Zwitterionic Surfactant Micelle-Directed Self-Assembly of Eu

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Interface-Rich Materials and Assemblies

Zwitterionic Surfactant Micelle-Directed Self-Assembly of Eu-Containing Polyoxometalate into Organized Nanobelts with Improved Emission and pH Responsiveness Nana Lei, Lei Feng, and Xiao Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00261 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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Zwitterionic Surfactant Micelle-Directed Self-Assembly of Eu-Containing Polyoxometalate into Organized Nanobelts with Improved Emission and pH Responsiveness Nana Lei, Lei Feng, Xiao Chen* Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, China

*Corresponding author: Xiao Chen Address: Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, China E–mail: [email protected]. Tel.: +86–531–88365420. Fax: +86–531–88564464.

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Abstract Recently, hybrid co-assembly between polyoxometalates (POMs) and cationic building blocks provides an efficient strategy to greatly optimize POMs’ functionality as well as their aggregate structural diversity. Adaptive hybrid supramolecular materials with enhanced luminescence have then been obtained from lanthanide-containing POMs. In this work, a commercially available and pH-switchable zwitterionic surfactant, tetradecyldimethylamine oxide (C14DMAO), was chosen to co-assemble with a lanthanide-containing anionic POM, (Na9(EuW10O36)·32H2O, abbreviated as EuW10) in water. The much improved red-emitting luminescent nanobelts at a C14DMAO/EuW10 molar ratio (R) of 20 were obtained, which exhibited longer luminescence lifetime and higher quantum yield compared with EuW10 aqueous solution. After careful characterization of morphology and structure of nanobelts, an unusual axial lamellar aggregation arrangement mechanism was proposed. It was the partial protonation of C14DMAO at the solution pH of about 6.5 that led to positively charged micelles, being bridged by anionic EuW10 clusters to aggregate into such novel nanobelts under the synergetic effects of appropriate electrostatic, hydrogen bonding and hydrophobic interactions. The resulted pH responsive luminescent nanobelts and their aggregation model should offer attractive references for preparing smart optical supramolecular materials. Keywords: Eu-containing polyoxometalate; zwitterionic surfactant; micelle; luminescent nanobelt; co-assembly;

pH


responsiveness

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1. Introduction Multifunctional materials can be easily fabricated through supramolecular self-assembly based on noncovalent forces, such as electrostatic, hydrogen bonding, π-π stacking, charge-transfer, and host-guest interactions.1,2 The rational combination of building blocks may result in smart nanostructures with multi-stimuli responsiveness (pH, light, magnet, temperature, and other external stimuli).3-5 Especially, hybrid co-assemblies between inorganic and organic components have received considerable attention recently for designing supramolecular materials with structural and functional diversity.6-8 As one type of inorganic luminophore, lanthanide compounds have gained extensive interests for preparing luminescent materials because of their narrow emission bands, long lifetime, high photoluminescent efficiencies, large Stokes shifts, and tunable emissions.9,10 Except for their widespread investigations in solid state,11 they also exhibited great potential applications in bulk solutions or supramolecular soft materials.12-14 Generally, organic ligands which can coordinate or chelate with lanthanide (III) ions are needed to activate the energy transfer due to the Laporte-forbidden for f-f transition of the latter.9,10 Therefore, lanthanide complexes have been well-investigated for their excellent photophysical properties compared to the commonly studied organic dyes or quantum dots.15,16 Meanwhile, the lanthanide-containing polyoxometalates (POMs) have been paid more and more attention now owing to their superior luminescent performances as well as excellent mechanical properties.17 As nanosized transition-metal oxide clusters, POMs are inorganic polyanions with rigid framework, water solubility, versatile performances, and biofunctionality as well as strong self-assembly ability.18,19 Nevertheless, the applications for POM hybrid materials are still limited due to their poor processability in crystalline or amorphous powders, easily quenched luminescent properties in water and incompatibility with hydrophobic 3 ACS Paragon Plus Environment

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organic matrices. Fortunately, taking advantage of the polyanion and oxygen enrichment nature of POMs, diverse POM hybrid supramolecular aggregates as well as films have been fabricated with the aid of biomolecules, polymers or surfactants mainly through electrostatic or hydrogen bonding interactions in aqueous solution or organic solvents.7,8,18-21 Similarly, the lanthanide-containing POMs hybrids can also be obtained through such co-assembling strategy and presented much enhanced luminescent properties owing to the effective shielding of coordinated water molecules.22 For examples, Wu’s group has performed systematic investigations on the co-assembly of biomolecules and POMs in water,23 where the luminescence enhancement was realized and then different human papillomaviruses (HPVs) subtype was discriminated.24,25 Additionally, Zhang and Wan et al. obtained improved luminescent hybrid hydrogels co-assembled from double hydrophilic block copolymers and lanthanide-containing POMs.26,27 Then, they also constructed a novel recyclable supramolecular optical sensor for efficiently detecting CO2 based on a well-designed and synthesized block copolymer with pH responsibility.28 Just recently, ionic surfactants were further focused on their induced self-assembly of Eu-containing POMs because of their structure diversity, strong self-assembling abilities and commercial availability. Yu et al. reported hybrid supramolecular spheres with pH-reversible self-assembly behavior and well-controlled luminescence responses from an elaborately synthesized cationic surfactant and a lanthanide-containing anionic POM, (Na9(EuW10O36)·32H2O, abbreviated as EuW10).29 The stability of such spheres, however, was limited due to the decreased electrostatic repulsion for the obtained spheres at the binding stoichiometry, where the strong electrostatic attraction of hydrophobic cations to anionic EuW10 nanocluesters existed. Zheng’s group fabricated an injectable and luminescent supramolecular hydrogel with multi-stimuli responsibility based on 4 ACS Paragon Plus Environment

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the aqueous co-assembly of EuW10 and a zwitterionic amphiphile with tethered cation/anion.30 However, the obtained hydrogel luminescence improvement was also restricted because the electrostatic attraction between EuW10 and the amphiphile head group was weak and there was still higher possibility for luminescence quenching from water molecules. To improve these drawbacks, we recently fabricated highly luminescent multilamellar spheres co-assembled from EuW10 and a commercially available cationic surfactant, myristoylcholine chloride (Myr) with higher Myr content, where excess Myr molecules were located on aggregate shell with hydrophilic groups towards outside to allow good dispersity of spheres.31 This inspired us to consider if we can tune the co-assembly by using a surfactant with pH-dependent protonation property, which might not only control the aggregate morphology, but also improve the aggregate luminescence. As a well-known zwitterionic surfactant, tetradecyldimethylamine oxide (C14DMAO) exhibits advantages in both fundamental researches and practical applications for its much high surface activity and commercially availability.32,33 Especially, C14DMAO is pH-reversible, easily protonated in acidic solutions or deprotonized in basic ones. Such a feature offers the possibility of reversibly regulating C14DMAO charges by solution pH. It has been reported that C14DMAO can exist either in a nonionic (pH > 9) or in a cationic (protonated) form (pH < 3) relying on the aqueous solution pH and the cationic-nonionic mixed status is obtained in between.34,35 Therefore, this zwitterionic surfactant may possess ability to initiate self-assembly of EuW10 and tune aggregate transition efficiently at different solution pH. Compared to literature reported pH-tuned POMs aggregates based on elaborately designed and synthesized building blocks,29 C14DMAO is expected to be an ideal alternative for its biocompatibility and abundant source. For this motivation, the co-assembly of C14DMAO and EuW10 in water has then been carefully investigated, where the enhanced red-emitting luminescent nanobelts with much longer lifetime and higher quantum yield 5 ACS Paragon Plus Environment

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were formed at C14DMAO/EuW10 molar ratio (R) of 20. Figure 1 presented the polyanionic EuW10 model, molecular structure of C14DMAO, and schematic description on its reversible protonation/deprotonation process. Such novel nanobelts were first found in POMs co-assembling systems and presented well-defined layer stacking structure along axial direction, with anionic EuW10 clusters serving as nano-bridges for C14DMAO bilayers under the synergistic effects of appropriate electrostatic, hydrogen bonding and hydrophobic interactions.

Figure 1. Schematic presentation of polyhedral anionic EuW10 nanocluster (a) and chemical structure of C14DMAO and its pH-reversible process (b).

2. Experimental Section 2.1. Materials Na9(EuW10O36)·32H2O (EuW10) was prepared based on the methods described by Sugesta and Yamase.36 Tetradecyldimethylamine oxide (C14DMAO) was obtained from Clariant Company (Germany) in China as 30 wt% aqueous solution. Such crude solution was then freeze-dried and crystallized in acetone for three times before use. Tetradecyltrimethylammonium bromide (TTAB) was purchased from Sigma-Aldrich, which was used without further purification. High-purity water (resistivity of 18.4 Mcm) was used from a FLOM water purification system (Qingdao). 2.2. Preparation of C14DMAO/EuW10 hybrid aggregates To obtain C14DMAO/EuW10 aggregates, the stock solutions of EuW10 (10 mM), C14DMAO (10 mM) were first prepared. Then, the appropriate stock solutions of EuW10, C14DMAO and water 6 ACS Paragon Plus Environment

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were added into sealed glass vials, with the final solution volume as 3 mL and EuW10 concentration as 0.05 mM. The samples were fabricated homogeneously through vortex mixing and then equilibrated in incubator at 25 °C for at least two days before following characterizations. Meanwhile, EuW10 aqueous solution (0.05 mM) without C14DMAO was also prepared for a comparison. 2.3. Characterization methods 2.3.1. Fluorescence spectroscopy The excitation/emission spectra, luminescence decay curves and absolute luminescence quantum yield of samples were measured on an Edinburgh Instruments FLS920 luminescence spectrometer attached with a 450 W xenon lamp and a μF920 microsecond flash lamp. The lifetime values were obtained from fitted decay curves based on a single-exponential function, with the corresponding goodness-of-fit, χ2, no more than 1.3. The absolute luminescence quantum yield was performed with a BaSO4 coated integrating sphere and calculated on the basis of the manual provided by the company. 2.3.2. Dynamic light scattering and zeta potential (DLS and ζ potential) The aggregate sample size distribution curves were obtained on a Zetasizer Nano-ZS instrument (ZEN3600, Malvern Instruments, UK) by DLS technology, under an incident laser beam at a wavelength of 632 nm (detection angle = 173 °) at 25 °C. Their zeta potential values were further collected on the same instrument. The samples were filtered through 0.45 μm filters before tests. 2.3.3. Isothermal titration calorimetry (ITC) ITC results were measured on a MicroCal VP-ITC apparatus. The C14DMAO aqueous solution (1.8 7 ACS Paragon Plus Environment

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mM) in a 300 μL syringe was added slowly into EuW10 aqueous solution (0.02 mM, 1.4 mL) at 25 °C. 2.3.4. Fourier transformed infrared spectroscopy (FT-IR) FT-IR spectra (400 ~ 4000 cm-1) were obtained with a resolution of 4 cm-1 on an Alpha-T spectrometer (Bruker). A small amount of solid powder was mixed well with KBr salt and the final powder was compressed into a transparent wafer for further measurements. 2.3.5. Transmission electron microscopy (TEM) TEM images were obtained from a Hitachi 100CX-II operated at 100 kV. The further high-resolution TEM (HR-TEM) images were performed on a HRTEM JEOL 2100 system operated at 200 kV. The sample was prepared by adding a drop of solution on the carbon-coated copper grid and removing the excess solution with filter paper after 5 min. Finally, it was dried for 30 min before observation. 2.3.6. Scanning electron microscopy (SEM) The morphology observations and composition results were also obtained by SEM and X-ray energy dispersive spectroscopy (EDS) on a Hitachi SU8010 operated at an accelerating voltage of 5.0 kV. The sample was placed on a silica wafer, and the wafer was dried for 30 min and then platinum sputter coating. 2.3.7. UV-vis spectroscopy Transmittance

spectra

of

C14DMAO/EuW10

solutions

were

recorded

on

a

UV-4100

spectrophotometer. The high-purity water was utilized as a blank in the measurements. 2.3.8. Small-angle X-ray scattering (SAXS) The SAXS spectra were obtained from a SAXSess mc2 X-ray scattering system (Anton Paar) 8 ACS Paragon Plus Environment

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operated at 50 kV and 40 mA. The distance from detector to sample was 264.5 mm and the wavelength of X-ray used in tests was 0.1542 nm (Cu K). The test exposure time for all samples was set as 15 minutes.

3. Results and Discussion 3.1. Luminescent C14DMAO/EuW10 aggregate formation The solution-based self-assembly is an excellent strategy to light up the luminescence of lanthanide (III) ions owing to the effective displacement of coordinated water molecules around them.37,38 Here, being polyanionic and highly water-soluble, EuW10 nanocluster also suffered serious fluorescence quenching when it was dissolved into water. Fortunately, its luminescence intensity exhibited a gradual enhancement upon adding C14DMAO surfactant into solution, which could be revealed by the emission spectra and inset sample appearance photos under UV irradiation in Figure 2a. Meanwhile, the corresponding intensity variation of 5D0→7F2 transition at different C14DMAO/EuW10 molar ratio (R) was recorded and shown in Figure 2b, which clearly illustrated such a luminescence enhancement and gradual stability at R value above 20. Then, to have a systematic knowledge for their photophysical property variations, the luminescence lifetime (τ) and absolute luminescence quantum yield (Qabs) for EuW10 solution and hybrid sample were also compared at R of 20. Their luminescence decay curves of 5D0→7F2 transition were shown in Figure 2c and the corresponding fitted τ as well as Qabs vales were all summarized in Table S1. As expected, the hybrid solution also possessed much higher τ and Qabs values (1094.5 μs and 13.3 %) than those for EuW10 solution (240.7 μs and 2.0 %).


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Figure 2. Emission spectra of C14DMAO/EuW10 solution (a) and corresponding 5D0→7F2 transition intensity (λem = 616 nm) variations (b) at different C14DMAO/EuW10 molar ratio (R), with luminescence decay curves for EuW10 and hybrid solution at R = 20, fitted based on a single-exponential function. (c). Inset solution visual appearance photos in (a) are for EuW10 (left) and hybrid at R of 20 (right) under UV irradiation (λex = 280 nm). All measurements were performed at 25 °C and EuW10 concentration was kept at 0.05 mM if not specially mentioned.

Accompanied by the enhanced luminescence, the obvious turbidity and Tyndall effect for hybrid solution at R of 20 could be observed (see inset photo of Figure 3a), indicating the formation of large supramolecular aggregates,31 which resulted in successfully shielding of part water molecules around EuW10 nanoclusters and enhanced the luminescent properties. Figure 3a illustrated the transmittance change curve for hybrid solutions at different R values. It can be seen that, upon gradually adding C14DMAO, the beginning clear and isotropic EuW10 solution (0.05 mM) became turbid and then the phase-separated precipitates at the bottom were noticed at 5  R  13. When R was further increased, the turbid solutions were formed again. To reveal the interactions between C14DMAO and EuW10 as well as their aggregate stability, zeta potential (ζ) changes with increasing R value were further measured with the result shown in Figure 3b. Clearly, upon the addition of little C14DMAO,  potential became more negative than that of EuW10 aqueous solution because of the aggregate formation.31,39 Then, with more C14DMAO adding,  potential value increased progressively and then became positive due to the neutralization of EuW10 negative charges by partially protonated C14DMAO. It should be noted that only a part of C14DMAO molecules could be protonated now at the solution pH value of about 6.5.32,40 Meanwhile, 10 ACS Paragon Plus Environment

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C14DMAO and its protonated form were in an acid-base equilibrium and they could couple into dimers through hydrogen bonding interaction.41 The protonated C14DMAO also easily formed hydrogen bonding with EuW10 via hydroxyl.42 Considering the much low critical micellar concentration (cmc = 0.075 mM) and then the high surface activity of C14DMAO,32,43 it can be understood that the surfactant first aggregated as micelles, carrying a small amount of positive charges. Therefore, the ζ potential variations here could confirm that the electrostatic interaction occurred between EuW10 and C14DMAO cationic micelles for aggregate formation. Additionally, consistent to the appearance observations, the stable turbid solutions could be obtained at R values below 5 or above 13 when  potential exhibited higher absolute values. The samples of R in between were unstable and easily formed precipitates. Further, the fact of partial protonated C14DMAO micelle at the investigated system pH of 6.5 was also rationalized by the following analysis. The pKa value of C14DMAO surfactant is about 5.0 but its apparent pKa value is complex and influenced by many factors.44 Compared with its pKa value, the corresponding apparent pKa value increased with addition of EuW10 nanoclusters or the formation of micelle in C14DMAO solution.45,46 Especially, the hydrogen bond interaction between EuW10 clusters and C14DMAO micelles favored protonation on the surface of micelle compared with that of monomer in the bulk solution. Meanwhile, the acidic pH of EuW10 solution also facilitated the protonation of C14DMAO. Based on above analysis, therefore, combining the observed  potential and fact of the micelle formation at the investigated system pH of 6.5, the partial protonation is recognized.


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Figure 3. Changes of transmittance (a) and ζ potential (b) against R for C14DMAO/EuW10 solutions. Inset photo in (a) is sample appearances for EuW10 (1), C14DMAO (2) and C14DMAO/EuW10 solution at R of 20 (3) to check Tyndall effect.

To quantify the interactions between two building blocks with opposite charges, the isothermal titration calorimetry (ITC) measurement was also performed. As shown in Figure S1 for calorimetric heat and corresponding integration curve, upon adding C14DMAO into EuW10 aqueous solution slowly, the obvious exothermic change was noticed and the binding constant was obtained as 4.66 × 105 M-1, suggesting the existence of significant electrostatic interaction between such two components.8,31 In addition, FT-IR measurement was an efficient method to reveal the structural variation and interaction among C14DMAO/EuW10 hybrid aggregates. Figure 4 illustrated the FT-IR spectra of EuW10, C14DMAO and their representative hybrid aggregates in above two stable turbid regions. For EuW10, there appeared four typical vibration bands at 943 cm-1 (ν (W=Od)), 844 cm-1 (ν (W−Ob−W)) and 780, 702 cm-1 (ν (W−Oc−W)), respectively, with Od denoting the terminal oxygen and Ob & Oc representing the bridged oxygen atoms of two octahedra.47 Clearly, these characteristic peaks were still maintained for C14DMAO/EuW10 aggregates, such as 943, 846, 785 and 713 cm-1 for R = 4 and 932, 846, 783 and 718 cm-1 for R = 20. Here, the slightly shifted peaks well indicated the keeping of EuW10 basic structure in hybrid aggregates and the occurrence of electrostatic interaction between EuW10 and C14DMAO.29,30,47 Meanwhile, the hydrophobic interaction should not be ignored. According to previous investigations,29,30,47 the hydrocarbon chains were relatively orderly packed and in all-trans conformation when νas (CH2) and νs (CH2) 12 ACS Paragon Plus Environment

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bands appeared at 2915–2920 cm-1 and 2846–2850 cm-1, respectively. For C14DMAO, these two characteristic peaks occurred at about 2918 and 2849 cm-1 suggesting an ordered trans-zigzag alkyl chain packing. Similarly, the CH2 vibration peaks of C14DMAO/EuW10 aggregates at R of 20 did not change obviously, demonstrating that their alkyl chains were also orderly packed and almost all in the trans-zigzag conformation. On the contrary, the hydrocarbon chains were not so orderly packed for C14DMAO/EuW10 aggregates at R of 4. Therefore, for aggregates at higher R values, C14DMAO molecules were packed with higher order via stronger hydrophobic force, resulting in better hydrophobic environment for EuW10 nanoclusters and more enhanced luminescence.

Figure 4. FT-IR spectra of EuW10, C14DMAO and their aggregates at R values of 4 and 20.

3.2. Structural characterization of C14DMAO/EuW10 aggregates The morphology and structure of C14DMAO/EuW10 hybrid aggregates at two stable turbid regions were carefully investigated based on DLS, TEM and SEM measurements. The aggregates at lower C14DMAO concentrations (R < 5) were first explored. As illustrated by inset photo of Figure 5a, the C14DMAO/EuW10 solution at R of 4 presented the visible Tyndall effect and such a stable turbid solution was immediately formed upon mixing the clear aqueous solutions of C14DMAO and EuW10, indicating aggregate formation owing to interactions between two components. DLS size 13 ACS Paragon Plus Environment

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distribution curve in Figure 5a revealed an aggregate size range of 50 ~ 210 nm and an average hydrodynamic diameter of about 106 nm. Further TEM and SEM images shown in Figure 5b&c demonstrated that such aggregates were practically supramolecular hybrid spheres, with diameters well consistent with the DLS results. Especially, the inset HR-TEM image in Figure 5b also confirmed the spherical structure of aggregates. And the clear dark dots of approximately 1 ~ 2 nm in size were attributed to EuW10 clusters for the high content of tungsten, surrounded by a C14DMAO shell with lower electron contrast. Meanwhile, as shown in Figure S2, the energy dispersive X-ray (EDX) spectrum exhibited the obvious signals of C, O, W and Eu for the hybrid spheres, which confirmed the successful co-assembling of C14DMAO with EuW10. Additionally, the aggregates at R of 2 or 3 were also investigated with their TEM images shown in Figure S3. The corresponding TEM images and the enlarged section illustrated the formation of regular spherical structures with average diameters of about 100 nm.

Figure 5. DLS size distribution curve (a), TEM or HR-TEM inset (b) and SEM (c) images for C14DMAO/EuW10 samples at R of 4. Inset photos in (a) are sample appearances for EuW10 (1), C14DMAO (2) and aggregates (3) to check Tyndall effect.

Then, we further focused on the aggregates at higher C14DMAO concentrations (R > 13). For the C14DMAO/EuW10 sample at R of 20, however, TEM and SEM images shown in Figure 6a&b displayed the well-defined nanobelts with length up to ten micrometers and widths of about 50 ~ 100 nm, which entangled with each other to form three-dimensional (3D) network structures. 14 ACS Paragon Plus Environment

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Similar to supramolecular spheres, the strong electronic contrast for nanobelts observed from TEM was also caused by the electron-dense EuW10 clusters, which was validated by the EDX spectrum (Figure S4). Furthermore, the HR-TEM image shown in Figure 6c clearly exhibited an ordered lamellar packing structure, with alternatively arranged bright and dark streaks along the axial direction to form nanobelts. Here, a regular repeated layer spacing of about 3.9 nm can be well interpreted as the alternate organic amphiphile layers (~ 2.2 nm) and inorganic EuW10 layers (~ 1.7 nm). The SAXS curve for aggregate was also measured and shown in Figure 6d. Consistent with the HR-TEM result, three obvious scattering peaks with their relative scattering factor ratio of 1 : 2 : 3 appeared, indicating a highly ordered lamellar structure existing in such C14DMAO/EuW10 nanobelts with the repeat layer spacing of 3.9 nm calculated from the scattering factor of the first peak.47 In addition, we also obtained the TEM images for C14DMAO/EuW10 aggregates at R of 13, 30 and 40 (Figure S5). Obviously, nanobelts were observed for all the samples and with increasing C14DMAO concentration, nanobelts became thinner and shorter. Such a phenomenon was the reason for the slightly weaker turbidity upon increasing R value in the range of 13 ~ 40 as seen in Figure 3a. Clearly, the presence of much excess C14DMAO and then the relatively decreased available number of EuW10 clusters for co-assembling eventually resulted in the collapse of nanobelts, responsible for the decreased turbidity for the samples.39,48

Figure 6. Images of TEM (a), SEM (b), HR-TEM (c) and SAXS curve (d) for C14DMAO/EuW10 nanobelt samples at R of 20.


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Based on all above results, the effects of C14DMAO/EuW10 molar ratio on the co-assembly behavior and their corresponding photophysical properties could be deduced as follows. With the addition of C14DMAO at R below 5, the transparent EuW10 solution became turbid and the luminescence has also been partly turned on. For these stable solutions, the hybrid spheres with ζ potential below -15 mV were obtained, indicating that the excess EuW10 nanoclusters were located on the shell of spheres to allow the spheres to disperse in water stably. As analyzed in Section 3.1, the strong electrostatic and hydrogen bonding interactions between EuW10 and C14DMAO cationic micelles, combined with the hydrophobic force among the long alkyl chains of C14DMAO molecules, drove the formation of hybrid spheres. The relatively less C14DMAO cationic micelles here could not separate EuW10 nanoclusters well and then disordered spheres packed by above two building blocks were formed as revealed by the inset HR-TEM image (Figure 6b). In this process, the appropriate shielding of part associated water molecules around EuW10 nanoclusters through electrostatically interacting with C14DMAO cationic micelles enhanced the luminescent properties. Upon further increasing C14DMAO to the region at 5  R  13, the precipitates were formed due to the decreased electrostatic repulsion among the primary aggregates at point near the stoichiometric charge ratio. Interestingly, with continually adding C14DMAO, the stable turbid solutions were re-formed, where novel nanobelts were obtained from effectively separated EuW10 nanoclusters by enough C14DMAO cationic micelles at R above 13. Considering the HR-TEM and SAXS results showing in Figure 6c&d, as well as the chemical structure information of two components, a possible

lamellar

stacking

model

of

the

molecular

aggregation

arrangement

inside

C14DMAO/EuW10 nanobelts was proposed in Figure 7. The thickness of organic layers (2.2 nm) obtained from HR-TEM was comparable to the molecular length (1.8 nm) of C14DMAO obtained from density functional theory (DFT) calculation at B3LYP/6-31G(d,p) level, representing an 16 ACS Paragon Plus Environment

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interdigitated bilayer packing of C14DMAO molecules. The thickness of inorganic layers (1.7 nm) was comparable to the size of one EuW10 cluster. Instead of symmetric distribution in organic solvent environment, the alkyl chains tended to rearrange and aggregate on the sides of EuW10 through hydrophobic force in aqueous solution.7,49 Therefore, large amounts of C14DMAO cationic micelles were bridged by enough EuW10 nanoclusters immediately through electrostatic and hydrogen bonding forces towards one dimensional direction, with C14DMAO molecules rearranging and aggregating on the sides of EuW10 nanoclusters. Meanwhile, to reduce the exposure of hydrophobic alkyl chains to water, the bilayer C14DMAO molecules between EuW10 nanoclusters tended to adopt interdigitated arrangement. Then, supramolecular polymerization occurred between C14DMAO cationic micelles and EuW10 nanoclusters and primary lamellar belts were gradually formed.8,43 The hydrophobic and intermolecular hydrogen bonding forces among partially protonated C14DMAO molecules enforced several primary lamellar belts to connect with each other laterally, forming an extended nanobelt core. The excess C14DMAO molecules at R of 20 had a strong propensity to adsorb on the surface of the nanobelt core with hydrophilic groups toward the aqueous solution, allowing the positively charged belts to disperse in water stably. When further increasing R value, the relatively decreased number of EuW10 clusters available for bridging C14DMAO cationic micelles finally led to the collapse of nanobelts. Figure 7 schematically illustrated such aggregate models with increasing C14DMAO concentration, where the enhanced and more stable hydrophobic microenvironment for EuW10 in nanobelts explained the much luminescence improvement.


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Figure 7. Illustration on the luminescent supramolecular spheres and nanobelts co-assembled from EuW10 and C14DMAO cationic micelles in aqueous solution, with their respective chemical structure information at the bottom.

Obviously, the clear lamellar structure along axial direction shown by HR-TEM image in Figure 6c indicated relatively weak lateral intermolecular interactions among nanobelt core during the formation of nanobelts. On the contrary, the stronger electrostatic and hydrogen bonding forces between C14DMAO cationic micelles and EuW10 clusters along axial direction led to the one-dimentional extended nanobelts with well periodic layered structure as revealed by the SAXS result. Additionally, as reported in recent investigations on the co-assembly of cationic surfactants and EuW10 nanoclusters in water, only hybrid spheres with weak stabilities were immediately formed through strong electrostatic and hydrophobic forces.29,31 However, in the situation here, the used zwitterionic surfactant, C14DMAO, with excellent surface activity and partial protonation property at the solution pH of about 6.5, formed micelles easily to carry a small amount of positive charges. They acted as primary building blocks to drive the self-assembly of EuW10 clusters into such novel nanobelts via a delicate synergetic effect of appropriate electrostatic, hydrophobic and hydrogen bonding interactions at R of 20 as carefully analyzed above. It’s well-known that C14DMAO can approach full protonation at pH below 3.34,35 Upon directly adding HCl to the nanobelt solution to tune pH from 6.5 to 2.5, the hybrid spheres with larger size and weaker stability 18 ACS Paragon Plus Environment

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were formed as shown by TEM image in Figure S6, further suggesting the key role of C14DMAO partial protonation on the formation of novel nanobelts. 3.3 pH responsiveness of C14DMAO/EuW10 nanobelts Considering

the

reversible

protonation/deprotonation

of

C14DMAO

and

sensitive

photophysical properties of EuW10 nanoclusters to surrounding environment,32,50,51 the hybrid nanobelts were expected to alter their aggregation and luminescent properties upon pH changes. Then, the typical nanobelt sample at R of 20 was focused on to explore the pH responsiveness owing to its better luminescence and stability. Figure 8a&b illustrated the emission spectra and the corresponding 5D0→7F2 transition intensity variations for hybrid nanobelts at different pH values. It was obvious that the luminescence intensity decreased immediately upon adding small amount of NaOH and became almost fully quenched with continually increasing pH to 7.0. On the contrary, upon gradually titrating HCl to hybrid sample, its emission intensity exhibited a further improvement at first until pH to 6.0 and then slowly decreased until pH of about 2.5. The inset sample appearance photos under UV irradiation in Figure 8a at three pH values directly revealed the luminescence quenching at too low or high pH values. Concomitantly, we also measured the pH-induced visible turbidity variations with result shown in Figure 8c, where the turbid nanobelt solution became almost transparent at pH above 7.0. However, upon adding HCl, its turbidity first increased and then gradually decreased at pH from 4.8 to 2.5. The inset sample appearance photos in Figure 8c clearly demonstrated the typical Tyndall effect at pH of 2.5 and 6.5, while the transparent solution at pH of 7.5 displayed no sign of aggregates. As the turbidity of colloidal solutions was a direct macroscopic reflection for the aggregation behavior,31 the aggregate morphologies at different pH values were 19 ACS Paragon Plus Environment

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also observed with their TEM images shown in Figure S7. Obviously, at pH of 6.5, lots of nanobelts were obtained. If pH was tuned to 7.0, cracked belts and small particles were formed (Figure S7b) which was consistent with the decreased turbidity. On the contrary, nanobelts with higher contrast and lots of spheres were captured at pH of 6.0 (Figure S7c), while TEM images in Figure S7d-f showed large spheres with size of 200-500 nm at pH of 4.8, 3.5 and 2.5, which were the reasons for the increased turbidity (Figure S7c&d&e). However, as shown in Figure S7f, smaller and few spheres were found upon further decreasing pH value, corresponding to the further decreased turbidity. Moreover, compared with the reported stabilized pH range of 5.5 ~ 8.5 for EuW10 cluster,52 such a range could be extended to 2 ~ 11 for SiO2 nanoparticle bridged EuW10 and 2.6 ~ 8.0 for hybrid spheres from cationic surfactant encapsulated EuW10.53,54 In the situation here, we confirmed the stability of EuW10 cluster in those pH values between 2.5 and 7.0 based on the IR spectra results (Figure S8) ， where the FT-IR spectra of hybrid aggregates all presented the four typical vibration bands of EuW10, effectively suggesting the maintenance of EuW10 basic structure.27,28,42

Figure 8. Emission spectra (a), corresponding 5D0→7F2 transition intensity (b), transmittance (c), and ζ potential (d) variations of C14DMAO/EuW10 solution (R = 20) at different pH values. Inset photos in (a) and (c) are respectively sample appearances at pH of 2.5 (1), 6.5 (2) and 7.5 (3) under UV irradiation (λex = 280 nm) or laser to check Tyndall effect.

To better understand the aggregate transition and the corresponding luminescent properties, we further recorded ζ potential variation at different pH values (Figure 8d). Expectedly, ζ potential increased directly with pH decreasing, well indicating the stronger protonation extent of C14DMAO. 20 ACS Paragon Plus Environment

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Additionally, Figure S9 exhibited the control experiments on luminescence and turbidity properties at different pH values, where C14DMAO was replaced by a tetradecyltrimethylammonium bromide (TTAB) surfactant with no pH responsiveness. As shown in Figure S9a&b for emission spectra and the 5D0→7F2 transition intensity variation for TTAB/EuW10 samples, the emission intensity decreased immediately upon adding small amount of HCl or NaOH and was almost quenched at stronger acidic or basic conditions. The inset sample appearance photos under UV irradiation for TTAB/EuW10 solutions at pH of 2.5, 6.5 and 7.5 directly confirmed the quenching effect from HCl or NaOH. At the same time, the corresponding transmittance variation curve in Figure S9c revealed the decreased turbidity with titrating HCl or NaOH, mainly resulted from the weakening of electrostatic interaction. However, their solutions at pH of 2.5 and 7.5 still presented obvious Tyndall effect as shown by inset photo in Figure S9c, suggesting the existence of aggregates in spite of decreased turbidity. Therefore, we can conclude that the aggregate morphology and luminescent variations for C14DMAO/EuW10 nanobelts upon adding HCl or NaOH were dependent on protonation extent of C14DMAO. Adding NaOH caused C14DMAO to present as more de-proton form and therefore weak interaction with EuW10, resulting in the disassembly of nanobelts and luminescence quenching. Upon decreasing pH value from 6.5, however, the increased protonation extent of C14DMAO enhanced its combination with EuW10 behaved just like common cationic surfactants27,29 and then the larger sphere aggregates were formed via strong electrostatic and hydrophobic forces. Correspondingly, higher turbidity as well as luminescent intensity were obtained at first owing to the more efficient shielding of water molecules. However, the much excess H+ might reduce the electrostatic force and quench the luminescence of EuW10 hybrids,29 leading to the decreased turbidity and luminescence quenching. Such results further confirmed the above deduction that the partial protonation of C14DMAO at the system pH of 6.5 was a key factor 21 ACS Paragon Plus Environment

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for the formation of stable nanobelts. Finally, the pH-stimulus reversibility of C14DMAO/EuW10 nanobelts was further investigated. As shown in Figure 9a, when adding appropriate amount of NaOH to tune pH from 6.5 to 7.5, the turbid nanobelt solution became transparent, accompanied by the luminescence quenching. If pH was tuned back to 6.5, the transparent solution turned turbid again and the luminescence was recovered. TEM image in Figure S10 confirmed the re-formation of nanobelts. In addition, Figure 9b exhibited that the cycled luminescent switching could be repeated for several times. The corresponding turbidity and ζ potential values were also reversible as shown in Figure 9c&d.

Figure 9. C14DMAO/EuW10 solution appearance changes under laser (left) or UV irradiation (λex = 280 nm) (right) upon addition of NaOH or HCl (a). Reversible variations of 5D0→7F2 transition intensity (b), transmittance (c), and  potential (d) at different pH values.

4. Conclusions In summary, the improved red-emitting luminescent hybrid nanospheres and novel nanobelts were prepared in water, which were co-assembled by Na9(EuW10O36)·32H2O (EuW10) and a pH-reversible zwitterionic surfactant, tetradecyldimethylamine oxide (C14DMAO) mainly via electrostatic, hydrogen bonding and hydrophobic interactions. The nanobelts with organized inner structure at C14DMAO/EuW10 molar ratio of 20 exhibited the optimal emission intensity, 22 ACS Paragon Plus Environment

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luminescence lifetime and absolute quantum yield owing to the efficient shielding of coordinated water molecules around EuW10 nanoclusters. The morphology and structure characterization of such nanobelts indicated that the partial protonation of C14DMAO at the system pH of 6.5 played the key role to cooperate with other intermolecular forces to induce nanobelt formation. A rational lamellar aggregation model for the nanobelts was proposed and the stronger electrostatic and hydrogen bonding forces between C14DMAO cationic micelles and EuW10 clusters along axial direction induced such nanobelts with well periodic layered structure, where the stable hydrophobic microenvironment for EuW10 well explained the much luminescence enhancement. Further, with the pH-reversibility of C14DMAO and the sensitive photophysical properties of Eu3+ to the environment, the luminescent nanobelts presented excellent pH responsiveness, thus offering an attractive reference to conveniently prepare smart optical supramolecular materials for applications in fluorescent sensing and detection.

Supporting Information Details on the comparison of photophysical properties between EuW10 and C14DMAO/EuW10 aggregates as well as ITC results of C14DMAO being titrated into EuW10 aqueous solution are provided. The EDX spectra, TEM images and FT-IR profiles for C14DMAO/EuW10 aggregates are also included. Additionally, control experiments on luminescence spectra and transmittance variations at different pH values for TTAB/EuW10 samples are offered.

Conflicts of interest The authors declare no competing financial interests.

Author information Corresponding Author 23 ACS Paragon Plus Environment

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*E–mail: [email protected].

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21373127 and 21673129).

References (1) Huang, X.; Li, M.; Green, D. C.; Williams, D. S.; Patil, A. J.; Mann, S. Interfacial assembly of protein–polymer nanoconjugates into stimulus-responsive biomimetic protocells. Nat. Commun. 2013, 4, 2239–2247. (2) Wang, A. D.; Huang, J. B.; Yan, Y. Hierarchical molecular self-assemblies: construction and advantages. Soft Matter 2014 , 10, 3362−3373. (3) Zhao, W. R.; Cui, J. W.; Hao, J. C.; Van Horn, J. D. Co-assemblies of polyoxometalate {Mo72Fe30}/double-tailed magnetic-surfactant for magnetic-driven anchorage and enrichment of protein. J. Colloid Interface Sci. 2019, 536, 88−97. (4) Xu, L.; Feng, L.; Hao, J. C.; Dong, S. L. Controlling the capture and release of DNA with a dual-responsive cationic surfactant. ACS Appl. Mater. Interfaces 2015, 7, 8876−8885. (5) Zhu, Q. D.; Zhang, L. H.; Van Vliet, K.; Miserez, A.; Holten-Andersen, N. White light-emitting multistimuli-responsive hydrogels with lanthanides and carbon dots. ACS Appl. Mater. Interfaces 2018, 10, 10409–10418. (6) Li, Z. Q.; Hou, Z. H.; Fan, H. X.; Li, H. R. Organic-inorganic hierarchical self-assembly into robust luminescent supramolecular hydrogel. Advanced Functional Materials 2017, 27, 1604379. (7) Wu, A. L.; Gao, X. P.; Sun, P. P.; Lu, F.; Zhen, L. Q. Co-assembly of polyoxometalates and zwitterionic amphiphiles into supramolecular hydrogels: from crystalline fibrillar to amorphous 24 ACS Paragon Plus Environment

Page 25 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

micellar networks. Angew. Chem., Int. Ed. 2018, 57, 4025–4029. (8) Li, J. F.; Chen, Z. G.; Zhou, M. C.; Jing, J. B.; Li, W.; Wang, Y.; Wu, L. X.; Wang, L. Y.; Wang, Y. Q.; Lee, M. Polyoxometalate-driven self-assembly of short peptides into multivalent nanofibers with enhanced antibacterial activity. Angew. Chem., Int. Ed. 2016, 55, 2592–2595. (9) Bruno, S. M.; Ferreira, R. A. S.; Almeida Paz, F. A.; Carlos, L. s. D.; Pillinger, M.; Ribeiro-Claro, P.; Goncalves, I. S. Structural and photoluminescence studies of a Europium(III) tetrakis(β-diketonate)

complex

with

tetrabutylammonium,

imidazolium,

pyridinium

and

silica-supported imidazolium counterions. Inorg. Chem. 2009, 48, 4882−4895. (10) Binnemans, K. Interpretation of Europium(III) spectra. Coord. Chem. Rev. 2015, 295, 1−45. (11) Wei, C.; Ma, L.; Wei, H. B.; Liu, Z. W.; Bian, Z. Q.; Huang, C. H. Advances in luminescent lanthanide

complexes

and

applications.

Sci.

China:

Technol.

Sci.

2018,

61,

DOI:

10.1007/s11431-017-9212-7. (12) Chen, P. K.; Li, Q. C.; Grindy, S.; Holten-Andersen, N. White-light-emitting lanthanide metallogels with tunable luminescence and reversible stimuli-responsive properties. J. Am. Chem. Soc. 2015, 137, 11590−11593. (13) Lei, N. N.; Shen, D. Z.; Wang, X. F.; Wang, J.; Li, Q. T.; Chen, X. Enhanced full color tunable luminescent lyotropic liquid crystals from P123 and ionic liquid by doping lanthanide complexes and AIEgen. J. Colloid Interface Sci. 2018, 529, 122–129. (14) Liu, J.; Morikawa, M.; Morikawa, N. Conversion of molecular information by luminescent nanointerface self-assembled from amphiphilic Tb(III) complexes. J. Am. Chem. Soc. 2011, 133, 17370–17374. (15) Lunstroot, K.; Driesen, K.; Nockemann, P.; Van Hecke, K.; Van Meervelt, L.; Gorller-Walrand, C.; Binnemans, K.; Bellayer, S.; Viau, L.; Le Bideau, J.; Vioux, A. Lanthanide-doped luminescent 25 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

ionogels. Dalton Trans. 2009, 0, 298–306. (16) Lunstroot, K.; Driesen, K.; Nockemann, P.; Viau, L.; Mutin, P. H.; Vioux, A.; Binnemans, K. Ionic liquid as plasticizer for Europium(III)-doped luminescent poly(methyl methacrylate) films. Phys. Chem. Chem. Phys. 2010, 12, 1879–1885. (17) Zhao, J. W.; Li, Y. Z.; Chen, L. J.; Yang, G. Y. Research progress on polyoxometalate-based transition-metal–rare-earth heterometallic derived materials: synthetic strategies, structural overview and functional applications. Chem. Commun. 2016, 52, 4418−4445. (18) Li, B.; Li, W.; Li, H. L.; Wu, L. X. Windsor, M. W. Ionic complexes of metal oxide clusters for versatile self-assemblies. Acc. Chem. Res. 2017, 50, 1391−1399. (19) Song, Y. F.; Tsunashima, R. Recent advances on polyoxometalate-based molecular and composite materials. Chem. Rev. 2007, 107, 2592−2614. (20) Yang, J.; Chen, M.; Li, P.; Cheng, F.; Xu, Y.; Li, Z. Q.; Wang, Y. G.; Li, H. R. Self-healing hydrogel containing Eu-polyoxometalate as acid-base vapor modulated luminescent switch. Sens. Actuators, B 2018, 273, 153–158. (21) Yang, J.; Wang, T. R.; Wang, Y. G.; Li, H. R. Reversible photoluminescence switching in supramolecular ultrathin films architecture. Dyes Pigm. 2018, 149, 902–907. (22) Zhang, J.; Liu, Y.; Li, Y.; Zhao, H. X.; Wan, X. H. Hybrid assemblies of Eu-containing polyoxometalates and hydrophilic block copolymers with enhanced emission in aqueous solution. Angew. Chem., Int. Ed. 2012, 51, 4598−602. (23) Gao, P. F.; Wu, Y. Q.; Wu, L. X. Co-assembly of polyoxometalates and peptides towards biological applications. Soft Matter 2016, 12, 8464−8479. (24) Li, H. W.; Wang, Y.; Zhang, T.; Wu, Y.; Wu, L. X. Selective binding of amino acids on Europium-substituted polyoxometalates and the interaction-induced luminescent enhancement 26 ACS Paragon Plus Environment

Page 27 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

effect. ChemPlusChem 2014, 79, 1208−1213. (25) Zhang, T.; Fu, D. Y.; Wu, Y. Q.; Wang, Y. Z.; Wu, L. X. Potential applications of polyoxometalates for the discrimination of human papillomavirus in different subtypes. Dalton Trans. 2016, 45, 15457−15463. (26) Wei, H. B.; Shi, N.; Zhang, J. L.; Guan, Y.; Zhang, J.; Wan, X. H. pH-responsive inorganic-organic hybrid supramolecular hydrogels with jellyfish-like switchable chromic luminescence. Chem. Commun. 2014, 50, 9333−9335. (27) Wei, H. B.; Du, S. M.; Liu, Y.; Zhao, H. X.; Chen, C. Y.; Li, Z. B.; Lin, J.; Zhang, Y.; Zhang, J.; Wan, X. H. Tunable, luminescent, and self-healing hybrid hydrogels of polyoxometalates and triblock copolymers based on electrostatic assembly. Chem. Commun. 2014, 50, 1447−1450. (28) Wei, H. B.; Zhang, J. L.; Shi, N.; Liu, Y.; Zhang, B.; Zhang, J.; Wan, X. H. A recyclable polyoxometalate-based supramolecular chemosensor for efficient detection of carbon dioxide. Chem. Sci. 2015, 6, 7201−7205. (29) Gong, Y. J.; Hu, Q. Z.; Wang, C.; Yu, L. Stimuli-responsive polyoxometalate/ionic liquid supramolecular spheres: fabrication, characterization, and biological applications. Langmuir 2016, 32, 421−427. (30) Wu, A. L.; Sun, P. P.; Sun, N.; Yu, Y.; Zhen, L. Q.. Co-assembly of polyoxometalate and zwitterionic amphiphile into luminescent hydrogel with excellent stimuli responsiveness. Chem. – Eur. J. 2018, 24, 16857–16864. (31) Lei, N. N.; Shen, D. Z.; Chen, X. Highly luminescent and multi-sensing aggregates co-assembled from Eu-containing polyoxometalate and an enzyme-responsive surfactant in water. Soft Matter, 2019,15, 399–407. (32) Feng, L.; Xu, L.; Dong, S. L.; Hao, J. C. Thermo-reversible capture and release of DNA by 27 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 41

zwitterionic surfactants. Soft Matter, 2016, 12, 7495–7504. (33) Zhao, W. R.; Feng, L.; Xu, L.; Xu, W. L.; Sun, X.; Hao, J. C. Chiroptical vesicles and disks that originated from achiral molecules. Langmuir 2015, 31, 5748–5757. (34) Lair, V.; Bouguerra, S.; Turmine, M.; Letellier, P. Thermodynamic study of the protonation of dimethyldodecylamine N-oxide micelles in aqueous solution at 298 K. Establishment of a theoretical relationship linking critical micelle concentrations and pH. Langmuir 2004, 20, 8490–8495. (35) Feng, L.; Xu, L.; Hao, J. C.; Dong, S. L. Controlled compaction and decompaction of DNA by zwitterionic surfactants. Colloids and Surfaces A 2016, 501, 65–74. (36) Sugeta, M.; Yamase, T. Crystal structure and luminescence site of Na9(EuW10O36)·32H2O. Bull. Chem. Soc. Jpn. 1993, 66, 444–449. (37) Morikawa, M.; Tsunofuri, S.; Kimizuka, N. Controlled self-assembly and luminescence characteristics of Eu(III) complexes in binary aqueous/organic media. Langmuir 2013, 29, 12930–12935. (38) Zhang, G. P.; Zhu, H. X.; Chen, M. J.; Li, H. G.; Hao, J. C. Aggregation-induced emission of Eu(III) complex balanced with a bulky and amphiphilic imidazolium cation in ethanol/water binary mixtures. Chem. – Eur. J. 2018, 24, 15912–15920. (39) Zhou, S. J.; Zhang, L. W.; Feng, Y. Q.; Li, H. G.; Chen, M. J.; Pan W.; Hao, J. C. Fullerenols revisit: highly monodispersed photoluminescent nanomaterials and ideal building blocks for supramolecular chemistry. Chem. – Eur. J. 2018, 24, 16609–16619. (40) Mel’nikov, Y. S.; Lindman, B. pH-controlled DNA condensation in the presence of dodecyldimethylamine oxide. Langmuir 2000, 16, 5871–5878. (41) Hoffmann, H.; Oetter, G.; Schwandner, B. The aggregation behaviour of tetradecyl28 ACS Paragon Plus Environment

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Langmuir

dimethylaminoxide. Prog. Colloid Polym. Sci. 1987, 73, 95–106. (42) Wang, Z. L.; Zhang, R. L.; Ma, Y.; Peng, A. D.; Fu, H. B.; Yao, J. N. Chemically responsive luminescent switching in transparent flexible self-supporting [EuW10O36]9--agarose nanocomposite thin films. J. Mater. Chem. 2010, 20, 271–277. (43) Sun, X. F.; Chen, M. J.; Zhang, Y. Q.; Yin, Y. J.; Zhang, L. W.; Li, H. G.; Hao J. C. Photoluminescent and pH-responsive supramolecular structures from co-assembly of carbon quantum dots and zwitterionic surfactant micelles. J. Mater. Chem. B 2018, 6, 7021–7032. (44) Weers, J. G.; Rathman, J. E.; Scheuing, D. R. Structure/performance relationships in long chain dimethylamine oxide/sodium dodecylsulfate surfactant mixtures. Colloid Polym Sci 1990, 268, 832–846. (45) Mel’nikova, Y. S.; Lindman, B. pH-controlled DNA condensation in the presence of dodecyldimethylamine oxide. Langmuir 2000, 16, 5871– 5878 (46) Maeda, H.; Kakehashi, R. Effects of protonation on the thermodynamic properties of alkyl dimethylamine oxides. Adv. Colloid Interface Sci. 2000, 88, 275–293. (47) Lei, N. N.; Yi, S. J.; Wang, J.; Li, Q. T.; Chen, X. Unusual aggregation arrangement of Eu-containing polyoxometalate hybrid in a protic ionic liquid with improved luminescence property. J. Phys. Chem. B, 2017, 121, 11528–11536. (48) Sun, X. F.; Zhang, Q. H.; Yin, K. Y.; Zhou, S. J.; Li, H. G. Fluorescent vesicles formed by simple surfactants induced by oppositely-charged carbon quantum dots. Chem. Commun. 2016, 52, 12024–12027. (49) Yan, Y.; Li, B.; Li, W.; Li, H. L.; Wu, L. X. Controllable vesicular structure and reversal of a surfactant-encapsulated polyoxometalate complex. Soft Matter 2009, 5, 4047−4053. (50) Kawasaki, H.; Sasaki, A.; Kawashima, T.; Sasaki, S.; Kakehashi, R.; Yamashita, I.; Fukada, K.; 29 ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 41

Kato, T.; Maeda, H. Protonation-induced structural change of lyotropic liquid crystals in oley- and alkyldimethylamine oxides/water systems. Langmuir 2005, 21, 5731−5737. (51) Goldsipe, A.; Blankschtein, D. Molecular-thermodynamic theory of micellization of pH-sensitive surfactants. Langmuir 2006, 22, 3547−3559. (52) Kim, H. S.; Hoa, D. T. M.; Lee, B. J.; Park, D. H.; Kwon, Y. S. Synthesis and photoluminescent property of Eu-containing organic–inorganic hybrid polyoxometalate. Curr. Appl. Phys. 2006, 6, 601–604. (53) Zhao, Y. Y.; Li, Y.; Li, W.; Wu, Y. Q.; Wu, L. X. Preparation, structure, and imaging of luminescent

SiO2

nanoparticles

by

covalently

grafting

surfactant-encapsulated

Europium-substituted polyoxometalates. Langmuir 2010, 26, 18430–18436. (54) Guo, Y. X.; Gong, Y. J.; Gao, Y. A.; Xiao, J. H.; Wang, T.; Yu, L. Multi-stimuli responsive supramolecular structures based on azobenzene surfactant-encapsulated polyoxometalate. Langmuir 2016, 32, 9293–9300.


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For Table of Contents Only

With much high surface activity and partial protonation property at the solution pH of about 6.5, the zwitterionic surfactant, tetradecyldimethylamine oxide (C14DMAO) first formed positively charged micelles and co-assembled with Na9(EuW10O36)·32H2O (EuW10) into enhanced luminescent multilamellar nanobelts at a C14DMAO/EuW10 molar ratio (R) of 20, which exhibited excellent pH responsiveness.


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Figure 1. Schematic presentation of polyhedral anionic EuW10 nanocluster (a) and chemical structure of C14DMAO and its pH-reversible process (b). 79x25mm (300 x 300 DPI)

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Figure 2. Emission spectra of C14DMAO/EuW10 solution (a) and corresponding 5D0→7F2 transition intensity (λem = 616 nm) variations (b) at different C14DMAO/EuW10 molar ratio (R), with luminescence decay curves for EuW10 and hybrid solution at R = 20, fitted based on a single-exponential function. (c). Inset solution visual appearance photos in (a) are for EuW10 (left) and hybrid at R of 20 (right) under UV irradiation (λex = 280 nm). All measurements were performed at 25 °C and EuW10 concentration was kept at 0.05 mM if not specially mentioned. 149x35mm (300 x 300 DPI)


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Figure 3. Changes of transmittance (a) and ζ potential (b) against R for C14DMAO/EuW10 solutions. Inset photo in (a) is sample appearances for EuW10 (1), C14DMAO (2) and C14DMAO/EuW10 solution at R of 20 (3) to check Tyndall effect. 119x43mm (300 x 300 DPI)


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Langmuir

Figure 4. FT-IR spectra of EuW10, C14DMAO and their aggregates at R values of 4 and 20. 79x64mm (300 x 300 DPI)


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Figure 5. DLS size distribution curve (a), TEM or HR-TEM inset (b) and SEM (c) images for C14DMAO/EuW10 samples at R of 4. Inset photos in (a) are sample appearances for EuW10 (1), C14DMAO (2) and aggregates (3) to check Tyndall effect. 139x38mm (300 x 300 DPI)


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Langmuir

Figure 6. Images of TEM (a), SEM (b), HR-TEM (c) and SAXS curve (d) for C14DMAO/EuW10 nanobelt samples at R of 20. 149x34mm (300 x 300 DPI)


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Figure 7. Illustration on the luminescent supramolecular spheres and nanobelts co-assembled from EuW10 and C14DMAO cationic micelles in aqueous solution, with their respective chemical structure information at the bottom. 149x59mm (300 x 300 DPI)


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Langmuir

Figure 8. Emission spectra (a), corresponding 5D0→7F2 transition intensity (b), transmittance (c), and ζ potential (d) variations of C14DMAO/EuW10 solution (R = 20) at different pH values. Inset photos in (a) and (c) are respectively sample appearances at pH of 2.5 (1), 6.5 (2) and 7.5 (3) under UV irradiation (λex = 280 nm) or laser to check Tyndall effect. 149x27mm (300 x 300 DPI)


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Figure 9. C14DMAO/EuW10 solution appearance changes under laser (left) or UV irradiation (λex = 280 nm) (right) upon addition of NaOH or HCl (a). Reversible variations of 5D0→7F2 transition intensity (b), transmittance (c), and ζ potential (d) at different pH values. 80x62mm (300 x 300 DPI)


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Graphic for TOC 82x36mm (300 x 300 DPI)


Zwitterionic Surfactant Micelle-Directed Self-Assembly of Eu

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