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Tiny crystals made of a waste chemical and an amino acid make these signs glow ghostly white and ghoulish green. Sensors, security inks, and glow-in-the-dark toys use fluorescent and phosphorescent materials\u2014compounds that absorb light, get excited, and then radiate as they release their energy. But conventional light emitters often contain toxic heavy metals or halogens.<\/p>\n
Hanno C. Erythropel<\/a>, Paul T. Anastas,<\/a> and team at Yale University instead made environmentally friendly emitters by sprinkling lignin<\/a>, a paper industry waste product, in a histidine matrix. Lignin\u2019s phenolic groups can absorb ultraviolet (UV) light and release protons. Imidazole groups in histidine then grab the protons, allowing the lignin to relax and fluoresce pale white under UV light (top).<\/p>\n When the researchers swapped one of histidine\u2019s hydrogen atoms for a methyl group, the material showed phosphorescence. This type of slightly delayed emission allowed the sign to glow green for a fraction of a second after the UV lamp was switched off.<\/p>\n For these signs to work, lignin molecules need to be separated from one another in a close-packed crystal structure. Others have tried to scatter lignin in synthetic polymers, but then \u201cyou need solvents and additives,\u201d Erythropel says. \u201cThis is a simpler way to do it, and we\u2019re mainly using waste or renewably sourced material.\u201d<\/p>\n Credit: Chem<\/p>\n Do science. Take pictures. Win money. Enter our photo contest<\/a>.<\/p>\n