Journal of Luminescence 141 (2013) 48–52

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Journal of Luminescence

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Thiosemicabazone based fluorescent chemosensor for transition metal ionsin aqueous medium

Duraisamy Udhayakumari, Sivalingam Suganya, Sivan Velmathi n

Organic and Polymer Synthesis Laboratory, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India

a r t i c l e i n f o

Article history:Received 16 October 2012Received in revised form8 March 2013Accepted 15 March 2013Available online 26 March 2013

Keywords:ThiosemicarbazoneTransition metalsFluorescence spectroscopyPET

13/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jlumin.2013.03.023

esponding author. Tel.: þ91 431 2503640, þ91 431 2500133.ail addresses: velmathis@nitt.edu, svelmathi@

a b s t r a c t

Highly efficient fluorescent chemosensors for metal ions have been synthesized by using thiosemicar-bazide and aromatic aldehydes. Detection of transition metal ions was performed via UV–vis andfluorescence spectroscopic methods. This is the first report on thiosemicarbazone based sensor capableof detecting transition metal ions in aqueous medium. The binding constant, stoichiometry of thecomplex were confirmed by using B–H plot and Job's plot method. The fluorescence enhancement ofthiosemicarbazones on binding with Hg2þ , Zn2þ , Co2þ , Ni2þ and Sn2þ ions is due to the inhibition ofphotoinduced electron transfer mechanism whereas, quenching of fluorescence is attributed to thephotoinduced electron transfer mechanism in case of Cu2þ and Mn2þ ions.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

The development of artificial receptors for monitoring heavy andtransition metal ions has received considerable attention owing tothe wide application in environmental and analytical chemistry[1–3]. Fluorescent chemosensor supplies high sensitivity towards apractical application in biological and cell imaging field and can bedirectly used as chemosensors with fiber optic systems [4,5]. Copperis the third most abundant metal existing in human cells, tissues andespecially in liver [6–8]. A high concentration of copper in neuronalcytoplasm may contribute to the etiology of Alzheimer's disease[9,10]. Cyanocobalamin, a natural organometallic compound whichplays a vital role in the metabolism contains cobalt as the centralmetal atom. Its deficiency in human body may lead to pathologicalconditions and acts also a significant environmental pollutant[11–15]. Nickel is a very important element among various heavymetals; it is used in many industries, catalytic process and present invarious effluents. Nickel (II) ion can cause disorders of centralnervous system and cancer in the nasal cavity and lungs. In 1990the International Agency for Research on Cancer (IARC) classifiednickel compounds as group I carcinogenic to humans [16–20].

Mercury is one of the toxic and heavy metals that are present inthe earth crust in 0.08 ppm. In the environment mercury exists inthree forms, elemental, organic and inorganic. High concentrationof mercury causes lot of human health problems [21–23]. Zinc is thesecond most abundant transition metal ion in the human biologicalsystem. Zinc (II) is present in approximately 300 enzymes, either fora structural purpose or as a part of a catalytic site. Zinc is also

ll rights reserved.

1 431 2500133;

hotmail.com (S. Velmathi).

known to have a role in neurological disorders, such as Alzheimer'sdisease, Epileptic seizures and Parkinson's disease [24–28]. Zinc andcadmium both have similar chemical properties. Therefore, Zn2þ

and Cd2þ cause alike spectral changes when coordinated with afluorosensor. Thus, it is necessary to develop a sensor that selec-tively senses Zn2þ in the presence of Cd2þ [29–31]. Developingfluorescent sensor that can detect different kinds of toxic metalcations in manner of simplicity, high sensitivity and time is ofconsiderable interest [32]. Here, we report the thiosemicarbazonebased compounds derived from the condensation of thiosemicar-bazide and substituted aldehydes as fluorescent and optical sensorsfor transition metal ions, whose synthesis procedure is welldocumented [33,34]. Thiosemicarbazones are a class of compoundsshowing promise in the treatment of many diseases, cancer inparticular, and their development is still in progress. The chemistryof thiosemicarbazone ligands has been receiving considerableattention primarily because of their bioinorganic relevance. Thoughthiosemicarbazones are widely used in the field of medicinalchemistry, their application as sensors has not been explored indetail. These sulfur and/or nitrogen heterocyclic azo dyes providestrong binding affinity prompted us to use thiosemicarbazonebased dyes as chemosensors. To the best of our knowledge, this isthe first report on R1, R2 and R3 as a fluorescent and optical sensorfor transition metal ions.

2. Experimental

2.1. Chemicals and spectroscopic measurements

Thiosemicarbazide, salicylaldehyde, 5-nitrosalicylaldehyde,2,4-dihydroxybenzaldehyde, iron (III) chloride, cobalt (II) chloride,

D. Udhayakumari et al. / Journal of Luminescence 141 (2013) 48–52 49

nickel (II) chloride, copper (II) chloride, zinc (II) acetate, cadmium(II) acetate, tin (II) chloride, lead (II) acetate, manganese (II)acetate, chromium (II) chloride, mercury (II) nitrate and analyticalgrade solvents such as acetonitrile (CH3CN) and ethanol (EtOH)were purchased from Sigma Aldrich and used as such. Proton NMRspectra were obtained using a Bruker 400 MHz spectrometer usingTetramethylsilane (TMS) as an internal standard, IR spectra wasrecorded in pellet mode on a Perkin-Elmer Spectrum One FTIRspectrometer. Shimadzu UV-2600 UV–vis spectrophotometer wasused to record UV-visible spectra using quartz cell with 1 cm pathlength. Fluorescence emission spectra were recorded in a Shi-madzu RF-5301 PC spectrofluorophotometer at a scan rate of500 nm/slit width Ex: 10 nm; Em: 10 nm. Excitation wavelengthwas set at 360 nm. 2.5�10−5 M solution of the receptors in CH3CNand 1.5�10−3 M aq. solutions of the cations were prepared. UVtitrations were carried out by the incremental addition of 0.2 eq.(10 μL)–2 eq. (100 μL) guest solutions to 3 ml of receptors in theUV cuvette. The same solutions were used for fluorescent titra-tions also. The receptors and cation solutions were prepared as2.5�10−5 M in CH3CN for Job's plot.

Table 1Electronic spectra of R1, R2 and R3.

S. no. Receptor λmax (nm)

1 R1 235, 300, 3302 R2 235, 300, 330, 4503 R3 240, 300, 330

2.2. Synthesis and characterization of sensors R1–R3

All the sensors were well characterized by 1H NMR, FT-IR andUV–vis spectroscopic techniques. A hot ethanolic solution ofthiosemicarbazide was slowly added to a solution of substitutedsalicylaldehyde in ethanol. The reaction mixture was reflux at75–80 1C for 3 h, yielding the precipitate of R1–R3 (Fig. 1). Afterevaporating the solvent in vacuum, the residue was filtered andrecrystallized with ethanol.

R1: yield: 80%; m.p. 220 1C; IR (KBr, cm−1) v: (C¼S) 949, (C¼N)1608, (N–H) 3122, (OH) 3424. 1H NMR (DMSO-d6 δppm) 6.78–6.86 m (aromatic H), 7.18–7.22 m (aromatic), 7.91 m (aromatic),8.09 s (NH2), 8.35 s (HC¼N), 9.87 s (NH), 11.36 s (OH).R2: Yield: 83%; m.p. 224 1C; IR (KBr, cm−1) v: (C¼S) 943, (C¼N)1603, (N–H) 3122, (OH) 3415. 1H NMR (DMSO-d6 δppm) 7.02–7.04 m (aromatic H), 8.08–8.11 m (aromatic), 8.19 m (aromatic),8.25 s (NH2), 8.36 s (HC¼N), 8.83 s (NH), 11.49 s (OH).R3: yield: 75%; m.p. 192 1C; IR (KBr, cm−1) v: (C¼S) 967, (C¼N)1620, (N–H) 3167, (OH) 3466. 1H NMR (DMSO-d6 δppm) 6.24–6.26 m (aromatic H), 6.28–6,29 m (aromatic), 7.64–7.66 m (aro-matic), 7.72 s (NH2), 7.92 s (OH) 8.23 s (HC¼N), 9.74 s (NH),11.15 s (OH). (For 1H NMR, FT-IR spectra see supportinginformation).

3. Results and discussion

3.1. UV–vis spectroscopic studies

Thiosemicarbazide with three different substituted aldehydescapitulate three sensors R1–R3 in good yield. The reason forchoosing NO2 as substituent (R2) is mainly because of its electronwithdrawing nature. It will enhance the acidity of the hydroxylproton present in the receptor 2 and form a strong bond with the

Fig. 1. Structure of receptor

cations. By increasing the electron withdrawing properties, we canbe able to get strong complexes with higher binding constants.And in R3 –OH group acts as an auxochrome. It has both n and πe-s, so that these show n-πn transition in addition to π-πn

transitions. Because of these reasons we expect the bindingaffinity of receptor with metal cations is increasing. The bindingaction of receptors with metal ions was investigated using UV–visspectroscopic titration method. The titrations were carriedout with receptors (2.5�10–5 M) in organic medium and cationsin aqueous medium (1.5�10–3 M). Electronic spectra of R1, R2 andR3 displayed three different transitions and the λmax valuesare listed in Table 1. Fig. 2a conceals the electronic spectrum ofR1 with different metal cations. In the gradual addition of Zn2þ

into R1, the band at 330 nm decreases and the band at 235 nmincreases with the appearance of new band around 400nm (Fig. 2b). A similar trend was observed upon the incrementaladdition of Co2þ , Ni2þ and Cu2þ (see supporting information).But, other guest solutions such as Fe3þ , Cd2þ , Sn2þ , Mn2þ , Pb2þ ,Hg2þ and Cr2þ showed zero effect with receptors' solutions.Interestingly, selective colorimetric response towards Hg2þ andSn2þ was observed in case of R2. The color change was visualizedby direct eye experiments, visual inspection of the R2 showed thecolor turn-off from green to colorless by adding 200 mL of Hg2þ

(Fig. 2c inset) and Sn2þ ions (Fig. S10 inset). The color change canbe attributed to the complex formation. During the incrementaladdition of Hg2þ into R2, the absorbance band at 210 nm wasgetting intensified and the band at 310 nm and 450 nmwere beingdiminished (Fig. 2c). Similar spectra were observed upon increas-ing the concentration of Sn2þ into R2 (see supporting informa-tion). There are no optical changes observed upon the excessaddition of other metal cations into R2. Fig. 2d accentuates theabsorbance changes of different metal ions into R3. With thecontinuing addition of Mn2þ into R3, the band at 400 nm gotamplified and the bands at 230 and 330 nm got reduced (Fig. 2e).The red shift in the absorption band can be attributed to the ligandto metal charge transfer (LMCT) complex formation. But no opticalchange was observed when other metal ions were added to R3. SoR3 was selectively sensing Mn2þ in the presence of other metalions. Fig. 2(f) and (g)shows the relative absorbance of R1 and R3with metal ions. As we expected, three receptors show their ownselectivity in sensing transition metal ions due to the difference inthe ring substitution.

3.2. Binding constant and Jobs plot studies

The binding constants (Kapp) of metal ions with receptors werecalculated using B–H plot method. From the non-linear least square

s R1, R2 and R3.

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Fig. 2. (a) UV–vis spectral changes of R1 (2.5�10–5 M, in CH3CN) upon titration with different metal ions (1.5�10–3 M, solution in H2O). (b) UV–vis spectral changes of R1(2.5�10–5 M, soln in CH3CN) upon titration with Zn2þ (1.5�10–3 M, solution in H2O) (inset: changes of absorbance upon addition of Zn2þ ion at 375 nm). (c) UV–visspectral changes of R2 (2.5�10–5 M, solution in CH3CN) upon titration with Hg2þ (1.5�10–3 M, solution in H2O) (inset: changes of absorbance upon addition of Hg2þ ion at450 nm). (d) UV–vis spectral changes of R3 (2.5�10–5 M, in CH3CN) upon titration with different metal cations (1.5�10–3 M, solution in H2O). (e) UV–vis spectral changes ofR3 (2.5�10–5 M, soln in CH3CN) upon titration with Mn2þ (1.5�10–3 M, solution in H2O) (Inset: changes of absorbance upon addition of Mn2þ ion at 400 nm). (f) Therelative absorbance of R1 (2.5�10−5 M) at 420 nm with various metal ions (2eq.). (g) The relative absorbance of R3 (2.5�10−5 M) at 420 nm with various metal ions (2eq.).

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fit graph, the binding constants calculated for R1 with Co2þ , Ni2þ ,Cu2þ and Zn2þ were 1.18�104, 1.27�104, 1.21�104 and 1.50�104

respectively. The binding constants of R2 with Hg2þ and Sn2þ were

2.32�104 and 1.48�104. Binding affinity of R3 with Mn2þ is3.17�104 (For B–H plot see supporting information). The Job's plotstudies revealed that the stoichiometry ratio of the complex formed

D. Udhayakumari et al. / Journal of Luminescence 141 (2013) 48–52 51

between R1 with Co2þ , Ni2þ , Cu2þ and Zn2þ is 2:1 whereas,stoichiometry ratio is 1:1 for all the metal ions under investigationin case of R2. The stoichiometry ratio of the complex formedbetween R3 with Mn2þ is 1:1 (see supporting information).

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Fig. 3. (a) Fluorescence titration spectrum of R1 upon the gradual addition of (0–100 mgradual addition of (0–100 mL) different metal ions in H2O. (c) Fluorescence titration specof fluorescence emission upon addition of Zn2þ ion at 475 nm) (excited at 360 nm). (d) Fions in H2O (inset: changes of fluorescence emission upon addition of Hg2þ ion at 450 naddition of (0–200 mL) Mn2þ ions in H2O (inset: changes of fluorescence emission uponemission of R1 (2.5�10−5 M) at 420 nm with various metal ions (2eq.) (g) The relative fl

3.3. Fluorescence spectroscopic studies

The fluorescence response of sensors R1–R3 towards variousmetal ions was investigated. Fluorescence spectra of free receptors

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L) different metal ions in H2O. (b) Fluorescence titration spectrum of R2 upon thetrum of R1 upon the gradual addition of (0–100 mL) Zn2þ ions in H2O (inset: changesluorescence titration spectrum of R2 upon the gradual addition of (0–200 mL) Hg2þ

m) (excited at 360 nm). (e) Fluorescence titration spectrum of R3 upon the gradualaddition of Mn2þ ion at 425 nm) (excited at 360 nm). (f) The relative fluorescenceuorescence emission of R2 (2.5�10−5 M) at 420 nm with various metal ions (2eq.).

Fig. 4. The proposed binding mechanism of R1, R2 and R3 with metal ions.

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R1–R3 showed emission bands at 475 nm, 415 nm and 450 nmrespectively. Fig. 3(a) and (b) displays the emission spectra uponthe addition of various metal ions in R1 and R2. R1 with anincrease in the concentration of Zn2þ ions the emission bandenhances at 475 nm (Fig. 3c). Similarly the stepwise addition ofCo2þ , Ni2þ ions into R1 displays fluorescence enhancement (seesupporting information). A solution of R1 showed no change inemission spectra upon addition of various metal ions (Fe3þ , Cd2þ ,Cr2þ , Sn2þ , Pb2þ , Hg2þ and Mn2þ). Due to the paramagneticnature of Cu (II) ion, the fluorescence quenching was observedwith the gradual addition of Cu2þ solution to R1 (see supportinginformation) On the successive addition of Hg2þ solution to R2,the emission band at 415 nm was intensified (Fig. 3d). Similaremission spectrumwas observed for Sn2þ with R2 (see supportinginformation). Other metal ions with R2 did not show any sig-nificant changes in the emission spectra. Fluorescence quenchingwas observed upon the step by step addition of Mn2þ ions into R3(Fig. 3e). The fluorescence enhanced response observed in case ofZn2þ , Ni2þ , Co2þ ions with R1 and Hg2þ , Sn2þ with R2 may bedue to the inhibition of the photoinduced electron transfermechanism in the complex. The fluorescence quenching responseof R1 with Cu2þ and R3 with Mn2þ could be explained by thephotoinduced electron transfer mechanism. Fig. 3f and g showsthe relative fluorescence emission of R1 and R2 with metal ions.The proposed binding mechanism and structure formed betweenthe metal ions and the receptors (R1–R3) are shown in Fig. 4.

4. Conclusion

In summary, optical and fluorescence emission properties of as-synthesised receptors R1–R3 were studied. Sensor R3 exhibitshighly selective and sensitive recognition towards Mn2þ in thepresence of the other metal ions. The complexation of the R1withCo2þ , Ni2þ , Zn2þ and R2 with Hg2þ , Sn2þ showed that anenhanced fluorescence change, is presumably due to the inhibitionof photo induced electron transfer mechanism. The complexationof R1 with Cu2þ and R3 with Mn2þ led to fluorescence quenching.The fluorescence turn-off is probably due to the photo inducedelectron transfer mechanism.

Acknowledgments

Authors express their thanks to DRDO (ERIP/ER/1006004/M/01/1333 dated 23–05–2011) for financial assistance in the form of amajor sponsored project.

Appendix A. Supplementary information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/%2010.1016/j.jlumin.2013.03.023.

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