Simple, inexpensive sensor can be configured based on a user’s skin tone. With a series of emoji-like faces, an inexpensive bracelet can tell users when they’ve been exposed to too much ultraviolet light (Nat. Comm. 2018, DOI: 10.1038/s41467-018-06273-3). The bracelets can be configured for different skin tones, allowing wearers to get the right dose of UV light for their personal pigmentation.

Although people know to seek shade and apply sunblock to avoid the sun’s damaging UV light, a little sunlight can be healthy. Some sun exposure helps our bodies produce vitamin D. The trick is to not to overdo it and risk skin cancer.

Another complication is that sunlight contains a mixture of UV light. “We can tolerate 1,000 times more UVA than we can UVB,” before risking sunburn, says Vipul Bansal of RMIT University, Melbourne. Electronic UV sensors can distinguish between UVA and UVB but can be bulky and expensive. Color-changing chemical UV sensors are smaller and less costly, but treat all UV light the same.

Bansal wanted a chemical that could differentiate lower-energy UVA from higher-energy UVB. “I was hunting for chemicals that could do this job for the past seven to eight years,” he says. He found a promising candidate in phosphomolybdic acid, a polyoxometalate with multiple redox states. In the presence of an electron donor, such as lactic acid, and UV light, phosphomolybdic acid gets reduced, changing from a colorless molecule to a deep blue one. The redox state and the extent of the color change is dependent, in part, on the energy of the UV light.

Using a mixture of phosphomolybdic and lactic acids, the researchers painted emoji-like faces onto four 15-mm-diameter circles of paper lined up on a wristband. The face on the farthest left had the most enthusiastic phosphomolybdic grin, followed to the right by less and less happy faces, and finally a serious frown on the far right. Also, moving from left to right, the researchers covered the faces with increasing numbers of light filters, such that the leftmost face was the most exposed to incoming light and the rightmost face was the most shielded.

In tests of the sensor, UVA produced minimal color changes and UVB generated significant dose-dependent changes. And as UV exposure increased, the faces became darker and darker blue, left to right, happy to sad.

By adding and removing the filters, the researchers could calibrate the sensor such that each face indicated a specific percentage of a person’s safe UV exposure level. And they could tailor the bracelets for people’s skin tone by increasing the number of filters for people with more skin pigmentation.

The sensor’s ability to respond to different types of UV light is the most significant advance, says John Rogers of Northwestern University, who has developed a UV-sensing tattoo. “It’s really important to know UVB because it is more carcinogenic.” He wonders, though, whether people would use the device, as is. “For devices that are worn, the aesthetics and the fashion aspect turn out to be super important in terms of compliance,” he says.

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