Coassembly of Mixed Weakley-Type ... - ACS Publications

Article Cite This: Langmuir 2018, 34, 6367−6375

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Coassembly of Mixed Weakley-Type Polyoxometalates to Novel Nanoflowers with Tunable Fluorescence for the Detection of Toluene Congxin Xia,† Shanshan Zhang,‡ Di Sun,*,‡ Baolai Jiang,† Wenshou Wang,† and Xia Xin*,† †

National Engineering Technology Research Center for Colloidal Materials, and ‡Key Lab for Colloid and Interface Chemistry of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, P. R. China S Supporting Information *

ABSTRACT: In this work, three-dimensional nanoflowers with tunable fluorescent properties constructed with mixed Weakley-type polyoxometalates (POMs, Na9[LnW10O36]· 32H 2 O, Ln = Eu, Tb, abbreviated to LnW 10 ) and tetraethylenepentamine (TEPA) have been successfully prepared through a facile ionic self-assembly (ISA) method. The shape and petal size of the nanoflower as well as its fluorescent behaviors can be tuned through varying the ratio of EuW10/TbW10. The varied-temperature emission behaviors at 80−260 K show that the fluorescent intensity of both Tb3+ and Eu3+ decreased with the increase in temperature, which makes them potential luminescent ratiometric thermometers. Moreover, after being mixed with polydimethylsiloxane (PDMS), the as-formed hybrid films showed stable fluorescence along with good transparency. The robustness of the hybrid films was also demonstrated by corrosion resistance upon treatment with strong acid and alkali and thus can be used as a sensor to detect toluene circularly. Our results provide a new avenue to the facile construction of fluorescent composites and demonstrate that the POM complexes can be further used in supramolecular chemistry and nanomaterials.

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INTRODUCTION

assembly of the complementary color components onto a quartz plate.9 Polyoxometalates (POMs) are a well-known class of metaloxide clusters on nanoscale with controllable size and shape and high negative charges.10−15 Furthermore, lanthanide-containing POMs attract more attention in the fluorescence field due to their narrow emission bands, large Stokes shift, long lifetime, and tunable emission.16 To the best of our knowledge, Na9[LnW10O36]·32H2O (abbreviated to LnW10, including EuW10 and TbW10) have excellent fluorescent properties, in which the inorganic W5O186− ions have influence on the coordination geometry of Ln3+ and thus affect its emission properties.17,18 Moreover, as another focus area of research, self-assembly is an advanced nanotechnology for constructing novel nanostructures through noncovalent interactions such as electrostatic interaction, hydrogen bonding, hydrophobic interaction, van der Waals force, and π−π stacking.19−24 Well-defined hybrid self-assembly, especially those constructed from bio- and inorganic compositions, has become more useful and popular to integrate functions of different components25,26 and exhibits widespread potential applications in biological detection, electro-optical materials, catalysis science, and smart microreactors.26−33 Because lanthanide-containing POMs have

The development of fluorescent materials has attracted particular attention in recent years. Fluorescent compounds can be divided into organic molecules and inorganic nanomaterials and have applications in photonics, optoelectronics, and lighting.1,2 However, inorganic nanomaterials possess higher photostability and lower photobleaching than that of organic fluorescent materials,3,4 making them suitable for many systems. Generally, inorganic fluorescent nanomaterials contain quantum dots, up-converting nanoparticles, and lanthanidedoped nanoparticles. Especially, Ln-doped nanoparticles are commonly prepared by doping Ln ions into an inorganic or organic matrix to endow fluorescent properties to the nanoparticles, and the photoluminescence of trivalent lanthanide ions (Ln3+) is sharp and has pure color. To date, Eu3+ and Tb3+ as red and green emission components have been doped into many systems and coordinate the luminescence.5−9 Ma et al. successfully prepared nanocrystals with chirality-dependent tunable fluorescent properties through the coordination between terbium and aspartic acid (Asp); the fluorescence intensity of Tb−Asp increases linearly with the increased content of D-Asp.8 Yang et al. utilized the dynamic nature of reversible coordination polymers to control the mixing of red emissive Eu and green emissive Tb based coordination composites and then constructed white-light-emitting thin films with layer-by-layer © 2018 American Chemical Society

Received: January 28, 2018 Revised: March 2, 2018 Published: May 14, 2018 6367

DOI: 10.1021/acs.langmuir.8b00283 Langmuir 2018, 34, 6367−6375

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Scheme 1. Formation of Fluorescent EuW10/TbW10/TEPA Nanoflowers and Their Use for the Detection of Toluene Circularly

Figure 1. SEM images of morphological changes with different concentrations of EuW10 and TbW10 while the concentration of TEPA was fixed at 0.5 mg mL−1: (a−c) 2.0 mg mL−1 EuW10, (d−f) 1.5 mg mL−1 EuW10/0.5 mg mL−1 TbW10, (g−i) 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10, (j−l) 0.5 mg mL−1 EuW10/1.5 mg mL−1 TbW10, (m−o) 2.0 mg mL−1 TbW10. Chemistry Co., Ltd., and used as received. SYLGARD silicone elastomer 184 and the curing agent were purchased from Dow Corning Corp. (Midland, MI) and used without further purification. All the organic solvents we used were analytical reagents purchased from Tianjin Fuyu Fine Chemical Co., Ltd. Water with a resistivity of 18.25 MΩ cm used in this experiment was obtained using a UPH-IV ultrapure water purifier (China). Methods and Characteristics. Transmission electron microscopy (TEM) observation was observed on a JEM-1011 (JEOL) instrument at an accelerating voltage of 100 kV. Field-emission scanning electron microscopy (FE-SEM) images and element mapping analysis were carried out on a Hitachi SU8010 at 5.0 or 8.0 kV. High-resolution transmission electron microscopy (HR-TEM) images were acquired from a HRTEM JEOL 2100 system operating at 200 kV. The X-ray powder diffraction patterns (XRD) were measured on a D8 ADVANCE (Germany Bruker) diffractometer equipped with a graphite monochromator and Cu Kα radiation. Fourier transform infrared (FT-IR) spectra were recorded on an AlPHA-T spectrometer (Bruker Optics, Germany) with a range from 7800 to 370 cm−1. Confocal laser scanning microscopy (CLSM) was performed with a Panasonic Super Dynamic II WV-CP460 with an excitation wavelength at 488 nm. Polarized optical microscopy was conducted using an Axio Scope.A1 (Germany) microscope. The fluorescence spectra were recorded on a Lumina fluorescence spectrometer (Thermo Fisher). X-ray photoelectron spectroscopy (XPS) was conducted on an X-ray photoelectron spectrometer (ESCALAB250) with a monochromatized Al Kα X-ray source (1486.71 eV). The solid-state absolute fluorescence quantum yields were determined on a spectrofluorometer (FLSP920, Edinburgh Instruments Ltd.) equipped

poor processability in the solid state and water-quenched emission in the solution state,34,35 their applications in practice were severely obstructed. It is important to assemble lanthanide-containing POMs with other components mainly through electrostatic interactions between the extra cations and anions of the POMs, from which the fluorescent properties of lanthanide-containing POMs were fully developed.36,37 Herein the hierarchical fluorescent nanoflowers were constructed by mixing two Weakley-type lanthanide-containing POMs (EuW10 and TbW10) and tetraethylenepentamine (TEPA) through an ionic self-assembly (ISA) strategy, and the properties of these nanoflowers (especially solid-state photoluminescence studies, including emission spectra and quantum yield) have been systematically characterized by various techniques. Our results revealed that the photoluminescence of these nanostructures can be tuned by adjusting the ratio of EuW10/TbW10. Moreover, by incorporating EuW10/TbW10/TEPA nanoflowers into a polydimethylsiloxane (PDMS) matrix, the films possessed long-term stability and good transparency and showed excellent detection ability toward toluene (Scheme 1), which will open up a new vista in fluorescent materials science.

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EXPERIMENTAL SECTION

Chemicals and Materials. EuW10 and TbW10 were synthesized as described by Sugeta and Yamase.38 TEPA was purchased from Aladdin 6368


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Figure 2. Detailed characterizations of the sample of 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA. (a, b) SEM images of nanospheres with aging for 1 day; panel b is the local enlarged image of panel a. (c) SEM image of nanoflowers with aging for 2 weeks. (d) TEM image of a single nanoflower. (e) Local enlarged image of panel d. (f) HR-TEM (inset is SAED pattern of nanoflower petal), (g) CLSM, and (h) polarized optical microscopy images of nanoflowers. (i) EDX spectrum. with an integrating sphere, which consisted of a 120 mm inside diameter spherical cavity. The temperature-dependent emission spectra (from 80 to 260 K) were measured on an Oxford Instruments MercuryiTC with Optistat DN2 model liquid nitrogen thermostat, and the temperature-dependent emission spectra excited at 378 nm were recorded on an Edinburgh FLS 920 spectrometer with a picosecond pulsed diode laser (EPL-375, Edinburgh Instruments Ltd.) as the excitation light source. Sample Preparation of EuW10/TbW10/TEPA Hybrid Nanostructures. In this experiment process, 0.25 mL of TEPA aqueous solution (2 mg mL−1) was dissolved in 0.55 mL of water and then 0.1 mL of EuW10 aqueous solution (10 mg mL−1) as well as 0.1 mL of TbW10 aqueous solution (10 mg mL−1) was added with stirring. Then the samples rested for 2 weeks in a thermostat at 20.0 ± 0.1 °C to react thoroughly. At last, the white powder was collected for characterization by centrifugation, removing the upper phase, washing with deionized water three times, and freeze-drying in a vacuum extractor at −60 °C for 1 day. Fabrication of EuW10/TbW10/TEPA/PDMS Film. PDMS was prepared by mixing SYLGARD silicone elastomer 184 and the curing agent at a mass ratio of 10:1. About 0.6 mg of composite powder with 0.5 mL of acetone solution, a carrier of the composite, dispersed homogeneously using ultrasonication, which was put into 1.5 mL of PDMS and mixed thoroughly in a culture dish. The samples should age for 3 h with heat treatment at 50 °C in a horizontal position to remove solvent and finish the cross-linking reaction. The different types of composite materials can easily be fabricated into an ideal model with this method.

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of EuW10 and TbW10 was adjusted, the morphologies of the nanoflowers were still maintained, but the ductility of nanoflower petals decreased gradually when the ratio of TbW10 increased, which indicated that the addition of TbW10 can make the nanoflowers increasingly rigid. To get more information about the properties of nanoflowers, 1 mg mL−1 EuW10/1 mg mL−1 TbW10/0.5 mg mL−1 TEPA was selected as an example for further study. First, the effect of aging time on nanoflower formation was investigated. The adhesive rough nanospheres were first formed when the cultivation time was 1 day (Figure 2a,b), then nanospheres gradually transformed to hierarchical nanoflowers with smooth surface after 2 weeks (Figure 2c,d). Figure 2e shows a local partial enlarged TEM image of the nanoflower, and the HRTEM results (Figure 2f) clearly reveal that there are lots of dark spots in the petal which are assumed to be the clusters of EuW10 and TbW10. Moreover, a set of dispersive diffraction spots in a ring distribution can be observed from the selected area electron diffraction (SAED) pattern (inset of Figure 2f), confirming the crystalline structures of nanoflowers and that the oriented alignment of nanocrystals induced the quasi-singlecrystalline structure.40 The CLSM image (Figure 2g) reveals red fluorescence because of the larger luminous intensity of EuW10 in the nanoflowers, and the polarized optical microscopy image illustrates the anisotropic growth for nanoflower structure (Figure 2h). The energy-dispersive X-ray (EDX) spectrum (Figure 2i) and EDX elemental analysis (Figure 3) demonstrate that the elements Eu, Tb, and W, as well as C and N, are distributed in the nanoflower structure, which proved the successful hybridization of EuW10, TbW10, and TEPA. Furthermore, the EDX spectra and EDX elemental analysis of nanoflowers with other ratios of EuW10/TbW10 (Figures S1 and S2, Supporting Information) were also measured to verify the successful hybridization of EuW10, TbW10, and TEPA. FT-IR, XRD, and XPS Analysis. FT-IR and XRD vibration spectroscopy are important tools to characterize the structural alteration and confirm the interactions. FT-IR spectra of TEPA,

RESULTS AND DISCUSSION

Synthesis and Characterization of EuW10/TbW10/TEPA Nanostructures. We have realized the construction of monodisperse nanoflowers from 2 mg mL−1 EuW10/0.5 mg mL−1 TEPA previously.39 Here TbW10 was further introduced into this system, and the morphology evolutions with different ratios of EuW10 and TbW10 were studied. We fixed the total concentration of EuW10 and TbW10 at 2 mg mL−1 and varied their concentration while the concentration of TEPA was fixed at 0.5 mg mL−1, as shown in Figure 1. No matter how the ratio 6369


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corner, Oc is the bridged oxygen of two octahedra sharing an edge, and Od is the terminal oxygen.41 After the interaction between TEPA and EuW10/TbW10, these peaks moved to 938, 838, 755, and 617 cm−1 for nanoflowers (Figure 4B(c)), proving that the powerful driving force in the system may be electrostatic interaction and hydrogen bonding.42 Then the XRD patterns of EuW10, TbW10, and as-prepared EuW10/ TbW10/TEPA nanoflowers (Figure S3) were collected, which indicated that the EuW10 and TbW10 had well-defined structure with sharp Bragg reflections. Furthermore, new peaks were observed for the nanoflowers after the assembly with TEPA, demonstrating that the coassembled composite had a novel crystalline phase structure.13 To further study the existing state of chemical composition of the hybrid materials, XPS spectra were also analyzed. The binding energy of Eu3d and Tb4d are 1134.6 and 149.8 eV (Figure 4C,D), respectively, proving the +3 valence of Eu and Tb atoms.43 Figure 4E indicates that W atoms maintained their hexavalent state with the binding energy of W4f at 35.2 and 37.1 eV.44 As for the N1s XPS spectrum (Figure 4F), the peak centered at 398.5 eV can be the signal of alkylamines,45 and the other peak at 400.5 eV demonstrates the protonation of the amine groups in TEPA, which further attests to the existence of electrostatic or hydrogen bonding interactions between EuW10/ TbW10 and TEPA.46 The XPS survey spectrum of 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA is shown in Figure S4. Fluorescence Properties of EuW10/TbW10/TEPA Hybrid Nanoflowers. Rare earth element doped POMs possess excellent photoluminescent properties.47 To study the influence of EuW10/TbW10 ratio to tune the fluorescence performance, fluorescent images under UV light and the emission spectra of various EuW10/TbW10/TEPA hybrid materials were investigated. POMs that were used in our work have orange (EuW10) and green (TbW10) emission (Figure 5A). After the assembly with TEPA, we obtained white powder for all samples; however, the samples under UV-light (Figure 5A) showed emission changes with different components. The emission changed from orange, red, yellow to green under 254 nm UV-light, whereas it changed from orange, pink, red, pink to green under 365 nm UV-light. Figure 5B shows the corresponding CIE chromaticity coordinates (x, y). The colortunable luminescence mainly originates from the simultaneous emissions of Tb3+ and Eu3+ in the system individually. Pure EuW10 powder displays four main sharp emission bands (Figure 5C), exhibiting the characteristic transitions of Eu3+ ions, between 550 and 750 nm as follows: 5D0→ 7F1 at 594 nm, 5 D0 → 7F2 at 623 nm, 5D0 → 7F3 at 651 nm, and 5D0 → 7F4 at 699 nm; in the fluorescence spectrum of the pure TbW10 powder (Figure 5D), there are also four main sharp emission bands from 450 to 700 nm, caused by f−f* transitions of Tb3+ and were ascribed to 5D4 → 7FJ (J = 6-3) transitions at 491, 548, 587, and 624 nm. The POM fluorescence emission can be attributed to photoexcitation of the ligand-to-metal chargetransfer (O → W LMCT) bands that leads to hopping of the d1 electron, and then the intramolecular energy transfers from the O → W LMCT state to the 5D0 emitting state of Eu3+ ions and 5D4 emitting state of Tb3+ ions.48 After the assembly with TEPA, three characteristic sharp emission bands of Eu3+ ions moved to 617 nm (5D0 → 7F2), 654 nm (5D0 → 7F3), and 704 nm (5D0 → 7F4) for EuW10/TEPA hybrid materials (Figure 5C), and two characteristic sharp emission bands of Tb3+ ions moved to 594 nm (5D4 → 7F4), 617 nm (5D4 → 7F3) for

Figure 3. (a) SEM image of 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA hybrid nanoflowers. Energy-dispersive Xray (EDX) mapping analyses of nanoflowers: (b) Eu, (c) Tb, (d) W, (e) C, (f) N.

EuW10, TbW10, and EuW10/TbW10/TEPA hybrid nanoflowers were performed as shown in Figure 4A. The symmetric and

Figure 4. (A) FT-IR spectra (a) TEPA, (b) EuW10, (c) 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA, (d) TbW10. (B) Partial enlarged (500−1000 cm−1) spectra of A; XPS spectra for nanoflowers of 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA: (C) Eu, (D) Tb, (E) W, (F) N.

asymmetric stretching vibrations of CH2 in the TEPA alkyl chains were situated at 2846 and 2949 cm−1.18 The bands slightly shift after assembly with POMs. The C−N stretching vibration (1476 cm−1) of TEPA was retained whereas the NH2 group scissoring vibration (1574 cm−1) of TEPA disappeared after assembly with POMs. Moreover, the characteristic vibration bands (WOd, W−Ob−W, and W−Oc−W) for EuW 10 and TbW10 were listed in Figure 4B(b−d), Ob represents the bridged oxygen of two octahedra sharing a 6370


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Figure 5. (A) Sample photographs under daylight (upper), 254 nm UV-light (the second row), and 365 nm UV-light (down) for (a) EuW10, (b) 2.0 mg mL−1 EuW10/0.5 mg mL−1 TEPA, (c) 1.5 mg mL−1 EuW10/0.5 mg mL−1 TbW10/0.5 mg mL−1 TEPA, (d) 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA, (e) 0.5 mg mL−1 EuW10/1.5 mg mL−1 TbW10/0.5 mg mL−1 TEPA, (f) 2.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA, (g) TbW10. (B) CIE diagram corresponding to the materials of A at 378 nm excitation. (C, D) Fluorescence spectra of pure POMs powder and hybrid nanoflowers of different ratio POMs with 0.5 mg mL−1 TEPA at 378 nm excitation under room temperature.

decrease. With regard to EuW10/TbW10/TEPA, the rule is similar to that for EuW10/TEPA when the ratio of EuW10/ TbW10 is 1:1. However, the fluorescent intensity of EuW10 is much higher than that of TbW10 which may explain why the emission spectra only displayed the characteristic peaks of Eu3+ ions. So we further adjusted the concentration of EuW10/ TbW10 to 0.014/1.986 mg mL−1 (mol ratio at 1:142). The sample could emit the characteristic peaks both of Eu3+ and Tb3+ ions (Figure 6D), but the emission of Eu3+ ions still dominates the whole spectrum although the percent of EuW10 in the composite is very low, possibly because Eu3+ alters the electronic band structure of the solid. The linear fitting curve of the integrated intensity ratio for TbW10/TEPA was analyzed in detail as shown in Figure 6F. There is an excellent linear relationship between the integrated intensity of 5D4 → 7F5 (Tb3+) and the temperature from 100 to 260 K with the coefficient of determination (R2) at 0.99165. The linear trend of the fluorescence intensity at each temperature (I) and the intensity at the lowest temperature (I0) can be fitted as eq 1, which indicates that the regular temperature-dependent fluorescent emissions of TbW10/TEPA have enabled it to be a prominent candidate for luminescent ratiometric thermometers.

TbW10/TEPA hybrid materials (Figure 5D). When EuW10 and TbW10 are both coordinated with TEPA, the color of composites can be tuned by adjusting the ratio of EuW10/ TbW10. It is worth noting that the fluorescent intensity of TbW10 is much lower than that of EuW10, which is why the 5D0 → 7F2 characteristic emission bands of Eu3+ ions played a dominant role in the EuW10/TbW10/TEPA composites. Furthermore, on the basis of the fact that the red luminescence of 5D0 → 7F2 is hypersensitive to chemical bonds of Eu3+ ions in the microenvironment,49 the fluorescent intensity of hybrid materials showed a downward trend as the ratio of EuW10 decreased, leading to the transformation of emission (see Figure 5B with the CIE diagram). To quantitatively analyze the differences in the fluorescent emission of the samples, the quantum yield of the samples were measured (Table S1). It can be seen that the measured quantum yield was gradually reduced with the decrease of EuW10 proportion, and the quantum efficiency loss is inescapable on account that those acceptors usually have low quantum efficiency values.50 Thermal stability is a key parameter for phosphors,51−53 which has been accelerated in the development of luminescent thermometers.54−58 Therefore, the effect of temperature (80− 260 K) on the photoluminescent (PL) properties of EuW10/ TEPA, TbW10/TEPA, and EuW10/TbW10/TEPA composites was investigated. As Figure 6A−D show, the fluorescent intensity of both Tb3+ and Eu3+ in all hybrid materials decreases with the increase in temperature, which can be explained by the thermal activation of nonradiative-decay pathways.59 However, there are still some visible distinctions for the variation rate of fluorescent intensity, which are directly expressed in the integrated intensities of the 5D0 → 7F2 (Eu3+) and 5D4 → 7F5 (Tb3+) transitions (Figure 6E,F). The luminescent intensity of EuW10/TEPA decreased on an even keel in the range of 80− 180 K and then almost remained unchanged (180−260 K), which indicated that EuW10/TEPA has good stability toward temperature after 180 K, while the change of fluorescent intensity with temperature for TbW10/TEPA is a regular

I /I0 = 1.40428 − 0.00394T

(1)

Properties of EuW10/TbW10/TEPA/PDMS Film. To further explore the properties of fluorescent nanoflowers, it is better to embed the nanoflowers into the matrix to investigate its application performance. PDMS as a kind of polymeric material has wide applications because of its hydrophobic property, stability against heat and oxidation, nontoxicity, and noninflammability.60 Thus, we dispersed our EuW10/TbW10/ TEPA nanoflowers in PDMS to improve the environmental stability for a practical application, using a casting method to obtain round plates.61 As shown in Figure 7A(a), due to the small size and monodispersity of the nanoflowers, a transparent film was constructed which possessed good fluorescence 6371


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Figure 6. Emission spectra of (A) 2.0 mg mL−1 EuW10/0.5 mg mL−1 TEPA, (B) 2.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA, (C) 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA, (D) 0.014 mg mL−1 EuW10/1.986 mg mL−1 TbW10/0.5 mg mL−1 TEPA recorded between 80 and 260 K (excited at 378 nm). (E) Temperature-dependent integrated intensity of the 5D0 → 7F2 transition of EuW10/TbW10/TEPA composites. (F) Fitted curve of the integrated intensity ratio of the 5D4 → 7F5 transition from 100 to 260 K.

fluorescence intensity of the films (Figure S7, Figure 7B), we can draw a conclusion that toluene manifests an efficient quencher for the fluorescence of nanoflower films while other solvents have little effect on quenching. We speculate that the presence of toluene blocks the intermolecular luminescent resonant energy transfer between W5O186− and Eu3+/Tb3+ ion. It is very interesting that when toluene volatilizes fully, the fluorescence of the films can be recovered. Moreover, using this phenomenon, the film for the detection of toluene can be reused for at least six cycles with only a slight decrease of fluorescence intensity, suggesting their promising applications in the sensing of toluene (Figure 7C). Furthermore, the detection of toluene still plays a role in mixtures of toluene and other solvents. For example, in a toluene/ethanol mixture, the degree of quenching gradually increased as the ratio of toluene in the mixed solvent increased (Figure S8), demonstrating that the detection of toluene in mixed solvents is selective. The

intensity under UV light with tunable colors (Figure 7A(b)). Furthermore, the durability of the film to withstand the corrosion of strong acid and alkali was also studied. When the films were soaked at pH = 0, 7, and 13 for 6 days, high fluorescence intensity was still maintained (Figure S6). It is known that the luminescence property of Eu3+/Tb3+ ion can be affected by the neighboring environment which is closely related to its symmetry,62 and maybe influences the fluorescence of our nanoflower films in the presence of various solvents. Consequently, the chemical-sensing performance of the nanoflower films was investigated to find the potential application as luminescent sensors.63 Take the 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA nanoflower films as an example: dipping the films in different organic solvents (DMF, DMSO, cyclohexane, CHCl3, methanol, ethanol, toluene, and acetone), by investigating the films under 254 nm UV-light (Figure 7A(c)) and measuring the 6372


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Figure 7. (A) Sample photographs of films under (a) daylight, (b) 254 nm UV-light, and (c) 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA/PDMS films after dipping with different organic solvents. (B) Fluorescence spectra of PDMS films and 1.0 mg mL−1 EuW10/1.0 mg mL−1 TbW10/0.5 mg mL−1 TEPA/PDMS films in the presence of different organic solvents at 260 nm excitation. (C) The modulation of fluorescence intensity at 617 nm for cycle detection of toluene using nanoflower films.

detection limit of toluene, which can be defined as a change of 10% in the fluorescence intensity,64 was calculated to be ca. 3.005 mg mL−1 according to the linear fitting of the relative fluorescence intensity (I/I0, I is the fluorescence intensity of different contents of toluene in ethanol solvent while I0 is the initial fluorescence intensity of films) with toluene content (Figure S9).

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AUTHOR INFORMATION

Corresponding Authors

CONCLUSION In summary, the construction of hierarchical nanoflowers by a cationic component (TEPA) and mixed Weakley-type POMs (EuW10/TbW10) through ISA strategy was reported, and the morphologies, fluorescence properties, and applications of nanoflower/PDMS films were investigated in detail. The results indicated that the fluorescent intensity showed a downward trend as the ratio of EuW10 decreased in the hybrid nanoflowers and that the emission can be tuned through varying the ratio of EuW10/TbW10. Moreover, the EuW10/TEPA composites displayed good stability toward temperatures above 180 K while the fluorescent emissions of TbW10/TEPA composites decreased linearly with an increase in temperature in the range of 100−260 K, which may be used as luminescent ratiometric thermometers. Most importantly, the transparent nanoflower/ PDMS films with corrosion resistance to strong acid and alkali provide huge advantages for the circular detection of toluene. Our results provide a new class of fluorescent materials with unfathomable potential in optical applications and chemical sensing.

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fluorescence spectra of PDMS films in the presence of different organic solvents, photographs of films at different pH values and different ratios of toluene in ethanol solvent, and the linear fitting of the relative fluorescence intensity (I/I0) with toluene content (PDF)

*E-mail: [email protected]. Phone: +86-531-88363597. Fax: +86-531-88361008. *E-mail: [email protected]. Phone: +86-531-88364218. Fax: +86-531-88564750. ORCID

Di Sun: 0000-0001-5966-1207 Xia Xin: 0000-0002-4886-6028 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21201110, 21571115) and Young Scholars Program of Shandong University (2016WLJH20).

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REFERENCES

(1) Akkerman, Q. A.; D’Innocenzo, V.; Accornero, S.; Scarpellini, A.; Petrozza, A.; Prato, M.; Manna, L. Tuning the Optical Properties of Cesium Lead Halide Perovskite Nanocrystals by Anion Exchange Reactions. J. Am. Chem. Soc. 2015, 137, 10276−10281. (2) Zhang, C.; Wang, B.; Li, W.; Huang, S.; Kong, L.; Li, Z.; Li, L. Conversion of invisible metal-organic frameworks to luminescent perovskite nanocrystals for confidential information encryption and decryption. Nat. Commun. 2017, 8, 1138. (3) Ma, B.; Zhang, S.; Qiu, J.; Li, J.; Sang, Y.; Xia, H.; Jiang, H.; Claverie, J.; Liu, H. Eu/Tb codoped spindle-shaped fluorinated hydroxyapatite nanoparticles for dual-color cell imaging. Nanoscale 2016, 8, 11580−11587.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b00283. EDX spectra and EDX elemental analysis of hybrid nanosflowers, XRD spectra, XPS survey spectrum, quantum yield, and fluorescence spectra of the samples, 6373


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