Water as a colorful ink: transparent, rewritable photonic coatings based on colloidal crystals embedded in chitosan hydrogel

The invention of paper has greatly contributed to the development of information storage and spreading. However, the vast production and consumption have also brought significant environment problems to our society. Although several kinds of rewritable papers have been reported, their practical application is rare. Herein, a new rewritable paper with water as a colorful ink is proposed and demonstrated successfully by using water as the trigger to tune the band-gap of photonic coatings on solid substrates. Various colors are achieved by adjusting the particle size of photonic coatings, or writing with aqueous solutions of certain pH. The photonic coatings can be erased and rewritten multiple times with no significant loss in color quality. Furthermore, the photonic coatings, which are transparent, enable fast and convenient visualization of the invisible photonic patterns with good tenability and reproducibility, offering potential applications for steganography, identification marking and anti-counterfeiting purposes. This transparent and rewritable paper can serve as an environmentally friendly information storage device to meet the sustainability of modern society.