A new selective fluorescent sensor for Zn(II) ions based on poly(azomethine-urethane)

Tetrahedron Letters xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet

A new selective fluorescent sensor for Zn(II) ions based on poly(azomethine-urethane) _ Ali Avcı a,b, Ismet Kaya b,⇑ a b

Celal Bayar University, Faculty of Sciences and Arts, Department of Chemistry, 45140 Manisa, Turkey Çanakkale Onsekiz Mart University, Faculty of Sciences and Arts, Department of Chemistry, Polymer Synthesis and Analysis Lab., 17020 Çanakkale, Turkey

a r t i c l e

i n f o

Article history: Received 4 August 2014 Revised 24 January 2015 Accepted 18 February 2015 Available online xxxx Keywords: Poly(azomethine-urethane) Zn(II) sensor Fluorescence sensor Optical sensor

a b s t r a c t A new and selective fluorescent sensor based on poly(azomethine-urethane) (PAMU) is synthesized and characterized by FT-IR, 1H NMR, and size exclusion chromatography (SEC) techniques. The new sensor shows a high selectivity for Zn(II) over other metal cations in DMF/deionized H2O (1:2, v/v). The fluorescence sensor gives a linearly and highly stable response to Zn(II) as an increasing emission peak at 595 nm. The sensitivity limit of the new sensor is found to be 11.4  103 mol L1. The results show that the proposed sensor can be efficiently used as a simple method for the detection of Zn(II) ions. Ó 2015 Elsevier Ltd. All rights reserved.

The design and synthesis of new chemosensors for the efficient detection of heavy or transition metal ions are important research topics in analytical, biomedical, and environmental sciences.1 These metal ions affect biological systems in many different ways ranging from being essential trace elements to acting as acute toxins.2,3 Among those ions, the zinc ion (Zn2+) is the second most abundant transition metal ion in the human body next to iron. It is indispensable for mediating many enzyme-catalyzed reactions, and therefore plays crucial roles in many important biological processes.4,5 Many enzymatic, cellular, and neurological functions are under the direct control of Zn2+ ions.6,7 The zinc ion is also known to have a role in neurological disorders such as Alzheimer’s disease, Parkinson’s disease, and epileptic seizures.8,9 There are several methods used for the determination of heavy metals such as atomic absorption spectrometry (AAS),10,11 fast neutron activation analysis,12 inductively coupled plasma atomic emission spectrometry,13 and electrochemical methods.14–16 However, these methods are relatively costly to employ and difficult to apply. As an alternative to these expensive methods, many optical sensors have been developed.17,18 Optical sensors have a number of advantages including low cost, high sensitivity, freedom from electrical interference, safety, and are easy to apply.19 In addition, many analytes which cannot be detected by other methods can be analyzed by optical sensors. Fluorescence signaling is highly sensitive and can be used directly for chemosensors with fiber ⇑ Corresponding author. Tel.: +90 286 218 00 18; fax: +90 286 218 05 33.

optic systems.20,21 Also, a significant advantage of this sensing method is the ease of handling. The development of a new generation of fluorescent agents containing chelating groups should enable the detection of heavy metals more efficiently. Polymers have also been used in this field to obtain more selective and sensitive sensors.22,23 Taking into consideration all the above factors, we describe an easy recognition and detection of Zn(II) in aqueous solutions via the fluorescence measurements employing a polyphenol derivative of poly(azomethine-urethane) (P-TP-4AP). The polymeric sensor was prepared according to the literature24,25 and was found to be a good candidate for optical detection of Zn(II). An important advantage of the proposed Zn(II) sensor is its ease of production and its fine-sensing and stable properties. The polyphenol derivative, poly(azomethine-urethane) (P-TP4AP), was synthesized in three steps.26 The first step consisted of the copolymerization reaction of 2,4-dihydroxybenzaldehyde (2,4-DHBA) with 2,4-toluenediisocyanate (TDI) to form a novel polyurethane (PU). The second step involved the graft copolymerization reaction of 4-aminophenol (4-AP) with the polyurethane to form a poly(azomethine-urethane) (PAMU) (TP-4AP).27 In the third step, the synthesized PAMU was converted into its polyphenol derivative (P-TP-4AP) through oxidative polycondensation in an aqueous alkaline medium using sodium hypochlorite (30%) as the oxidant, as reported in the literature.28 The yield of P-TP-4AP was found to be 70%.29 All the synthetic procedures are summarized in Scheme 1.

_ Kaya). E-mail address: [email protected] (I. http://dx.doi.org/10.1016/j.tetlet.2015.02.079 0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

_ Please cite this article in press as: Avcı, A.; Kaya, I.Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.02.079

_ Kaya / Tetrahedron Letters xxx (2015) xxx–xxx A. Avcı, I.

2

O

O HO

OCN

OH

C 2,4-DHBA O

NCO

+

H

TDI

Step I

PU

CH 3

O C N

O

THF reflux, Ar

CH 3

H

N C H

C

H

n

O DMF / MeOH 4-aminophenol

O

O

O

O C N

N C n H

Step III

CH 3

KOH(aq) NaOCl

H C N H

Step II

O O

O N C n H

O C N H

CH 3

C N H

OH

OH TP-4AP

P-TP-4AP Scheme 1. Synthesis of TP-4AP and P-TP-4AP.

O

O

C

H

H

O

N

N

C

O

C

CH3

N

H

OH Figure 1. Possible structure of the Zn2+ complex of P-TP-4AP formed after exposure to Zn2+ ions.

400 350

Zn2+ Pb2+ Ba2+ host Cd2+ Mn2+ Ni2+ Zr4+ Co2+ Cu2+ Cr3+ Fe2+

PL intensity (a.u.)

300 250 200 150 100 50 0 565

585

605

625 645 665 Wavelength (nm)

685

705

Figure 2. Fluorescence emission spectra (kex = 555 nm) of P-TP-4AP (0.333 g L1) in the presence of 6.66  102 mol L1 solutions of metal ions in 33% aqueous DMF (v/v). The excitation and emission slit widths were 5 nm.

The optimum excitation and emission wavelengths of P-TP-4AP (0.333 g L1) in DMF were found to be 528 and 591 nm, respectively. Also, the value of Stoke’s shift of P-TP-4AP was found to be 63 nm. Stoke’s shift is important for a fluorescence sensor. Higher Stoke’s shift values result in very low background signals, and accordingly indicate that the material can be employed as a fluorescence sensor.24 P-TP-4AP has several chelating groups including the imine nitrogens and urethane linkages for complexation with metal ions. The Huckel method was used to calculate the charge density on P-TP-4AP.24,25 Also, the possible structure of P-TP-4AP with a Zn(II) atom is shown in Figure 1. According to the Huckel calculations and Figure 1, the charges on the nitrogen atom of the imine group and the oxygen atom of the urethane carbonyl (C@O) were calculated as 0.419 and 0.979, respectively. Also, the charge on the nitrogen atom of the urethane linkage was calculated as 0.242. According to these results, the imine nitrogen possesses a somewhat higher negative charge than that in the urethane group. This indicates that possible complexation could be mainly accomplished between the nitrogen atom of the imine group, the oxygen atom, and the Zn(II) ion.1,5,7 The polymer synthesized was exposed to different metal ions including Ba(II), Cd(II), Co(II), Cr(III), Cu(II), Fe(II), Mn(II), Ni(II), Pb(II), Zn(II), and Zr(IV), and the changes in the emission spectra were recorded (Fig. 2). A very low polymer concentration was chosen to minimize the photoluminescence (PL) intensity of the metal-free polymer solution over the working range. The results are summarized in Table 1. The results obtained show that when the polymer was exposed to Ba(II), Pb(II), and Zn(II) ions, the emission peak at 595 nm increased with respect to that of the free host. On the other hand, the same emission peak decreased when the polymer was exposed to Cd(II), Co(II), Zr(IV), Mn(II), Cu(II), Fe(II), Ni(II), and Cr(III) ions. The peak intensity increases on increasing the Zn(II) concentration. This result indicates that P-TP-4AP could be used as a very selective Zn(II) sensor in aqueous solutions. The effect of the Zn(II) ion concentration on the emission spectrum of the polymer is shown in Figure 3. The relative emission intensities (I  I0/I0) at 595 nm obtained from Figure 3 are plotted

Table 1 PL intensities of the polymer solutions in the presence of different cations Metal content

Zn(II)

Pb(II)

Ba(II)

Host

Cd(II)

Mn(II)

Ni(II)

Zr(IV)

Co(II)

Fe(II)

Cu(II)

Cr(III)

Iem (595)a

377

104

93

91

47

41

20

15

12

4

3

3

kex = 555 nm, slit width = 5 nm. Polymer concentration = 0.333 g L1 and metal concentration = 6.66  102 mol L1 [33% aqueous DMF (v/v)]. a Emission intensity at 595 nm.

_ Please cite this article in press as: Avcı, A.; Kaya, I.Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.02.079

500

400

450

350

400

300

PL intensity (a.u.)

PL intensity (a.u.)

_ Kaya / Tetrahedron Letters xxx (2015) xxx–xxx A. Avcı, I.

350 Increasing Zn2+ concentration

300 250 200 150

Zn2+ Zn2+- Pb2+ Zn2+- Mn2+ Zn2+ - Co2+ Zn2+ - Ni2+ Zn2+ - Zr4+ Zn2+ - Cu2+ Zn2+ - Cr3+

250 200 150 100

100

50

50

0 575

0 565

585

605

625

645

665

685

Wavelength (nm) Figure 3. Emission spectra of P-TP-4AP (0.333 g L1) solutions in 33% aqueous DMF (v/v) on exposure to different concentrations of Zn2+ solutions (mol L1, from bottom to top): 4.44  103, 6.05  103, 7.40  103, 1.11  102, 3.33  102, 6.66  102, and 1.33  101. Conditions: slit width = 5 nm, kex = 555 nm.

3.1

3

595

615

635 655 Wavelength (nm)

675

695

Figure 6. Fluorescence emission spectra of P-TP-4AP in 33% aqueous DMF solution (v/v) in the presence of Zn2+ ions and Zn2+ ion with other quenching ions: Pb2+, Mn2+, Co2+, Ni2+, Zr4+, Cu2+, and Cr3+. Conditions: slit width = 5 nm, kex = 555 nm, concentration of P-TP-4AP = 0.333 g L1.

a

I-I0/I0

2.7

2.1 y = 155.24x + 1.1155 R2= 0 .9803

1.5 0.008

0.004

0.012

[ Zn2+] 4.3

b I-I0/I0

3.7 Figure 7. The photostability results of P-TP-4AP and its Zn(II) complex in DMF.

y = 9.6149x + 2.7749 R2= 0 .9796

3.1

2.5 0.0

0.075 [ Zn2+]

0.15

Figure 4. Linearized responses of P-TP-4AP to Zn2+ ions [concentration range between 4.44  103 to 1.11  102 mol L1 (a) and 1.11  102 to 1.33  101 mol L1 (b)].

Zn2+

3.5 3.0 2.5 2.0 1.5 I-I0/I0

1.0 0.5

Pb2+

Ba2+

0 - 0.5 - 1.0 - 1.5

Cd2+

Mn2+ Co2+ Cr3+ Cu2+ Fe2+

Ni2+

Zr 4+ 2

Figure 5. Relative intensity changes of P-TP-4AP on exposure to 6.66  10 mol L1 solutions of different metal cations in 33% aqueous DMF (v/v).

-

Figure 8. Fluorescence emission spectra (kex: 555 nm) of P-TP-4AP homogenous film in the presence of 6.66  102 mol L1 solutions of metal ions in 33% aqueous DMF (v/v). The photograph of the film (top) is taken under sunlight. (Slit widths were 5 nm).

_ Please cite this article in press as: Avcı, A.; Kaya, I.Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.02.079

_ Kaya / Tetrahedron Letters xxx (2015) xxx–xxx A. Avcı, I.

4 Table 2 TGA data of P-TP-4AP First degradation step (°C)

P-TP-4AP a b c d

Tona

Tmax

261

296

b

Tend

c

396

Second degradation step (°C)

Weight loss (%)

Tstartd

Tmax

Tend

Weight loss (%)

40

396

437

1000

24

Residue at 1000 °C (wt%)

Losses of solvent/absorbed water (%)

24

12

Thermal degradation onset temperature. Maximum weight loss temperature. Thermal degradation end temperature. Thermal degradation start temperature.

versus the Zn(II) concentration and a linear spectrofluorometric response for Zn(II) concentration at 595 nm was obtained with very good regression coefficients, R = 0.9803 (Fig. 4a) and R = 0.9796 (Fig. 4b). This linear response could be used for the detection of Zn(II) concentration using the following equations for the concentration range between 4.44  103 to 1.11  102 mol L1 (a) and 1.11  102 to 1.33  101 mol L1 (b), Eqs. 1 and 2, respectively:

I  I0 ¼ 155:24 ½Zn2þ  þ 1:115 I0

ð1Þ

I  I0 ¼ 9:6169 ½Zn2þ  þ 2:7749 I0

ð2Þ

where I is the emission intensity of the tested sample at 595 nm, and I0 is the emission intensity of the metal-free polymer solution. The detection limit of the proposed sensor was determined by using these two equations and the intersection of the extrapolated linear regions. Firstly, these two equations were equalized with respect to each other, and then the detection limit was calculated. According to the calculations, the detection limit of the sensor was 11.4  103 mol L1.25 Other Zn(II) sensors with lower detection limits have been reported in the literature.1,7 However, the selectivity, easy and cheap production, as well as the facile sensing application make the present sensor a potentially superior candidate for Zn(II) sensing. The possible interference from other metal ions at the same wavelength (595 nm) is shown in Figure 5. Measurements were carried out using the same concentration (6.66  102 mol L1) of metal ions and a polymer concentration of 0.333 g L1. As seen in Figure 5, with the exception of Zn(II), the other metal ions gave no responses at the employed wavelength. However, Zn(II) shows a relatively high response in this region allowing the synthesized polymer to be used as a highly selective Zn(II) sensor. Reversibility is another important property for a sensor. Ion binding reactions based on chelators are reversible in principle and this is desired for continuous monitoring. In practice, however, most chelating reactions are irreversible. Reversible ion sensors are generally developed as active-sensing layers immobilized on a polymeric membrane. Optical sensors can be applied to flow systems (with the analyte solution passing a detection cell) or in a batch mode (where the sensor is exposed to a fixed volume of the analyte solution).30 The reversibility of a sensor can be determined using flow systems. In this study we envisaged a disposable sensor, which would be useful in solution form and that employed the batch mode. However, as mentioned above, a very low concentration of polymer solution is needed for the detection of Zn(II) content, resulting in a new, cheap and potentially more favorable method due to the selectivity, low-cost and easy production of the polymer, as well as the easy sensing application, making the present sensor a superior candidate for Zn(II). The fluorescence emission spectra of P-TP-4AP in the presence Zn2+ ions and with Zn2+ ions and other quenching ions including Pb2+, Mn2+, Co2+, Cu2+, Ni2+, Cr2+, and Zr4+ are shown in Figure 6. These spectra were obtained using a 0.333 g L1 P-TP-4AP

concentration, a 6.66  102 mol L1 Zn2+ concentration, and 3.33  102 mol L1 Pb(CH3COO)2.3H2O, Mn(CH3COO)2.H2O, Co(CH3COO)2.4H2O, Cu(CH3COO)2.H2O, Ni(CH3COO)2.4H2O, ZrCl4, or CrCl3. According to Figure 6, despite there being a decrease in the emission intensity, selectivity was observed. The photostability of the P-TP-4AP sensor was tested in DMF using a xenon arc lamp and a steady state spectrofluorometer in the ‘Time Based Measurements’ mode. The data were acquired at their maximum emission wavelengths during monitoring over 30 min. The acquired data for P-TP-4AP are shown in Figure 7. The fluorescence intensity of the sensor did not change over the 30-min period. The results show that the complex formed between P-TP-4AP and Zn(II) was stable and that no reaction took place within the first 30 min. Solid state fluorescence measurements of P-TP-4AP were recorded in PMMA gel matrix according to the literature.31 The fluorescence emission spectra of P-TP-4AP homogenous film in the presence of 6.66  102 mol L1 metal ions are shown in Figure 8. The color of the film of TP-4AP deposited onto the transparent polyester surface was light brown under sunlight. The obtained results show that when the polymer was exposed to each of the metal ions, the emission peak at 480 nm decreased. For this reason, P-TP-4AP is not suitable as a Zn(II) sensor in the solid state. The thermogravimetric analysis (TGA) results are summarized in Table 2. According to thermal analysis results, P-TP-4AP decomposes in two steps between 20 and 1000 °C. The first and second degradation temperature ranges (°C) are 200–396 and 396–1000 for P-TP-4AP, respectively. The initial degradation temperature and percentage carbine residue are 261 °C and 24% for P-TP-4AP. Also, the 12% weight loss between 20 and 125 °C is attributed to losses of moisture, adsorbed solvent, or monomer.26 In conclusion, the polymeric sensor produced a linear response over a Zn(II) concentration range of 4.44  103 to 1.33  101 mol L1 with a detection limit of 11.4  103 mol L1. The new sensor is cheap to prepare and highly selective for the determination of Zn(II). Selectivity studies were carried out with various other transition metal cations at the same concentration. It was found that P-TP-4AP showed high selectivity toward Zn(II) ions in solution. The stability of the sensor was determined using the fluorescence lifetime data and was found to be relatively high. As a result, P-TP-4AP can be used for the quantitative determination of Zn(II) ions in solution according to changes in the fluorescence. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.02. 079. References and notes 1. Hsieh, W. H.; Wan, C. F.; Liao, D. J.; Wu, A. T. Tetrahedron Lett. 2012, 53, 5848. 2. Park, G. J.; Lee, M. M.; You, G. R.; Choi, Y. W.; Kim, C. Tetrahedron Lett. 2014, 55, 2517.

_ Please cite this article in press as: Avcı, A.; Kaya, I.Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.02.079

_ Kaya / Tetrahedron Letters xxx (2015) xxx–xxx A. Avcı, I. 3. Gong, H. Y.; Zheng, Q. Y.; Zhang, X. H.; Wang, D. X.; Wang, M. X. Org. Lett. 2006, 8, 4895. 4. Shen, K.; Yang, X.; Cheng, Y.; Zhu, C. Tetrahedron 2012, 68, 5719. 5. Helal, A.; Rashid, M. H.; Choi, C. H.; Kim, H. S. Tetrahedron 2012, 68, 647. 6. Ngwendson, J. N.; Banerjee, A. Tetrahedron Lett. 2007, 48, 7316. 7. Mashraqui, S. H.; Betkar, R.; Ghorpade, S.; Tripathi, S.; Britto, S. Sensor Actuators B-Chem. 2012, 174, 299. 8. Frederickson, C. J.; Koh, J. Y.; Bush, A. I. Nat. Rev. Neurosci. 2005, 6, 449. 9. Ma, Y.; Chen, H.; Wang, F.; Kambam, S.; Wang, Y.; Mao, C.; Chen, X. Dyes Pigm. 2014, 102, 301. 10. Verma, R.; Gupta, B. D. Food Chem. 2015, 166, 568. 11. Zhua, H.; Xua, Y.; Liu, A.; Kong, N.; Shan, F.; Yang, W.; Barrow, C. J.; Liu, J. Sensor Actuators B-Chem. 2015, 206, 592. 12. Nunes, L. C.; Carvalho, G. G. A.; Júnior, D. S.; Krug, F. J. Spectrochim. Acta B 2014, 97, 42. 13. Peng, X.; He, H.; Xia, J.; Lou, Z.; Chang, G.; Zhang, X.; Wang, S. Tetrahedron Lett. 2014, 55, 3541. 14. Xiao, L.; Xu, H.; Zhou, S.; Songa, T.; Wang, H.; Li, S.; Gan, W.; Yuan, Q. Electrochim. Acta 2014, 143, 143. 15. Cui, L.; Wu, J.; Ju, H. Biosens. Bioelectron. 2015, 63, 276. 16. Xu, X.; Duan, G.; Li, Y.; Liu, G.; Wang, J.; Zhang, H.; Dai, Z.; Cai, W. ACS Appl. Mater. Interfaces 2014, 6, 65.

5

17. Wang, Y. W.; Shi, Y. T.; Peng, Y.; Zhang, A. J.; Ma, T. H.; Dou, W.; Zheng, J. R. Spectrochim. Acta A 2009, 72, 322. 18. Oter, O.; Ertekin, K.; Kılıncarslan, R.; Ulusoy, M.; Çetinkaya, B. Dyes Pigm. 2007, 74, 730–735. 19. Safavi, A.; Abdollahi, H. Anal. Chim. Acta 1998, 367, 167. 20. Kramer, R. Angew. Chem., Int. Ed. 1998, 37, 772. 21. Derinkuyu, S.; Ertekin, K.; Oter, O.; Denizaltı, S.; Çetinkaya, E. Anal. Chim. Acta 2007, 588, 42. 22. Adhikari, B.; Majmdar, S. Prog. Polym. Sci. 2004, 29, 699. 23. Li, N.; Xu, Q.; Xia, X.; Wang, L.; Lu, J.; Wen, X. Mater. Chem. Phys. 2009, 114, 339. _ Yıldırım, M.; Kamacı, M. Synth. Met. 2011, 161, 2036. 24. Kaya, I.; _ Kamacı, M. J. Fluoresc. 2013, 23, 115. 25. Kaya, I.; _ Avcı, A. Mater. Chem. Phys. 2012, 133, 269. 26. Kaya, I.; _ Yıldırım, M.; Avcı, A.; Kamacı, M. Macromol. Res. 2011, 19, 286. 27. Kaya, I.; _ Avcı, A.; Gültekin, Ö. Chin. J. Polym. Sci. 2012, 30, 796. 28. Kaya, I.; _ Avcı, A. J. Polym. Res. 2012, 19, 9780. 29. Kaya, I.; 30. Oehme, I.; Wolfbeis, O. S. Microchim. Acta 1997, 126, 177. _ Turk. J. Chem., in press, http://dx.doi.org/10.3906/kim31. Yıldırım, M.; Kaya, I. 1405-66.

_ Please cite this article in press as: Avcı, A.; Kaya, I.Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.02.079