Several different types of nanomaterials have been utilized as pigments since time immemorial. For instance, in ancient Egypt, the soot from oil lamps was used to make black pigments of high opacity and stability. These were ideal as black ink for writing on papyrus scrolls. These soot particles or pigments, as we would call them today, were composed of carbon nanoparticles. Even today, in many parts of India, soot from metallic oil lamps is “designed” by varying the proportions and types of oils. This soot is then used as eyeliners that are even applied on infants and adults alike. In infants, application of these eyeliners is customary as they are believed to improve eyesight and protect against diseases. Recent research shows that these soot eyeliners contain various metals and carbon in their nanoforms. The nanoform of carbon gives the dark black colour, while the metal nanoparticles provide antimicrobial activity (Gargade and Chandrasekhar, 2019; Mohanty et al., 2019). Similarly, nanomaterials, such as colloidal gold particles for staining glass have been used for hundreds of years and can be found in the stained-glass windows of many historical buildings throughout Europe.
Such “nano” pigments can be currently found in several products, paints, coatings, printing inks, and cosmetics. Metallic nanoparticles of various geometries, sizes and crystallinity can all provide a range of extremely specific colours, which has opened newer vistas for the utilization of nanoparticles as pigments. Currently, in the EU, as part of a study commissioned by the EUON to investigate the safe use of nano-sized pigments in consumer products, a list of nanomaterials used in pigments currently known to be on the EU market was established. The list consists of 81 substances, and the details of the members in the list were procured from the European Chemicals Agency’s (ECHA’s) chemicals database, Belgian and French national inventories, and the current EU catalogue of nanomaterials used in cosmetic products. Data from the Danish Product Register was also used. A complete and interactive list, along with the safety data of these pigments can be obtained here.
The advantages of using nanoparticles as pigments are multiple and can be witnessed in several industries. Titania and silica nanoparticles are commonly used ingredients in paints. They function as UV filters and can also provide antimicrobial and self-cleaning properties. Depending on the type of paint and the desired functionality, nanomaterials can be integrated as free powders or as stabilised particles in colloidal form resulting in the nanomaterial being firmly embedded in the paint matrix.
Application of nanoparticles as pigments also can provide “special effects”. These are particularly helpful in jewellery and cosmetics, where unusual, shimmering or glittery effects are required. A combination or specifically oriented nanocrystals are used to provide these effects. Structural colour is another class of pigments where the colour is obtained through the periodic ordering of nanoscale features. These provide effects similar to the opalescence of a butterfly wing, which is one of the newest trends in eye and nail make-up industry. Nanomaterials also provide hydrophobicity for “long lasting” effects in cosmetics.
Along with the exciting possibilities, there is also the risk associated with nanoparticles, which is not overlooked when these are used as pigments. The same regulatory aspects applicable to nanomaterials also are applicable to nanomaterials as pigments. Therefore, these pigments are safe for use albeit some stigma that arises mainly from the “nano” tag. Particularly, nanoparticles are small enough to pass through cell membranes. Therefore, the REACH and the specific regulations of the cosmetic industry has brought out specific guidelines to understand the effects of these nanoparticles to humans. Physico-chemical characteristics, such as, size and shape of individual nanoparticles as well as their agglomerates must be tested and reported for pigments, during REACH registration. Furthermore, they are specifically tested for dustiness properties to ensure that the nanoparticles or their agglomerates do not cause any inhalation related toxicity in people coming in contact with products containing nanomaterial pigments. Likewise, a complete life cycle assessment of these pigments is often part of the REACH registration process. This eliminates human or environmental health risks that may potentially arise from their use.
We, at nEcoTox are experts at characterization of pigments according to the new REACH registration requirements. Additionally, we also provide aquatic fate analyses of pigments containing nanomaterials, which are crucial for REACH registration.
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Gargade, V. A., & Chandrasekhar, S. (2019). Nanotechnology in Ancient India. History of Nanotechnology: From Pre‐Historic to Modern Times, 37-55.
Mohanty, B., Verma, A. K., Claesson, P., & Bohidar, H. B. (2007). Physical and anti-microbial characteristics of carbon nanoparticles prepared from lamp soot. Nanotechnology, 18(44), 445102.
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