Nanocrystal Liquid Identification

By: Katie Anderson (Schaefer), Cosmetics & Toiletries magazine

Posted: October 5, 2011, from the October 2011 issue of Cosmetics & Toiletries.

Ian Burgess

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The need for liquid identification exists in almost all industries. In the petroleum industry, it can be used to verify fuel grade; in the pharmaceutical industry, it can be used to validate a liquid medication; and in the personal care industry, it can be used to identify materials in a lab, check for contaminants or variations in scale-up, or possibly to research the components in benchmark products. Currently, there are chemical methods used to identify liquids; however, Ian Burgess and his team at Harvard University’s School of Engineering and Applied Sciences (SEAS)* have developed a device that identifies unknown liquids on the go and with no power source.

Watermark Ink (W-Ink) utilizes chemical and optical properties of nanostructured materials to distinguish liquids based on surface tension. The nanostructured material is called an inverse opal, which is a layered glass structure with an internal network of ordered, interconnected air pores.

Components of W-Ink

The device simply is comprised of the inverse opal; however, according to Burgess, the composition of the opal makes it truly unique. “[The inverse opal] is a slab of silica that has the refractive index of glass. It has an impeccably highly ordered array of air spherical pores. We were not the first group to make [a photonic crystal], but in comparison to other photonic crystals, this inverse opal is really highly ordered. It has a face-centered cubic (FCC) lattice and a single orientation in the growth direction. We grow it vertically by evaporation,” he says.

To grow the crystal, the team begins with an aqueous suspension containing polymer micropheres and a silica precursor. The team then selects a flat hydrophilic surface on which to grow the crystals.According to Burgess, any flat hydrophilic surface will do, such as glass or plastic; however, the team chose silicon wafers, as they are extremely flat, hydrophilic and dark, which offers better contrast for the iridescent colors that result.

The team combines the mixture in a pot and heats it to let the water evaporate. “As the water evaporates, the colloidal crystal grows which will eventually become your air pores,” Burgess adds. The silica grows between the spheres; therefore, after the team burns off the spheres the silica inverse opal is left behind. Burgess explains, “If you take it up to 500°C in an oxygen atmosphere, you can burn off the polymer but the silica stays. Not only are the pores uniform size, but because they are so ordered in terms of their orientation, the little openings between two of these spheres are uniform in size as well.”