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The co-luminescence effect of a europium (III)–lanthanum (III)–gatifloxacin–sodium dodecylbenzene sulfonate system and its application for the determination of trace amount of europium (III)
ARTICLE IN PRESS Journal of Luminescence 130 (2010) 591–597
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The co-luminescence effect of a europium (III)–lanthanum (III)–gatifloxacin– sodium dodecylbenzene sulfonate system and its application for the determination of trace amount of europium (III) Changchuan Guo a, Aidong Lang a, Lei Wang a,n, Wei Jiang b,nn a b
School of Pharmaceutical Sciences, Shandong University, Jinan, Shandong 250012, China School of Chemistry & Chemical Engineering, Shandong University, Jinan, Shandong 250012, China
a r t i c l e in fo
abstract
Article history: Received 20 July 2009 Received in revised form 8 October 2009 Accepted 3 November 2009 Available online 10 November 2009
A novel co-luminescence system based on the formation of a complex between europium (III) (Eu3 + ) and gatifloxacin (GFLX) in sodium dodecylbenzene sulfonate (SDBS) micelle solution containing lanthanum (III) (La3 + ) has been developed for the determination of Eu3 + . The experimental results show that the complex formed by Eu3 + and GFLX here can emit the characteristic luminescence of Eu3 + . With the addition of La3 + , the luminescence intensity of the system was enhanced about 7-fold compared with that without La3 + . Under the optimal conditions, the luminescence intensity exhibits an excellent linear relationship with Eu3 + concentration in the range of 1.0 10 10–5.0 10 8 mol L 1. The correlation coefficient (r) is 0.9998, and the detection limit (3s) is 7.0 10 14 mol L 1. A test method with satisfactory accuracy based on this system was applied to determine trace amounts of Eu3 + in rare earth samples. In addition, the detailed luminescence mechanism of this system was investigated by analyzing the ultraviolet absorption spectra, surface tension, fluorescence polarization, quantum yield, and the number of water molecules in the first coordination sphere of the Eu3 + complex. & 2009 Elsevier B.V. All rights reserved.
Keywords: Europium Lanthanum Gatifloxacin Determination Luminescence
1. Introduction The need to determine trace levels of europium is encountered more and more in environmental as well as material and mineral resources. However, the chemical properties of the rare earth elements are so similar that it is difficult for specific detection of europium with common chemical analysis methods, especially in their mixtures [1]. To solve this problem, complexes formed by europium and organic ligands with good luminescent properties of high sensitivity and good selectivity could be utilized for the specific detection of europium. Ligand-sensitized luminescence of lanthanide is of great interest to analytical chemists due to its potential applications in biochemistry and luminescent analysis of biochemical drugs [2–5], as well as for the trace determination of lanthanide ions [6,7] and some organic analytes [8,9]. Trivalent lanthanide ions are weak luminescent species in water due to their low molar absorptivities and poor quantum yields [10]. However, the drawback of weak light absorption can be avoided by the
n
Corresponding author. Tel.: + 86 531 88382330; fax: + 86 531 8856 5167. Corresponding author. Tel.: + 86 531 88363888. E-mail addresses: [email protected] (L. Wang), [email protected] (W. Jiang). nn
0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.11.001
so-called ‘‘antenna effect’’ [11], which takes place in three steps. First, light is absorbed by the immediate environment of the Ln (III) ion through the attached organic ligands (chromophores). Energy is then transferred onto one or several excited states of the metal ion. Finally, the metal ion emits light [12]. Another phenomenon of analytical interest is the co-luminescence effect [13–20], which was first found and studied by Yang and Zhu in 1986 [21]. According to this method, addition of certain lanthanide ions, such as La3 + , Lu3 + , Gd3 + , Y3 + , and Tb3 + , could greatly enhance the luminescence intensity of the Eu3 + , Tb3 + , and Sm3 + chelates in solution. The co-luminescence effect which involves an intermolecular energy transfer process has become an important way to improve the luminescence detection sensitivity of lanthanide ions. As widely used antibacterial agents in clinic, quinolones are a group of compounds often utilized as sensitizing ligands, which have suitable functional groups to form stable complexes with lanthanide ions. The presence of these ions in quinolone solutions leads to the formation of complexes that absorb energy at the characteristic wavelength of the organic ligand and emit radiation of the characteristic wavelength of lanthanide ions (Tb3 + or Eu3 + ). These complexes show a large Stokes shift, long lifetime, and narrow emission bands. Highly sensitive methods based on quinolone complexes have been reported for the determination of quinolones ciprofloxacin [22], pipemidic acid [23], lomefloxacin
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[23], levofloxacin [24], gatifloxacin [25], trovafloxacin [26], and garenoxacin [27]. Gatifloxacin[(7)-1-cyclopropyl-6-fluoro-1,4-dihydro-8-methoxy-7-(3-methyl-1-piperazinyl)-4-oxo-3-quinoline carboxylic acid, GFLX] is the fourth generation of a new class of synthetic fluoroquinolone antibacterial agents. Like other quinolones, GFLX also has a b-keto acid group to make it still behave as a b-keto acid ligand for lanthanide ions [28]. In this work, by using GFLX as an organic ligand, a novel co-luminescence system Eu3 + –La3 + – GFLX–sodium dodecylbenzene sulfonate (SDBS) is developed. The characteristics of its spectra and the effects of various experimental conditions on the luminescence intensity were investigated. This method has been successfully applied to determine trace amount of europium in both synthetic and commercial samples of lanthanide oxides. Compared with most of other methods reported for the determination of Eu3 + , the present paper provides a much more thorough understanding on the luminescence mechanism. Furthermore, by investigation of the number of water molecules in the first coordination sphere of the lanthanide complex, we have verified that SDBS molecules could block the coordination of water molecules to the lanthanide–quinolone complex and therefore result in the luminescence enhancement, which is usually discussed merely by hypothesis or speculation in some literatures [29,30].
SDBS solution (1.2 10 3 mol L 1) was prepared by dissolving 0.0929 g SDBS (90.0%) in water and then diluting to 200 mL. NH4Ac–NH3 H2O buffer solution (pH 7.0, 0.5 mol L 1) was prepared by dissolving 9.8316 g NH4Ac in 250 mL water and then adjusting pH to 7.0 with NH3 H2O. All reagents used were of analytical grade unless otherwise indicated. In-house redistilled water was used throughout the study. 2.3. Methods To a 10 ml volumetric flask, the following solutions were added in sequence: 1.0 mL of 0.5 mol L 1 NH4Ac–NH3 H2O (pH 7.0), 1.0 mL of 2.0 10 4 mol L 1 La3 + , 1.0 mL of 1.2 10 3 mol L 1 SDBS, an aliquot of Eu3 + working standard solution, 0.4 mL of 1.0 10 3 mol L 1 GFLX. The solution was then diluted to the mark with water and mixed thoroughly by shaking, and then allowed to stand for 15 min at room temperature. The range of the final Eu3 + concentrations was 1.0 10 10–5.0 10 8 mol L 1. The luminescence intensity was measured in a 10 mm pathlength quartz cell with excitation and emission wavelengths of 336 nm and 616 nm, respectively.
3. Results and discussion 2. Experimental
3.1. Excitation and emission spectra
2.1. Apparatus
The excitation and emission spectra of Eu3 + (1, 10 ), GFLX (2, 20 ), Eu –GFLX (3, 30 ), Eu3 + –GFLX–SDBS (4, 40 ), and Eu3 + –La3 + –GFLX– SDBS (5, 50 ) systems are shown in Fig. 1. From curve 5, it can be seen that the excitation peak of Eu3 + –La3 + –GFLX–SDBS system is at 336 nm, which was therefore selected as the excitation wavelength. The emission spectra show that none of Eu3 + (curve 10 ), GFLX (curve 20 ), and Eu3 + –GFLX (curve 30 ) aqueous solutions could display the characteristic luminescence signal of Eu3 + at 616 nm. Curve 40 is the emission spectrum of Eu3 + –GFLX– SDBS system showing a weak emission peak at 616 nm. Curve 50 is the emission spectrum of Eu3 + –La3 + –GFLX–SDBS system with an excitation wavelength of 336 nm. Strong emission peaks were observed at 594 and 616 nm, corresponding to transitions of the Eu3 + 5D0–7F1 and 5D0–7F2, respectively. Compared to the 3+
Normal luminescence measurements were recorded with a Hitachi F–2500 Fluorescence Spectrophotometer (Hitachi, Japan), using a standard 10 mm pathlength quartz cell with 10 nm bandwidths for both the excitation and emission monochromators. All absorption spectra were measured on a UV–2401PC spectrophotometer (Shimadzu, Japan) equipped with 10 mm pathlength quartz cells. The fluorescence polarization data were obtained using a LS–55 Luminescence Spectrometer (PerkinElmer, USA). The luminescence lifetime was determined using a FLS920 fluorescence spectrometer (Edinburg, England). The surface tension was measured on a Processor Tensi¨ ometer–K12 (KRUSS Corp, Germany) with the precise degree of the measurement 0.01 mN m 1 by the Wilhelmy plate. All pH measurements were made with a Lei Ci pHs-3C pH-meter (Lei Ci, China).
2.2. Reagents The stock standard solutions of Eu3 + (1.0 10 3 mol L 1) or La3 + (4.0 10 3 mol L 1) were prepared, respectively, by dissolving 0.0176 g Eu2O3 (purity, 99.99%) or 0.0652 g La2O3 (purity, 99.99%) in 1:1 HCl and evaporating the solutions to near dryness before diluting to 100 mL with water. The stock standard solutions were kept at 4 1C. The working standard solutions were prepared by making appropriate dilutions with water. A stock standard solution (1.0 10 2 mol L 1) of GFLX (99.0%, Shandong Luoxin Pharmacy Stock Co., Ltd. Linyi, China) was prepared by dissolving 0.3792 g GFLX with an appropriate NaOH solution (0.1 mol L 1) and diluting it to 100 mL with water. The solution was kept at 4 1C in the refrigerator and protected from light. Working standard solutions were obtained by making appropriate dilutions of the stock standard solution with water.
Fig. 1. Excitation and emission spectra. 1, 10 Eu3 + ; 2, 20 GFLX; 3, 30 Eu3 + –GFLX; 4, 40 Eu3 + –GFLX–SDBS 5, 50 Eu3 + –La3 + –GFLX–SDBS. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; SDBS, 1.2 10 4 mol L 1; La3 + , 2.0 10 5 mol L 1; NH4Ac–NH3 H2O, 0.05 mol L 1, pH 7.0.
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Eu3 + –GFLX–SDBS system, the luminescence intensity was enhanced 7-fold with the addition of co-luminescence lanthanide ion La3 + .
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Table 1 Effect of different co-luminescence lanthanide ions. Co-luminescence lanthanide ions and the optimal concentration (mol L 1)
Relative luminescence intensity (%)
Lu3 + , 2.0 10 6 Gd3 + , 1.0 10 6 Y3 + , 2.0 10 6 La3 + , 2.0 10 5
23.6 12.4 24.4 100
3.2. Effect of pH and buffers Change of luminescence intensity with pH is shown in Fig. 2. The optimal pH value was in the range of 5.5–7.5, and pH 7.0 was selected for the recommended procedure. The effects of following buffer solutions on the luminescence intensity were then examined: NH4Ac–NH3 H2O, NaAc–HAc, NH4Cl–NH3 H2O, Tris– HCl, and KH2PO4–NaOH. It was found that the luminescent system in a 0.05 mol L 1 NH4Ac–NH3 H2O medium exhibited the highest sensitivity. Therefore, the NH4Ac–NH3 H2O buffer solution of pH 7.0 was chosen for further study. 3.3. Effect of GFLX concentration The GFLX concentration exerted a significant effect on the luminescence intensity of the system. The GFLX concentration was varied while the concentration of europium was fixed. The maximum and constant luminescence intensity was obtained at a GFLX concentration range of 3.5 10 5–5.5 10 5 mol L 1. Consequently, the GFLX concentration was selected at 4.0 10 5 mol L 1 for further experiments.
Table 2 Effect of different surfactants. Surfactants and the optimal concentration
Relative luminescence intensity (%)
Cetrimonium bromide (CTAB), 2.5 10 5 mol L 1 PEG 40, 2.0 10 3 g ml 1 Pluronic F68, 5.0 10 4 g ml 1 Arabic gum (GA), 5.0 10 3 g ml 1 Polyoxyethylene nonylphenol ether (OP), 2.0% (V/V) b-cyclodextrin (b-CD), 4.0 10 4 mol L 1 Sodium dodecyl sulfate (SDS), 1.0 10 4 mol L 1 Sodium dodecylbenzene sulfonate (SDBS), 1.2 10 4 mol L 1
0.2 3.6 0.5 2.4 3.3 0.5 40.6 100
3.4. Effect of La3 + concentration Effects of the co-luminescence lanthanide ions such as La3 + , Lu3 + , Y3 + , and Gd3 + were studied. The results are shown in Table 1, from which we can see that La3 + is the most effective one. The change of system luminescence intensity with different La3 + concentration was investigated, with a result showing that the enhanced luminescence intensity reaches its maximum value when the concentration of La3 + is in the range of 1.0 10 5– 2.4 10 5 mol L 1. Hence, 2.0 10 5 mol L 1 La3 + was chosen for further studies. 3.5. Effect of surfactants Various types of surfactants luminescence intensity of the influence are shown in Table 2. PEG 40, pluronic F68, arabic
have different effects on the system. The results of such Nonionic surfactants including gum (GA), polyoxyethylene Fig. 3. Effect of SDBS concentration. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; La3 + , 2.0 10 5 mol L 1; NH4Ac–NH3 H2O, 0.05 mol L 1, pH 7.0.
nonylphenol ether (OP), b-cyclodextrin (b-CD), and cationic surfactant cetrimonium bromide (CTAB) have little effect, while the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS) significantly increase the luminescence intensity of the system with the latter being more effective. Accordingly, SDBS was selected for further studies. The effect of SDBS concentration on the luminescence intensity is shown in Fig. 3. Maximum luminescence intensity was observed for an SDBS concentration of 1.2 10 4 mol L 1, which was chosen for further experiments.
3.6. Effect of reagent addition sequence and the system stability
Fig. 2. Effect of pH. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; SDBS, 1.2 10 4 mol L 1; La3 + , 2.0 10 5 mol L 1.
The effect of reagent addition sequence on the luminescence intensity was studied. The result showed that the optimal addition sequence was NH4Ac–NH3 H2O, La3 + , SDBS, Eu3 + , and
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GFLX. Therefore, this addition order was chosen for further research. Under the optimal conditions, the stability of the luminescence intensity was investigated. The results showed that the luminescence intensity reached a maximum after all the reagents had been added and remained stable for over 2.5 h, indicating a good stability. 3.7. Interference of coexisting foreign substances The interference of possible coexisting foreign substances was tested at the concentration of Eu3 + 1.0 10 6 mol L 1 and the result is shown in Table 3. It was found that most coexisting foreign substances except for Cu2 + , Fe3 + and several other lanthanide ions had little interference on the determination of Eu3 + , within the acceptable accuracy of 75% relative errors. 3.8. Analytical application 3.8.1. Calibration curve and detection limit Under optimal conditions, the enhanced luminescence intensity of the system (DIL) shows an excellent linear relationship to the Eu3 + concentration in the range of 1.0 10 10–5.0 10 8 mol L 1. The linear equation is DIL = 4.46 1010CEu + 39.7 with a correlation coefficient of 0.9998. The detection limit (3s) was determined as 7.0 10 14 mol L 1. Table 3 Maximum permissible concentrations of coexisting foreign substances. Coexisting foreign substances
Maximum permissible concentration (mol L 1)
Change of DIf (%)
Pr3 + Nd3 + Tm3 + Er3 + Ho3 + Sm3 + Tb3 + Yb3 + Dy3 + Sc3 + Y3 + Gd3 + Lu3 + Cu2 + , SO24 Fe3 + , Cl Al3 + , Cl Mg2 + , Cl Zn2 + , SO24 Ca2 + , Cl K + , Cl Na + , Cl
2.0 10 7 2.0 10 7 3.5 10 7 6.0 10 7 6.0 10 7 7.0 10 7 9.0 10 7 9.0 10 7 1.0 10 6 2.0 10 6 2.0 10 6 3.5 10 6 4.0 10 6 2.0 10 7 8.0 10 7 4.5 10 6 9.0 10 4 1.0 10 3 2.5 10 3 6.0 10 2 9.5 10 2
4.5 4.5 4.8 4.8 4.1 4.4 4.5 4.6 5.0 5.0 4.6 4.5 4.7 4.3 5.0 4.4 5.0 4.4 4.9 4.2 4.3
In comparison with most of other methods reported for the determination of Eu3 + , as shown in Table 4, the rapid and simple method proposed in this paper offers a lower detection limit and a comparable linear range. 3.8.2. Sample determination and the recovery tests In order to verify the validity of the method, a synthetic sample was prepared according to the abundance of each lanthanide in the earth crust (sample 1). In addition, the amount of europium in a reference material obtained from Baotou Research Institute of Rare Earths, China, was also analyzed (sample 2). The analysis results are given in Table 5, which indicate that the proposed method based on the co-luminescence system Eu3 + – La3 + –GFLX–SDBS is suitable for the determination of trace amounts of europium in lanthanides mixtures. The recovery experiments of europium in both synthetic and commercial samples of lanthanide oxides are also examined and summarized in Tables 6 and 7. All recoveries are in the range of 96.1–101.7%. 3.9. Luminescence mechanism 3.9.1. Formation of Eu3 + –La3 + –GFLX–SDBS complex In order to study the interaction between Eu3 + , La3 + , GFLX, and SDBS, several ultraviolet absorption spectra were recorded and are shown in Fig. 4. It can be seen that there are two absorption peaks of GFLX at 285 and 331 nm. After the addition of Eu3 + or Eu3 + along with SDBS, the absorption peak positions of GFLX do not change and only the absorbance at 285 nm decreases slightly. However, the absorbance of GFLX increases remarkably when Eu3 + along with La3 + and SDBS is added, in agreement with the luminescence enhancement of the excitation spectra in the Eu3 + – La3 + –GFLX–SDBS system (see Fig. 1), and the maximum absorption wavelength underwent a red shift from 285 to 289 nm, indicating the formation of a Eu3 + –La3 + –GFLX–SDBS quaternary complex [23]. 3.9.2. Luminescence enhancement mechanism The mechanism of SDBS effect was studied by investigating the surface tension change of the luminescence system with SDBS concentration. The results are shown in Fig. 5. It can be seen that the surface tension first decreases sharply with increasing SDBS concentration. Soon an equilibrium point is reached and then it decreases very slowly. The concentration of 1.8 10 5 mol L 1 could be considered as the apparent critical micelle concentration (CMC) of SDBS in this system. Since the SDBS concentration selected for the study (1.2 10 4 mol L 1) is well above the CMC, it can be concluded that the formation of micelles has a great impact on the enhancement of the luminescence intensity of the system.
Table 4 Comparison of methods for the determination of europium. Method
Detection limit (mol L 1)
Linear range (mol L 1)
References
Europium-sensitized luminescence Europium-sensitized luminescence Co-luminescence Co-luminescence Co-luminescence Co-luminescence Co-luminescence Time-resolved co-luminescence Time-resolved co-luminescence Time-resolved co-luminescence Co-luminescence
1.0 10-10 1.0 10 10 2.0 10 11 1.0 10 13 2.0 10 13 4.0 10 13 1.46 10 10 4 10 15 1.9 10 14 3.5 10 14 7.0 10 14
5.0 10 9 1.0 10-6 5.0 10 9 2.5 10 6 1.0 10 9 2.0 10 7 5.0 10 11 5.0 10 7 1.0 10 10 5.0 10 7 2.0 10 10 2.0 10 7 5.0 10 10 2.0 10 7
[6] [7] [15] [16] [17] [19] [20] [31] [32] [33] Present method
1.0 10 10 5.0 10 8
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595
Table 5 Determination of europium in standard samples in lanthanide oxides. Sample
Present
Found
Mean7 SD
1 2
7.24 ( 10 7 mol L 1) 0.23 (%)
7.26, 6.98, 7.16, 6.92, 7.31 ( 10 7 mol L 1) 0.21, 0.21, 0.20, 0.21, 0.20 (%)
7.13 70.17 0.217 0.0055
Sample 1: Y 3.49 10 5 mol L 1, La 1.37 10 5 mol L 1, Ce 3.14 10 5 mol L 1, Pr 3.97 10 6 mol L 1, Nd 1.66 10 5 mol L 1, Sm 6.98 10 6 mol L 1, Eu 7.24 10 7 mol L 1, Gd 4.01 10 6 mol L 1, Tb 6.29 10 7 mol L 1, Dy 2.65 10 6 mol L 1, Ho 7.28 10 7 mol L 1, Er 1.43 10 6 mol L 1, Tm 1.24 10 6 mol L 1, Yb 1.50 10 6 mol L 1, Lu 4.00 10 7 mol L 1. Sample 2: Y2O3 0.27%, La2O3 27.11%, CeO2 49.21%, Pr6O11 5.18%, Nd2O3 16.75%, Sm2O3 1.29%, Eu2O3 0.23%, Gd2O3 0.40%, Tb4O7 0.03%, Dy2O3 0.09%, Ho2O3 0.023%, Er2O3 0.027%, Tm2O3 0.0095%, Yb2O3 0.013%, and Lu2O3 0.003%.
Table 6 Recoveries of europium in synthetic samples of lanthanide oxides. Europium added ( 10 8 mol L 1)
Europium found ( 10 8 mol L 1)
Recoveries (%)
0.500
0.482 0.488 0.484 0.504 0.514
96.5 97.7 96.7 100.9 102.8
98.9 7 2.8
1.00
1.024 1.028 1.015 0.988 1.032
102.4 102.8 101.5 98.8 103.2
101.7 7 1.8
Mean 7SD (%)
Table 7 Recoveries of europium in commercial samples of lanthanide oxides. Europium added ( 10 8 mol L 1)
Europium found ( 10 8 mol L 1)
Recoveries (%)
Mean7 SD (%)
0.500
0.479 0.487 0.473 0.473 0.490
95.8 97.5 94.7 94.7 98.0
96.1 7 1.5
1.00
0.970 0.951 0.976 0.987 0.973
97.0 95.1 97.6 98.7 97.3
97.2 7 1.3
Fig. 4. Ultraviolet absorption spectra. 1, Eu3 + ; 2, GFLX; 3, Eu3 + –GFLX; 4, Eu3 + – GFLX–SDBS 5, Eu3 + –La3 + –GFLX–SDBS. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; SDBS, 1.2 10 4 mol L 1; La3 + , 2.0 10 5 mol L 1; NH4Ac–NH3 H2O, 0.05 mol L 1, pH 7.0.
Fig. 5. Effect on solution surface tension with the addition of SDBS. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; La3 + , 2.0 10 5 mol L 1; NH4Ac–NH3 H2O, 0.05 mol L 1, pH 7.0.
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Table 8 The number of water molecules in the first coordination sphere of the Eu3 + complex. System
Lifetime in H2O (ms) Lifetime in D2O (ms)
Eu3 + –GFLX Eu3 + –GFLX–SDBS Eu3 + –GFLX–La3 + Eu3 + –La3 + –GFLX– SDBS
0.1906 0.5436 0.2283 0.9733
1.398 0.6041 1.087 1.120
n 4.627 0.06211 3.502 0.01024
The microviscosity (Z) of different luminescence systems can be calculated using the equation developed by Shinitzky and Barenholz [34]:
Z¼
2P 0:46 P
where P refers to the fluorescence polarization. The P of different systems were determined and then the Z values of different systems were calculated acoording to the above equation. The results indicates that the Z value increases significantly with the addition of SDBS and La3 + to the Eu3 + –GFLX system. Therefore, it can be concluded that La3 + and SDBS could provide a high microviscosity for Eu3 + –GFLX complexes and contribute to the luminescence enhancement of the complexes. In addition, the greater microviscosity environment provided by La3 + and SDBS can also prevent the collision of the complex and water, leading to the reduction in the energy loss of the Eu3 + –La3 + –GFLX–SDBS system. Consequently, a significant enhancement of the luminescence intensity is observed. The number of water molecules in the first coordination sphere of a Eu3 + complex, n, in aqueous solution, was determined 1 according to the equation n ¼ 1:05 ½t1 H2 O tD2 O 0:125[35]. The results are shown in Table 8. In aqueous solutions, interaction with water (both inside and outside the coordination sphere of Eu3 + ) may lead to a severe quenching of the Eu3 + luminescence via O–H vibrations and other de-activating vibrations (e.g. N–H or C–H oscillators) [12]. However, GFLX– Eu3 + complex could reside inside the micelles with the presence of surfactant SDBS. Thus, the GFLX–Eu3 + complex is free of highenergy vibrations and solvent interaction. From Table 8, it can be seen that the SDBS medium could provide an optimal hydrophobic coordination environment of Eu3 + and GFLX and decrease the interaction between the water molecules and Eu3 + – GFLX complex. The quantum yield (Ff) of the luminescence systems in various media was determined. Quantum yield is one of the most basic and significant parameters in all the characters of luminescence substance [36] and represents the ability of a complex to translate absorption energy to luminescence. In this paper, the quantum yield was measured according to the equation F2 = (F1A1F2)/ F1A2[37], using quinine sulfate as the reference standard. The results show that the quantum yield Ff of Eu3 + –La3 + –GFLX–SDBS is higher than any of the other three systems, proving that its ability of translating absorption energy to luminescence is stronger than the other three, and therefore, the enhancement of the luminescence intensity.
4. Conclusion This paper describes a novel co-luminescence system for the determination of trace amounts of europium. Under optimal conditions, the enhanced luminescence intensity is proportional to the europium concentration in the range of 1.0 10 10–
5.0 10 8 mol L 1. The detection limit (3s) is 7.0 10 14 mol L 1. The proposed method has been successfully applied to the determination of trace amounts of europium in rare earth samples. The luminescence mechanism is also discussed in detail. It’s been demonstrated that SDBS exists in the form of micelles in solution and can provide a hydrophobic environment with high viscosity, which is beneficial to the luminescence enhancement. SDBS medium can also provide an optimal hydrophobic coordination environment of Eu3 + and GFLX and decrease the interaction between the water molecules and the Eu3 + –GFLX complex. With the formation of Eu3 + –La3 + –GFLX–SDBS complex, the quantum yield increases enormously compared with the Eu3 + –GFLX binary complex, resulting in significant enhancement of the luminescence intensity. In comparison with other reported methods, the rapid and simple procedure proposed in the text offers comparable sensitivity and better stability with a much thorough understanding of the luminescence mechanism.
Acknowledgements This project was supported by the National Natural Science Foundations of China (Nos. 20875056, 20775043). The authors would like to thank the Key Laboratory of Education Ministry on Colloid & Interface Chemistry at Shandong University for kindly loaning Processor Tensiometer–K12 and LS–55 Luminescence Spectrometer for the determination of surface tension and fluorescence polarization. Also, the authors would like to thank the State Key Laboratory for Crystal Materials of Shandong University for kindly supplying Edinburg FLS920 for the determination of the luminescence lifetime.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
The co-luminescence effect of a europium (III)–lanthanum (III)–gatifloxacin– sodium dodecylbenzene sulfonate system and its application for the determination of trace amount of europium (III) Changchuan Guo a, Aidong Lang a, Lei Wang a,n, Wei Jiang b,nn a b
School of Pharmaceutical Sciences, Shandong University, Jinan, Shandong 250012, China School of Chemistry & Chemical Engineering, Shandong University, Jinan, Shandong 250012, China
a r t i c l e in fo
abstract
Article history: Received 20 July 2009 Received in revised form 8 October 2009 Accepted 3 November 2009 Available online 10 November 2009
A novel co-luminescence system based on the formation of a complex between europium (III) (Eu3 + ) and gatifloxacin (GFLX) in sodium dodecylbenzene sulfonate (SDBS) micelle solution containing lanthanum (III) (La3 + ) has been developed for the determination of Eu3 + . The experimental results show that the complex formed by Eu3 + and GFLX here can emit the characteristic luminescence of Eu3 + . With the addition of La3 + , the luminescence intensity of the system was enhanced about 7-fold compared with that without La3 + . Under the optimal conditions, the luminescence intensity exhibits an excellent linear relationship with Eu3 + concentration in the range of 1.0 10 10–5.0 10 8 mol L 1. The correlation coefficient (r) is 0.9998, and the detection limit (3s) is 7.0 10 14 mol L 1. A test method with satisfactory accuracy based on this system was applied to determine trace amounts of Eu3 + in rare earth samples. In addition, the detailed luminescence mechanism of this system was investigated by analyzing the ultraviolet absorption spectra, surface tension, fluorescence polarization, quantum yield, and the number of water molecules in the first coordination sphere of the Eu3 + complex. & 2009 Elsevier B.V. All rights reserved.
Keywords: Europium Lanthanum Gatifloxacin Determination Luminescence
1. Introduction The need to determine trace levels of europium is encountered more and more in environmental as well as material and mineral resources. However, the chemical properties of the rare earth elements are so similar that it is difficult for specific detection of europium with common chemical analysis methods, especially in their mixtures [1]. To solve this problem, complexes formed by europium and organic ligands with good luminescent properties of high sensitivity and good selectivity could be utilized for the specific detection of europium. Ligand-sensitized luminescence of lanthanide is of great interest to analytical chemists due to its potential applications in biochemistry and luminescent analysis of biochemical drugs [2–5], as well as for the trace determination of lanthanide ions [6,7] and some organic analytes [8,9]. Trivalent lanthanide ions are weak luminescent species in water due to their low molar absorptivities and poor quantum yields [10]. However, the drawback of weak light absorption can be avoided by the
n
Corresponding author. Tel.: + 86 531 88382330; fax: + 86 531 8856 5167. Corresponding author. Tel.: + 86 531 88363888. E-mail addresses: [email protected] (L. Wang), [email protected] (W. Jiang). nn
0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.11.001
so-called ‘‘antenna effect’’ [11], which takes place in three steps. First, light is absorbed by the immediate environment of the Ln (III) ion through the attached organic ligands (chromophores). Energy is then transferred onto one or several excited states of the metal ion. Finally, the metal ion emits light [12]. Another phenomenon of analytical interest is the co-luminescence effect [13–20], which was first found and studied by Yang and Zhu in 1986 [21]. According to this method, addition of certain lanthanide ions, such as La3 + , Lu3 + , Gd3 + , Y3 + , and Tb3 + , could greatly enhance the luminescence intensity of the Eu3 + , Tb3 + , and Sm3 + chelates in solution. The co-luminescence effect which involves an intermolecular energy transfer process has become an important way to improve the luminescence detection sensitivity of lanthanide ions. As widely used antibacterial agents in clinic, quinolones are a group of compounds often utilized as sensitizing ligands, which have suitable functional groups to form stable complexes with lanthanide ions. The presence of these ions in quinolone solutions leads to the formation of complexes that absorb energy at the characteristic wavelength of the organic ligand and emit radiation of the characteristic wavelength of lanthanide ions (Tb3 + or Eu3 + ). These complexes show a large Stokes shift, long lifetime, and narrow emission bands. Highly sensitive methods based on quinolone complexes have been reported for the determination of quinolones ciprofloxacin [22], pipemidic acid [23], lomefloxacin
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[23], levofloxacin [24], gatifloxacin [25], trovafloxacin [26], and garenoxacin [27]. Gatifloxacin[(7)-1-cyclopropyl-6-fluoro-1,4-dihydro-8-methoxy-7-(3-methyl-1-piperazinyl)-4-oxo-3-quinoline carboxylic acid, GFLX] is the fourth generation of a new class of synthetic fluoroquinolone antibacterial agents. Like other quinolones, GFLX also has a b-keto acid group to make it still behave as a b-keto acid ligand for lanthanide ions [28]. In this work, by using GFLX as an organic ligand, a novel co-luminescence system Eu3 + –La3 + – GFLX–sodium dodecylbenzene sulfonate (SDBS) is developed. The characteristics of its spectra and the effects of various experimental conditions on the luminescence intensity were investigated. This method has been successfully applied to determine trace amount of europium in both synthetic and commercial samples of lanthanide oxides. Compared with most of other methods reported for the determination of Eu3 + , the present paper provides a much more thorough understanding on the luminescence mechanism. Furthermore, by investigation of the number of water molecules in the first coordination sphere of the lanthanide complex, we have verified that SDBS molecules could block the coordination of water molecules to the lanthanide–quinolone complex and therefore result in the luminescence enhancement, which is usually discussed merely by hypothesis or speculation in some literatures [29,30].
SDBS solution (1.2 10 3 mol L 1) was prepared by dissolving 0.0929 g SDBS (90.0%) in water and then diluting to 200 mL. NH4Ac–NH3 H2O buffer solution (pH 7.0, 0.5 mol L 1) was prepared by dissolving 9.8316 g NH4Ac in 250 mL water and then adjusting pH to 7.0 with NH3 H2O. All reagents used were of analytical grade unless otherwise indicated. In-house redistilled water was used throughout the study. 2.3. Methods To a 10 ml volumetric flask, the following solutions were added in sequence: 1.0 mL of 0.5 mol L 1 NH4Ac–NH3 H2O (pH 7.0), 1.0 mL of 2.0 10 4 mol L 1 La3 + , 1.0 mL of 1.2 10 3 mol L 1 SDBS, an aliquot of Eu3 + working standard solution, 0.4 mL of 1.0 10 3 mol L 1 GFLX. The solution was then diluted to the mark with water and mixed thoroughly by shaking, and then allowed to stand for 15 min at room temperature. The range of the final Eu3 + concentrations was 1.0 10 10–5.0 10 8 mol L 1. The luminescence intensity was measured in a 10 mm pathlength quartz cell with excitation and emission wavelengths of 336 nm and 616 nm, respectively.
3. Results and discussion 2. Experimental
3.1. Excitation and emission spectra
2.1. Apparatus
The excitation and emission spectra of Eu3 + (1, 10 ), GFLX (2, 20 ), Eu –GFLX (3, 30 ), Eu3 + –GFLX–SDBS (4, 40 ), and Eu3 + –La3 + –GFLX– SDBS (5, 50 ) systems are shown in Fig. 1. From curve 5, it can be seen that the excitation peak of Eu3 + –La3 + –GFLX–SDBS system is at 336 nm, which was therefore selected as the excitation wavelength. The emission spectra show that none of Eu3 + (curve 10 ), GFLX (curve 20 ), and Eu3 + –GFLX (curve 30 ) aqueous solutions could display the characteristic luminescence signal of Eu3 + at 616 nm. Curve 40 is the emission spectrum of Eu3 + –GFLX– SDBS system showing a weak emission peak at 616 nm. Curve 50 is the emission spectrum of Eu3 + –La3 + –GFLX–SDBS system with an excitation wavelength of 336 nm. Strong emission peaks were observed at 594 and 616 nm, corresponding to transitions of the Eu3 + 5D0–7F1 and 5D0–7F2, respectively. Compared to the 3+
Normal luminescence measurements were recorded with a Hitachi F–2500 Fluorescence Spectrophotometer (Hitachi, Japan), using a standard 10 mm pathlength quartz cell with 10 nm bandwidths for both the excitation and emission monochromators. All absorption spectra were measured on a UV–2401PC spectrophotometer (Shimadzu, Japan) equipped with 10 mm pathlength quartz cells. The fluorescence polarization data were obtained using a LS–55 Luminescence Spectrometer (PerkinElmer, USA). The luminescence lifetime was determined using a FLS920 fluorescence spectrometer (Edinburg, England). The surface tension was measured on a Processor Tensi¨ ometer–K12 (KRUSS Corp, Germany) with the precise degree of the measurement 0.01 mN m 1 by the Wilhelmy plate. All pH measurements were made with a Lei Ci pHs-3C pH-meter (Lei Ci, China).
2.2. Reagents The stock standard solutions of Eu3 + (1.0 10 3 mol L 1) or La3 + (4.0 10 3 mol L 1) were prepared, respectively, by dissolving 0.0176 g Eu2O3 (purity, 99.99%) or 0.0652 g La2O3 (purity, 99.99%) in 1:1 HCl and evaporating the solutions to near dryness before diluting to 100 mL with water. The stock standard solutions were kept at 4 1C. The working standard solutions were prepared by making appropriate dilutions with water. A stock standard solution (1.0 10 2 mol L 1) of GFLX (99.0%, Shandong Luoxin Pharmacy Stock Co., Ltd. Linyi, China) was prepared by dissolving 0.3792 g GFLX with an appropriate NaOH solution (0.1 mol L 1) and diluting it to 100 mL with water. The solution was kept at 4 1C in the refrigerator and protected from light. Working standard solutions were obtained by making appropriate dilutions of the stock standard solution with water.
Fig. 1. Excitation and emission spectra. 1, 10 Eu3 + ; 2, 20 GFLX; 3, 30 Eu3 + –GFLX; 4, 40 Eu3 + –GFLX–SDBS 5, 50 Eu3 + –La3 + –GFLX–SDBS. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; SDBS, 1.2 10 4 mol L 1; La3 + , 2.0 10 5 mol L 1; NH4Ac–NH3 H2O, 0.05 mol L 1, pH 7.0.
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Eu3 + –GFLX–SDBS system, the luminescence intensity was enhanced 7-fold with the addition of co-luminescence lanthanide ion La3 + .
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Table 1 Effect of different co-luminescence lanthanide ions. Co-luminescence lanthanide ions and the optimal concentration (mol L 1)
Relative luminescence intensity (%)
Lu3 + , 2.0 10 6 Gd3 + , 1.0 10 6 Y3 + , 2.0 10 6 La3 + , 2.0 10 5
23.6 12.4 24.4 100
3.2. Effect of pH and buffers Change of luminescence intensity with pH is shown in Fig. 2. The optimal pH value was in the range of 5.5–7.5, and pH 7.0 was selected for the recommended procedure. The effects of following buffer solutions on the luminescence intensity were then examined: NH4Ac–NH3 H2O, NaAc–HAc, NH4Cl–NH3 H2O, Tris– HCl, and KH2PO4–NaOH. It was found that the luminescent system in a 0.05 mol L 1 NH4Ac–NH3 H2O medium exhibited the highest sensitivity. Therefore, the NH4Ac–NH3 H2O buffer solution of pH 7.0 was chosen for further study. 3.3. Effect of GFLX concentration The GFLX concentration exerted a significant effect on the luminescence intensity of the system. The GFLX concentration was varied while the concentration of europium was fixed. The maximum and constant luminescence intensity was obtained at a GFLX concentration range of 3.5 10 5–5.5 10 5 mol L 1. Consequently, the GFLX concentration was selected at 4.0 10 5 mol L 1 for further experiments.
Table 2 Effect of different surfactants. Surfactants and the optimal concentration
Relative luminescence intensity (%)
Cetrimonium bromide (CTAB), 2.5 10 5 mol L 1 PEG 40, 2.0 10 3 g ml 1 Pluronic F68, 5.0 10 4 g ml 1 Arabic gum (GA), 5.0 10 3 g ml 1 Polyoxyethylene nonylphenol ether (OP), 2.0% (V/V) b-cyclodextrin (b-CD), 4.0 10 4 mol L 1 Sodium dodecyl sulfate (SDS), 1.0 10 4 mol L 1 Sodium dodecylbenzene sulfonate (SDBS), 1.2 10 4 mol L 1
0.2 3.6 0.5 2.4 3.3 0.5 40.6 100
3.4. Effect of La3 + concentration Effects of the co-luminescence lanthanide ions such as La3 + , Lu3 + , Y3 + , and Gd3 + were studied. The results are shown in Table 1, from which we can see that La3 + is the most effective one. The change of system luminescence intensity with different La3 + concentration was investigated, with a result showing that the enhanced luminescence intensity reaches its maximum value when the concentration of La3 + is in the range of 1.0 10 5– 2.4 10 5 mol L 1. Hence, 2.0 10 5 mol L 1 La3 + was chosen for further studies. 3.5. Effect of surfactants Various types of surfactants luminescence intensity of the influence are shown in Table 2. PEG 40, pluronic F68, arabic
have different effects on the system. The results of such Nonionic surfactants including gum (GA), polyoxyethylene Fig. 3. Effect of SDBS concentration. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; La3 + , 2.0 10 5 mol L 1; NH4Ac–NH3 H2O, 0.05 mol L 1, pH 7.0.
nonylphenol ether (OP), b-cyclodextrin (b-CD), and cationic surfactant cetrimonium bromide (CTAB) have little effect, while the anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS) significantly increase the luminescence intensity of the system with the latter being more effective. Accordingly, SDBS was selected for further studies. The effect of SDBS concentration on the luminescence intensity is shown in Fig. 3. Maximum luminescence intensity was observed for an SDBS concentration of 1.2 10 4 mol L 1, which was chosen for further experiments.
3.6. Effect of reagent addition sequence and the system stability
Fig. 2. Effect of pH. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; SDBS, 1.2 10 4 mol L 1; La3 + , 2.0 10 5 mol L 1.
The effect of reagent addition sequence on the luminescence intensity was studied. The result showed that the optimal addition sequence was NH4Ac–NH3 H2O, La3 + , SDBS, Eu3 + , and
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GFLX. Therefore, this addition order was chosen for further research. Under the optimal conditions, the stability of the luminescence intensity was investigated. The results showed that the luminescence intensity reached a maximum after all the reagents had been added and remained stable for over 2.5 h, indicating a good stability. 3.7. Interference of coexisting foreign substances The interference of possible coexisting foreign substances was tested at the concentration of Eu3 + 1.0 10 6 mol L 1 and the result is shown in Table 3. It was found that most coexisting foreign substances except for Cu2 + , Fe3 + and several other lanthanide ions had little interference on the determination of Eu3 + , within the acceptable accuracy of 75% relative errors. 3.8. Analytical application 3.8.1. Calibration curve and detection limit Under optimal conditions, the enhanced luminescence intensity of the system (DIL) shows an excellent linear relationship to the Eu3 + concentration in the range of 1.0 10 10–5.0 10 8 mol L 1. The linear equation is DIL = 4.46 1010CEu + 39.7 with a correlation coefficient of 0.9998. The detection limit (3s) was determined as 7.0 10 14 mol L 1. Table 3 Maximum permissible concentrations of coexisting foreign substances. Coexisting foreign substances
Maximum permissible concentration (mol L 1)
Change of DIf (%)
Pr3 + Nd3 + Tm3 + Er3 + Ho3 + Sm3 + Tb3 + Yb3 + Dy3 + Sc3 + Y3 + Gd3 + Lu3 + Cu2 + , SO24 Fe3 + , Cl Al3 + , Cl Mg2 + , Cl Zn2 + , SO24 Ca2 + , Cl K + , Cl Na + , Cl
2.0 10 7 2.0 10 7 3.5 10 7 6.0 10 7 6.0 10 7 7.0 10 7 9.0 10 7 9.0 10 7 1.0 10 6 2.0 10 6 2.0 10 6 3.5 10 6 4.0 10 6 2.0 10 7 8.0 10 7 4.5 10 6 9.0 10 4 1.0 10 3 2.5 10 3 6.0 10 2 9.5 10 2
4.5 4.5 4.8 4.8 4.1 4.4 4.5 4.6 5.0 5.0 4.6 4.5 4.7 4.3 5.0 4.4 5.0 4.4 4.9 4.2 4.3
In comparison with most of other methods reported for the determination of Eu3 + , as shown in Table 4, the rapid and simple method proposed in this paper offers a lower detection limit and a comparable linear range. 3.8.2. Sample determination and the recovery tests In order to verify the validity of the method, a synthetic sample was prepared according to the abundance of each lanthanide in the earth crust (sample 1). In addition, the amount of europium in a reference material obtained from Baotou Research Institute of Rare Earths, China, was also analyzed (sample 2). The analysis results are given in Table 5, which indicate that the proposed method based on the co-luminescence system Eu3 + – La3 + –GFLX–SDBS is suitable for the determination of trace amounts of europium in lanthanides mixtures. The recovery experiments of europium in both synthetic and commercial samples of lanthanide oxides are also examined and summarized in Tables 6 and 7. All recoveries are in the range of 96.1–101.7%. 3.9. Luminescence mechanism 3.9.1. Formation of Eu3 + –La3 + –GFLX–SDBS complex In order to study the interaction between Eu3 + , La3 + , GFLX, and SDBS, several ultraviolet absorption spectra were recorded and are shown in Fig. 4. It can be seen that there are two absorption peaks of GFLX at 285 and 331 nm. After the addition of Eu3 + or Eu3 + along with SDBS, the absorption peak positions of GFLX do not change and only the absorbance at 285 nm decreases slightly. However, the absorbance of GFLX increases remarkably when Eu3 + along with La3 + and SDBS is added, in agreement with the luminescence enhancement of the excitation spectra in the Eu3 + – La3 + –GFLX–SDBS system (see Fig. 1), and the maximum absorption wavelength underwent a red shift from 285 to 289 nm, indicating the formation of a Eu3 + –La3 + –GFLX–SDBS quaternary complex [23]. 3.9.2. Luminescence enhancement mechanism The mechanism of SDBS effect was studied by investigating the surface tension change of the luminescence system with SDBS concentration. The results are shown in Fig. 5. It can be seen that the surface tension first decreases sharply with increasing SDBS concentration. Soon an equilibrium point is reached and then it decreases very slowly. The concentration of 1.8 10 5 mol L 1 could be considered as the apparent critical micelle concentration (CMC) of SDBS in this system. Since the SDBS concentration selected for the study (1.2 10 4 mol L 1) is well above the CMC, it can be concluded that the formation of micelles has a great impact on the enhancement of the luminescence intensity of the system.
Table 4 Comparison of methods for the determination of europium. Method
Detection limit (mol L 1)
Linear range (mol L 1)
References
Europium-sensitized luminescence Europium-sensitized luminescence Co-luminescence Co-luminescence Co-luminescence Co-luminescence Co-luminescence Time-resolved co-luminescence Time-resolved co-luminescence Time-resolved co-luminescence Co-luminescence
1.0 10-10 1.0 10 10 2.0 10 11 1.0 10 13 2.0 10 13 4.0 10 13 1.46 10 10 4 10 15 1.9 10 14 3.5 10 14 7.0 10 14
5.0 10 9 1.0 10-6 5.0 10 9 2.5 10 6 1.0 10 9 2.0 10 7 5.0 10 11 5.0 10 7 1.0 10 10 5.0 10 7 2.0 10 10 2.0 10 7 5.0 10 10 2.0 10 7
[6] [7] [15] [16] [17] [19] [20] [31] [32] [33] Present method
1.0 10 10 5.0 10 8
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595
Table 5 Determination of europium in standard samples in lanthanide oxides. Sample
Present
Found
Mean7 SD
1 2
7.24 ( 10 7 mol L 1) 0.23 (%)
7.26, 6.98, 7.16, 6.92, 7.31 ( 10 7 mol L 1) 0.21, 0.21, 0.20, 0.21, 0.20 (%)
7.13 70.17 0.217 0.0055
Sample 1: Y 3.49 10 5 mol L 1, La 1.37 10 5 mol L 1, Ce 3.14 10 5 mol L 1, Pr 3.97 10 6 mol L 1, Nd 1.66 10 5 mol L 1, Sm 6.98 10 6 mol L 1, Eu 7.24 10 7 mol L 1, Gd 4.01 10 6 mol L 1, Tb 6.29 10 7 mol L 1, Dy 2.65 10 6 mol L 1, Ho 7.28 10 7 mol L 1, Er 1.43 10 6 mol L 1, Tm 1.24 10 6 mol L 1, Yb 1.50 10 6 mol L 1, Lu 4.00 10 7 mol L 1. Sample 2: Y2O3 0.27%, La2O3 27.11%, CeO2 49.21%, Pr6O11 5.18%, Nd2O3 16.75%, Sm2O3 1.29%, Eu2O3 0.23%, Gd2O3 0.40%, Tb4O7 0.03%, Dy2O3 0.09%, Ho2O3 0.023%, Er2O3 0.027%, Tm2O3 0.0095%, Yb2O3 0.013%, and Lu2O3 0.003%.
Table 6 Recoveries of europium in synthetic samples of lanthanide oxides. Europium added ( 10 8 mol L 1)
Europium found ( 10 8 mol L 1)
Recoveries (%)
0.500
0.482 0.488 0.484 0.504 0.514
96.5 97.7 96.7 100.9 102.8
98.9 7 2.8
1.00
1.024 1.028 1.015 0.988 1.032
102.4 102.8 101.5 98.8 103.2
101.7 7 1.8
Mean 7SD (%)
Table 7 Recoveries of europium in commercial samples of lanthanide oxides. Europium added ( 10 8 mol L 1)
Europium found ( 10 8 mol L 1)
Recoveries (%)
Mean7 SD (%)
0.500
0.479 0.487 0.473 0.473 0.490
95.8 97.5 94.7 94.7 98.0
96.1 7 1.5
1.00
0.970 0.951 0.976 0.987 0.973
97.0 95.1 97.6 98.7 97.3
97.2 7 1.3
Fig. 4. Ultraviolet absorption spectra. 1, Eu3 + ; 2, GFLX; 3, Eu3 + –GFLX; 4, Eu3 + – GFLX–SDBS 5, Eu3 + –La3 + –GFLX–SDBS. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; SDBS, 1.2 10 4 mol L 1; La3 + , 2.0 10 5 mol L 1; NH4Ac–NH3 H2O, 0.05 mol L 1, pH 7.0.
Fig. 5. Effect on solution surface tension with the addition of SDBS. Experimental conditions: Eu3 + , 1.0 10 6 mol L 1; GFLX, 4.0 10 5 mol L 1; La3 + , 2.0 10 5 mol L 1; NH4Ac–NH3 H2O, 0.05 mol L 1, pH 7.0.
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Table 8 The number of water molecules in the first coordination sphere of the Eu3 + complex. System
Lifetime in H2O (ms) Lifetime in D2O (ms)
Eu3 + –GFLX Eu3 + –GFLX–SDBS Eu3 + –GFLX–La3 + Eu3 + –La3 + –GFLX– SDBS
0.1906 0.5436 0.2283 0.9733
1.398 0.6041 1.087 1.120
n 4.627 0.06211 3.502 0.01024
The microviscosity (Z) of different luminescence systems can be calculated using the equation developed by Shinitzky and Barenholz [34]:
Z¼
2P 0:46 P
where P refers to the fluorescence polarization. The P of different systems were determined and then the Z values of different systems were calculated acoording to the above equation. The results indicates that the Z value increases significantly with the addition of SDBS and La3 + to the Eu3 + –GFLX system. Therefore, it can be concluded that La3 + and SDBS could provide a high microviscosity for Eu3 + –GFLX complexes and contribute to the luminescence enhancement of the complexes. In addition, the greater microviscosity environment provided by La3 + and SDBS can also prevent the collision of the complex and water, leading to the reduction in the energy loss of the Eu3 + –La3 + –GFLX–SDBS system. Consequently, a significant enhancement of the luminescence intensity is observed. The number of water molecules in the first coordination sphere of a Eu3 + complex, n, in aqueous solution, was determined 1 according to the equation n ¼ 1:05 ½t1 H2 O tD2 O 0:125[35]. The results are shown in Table 8. In aqueous solutions, interaction with water (both inside and outside the coordination sphere of Eu3 + ) may lead to a severe quenching of the Eu3 + luminescence via O–H vibrations and other de-activating vibrations (e.g. N–H or C–H oscillators) [12]. However, GFLX– Eu3 + complex could reside inside the micelles with the presence of surfactant SDBS. Thus, the GFLX–Eu3 + complex is free of highenergy vibrations and solvent interaction. From Table 8, it can be seen that the SDBS medium could provide an optimal hydrophobic coordination environment of Eu3 + and GFLX and decrease the interaction between the water molecules and Eu3 + – GFLX complex. The quantum yield (Ff) of the luminescence systems in various media was determined. Quantum yield is one of the most basic and significant parameters in all the characters of luminescence substance [36] and represents the ability of a complex to translate absorption energy to luminescence. In this paper, the quantum yield was measured according to the equation F2 = (F1A1F2)/ F1A2[37], using quinine sulfate as the reference standard. The results show that the quantum yield Ff of Eu3 + –La3 + –GFLX–SDBS is higher than any of the other three systems, proving that its ability of translating absorption energy to luminescence is stronger than the other three, and therefore, the enhancement of the luminescence intensity.
4. Conclusion This paper describes a novel co-luminescence system for the determination of trace amounts of europium. Under optimal conditions, the enhanced luminescence intensity is proportional to the europium concentration in the range of 1.0 10 10–
5.0 10 8 mol L 1. The detection limit (3s) is 7.0 10 14 mol L 1. The proposed method has been successfully applied to the determination of trace amounts of europium in rare earth samples. The luminescence mechanism is also discussed in detail. It’s been demonstrated that SDBS exists in the form of micelles in solution and can provide a hydrophobic environment with high viscosity, which is beneficial to the luminescence enhancement. SDBS medium can also provide an optimal hydrophobic coordination environment of Eu3 + and GFLX and decrease the interaction between the water molecules and the Eu3 + –GFLX complex. With the formation of Eu3 + –La3 + –GFLX–SDBS complex, the quantum yield increases enormously compared with the Eu3 + –GFLX binary complex, resulting in significant enhancement of the luminescence intensity. In comparison with other reported methods, the rapid and simple procedure proposed in the text offers comparable sensitivity and better stability with a much thorough understanding of the luminescence mechanism.
Acknowledgements This project was supported by the National Natural Science Foundations of China (Nos. 20875056, 20775043). The authors would like to thank the Key Laboratory of Education Ministry on Colloid & Interface Chemistry at Shandong University for kindly loaning Processor Tensiometer–K12 and LS–55 Luminescence Spectrometer for the determination of surface tension and fluorescence polarization. Also, the authors would like to thank the State Key Laboratory for Crystal Materials of Shandong University for kindly supplying Edinburg FLS920 for the determination of the luminescence lifetime.
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