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Intelligent paint

Researchers use nanotechnology to create a pigment that changes color

Institute of Chemistry/UspPigment made with nanoparticles of gold gradually changes color as it is heatedInstitute of Chemistry/Usp

A pigment that has the curious characteristic of changing color after being exposed to light and a laser was developed by researchers from the Institute of Chemistry of the University of São Paulo (USP). The research won first prize in the Abrafati-Petrobras  Paint Science Awards, 2006, the most important in the sector, and bestowed by the Brazilian Association of Paint Manufacturers in partnership with Petrobras Distribuidora. The substance, developed using technology on a nano-metric scale (1 nanometer is equal to 1 mm divided by 1 million), consists of nano-particles of gold deposited on hydrotalcites, a white powder formed from magnesium hydroxide and aluminum, with its structure of small layers.

Gold nanoparticles, when suspended in water, form reddish solutions. This color results from the interaction of light with the electrons on the surface of the nanoparticles that move like waves. When the nanoparticles approach each other a sort of interference between the waves occurs, giving rise to a violet tint. This allows them to be used as chemical and biological sensors because of the modification of their surfaces, so that they can indicate the presence of molecules and antibodies by changing color.

In a pigment that has been developed at normal room temperature the gold nanoparticles   are deposited on the edges of the layers and interact, giving rise to a bluish violet color. When the pigment is heated the color changes gradually to red, indicating the formation of isolated nanoparticles. The change in color can also be caused by a laser, which heats the bluish pigment, causing the nanoparticles of gold to melt. The starting point of the work, developed by a group led by Professor Henrique Eisi, was the Masters’ degree thesis of Leonardo da Silva Bonifácio, who after finishing his research went to work at the University of Toronto in Canada.

Before considering marketing the product, various tests will still be necessary in order to measure variations in the pigment response according to the laser used and the material agent employed. Toma explains that it can be used for various technological purposes and nothing prevents it from being applied in wall paint. However, most probably the gold-based paint will have a use that is more restricted to works of art or other objects whose painting needs to last longer. Gold is inorganic, resists high temperatures (it can withstand up to 900°C) and does not fade or lose its shine easily. As compared to organic colorants and other cadmium sulfate-based pigments that are toxic and potentially carcinogenic, the invention has the advantage of not posing any risks to the human body. Furthermore, the pigment is prepared in water, without the use of chemical solvents, and therefore there is no residual waste to harm the environment.

Despite the discovery’s contribution to the paint sector, there are still great things to come, in Toma’s opinion. “This was just the first step. Using nanotechnology that allows the molecular control of substances, it is possible to construct an ‘intelligent pigment’ that will have the most varied range of uses”, he says. To continue with the research he is already thinking of the possibility of creating a pigment capable of absorbing sunlight and converting it into electrical energy, or using it to decompose dirt molecules.