Inks that are inexpensive to make and shine brightly under ultraviolet light could be used to manufacture colourful coatings for light-emitting diodes.
To make a brilliantly glowing ink, Hong-Bin Yao at the University of Science and Technology of China in Hefei and his team devised nanoparticles that luminesce when they crystallise, a phenomenon called aggregation-induced emission. Most materials that exhibit such behaviour are based on expensive metals, but the researchers used molecules that contain much cheaper elements — copper and iodine. The team produced inks in a wide palette of colours by adding various organic groups to the main structure.
A QR code printed using the inks was invisible until illuminated with UV light. The inks can even adhere to surfaces underwater.
As a coating on LEDs, the inks can create lights that shine in a rainbow of colours, the authors show.