Ink used in daily life contains dye, film formers, and dispersant substances leading to certain toxicity and pollution. For example, some dyes are toxic to marine life or will react with disinfectants such as chlorine to produce similar harmful byproducts.

Therefore an alternative dye emerged recently. The research team from the University of St. Petersburg developed a colorless, non-toxic ink. This ink does not subside with the time, but also it's ecosystem absolutely safe. The research team used nano-titanium dioxide as raw material, by varying the nanostructured material, so that they can reflect light, light interference occurs on a particular route, you will be greeted by a variety of colors. By changing the thickness of the coating, the color change can be achieved. In fact, such phenomenon is common in the life. When we play with soap bubbles,  the thickness of other films would determine  the color we can see.

It is worth mentioning that this ink on the printing conditions is not high. It does not require special printing equipment, or need any high temperature treatment.

Below is a squirrel image printed by the new ink.

 While this work is still in the validation phase principle, the use of this method allows the red looks lifelike yet be a challenge. However the ink is  friendly to ecosystem and  even under exposure to UV light, it is still stable, which is unprecedented.

The study has also been reported in ACS Nano journal.