“Our new system combines the redox-driven colour-switching property of Prussian blue nanoparticles or their analogues (the well-known synthetic coordination compound of transition-metal hexacyanometalates) with the photocatalytic activity of titania nanoparticles,” explains team leader Yadong Yin. “The film produced can be cast onto conventional paper to produce an ink-free, light, printable rewriteable paper that has the same feel and appearance as conventional paper.”

So how does the system work? “When we shine UV light onto the TiO2 nanoparticles, photoexcited electrons are generated that can then catalytically reduce the Prussian blue nanoparticles present in the film into the Prussian white phase,” says Yin. “The colour thus switches from blue to white. The paper can be decoloured much more slowly too (in around five days), since it oxidizes when exposed to ambient oxygen.”

High photochemical stability

Letters and patterns can be effectively printed onto the paper by UV irradiation through a mask or printed onto it by direct scanning with a focused light beam, he adds. The written content remains legible for more than five days but it can be quickly erased if wished by heating the film to around 120°C.

“Compared to previous such colour-switching systems based on photoisomerization of chromophores that can only operate over several cycles, or the one recently fabricated by our team that consisted of organic dyes that significantly deteriorate after just 20 switching cycles, one of the most important advantages of the new Prussian blue-based rewritable paper is its high photochemical stability,” says Yin. “This means that it can be written onto and erased more than 80 times without any apparent changes to the paper’s properties (such as colour intensity or switching rate). And, since many Prussian blue analogues can be employed, this new version of our rewritable paper can work in many different colours.”

Many practical applications

And that is not all: other advantages include easy processing, low cost, high printing resolution and low toxicity, since Prussian blue is widely used as a pigment in commercial paints and inks.

According to the researchers, the new system could find use in many practical applications, including newspapers, magazines, posters, notepads, writing easels, product life indicators, oxygen sensors and various rewritable labels.

The team, reporting its work in Nano Letters DOI: 10.1021/acs.nanolett.6b03909, says that it is now busy constructing a laser printer to work with the rewriteable paper. “We will also be looking into full-colour printing onto the paper,” Yin tells nanotechweb.org.