Researchers at Harvard’s School of Engineering and Applied Sciences (SEAS) have developed a device that can identify an unknown liquid. The researchers' findings were published in the Journal of the American Chemical Society in the article, "Encoding Complex Wettability Patterns in Chemically Functionalized 3D Photonic Crystals."
The device, named Watermark Ink (W-Ink), utilizes the chemical and optical properties of precisely nanostructured materials to distinguish liquids by their surface tension. The nanostructured material is called an inverse opal. The inverse opal is a layered glass structure with an internal network of ordered, interconnected air pores. It can be used to diagnose liquid contaminants in the field, as it is portable and requires no power source.
Similar to litmus paper, W-Ink changes color when it encounters a liquid with a particular surface tension. A single chip can react differently to a wide range of substances; it is also sensitive enough to distinguish between two closely related liquids. A hidden message can be “written” on a chip, revealing itself only when exposed to exactly the right substance. Dipped in another substance, the chip can display a different message altogether.
The color will change based on the surface chemistry and the degree of order in the pore structure. A more ordered structure can control whether or not the liquid enters certain pores by changing their surface chemistry.
Treating parts of the inverse opal with vaporized chemicals and oxygen plasma creates variations in the reactive properties of the pores and channels, letting certain liquids pass through while excluding others. Allowing liquid into a pore changes the material’s optical properties, so the natural color of the inverse opal shows up only in the dry regions. Each chip is calibrated to recognize only certain liquids, but it can be used over and over (provided the liquid evaporates between tests).
The researchers believe that the device has a wide range of applications. They are currently developing more precisely calibrated chips and conducting field tests with government partners for applications in quality assurance and contaminant identification. According to the researchers, W-Ink can be tuned to be sensitive to one formulation, thereby turning a different color if the formulation varies. Suggested application include: verifying the fuel grade of gasoline at the pump, testing bootleg liquor for toxic levels of methanol and identifying chemical spills quickly.
The researchers noted that W-Ink was not created to rival gas chromatography–mass spectrometry, rather it was created to offer formulation analysis in a portable fashion. Perhaps the device could find application in personal care product analysis, either in scale-up or in testing products on the market.