Eu3+-Controlled Fluorescent Bilayer Vesicles - Langmuir (ACS

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Eu3+-Controlled Fluorescent Bilayer Vesicles Jin Yuan, Ling Wang, Yitong Wang, Shuli Dong, and Jingcheng Hao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00302 • Publication Date (Web): 16 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019

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Eu3+-Controlled Fluorescent Bilayer Vesicles Jin Yuan, Ling Wang, Yitong Wang, Shuli Dong, and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, China

* To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +86-531-88366074. Fax: +86-531-8856-4750 1


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ABSTRACT: By appropriate substitution, polyoxometalates (POMs) can be modified to be organic-inorganic supramolecules (OISMs) that are nonaqueous or water soluble and form aggregates in solution. Here we report a new OISM, (TBA)3POM-PPCT, that can self-assemble to form bilayer vesicles controlled by Eu3+ in nonaqueous solution. Dynamic laser light scattering (DLS), transmission and scanning electron microscopies (SEM and TEM) and atomic force microscopy (AFM) clearly demonstrated the controllable formation of stable bilayer vesicles with an average hydrodynamic radius of about 510 nanometers. Because of the coordination between (TBA)3POM-PPCT and Eu3+, the stable vesicles possess fluorescence by studying fluorescence spectra and show highly selective response to Cu2+ allowing function as an ion detecting platform of Cu2+.

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1. INTRODUCTION Over the past few decades, supramolecular self-assembly has generated an ever-increasing number of elegant and intricate functional structures with sizes of nanoscopic dimensions.1 Supramolecular assemblies are accessible using amphiphilic constructive blocks of hydrophobic and hydrophilic portions through noncovalent interactions including complementary hydrogen bonding,2,3 van der Waals force,4 hydrophobic interactions,5 electrostatic interactions6 and compound effects.7 Amphiphilic POMs with a large group of structurally well-defined metal-oxide clusters (ca. 1-6 nm) are favored as ideal building blocks for targeting new amphiphiles due to their diverse physical properties and applications.8-13 POM-organic hybrids have organic ligands or chains chemically grafted onto the POM surface and are amphiphilic OISMs.4-6 Liu et al. first synthesized POM-organic hybrids with two alkyl chains. This POM-organic hybrid can assemble into vesicles in mixed solutions.14 A series of POM-containing hybrids with different shapes showed typical surfactant properties and can slowly self-assemble into large, hollow, spherical structures in polar solvents.15-17 Wang et al. selected diverse functional organic groups to promote the relevant properties. They synthesized hybrids by covalently grafting two cholesterols onto the two sides of Anderson-type POMs to enhance the thermal stability.18 One can skillfully select POM-organic hybrid ligands and metal ions for fabricating well-defined supramolecular structures. Kurth et al. synthesized a series of dynamic coordination polymers based on the bis(terpyridine) ligand with rigid benzenyl 3


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spacers and Fe2+, Co2+ and Ni2+ ions. They obtained different supramolecular structures including chains, rings, and branched structures.19-21 Compared with the transition metal coordination compounds, lanthanide-based analogues possess the metal-centered photoluminescent properties.22-24 Luminescence of these complexes predominantly stems from lanthanide metal sources in the presence of UV-light-absorbing functional groups, the so-called “antenna effect.” These organic sensitizers have already been utilized as luminescent probes,25 sensors for analytes,26,27 and bioimaging probes28 because of their fascinating optical properties. Andersen et al. used the lanthanide-terpyridine complex mixture to engineer white luminescent materials in organic/aqueous solutions, showing high sensitivity to variations in temperature, pH, mechanical force, and presence of chemical anions.29 Maji et al. synthesized a new low molecular weight gelator having a terminal terpyridine (tpy) to act as a sensitizer for lanthanide ions. A coordination-driven self-assembly of the gelator with lanthanide ions (Tb3+/Eu3+) resulted in bright luminescent coordination polymer gels in different colors.30 The reported functional materials with lanthanide can emit glorious fluorescence that can be mostly used as white-light-emitting materials, stimuli-responsive luminescent materials, smart coatings or paints and for biological imaging.31 Herein, by grafting an Anderson-type POM, we synthesized a new ligand POM-organic

hybrid,

[N(C4H9)4]3[MnMo6O18{(OCH2)3CNHCO(C21H15N3)}2]

(POM-PPCT) (Figures S (1-4) in the Supporting Information (SI)).32 Both 4’-para-phenylcarboxyl-2,2’:6’,2”-terpyridine (PPCT) groups are at opposite ends. 4


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PPCT groups are linked by a Mn-Anderson-type POM. The bulky organic cation, tetrabutylammonium (TBA+), is the original counterion of the anionic polar head group. The length of the POM-PPCT molecule is ca. 3.1 nm calculated from Materials Studio (MS) and its molecular weight is 2263 g∙mol-1. POM-PPCT can self-assemble into vesicles in organic solvent through the coordination of terpyridine and Eu3+, as shown in Figure 1. The bilayer vesicles were endowed with the excellent fluorescence properties by resonance energy transfer between terpyridine as donor and Eu3+ as acceptor. The assembled structures show high selectivity for Cu2+ due to the fluorescence quenching effect, which may enable its use in potential Cu2+ detection applications.

Figure 1. Schematic representation of Eu3+/POM-PPCT, showing the molecular of POM-PPCT and energy transfer process from tpy to Eu3+.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. [N(C4H9)4]3[MnMo6O18{(OCH2)3CNHCO(C21H15N3)}2]

(POM-PPCT)

and

4-([2,2':6',2''-terpyridin]-4'-yl)benzoic acid (PPCT). Details of the synthetic procedure of the PPCT and POM-PPCT conjugates have been reported in our previous study and it can be seen in the ESI.32 Europium(III) nitrate hexahydrate (Eu(NO3)3·6H2O, 99.9%) was purchased from J&K Scientific Ltd. Sodium nitrate (NaNO3, > 99.0%), 5


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ammonium

nitrate

(NH4NO3,

>

99.0%),

copper(II)

nitrate

trihydrate

(Cu(NO3)2·3H2O, > 99.0%) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, > 99.0%), iron(III) nitrate nonahydrate (Fe(NO3)3·6H2O, >98.5%), lead nitrate (Pb(NO3)2, > 99.0%), chromium nitrate (Cr(NO3)3, > 99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Potassium nitrate (KNO3, > 99.0%) and aluminum nitrate nonahydrate (Al(NO3)3·9H2O, > 99.0%) were purchased from Tianjin Guangcheng Chemical Reagent Co., Ltd. (Tianjin, China). Other organic reagents were of analytical grade and were received without further purification. The water utilized in the experiments was obtained using a UPH-IV ultrapure water apparatus (China) with a resistivity of 18.25 MΩ·cm. 2.2. Sample Preparation Eu3+/POM-PPCT complexes were prepared by dissolving POM-PPCT mixing appropriate amounts of Eu(NO3)3·6H2O in N,N-Dimethylformamide (DMF) with continuous stirring at room temperature. The sample solution was put in a constant temperature incubator for about 10 days to reach equilibrium. 2.3. Characterizations 2.3.1. Transmission Electron Microscopy (TEM). About 6 μL of sample solution was placed on carbon-coated copper grids and then dried. The morphologies of samples were studied on a JEOL JEM-1400 TEM (acceleration voltage, 120 kV) with a Gatanmultiscan CCD for collecting images. 2.3.2. Field-Emission Scanning Electron Microscopy (FE-SEM). 6


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For SEM observation, 5 μL of solution was dropped on a silica wafer surface, and most of solvent was removed with nitrogen to form a thin film. The samples were observed on JEOL JSM-6700F FE-SEM at 3 kV. 2.3.3. Atomic Force Microscopy (AFM). A droplet of sample solution was placed on a silica wafer surface, and the excess solution was evaporated with nitrogen in order to obtain a thin film. Images were recorded using a Cypher ES AFM (Asylum Research). 2.3.4. Spectroscopy Characterization. The FT-IR spectra were obtained from a VERTEX-70/70v FT-IR spectrometer (Bruker Optics, Germany). By taking 32 scans with a resolution of 4 cm−1, spectra from 4000 to 400 cm−1 were measured. We performed the spectral manipulation by using the OPUS 6.5 software package (Bruker Optics, Germany). UV/Vis measurements were performed using a HITACHIU-4100 spectrophotometer (Hitachi, Japan). Fluorescence emission spectra were measured on a PerkinElmer LS55 fluorophotometer (Perkin ELmer, U.K.). The fluorescence lifetime, quantum yields and time resolved fluorescence spectra of POM-PPCT/Eu(III) complex system by using a FLS-920 Fluorescent Spectrometer (Edinburgh Instrument), and the measurement of quantum yields were taken in an integrating sphere, which consists of a 120 mm inside diameter spherical cavity, and the same volume of solvent was used as the blank control. 2.3.5. Dynamic Laser Light Scattering (DLS) Measurements.33 To prepare dust-free solutions for dynamic light scattering (DLS) measurements, the 7


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sample solutions were filtered directly into dust-free light scattering cells through Millipore sterile membrane filter depending on the concentrations and the sizes of the aggregates. The light-scattering cells had been rinsed inside and outside with distilled (dust-free) acetone to ensure a dust-free condition before use. A standard laboratory-built laser light scattering spectrometer equipped with a Coherent Radiation 200 mW diode pumped solid-state (DPSS) 532 laser, operating at 532 nm and a Brookhaven Instruments (BI-9000AT) correlator was used for the DLS measurements. The spectrometer is capable of making measurements of both the angular dependence of absolute integrated scattered intensity over a scattering angular range of 20o to 140o and of intensity-intensity digital photon correlation over a similar angular range (DLS and depolarized DLS). About 2~3 mL of sample solutions were transferred into a special dust-free light scattering cell for light scattering measurements. The scattering cells were held in a brass thermostat block filled with refractive index-matching silicone oil. The temperature was controlled to within ±0.05oC. 3. RESULTS AND DISCUSSION Tetrabutylammonium (TBA+) as the counter-ion of POM-PPCT ensures that the molecules are quite stable due to the strong electrostatic association between polar heads and TBA+. This stability can be used to control the self-assembly behavior in solvent.11 The nitrogen atom in the three pyridine rings of tpy in PPCT possesses partly negative charge, δ-, which makes it a superior π-electron conjugated group and has a high ability of coordination with metal ions. It has been reported that the tpy 8


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group exhibits a specific interaction with Na+ to form a hydrogel.34 Interacting with K+, PPCT was applied for removing formadehyde.35 The tpy group has been used as building blocks toward self-assembly into gels, functional interface assemblies and well-defined architectures.36-38 The Eu3+/POM-PPCT supramolecular structures were prepared by a facile method; a certain amount of Eu(NO3)3 was added into POM-PPCT (1 mg/mL) DMF solution with stirring at a high speed. The complexes are in a colorless, transparent, and clear solution and emit strong red fluorescence under 365 nm UV-irradiation. The nonaqueous Eu3+/POM-PPCT solution manifests an obvious Tyndall effect (Figure S5 in the SI), indicating a colloidal dispersion system. The morphology and size of the aggregates were determined by DLS, TEM (Figure 2), SEM and AFM (Figure S6 in the SI). The formation of stable bilayer vesicles of the POM-PPCT/Eu3+ mixtures in DMF solution was first determined by means of DLS data.33 Figure 2 (a and b) shows the apparent hydrodynamic radius (Rh = kBT/6D, where kB is the Boltzmann constant, T is absolute temperature,  is the solvent viscosity, and D is the translational diffusion coefficient) distributions as determined from DLS of POM-PPCT/Eu3+ mixtures in DMF at different concentrations and at different scattering angles. At the concentration of 1 mg/mL POM-PPCT and 80 mg/mL Eu3+ in DMF solution (Figure 2a), the size distribution consists only of large structures with an average Rh of about 514.3 nm and a polydispersity of

2/2 ≈ 0.17, which is fairly narrow. The size

and the size distribution do not show an apparent angular dependence, indicating 9


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impermeable spherical shells, i.e., hollow spheres, in the limit of infinitely thin wall thickness.33 For the solution of 1 mg/mL POM-PPCT and 80 mg/mL Eu3+ in DMF diluted 20 times, similar results (Figure 2b) were observed. The size distribution peaked around Rh ≈ 515.6 nm with 2/2 ≈ 0.21, again with no apparent angular dependence. Both Rh values of DLS data also prove that the angular dependence of the DLS was small at these concentrations. The TEM images show that the regular spheroidal aggregations with a clear contrast between the center and the periphery can be clearly observed, indicating the typical thin-layered and collapsed vesicle morphology (Figure 2c-e). 1.0 0.8

1.0

o 90 , Rh = 508.3 nm

60o, Rh = 518.3 nm

0.8

•G ()

0.6 0.4

b

o 90 , Rh = 512.9 nm

a

45o, Rh = 516.4 nm

•G ()

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

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0.6

60o, Rh = 519.6 nm 45o, Rh = 514.4 nm

0.4 0.2

0.2

0.0

0.0 10

100

1000

10000

10

Rh / nm

100

1000

10000

Rh / nm

Figure 2. Apparent hydrodynamic radius distribution of Eu3+/POM-PPCT mixtures in DMF solution, the relative intensity contribution G∙G(G) as a function of the apparent

10


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hydrodynamic radius Rh. (a) 1 mg/mL POM-PPCT and 80 mg/mL Eu3+ in DMF solution, slight pink color, (b) above solution was diluted 20 times, colorless. (c, d, and e) TEM images of Eu3+/POM-PPCT mixtures in DMF solution. This result of height signifies the hollow structure of the vesicles rather than sphere structure. The

adsorption

spectra

of

Eu(NO3)3,

Eu3+/PPCT,

POM-PPCT,

and

Eu3+/POM-PPCT in DMF are shown in Figure 3a. The adsorption peak at 290 nm is attributed to benzene rings of the POM-PPCT. The one at 277 nm belongs to the interaction between Eu3+ and solvent molecules. After adding Eu3+ into the solution, a shoulder peak was observed at 298 nm. It is presumably associated with complex formation between Eu3+ and POM-PPCT. In order to prove the interaction site is the terpyridine group rather than amide linkage, Eu3+ was introduced into the PPCT solution. A shoulder peak at 298 nm was observed. The molecular-level interaction between the metal ion and ligand was verified through the FT-IR spectrum of the solution in the presence and absence of Eu3+. In the absence of Eu3+, the absorption peaks at 1256 and 1667 cm-1 are attributed to C–N and C=N stretching vibrations, respectively. Adding Eu3+, the absorption peaks were shifted to 1300 and 1703 cm-1, respectively, demonstrating the coordination between Eu3+ and tpy through the Eu3+-Ntpy ligand (Figure 3b). The energy-dispersive X-ray spectrum (EDS) analysis (Figure S7 in the SI) of the vesicles shows the characteristic X-ray energies of manganese (Mn), molybdenum (Mo) and europium (Eu), confirming the existence of the POM cluster and Eu3+ in the stable bilayer vesicles. The coordination of Eu3+ to the Ntpy of the ligand and a π-π stacking interaction are well illustrated to be the mechanisms for assembly of POM-PPCT and Eu3+. Both are key points for Eu3+/POM-PPCT vesicle formation. First, the π-π stacking interaction 11


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is attributed to the superior π-electron conjugation group. The formation of a cyclic structure rather than planar construction may be ascribed to TBA+. Second, the nitrogen atoms of tpy have a lone pair of electrons and a strong ability to bond with Eu3+. By coordination interaction, the molecules form a ring structure extending outward first, and then turned into vesicles. The length of the POM-PPCT molecule is determined as about 3.1 nm by the theoretical simulations of Materials Studio (MS). Combining the data analysis of AFM and TEM (Figures S6 and S8 in the SI), the average aggregate number of POM-PPCT in bilayer vesicles should be N = 7~15 via dividing the membrane thickness by the molecular length. Bilayer vesicles of Eu3+/POM-PPCT satisfy the experimental observations was illustrated in Figure 3c.

a

b

Transmittance

POM-PPCT 3+ PPCT/Eu Eu(III) 3+ POM-PPCT/Eu

Absorbance (a.u.)

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

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POM-PPCT 3+ POM-PPCT/Eu

1256

1667

1300 1703

280

300

320

340

360

380

400

Wavelength (nm)

4000

3500

3000

2500

2000

1500 -1

1000

500

Wavenumber (cm )

Figure 3. UV-vis absorption spectra of (a) Eu3+, Eu3+/PPCT, POM-PPCT and Eu3+/POM-PPCT in DMF, cEu3+ = 80 mg/mL. (b) FTIR spectra of POM-PPCT and

12


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Eu3+/POM-PPCT solutions. (c) Schematic illustration of the formation mechanism of the bilayer vesicles constructed by POM-PPCT and Eu3+. Eu3+ is one of the widely used lanthanides because it exhibits an intense red photoluminescence upon irradiation with UV radiation and has narrow transitions in the absorption and luminescence spectra.39 The fascinating fluorescence properties of bilayer vesicles induced by Eu3+ are shown in Figure S9 in the SI. From Figure 4a in the SI, the emission peak at 591 nm was assigned to the 5D0 → 7F1 transition, the emission peak at 616 nm to the 5D0 → 7F2 transition, and the emission peak at 367 nm to the fluorescence emission of POM-PPCT. Figure S10a in the SI shows the fluorescence emission spectrum of POM-PPCT. Adding Eu3+, a significant reduction of the emission intensity of POM-PPCT and a concomitant increase in fluorescence intensity from Eu3+ were observed, attributed to resonance energy transfer. With the increase of Eu3+ concentration, the intensity of the characteristic peaks at 519 and 616 nm gradually increase (Figure 4b). To further confirm energy transfer process and the sensitization of ligand molecule, we measured the fluorescence lifetime of the POM-PPCT in the absence and presence of Eu(III) in DMF, as shown in Figure 4c. The excited state decay profile showed the fluorescence lifetime of the donor (POM-PPCT) was quenched in the presence of acceptor (Eu(III)), substantiating the resonance energy transfer process. The fluorescence intensity of Eu(III) decreased and the fluorescence lifetime of the Eu(III) increased in the presence of POM-PPCT. We analyzed that the reason is the competition between POM-PPCT and solvent molecules could reduce the stability of 13


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the coordination environment and eventually lead to a decrease in fluorescence lifetime (Figure S10 and Table S1 in the SI). The sensitization efficiency ηsens. calculated by using the relationships developed by Werts et al40 is 7.2%. Details of the calculation procedure of the sensitization efficiency can be seen in the ESI (Figure S11 and Table S2 in the SI). We also measured the time-resolved fluorescence spectra of Eu(III) in the absence and presence of POM-PPCT, as shown in Figure S12 in the SI. From the test results, in the case of a certain light source, we found that POM-PPCT/Eu(III) showed three characteristic emission, and the emission peak at 591 nm was assigned to the 5D0 → 7F1 transition which could not observed in Eu(III) in DMF. We believe that it is possible to prove the sensitization of the ligand from the side. 350

3+

500

3+

POM-PPCT/Eu , cEu = 80 mg/mL 3+

400 300 200 100 0 350

b

cEu3+ / mg/mL

300

0 10 20 30 60 80 100

250 200 150

1.0x10

3

8.0x10

2

6.0x10

2

4.0x10

2

2.0x10

2

100

c

POM-PPCT POM-PPCT/Eu(III)

Counts

600

a

Counts

3+

Eu POM-PPCT 3+ POM-PPCT/Eu , cEu = 20 mg/mL

Fluorescence Intensity

700


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e

7

e

6

e

5

e

4

e

3

e

2

e

1

e

0

POM-PPCT POM-PPCT/Eu(III)

0

20

40

60

80

100

Time (ns)

50 0.0

400

450

500

550

Wavelength (nm)

600

0

580

590

600

610

620

630

Wavelength (nm)

5

10

15

20

25

30

35

40

45

50

Time (ns)

Figure 4. Fluorescence spectra of (a) POM-PPCT in the DMF solution in the absence and presence of Eu3+, (b) with different Eu3+ concentrations, (c) fluorescence lifetime measurements from the decay profile of excited states at 375 nm for POM-PPCT in the absence and presence of Eu(III) in DMF. T = 298.0 ± 0.1 K, λexe = 333 nm, cPOM-PPCT = 1 mg/mL. Fluorescent materials have drawn considerable attention owing to their potential applications in detection of heavy metal ions such as Cu2+.41,42 Bilayer vesicles of Eu3+/POM-PPCT are highly sensitive to Cu2+, which can be seen from Figure S13 (a 14


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and b) in the SI. The fluorescence emission (excitation wavelength, 333 nm) of bilayer vesicles was evidently quenched in the presence of Cu2+. The fluorescence emission spectra indicated that the fluorescence intensities at 591 and 616 nm were gradually weakened by increasing the amount of Cu2+, and the intensity was reduced to 50% at a concentration about 10 μM Cu(NO3)2. FTIR spectra provide the structural change of Eu/POM-PPCT bilayer vesicles. As shown in Figure S14 in the SI, the characteristic bands of C–N and C=N bonds of the POM-PPCT at 1300 and 1703 cm-1 shift to 1677 and 1256 cm-1, providing the evidence of the involvement of the nitrogen atom in disorganization with Eu3+. The infrared spectrum is quite similar to the POM-PPCT solution without the incorporation of Eu3+, proving that Cu2+ breaks the dynamic Eu3+-Ntpy bonds, i.e., the Eu3+/POM-PPCT coordination complexes were destroyed. We also measured the fluorescence lifetime of the Eu(III) of POM-PPCT/Eu(III) in the presence of Cu(II) with different concentration in DMF (Figure S15 and Table S3 in the SI). When the concentration of Cu(II) is low, fluorescence lifetime remains unchanged, substantiating there is only static quenching. When the concentration of Cu(II) is high, fluorescence intensity decreased and fluorescence lifetime increase, substantiating most of Eu(III) enter the DMF solution. This is the synergy between static quenching and dynamic quenching. This is also consistent with the result of a fitted curve with two slopes in our steady-state spectrum. Figure 5a shows a gradual decrease in fluorescence intensity (FI) at 616 nm with the increase of Cu2+ concentration, revealing that the bilayer vesicles are sensitive to Cu2+. The fluorescent quenching data were analyzed by plotting FI vs. log cCu2+. A good linearity between FI at 616 nm and log cCu2+ in the range from 30 to 300

15


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mmol/mL was obtained with a correlation coefficient of 0.994 (Figure 5b). The detection limit was calculated to be 24 nM, which was evaluated using 3σ/S (S is the slope of the calibration curve and σ is the standard deviation of the blank solution).43,44 The fluorescent chemosensors are compared in Table 1, indicating that Eu3+/POM-PPCT bilayer vesicles for Cu2+ detection are close to the previously fluorescent assay. The detection platform designed by bilayer vesicles possesses a high sensitivity for Cu2+, which may open up a new avenue for designing a high-efficiency detection platform of heavy metal ions.

a

cCu2+ / mol L

0 0.1 0.2 0.3 0.5 1.0 2.0 3.0 10 20 30 50 80 100 200 300 500

200 150 100

100

50 0

0 100 200 300 400 500 CCu (mol/L) 2+

50

0 580

590

600

610

620

200


150

−

Intensity

200


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

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b

150

100

50

0 0.1

630

1

10

100 -1

Log cCu2+ (μmol L )

Wavelength (nm)

Figure 5. (a) Fluorescence spectra of Eu3+/POM-PPCT in the presence of different concentrations of Cu2+, and (b) calibration curve for Cu2+ detection. Excitation and emission wavelength are 333 nm and 616 nm, respectively. Table 1. Comparison of the present approach with other fluorescent chemosensors for the detection of Cu2+ Sensing Materials Polyamine-Functionalized Quantum dots

Carbon quantum dots Graphene quantum dots

UCNPs Nanoparticles

UCNPs DNA-Silver nanoclusters

Detection

Correlation

Limit

coefficient

6.0×10-9 M

0.998

45

2.3×10-7 M

0.9991

46

1.0×10-6

0.99652

47

0.99

48

M

8.0×10-9 M 16


References

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CdTe nanoparticles-based

1.8×10-6 M

0.990

49

1.5×10-6 M

-

50

3×10-7 M

0.996

51

9×10-7 M

0.996

Benzimidazole-based

1.82×10-8M

0.99519

52

Sugar-rhodamine

0.20×10-6 M

0.9940

53

POM-hybrids

2.4×10-8 M

0.994

superparticles Zinc porphyrin-dipyridylamino Coordination compound

This method

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2,2′-dipyridylaminoanthracene

We examined the effect of cations on FI of bilayer vesicles in the presence of Cu2+. Then, NaNO3, KNO3, Mg(NO3)2, Pb(NO3)2, Al(NO3)3, Cr(NO3)3, Cu(NO3)2, and NH4NO3 at c = 10-3 mol/L were added into Eu/POM-PPCT bilayer vesicles. Because of the purple color for Fe(NO3)3 sample, cFe3+ = 10-6 mol/L was studied. From the fluorescence spectra in Figure 6 (a and b), one can see that Cu2+ shows a significant quenching effect on the fluorescence intensity at 593 and 616 nm. Fluorescence intensity of other metal ion systems all showed less enhancement or less decrease, indicating the highly selective sensitivity of Eu3+/POM-PPCT bilayer vesicles for Cu2+. The obvious fluorescence quenching with the addition of Cu2+ could be facilely observed by a simple visual detection under the ultraviolet lamp (Figure S16 in the SI). A significantly weaker red fluorescence of bilayer vesicles with Cu2+ is visible by naked eyes, providing a real-time monitoring. The quenching efficiency of Cu2+ was verified by monitoring the fluorescence intensity of the Eu3+/POM-PPCT system with addition of Cu2+. Polyionic mixed solutions including Na+, K+, NH4+, Mg2+, Al3+, Pb2+, Cr3+, Fe3+ were also studied. As illustrated in Figure 6 (c and d), Cu2+ exhibits a more significant quenching effect. After Cu2+ was incorporated in the mixed ions, the luminescence intensity of the samples was nearly completely quenched, further 17


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demonstrating the high selectivity of Eu/POM-PPCT bilayer vesicles for Cu2+.

a

2+

Cu + NH4

180

+

Na + K 2+ Mg 3+ Al 3+ Fe 2+ Pb 3+ Cr DMF

150

100

50

0 580

600

620



200

b

160 140 120 100 80 60 40 20 0

640

2+

Cu

Pb

2+

2+

Cr

2+

DMF Mg

Wavelength (nm) 120

80

c

100 DMF mix ions mix ions + Cu2+


100


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

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60 40 20 0 580

600

620

640

+

NH4

3+

Fe

Al

3+

K

+

+

Na

d

80 60 40 20 0

Wavelength (nm)

DMF

2+

Cu

mixed ions

mixed ions + Cu

2+

Figure 6. (a) Fluorescence spectra of Eu3+/POM-PPCT in DMF solutions with different ions. (b) Fluorescence intensity at 616 nm of Eu3+/POM-PPCT in DMF solutions with different ions. (c) Fluorescence spectra of Eu/POM-PPCT in DMF solutions and Eu3+/POM-PPCT in DMF with Cu2+ (10-3 M), mixed ions (Na+, K+, Mg2+, Al3+, Pb2+, Cr3+, NH4+, 10-3 M, Fe3+, 10-6 M), mixed ions + Cu2+ (10-3 M). (d) Fluorescence intensity at 616 nm of Eu3+/POM-PPCT in DMF and Eu/POM-PPCT in DMF with Cu2+ (10-3 M), mixed ions (Na+, K+, Mg2+, Al3+, Pb2+, Cr3+, NH4+, 10-3 M, Fe3+, 10-6 M), mixed ions + Cu2+ (10-3 M).

4. CONCLUSIONS In summary, a new Anderson-type POM-organic hybrid can self-assemble to form well-defined bilayer vesicles in DMF by controlling Eu3+. In contrast to lipid 18


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membranes or liposome vesicles, the well-defined POM-based “surfactants” have a distinct feature; instead of flexible hydrophobic tails, the modified POM has a rigid and confirmed-size cluster with a dominant intrinsic geometric constraint. Furthermore, chemistry permits rational design on the chemical nature of these inorganic and organic clusters, in terms of hydrophobicity, geometric constraint (size and shape), and charge (cationic, anionic, or neutral; univalent or multivalent) as well as spacing between the charges and the hydrophobic part. Because of its close control of the modified structures, this versatile synthetic bilayer vesicles controlled by lanthanide ions might find diverse applications in detection of heavy metal ions, biology and medicine as an alternative to lipids and liposomes.

ASSOCIATED CONTENT The supporting information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: (+86) 531-8856-4750 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the NSFC (Nos. 21420102006 & 21773144) and by the NSF of Shandong Province (No. ZR2018ZA0547).

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