- Email: [email protected]
A novel fluorescent Fe3+ sensor based on a europium complex
Optical Materials 35 (2013) 833–836
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
A novel fluorescent Fe3+ sensor based on a europium complex Ruichun Bai a, Wei Gao b, Shengdi Bai c,⇑, Fengling Yang d, Huili Chen a,e,⇑ a
Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Shanxi University, Taiyuan 030006, PR China Department of Chemistry, Changzhi University, Shanxi, Changzhi 046011, PR China c Research Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, PR China d Institute of Resources and Environment Engineering, Shanxi University, Taiyuan 030006, PR China e State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China b
a r t i c l e
i n f o
Article history: Received 19 June 2012 Received in revised form 11 October 2012 Accepted 12 October 2012 Available online 28 November 2012 Keywords: Fe(III) sensor Rare earth complex Fluorescent sensor
a b s t r a c t A novel Eu3+ complex (EuL) based on 1,10-phenanthroline-2-carboxylic was successfully synthesized, and gave a characteristic red emission. The complex was shown to act as a selective luminescence quencher for Fe3+, as shown by the ‘‘on/off’’ switch phenomenon. The quenching curve showed a double-exponential well decay with the increase of Fe3+ ions. The stability constant of EuL/Fe3+ was calculated as 8.6 103 L mol 1(lg b/L mol 1 = 3.93). The response showed high selectivity for Fe3+ compared with other metal ions; the complex therefore has the potential to be applied as a fluorescent sensor for Fe3+. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Iron is an essential element for the human body, and for plant growth. However, excessive amounts of iron can do great harm to the body. Iron metabolism disorders have been reported to cause anemia, as well as liver and kidney damage that might ultimately cause liver cancer, liver cirrhosis, arthritis, diabetes, or heart failure [1]. Hence, the detection of iron ions has always been an important consideration in the environmental protection and human health fields. At present, a variety of methods exist for the detection of iron ions. Fluorescent detection has been used in a wide range of applications [2–7]. Most sensors are based on organic compounds. However, organic compounds typically hold some disadvantages as fluorescent emitters, including short fluorescence lifetimes, and photobleaching. Rare earth complexes typically have a longer fluorescence lifetime, and can be used in time-resolved fluorescent detection [8,9]. Recently, a number of fluorescent sensors based on rare earth complexes have been reported [10–13]. The coordination of aromatic carboxylic acids towards rare earth complexes has received considerable attention, due to the strong coordination abilities and variety of bridging modes of the carboxylate group for the construction of metal-organic framework (MOF) structures [14,15]. With their higher luminous efficiency and narrower spec⇑ Corresponding authors. Address: Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Shanxi University, Taiyuan 030006, PR China (H. Chen). Tel.: +86 3517010699. E-mail addresses: [email protected] (H. Chen), [email protected] (S. Bai). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.10.030
trum width, rare earth complexes have significant potential for applications. In this paper, a novel Eu3+ complex based on a phenanthroline carboxylic acid derivative (EuL) was developed to detect Fe3+ via the fluorescence method. The complex emitted strong fluorescence in the red region of the spectrum when solved in methanol–water (V:V, 5:1). With the incremental addition of Fe3+, the fluorescence of the EuL complex was quenched gradually. The response showed high selectivity for Fe3+ compared with other metal ions, and could be used to detect Fe3+.
2. Experimental details 2.1. Reagents and instruments ESI-MS spectra were measured on a Finnigan LCQ system. UV–Vis spectra were recorded on a Hewlett Packard HP-8453 spectrophotometer. Thermal analysis was carried out with HCT-1/2 TGDTA analyzer (from HENVEN.Co, Beijing) using 3–5 mg samples that were heated at a rate of 10 K/min from ambient to 973 K in air. Luminescence spectra were recorded on Hitachi F-4600 fluorescence spectrophotometer. Both the excitation and emission slits are 2.5 nm. All experiments were carried out in methanol–water (V:V, 5:1) in pH 6.2. All solvents were purchased from Tianjin Fengchuan Chemical Reagent Technology Co. Ltd. 2-Methyl-1,10-phenanthroline was synthesized according to literature [16]. 2-Carboxyl-1,10-phenanthroline was obtained
834
R. Bai et al. / Optical Materials 35 (2013) 833–836
through two steps by the oxidation of SeO2 and concentrated HNO3 [17] .
L EuL
60
2.2. Synthesis and characterization of europium(III) complex of 1,10phenanthroline-2-carboxylic (EuL) 40
T%
EuCl36H2O (0.1 m mol) in water was added into 0.2 m mol 2carboxyl-1,10-phenanthroline in methanol–H2O–NaOH. The solution was refluxed for 4 h and cooled to room temperature. The result precipitation was collected and washed with water and ethanol to give the titled complex. Yield, 79%. Elemental Analysis Calcd for C26H22ClEuN4O8: C: 44.24; H: 3.14; N: 7.94. Found: C: 44.46; H: 3.25; N: 8.57 (see Fig. 1).
ν (COO-)
ν (COOH)
s
20
ν (COO-)
as
0
3. Results and discussion
1200
1800
Fig. 2. IR spectra of L and EuL.
100
Exo
743 K loss 10.4%
80 60
DTA
TG W/%
40 loss 75.1%
20
Endo
Fig. 2 shows FTIR absorption spectra for L and EuL. For L, a carbonyl group (ACOOH) stretching vibration peak appeared at 1746 cm 1. The hydroxyl bending vibration peaks appeared at 1210 cm 1 and 871 cm 1. These characteristic mCOOH peaks disappeared when EuL was formed. The asymmetric and symmetric stretching vibrations of the carboxylic groups in EuL appeared at 1631 cm 1 and 1398 cm 1, respectively. The difference between the two peaks was more than 200 cm 1, indicating that one oxygen atom in the carboxylic group participated in the coordination with Eu [18]. The C@N stretching vibration peak for L at 1616 cm 1 shifted to the low frequency region after the coordination with Eu, which indicated that the bonds were formed between Eu and the nitrogen atoms of L [19]. The thermal stability and structure of EuL were examined using TG and DTA analysis. The curves are shown in Fig. 3. There were three weight loss peaks in the curve. The DTA curve showed a weak endothermic peak at 423 K, and a strong exothermic peak at 743 K. From the TG curve, the first decomposition process involved the loss of 4 molecules of water at 423 K, with a mass loss of 10.4% (Cal. 10.2%). The second stage consisted of the loss and combustion of the ligand L, accompanied by a violent exothermic process. The final residue was Eu2O3, with a final mass loss of 74.5% (Cal. 75.1%). In methanol–water (V:V, 5:1), the UV-Vis absorbance spectrum for EuL showed strong absorbance at 231 nm (e = 3.56 104 cm 1 mol 1) and 283 nm (e = 3.54 104 cm 1 mol 1) in the UV region, which was ascribed to the p–p transition of 2-carboxyl-1,10-phenanthroline. Under these conditions, the excitation in the phen absorption band at 283 nm caused the well known, structured emission of the Eu3+ ion, confirming that the energy transfer from the ligand-centered levels to the metal-centered levels took place. The relatively high intensity of the 5D0 ? 7F2 transition indicated that the Eu3+ ion did not lie in a centrosymmetric coordinate site. The complex showed a striking red luminescence. Upon the addition of Fe3+, a rapid decrease in intensity of the whole emission band occurred (saturated at 10 lM Fe3+ with a 60-fold decrease), and the fluorescent color of the solution turned
1500
Wavenumbers (cm-1)
0 423 K
400
600
800
T/K Fig. 3. TG-DTA curves of EuL.
from red to colorless. A detailed spectrofluorometric titration was performed (Fig. 4A). The titration curve at 618 nm fitted well a fluorescence double exponential decay. The effect stability constant (b) was obtained using the Hildebrand–Bebesi equation [20]. A linear least-squares curve fitting of the plot with I0/(I I0) versus [Fe3+] 1 indicated a 1:1 binding model between the Eu3+ complex and the Fe3+ ion (Fig. 4B). The lg b/L mol 1 value was 3.93. The fluorescent titration by other metal ions from chloride salts revealed excellent selectivity for Fe3+. For example, physiologically important metal ions that exist in living cells, such as Ca2+, Mg2+, Na+, and K+, did not give any response at a 100-fold excess concentration. Heavy and transition metal ions such as Cd2+, Cr3+, Mn2+,
Fig. 1. Synthesis of europium(III) complex of 1,10-phenanthroline-2-carboxylic (EuL).
835
R. Bai et al. / Optical Materials 35 (2013) 833–836
I 618 nm
0
3000
-1.2 1000 0
3+
Fe
2000
R=0.99994
2000
0
50
100
-1.4
150
equiv of Fe3+
I0/(I-I0)
Fluorescence intensity
-1.0
3000
J=2
J=1 170 eq.
1000
-1.6
-1.8
J=4 -2.0
J=3 0 600
650
700
0
0.2
0.4
0.6
Wavelength/nm
[Fe ] (10 M )
(A)
(B)
3+ -1
4
0.8
1.0
-1
Fig. 4. Emission spectra of EuL (10 lM) with various amounts of Fe3+ (0–170 equiv.) (kex = 283 nm). Inset: titration curve of EuL with Fe3+ (0–17 lM).
Emission Intensity
3000 2500 2000 1500 1000 500
L Eu
+
2+
Fe
N a+ N 2 i + Zn 2
+
K
C 3 r + M n 2+
2+
2+
2+
d
C o C u
C
g 2+ C a 2+
M
Fe
3+
0
Fig. 5. Fe3+-selectivity profile of EuL in methanol–water(V:V, 5:1): (black bars) emission intensity of EuL + 100 equiv. Mn+; (gray bars) emission intensity of EuL + 100 equiv. Mn+, followed by 100 equiv. Fe3+. Conditions: [EuL] = 10 lM; kex = 283 nm.
and Fe2+ also showed no interference. Other paramagnetic ions such as Cu2+, Co2+, and Ni2+ could not quench the fluorescence. Fig. 5 shows the dependence of the intensity at 618 nm on the cations. It is clear that the high selectivity for Fe3+ resulted from the presence of EuL. Competition experiments in which Fe3+ was mixed with other metal ions showed an obvious quenching in the fluorescence intensity. These results showed that EuL could be used to detect Fe3+. A control experiment was designed to explore the quenching mechanism. It is known that the reaction of potassium thiocyanate (KSCN) with free Fe3+ produces a blood-red complex Fe(SCN)3. Here, this complexation reaction was performed in the absence (a) and presence (b) of EuL (see Fig. 6). 0.1 mM Fe3+ in a methanol–and–water solution (V:V, 5:1) was added to the two cells (a) and (b). Cell (b) contained an additional 0.2 mM of EuL. The same amount of KSCN was then added to the two cells. Solution (a) immediately changed from colorless to red, while the color of the solution (b) showed no obvious change (Fig. 6.). This result indicated that the EuL–Fe3+ complex (lg b/L mol 1 = 3.93) was more stable than the Fe(SCN)2+ (lg b/L mol 1 = 2.94) [21] in aqueous solution, and that there were no free Fe3+ ions in solution (b).
Emission Intensity
10000
EuL 3+ EuL+Fe 3+ EuL+Fe +KSCN
8000
6000
4000
2000
0 550
600
650
700
750
Wavelength/nm
(A)
(B)
Fig. 6. (A) The photograph of the control experiment: (a) 0.1 mM Fe3+ + 0.3 mM KSCN; (b) 0.1 mM Fe3+ + 0.2 mM EuL + 0.3 mM KSCN. (B) The corresponding emission spectra of experiment (A).
836
R. Bai et al. / Optical Materials 35 (2013) 833–836
The corresponding emission spectra were recorded simultaneously; the results are shown in Fig. 6B, and indicated that the addition of KSCN to EuL–Fe3+ did not cause any change in the emission. Due to the paramagnetic quenching of high concentrations of Fe3+, the addition of 0.5 equiv. of Fe3+ led to complete fluorescence quenching of EuL. Since the other transition metal ions such as Zn2+, Cd2+, and Cu2+ have a similar affinity to N-phen [22–24], the quenching is not related to Mn+-N bond affinity. Furthermore, a reviewer has provided a possible explanation for the selective quenching. The 4T1(G) and 4T2(G) energy levels of Fe3+ are located roughly at 9500 cm 1 and 14,500 cm 1, respectively. Since 5D0 is at 17,200 cm 1, and the 7FJ levels of Eu3+ extend above 5000 cm 1, the cross-relaxation nonradiative deactivation of 5D0 to 7FJ leads to the population of 4T2(G) and the quenching of visible emission. 4. Conclusions A novel Fe3+ fluorescent sensor was developed based on a lanthanide luminescent switch. The sensor exhibited a high selectivity for Fe3+, compared with other metal ions examined. We believe that the response mechanism was due to the energy transfer from Eu3+ to Fe3+. These studies provide a foundation for the development of fluorescent switches and sensors using lanthanide ions. Acknowledgments The authors gratefully acknowledge the beneficial discussion with Prof. Yangfang, Fan, and the financial support of this work by the National Natural Science Foundation of China (Nos. 20601018, 21071092), the Shanxi Provincial Natural Science Foundation for Youth (No. 2009021006-3) and Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi (TYAL). References [1] C. Brugnara, Iron deficiency and erythropoiesis: new diagnostic approaches, Clin. Chem. 49 (2003) 1573–1578. [2] L. Jing, C. Liang, X.H. Shi, S.Q. Ye, Y.Z. Xian, Fluorescent probe for Fe(III) based on pyrene grafted multiwalled carbon nanotubes by click reaction, Analyst 137 (2012) 1718–1722. [3] M. Kumar, R. Kumar, V. Bhalla, P.R. Sharma, T. Kaurb, Y. Qurishi, Thiacalix[4]arene based fluorescent probe for sensing and imaging of Fe3+ ions, Dalton Trans. 41 (2012) 408–412. [4] W.T. Yin, H. Cui, Z. Yang, C. Li, M.Y. She, B. Yin, J.L. Li, G.F. Zhao, Z. Shi, Facile synthesis and characterization of rhodamine-based colorimetric and ‘‘off on’’ fluorescent chemosensor for Fe3+, Sensor Actuat. B – Chem. 157 (2011) 675– 680.
[5] D.P. Kenndy, C.D. Incarvilo, S.C. Burdette, FerriCast: a macrocyclic photocage for Fe3+, Inorg. Chem. 49 (2010) 916–923. [6] X.B. Zhang, G. Cheng, W.J. Zhang, G.L. Shen, R.Q. Yu, A fluorescent chemical sensor for Fe3+ based on blocking of intramolecular proton transfer of a quinazolinone derivative, Talanta 71 (2007) 171–177. [7] K. Suganandam, P. Santhosh, M. Sankarasubramanian, A. Gopalan, T. Vasudevan, K.P. Lee, Fe3+ ion sensing characteristics of polydiphenylamineelectrochemical and spectroelectrochemical analysis, Sensor Actuat. B – Chem. 105 (2005) 223–231. [8] D.Y. Jin, Background-free cytometry using rare earth complex bioprobes, Methods Cell Biol. 102 (2011) 479–513. [9] H. John, S. Schulz, W.G. Forssmann, L. Standker, Time-resolved fluorometric assay for the detection of endostatin in chromatographically separated extracts of natural peptides, J. Immunol. Methods 268 (2002) 233–237. [10] B. Li, Q.Y. Chen, Y.C. Wang, J. Huang, Y. Li, Synthesis, characterization and fluorescence studies of a novel europium complex based sensor, J. Lumin. 130 (2010) 2413–2417. [11] L.M. Zhang, B. Li, Z.M. Su, S.M. Yue, Novel rare-earth(III)-based water soluble emitters for Fe(III) detection, Sensor Actuat. B – Chem. 143 (2010) 595–599. [12] D.Y. Liu, K.Z. Tang, W.S. Liu, C.Y. Su, X.H. Yan, M.Y. Tan, Y. Tang, A novel luminescent chemosensor for detecting Hg2+ based on the pendant benzo crown ether terbium complex, Dalton Trans. 39 (2010) 9763–9765. [13] M.L. Cable, D.J. Levine, J.P. Kirby, H.B. Gray, A. Ponce, Luminescent lanthanide sensors, Adv. Inorg. Chem. 63 (2011) 1–45. [14] Y. Li, F.K. Zheng, X. Liu, W.Q. Zou, G.C. Guo, C.Z. Lu, J.S. Huang, Crystal structures and magnetic and luminescent properties of a series of homodinuclear lanthanide complexes with 4-cyanobenzoic ligand, Inorg. Chem. 45 (2006) 6308–6316. [15] T. Fiedler, M. Hilder, P.C. Junk, U.H. Kynast, M.M. Lezhnina, M. Warzala, Synthesis, structural and spectroscopic studies on the lanthanoid paminobenzoates and derived optically functional polyurethane composites, Eur. J. Inorg. Chem. 2 (2007) 291–301. [16] A.P. Robert, B. Gabriell, J.C. Martin, C.F. Juan, P. David, D.P. Robert, Synthesis and characterisation of highly emissive and kinetically stable lanthanide complexes suitable for usage ‘in cellulo’, Org. Biomol. Chem. 3 (2005) 1013– 1024. [17] G.B. Deacon, R.J. Phillips, Relationships between the carbon-oxygen stretching frequencies of carboxylate complexes and the type of carboxylate coordination, Coord. Chem. Rev. 33 (1980) 227. [18] G.D. Qian, M.Q. Wang, S.Z. Lv, S.H. Huang, Synthesis, characterization and fluorescence of Eu3+, Tb3+ complexes with heterocyclic ligands containing nitrogen, Chinese J. Lumin. 19 (1998) 60–65. [19] G. Lourdes, F. Mari9 a-José, B.G. Kathryn, L. Antonio, Syntheses and copper(II)dependent DNA photocleavage by acridine and anthracene 1,10phenanthroline conjugate systems, Org. Biomol. Chem. 3 (2005) 1856–1862. [20] H.A. Benesi, J.H. Hildebrand, The Benesi–Hildebrand method for determination of Kf for DA association and e values for DA CT absorption, J. Am. Chem. Soc. 71 (1949) 2703. [21] A. Bahta, G.A. Parker, D.G. Tuck, Critical evaluation of stability constants of metal complexes in solution, Pure Appl. Chem. 69 (1997) 1489–1548. [22] J. Hernandez-Gil, S.F. Llusar, C.R. Maldonado, J.C. Mareque-Rivas, Synergy between quantum dots and 1,10-phenanthroline-copper(II) complex towards cleaving DNA, Chem. Commun. 47 (2011) 2955–2957. [23] H.L. Chen, W. Gao, M.L. Zhu, H.F. Gao, J.F. Xue, Y.Q. Li, A highly selective off–on fluorescent sensor for zinc in aqueous solution and living cells, Chem. Commun. 46 (2010) 8389–8391. [24] A. Benvidi, S.N. Lanjwani, Z.F. Ding, Cd2+ transfer across water/1,2dichloroethane microinterfaces facilitated by complex formation with 1,10Phenanthroline, Electrochim. Acta 55 (2010) 2196–2200.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
A novel fluorescent Fe3+ sensor based on a europium complex Ruichun Bai a, Wei Gao b, Shengdi Bai c,⇑, Fengling Yang d, Huili Chen a,e,⇑ a
Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Shanxi University, Taiyuan 030006, PR China Department of Chemistry, Changzhi University, Shanxi, Changzhi 046011, PR China c Research Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, PR China d Institute of Resources and Environment Engineering, Shanxi University, Taiyuan 030006, PR China e State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China b
a r t i c l e
i n f o
Article history: Received 19 June 2012 Received in revised form 11 October 2012 Accepted 12 October 2012 Available online 28 November 2012 Keywords: Fe(III) sensor Rare earth complex Fluorescent sensor
a b s t r a c t A novel Eu3+ complex (EuL) based on 1,10-phenanthroline-2-carboxylic was successfully synthesized, and gave a characteristic red emission. The complex was shown to act as a selective luminescence quencher for Fe3+, as shown by the ‘‘on/off’’ switch phenomenon. The quenching curve showed a double-exponential well decay with the increase of Fe3+ ions. The stability constant of EuL/Fe3+ was calculated as 8.6 103 L mol 1(lg b/L mol 1 = 3.93). The response showed high selectivity for Fe3+ compared with other metal ions; the complex therefore has the potential to be applied as a fluorescent sensor for Fe3+. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Iron is an essential element for the human body, and for plant growth. However, excessive amounts of iron can do great harm to the body. Iron metabolism disorders have been reported to cause anemia, as well as liver and kidney damage that might ultimately cause liver cancer, liver cirrhosis, arthritis, diabetes, or heart failure [1]. Hence, the detection of iron ions has always been an important consideration in the environmental protection and human health fields. At present, a variety of methods exist for the detection of iron ions. Fluorescent detection has been used in a wide range of applications [2–7]. Most sensors are based on organic compounds. However, organic compounds typically hold some disadvantages as fluorescent emitters, including short fluorescence lifetimes, and photobleaching. Rare earth complexes typically have a longer fluorescence lifetime, and can be used in time-resolved fluorescent detection [8,9]. Recently, a number of fluorescent sensors based on rare earth complexes have been reported [10–13]. The coordination of aromatic carboxylic acids towards rare earth complexes has received considerable attention, due to the strong coordination abilities and variety of bridging modes of the carboxylate group for the construction of metal-organic framework (MOF) structures [14,15]. With their higher luminous efficiency and narrower spec⇑ Corresponding authors. Address: Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Shanxi University, Taiyuan 030006, PR China (H. Chen). Tel.: +86 3517010699. E-mail addresses: [email protected] (H. Chen), [email protected] (S. Bai). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.10.030
trum width, rare earth complexes have significant potential for applications. In this paper, a novel Eu3+ complex based on a phenanthroline carboxylic acid derivative (EuL) was developed to detect Fe3+ via the fluorescence method. The complex emitted strong fluorescence in the red region of the spectrum when solved in methanol–water (V:V, 5:1). With the incremental addition of Fe3+, the fluorescence of the EuL complex was quenched gradually. The response showed high selectivity for Fe3+ compared with other metal ions, and could be used to detect Fe3+.
2. Experimental details 2.1. Reagents and instruments ESI-MS spectra were measured on a Finnigan LCQ system. UV–Vis spectra were recorded on a Hewlett Packard HP-8453 spectrophotometer. Thermal analysis was carried out with HCT-1/2 TGDTA analyzer (from HENVEN.Co, Beijing) using 3–5 mg samples that were heated at a rate of 10 K/min from ambient to 973 K in air. Luminescence spectra were recorded on Hitachi F-4600 fluorescence spectrophotometer. Both the excitation and emission slits are 2.5 nm. All experiments were carried out in methanol–water (V:V, 5:1) in pH 6.2. All solvents were purchased from Tianjin Fengchuan Chemical Reagent Technology Co. Ltd. 2-Methyl-1,10-phenanthroline was synthesized according to literature [16]. 2-Carboxyl-1,10-phenanthroline was obtained
834
R. Bai et al. / Optical Materials 35 (2013) 833–836
through two steps by the oxidation of SeO2 and concentrated HNO3 [17] .
L EuL
60
2.2. Synthesis and characterization of europium(III) complex of 1,10phenanthroline-2-carboxylic (EuL) 40
T%
EuCl36H2O (0.1 m mol) in water was added into 0.2 m mol 2carboxyl-1,10-phenanthroline in methanol–H2O–NaOH. The solution was refluxed for 4 h and cooled to room temperature. The result precipitation was collected and washed with water and ethanol to give the titled complex. Yield, 79%. Elemental Analysis Calcd for C26H22ClEuN4O8: C: 44.24; H: 3.14; N: 7.94. Found: C: 44.46; H: 3.25; N: 8.57 (see Fig. 1).
ν (COO-)
ν (COOH)
s
20
ν (COO-)
as
0
3. Results and discussion
1200
1800
Fig. 2. IR spectra of L and EuL.
100
Exo
743 K loss 10.4%
80 60
DTA
TG W/%
40 loss 75.1%
20
Endo
Fig. 2 shows FTIR absorption spectra for L and EuL. For L, a carbonyl group (ACOOH) stretching vibration peak appeared at 1746 cm 1. The hydroxyl bending vibration peaks appeared at 1210 cm 1 and 871 cm 1. These characteristic mCOOH peaks disappeared when EuL was formed. The asymmetric and symmetric stretching vibrations of the carboxylic groups in EuL appeared at 1631 cm 1 and 1398 cm 1, respectively. The difference between the two peaks was more than 200 cm 1, indicating that one oxygen atom in the carboxylic group participated in the coordination with Eu [18]. The C@N stretching vibration peak for L at 1616 cm 1 shifted to the low frequency region after the coordination with Eu, which indicated that the bonds were formed between Eu and the nitrogen atoms of L [19]. The thermal stability and structure of EuL were examined using TG and DTA analysis. The curves are shown in Fig. 3. There were three weight loss peaks in the curve. The DTA curve showed a weak endothermic peak at 423 K, and a strong exothermic peak at 743 K. From the TG curve, the first decomposition process involved the loss of 4 molecules of water at 423 K, with a mass loss of 10.4% (Cal. 10.2%). The second stage consisted of the loss and combustion of the ligand L, accompanied by a violent exothermic process. The final residue was Eu2O3, with a final mass loss of 74.5% (Cal. 75.1%). In methanol–water (V:V, 5:1), the UV-Vis absorbance spectrum for EuL showed strong absorbance at 231 nm (e = 3.56 104 cm 1 mol 1) and 283 nm (e = 3.54 104 cm 1 mol 1) in the UV region, which was ascribed to the p–p transition of 2-carboxyl-1,10-phenanthroline. Under these conditions, the excitation in the phen absorption band at 283 nm caused the well known, structured emission of the Eu3+ ion, confirming that the energy transfer from the ligand-centered levels to the metal-centered levels took place. The relatively high intensity of the 5D0 ? 7F2 transition indicated that the Eu3+ ion did not lie in a centrosymmetric coordinate site. The complex showed a striking red luminescence. Upon the addition of Fe3+, a rapid decrease in intensity of the whole emission band occurred (saturated at 10 lM Fe3+ with a 60-fold decrease), and the fluorescent color of the solution turned
1500
Wavenumbers (cm-1)
0 423 K
400
600
800
T/K Fig. 3. TG-DTA curves of EuL.
from red to colorless. A detailed spectrofluorometric titration was performed (Fig. 4A). The titration curve at 618 nm fitted well a fluorescence double exponential decay. The effect stability constant (b) was obtained using the Hildebrand–Bebesi equation [20]. A linear least-squares curve fitting of the plot with I0/(I I0) versus [Fe3+] 1 indicated a 1:1 binding model between the Eu3+ complex and the Fe3+ ion (Fig. 4B). The lg b/L mol 1 value was 3.93. The fluorescent titration by other metal ions from chloride salts revealed excellent selectivity for Fe3+. For example, physiologically important metal ions that exist in living cells, such as Ca2+, Mg2+, Na+, and K+, did not give any response at a 100-fold excess concentration. Heavy and transition metal ions such as Cd2+, Cr3+, Mn2+,
Fig. 1. Synthesis of europium(III) complex of 1,10-phenanthroline-2-carboxylic (EuL).
835
R. Bai et al. / Optical Materials 35 (2013) 833–836
I 618 nm
0
3000
-1.2 1000 0
3+
Fe
2000
R=0.99994
2000
0
50
100
-1.4
150
equiv of Fe3+
I0/(I-I0)
Fluorescence intensity
-1.0
3000
J=2
J=1 170 eq.
1000
-1.6
-1.8
J=4 -2.0
J=3 0 600
650
700
0
0.2
0.4
0.6
Wavelength/nm
[Fe ] (10 M )
(A)
(B)
3+ -1
4
0.8
1.0
-1
Fig. 4. Emission spectra of EuL (10 lM) with various amounts of Fe3+ (0–170 equiv.) (kex = 283 nm). Inset: titration curve of EuL with Fe3+ (0–17 lM).
Emission Intensity
3000 2500 2000 1500 1000 500
L Eu
+
2+
Fe
N a+ N 2 i + Zn 2
+
K
C 3 r + M n 2+
2+
2+
2+
d
C o C u
C
g 2+ C a 2+
M
Fe
3+
0
Fig. 5. Fe3+-selectivity profile of EuL in methanol–water(V:V, 5:1): (black bars) emission intensity of EuL + 100 equiv. Mn+; (gray bars) emission intensity of EuL + 100 equiv. Mn+, followed by 100 equiv. Fe3+. Conditions: [EuL] = 10 lM; kex = 283 nm.
and Fe2+ also showed no interference. Other paramagnetic ions such as Cu2+, Co2+, and Ni2+ could not quench the fluorescence. Fig. 5 shows the dependence of the intensity at 618 nm on the cations. It is clear that the high selectivity for Fe3+ resulted from the presence of EuL. Competition experiments in which Fe3+ was mixed with other metal ions showed an obvious quenching in the fluorescence intensity. These results showed that EuL could be used to detect Fe3+. A control experiment was designed to explore the quenching mechanism. It is known that the reaction of potassium thiocyanate (KSCN) with free Fe3+ produces a blood-red complex Fe(SCN)3. Here, this complexation reaction was performed in the absence (a) and presence (b) of EuL (see Fig. 6). 0.1 mM Fe3+ in a methanol–and–water solution (V:V, 5:1) was added to the two cells (a) and (b). Cell (b) contained an additional 0.2 mM of EuL. The same amount of KSCN was then added to the two cells. Solution (a) immediately changed from colorless to red, while the color of the solution (b) showed no obvious change (Fig. 6.). This result indicated that the EuL–Fe3+ complex (lg b/L mol 1 = 3.93) was more stable than the Fe(SCN)2+ (lg b/L mol 1 = 2.94) [21] in aqueous solution, and that there were no free Fe3+ ions in solution (b).
Emission Intensity
10000
EuL 3+ EuL+Fe 3+ EuL+Fe +KSCN
8000
6000
4000
2000
0 550
600
650
700
750
Wavelength/nm
(A)
(B)
Fig. 6. (A) The photograph of the control experiment: (a) 0.1 mM Fe3+ + 0.3 mM KSCN; (b) 0.1 mM Fe3+ + 0.2 mM EuL + 0.3 mM KSCN. (B) The corresponding emission spectra of experiment (A).
836
R. Bai et al. / Optical Materials 35 (2013) 833–836
The corresponding emission spectra were recorded simultaneously; the results are shown in Fig. 6B, and indicated that the addition of KSCN to EuL–Fe3+ did not cause any change in the emission. Due to the paramagnetic quenching of high concentrations of Fe3+, the addition of 0.5 equiv. of Fe3+ led to complete fluorescence quenching of EuL. Since the other transition metal ions such as Zn2+, Cd2+, and Cu2+ have a similar affinity to N-phen [22–24], the quenching is not related to Mn+-N bond affinity. Furthermore, a reviewer has provided a possible explanation for the selective quenching. The 4T1(G) and 4T2(G) energy levels of Fe3+ are located roughly at 9500 cm 1 and 14,500 cm 1, respectively. Since 5D0 is at 17,200 cm 1, and the 7FJ levels of Eu3+ extend above 5000 cm 1, the cross-relaxation nonradiative deactivation of 5D0 to 7FJ leads to the population of 4T2(G) and the quenching of visible emission. 4. Conclusions A novel Fe3+ fluorescent sensor was developed based on a lanthanide luminescent switch. The sensor exhibited a high selectivity for Fe3+, compared with other metal ions examined. We believe that the response mechanism was due to the energy transfer from Eu3+ to Fe3+. These studies provide a foundation for the development of fluorescent switches and sensors using lanthanide ions. Acknowledgments The authors gratefully acknowledge the beneficial discussion with Prof. Yangfang, Fan, and the financial support of this work by the National Natural Science Foundation of China (Nos. 20601018, 21071092), the Shanxi Provincial Natural Science Foundation for Youth (No. 2009021006-3) and Program for the Top Young Academic Leaders of Higher Learning Institutions of Shanxi (TYAL). References [1] C. Brugnara, Iron deficiency and erythropoiesis: new diagnostic approaches, Clin. Chem. 49 (2003) 1573–1578. [2] L. Jing, C. Liang, X.H. Shi, S.Q. Ye, Y.Z. Xian, Fluorescent probe for Fe(III) based on pyrene grafted multiwalled carbon nanotubes by click reaction, Analyst 137 (2012) 1718–1722. [3] M. Kumar, R. Kumar, V. Bhalla, P.R. Sharma, T. Kaurb, Y. Qurishi, Thiacalix[4]arene based fluorescent probe for sensing and imaging of Fe3+ ions, Dalton Trans. 41 (2012) 408–412. [4] W.T. Yin, H. Cui, Z. Yang, C. Li, M.Y. She, B. Yin, J.L. Li, G.F. Zhao, Z. Shi, Facile synthesis and characterization of rhodamine-based colorimetric and ‘‘off on’’ fluorescent chemosensor for Fe3+, Sensor Actuat. B – Chem. 157 (2011) 675– 680.
[5] D.P. Kenndy, C.D. Incarvilo, S.C. Burdette, FerriCast: a macrocyclic photocage for Fe3+, Inorg. Chem. 49 (2010) 916–923. [6] X.B. Zhang, G. Cheng, W.J. Zhang, G.L. Shen, R.Q. Yu, A fluorescent chemical sensor for Fe3+ based on blocking of intramolecular proton transfer of a quinazolinone derivative, Talanta 71 (2007) 171–177. [7] K. Suganandam, P. Santhosh, M. Sankarasubramanian, A. Gopalan, T. Vasudevan, K.P. Lee, Fe3+ ion sensing characteristics of polydiphenylamineelectrochemical and spectroelectrochemical analysis, Sensor Actuat. B – Chem. 105 (2005) 223–231. [8] D.Y. Jin, Background-free cytometry using rare earth complex bioprobes, Methods Cell Biol. 102 (2011) 479–513. [9] H. John, S. Schulz, W.G. Forssmann, L. Standker, Time-resolved fluorometric assay for the detection of endostatin in chromatographically separated extracts of natural peptides, J. Immunol. Methods 268 (2002) 233–237. [10] B. Li, Q.Y. Chen, Y.C. Wang, J. Huang, Y. Li, Synthesis, characterization and fluorescence studies of a novel europium complex based sensor, J. Lumin. 130 (2010) 2413–2417. [11] L.M. Zhang, B. Li, Z.M. Su, S.M. Yue, Novel rare-earth(III)-based water soluble emitters for Fe(III) detection, Sensor Actuat. B – Chem. 143 (2010) 595–599. [12] D.Y. Liu, K.Z. Tang, W.S. Liu, C.Y. Su, X.H. Yan, M.Y. Tan, Y. Tang, A novel luminescent chemosensor for detecting Hg2+ based on the pendant benzo crown ether terbium complex, Dalton Trans. 39 (2010) 9763–9765. [13] M.L. Cable, D.J. Levine, J.P. Kirby, H.B. Gray, A. Ponce, Luminescent lanthanide sensors, Adv. Inorg. Chem. 63 (2011) 1–45. [14] Y. Li, F.K. Zheng, X. Liu, W.Q. Zou, G.C. Guo, C.Z. Lu, J.S. Huang, Crystal structures and magnetic and luminescent properties of a series of homodinuclear lanthanide complexes with 4-cyanobenzoic ligand, Inorg. Chem. 45 (2006) 6308–6316. [15] T. Fiedler, M. Hilder, P.C. Junk, U.H. Kynast, M.M. Lezhnina, M. Warzala, Synthesis, structural and spectroscopic studies on the lanthanoid paminobenzoates and derived optically functional polyurethane composites, Eur. J. Inorg. Chem. 2 (2007) 291–301. [16] A.P. Robert, B. Gabriell, J.C. Martin, C.F. Juan, P. David, D.P. Robert, Synthesis and characterisation of highly emissive and kinetically stable lanthanide complexes suitable for usage ‘in cellulo’, Org. Biomol. Chem. 3 (2005) 1013– 1024. [17] G.B. Deacon, R.J. Phillips, Relationships between the carbon-oxygen stretching frequencies of carboxylate complexes and the type of carboxylate coordination, Coord. Chem. Rev. 33 (1980) 227. [18] G.D. Qian, M.Q. Wang, S.Z. Lv, S.H. Huang, Synthesis, characterization and fluorescence of Eu3+, Tb3+ complexes with heterocyclic ligands containing nitrogen, Chinese J. Lumin. 19 (1998) 60–65. [19] G. Lourdes, F. Mari9 a-José, B.G. Kathryn, L. Antonio, Syntheses and copper(II)dependent DNA photocleavage by acridine and anthracene 1,10phenanthroline conjugate systems, Org. Biomol. Chem. 3 (2005) 1856–1862. [20] H.A. Benesi, J.H. Hildebrand, The Benesi–Hildebrand method for determination of Kf for DA association and e values for DA CT absorption, J. Am. Chem. Soc. 71 (1949) 2703. [21] A. Bahta, G.A. Parker, D.G. Tuck, Critical evaluation of stability constants of metal complexes in solution, Pure Appl. Chem. 69 (1997) 1489–1548. [22] J. Hernandez-Gil, S.F. Llusar, C.R. Maldonado, J.C. Mareque-Rivas, Synergy between quantum dots and 1,10-phenanthroline-copper(II) complex towards cleaving DNA, Chem. Commun. 47 (2011) 2955–2957. [23] H.L. Chen, W. Gao, M.L. Zhu, H.F. Gao, J.F. Xue, Y.Q. Li, A highly selective off–on fluorescent sensor for zinc in aqueous solution and living cells, Chem. Commun. 46 (2010) 8389–8391. [24] A. Benvidi, S.N. Lanjwani, Z.F. Ding, Cd2+ transfer across water/1,2dichloroethane microinterfaces facilitated by complex formation with 1,10Phenanthroline, Electrochim. Acta 55 (2010) 2196–2200.