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Horizontal filler.printable lined paper for home and classroom, free from activity village. So we would always search online for preschool or kindergarten handwriting paper for the youngest and smaller handwriting. .writing paper kindergarten printable mb blue, oxidized form and lmb colourless, reduced form molecules are stabilized by surrounding hec molecules through hydrogen bonding. a uv–vis spectra showing the decolouration process under uv irradiation b uv–vis spectra showing the recolouration of the film at room temperature under ambient air c uv–vis spectra showing the recolouration process on heating at 115 °c in air and d the absorption intensity of a solid film recorded continuously in 20 cycles of switching between colour and colourless states. The hec film turned to slightly yellow on heating at 115 °c, and therefore increased the intensity of the background absorption. Plots of the percent of mb recovered from lmb in solid films by monitoring the absorbance of mb after uv light irradiation as a function of time in ambient air: a tio₂ nanocrystals/ mb.

B tio₂ nanocrystals/ mb /hec, c tio₂ nanocrystals/ mb /hec with additional hec film on top surface and d tio₂ nanocrystals/ mb /hec solid film with concentration of hec doubled from the case in b. The contribution of hec to the absorption background was subtracted for all samples. a schematic representation of writing letters on the rewritable paper using photomask on uv light irradiation, b digital images of writing and erasing letters on the rewritable paper, c –f digital images of rewritable paper maintaining in ambient air after writing of c 10 min, d 1 day, e 3 days and f 5 days. The slight variation in the background was due to the uneven thickness of the film resulted from the manual drop casting. The microscale patterns were photoprinted on a rewritable film using a laboratory 365 nm uv lamp through a chrome photomask. Although we now live in an age dominated by electronic media, paper still plays a very important role in communication and information storage, as evidenced by its tripled global consumption over the past three decades 1. According to recent international surveys, 90% of all information in businesses is currently retained on paper however, most of the prints are disposed after only one time reading, which not only significantly increases business operating cost on both paper and ink cartridges but also creates huge environmental problems including deforestation, solid waste and chemical pollution to air, water and land 3.

Rewritable paper that can be used multiple times and does not require additional inks for printing is therefore an attractive alternative that can have enormous economical and environmental merits to modern society 5. Conventionally, organic dyes capable of undergoing reversible colour switching based on the photoisomerization of constitute chromophores were proposed for potential use as the imaging layer in rewritable printing media 12. However, only limited progresses have been made in this area because of some major challenges 20. I colour switching often becomes much slower when dyes are present in solid media instead of solution, as their molecular mobility is markedly restricted 23. Ii many switchable dyes retain their colour for only several hours under ambient conditions, which is too short for reading, i the toxicity of switchable dyes is often an issue for daily use and iv most switchable dyes involve complex synthesis and are therefore expensive. As a result, it is of high interest to develop rewritable papers based on new colour switching mechanisms 24.

They may serve as promising imaging media for the development of rewritable paper if their redox reactions can be manipulated properly. � methylene blue mb , for example, can be switched between blue colour in an oxidizing environment and colourless leuco form, lmb in a reducing environment 26. It is a dye of low toxicity broadly used in biology and medicine, with typical applications include being an antidote for cyanide and, most commonly, in vitro diagnostic in biology, cytology, haematology and histology 29. A photocatalytically active material, could be used to enable the decolouration of mb under uv irradiation 26. Were usually used as sacrificial electron donor sed to scavenge the holes produced from the excitation of tio₂ under uv irradiation, and leave photo generated electrons for reducing mb to lmb in solution. However, there is no report about using tio₂ and mb as an imaging layer for the fabrication of light switchable rewritable paper, because the recolouration process could not be initiated by any convenient means due to the presence of excessive reducing agents in the system.

� tio₂ nanocrystals capped with appropriate ligands have been recently used to promote the decolouration of an aqueous solution of mb from blue to colourless under uv irradiation, and the system can recover to its original blue colour on visible light irradiation 32. The decolouration is mainly driven by the reduction of mb to lmb by photo generated electrons from tio₂ nanocrystals under uv irradiation, and the recolouration process operates by the tio₂ induced self catalysed oxidation of lmb by ambient o₂ under visible irradiation. Compared with photoisomerizable chromophores, the tio₂ / mb /water system can rapidly switch colour with high reversibility and excellent repeatability.

It also has the merits of low toxicity and low cost as both tio₂ particles and mb have already been widely used in cosmetic, medical and other industries. However, the spontaneous recolouration process under visible light makes this system incompatible with its potential use as an imaging layer in the rewritable paper. Indeed, simply depositing the solution of tio₂ nanocrystals and mb on a solid substrate led to a flaky film, which, after decolouration by uv, can only remain colourless for x3c 6 h, mainly due to the quick oxidation of lmb by ambient oxygen. For using as the imaging layer in the rewritable paper, a new mechanism is highly desirable to effectively stabilize the lmb and maintain the colourless state for significantly longer periods. For a redox dye based rewritable paper, the recolouration should be slow enough to retain the printed information under ambient conditions, but sufficiently fast when external stimulation for switching is applied.

In this work, we report the fabrication of a solid composite film to which letters and patterns can be repeatedly printed using uv light, retained for days and then erased by simple heating. The imaging layer of the rewritable film is composed of tio₂ nanocrystals, a redox dye, and hydroxyethyl cellulose hec. This new rewritable paper can be erased and rewritten x03e 20 times with no significant loss in resolution. In addition, rewritable paper with three primary colours blue, red and green can be produced by using various commercial redox dyes, such as mb. The excellent performance of these novel rewritable paper promises their potential use as an attractive alternative to a regular paper to meet our society’s increasing needs for sustainability and environmental protection. Figure 1: reversible redox reactions involved in the colour switching of a tio₂ / mb /hec composite film.

� mb blue, oxidized form and lmb colourless, reduced form molecules are stabilized by surrounding hec molecules through hydrogen bonding. Colour switching tests of tio₂ / mb solid film were conducted first by directly drying tio₂ nanocrystals/ mb /water solution on a glass substrate supplementary fig. The absorption spectrum of the solid film appeared very different from that of the aqueous mixture, in which the absorption at 660 nm decreased while the absorption at 590 nm increased drastically, indicating the conversion of monomers of mb to dimers and some trimers due to the increased mb concentration during drying.

The blue coloured mb switched rapidly to colourless lmb under uv irradiation, suggesting the effective photocatalytic reduction of mb by tio₂ nanocrystals. As expected, lmb switched back to mb completely under ambient conditions in x3c 6 h, making the system difficult for practical applications in the rewritable paper. The stabilization effect can be attributed to the hydrogen bonding between the abundant −oh groups on hec molecules and the −n ch₃ ₂ groups on mb and lmb, as schematically shown in fig. Although therein the interaction between hec and mb was believed to be electrostatic.

The stabilization effect can also be found even in solution, where introducing hec to an mb solution can promote the transition of mb monomers to their dimeric form supplementary fig. 3 , as evidenced by the progressive enhancement in the intensity of the peak at 610 nm with increasing concentration of hec in the solution. � tio₂ nanocrystals, hec and eg homogenously in water, the mixture remained a stable blue dispersion and no precipitation could be observed even after 3 months supplementary fig. Hec also greatly helped with the film formation: the mixture could be conveniently drop casted on a glass or plastic substrate and produce a solid film with a homogeneous blue colour and a smooth surface, which remained unchanged in topography even after being heated to 150 °c. In addition, hec has been widely used in cosmetics and household products as a gelling and thickening agent due to its low toxicity and cost.

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The introduction of hec also helps in preventing potential exposure of human body directly to dyes, again benefiting the practical usage of the system. Incorporating hec into tio₂ / mb /hec solid film did not show obvious effect on the decolouration rate under uv light irradiation, further indicating the effectiveness of the photoexcited tio₂ nanocrystals in reducing mb. The absorption peak of solid film main peak at 660 nm disappeared completely after 1 min of uv irradiation. However, it took 6 days to re oxidize 20% lmb back to mb under ambient conditions, which was substantially slower than the case without hec fig.

It is therefore clear that hec can greatly inhibit the oxidation of lmb, making the system compatible with the needs in practical applications. In contrast, although the stabilization effect of hec is very effective at ambient conditions, heating the colourless solid film in air at 115 °c can markedly enhance the recolouration rate. When the colourless solid film containing lmb was heated in air at 115 °c, the absorption of mb monomers at 660 nm gradually increased and fully recovered to the original intensity after 8 min. Interestingly, the heating process also indicates that monomer to dimer conversion is an exothermic process: after cooling in air for 40 min, some of the mb monomers converted to dimers again with absorption peak partially shifting from 610 nm supplementary fig.

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