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Stimuli-Responsive Organic Ultralong Phosphorescent Materials with Complete Biodegradability for Sustainable Information Encryption

2022; Chinese Chemical Society; Volume: 5; Issue: 9 Linguagem: Inglês

10.31635/ccschem.022.202202388

2096-5745

Xin Zhang, Jingxuan You, Jinming Zhang, Chunchun Yin, Yirong Wang, Ruiqiao Li, Jun Zhang,

Sulfur-Based Synthesis Techniques

Open AccessCCS ChemistryRESEARCH ARTICLES1 Dec 2022Stimuli-Responsive Organic Ultralong Phosphorescent Materials with Complete Biodegradability for Sustainable Information Encryption Xin Zhang, Jingxuan You, Jinming Zhang, Chunchun Yin, Yirong Wang, Ruiqiao Li and Jun Zhang Xin Zhang CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Jingxuan You CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Jinming Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190 , Chunchun Yin CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Yirong Wang CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Ruiqiao Li CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190 and Jun Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.022.202202388 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Constructing eco-friendly stimuli-responsive phosphorescence materials remains challenging and fascinating. Herein, we use natural cellulose as the raw material to prepare pH-responsive room-temperature phosphorescent (RTP) materials with excellent biodegradability by introducing anionic structures. The introduction of a phenylcarboxylate substituent not only promotes intersystem crossing but also brings about electrostatic-attractive and strong hydrogen-bonding interactions, which enhance the intermolecular chain interactions. Therefore, the obtained anionic cellulose derivatives containing phenylcarboxylate groups exhibit ultra-long RTP. More intriguingly, these cellulose-based phosphorescent materials have a distinctive pH-responsive behavior. Under acidic conditions, the carboxylate is converted into the carboxylic acid, resulting in phosphorescence quenching. This process is reversible. Moreover, the obtained cellulose-based phosphorescent materials have excellent processability and can be easily processed into various material forms, such as film, coating, and pattern, by using eco-friendly aqueous solution processing strategies. Such proof-of-concept biomass-based phosphorescent materials with unique pH-responsive behavior and excellent processability have a huge potential in information encryption, advanced anti-counterfeiting, and food monitoring. Download figure Download PowerPoint Introduction Organic room-temperature phosphorescent (RTP) materials exhibit many attractive advantages, especially no interference from background fluorescence and scattered light. Thus, they are widely used in information encryption,1 anti-counterfeiting,2 biological imaging,3 optical sensing,4 and so on. The preparation of organic RTP materials is still challenging because most organic materials lack efficient intersystem crossing (ISC) and/or have strong non-radiative transitions. Therefore, to obtain organic RTP materials, two essential prerequisites must be satisfied, including efficient ISC and weak non-radiative transitions.5,6 The ISC can be promoted through the heavy-atom effect,7 molecular aggregation,8 lone-pair electron incorporation,9 energy-gap narrowing,10 and so on. The non-radiative transitions can be suppressed by crystallization,11 polymerization,12 crosslinking,13 complexation,14 and so on. More attempts are being made to obtain organic RTP materials with high-performance phosphorescence. Stimuli-responsive optical materials can reflect the changes of the external environment, such as heat,15 light,16 force,17 pH,18–20 and oxygen,21 and supply abundant information. Among them, pH-responsive materials are especially suitable in advanced anti-counterfeiting and information encryption because the pH-responsive behavior is hidden, hardly disturbed, and easily implemented. Furthermore, pH change is closely related to biochemical processes,22 chemical reaction processes,23 and ecological environments.24 Therefore, it is significantly important to develop pH-responsive optical materials. Organic pH-responsive visualization sensors have attracted much attention, due to their low toxicity, low cost, high sensitivity, and high specificity.25 Most of these organic pH-responsive luminescent materials exhibit a change in fluorescence. pH-Responsive RTP materials are currently in a preliminary research stage. Zhang et al.26 prepared RTP thioether molecules that respond to volatile acids such as HCl. Ma et al.27 prepared a fluorescein-based polymer, which can output an acid- or base-reversible RTP signal due to the reversible protonation and deprotonation of fluorescein. In addition, RTP carbon dots with complex structure and special composition can give different RTP behavior as the pH changes, due to the change of the chemical structure.28–30 Based on a host–guest complexation, Gong et al.31 prepared pH-responsive RTP materials from cucurbituril and isoquinoline. The RTP emission could be significantly promoted by enhancing the binding force between cucurbituril and isoquinoline in an alkaline environment. Wang et al.32 prepared phosphorescent supramolecular gels, in which their microstructures could be affected by the pH, indicating a change in phosphorescence. Currently, the pH-responsive phosphorescent materials are extremely limited and have many problems, such as complex preparation processes, weak responsiveness, and poor formability. Methods to construct stimuli-responsive RTP remain challenging and fascinating. Cellulose is the most extensive and abundant biomass resource in nature. Along the cellulose chains, there are numerous hydroxyl groups, which impart cellulose with strong interchain hydrogen-bonding interactions.33,34 Therefore, cellulose is an ideal substrate for RTP.35 Moreover, due to the formation of clusters, natural cellulose has a weak phosphorescence emission at room temperature.36,37 However, it is difficult to process cellulose via the conventional polymer processing technology because cellulose is infusible and insoluble. Furthermore, the processing treatment will disrupt the natural aggregation structure of cellulose with the strongest hydrogen-bonding network, resulting in the deterioration of RTP performance. The cellulose derivatives obtained by the chemical modification have excellent processing performance because the introduction of substituents destroys the hydrogen-bonding network in cellulose. However, due to the broken hydrogen-bonding network, common cellulose derivatives have no or very weak phosphorescence emission at room temperature.38,39 Therefore, it is challenging to construct organic RTP materials from cellulose. In this work, we demonstrate a simple strategy to prepare cellulose-based pH-responsive ultra-long RTP materials by enhancing the interactions between polymer chains after a homogeneous modification of cellulose. In general, derivatization will destroy the original hydrogen-bonding network of cellulose. The higher the degree of substitution (DS), the higher the damage degree of the hydrogen-bonding network. Herein, we ingeniously modified cellulose with anionic substituents containing phenylcarboxylate groups, which not only promoted ISC but also improved the interchain interactions. The enhanced interchain interactions can effectively suppress the non-radiative transitions of triplet excitons. Therefore, cellulose-based materials with ultra-long RTP properties were obtained (Figure 1). Moreover, the resultant cellulose-based RTP materials have reversible pH-responsive behavior and can be used in advanced anti-counterfeiting, information encryption, environment, food monitoring, and so on. Figure 1 | Schematic illustration of the chemical structure and advanced anti-counterfeiting application of the anionic cellulose derivative with RTP property. Download figure Download PowerPoint Experimental Section Materials Cellulose (microcrystalline cellulose, PH-101) with an average degree of polymerization of 220 was purchased from Beijing Fengli Jingqiu Commerce and Trade Company (China). It was dried under vacuum at 80 °C for 24 h. 1-Allyl-3-methylimidazolium chloride (AmimCl) was synthesized in the laboratory. The water content in AmimCl determined by Karl Fischer method was less than 0.3 wt %. Phthalic anhydride, trimellitic anhydride, 4-dimethylaminopyridine (DMAP), lithium carbonate (Li2CO3), sodium bicarbonate (NaHCO3), potassium carbonate (K2CO3), cesium carbonate (Cs2CO3), ammonium bicarbonate (NH4HCO3), guanidine carbonate ((C(NH2)3)2CO3), and lithium benzoate (PhCOOLi) were purchased from Innochem and J&K Scientific. N,N′-dimethylformamide and ethanol were received from Tianjin Concord Technology Co., Ltd. Concentrated hydrochloric acid was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The dialysis bag with a molecular weight cut-off of 3500 was purchased from Beijing Ruida Henghui Technology Development Co., Ltd. (China). Double-distilled water (>18.2 MΩ cm−1) from the Millipore Milli-Q system was used in all experiments. Synthesis of cellulose phthalate Three grams (18.52 mmol) of cellulose was completely dissolved in 57 g of the ionic liquid AmimCl at 80 °C. Then, DMAP (226 mg, 1.85 mmol) and phthalic anhydride (2.74–19.20 g, 18.52–129.64 mmol) were added into the cellulose/AmimCl solution at 70 °C for 6.0–8.0 h. Ethanol was added to the reaction system to remove unreacted phthalic anhydride. Subsequently, the reaction solution was precipitated in ethanol (300 mL) with 1 mL of concentrated hydrochloric acid. The precipitate was filtered and washed three times with HCl/ethanol (0.3%) solution. Finally, the product was filtered and dried under vacuum at 60 °C for 24 h before characterization. Synthesis of cellulose trimelliticate Three grams (18.52 mmol) of cellulose was completely dissolved in 57 g of the ionic liquid AmimCl at 80 °C. Then, DMAP (226 mg, 1.85 mmol) and trimellitic anhydride (3.56–16.01 g, 18.52–83.34 mmol) were added into the cellulose/AmimCl solution at 70 °C for 9.0–12.0 h. Ethanol was added to the reaction system to remove unreacted trimellitic anhydride. Then, the reaction solution was precipitated in ethanol (300 mL) with 1 mL of concentrated hydrochloric acid. The precipitate was filtered and washed three times with HCl/ethanol (0.3%) solution. Finally, the product was filtered and dried under vacuum at 60 °C for 24 h before characterization. Preparation of RTP patterns Cellulose trimelliticate carboxylate lithium (CBtCOOLi) (DS = 0.61)/H2O solution (100 mmol/L) and cellulose 1-(hydroxyethyl) imidazolium chloride (COHimCl)/H2O solution (100 mmol/L) were used to prepare phosphorescent patterns by inkjet printing. Results and Discussion Preparation and properties of cellulose-based RTP materials We used cellulose as the raw material to synthesize a series of cellulose phthalates (CPhCOOH) with DS values from 0.52 to 2.71 in AmimCl. After a neutralization reaction between CPhCOOH and different carbonates or bicarbonates, anionic cellulose derivatives containing phenylcarboxylate groups (CPhCOOX) were obtained (Figure 2a and Supporting Information Table S1). In the 1H NMR spectra of CPhCOOH ( Supporting Information Figure S1a), the peaks at 7.1–8.2 ppm were assigned to the protons on the benzene ring, and the peaks at 2.8–5.5 ppm belonged to the protons of the cellulose backbone. The DS of phthalate substituent in CPhCOOH can be calculated from the 1H NMR spectra. In the Fourier transform infrared (FTIR) spectra ( Supporting Information Figure S1b), a new peak at 1701 cm−1 was the carbonyl stretching vibration peak. In the X-ray photoelectron spectroscopy (XPS) curves of CPhCOOX ( Supporting Information Figure S2), the element peaks of the corresponding cations appeared. These above results confirm that cellulose derivatives CPhCOOH and CPhCOOX were successfully synthesized. Figure 2 | RTP performance of CPhCOOX (DS = 1.24). (a) Synthetic route of CPhCOOH and CPhCOOX; (b) RTP spectra of CPhCOOX (Ex = 330 nm; delay time = 0.1 ms); (c) photoluminescence quantum yield of CPhCOOX (Ex = 330 nm); (d) RTP lifetime spectra of CPhCOOX (Ex = 330 nm; detection wavelength = 500 nm); (e) photographs of CPhCOOX taken under 365 nm lamp and with the lamp off. Download figure Download PowerPoint CPhCOOH has no RTP emission ( Supporting Information Figure S3), whereas CPhCOOX has a pronounced RTP phenomenon (Figure 2b–e). The phosphorescent properties of CPhCOOX strongly depend on the cation structure. The smaller the cation size, the stronger the RTP intensity, quantum yield, and RTP lifetime of CPhCOOX (Figure 2b–e). Because a smaller cation is beneficial to tighter chain packing, the non-radiative transitions of triplet excitons is effectively inhibited. The Li+ ion is the smallest cation, thus CPhCOOLi has the best phosphorescence performance. CPhCOOLi exhibits the highest phosphorescence intensity and the longest persistent luminescence after turning off the UV lamp (Figure 2b,e). In addition, the phosphorescence performance of CPhCOOLi is pH-responsive. As the ratio of carboxylate/carboxylic acid increases, the RTP emission intensity increases markedly ( Supporting Information Figure S3e). In comparing the FTIR spectra of CPhCOOH and CPhCOOLi ( Supporting Information Figure S3f), the hydroxyl peak at 3400 cm−1 broadens and significantly shifts to a lower wavenumber region when CPhCOOH is transformed into CPhCOOLi, indicating that there are stronger hydrogen-bonding interactions in CPhCOOLi than those in CPhCOOH. Moreover, the anions and cations in CPhCOOLi easily dissociate from each other, which leads to the formation of electrostatic attractive interactions between cellulose chains. Therefore, the anionic cellulose derivatives CPhCOOX with phenylcarboxylate groups, especially CPhCOOLi, exhibit excellent RTP performance. Phosphorescent mechanism of cellulose-based RTP materials The hydrogen bonding network in natural cellulose is an optimized structure with the strongest hydrogen-bonding interactions. After a physical treatment or chemical modification of cellulose, the interchain hydrogen-bonding interactions become weak due to the semi-rigid nature of the cellulose chain, resulting in the deterioration of phosphorescence performance of the treated cellulose. In addition, the bulky aromatic substituents have an obvious steric hindrance effect. Inspired by the RTP materials from alginate,40 carboxylate anionic groups were introduced into the cellulose chains because they have a stronger hydrogen-bonding capability than the hydroxyl groups. The introduction of carboxylate anionic groups to partially replace the hydroxyl groups not only enhances the hydrogen-bonding interactions between cellulose chains but also delivers electrostatic attractive interactions. As a result, the interactions between cellulose chains are significantly enhanced, and the non-radiative transitions of triplet excitons are efficiently suppressed (Figure 3a). Therefore, the obtained anionic cellulose derivatives CPhCOOX exhibit excellent RTP performance (Figure 3b). The CPhCOOLi (DS = 0.52) was used as the model to research the RTP photophysical properties of CPhCOOX. The difference between fluorescence and phosphorescence emission maxima of CPhCOOLi is about 75 nm ( Supporting Information Figure S4), and the energy gap is 0.73 eV between the lowest excited singlet state (S1) and the lowest triplet excited triplet state (T1) (Figure 3b). The phosphorescence emission of CPhCOOLi has a weak excitation dependence (Figure 3c). The CPhCOOLi aqueous solution gives a green phosphorescence with an emission peak of 502 nm at 77 K ( Supporting Information Figure S5). The PhCOOLi aqueous solution (1 mg/mL) exhibits phosphorescence at 77–160 K. The phosphorescence emission wavelength is 455 nm, and the phosphorescence lifetime at 77 K is the longest, which is 573 ms (Figure 3d, Supporting Information Figure S6). The fluorescence emission wavelength of PhCOOLi solid powder is 405 nm, and the phosphorescence emission wavelength of PhCOOLi powder is 500 nm (Figure 3e), which is similar to that of CPhCOOLi. In addition, the RTP lifetime of PhCOOLi powder is as high as 1408 ms ( Supporting Information Figure S7), and the afterglow can last up to 5.6 s at room temperature ( Supporting Information Figure S8). Therefore, the RTP emission of CPhCOOLi originates from the PhCOOLi group. The PhCOOLi group not only promotes ISC but also significantly improves the interactions between cellulose chains, triggering the phosphorescence emission. Figure 3 | RTP mechanism and performance of CPhCOOLi. (a) Schematic illustration of the RTP mechanism of CPhCOOLi; (b) energy gap of CPhCOOLi (DS = 0.52); (c) normalized phosphorescence spectra of CPhCOOLi at different excitation wavelengths (DS = 0.52); (d) phosphorescence spectra of PhCOOLi aqueous solution (1 mg/mL) at different temperatures (Ex = 330 nm; delay time = 0.1 ms); (e) normalized fluorescence and phosphorescence spectra of PhCOOLi powder (delay time for phosphorescence spectrum = 0.1 ms); (f) RTP spectra of CPhCOOLi with different DS (Ex = 330 nm; delay time = 0.1 ms); (g) photoluminescence quantum yield of CPhCOOLi with different DS (Ex = 330 nm); (h) RTP lifetime spectra of CPhCOOLi with different DS (Ex = 330 nm; detection wavelength = 502 nm); (i) photographs of CPhCOOLi with different DS taken under 365 nm lamp and with the lamp off. FL, fluorescence; Exc., excitation. Download figure Download PowerPoint To further verify the mechanism and adjust RTP performance, we changed the DS of PhCOOLi in CPhCOOLi ( Supporting Information Table S1). When the DS increases from 0.52 to 2.71, the RTP intensity, quantum yield, and RTP lifetime of CPhCOOLi change remarkably. The CPhCOOLi with a DS of 0.52 has the best RTP performance (Figure 3f–h). The afterglow can last for more than 2 s after the 365 nm UV lamp is turned off (Figure 3i). When the DS is less than 1.65, the RTP properties of CPhCOOLi are similar as the DS increases. When the DS is higher than 1.65, the RTP properties of CPhCOOLi deteriorate obviously (Figure 3f–i). The CPhCOOLi with a DS of 2.71 exhibits the worst RTP performance with the afterglow lasting for less than 1 s (Figure 3i). As the DS of PhCOOLi increases, the steric hindrance between cellulose chains increases, the hydrogen-bonding interactions weaken, and the electrostatic-repulsive interactions between ions (cation-cation and anion-anion) improve, thus the interchain attractive interactions in CPhCOOLi evidently decrease. Thus, the RTP performance of CPhCOOLi deteriorates as the DS increases. High-performance cellulose-based RTP materials Based on the above mechanism, we hoped to further improve the performance of cellulose-based phosphorescent materials by changing the chemical structure. We replaced phthalic anhydride with trimellitic anhydride to prepare cellulose trimelliticate (CBtCOOH) containing more carboxylic acid groups. Then, after a neutralization reaction, CBtCOOLi was obtained (Figure 4a and Supporting Information Table S2). In the 1H NMR spectra of CBtCOOH ( Supporting Information Figure S9a), the peaks at 7.3–8.7 ppm were assigned to the protons on the benzene ring, and the peaks at 2.8–5.5 ppm were the protons of cellulose backbone. In the FTIR spectra ( Supporting Information Figure S9b), the new peak at 1702 cm−1 was the carbonyl stretching vibration peak. In the XPS curve of CBtCOOLi ( Supporting Information Figure S9c), the element peak of lithium appeared. The above results prove that cellulose derivatives CBtCOOH and CBtCOOLi were successfully synthesized. Figure 4 | Synthesis and RTP performance of CBtCOOLi. (a) Synthetic route of CBtCOOLi; (b) RTP spectra of CBtCOOLi (Ex = 350 nm; delay time = 0.1 ms); (c) photoluminescence quantum yield of CBtCOOLi (Ex = 350 nm); (d) RTP lifetime spectra of CBtCOOLi (Ex = 350 nm; detection wavelength = 509 nm); (e) photographs of CBtCOOLi taken under 365 nm lamp and with the lamp off. Download figure Download PowerPoint The increase of carboxylate groups in anionic cellulose derivatives can markedly enhance the hydrogen-bonding interactions and electrostatic attractive interactions, so the obtained CBtCOOLi exhibits better RTP performance. Compared with the above-mentioned CPhCOOLi, the RTP intensity, quantum yield, and RTP lifetime of CBtCOOLi are significantly higher. For example, the quantum yield of CBtCOOLi (DS = 0.61) is 6.04%, and the RTP lifetime is 433 ms, which is much higher than the quantum yield (2.96%) and RTP lifetime (254 ms) of CPhCOOLi (DS = 0.52) with the best phosphorescence performance (Figure 4b–e and Supporting Information Table S3). The difference of emission peaks is about 80 nm between the phosphorescence and fluorescence emission of CBtCOOLi (DS = 0.61), and the energy gap between S1 and T1 is 0.70 eV ( Supporting Information Figure S10). As the DS of BtCOOLi increases from 0.36 to 1.94, the RTP intensity, quantum yield, and RTP lifetime of CBtCOOLi first increase and then decrease. The CBtCOOLi with a DS of 0.61 exhibits the optimal RTP performance (Figure 4b–e). After the 365 nm UV lamp is turned off, the afterglow can last for more than 4 s (Figure 4e). In summary, the introduction of phenylcarboxylate groups into the cellulose chains can promote ISC. Meanwhile, the carboxylate anions not only enhance the hydrogen-bonding interactions but also bring about electrostatic-attractive interactions, which improve the interchain interactions to effectively suppress the non-radiative transitions of phosphorous photons. Therefore, the obtained anionic cellulose derivatives, CPhCOOX and CBtCOOLi, exhibit excellent RTP performance. More carboxylate groups on the substituent promote the intermolecular hydrogen-bonding interactions and electrostatic-attractive interactions, which facilitates tighter packing of the cellulose chains. As a result, better RTP materials can be obtained. However, if there are too many phenylcarboxylate groups on the cellulose chains, the steric hindrance effect and electrostatic-repulsive interactions will be obviously enhanced to prevent the tight packing of cellulose chains, so the RTP properties will decrease. Thus, for different anionic substituents, the optimal RTP performance can be obtained by adjusting the appropriate DS of the anionic cellulose derivatives. Responsiveness and processability of cellulose-based RTP materials The above phosphorescent CPhCOOX and CBtCOOLi exhibit novel reversible pH-responsive behavior (Figure 5a). Under acidic conditions, such as HCl or HOAc vapor, the carboxylate groups are transformed into carboxylic acids. The electrostatic interactions and hydrogen-bonding interactions between cellulose chains are weakened, and the non-radiative transitions are ineffectively suppressed, resulting in the phosphorescence quenching. Under basic conditions, such as NH3·H2O vapor, the carboxylic acids are changed back into the carboxylate groups. Thus, the electrostatic interactions and hydrogen bonding-interactions between cellulose chains are prominently improved, and the non-radiative transitions are effectively suppressed. As a result, the phosphorescence emission is restored. This pH-responsive process can be repeated multiple times. Figure 5 | pH-Responsive behavior and processability of CBtCOOLi. (a) Photographs of pH responsive behavior of CBtCOOLi powder; (b) photographs of CBtCOOLi film and patterns (Ex = 365 nm). Download figure Download PowerPoint The phosphorescent CPhCOOLi and CBtCOOLi are soluble in water because they have anionic carboxylate groups. They can be easily processed into different kinds of phosphorescent materials, including films, coatings, and patterns, by using various solution processing methods, such as blade coating, inkjet printing, and screen printing (Figure 5b). In particular, water is the solvent. During the entire preparation process, only the volatilization of water rather than the volatile organic compounds is necessary, which is beneficial to environmental protection. Such unique pH-responsive behavior and excellent processability make the phosphorescent CPhCOOLi and CBtCOOLi applicable to anti-counterfeiting, information storage, smart label, functional packaging, and visualization sensors. Moreover, the CBtCOOLi exhibits excellent biodegradability ( Supporting Information Figure S11). The degradation rate of CBtCOOLi (DS = 0.61) is more than 88.0% after cellulase treatment for 3 days. Under the synergic enzymatic degradation of lipase and cellulase, the degradation rate of CBtCOOLi (DS = 0.61) achieves 98.2% after 5 days. Such unique pH-responsive behavior, excellent processability, and complete biodegradation make the phosphorescent CPhCOOLi and CBtCOOLi expected to be used in eco-friendly smart label, functional packaging, and visualization sensors. Applications of cellulose-based RTP materials Based on the unique pH-responsive phosphorescence and excellent processability, the anionic cellulose derivatives were used as water-based inks to prepare various patterns for advanced anti-counterfeiting and information encryption by inkjet printing. For example, we chose pH-responsive CBtCOOLi (DS = 0.61) as the phosphorescent ink A and non-pH-responsive COHimCl as the phosphorescent ink B41 to prepare the anti-counterfeiting patterns (Figure 6a). Both CBtCOOLi and COHimCl are water-soluble cellulose derivatives, and they exhibit green phosphorescence emission. The number "88" was printed with CBtCOOLi and COHimCl. The resultant pattern is a blue number "88" under 365 nm UV light. After turning off the UV lamp, the blue number "88" changes into the green number "88," which is false information (Figure 6b). Afterwards, the pattern was fumigated with HCl vapor and then dried. The fluorescence pattern is still the blue number "88" under 365 nm UV light. When the UV lamp is turned off, the number changes to a green number "69" which is true information, due to the phosphorescence quenching of CBtCOOLi. Figure 6 | Application of cellulose-based phosphorescent materials. (a) Schematic diagram of inkjet printing and the chemical structures of two phosphorescent inks; (b) design and responsiveness of phosphorescent digital patterns; (c) hidden phosphorescent patterns (Ex = 365 nm). Download figure Download PowerPoint For another example, we used CBtCOOLi to print the pattern on a blue-fluorescent substrate (A4 paper), and then fumigated the paper with HCl vapor to hide the pattern (Figure 6c). When the paper was checked by UV lamp, there was no pattern; the information hiding was achieved. After the treatment of the paper with NH3·H2O vapor, the paper did not display the pattern under 365 nm ultraviolet light because the substrate has a strong blue fluorescence which covers the blue fluorescence of the CBtCOOLi pattern. After the UV lamp was turned off, the pattern with green phosphorescence emerged, and the information was visible. The pattern can be hidden again with a HCl vapor treatment after reading the information. Moreover, the pattern can be destroyed by adding a few drops of water after reading. These triggering switches are facile and convenient for information encryption and protection. It should be noted that the pH-responsive optical materials are very suitable in the fields of advanced anti-counterfeiting and information encryption because the pH regulation is hidden, hardly disturbed, and easily implemented. Therefore, considering the excellent biodegradability, nontoxicity, low-cost, and good formability of cellulose-based derivatives, such pH-responsive RTP materials have great application prospects in advanced anti-counterfeiting, information encryption, and environment and food monitoring. Conclusion Cellulose with a strong hydrogen-bonding network is an ideal substrate to fabricate RTP materials. However, due to the semi-rigid nature of cellulose chains, it is extremely difficult to construct high-performance RTP materials from cellulose, especially responsive phosphorescent materials. In this work, taking advantage of the polyhydroxy characteristic of cellulose, we fabricated novel pH-responsive RTP materials by chemically immobilizing anionic aromatic substituents into the cellulose chain. The introduction of substituents containing phenylcarboxylate groups not only promotes ISC but also brings about electrostatic-attractive interactions and strong hydrogen-bonding interactions, which significantly enhance the intermolecular chain interactions and inhibit the non-radiative transitions. In addition, under acidic conditions, the carboxylate group can be transformed into the carboxylic acid group. As a result, the electrostatic and hydrogen-bonding interactions between cellulose chains are weakened, and the non-radiative transitions are ineffectively suppressed, resulting in the phosphorescence quenching. Under basic conditions, the carboxylic acid groups can be converted into the carboxylate groups, so the electrostatic and hydrogen-bonding interactions between cellulose chains are enhanced, and the non-radiative transitions are effectively suppressed, resulting in the phosphorescence restoration. Such novel pH-responsive behavior is reversible. The resultant cellulose-based stimuli-responsive RTP materials are soluble in water, and thus they can be easily processed to phosphorescent films, coatings, and patterns by the environmentally-friendly aqueous solution processing methods. Such proof-of-concept biomass-based phosphorescent materials with special stimuli-responsive behavior and excellent processability have been used in information encryption, advanced anti-counterfeiting, and environmental monitoring. Conflict of Interest The authors declare no conflict of interest. Supporting Information Supporting Information is available and includes measurements; synthesis conditions; 1H NMR spectra; FTIR spectra; XPS curves; RTP performance; fluorescence spectra; photographs; and biodegradation data. Author Contributions X.Z. and J.M.Z. conceived the idea and designed the experiments. J.M.Z. and J. Z. supervised the project. X.Z. performed the experiments. X.Z., J.X.Y., C.C.Y., Y.L.W. and R.Q.L. analyzed the data. X.Z. wrote the manuscript. J.M.Z. and J.Z. revised the manuscript. Acknowledgments This work was supported by the Youth Innovation Promotion Association CAS (grant no. 2018040) (J.M.Z.), the National Natural Science Foundation of China (grant nos. 52173292 and U2004211) (J.M.Z. and J.Z.), and the National Key Research and Development Project (grant no. 2020YFC1910303) (J.Z.). References 1. 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