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Novel fluorescent vanadylmoxifloxacinato complexes as sensors for Cu2+
Journal of Photochemistry and Photobiology A: Chemistry 335 (2017) 211–216
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
Novel fluorescent vanadylmoxifloxacinato complexes as sensors for Cu2+ Debjani Chakraborty* , Rinky Singh Department of Chemistry, Faculty of Science, The M .S .University of Baroda, Vadodara, 390002, Gujarat, India
A R T I C L E I N F O
Article history: Received 24 August 2016 Received in revised form 30 November 2016 Accepted 2 December 2016 Available online 7 December 2016 Keywords: VO (II) complexes Cu2+ detection Fluorescent sensor Moxifloxacin Dipeptide
A B S T R A C T
Three novel mixed ligands complexes of VO2+, (C1–C3) with the ligands moxifloxacin and dipeptides have been synthesised and characterised by Infraredand Ultraviolet-visible spectroscopy, Mass spectrometry, Electron spin resonance spectroscopyand elemental analysis.The fluorescent spectral measurements revealed that complexes C1–C3 are selective fluorescent sensors for Cu2+ but not for metal ions such as Na+, K+, Al3+, Fe3+, Co2+, Ni2+, Zn2+ and Cd2+. The detection limit for the determination of the Cu2+ ion was estimated as low as 2.0 mM. Furthermore, EDTA cannot turn on the quenched fluorescence of complexes C1–C3 induced by Cu2+ (“off”type sensor). © 2016 Elsevier B.V. All rights reserved.
1. Introduction Luminescent metal complexes have attracted increasing attention in the literature over the last few decades. Spearheaded by pioneering developments in ligand field theory and in the understanding of electronic transitions and spectroscopy of transition metal complexes, these compounds have enjoyed widespread application in photochemistry [1,2], organic optoelectronics [3,4] and luminescent sensors [5,6]. In contrast to conventional organic fluorophores, which are singlet emitters, transition metal complexes display triplet emission due to spin– orbit coupling imparted by the heavy atom effect, which leads to efficient singlet–triplet state mixing and enhancement of phosphorescence quantum efficiency. The phosphorescence behaviour of metal complexes has found potential use in the construction of organic light-emitting diodes (OLEDs) for display or lighting applications [7–10]. In the context of luminescent sensing, transition metal complexes have unique advantages that make them suitable for chemosensing or biosensing applications. These include their (i) high luminescence quantum yield, (ii) long phosphorescence lifetime that allows their emission to be distinguished from a fluorescent background, (iii) large Stokes shift for effective discrimination of excitation and emission wavelengths, (iv) sensitivity of their emissive properties to subtle changes in the
* Corresponding author. E-mail address: [email protected] (D. Chakraborty). http://dx.doi.org/10.1016/j.jphotochem.2016.12.004 1010-6030/© 2016 Elsevier B.V. All rights reserved.
local environment, and (v) modular synthesis that allows facile synthesis of analogues for fine-tuning of their chemical and/or photophysical properties [11]. In light of these advantages, transition metal compounds have been widely studied for luminescent sensing applications, particularly the d6, d8or d10 electron complexes based on ruthenium(II), platinum(II), iridium (III), osmium(II), gold(I), zinc (II) and rhenium(I) [11]. Compared with organic fluorophores, the excited state properties of transition metal complexes are complicated and can include metal-to-ligand charge-transfer (MLCT), ligand-to-ligand charge transfer (LLCT), intraligand charge-transfer (ILCT), ligand-to-metal charge transfer (LMCT), metal–metal-to-ligand charge-transfer (MMLCT), ligand-to-metal–metal charge-transfer (LMMCT) and metal-to-ligand–ligand charge-transfer (MLLCT) states [12,13]. The properties of the excited states are highly sensitive to the metal centre, the type of ligands, and the nature of the local environments, allowing the photo physical properties (such as emission wavelength, lifetime, and intensity) of metal complexes to be tailored for specific applications. Copper is an essential trace element for growth and development in all forms of life. It is the second most important trace metal in the human body and tends to be an integral part of a number of metalloenzymes covering the whole range of functionality [14]. However excessive amount of copper can cause toxicity and do great harm to the body. Cu2+ metabolism disorders have been reported to cause Wilson’s disease which leads to excessive Cu accumulation in vital organs like kidneys, liver, lungs and brain causing malfunctioning of these organs [15]. This potential hazard makes it indispensable to monitor the amount of copper in
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environmental water. Toward alleviating this issue, the selective detection of Cu2+ ions in aqueous solutions with high sensitivity is of great importance. To ensure safety, a detection limit as low as 20 mM (1.3 ppm) is recommended by the Environmental Protection Agency (EPA) [16]. Recently, ZnO@ZnS core-shell nanoparticles [17] and fluorescent gold nanoclusters [18] have been introduced to achieve efficient and selective detection of Cu2+ ions. Functionalized gold nanoparticles (AuNPs) [19] were also used for the detection of metal ions including Cu2+. SERS (Surfaceenhanced Raman spectroscopy) has also been used to discriminate Cu2+ ions in aqueous solutions with an interference of Hg2+and Co2 ~ + [20,21]. Nguyê n Hoàng Ly et al. have reported detection of copper (II) ions using glycine on hydrazine-adsorbed gold nanoparticles using Raman spectroscopy [22]. Another report has been published by J. Hwang et al. on sensitive detection of copper ions via ionresponsive fluorescence quenching of engineeredporous silicon nanoparticles [23]. The standard method of measuring Cu2+ ions, however, employs ion-selective electrodes, which presents difficulties when considering the presence of various non-specific biomolecules (i.e.proteins, enzymes, other ions). From this perspective, a simple and convenient method to detect Cu2+ ions with high specificity and sensitivity is desirable. In the present work, novel VO2+ mixed ligand complexes C1–C3 based on an antibiotic drug moxifloxacin and the dipeptidesglycyleucine (C1), glycylglycine(C2) and glycylalanine (C3) were synthesized and characterized by infrared (IR) and ultravioletvisible (UV–vis) spectroscopy, mass spectrometry (MS), electron spin resonance (ESR) spectroscopy and elemental analysis, and their potential to detect Cu2+ ions through UV–vis absorption and fluorescence spectroscopy is reported. The complexes emitted strong fluorescence at 450 nm and with the incremental addition of Cu2+, the fluorescence of the complexes were quenched gradually. The response showed high selectivity for Cu2+ compared with other metal ions and could be used to detect Cu2+.
grade and were used as purchased. MFL (Moxifloxacin) was kindly donated by Alembic Research Centre (Gujarat, India). The metal salts and solvents were purchased from Merck. Dipeptides were purchased from SRL (Sisco research laboratory, Mumbai, India.). Infrared (IR) spectra (400–4000 cm1) were recorded on Perkin Elmer RX-1 FTIR with samples prepared as KBr disks. ESI Mass spectra of the ligands were recorded on Thermoscientific DSQ – II Mass spectrometer and those of the complexes were recorded on Applied Biosystem API 2000 Mass spectrometer. C, H and N elemental analysis were performed on a Perkin-Elmer 240 B elemental analyzer. UV–vis absorption spectra were recorded in DMSO solution on Perkin Elmer Lambda-35 dual beam UV–vis spectrophotometer. EPR spectra at liquid nitrogen temperature were recorded on JES – FA200 ESR Spectrometer. Fluorescence spectra were recorded in solution on JASCO FP-6300 fluorescence spectrophotometer. The% metal content in each of the complexes was determined by gravimetric/complexometric titration method after decomposition of the complexes.
2. Experimental
2.3.1. [VO(glyleu)(MFL)](C1) Yield: 75%, Molecular weight: 655.59 g/mol, Molecular formula: C29H38FN5O8V, Calc. (%): C, 53.21; H, 5.85; N, 10.70.; V, 7.78., Found (%): C, 54.21; H, 5.85; N, 10.60.; V, 7.8, lmax: 224, 288 (e = 16305 L mol1 cm1), 333 nm, ESI–MS [Me OH, m/z]: 655.8 [M+].
2.1. Reagents and instrumentation All the chemicals and solvents used for synthesis and characterization of the complexes were of analytical reagent
2.2. Synthesis of complexes To a stirred methanolic solution (10 mL) of oxovanadium(IV) sulfate (0.25 g, 1.0 mmol) was added an aqueous solution of the dipeptide (1.0 mmol) followed by addition of KOH (0.056 g, 1.0 mmol). The reaction mixture was stirred for 2 h at room temperature to obtain a clear green color solution. A methanolic solution (20 mL) of moxifloxacin (MFL) (0.401 g, 1.0 mmol) was added to the above reaction mixture and refluxed for another 2 h. The green colored solid was filtered, thoroughly washed with ice cold methanol and dried in vacuum over anhydrous CaCl2 (Scheme 1). The complexes were characterized by IR, ESR, Mass spectral and elemental analysis techniques. 2.3. Physicochemical properties of the complexes
Scheme 1. Synthetic scheme of complexes C1–C3.
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2.3.2. [VO(glygly)(MFL)](C2) Yield: 78%, Molecular weight: 598.45 g/mol, Molecular formula: C25H30FN5O8V, Calc. (%): C, 50.17; H, 5.05; N, 11.70.; V, 8.51, Found (%): C, 51.0; H, 5.10; N, 11.60.; V, 8.5, lmax: 224, 293 (e = 17008 L mol1 cm1), 344 nm. ESI–MS [Me OH, m/z]: 597.6 [M1]. 2.3.3. [VO(glyala)(MFL)](C3) Yield: 71%, Molecular weight: 612.50 g/mol, Molecular formula: C26H32FN5O8V, Calc (%): C, 50.98; H, 5.27; N, 11.43.; V, 8.32, Found (%): C, 51.0; H, 5.26; N, 11.50.; V, 8.30, lmax: 231, 293 (e = 18000 L mol1 cm1), 344 nm. ESI–MS [Me OH, m/z]: 613.9[M+1]. 2.4. Results and discussion Mixed ligand vanadyl (II) complexes with the fluoroquinolone moxifloxacin (MFL), and the dipeptides, [VO(glyleu)(MFL)] (C1), [VO(glygly)(MFL)] (C2) and [VO(glyala)(MFL)] (C3) were obtained from methanol solution, by reaction of vanadyl sulphate with the dipeptides and MFL in 1:1:1 molar ratio (Scheme 1) [24]. The isolated compounds were characterized by infrared and ultraviolet-visible spectroscopy, mass spectrometry, electron spin resonance spectroscopy and elemental analysis. The ESI–MS spectra of complexes C1–C3 (Fig. S1) showed molecular ion peaks at m/z 655.6[M+], 597.6 [M1] and 613.9 [M +1] respectively with their molecular weights matching the calculated values, confirming thereby the composition of the complexes. Furthermore the composition and purity of the complexes have been confirmed by their C, H, N elemental analysis. The FTIR spectra of C1-C3 were recorded in the region 4000– 400 cm1 (Fig. S2) and analyzed in comparison to the spectra of the free ligands. In the IR spectra, the absorption bands at 1374– 1379 cm1 are the characteristic C N stretch of the deprotonated peptide nitrogen bound to the vanadyl ion. The N H bending vibration (amide II band) observed at 1575 cm1 in the free dipeptides has disappeared in the complexes due to deprotonation and coordination of the peptide nitrogen. The n(CO)pepband (amide I) is shifted to 1623 cm1 due to the involvement of the depronated peptide nitrogen in bonding with vanadyl ion, which lowers the bond order of the n(CO)amide group due to resonance stabilization and further confirms the coordination of the metal ion through peptide-N atom [25]. The shift in the bands corresponding to nasym(COO)pep and nsym(COO)pep to1518 cm1 and 1445 cm1 respectively, suggests the involvement of the carboxylic group of the dipeptides in complex formation. A broad band at 3223– 3240 cm1 observed in the IR spectra of complexes is attributed to N H stretch of the terminal amino group of the dipeptides which is shifted to lower frequency upon coordination [26]. Similarly the shifts in the pyridone carbonyl n(CO)MFL and carboxylate n(COO)MFL stretching frequencies of moxifloxacin in complexes indicate the binding of these groups with the metal ion. The IR data have been presented in table S1.
Fig. 1. Electronic spectra of C2 in DMSO. Inset shows the visible spectra of the complex C2.
The electronic absorption spectra of C1–C3 in freshly prepared DMSO solutions was observed in the region 200–900 nm at room temperature (Fig. 1, Table 1). The electronic spectra of free dipeptides display intense absorption bands at 220 nm due n-p* transition, which were not observed in the spectra of complexes. The bands observed between 224 and 231 nm in the complexes have been assigned to N(dipeptide) ! VO(IV) charge transfer (LMCT) transitions [27]. Broad bands at 288–293 nm (e = 16305– 18000 L mol1 cm1) and 333–344 nm (e = 10800–11958 L mol1 cm1) for C1–C3 were attributed to p ! p* and n ! p* transitions of moxifloxacinato ligand [28]. Intense bands observed at 398– 406 nm are attributed to the moxifloxacin ligand-to-metal chargetransfer transitions for C1–C3. In the visible region, low-intensity absorption bands at 851–861 cm1 assigned to a b2 (dxy) ! ep* (dxz; dyz) transitions of the VO2+ion are observed. These absorption bands are typical for distorted octahedral VO2+ complexes [27]. The X-band EPR spectra of C1–C3 were recorded in frozen DMSO solution at 110 K (Fig. 2). The spectra exhibit a hyperfine eight-line pattern, characteristic of an unpaired electron being coupled with a vanadium nuclear spin (I = 7/2). The VO2+ ion belongs to the 3d1 system and the unpaired electron is in the vicinity of I = 7/2 of its own mother nucleus [29]. Thus, the spectra display well-resolved hyperfine lines and the signal parameters for complexes are given in Table 2. The g// >> g?and A// >> A? relationships are characteristic of an axially compressed dxy1 configuration [30].
Table 1 Characteristic bands lmax (nm) in the UV–vis spectra of complexes C1–C3. Ligands
Intraligand band p ! p* (nm)
Intraligand band n ! p* (nm)
LMCTa N(pep) ! VO2+ (nm)
LMCTa MFL ! VO2+ (nm)
d-d transition (nm)
Dipeptides MFL
– 290
220 337
– –
– –
– –
Complexes C1 C2 C3
288 293 293
333 344 344
224 231 231
406 398 399
861 858 851
a
LMCT = Ligand to metal charge transfer.
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solution. The EPR hyperfine profile is similar to those of many other oxovanadium(IV) complexes reported earlier [33]. 2.5. VO (II) complexes as sensors for Cu2+
Fig. 2. ESR Spectraof C1 in DMSO. EPR conditions: Temperature, 10 K; microwave power, 5.0 mW; Modulation amplitude, 1G; microwave frequency, 9.1 GHz.
Table 2 ESR parameters of C1–C3. Compounds
g
g?
G*
A
A?
C1 C2 C3
2.04 2.07 2.03
2.002 2.008 2.006
6.3 6.1 5.5
164*104 167*104 165*104
35.3*104 33.3*104 36.2*104
G* = Exchange interaction parameter. EPR conditions: Temperature, 10 K; microwave power, 5.0 mW; Modulation amplitude, 1G; microwave frequency, 9.1 GHz.
The parallel component of the hyperfine coupling constant, A//is sensitive to the donor type in the “equatorial” coordination sphere. From this knowledge, the empirical additivity relationship has been developed and frequently used as a means of determining, to a first approximation, the identity of the equatorial lignds in vanadyl complexes [31]. On the basis of the additivity relationship, the experimental A//values were found to be close to the calculated A//value for the oxovanadium complexes [32]. Thus, the most reasonable equatorial donor atom set is N2O2 (Ocarboxylate, Opyridone, Nimine and Namine)for the oxovanadium complexes in DMSO
2.5.1. UV–vis titrations The absorption spectrum of C1 (20 mM) in buffer: DMSO (9:1) solution exhibits bands at 288 nm and 340 nm assigned to a p-p* and n-p* transitions of the moxifloxacinato ligands, respectively [28]. To investigate the cation response of C1, two equivalence (40 mM) of the metal salts (Na+, Al3+, Co2+, K+, Zn2+, Cd2+ and Cu2+) were added to the solution of C1 and the spectrum was recorded in the presence of the metal ions. The addition of metal ions other than Cu2+ induces a slight red shift (4 nm) of the p-p* band and slight increase in absorption intensity (Fig. 3a). In contrast, the addition of Cu2+ leads to a remarkable increase in absorption intensity along with a slight shift in the absorption peak. To further establish the Cu2+ response of C1, UV-titration of a solution of C1 (20 mM) with increasing concentration (0–600 mM) of Cu2+ was performed. A steady increase in the absorption intensity of the peak at 288 nm (Fig. 3b) was observed. UV-titration of complexes C2 and C3 with increasing concentrations of Cu2+ also showed an increase in the absorption intensity of the peak at 293 nm (p-p*)indicating the selective sensing capability of the complexes C1-C3 for Cu2+. 2.5.2. Fluorescence measurements The excitation of the absorption band at lmax = 288 nm caused emissions at 470 nm for C1. Fluorescence titrations of C1 with different metal cations were carried out and the results are shown in Fig. 4a. The addition of Cd2+, Zn2+, Na+ or K+ induced a very minor change in the emission intensity of C1, whereas other cations such as Al3+, Fe3+, Ni2+ and Co2+ triggered partial emission quenching through electron and/or energy transfer process [34]. In contrast, Cu2+ quenched the fluorescence completely (Fig. 4a). A detailed spectrofluorometric titration was performed by adding 0–600 mM of Cu2+ to solutions of C1 (20 mM) in Tris–HCl buffer at 25 1 C .Upon addition of increasing amounts of Cu2+, a rapid decrease in the intensity of the emission band occurred (Fig. 4b). Similar results were obtained for C2 and C3. The photograph in Fig. 5a shows the quenching of fluorescence that occurs when Cu2+ (400 mM) was added to a solution of C1 (20 mM). No such quenching was observed on the addition of other
Fig. 3. a) Absorption spectra of C1 (20 mM) in buffer-DMSO (v/v = 9:1), in presence of different metalcations (2 equiv.). b) Absorption spectra of C1 (20 mM) in buffer-DMSO (v/ v = 9:1), in presence of different equivalence of Cu2+ cation (0–600 mM).
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Fig. 4. a) Emission spectra of C1 (20 mM) upon addition of 2 equivalents of different metal ions in buffer-DMSO (v/v = 9:1) solutions when excited at 288 nm. b) Emission spectra of C1 (20 mM) with various amounts of Cu2+ (0–600 mM) in buffer-DMSO (v/v = 9:1) solutions (lex = 288 nm).
metal ions. A gradual decrease in fluorescence intensity was also observed on addition of increasing amounts of Cu2+(0–600 mM) as shown in Fig. 5b .The detection limit for detection of Cu2+ ions is as low as 2.0 mM. The detection limit was estimated from the linear fitting curve,where the spectral change could begin to occur based on the linear equation of Y (lem of C1) = B(intercept) + A (slope) x X (Cu 2+concentrations). 2.6. Evidence for replacement of VO2+ by Cu2+ To get deep insight into the changes taking place after addition of Cu2+ that results in the quenching of fluorescence of the complexes, electrospray ionization (ESI) mass spectra of complexes C1-C3 in the presence of Cu2+ were recorded. The mass
spectra exhibited m/z peaks corresponding to the mixed ligand [Cu (dipeptide)(MFL)(H2O)] complexes indicating complete replacement of VO2+ and complexation by Cu2+ (Fig. S4). Replacement of the VO2+ by Cu2+ and formation of [Cu (glyleu) (MFL)(H2O)] complexes was also confirmed by the absence of V¼O peak [y(V=O) = 976 cm1) in the FTIR spectrum of C1 in the presence of Cu2+ (Fig. 6). Similar results were obtained for complexes C2 and C3. The quenching of fluorescence upon complexation with Cu2+ originate partly due to interactions between the excited MFL ligand and the non-excited Cu2+.The electronic excited energy may be transferred from MFL p molecular orbital to the Cu2+ dp molecular orbital by energy transfer and electron transfer process. The Cu2+ ion is formally reduced and the ligand MFL is formally oxidised. The p electron in the higher energy orbital of the ligand is transferred to an electron deficient Cu2+ ion quencher. The hole left due to promotion of an electron, accepts an electron from the electron rich quencher [35]. Other than electron transfer process, energy transfer process can also cause quenching of fluorescence [35]. Energy is transferred from the excited state of the complexes C1– C3 to the non-excited Cu2+ ion on complexation, by non-radiative decay thus quenching fluorescence. The results imply that the complexes can act as chemosensors for detecting Cu2+ due to replacement of the VO2+ by Cu2+. 2.7. Conclusion
Fig. 5. (a) Photograph of C1 (20 mM) solutions containing different metal ions (400 mM) at lex = 254 nm. (b) Photograph of C1 (20 mM) solutions containing Cu2+ (0–600 mM). (0–30 equivalence).
Three novel mixed ligands complexes of VO2+, (C1–C3) with the ligands moxifloxacin and dipeptides have been synthesised and studied as selective probes for Cu2+ ions. The complexes emit at 450 nm on excitation at 288/293 nm. Presence or addition of Cu2+ ions show quenching of fluorescence intensity of the complexes due to replacement of VO2+ ion from the complexes and form the Cu2+ mixed ligand complexes as evident from ESI mass spectral and IR spectral studies. The quenching of fluorescence is attributed to electron and or energy transfer from the excited fluorophore to the non-excited Cu2+ ion. The present study demonstrates that the mixed ligand vanadyl complexes (C1–C3) can be used as chemosensors for selective detection of Cu2+.
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Fig. 6. IR spectral changes of C1 on addition of Cu2+ solution.
Acknowledgements The authors acknowledge University Grants Commission, New Delhi, India for providing financial support and Department of Science and Technology(DST) for DST INSPIRE Fellowship. Authors are grateful to the Head, Department of Chemistry, The Maharaja Sayajirao University of Baroda, for providing with the necessary laboratory facilities. 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. jphotochem.2016.12.004. References [1] Y. Yamaguchi, W. Ding, C.T. Sanderson, M.L. Borden, M.J. Morgan, C. Kutal, Coord. Chem. Rev. 251 (2007) 515. [2] K.M.C. Wong, V.W.-W. Yam, Coord. Chem. Rev. 251 (2007) 2477. [3] K.E. Augustyn, E.D.A. Stemp, J.K. Barton, Inorg. Chem 46 (2007) 9337. [4] S.R. Forrest, M.E. Thompson, Chem. Rev. 107 (2007) 923. [5] Q. Zhao, F. Li, C. Huang, Chem. Soc. Rev. 39 (2010) 3007. [6] J. Zhao, S. Ji, W. Wu, W. Wu, H. Guo, J. Sun, H. Sun, Y. Liu, Q. Li, L. Huang, RSC Adv. 2 (2012) 1712. [7] G. Zhou, W.-Y. Wong, X. Yang, Chem. Asian J. 6 (2011) 1706. [8] B. Happ, A. Winter, M.D. Hager, U.S. Schubert, Chem. Soc. Rev. 41 (2012) 2222. [9] J. Kalinowski, V. Fattori, M. Cocchi, J.A.G. Williams, Coord. Chem. Rev. 255 (2011) 2401. [10] Y. You, S.Y. Park, Dalton Trans. (2009) 1267. [11] D.L. Maa, V.P.Y. Maa, D.S.H. Chan, K.H. Leung, H.Z. He, C.H. Leung, Coord. Chem. Rev. 256 (2012) 3087. [12] (a) Q. Wu, E.V. Anslyn, J. Am. Chem. Soc. 126 (2004) 14682; (b) Z. Xu, X. Qian, J. Cui, Org. Lett. 7 (2005) 3029; (c) Y.Q. Weng, F. Yue, Y.-R. Zhong, B.H. Ye, Inorg. Chem. 46 (2007) 7749. [13] E.M. Zoupa, S.P. Perlepes, V. Hondrellis, J.M. Tsangaris, J. Inorg. Biochem. 55 (1994) 217.
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
Novel fluorescent vanadylmoxifloxacinato complexes as sensors for Cu2+ Debjani Chakraborty* , Rinky Singh Department of Chemistry, Faculty of Science, The M .S .University of Baroda, Vadodara, 390002, Gujarat, India
A R T I C L E I N F O
Article history: Received 24 August 2016 Received in revised form 30 November 2016 Accepted 2 December 2016 Available online 7 December 2016 Keywords: VO (II) complexes Cu2+ detection Fluorescent sensor Moxifloxacin Dipeptide
A B S T R A C T
Three novel mixed ligands complexes of VO2+, (C1–C3) with the ligands moxifloxacin and dipeptides have been synthesised and characterised by Infraredand Ultraviolet-visible spectroscopy, Mass spectrometry, Electron spin resonance spectroscopyand elemental analysis.The fluorescent spectral measurements revealed that complexes C1–C3 are selective fluorescent sensors for Cu2+ but not for metal ions such as Na+, K+, Al3+, Fe3+, Co2+, Ni2+, Zn2+ and Cd2+. The detection limit for the determination of the Cu2+ ion was estimated as low as 2.0 mM. Furthermore, EDTA cannot turn on the quenched fluorescence of complexes C1–C3 induced by Cu2+ (“off”type sensor). © 2016 Elsevier B.V. All rights reserved.
1. Introduction Luminescent metal complexes have attracted increasing attention in the literature over the last few decades. Spearheaded by pioneering developments in ligand field theory and in the understanding of electronic transitions and spectroscopy of transition metal complexes, these compounds have enjoyed widespread application in photochemistry [1,2], organic optoelectronics [3,4] and luminescent sensors [5,6]. In contrast to conventional organic fluorophores, which are singlet emitters, transition metal complexes display triplet emission due to spin– orbit coupling imparted by the heavy atom effect, which leads to efficient singlet–triplet state mixing and enhancement of phosphorescence quantum efficiency. The phosphorescence behaviour of metal complexes has found potential use in the construction of organic light-emitting diodes (OLEDs) for display or lighting applications [7–10]. In the context of luminescent sensing, transition metal complexes have unique advantages that make them suitable for chemosensing or biosensing applications. These include their (i) high luminescence quantum yield, (ii) long phosphorescence lifetime that allows their emission to be distinguished from a fluorescent background, (iii) large Stokes shift for effective discrimination of excitation and emission wavelengths, (iv) sensitivity of their emissive properties to subtle changes in the
* Corresponding author. E-mail address: [email protected] (D. Chakraborty). http://dx.doi.org/10.1016/j.jphotochem.2016.12.004 1010-6030/© 2016 Elsevier B.V. All rights reserved.
local environment, and (v) modular synthesis that allows facile synthesis of analogues for fine-tuning of their chemical and/or photophysical properties [11]. In light of these advantages, transition metal compounds have been widely studied for luminescent sensing applications, particularly the d6, d8or d10 electron complexes based on ruthenium(II), platinum(II), iridium (III), osmium(II), gold(I), zinc (II) and rhenium(I) [11]. Compared with organic fluorophores, the excited state properties of transition metal complexes are complicated and can include metal-to-ligand charge-transfer (MLCT), ligand-to-ligand charge transfer (LLCT), intraligand charge-transfer (ILCT), ligand-to-metal charge transfer (LMCT), metal–metal-to-ligand charge-transfer (MMLCT), ligand-to-metal–metal charge-transfer (LMMCT) and metal-to-ligand–ligand charge-transfer (MLLCT) states [12,13]. The properties of the excited states are highly sensitive to the metal centre, the type of ligands, and the nature of the local environments, allowing the photo physical properties (such as emission wavelength, lifetime, and intensity) of metal complexes to be tailored for specific applications. Copper is an essential trace element for growth and development in all forms of life. It is the second most important trace metal in the human body and tends to be an integral part of a number of metalloenzymes covering the whole range of functionality [14]. However excessive amount of copper can cause toxicity and do great harm to the body. Cu2+ metabolism disorders have been reported to cause Wilson’s disease which leads to excessive Cu accumulation in vital organs like kidneys, liver, lungs and brain causing malfunctioning of these organs [15]. This potential hazard makes it indispensable to monitor the amount of copper in
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environmental water. Toward alleviating this issue, the selective detection of Cu2+ ions in aqueous solutions with high sensitivity is of great importance. To ensure safety, a detection limit as low as 20 mM (1.3 ppm) is recommended by the Environmental Protection Agency (EPA) [16]. Recently, ZnO@ZnS core-shell nanoparticles [17] and fluorescent gold nanoclusters [18] have been introduced to achieve efficient and selective detection of Cu2+ ions. Functionalized gold nanoparticles (AuNPs) [19] were also used for the detection of metal ions including Cu2+. SERS (Surfaceenhanced Raman spectroscopy) has also been used to discriminate Cu2+ ions in aqueous solutions with an interference of Hg2+and Co2 ~ + [20,21]. Nguyê n Hoàng Ly et al. have reported detection of copper (II) ions using glycine on hydrazine-adsorbed gold nanoparticles using Raman spectroscopy [22]. Another report has been published by J. Hwang et al. on sensitive detection of copper ions via ionresponsive fluorescence quenching of engineeredporous silicon nanoparticles [23]. The standard method of measuring Cu2+ ions, however, employs ion-selective electrodes, which presents difficulties when considering the presence of various non-specific biomolecules (i.e.proteins, enzymes, other ions). From this perspective, a simple and convenient method to detect Cu2+ ions with high specificity and sensitivity is desirable. In the present work, novel VO2+ mixed ligand complexes C1–C3 based on an antibiotic drug moxifloxacin and the dipeptidesglycyleucine (C1), glycylglycine(C2) and glycylalanine (C3) were synthesized and characterized by infrared (IR) and ultravioletvisible (UV–vis) spectroscopy, mass spectrometry (MS), electron spin resonance (ESR) spectroscopy and elemental analysis, and their potential to detect Cu2+ ions through UV–vis absorption and fluorescence spectroscopy is reported. The complexes emitted strong fluorescence at 450 nm and with the incremental addition of Cu2+, the fluorescence of the complexes were quenched gradually. The response showed high selectivity for Cu2+ compared with other metal ions and could be used to detect Cu2+.
grade and were used as purchased. MFL (Moxifloxacin) was kindly donated by Alembic Research Centre (Gujarat, India). The metal salts and solvents were purchased from Merck. Dipeptides were purchased from SRL (Sisco research laboratory, Mumbai, India.). Infrared (IR) spectra (400–4000 cm1) were recorded on Perkin Elmer RX-1 FTIR with samples prepared as KBr disks. ESI Mass spectra of the ligands were recorded on Thermoscientific DSQ – II Mass spectrometer and those of the complexes were recorded on Applied Biosystem API 2000 Mass spectrometer. C, H and N elemental analysis were performed on a Perkin-Elmer 240 B elemental analyzer. UV–vis absorption spectra were recorded in DMSO solution on Perkin Elmer Lambda-35 dual beam UV–vis spectrophotometer. EPR spectra at liquid nitrogen temperature were recorded on JES – FA200 ESR Spectrometer. Fluorescence spectra were recorded in solution on JASCO FP-6300 fluorescence spectrophotometer. The% metal content in each of the complexes was determined by gravimetric/complexometric titration method after decomposition of the complexes.
2. Experimental
2.3.1. [VO(glyleu)(MFL)](C1) Yield: 75%, Molecular weight: 655.59 g/mol, Molecular formula: C29H38FN5O8V, Calc. (%): C, 53.21; H, 5.85; N, 10.70.; V, 7.78., Found (%): C, 54.21; H, 5.85; N, 10.60.; V, 7.8, lmax: 224, 288 (e = 16305 L mol1 cm1), 333 nm, ESI–MS [Me OH, m/z]: 655.8 [M+].
2.1. Reagents and instrumentation All the chemicals and solvents used for synthesis and characterization of the complexes were of analytical reagent
2.2. Synthesis of complexes To a stirred methanolic solution (10 mL) of oxovanadium(IV) sulfate (0.25 g, 1.0 mmol) was added an aqueous solution of the dipeptide (1.0 mmol) followed by addition of KOH (0.056 g, 1.0 mmol). The reaction mixture was stirred for 2 h at room temperature to obtain a clear green color solution. A methanolic solution (20 mL) of moxifloxacin (MFL) (0.401 g, 1.0 mmol) was added to the above reaction mixture and refluxed for another 2 h. The green colored solid was filtered, thoroughly washed with ice cold methanol and dried in vacuum over anhydrous CaCl2 (Scheme 1). The complexes were characterized by IR, ESR, Mass spectral and elemental analysis techniques. 2.3. Physicochemical properties of the complexes
Scheme 1. Synthetic scheme of complexes C1–C3.
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2.3.2. [VO(glygly)(MFL)](C2) Yield: 78%, Molecular weight: 598.45 g/mol, Molecular formula: C25H30FN5O8V, Calc. (%): C, 50.17; H, 5.05; N, 11.70.; V, 8.51, Found (%): C, 51.0; H, 5.10; N, 11.60.; V, 8.5, lmax: 224, 293 (e = 17008 L mol1 cm1), 344 nm. ESI–MS [Me OH, m/z]: 597.6 [M1]. 2.3.3. [VO(glyala)(MFL)](C3) Yield: 71%, Molecular weight: 612.50 g/mol, Molecular formula: C26H32FN5O8V, Calc (%): C, 50.98; H, 5.27; N, 11.43.; V, 8.32, Found (%): C, 51.0; H, 5.26; N, 11.50.; V, 8.30, lmax: 231, 293 (e = 18000 L mol1 cm1), 344 nm. ESI–MS [Me OH, m/z]: 613.9[M+1]. 2.4. Results and discussion Mixed ligand vanadyl (II) complexes with the fluoroquinolone moxifloxacin (MFL), and the dipeptides, [VO(glyleu)(MFL)] (C1), [VO(glygly)(MFL)] (C2) and [VO(glyala)(MFL)] (C3) were obtained from methanol solution, by reaction of vanadyl sulphate with the dipeptides and MFL in 1:1:1 molar ratio (Scheme 1) [24]. The isolated compounds were characterized by infrared and ultraviolet-visible spectroscopy, mass spectrometry, electron spin resonance spectroscopy and elemental analysis. The ESI–MS spectra of complexes C1–C3 (Fig. S1) showed molecular ion peaks at m/z 655.6[M+], 597.6 [M1] and 613.9 [M +1] respectively with their molecular weights matching the calculated values, confirming thereby the composition of the complexes. Furthermore the composition and purity of the complexes have been confirmed by their C, H, N elemental analysis. The FTIR spectra of C1-C3 were recorded in the region 4000– 400 cm1 (Fig. S2) and analyzed in comparison to the spectra of the free ligands. In the IR spectra, the absorption bands at 1374– 1379 cm1 are the characteristic C N stretch of the deprotonated peptide nitrogen bound to the vanadyl ion. The N H bending vibration (amide II band) observed at 1575 cm1 in the free dipeptides has disappeared in the complexes due to deprotonation and coordination of the peptide nitrogen. The n(CO)pepband (amide I) is shifted to 1623 cm1 due to the involvement of the depronated peptide nitrogen in bonding with vanadyl ion, which lowers the bond order of the n(CO)amide group due to resonance stabilization and further confirms the coordination of the metal ion through peptide-N atom [25]. The shift in the bands corresponding to nasym(COO)pep and nsym(COO)pep to1518 cm1 and 1445 cm1 respectively, suggests the involvement of the carboxylic group of the dipeptides in complex formation. A broad band at 3223– 3240 cm1 observed in the IR spectra of complexes is attributed to N H stretch of the terminal amino group of the dipeptides which is shifted to lower frequency upon coordination [26]. Similarly the shifts in the pyridone carbonyl n(CO)MFL and carboxylate n(COO)MFL stretching frequencies of moxifloxacin in complexes indicate the binding of these groups with the metal ion. The IR data have been presented in table S1.
Fig. 1. Electronic spectra of C2 in DMSO. Inset shows the visible spectra of the complex C2.
The electronic absorption spectra of C1–C3 in freshly prepared DMSO solutions was observed in the region 200–900 nm at room temperature (Fig. 1, Table 1). The electronic spectra of free dipeptides display intense absorption bands at 220 nm due n-p* transition, which were not observed in the spectra of complexes. The bands observed between 224 and 231 nm in the complexes have been assigned to N(dipeptide) ! VO(IV) charge transfer (LMCT) transitions [27]. Broad bands at 288–293 nm (e = 16305– 18000 L mol1 cm1) and 333–344 nm (e = 10800–11958 L mol1 cm1) for C1–C3 were attributed to p ! p* and n ! p* transitions of moxifloxacinato ligand [28]. Intense bands observed at 398– 406 nm are attributed to the moxifloxacin ligand-to-metal chargetransfer transitions for C1–C3. In the visible region, low-intensity absorption bands at 851–861 cm1 assigned to a b2 (dxy) ! ep* (dxz; dyz) transitions of the VO2+ion are observed. These absorption bands are typical for distorted octahedral VO2+ complexes [27]. The X-band EPR spectra of C1–C3 were recorded in frozen DMSO solution at 110 K (Fig. 2). The spectra exhibit a hyperfine eight-line pattern, characteristic of an unpaired electron being coupled with a vanadium nuclear spin (I = 7/2). The VO2+ ion belongs to the 3d1 system and the unpaired electron is in the vicinity of I = 7/2 of its own mother nucleus [29]. Thus, the spectra display well-resolved hyperfine lines and the signal parameters for complexes are given in Table 2. The g// >> g?and A// >> A? relationships are characteristic of an axially compressed dxy1 configuration [30].
Table 1 Characteristic bands lmax (nm) in the UV–vis spectra of complexes C1–C3. Ligands
Intraligand band p ! p* (nm)
Intraligand band n ! p* (nm)
LMCTa N(pep) ! VO2+ (nm)
LMCTa MFL ! VO2+ (nm)
d-d transition (nm)
Dipeptides MFL
– 290
220 337
– –
– –
– –
Complexes C1 C2 C3
288 293 293
333 344 344
224 231 231
406 398 399
861 858 851
a
LMCT = Ligand to metal charge transfer.
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solution. The EPR hyperfine profile is similar to those of many other oxovanadium(IV) complexes reported earlier [33]. 2.5. VO (II) complexes as sensors for Cu2+
Fig. 2. ESR Spectraof C1 in DMSO. EPR conditions: Temperature, 10 K; microwave power, 5.0 mW; Modulation amplitude, 1G; microwave frequency, 9.1 GHz.
Table 2 ESR parameters of C1–C3. Compounds
g
g?
G*
A
A?
C1 C2 C3
2.04 2.07 2.03
2.002 2.008 2.006
6.3 6.1 5.5
164*104 167*104 165*104
35.3*104 33.3*104 36.2*104
G* = Exchange interaction parameter. EPR conditions: Temperature, 10 K; microwave power, 5.0 mW; Modulation amplitude, 1G; microwave frequency, 9.1 GHz.
The parallel component of the hyperfine coupling constant, A//is sensitive to the donor type in the “equatorial” coordination sphere. From this knowledge, the empirical additivity relationship has been developed and frequently used as a means of determining, to a first approximation, the identity of the equatorial lignds in vanadyl complexes [31]. On the basis of the additivity relationship, the experimental A//values were found to be close to the calculated A//value for the oxovanadium complexes [32]. Thus, the most reasonable equatorial donor atom set is N2O2 (Ocarboxylate, Opyridone, Nimine and Namine)for the oxovanadium complexes in DMSO
2.5.1. UV–vis titrations The absorption spectrum of C1 (20 mM) in buffer: DMSO (9:1) solution exhibits bands at 288 nm and 340 nm assigned to a p-p* and n-p* transitions of the moxifloxacinato ligands, respectively [28]. To investigate the cation response of C1, two equivalence (40 mM) of the metal salts (Na+, Al3+, Co2+, K+, Zn2+, Cd2+ and Cu2+) were added to the solution of C1 and the spectrum was recorded in the presence of the metal ions. The addition of metal ions other than Cu2+ induces a slight red shift (4 nm) of the p-p* band and slight increase in absorption intensity (Fig. 3a). In contrast, the addition of Cu2+ leads to a remarkable increase in absorption intensity along with a slight shift in the absorption peak. To further establish the Cu2+ response of C1, UV-titration of a solution of C1 (20 mM) with increasing concentration (0–600 mM) of Cu2+ was performed. A steady increase in the absorption intensity of the peak at 288 nm (Fig. 3b) was observed. UV-titration of complexes C2 and C3 with increasing concentrations of Cu2+ also showed an increase in the absorption intensity of the peak at 293 nm (p-p*)indicating the selective sensing capability of the complexes C1-C3 for Cu2+. 2.5.2. Fluorescence measurements The excitation of the absorption band at lmax = 288 nm caused emissions at 470 nm for C1. Fluorescence titrations of C1 with different metal cations were carried out and the results are shown in Fig. 4a. The addition of Cd2+, Zn2+, Na+ or K+ induced a very minor change in the emission intensity of C1, whereas other cations such as Al3+, Fe3+, Ni2+ and Co2+ triggered partial emission quenching through electron and/or energy transfer process [34]. In contrast, Cu2+ quenched the fluorescence completely (Fig. 4a). A detailed spectrofluorometric titration was performed by adding 0–600 mM of Cu2+ to solutions of C1 (20 mM) in Tris–HCl buffer at 25 1 C .Upon addition of increasing amounts of Cu2+, a rapid decrease in the intensity of the emission band occurred (Fig. 4b). Similar results were obtained for C2 and C3. The photograph in Fig. 5a shows the quenching of fluorescence that occurs when Cu2+ (400 mM) was added to a solution of C1 (20 mM). No such quenching was observed on the addition of other
Fig. 3. a) Absorption spectra of C1 (20 mM) in buffer-DMSO (v/v = 9:1), in presence of different metalcations (2 equiv.). b) Absorption spectra of C1 (20 mM) in buffer-DMSO (v/ v = 9:1), in presence of different equivalence of Cu2+ cation (0–600 mM).
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Fig. 4. a) Emission spectra of C1 (20 mM) upon addition of 2 equivalents of different metal ions in buffer-DMSO (v/v = 9:1) solutions when excited at 288 nm. b) Emission spectra of C1 (20 mM) with various amounts of Cu2+ (0–600 mM) in buffer-DMSO (v/v = 9:1) solutions (lex = 288 nm).
metal ions. A gradual decrease in fluorescence intensity was also observed on addition of increasing amounts of Cu2+(0–600 mM) as shown in Fig. 5b .The detection limit for detection of Cu2+ ions is as low as 2.0 mM. The detection limit was estimated from the linear fitting curve,where the spectral change could begin to occur based on the linear equation of Y (lem of C1) = B(intercept) + A (slope) x X (Cu 2+concentrations). 2.6. Evidence for replacement of VO2+ by Cu2+ To get deep insight into the changes taking place after addition of Cu2+ that results in the quenching of fluorescence of the complexes, electrospray ionization (ESI) mass spectra of complexes C1-C3 in the presence of Cu2+ were recorded. The mass
spectra exhibited m/z peaks corresponding to the mixed ligand [Cu (dipeptide)(MFL)(H2O)] complexes indicating complete replacement of VO2+ and complexation by Cu2+ (Fig. S4). Replacement of the VO2+ by Cu2+ and formation of [Cu (glyleu) (MFL)(H2O)] complexes was also confirmed by the absence of V¼O peak [y(V=O) = 976 cm1) in the FTIR spectrum of C1 in the presence of Cu2+ (Fig. 6). Similar results were obtained for complexes C2 and C3. The quenching of fluorescence upon complexation with Cu2+ originate partly due to interactions between the excited MFL ligand and the non-excited Cu2+.The electronic excited energy may be transferred from MFL p molecular orbital to the Cu2+ dp molecular orbital by energy transfer and electron transfer process. The Cu2+ ion is formally reduced and the ligand MFL is formally oxidised. The p electron in the higher energy orbital of the ligand is transferred to an electron deficient Cu2+ ion quencher. The hole left due to promotion of an electron, accepts an electron from the electron rich quencher [35]. Other than electron transfer process, energy transfer process can also cause quenching of fluorescence [35]. Energy is transferred from the excited state of the complexes C1– C3 to the non-excited Cu2+ ion on complexation, by non-radiative decay thus quenching fluorescence. The results imply that the complexes can act as chemosensors for detecting Cu2+ due to replacement of the VO2+ by Cu2+. 2.7. Conclusion
Fig. 5. (a) Photograph of C1 (20 mM) solutions containing different metal ions (400 mM) at lex = 254 nm. (b) Photograph of C1 (20 mM) solutions containing Cu2+ (0–600 mM). (0–30 equivalence).
Three novel mixed ligands complexes of VO2+, (C1–C3) with the ligands moxifloxacin and dipeptides have been synthesised and studied as selective probes for Cu2+ ions. The complexes emit at 450 nm on excitation at 288/293 nm. Presence or addition of Cu2+ ions show quenching of fluorescence intensity of the complexes due to replacement of VO2+ ion from the complexes and form the Cu2+ mixed ligand complexes as evident from ESI mass spectral and IR spectral studies. The quenching of fluorescence is attributed to electron and or energy transfer from the excited fluorophore to the non-excited Cu2+ ion. The present study demonstrates that the mixed ligand vanadyl complexes (C1–C3) can be used as chemosensors for selective detection of Cu2+.
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Fig. 6. IR spectral changes of C1 on addition of Cu2+ solution.
Acknowledgements The authors acknowledge University Grants Commission, New Delhi, India for providing financial support and Department of Science and Technology(DST) for DST INSPIRE Fellowship. Authors are grateful to the Head, Department of Chemistry, The Maharaja Sayajirao University of Baroda, for providing with the necessary laboratory facilities. 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. jphotochem.2016.12.004. References [1] Y. Yamaguchi, W. Ding, C.T. Sanderson, M.L. Borden, M.J. Morgan, C. Kutal, Coord. Chem. Rev. 251 (2007) 515. [2] K.M.C. Wong, V.W.-W. Yam, Coord. Chem. Rev. 251 (2007) 2477. [3] K.E. Augustyn, E.D.A. Stemp, J.K. Barton, Inorg. Chem 46 (2007) 9337. [4] S.R. Forrest, M.E. Thompson, Chem. Rev. 107 (2007) 923. [5] Q. Zhao, F. Li, C. Huang, Chem. Soc. Rev. 39 (2010) 3007. [6] J. Zhao, S. Ji, W. Wu, W. Wu, H. Guo, J. Sun, H. Sun, Y. Liu, Q. Li, L. Huang, RSC Adv. 2 (2012) 1712. [7] G. Zhou, W.-Y. Wong, X. Yang, Chem. Asian J. 6 (2011) 1706. [8] B. Happ, A. Winter, M.D. Hager, U.S. Schubert, Chem. Soc. Rev. 41 (2012) 2222. [9] J. Kalinowski, V. Fattori, M. Cocchi, J.A.G. Williams, Coord. Chem. Rev. 255 (2011) 2401. [10] Y. You, S.Y. Park, Dalton Trans. (2009) 1267. [11] D.L. Maa, V.P.Y. Maa, D.S.H. Chan, K.H. Leung, H.Z. He, C.H. Leung, Coord. Chem. Rev. 256 (2012) 3087. [12] (a) Q. Wu, E.V. Anslyn, J. Am. Chem. Soc. 126 (2004) 14682; (b) Z. Xu, X. Qian, J. Cui, Org. Lett. 7 (2005) 3029; (c) Y.Q. Weng, F. Yue, Y.-R. Zhong, B.H. Ye, Inorg. Chem. 46 (2007) 7749. [13] E.M. Zoupa, S.P. Perlepes, V. Hondrellis, J.M. Tsangaris, J. Inorg. Biochem. 55 (1994) 217.
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