Novel room temperature ionic liquid for fluorescence enhancement of Eu3+ and Tb3+

Journal of Luminescence 131 (2011) 739–748

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Full Length Article

Novel room temperature ionic liquid for fluorescence enhancement of Eu3 + and Tb3 + V. Shyamala Devi, S. Maji, K.S. Viswanathan n Materials Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

a r t i c l e in f o

abstract

Article history: Received 15 February 2010 Received in revised form 16 November 2010 Accepted 26 November 2010 Available online 7 December 2010

The newly prepared ionic liquid, 1-butyl-3-methylimidazolium benzoate, ([bmim][BA]), was found to enhance the fluorescence of Eu3 + and Tb3 + . The fluorescence enhancement resulted from a sensitization of the lanthanide fluorescence by the benzoate anion of the ionic liquid, [bmim][BA], and a reduction in the non-radiative channels in the non-aqueous environment provided by the ionic liquid. However, the fluorescence enhancement of the lanthanides in the ionic liquid was limited due to the operation of the inner filter effect, which resulted from the strong absorption of the benzoate. The inner filter effect was minimized by observing the Eu3 + fluorescence using a front face geometry and also by diluting the lanthanide–[bmim][BA] system, using another ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]), as a solvent. In the case of Tb3 + , the emission from the lanthanide was masked by the strong emission from the ionic liquid in the region 450–580 nm. The long lived Tb3 + emission was therefore observed using delayed gated detection, where an appropriate delay was used to discriminate against the short lived emission from the ionic liquid. The large fluorescence enhancement due to ligand sensitized fluorescence observed with [bmim][BA] diluted in [bmim][Tf2N], leads to nanomolar detection of the lanthanides. This is, to the best of our knowledge, the first report of an ionic liquid being employed for ligand sensitized fluorescence enhancement of lanthanides. & 2010 Elsevier B.V. All rights reserved.

Keywords: Ionic liquid 1-Butyl-3-methylimidazolium benzoate Fluorescence enhancement Eu3 + Tb3 +

1. Introduction Lanthanides are weakly fluorescing species, due to their low molar absorptivities and poor quantum yields [1]. To overcome the disadvantage of low absorptivities of the lanthanides, various ligands have been employed to sensitize the fluorescence of the lanthanides [2–7]. The sensitization, arising from an intramolecular energy transfer from the ligand to the metal, results in a fluorescence enhancement of the lanthanides. In this process, the ligand, which absorbs the light, is first excited to the singlet state; an intersystem crossing then takes the ligand to a long lived triplet state. Energy is then transferred from the triplet state of the ligand to the lanthanide, resulting in the excitation of the lanthanide, which then fluoresces; a process referred to as ligand sensitized fluorescence. If the absorption of the ligand is strong and the energy transfer to the lanthanide efficient, the resulting lanthanide fluorescence due to ligand sensitization would be orders of magnitude greater than that resulting from the direct excitation of the lanthanide. The problem of poor quantum yield of the lanthanides in aqueous media is usually addressed using synergistic ligands, such as trioctylphosphine oxide (TOPO). In this process, TOPO displaces the water coordinated to the metal,

n

Corresponding author. E-mail address: [email protected] (K.S. Viswanathan).

0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.11.029

thereby reducing the quenching of the lanthanide emission due to water. The fluorescence is therefore increased by the reduction of non-radiative processes [8,9]. In these experiments, the TOPO, which is insoluble in water, is solubilized in the aqueous phase through the use of surfactants, such as Triton X-100. These experiments are therefore conducted in two stages; the lanthanide is first coordinated to a sensitizing ligand, which results in fluorescence enhancement. The lanthanide–ligand complex is then treated with a synergistic ligand in a surfactant medium, leading to a further enhancement. In recent times, ionic liquids have emerged as alternatives to the aqueous phase for many chemical studies. Ionic liquids are generally composed of large asymmetric cations, such as 1,3dialkylimidazolium, N-alkylpyridinium, tetraalkylammonium, and phosphonium core with different anions, such as BF4 , PF6 , CF3SO,3 (CF3SO2)2N  , NO3 , ClO4 and alkyl sulfate. These ionic liquids serve as novel solvents in various applications, such as liquid–liquid biphasic extraction processes [10,11], organic synthesis [12,13], electrochemical studies [14] and catalysis [15]. The properties that make these liquids unique and versatile as solvents, are the wide temperature range over which they exist as liquids, negligible vapor pressure, thermal stability, high ionic conductivity and recyclable nature. Ionic liquids are also known as ‘designer solvents’, as the cation and anion can be suitably tailored for a desired application [16]. For example in biphasic liquid–liquid extractions, the imidazolium

740

V. Shyamala Devi et al. / Journal of Luminescence 131 (2011) 739–748

cation is derivatized to include task-specific functionalities, such as urea, thiourea, thioether, aza-crown ether, to extract metal ions, such as Cs + , Sr2 + , Cd2 + , Hg2 + , from an aqueous phase to an ionic liquid [17,18]. Similar biphasic extractions of alkali metal ions from an aqueous phase, by task-specific ionic liquid with a thiolcontaining anion, have also been reported [19]. In this work, we report the use of a tailored ionic liquid, as an alternative to the aqueous phase, for the study of lanthanide fluorescence. We report, for the first time, the preparation and application of an imidazolium cation based ionic liquid, with benzoate as the anion, to effect fluorescence enhancement of lanthanides. We have shown in earlier works that aromatic carboxylic acids serve as excellent ligands for the sensitization and enhancement of lanthanide fluorescence [20] in the aqueous phase, which prompted us to use the benzoate anion in the preparation of the ionic liquid. The idea was to use the anion of the ionic liquid as a sensitizing agent for lanthanide fluorescence. In addition to the sensitization of lanthanide fluorescence by the benzoate, the non-aqueous medium provided by the ionic liquid can also be expected to simultaneously reduce the rates of nonradiative decay, which in aqueous media are dominant fluorescence quenching channels. Consequently, a large enhancement in lanthanide fluorescence can be expected with the ionic liquid. To the best of our knowledge, this is first such application of an ionic liquid. While ionic liquids have been used in fluorescence studies, earlier works have explored other aspects, such as determination of the polarity of the ionic liquid using fluorescent probes [21], the excitation wavelength dependent behavior of dipolar molecules in ionic liquid medium [22], the lanthanide fluorescence in ionic liquid [23] and the effect of H2O on the Eu3 + fluorescence in [Tf2N] anion based ionic liquids [24]. The fluorescence of Eu3 + and Tb3 + in the task-specific room temperature ionic liquid, 1-butyl-3-methylimidazolium benzoate [bmim][BA], the structure of which is shown in Fig. 1, has been discussed. Both steady state and lifetime measurements were performed, which are presented. In addition, we have also studied the effect of water in the ionic liquid, on the fluorescence of the lanthanides; these studies were necessitated as water turns out to be an inevitable impurity in the ionic liquid.

2. Experimental section 2.1. Apparatus Fluorescence spectra were recorded using a Edinburgh FLS920 spectrofluorimeter, with a 450 W Xe lamp as the excitation source and a 2 nm band pass for both excitation and emission monochromators. A few spectra were also recorded using a Shimadzu 5000 spectrofluorimeter, with a 150 W Xe lamp for excitation. Solutions were taken in quartz cuvettes, with path lengths of 10 and 3 mm. While in all cases, fluorescence spectra were recorded at room temperature, with the ionic liquid, 1-butyl-3-methylimidazolium chloride ([bmim][Cl]), fluorescence spectra were recorded using a thermoelectric temperature controlled cuvette holder maintained at a temperature of 70 1C, which is 5 1C above the melting point of [bmim][Cl]. A long-wavelength pass filter (UV-39,

6

+ 5 CH3

N

N 8

O

9

7 3 4

1 2

11

15 14

CH3

10

13 12

Fig. 1. Structure of [bmim][BA] showing atom numbering.

O

-

Shimadzu) with a maximum and uniform transmittance ( 485%) above 400 nm was placed in front of the emission monochromator, in order to reduce the scatter of the incident beam into the emission monochromator. Due to the operation of the inner filter effect, which will be discussed later, Eu3 + and Tb3 + fluorescence in neat [bmim][BA] was also recorded using a front face geometry. The Edinburgh FLS920 was also used, to measure lifetimes using a microsecond flashlamp as an excitation source. A single exponential fit was found adequate for the lanthanide–[bmim][BA] complexes, giving a chi-square of near unity. The relative standard deviation of the lifetime values was less than 8%. In the case of Tb3 + –[bmim][BA] system, the strong emission from the ionic liquid masked the emission of Tb3 + at 544 nm, (which was not a problem in the case of Eu3 + , where the fluorescence occurred at longer wavelengths). In order to observe the Tb3 + emission, a delayed gated detection scheme was used. The emission from the Tb3 + -ionic liquid was excited using the fourth harmonic of a Nd:YAG laser (Brilliant b,Quantel) operated at a repetition rate of 10 Hz. The emission from the sample was focused onto a monochromator (Model TRIAX 550, Jobin Yvon) fitted with a 600 g/mm grating and an ICCD detector. The detection was delayed by 300 ns after the excitation pulse, after which time, the short-lived emission from the ionic liquid was insignificant. A gate width of 1000–2000 ms was used subsequent to this delay, to observe the long lived Tb3 + emission. The entrance and exit slit widths of the monochromator were set at 30 mm. Absorption spectra of the ionic liquid were recorded using a Thermo Electron Corporation 500 UV–Visible spectrophotometer. Infrared spectra were recorded using a BOMEM MB 100 FTIR spectrometer over the range 4000–650 cm  1, with a resolution of 4 cm  1. A thin film of the ionic liquid was taken between two ZnSe windows and the infrared spectrum of the resultant film was recorded. To estimate the water content in the ionic liquids, a Karl Fischer titration was conducted on a 831 KF Coulometer, with Hydranal Coulomat AG as a reagent. Duplicate measurements were performed on each sample and the relative standard deviation in the measured water content was less than 5%. 1 H and 13C NMR spectra were recorded on a Bruker AVANCE III 500 MHz (AV 500) NMR instrument, using CDCl3 as the solvent. Mass spectra were recorded on a Thermo Scientific DSQ II single quadrupole GC/MS by direct exposure probe (DEP) method. 1 ml of [bmim][BA], diluted in methanol, was placed on the filament tip of a probe, inserted into the mass spectrometer, and a mass spectrum then recorded.

2.2. Reagents Standard aqueous stock solution of Eu3 + was prepared by dissolving the required amount of Eu2O3 (99.9%, Indian Rare Earths) in distilled HCl and evaporated to dryness. The residue was then dissolved in water to get the required concentration of 10  1 M Eu3 + . Standard Stock solution of 10  1 M Tb3 + was prepared by dissolving the required amount of Tb(NO3)3 (99.9%, Indian Rare Earths) in triple distilled water. Triple distilled water was used in preparing all aqueous solutions. For fluorescence experiments in aqueous solutions and ionic liquids, stock solution of benzoic acid (Fluka) was prepared by dissolving the acid in distilled water. Required amount of 1 M NaOH was added to maintain a pH of 6, which ensured dissolution of the acid. Stock solution of TOPO (Merck) was prepared by dissolving it in 10% Triton X-100 (Loba Chemie) and then diluting with water. All chemicals procured commercially, such as [bmim][Cl], were used as such.

V. Shyamala Devi et al. / Journal of Luminescence 131 (2011) 739–748

2.3. Synthesis procedure of [bmim][BA]and [bmim][Tf2N] The ionic liquid [bmim][BA] was prepared from [bmim][Cl] (499% pure, Fluka) using a procedure similar to that outlined in Ref. [25], for the preparation of ionic liquids such as [bmim][acetate]. Approximately 9 g (39 mmol) of [bmim][Cl] crystals were dissolved in 60 mL of high pure methanol (499%, Qualigens Company) and stirred for 5–10 min. About 6.8 g (39 mmol) of silver benzoate (99%, Aldrich Chemical Company) was added to the solution of [bmim][Cl] in methanol, under vigorous stirring of the reaction mixture. The flask containing the reaction mixture was wrapped with Al foil, as the byproduct, AgCl, is photosensitive. The reaction mixture was stirred for 3 h at room temperature. At the end of the reaction period, the AgCl precipitate was filtered and weighed, to ensure stoichiometric yield of the reaction. The filtrate of the reaction mixture was concentrated using a rotary evaporator, when a clear, colorless and viscous liquid was obtained (yield 490%). This liquid was first dried for 10 h at room temperature (25 1C) and then for 40 h at 105 1C under reduced pressure, to remove traces of dissolved solvent and water. This drying procedure has been shown to be essential in preparing ionic liquids with minimum water content [26]. After drying, the ionic liquid, [bmim][BA], was stored in a desiccator, which was flushed with Ar, prior to storing the product. The ionic liquid [bmim][Tf2N] was synthesized on the lines of the procedure reported in Ref. [27]. About 31.6 g (110 mmol) of lithium bis(trifluoromethylsulfonyl)imide salt (Aldrich Chemical Company) was dissolved in approximately 45 ml of water and added in drops to a homogeneous solution of [bmim][Cl] (17.4 g, 100 mmol) in water, under constant stirring. After the aqueous solution of the lithium salt was completely added, the reaction mixture was vigorously stirred for 3 h at room temperature. At this time, a separate phase consisting of the ionic liquid, [bmim][Tf2N], was observed at the bottom of the flask. The ionic liquid was then separated and washed several times with small aliquots of water, until the washings did not show any halide, as checked by the addition of a solution of AgNO3. The clear, colorless ionic liquid was dried as before, first for 10 h at room temperature and then for 40 h at 135 1C under reduced pressure, to remove any traces of water and volatile organic impurities. The ionic liquid (yield495%) was stored in a desiccator, which had been purged with Ar, prior to storage of the product. 2.4. Characterization of the Ionic liquids As the ionic liquid [bmim][BA] was prepared for the first time, it was necessary to characterize this product before use, which is discussed below. Table 1 Vibrational frequencies of [bmim][BA] together with assignments. Mode assignments

Wavenumber (cm  1)

nC–H (imidazole ring) nC–H (aliphatic) nH–C–C (imidazole ring) nC¼O nCOO (asymm) nCOO (symm)

3091.7 2962.4, 2875.6 1168.8 1595.1 1552.5 1384.8

741

2.4.1. Infrared spectrum The infrared spectrum of the product can be seen to carry signatures of both the imidazole ring and the benzoate moiety, the assignments of which have been shown in Table 1. In summary, the prominent bands due to the C–H stretch of the imidazole ring, the aliphatic chain, H–C–C deformation of the ring, and the C¼O stretch of the benzoate are clearly observed. 2.4.2. 1H and 13C NMR spectra The 1H and 13C NMR chemical shifts of [bmim][BA] and [bmim][Tf2N] are given in Tables 2 and 3, respectively, together with their assignments. The chemical shifts of the protons assigned to the [bmim] group agree well with that reported in the literature by Bonhote et al. [28] (except that for C8) and clearly confirms the presence of this cation group. The chemical shift for the proton on C8 occurs at 11.51 ppm, which occurs significantly downfield compared with that observed for [bmim][Cl]. The downfield shift may be due to the deshielding effect of the p-electrons of the benzoate group. The chemical shifts for the 1H in the benzoate group are also clearly evident. 13C chemical shifts also support the characterization of the compound to be [bmim][BA]. Likewise, 1H NMR chemical shifts and assignments of [bmim][Tf2N] shown in Table 2 also confirm the characterization of this ionic liquid. 2.4.3. Mass spectra The mass spectrum of [bmim][BA] shows features at m/z of 105, 122, 77, 96, 82, 137 given in order of decreasing intensity. The strongest peak at 105 corresponds to the C6H5CO fragment from the benzoate group of the ionic liquid. The feature at 122 was due to C6H5COOH, while the feature at 77 was due to C6H5. These features (at m/z 105, 122 and 77) were also in the mass spectra of silver benzoate, which supports the assignments of these fragments to the benzoate group. The features observed at m/z 96, 82 and 137, were also observed for [bmim][Cl], which indicates that these features must be due to the [bmim] group. It must be noted that in both [bmim][BA] and [bmim][Cl], no strong feature was observed, due to the bmim ion, at m/z 139. It is likely that electron impact ionization does not allow for this feature to be observed. It may be relevant to note that only field desorption [29] and MALDI-TOF spectra [30] of [bmim] containing ionic liquids, show the feature at m/z 139. 2.4.4. Determination of water content in the Ionic liquids The water content in the ionic liquid was determined using the method of Karl Fischer titration. The concentration of water in [bmim][BA] was estimated to be 1050 mg/mL and that in [bmim][Tf2N] was estimated to be 700 mg/mL. 2.5. Preparation of lanthanide–[bmim][BA] solution Solutions of the required concentration of Eu3 + and Tb3 + in [bmim][BA] were prepared by adding appropriate volumes, typically 1–2 mL of aqueous solutions of Eu3 + and Tb3 + of known concentration, to 500 mL of [bmim][BA] and ultrasonicated.

Table 2 1 H NMR chemical shifts (d/ppm relative to TMS) of [bmim][BA] and [bmim][Tf2N], in chloroform-d. dn is the chemical shift of H on carbon atom ‘n’ shown in Fig. 1. For [bmim][Tf2N], the 1H chemical shifts for the [bmim] group alone is shown.

d1

d2

d3

d4

d5

d6

d7

d8

d9,10

d 11,12,13

[bmim][BA] 0.92 (t, 3H) 1.30–1.36 (m,2H) 1.81–1.85 (m,2H) 4.29 (t,2H) 4.08 (s,3H) 7.10 (d,1H) 7.12 (d,1H) 11.51 (s,1H) 8.09–8.11 (m,2H) 7.33–7.36 (m,3H) [bmim][Tf2N] 0.88 (t,3H) 1.27–1.32 (m,2H) 1.75–1.81 (m,2H) 4.10 (t,2H) 3.86 (s,3H) 7.30 (d,2H) 7.30 (d,2H) 8.58 (s,1H) – –

742

V. Shyamala Devi et al. / Journal of Luminescence 131 (2011) 739–748

Table 3 13 C NMR chemical shifts (d/ppm relative to TMS) of [bmim][BA], in chloroform-d. dn is the chemical shift on carbon atom ‘n’ shown in Fig. 1.

[bmim][BA]

d1

d2

d3

d4

d5

d6

d7

d8

d9, 10

d11,12

d13

d14

d15

13.35

19.46

32.11

49.64

36.30

122.68

121.09

140.39

129.29

127.44

129.36

139.22

172.60

Eu3+-aquo [Eu3+] = 1.0 x 10-1 M λex = 395 nm

Eu3+-aquo 3+] = 1.0

x10-1 M [Eu λem = 592 nm

Eu3+-benzoate in water [Eu3+] = 7.6 x 10-4 M λex = 277 nm; pH = 6

Intensity (arb. unit)

Eu3+-neat [bmim][BA] [Eu3+] = 7.0 x 10-5 M λem = 615 nm

Intensity (arb. unit)

Eu3+-benzoate in water [Eu3+] = 7.6 x 10-4 M; pH = 6 λem = 615 nm

Eu3+-neat [bmim][BA] [Eu3+] = 7.0 x 10-5 M λex= 312 nm

Eu3+-[bmim][BA]//[bmim][Tf2N] [Eu3+] = 3.0 x 10-6 M λex = 278 nm

Eu3+-[bmim][BA]//[bmim][Tf2N] [Eu3+] = 3.0 x 10-6 M λem = 615 nm

Eu3+-benzoate-TOPO/Triton X-100 (aq) [Eu3+] = 8.0 x 10-6 M λex = 285 nm; pH = 6

Eu3+-benzoate-TOPO/Triton X-100 (aq) [Eu3+] = 8.0 x 10-6 M; pH = 6 λem = 615 nm

Eu3+-benzoate//[bmim][Tf2N] [Eu3+] = 1.0 x 10-5 M λem = 615 nm 220

270 320 370 Wavelength (nm)

420

Fig. 2. Excitation spectra (a) Eu3 + (1.0  10  1 M)-aquo; (b) Eu3 + (7.6  10  4 M)– benzoate (5  10  3 M) in water; (c) Eu3 + (7.0  10  5 M)–[bmim][BA]; (d) Eu3 + (3.0  10  6 M)–[bmim][BA] (1  10  3 M) in [bmim][Tf2N]; (e) Eu3 + (8.0  10  6 M)– benzoate (1  10  3 M)–TOPO (10  4 M)/Triton X-100 in water; (f) Eu3 + (1.0  10  5 M)–benzoate (3  10  3 M) in [bmim][Tf2N]. The emission wavelength used to record each of the spectra is shown alongside.

3. Results and discussion 3.1. Eu3 + fluorescence in [bmim][BA] The excitation and emission spectrum of Eu3 + in various media are shown in Figs. 2 and 3, respectively. In particular, the excitation and emission spectra of Eu3 + in Eu3 + -[bmim][BA] are shown in Figs. 2c and 3c. Also shown are spectra recorded for Eu3 + in water, which will henceforth be referred to as the Eu3 + –aquo complex

Eu3+-benzoate//[bmim][Tf2N] [Eu3+] = 1.0 x 10-5 M λex = 280 nm 575

600 625 Wavelength (nm)

650

Fig. 3. Emission spectra. (a) Eu3 + (1.0  10  1 M)–aquo; (b) Eu3 + (7.6  10  4 M)– benzoate (5  10  3 M) in water; (c) Eu3 + (7.0  10  5 M)–[bmim][BA]; (d) Eu3 + (3.0  10  6 M)–[bmim][BA] (1  10  3 M) in [bmim][Tf2N]; (e) Eu3 + (8.0  10  6 M)– benzoate (1  10  3 M)–TOPO (10  4 M)/Triton X-100 in water; (f) Eu3 + (1.0  10  5 M)–benzoate (3  10  3 M) in [bmim][Tf2N]. The excitation wavelength used to record each of the spectra is shown alongside.

(Figs. 2a and 3a). The emission spectra in Fig. 3a and c show features due to Eu3 + at 615 and 592 nm, but with different relative intensities of the two features, which will be discussed later. Even though the emission spectra shown in Fig. 3a and c are similar, the excitation spectrum observed for Eu3 + –[bmim][BA] system (Fig. 2c) is clearly different from that observed for the Eu3 + –aquo complex (Fig. 2a), indicating that the absorbers are different in the two experiments. The shape of the excitation spectrum observed for Eu3 + –[bmim][BA] bears a resemblance to that observed for Eu3 + – benzoate (Fig. 2b) in aqueous medium. In an earlier work we have shown that in aqueous Eu3 + –benzoate experiments, the benzoate is the absorber, which then transfers energy to the Eu3 + [31]. It is

V. Shyamala Devi et al. / Journal of Luminescence 131 (2011) 739–748

therefore indicated that in [bmim][BA] too, the benzoate anion effects sensitization of the Eu3 + fluorescence. It may be noted that the peak maximum in Fig. 2c is somewhat shifted to the red from that observed in Fig. 2b, which will be discussed later. The fluorescence enhancement in the Eu3 + –[bmim][BA] relative to that in the aqueous phase is obvious, from the traces shown in the emission spectra of Fig. 3. While the intensities of the emission features of Eu3 + shown in each trace of Fig. 3 are approximately the same, the concentrations of Eu3 + used to obtain these spectra are different. The concentration of Eu3 + in the [bmim][BA] experiment is 1310 times smaller than that in the Eu3 + –aquo complex, thus implying the fluorescence enhancement of Eu3 + in the experiments with the ionic liquid. In the rest of the discussion we will use the term enhancement factor to indicate the fluorescence enhancement; enhancement factor being defined as the ratio [Eu3 + ]aq/[Eu3 + ]exp, where [Eu3 + ]aq and [Eu3 + ]exp are the concentrations of Eu3 + in the aquo complex and in the experiment in question, that gave the same fluorescence intensity. When comparing the fluorescence intensities, the peak at 592 nm was used for the aquo complex and the peak at 615 nm was used in the other experiments, as these were the strongest peaks in the respective experiments. For example, the enhancement factor of Eu3 + –[bmim][BA] in Table 4 is indicated to be 1310, implying that the Eu3 + concentration in the Eu3 + –[bmim][BA] system was 1310 times smaller than that of Eu3 + in the aquo complex, and yet gave the same Eu3 + fluorescence intensity for the 615 nm feature in the Eu3 + –[bmim][BA] system and the 592 nm feature for the aquo system. It must be noted that the fluorescence of Eu3 + in neat [bmim][BA] was observed using a front face reflection geometry, as a strong inner filter effect was operative in this medium. The existence of an inner filter effect in these solutions was recognized when it was observed that a 3 mm path length cell yielded a fluorescence that about 7 times larger than that obtained with a 10 mm path length cell. The operation of the inner filter effect is due to the following reason. In the experiment which uses neat ionic liquid, [bmim][BA], as the medium, the benzoate concentration turns out to be  4 M (density of [bmim][BA] is 1.05 g/cm  3), which is at least 4 orders of magnitude larger than the concentration of 10  3 M used in experiments involving Eu3 + –benzoate in water. The larger concentration of the absorber, therefore leads to an inner filter effect. This effect can be reduced by either using a front face collection geometry or by diluting the fluorophore in a non-absorbing diluent. The front face geometry has been shown to yield an enhancement factor of 1310. We also wanted to examine if diluting the Eu3 + – [bmim][BA] in an inert solvent provides any further improvement in fluorescence enhancement. Since the idea was to maintain a nonaqueous medium, it was considered prudent to dilute the Eu3 + – [bmim][BA] in another ionic liquid that has significantly smaller absorption in the region of interest. The ionic liquid [bmim][Tf2N]

743

was chosen as the diluent ionic liquid to reduce the inner filter effect, as it has an absorbance that is about eight times smaller than [bmim][BA], in the wavelength region of interest. It is the operation of the inner filter that resulted in the shift of the peak maximum in Fig. 2c relative to that observed in Fig. 2b, which has been referred to earlier.

3.2. Eu3 + –[bmim][BA] fluorescence in [bmim][Tf2N] To conduct the fluorescence studies of Eu3 + –[bmim][BA] in [bmim][Tf2N], the working concentrations of Eu3 + and [bmim][BA] in [bmim][Tf2N] were typically prepared as follows. A solution (A) of Eu3 + in [bmim][Tf2N] was prepared by adding 1 mL of aqueous Eu3 + solution, in 0.5 mL of [bmim][Tf2N]. (The concentration of Eu3 + in the aqueous solution was ranged from 1.5  10  3 to 5  10  6 M, depending on the targeted concentration of Eu3 + in the final working solution.) A stock solution (B) was then prepared by adding 4.1 mL of [bmim][BA] to 0.5 mL of [bmim][Tf2N]. 15 mL of the stock solution B was then added to the solution ‘A’ of Eu3 + in [bmim][Tf2N], which served as the working solution. In this working solution, the concentration of [bmim][BA] was 10  3 M. The concentration of Eu3 + was ranged from 3  10  6 to 10  8 M depending on the concentration of Eu3 + in the original aqueous stock solution. The final working solution of Eu3 + –[bmim][BA] in [bmim][Tf2N] was ultrasonicated for 2 min, to ensure complete dissolution. In all future discussions, [bmim][BA] was used as the sensitizing ionic liquid and [bmim][Tf2N] as the diluent ionic liquid and will be indicated as Eu3 + –[bmim][BA]//[bmim][Tf2N]. At the outset, the Eu3 + fluorescence in [bmim][Tf2N] was measured as a function of [bmim][BA] concentration (Fig. 4). For the concentrations of Eu3 + used in this study (10  7 M), the optimum concentration of the sensitizing ionic liquid was found to be 10  3 M and this concentration was used in all subsequent experiments. It can be seen from Fig. 2 that the excitation spectra of Eu3 + [bmim][BA]//[bmim][Tf2N] (Fig. 2d) is similar to Eu3 + –benzoate in water (Fig. 2b), and Eu3 + –benzoate–TOPO/Triton X-100 in water (Fig. 2e), clearly indicating that the benzoate is the sensitizing species in this experiment, as in the Eu3 + –[bmim][BA] system. It can be seen that with the suppression of the inner filter effect, the maximum in the excitation spectrum shown in Fig. 2d is almost similar to that observed in Fig. 2b. It may be recalled that due to inner filter effect, we observed a shift in the maximum in the excitation spectrum with neat Eu3 + –[bmim][BA] (Fig. 2c) relative to that observed in Fig. 2b. Emission spectra of the Eu3 + –[bmim][BA]//[bmim][Tf2N] complex is shown in Fig. 3d. Table 4 also shows the Eu3 + –[bmim][BA]// [bmim][Tf2N] system to yield the largest enhancement factor of 37,970. The Eu3 + fluorescence in [bmim][BA]//[bmim][Tf2N] is more than twice as that seen in the TOPO/Triton X-100

Table 4 Enhancement factors (EF)a, lifetimes (t) in ms and asymmetry factor (R)b for Eu3 + in different systems.

Enhancement factorc (EF) Lifetime (in ms) Asymmetry factor (R)

Eu3 + (aq)

Eu3 + –benzoate (aq)

Eu3 + –neat [bmim][BA]

Eu3 + –[bmim][BA]// [bmim][Tf2N]

Eu3 + –benzoate-TOPO/Triton X-100 (aq)

Eu3 + –benzoate// [bmim][Tf2N]

1 110d 0.5

130 122e 1.6

1310 782 12.0

37970 979 9.6

13320 1500 5.1

11870 830 9.3

a Enhancement factor (EF) is defined as the ratio [Eu3 + ]aq/[Eu3 + ]exp, where [Eu3 + ]aq and [Eu3 + ]exp are the concentrations of Eu3 + in the aquo complex and that in the experiment in question, that gave the same fluorescence intensity. See text for details. b The asymmetry factor, R, is the ratio of the areas of the 615 nm (5D0-7F2) and 592 nm (5D0-7F1) feature. c The excitation wavelengths used to record Eu3 + fluorescence in the different systems are given in Fig. 2. d Reported in Ref. [36]. e Reported in Ref. [37].

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Eu3+ intensity at 615 nm (arb. unit)

experiments and certainly much larger (  250 times) than benzoate in water. It is apparent that the use of a diluent ionic liquid has significantly increased the fluorescence intensities of Eu3 + . A separate experiment was also performed where Eu3 + -BA was taken to [bmim][Tf2N] to observe the Eu3 + fluorescence (Figs. 2f and 3f). At the outset, Eu3 + fluorescence in [bmim][Tf2N] was measured as a function of the benzoate concentration. The benzoic acid concentration in the working solution was varied from 1  10  3 to 5  10  3 M; eventually, 3  10  3 M was found to be optimal. In these experiments, the working solution was prepared by adding 1 mL of 5  10  3 M aqueous Eu3 + solution, and 1 mL of 1.5 M aqueous benzoic acid to 0.5 mL of [bmim][Tf2N]. The enhancement factor in this case was observed to be 11,870, smaller than that observed for the Eu3 + –[bmim][BA]//[bmim][Tf2N] case. This experiment shows that the ionic liquid [bmim][BA] in [bmim][Tf2N] is clearly superior to just dissolving the Eu3 + – benzoate in [bmim][Tf2N]. The luminescence lifetimes of an excited state of the Ln (III) ion in complexes have served to determine the extent of non-radiative processes in these complexes and the inner sphere hydration number of lanthanides [32,33]. Hence fluorescence lifetime measurements of Eu3 + were also in the different media discussed above. Table 4 presents our results on the lifetimes of Eu3 + fluorescence in the various experiments. Even though the benzoate complex in water yields enhancement of the Eu3 + fluorescence, the lifetimes in the aquo complex and in the benzoate complex in water are not very different at 110 and 122 ms, respectively. This observation indicates that the enhancement results from a fluorescence

0

4x10-3 6x10-3 8x10-3 1x10-2 2x10-3 Concentration of [bmim][BA] in [bmim][Tf2N] (M) 3+

Fig. 4. Plot showing the variation of Eu emission intensity at 615 nm as a function of [bmim][BA] concentration over the range 4.0  10  4–1.0  10  2 M, in [bmim][Tf2N]; excitation wavelength¼ 276 nm, [Eu3 + ]¼ 1  10  7 M.

sensitization process, while the non-radiative processes through collisions with water are still operative at about the same rate in the two complexes. The lifetime of Eu3 + in the [bmim][BA] medium was, on the contrary, considerably longer at 782 ms, clearly indicating the reduction in the rates of non-radiative processes in the ionic liquid relative to that in aqueous systems. Likewise, the lifetime in Eu3 + –[bmim][BA]//[bmim][Tf2N] was also considerably long (979 ms). Similarly, the lifetime of Eu3 + in the benzoate// [bmim][Tf2N] medium was observed to be 830 ms. A study of the asymmetry factor is also illustrative. The asymmetry factor, R, is the ratio of the intensities of the 615 nm (5D0-7F2) to the 592 nm (5D0-7F1) feature. While the intensity of transition 5D0-7F1 (592 nm) is primarily magnetic dipole in nature and only weakly dependent on crystal field effect, the intensity of transition 5D0-7F2 (615 nm) is electric dipole in nature and strongly dependent on the crystal field. The extreme sensitivity of the 5D0-7F2 transition strength to the nature of the environment of the Eu3 + site makes this a hypersensitive transition. The asymmetry factor (R) has therefore been shown to be useful for making qualitative comparisons of Eu3 + site symmetry [34]. From Table 4, it can be seen that the value of R (calculated as the ratio of the areas of the 615 and 592 nm features) for the Eu3 + –aquo complex is the smallest with a value of 0.5, in agreement with literature value [35], increasing to 1.6 for Eu3 + –benzoate in water and further increasing to  12.0 for Eu3 + –[bmim][BA]. The value of R in the [bmim][BA] experiment is the largest observed for a Eu3 + complex. Likewise the R values for the Eu3 + emission in the [bmim][BA]//[bmim][Tf2N] and benzoate//[bmim][Tf2N] media are also high with a value of  9.6, indicating a near similarity in the environment of Eu3 + in the ionic liquid experiments and which, in turn, are different from that prevailing in the Eu3 + –aquo complex, Eu3 + –benzoate complex and Eu3 + –benzoate–TOPO/Triton X-100 in water. Eu3 + fluorescence was also studied in [bmim][Cl] and [bmim][Tf2N] for comparison. The enhancement factor for Eu3 + fluorescence in the case of [bmim][Cl] was only 150, which is less than that in [bmim][BA] or [bmim][BA]//[bmim][Tf2N]. This indicates that the [bmim] cation does not yield any significant enhancement and the enhancement of 150 seen in [bmim][Cl] is probably due to the suppression of non-radiative decay mechanisms typically observed in non-aqueous systems, which is indicated by the long lifetime of 2719 ms of Eu3 + fluorescence observed in this ionic liquid. Likewise, [bmim][Tf2N] also causes enhancement of Eu3 + fluorescence by a factor of 730 due to the hydrophobic environment that it presents and not due to any sensitization. The absence of Eu3 + sensitization in [bmim][Tf2N] has also been indicated by Billard et al. [26]. 3.3. Tb3 + fluorescence The excitation spectrum of Tb3 + in [bmim][BA] was similar to that of Eu3 + in [bmim][BA] with an excitation maxima at 312 nm, which suggests a common absorber in the two system, Eu3 + and Tb3 + . However, the enhancement factor for Tb3 + in [bmim][BA] and

Table 5 Enhancement factors (EF)a and lifetimes (t) in ms, for Tb3 + in different systems.

Enhancement factor (EF) Lifetime (in ms)

Tb3 + (aq)

Tb3 + –benzoate (aq)

Tb3 + –[bmim][BA]

Tb3 + –benzoate–TOPO/ Triton X-100 (aq)

Tb3 + –[bmim][BA]// [bmim][Tf2N]

1 409

1030 403

1060b 900

43,660 1920

5080b 1270

a Enhancement factor (EF) is defined as the ratio [Tb3 + ]aq/[Tb3 + ]exp, where [Tb3 + ]aq and [Tb3 + ]exp are the concentrations of Tb3 + in the aquo complex and that in the experiment in question, that gave the same fluorescence intensity at 544 nm. b This enhancement factor has been derived from steady state experiments, where the background emission from the ionic liquid has not been discriminated against.

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745

Td = 50 ns

Fig. 5. Emission spectrum of Tb3 + –[bmim][BA]//[bmim][Tf2N]; excitation wavelength was 276 nm; [Tb3 + ]¼ 5.0  10  6 M; [bmim][BA]¼ 1.0  10  3 M.

Tb3 + –benzoate in water are almost the same (Table 5). In addition and more importantly, the 544 nm emission from Tb3 + was masked by a strong emission from the ionic liquid over the region 450–580 nm. The emission from the ionic liquid was not a problem with the Eu3 + emission, which occurs at 615 nm, well beyond the emission of the ionic liquid. The large background in the emission spectrum of Tb3 + – [bmim][BA]//[bmim][Tf2N], due to organic emission, is obvious in Fig. 5. It may be recognized that the background signal from the ionic liquid would be considerably short-lived relative to the Tb3 + emission. The problem of the strong background emission from the ionic liquid was addressed by using time-gated detection of the Tb3 + fluorescence. The detection of time-gated fluorescence of Tb3 + –[bmim][BA] was performed using an ICCD, by delaying the observation of the fluorescence by  300 ns (Td) after the excitation pulse, by which time the emission from the ionic liquid would be insignificant. Subsequent to this delay, the Tb3 + fluorescence was observed using a gate width of 1000–2000 ms (Tg), to selectively observe the Tb3 + emission. This scheme discriminated against background emission and improved the detection sensitivity of the Tb3 + emission. Fig. 6 shows the Tb3 + emission (5  10  8 M) as a function of the delay, to depict the efficacy of the above described detection scheme. Each spectrum was averaged over 15 laser shots. As a result of the time-gated detection, the fluorescence of Tb3 + in the [bmim][BA]//[bmim][Tf2N] was significant enough to allow for trace detection of the lanthanide. The lifetime of Tb3 + was measured to be 900 ms in [bmim][BA], which was higher than that of Tb3 + –aquo and Tb3 + –benzoate in an aqueous medium, where the lifetime was  400 ms (Table 5), which agrees with that reported in Ref. [9]. The lifetime of Tb3 + in [bmim][BA]//[bmim][Tf2N] increased to 1270 ms. 3.4. Effect of water on the Eu3 + -[bmim][BA]//[bmim][Tf2N] system The strong influence of water on various physical and spectroscopic properties measured in ionic liquids has been widely discussed in literature [24,25]. This point becomes even more relevant when one recognizes that in some hydrophilic ionic liquids, it is almost impossible to remove all of the water and all

Intensity (arb. unit)

70 ns

80 ns

200 ns

535

545 Wavelength (nm)

555

Fig. 6. Time-gated emission spectra of Tb3 + in [bmim][BA]//[bmim][Tf2N] as a function of delay and a constant gate width of 2000 ms. Excitation wavelength ¼ 266 nm, [Tb3 + ] ¼ 5  10  8 M.

measured properties are necessarily influenced by the presence of the unavoidable residual water content in the ionic liquids [38]. This has therefore necessitated a study and discussion on various drying procedures that have been adopted [26,39]. In the present study, we have adopted a drying procedure for [bmim][Tf2N] and [bmim][BA] as mentioned in Section 2.3. With this rather elaborate drying methods adopted, the water content in [bmim][BA] and [bmim][Tf2N] was estimated to be 1050 and 700 mg/mL, respectively. It was therefore considered necessary to study the effect of water on the fluorescence of lanthanides in [bmim][BA]// [bmim][Tf2N]. To study the effect of water, steady state and lifetime measurements were performed after deliberate addition of H2O and D2O to the Eu3 + -[bmim][BA]//[bmim][Tf2N] system. It must be noted that in all

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our experiments of Eu3 + -[bmim][BA]//[bmim][Tf2N], 1360 mg water is necessarily present to begin with; 360 mg of H2O being the amount present in the [bmim][BA]//[bmim][Tf2N] system taken for each experiment and 1000 mg (1 mL) of H2O in the aqueous Eu3 + solution added. Starting with this solution, 5 mL of H2O was successively added and the fluorescence measured. Fig. 7a shows the fluorescence intensity of Eu3 + in the Eu3 + -[bmim][BA]//[bmim][Tf2N] system, as a function of systematically added H2O. The fluorescence intensity has been plotted against H2O to [bmim][Tf2N] mole ratios. The corresponding H2O to Eu3 + mole ratios are also shown in the same plot. It is clear from the Fig. 7a, that the fluorescence intensity decreased systematically with successive addition of H2O, which implies that the Eu3 + in the original Eu3 + -[bmim][BA]//[bmim][Tf2N] solution was not coordinately saturated with the initially present H2O. If they had been so, any further addition of H2O would have had little effect on the Eu3 + fluorescence intensity.

moles of H2O/moles of Eu3+ 5000

2.0x106

4.0x106

6.0x106

Intensity at 615 nm (arb. unit)

4500 4000 3500 3000 2500 2000 1500

0.05

0.10 0.15 0.20 moles of H2O/moles of [bmim][Tf2N] moles of H2O/moles of Eu3+

2.0x106

4.0x106

6.0x106

1000

Lifetime (μs)

900

800

700

600

500

0.05

0.10 0.15 0.20 moles of H2O/moles of [bmim][Tf2N]

Fig. 7. Plots showing the (a) Eu3 + intensity and (b) Eu3 + lifetime as a function of the H2O to [bmim][Tf2N] mole ratios and H2O to Eu3 + mole ratios in the system Eu3 + – [bmim][BA]//[bmim][Tf2N]; excitation wavelength ¼276 nm, [Eu3 + ] ¼1  10  7 M; [bmim][BA]¼ 1.0  10  3 M.

Similarly, lifetime measurements, as a function of added H2O, were also done (Fig. 7b). The lifetime was found to decrease from a value of 979 ms to 562 ms as the H2O to [bmim][Tf2N] mole ratio was increased from 0.045 to 0.21 in the Eu3 + - [bmim][BA]// [bmim][Tf2N]. Both values of lifetimes are significantly higher than the value of 110 ms observed in aqueous Eu3 + , where the ion is coordinately saturated with H2O. The longer lifetimes in the ionic liquids imply that the Eu3 + in the ionic liquid systems that we have studied are not coordinately saturated with H2O, even though the Eu3 + to H2O ratios in the ionic liquids are in the realm of 1:106. If the plot of lifetimes of Eu3 + against H2O concentration were to be extrapolated to the hypothetical ‘zero-water’ concentration a lifetime of 1044 ms was obtained. While the values actually measured in the presence of the realizable water contents are less than the hypothetical ‘‘dry’’ Eu3 + lifetimes, the fluorescence intensities and lifetimes observed in the ionic liquid systems are still significantly larger than that obtained in aqueous systems, which allows for the use of the ionic liquid systems as useful fluorescence enhancers of lanthanide fluorescence. In addition to the experiments with H2O, similar steady state and lifetime measurements as a function of added D2O were also studied. In this case, the solution preparation is similar to that mentioned in Section 3.2, except for the addition of 1 mL of appropriate concentrations of Eu3 + in D2O instead of Eu3 + in H2O. A plot of fluorescence intensity and lifetime of Eu3 + as a function of D2O to [bmim][Tf2N] mole ratios and D2O to Eu3 + mole ratios in Eu3 + -[bmim][BA]// [bmim][Tf2N] are shown in Figs. 8a and b respectively. It can be seen that the lifetimes and the intensities of Eu3 + fluorescence decrease to a smaller extent when D2O was added, compared with the values obtained when H2O was added. This clearly indicates that Eu3 + is not coordinately saturated with water in these experiments, which if it were, would have shown only marginal changes in the lifetimes whether H2O or D2O were added. In all these experiments, we also calculated the number of H2O molecules coordinated to the Eu3 + in Eu3 + -[bmim][BA]// [bmim][Tf2N] as a function of added H2O. The average number of H2O molecules, NH2O, in the first coordination sphere of Eu3 + is calculated from the equation, NH2O ¼1.05  10  3 (kH2O–kD2O); where kH2O and kD2O are the luminescence decay constant (s  1) for the H2O and D2O experiments [33]. The value of NH2O was found to be in the range 0.1–0.5 as the H2O to [bmim][Tf2N] mole ratio was increased from 0.045 to 0.21 in Eu3 + -[bmim][BA]//[bmim][Tf2N]. Taking into account the reported uncertainty in the calculated NH2O values of 70.5 (as quoted in Ref. [33], it can be concluded that the number of H2O molecules coordinated to Eu3 + in these experiments cannot be significantly greater than 1. In Eu3 + -[bmim][Cl] system, Samikkanu et al. [33], reported a value of NH2O less than 1, when the H2O to [bmim][Cl] mole ratios were  0.5, which agrees with our findings. It may be noted that in an aqueous medium Eu3 + is coordinated by approximately nine H2O molecules [37]. It was also reported by Nagaishi et al. [24] that a value of 9 for NH2O was observed for Eu3 + in H2O saturated [bmim][Tf2N] medium. All these observations indicate that the inevitable water content present in these experiments, does not predominantly bind to Eu3 + . This observation is also consistent with the significantly large values of R obtained for the Eu3 + emission in the ionic liquids. It is therefore reassuring to realize that while water cannot be completely removed from the ionic liquid, the effect of the initially present water is still not serious enough, so that the ionic liquid serves as a good medium for fluorescence enhancement.

3.5. Dynamic range and detection limit A plot of intensity of Eu3 + fluorescence in Eu3 + -[bmim][BA]// [bmim][Tf2N] as a function of Eu3 + concentration, over the range

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950

providing a non-quenching medium, which is evidenced by the increase in the fluorescence lifetimes of the lanthanides in the ionic liquid, compared with those observed in its aquo complex and that in its benzoate complex in water. The fluorescence of the lanthanides in [bmim][BA] suffered from inner filter effect which was minimized by dilution with [bmim][Tf2N], following which the fluorescence enhancement was substantial enough to allow for the trace level detection of the lanthanides. A front face collection geometry was also used to observe the fluorescence of Eu3 + –[bmim][BA] to minimize the effects of the inner filter process, but dilution of Eu3 + –[bmim][BA] with [bmim][Tf2N] yielded a higher fluorescence intensity for Eu3 + . The Eu3 + environment in the ionic liquids yielded one of the largest values for the asymmetry factor R (  12) observed thus far. In the case of Tb3 + , the strong emission from the ionic liquid masked the emission of the lanthanide. This problem was overcome by the use of time-gated fluorescence detection, as the fluorescence of Tb3 + was found to be substantially longer than that of the background emission. Water was found to be an inevitable impurity in the ionic liquids; consequently experiments were performed by deliberately adding water to the ionic liquid system to study the influence of water on the fluorescence of Eu3 + in the ionic liquid systems. The results showed that, while water did have a detrimental effect on the fluorescence of Eu3 + as can be expected, the amount of water present in the ionic liquid does not pose serious problems and allows for the use of the ionic liquids as effective fluorescence enhancing medium for lanthanide fluorescence. This work highlights the possibility of suitably tailoring the ionic liquids to serve as fluorescence enhancing agents for pushing limits of detection.

900

Acknowledgements

moles of D2O/moles of Eu3+ 2.0x106

4.0x106

6.0x106

5200

Intensity at 615 nm (arb. unit)

5000 4800 4600 4400 4200 4000 3800 3600 3400 3200

0.05

0.10 0.15 0.20 moles of D2O/moles of [bmim][Tf2N] moles of D2O/moles of Eu3+

2.0x106

4.0x106

6.0x106

1050

Lifetime (μs)

1000

850 800 750 0.05

0.10 0.15 0.20 moles of D2O/moles of [bmim][Tf2N]

Fig. 8. Plots showing the (a) Eu3 + intensity and (b) Eu3 + lifetime as a function of the D2O to [bmim][Tf2N] mole ratios and D2O to Eu3 + mole ratios in the system Eu3 + – [bmim][BA]//[bmim][Tf2N]; excitation wavelength ¼276 nm, [Eu3 + ] ¼1  10  7 M; [bmim][BA]¼ 1.0  10  3 M. 8

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6

We acknowledge the assistance of Mrs. K.V. Syamala and Mr. K.A. Venkatesan, Chemistry Group, IGCAR for Karl-Fisher titration and providing certain starting materials for the synthesis of the ionic liquids. We thank Dr. M.T. Jose, Safety Group for help in recording some fluorescence spectra, Dr. D. Ponraju, Safety Group, IGCAR for providing Electron-Impact Ionization Mass Spectral data and SAIF, Indian Institute of Technology Madras, Chennai for providing NMR spectra. We also thank Dr. K.S. Krishna Rao, MAPS for providing high purity D2O. VSD gratefully acknowledges the grant of a research fellowship from the IGCAR, Department of Atomic Energy, India. References

3+

10 to 10 M shows linear plot. The detection limit of Eu in [bmim][BA]//[bmim][Tf2N] was calculated to be 5  10  9 M, using the criterion of 3s of the noise to be equal to the detection limit. Similarly the detection limit for the determination of Tb3 + in [bmim][BA]//[bmim][Tf2N] was 5  10  9 M. It must be noted that this detection limit was possible only when the Tb3 + fluorescence was measured by the time-gated fluorescence measurement described earlier to discriminate against ionic liquid background emission.

4. Conclusion This work reports for the first time the use of the newly synthesized ionic liquid [bmim][BA] to sensitize and enhance the fluorescence of Eu3 + and Tb3 + . In addition to the sensitization, the ionic liquid simultaneously enhances the fluorescence by

[1] J.-C.G. Bunzli, G.R. Choppin, Lanthanide Probes in Life Chemical and Earth Sciences: Theory and Practice, Elsevier, Amsterdam, 1983. [2] Z. Xuehui, H. Kelong, J. Feipeng, L. Zhiaojianl, P. Xiahui, Rare Metals 25 (2006) 144. [3] P. Juan, G. Xiaotian, Y. Jianbo, Z. Yanhui, Z. Ying, W. Yunyou, S. Bo, J. Alloys Compd. 426 (2006) 363. [4] B.S. Panigrahi, J. Lumin. 82 (1999) 121. [5] H.G. Brittain, J. Lumin. 17 (1978) 411. [6] S.V. Beltyukova, E.I. Tselik, A.V. Egorova, J. Pharm, Biomedical Anal. 18 (1998) 261. [7] S. Maji, K.S. Viswanathan, Spectrochim. Acta A 64 (2006) 972. [8] B.S. Panigrahi, S. Peter, K.S. Viswanathan, C.K. Mathews, Anal. Chim. Acta 282 (1993) 117. [9] S. Maji, K. Sundararajan, K.S. Viswanathan, Spectrochim. Acta A 59 (2003) 455. [10] G.-T. Wei, Z. Yang, C.J. Chen, Anal. Chim. Acta 488 (2003) 183. [11] J.G. Huddleston, H.G. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Chem. Commun. 16 (1998) 1765. [12] H.-P. Zhu, F. Yang, J. Tang, M.-Y. He, Green Chem. 5 (2003) 38. [13] L.D.S. Yadav, C. Awasthi, A. Rai, Tetrahedron Lett. 49 (2008) 6360. [14] C. Villagra´n, L. Aldous, M.C. Lagunas, R.G. Compton, C. Hardacre, J. Electroanal. Chem. 588 (2006) 27. [15] T. Welton, Coord. Chem. Rev. 248 (2004) 2459.

748

V. Shyamala Devi et al. / Journal of Luminescence 131 (2011) 739–748

[16] K. Mikami, Green Reaction Media in Organic Synthesis, Blackwell publishers, 2005. [17] H. Luo, S. Dai, P.V. Bonnesen, A.C. Buchanan, J. Alloys Compds. 418 (2006) 195. [18] A.E. Visser, R.P. Swatloski, W.M. Reichert, R. Mayton, S. Sheff, A. Wierzbicki, A.H. Davis, R.D. Rogers, Chem. Commun. 1 (2001) 135. [19] D. Kogelnig, A. Stojanovic, M. Galanski, M. Groessl, F. Jirsa, R. Krachler, B.K. Keppler, Tetrahedron Lett. 49 (2008) 2782. [20] B.S. Panigrahi, S. Peter, K.S. Viswanathan, C.K. Mathews, Spectrochim. Acta A 51 (1995) 2289. [21] P.K. Mandal, A. Samanta, J. Phys. Chem. B 109 (2005) 15172. [22] P.K. Mandal, A. Paul, A. Samanta, J. Photochem. Photobiol. A: Chem. 182 (2006) 113. [23] S. Arenz, A. Babai, K. Binnemans, K. Driesen, R. Giernoth, A.-V. Mudring, P. Nockemann, Chem. Phys. Lett. 402 (2005) 75. [24] R. Nagaishi, M. Arisaka, T. Kimura, Y. Kitatsuji, J. Alloys Compds. 431 (2007) 221. [25] K.R. Seddon, A. Stark, M.-J. Torres, Pure Appl. Chem. 72 (2000) 2275. [26] I. Billard, S. Mekki, C. Gaillard, P. Hesemann, C. Mariet, G. Moutiers, A. Labet, ¨ J.-C.G. Bunzli, Eur. J. Inorg. Chem. 6 (2004) 1190.

[27] V.R. Koch, C. Nanjundiah, G. Battista Appetecchi, B. Scrosati, J. Electrochem. Soc. 142 (1995) L116. [28] P. Bonhote, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Gratzel, Inorg. Chem. 35 (1996) 1168. [29] J.H. Gross, J. Am., Soc. Mass Spectrom. 18 (2007) 2254. [30] M. Zabet-Moghaddam, R. Kruger, E. Heinzle, A. Tholey, J. Mass Spectrom. 39 (2004) 1494. [31] S. Peter, B.S. Panigrahi, K.S. Viswanathan, C.K. Mathews, Anal. Chim. Acta 260 (1992) 135. [32] T. Kimura, Y. Kato, J. Alloys Compds. 278 (1998) 92. [33] S. Samikkanu, K. Mellem, M. Berry, P.S. May, Inorg. Chem. 46 (2007) 7121. [34] A.F. Kirby, D.F. Foster, F.S. Richardson, Chem. Phys. Lett. 95 (1983) 507. [35] S. Maji, K.S. Viswanathan, J. Lumin. 128 (2008) 1255. [36] S. Lis, G.R. Choppin, J. Alloys Compds. 225 (1995) 257. [37] Z.-M. Wang, L.J. Van de Burgt, G.R. Choppin, Inorg. Chim. Acta 293 (1999) 167. [38] J.G. Huddleston, A.E. Visser, M. Reichert, H.D. Willauer, G.A. Broker, R.D. Rogers, Green Chem. 3 (2001) 156. [39] C. Gaillard, I. Billard, A. Chaumont, S. Mekki, A. Ouadi, M.A. Denecke, G. Moutiers, G. Wipff, Inorg. Chem. 44 (2005) 8355.