Tetraphenylethylene @ Graphene Oxide with Switchable

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Tetraphenylethylene @ Graphene Oxide with Switchable Fluorescence Triggered by Mixed Solvents for the Application of Repeated Information Encryption and Decryption Mengmeng Qin, Yuxiao Xu, H. Gao, Guoying Han, Rong Cao, Peili Guo, Wei Feng, and Li Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12421 • Publication Date (Web): 02 Sep 2019 Downloaded from pubs.acs.org on September 2, 2019

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ACS Applied Materials & Interfaces

Tetraphenylethylene @ Graphene Oxide with Switchable Fluorescence Triggered by Mixed Solvents

for

the

Application

of

Repeated

Information Encryption and Decryption Mengmeng Qin,*,†,‡,§ Yuxiao Xu,† H. Gao, ⊥ Guoying Han,† Rong Cao,† Peili Guo,† Wei Feng,*,‖ and Li Chen*,†,‡,§ †School

of Materials Science and Engineering, Tianjin University of Technology, Tianjin

300384, P. R. China ‡Tianjin

Key Laboratory for Photoelectric Display Materials and Devices, Tianjin 300384,

China §Key

Laboratory of Photoelectric Display Materials and Devices, Ministry of Education,

Tianjin 300384, P. R. China ‖School

of Materials Science and Engineering, Tianjin University, Tianjin 300072, P. R.

China ⊥ School

of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin

300384, P. R. China KEYWORDS: graphene oxide, tetraphenylethylene, composite, switchable microstructure and fluorescence, solvents treating

ACS Paragon Plus Environment

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ABSTRACT: Aggregation-induced emission (AIE) materials present unique solid-state fluorescent. However, there remains challenge in the switching of fluorescence quenching/emitting of AIE materials, limiting the application in information encryption. Herein, we report a composite of tetraphenylethylene@graphene oxide (TPE@GO) with switchable microstructure and fluorescence. We choose GO as fluorescence quencher to control the fluorescence of TPE by controlling the aggregation structure. Firstly, TPE coating with average thickness of about 31 nm was deposited at GO layer surface, which is the critical thickness that the fluorescence can be largely quenched due to the fluorescence resonance energy transfer. After spraying a mixed solvent (good and poor solvent of TPE) on the TPE@GO, blue fluorescence of TPE can be emitted in the drying process. During the treating of mixed solvents, the planar TPE coating was dissolved in THF firstly, and then the TPE molecules aggregated into nanoparticles (average diameter of 65 nm) in H2O during the volatilization of THF. We found the fluorescence switching of the composite is closely related to the microstructure change of TPE between planar and granular structures, which could make the upper TPE molecules in and out of the effective quenching region of GO. This composite, along with the treating method was used as invisible ink in repeated information encryption and decryption. Our work not only provides a simple strategy to switch the fluorescence of solid-state fluorescent materials but also demonstrates the potential for obtaining diverse material structures through compound solvents treating.


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INTRODUCTION Invisible inks have become of increasing importance in the field of information security.1-3 They are usually composed of invisible substances that provide painted images that are not able to be seen with naked eye, and are readable only under special environments.4, 5 Thanks to the development of small-molecule fluorescent materials, various types of materials have been reported in the field of information encryption, such as quantum dots, metal-organic frameworks, and other solid-state fluorescent materials.6-13 The fluorescence was usually controlled by changing the chemical structures or dispersion state of the molecules.14-18 However, most fluorescent molecules are prone to aggregate, which usually leads to fluorescence quenching.19, 20 Consequently, the development of new invisible ink with novel and simple encryption/decryption mechanism is increasingly necessary. Aggregation-induced emission (AIE) presents unique fluorescence phenomenon that the molecule is non-fluorescent in the dissolved state, but the fluorescence can be induced after forming aggregate state.21, 22 AIE solves the problem of aggregation-caused quenching effects of organic materials in the aggregated state, and it has been widely studied as outstanding organic light-emitting diodes, chemical sensing, biomedical applications, and so on.15,

23-26

However, the unique fluorescence performance of AIE materials, such as tetraphenylethene (TPE), has been rarely reported for the application in information encryption and decryption. And one reason could be the difficulty in the simple switching of fluorescence quenching and emitting. As reported, graphene oxide (GO) featured sp2, sp3-hybridized domains and large planar structure usually served as substrate material,27,

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and could quench the fluorescence of

nearby dye molecules including AIE materials, owing to the effect of fluorescence resonance energy transfer (FRET).29-31 This property of GO has been used as fluorescence probe in


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many fields,32, 33 such as the quenching platform for the sensitive and selective detection of DNA and proteins.34, 35 Koppens et al.36 also reported that the observed energy transfer rate of graphene is in agreement with a d-4 (d, emitter-graphene distance) dependence. And the reported effective quenching distance for GO is about 0-30 nm.36,

37

In other words, the

fluorescent molecule cannot emit fluorescence when it is within the quenching region of GO, while the fluorescence can be triggered when fluorescent molecule is beyond the quenching region.38, 39 Inspired by these results, we attempted to develop a platform, of which TPE is closely attached on the surface of GO sheet with the π-π stacking interaction, and to switch the fluorescence quenching and emitting of TPE by reversibly changing the average distance between GO and TPE. The peak-forest of China Guilin Karst Landform was formed from the etching of acid rain on limestone during the crustal movement (detail in supporting information). Enlightened by the morphology changing and solvent etching process, we proposed to use the dissolution and precipitation behavior to control the aggregation structures (planar or granular) of TPE, which could change the average distance between GO and TPE. To our knowledge, this concept has not been realized yet because of the difficulty in controlling the distance between two solid substances, although there exist some studies of preparation or modification of the nanoparticles or patterns on graphene platform.40-44 In this paper, aiming at the necessary of repeated information encryption and decryption material, we developed a novel fluorescence control platform of TPE@GO composite with switchable fluorescence and microstructure. After investigating the solubility of TPE in different solvents, we found tetrahydrofuran (THF) could dissolve TPE most quickly (Movie S1, supporting information). And THF was also the commonly used good solvent for TPE derivatives.45 Thus we choose THF and H2O as the good and poor solvents for TPE,


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respectively. The preparation of TPE@GO and the process of information encryption and decryption are schematically illustrated in Figure 1. In a typical experiment, GO dispersion was sprayed on the substrate of glass sheet or plastic film to get a GO coating, on which TPE dispersion was sprayed afterwards. After the volatilization of solvent, an imprint of TPE@GO with no fluorescence was formed and the information was encrypted. To decrypt the information, mixed solvents of THF and H2O were sprayed on the TPE@GO. After drying, fluorescence of TPE@GO could be triggered under UV-light and the information can be obtained. Additionally, the fluorescence can be further quenched and emitted again after spraying different solvents. The fluorescence and microstructure of TPE@GO samples with different treating conditions were characterized. The aggregation structure evolution and fluorescence switching mechanism of TPE, and their relationship was revealed. Owing to the fluorescence switching properties, this composite holds great potential as invisible ink in the field of information security.

Figure 1. Schematic illustration of the preparation of TPE@GO as invisible ink, and the process of the information encryption and decryption.


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RESULTS AND DISCUSSION Synthesis and Characterization of TPE@GO The TPE was synthesized by McMurray coupling method followed with purification, and the chemical structure of TPE was investigated by Fourier transform infrared (FTIR) spectroscopy. The spectra shown in Figure S1 exhibits a peak at 1598 cm-1 ascribed to C=C, which is transformed from C=O (1665 cm-1) in benzophenone.46 The four benzene rings of TPE conjugated with the center C=C bond were further confirmed by 1H NMR spectroscopy (Figure 2a). As shown in Figure 2b, the TPE in THF exhibits a maximum absorption at 310 nm in UV-vis absorption spectrum, and the aggregated TPE exhibits a maximum fluorescence emission spectrum at 465 nm. Additionally, we can find the fluorescence emission spectrum of TPE has a large overlap with the UV-vis absorption spectrum of GO, making the opportunity for the occurrence of FRET.26 In order to preliminarily get the relationship between fluorescence and dispersion state of TPE, the optical images of TPE powder, TPE dispersion (THF), GO/TPE dispersion (THF), TPE nanoparticles dispersion (H2O), and TPE nanoparticles/GO dispersion (H2O) under UVlight were shown in Figure 2d. The TPE powder exhibited a blue fluorescence while TPE and GO/TPE dispersion (THF) showed no fluorescence.47,

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After the adding of H2O and

volatilizing of THF in TPE dispersion (THF), the dispersion emitted blue fluorescence owing to the formation of TPE nanoparticles (Figure 2e inset), which has AIE effect.14 After adding GO into TPE nanoparticles dispersion, the fluorescence was weakened. These results provided idea for fluorescence switching by controlling the packing structure of TPE and GO. Additionally, the dispersibility of GO film and TPE powder in THF/H2O mixture (VTHF = 60%) was presented in Figure S2, supporting information. When the mixture (0.5 mL) was dripped on the TPE powder (2 mg), it dissolved quickly in 1 s. However, the GO film showed


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almost no change after being soaked in the mixture for 25 s in static state, owing to the strong interlayer interaction. The big difference in solubility of TPE and GO make it easy to change the aggregation structure of TPE on the quenching platform of GO film.

Figure 2. (a) 1H NMR spectra of TPE in CDCl3. (b) UV-vis absorption spectra of GO and TPE, and solid state fluorescence emission spectrum of TPE. (c) Contact angle of TPE, GO, and TPE@GO as a function of contact time (the droplet composition is H2O and THF/H2O with VTHF of 60%, respectively). (d) Optical images of TPE powder, TPE dispersion in THF, GO/TPE dispersion in THF, TPE nanoparticles dispersion in H2O, and TPE nanoparticles/GO dispersion in H2O under UV-light (concentration of all the TPE and GO dispersion was 0.05 mg/mL). SEM images of (e) TPE@GO on glass substrate and (inset) the TPE nanoparticles. (f) SEM and EDS mapping of the TPEDBr@GO. The microstructure of GO on glass sheet exhibited membrane structure with silk-like wrinkle (Figure S3, supporting information). After the spraying and solvent volatilization of TPE dispersion, TPE coating on GO surface (TPE@GO) was formed. The TPE@GO showed similar structure with GO, but the surface was rough, indicating the well coating of TPE layer (Figure 2e). Additionally, we studied the contact angle between the droplets (H2O and THF/H2O) and the TPE@GO, as well as pure TPE and GO. As reported, GO has good hydrophilicity, while H2O is a poor solvent for TPE.51, 52 Figure 2c shows that the contact


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angle of H2O with pure GO and TPE is 33° and 63°, respectively, at the contact time of 19 s. While the contact angle of H2O with the original TPE@GO was 61° at the same contact time, which is very close to that of TPE, further indicating that the surface of TPE@GO was uniformly coated with TPE. Additionally, when the droplet was THF/H2O with the VTHF of 60%, the contact angle decreased quickly for all the three basements, indicating the low surface tension and strong solubility of THF. (Figure 2c, Movie S2, supporting information). The good wettability could facilitate the rapid dissolution of TPE in THF. To further examine the distribution of TPE on GO surface, we synthesized the bromine substituted TPE (TPEDBr), which was characterized by 1H NMR spectroscopy (Figure S4, supporting information). The TPEDBr@GO was prepared and characterized by SEM with EDS mapping. Figure 2f shows that the elements of C, O, and Br distribute evenly in the testing area, indicating the uniform distribution of the AIE molecules. Owing to the existence of multi benzene ring structure, the TPE can conjugate with GO to form a composite with good compatibility.32 The good compatibility is also conductive to control the microstructure of TPE on GO, as well as the fluorescence. Fluorescence of the Original TPE@GO For invisible ink application, whether the information is successfully encrypted depends on the degree of fluorescence quenching. As reported above, the TPE in the as-prepared TPE@GO was evenly coated on GO film. Thus the fluorescence of TPE@GO would be only dependent on the thickness of TPE layer, which can be controlled by the spraying volume. Accordingly, the relationship between fluorescence intensity and the content of TPE in TPE@GO was investigated. The spraying volume of GO (0.05 mg/mL) and TPE (0.05 mg/mL) dispersion, and the density of the obtained solids were shown in Table 1. The


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samples were prepared by the spraying of the dispersions on glass sheets (1 cm2), and the density ρ was defined as the mass per unit area. Table 1. The spraying volume of GO and TPE dispersions, and the density of the obtained solids. No . 0 1 2 3 4 5

GO&TPE dispersions

TPE@GO composite

VGO (mL)

VTPE (mL)

ρGO (mg/cm2)

ρTPE (mg/cm2)

0 0.05 0.05 0.05 0.05 0.05

0.25 0.05 0.10 0.15 0.20 0.25

0 0.0025 0.0025 0.0025 0.0025 0.0025

0.0125 0.0025 0.0050 0.0075 0.0100 0.0125

As shown in Figure 3a, the fluorescence emission spectrum of pure TPE (ρTPE= 0.0125 mg/cm2) shows an obvious fluorescence emission band (peak maximum at 465 nm). However, the spectra of the TPE@GO (No. 1, ρTPE= 0.0025 mg/cm2, ρGO= 0.0025 mg/cm2) shows almost no fluorescence, which is the same to that of pure GO (ρGO= 0.0025 mg/cm2). Notably, when the ρTPE was increased to 0.0050 mg/cm2, a weak fluorescence emission band appeared in the sample of TPE@GO (No. 2). And the emission peak further increased with the increasing of ρTPE (No. 3, 4, 5). The change of fluorescence was related with the fluorescence quenching effect, which was controlled by the relative distance between TPE and GO. As previously mentioned, the GO has FRET effect which can lead to fluorescence quenching of dye molecules.37, 38 As shown in Figure 3b and 3d, when the amount of TPE was in low degree (No. 1), the average thickness of TPE coating on the surface of GO was 21 nm, which was within the effective fluorescence quenching region of GO. For the sample of No. 2, the average thickness of TPE coating was 31 nm, and some TPE molecules were at the critical quenching distance of GO, leading to the weak fluorescence in Figure 3a. With the


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increase of TPE density, the thickness of the TPE layer also increased, and thus some TPE molecules on top surface would be beyond the quenching distance of GO. Additionally, the emission peak shows a red shift with the increasing of ρTPE from 0.0050 to 0.0125 mg/cm2. This is ascribed to inner filter effect, which caused by the partial overlap of absorption and emission spectra of TPE.49, 50

Figure 3. (a) Fluorescence emission spectra of GO, TPE, and TPE@GO with different TPE content. (b) Thickness of the GO film and TPE@GO. (c) Photoluminescence decay curves of pure TPE and TPE@GO. (d) Illustration of the fluorescence quenching and emission of TPE@GO with different TPE thickness. (e) Optical images of pure TPE and TPE@GO deposited on a glass sheet under UV-light with a wavelength of 365 nm. (f) Optical images TPE@GO deposited on plastic film which was placed on white and black basement. To further confirm the quenching effect of GO, the fluorescence lifetime of TPE and TPE@GO was examined by the fluorescence decay curves (Figure 3c). The pure TPE showed an average fluorescence lifetime of 1.86 ns, which is higher than that of TPE@GO. And the higher the ρTPE, the longer the lifetime of TPE@GO (1.53 and 1.69 ns for TPE@GO No.1 and No.2, respectively). This result further demonstrated that the GO could only quench the nearby TPE and thus the fluorescence of TPE@GO could be controlled by regulating the TPE content.


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Additionally, to examine the visual fluorescence of TPE@GO, we took the optical images of pure TPE and TPE@GO deposited on glass sheet under UV-light. As shown in Figure 3e, the dispersions were uniformly sprayed on the upper part of glass sheets, and the lower part was used as a blank control. The sample of pure TPE (ρTPE= 0.0125 mg/cm2) showed a bright blue fluorescence owing to the aggregation induced emission. Consistent with the fluorescence spectra in Figure 3a, the optical image of TPE@GO (No.1) was black under UV-light. However, when the ρTPE of TPE@GO (No.2) was increased to 0.0050 mg/cm2, the sample still showed a black image, which is inconsistent with the spectra result. And there was almost no difference in the visual fluorescence between the upper and lower sides of the glass sheet. The reason is that the fluorescence was so weak that cannot be observed by naked eye. When the ρTPE was further increased, the samples (No.3, 4, 5) showed improved visual fluorescence. The imprint of the TPE@GO was also related to the encryption effect. As shown in Figure 3f, we made a comparison of the optical images of different samples deposited on plastic film, which was placed on white and black basement. The samples of No. 1 and No. 2 were close to colorless and transparent. However, the samples of No. 3 and No. 4 exhibited obvious patterns on black basement. Based on the data of spectra, visual fluorescence, and optical color of the samples, we believe that the TPE@GO (No.2) with ρTPE of 0.0050 mg/cm2 is at the critical state of visual fluorescence, of which we could switch the on/off of the visual fluorescence. Effect of Solvents Treating on Microstructure and Fluorescence of TPE@GO Based on the different solubility of TPE in THF and H2O, we tried to control the fluorescence and microstructure of the TPE@GO sample (No. 2) by the treating of different solvents. The samples were treated by the spraying and volatilization of THF, H2O, and THF/H2O (volume


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ratio of 3/2, 0.05 mL) on the surface. Figure 4a shows the fluorescence emission spectra of the sample treated with different solvents. Compared with the original TPE@GO, the fluorescence intensity of the samples showed almost no change after spraying pure H2O or THF. However, the fluorescence intensity enhanced significantly after being treated with THF/H2O mixture, indicating that the THF/H2O mixture can change the microstructure of the TPE@GO. Additionally, when the THF/H2O treated TPE@GO was further treated with pure THF, the fluorescence intensity showed an obvious decrease.

Figure 4. (a) Fluorescence emission spectra of TPE@GO treated with different solvents. Fluorescence lifetime micro-images of TPE@GO treated with (b) THF, (c) H2O, (d) THF/H2O mixture, and (e) THF/H2O mixture and THF in turn. (f) Optical images of pure TPE and TPE@GO treated with different solvents under UV-light. SEM images of TPE@GO treated with (g) THF, (h) H2O, (i) THF/H2O mixture, and (j) THF/H2O mixture following by THF. The inset in (i) is the particle size distribution of the TPE@GO. To further study the fluorescence changing mechanism of the TPE@GO, the samples were characterized by fluorescence microscope. As shown in Figure 4b and 4c, there exists no fluorescence in the TPE@GO treated with pure THF or H2O. While there exist many luminescent spots in the sample treated with mixed solvent, indicating the fluorescence emission of TPE nanoparticles (Figure 4d). When THF was sprayed again, the spots become


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weak and sparse (Figure 4e). The evolution of microphotographs is also consistent with the changes in visual fluorescence of the samples (Figure 4f), which demonstrates the preliminary realization of the on and off of the visible fluorescence. To explore the changing mechanism of the fluorescence, we characterized the microstructure of the samples by SEM images (Figure 4g-4j). A large amount of nanoparticles, exhibited a uniform distribution with a size ranging mainly from 50 to 80 nm and an average diameter of about 65 nm, could be found on the surface of TPE@GO treated with mixed solvents, while all the other samples showed planar structure. Considering that the fluorescence of the composite was changed after spraying THF/H2O mixture, while the chemical structure of the TPE did not change, we believe that the generation of the large number of nanoparticles is the key factor triggering the fluorescence, and the nanoparticle composition is TPE.

Figure 5. Illustration of the changing mechanism of the microstructure and fluorescence of TPE@GO treated with mixed solvents. The changing mechanism of the microstructure and fluorescence of TPE@GO was illustrated in Figure 5. For the original TPE@GO (No. 2), the TPE was coating on the surface of GO layer. And all the TPE molecules were within the quenching distance of GO. Thus the


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fluorescence of TPE was quenched. After the spraying of THF/H2O mixture, the TPE layer was quickly dissolved in THF of the mixture. We know that THF is more volatile than H2O, and thus the TPE was stranded in H2O during the volatilization of THF. Considering that H2O is poor solvent for TPE, the TPE would be aggregated in H2O, followed by the formation of TPE nanoparticles during the volatilization of H2O. This process is similar to the formation of peak-forest of Karst Landform in Guilin, China. Consequently, the upper part of TPE nanoparticles was beyond the quenching distance of GO, and the fluorescence can be emitted. Additionally, when the THF was sprayed again, the TPE nanoparticles would be dissolved and TPE layer was formed during the volatilization of THF, and thus the fluorescence was quenched again. We know that N, N-Dimethylformamide (DMF) has a high boiling point of 152.8℃ and it is also good solvent for TPE. The original TPE@GO (No. 2) was treated with a mixture of DMF and H2O (the volume fraction of DMF was 60%) by the same method, and was then dried at 80℃. The obtained sample exhibited no visual fluorescence under UV-light. The SEM image was shown in Figure S5, supporting information, and there existed no particles but some wrinkle on the surface of the sample. These results demonstrate that both the good solubility and volatility of TPE played important role in the formation of TPE nanoparticles. Effect of the Volume Ratio of Mixed Solvents on Microstructure and Fluorescence of TPE@GO Based on the analysis above, we got the appropriate TPE content in TPE@GO, which could realize the on/off of fluorescence. What’s more, we can confirm that the THF/H2O mixture could significantly affect the microstructure and fluorescence of TPE@GO. To further improve the fluorescence switching property of the as prepared TPE@GO (No. 2), we studied the effect of volume ratio of THF and H2O on the microstructure and fluorescence of the


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composite. Figure 6a shows the fluorescence emission spectra of the sample treated with mixed solvents with different volume ratio. The fluorescence intensity increased with the increasing of the volume fraction of THF (VTHF) from 20 to 60%, and reached the max value when the VTHF was 60%. Noteworthily, the fluorescence intensity showed a slow decrease when the VTHF reached 80%. This result was further confirmed by the visual fluorescence image of the composites under UV-light (Figure 6e).

Figure 6. (a) Fluorescence emission spectra of TPE@GO treated with THF/H2O mixture with different VTHF. SEM images of TPE@GO treated with THF/H2O mixture with VTHF of (b) 20%, (c) 60%, and (d) 80%. (e) Optical image TPE@GO treated with THF/H2O mixture with different VTHF under UV-light. Fluorescence lifetime micro-images of TPE@GO treated with THF/H2O mixture with VTHF of (f) 20%, (g) 60%, and (h) 80%. (i) Illustration of the evolution of the nanoparticles and fluorescence of TPE@GO treated with THF/H2O mixture with different VTHF.


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In order to understand the effect of VTHF on the microstructure and fluorescence of TPE@GO, the morphology was characterized by SEM. When the VTHF was 20%, the sample showed a rough surface on which there exist sparse nanoparticles (Figure 6b). With the increasing of VTHF from 20 to 60%, much more nanoparticles were formed (Figure 6c). While when the VTHF was increased to 80 vol%, much of the nanoparticles disappeared and the surface of the sample became rough with some wrinkle structure (Figure 6d). The fluorescence lifetime micro-image of the TPE@GO samples was shown in Figure 6f-6h. When the VTHF was 20%, the sample showed a sparse spots of blue light. And the spot became denser when VTHF was increased to 60%. When the VTHF was 80%, although the light point was very dense, much of them were small weak light with low fluorescence lifetime. Based on the above result, the mechanism of the effect of VTHF on microstructure and fluorescence of TPE@GO was presented in Figure 6i. When the VTHF was in low degree, it is difficult to completely dissolve the TPE, and thus only the dissolved part of TPE can participate in the formation TPE nanoparticles, leading to the sparse fluorescence point and weak visual fluorescence. When the VTHF was in high degree, all the TPE was dissolved in THF. However, there exist not enough H2O for them to aggregated, leading to the formation of wrinkle layer structure. Consequently, too much or too little VTHF was easy to facilitate the formation of TPE layer, large part of which was within the quenching scope of GO layer, leading to the weak fluorescence. However, appropriate VTHF was helpful to the formation of TPE nanoparticles, much of which (upper part) was beyond the quenching scope of GO, thus the fluorescence of the sample was largely emitted. Application of Invisible Ink for Repeated Information Encryption and Decryption The GO and TPE dispersion was used as invisible ink to paint image on plastic film, which didn’t emit fluorescence (Figure 7c). The pattern of “Tai Chi” and “QR code” was sprayed


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with the GO and TPE dispersion in turn. After the drying, no ink marks and fluorescence under UV-light can be seen with naked eye, indicating the encryption of information (Figure 7a, 7b and 7d). After the spraying and drying of the THF/H2O mixture and pure THF in turn, the light fluorescent patterns appeared and disappeared in turn, demonstrating the ability of repeated information encryption and decryption (Figure 7e and 7f). For comparison, the pattern of “PURE TPE” was sprayed with pure TPE dispersion. The pattern exhibited strong fluorescence throughout the processes no matter what solvents were used for spraying. The fluorescence switching property makes this composite hold great potential as invisible ink in the field of information security.

Figure 7. (a, b) Optical images of the plastic film on which “Tai Chi” and “QR code” patterns were sprayed with GO and TPE dispersion in turn, and “PURE TPE” pattern was sprayed with pure TPE dispersion. Optical images of (c) the empty film, (d) original sprayed film, and the sprayed film treated by (e) THF/H2O mixture, (f) THF/H2O mixture and pure THF in turn under UV-light. CONCLUSION The as prepared TPE exhibited a maximum fluorescence emission spectrum at 465 nm, which overlaps with the absorption spectrum of GO, making it possible of the fluorescence resonance energy transfer. Owing to the conjugated structure and good solubility in THF,


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TPE exhibited a planar structure at GO layer surface in TPE@GO. For a constant GO film, the fluorescence emission intensity of TPE@GO enhanced with the increasing of TPE content, resulting from an increasing amount of TPE molecules beyond the quenching region of GO. Thus, we got the critical planar density (0.0050 mg/cm2) and thickness (31 nm) of TPE coating, fluorescence of which was hard to be seen under UV-light with naked eye. Then, we treated the surface of the TPE@GO with THF/H2O mixture by spraying. The THF served as good solvent to dissolve TPE while H2O could cause the aggregation, and their volume ratio was related to the microstructure of TPE. At an appropriate ratio (VTHF= 60%), the THF/H2O mixture could facilitate the forming of TPE nanoparticles (mainly 50-80 nm in diameter), leading to the emission of fluorescence. During the treating of mixed solvents, the planar TPE coating was dissolved in THF firstly, and then the TPE molecules aggregated into nanoparticles in H2O during the volatilization of THF. The forming of particle increased the distance between GO and the upper part of TPE nanoparticle, which was beyond the quenching region of GO and emitted the fluorescence. Moreover, we found that fluorescence could be quenched again after the spraying of pure THF again, since the aggregated TPE nanoparticle could be dissolved in THF. Finally, the GO and TPE dispersions were used as invisible ink to paint patterns, and the information decryption or encryption can be conducted by the spraying of the THF/H2O mixture or pure THF, respectively. The work has successfully opened up a mixed solvents triggered switching of microstructure and fluorescence strategy, which can be used in the field of information security and so on. EXPERIMENTAL SECTION Materials: Benzophenone, K2CO3, dichloromethane (CH2Cl2), and tetrahydrofuran (THF) were purchased from Aladdin Reagent (China) Co., Ltd. Zine powder and Titanium tetrachloride (TiCl4) were purchased from Macklin Biochemical Co., Ltd. KMnO4, NaNO3,


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concentrated H2SO4 (98%), and H2O2 (30%) were purchased from J&K Scientific Co., Ltd. Nature graphite was purchased from Qingdao Huatai Graphite Co., Ltd. Unless otherwise mentioned, all of the reagents were obtained from commercial sources and used directly without further purification. Synthesis of TPE and TPE Nanoparticle Dispersions: TPE was synthesized by McMurray coupling method. Briefly, Zinc powder (3.93 g, 60 mmol) and benzophenone (3.64 g, 20 mmol) were placed in a 250 mL round bottom flask. THF (60 mL) was added to the flask under nitrogen atmosphere. The reaction mixture was cooled to 0°C, and TiCl4 (3 mL) was slowly added while maintaining the temperature under 0°C. The mixture was warmed up during 1.5 hour, and then heated to 78℃ and reflux for 12 h. After cooling to room temperature, the reaction mixture was quenched with a 30% K2CO3 solution and extracted in CH2Cl2. The crude product was purified by column chromatography. The TPE nanoparticle dispersion was prepared by nano-coprecipitation method. Briefly, TPE dispersion (THF) was prepared by dissolving TPE (3 mg) in THF (10 mL). The poor solvent H2O (30 mL) was dripped into the dispersion and stirred continuously (300 r/min). TPE nanoparticle dispersion was formed after the volatilization of THF. Preparation of GO and TPE@GO: GO was prepared from nature graphite using the mild Hummer’s method.31 The freeze-dried GO was dispersed in H2O via ultrasonication to form a uniform GO dispersion (0.05 mg/mL), and the TPE was dissolved in THF to prepare TPE dispersion (0.05 mg/mL). The GO dispersion (0.05 mL) was sprayed on the glass sheet (1 cm2) by atomizer to get GO coating (0.0025 mg/cm2) after drying, on which the TPE solution (0.05, 0.10, 0.15, 0.20, 0.25 mL) was sprayed, and the TPE@GO was formed after the volatilization of THF solvent at room temperature. Solvent Treating of TPE@GO: TPE@GO was treated with different solvents at room


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temperature by spraying method. The solvents were the mixture of THF and H2O with VTHF ranging from 0 to 80%. The solvent spraying speed and amount was 0.05 mL/min and 0.05 mL/cm2, respectively. After the spraying, the samples were dried at room temperature during the volatilization of solvents. Additionally, the original TPE@GO (No. 2) was also treated with a mixture of DMF and H2O (the volume fraction of DMF was 60%) by the same method, which was dried at 80℃. Fluorescence Switching of the Sprayed Patterns: GO dispersion (2.5 mL) and TPE dispersion (5 mL), as the invisible ink, were sprayed on commercial non-fluorescent fluorinated ethylene propylene film (spray size was 5 cm x10 cm) in turn. The patterns were sprayed with the assistance of mask. Then, the THF/H2O mixture (VTHF = 60%, 2.5 mL) was sprayed on the patterns. After drying, the solvent of pure THF (2.5 mL) was sprayed on the patterns. As a contrast, pure TPE dispersion (2.5 mL) was also sprayed (spray size was 2.5 cm х10 cm) and treated by the solvents. Notes: Some commercial plastic films contain fluorescent brighteners, which could interfere the experiment. Also, the problem of additives precipitation during THF treating exists in the general plastic film (such as PE, PVC, PET). Characterizations: Chemical structures of TPE were studied by Fourier transform infrared (FTIR) spectra (Bruker Tensor 27, using KBr pellets). UV-vis absorption spectra of the TPE were measured using Lambda 35 (PerkinElmer), and fluorescence spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer. The visual fluorescence photographs were taken by camera under the UV lamp (LED-200, Electro-lite) with UV power of 2.5 W/cm2. The fluorescence micrographs and photoluminescence decay curves were characterized by confocal fluorescence microscopy (Microtime 200) with excitation wavelength at 375 nm. Scanning electron microscope (SEM) and EDS mapping was carried out on verios 460L (FEI). 1H NMR spectra were carried out on AVANCE III HD 400MHz spectrometer. The


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contact angles were measured by a contact angle goniometer (Powereach, JC2000D) using liquid droplets of 1 μL in volume for three times, and the average values were calculated. The thickness of the samples was characterized by a step profiler (Dektak 150) at a contact pressure of 4 mg. ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Text: The introduction of peak-forest of China Guilin Karst Landform. Figures: FTIR spectra of TPE and Benzophenone. The dispersibility of GO film and TPE powder in THF/H2O (VTHF = 60%). SEM and TEM images of GO. 1H NMR spectrum of TPEDBr in CDCl3. SEM image of TPE@GO treated with DMF/H2O. Movie S1: Dissolution process of TPE in solvent of THF, acetone, CH2CL2, petroleum ether, ethanol, and ethyl acetate. (MP4) Movie S2: Contact angle measurement process of TPE, GO, and TPE@GO (the droplet composition is H2O and THF/H2O with VTHF of 60%, respectively). (MP4) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (No.


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Efficient

Red

AIEgens

Based

on

Tetraphenylethene:

Synthesis,

Structure,

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Tetraphenylethylene @ Graphene Oxide with Switchable

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