The researchers coated conventional paper with nanoparticles of two chemicals: Prussian blue, the pigment that gives blueprints their characteristic color, and titanium dioxide, a substance used in sunscreens. A blast of ultraviolet light makes the titanium dioxide nanoparticles donate electrons to their Prussian blue neighbors. That jolts the pigment into shifting its color from midnight blue to milky white.
By shining that UV light through a transparent screen marked with black text, the researchers “printed” blue text on a white background. The text lasts about five days and then spontaneously fades away: “Every morning I could just push a button, and a printer would give me a fresh newspaper to read over breakfast,” Yin says.
Since reporting their invention in Nano Letters early this year, the scientists have MacGyvered a digital projector to replace their transparent screen. They are now working on increasing the number of times the paper can be reused. Zhang says a key hurdle will be persuading companies to develop the unconventional UV zappers needed for widespread use. Even though commercialization could be a few years away, Yin says, “We’ve had a lot of discussions with industry investors.”
