Bioinspired Fluorescent Nanosheets for Rapid and Sensitive

Sep 15, 2016 - Detection of organic pollutants in aqueous media is crucial for guaranteeing water safety. Conventional methods for organic pollutant d...
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Bioinspired Fluorescent Nanosheets for Rapid and Sensitive Detection of Organic Pollutants in Water Guodong Liang,*,† Feng Ren,† Haiyang Gao,† Qing Wu,† Fangming Zhu,† and Ben Zhong Tang*,‡ †

PCFM and GDHPPC Lab, School of Materials Science and Engineering, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China ‡ Department of Chemistry, Institute for Advanced Study, Division of Biomedical Engineering, State Key Laboratory of Molecular, Neuroscience and Institute of Molecular Functional Materials, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China S Supporting Information *

ABSTRACT: Detection of organic pollutants in aqueous media is crucial for guaranteeing water safety. Conventional methods for organic pollutant detection suffer from time-consuming operation procedures (on the order of hours) and expensive devices. Inspired by dog noses, herein, we constructed self-assembled fluorescent nanosheets for rapid and sensitive detection of organic pollutants based on the grasp-report strategy. Tetraphenylethene decorated cyclodextrins (TPE-CDs) self-assembled into nanosheets with hydrophobic TPE layers sandwiched between two hydrophilic cyclodextrin layers. The hydrophobic cavity of the outer cyclodextrin layers grasped and collected organic pollutants, and subsequently transported them to the TPE layers and quenched the fluorescence emission of TPE layers. Such nanosheets allowed rapid detection of xylene (on the order of seconds) at a concentration of 5 μg/L. With the merits of the ease of synthesis, simple operation, and high sensitivity, the fluorescent nanomaterials provide a promising candidate for rapid and sensitive detection of organic pollutants. KEYWORDS: fluorescence, nanosheets, organic pollutants, aggregation-induced emission, bioinspired

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promising candidates for VOC detection.4−17 Currently, a few methods are available to construct organic fluorescent materials for VOC detection. Fluorescent dyes were encapsulated in metal−organic frameworks (MOFs) to generate fluorescent MOFs.18−20 Upon exposure to VOCs, fluorescence emission of the MOFs changed due to the interaction between the dyes and the VOCs. However, the fluorescence intensity varied only 2-fold even when they were immersed in benzene derivates.21 The low sensitivity hindered them from practical application in VOC detection. An alternative approach to fluorescent VOC materials is doping polymers with fluorescent dyes. Upon exposure to VOCs, VOC molecules diffuse into polymer films, and mobility of polymers is enhanced, which induces changes in fluorescence emission. However, it takes time for polymers to swell in VOCs, and the polymer chains creep slowly. This leads to weak and slow fluorescence response to VOCs.22,23 Dogs have an extremely sensitive sense of smell, because dogs’ noses have a bony structure inside, which allows sniffed air to pass through a bony shelf and odor molecules to stick to it. The stuck odor molecules are not washed out when dogs breathe, so the scent molecules accumulate in the nasal

ualified and safe water supplies are imperative for all known forms of life. However, research has shown that water pollution by volatile organic compounds (VOCs) is over 30% worldwide. 64% of lakes are polluted in the U.S. The situation is even worse in developing countries. 14,000 innocent people daily die from water pollution. Water safety and VOC detection have attracted great concern from the public. Among VOCs, benzene derivates such as benzene, toluene, and xylene are the most dangerous and notorious because these toxic compounds, which pose tremendous risks to human health, are widely used as solvents in the industrial sector. Waste water from factories has become the main pollution source. The conventional method for VOC detection is gas chromatography−mass spectrometry (GC-MS), which suffers from time-consuming and complicated operation procedures, bulky and expensive devices, and the need for specialized operators.1−3 Due to these limitations, the bulky and expensive GC-MS instruments are sparsely located in government buildings, which are inaccessible to the public. A wise way to counter water pollution is to arouse the public to join. People are encouraged to test water specimens in a timely manner. This requires development of new techniques, which should be rapid, simple and easy, low cost, and highly efficient for VOC detection. Because they meet all the requirements mentioned in the preceding paragraph, fluorescent materials appear to be © 2016 American Chemical Society

Received: August 26, 2016 Accepted: September 15, 2016 Published: September 15, 2016 1272

DOI: 10.1021/acssensors.6b00530 ACS Sens. 2016, 1, 1272−1278

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chambers and the scent builds with intensity, allowing dogs to detect the faintest odors. Inspired by dog noses, to develop a highly sensitive detection system for VOCs, two prerequisites must be satisfied. First of all, the grasp unit which spontaneously collects VOCs at low concentrations should be integrated in the system. Second, the collected VOCs should be quickly transported to report units.24 Herein, imitating dog noses, we constructed a type of sensitive fluorescent nanomaterial for VOC rapid detection based on the grasp−report strategy. As a proof-of-concept demonstration, cyclodextrins (CDs) with water-soluble shell and hydrophobic cavity were selected as the grasp unit.25−29 Tetraphenylethene (TPE) showing an intense fluorescent emission in aggregate state was used as a report unit.30−36 To shorten the distance between the grasp and report units, TPE was covalently bonded with cyclodextrin to generate a type of amphiphilic fluorescent compound (TPE-CDs). The amphiphilic TPE-CDs self-assembled into nanosheets in aqueous media, in which hydrophobic TPE layers were sandwiched between two hydrophilic cyclodextrin layers, as illustrated in Scheme 1. The hydrophobic cavity of cyclodextrin grasped and

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EXPERIMENTAL SECTION

Materials and Characterization. Tetraphenylethene decorated cyclodextrins (TPE-CDs) were synthesized through the esterification reaction of monocarboxyl acid-substituted TPE with cyclodextrins.26 Fluorescence (FL) spectra were recorded on a PerkinElmer LS 55 spectrofluorometer. A general process of detection of organic pollutants in water was shown as follows. TPE-CDs were dissolved in distilled water at room temperature. FL spectra of the TPE-CD aqueous solutions were scanned. To the TPE-CD aqueous solutions were added organic pollutants. The mixtures were stirred for 1 min. FL spectra of the mixtures were scanned immediately. Tapping mode atomic force microscopy (AFM) to investigate the three-dimensional morphology of nanosheets was performed using a commercial atomic force microscope (SPM-9500J3) with a silicon microcantilever (spring constant 30 N/m and resonance frequency ∼270 kHz). The scan rate varied from 0.1 to 2.0 Hz to optimize the image quality. The sample was prepared by spin coating TPE-β-CD aqueous suspension (0.1 mg/ mL) onto a mica wafer. The top layer of mica wafers was removed prior to spin coating. Rotation speed was 900 rpm for 10 s and 3000 rpm for 45 s. For TPE-β-CD nanosheets, absorbed xylene, TPE-β-CD aqueous suspension containing 50 mg/L xylene was spin coated onto a fresh mica wafer under identical conditions. A transmission electron microscope (TEM) (JEM1020) was used to characterize the microstructure of TPE-CDs nanosheets. Acceleration voltage was 120 kV. The sample was prepared by drying a drop of TPE-CD aqueous solution (0.1 mg/mL) on a carbon-coated copper grid. After solvent was evaporated at room temperature, the grid was then annealed at 40 °C overnight. Dynamic light scattering (DLS) measurement was performed using a Brookhaven instrument. Scattering angle was fixed at 90°.

Scheme 1. Schematic Illustration of Nanosheets of TPE-CDs for Sensing VOCs

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RESULTS AND DISCUSSION Tetraphenylethene decorated cyclodextrins (TPE-CDs) were synthesized through the esterification reaction of monocarboxyl acid-substituted TPE with cyclodextrins in our lab previously.26 TPE functionalized cyclodextrins (TPE-CDs) are amphiphilic. Fluorescence (FL) spectroscopy was used to determine the critical aggregation concentration (CAC) of TPE-CDs in aqueous solution. The aqueous solutions of TPE-β-CD emitted weakly at low concentrations (below 0.05 mg/mL), while fluorescence intensity increased rapidly with concentration when concentration exceeded 0.05 mg/mL (Figure 1). The CAC of TPE-β-CD was determined to be 0.05 mg/mL. Morphology Characterization. TPE functionalized cyclodextrins (TPE-CDs) are typical amphiphilic molecules. In aqueous media, hydrophobic TPE forms cores stabilized by hydrophilic cyclodextrin as shells. Morphology of TPE-β-CD

collected VOCs due to the hydrophobic interactions between them. The collected VOC molecules were simultaneously transported to fluorescent TPE layers, and their fluorescence emission quenched, allowing rapid and sensitive detection of VOCs. Moreover, nonplanar TPE moieties are loosely packed in report layers, which facilitated diffusion of VOCs and subsequent quenching of fluorescence emission. With the merits of ease of synthesis, low cost, and high sensitivity, such fluorescent nanomaterials open new access to facile detection of VOCs.

Figure 1. (a) Fluorescence spectra of TPE-β-CD aqueous solutions at various concentrations and (b) the plot of fluorescence intensity (470 nm) against concentration. Excitation: 350 nm. The critical aggregation concentration (CAC) of TPE-β-CD in aqueous solutions was 0.05 mg/mL. Inset showed digital images of TPE-β-CD aqueous solutions at various concentrations under UV radiation (365 nm). 1273

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ACS Sensors was investigated using tapping mode atomic force microscopy (AFM) and transmission electron microscopy (TEM) (Figure 2). AFM height images showed sheet-like particles of TPE-β-

Table 1. Summary of Geometric Parameters of TPE-β-CD Nanosheets surface area (m2/g)

pore volume in a nanosheet (10−23 m3)

cavity of β-CD 1013 TPE layer 3004 TPE volume fraction in TPE layer was 4.2%

1.03 7.59

to absorb organic pollutants in aqueous media. Each TPE-β-CD nanosheet consisted of approximately 27 700 TPE-β-CD molecules (see SI). VOC Detection. Cyclodextrins possess hydrophobic cavities, which show specific interaction with benzene derivates.37−43 Thus, TPE-β-CD nanosheets were used to sense toxic VOCs. TPE-β-CD nanosheets emitted efficiently at a wavelength of 470 nm upon UV radiation (365 nm). The fluorescence intensity at wavelength of 470 nm dropped abruptly upon the addition of a trace amount of para-xylene (Figure 3). The possible reason is that TPE moieties are packed

Figure 2. Tapping mode atomic force microscopy (AFM) height image (a) and transmission electron microscopy (TEM) image (b) of TPE-β-CD. The insets in left and right panels showed the crosssectional profile and dynamic light scattering (DLS) curve, respectively.

CD with the lateral size of approximately 180 nm. TEM images further confirmed the formation of TPE-β-CD nanosheets. Dynamic light scattering (DLS) results revealed TPE-β-CD nanoparticles of 190 nm, in agreement with AFM and TEM results. The thickness of TPE-β-CD nanosheets was 4 nm, double of the full length of TPE-β-CD (Scheme S1 and S2, Supporting Information (SI)). This suggested that the nanosheets consisted of two layers of TPE-β-CD, where hydrophobic TPE layers were sandwiched between two hydrophilic CD layers, as illustrated in Schemes 1 and 2.

Figure 3. (a) Fluorescence (FL) spectra of TPE-β-CD aqueous solutions containing various amounts of para-xylene and (b) variations of FL intensity of TPE-β-CD aqueous solutions as a function of paraxylene concentration. Concentration of TPE-β-CD was 0.1 mg/mL. Detection limit: 5 ug/L.

Scheme 2. Schematic Depiction of the Packed Structures of TPE-β-CD Nanosheets

and solidified into TPE-β-CD nanosheets. The intramolecular motions (i.e., rotation, vibration, bending, and so on) of phenyl rings of TPE are restricted, giving rise to intense fluorescence emission.30,44 In the presence of xylene, a good solvent of TPE, xylene molecules activate intramolecular motion of TPE due to strong π−π interaction between them, resulting in a weak fluorescence emission of TPE layers. The value of I0/I − 1 increased linearly with increasing xylene concentration, following the Stern−Volmer relation: I0/I = 1 + K[Q ]

where I0 and I are the steady-state fluorescence intensities in the absence and in the presence of quencher, respectively. K denotes the Stern−Volmer constant, being 16 000 L/mol. The detection limit was determined to be 5 μg/L, by far lower than the water quality standard by the World Health Organization (WHO) (500 μg/L for xylene). The TPE-β-CD fluorescent nanosheets are much more sensitive than the fluorescent materials reported in the literature.21,22 When 5 μg/L of paraxylene was added, the molar ratio of para-xylene to TPE-β-CD was 7 × 10−4 (Table 2), much smaller than 1:1 for the inclusion complex formation. In this case, each nanosheet contained only 19 para-xylene molecules, which generated a detectable variation in the fluorescence emission intensity of TPE layers. It is noted that only when para-xylene interacts with TPE

TPE moieties were loosely packed in the middle layer due to the nonplanar nature (Scheme S2, SI). The volume fraction of TPE moiety in the middle layers of TPE-β-CD nanosheets was only 4.2% (Table 1, and SI), suggesting that large volume of hydrophobic pores remained in the middle layers. On the other hand, in the outer cyclodextrin layer of TPE-β-CD nanosheets, the surface area of the hydrophobic cavity of β-CD was estimated to be 1013 m2/g (Table 1). Such large hydrophobic surface area of β-CD is preferential for TPE-β-CD nanosheets 1274

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ACS Sensors Table 2. Calculation of the Molar Ratio of Xylene to TPE-βCD xylene concentration xylene/TPE-β-CDa molar ratio number of xylene molecules in a nanosheet a

100 mg/L

7.1 mg/L

5 μg/L

14 3.88 × 105

1 2.77 × 104

7 × 10−4 19

Concentration of TPE-β-CD was 0.1 mg/mL.

chromophores, does the fluorescence of TPE layers change. The fluorescent intensity of TPE-β-CD nanosheets decrease upon the addition of xylene implied that the xylene molecules grasped by cyclodextrin were subsequently transported to TPE layers, which induced the variation in the fluorescence intensity of the TPE layers. When 100 mg/L of xylene was added, the molar ratio of xylene to TPE-β-CD was 14:1, much greater than 1:1 for the inclusion complex formation. This further indicated that the absorbed xylene by cyclodextrin was transported to TPE layers because the cavity of cyclodextrin cannot accommodate so much xylene based on the formation of the host−guest inclusion complex. The transportation of xylene from the cavity of cyclodextrin to TPE layers is possibly driven by the preferential π−π interaction of xylene with TPE, overcoming the hydrophobic interaction of xylene with cyclodextrin. Moreover, the cavity of cyclodextrin is closely connected with TPE layers through covalent bonds, which facilitates the xylene transportation from cyclodextrin to TPE layers. To further understand the xylene transportation from cyclodextrin to TPE layers, the pore volume of the cavity of cyclodextrin and TPE layers in TPE-β-CD nanosheets was estimated, respectively. The pore volume of TPE layers is 7-fold that of cyclodextrin. Moreover, the surface area of TPE layers is 3 times that of the cavity of cyclodextrin. Such TPE layers with huge pore volume and surface area, as well as preferential interaction with xylene, show a stronger capability to absorb xylene than cyclodextrin. On the other hand, the TPE layers cannot directly absorb xylene from aqueous media since hydrophobic TPE layers are encapsulated by water-soluble cyclodextrin. Thus, cyclodextrin layers absorb xylene from aqueous media, which is subsequently transported to TPE layers and quenches the fluorescence emission of TPE layers. We further checked the morphology of xylene-absorbed TPE-βCD nanosheets using AFM (Figure 4a). TPE-β-CD nanosheets after absorption of xylene still retained a sheet-like morphology. The xylene-absorbed nanosheets had a thickness of 6 nm, much larger than that of pristine nanosheets (4 nm). Given that encapsulation of xylene by the cavity of cyclodextrin contributes little to the thickness of nanosheets (Scheme 1), the significantly enhanced thickness of xylene-absorbed nanosheets shows that xylene is located in TPE layers. Xylene is a good solvent of TPE due to strong π−π interaction between them. Xylene swells TPE layers, leading to an increased thickness of TPE-β-CD nanosheets. This result further supported the transportation of xylene to TPE layers. More interestingly, TPE-β-CD nanosheets responded quickly to xylene. Upon the addition of para-xylene, the fluorescence of the nanosheets dropped immediately on the order of seconds (Figure 4), much more quickly than that of the fluorescent polymers reported in the literature (more than 30 min)22 and conventional GC-MS (on the order of hours). Such a rapid response of TPE-β-CD nanosheets to xylene suggested nearinstantaneous absorbance of xylene by cyclodextrin units

Figure 4. (a) AFM height image of xylene-absorbed TPE-β-CD nanosheets and (b) fluorescence response of TPE-β-CD nanosheets to xylene as a function of time. The insets show the cross-sectional profile. Concentration of TPE-β-CD was 0.1 mg/mL. Concentration of para-xylene was 200 mg/L.

possibly due to large surface area, and rapid transportation of collected xylene to TPE layers. The quick response of the nanosheets efficiently prompts the process of VOC detection. We investigated the fluorescence response of TPE-β-CD nanosheets to various VOCs. Blank TPE-β-CD nanosheets radiated intensely. The addition of a small amount of paraxylene resulted in quenching of their fluorescence emission. While in the presence of equivalent amounts of other VOCs such as toluene, benzene, dichloromethane (DCM), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), acetone, methanol, hexane, and ethanol, fluorescence of TPE-β-CD nanosheets still can be observed by the naked eye. The values of I0/I − 1 of TPE-β-CD nanosheets in the presence of various VOCs were summarized in Figure 5. para-Xylene showed a by far larger value of I0/I − 1, demonstrating superior specificity of TPE-β-CD nanosheets toward para-xylene. The interference of various VOCs on the sensing process was also tested. Even TPE-β-CD aqueous suspension contained various VOCs, similar values of I0/I − 1 were obtained, verifying the high specificity of TPE-β-CD nanosheets toward para-xylene. Two possible reasons for the

Figure 5. Fluorescence responses of TPE-β-CD nanosheets to various VOCs. Cyan rods represented the addition of different VOCs (200 mg/L) to the aqueous solution of the TPE-β-CD nanosheets, while the red rods represented the subsequent addition of para-xylene (200 mg/L) to the solution containing a specific VOC. Excitation: 350 nm. Emission: 470 nm. 1275

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ACS Sensors specific response of TPE-β-CD nanosheets toward para-xylene are considered: (1) specific interaction between cyclodextrin and para-xylene due to good size match between them (Table S1 and Scheme S3, SI) (For instance, the inner size of β-CD was 0.78 nm, close to the full length of para-xylene (0.68 nm), but much larger than those of toluene and benzene (0.59 and 0.49 nm, respectively); such a good size match between β-CD and xylene may favor their efficient contact, maximizing the interaction between them.) (2) good compatibility among xylene, cyclodextrin cavity, and TPE moiety. The fluorescence response of TPE-β-CD nanosheets to paraxylene analogues such as ortho-xylene, meta-xylene, and ethylbenzene was further investigated (Figure 6). para-Xylene

Figure 7. Fluorescence responses of TPE-β-CD solutions at various concentrations (a) and its counterparts at concentration of 0.1 mg/mL (b) to para-xylene.

Figure 6. Fluorescence responses of TPE-β-CD nanosheets to paraxylene and its analogues.

intensity of TPE-β-CD solutions by regulating their concentrations without a loss in response sensitivity. The fluorescence response of TPE-CDs to para-xylene was dependent on the nature of cyclodextrins. In contrast to TPE-βCD, TPE modified α-cyclodextrin and γ-cyclodextrin (TPE-αCD and TPE-γ-CD, respectively) showed much weaker responses to para-xylene (Figure 7b), likely due to size mismatch of cavities of α- and γ-cyclodextrin with para-xylene. For instance, the full length of para-xylene was approximately 0.68 nm, close to the inner size of β-CD (0.78 nm), but quite different from those of α-CD and TPE-γ-CD (0.57 and 0.95 nm, respectively, Table S1 and Scheme S3, SI). Moreover, high CAC for TPE-α-CD and TPE-γ-CD (Figure S1, SI) may also be associated with their weak response to xylene. For example, CAC of TPE-γ-CD was 0.25 mg/mL. TPE-γ-CD existed as monomers in its aqueous solution of 0.1 mg/mL, leading to a weak response to xylene. On the other hand, the mixture of βCD and TPE also showed a weak fluorescence response to para-xylene, revealing that it was necessary to covalently bind the grasp unit of cyclodextrin with the report unit of TPE. The possible reason is that covalent bonding efficiently shortens the distance between them, which facilitates quick transportation of collected xylene to report units of TPE layers.

exhibited a larger value of I0/I − 1 than its analogues including ortho-xylene, meta-xylene, and ethylbenzene. Superior specificity of TPE-β-CD nanosheets toward para-xylene is possibly due to good size matching between the inner pore of β-CD and para-xylene (Table S1 and Scheme S3, SI). Effect of Morphology. To verify that the nanosheet morphology has an effect on the fluorescence response to xylene, the effect of the concentration of TPE-β-CD aqueous solutions on the fluorescence response was investigated. Diluted TPE-β-CD solutions (≤0.05 mg/mL) showed small values of I0/I − 1, revealing weak fluorescence responses to para-xylene (Figure 7a), while the concentrated solutions (≥0.1 mg/mL) exhibited sensitive responses to para-xylene. When the TPE-β-CD concentration was 0.1 mg/mL, the value of I0/I − 1 reach 15.5. After further increasing TPE-β-CD concentration to 0.2 and 0.5 mg/mL, the value of I0/I − 1 did not increase, but decreased slightly. Therefore, the optimized TPE-β-CD concentration of 0.1 mg/mL existed for the fluorescence response of TPE-β-CD aqueous solutions to xylene. Below CAC (0.05 mg/mL), TPE-β-CD existed as monomers, while nanosheets formed above CAC. This demonstrated that the formation of nanosheets was crucial to the sensitive fluorescence response of TPE-β-CD to paraxylene. TPE moiety packs together to form a middle layer in TPE-β-CD nanosheets. Once xylene molecules are transported to TPE layers, xylene quenches the fluorescence emission of a few adjacent TPE moieties, leading to a sensitive response of TPE-β-CD nanosheets to xylene. Moreover, the formation of the nanosheets, especially TPE layers, significantly enhances the absorption capability of TPE-β-CD toward xylene. On the other hand, the fluorescence intensity of TPE-β-CD solutions increased with concentration above CAC (Figure 1), contrary to the conventional dyes showing an aggregation-caused quenching (ACQ) effect.44 This allows optimizing fluorescence

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CONCLUSION In summary, we construct a type of bioinspired fluorescent nanosheets for rapid and sensitive detection of organic pollutants based on the grasp−report strategy. Tetraphenylethene decorated cyclodextrins (TPE-CDs) self-assemble into fluorescent nanosheets, in which hydrophobic TPE layers are sandwiched between two hydrophilic cyclodextrin layers. The hydrophobic cavity of cyclodextrin grasps and collects organic pollutants due to hydrophobic interactions between them. Collected organic pollutants are simultaneously transported to fluorescent TPE layers, and quench their fluorescence emission. 1276

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Such nanosheets allow rapid detection of xylene (on the order of seconds) at a concentration of 5 μg/L. Given the ease of synthesis, low cost, and high sensitivity, the fluorescent nanomaterials provide a promising candidate for the facile detection of VOCs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.6b00530. Molecular structure of TPE-β-CD, calculation of aggregation number, surface area, and pore volume of TPE-β-CD nanosheets, fluorescence intensity of TPE-αCD and TPE-γ-CD as a function of concentration (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21074151 and 21374136) for financial support. REFERENCES

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