Color printing goes green

Researchers have developed a completely new way of printing color images using nanostructures, which they believe could be more efficient and environmentally friendly than current methods for certain applications [Yakovlev et al., ACS Nano 10 (2016) 3078].

Color printing has transformed in less than half a century from simple dot matrix printers to devices capable of producing three-dimensional structures. But what has not changed in that time is the use of dye inks for color printing. These inks are often toxic or environmentally unfriendly and require high-temperature fixing.

Now a team from ITMO University in Saint Petersburg, Russia has come up with a new strategy based on inkjet printing of nanostructures, which interact with light to produce visible color in a similar way to the rainbow hues seen on a soap bubble. In this fundamentally different approach to color printing, a transparent, bio-friendly nanocrystalline TiO2-based ink is deposited by conventional inkjet technology in layered structures on a polymer substrate. By controlling the thickness of the deposited layers accurately, the difference in refractive index creates interference effects that produce different visible colors from a single colorless ink.

‘‘The specially designed inks are based on the sol—gel transition of crystalline nanoparticles into a nanodimensional solid ceramic film without annealing,’’ explain researchers Alexandr V. Yakovlev and Alexandr V. Vinogradov.

The technique relies on the ability of inkjet technology to emit drops of a dispersion of nanoparticles in ethanol accurately and consistently through a nozzle head. As the ethanol gradually evaporates, the TiO2 nanoparticles settle into a thin film, which eventually crystallizes into a nanoscale, highly refractive layer. As well as the thickness of the layer, the concentration of nanoparticles and number of layers contribute to the ability of the approach to produce a range of colors.

Unlike dye-based colors, the TiO2-based ink does not fade in sunlight or over time. Moreover, TiO2 is nontoxic and biologically inert. The researchers believe that their new approach could be interesting for the long-term storage of images or information.

‘‘Another practical application could be security printing on documents, for example hidden marks for banknotes,’’ add Yakovlev and Vinogradov.

The team is now working on increasing the printing area and developing a desktop nanofabrication system for researchers.

This article was originally published in Nano Today (2016), doi: 10.1016/j.nantod.2016.04.001