Solid-State Fluorescence of Fluorine-Modified Carbon Nanodots

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Solid-state Fluorescence of Fluorine-modified Carbon Nanodots Aggregates Triggered by Polyethylene Glycol Peng Long, Yiyu Feng, Yu Li, Chen Cao, Shuangwen Li, Haoran An, Chengqun Qin, Junkai Han, and Wei Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13138 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Applied Materials & Interfaces

Solid-state Fluorescence of Fluorine-modified Carbon Nanodots Aggregates Triggered by Polyethylene Glycol Peng Long1, Yiyu Feng1,3,4, Yu Li1, Chen Cao1, Shuangwen Li1, Haoran An1, Chengqun Qin1, Junkai Han1, Wei Feng1,2,3,4* 1

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

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Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin

300072, P. R. China. 3

Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education,

Tianjin 300072, P. R. China. 4

Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, P. R. China.

ACS Paragon Plus Environment

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ABSTRACT

Solid-state fluorescent carbon quantum dots (QDs) can be used for the encryption of security information. Controlling the dispersion and aggregation of the QDs is crucial for switching their solid-state fluorescence “on” and “off”. It has been proposed to use polymers to slightly separate the QDs inside aggregates in order to trigger their fluorescence. However, the complex interactions between the QDs and flexible polymer chains make this process challenging. Here, fluorine-modified carbon nanodots (FCDs) were used in solution as printing ink. After printing, the FCDs were aggregated on paper via hydrogen bonds, thereby quenching the fluorescence. After a polyethylene glycol (PEG) treatment, the FCDs exhibited yellow solid-state fluorescence due to an increased interdot spacing. The fluorescence intensity and emission wavelength could be tuned by varying the molecular weight and quantity of PEG used. Finally, we demonstrated a high-resolution encryption and decryption system based on the PEG-triggered fluorescence of FCDs.

Keywords: solid-state fluorescence, fluorine-modified carbon nanodots, interdot spacing, aggregates, polyethylene glycol


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INTRODUCTION Solid-state fluorescent materials (SFMs) are regarded as outstanding materials for organic light emitting diodes (OLEDs)1-4, fluorescent sensors5, data recording and storage6-8, dye lasers9-11, and security printing12-15. Recently, fluorescent carbon dots (CDs) have attracted considerable attention as SFMs owing to their small size, tunable properties, biocompatibility and high photostability.16-20 However, solid-state CDs are prone to aggregate, which usually leads to fluorescence quenching.21 Thus, controlling the dispersion or individually isolated CDs is crucial in switching the fluorescence on. Two strategies have been used to disperse solid-state CDs. The first one consists in coating the CDs with a polymer, which leads to surface passivation through the formation of bonds between the CDs and the polymer.22,23 The solid-state CDs deposited from solution on a substrate are then well dispersed and usually show high fluorescence. However, as the fluorescence of the CDs depends on their size and electronic interactions with the polymer, it is difficult to modify it once the CDs are coated by the polymer. The second strategy is based on the decomposition of CDs aggregates into individually isolated CDs caused by the hydrophobic and electrostatic/dipole interactions between CDs and water24 or organic solvent16. Compared with water and organic solvent, polymer chains uniformly coat the CDs surface, increasing the distance between two neighboring nanodots. When the interdot distance is larger than the Förster distance (R0), the fluorescence is dramatically enhanced due to a substantial reduction of Förster resonance energy transfer (FRET) interactions.16,25,26 This effect thus offers a great promise to switch on the fluorescence by disaggregating CDs aggregates.


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The interactions of CDs with molecules or polymers depend on the functional groups and substituted atoms on their surface.20,27-29 Fluorine-modified CDs (FCDs) with electronegative CF bonds show a great potential for interacting with diverse molecules owing to their hydrophobic nature and low surface energy.30-34 The polarity of the C-F bond with a significant electrostatic character enables electrostatic (dipole-dipole) interactions (C-Fδ-•••Xδ+) between the polarised CF bond and electropositive groups.35 For instance, Zhao et al. reported that strongly polar C-F bonds promoted CO2 adsorption based on electrostatic (dipole–dipole) interactions between electropositive carbon atoms of CO2 and electronegative fluorine.36 This interaction can facilitate the coating of polymers on FCDs surface for separating nanodots aggregates. When the fluorescence is triggered by polymers, FCDs retain their original macro-morphology but with an increased interdot distance, in contrast to a random dispersion in the polymer matrix due to the hydrophobicity of FCDs. To our knowledge, polymer-triggered solid-state fluorescence of CDs has not been realized yet because of the complex interactions between CDs and flexible polymer chains. Thus, optimising these interactions (e.g., electrostatic/dipole and hydrophobic interactions, hydrogen bond, van der Waals force) is central to switching solid-state fluorescence on. In this paper, FCDs were prepared by solvothermal fluorination (Figure 1a) and fully characterised in terms of morphology, fluorescence, and electronic properties. They were then used in solution as printing ink, and polyethylene glycol (PEG) treatment of printed paper was shown to trigger their fluorescence. The fluorescence intensity and emission wavelength could be tuned by varying the molecular weight and quantity of PEG. FCDs-painted paper shows high resolution fluorescence imaging of various patterns by controlling the coating and interaction


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with PEG. We finally demonstrated an information encryption system based on this phenomenon (Figure 1b).

RESULTS AND DISSCUSION As an optimized example (see below), the FCDs were prepared by a two-step solvothermal method (Figure 1, and see the Methods section for further details). A brown FCD powder was obtained after separation and purification. The hydrophobic FCDs showed a good dispersibility and stability in diverse organic solvents including dichloromethane (CH2Cl2), tetrahydrofuran (THF), trichloromethane (CHCl3), acetone, DMF, and dimethylsulfoxide (DMSO), but not in water (Figure S1). These solvents molecules have free pz orbital, which behaves as an electron acceptor, and enable the pseudo-hydrogen bonds with the C-F bonds.37 Because the fluorescence of QDs mainly depends on their microstructure in terms of size and interdot distance in the QDs aggregates or clusters38,39, the structural characterisation of the QDs is of paramount importance before studying their fluorescence. The morphology of the CDs and FCDs was observed by transmission electron microscopy (TEM). The CDs were well dispersed, with the average diameter of ~3.3 nm (Figure 2a), while the FCDs (Figure 2b) showed a slightly larger average diameter of ~3.6 nm. The high-resolution (HR) TEM lattice fringe images (inset of Figure 2a and Figure 2b) showed that the CDs and FCDs exhibited identical well-resolved lattice fringes with an interplanar spacing of 0.21 nm, which is close to that of the (100) facet of graphitic carbon.24,40 Further, the results reveal that C-F bonds formed on the surface rather than in the inner core. More importantly, compared with the CDs, the FCDs formed some aggregates (with a size of ~100 nm) (Figure S2) composed of close-packed nanodots without interdot


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spacing (Figure 2c) due to interdot hydrogen bonds (H-bonds) of C-F•••H-C.41,42 The presence of H-bonds was confirmed by differential scanning calorimetry (DSC) measurements (Figure S3). The FCDs aggregates showed a small endothermic transition at 94.5 ºC in the heating phase due to the breakdown of H-bonds, and an exothermic heat flow at 74.5 ºC in the cooling phase, which indicates the formation of H-bonds. The C-F bonds of the FCDs were characterised by Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR). The spectra of CDs shown in Figure S4 exhibited several broad peaks at 3400, 1720, and 1080 cm-1 ascribed to -OH, C=O, and C-O, respectively. The intensity of these oxygen-containing groups decreased with surface fluorination. Simultaneously, two new peaks at 1258 and 662 cm-1 attributed to the stretching vibration of C-F bond and symmetrical stretching vibration of CF2 were observed. Moreover, the peaks corresponding to phenols (-OH at 5.5~9.0 ppm and -COOH at > 9.0 ppm), disappeared, and that corresponding to alcohols (-OH at 2~5.5 ppm) greatly decreased (Figure S5) after fluorination. This result indicates that the oxygen-containing groups of the CDs were fluorinated to form the respective C-F bonds (i.e., -CF and -CF2) in the FCDs.34 The C-F bonds were further investigated by 19F NMR spectroscopy (Figure S6). Two strong and broad signals at ~188 and ~168 ppm in Figure S6 indicate the presence of C(sp3)-F and C(sp2)-F bonds, respectively. A weak peak at ~108 ppm was also observed, and is attributed to C-F2 bonds, probably originating from the difluorinated substitution of carbonyl and difluorination of alkene carbons.43,44 The chemical bonds and composition of the FCDs were further analysed by high-resolution Xray photoelectron spectroscopy (XPS) (Table S1 and Figure S7). Compared with the CDs being composed of 75.61% C and 24.39% O, the chemical composition of the FCDs was 80.37% C, 17.04% O, and 2.59% F. The binding energy peak at about 687 eV (Figure S7a) is attributed to


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highly polar C-F bonds. The high-resolution spectra for F 1s (Figure S7d) revealed the presence of both C(sp3)-F (689.2 eV) and C(sp2)-F (686.9 eV) bonds.34 The C 1s spectrum of the CDs (Figure S7b) was deconvoluted into four peaks, corresponding to C=C (284.6 eV), C−C (285.1 eV), C-O (285.9 eV), and C=O (287.5 eV). Compared with the C 1s spectrum of CDs, two new binding energies corresponding to C-F (288.2 eV) and C-F2 (289.3 eV) bonds appeared in the C 1s spectrum of the FCDs (Figure S7e), attributed to the monofluorinated substitution of oxygencontaining groups and difluorination of alkene and carbonyl.45-48 The peak corresponding to C=C and C-O significantly reduced after the fluorination due to the alkene fluorination and fluorine substitution.48-50 While, the FCDs showed a much smaller peak corresponding to C-O with respect to that of the CDs in the high-resolution O 1s spectra (Figure S7c and Figure S7f). In conclusion, these spectra indicate the existence of many types of C-F bonds (e.g., -CF and -CF2) due to the fluorination of oxygen-containing groups (hydroxyl or carbonyl/carboxylic). The fluorescence of the FCDs in DMF solution was investigated by fluorescence spectroscopy. The 3D-fluorescence plots of both the CDs (Figure 3a) and FCDs (Figure 3b) display a single broad emission peak deriving from homogeneous CD and FCD species, respectively. Compared with the maximum fluorescence peak at around 500 nm for the CDs, the fluorescence peak of the FCDs was red-shifted at around 580 nm. This feature is confirmed by the excitation-dependent fluorescence emission spectra shown in Figure S8 and by the fact that the FCDs solution showed a fluorescence emission band continuously red-shifted from 500 to 595 nm with an excitation at 365 nm when the concentration increased from 0.01 to 1.0 mg/mL (Figure 3d and Figure 3e). Moreover, the optimal excitation wavelength of the FCDs increased from 440 to 500 nm with the fluorination temperature from 120 to 180 ºC (Table S2); and the optimal emission wavelength of the FCDs red shifted from 533 to 580 nm (Figure S8). The quantum yield also increased from


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7.4% to 16.1% with the fluorination temperature increasing, which ascribe to the F edge modification.21,29 The fluorescence lifetime also changed by surface fluorination, as evidenced by the fluorescence decay curves fitted using a single-exponential function (Figure 3c). The FCDs showed an average fluorescence lifetime of 5.31 ns, compared with 3.17 ns for the CDs. The increase in lifetime arises from the transition of an increasing amount of FCDs from the ground state (S0) to the excited state (S1).51 The UV-vis absorption spectra shown in Figure 3f allowed us to gain further insight into the electronic property changes undergone by the CDs upon fluorination. Compared with the CDs, the FCDs showed an increased absorption in the range of 310 ~ 500 nm, and a remarkably reduced peak at 277 nm. All the results presented in this subsection demonstrate some dramatic changes in the conjugated nanocarbon electronic structure upon surface fluorination, which led to new fluorescence properties of the FCDs with respect to those of the CDs. The increase in light absorption is associated with the n-π* transition created by the C(sp3)-F bond28,52, while the reduction of the absorption peak at 277 nm indicates reduced π-π* transitions.53,54 Furthermore, we believe that the red-shifted fluorescence emission of the FCDs arose from a decrease in the πelectron concentration and the strong polarity of the C-F bonds on the surface which exhibited a longer wavelength emission.55 A higher concentration of FCDs resulted in the formation of larger FCDs clusters in solution, which showed more red-shifted fluorescence.16 As shown in Figure 2b (TEM), the FCDs were prone to aggregate due to H-bonds, which resulted in solid-state fluorescence quenching (Figure S10).56,57 However, the strong polarity of


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the C-F bonds and the hydrophobic surface of the FCDs are expected to facilitate the uniform coating of the FCDs by PEG, a polymer soluble in water and having electropositive carbon atoms on its (-CH2-CH2-O-)n chains. The FCDs and CDs were dip-coated on a filter paper (rounded size with diameter of 1.0 cm) using a uniform FCDs or CDs solution (2.0 mg/mL in DMF solution). Before the treatment by PEG, the fluorescence of papers coated by either CDs or FCDs was investigated. The CD-coated paper showed bright cyan fluorescence (Figure 4a) at around 500 nm (Figure S11) in agreement with that in a DMF solution (Figure 3a), suggesting that the CDs were individually dispersed on the paper. This dispersion can be explained by the presence of oxygen-containing groups on the CDs surface that hindered aggregation.16 By contrast, the FCDs-coated paper showed fluorescence quenching due to the formation of large aggregates (Figure 4b). However, after spraying the FCDs-coated paper with a PEG aqueous solution (200 mg/mL) and complete drying, a yellow fluorescence at around 575 nm was measured (Figure 4b and Figure S11) with a high quantum yield of 10.4%. This peak is consistent with that found in DMF solution (Figure 3e). The microstructure of the FCDs coated by PEG was observed by TEM (Figure 4c). Compared with the aggregated FCDs shown in Figure 2c, the PEG-coated FCDs displayed an increased interdot spacing, as evidenced by the spacing distribution shown in Figure 4d, demonstrating an average spacing of 1.9 nm. Specifically, according to the proposed model in Figure 4e, the fluorescence emission in solid-state FCDs system is related to the interdot distance between two neighboring FCDs. The FCDs are susceptible to aggregate and collide due to H-bonds after volatilization of DMF, leading to FRET interactions and solid-state fluorescence quenching. Furthermore, after PEG chain permeating and coating on the FCDs surface, the fluorescence is dramatically enhanced due to the increased interdot distance and substantial reduction of FRET


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interactions. This feature arises from the electrostatic (dipole-dipole) interaction of C-Fδ-•••Xδ+ between PEG and FCDs (Figure 4f). The electrostatic (dipole-dipole) interaction of C-Fδ-•••Xδ+ between PEG and FCDs was further verified by the measurement of zeta potential (Figure 4g). FCDs shows more negative surface with the zeta potential of -45.3 mV due to the existence of electronegative fluorine compared with CDs (-1.84 mV). The negative surface of FCDs facilitates the PEG coating based on the electrostatic (dipole-dipole) interaction of C-Fδ-•••Xδ+ , leading to a positive zeta potential of +1.31 mV. The FT-IR spectra (Figure S12) of PEG-coated FCDs display the three peaks attributed to the bending vibrations of -CH2- groups in PEG shifted from 1344, 1280 and 1242 cm-1 for pure PEG to 1352, 1297 and 1254 cm-1, respectively. This shift is attributed to the electrostatic bonds C-Fδ-•••Xδ+ that hinder to some extent the bending vibrations of the -CH2- groups in PEG. The electrostatic interaction also weakens the stretching vibration (1061 cm-1) of C-O-C in PEG and symmetrical stretching vibration of CF2 (662 cm-1). In addition, the endothermic and exothermic peaks of the FCDs aggregates are no longer observed in the DSC curve (Figure S13), indicating that no H-bonds formed between FCDs due to the interactions between the FCDs and PEG. These interactions are further evidenced by the shift of the PEG glass transition temperature from 63.5 ºC to 61.5 ºC in the heating period. Finally, it is noteworthy that the FCDs showed no fluorescence just after having been sprayed with PEG, i.e., before water evaporation. This can be attributed to strong van der Waals forces between water and PEG that limited the motion of PEG chains and hence the coating of the FCDs by PEG. The influence of the molecular weight of PEG on the FCDs fluorescence was investigated by measuring the solid-state fluorescence of FCDs coated by PEG (PEO is polymer of ethylene oxide with a molecular mass above 2 × 104 g/mol; so we both consider PEG and PEO as PEG in


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this paper) with a molecular weight in the range of 1 × 103 ~ 1 × 106 g/mol. As shown in Figure 5a, all the papers printed with FCDs and then treated with PEG of different molecular weight displayed yellow fluorescence (peak maximum at 575 nm), whose intensity depended on the PEG molecular weight, i.e., on the length of the ethylene oxide chains. The fluorescence intensity increased when the molecular weight of PEG increased from 1 × 103 to 2 × 104 g/mol, and decreased when the molecular weight further increased. PEG with a relatively low molecular weight has short ethylene oxide chains that easily penetrated the aggregates owing to the electrostatic interactions between their positively charged carbon atoms and the negatively charged F atoms on the FCDs surface. However, the increase in the interdot spacing was limited because the chains were too small to coat the FCDs uniformly that do not sufficiently screen Hbonds.58 A uniform coating leading to an interdot spacing higher than R0 was obtained with PEG with longer chains (molecular weight of 2 × 104 g/mol), yielding the highest fluorescence intensity (Figure S14a). However, PEG with very long chains (molecular weight higher than 2 × 104 g/mol) has long and entangled PEG chains that only covered the surface of the FCDs aggregates without penetrating inside the aggregates or adsorbing onto the surfaces.59-61 As a result, only a small amount of FCDs were individually coated by PEG, and the effect on the FCDs fluorescence was reduced. The effect of the quantity of PEG used in the PEG treatment on the fluorescence was also analysed using an aqueous solution of PEG with a molecular weight of 2 × 104 g/mol (hereafter referred to as PEG20000) at different concentrations (25 ~ 800 mg/mL). The FCDs-coated papers exhibited increased fluorescence with increasing quantity of PEG20000 (from 2.5 to 80 mg as 100 µL of solution were used) used to coat the FCDs (Figure 5b). The fluorescence intensity was saturation when the FCDs had been coated with 20 mg of PEG (Figure S14b). In addition, with


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an excitation at 365 nm, the maximum fluorescence peak was blue-shifted from 605 nm to 575 nm when the quantity of PEG20000 used to coat the FCDs increased from 2.5 to 80 mg. This blueshift indicates that more FCDs were disaggregated into small clusters by the effect of PEG when a larger quantity of PEG was used.62,63 This peaks are agree with that found in DMF solution with different concentration of FCDs (Figure 3d and Figure 3e). To further illustrate the mechanism of the PEG coating, we used diverse water-soluble polymers with a similar chemical structure as PEG to treat FCDs aggregates: polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP) and polyacrylamide (PAM). These hydrophilic polymers not only show excellent dispersibility in water but have also numerous functional groups such as carboxyl, hydroxyl, amino, and amide groups. Figure 5c shows that in spite of their hydrophilic properties, none of these polymers was able to switch on the solid-state fluorescence of papers printed with FCDs, whatever their molecular weight and the quantity of polymer used to coat the FCDs. This result is further evidenced by the optical images (Figure 5d) of the papers on which the logo of “Tianjin University” had been printed with FCDs ink (see the Methods section for further details on the preparation of the FCDs ink) and subsequently treated with different types of polymer. The resolution of the image directly depends on the intensity of fluorescence. It can be seen that the paper treated by PAA, PVA, PAM and PVP showed no fluorescence. This result is attributed to the poor wettability between the FCDs and these polymers, as show the contact angle measurements between the polymer solution and the paper printed with FCDs (Figure 6), which give a high contact angle of 112°, 109°, 108°, 118°, and 125° between the paper printed with FCDs and PAA, PVA, PVP, PAM and water, respectively. By contrast, PEG showed good wettability (with a contact angle of 13°) with the FCDs, and Movie S1 shows that PEG penetrated into the FCDs aggregates when the


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solution was dipped on the paper. This poor wettability indicates weak interactions between the polymers and the FCDs owing to the abundance of hydrophilic groups (carboxyl, hydroxyl, amino and amide, respectively) on the polymer chains that limited their interactions with C-F bonds on the FCDs surface.57,64 As a result, these polymers only covered the surface of the aggregates but did not coat individual FCDs and hence no fluorescence was observed. Figure 5d shows also optical images of papers printed with FCDs coated with PEG of different molecular weights. The paper treated with PEG20000 showed the highest fluorescence (hence the better image resolution), in agreement with the results shown in Figure 5a. The paper printed with FCDs and then treated by PEG with a molecular weight larger than 2 × 104 g/mol showed weak fluorescence, and the logo cannot be recognised. Interestingly, the fluorescence could be switched off by immersing the paper in deionised water (Figure S15). After drying, the paper showed no fluorescence because the PEG-coated FCDs were soluble in water. When treated with PEG again, the paper did not recover its fluorescence. The results presented above show that solid-state fluorescence of FCDs on paper can be switched on by a PEG treatment, and then definitively switched off with water, enabling optical encryption and decryption of written information, as schematically illustrated in Figure 7a. The word “FCDs” was written on a paper with a marker pen filled with FCDs ink (Figure 7b and Movie S2). The word “FCDs” could not be observed under UV irradiation due to fluorescence quenching of the FCDs aggregates. This stage corresponds to information encryption and storage. Dipping the paper in a PEG20000 aqueous solution (200 mg/mL, 100 µL) enabled the decryption of the information by switching on the fluorescence, as demonstrates the last optical image in Figure 7b where “FCDs” can be read under UV irradiation. The word “FCDs” was erased by immersing it in water. To further demonstrate the versatility and high-resolution of the


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switchable fluorescence for information encryption and decryption, we printed many complicated patterns on the paper with an inkjet printer using FCDs ink. After treatment with PEG20000 and drying, the paper showed a high-resolution image of a Tai Chi symbol and Chinese dragon under UV irradiation (Figure 7c). Clear boundaries were observed in fluorescence microscopy images (Figure S16), confirming that the fluorescence originated from the patterns printed with FCDs ink. The detailed micrometer-scale information can thus be stored and encrypted by controlling the microstructure of the FCDs on the paper. CONCLUSIONS FCDs with an average diameter of 3.6 nm were prepared by a solvothermal fluorination. In DMF, they showed a continuously red-shifted fluorescence from 500 nm to 595 nm when the concentration of FCDs increased from 0.01 to 1.0 mg/mL, indicating that the FCDs formed large conjugated clusters with increasing concentration. In contrast to the CDs, the FCDs formed aggregates (with a size of ~ 100 nm) and showed fluorescence quenching due to interdot Hbonds. The solid-state fluorescence was remarkably switched on by PEG treatment. PEG disaggregated the FCDs aggregates and coated the surface of individual FCDs due to the electrostatic interactions between the FCDs surface and the PEG molecules, hence increasing the interdot spacing (1.9 nm in average in TEM images). As a result, the FCDs exhibited bright yellow fluorescence with an emission band maximum at 575 nm under UV illumination at 365 nm. The wavelength and intensity of this polymer-triggered solid-state FCDs fluorescence can be furthermore controlled by the amount and molecular weight of the PEG coating. The FCDs exhibited maximum fluorescence intensity at 575 nm when treated by PEG with a molecular weight of 2 × 104 g/mol (PEG20000). This maximum fluorescence intensity is attributed to a uniform PEG coating of individual FCDs. Moreover, the fluorescence band blue-shifted from


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605 nm to 575 nm when an increasing quantity of PEG20000 was coated on the FCDs. We demonstrated that good wettability between PEG and FCDs based on strong interactions between both materials is crucial in switching the solid-state fluorescence “on”. Finally, we realised a high-resolution encryption of information by printing paper with FCDs ink, and decryption based on the PEG treatment of the paper to switch on the fluorescence, allowing us to read the printed information under UV illumination. The success of this pilot opens up the way to developing PEG-triggered fluorescent FCDs for dual-mode encryption of security information. EXPERIMENTAL SECTION Materials The organic solvents, i.e. n-hexane, cyclohexane, 1,2-dichlorobenzene, isopropanol, dichloromethane (CH2Cl2), tetrahydrofuran (THF), ethyl acetate, trichloromethane (CHCl3), ethanol, acetone, acetonitrile (ACN), N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were analytical reagents purchased from Aladdin Reagent (China) Co., Ltd. Glucose (99%) and HF(40%) were purchased from TCI, Japan. The polymer materials PVA (avg. Mw ~ 72000), PAA (avg. Mw ~ 50000), PVP (avg. Mw ~ 40000), PAM (avg. Mw ~ 40000) and PEG/PEO (various avg. Mw ~ 1000, 2000, 4000, 10000, 20000, 100000, 200000, 400000, 1000000) were purchased from Sigma-Aldrich and used as received. Deionized water was used in the experiments. Unless otherwise noted, all reagents and solvents were obtained from commercial sources and used directly without further purification. Synthesis of CDs


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The raw CDs were synthesised by a simple hydrothermal method as follows: 2.0 g of glucose was added to 80 mL of ethanol in a Teflon lined autoclave and then heated at 200 °C for 12 h. After the hydrothermal carbonization process, a dark brown dispersion was obtained and the solvent was removed by vacuum rotary evaporation at 60 °C. The resulting black viscous solid was added into 300 mL of dichloromethane to extract the CDs. The dichloromethane phase was filtered out using a microporous membrane (0.22 µm PTFE membrane), and dried to powder for further use. Synthesis of FCDs The FCDs were prepared by solvothermal fluorination. Typically, 500 mg of raw CDs and 38 mL of ethanol were mixed by ultrasonication for 30 min. The mixture was put into a sealed 50 mL Teflon lined autoclave containing 2 mL of a 40% HF solution as the fluorine source. The autoclave was heated to 160 °C for 12 h. After cooling to room temperature, the mixture was added into a polyethylene beaker containing 150 mL of deionized water and 50 mL of a saturated aqueous solution of NaHCO3 to remove unreacted HF. The product was filtered out using a microporous membrane (0.22 µm PTFE membrane) and throughout washed with deionized water, and finally dried by freeze drying. Brown FCDs powders were thusly obtained. Paper printed with FCDs ink FCDs were dispersed in DMF/ethanol (V/V = 2:1) via ultrasonication to form a uniform solution (2.0 mg/mL). This solution was used as printing ink (hereafter referred to as FCDs ink) and injected into a vacant cartridge of a commercial ink-jet printer (HP deskjet 2020 printer). The desired patterns or words were printed onto a piece of commercial brown paper. For the pen-


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on-paper procedure, the characters “FCDs” were written on paper with a marking pen filled with the FCDs ink. Coating PEG of paper printed with FCDs ink for switching on the fluorescence A certain quantity of PEG (2.5 ~ 80 g) with a molecular weight in the range of 1 × 103 to 1 × 106 g/mol was added to water (100 mL), forming a solution of mass concentration in the range of 25 ~ 800 mg/mL. The PEG solution was dipped on the paper after different words or patterns had been printed on it with FCDs ink. After coating, fluorescence emission was recorded by a Canon 5D digital camera under UV illumination at 365 nm. Characterisation Transmission electron microscopy (TEM) was carried out on a high-resolution JEOL 2010F TEM/STEM equipped with a high-angle annular dark-field (HAADF) detector. Fourier transform infrared (FTIR) spectra were obtained on a Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA). UV-vis absorption spectra of the CDs were measured using Lambda 35 (PerkinElmer, USA) and fluorescence spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer at room temperature. The photoluminescence lifetime and quantum yield were measured by time correlated single-photon counting (TCSPC) on a FLS920 spectrometer. The X-ray photoelectron spectra were obtained using PERKIN ELMZR PHI 3056 with an Al anode source operated at 15 kV to analyse the chemical composition of the materials. 1H NMR and 19F NMR spectra were carried out on a Varian INOVA 500 MHz spectrometer with DMSOd6 as internal standard. The contact angles were measured by a contact angle goniometer (POWEREACH, JC2000D) using liquid droplets of 6 µL in volume. All contact angles were recorded after 2 s of contact between the liquid and the solid-state FCDs. Elemental analyses


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were performed on a Vario EL elemental analysis instrument (Elementar Co.). Differential scanning calorimetry (DSC) was studied using a TA Q20 differential scanning calorimeter with a scan rate of 2 °C/min. Zeta potential measurements were carried out on an Zetasizer nano ZS90 (Malvern Instruments S.A.). ASSOCIATED CONTENT Supporting Information Additional figures, tables and movies. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Dispersibility, DSC, FTIR, NMR, XPS and Elemental analysis data; spectroscopic data, quantum yield, digital photographs of FCDs-painted papers and fluorescence microscopy image (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest ACKNOWLEDGMENT


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This work was financially supported by National Key R&D Program of China (No. 2016YFA0202302), National Natural Science Funds for Distinguished Young Scholars (No. 51425306), the State Key Program of National Natural Science Foundation of China (No. 51633007), and National Natural Science Foundation of China (No. 51573125 and 51773147). REFERENCES 1.

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FIGURES

Figure 1. Schematic illustration of (a) the preparation of FCDs and (b) encryption based on PEG-triggered fluorescence.


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Figure 2. TEM images of CDs (a) and FCDs (b). Insets: HRTEM lattice fringe images (top) and size distribution (bottom). (c) Magnified image of the marked area in (b).


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b

a

c CDs FCDs-Y

Intensity

103

102

101

100 0

10

20

30

40

50

60

70

80

Time (ns)

0.5

f

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CDs

580 560

Abs

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e

0.2 mg/mL 0.5 mg/mL 1.0 mg/mL

0.01 mg/mL 0.05 mg/mL 0.1 mg/mL

Emission maximum (nm)

d

Normalized FL Intensity (a.u.)

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540

FCDs

520 500

0.0 400

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0.2

0.4

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0.8

1.0

Concentration of FCDs in DMF (mg/mL)

Wavelength (nm)

200

300

400

500

600

Wavelength (nm)

Figure 3. 3D-luminescence plots of (a) CDs and (b) FCDs at 0.5 mg/mL. (c) Photoluminescence decay curves of the CDs and FCDs. (d) Fluorescence spectra of a FCDs solution with a concentration varying from 0.01 to 1.0 mg/mL. (e) Position of the maximum peak in the curves shown in (d) as a function of the solution concentration (recorded under UV light excitation with a wavelength of 365 nm). (f) UV-Vis absorption spectra of the CDs and FCDs. All samples were measured in DMF at room temperature.


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Figure 4. Optical images of (a) CDs and (b) FCDs deposited on a commercial filter paper under UV illumination with a wavelength of 365 nm. (c) Schematic illustration of the PEG-triggered fluorescence of FCDs based on the electrostatic interactions between FCDs and PEG that increased the interdot spacing. (d) The zeta potentials of CDs, FCDs and FCDs@PEG20000 in ethanol. (e) The proposed mechanism of PEG-switched fluorescence emission of solid-state FCDs. (f) High-resolution TEM image of dispersed FCDs coated with PEG. (g) Distribution of the interdot spacing.


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Figure 5. Fluorescence emission spectra of papers printed with FCDs and then (a) coated by PEG with different molecular weights and (b) coated with PEG20000 at different concentrations. (c) Fluorescence emission spectra of papers printed with FCDs and then coated by PAA, PVA, PAM and PVP, respectively. (d) Optical images of the papers on which the logo of “Tianjin University” had been printed with FCDs ink and subsequently coated by PAA, PVA, PAM, PVP, PEG1000, PEG4000, PEG10000, PEG20000, PEG100000, PEG200000, PEG400000, and PEG1000000 (scale bar = 1 cm). All spectra were recorded under UV light excitation with a wavelength of 365 nm.


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Figure 6. Contact angle of PAA, PVA, PAM and PVP aqueous on FCDs-coated paper. (The concentration of these polymers was 5 wt% and the molecular weight of PEG was 20000 g/mol)


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Figure 7. Encryption and decryption of information printed on paper with FCDs ink. (a) Schematic illustration and (b) optical images of a paper on which “FCDs” had been written with FCDs ink and encrypted by fluorescence quenching, and then decrypted by PEG-triggered fluorescence under UV illumination (scale bar = 2 cm). (c) Fluorescence images of the Tai Chi symbol and drawing of a Chinese Dragon printed on paper with an inkjet printer using FCDs ink (scale bar = 1 cm).


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TOC


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Solid-State Fluorescence of Fluorine-Modified Carbon Nanodots

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