of surface coatings of silver nanoparticles on the bioavailability and toxicity to elliptio complanata mussels Auclair J et al, Journal of Nanomaterials, 2019 : Silver nanoparticles harm mussels: high silver levels in freshwater mussels are linked to reduced survival time in air, weight loss during air exposure, and DNA damage.. The toxic action can also be more important on one of the facets, for example for nanomaterials of complex shape and nature (but again, this will depend on the type of nanomaterial).

– Their specific surface :

This is the surface area of a particle or a material in relation to its volume. It has an important role in explaining certain changes in the behavior of the same material (e.g. powdered sugar will melt more quickly in hot tea than a large lump of sugar).

– Their surface reactivity / surface chemistry (and if necessary, their outer cover: coating or encapsulation)

– Their state of charge

– Their degrees of agglomeration / aggregation:

• Note: Research conducted in Belgium and published in 2020 shows that aggregates larger than 100 nm should not necessarily be considered less toxic than their nanometric counterparts, whether they are silica nanoparticles⁴Cf. Assessing the Toxicological Relevance of Nanomaterial Agglomerates and Aggregates Using Realistic Exposure In Vitro, Murugadoss S et al, Nanomaterials, 11, 1793, 2021 and Is aggregated synthetic amorphous silica toxicologically relevant, Murugadoss S et al, Particle and Fibre Toxicology, 17(1), 2020 or titanium dioxide nanoparticles⁵Cf. Agglomeration of titanium dioxide nanoparticles increases toxicological responses in vitro and in vivo, Murugadoss S et al, Particle and Fibre Toxicology, 17(10), 2020.

– Their solubility (in water, biological fluids, …)

– Their crystallinity

– Their powderiness