Prototype paper printed with ultraviolet light instead of ink

According to the journal Nano Letters, scientists from Shandong University in China as well as Lawrence Berkeley National Laboratory have figured out a way to make paper that can be printed with ultraviolet light, erased by heating it and then rewritten more than 80 times. That’s a solution for saving trees, as paper production is still going strong despite modern society existing in a full-on digital age.
The researchers found a low-cost, environmentally friendly way to coat conventional paper with a thin layer of nanoparticles that respond to light. Thus, they used two types of nanoparticles in order to make the inkless paper: those made of Prussian blue, a common, nontoxic blue pigment, and titanium dioxide, a nontoxic chemical used to, among other things, print the white M&Ms label on the candies. They mixed these two kinds of nanoparticles in a solution and then applied it to a conventional piece of paper which turned a deep blue.
To print text on it, the scientists used a mask with the letters etched out and placed it on top of the coated paper.
Next, they exposed the paper and mask to ultraviolet light. When the photons in the light hit the titanium dioxide nanoparticles through the cut-out holes in the mask, the nanoparticles released electrons. Those electrons got picked up by the Prussian blue nanoparticles, which then turned colorless. In short, some spaces stayed blue and other spaces became colorless, producing the printed text. The inkless letters remained visible for at least five days before they faded away. The team found that the letters could be erased faster by exposing the page to heat for 10 minutes. The whole process can be repeated more than 80 times.
As Yin and his scientists-colleagues think their rewritable paper could be used to make newspapers, notepads and price tags, which can be rewritten when the amount changes, the next step for the team is to develop a printer that’s able to print on a standard-size sheet of paper and then take the system to full color.

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