Oxygen-Deficient TiO_2 – x/Methylene Blue Colloids: Highly Efficient Photoreversible Intelligent Ink

Abstract

Oxygen-sensitive photoreversible intelligent ink capable of assessment with the human eye is an ongoing demand in the modern era. In the food industry, redox-dye-based oxygen indicator films have been proposed, but the leaching of dyes from the film that contaminates the food is one unsolved issue. On the other hand, it is also highly desirable to develop rewritable paper that significantly reduces the pressure on modern society for the production and consumption of paper. Herein, we have developed an oxygen-deficient TiO2 - x/methylene blue (MB) sol without relying on external sacrificial electron donors (SEDs) for photoreversible color switching. Oxygen vacancies in TiO2 - x can work as electron donor to favor the adsorption of the substrate and improve the charge separation that is required for the redox-based color-switching system. The problems of rewriteable paper and food packaging are addressed as two sides of a single coin in this article. We have used hydroxyethyl cellulose (HEC) for rewritable paper that can significantly delay the oxidation of leuco-MB (LMB) through hydrogen bonding and retain the printed information for a long time. The dye leaching from oxygen indicator films is also significantly reduced (only 1.54%) by using furcelleran as the coating polymer that is extracted from edible red seaweed.

Photocatalytic Self‐Doped SnO2−x Nanocrystals Drive Visible‐Light‐Responsive Color Switching

“…Theenhanced recoloration rate under red light illumination could be attributed to the weak absorption of the SnO 2Àx NCs at this wavelength (Figure 1b), which promoted the self-catalyzed oxidation of LMB while restricting the competing reduction reaction of MB. [5,11] Reversibility and stability are critically important in PCSS,i np articular for applications that rely on recyclability of the system, such as rewritable media and sensors. Figure 2e shows the intensity of the main absorption peaks of the system in the colored and colorless states,demonstrating high reversibility and repeatability in color switching (Figure 2e).…”

“…The colorless SnO 2Àx /LMB/H 2 Os ystem switched back to its original color through oxidation in ambient air in about two hours (Figure 2c), while illumination with red light (630 nm) can significantly accelerate the recoloration to the original state in three minutes (Figure 2d). [5,11] Reversibility and stability are critically important in PCSS,i np articular for applications that rely on recyclability of the system, such as rewritable media and sensors. [5,11] Reversibility and stability are critically important in PCSS,i np articular for applications that rely on recyclability of the system, such as rewritable media and sensors.…”

Hydrogenation/oxidation induced efficient reversible color switching between methylene blue and leuco-methylene blue

“…The inclusion of this color switching system in food packaging could serve as oxygen indicator to ensure the packaging is properly sealed and the quality of food can be guaranteed. 35 This color switching system can also be used as security ink for anti-counterfeiting, information coding applications. The information can be written by using aqueous dispersion of Pd/g-C 3 N 4 /MB/HEC which will become colorless aer hydrogenation process, while the visible images or words can be regenerated from the printed ink upon exposure to oxidative environment for a period of time.…”

Multi-color Reversible Photochromisms via Tunable Light-Dependent Responses

“…The ISED scavenge holes left behind by the photoexcitation of valence band electrons in order to prevent irreversible oxidation of the organic dyes. [31][32][33] To improve the photocatalytic activity of the color-changing reaction, we couple reduced graphene oxide (rGO) with the TiO 2Àx nanocrystals by an in situ reduction of graphene oxide (GO) powders during the hydrothermal TiO 2 crystallization, which has been shown to improve the charge separation in TiO 2 semiconductors. [34][35][36][37][38] This resulting semiconductor nanocomposite shows fast switching rates and excellent cyclability, and is used in the design of four different reversible chromic systems that each show distinct multicolor responses, namely (1) multi-wavelength photochromism (MWP), (2) intensitydependent photochromism (IDP), (3) time-dependent photochromism (TDP), and (4) dual-response hydro-/photochromism (DRHP).…”