If you want to know exactly what a substance is, your best bet is to use something like a gas chromatographer. The problem is, such machines tend to be large, lab-based and expensive - not the greatest for use in the field, or by people who aren't connected with a research institute. Researchers from Harvard University's School of Engineering and Applied Sciences, however, have created inexpensive, portable 3D-nanostructured chips, that can instantly identify any liquid via its surface tension.
The chips utilize something known as the "Watermark Ink" or "W-Ink" concept. W-Ink is based around a fabricated material called inverse opal, which has a layered glass structure with an internal network of ordered, interconnected air pores. By selectively treating certain parts of the inverse opal with vaporized chemicals and oxygen plasma, the Harvard researchers have been able to alter the properties of those pores and the channels between them. Depending on how those pores are altered, they will only allow liquid with a specific surface tension to flow into them. If that liquid is able to enter the pores, it alters the opal's optical properties, causing it to change color.
Single chips can be "tuned" to test for multiple liquids, producing a different type of color change in the presence of each one. They require no power source, and can be reused many times.
Old school spies could even find them useful, as they can be treated to reveal written messages when subjected to the right liquids - they can even contain several messages, each one becoming visible using a different solution.
The Harvard team are hoping to commercialize the W-Ink technology, and are currently working on calibrating the chips for use in quality assurance and contaminant identification. Their research was recently published in the Journal of the American Chemical Society.
The video below shows how the chips can react in different ways, to different liquids.