Article pubs.acs.org/Langmuir
Multi-stimuli Responsive Supramolecular Structures Based on Azobenzene Surfactant-Encapsulated Polyoxometalate Yongxian Guo,† Yanjun Gong,† Yan’an Gao,‡ Jianhong Xiao,§ Tao Wang,§ and Li Yu*,† †
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, P.R. China China Ionic Liquid Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China § Petroleum Engineering Technology Research Institute of Shengli Oilfield, Sinopec, Dongying 257000, P.R. China ‡
S Supporting Information *
ABSTRACT: Multi-stimuli responsive materials have attracted intense attention as extensive application prospect in many fields, yet achievement of multi-stimuli responsiveness remains a challenge. Herein, we report a tri-stimuli responsive supramolecular structure fabricated by a cationic surfactant, 4ethyl-4′-(trimethylaminohexyloxy) azobenzene bromide (ETAB), and anionic Eu-containing polyoxometalates (EuPOM), based on an ionic self-assembly (ISA) strategy. Following different responsive mechanisms, the resultant ETAB/Eu-POM supramolecular materials are responsive to UV light, pH, and Cu2+, respectively. The response to UV irradiation is based on the configuration change of azobenzene molecules. The response to H+ can be attributed to the formation of a hydrogen bond W−O···H···O−H among Eu-POM, H+, and H2O, which blocks the energy transfer pathway from O → W, while the coordination interaction between Cu2+ and Oc (bridged oxygen of two octahedra sharing an edge in the EuPOM molecule) causes the response to Cu2+. The multi-stimuli responsive characteristics for the ETAB/Eu-POM supramolecular structures maybe provide a potential application in ultraviolet detection, optical storage devices, and chemical substance sensors, etc.
1. INTRODUCTION Smart materials have aroused considerable attention because of their promising application in drug delivery, sensors, probes, memory storage devices, etc.1−6 Recently, there have been many reports on smart materials whose properties have been established on the responsiveness to surrounding environmental changes, including light, temperature, redox agents, pH, CO2, magnetism, and enzymes.7−11 The responsiveness is benefited from the particular functional groups, such as styryl, polyethylene glycol, ferrocenyl, and carboxyl groups, and so on.12−15 Compared to the singular- or dual-responsive materials, smart materials with multi-stimuli responsiveness would have superior adaptability to various complex environments.16 However, the fabrication of multi-stimuli responsive materials still presents some challenges. To date, most multi-stimuli responsive materials have been constructed mainly via covalent bonds. These synthetic processes are generally expensive or time-consuming to some extent.17,18 On the basis of electrostatic interaction and other weak interactions (e.g., π−π stacking interaction, H-bond, charge transfer interaction, etc.), the ionic self-assembly (ISA) strategy provides a potent and versatile pathway to fabricate well-defined, discrete supramolecular architectures from oppositely charged molecules.19,20 It has made a great influence due to generalizability, simplicity, and cheapness. © 2016 American Chemical Society
Among the responses of supramolecular assemblies to diversified external stimuli, light-responsive supramolecular materials have attracted much attention, as light is a rapid, facile, reversible, and noninvasive external stimulus that endows possibilities for special functional materials, for example, selfhealing and shape-memory materials.21 There are various photoresponsive organic molecules (e.g., azobenzene, stilbene, spiropyran, and their derivatives) that act as photoswitching building blocks to manipulate assemblies in many systems.4 Among them, azobenzene-based molecules could undergo reversible trans- to cis-conformational change triggered by UV and visible light irradiation without any side reactions. Therefore, they have been extensively used in constructing various supramolecular functional architectures with the variable structures and properties.22,23 Polyoxometalates (POMs), as a large family of polyanion transition metal oxide clusters, are widely employed in fabricating various supramolecular assemblies with unique catalytic, electrochemical, and optical properties.24 Recently, lanthanide-containing polyoxometalates have aroused progressive attention and interest especially due to their excellent Received: July 9, 2016 Revised: August 17, 2016 Published: August 22, 2016 9293
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used without further purification. Triply distilled water was used to prepare aqueous solutions. 2.2. Preparation of ETAB/Eu-POM Supramolecular Structures. The 4-ethyl-4′-(trimethylaminohexyloxy) azobenzene bromide (ETAB) molecule was synthesized by a modified procedure according to the published work.30,31 On the basis of the processes reported by Sugesta and Yamase,32 Na9EuW10O36·32H2O (Eu-POM) was synthesized. The supramolecular structures were prepared by a one-step synthetic method: mixing the aqueous solutions of ETAB (0.5 mM, 9 mL) and Eu-POM (0.5 mM, 1 mL) under stirring for 10 min. A colloidal solution was obtained. The solid products were collected by centrifuging the complex solution and washed three times with water to remove the salts and possible precursors. Then, the solid products were dried under a vacuum at 55 °C for 24 h. The UV irradiation (365 nm) experiments were carried out using a light source of high-pressure mercury lamp (500 W), and the distance of the lamp from the sample was kept at 10 cm. Samples were exposed to air during irradiation. The experiments of pH response were performed using a pH meter (Shanghai INESA Scientific Instrument Co., Ltd.) to measure the real pH value. The pH was adjusted by diluted hydrochloric acid. 2.3. Characterization and Measurement of Supramolecular Structures. Transmission electron microscopy (TEM, JEM-100CX II (JEOL)) was used to characterize the microcosmic structures. The sample was made by putting a drop of solution on the carbon-coated copper grid and removing the excess solution with filter paper after 5 min. 1H NMR was measured using a Bruker AV-300 NMR spectrometer at 25 °C. Tetramethylsilane and deuterated dimethyl sulfoxide (DMSO) acted as an internal reference and solvent, respectively. Field emission scanning electron microscopy (FE-SEM, Hitachi SU8010) was used to observe the morphology of supramolecular structures. A layer of gold was sputtered on the samples to enhance the conductivity. UV−vis spectra were measured by a U-4100 (Hitachi) instrument in a quartz cell, and the light path was 1 mm. FTIR spectra were measured by a VERTEX-70/70v FTIR spectrometer (Bruker Optics, Germany), and the data were recorded between 4000 and 400 cm−1 on pressed thin KBr sample disks. Smallangle X-ray scattering (SAXS) was performed with an Anton-paar SAX Sess mc2 system with Ni-filtered Cu Kα radiation (1.5406 Å) operated at 50 kV and 40 mA. The fluorescence measurements were conducted on a Hitachi F-4500 fluorospectro photometer at room temperature. The excitation source was a Xe lamp, and the excitation wavelength was 280 nm. The diameter of particles was obtained by a laser particle analyzer (Delsa Nano_C, Beckmann, US) at 25 °C.
photoluminescent properties, such as narrow emission bands, large Stokes shift, long lifetime, and tunable emission.25,26 However, the super water solubility of POMs limits their processability in material science, so organic moleculeencapsulated polyoxometalates were investigated. Yao’s group constructed a transparent and flexible self-supporting [EuW10O36]9−-agarose nanocomposite thin film, which exhibited chemically responsive luminescent switching for acid/ base gas.27 Wu and his partners prepared polymerizable surfactant-encapsulated POM clusters (SECs), and after the subsequent copolymerization, they obtained a POM-based hybrid polymer with highly transparent and luminescent properties.28 Wan et al. fabricated core-shell nanostructures using Eu-containing POM and double-hydrophilic block copolymers in aqueous solution.29 Herein, tri-stimuli responsive supramolecular nanostructures were designed on the basis of a cationic azobenzene-containing surfactant, viz., 4-ethyl-4′-(trimethylaminohexyloxy) azobenzene bromide (ETAB) and a sandwich-type Eu-containing polyoxometalate (Eu-POM), via the ionic self-assembly (ISA) strategy. The resulting hybrid supramolecular structures integrate the excellent properties of ETAB and Eu-POM, and demonstrate three responses: UV light irradiation, pH, and Cu2+ (Scheme 1). The photoresponsive property of the ETAB/ Scheme 1. Schematic Diagram of the Formation of ETAB/ Eu-POM Supramolecular Materials and Their Responsiveness to UV Light, Cu2+, and H+
3. RESULTS AND DISCUSSION 3.1. Assembly of ETAB/Eu-POM Supramolecular Hybrids. The Eu-containing polyoxometalate (Eu-POM) with sandwich structure possesses some anionic sites to interact with azobenzene-containing cationic surfactant, ETAB. The ETAB/Eu-POM hybrid supramolecular structures were obtained by a one-step method: mixing an appropriate volume ratio of ETAB (0.5 mM) and Eu-POM (0.5 mM) aqueous solutions under stirring. The complex solution manifested a typical colloidal state and an obvious Tyndall effect (Figure 1a). However, when the volume ratio was inadequate, only some precipitates were obtained. The microscopic morphology of the colloidal solution was observed by transmission electron microscopy (TEM). The obtained monodisperse nanoparticles have a diameter of ca. 200 nm (Figure 1b). A laser particle analyzer was then used to monitor the size distribution of supramolecular nanoparticles in solution. As shown in Figure 1c (black plots), the distribution of the nanoparticle diameter was centralized at about 200 nm, which was in agreement with the results of TEM. To explore the interaction during the formation of the supramolecular
Eu-POM composite materials is caused by the cationic azobenzene-containing surfactant ETAB, whose trans/cis isomerization behavior is responsible for the variation of morphology and luminescence intensity upon UV light irradiation. Beyond that, with increasing acidity of the aqueous solution, the morphology of the structures translated from monodisperse nanoparticles to continuous necklace-like crosslinked nanoparticles, and the fluorescence signal was quenched distinctly. Moreover, by adding Cu2+ into the hybrid solution, the fluorescence intensity was also reduced obviously. The latter two responses can be attributed to the existence of EuPOM, as a result, the generation of hydrogen bond and coordination interaction between Oc of Eu-POM and Cu2+.
2. EXPERIMENTAL SECTION 2.1. Materials. 4-Ethylaniline (99%), 1,6-dibromohexane (97%), and NaNO2 (99%) were purchased from Aladdin Chemistry Co., Ltd. NaOH, HCl, ethanol, and phenol were obtained from Sinopharm Chemical Reagent Co., Ltd. Hexadecyl trimethylammonium bromide (99%), europoum nitrate hexanydrate (99%), and sodium tungstate dehydrate (99%) were all purchased from J&K Chemical Technology, China. All of the above chemical reagents were analytical grade and 9294
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Figure 1. (a) Digital photo of the ETAB/Eu-POM complex solution with the Tyndall effect before (left) and after (right) UV light irradiation for 30 min. The red pathway is caused by laser pointer irradiation. (b) TEM image of ETAB/Eu-POM supramolecular materials at pH 8.0. (c) Laser particle analyzer spectra before and after UV light irradiation for the ETAB/Eu-POM hybrids. (d) UV−vis absorption spectra of ETAB and ETAB/ Eu-POM complexes.
Figure 2. (a) Chemical structures of ETAB (left) and Eu-POM (right) molecules. (b) SAXS spectrum of ETAB/Eu-POM hybrid materials. (c) The simulated internal structure of ETAB/Eu-POM particles.
inexistence of π−π stacking interaction during the construction process of nanoparticles. Fourier transform infrared (FTIR) spectroscopy is a powerful tool to evaluate the possible structural change of molecules and arrangement of hydrocarbon chains in the composite. Figure S1 shows the FTIR spectra of Eu-POM (black line) and ETAB/Eu-POM (red line). The characteristic vibration bands of Eu-POM at 945 and 843 cm−1 are assigned to ν (WOd) and ν (W−Ob−W), respectively, and 783 together with 705 cm−1 are ascribed to ν (W−Oc−W), where Ob and Oc represent the bridged oxygen atoms of two octahedra sharing a corner and an edge, respectively, and Od denotes the terminal oxygen.35 For the ETAB/Eu-POM
structures, UV−vis absorption, Fourier transform infrared (FTIR), and small-angle X-ray scattering (SAXS) spectroscopies were used. Since azobenzene-derived surfactants have a superior πelectron conjugated group, there are many reports to investigate π−π stacking interaction during the formation of different nanostructures.33 To testify if π−π stacking interaction exists during the assembly process of this system, UV−vis absorption spectra (Figure 1d) were determined. For ETAB solution and ETAB/Eu-POM hybrid solution, absorption peaks at 350 and 420 nm are corresponding to the π−π* and n−π* electron transitions, respectively.34 There is no obvious shift observed in the absorption band at 350 nm, revealing the 9295
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Figure 3. (a) TEM images of ETAB/Eu-POM supramolecular materials after UV light irradiation for 30 min. Fluorescence (b) and UV−vis absorbance (c) spectra of ETAB/Eu-POM composites before and after UV irradiation with different time intervals. (d) Fluorescence spectra of the CTAB/Eu-POM complex before and after UV irradiation for 30 min.
supramolecular assemblies, the characteristic bands of Eu-POM still exist, suggesting that its structure remains intact in the hybrid materials. However, these bands shift to 936, 846, 788, and 709 cm−1, respectively. The slight shift illustrates the intense electrostatic interaction between Eu-POM and ETAB molecules. In addition, the bands appearing at 2939 and 2873 cm−1 can be ascribed to νas(CH2) and νs(CH2), indicative of the weak alignment of hydrophobic chains.36 To further identify the arrangement of hydrophobic chains and the interlamellar distance of the ETAB/Eu-POM hybrid materials, the small-angle X-ray scattering (SAXS) spectrum was measured. As illustrated in Figure 2b, one sharp peak and a broad peak appear, which further prove that orientation of the hydrophobic chains of ETAB in the supramolecular hybrids is weak. This result is identical to that of FTIR spectra. Additionally, we can calculate the lamellar distance for the ETAB/Eu-POM supramolecular materials is about 4.49 nm, based on the sharp peak at 1.40 nm−1. In our previous work, the length of the trans-ETAB molecule evaluated by the density functional theory (DFT) method is about 2.23 nm.31 The longitudinal and transverse lengths of Eu-POM are about 1.44 and 0.8 nm, respectively. Then, the highest outstretched length value of the ETAB/Eu-POM supramolecule is calculated to be about 5.26 nm, which is higher than the lamellar distance calculated from the SAXS results. This illustrates that there are overlapping regions between two adjacent supramolecules, but it is insufficient to provide a condition of π−π stacking interaction, which has been identified by UV−vis absorption spectra. Besides, the azobenzene surfactant-encapsulated Eu-POM materials also display a superior emission property. The photoluminescent property of the hybrid supramolecular structures was measured by the excitation wavelength at 280
nm, corresponding to intramolecular energy transfer from the ligand-to-metal charge transfer (LMCT) band of O → W to the photoluminescent Eu3+ core.29 As shown in Figure S2, the emission intensity of ETAB/Eu-POM solution is much stronger than that of Eu-POM aqueous solution. As reported,32 the emission intensity of the Eu-POM aqueous solution is very weak. The main reason is that the emission of Eu3+ is highly related with its coordinated water, the radiationless deactivation of the 5D0 excited state through coupling with the vibrational states of high-frequency O−H oscillators of water ligands. However, when ETAB-encapsulated Eu-POM hybrids are produced, the emission intensity increases obviously, because the electrostatic interaction of cationic ETAB with Eu-POM molecules is strong enough to replace water ligands around Eu3+. Additionally, due to the hydrophobic interaction among the alkyl chains of ETAB molecules, a relative hydrophobic environment for Eu-POM can also be provided to remarkably enhance the fluorescence intensity of the hybrids. 3.2. UV Light-Induced Morphology and Fluorescence Change of ETAB/Eu-POM Hybrids. It is well-known that the azobenzene-containing molecules have a superior responsive property to UV light irradiation; we wondered whether the supramolecular structures still maintained this property. After irradiation for 30 min in UV light (365 nm), it was observed that the color of the ETAB/Eu-POM solution turned deeper, and the turbidity decreased (see Figure 1a). Parts a and b of Figure 3 illustrate the change of morphology and emission property after UV light irradiation. TEM images (Figure 3a) show the as-prepared ETAB/Eu-POM nanospheres with a diameter of about 180 nm (the inset representing the magnified image). The particle size was also verified by the laser particle analyzer spectra (Figure 1c, red points and line). Obviously, the shape of the ETAB/Eu-POM supramolecular materials after 9296
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3.3. pH-Induced Morphology and Fluorescence Change of ETAB/Eu-POM Hybrids. There are some reports about the hybrid materials built by organic substances and POMs, which always possess the quality of response to H+.35,38 Therefore, we then investigated if the ETAB/Eu-POM hybrids fabricated in this work could undergo variations in the presence of H+. As depicted in Figure 4, the morphology of ETAB/Eu-
UV light irradiation is similar to that without UV light irradiation (Figure 1b). However, there are discrepancies in the size and dispersity of the nanoparticles. After UV irradiation for 30 min, it is observed that the diameter of the spherical aggregates decreased from ∼200 to ∼180 nm, accompanied by an increasing dispersity (Figure 1c). Therefore, the ETAB/EuPOM supramolecular structures still possess the photoresponsive property, and their micromorphology can be distinctly affected by UV light irradiation. Variation in the hybrid nanostructures can feasibly give rise to changes in their properties. Figure 3b illustrates the remarkable difference in the fluorescence property of ETAB/ Eu-POM hybrids before and after UV irradiation. More specifically, with a prolonged irradiation time of UV light, the emission intensity of the supramolecular structures decreases acutely. The extent of fluorescence quenching is close to equilibrium with 5 min of UV irradiation. Absorption spectra were used to monitor the transformation of the azobenzene group when the supramolecular structures were irradiated by UV light (Figure 3c). As can be seen from Figure 3c, the absorption peak of ETAB around 350 nm, which can be assigned to π−π* electron transition, decreases dramatically after UV irradiation, but the band intensity at about 420 nm ascribed to n−π* electron transition is strengthened slightly. The absorbance change of characteristic bands for the supramolecular hybrids indicates the transformation of ETAB molecules from trans- to cis-states after UV irradiation. Additionally, when the UV irradiation time was elongated to more than 5 min, the absorption spectra were almost constant, which suggests a photostationary state was established within a certain range of time. From the foregoing, changes in the morphology and properties of the ETAB/Eu-POM hybrids are due to the UVinduced photoisomerization behavior of ETAB molecules. As reported previously,37 trans-ETAB is more hydrophobic than cis-ETAB, which can reduce the hydrophobic interaction among the hydrophobic chains in the supramolecular units. Furthermore, compared to the trans-state, steric hindrance of the cis-conformation results in a looser alignment of ETAB molecules under UV light irradiation. The two reasons mentioned above lead to the more hydrophilic microenvironment around the luminescence center Eu3+, concomitantly with the quenched fluorescence intensity of ETAB/Eu-POM. According to the report in ref 27, the photoreduction of the W element in POMs can give rise to the occurrence of fluorescence quenching upon UV irradiation. Therefore, it is necessary to elucidate whether it was for this reason or not that the fluorescence of ETAB/Eu-POM supramolecular structures was quenched. Subsequently, we used cetyltrimethylammonium bromide (CTAB), an azobenzene-free cationic surfactant with similar chemical structure to ETAB, in substitution for ETAB to carry out a contrast test. CTAB/Eu-POM supramolecular materials were constructed according to the same preparation method as ETAB/Eu-POM hybrid structures. As depicted in Figure 3d, the fluorescence spectrum of CTAB/Eu-POM assemblies shows no obvious change after UV irradiation, which indicates that the photoreduction of W (VI) to W (V) in Eu-POM cannot trigger a reduction in fluorescence intensity. This provides indirect evidence that the photoisomerization of azobenzene groups was mainly responsible for the fluorescence quenching mechanism of ETAB/Eu-POM supramolecular structures upon UV irradiation.
Figure 4. TEM and SEM images of ETAB/Eu-POM hybrids obtained at pH 5.0 (a, c) and pH 3.8 (b, d).
POM hybrids changed considerably with the decreasing pH value of aqueous solution. The ETAB/Eu-POM disperse nanoparticles formed at pH 8.0 (Figure 1b) coalesced by twos and threes at pH 5.0 (Figure 4 a, c), and further transformed to continuous necklace-like nanoparticles at pH 3.8 (Figure 4b,d and Figure S3). Furthermore, as can be seen from Figure 5, when the pH of the supramolecular solution was adjusted with diluted hydro-
Figure 5. Fluorescence spectra of ETAB/Eu-POM at different pH values.
chloric acid, the fluorescence peaks located at 590 and 620 nm of ETAB/Eu-POM structures were quenched distinctly, respectively. The correlation between the luminescent intensity of ETAB/Eu-POM hybrids at 620 nm and the pH value of aqueous solution is shown in Figure S4. Obviously, the degree of fluorescence quenching is very remarkable when the pH value is adjusted from 8.0 to 6.0, and after that, it changes gently with the reduction of pH value. 9297
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Langmuir As we all know, most azobenzene-based molecules could accept H+ and change their structures and colors, such as Methyl Orange. Polyoxometalate, as a transition metal cluster, is unstable in acidic solution, and easily decomposed.39 Therefore, for ETAB/Eu-POM hybrids, either ETAB or Eucontaining polyoxometalate molecules may be affected by pH. First, to distinguish whether the pH-responsive fluorescence properties of ETAB/Eu-POM materials were actually triggered by ETAB molecules, we measured the UV−vis absorption spectra of ETAB/Eu-POM hybrids under different pH conditions. In Figure S5, the absorption spectra at different pH values around 300−600 nm are basically similar, only with a slight decline in the characteristic band (365 nm), which may be caused by the addition of a small amount of water during adjustment of the pH. These results proved that the azobenzene groups in these supramolecular hybrids have not accepted H+ and changed their structures during the pH adjustment. Then, to research if the structure of Eu-POM in ETAB/Eu-POM supramolecular structures persisted during pH alteration, we obtained the supramolecular structures solid altering with H+ by the centrifugal method. From the FTIR spectra of ETAB/Eu-POM materials at different pH values (Figure S6), the four characteristic bands of Eu-POM exist just with a slight shift, which implies that the Eu-POM structure still remained in the hybrids. The slight shift can be attributed to the formation of a hydrogen bond W−O···H···O−H among Eu-POM, H+, and H2O. Meanwhile, the bands at 2873 and 2939 cm−1 assigned to vibration peaks of CH2 maintain well, indicative of the unchanged arrangement of hydrophobic chains in the hybrid composites.36 In this case, then how does H+ affect the fluorescence property of the system? As reported,40 the emission mechanism of Eu-POM could be divided into three steps. In the first step, the O → W ligand metal charge transfer (LMCT) upon photoexcitation leads to the hopping of d1 electron, coupled with released energy through the deactivated recombination by electron and hole.25 Then, the newly formed hydrogen bond of W−O···H···O−H could block the second step of the photoluminescent process, viz., the energy transfer pathway from the O → W LMCT state to the 5D0 emitting state of Eu3+.25,38 Consequently, fluorescence quenching occurs when regulating the pH value of the supramolecular solution with diluted hydrochloric acid. 3.4. Cu2+-Induced Fluorescence Quenching of ETAB/ Eu-POM Hybrids. It is known that Cu2+ is a good quencher due to the easy occurrence of energy/electron transfer.41 In addition to the dual responses to UV light and pH values, the ETAB/Eu-POM supramolecular structures were also found to be sensitive to Cu2+. To minimize the effect of counterions, we chose CuBr2 solution as the addition agent. Just as depicted in Figure 6, the fluorescence spectra (excitation wavelength, 280 nm) of supramolecular structures were evidently quenched in the presence of Cu2+. The intensity of bands at 590 and 610 nm decreases rapidly when the concentration of Cu2+ increases from 1.4 μM to 1.4 mM. To reveal the quenching mechanism, FTIR spectra were used to discuss the structural change of ETAB/Eu-POM supramolecular hybrids after addition of Cu2+. As shown in Figure S7, the characteristic bands of Eu-POM at 846 and 936 cm−1 are unchanged, while the patterns at 788 and 709 cm−1 assigned to ν (W−Oc−W) shift to 791 and 710 cm−1, respectively. The small shift may be caused by the coordination interaction between Cu2+ and Oc (the bridged oxygen of two octahedra sharing an edge). On the basis of the
Figure 6. Fluorescence spectra of ETAB/Eu-POM hybrid solution in the presence of different concentrations of Cu2+ (from top to bottom, in order, the concentrations are 0, 1.4 μM, 14 μM, 140 μM, and 1.4 mM, respectively).
luminescence principle of Eu-POM, we could surmise the fluorescence quenching mechanism of ETAB/Eu-POM supramolecular structures upon addition of Cu2+ as follows: the energy released from the O → W LMCT excited state transfers to Cu2+ instead of the 5D0 emitting state of Eu3+, and the next step of the 5D0 excited state relaxing to the 7Fj ground state will be prevented.32,40
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CONCLUSIONS In summary, the tri-stimuli responsive ETAB/Eu-POM supramolecular materials were successfully fabricated in aqueous solution by the ionic self-assembly (ISA) strategy. Electrostatic interaction, van der Waals force, and hydrophobic interactions are proved to be the main driving forces during the formation of ETAB/Eu-POM nanoparticles. Compared to Eu-POM, the resultant materials display enhanced photoluminescence property due to the hydrophobic environment supplied by the ETAB molecules encapsulated on the surfaces of Eu-POM. The fluorescence quenching of ETAB/Eu-POM upon UV irradiation can be ascribed to the photoresponsive behavior of ETAB molecules, leading to their weakened hydrophobic interaction and alignment tightness. As for ETAB/Eu-POM hybrids responding to pH and Cu2+, the nature of the fluorescence intensity reduction was attributed to the formation of hydrogen bond and coordination interaction, respectively, as a result of structural alteration of the assemblies. On the basis of the multiresponsive properties, the supramolecular materials investigated in this work may open a pathway for their potential applications in some fields, such as ultraviolet detector, optical storage devices, and chemosensors.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02539. Additional figure and supporting data (PDF)
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Corresponding Author
*Phone: +86-531-88364807. Fax: +86-531-88564750. Notes
The authors declare no competing financial interest. 9298
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 21373128), Scientific and Technological Projects of Shandong Province of China (No. 2014GSF117001), and the Natural Science Foundation of Shandong Province of China (No. ZR2011BM017).
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DOI: 10.1021/acs.langmuir.6b02539 Langmuir 2016, 32, 9293−9300
