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A fluorimetric study of terbium, europium and dysprosium in aqueous solution using pyridine carboxylic acids as ligands
Journal of Alloys and Compounds 334 (2002) 228–231
L
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A fluorimetric study of terbium, europium and dysprosium in aqueous solution using pyridine carboxylic acids as ligands B.S. Panigrahi* Reactor Operations Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamilnadu 603 102, India Received 11 January 2001; received in revised form 16 May 2001; accepted 6 July 2001
Abstract Fluorescence enhancement of Tb 31 , Eu 31 and Dy 31 was studied in aqueous solution using pyridine carboxylic acids (nicotinic, picolinic and dipicolinic acids) as ligands. The fluorescence intensity of Tb 31 , Eu 31 and Dy 31 could be enhanced, contrary to an earlier report by more than two to three orders of magnitude using these acids as ligands. To further enhance the lanthanide fluorescence, a neutral ligand, trioctyl phosphine oxide (TOPO), in Triton X-100 micellar medium was added to lanthanide–pyridine carboxylic acid complexes. However, the additional fluorescence enhancement of lanthanides, following the addition of TOPO was observed only with nicotinic acid. 2002 Elsevier Science B.V. All rights reserved. Keywords: Lanthanide fluorescence; Fluorescence enhancement; Pyridine carboxylic acids
1. Introduction Lanthanides in aqueous solution are known to be either non-fluorescent or weakly fluorescent due to their low molar absorptivities and poor quantum yields [1,2]. The problem due to low molar absorptivity is overcome by employing the technique of ligand sensitised fluorescence. In this technique, the weakly fluorescent lanthanide ion is complexed with a ligand having higher molar absorptivity. Then the ligand is excited in its absorption band. A part of the excited energy from ligand is transferred by intramolecular energy transfer mode to lanthanide ion resulting in fluorescence enhancement of the lanthanide ion. Various ligands have been used to sensitise and enhance the lanthanide fluorescence [3–9]. The problem due to poor quantum yield is tackled to some extent by using a neutral ligand like trioctyl phosphine oxide (TOPO) as the secondary ligand. Such a ligand reduces the probability of non-radiative decay in the lanthanide ion by forming an insulating sheath around the lanthanide complex [10]. In the study of energy transfer from Tb 31 to Eu 31 complexes of pyridine carboxylic acids (picolinic acid, 2-pyridine carboxylic acid (PA), dipicolinic acid, 2,6pyridine dicarboxylic acid (DPA) and nicotinic acid, 3*Tel.: 191-4114-80358; fax: 191-4114-80336. E-mail address: [email protected] (B.S. Panigrahi).
pyridine carboxylic acid (NA)), by Brittain [11], it has been reported that no energy transfer work was possible with nicotinic acid complexes in aqueous solution since neither terbium nor europium was found to emit when bound to nicotinic acid. Similarly, he has also reported there that Eu 31 did not show any measurable fluorescence when complexed with any of these three acid ligands. However, Baker et al. [12] have reported terbium luminescence from terbium nicotinate dihydrate complex. Therefore, in this study, we have tried to find out the effect of the presence of these acids on lanthanide fluorescence in aqueous medium. We have also studied the effect of addition of TOPO in a micellar medium to the lanthanide– pyridine carboxylic acid complexes in aqueous solution. The micellar medium chosen in this study was a non-ionic surfactant, Triton X-100. The reason for choosing Triton X-100 to provide the required micellar medium is discussed in our earlier report [13].
2. Experimental All fluorescence measurements were made with a Shimadzu RF 5000 spectrofluorimeter with a 150 W xenon lamp as the excitation source. The bandwidth of the excitation and emission monochromators was set at 5 nm. Solutions were taken in a 1-cm path length quartz cell for
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01761-3
B.S. Panigrahi / Journal of Alloys and Compounds 334 (2002) 228 – 231
229
To begin with, the lanthanide fluorescence in lanthanide–acid complexes was measured as a function of pH and acid concentration. It was found that the optimum pH was 6 for nicotinic and picolinic acids whereas it was 8 for dipicolinic acid. The optimum acid concentration for all acids was found to be 1310 23 M. This acid concentration was used in all subsequent experiments. The excitation spectra of bare Tb 31 , Eu 31 and Dy 31 in water (without containing any complexing agents) for respective emission line are shown in Fig. 1a, c and e, respectively. The excitation spectra of Tb 31 , Eu 31 and Dy 31 –nicotinic acid complexes are shown in Fig. 1b, d and f, respectively. The excitation spectra of bare lanthanides were widely different from each other whereas the excitation spectra of the complexed lanthanides were all found to be similar, having excitation maxima around 274 nm. Observation of such a similar excitation spectrum among different lanthanide–nicotinic acid complexes points to the fact that nicotinic acid was the common absorber in all lanthanide complexes of nicotinic acid. This proves, contrary to the observation made by Brittain, that
Tb 31 , Eu 31 and Dy 31 fluorescence could be sensitised by using nicotinic acid as the ligand. The emission spectra of Tb 31 , Eu 31 and Dy 31 -nicotinic acid complexes are shown in Fig. 2b, d and f, respectively. All these spectra were recorded by exciting these complexes at 274 nm. The lanthanide concentrations used here: [Tb 31 ]51310 26 M, [Eu 31 ]51310 25 M and [Dy 31 ]513 10 25 M. At this level of these lanthanide concentrations, no perceptible fluorescence was observed from the respective bare lanthanides on excitation at 274 nm. Therefore, from these figures it becomes clear that apart from sensitising lanthanide fluorescence, nicotinic acid has also enhanced the lanthanide fluorescence. For a better comparison, the emission spectra of bare Tb 31 (4310 24 M), Eu 31 (1310 23 M) and Dy 31 (5310 23 M) in water are shown in Fig. 2a, c and e, respectively. The fluorescence intensities shown here for the bare and complexed lanthanide ions were comparable. However, the concentrations used to record the emission spectra of complexed lanthanide ions were nearly two orders of magnitude less than that used for bare lanthanides. Therefore it is obvious that in the presence of nicotinic acid, the Tb 31 , Eu 31 and Dy 31 lanthanide fluorescence was enhanced by nearly two orders of magnitude. The excitation spectra of Eu 31 –picolinic and Eu 31 – dipicolinic acid complexes are shown in Fig. 3a and b, respectively. Comparison of these excitation spectra with the excitation spectrum of bare Eu 31 (Fig. 1c) brings out the fact that like nicotinic acid, picolinic and dipicolinic acids also have sensitised Eu 31 fluorescence. The emission spectra of bare Eu 31 , Eu 31 –picolinic and Eu 31 –di-
Fig. 1. Excitation spectra of (a) Tb 31 (4310 24 M), lemi 5544 nm; (b) Tb 31 (1310 26 M)–NA (1310 23 M), lemi 5544 nm; (c) Eu 31 (1310 23 M), lemi 5614 nm; (d) Eu 31 (1310 25 M)–NA (1310 23 M), lemi 5614 nm; (e) Dy 31 (5310 23 M), lemi 5572 nm; and (f) Dy 31 (1310 25 M)–NA (1310 23 M), lemi 5572 nm.
Fig. 2. Emission spectra of (a) Tb 31 (4310 24 M), lexc 5348 nm; (b) Tb 31 (1310 26 M)–NA (1310 23 M), lexc 5274 nm; (c) Eu 31 (1310 23 M), lexc 5390 nm; (d) Eu 31 (1310 25 M)–NA (1310 23 M), lexc 5274 nm; (e) Dy 31 (5310 23 M), lexc 5348 nm; and (f) Dy 31 (1310 25 M)–NA (1310 23 M), lexc 5274 nm.
fluorescence measurements. The details about experimental conditions are described elsewhere [14]. All the reagents used here were of analytical reagent grade.
3. Results and discussion
3.1. Lanthanide–acid system
230
B.S. Panigrahi / Journal of Alloys and Compounds 334 (2002) 228 – 231
Fig. 3. Excitation spectra of (a) Eu 31 (1310 26 M)–PA (1310 23 M), lemi 5614 nm; (b) Eu 31 (1310 26 M)–DPA (1310 23 M), lemi 5614 nm and emission spectra of; (c) Eu 31 (1310 26 M)–PA (1310 23 M), lexc 5 276 nm; and (d) Eu 31 (1310 26 M)–DPA (1310 23 M), lexc 5273 nm.
picolinic acid complexes are shown (on the similar intensity scale) in Fig. 3c and d, respectively. Considering the Eu 31 concentration employed in each case to record these spectra it becomes evident that, Eu 31 fluorescence was enhanced by two to three orders of magnitude in the presence of picolinic and dipicolinic acids. Similarly, use of picolinic and dipicolinic acids as ligands has enhanced the fluorescence of Dy 31 . It can be seen from the emission spectra (Figs. 2 and 3) that the relative intensities of Eu 31 emission peaks at 592 nm ( 5 D 0 – 7 F 1 ) and 614 nm ( 5 D 0 – 7 F 2 ) change on addition of acid ligands. The ( 5 D 0 – 7 F 2 ) transition exhibits hypersensitivity. This results in a change in the relative intensities of the peaks mentioned above. Such changes in relative intensities of these peaks as a result of change in the ligand environment have been reported [15,16]. These results, as discussed above, have clearly demonstrated that nicotinic acid, picolinic acid and dipicolinic acid can sensitise and enhance the fluorescence of Tb 31 , Eu 31 and Dy 31 by more than two orders of magnitude. These results are at variance with the results reported by Brittain. He could get Tb 31 fluorescence from terbium– picolinate and terbium–dipicolinate complex but not from
terbium–nicotinate. Based on this, he concluded that to get Tb 31 fluorescence from terbium–pyridine carboxylic acid complexes, the ligand should bind the lanthanide ion in a bidentate fashion. According to him, Tb 31 fluorescence could be seen in the case of picolinic acid and dipicolinic acid complexes since here terbium can enter into bonding with both the carboxylate and nitrogen (placed at ortho position to the carboxylic group). Such a bidentate bonding was not possible with nicotinic acid as the nitrogen is positioned meta to the carboxylate group and therefore, as per him, no Tb 31 fluorescence was seen from terbium– nicotinate complex. However, the present study has clearly brought out the fact that it is not an absolute necessity for the pyridine carboxylic acid ligand to have a bidentate bonding with the lanthanide to emit lanthanide fluorescence. This observation is consistent with our earlier observation of strong lanthanide fluorescence from lanthanide–benzoate complexes [6]. Recently Amanda et al. [17], in their study of ultra trace determination of selected lanthanides by luminescence enhancement, have reported that lanthanides like Tb, Eu, Dy and Sm can be estimated at ultra trace level by using dipicolinic acid as the ligand in the presence of La, Gd or Y. They have reported therein that the optimal lanthanide luminescence occurred not when the ligand band was excited, but when a specific lanthanide absorbance was excited directly. Therefore, they concluded that the accepted mechanism of ligand sensitised luminescence was not operated in their case. However, we have observed in the present study that the lanthanide fluorescence was maximum only when the ligand band was excited (Fig. 3). Hence, we believe that the accepted mechanism of intramolecular energy transfer from the ligand to the lanthanide ion is operative in lanthanide–dipicolinate complexes also.
3.2. Lanthanide–acid–TOPO–Triton X-100 systems In our earlier studies with aromatic carboxylic acids [14], we have seen that on addition of TOPO–Triton X-100 to the lanthanide–acid complexes, the observed synergism was not uniform with all the acid ligands. The synergistic fluorescence enhancement of lanthanides was dependent on the structure of sensitising ligands. In the present study also the position of carboxylic group in the pyridine ring is different for different acids that are studied here. In case of nicotinic acid, the carboxylic group is meta positioned to the nitrogen present in the pyridine ring. Whereas in case of picolinic acid the carboxylic group is ortho positioned and in dipicolinic acid there are two ortho positioned carboxylic groups. Therefore, we have studied the effect of addition of TOPO–Triton X-100 to the lanthanide–pyridine carboxylic acid complexes in aqueous medium. The concentrations of TOPO and Triton X-100 were maintained at 1310 24 M and 0.1%, respectively as these
B.S. Panigrahi / Journal of Alloys and Compounds 334 (2002) 228 – 231 Table 1 Optimum pH, excitation wavelengths and relative fluorescence intensity a for the different Tb 31 (10 26 M)–pyridine carboxylic acid complexes with and without TOPO–Triton X-100 Acid
Nicotinic Picolinic Dipicolinic
Tb–acid
Tb–acid–TOPO
pH
lexc (nm)
RF
pH
lexc (nm)
RF
6.0 6.0 8.0
273 273 273
100 1027 561
6.0 6.0 8.0
288 288 288
404 228 322
231
in aqueous solution. The fluorescence intensities of Tb 31 , Eu 31 and Dy 31 were enhanced by more than two to three orders of magnitude in the presence of these acids. On addition of TOPO in Triton X-100 to lanthanide–acid complexes, a synergistic lanthanide fluorescence enhancement was observed with nicotinic acid whereas an antisynergism was observed with picolinic and dipicolinic acids.
a
Relative fluorescence (RF) intensity is the fluorescence intensity of the complex relative to that of Tb–nicotinic acid complex (without TOPO–Triton X-100), which is taken as 100.
concentrations were found as optimum earlier. The concentration for all the acid ligands was maintained at 13 10 23 M. Table 1 gives the fluorescence enhancement of Tb 31 in different Tb 31 –pyridine carboxylic acid complexes and the effect of addition of TOPO–Triton X-100 to the Tb 31 – acid complexes. It was observed that on addition of TOPO–Triton X-100 to the terbium–acid system in aqueous solution, the terbium fluorescence was further enhanced only with nicotinic acid. Such an enhancement in terbium fluorescence in the case of nicotinic acid following the addition of TOPO–Triton X-100 is consistent with the effect seen earlier with lanthanide–benzene monocarboxylic acid ligands [6,13]. However, the magnitude of synergistic enhancement observed with Ln–nicotinate was significantly less compared with that seen in the case of Ln–benzoate. With picolinic and dipicolinic acids, addition of TOPO–Triton X-100 did not result in any synergism; rather it has decreased the intensity of terbium fluorescence. The reason for such an anti-synergism is yet to be understood.
4. Conclusion Nicotinic, picolinic and dipicolinic acids are found to sensitise and enhance Tb 31 , Eu 31 and Dy 31 fluorescence
Acknowledgements The author thanks Mr K. Sundarrajan and Dr K.S. Viswanathan for useful discussions and comments.
References [1] G. Stein, E. Wurzberg, J. Chem. Phys. 62 (1975) 208. [2] W.T. Carnall, Handbook of the Physics and Chemistry of Rare Earths, Vol. 3, Elsevier, Amsterdam, 1978, Chapter 24. [3] J.-H. Yang, G.-Y. Zhu, B. Wu, Anal. Chim. Acta 198 (1987) 287. [4] L.M. Perry, J.D. Winefordner, Anal. Chim. Acta 237 (1990) 273. [5] M. Morin, R. Bador, H. Dechaud, Anal. Chim. Acta 260 (1989) 67. [6] S. Peter, B.S. Panigrahi, K.S. Viswanathan, C.K. Mathews, Anal. Chim. Acta 260 (1992) 135. [7] B.S. Panigrahi, S. Peter, K.S. Viswanathan, C.K. Mathews, Anal. Chim. Acta 282 (1993) 117. [8] T. Taketasu, Talanta 29 (1982) 397. [9] Y. Hass, G. Stein, E. Wurzberg, J. Chem. Phys. 60 (1974) 258. [10] F. Halverson, J. Brinen, J. Leto, J. Chem. Phys. 41 (1964) 157, 2752. [11] H.G. Brittain, Inorg. Chem 17 (1978) 2762. [12] J.M. Baker, C.A. Hutchison Jr., M.J.M. Leask, P.M. Martineau, M.G. Robinson, M.R. Wells, Proc. R. Soc. Lond. A 413 (1987) 515. [13] B.S. Panigrahi, Spec. Chim. Acta (A) 56 (2000) 1337. [14] B.S. Panigrahi, S. Peter, K.S. Viswanathan, C.K. Mathews, Spec. Chim. Acta (A) 51 (1995) 2289. [15] J.-C.G. Bunzil, in: J.-C.G. Bunzil, G.R. Choppin (Eds.), Lanthanide Probes in Life, Chemical and Earth Sciences, Elsevier, New York, 1989, Chapter 7. [16] F.S. Richardson, Chem. Rev. 82 (1982) 541. [17] L.J. Amanda, G.M. Murray, Anal. Chem. 68 (1996) 2974.
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A fluorimetric study of terbium, europium and dysprosium in aqueous solution using pyridine carboxylic acids as ligands B.S. Panigrahi* Reactor Operations Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamilnadu 603 102, India Received 11 January 2001; received in revised form 16 May 2001; accepted 6 July 2001
Abstract Fluorescence enhancement of Tb 31 , Eu 31 and Dy 31 was studied in aqueous solution using pyridine carboxylic acids (nicotinic, picolinic and dipicolinic acids) as ligands. The fluorescence intensity of Tb 31 , Eu 31 and Dy 31 could be enhanced, contrary to an earlier report by more than two to three orders of magnitude using these acids as ligands. To further enhance the lanthanide fluorescence, a neutral ligand, trioctyl phosphine oxide (TOPO), in Triton X-100 micellar medium was added to lanthanide–pyridine carboxylic acid complexes. However, the additional fluorescence enhancement of lanthanides, following the addition of TOPO was observed only with nicotinic acid. 2002 Elsevier Science B.V. All rights reserved. Keywords: Lanthanide fluorescence; Fluorescence enhancement; Pyridine carboxylic acids
1. Introduction Lanthanides in aqueous solution are known to be either non-fluorescent or weakly fluorescent due to their low molar absorptivities and poor quantum yields [1,2]. The problem due to low molar absorptivity is overcome by employing the technique of ligand sensitised fluorescence. In this technique, the weakly fluorescent lanthanide ion is complexed with a ligand having higher molar absorptivity. Then the ligand is excited in its absorption band. A part of the excited energy from ligand is transferred by intramolecular energy transfer mode to lanthanide ion resulting in fluorescence enhancement of the lanthanide ion. Various ligands have been used to sensitise and enhance the lanthanide fluorescence [3–9]. The problem due to poor quantum yield is tackled to some extent by using a neutral ligand like trioctyl phosphine oxide (TOPO) as the secondary ligand. Such a ligand reduces the probability of non-radiative decay in the lanthanide ion by forming an insulating sheath around the lanthanide complex [10]. In the study of energy transfer from Tb 31 to Eu 31 complexes of pyridine carboxylic acids (picolinic acid, 2-pyridine carboxylic acid (PA), dipicolinic acid, 2,6pyridine dicarboxylic acid (DPA) and nicotinic acid, 3*Tel.: 191-4114-80358; fax: 191-4114-80336. E-mail address: [email protected] (B.S. Panigrahi).
pyridine carboxylic acid (NA)), by Brittain [11], it has been reported that no energy transfer work was possible with nicotinic acid complexes in aqueous solution since neither terbium nor europium was found to emit when bound to nicotinic acid. Similarly, he has also reported there that Eu 31 did not show any measurable fluorescence when complexed with any of these three acid ligands. However, Baker et al. [12] have reported terbium luminescence from terbium nicotinate dihydrate complex. Therefore, in this study, we have tried to find out the effect of the presence of these acids on lanthanide fluorescence in aqueous medium. We have also studied the effect of addition of TOPO in a micellar medium to the lanthanide– pyridine carboxylic acid complexes in aqueous solution. The micellar medium chosen in this study was a non-ionic surfactant, Triton X-100. The reason for choosing Triton X-100 to provide the required micellar medium is discussed in our earlier report [13].
2. Experimental All fluorescence measurements were made with a Shimadzu RF 5000 spectrofluorimeter with a 150 W xenon lamp as the excitation source. The bandwidth of the excitation and emission monochromators was set at 5 nm. Solutions were taken in a 1-cm path length quartz cell for
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01761-3
B.S. Panigrahi / Journal of Alloys and Compounds 334 (2002) 228 – 231
229
To begin with, the lanthanide fluorescence in lanthanide–acid complexes was measured as a function of pH and acid concentration. It was found that the optimum pH was 6 for nicotinic and picolinic acids whereas it was 8 for dipicolinic acid. The optimum acid concentration for all acids was found to be 1310 23 M. This acid concentration was used in all subsequent experiments. The excitation spectra of bare Tb 31 , Eu 31 and Dy 31 in water (without containing any complexing agents) for respective emission line are shown in Fig. 1a, c and e, respectively. The excitation spectra of Tb 31 , Eu 31 and Dy 31 –nicotinic acid complexes are shown in Fig. 1b, d and f, respectively. The excitation spectra of bare lanthanides were widely different from each other whereas the excitation spectra of the complexed lanthanides were all found to be similar, having excitation maxima around 274 nm. Observation of such a similar excitation spectrum among different lanthanide–nicotinic acid complexes points to the fact that nicotinic acid was the common absorber in all lanthanide complexes of nicotinic acid. This proves, contrary to the observation made by Brittain, that
Tb 31 , Eu 31 and Dy 31 fluorescence could be sensitised by using nicotinic acid as the ligand. The emission spectra of Tb 31 , Eu 31 and Dy 31 -nicotinic acid complexes are shown in Fig. 2b, d and f, respectively. All these spectra were recorded by exciting these complexes at 274 nm. The lanthanide concentrations used here: [Tb 31 ]51310 26 M, [Eu 31 ]51310 25 M and [Dy 31 ]513 10 25 M. At this level of these lanthanide concentrations, no perceptible fluorescence was observed from the respective bare lanthanides on excitation at 274 nm. Therefore, from these figures it becomes clear that apart from sensitising lanthanide fluorescence, nicotinic acid has also enhanced the lanthanide fluorescence. For a better comparison, the emission spectra of bare Tb 31 (4310 24 M), Eu 31 (1310 23 M) and Dy 31 (5310 23 M) in water are shown in Fig. 2a, c and e, respectively. The fluorescence intensities shown here for the bare and complexed lanthanide ions were comparable. However, the concentrations used to record the emission spectra of complexed lanthanide ions were nearly two orders of magnitude less than that used for bare lanthanides. Therefore it is obvious that in the presence of nicotinic acid, the Tb 31 , Eu 31 and Dy 31 lanthanide fluorescence was enhanced by nearly two orders of magnitude. The excitation spectra of Eu 31 –picolinic and Eu 31 – dipicolinic acid complexes are shown in Fig. 3a and b, respectively. Comparison of these excitation spectra with the excitation spectrum of bare Eu 31 (Fig. 1c) brings out the fact that like nicotinic acid, picolinic and dipicolinic acids also have sensitised Eu 31 fluorescence. The emission spectra of bare Eu 31 , Eu 31 –picolinic and Eu 31 –di-
Fig. 1. Excitation spectra of (a) Tb 31 (4310 24 M), lemi 5544 nm; (b) Tb 31 (1310 26 M)–NA (1310 23 M), lemi 5544 nm; (c) Eu 31 (1310 23 M), lemi 5614 nm; (d) Eu 31 (1310 25 M)–NA (1310 23 M), lemi 5614 nm; (e) Dy 31 (5310 23 M), lemi 5572 nm; and (f) Dy 31 (1310 25 M)–NA (1310 23 M), lemi 5572 nm.
Fig. 2. Emission spectra of (a) Tb 31 (4310 24 M), lexc 5348 nm; (b) Tb 31 (1310 26 M)–NA (1310 23 M), lexc 5274 nm; (c) Eu 31 (1310 23 M), lexc 5390 nm; (d) Eu 31 (1310 25 M)–NA (1310 23 M), lexc 5274 nm; (e) Dy 31 (5310 23 M), lexc 5348 nm; and (f) Dy 31 (1310 25 M)–NA (1310 23 M), lexc 5274 nm.
fluorescence measurements. The details about experimental conditions are described elsewhere [14]. All the reagents used here were of analytical reagent grade.
3. Results and discussion
3.1. Lanthanide–acid system
230
B.S. Panigrahi / Journal of Alloys and Compounds 334 (2002) 228 – 231
Fig. 3. Excitation spectra of (a) Eu 31 (1310 26 M)–PA (1310 23 M), lemi 5614 nm; (b) Eu 31 (1310 26 M)–DPA (1310 23 M), lemi 5614 nm and emission spectra of; (c) Eu 31 (1310 26 M)–PA (1310 23 M), lexc 5 276 nm; and (d) Eu 31 (1310 26 M)–DPA (1310 23 M), lexc 5273 nm.
picolinic acid complexes are shown (on the similar intensity scale) in Fig. 3c and d, respectively. Considering the Eu 31 concentration employed in each case to record these spectra it becomes evident that, Eu 31 fluorescence was enhanced by two to three orders of magnitude in the presence of picolinic and dipicolinic acids. Similarly, use of picolinic and dipicolinic acids as ligands has enhanced the fluorescence of Dy 31 . It can be seen from the emission spectra (Figs. 2 and 3) that the relative intensities of Eu 31 emission peaks at 592 nm ( 5 D 0 – 7 F 1 ) and 614 nm ( 5 D 0 – 7 F 2 ) change on addition of acid ligands. The ( 5 D 0 – 7 F 2 ) transition exhibits hypersensitivity. This results in a change in the relative intensities of the peaks mentioned above. Such changes in relative intensities of these peaks as a result of change in the ligand environment have been reported [15,16]. These results, as discussed above, have clearly demonstrated that nicotinic acid, picolinic acid and dipicolinic acid can sensitise and enhance the fluorescence of Tb 31 , Eu 31 and Dy 31 by more than two orders of magnitude. These results are at variance with the results reported by Brittain. He could get Tb 31 fluorescence from terbium– picolinate and terbium–dipicolinate complex but not from
terbium–nicotinate. Based on this, he concluded that to get Tb 31 fluorescence from terbium–pyridine carboxylic acid complexes, the ligand should bind the lanthanide ion in a bidentate fashion. According to him, Tb 31 fluorescence could be seen in the case of picolinic acid and dipicolinic acid complexes since here terbium can enter into bonding with both the carboxylate and nitrogen (placed at ortho position to the carboxylic group). Such a bidentate bonding was not possible with nicotinic acid as the nitrogen is positioned meta to the carboxylate group and therefore, as per him, no Tb 31 fluorescence was seen from terbium– nicotinate complex. However, the present study has clearly brought out the fact that it is not an absolute necessity for the pyridine carboxylic acid ligand to have a bidentate bonding with the lanthanide to emit lanthanide fluorescence. This observation is consistent with our earlier observation of strong lanthanide fluorescence from lanthanide–benzoate complexes [6]. Recently Amanda et al. [17], in their study of ultra trace determination of selected lanthanides by luminescence enhancement, have reported that lanthanides like Tb, Eu, Dy and Sm can be estimated at ultra trace level by using dipicolinic acid as the ligand in the presence of La, Gd or Y. They have reported therein that the optimal lanthanide luminescence occurred not when the ligand band was excited, but when a specific lanthanide absorbance was excited directly. Therefore, they concluded that the accepted mechanism of ligand sensitised luminescence was not operated in their case. However, we have observed in the present study that the lanthanide fluorescence was maximum only when the ligand band was excited (Fig. 3). Hence, we believe that the accepted mechanism of intramolecular energy transfer from the ligand to the lanthanide ion is operative in lanthanide–dipicolinate complexes also.
3.2. Lanthanide–acid–TOPO–Triton X-100 systems In our earlier studies with aromatic carboxylic acids [14], we have seen that on addition of TOPO–Triton X-100 to the lanthanide–acid complexes, the observed synergism was not uniform with all the acid ligands. The synergistic fluorescence enhancement of lanthanides was dependent on the structure of sensitising ligands. In the present study also the position of carboxylic group in the pyridine ring is different for different acids that are studied here. In case of nicotinic acid, the carboxylic group is meta positioned to the nitrogen present in the pyridine ring. Whereas in case of picolinic acid the carboxylic group is ortho positioned and in dipicolinic acid there are two ortho positioned carboxylic groups. Therefore, we have studied the effect of addition of TOPO–Triton X-100 to the lanthanide–pyridine carboxylic acid complexes in aqueous medium. The concentrations of TOPO and Triton X-100 were maintained at 1310 24 M and 0.1%, respectively as these
B.S. Panigrahi / Journal of Alloys and Compounds 334 (2002) 228 – 231 Table 1 Optimum pH, excitation wavelengths and relative fluorescence intensity a for the different Tb 31 (10 26 M)–pyridine carboxylic acid complexes with and without TOPO–Triton X-100 Acid
Nicotinic Picolinic Dipicolinic
Tb–acid
Tb–acid–TOPO
pH
lexc (nm)
RF
pH
lexc (nm)
RF
6.0 6.0 8.0
273 273 273
100 1027 561
6.0 6.0 8.0
288 288 288
404 228 322
231
in aqueous solution. The fluorescence intensities of Tb 31 , Eu 31 and Dy 31 were enhanced by more than two to three orders of magnitude in the presence of these acids. On addition of TOPO in Triton X-100 to lanthanide–acid complexes, a synergistic lanthanide fluorescence enhancement was observed with nicotinic acid whereas an antisynergism was observed with picolinic and dipicolinic acids.
a
Relative fluorescence (RF) intensity is the fluorescence intensity of the complex relative to that of Tb–nicotinic acid complex (without TOPO–Triton X-100), which is taken as 100.
concentrations were found as optimum earlier. The concentration for all the acid ligands was maintained at 13 10 23 M. Table 1 gives the fluorescence enhancement of Tb 31 in different Tb 31 –pyridine carboxylic acid complexes and the effect of addition of TOPO–Triton X-100 to the Tb 31 – acid complexes. It was observed that on addition of TOPO–Triton X-100 to the terbium–acid system in aqueous solution, the terbium fluorescence was further enhanced only with nicotinic acid. Such an enhancement in terbium fluorescence in the case of nicotinic acid following the addition of TOPO–Triton X-100 is consistent with the effect seen earlier with lanthanide–benzene monocarboxylic acid ligands [6,13]. However, the magnitude of synergistic enhancement observed with Ln–nicotinate was significantly less compared with that seen in the case of Ln–benzoate. With picolinic and dipicolinic acids, addition of TOPO–Triton X-100 did not result in any synergism; rather it has decreased the intensity of terbium fluorescence. The reason for such an anti-synergism is yet to be understood.
4. Conclusion Nicotinic, picolinic and dipicolinic acids are found to sensitise and enhance Tb 31 , Eu 31 and Dy 31 fluorescence
Acknowledgements The author thanks Mr K. Sundarrajan and Dr K.S. Viswanathan for useful discussions and comments.
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