### New rewritable paper has been developed using photoreversible color-switching systems that can be applied to the surface of existing paper. This produces an ink-free rewritable paper that can be printed on using light.

<center><img src="http://i.imgur.com/XOFdLyH.jpg" /><br/><em><a href="https://phys.org/news/2017-02-ink-required-paper.html">source</a></em></center>

Current paper production has negative impacts on our environment. The process involves creating **pollution**, with 40% of our discarded paper going to **landfills** rather than recycling distribution centers. Even the ink removal process in recycling centers contributes to pollution. And this is all without the continuing **deforestation** occurring. About one-third of all trees harvested in the US are used for paper and cardboard production.

This new technology is an **eco-friendly** alternative to using regular standard paper just once before needing to be recycled. This new technology fits in line with global needs for **environmental protection and resource sustainability**. This is all thanks to researchers from the Shandong University in China, University of California, Riverside and awrence Berkeley National Laboratory. Their results have been [published](http://pubs.acs.org/doi/pdf/10.1021/acs.nanolett.6b03909) in *Nano Letters*.

In this newly developed process, Prussian blue can be integrated with the ***photocatalytic activity of titanium dioxide nanoparticles***. The light-printable paper is printed with UV light, and erased with heat at 120°C (250°F), and has an has outstanding rewriting performance with greater than 80 times reversibility, with the legibility lasting at least five days and resolution over 5 µm. This is made possible by the color changing chemistry of the the titanium dioxide nanoparticles

Yadong Yin, Chemistry Professor at the University of California says the new rewritable paper ***"has the same feel and appearance as conventional paper, but can be printed and erased repeatedly without the need for additional ink."***

Evenly mixing the titanium dioxide nanoparticles with Prussian blue results in a solid blue appearance on the plane unprinted paper.

To print onto the paper, exposure to UV light is required, which photo excites the titanium dioxide nanoparticles which release electrons that are picked up by the Prussian blue nanoparticles, which in turn change from blue to colorless. 

<center><img src="http://i.imgur.com/NlWxJr1.jpg" /><br/><em><a href="http://pubs.acs.org/doi/pdf/10.1021/acs.nanolett.6b03909">source</a></em></center>

Instead of the letters being printed, it's actually the background that is made colorless so that the colored text can be read. The reverse can also be done, with colorless text on a blue background.

Other colors beside blue can also be used for mixing of other colors with the Prussian blue.

<center><img src="http://i.imgur.com/ZD7SM9K.jpg" /><br/><em><a href="http://pubs.acs.org/doi/pdf/10.1021/acs.nanolett.6b03909">source</a></em></center>

**Paper slowly returns to solid blue through oxidation after five days. Heating the paper for 10 minutes will also return it to its solid blue state.**

This is a cost competitive way of coating materials for printing. The printing process is also cheaper than conventional printing since no inks are used. And, not to mention that it can be reused over 80 times. All this adds up to significant cost reduction.

Obviously five days of use is not practical at this point. The researchers are working to make the technology more convenient and have a wider range of applicability. Quicker laser-based printing, as well as a fuller range of colors, is on the books to be developed as well.

### "We believe the rewritable paper has many practical applications involving temporary information recording and reading, such as newspapers, magazines, posters, notepads, writing easels, product life indicators, oxygen sensors, and rewritable labels for various applications."

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References:
- [No ink required: paper can be printed with light](https://phys.org/news/2017-02-ink-required-paper.html)
- [Photocatalytic Color Switching of Transition Metal Hexacyanometalate Nanoparticles for High-Performance Light-Printable Rewritable Paper](http://pubs.acs.org/doi/pdf/10.1021/acs.nanolett.6b03909)

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2017-02-02, 5:01pm