Methylene Blue Colloids: Highly Efficient

Article pubs.acs.org/Langmuir

Oxygen-Deficient TiO2 − x/Methylene Blue Colloids: Highly Efficient Photoreversible Intelligent Ink M. Imran, Ammar B. Yousaf, Xiao Zhou, Kuang Liang, Yi-Fan Jiang, and An-Wu Xu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, PR China S Supporting Information *

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 redoxbased 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.

1. INTRODUCTION Oxygen is the main cause of most food spoilage because foodspoiling microorganisms usually grow and thrive under aerobic conditions. Enzyme-catalyzed reactions are also favored in the presence of oxygen, as in the browning of vegetables and fruits, the oxidation of a wide range of flavors, and the destruction of ascorbic acid.1,2 Food products are often packaged in modified atmosphere (MA) packaging, involving an oxygen-free, high carbon dioxide concentration environment to limit microbial and oxidative spoilage of the products.3 A printing ink with color-switching properties incorporated into food packaging has been suggested as a cheap and reliable method of producing an oxygen indicator.4−10 Many redox-dye-based oxygen indicator films have been proposed for inclusion in food packaging, but limitations are clear because of the leaching of dyes from the films, which not only reduces the efficiency but also contaminates the food and leads to health problems.11−17 Methylene blue (MB) is also one such oxygen indicator which can be reduced to a colorless form (leuco-methylene blue, LMB) and reoxidize back to its blue form (MB) in an oxidizing environment.18,19 The decoloration of methylene blue could be enabled by the use of TiO2 nanoparticles under UV light irradiation.20 Upon UV irradiation, the photogenerated holes by TiO2 nanoparticles are captured by sacrificial electron donors, and the surviving electrons reduce MB to its colorless state (LMB). In the presence of oxygen, LMB is oxidized back to blue MB, and this phenomenon could be enhanced by using visible light. By effectively stabilizing the LMB and prolonging © XXXX American Chemical Society

the colorless state, the system can be used for rewriteable paper, and by providing an oxygen-free environment and solving the dye-leaching issue, the system can be used as an oxygen indicator for food packaging. The interfacial transfer of electrons is directly affected by the surface chemistry of TiO2; strong specific adsorption results in the direct transfer of electrons, but indirect transfer is favored by weak adsorption. The adsorption of reactants can be enhanced by creating oxygen vacancies that may serve as electron donors to capture the photogenerated holes and limiting the use of external SEDs (sacrificial electron donors). Our recent work on ZnO1 − x nanosheets has shown a significant increase in photocatalytic properties due to oxygen vacancies.21 In the stoichiometric crystal, oxygen ions take two electrons each (O2−). However, if an oxygen atom is missing from its place in the crystal (vacancy), then there are two extra electrons that can act as a double electron donor.22 We are reporting a simple, one-step, convenient method to synthesize oxygen-deficient TiO2 − x nanoparticles with enhanced colorswitching properties in rewriteable paper and food packaging. To retain the printed information for rewritable paper, the recoloration of LMB to MB should be slow enough. Hydroxyethyl cellulose (HEC) can significantly slow the oxidation process of LMB through hydrogen bonding between Received: July 19, 2016 Revised: August 8, 2016

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Figure 1. (a) TEM image of TiO2 − x nanoparticles. (b) High-resolution TEM image of TiO2 − x nanoparticles. 0.3 mL of H2O. The mixture was held at 230 °C for 4 h. After cooling to room temperature, a brown product was obtained by centrifugation and washed with acetone and ethanol several times and then redispersed in water. Color-Switching Reaction. In a typical color-switching reaction, a TiO2 − x aqueous dispersion (1 mL, 10 mg mL−1) and methylene blue (24 mL, 2 × 10−5 M) were added to a 50 mL quartz cell. The quartz cell was sealed with a rubber cap and purged with nitrogen for 20 min before irradiation. A UV lamp (300 W Hg lamp) with a 365 nm bandpass filter was used for the UV irradiation and for visible light irradiation, and a quartz cell was directly irradiated with a visible lamp (Utilitech-WL250CL4-L, 250 W). The concentration of MB in the reaction system was measured by UV−vis spectrophotometer at different time intervals. Preparation of a TiO2 − x/MB/HEC Solid Film for Rewriteable Paper. A solid blue film for rewritable paper was prepared by drying an aqueous dispersion of TiO2 − x/MB/HEC on a glass substrate. In a typical procedure, a TiO2 − x aqueous dispersion (1 mL, 10 mg mL−1), an MB/H2O solution (300 μL, 0.01 M), a 5 wt % aqueous solution of hydroxyethyl cellulose (HEC) (1 mL), and ethylene glycol (0.3 mL) were mixed and sonicated to form a homogeneous solution. The solution was drop cast on a glass substrate (5−6.5 cm2) and dried in an oven at 80 °C for 12 h. Ethylene glycol was included in the mixture to improve the smoothness of the film. Photoprinting. Photoprinting on the rewritable paper was obtained by UV light irradiation (300 W Hg lamp) through a photomask that was premade by inkjet printing on a plastic transparency film. Fabrication of Oxygen Indicator Films for Food Packaging. A typical procedure for the preparation of an oxygen indicator film is as follows. A TiO2 − x/H2O dispersion (1 mL, 5 mg mL−1) and an MB/ H2O solution (200 μL, 0.01 M) were well mixed and sonicated to form a homogeneous solution. A drop of the above ink was smeared on a glass slide and allowed to dry. The resultant film was then dipped into furcelleran solutions (1, 2, 3, and 4%, w/v) and allowed to dry, generating a very effective oxygen indicator; the final appearance of the film is blue. Dye-Leaching Behavior of Oxygen Indicator Films. The dyeleaching behavior of oxygen indicator films was quantified by immersing the film in distilled water and measuring the absorbance at 665 nm with the UV−vis spectrophotometer for different time intervals. The dye leakage (%) was defined as the ratio of the amount of dye leaching into water for a given time to the initial amount of the coated dye.

the −N(CH3)2 groups on LMB and −OH groups on HEC molecules.23 Herein we report an oxygen-deficient TiO2 − x/ MB/HEC color-switching system with enhanced recyclability for rewriteable paper. Oxygen vacancies in TiO2 − x nanoparticles can act as effective electron donors to capture photogenerated holes and enable the highly reversible photoresponsive color switching of MB. Oxygen vacancies in TiO2 − x can scavenge the photogenerated holes under UV light irradiation. This color-switching system can now be enabled without relying on external sacrificial electron donors, and the activity and recyclability can also be greatly improved by oxygen vacancies in TiO2 − x. In this study, we have also used water-resistant furcelleran as the encapsulating polymer to address the dye-leaching issue in oxygen indicator films. Furcelleran extracted from edible red seaweed is a natural sulfated polysaccharide and is used extensively in the food industry as edible films and as coating, thickening, and gelling agents.24−26 We present the UVactivated TiO2 − x/MB/furcelleran oxygen indicator for food packaging, which displays a rapid color change in the presence of oxygen and a high resistance to dye leaching.

2. EXPERIMENTAL SECTION Chemicals. Titanium(IV) chloride (TiCl4), methylene blue (MB), 2-hydroxyethyl cellulose (HEC), ethylene glycol (EG), and diethylene glycol (DEG) were purchased from Sigma-Aldrich. All other chemical reagents were of analytical grade and used as received without further purification. Characterization. Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were recorded on a JEM-2010 transmission electron microscope at an accelerating voltage of 200 kV. The X-ray powder diffraction (XRD) patterns of the products were performed on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). The operation voltage was maintained at 40 kV, and the current, at 200 mA. X-ray photoelectron spectroscopy (XPS) was carried out on a PerkinElmer RBD upgraded PHI-5000C ESCA system. The UV−visible spectra at room temperature were recorded for the samples in the 200 to 800 nm wavelength range (Shimadzu UV-2550 UV−visible spectrometer). The electron paramagnetic resonance (EPR) spectra were recorded on a JEOL JES-FA200 EPR spectrometer (140 K, 9064 MHz, 0.998 mW, X-band). Synthesis of Oxygen-Deficient TiO2 − x Nanoparticles. Oxygen-deficient TiO2 − x nanoparticles were synthesized by a simple onestep method. In a typical synthesis, 0.3 mL of titanium tetrachloride (TiCl4) was introduced into 30 mL of diethylene glycol (DEG) and

3. RESULTS AND DISCUSSION Many methods were reported to prepare anatase-phase titania at low temperature.27 The polyol synthesis route provides a B

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and a reducing agent to synthesize oxygen-deficient TiO2 − x nanoparticles at 230 °C (Experimental Section). The morphology and particle size of TiO2 − x nanoparticles were characterized by transmission electron microscopy (TEM) and high-resolution TEM and are shown in Figure 1. The particles are irregularly shaped with an average size of approximately 2−3 nm, which is in agreement with the XRD results. The HRTEM image in Figure 1b shows an interplanar spacing of 0.352 nm, which corresponds to the (101) lattice plane of the anatase phase of TiO2. All of the diffraction peaks in the XRD pattern of TiO2 − x nanoparticles (Figure 2) are also in complete agreement with anatase-phase TiO2 (JCPDS no. 21-1272) having lattice parameters of a = 3.786 Å and c = 9.499 Å.30 The crystal size of TiO2 − x nanoparticles was calculated using Scherrer’s equation, with the most intense anatase (101) plane peak observed at 2θ = 25.5°, and was found to be 1.9 nm. The small size of TiO2 − x nanoparticles determined by HRTEM is also consistent with the broadening of the XRD peaks. X-ray photoelectron spectroscopy (XPS) analysis of the sample was also performed to reveal the presence of titanium and oxygen in TiO2 − x nanoparticles. The survey XPS spectrum contains Ti and O signals as shown in Figure 3a. The Ti 2p spectrum depicts peaks at binding energies of 457.2 and 463.01 eV for Ti 2p3/2 and Ti 2p1/2, respectively (Figure 3b).31 Ti 2p3/2 and Ti 2p1/2 can be further deconvoluted into Gaussian curves of Ti3+ and Ti4+ with the major fraction being Ti3+, which

Figure 2. XRD pattern of as-synthesized TiO2 − x nanoparticles and anatase TiO2 (JCPDS no. 21-1272).

simple and versatile approach to control the size, shape, and properties of metal oxide nanostructures.28 The fundamental aspect of polyols is known to reduce metal salts to metal nuclei that then nucleate to form nanoparticles.29 Here, we used diethylene glycol (DEG) as a polyol that acts both as a solvent

Figure 3. (a) Survey XPS spectrum of as-synthesized TiO2 − x nanoparticles. High-resolution XPS spectra of (b) Ti and (c) O. (d) EPR spectra of assynthesized TiO2 − x and commercial TiO2 recorded at T = 130 K. C

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Figure 4. (a) UV−visible spectra showing the decoloration of the TiO2 − x nanoparticles/MB/water system upon UV irradiation. UV/vis spectra showing the recoloration process under (b) visible-light irradiation, (c) dark conditions, and (d) ambient laboratory conditions.

Figure 5. Digital images of rewritable paper obtained by casting the TiO2 − x/MB/HEC mixture on a glass substrate. (a) Original film after drying. Writing letters on the film using a photomask under UV light irradiation and maintained in the ambient air for (b) 5 min, (c) 1 day, and (d) 2 days.

confirms the presence of oxygen vacancies. Figure 3c shows the O 1s spectral binding energy region in which the lattice oxygen

of TiO2 − x exhibits a peak at a binding energy of 528.8 eV. The relative atomic percentage of Ti to O derived from XPS data D

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Figure 6. Effect of the coating polymer concentration (w/v %) on MB leakage. The furcelleran-based oxygen indicator films were dipped in water for 24 h.

Figure 8. Absorbance−time relationship of the TiO2 − x/MB/ furcelleran oxygen indicator: before UV irradiation, after 48 h of storage in an oxygen-free environment, and after further oxidation of LMB in air.

was approximately 1:1.8, further indicating the formation of oxygen-deficient TiO2 − x (x = 0.2). To examine the paramagnetic characteristics of the TiO2 − x nanoparticles, EPR spectroscopy was performed. Defect-free commercial TiO2 is antiferromagnetic, and no signal can be seen in the EPR spectrum, as shown in Figure 3d. The brown TiO2 − x sample gives rise to a very strong EPR signal at g = 1.99, which has previously been identified as electrons trapped on oxygen vacancies.32 Oxygen vacancies are prone to adsorb atmospheric

O2 molecules, which would be reduced to ·O2−, thus generating an EPR signal at g = 1.99.33 Figure S1 shows the UV−visible absorption spectra for commercial anatase TiO2 and our obtained TiO2 − x nanoparticles. TiO2 exhibits its absorption maximum in the 230− 330 nm range with a long shoulder that extends to 700 nm as

Figure 7. Photographs of two typical blue-colored oxygen indicators with the formulation TiO2 − x/MB/furcelleran, one placed inside and the other placed outside a plastic package flushed with CO2 and sealed. In (a), the package had just been sealed and the two indicators are blue. (b) The two indicators were then irradiated with UV light (2 min). (c) The indicator outside the package regained its original color within approximately 3 h, whereas the indicator inside remained colorless even after 48 h. (d) Finally, upon opening the package, the inside indicator regained its original color in response to the ingress of oxygen. E

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The reversibility of the TiO2 − x/MB system was also evaluated for practical applications. The decoloration (MB → LMB) by UV light for 30 s and recoloration (LMB → MB) by visible light irradiation for 6 min was first repeated 10 times, and the system showed full reversibility with a small decrease in the absorption intensity of MB. The decreased absorption intensity was slightly recovered after resting for approximately 24 h in air as a result of the continuous oxidation of remaining LMB. The color-switching process was then repeated for another 10 times, in which the system displayed good reversibility with further decrease in MB absorption. The catalytic performance of the TiO2 − x/MB system was studied again after approximately 1 week of rest in air. The gradual decrease in absorption intensity was also observed owing to the consumption of oxygen vacancies (Figure S3), which is consistent with the XPS results after 30 consecutive UV and visible light irradiation cycles (Figure S4). The Ti 2p spectrum shows a shift toward the Ti4+ region and depicts peaks for Ti 2p3/2 and Ti 2p1/2 at 458.7 and 464.5 eV, respectively. Ti 2p3/2 and Ti 2p1/2 can be further deconvoluted into Gaussian curves of Ti3+ and Ti4+, and the molar ratio of Ti4+/Ti3+ is much higher compared to that of the initial sample, suggesting the consumption of oxygen vacancies indeed occurs during the color-switching reaction. However, our TiO2 − x/MB system showed better reversible and repeatable performances without the use of external SEDs as compared to other photoreversible color-switching systems.51,52 The color-switching experiments of an aqueous dispersion of ordinary TiO2/MB were also studied to compare the results with oxygen-deficient material (Figure S5). The TiO2/MB system could not be able to bleach the blue color of MB even after 30 min of UV irradiation (Figure S5a), which further supports oxygen vacancies as SED in TiO2 − x nanoparticles playing a key role in capturing the photogenerated holes, leading to a fast color-switching reaction. Upon visible light irradiation for 30 min, the TiO2/MB system was also not able to recover its complete blue color (Figure S5b), suggesting the degradation of methylene blue during long-time UV irradiation. The recoloration process occurs slowly under ambient laboratory (Figure S 5d) and dark conditions (Figure S5c). Our TiO2 − x/MB system may find a wide range of practical applications, especially as a rewritable paper and oxygen indicator for food packaging. The key challenge of this system for rewritable paper is the stabilization of the colorless LMB form to retain the printed information for a long-enough time. It has been reported that hydroxyethyl cellulose (HEC) can significantly delay the oxidation of LMB through hydrogen bonding.53,54 Herein, we used HEC for the stabilization of LMB, and ethylene glycol was also used to increase the smoothness of the film.23 We drop-casted a mixture of TiO2 − x, MB/H2O, HEC, and EG on a glass substrate for the fabrication of rewriteable paper. The film was then dried at 60 °C for 12 h and further annealed at 120 °C for 30 min before use. A transparent plastic sheet was used to produce a photomask by inkjet printing. After UV irradiation for 3 min, the exposed regions turned colorless while the unexposed regions retained a blue color, replicating letters from the photomask to the film (Figure 5a). The replicating letters showed good resolution and were stable under ambient conditions for at least 3 days as clearly shown in Figure 5b−d. The TiO2 − x/MB system was also evaluated as an oxygen indicator for food packaging. The key challenge here is the water solubility of MB because without protective polymer the dye leaches out into water very

compared to that for TiO2 − x nanoparticles, which showed a prominent shift toward shorter wavelengths. The wavelength shift in the UV region will subdue the reduction of methylene blue to its leuco form (LMB) under visible light, which is essential for the system to be used in photoreversible color switching. The blue shift in TiO2 − x nanoparticles is due to the quantum-size (Q-size) effect that raises the conduction band and lowers the valence band, resulting in enhanced catalytic activity.34,35 We have examined the behavior of aqueous dispersion of TiO2 − x nanoparticles and methylene blue under UV and visible light irradiation. When the system was irradiated with UV light, the blue color of MB disappeared within 30 s. Figure 4a shows the time-dependent UV/vis spectra of the system; the absorption peak for MB at 665 nm decreased in intensity after UV exposure. Under visible light irradiation, the system recovered its blue color within a short time of 6 min (Figure 4b). When the system was irradiated with UV light, the electron−hole pairs were generated, and the photogenerated electrons reduced the blue MB to its colorless form (leucoMB). Upon visible light irradiation, a residual small number of MB molecules absorb energy and transfer the LMB to its excited state (LMB*), which then reacts with dissolved oxygen and enhances the self-catalyzed oxidation reaction.36 Oxygen in the reaction system is important for the conversion of LMB because in the absence of oxygen the reverse reduction of MB to LMB may take place.37 The recoloration process occurring under ambient laboratory (90 min, Figure 4d) and dark conditions (3.5 h, Figure 4c) required a much longer time, which also supports the fact that visible light illumination can promote the self-catalyzed oxidation of LMB. Oxygen vacancies play the most important role in enhancing the catalytic activity. Oxygen vacancies are the most reactive sites on the surfaces of metal oxides and are proposed to participate in many chemical reactions because oxygen-deficient tungsten oxide and MnO2 showed higher catalytic performance for hydrogenation because the defective surface activates molecular H2 easily.38,39 STM images of TiO2 − x surfaces have uncovered a surprising amount of detail about oxygen vacancies for adsorption and subsequent catalytic reactions.40−44 Oxygen vacancies in TiO2 − x play a vital role in photocatalytic reactions because the oxygen vacancies carry unpaired electrons and their charged nature controls the band-bending and thus electron−hole pair separation.45−49 The photogenerated holes created upon UV irradiation usually readily and reversibly oxidize sacrificial electron donors (SEDs) present in the color-switching mixture. Although no external SED is present in our system, the oxygen vacancies can act as effective electron donors to directly capture the photogenerated holes leading to fast MB reduction by the remaining photogenerated electrons. Therefore, there is no need for external SED molecules that are usually consumed after a few cycles and reduce the color-switching reversibility.50 Another advantage of as-synthesized TiO2 − x nanoparticles is the shift in absorption toward shorter wavelengths, which minimizes the reduction of MB under visible light, leading to a fast recoloration rate. When the aqueous dispersion of TiO2 − x nanoparticles and MB was initially purged with nitrogen to remove oxygen and then irradiated with visible light, the absorption intensity of MB remains almost the same, which further confirms that the system could not be excited with visible light (Figure S2). F

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Langmuir quickly and contaminates the food.55 Herein we propose the use of furcelleran as the coating polymer that is extracted from edible red seaweed. We prepared oxygen indicator films by mixing aqueous dispersion of TiO2 − x with MB and a smeared drop onto a glass slide and drying; the slide was then dipped into different concentration solutions (1, 2, 3, 4%, w/v) of furcelleran. The dye leakage from the furcelleran-based oxygen indicator was studied by dipping the film in water for different time intervals as presented in Figure 6. Surprisingly, only 1.54% MB leached into water after 8 h from the film, which was dipped with 4% furcelleran solution with no significant change even after 24 h. This high resistance to dye leaching may be attributed to the electrostatic interaction between the cationic dye and anionic polymer.56−58 Furcelleran is a natural polysaccharide having negatively charged sulfonate groups that readily binds with the cationic MB molecules. We also explored a simple laboratory experiment for the possible use of our oxygen indicator in food packaging, in which a piece of fresh meat was packaged and sealed in a carbon dioxide environment with one glass slide of oxygen indicator inside and one outside (Figure 7a). Upon UV irradiation, both slides become colorless as a result of the conversion of MB to its colorless form (LMB), as clearly shown in Figure 7b. Because the LMB is stable only in an oxygen-free environment, the slide outside the package regained its blue color within approximately 3 h, but the slide inside the package remained in its colorless state even after 48 h (Figure 7c).19 It is noted that, after opening the package, the inside slide also gained its blue color within a short period of time (Figure 7d). Figure 8 shows the time-dependent absorbance change in the above-mentioned conditions. The TiO2 − x/MB/furcelleran-based oxygen indicator showed superior performance with negligible dye leaching, and the system can also be reused for a number of times without relying on external SEDs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-551-3600246. Tel: +86-551-3602346. Author Contributions

M.I. and A.B.Y. contributed equally to this work Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Basic Research Program of China (2011CB933700) and the National Natural Science Foundation of China (51561135011, 51572253, and 21271165). This work is also supported by the CAS-TWAS President’s Fellowship programme.

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REFERENCES

(1) Rooney, M. L. Overview of Active Food Packaging; In Active Food Packaging; Rooney, M. L., Ed.; Blackie Academic &Professional, London, 1995. (2) Brody, A. L.; Strupinsky, B. R.; Kline, L. R. Active Packaging for Food Applications; Technomic Publishing Co.: Lancaster, PA, 2001. (3) Brody, A. L.; Bugusu, B.; Han, J. H.; Sand, C. K. Scientific Status Summary: Innovative Food Packaging Solutions. J. Food Sci. 2008, 73, R107−R116. (4) McMillin, K. W. Where is MAP Going? A review and future potential of modified atmosphere packaging for meat. Meat Sci. 2008, 80, 43−65. (5) Mills, A. Oxygen indicators and intelligent inks for packaging food. Chem. Soc. Rev. 2005, 34, 1003−1011. (6) Yoshikawa, Y.; Nawata, T.; Goto, M.; Fujii, Y. Mitsubishi Gas Chemical Company. U.S. Patent 4,169,811, 1979. (7) Eilamo, M.; Ahvenainen, R.; Hurme, E.; Heiniö, R.-L.; MattilaSandholm, T. The effect of package leakage on the shelf-life of modified atmosphere packed minced meat steaks and its detection. Food Sci. Technol. 1995, 28, 62−71. (8) Perlman, D.; Linschitz, H. U.S. Patent 4,526,752, 1985. (9) Hatakeyama, H.; Tobari, S.; Iwauchi, S.; Ohsawa, M. Mitsubishi Gas Company, U.S. Patent 6,676,901, 2004. (10) Piletsky, S.; Higson, S. P. J.; Davis, F. Cranfield University. Int. Patent 06/024848, 2006. (11) Barmore, C. R.; Speer, D. V.; Kennedy, T. D.; Havens, M. R. Cryovac. Inc. Int. Patent 04/052644, 2004. (12) Putnam, D. L.; Hubbard, T. U.S. Patent 6,794,191, 2004. (13) Saaski, E. W.; McCrae, D. A.; Lawrence, D. M. Metri. Cor. Inc., U.S. Patent 5,039,491, 1991. (14) Mills, A.; Lee, S. K. Strathclyde University, Int. Patent WO 03/ 021252, 2003. (15) Blinka, T. A.; Bull, C.; Barmor, C. R.; Speer, D. V.; Grace, Co. U.S. Patent 5,583,047, 1996. (16) Oster, G.; Wotherspoon, N. Photobleaching and photorecovery of dyes. J. Chem. Phys. 1954, 22, 157−158. (17) Mills, A.; Hazafy, D. Nanocrystalline SnO2-based, UVBactivated, colourimetric oxygen indicator. Sens. Actuators, B 2009, 136, 344−349.

4. CONCLUSIONS In this work, we have developed oxygen-deficient TiO2 − x/MB colloids without relying on external SEDs for photoreversible color switching in rewritable paper and food packaging. The oxygen vacancies in TiO2 − x act as effective electron donors to enhance the substrate adsorption and capture the photogenerated holes, thus the remaining electron reduces methylene blue to colorless leuco-methylene blue effectively. For rewritable paper, hydroxyethyl cellulose is used to slow the oxidation process of LMB under ambient conditions and retain the printed information for a long time. In food packaging, because the oxygen indicator will be placed inside the packaging, the ideal oxygen indicator would be nontoxic and water-insoluble. We have used furcelleran (water-resistant natural sulfated polysaccharide) as an encapsulating polymer to stop dye leaching from TiO2 − x/MB films. Furcelleran extracted from edible red seaweed is extensively used in the food industry as thickening, stabilizing, and gelling agents. The dye leaching from furcelleran-coated films is only 1.54% after 8 h, suggesting the good stability for potential applications.

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UV−visible absorbance spectra of TiO2 − x and anatase TiO2. Absorption spectra of the TiO2 − x/MB system initially purged with nitrogen. Recyclability test of TiO2 − x/MB for up to 30 cycles. XPS spectra of TiO2 − x nanoparticles after 30 consecutive color-switching cycles. UV/vis spectra showing the decoloration and recoloration processes of the TiO2/MB system under different conditions. (PDF)

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02676. G

DOI: 10.1021/acs.langmuir.6b02676 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b02676 Langmuir XXXX, XXX, XXX−XXX