Fluorescence and co-fluorescence of Tb3+ and Eu3+ in acetonitrile using 2,6-pyridine dicarboxylic acid as ligand

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 405–409

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Fluorescence and co-fluorescence of Tb3+ and Eu3+ in acetonitrile using 2,6-pyridine dicarboxylic acid as ligand S. Maji ⇑, Satendra Kumar, K. Sankaran Material Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

h i g h l i g h t s 3+ and Eu3+ ion complexed with 2,6-pyridine dicarboxylic acid is studied in acetonitrile medium.  Both the lanthanides show cofluorescence with La3+.  This study represents the first report of co-fluorescence in acetonitrile medium.  Detection limit of both the lanthanides is at the level of 10 11 M.

g r a p h i c a l a b s t r a c t

 Fluorescence of Tb

With La

3+

Without La

3+

3+

Eu

3+

Tb

-10

5x10 M

-10

Intensity

2.5x10 M

-8

-8

2.5x10 M

5x10 M

475

500

525

550

575

600

625

650

Wavelength (nm)

a r t i c l e

i n f o

Article history: Received 30 May 2014 Received in revised form 4 July 2014 Accepted 15 July 2014 Available online 23 July 2014 Keywords: Fluorescence Co-fluorescence 2,6-Pyridine dicarboxylic acid Terbium Europium Acetonitrile

a b s t r a c t Fluorescence from Tb3+ and Eu3+ complexed with 2,6-pyridine dicarboxylic acid (PDA) has been studied using acetonitrile (MeCN) as solvent. The enhancement in fluorescence intensity because of non-aqueous environment provided by the MeCN is less significant, where as fluorescence enhancement of more than two orders of magnitude has been observed with the addition of La3+; a process known as co-fluorescence in MeCN. The present study demonstrates for the first time co-fluorescence of Tb3+ and Eu3+ with excitation through the absorption of PDA. Intermolecular energy transfer is believed to be responsible for co-fluorescence enhancement and it becomes possible as the quenching due to water at the secondary coordination spheres of Tb3+ and Eu3+ is reduced when MeCN is used as solvent. Ó 2014 Published by Elsevier B.V.

Introduction The estimations of concentrations of Tb3+ and Eu3+ ions are important in different areas such as nuclear industry, geochemistry [1–8], in addition to its vast application in biological samples [9–18]. As these ions are weakly fluorescing species due to their low quantum yields and poor molar absorptivities [19–20], ligand sensitized fluorescence (LSF) [21–33] is employed to overcome ⇑ Corresponding author. E-mail address: [email protected] (S. Maji). http://dx.doi.org/10.1016/j.saa.2014.07.022 1386-1425/Ó 2014 Published by Elsevier B.V.

these drawbacks. LSF is an indirect way of enhancement in fluorescence where the ligand with higher molar absorptivity transfers its absorbed energy to the lanthanides complexed to it and thereby results in the enhancement of intensity of lanthanide fluorescence. A number of organic ligands like b-diketones, aromatic acids are reported to enhance fluorescence of lanthanides by nearly three orders of magnitude [21–26]. It has also been shown that LSF can be further enhanced by the addition of certain ions such as La3+, Gd3+, Lu3+, Y3+ to the solution [34–46]. This phenomenon is known as co-fluorescence and it has been shown as an efficient way of improving the sensitivity of the fluorimetric determination of

S. Maji et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 405–409

lanthanides. Another way of improving the sensitivity of LSF is to protect the lanthanides from collisions with water molecules. Collision with water molecules leads to quenching of fluorescence intensity by non-radiative deactivation of electronic energy of lanthanides to the high vibrational modes of OAH bond [47–49] of water molecules. To minimize these collisions, neutral ligands like trioctyl phosphine oxide (TOPO), organic phosphates and sulphoxides are used. These ligands replace the water molecules present in the inner sphere of lanthanide complex and coordinate with it [50– 53]. As a result further enhancement in LSF has been achieved. Recent studies on the fluorescence of Tb3+ and Eu3+ in 1-butyl3-methylimidazolium benzoate [54] have reported that the enhancement in fluorescence was due to sensitization and reduction of non-radiative channels in non-aqueous environment provided by the ionic liquid. In the present study, the LSF of Tb3+ and Eu3+ using acetonitrile (MeCN) as solvent and 2,6-pyridine dicarboxylic acid (PDA) as ligand is reported. Earlier studies have reported an enhancement in the fluorescence of Tb3+ and Eu3+ by about three orders of magnitude using PDA as ligand in aqueous medium [26,55]. As MeCN provides a non-aqueous medium, it is expected to reduce the rates of non-radiative decay processes [47–49], and hence an enhancement in fluorescence can be expected apart from PDA enhanced fluorescence. Co-fluorescence using La3+ as a co-fluorescing ion has also been carried out, which are presented. To the best of our knowledge, there is no other report of co-fluorescence enhancement of Tb3+ and Eu3+ with PDA as ligand in a solvent other than water. The mechanism for energy transfer and fluorescence enhancement is also suggested. Experimental Apparatus All fluorescence spectra were recorded using an Edinburgh FLS920 spectrofluorimeter, with a 450 W xenon lamp as an excitation source. Solutions were taken in a 2 mm path length fused silica cell. The band pass for the excitation and emission monochromators were set at 3 nm each. A long-wavelength pass filter, (UV – 39, Shimadzu) with a maximum and uniform transmittance (>85%) above 400 nm, was placed in front of the emission monochromator to reduce the scatter of the incident beam into the emission monochromator. Spectra were recorded at room temperature with a 90° collection geometry. All spectra were blank subtracted: a blank spectrum was recorded using identical experimental conditions but without the lanthanide in the solution. Excitation spectra presented are after spectral correction for instrument response. Lifetimes of Eu3+ and Tb3+ in various complexes were also recorded using a ls Xe-flash lamp, as an excitation source. Fluorescence life times were determined by fitting the observed time resolved fluorescence signals to an exponential decay function. A single exponential fit was found to be adequate for the decay processes observed in this study. The v2 values of all the fits ranged between 1.0 and 1.2. The lifetimes were extracted through a tailfit, where the data points in the decay profile extending to long temporal regions were used in the fitting procedure. The error of determination in lifetime was about 10%. Absorption spectra were recorded using a Thermo Electron corporation spectrophotometer, model Evolution 500 with 10 mm path length cell.

(Aldrich, 99%) were prepared by dissolving the required amounts of the reagents in water. Acetonitrile procured from Himedia (India, 99.5%) was used as such. Other solvents used in our experiments were of analytical grade and used as such. De-ionized water (18 MX) obtained with a Milli-Q (Millipore) system was used in preparing all solutions. To record the spectrum in MeCN, solutions were prepared by adding appropriate volume, typically 2.5–5 lL of aqueous solutions of Tb3+, Eu3+, La3+ and PDA of known concentration, to 0.5 mL of MeCN to achieve the required final concentration. The aqueous solution of PDA was adjusted to a known pH value by the addition of sodium hydroxide/quartz distilled nitric acid before it was added to the solvent MeCN. Results and discussion Fluorescence of Tb3+-PDA and Eu3+-PDA complex At the outset, the fluorescence of Tb3+ and Eu3+ was recorded in MeCN as a function of the PDA concentration and pH. In the following text the term ‘pH’ implies the aqueous solution of PDA prepared at a given pH, from which 2.5 lL was taken and dissolved in 0.5 mL of MeCN. The maximum fluorescence intensity was observed at pH 7 when PDA concentration was 1  10 4 M in acetonitrile and hence these values were used in subsequent experiments. Figs. 1b and 2b show the emission spectra of Tb3+-PDA and 3+ Eu -PDA systems respectively, recorded in MeCN with an excitation wavelength of 271 nm. The excitation maxima for both Tb3+ and Eu3+ was found to occur at 271 nm indicating clearly a common absorber, namely PDA and these observations are similar to that in aqueous medium experiments [55]. The emission spectra show characteristic sharp bands of Tb3+ and Eu3+. For comparison, the emission spectrum for both the systems recorded in water is shown in Figs. 1a and 2a respectively. About two times increase in fluorescence intensity is found when MeCN was used as the solvent compared to water for both the lanthanides.

4

3x10

c 4

2x10

Tb3+-La3+- PDA in MeCN [Tb3+] = 2.5x10-10 M

4

1x10 0

4

2x10

Intensity

406

b

Tb3+-PDA in MeCN [Tb3+] = 5.0x10-8 M

4

1x10

0 4

2x10

a

Tb3+-PDA in H 2O [Tb3+] = 5.0x10-8 M

4

1x10

0

Reagents

475

500

525

550

Wavelength (nm) Stock solutions of lanthanides were prepared by dissolving the required amount of their oxides (Indian Rare Earths, 99.9%) in distilled HNO3 and evaporated to dryness. Stock solutions of PDA

Fig. 1. Emission spectra of (a) Tb3+ (5.0  10 8 M)-PDA (1  10 4 M) in water; (b) Tb3+ (5.0  10 8 M)-PDA (1  10 4 M) in MeCN and (c) Tb3+ (2.5  10 10 M)-La3+ (1.0  10 5 M)-PDA (5.0  10 5 M) in MeCN. kex = 271 nm.

407

S. Maji et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 405–409

4

2x10

c Eu3+-La3+- PDA in MeCN [Eu3+] = 5.0x10-10 M

4

1x10

0 4

Intensity

2x10

b Eu3+-PDA in MeCN [Eu3+] = 2.5x10-8 M

X3

4

1x10

0 4

2x10

a

Eu3+-PDA in H 2O [Eu3+] = 2.5x10-8 M

4

1x10

X3

0

575

600

625

650

Wavelength (nm) Fig. 2. Emission spectra of (a) Eu3+ (2.5  10 8 M)-PDA (1  10 4 M) in water; (b) Eu3+ (2.5  10 8 M)-PDA (1  10 4 M) in MeCN and (c) Eu3+ (5.0  10 10 M)-La3+ (1.0  10 5 M)-PDA (5.0  10 5 M) in MeCN. kex = 271 nm.

The lifetimes of Tb3+ and Eu3+ recorded for these systems are given in Table 1. The lifetime of PDA complexes of Tb3+ was slightly increased to 2100 from 2000 ls as measured in water and the lifetime of PDA complexes of Eu3+ was increased from 1460 (in water) to 1890 ls when MeCN was used as solvent. Similar observations were reported for Sm3+ complexes when solvent was changed from water to imidazolium ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [56]. The lifetime of Sm3+ was 21 ls when Sm3+-PDA complex was in water whereas it was 61 ls when the complex was dissolved in imidazolium. The larger increase in lifetime observed for Eu3+ compared to Tb3+ in our work can be explained by calculating the number of coordinated water molecules (NH2O) [48,57] to the lanthanide ions. The calculated value of NH2O is 1.2 and 0.3 for PDA complexes of Tb3+ and Eu3+ respectively in water. In aqueous medium, PDA replaces almost all the water molecules coordinated to Eu3+ but in case of Tb3+ still about one water molecule remains coordinated. When MeCN was used as solvent the values of NH2O were calculated to be 1.1 and 0.1 for PDA complexes of Tb3+ and Eu3+ respectively (Table 1). The inner coordination sphere of Tb3+ is still coordinated with one water molecule. We believe that when MeCN is used as solvent, the water molecules from secondary coordination spheres are replaced by MeCN. The one water molecule present in the inner coordination sphere of Tb3+ still quenches the fluorescence intensity. Hence the lifetime of Tb3+ is increased by lesser amount than Eu3+. Larger values of lifetimes in MeCN compared to aqueous medium suggests that fluorescence enhancement observed for Tb3+/Eu3+-PDA complex in MeCN is due to reduction in nonradiative emission from the secondary sphere of Tb3+/Eu3+. This is also clear from the absorption spectra shown in Fig. 3. The

absorption spectrum of PDA recorded in water and MeCN is shown in Fig. 3p and q respectively. Even though absorption of PDA at 271 nm in MeCN is less compared to aqueous medium, the fluorescence intensity of Tb3+/Eu3+-PDA complex in MeCN is more. Effect of water on the Tb3+/Eu3+-PDA fluorescence As water is a strong quencher of fluorescence of lanthanides [47], it was therefore considered to study the effect of water on the fluorescence of Tb3+/Eu3+-PDA complexes in MeCN. To begin with, our experimental solutions contain 0.75% (v/v) water that comes from the added lanthanide and PDA solutions which are prepared in aqueous medium. At this experimental condition, as discussed earlier, the inner coordination sphere remains nearly same as that of water used as solvent. So addition of water further to the system in MeCN is likely to affect the secondary sphere of Tb3+/Eu3+. To study the effect of water, steady state and lifetime measurements were performed after deliberately adding water to the Tb3+/Eu3+-PDA system in MeCN. Fig. 4a and b shows the variations of fluorescence intensity and lifetime of Tb3+ and Eu3+ in their PDA complexes respectively, as a function of water concentration. As can be seen that both the fluorescence intensity and lifetime remain nearly constant between 0.75% and 2% (v/v) of water content in the systems studied. It suggests that the secondary coordination sphere in MeCN does not change even when water content is about 2% (v/v). Effect of solvents Apart from MeCN, fluorescence studies of Eu3+/Tb3+-PDA complexes were carried out using other solvents like Dimethyl Formamide (DMF), Dimethyl Sulfoxide (DMSO), Acetone and Methanol. Fluorescence intensities are found to be different in different systems. Compared to water, Eu3+-PDA complex showed more intense fluorescence in MeCN, DMSO, DMF and Methanol whereas Tb3+-PDA complex showed more fluorescence in MeCN and Methanol with the former one much stronger than the latter. Consequently MeCN was chosen as the solvent in our experiments to study both the lanthanides. Different enhancement factors in different solvents for Eu3+ and Tb3+ complexes could be due to different co-ordinating ability of solvents to Tb3+/Eu3+ [8,58,59]. Co-fluorescence When La3+ was added to Tb3+/Eu3+-PDA complexes in MeCN, cofluorescence was observed with enhanced fluorescence intensity of Tb3+/Eu3+. To optimize the co-fluorescence experimental conditions, the fluorescence intensity was measured as a function of La3+ concentration, for a fixed concentration of Tb3+/Eu3+. Similarly experimental conditions were also optimized for PDA concentration. The enhancement due to co-fluorescence was found to be maximum for 1  10 5 M of La3+ and 5  10 5 M of PDA. Hence these concentrations of La3+ and PDA were used in the subsequent experiments. The fluorescence intensity was also measured as a function of time. The result showed that the fluorescence intensity reached a maximum after 25 min of preparation of the sample and remained constant for more than 1 h. In all our further

Table 1 Enhancement factor, lifetime and number of coordinated water molecules, NH2O, for Tb3+ and Eu3+ in different systems.

Enhancement factora Lifetime (in ls) NH2O (±0.5) a

Tb3+-PDA in water

Tb3+-PDA in MeCN

Tb3+-La3+-PDA in MeCN

Eu3+-PDA in water

Eu3+-PDA in MeCN

Eu3+-La3+-PDA in MeCN

1 2000 1.2

2 2100 1.1

532 1700 1.6

1 1460 0.3

1.6 1890 0.1

260 1700 0.2

Enhancement factor is the fluorescence intensity of the complex relative to that of Tb3+/Eu3+-PDA complex in water, which is taken as 1.

408

S. Maji et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 405–409

excitation path was through a lanthanide ion excitation rather than the excitation through the absorption band of the ligand PDA. However, in our study we have observed that the fluorescence and co-fluorescence of Tb3+ and Eu3+ were maximum only when PDA band was excited in MeCN. In fact in water both the lanthanides did not show any co-fluorescence, similar observations were also made by Panigrahi [26].

Absorbance

0.6

0.4

Calibration curve and detection limit Under optimal experimental conditions, the fluorescence intensity of the co-fluorescence systems showed linear relationship in the concentration range of 2.5  10 10–1  10 8 M for Tb3+ and 5  10 10–1  10 8 M for Eu3+ ions. The detection limit (3r) was determined to be 1.5  10 11 M for Tb3+ and 4  10 11 M for Eu3+ respectively.

s r

0.2

q

p 0.0 220

240

260

280

300

320

Wavelength (nm) Fig. 3. Absorption spectra of (p) PDA in water; (q) PDA in MeCN; (r) Tb3+-PDA in MeCN and (s) Tb3+-La3+-PDA in MeCN. [PDA] = 1  10 4 M, [Tb3+] = 2.5  10 7 M and [La3+] = 1  10 5 M.

4

2200 3+

Tb

a

2.0x10

4

1.5x10

4

Intensity Life time 2000

1800 3+

1.2x10

4

8.0x10

3

2100

Eu

b

Life time (µs)

Intensity at 615 nm

Intensity at 544 nm

2.5x10

1800

1500

0.8

1.2

1.6

2.0

Water content (%)

Co-fluorescence mechanism Absorption spectrum of Tb3+-La3+-PDA shown in Fig. 3s, nearly follows the absorption spectrum of PDA (Fig. 3q) and Tb3+-PDA (Fig. 3r). With the addition of La3+ to Tb3+-PDA, neither the absorbance nor the excitation maximum changes (Fig. 5). Similar observations were found for Eu3+ also. The structure of PDA and this result together suggest that no polynuclear complex is formed when La3+ is added to Tb3+/Eu3+-PDA complex. It is likely that coprecipitate of Tb3+/Eu3+-PDA and La3+-PDA complexes is formed and intermolecular energy transfer is taking place from La3+-PDA complex to Tb3+/Eu3+-PDA complex and hence enhancement in fluorescence. Energy transfer is possible as the water molecules in the secondary sphere are replaced by MeCN whose vibrational mode (CBN, 2253 cm 1) is lesser than OH vibrational mode (OAH, 3450 cm 1) and hence it is less effective to fluorescence quenching. It is well reported that OAH vibration is more effective quencher than other vibrations like CAH, C@O, S@O etc. [47–49]. The OAH vibrations of water molecules present at secondary coordination sphere of Tb3+/Eu3+ take away the energy transferred by La3+-PDA to Tb3+/Eu3+-PDA complex and hence inhibit further enhancement in fluorescence and also result in the absence of co-fluorescence in aqueous medium. Co-fluorescence using methanol as a solvent was also studied and found to be nil, result similar to the one seen in aqueous medium. Formation of co-precipitate in most of the co-fluorescence system leads to the increase in lifetime as well as increase in absorbance [42–46]. According to these studies the fluorescing and co-fluorescing ions exist as tiny particle with the former ion

Fig. 4. Plots showing the fluorescence intensity and lifetime of (a) Tb3+ and (b) Eu3+ in there PDA complexes in MeCN as a function of water content in the system; kex = 271 nm, [Tb3+] = 5.0  10 8 M; [Eu3+] = 2.5  10 8 M; [PDA] = 1.0  10 4 M.

5

5x10

271 nm 4x10

b

5

Intensity

experiments, therefore, the fluorescence intensity was measured 30 min after the reagents were mixed. Figs. 1c and 2c show the emission spectra of Tb3+-PDA-La3+ and 3+ Eu -PDA-La3+ system respectively, recorded with excitation wavelength of 271 nm. The concentration of Tb3+ and Eu3+ used to record these emission spectra was 2.5  10 10 M and 5.0  10 10 M respectively. These concentrations were about 2 orders less than the concentration used to record the emission spectra without La3+ (Figs. 1b and 2b). Enhancement in fluorescence by the addition of La3+ is therefore obvious. The enhancement factors calculated to be about 260 and 160 for Tb3+ and Eu3+ respectively (Table 1) with respect to when no La3+ is added. In the study of ultra trace determination of lanthanides by fluorescence enhancement, Jenkins and Murray [60] had reported that the fluorescence of Tb3+ and Eu3+ co-ordinated to PDA in water was further enhanced by the addition of co-fluorescent ions such as La3+, Y3+, Tb3+ and Gd3+. They explained that the most efficient

5

3x10

5

2x10

a 5

1x10

0 220

240

260

280

300

320

Wavelength (nm) Fig. 5. Excitation spectra of (a) Tb3+ (5.0  10 8 M)-PDA (1  10 4 M) and (b) Tb3+ (2.5  10 10 M)-La3+ (1.0  10 5 M)-PDA (5.0  10 5 M) in MeCN. kem = 544 nm.

S. Maji et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 405–409

surrounded by more ions of the latter and hence increase in absorbance and lifetime. In the present case neither absorbance (Fig. 3) nor the lifetime increases (Table 1). In fact lifetime of the co-fluorescence system decreased by about 190–400 ls. Similar decay time shortening was also reported for the co-fluorescence system of Tb3+ where lifetime of it decreased from 400 ls to 323 ls [35]. In the present study, formations of tiny particles were not observed to naked eye. But the fact that the co-fluorescence intensity is time dependent, suggest the formation of coprecipitate of Tb3+/Eu3+-PDA and La3+-PDA. As MeCN is less polar than water, it favors the close proximity between the co-fluorescing ion and the fluorescing ion by less solvating. This close proximity might favor in efficient energy transfer but does not promote increase in lifetime. Conclusion This works reports the fluorescence of Tb3+ and Eu3+ complexes with PDA in MeCN. Both the complexes exhibited strong cofluorescence with the addition of La3+. The solvent MeCN helps in reducing non-radiative deactivation processes as well as favors the existence of close proximity between the donor and acceptor complexes. Thereby the energy absorbed by La3+-PDA complex could be transferred to Tb3+/Eu3+-PDA complex. This cofluorescence enhancement observed in the present study is different from that observed with other aromatic acid ligands in water where intra-molecular energy transfer was believed to occur. The enhancement was sufficient to lower the detection limits of these lanthanides to the 10 11 M level using PDA as a sensitizing ligand. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

R.K. Malhotra, K. Satyanarayana, Talanta 50 (1999) 601–608. Ki-Soo Cho, In-Suck Suh, J. Korean Nucl. Soc. 15 (1983) 142–148. V.N. Mitkin, B.M. Shavinsky, J. Fluorine Chem. 130 (2009) 117–121. F. Thevenet, G. Panczer, P. Jollivet, B. Champagnon, J. Non-Cryst. Solids 351 (2005) 673–677. N. Ollier, G. Panczer, B. Champagnon, G. Boulon, P. Jollivet, J. Lumin. 94–95 (2001) 197–201. E. Orvini, M. Speziali, A. Salvini, C. Herborg, Microchem. J. 67 (2000) 97–104. M. Hosseini, M.R. Ganjali, B. Veismohammadi, F. Faridbod, S.D. Abkenar, P. Norouzi, Anal. Chim. Acta 664 (2010) 172–177. L. Zhang, X. Zheng, W. Ahmad, Y. Zhou, Y. An, Spectrochim. Acta A 104 (2013) 243–249. S.M. Wabaidur, Z.A. Alothman, Mu. Naushad, Spectrochim. Acta A 93 (2012) 331–334. P. Dong, Na Xu, Bo Fu, L. Wang, J. Pharm. Anal. 1 (2011) 46–50. Belal H.M. Hussein, Hassan A. Azab, Walid Fathalla, Sherin A.M. Ali, J. Lumin. 134 (2013) 441–446. J.A. Ocana, F.J. Barragan, M. Callejon, Talanta 63 (2004) 691–697. K. Kaur, S.S. Saini, A.K. Malik, B. Sing, Spectrochim. Acta A 96 (2012) 790–795. J.L. Manzoori, A. Jouyban, M. Amjadi, V.P. Azar, A.R.K. Bonari, E. Tamizi, Food Chem. 126 (2011) 1845–1849.

409

[15] H.A. Azab, A. Duerkop, E.M. Saad, F.K. Awad, R.M. Abd El Aal, R.M. Kamel, Spectrochim. Acta A 97 (2012) 915–922. [16] S.M.Z. Al-Kindy, F.E.O. Suliman, A.A. Al-Wishahi, H.A.J. Al-Lawati, M. Aoudia, J. Lumin. 127 (2007) 291–296. [17] F. Gao, F. Luo, L. Tang, Lu Dai, L. Wang, J. Lumin. 128 (2008) 462–468. [18] T. Wang, C. Jiang, Anal. Chim. Acta 561 (2006) 204–209. [19] G. Stein, E. Wurzberg, J. Chem. Phys. 62 (1975) 208–213. [20] W.T. Carnall, Handbook of the Physics and Chemistry of rare earths, vol. 3, Elsevier, Amsterdam, 1979 (Chapter 24). [21] J.-H. Yang, G.-Y. Zhu, B. Wu, Anal. Chim. Acta 198 (1987) 287–292. [22] L.M. Perry, J.D. Winefordner, Anal. Chim. Acta 237 (1990) 273–283. [23] M. Morin, R. Bador, H. Dechaud, Anal. Chim. Acta 219 (1989) 67–77. [24] B.S. Panigrahi, S. Peter, K.S. Viswanathan, C.K. Mathews, Spectrochim. Acta A 51 (1995) 2289–2300. [25] T. Taketatsu, Talanta 29 (1982) 397–400. [26] B.S. Panigrahi, J. Alloys Compd. 334 (2002) 228–231. [27] H.A. Azab, A. Duerkop, E.M. Saad, F.K. Awad, R.M. Abd El Aal, R.M. Kamel, Spectrochim. Acta A 97 (2012) 915–922. [28] Y. Binsheng, F. Hoegy, G.L.A. Mislin, P.J. Mesini, I.J. Schalk, J. Inorg. Biochem. 105 (2011) 1293–1298. [29] L.J. Charbonniere, R. Ziessel, M. Montalti, L. Prodi, N. Zaccheroni, C. Boehme, G. Wipff, J. Am. Chem. Soc. 124 (2002) 7779–7788. [30] B.-L. An, M.-L. Gong, K.-W. Cheah, J.-M. Zhang, K.-F. Li, Chem. Phys. Lett. 385 (2004) 345–350. [31] Amr A. Essawy, Sens. Actuators B 196 (2014) 640–646. [32] P. Juan, G. Xiaotian, Y. Jianbo, Z. Yanhui, Z. Ying, W. Yunyou, S. Bo, J. Alloys Compd. 426 (2006) 363–367. [33] Y. Shiraishi, Y. Furubayashi, G. Nishimura, T. Hirai, J. Lumin. 126 (2007) 68–76. [34] Y X Ci, Z.-H. Lan, Anal. Chem. 61 (1989) 1063–1069. [35] Y.-Y. Xu, I.A. Hemmila, Anal. Chim. Acta 256 (1992) 9–16. [36] Y. Jinghe, R. Xuezhen, Z. Huabin, S. Ruiping, Analyst 115 (1990) 1505–1508. [37] Y.-X. Ci, Z.-H. Lan, Analyst 113 (1988) 1453–1457. [38] B.S. Panigrahi, S. Peter, K.S. Viswanathan, Spectrochim. Acta A 53 (1997) 2579– 2585. [39] C.-J. Xu, F. Xie, X.-Z. Guo, H. Yang, Spectrochim. Acta A 61 (2005) 2005–2008. [40] Z. Xue-hui, H. Ke-long, L. Su-qin, J. Fei-peng, L. Zhi-guo, H. Shun-qin, L. Zhaojian, Trans. Nonferr. Metals Soc. China 17 (2007) 638–643. [41] Y.-Y. Xu, I.A. Hemmila, T.N.-E. Lovgren, Analyst 117 (1992) 1061–1069. [42] J. Yang, H. Zhou, X. Ren, C. Li, Anal. Chim. Acta 238 (1990) 307–315. [43] W. Li, W. Li, G. Yu, Q. Wang, Y. Jin, J. Alloys Compd. 191 (1993) 107–110. [44] J. Yang, H. Ge, N. Jie, X. Ren, N. Wang, H. Zou, Spectrochim. Acta A 51 (1995) 185–195. [45] C. Sun, J. Yang, X. Wu, S. Liu, B. Su, Biochimie 86 (2004) 569–578. [46] S. Liu, J. Yang, X. Wu, B. Su, C. Sun, F. Wang, Talanta 64 (2004) 387–394. [47] A.I. Voloshin, N.M. Shavaleev, V.P. Kazakov, J. Lumin. 93 (2001) 191–197. [48] T. Kimura, R. Nagaishi, Y. Kato, Z. Yoshida, J. Alloys Compd. 323–324 (2001) 164–168. [49] Y.-L. Zhang, W.-W. Qin, W.-S. Liu, M.-Y. Tan, N. Tang, Spectrochim. Acta A 58 (2002) 2153–2157. [50] F. Halverson, J.S. Brinen, J.R. Leto, J. Chem. Phys. 41 (1964) 157–163. [51] F. Halverson, J.S. Brinen, J.R. Leto, J. Chem. Phys. 41 (1964) 2752–2760. [52] R. Brennetot, J. Georges, Spectrochim. Acta A 56 (2000) 703–715. [53] S. Maji, K.S. Viswanathan, J. Lumin. 128 (2008) 1255–1261. [54] V. Shyamala Devi, S. Maji, K.S. Viswanathan, J. Lumin. 131 (2011) 739–748. [55] S. Maji, K.S. Viswanathan, J. Lumin. 129 (2009) 1242–1248. [56] K. Lunstroot, P. Nockemann, K.V. Hecke, L.V. Meervelt, C.G. Walrand, K. Binnemans, K. Driesen, Inorg. Chem. 48 (2009) 3018–3026. [57] T. Kimura, R. Nagaishi, Y. Kato, Z. Yoshida, Radiochim. Acta. 89 (2001) 125– 130. [58] L. Zhang, Y. An, W. Ahmad, Y. Zhou, Z. Shi, X. Zheng, J. Photochem. Photobiol. A: Chem. 252 (2013) 167–173. [59] M.S. Attia, A.O. Youssef, Amr A. Essawy, M.S.A. Abdel-Mottaleb, J. Lumin. 132 (2012) 2741–2746. [60] A.L. Jenkins, G.M. Murray, Anal. Chem. 68 (1996) 2974–2980.