Challenges resulting from the transformation of nanomaterials
Nanomaterials (NMs), more and more commonly used in consumer products, medicine and other areas of human activity, are considered as one of the most innovative areas of science and industry, but they also raise some concerns in terms of safety. Having unique properties due to their size, shape, composition and other characteristics, they are often treated as a separate category of materials, demanding specific approaches (for example risk assessment and regulation, Nowack, 2012). Indeed, numerous studies found that engineered NMs have very specific properties and that in many aspects (uptake by organisms, environmental fate) they are radically different than bulk materials. But key questions from a risk assessment perspective are: do NMs remain the same throughout their life cycle? Do they retain the same properties?
NMs are currently used in many consumer products, such as cosmetics, textiles, biomedical devices and others. In some of those products, NMs have to retain their nano-character – when they lose it, they lose also their desired features. On the other hand, some NMs are supposed to overcome significant transformation – for example in the environment or within the human body – and the products of this transformation cause the desired effect. Silver nanoparticles, widely used in many categories of consumer products, are a good example of the second case – their (partial) dissolution leads to release of silver ions, which have strong antibacterial properties. A separate, but related, issue is that of mislabelling of nano-products, which actually occurs quite often, especially in the case of more commercialised NMs. In some cases ‘nano’ labelled products actually do not contain NMs (Lorenz, 2011).
All nanoparticles, regardless of their shape, size, material they are made from, surface functionalization, are transformed once they are released into the environment or the products containing NMs come into contact with the environment (Nowack, 2012). Release of NMs into the environment – from where they can in turn enter living organisms – occurs at each stage of their life cycle, beginning from the production processes, transport, intentional and non-intentional use, and disposal. ‘Transformation’ is a very general term, and many different processes can fall into this category (Nowack, 2012). As mentioned before, we can divide them into two groups: processes, due to (or despite) which NMs s retain their nanoparticulate character (i. e. NMs used as drug carriers (Malam, 2009), and processes in which NMs lose their nano-character (i. e. dissolution of silver nanoparticles to antibacterial silver ions (Choi, 2008). Some transformation processes (like dissolution) can belong to both groups, depending on the initial size of the nanoparticles, and the extent and kinetics of the process. Another classification is based on whether the NMs increase or decrease their toxicity after being released into environment (in comparison with the pristine NPs)(Nowack, 2012).
Nanoparticles undergo significant transformations even in conditions which are supposedly mild and stable. For example, humidity-dependent dissolution and re-formation of silver nanoparticles exposed to air has been found by Glover et al. Newly formed nanoparticles were also found on the surface of common silver and copper objects (jewellery, spoons etc.) (Glover, 2011). Spontaneous formation of silver nanoparticles was also observed by other authors (Akaighe, 2011). It seems that transformation, degradation and re-formation of nanoparticles takes place in common environmental conditions in an uncontrolled way and often without our knowledge.
Silver nanoparticles are generally well studied in terms of their transformation in the environment and in their interactions with biological systems (Monteiro-Riviere, 2013). They are widely commercialised and thus produced in large quantities, and their chemistry in the environment is complex and can be at least party based on previous, well established knowledge of different silver compounds (based for example on toxicological studies of silver compounds used in photographic industry, Purcell and Peters, 1998) .
There are many different environmental factors which can have an impact on silver NPs transformation in the environment: temperature, pH, size, surface charge and stabilisers used in NPs synthesis, air humidity, and the presence of certain ions (Glover, 2011, Stebounova, 2011, Adams, 1999). For example, chloride ions (which are present in almost any environment and within organisms, although their concentration may vary) are well known to have an impact on both silver NPs aggregation (as chloride concentration increases, more aggregates are formed) and dissolution (which strongly increases under increased chloride conditions). Soluble silver-chloride species formed in those processes were found to be less toxic to certain bacteria than silver ions (Chambers, 2014), and thus information on chloride ions concentration is important for predicting antibacterial properties of silver-containing NMs. Sulfide ions, which were found to form very stable, insoluble complexes with silver, are the other example of substances which have a profound impact on the transformation of silver NPs. Formation of silver-sulfide compounds (i.e. sulfidation) also results in decreased toxicity of silver compounds, and has to be taken into account when discussing toxicity of silver compounds. Sulfidation takes place also within cells, and is one of the ways in which organisms detoxify harmful silver ions. Due to the detoxifying properties of substances containing sulfide ions or thiol groups, they may be used as a ‘pharmacological rescue’ method (Yang 2012).
As can be seen even in the very limited examples presented above, transformation of silver NMs takes place spontaneously, in any given environment, and thus the toxicity predicted for pristine, just synthesized NMs may not be valid for those NPs present in any natural system. Indeed, more toxicological studies on products of NMs transformation, as well as better understanding of those processes, are needed (Levard, 2012). This knowledge is lacking for all NMs, despite the fact that it is much more relevant for safety than knowledge on pristine NMs toxicity (Nowack, 2012). This issue will be discussed later in more detail.
Environmental transformation of silver NPs and the associated complex chemistry is relatively well studied, although new, interesting papers on this topic are still being published (Liu, 2012). Other types of NMs transformations are also extensively studied. For example, photochemical transformation of aqueous nC60 has been studied by Hwang and Li. It has been found that the studied NPs undergo chemical transformation when exposed to UVA irradiation, although the core of NPs remains intact even after 21 days of the experiment. It has also been found that the presence of aquatic humic acid reduced the transformation kinetics, which has great implications for studying environmental samples of those NPs (as changes on nC60 have an impact of effectiveness of the currently used analytic methods (Hwang and Li, 2010).
In a broad, critical review (Nowack, 2012) several hypothetical case studies are analysed, among them those of titanium dioxide in sunscreen and paint, composite structures containing carbon nanotubes, and cerium oxide in the fuel combusted in diesel engines. All those NMs can be found in products commonly used in everyday life, but they vary significantly in their properties and in terms of the ways in which they enter the environment. The authors propose a new way of categorising NMs, which takes into account the changing of their properties through their life cycle. Detailed analysis of different process which may be involved in the transformation of NMs in the environment is provided, as well as materials flow diagrams (release of NMs) from different products and the transformation reactions in the different environmental compartments.
Looking at the examples presented above, one may ask if it is not better to study NMs in forms in which they are actually present in the environment, and disregard pristine NMs not fitting into realistic scenarios of toxicity and other impacts they may have. Obviously, such an approach has its advantages, but at the same time presents certain challenges. Transformation of NMs is a complex process depending on many factors, and its final result (i. e. the exact form and concentration in which certain transformed NMs are present in the environment) may be hard to predict or reproduce (at least based on current knowledge). At the same time, products of NMs’ transformation may be difficult to separate from the environment (and during the separation processes further transformation can take place).
To conclude, it is important to remember that all NMs are transformed in the environment, and multiple and diverse processes are involved. As a result, properties of NMs (including their toxicity) may change significantly. It is thus important to plan all nanotoxicity studies very carefully, trying to mimic the naturally occurring processes and conditions as closely as possible. At the same time, knowledge on the impact of certain environmental conditions and processes on NMs properties should be greatly extended, as it may allow prediction of realistic scenarios of NMs transformation and ultimate environmental fate and behaviour.
It should be also remembered that NMs are not a uniformed group of materials, and extensive studies are needed to link different scenarios of their life cycle with their possible impact on the environment, and on organisms. And one more thing to remember – some NMs may retain their nanoparticulate character, but some of them may be even not present in the environment as NMs!
- Adams NWH, Kramer, J.R. 1999. Silver speciation in wastewater effluent, surface waters, and pore waters. Environ Toxicol Chem, 18, 2667–2673.
- Akaighe N, MacCuspie, R. I., Navarro, D. A., Aga, D. S., Banerjee, S., Sohn, M., Sharma, V. K. 2011. Humic acid-induced silver nanoparticle formation under environmentally relevant conditions. Environ Sci Technol, 45, 3895–3901.
- Chambers BA, Afrooz AN, Bae S, Aich N, Katz LE, Saleh NB & Kirisits MJ 2014. Effects of Chloride and Ionic Strength on Physical Morphology, Dissolution, and Bacterial Toxicity of Silver Nanoparticles. Environmental science & technology.
- Choi O, Deng KK, Kim N-J, Ross Jr L, Surampalli RY & Hu Z 2008. The inhibitory effects of silver nanoparticles, silver ions, and silver chloride colloids on microbial growth. Water research, 42, 3066-3074.
- Glover RD, Miller, J. M., Hutchison, J. E. 2011. Generation of metal nanoparticles from silver and copper objects: nanoparticle dynamics on surfaces and potential sources of nanoparticles in the environment. ACS Nano, 5, 8950–8957.
- Hwang YS & Li Q 2010. Characterizing photochemical transformation of aqueous nC60 under environmentally relevant conditions. Environmental science & technology, 44, 3008-3013.
- Levard C, Hotze, E. M., Lowry, G. V., Brown, Jr., G. E. 2012. Environmental transformations of silver nanoparticles: impact on stability and toxicity. Environ Sci Technol, 46, 6900−6914.
- Liu J, Wang, Z., Liu, F. D., Kane, A. B., Hurt, R. H. 2012. Chemical transformations of nanosilver in biological environments. ACS Nano, 6, 9887–9899.
- Lorenz C, Hagendorfer, H., von Goetz, N., Kaegi, R., Gehrig, R., Ulrich, A., Scheringer, M., Hungerbühler, K. 2011. Nanosized aerosols from consumer sprays: experimental analysis and exposure modeling for four commercial products. J Nanopart Res, 13, 3377–3391.
- Malam Y, Loizidou M & Seifalian AM 2009. Liposomes and nanoparticles: nanosized vehicles for drug delivery in cancer. Trends in pharmacological sciences, 30, 592-599.
- Monteiro-Riviere NA, Samberg ME, Oldenburg SJ & Riviere JE 2013. Protein binding modulates the cellular uptake of silver nanoparticles into human cells: Implications for in vitro to in vivo extrapolations? Toxicology Letters, 220, 286-293.
- Nowack B, Ranville JF, Diamond S, Gallego‐Urrea JA, Metcalfe C, Rose J, Horne N, Koelmans AA & Klaine SJ 2012. Potential scenarios for nanomaterial release and subsequent alteration in the environment. Environmental Toxicology and Chemistry, 31, 50-59.
- Purcell TW & Peters JJ 1998. Sources of silver in the environment. Environmental Toxicology and Chemistry, 17, 539-546.
- Stebounova LV, Guio, E., Grassian, V. H. 2011. Silver nanoparticles in simulated biological media: a study of aggregation, sedimentation, and dissolution. J Nanopart Res, 13, 233–244.