Synthesis of Lanthanide-Based Room Temperature Ionic Liquids with

Feb 9, 2016 - ... an outstanding application prospect in “green” fluorescent sensor and can ... Fangyuan Gai , Li Yin , Mengmeng Fan , Ling Li , J...
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Research Note/Communication

Synthesis of lanthanide-based room temperature ionic liquids with strong luminescence and selective sensing of Fe(III) over mixed metal ions Ben-Han Fan, Jie Wei, Xiao-Xue Ma, Xiao-Xue Bu, Nan-Nan Xing, Yi Pan, Ling Zheng, and Wei Guan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03947 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 15, 2016

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Synthesis of lanthanide-based room temperature ionic liquids with strong

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luminescence and selective sensing of Fe(III) over mixed metal ions

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Benhan Fan, Jie Wei, Xiaoxue Ma, Xiaoxue Bu, Nannan Xing, Yi Pan, Ling Zheng, Wei Guan*

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College of Chemistry, Liaoning University, Shenyang 110036, P. R. C.

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Abstract: Two new lanthanide-based room temperature ionic liquids [Bmim][Ln(NO3)4] (Bmim =

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1-butyl-3-methylimidazolium; Ln = Dy, Sm) were synthesized and characterized. They show good

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luminescent properties and could be used as good soft luminescent materials. This kind of hydrostable

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and eco-friendly ionic liquids were firstly used to explore the sensing behavior for Fe(III) ions, exhibit

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a highly specific recognition and not interfered by the following common mental ions: Ca(II), Al(III),

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Zn(II), Cu(II), Pb(II), Hg(II), Cd(II), Co(II), Fe(II), Ni(II) and Cr(III). These lanthanide-based ionic

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liquids with strong luminescence have an outstanding application prospect in “green” fluorescent

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sensor and could be widely used in actual detection of Fe(III) in the aqueous solutions.

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Keywords: Ionic liquid; Lanthanide; Luminescence; Sensors; Fe(III).

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1 Introduction

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Ionic liquids (ILs) have several unique properties including generally negligible vapor pressure,

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good electric conductivity, wide liquid ranges and large electrochemical window1-3, which make them

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widely used in many physical and chemical fields, such as liquid-liquid extraction4, 5, CO2 capture6, 7,

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green solvents8-10 and separations11.

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Metal-containing ionic liquids based on imidazolium salts are promising new materials which can

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favorably combine the properties of ionic liquids with additional magnetic, spectroscopic and catalytic

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properties depending on the metal incorporated12-21. Recently, several types of ionic liquids containing

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lanthanide have been reported as promising new materials in terms of luminescent materials

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magnetic materials28, 29, and energetic materials30, 31.

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,

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Iron is a necessary microelement in human body and its content is regulated to maintain a normal

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physiological function. Fe(III) ions play an essential role in vital cell functions such as hemoglobin

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formation, muscle and brain function, and electron transfer processes in DNA and RNA synthesis.

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However, studies have indicated that excess levels of Fe(III) in the blood may be regarded as the

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pathogenic factor of bladder cancer and esophageal cancer. Hence, selective detection of Fe(III) ions is

*

Corresponding author. Tel./fax: + 86 24 62207797. E-mail: [email protected] (W Guan)

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very important for human health32-35.

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Traditional qualitative or quantitative analytical methods of Fe(III) ions include atomic absorption,

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colorimetry, spectrophotometry and voltammetry, etc. However, these analytical techniques not only

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require complicated equipments but also need cumbersome sample pretreatments, which make the

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detection processes more time-consuming and effort-consuming. In the past decade, fluorescent probes

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such as Rhodamin B and metal-orgainc framework (MOF) have been developed significantly for

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sensing metal ions due to their high sensitivity and low detection limit. However, experimental acute

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poisoning in mice has demonstrated that Rhodamine B can lead to subcutaneous tissue sarcoma, so

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they were considered to be carcinogens. Majority of luminescent lanthanide–organic frameworks

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(Ln–MOFs) can identify Fe(III) ions due to their fluorescence quenching effect, nevertheless, other

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metal ions such as Cu(II), Al(III) and Fe(II) can also quench the fluorescence meanwhile Ca(II) has the

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function of enhanced fluorescence. As a result, Ln–MOFs do not performed very well in Fe(III)

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specificity recognition because some of the detection results are interfered by some other metal ions

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(such as Cu(II), Al(III), Fe(II) and Ca(II))36-40. On the other side, most of the Ln–MOFs were unstable

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in aqueous solution, therefore, this characteristic will limit the applications of Ln–MOFs in actual

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detection. Above all, searching for a new kind of hydrostable and environment-friendly fluorescent

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probes which can recognize Fe(III) in a highly effective and specific manner is of great importance for

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promoting the development of “green” chemistry.

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2 Material and methods

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The chemical materials, instrumentations, synthetic process and characterizations are described

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particularly in supporting information.

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3 Results and discussion

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The luminescent properties of the two compounds were investigated at room temperature. The

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Dy(III)-based room-temperature ionic liquid had an intense kelly emission and exhibited luminescence

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peak at 481, 574, 662 and 751 nm. The characteristic transitions of Dy(III) were detected in both

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excitation spectra and emission spectra which could be assigned to 4F9/2→6H15/2, 4F9/2→6H13/2,

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4

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among all the fluorescence peaks, with an extremely sharp peak, which indicated high color purity, as

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shown in the insert of Fig. 1(a).

F9/2→6H11/2 and 6F9/2→6H15/2 transitions, respectively. The 4F9/2→6H13/2 transition was the most intense

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Fig. 1(b) shows the excitation spectrum of Sm(III)-based ionic liquid at λem = 596 nm together

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with the emission spectra recorded for the excitation wavelengths λex = 478 at room temperature. The

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narrow emission bands from 550 to 750 nm which exhibited the characteristic transitions of Sm(III)

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ions and they are attributed to 4G5/2→6H5/2 (565 nm), 4G5/2→6H7/2 (596 nm) and 4G5/2→6H9/2 (648 nm),

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respectively. Under UV-light irradiation, Sm-based ionic liquid shows a strong red luminescence,

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which could be readily observed by the naked eye, as shown in the insert of Fig. 1(b).

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The excitation spectra (Fig. 1) indicate that the two compounds belong to the luminescence

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materials system with broad excitation bands, which extend to the visible-light region and could be

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used as good soft luminescent materials.

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The rare-earth ionic liquids which were used to explore the sensing behavior for Fe(III) ions have

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never been explored before. So we firstly investigated the application of lanthanide-based room

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temperature ionic liquid in the field of fluorescent probe. To examine the potential recognition of

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mental ions, the two samples were dissolved in deionized water containing M(NO3)x (M = Ca(II),

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Al(III), Zn(II), Cu(II), Pb(II), Hg(II), Cd(II), Co(II), Fe(II), Ni(II), Cr(III) and Fe(III)). The luminescent

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properties were recorded and compared in Fig. 2.

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Interestingly, the results revealed that all the metal ions except Fe(III) have minuteness degrees of

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quenching effects on luminescence intensity. Different quenching effects led to the changes of emitting

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color under UV-light irradiation, and the luminescence colors of the other metal ions (except Fe(III))

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could hardly distinguished from the original color by the naked eye. In contrast, Fe(III) ions can quench

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the emission of the two ionic liquids completely and their emission colors under UV light are dark. As

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shown in the inserts of Fig. 2. The concentration of Dy-IL complex was 0.1 M and the detection limit

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of Fe(III) at this concentration was calculated to be 9.4 µM from 3σ/k, where σ was the standard

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deviation of the blank solutions (n=11) and k was the slope of the calibration curve (see Fig.S10 and

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11). Use the same method, the detection limit of Sm-IL complex was calculated to be 86.8 µM (see

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Fig.S12 and 13).

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In order to check the high selectivity to Fe(III) ions over other metal ions, mixed metal ions (the

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mixed ions are Ca(II), Al(III), Zn(II), Cu(II), Pb(II), Hg(II), Cd(II), Co(II), Fe(II), Ni(II) and Cr(III))

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were added into the Dy(III)-based ionic liquid aqueous solution. Under the irradiation of UV light of

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365 nm, by the naked eye, the test sample shows difference in color change from that of the original

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one. The emission spectrum of the mixed-ion-loaded sample decreased marginally compared to the

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original one. However, when the test sample contained mixed metal ions including Fe(III) ions, the

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sample shows a dark color which was easily distinguished by the naked eye.(See the insert of Fig. 3(a))

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The measurement of the emission spectrum shows that the luminescence is quenched completely,

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indicated that the selectivity for Fe(III) ions is not interfered by the existence of other common metal

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ions. A block diagram showing the method of detecting Fe(III) is shown in Fig. 4.

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The same method was applied to examine the selectivity of the Sm(III)-based ionic liquid. Fig.

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3(b) shows that the Sm(III)-based ionic liquid containing mixed-ions (without Fe(III)) displayed a red

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color under UV light irradiation at 365 nm, but this color was absent when Fe(III) was added to the

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sample. The results suggest that two ionic liquids may have a strong selectivity for Fe(III) and this

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selectivity could not be interfered by the presence of other common metal ions (Ca(II), Al(III), Zn(II),

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Cu(II), Pb(II), Hg(II), Cd(II), Co(II), Fe(II), Ni(II) and Cr(III)). Comparing with the traditional

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detection methods, these two rare-earth ionic liquids as fluorescent probes to detect Fe(III) ions have

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the excellent characteristics such as simple operation, swift response, convenient observation, high

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sensitivity, high selectivity and low detection limit, especially, the lanthanide-based room temperature

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ionic liquids are hydrostable and eco-friendly which can be used in “green” fluorescence probes fields

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and be widely used to detect Fe(III) ions in aqueous solution.

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4 Conclusion

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Two new lanthanide-based room temperature ionic liquids [Bmim][Ln(NO3)4] (Bmim =

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1-butyl-3-methylimidazolium; Ln = Dy, Sm) were synthesized and characterized by Fourier Transform

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Infrared Spectroscopy (FTIR), Proton Nuclear Magnetic Resonace (1H NMR), Mass Spectrum (MS),

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Thermal Gravimetric Analyzer (TGA) and Elemental Analysis (EA). The photoluminescence properties

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of the lanthanide-containing ionic liquids were studied at room temperature in deionized water by

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measuring emission and excitation spectra. Under UV lamp irradiated at 365 nm, [Bmim][Dy(NO3)4]

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and [Bmim][Sm(NO3)4] shows strong fluorescence, which shows that these ionic liquids could be used

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as good soft luminescent materials. Furthermore, the lanthanide-based room temperature ionic liquids

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were firstly used to explore the sensing behavior of metal ions. Most interestingly, the two compounds

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are rare examples of a highly selective and sensitive luminescence probes for Fe(III) ions. The emitting

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color change of these probes before and after containing Fe(III) is easily distinguishable even with the

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naked eye under UV-light irradiation of 365 nm. The significant property of the two ionic liquids is that

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the selectivity to Fe(III) ions is not influenced by other mixed common metal ions (Ca(II), Al(III),

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Zn(II), Cu(II), Pb(II), Hg(II), Cd(II), Co(II), Fe(II), Ni(II) and Cr(III)). More than that, these

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lanthanide-containing ionic liquids could be widely used in actual detection because they are

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hydrostable and environment-friendly fluorescent probes. The present results may provide a novel

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facile route to design and synthesize functional ionic liquids with applications in fluorescent sensors.

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Acknowledgements

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The project was supported by the National Natural Science Foundation of China (21173107).

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Supporting Information

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The chemical materials, instrumentations, synthetic process and characterizations. This material is

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available free of charge via the Internet at http://pubs.acs.org.

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a

λ ex=351nm

λ em=574nm

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Relative Intensity/a.u.

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F9/2→ H15/2 6

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Fig. 1

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λex=478nm

G5/2→ H7/2

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Relative Intensity/a.u.

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G5/2→ H9/2

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F9/2→ H11/2 6F → 6H 5/2 15/2 6

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Excitation and emission spectra of [Bmim][Dy(NO3)4] (a) and [Bmim][Sm(NO3)4] (b), the insets are the

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corresponding luminescence images under UV-light irradiation of 365 nm.

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H2O Al(Ⅲ) Ca(Ⅱ) Zn(Ⅱ) Cu(Ⅱ) Pb(Ⅱ) Hg(Ⅱ) Cd(Ⅱ) Co(Ⅱ) Fe(Ⅱ) Ni(Ⅱ) Cr(Ⅲ) Fe(Ⅲ Fe Ⅲ)

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Relative Intensity/a.u.

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a

Relative Intensity/a.u.

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H2O Al(Ⅲ) Ca(Ⅱ) Zn(Ⅱ) Cu(Ⅱ) Pb(Ⅱ) Hg(Ⅱ) Cd(Ⅱ) Co(Ⅱ) Fe(Ⅱ) Ni(Ⅱ) Cr(Ⅲ) Fe(Ⅲ Fe Ⅲ) 500

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Relative intensity/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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238 239

H2O Al

Fig. 2

3+

Ca

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Zn Cu

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Pb

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2+

2+

2+

Hg Cd Co2+ Fe Ni

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Cr

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Fe

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H2O Al3+ Ca2+ Zn2+ Cu2+ Pb2+ Hg2+ Cd2+ Co2+ Fe2+ Ni2+ Cr2+ Fe3+

Emission spectra of [Bmim][Dy(NO3)4] (a) and [Bmim][Sm(NO3)4] (b) with different mental ions when excited at 351

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nm and 478 nm respectively, the insets are the corresponding luminescence images under UV-light irradiation of 365 nm;

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Photoluminescence relative intensity of the 4F9/2→6H13/2 transition (574 nm) of [Bmim][Dy(NO3)4] (c) and the 4G5/2→6H7/2

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transition (596 nm) of [Bmim][Sm(NO3)4] (d) treated with different metal ions.

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a

Dy-origin n+ M n+ 3+ M +Fe

140 120

b

Sm-origin n+ M n+ 3+ M +Fe

50

40 100

Fluorescence intensity

Fluorescence intensity

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Wavelength/nm

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Fig. 3 (a) Comparison of photoluminescence intensity of [Bmim][Dy(NO3)4] (a) and [Bmim][Sm(NO3)4] (b) mixed with

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Mn+(Ca(II), Al(III), Zn(II), Cu(II), Pb(II), Hg(II), Cd(II), Co(II), Fe(II), Ni(II) and Cr(III)) in the absent and presence of Fe(III),

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the insets are the corresponding luminescence images under UV-light irradiation of 365 nm.

247 248 249

Fig. 4

Schematic diagram showing method for detecting Fe(III) in the aqueous samples.

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