Tunable Aggregation-Induced Emission of Polyoxometalates via

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Tunable Aggregation-Induced Emission of Polyoxometalates via Amino Acid-Directed Self-Assembly and Their Application in Detecting Dopamine Han Zhang, Ling-Yu Guo, Zengchun Xie, Xia Xin, Di Sun, and Shiling Yuan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03709 • Publication Date (Web): 19 Nov 2016 Downloaded from http://pubs.acs.org on November 23, 2016

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Tunable Aggregation-Induced Emission of Polyoxometalates via Amino Acid-Directed Self-Assembly and Their Application in Detecting Dopamine Han Zhang a, Lingyu Guo a, Zengchun Xie b, Xia Xin a, b *, Di Sun a *, Shiling Yuan a * a

Key Lab for Colloid and Interface Chemistry of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, P. R. China.

b

National Engineering Technology Research Center for Colloidal Materials, Shandong University, Jinan, 250100, P. R. China.

*

Author to whom correspondence should be addressed, E-mail: [email protected].

Phone: +86-531-88363597. Fax: +86-531-88361008 *Author to whom correspondence should be addressed, E-mail: [email protected]. Phone: +86-531-88364218. Fax: +86-531-88564750 *

Author to whom correspondence should be addressed, E-mail: [email protected].

Phone: +86-531-88365896. Fax: +86-531-88564750 1

ACS Paragon Plus Environment

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Abstract In this work, through the aqueous phase self-assembly of an Eu-containing polyoxometalate (POM), Na9[EuW10O36]·32H2O (EuW10) and different amino acids, we obtained the spontaneous formed vesicles which showed the luminescence enhancement for EuW10 and arginine (Arg), lysine (Lys) or histidine (His) complexes, but luminescence quenching for EuW10 and glutamic acid (Glu) or aspartic acid (Asp) complexes. The binding mechanisms between them have been explored at the molecular level by using different characterization techniques. It was found that EuW10 acted as the polar head groups interacted with the positively charged residues for alkaline amino acids, protonated amide groups for acidic amino and nonpolar acid aminos through electrostatic interactions, and the rest segments of amino acids served as relatively hydrophobic parts aggregated together forming bilayer membrane structures. Moreover, the different influences of amino acids on the fluorescence property of EuW10 was revealed that is the electrostatic interaction between the positive charged group of amino acid and the polyanionic cluster dominates the fluorescence properties of assembles. Furthermore, a turn-off sensing application of the EuW10/Arg platform to probe dopamine (DA) against various other biological molecules such as neurotransmitters or amino acids was also established. The concept of combining POMs with amino acids extends the research category of POM-based functional materials and devices. Keywords: Eu-containing polyoxometalate, amino acids, dopamine, vesicle, fluorescence, detection.

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Introduction Polyoxometalates (POMs), one kind of inorganic clusters, are a well-known class of polyanions with a rigid framework, monodispersed size, water solubility, versatile properties, and biofunctionality [1-3]. With these superior structural and physicochemical properties, POMs have shown various applications in the fields of catalysis, biomedicine, and materials science [4-7]. The direct use of POM-containing materials can provide a simpler way to construct POM-based self-assemblies, in which the functionalities of POMs may be largely amplified or modified. Among various POMs, lanthanide-containing POMs that possess excellent photoluminescent properties with narrow emission bands, large Stokes shift, high fluorescence quantum yield and long emission timescale (micro- to milliseconds) have aroused progressively attentions and interests in recent years [8-11]. However, incompatibility with hydrophobic organic materials and weakened luminescence functionality in aqueous solution extremely suppressed their practical utilization. Therefore, further fabrication of POM-based materials and devices requires the manipulation of the POM clusters on the nanoscale through solution-based self-assembly [12]. For this purpose, cationic surfactants, polymer or biomolecules were used to replace the counter-cations of POMs to modify the surface properties of POMs and different kinds of aggregates such as micelles, vesicles, hydrogel, lyotropic phases and films consequently were formed in the assembly processes [13-16]. For example, for surfactant-encapsulated complexes (SECs), Wu et al. investigated an unusual vesicular assembly of (DODA)4H[Eu(H2O)2SiW11O39] (DODA= dimethyldioctadecylammonium) in a solution environment [17] and stable assemblies of (DODA)4SiW12O40 as onion-like spheres in chloroform [18]. They also fabricated the hybrid nanospheres from the co-assembly of Eu-containing POM (EuW10) and arginine/lysine-rich peptide 3


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in aqueous solution, the luminescence of EuW10 enhanced as a consequence [19]. Besides, 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 [20]. Wan and co-workers prepared self-healable supramolecular luminescent hydrogels from Eu-containing POM and cationic-neutral-cationic ABA triblock copolymers [21]. Among above various cationic molecules, amino acids are considered as the major building blocks of all naturally occurring peptides and proteins. The side chains of amino acids vary a lot from each other, making them have the potential usefulness in chiral molecular recognition and selection processes [22, 23]. Furthermore, among different kinds of aggregates, especially for vesicles, they are receiving considerable attention because of their potential in biological field as novel compartments, artificial cell membranes, especially drug and gene delivery agents [24-26]. There are many POMs formed vesicle in various solvents [27-31], however, the vesicles formed in organic

solvent

would

limit

their

applications

in

vivo

and

vesicle

constructed

by

lanthanide-containing POMs in aqueous solution is also very rare [32-35]. Thus, in this work, a Weakley-type POM (a europium ion sandwiched by two Lindqvist-type POM, Na9[EuW10O36]32H2O (EuW10)) assembled with various amino acids into vesicles in aqueous solution were reported. The microstructures and properties of EuW10/amino acids mixed systems were characterized using transmission electron microscopy (TEM), freeze fracture transmission electron microscope (FF-TEM), Atomic force microscopy (AFM), confocal laser scanning microscope (CLSM), X-Ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, ζ-potential, and fluorescence spectra. It was found that the luminescence of EuW10 was effectively enhanced after combining with arginine (Arg), lysine (Lys) and histidine (His), while 4


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quenched after combining with glutamic acid (Glu) and aspartic acid (Asp). The enhanced or quenched emission was dependent on the electrostatic interaction between the polyanionic clusters and positive charged groups. The hydrophobic force derived from amino acids induced the formation of the vesicle structure. Moreover, the EuW10/Arg system can act as an excellent sensing platform to sense dopamine (DA) with high sensitivity and selectivity compared to other biomolecules. Experimental section Materials Na9(EuW10O36)·32H2O was prepared as described by Sugesta and Yamase [36]. Arginine (Arg), lysine (Lys), histidine (His), Alanine (Ala), phenylalanine (Phe), aspartic acid (Asp), glutamic acid (Glu), acetylcholine chloride (AC), ascorbic acid (AA), dopamine (DA) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ultrapure water with a resistivity of 18.25 MΩ cm was obtained using a UPH-IV ultrapure water purifier (China). Sample preparation Sample solutions were prepared by adding 0.4 mL EuW10 aqueous solution (1.75 mM) to 0.6 mL amino acids aqueous solution with stirring. Samples were allowed to equilibrate in a thermostat under incubation at 20.0 ± 0.1 °C for at least 3 days before other characterizations performance. Dopamine detection 20 µL DA solution of different concentration were added stepwise to 3 mL 0.7 mM EuW10/1.5 Arg solution. As a very small amount of DA was added, the final volume of the solution was nearly unchanged. After the mixture was incubated within 30 min, the fluorescence spectra of the mixture were recorded. The selectivity for DA was confirmed by adding other related analogues instead of 5


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DA in a similar way. All experiments were performed at room temperature Methods and characterizations For transmission electron microscopy (TEM) observations, about 5 µL of solution was placed on a carbon-coated copper grids and the excess solution was wicked away with a piece of filter paper. The copper grids were freeze-dried and observed on a JEOL JEM-100 CXII (Japan) at an accelerating voltage of 80 kV with a Gatan multiscan CCD for collecting images. For Freeze Fracture Transmission Electron Microscope (FF-TEM) characterizations, a small amount of sample solution was placed on a 0.1 mm thick copper disk covered with a second copper disk. Then the copper sandwiched with the sample was plunged into liquid propane cooled by liquid nitrogen. Fracturing and replication were carried out on a Balzers BAF-400D equipment at 150 C. Pt/C was deposited at an angle of 45. The replicas were examined on a JEOL JEM-100 TEM operated at 80 kV. High-resolution transmission electron microscopy (HRTEM) images were recorded on a HRTEM JEOL 2100 system operating at 200 kV. FE-SEM observations were carried out on a JSM-6700F. Atomic force microscopy (AFM) images were examined on Dimension Icon (American) equipped with SCANASYST-AIR silicon nitride probe with scan Asyst. Samples were applied to silica wafers, which were subjected to freeze-drying in a vacuum extractor at -60 ℃ for one day and the observations was operated. The fluorescent images were acquired by confocal laser scanning microscope (CLSM) (Panasonic Super Dynamic II WV-CP460) with excitation wavelength at 488nm. FT-IR spectrum was recorded on a VERTEX-70/70v spectrometer (Bruker Optics, Germany). A microelectrophoresis meter (JS94H, Zhongchen Ltd. Co., Shanghai) was employed to determine the zeta potential and the particle size. The fluorescence spectra were performed on a LS-55 6


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spectrofluorometer (PerkinElmer, Waltham, MA, USA) with a quartz cell (1×1 cm). The fluorescence lifetimes were performed on an Edinburgh Instruments FLS920 luminescence spectrometer (xenon lamp, 450 W). The measurement of luminescence lifetime was performed by monitoring the luminescence intensity decay at 618 nm with time excited by a mF920 microsecond flash lamp. X-Ray photoelectron spectroscopy (XPS) data were collected by an X-ray photoelectron spectrometer (ESCALAB 250) with a monochromatized Al Ka X-ray source (1486.71 eV). Results and Discussion The self-assembled structures of EuW10/Arg system The self-assembled morphologies of 0.7 mM EuW10/1.5 mM Arg in aqueous solution were firstly studied with transmission electron microscopy (TEM). TEM micrograph (Fig. 1a) shows that the aggregates have a loose appearance, more important, this image shows a clear contrast between the center and the periphery of the spheres, indicating a typical vesicular morphology. Moreover, the EuW10/Arg colloidal suspension presented a clear Tyndall scatter phenomenon (inset in Fig. 1a) under the laser illumination (λ = 635 nm), confirming the presence of abundant of nanosiszed vesicles. The average diameter of vesicles is about 78 nm by the statistical analysis of 100 vesicles from the TEM images. However, the vesicles size measured by TEM is smaller than that derived from DLS measurements (Dh = 122 nm) since the vesicles were observed in a dried state during the TEM measurement whereas the hydrodynamic radius was obtained from DLS (inset in Fig. 1a). As the concentration of Arg increased from 0.5 to 1.5 mM, the vesicles grow gradually which was confirmed by DLS measurement with Dh values varied from 48 nm to 70 nm, and to 122 nm ultimately (Fig. S1). During the loss of solvent molecules, some vesicles collapsed with the enlarged

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height-to-diameter ratio nearly up to 1:10 (Horiz distance: 75 nm; Vert distance: 7.4 nm), which can be clearly observed in AFM images (Fig. 1b and c). and also prove the hollow structure of the vesicles. Further increasing the concentration of Arg caused the sample become to the precipitated phase, and the particles shrinked to 55 nm with coarse surfaces (Fig. S2 a and b). The particles possess the order lattice structure and fluorescence property (Fig. S2 c-e). The change in the morphology is probably explained as follows: an increase of the concentration of Arg leads to the increase of the electrostatic force and interfacial tension, and further expansion of the vesicles in the solution is impossible, thus the morphology changes from vesicles to precipitates [37]. FF-TEM as a powerful technique can provide the direct information on assembled structures in their hydrated state. The vesicles are found with smaller diameters and higher curvatures, and the bilyers are scarcely deformed, which correlates the stiff bilayer membranes (Fig. 1d). The vesicles are comprised of the giant POM head group as the hydrophilic head and Arg as the relatively hydrophobic part. This combination favors to form unsymmetrical membranes with high curvature [38]. From CLSM (Fig. 1e), we observed the vesicles with fluorescence, which indicated EuW10 was successfully incorporated in the vesicles. Energy-dispersive X-ray (EDX) measurements (Fig. 1f) also showed europium and tungsten elements from EuW10, carbon and nitrogen elements from Arg which confirmed both EuW10 and Arg participated in the construction of vesicles.

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Fig. 1 (a) TEM image of 0.7 mM EuW10/1.5 mM Arg (insets are the pictures showing the Tyndall effect under the irradiation by a light beam and the hydrodynamic radius of vesicles observed by DLS), (b) AFM height image of 0.7 mM EuW10/1.5 mM Arg. (c) Sectional height profile of two collapsed vesicles for the rectangular region in c. (d) FF-TEM image, (e) CLSM image and (f) EDS spectrum of 0.7 mM EuW10/1.5 mM Arg. XPS, FTIR and ζ-potential analysis for EuW10/Arg system In order to further investigate the properties of EuW10/Arg system, Eu3+ and W6+ in the hybrid material of EuW10/Arg were characterized at the molecular level by XPS spectroscopy (Fig. 2 and Fig. S3). The binding energy of Eu3d in EuW10/Arg mixed system is 1133.6 eV (Fig. 2A) similar to that of in EuW10 of 1134.0 eV (Fig. 2B), which indicates the Eu atoms remained its +3 valence in the EuW10/Arg hybrid material [39]. Similarly, as shown in Fig. 2C, the W4f doublet (W4f7/2 and W4f5/2) for EuW10 can be observed at 35.3 and 37.4 eV, which corresponds to the hexavalent state W6+ [40], whereas the W4f doublet (W

4f7/2

and W

4f5/2)

in EuW10/Arg mixed system can be observed at 35.4

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and 37.5 eV which do not change so much (Fig. 2D). Based on above analysis, the binding energies of Eu3+ and W6+ do not obviously change after the self-assembly of EuW10 and Arg which suggests that the basic structure of the EuW10 is retained after assembly.

A

B

Eu3d5/2

Intensity

Intensity

Eu3d5/2

1140

1138

1136

1134

1132

1130

1128

1126

1140

1138

1136

C

1134

1132

1130

1128

1126

Bing Energy (ev)

Bing Energy (ev)

D

W4f 7/2

W4f 7/2

W4f 5/2

W4f 5/2

Intensity

Intensity

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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38

36

34

38

Bing Energy (ev)

36

34

Bing Energy (ev)

Fig. 2 Eu3d XPS spectra of (A) EuW10 and (B) 0.7 mM EuW10/1.5 mM Arg. W4f XPS spectra of (C) EuW10 and (D) 0.7 mM EuW10/1.5 mM Arg. FT-IR spectroscopy is used to characterize the atomic interaction vibration

within the

supramolecular assemblies. In Fig. 3A, EuW10 showed four main vibration peaks of ν(W=Od), ν(W–Ob–W), and ν(W–Oc–W) at 942, 841, and 788 and 703 cm-1, where Ob is the bridged oxygen of two octahedra sharing a corner, Oc is the bridged oxygen of two octahedra sharing an edge, and Od is the terminal oxygen [41]. Arg exhibited νs(CH2), νas(CH2), and ν(C-N) at 2945, 2868, and 1326 and 1131 cm-1. After EuW10 incorporated with Arg, the peaks of ν(W=Od), ν(W–Ob–W), and ν(W–Oc–W) 10


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shift to 928, 841, and 784 and 708 cm-1, respectively. In addition, the bands of νas(CH2), νs(CH2), and ν(C-N) move to 2927, 2858, and 1274 and 1121 cm−1. These results illustrated EuW10 and Arg are successfully incorporated in the vesicles and the peaks shifts indicated there may be electrostatic and hydrogen bond interactions between EuW10 and Arg. The results of ζ-potential were shown in Fig. 3B. The ζ-potential of EuW10 in aqueous solution changed along adding Arg into it. In detail, when the negatively charged EuW10 interacted with the positively charged Arg, the ζ-potential values increased progressively. The linear increase of the ζ-potential values reflected the interaction between EuW10 and Arg was essentially dominated by electrostatic interactions, which neutralizing the net values of the negative charge of EuW10 with more Arg [19].

B

a

-25

1131

b

2945

2868

1326

c

942 2927

703 788 841

1274 1121 928 784708 841

2858

ξ-Potential/mV

A Absorbance

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-30

-35

-40 3500

3000

2500

2000

1500

1000

500

0.0

0.3

0.6

Wavenumber(cm-1)

0.9

1.2

1.5

CArg/mM

Fig. 3 (A) FTIR spectra of (a) Arg, (b) EuW10 and (c) 0.7 mM EuW10/1.5 mM Arg. (B) The ζ-potential distribution of EuW10 (0.7 mM) binding with Arg of different concentrations. The measurement was performed at room temperature for three times (25 °C). The influence of Arg on the fluorescent properties of EuW10 EuW10 possesses excellent photoluminescent properties with narrow emission bands, large Stokes shift, high fluorescence quantum yield and long lifetime. The emission of EuW10 displayed four 11


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bands at 591, 618, 651, and 696 nm, which were assigned to the transition of 5D0→7 F1 (591 nm), 5D0

→7F2 (618 nm), 5D0→7F3 (651 nm), 5D0→7F4 (696 nm) transition of Eu3+, respectively. The 5D0→7F1 transition is not affected by the local chemical environment of Eu ion because of its magnetic-dipolar origin. On the other hand, the 5D0→7F2 transition is very sensitive to the local environment due to its electric-dipolar origin: The transition intensity increases with the decrease of Eu3+ symmetry. Therefore, the ratio between these two transitions (5D0→7F2)/(5D0→7F1) is normally used as an indicator of the coordination state and site symmetry of Eu3+ ions and is a very good index for the change of the micro-environments of the POMs [42, 43]. It is also known that the luminescence of Eu3+ ions in aqueous solutions is strongly quenched by water molecules, since the 5D0 state is affected by coupling with OH oscillators [44]. Thus, the influence of Arg on the fluorescence property of EuW10 was investigated as below. Fig. 4A shows that the luminescence intensity of EuW10 enhanced gradually as the addition of Arg. Such enhancement can be detected by the naked eye (the inset of Fig. 4A). Fig. 4B clearly shows that the ratio between the two typical transitions,

I618/I591, increased from 1.54 to 2.22 upon successive addition of Arg (max. to 1.5 mM) and finally reached to a doubled intensity compared to EuW10, indicating the micro-environments around EuW10 changed with the addition of Arg. Moreover, it can be seen directly that the luminescence intensity of EuW10 at 618 nm increased in the process (inset of Fig. 4B). In the current case, Arg served as ligands interacted with EuW10 expelling water molecules surrounding EuW10, and finally induced a significant enhancement to the emission intensity [45]. To verify this assumption, we further performed time-resolved fluorescence decay to investigate the mechanism of the enhanced response of EuW10 toward Arg. The luminescence lifetime of EuW10 before and after binding with Arg were measured, as shown 12


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in Fig. 4C and Table 1. The luminescence decay curve of the highly concentrated EuW10 (0.7 mM) solution was found to be bi-exponential decay and supplied two different lifetimes and proportions [46]. The shorter-lived component (0.122 ms) is assigned to Eu3+ emission influenced by the aqueous environment similar to that reported earlier, while the longer-lived component (1.042 ms) is indicative of a reduction in the nonradiative pathway, possibly by the lack of coupling to water [47]. After incorporation with Arg, a decrease in the amount of the shorter-lived component (0.078 ms) and a concomitant increase in the longer-lived component (1.843 ms) gave an overall longer lifetime. All these together with the time-resolved fluorescence spectra suggested the strong interaction between Arg and EuW10 is responsible for the effective luminescence enhancement [48]. Moreover, the influence of pH on the fluorescent properties of EuW10/Arg assemble was also investigated (Fig. S4). The hydrochloric acid or sodium hydroxide was added to 0.7 mM EuW10/0.9 mM Arg (pH = 6.6) to adjust the content of the positive charge of Arg. It was observed that the fluorescence was quenched at pH = 7.2 but enhanced at pH 6.0. It can be rationalized that Arg act as the cation here could interact with EuW10 cluster through electrostatic interaction. Adding acid will protonate the amino group making Arg more positive, then strengthening the electrostatic interaction and expelling more water molecules, thus the fluorescence was enhanced. When adding alkali to this system, the opposite variation trend of luminescence was observed which further indicated that our system is mainly assembled by the electrostatic interaction.

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7F4 5D0

800

600 3

10 2.0

400 0.0

0.4

0.8

1.2

CArg mM (

1.6

)

log(counts)

7F3 5D0

7F1

600

C 2.2

I 618

5D0

900

B

I 618 /I 591

CArg/mmol L-1 0 0.20 0.40 0.60 0.90 1.05 1.30 1.50 1.60

7F2

A

5D0

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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1.8

2

10

b

300 1.6

1

10 0 550

600

650

700

Wavelength (nm)

0.0

0.3

0.6

0.9

1.2

1.5

CArg(mM)

a

0

5

10

15

Time/ms

Fig. 4 (A) Variation of the fluorescence spectra of EuW10 (0.7 mM) upon the titration of Arg. The inset is the image of 0.7 mM EuW10 (left) and 0.7 mM EuW10/1.5 mM Arg (right) under UV light. (B) The intensity ratios of 618 to 591 nm (I618/I591) along the gradually addition of Arg. Inset is the intensity change of EuW10 at 618 nm in the process. (C) Time-resolved decay of EuW10 luminescence of (a) 0.7 mM EuW10, (b) 0.7 mM EuW10/1.5 mM Arg. The POM was excited at 260 nm and monitored at 618 nm. Table 1 Time-resolved decay data of EuW10 and EuW10/Arg. Sample

τ1/µs (%)

τ2/µs (%)

0.7 mM EuW10

249.8954 (48.91)

2037.7440 (51.09)

0.7 mM EuW10/1.5 mM Arg

252.7114 (30.76)

2660.8237 (69.24)

The influence of Asp on the morphology and fluorescence properties of EuW10 In order to reveal the effects of various amino acids on the assemble behavior of EuW10, Asp, were then chosen to form assemble with EuW10 for comparison. Firstly, TEM and AFM were employed to investigate the assembled aggregates of EuW10/Asp. It can be seen that vesicles were 14


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also observed in TEM images (Fig. 5a). The vesicular nature is evidenced by the higher transmission in the center than around the periphery of the dark capsule. The POMs should be located at the vesicle surface based on the observation of a very narrow border and high imaging contrast. Some vesicle aggregated to the necklace-like structure also can be observed which ascribes to the concentration induced aggregation behavior [49]. The vesicles structure was further investigated by AFM (Fig. 5b). The AFM images of the dried particles showed that their horizontal sizes are much larger than the corresponding vertical heights by ten times (Horiz distance: 81 nm; Vert distance: 7.6 nm) (Fig. 5c), indicating a thin layered and collapsed vesicle structure. Fig. 9 showed a variation of the fluorescence of EuW10 depending on the amount of added Asp. With the addition of Asp, the emissions of EuW10 were gradually quenched (Fig. 5d) which also can be directly observed in the inset of Fig. 5e. I618 /I591 increased from 1.54 to 1.76 along with the addition of Asp (Fig. 5e). With the addition of Asp, more protonated amino group and deprotonated carboxy group existed but the decrease of degree of ionization rendered the pH of the sample changed from 6.2 to 5.0. Therefore, in this case, the electrostatic interaction between protonated amino group and EuW10 cluster is weaker than that of EuW10/Arg assembly. The hydrated protons could approach polyanionic cluster driven by the electrostatic attraction, yielding a hydrated microenvironment around the cluster that is why the emission of EuW10 was quenched [50]. In time-resolved fluorescence spectra (Fig. 5f and Table S1), the biexponential decay for EuW10 becomes to monoexponential upon incorporation with Asp. It is well known that the high-energy vibrations associated with the hydroxyl groups can couple with the excited electronic states of the lanthanide ions lead to quenching the luminescence of the europium ion and shortening the luminescence lifetime. In our case, EuW10 clusters interact with the positively charged parts of Asp 15


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to reduce the interactions of POM–POM in this medium [51], and the hydrated protons produce a hydrated microenvironment around POM enhanced deactivation of the excited state via nonradiative relaxation [52]. Thus, the short mono-exponential behaviour was observed after the addition of Asp.

Fig. 5 (a) TEM image and (b) AFM image of 0.7 mM EuW10/5.0 mM Asp. (c) Corresponding height graph of (b). (d) Fluorescence spectra of EuW10 incorporated with Asp. The concentration of EuW10 was fixed at 0.7 mM while the concentrations of Asp varied. (e) The intensity ratios of 618 to 591 nm (I618 /I591) with the gradual addition of Asp. Inset is intensity variation of the peak at 618 nm. (f) Time-resolved decay curves of (1) 0.7 mM EuW10, (2) 0.7 mM EuW10/5.0 mM Asp. The POM was excited at 260 nm and monitored at 618 nm. The mechanism for the EuW10/Amino acids assembly To systematically investigate the influence of amino acids on the self-assembly of EuW10, several other acidic amino acid (Glu), alkaline amino acids (Lys and His) and nonpolar amino acids (Leu, Ala and Phe) were selected. The morphology of typical samples was characterized by TEM (Fig 6 16


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a-d), it can be seen that all amino acids could interact with EuW10 to form vesicles with different contrasts. The tunable ability of amino acids on the fluorescence properties were also performed in detail. All results indicated that the emission of 0.7 mM EuW10 can be enhanced by adding alkaline amino acids (Arg, Lys and His) while quenched by acidic amino acids (Glu, Asp) and not obviously influenced by nonpolar amino acids (Leu, Ala and Phe) at the same concentration (Fig. 6e). The intensity of the band at 618 nm for EuW10/amino acids can be directly seen in Fig. 6f.

Fig. 6 TEM images of (a) 0.7 mM EuW10/1.5 mM Lys, (b) 0.7 mM EuW10/1.5 mM His, (c) 0.7 mM EuW10/1.5 mM Phe and (d) 0.7 mM EuW10/1.5 mM Glu. (e) Fluorescence spectra of EuW10 incorporated different amino acids. The concentration of EuW10 and amino acid is fixed at 0.7 mM and 1.5 mM, respectively. (f) The intensity of the bands at 618 nm in Fig. 6e. The binding sites of amino acids was analysed. Addition amino acids to 0.7 mM EuW10 (pH = 5.8 ), they will carry both protonated –NH2 (α) and ionized –COOH (α) but with different charged R groups. For alkaline amino acids, Arg (pI = 10.76), Lys (pI = 9.60) and His(pI = 7.60) with positively charged R groups interact with anionic clusters easily through electrostatic interaction, which was 17


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demonstrated in the work of Li et al in detail [45]. For nonpolar amino acids, Leu (pI = 6.01), Ala (pI = 6.11) and Phe (pI = 5.49), and acidic amino acids, Glu (pI = 3.15) and Asp (pI = 2.85), the protonated –NH2 (α) is the possible binding site for anionic cluster. However, the interaction between protonated –NH2 (α) and EuW10 for acidic amino acids is weaker than nonpolar amino acids due to the negatively charged R groups impose repulsive force on the anionic clusters. From the above results and considering the possible binding sites, the similar aggregates and their different influences on the fluorescence of EuW10. The assembly mechanism can be reasonalized as following: EuW10 with the electron-donating nature acted as the large polar head groups interacted with the positively charged residues of alkaline amino acids, while binded to protonated amide groups for acid and nonpolar amino acids through electrostatic interaction, meanwhile the rest parts served as relatively hydrophobic part aggregated together forming a bilayer membrane structure. The assembly process was shown in Scheme 1. Furthermore, the influences of amino acids on the fluorescence behavior of EuW10 intensively depend on their electrostatic interactions. The interaction between the positively charged group of amino acid and the polyanionic cluster is strong for alkaline amino acid, moderate for non-charged amino acid, and weak for acidic amino acid. Therefore, less water molecules were expelled and more hydrated protons approached the polyanionic cluster which induced the fluorescence from enhancement to quenching.

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Scheme 1. Schematic illustration of the vesicles formed by EuW10 and amino acids. Detection of DA using EuW10/Arg platform Fluorescent materials have drawn considerable attention owing to their potential applications in a variety of fields, such as bioimaging, optoelectronic devices, and sensing [53–56]. Dopamine (DA) is one of the most important catecholamine neurotransmitters in the mammalian central nervous system. Abnormal DA concentration in the brain may result in serious disease, such as Parkinsons disease [57]. In the current study, we investigated the sensing application of EuW10/Arg platform to probe DA. Just as depicted in Fig. 7A, the fluorescence spectra of supramolecular structures were evidently quenched in the presence of DA. The decreased emission at 618 nm is proportional to the DA concentration in the range from 0–20 µM, and the detection limit is 3.2 µM based on a 3δ/slope (Fig. 7B). The fluorescence quenching, in here, is based on a competition mechanism. It has been proved that hydrogen bonds acted as a bridge between EuW10 and other proton acceptors which blocked the intramolecular luminescent resonant energy transfer between WO6 octahedron and Eu3+ ion, induced fluorescence quenching [58, 59]. Thus, it is speculated that the DA can substitute Arg to form assemble with EuW10 through hydrogen bond interaction between the ammonium group of DA and 19


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the oxygen atom of EuW10, leading to the fluorescence quenching. To determine whether our system was specific for DA, the influence of possible foreign substances on the present assay was tested, including Ala, Phe, Asp, Glu, AC, AA, DA. It could be seen that only DA effectively quenched the fluorescence of EuW10/Arg system (Fig. 7C). The results clearly demonstrated that the EuW10/Arg based fluorescent sensor was highly selective toward DA over the other biomolecules. Our method provides simplicity, convenience, and rapid implementation advantages and has potential application for the detection of DA in a complex environment.

300

C

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Phe

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VC

DA

Fig. 7 (A) Variation of the fluorescence spectra of 0.7 mM EuW10/1.5 mM Arg with the addition of DA (0 to 1000 µM). (B) The relationship between (I0-I)/I0 and DA. Inset is a linear region. I is the fluorescence intensity of EuW10/Arg in the presence of DA, I0 is the fluorescence intensity of EuW10/Arg at 618 nm, respectively. (C) Fluorescence response of the EuW10/Arg system to DA; All the concentrations of the biological molecules were 500 µM. Conclusion In summary, we fabricated inorganic–organic hybrid vesicles using EuW10 as the inorganic component and different natural amino acids (including Arg, Lys, His, Glu, Asp, Leu, Ala and Phe) 20


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as the organic component. It can be observed that all the amino acids could interact with EuW10 to form vesicles. The electrostatic interaction between the positively charged residues for alkaline amino acids, protonated amide groups for acidic and nonpolar amino acids and the polyanionic cluster combined the hydrophobic effect from the rest part of amino acid derived above assembly. All results indicated that the emission of 0.7 mM EuW10 can be enhanced by adding alkaline amino acids (Arg, Lys and His) while quenched by adding acidic amino acids (Glu, Asp) and not obviously influenced by adding nonpolar amino acids (Leu, Ala and Phe) at the same concentration. Furthermore, the hybrid material served as a fluorescence sensor with the ability to detect DA versus various biomolecules efficiently. We expect that these amino acid-inorganic hybrids will have important applications in biosensors, bioanalytical devices, pharmaceutical applications and industrial biocatalysis. Supporting Information Further characterizations (DLS, TEM, HR-TEM, CLSM image, XPS, the influence of pH on the fluorescence spectra and Time-resolved decay data) are reported. Acknowledgement We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21573130, 21173128, 21201110 and 21571115) and Young Scholars Program of Shandong University (2016WLJH20). References (1) Long, D. L.; Burkholder, E.; Cronin, L. Polyoxometalate clusters, nanostructures and materials: from self-assembly to designer materials and devices. Chem. Soc. Rev. 2007, 36, 105-121. 21


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Tunable Aggregation-Induced Emission of Polyoxometalates via

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