Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 7–11
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Highly selective and sensitive fluorescence chemosensor for the detection of palladium species based on Tsuji–Trost reaction Zhong-Yong Xu a, Jing Li a, Su Guan a, Lei Zhang a,⇑, Chang-Zhi Dong b a b
School of Bioscience and Bioengineering, South China University, Guangzhou 510006, PR China Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baïf, 75205 Paris Cedex 13, France
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We have designed a highly selective
and sensitive chemosensor for palladium. Fluorescence intensity of NBDTC shows a large enhancement with turn-on over 50-fold in PEG400 under palladium species. NBDTC has lower detection limit of 1.13 10 9 M based on Tsuji–Trost reaction.
a r t i c l e
i n f o
Article history: Received 1 December 2014 Received in revised form 11 March 2015 Accepted 30 March 2015 Available online 4 April 2015 Keywords: Palladium Fluorescence sensor NBD Tsuji–Trost reaction
a b s t r a c t A new chemosensor 7-nitro-2,1,3-benzoxadiazole-4-allyl-N-(thiophen-2-ylmethyl)carbamate (NBDTC) was synthesized and utilized for palladium detection based on the Tsuji–Trost reaction. NBDTC displayed specific and ratiometric fluorescent responses toward palladium species. The chemosensor showed more than 50-fold enhancement in fluorescence intensity with the presence of PEG400 and palladium because NBDTC can be transformed to NBDT under palladium-catalyzing Tsuji–Trost reaction. NBDTC displayed high selectivity and sensitivity for palladium species with the detection limit of 1.13 10 9 M. Ó 2015 Published by Elsevier B.V.
Introduction Palladium is an important inert element in the platinum group elements, which is widely used in various materials such as dental crowns, catalysts, medicinal devices, automobile and so on [1–4]. In the fine chemical and pharmaceutical industries, the palladium species are widely used as catalysts in directed C–H functionalization in the synthesis of various drugs and fine chemicals [5–7]. Because palladium species have an influence on human health as a thiophilic element in an adverse way, the simple and reliable ⇑ Corresponding author at: School of Bioscience and Bioengineering, South China University of Technology, Guangzhou 510006, PR China. Tel.: +86 20 39380678. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.saa.2015.03.130 1386-1425/Ó 2015 Published by Elsevier B.V.
detection methods for palladium species have attracted tremendous attentions in the quantitative analyses of palladium residues in the drugs [8–10]. Among the known methods including atomic absorption spectrometry, plasma emission spectroscopy, solid phase micro-extraction high performance liquid chromatography, X-ray fluorescence, fluorescence probe, etc [11–13], fluorescent chemosensors for palladium species with high sensitivity and selectivity have been reported [14–20], most of reported fluorescence sensors are based on naphthalimide, anthracenophane, boradiazaindacene and rhodamine derivatives and feature high selectivity (Pd2+ or Pd0) and sensitivity (5 nM). In these systems, most of the fluorescence is enhanced or weakened (turn-on/turnoff) through the coordination of Pd2+ and chelate fluorescence sensors, and a new turn-on fluorescent system was recently developed
Z.-Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 7–11
400
400
DEG TEG PEG200 PEG400 PEG600
350
350 300
Intensity(a.u)
for the simultaneous detection of palladium species (Pd2+ or Pd0) [21]. Herein, we have developed a new ratiometric fluorescent chemosensor, 7-nitro-2,1,3-benzoxadiazole-4-allyl-N-(thiophen2-ylmethyl)carbamate (NBDTC) for palladium detection based on NBD (7-nitro-2,1,3-benzoxadiazole) derivative. 4-Amino-7-nitro2,1,3-benzoxadiazole (ANBD) is an excellent intramolecular charge transfer fluorophore with large Stokes shift, which can be applied in the molecular fluorescent sensors for small molecular and biological detections [22–29]. Under the presence of PEG400, palladium species can catalyze the departure of allyl formate based on Tsuji–Trost reaction principle [29]. By using this mechanism, the new chemosensor NBDTC can transform to NBDT with a large fluorescence enhancement over 50-fold under palladium species and PEG400.
Intensity(a.u)
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3:7
Experimental
4:6
5:5
6:4
7:3
8:2
9:1 10:0
PEG400:PBS(v:v)
General All solvents and reagents (analytical grade) were obtained commercially and used as received unless otherwise mentioned. NMR spectra were recorded on the Bruker 400 MHz instrument using TMS as an internal standard. MS spectra were recorded on the Agilent 100-SLzon Trep (Agilent, USA). UV absorption spectra were recorded on a UV-2600 UV–VIS spectrophotometer (Shimadzu, Japan). Fluorescence measurements were performed using an F-4500 fluorescence spectrophotometer (Hitachi, Japan) equipped with quartz cell of 1 cm path length.
Fig. 1. The influence of PEG400-PBS (0.1 M, pH = 7.40) buffers in different compositions. Insert spectra: the fluorescence intensity of NBDTC with the addition of different PEG series. The test conditions: NBDTC: 1lM, Pd2+:1 lM, in solutions incubated 2 h at 55 °C.
The chemosensor NBDTC was synthesized according to the route as shown in Scheme 1. The total yield was 46%.
(323 mg 2.5 mmol) were added using a syringe. After 5 min, the nucleophile allyl chloroformate (600 mg, 5 mmol) were added using a syringe. The reaction mixture was stirred for 12 h at room temperature under nitrogen gas. The reaction solution was filtered and the filtrate was evaporated under reduced pressure. The residue was chromatographed on silica gel with ethyl acetate– petroleum ether (1:9) to afford NBDTC (134 mg, 89%) as brow oils. ESI-MS: m/z 360.53 [M+H]+; 1H NMR: (400 MHz, CD3OD) d 8.57 (d, J = 7.9 Hz, Ha), 7.56 (d, J = 7.9 Hz, Hb), 7.27 (d, J = 5.1 Hz, Hf), 6.90 (d, J = 3.1 Hz, Hd), 6.85 (t, J = 4.1 Hz, He), 5.90 (m, J = 11.0, 5.8 Hz, Hh), 5.40 (s, Hc), 5.23 (dd, J = 19.1, 13.9 Hz, Hg), 4.70 (d, J = 5.6 Hz, Hi).
Synthesis of 7-nitro-N-(thiophen-2-ylmethyl)-2,1,3-benzoxadiazole4-amine (NBDT)
Results and discussion
Synthesis
4-Chloro-7-nitro-2,1,3-benzoxadiazole (NBD-Cl) (500 mg, 2.5 mmol) was dissolved in acetonitrile (30 mL). After the addition of 2-thiophenemethylamine (282 mg, 2.5 mmol), the solution was stirred at room temperature for 30 min. the reaction mixture was evaporated to dryness under reduced pressure, and the residue was chromatographed on silica gel with ethyl acetate–petroleum ether (2:8) to afford NBDT (329 mg, yield 52%) as brow crystals. ESI-MS: m/z 277.04 [M+H]+; 1H NMR: (400 MHz, CD3OD) d 8.50 (d, J = 8.7 Hz, Ha), 7.34 (d, J = 5.1 Hz, Hf), 7.16 (d, J = 2.6 Hz, Hd), 6.99 (t, J = 4.1 Hz, He), 6.43 (d, J = 8.8 Hz, Hb), 4.94 (s, Hc).
NO 2 N
N O N Cl
N NH2
HN
S
N
O
N
O
(II)
(I)
O
b
Cl
O
h g
i
O
c d
S f
NBDT
NBDT NBDTC+Pd(0) NBDTC+Pd(II) NBDTC
350
250 200 150 100
N
a
O
S
400
Intensity(a.u)
Under a nitrogen atmosphere, compound NBDT (138 mg, 0.5 mmol) was dissolved in anhydrous THF (25 mL) and DIEA
NO2
For the previous chemosensor system, tris(2-furyl)phosphine (TFP) or beta-4-platinum were used as palladium ligands for the Tsuji–Trost reaction [31]. Their poor solubilities in aqueous media
300
Synthesis of 7-nitro-2,1,3-benzoxadiazole-4-allyl-N-(thiophen-2ylmethyl) carbamate (NBDTC)
NO 2
Optimization of the experimental conditions
e
50 0 500
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560
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640
Wavelength/nm
NBDTC
Scheme 1. Synthesis of NBDTC. (I) acetonitrile, room temperature, 30 min, yield 52%; (II) anhydrous THF, DIEA, room temperature, 12 h, yield 89%.
Fig. 2. The fluorescence intensity of NBDT (1 lM), NBDTC (1 lM) and NBDTC (1 lM) with the addition different state of palladium (1 lM) in PEG400 solutions incubated 2 h at 55 °C.
9
Z.-Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 7–11
1.4
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NBDT NBDTC
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2+
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Fig. 3. Absorption spectra of NBDT and NBDTC (50 lM); (a) in DMSO and (b) in PEG400 within different concentrations of Pd2+ (0, 5, 10, 20, 30, 40, 50 lM) incubated for 2 h at 55 °C.
400
a
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Pd
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2+
2+
2+
Intensity(a.u)
2+
Mn , Co , Ba , Cu , + 2+ 2+ 2+ Ag , Hg , Mg , Ca , 3+ 2+ + 2+ Fe , Cd , K , Ni , + 2+ 2+ Na , Zn , Pb
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2+
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50
50
2+
Intensity(a.u)
b
350
Fig. 4. (a) Fluorescent spectra of probe NBDTC (1 lM) in the presence of various metal ions (1 lM) in PEG400 solutions incubated for 2 h at 55 °C. (b) The emission intensity at 526 nm.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2 3
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0 0.0
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1.6 2+
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-6
[Pd] /10 M
Fig. 5. (a) The fluorescence changes of NBDTC (1 lM) with the addition of palladium ion (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0, 3.0 equiv) in PEG400 solution incubated for 2 h at 55 °C. (b) Plot of emission intensity at 526 nm as a function of Pd2+ concentration.
Z.-Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 7–11
400 350
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300
-
HCO3
2-
HPO4
-
H2PO4
Cl
3+
Ions
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0
PO4
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-
50
Fe 2+ Pb 2+ Mn 2+ Cu 2+ Zn 2+ Mg 2+ Ni 2+ Co 2+ Cd
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-
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ClO4
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SO4
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250
2-
250
SO3
Intensity(a.u)
350
b
-
a
NBDTC 2+ Pd + K + Na + Ag 2+ Hg 2+ Ca 2+ Ba
Intensity(a.u)
400
NO3
10
Anions
Fig. 6. (a) Fluorescent intensities of the probe NBDTC (1 lM) to Pd2+ (1 lM) in the presence of (1 lM) other cations; (b) anions (1 lM) in PEG400 solutions incubated for 2 h at 55 °C.
were found to affect the sensor performance. Peng and co-workers found that PEG was more efficient for palladium-catalyzed coupling reactions than phosphine ligand besides it was non-toxic and widely used in biological application [20]. In our research system, PEG400 did exhibit better performance as solution and reductant than other PEG series shown in Fig. 1. In the previous literatures, phosphate-buffered saline buffer (PBS) was often used in biological system, Although PEG400-PBS buffers with different proportions were tested, the fluorescence intensity showed better performance in PEG400 as a sole solvent than PEG400-PBS mixed buffers with different proportions. In the PEG400 solution, the fluorescence intensity varied with temperature and incubation time. The optimization conditions were temperature at 55 °C and incubation time for 2 h (Figs. S1 and S2). For the Tsuji–Trost reaction system, it is well known that Pd(0) is the active catalytic species to promote the removal of the allyl formate. In the NBDTC system investigated, PEG400 showed the potency to reduce Pd2+ state to Pd0 state, which is also testified in the previous literatures[30]. When the Pd0 species was applied as the catalyst in the Tsuji–Trost reaction, the fluorescence intensity shown in Fig. 2 increased largely as found in Pd2+-PEG400 system. Therefore, in the PEG400 system, NBDTC became an important fluorescence chemosensor for both Pd2+ species and Pd0 species.
Optical absorption studies The optical absorption spectra of NBDT and NBDTC in DMSO solvent were recorded at room temperature in the range of 260–600 nm as shown in Fig. 3a. NBDT showed two strong absorption peaks at 338 (e = 8.82 103 M 1cm 1) and 473 nm (e = 2.16 104 M 1cm 1), in which the later was the typical absorption peak of ANBD. The chemosensor NBDTC showed also two absorption peaks at 278 nm (e = 1.44 104 M 1cm 1) and at 382 nm (e = 2.05 104 M 1cm 1). When NBDTC was titrated by Pd2+ ion in PEG400 solution, the absorption peaks at 382 and 278 nm gradually decreased, accompanied with two new absorption peaks at 338 and 473 nm gradually produced with the addition of Pd2+ as shown in Fig. 3b. The spectra obtained during the stepwise addition showed one isosbestic point indicates a nearly clean conversion of NBDTC into the corresponding NBDT. This result suggested that the chemosensor NBDTC can be converted into NBDT at the presence of Pd2+ in PEG400 solution.
Fluorescence study At first we have investigated the fluorescence properties of chemosensor NBDTC and its fluorescence response to various metal cations in PEG400 solvent. NBDTC itself shows negligible fluorescence at 526 nm upon exciting at 480 nm. However, the fluorescence intensity at 526 nm is strongly enhanced with the addition of 1 equiv of Pd2+. The native fluorescence of NBDTC is minimal, and the fluorescence intensity increases and reaches a maximum on the addition of equivalent molecule of Pd2+ ion as shown in Fig. 4. The fluorescence intensities increase from 7.6 to 385 upon the addition of equivalent molecule of Pd2+, which indicates 50-fold enhancement in fluorescence intensity. No fluorescence response occurred for Mn2+, Co2+, Ba2+ Cu2+, Ag+, Hg2+, Mg2+, Ca2+, Fe3+, Cd2+, K+, Ni2+, Na+, Zn2+ and Pb2+ ions. Then, the results indicate that NBDTC is a reliable highly selective chemosensor for Pd2+ ion. The responsive properties of chemosensor NBDTC to palladium ion were further examined by fluorescence titration in PEG400 solution (Fig. 5a). No more further change is observed upon increasing Pd2+ concentrations up to 1 equiv (Fig. 5b). It can be found that the fluorescence intensities were linearly proportional to Pd2+ concentrations in the range of 0.1–0.9 equivalent molecule, which is an important characteristic of chemosensors (Fig. S3). The detection limit for Pd2+ was deduced to be 1.13 10 9 M with a correlation coefficient of 0.9926, thus providing an efficient system for monitoring traces of palladium in environmental and pharmaceutical samples. In order to further explore the selectivity towards a matrix containing other competitive stimuli, the fluorescence chemosensor NBDTC exhibits an excellent selectivity for Pd2+ ion at the presence of 1 equiv concentrations of other metal ions and various anions including SO24 , ClO4 , PO34 , HPO24 , HCO3 , NO3 , Cl , H2PO4 and SO23 (Fig. 6). Although the fluorescence intensities is slightly weaker for Co2+, Hg2+ and Ag+ than those of sole Pd2+ ion, the lowest fluorescence intensity is 259 for Co2+, which is 34 folds enhancement compared to that of NBDTC chemosensor. These results further demonstrate that the chemosensor NBDTC can specifically recognize Pd2+ ion with high selectivity. Conclusions In conclusion, we have successfully developed a new ‘‘turn-on’’ fluorescence chemosensor NBDTC for palladium species based on
Z.-Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 7–11
the Tusji–Trost reaction in PEG400 solvent. NBDTC exclusively detects palladium species over a variety of other metal ions. Further, the visible detection of palladium species in PEG400 solution also exhibits high sensitivity. More importantly, in this sensing system, both Pd2+ and Pd0 could be detected at nM level, and this may develop a new method for improving the palladium detection in the environmental and pharmaceutical industries. Acknowledgments This work is supported by Guangdong Provincial Department of Science and Technology (Contract Grant No.: 2012A080800002) and Science and Information Technology of Guangzhou (2014J4100015). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.03.130. References [1] [2] [3] [4] [5] [6] [7]
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[8] T.Z. Liu, S.D. Lee, R.S. Bhatnagar, Toxicol. Lett. 4 (1979) 469. [9] J.S. Carey, D. Laffan, C. Thomson, M.T. Williams, Org. Biomol. Chem. 4 (2006) 2337. [10] C.D. Spicer, T. Triemer, B.G. Davis, J. Am. Chem. Soc. 134 (2012) 800. [11] B. Dimitrova, K. Benkhedda, E. Ivanova, F.J. Adams, Anal. Atom. Spectrom. 19 (2004) 1394. [12] C. Locatelli, D. Melucci, G. Torsi, Anal. Bioanal. Chem. 382 (2005) 1567. [13] K. Van Meel, A. Smekens, M. Behets, P. Kazandjian, R. Van Grieken, Anal. Chem. 79 (2007) 6383. [14] J.S. Kim, D.T. Quang, Chem. Rev. 107 (2007) 3780. [15] X. Li, X. Gao, W. Shi, H. Ma, Chem. Rev. 114 (2013) 590. [16] C.W. Rogers, M.O. Wolf, Coord. Chem. Rev. 233–234 (2002) 341. [17] L. Duan, Y. Xu, X. Qian, Chem. Commun. (2008) 6339. [18] H. Li, J. Fan, F. Song, H. Zhu, J. Du, S. Sun, X. Peng, Chem. Eur. J. 16 (2010) 12349. [19] S.G. Sun, B. Qiao, N. Jiang, J.T. Wang, S. Zhang, X.J. Peng, Org. Lett. 16 (2014) 1132. [20] J.Y. Wang, F.L. Song, J.Y. Wang, X. Peng, J. Analyst 138 (2013) 3667. [21] A.L. Garner, K. Koide, J. Am. Chem. Soc. 130 (2008) 16472. [22] K. Yoshioka, H. Takahashi, T. Homma, M. Saito, K.B. Oh, Y. Nemoto, H. Matsuoka, Biochim. Biophys. Acta 1289 (1) (1996) 5. [23] N. Nitin, A.L. Carlson, T. Muldoon, A.K. El-Naggar, A. Gillenwater, R. RichardsKortum, Int. J. Cancer 124 (11) (2009) 1. [24] R.J. Langsner, L.P. Middleton, J. Sun, F. Meric-Bernstam, K.K. Hunt, R.A. Drezek, T.K. Yu, Biomed. Opt. Express 2 (6) (2011) 1514. [25] S.R. Liu, S.P. Wu, J. Fluoresc. 21 (4) (2011) 1599. [26] C. Wei, Q. Zhu, W.W. Liu, W.B. Chen, Z. Xi, L. Yi, Org. Biomol. Chem. 12 (2014) 479. [27] Z.C. Xu, G.H. Kim, S.J. Han, M.J. Jou, C. Lee, I. Shin, J. Yoon, Tetrahedron 65 (2009) 2307. [28] W. Jiang, Q.Q. Fu, H.Y. Fan, W. Wang, Chem. Commun. (2008) 259. [29] F. Qian, C.L. Zhang, Y.M. Zhang, W.J. He, X. Gao, P. Hu, Z.J. Guo, J. Am. Chem. Soc. 131 (2009) 1460. [30] L. Nagarapu, R. Mallepalli, U. Nikhil Kumar, P. Venkateswarlu, R. Bantu, L. Yeramanchi, Tetrahedron Lett. 53 (2012) 1699. [31] B.M. Trost, D.L. Van Vranken, Chem. Rev. 96 (1996) 395.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Highly selective and sensitive fluorescence chemosensor for the detection of palladium species based on Tsuji–Trost reaction Zhong-Yong Xu a, Jing Li a, Su Guan a, Lei Zhang a,⇑, Chang-Zhi Dong b a b
School of Bioscience and Bioengineering, South China University, Guangzhou 510006, PR China Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baïf, 75205 Paris Cedex 13, France
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We have designed a highly selective
and sensitive chemosensor for palladium. Fluorescence intensity of NBDTC shows a large enhancement with turn-on over 50-fold in PEG400 under palladium species. NBDTC has lower detection limit of 1.13 10 9 M based on Tsuji–Trost reaction.
a r t i c l e
i n f o
Article history: Received 1 December 2014 Received in revised form 11 March 2015 Accepted 30 March 2015 Available online 4 April 2015 Keywords: Palladium Fluorescence sensor NBD Tsuji–Trost reaction
a b s t r a c t A new chemosensor 7-nitro-2,1,3-benzoxadiazole-4-allyl-N-(thiophen-2-ylmethyl)carbamate (NBDTC) was synthesized and utilized for palladium detection based on the Tsuji–Trost reaction. NBDTC displayed specific and ratiometric fluorescent responses toward palladium species. The chemosensor showed more than 50-fold enhancement in fluorescence intensity with the presence of PEG400 and palladium because NBDTC can be transformed to NBDT under palladium-catalyzing Tsuji–Trost reaction. NBDTC displayed high selectivity and sensitivity for palladium species with the detection limit of 1.13 10 9 M. Ó 2015 Published by Elsevier B.V.
Introduction Palladium is an important inert element in the platinum group elements, which is widely used in various materials such as dental crowns, catalysts, medicinal devices, automobile and so on [1–4]. In the fine chemical and pharmaceutical industries, the palladium species are widely used as catalysts in directed C–H functionalization in the synthesis of various drugs and fine chemicals [5–7]. Because palladium species have an influence on human health as a thiophilic element in an adverse way, the simple and reliable ⇑ Corresponding author at: School of Bioscience and Bioengineering, South China University of Technology, Guangzhou 510006, PR China. Tel.: +86 20 39380678. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.saa.2015.03.130 1386-1425/Ó 2015 Published by Elsevier B.V.
detection methods for palladium species have attracted tremendous attentions in the quantitative analyses of palladium residues in the drugs [8–10]. Among the known methods including atomic absorption spectrometry, plasma emission spectroscopy, solid phase micro-extraction high performance liquid chromatography, X-ray fluorescence, fluorescence probe, etc [11–13], fluorescent chemosensors for palladium species with high sensitivity and selectivity have been reported [14–20], most of reported fluorescence sensors are based on naphthalimide, anthracenophane, boradiazaindacene and rhodamine derivatives and feature high selectivity (Pd2+ or Pd0) and sensitivity (5 nM). In these systems, most of the fluorescence is enhanced or weakened (turn-on/turnoff) through the coordination of Pd2+ and chelate fluorescence sensors, and a new turn-on fluorescent system was recently developed
Z.-Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 7–11
400
400
DEG TEG PEG200 PEG400 PEG600
350
350 300
Intensity(a.u)
for the simultaneous detection of palladium species (Pd2+ or Pd0) [21]. Herein, we have developed a new ratiometric fluorescent chemosensor, 7-nitro-2,1,3-benzoxadiazole-4-allyl-N-(thiophen2-ylmethyl)carbamate (NBDTC) for palladium detection based on NBD (7-nitro-2,1,3-benzoxadiazole) derivative. 4-Amino-7-nitro2,1,3-benzoxadiazole (ANBD) is an excellent intramolecular charge transfer fluorophore with large Stokes shift, which can be applied in the molecular fluorescent sensors for small molecular and biological detections [22–29]. Under the presence of PEG400, palladium species can catalyze the departure of allyl formate based on Tsuji–Trost reaction principle [29]. By using this mechanism, the new chemosensor NBDTC can transform to NBDT with a large fluorescence enhancement over 50-fold under palladium species and PEG400.
Intensity(a.u)
8
250
300 250 200 150 100 50 0 500
525
200
550
575
600
625
650
Wavelength/nm
150 100 50 0
0:10 1:9
2:8
3:7
Experimental
4:6
5:5
6:4
7:3
8:2
9:1 10:0
PEG400:PBS(v:v)
General All solvents and reagents (analytical grade) were obtained commercially and used as received unless otherwise mentioned. NMR spectra were recorded on the Bruker 400 MHz instrument using TMS as an internal standard. MS spectra were recorded on the Agilent 100-SLzon Trep (Agilent, USA). UV absorption spectra were recorded on a UV-2600 UV–VIS spectrophotometer (Shimadzu, Japan). Fluorescence measurements were performed using an F-4500 fluorescence spectrophotometer (Hitachi, Japan) equipped with quartz cell of 1 cm path length.
Fig. 1. The influence of PEG400-PBS (0.1 M, pH = 7.40) buffers in different compositions. Insert spectra: the fluorescence intensity of NBDTC with the addition of different PEG series. The test conditions: NBDTC: 1lM, Pd2+:1 lM, in solutions incubated 2 h at 55 °C.
The chemosensor NBDTC was synthesized according to the route as shown in Scheme 1. The total yield was 46%.
(323 mg 2.5 mmol) were added using a syringe. After 5 min, the nucleophile allyl chloroformate (600 mg, 5 mmol) were added using a syringe. The reaction mixture was stirred for 12 h at room temperature under nitrogen gas. The reaction solution was filtered and the filtrate was evaporated under reduced pressure. The residue was chromatographed on silica gel with ethyl acetate– petroleum ether (1:9) to afford NBDTC (134 mg, 89%) as brow oils. ESI-MS: m/z 360.53 [M+H]+; 1H NMR: (400 MHz, CD3OD) d 8.57 (d, J = 7.9 Hz, Ha), 7.56 (d, J = 7.9 Hz, Hb), 7.27 (d, J = 5.1 Hz, Hf), 6.90 (d, J = 3.1 Hz, Hd), 6.85 (t, J = 4.1 Hz, He), 5.90 (m, J = 11.0, 5.8 Hz, Hh), 5.40 (s, Hc), 5.23 (dd, J = 19.1, 13.9 Hz, Hg), 4.70 (d, J = 5.6 Hz, Hi).
Synthesis of 7-nitro-N-(thiophen-2-ylmethyl)-2,1,3-benzoxadiazole4-amine (NBDT)
Results and discussion
Synthesis
4-Chloro-7-nitro-2,1,3-benzoxadiazole (NBD-Cl) (500 mg, 2.5 mmol) was dissolved in acetonitrile (30 mL). After the addition of 2-thiophenemethylamine (282 mg, 2.5 mmol), the solution was stirred at room temperature for 30 min. the reaction mixture was evaporated to dryness under reduced pressure, and the residue was chromatographed on silica gel with ethyl acetate–petroleum ether (2:8) to afford NBDT (329 mg, yield 52%) as brow crystals. ESI-MS: m/z 277.04 [M+H]+; 1H NMR: (400 MHz, CD3OD) d 8.50 (d, J = 8.7 Hz, Ha), 7.34 (d, J = 5.1 Hz, Hf), 7.16 (d, J = 2.6 Hz, Hd), 6.99 (t, J = 4.1 Hz, He), 6.43 (d, J = 8.8 Hz, Hb), 4.94 (s, Hc).
NO 2 N
N O N Cl
N NH2
HN
S
N
O
N
O
(II)
(I)
O
b
Cl
O
h g
i
O
c d
S f
NBDT
NBDT NBDTC+Pd(0) NBDTC+Pd(II) NBDTC
350
250 200 150 100
N
a
O
S
400
Intensity(a.u)
Under a nitrogen atmosphere, compound NBDT (138 mg, 0.5 mmol) was dissolved in anhydrous THF (25 mL) and DIEA
NO2
For the previous chemosensor system, tris(2-furyl)phosphine (TFP) or beta-4-platinum were used as palladium ligands for the Tsuji–Trost reaction [31]. Their poor solubilities in aqueous media
300
Synthesis of 7-nitro-2,1,3-benzoxadiazole-4-allyl-N-(thiophen-2ylmethyl) carbamate (NBDTC)
NO 2
Optimization of the experimental conditions
e
50 0 500
520
540
560
580
600
620
640
Wavelength/nm
NBDTC
Scheme 1. Synthesis of NBDTC. (I) acetonitrile, room temperature, 30 min, yield 52%; (II) anhydrous THF, DIEA, room temperature, 12 h, yield 89%.
Fig. 2. The fluorescence intensity of NBDT (1 lM), NBDTC (1 lM) and NBDTC (1 lM) with the addition different state of palladium (1 lM) in PEG400 solutions incubated 2 h at 55 °C.
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Z.-Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 7–11
1.4
1.4
NBDT NBDTC
a
1.2
b
1.2
2+
1.0
1.0
Absorbance(a.u)
Absorbance(a.u)
[Pd]
0.8 0.6 0.4
50µM 0µM
0.8 0.6 0.4 0.2
0.2 0.0
280
320
360
400
440
480
520
0.0
560
280
320
360
wavelength/nm
400
440
480
520
560
Wavelength/nm
Fig. 3. Absorption spectra of NBDT and NBDTC (50 lM); (a) in DMSO and (b) in PEG400 within different concentrations of Pd2+ (0, 5, 10, 20, 30, 40, 50 lM) incubated for 2 h at 55 °C.
400
a
400 350
300
300
2+
Pd
250
2+
2+
2+
Intensity(a.u)
2+
Mn , Co , Ba , Cu , + 2+ 2+ 2+ Ag , Hg , Mg , Ca , 3+ 2+ + 2+ Fe , Cd , K , Ni , + 2+ 2+ Na , Zn , Pb
200 150 100
250 200 150 100
575
600
625
650
Wavelength/nm
Ions
NBDTC
550
2+
0 525
Mn 2+ Co 2+ Ba 2+ Cu + Ag
0 500
Hg 2+ Mg 2+ Ca 3+ Fe 2+ Cd + K 2+ Ni + Na 2+ Zn 2+ Pb 2+ Pd
50
50
2+
Intensity(a.u)
b
350
Fig. 4. (a) Fluorescent spectra of probe NBDTC (1 lM) in the presence of various metal ions (1 lM) in PEG400 solutions incubated for 2 h at 55 °C. (b) The emission intensity at 526 nm.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2 3
a
350
Intensity(a.u)
300 250 200 150 100 50 0 500
525
550
575
Wavelength/nm
600
625
400
b
350 300
Intensity(a.u)
400
250 200 150 100 50
650
0 0.0
0.4
0.8
1.2
1.6 2+
2.0
2.4
2.8
3.2
-6
[Pd] /10 M
Fig. 5. (a) The fluorescence changes of NBDTC (1 lM) with the addition of palladium ion (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0, 3.0 equiv) in PEG400 solution incubated for 2 h at 55 °C. (b) Plot of emission intensity at 526 nm as a function of Pd2+ concentration.
Z.-Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 7–11
400 350
300
300
-
HCO3
2-
HPO4
-
H2PO4
Cl
3+
Ions
3-
0
PO4
0
-
50
Fe 2+ Pb 2+ Mn 2+ Cu 2+ Zn 2+ Mg 2+ Ni 2+ Co 2+ Cd
50
-
100
ClO4
100
150
2-
150
200
SO4
200
250
2-
250
SO3
Intensity(a.u)
350
b
-
a
NBDTC 2+ Pd + K + Na + Ag 2+ Hg 2+ Ca 2+ Ba
Intensity(a.u)
400
NO3
10
Anions
Fig. 6. (a) Fluorescent intensities of the probe NBDTC (1 lM) to Pd2+ (1 lM) in the presence of (1 lM) other cations; (b) anions (1 lM) in PEG400 solutions incubated for 2 h at 55 °C.
were found to affect the sensor performance. Peng and co-workers found that PEG was more efficient for palladium-catalyzed coupling reactions than phosphine ligand besides it was non-toxic and widely used in biological application [20]. In our research system, PEG400 did exhibit better performance as solution and reductant than other PEG series shown in Fig. 1. In the previous literatures, phosphate-buffered saline buffer (PBS) was often used in biological system, Although PEG400-PBS buffers with different proportions were tested, the fluorescence intensity showed better performance in PEG400 as a sole solvent than PEG400-PBS mixed buffers with different proportions. In the PEG400 solution, the fluorescence intensity varied with temperature and incubation time. The optimization conditions were temperature at 55 °C and incubation time for 2 h (Figs. S1 and S2). For the Tsuji–Trost reaction system, it is well known that Pd(0) is the active catalytic species to promote the removal of the allyl formate. In the NBDTC system investigated, PEG400 showed the potency to reduce Pd2+ state to Pd0 state, which is also testified in the previous literatures[30]. When the Pd0 species was applied as the catalyst in the Tsuji–Trost reaction, the fluorescence intensity shown in Fig. 2 increased largely as found in Pd2+-PEG400 system. Therefore, in the PEG400 system, NBDTC became an important fluorescence chemosensor for both Pd2+ species and Pd0 species.
Optical absorption studies The optical absorption spectra of NBDT and NBDTC in DMSO solvent were recorded at room temperature in the range of 260–600 nm as shown in Fig. 3a. NBDT showed two strong absorption peaks at 338 (e = 8.82 103 M 1cm 1) and 473 nm (e = 2.16 104 M 1cm 1), in which the later was the typical absorption peak of ANBD. The chemosensor NBDTC showed also two absorption peaks at 278 nm (e = 1.44 104 M 1cm 1) and at 382 nm (e = 2.05 104 M 1cm 1). When NBDTC was titrated by Pd2+ ion in PEG400 solution, the absorption peaks at 382 and 278 nm gradually decreased, accompanied with two new absorption peaks at 338 and 473 nm gradually produced with the addition of Pd2+ as shown in Fig. 3b. The spectra obtained during the stepwise addition showed one isosbestic point indicates a nearly clean conversion of NBDTC into the corresponding NBDT. This result suggested that the chemosensor NBDTC can be converted into NBDT at the presence of Pd2+ in PEG400 solution.
Fluorescence study At first we have investigated the fluorescence properties of chemosensor NBDTC and its fluorescence response to various metal cations in PEG400 solvent. NBDTC itself shows negligible fluorescence at 526 nm upon exciting at 480 nm. However, the fluorescence intensity at 526 nm is strongly enhanced with the addition of 1 equiv of Pd2+. The native fluorescence of NBDTC is minimal, and the fluorescence intensity increases and reaches a maximum on the addition of equivalent molecule of Pd2+ ion as shown in Fig. 4. The fluorescence intensities increase from 7.6 to 385 upon the addition of equivalent molecule of Pd2+, which indicates 50-fold enhancement in fluorescence intensity. No fluorescence response occurred for Mn2+, Co2+, Ba2+ Cu2+, Ag+, Hg2+, Mg2+, Ca2+, Fe3+, Cd2+, K+, Ni2+, Na+, Zn2+ and Pb2+ ions. Then, the results indicate that NBDTC is a reliable highly selective chemosensor for Pd2+ ion. The responsive properties of chemosensor NBDTC to palladium ion were further examined by fluorescence titration in PEG400 solution (Fig. 5a). No more further change is observed upon increasing Pd2+ concentrations up to 1 equiv (Fig. 5b). It can be found that the fluorescence intensities were linearly proportional to Pd2+ concentrations in the range of 0.1–0.9 equivalent molecule, which is an important characteristic of chemosensors (Fig. S3). The detection limit for Pd2+ was deduced to be 1.13 10 9 M with a correlation coefficient of 0.9926, thus providing an efficient system for monitoring traces of palladium in environmental and pharmaceutical samples. In order to further explore the selectivity towards a matrix containing other competitive stimuli, the fluorescence chemosensor NBDTC exhibits an excellent selectivity for Pd2+ ion at the presence of 1 equiv concentrations of other metal ions and various anions including SO24 , ClO4 , PO34 , HPO24 , HCO3 , NO3 , Cl , H2PO4 and SO23 (Fig. 6). Although the fluorescence intensities is slightly weaker for Co2+, Hg2+ and Ag+ than those of sole Pd2+ ion, the lowest fluorescence intensity is 259 for Co2+, which is 34 folds enhancement compared to that of NBDTC chemosensor. These results further demonstrate that the chemosensor NBDTC can specifically recognize Pd2+ ion with high selectivity. Conclusions In conclusion, we have successfully developed a new ‘‘turn-on’’ fluorescence chemosensor NBDTC for palladium species based on
Z.-Y. Xu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 7–11
the Tusji–Trost reaction in PEG400 solvent. NBDTC exclusively detects palladium species over a variety of other metal ions. Further, the visible detection of palladium species in PEG400 solution also exhibits high sensitivity. More importantly, in this sensing system, both Pd2+ and Pd0 could be detected at nM level, and this may develop a new method for improving the palladium detection in the environmental and pharmaceutical industries. Acknowledgments This work is supported by Guangdong Provincial Department of Science and Technology (Contract Grant No.: 2012A080800002) and Science and Information Technology of Guangzhou (2014J4100015). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.03.130. References [1] [2] [3] [4] [5] [6] [7]
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