Colorimetric chemosensor for multi-signaling detection of metal ions using pyrrole based Schiff bases Duraisamy Udhayakumari, Sivan Velmathi ⇑ Organic and Polymer Synthesis Laboratory, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India highlights R1 and R2 acts as fluorescent and colorimetric sensor for Fe 3+ , Cu 2+ , Hg 2+ and Cr 3+ . The binding constant of R2 was higher than R1 with 2:1 stoichiometry. R1 and R2 detect Fe 3+ , Cu 2+ , Hg 2+ and Cr 3+ metals at micromolar levels. R1 and R2 exhibits fluorescence quenching via PET mechanism. graphical abstract NH N NH 2 R R= H, R=-NO 2 article info Article history: Received 6 September 2013 Received in revised form 6 November 2013 Accepted 13 November 2013 Available online 21 November 2013 Keywords: Pyrrole Schiff bases Chemosensor Micromolar Quenching PET abstract Pyrrole based Schiff bases act as a highly sensitive probe for metal ions in aqueous medium. Both receptors R1 and R2 are sensitive towards Fe 3+ , Cu 2+ , Hg 2+ and Cr 3+ among the other metal ions. The sensing ability of the receptors are investigated via colorimetric, optical and emission spectroscopic studies. The binding stoichiometries of R1 and R2 with metal ions have been determined as 2:1 by using Job’s plot. The colorimetric receptors exhibited high sensitivity with a low detection limit of lM levels. In the presence of metal ions both receptors shows fluorescence quenching. This might be due to the photo induced electron transfer mechanism. The quenching constant was further determined using Stern–Volmer plot. Ó 2013 Elsevier B.V. All rights reserved. Introduction The development of new molecular systems for the colorimetric detection of cations has gained prime importance due to their significance in biological and environmental processes [1–3]. Currently, there is an active effort to develop molecular complexa- tion systems that binds with cation. Generally, pyrrole, –OH, –NH 2 , urea, thiourea, –CONH centers etc., act as binding sites for cations [4–16]. In particular, the sensing of metal ions has attracted grow- ing attention because of its great potential for biological and indus- trial applications. During recent years, there is an upsurge in the field of colorimetric sensing of alkali, alkaline–earth and transition metal ions by organic molecules [17,18]. Among the cations, special attention is devoted to develop chemo sensors for transi- tion metal ions and toxic metal ions: usually they address an envi- ronmental concern when present in uncontrolled amount but at the same time some of them like iron, cobalt, copper and zinc are present as essential elements in a biological system where mercury, chromium are very toxic metal ions. Iron is one of the important metal ions for most organisms, plays an important role in many biological processes and electron transfer processes in DNA and RNA synthesis [19,20]. Furthermore, iron homeostatic is an important factor involved in neuro-inflammation and progres- sion of Alzheimer’s disease and iron deficiency (hypoferremia) can be harmful [21–27]. Copper is a third most essential element, present in plants, animals and human. When the concentration of Cu 2+ increases its limit it causes Alzheimer’s, Parkinson’s, and Wilson diseases [28–30]. As mercury has an extremely toxic 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.083 ⇑ Corresponding author. Tel.: +91 431 2503640, +91 09486067404; fax: +91 431 2500133. E-mail addresses: [email protected], [email protected](S. Velmathi). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 428–435 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 428–435
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy
journal homepage: www.elsevier .com/locate /saa
Colorimetric chemosensor for multi-signaling detection of metal ionsusing pyrrole based Schiff bases
1386-1425/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.saa.2013.11.083
Duraisamy Udhayakumari, Sivan Velmathi ⇑Organic and Polymer Synthesis Laboratory, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
h i g h l i g h t s
� R1 and R2 acts as fluorescent andcolorimetric sensor for Fe3+, Cu2+,Hg2+ and Cr3+.� The binding constant of R2 was
higher than R1 with 2:1stoichiometry.� R1 and R2 detect Fe3+, Cu2+, Hg2+ and
Cr3+ metals at micromolar levels.� R1 and R2 exhibits fluorescence
quenching via PET mechanism.
g r a p h i c a l a b s t r a c t
NH
N
NH2
R
R= H,R=-NO2
a r t i c l e i n f o
Article history:Received 6 September 2013Received in revised form 6 November 2013Accepted 13 November 2013Available online 21 November 2013
Pyrrole based Schiff bases act as a highly sensitive probe for metal ions in aqueous medium. Bothreceptors R1 and R2 are sensitive towards Fe3+, Cu2+, Hg2+ and Cr3+ among the other metal ions. Thesensing ability of the receptors are investigated via colorimetric, optical and emission spectroscopicstudies. The binding stoichiometries of R1 and R2 with metal ions have been determined as 2:1 by usingJob’s plot. The colorimetric receptors exhibited high sensitivity with a low detection limit of lM levels. Inthe presence of metal ions both receptors shows fluorescence quenching. This might be due to thephoto induced electron transfer mechanism. The quenching constant was further determined usingStern–Volmer plot.
� 2013 Elsevier B.V. All rights reserved.
Introduction metal ions by organic molecules [17,18]. Among the cations,
The development of new molecular systems for the colorimetricdetection of cations has gained prime importance due to theirsignificance in biological and environmental processes [1–3].Currently, there is an active effort to develop molecular complexa-tion systems that binds with cation. Generally, pyrrole, –OH, –NH2,urea, thiourea, –CONH centers etc., act as binding sites for cations[4–16]. In particular, the sensing of metal ions has attracted grow-ing attention because of its great potential for biological and indus-trial applications. During recent years, there is an upsurge in thefield of colorimetric sensing of alkali, alkaline–earth and transition
special attention is devoted to develop chemo sensors for transi-tion metal ions and toxic metal ions: usually they address an envi-ronmental concern when present in uncontrolled amount but atthe same time some of them like iron, cobalt, copper and zincare present as essential elements in a biological system wheremercury, chromium are very toxic metal ions. Iron is one of theimportant metal ions for most organisms, plays an important rolein many biological processes and electron transfer processes inDNA and RNA synthesis [19,20]. Furthermore, iron homeostatic isan important factor involved in neuro-inflammation and progres-sion of Alzheimer’s disease and iron deficiency (hypoferremia)can be harmful [21–27]. Copper is a third most essential element,present in plants, animals and human. When the concentrationof Cu2+ increases its limit it causes Alzheimer’s, Parkinson’s, andWilson diseases [28–30]. As mercury has an extremely toxic
D. Udhayakumari, S. Velmathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 428–435 429
impact on the environment and human health. It is well-known;mercury can lead to dysfunctions of the brain, kidney, stomach,and central nervous systems [31–33]. Trivalent chromium playsan important role in the metabolism of carbohydrates, proteins,lipids and nucleic acids. The excess of chromium causes genotoxiceffects and the deficiency of chromium increases the risk for diabe-tes and cardiovascular diseases [34,35]. Many spectroscopic tech-niques are used for detecting metal ions. However, thesemethods are complicated and expensive. The chemosensors arevery convenient, sensitive detection, low-cost of equipment anddirect visual perception. In addition, some sensors can only be ap-plied in organic solvents, which limit their application in environ-mental systems. Therefore, it is essential to develop visual eyesensors for sensitive detection of Fe3+, Cu2+, Hg2+ and Cr3+ in phys-iologically suitable solvents. R1 and R2 were earlier reported by usas colorimetric and fluorescent chemosensors for fluoride andhydroxide anions [36]. Herein, we report the detection of Fe3+,Cu2+, Hg2+ and Cr3+ ions in aqueous analyte solution by R1 andR2. They exhibit a pronounced absorbance behavior toward Fe3+,Cu2+, Hg2+ and Cr3+ over other common interfering metal ions. R1and R2 with Fe3+, Cu2+, Hg2+ and Cr3+ shows higher binding con-stant with 2:1 stoichiometry. The receptors R1 and R2 detect themetal ions in micromolar levels and they exhibits fluorescencequenching, this might be due to the photoinduced electron transfermechanism.
NH
N
NH2
R= H (Receptor 1),R=-NO2 (Receptor 2)
R
Fig. 1. Structure of R1 and R2.
Experimental
Materials and methods
Pyrrole-2-carboxaldehyde, 2-Nitro-1,4-phenylene diamine, 1,4-phenylene diamine, iron (III) chloride, cobalt (II) chloride, nickel (II)chloride, copper (II) chloride, zinc (II) chloride, cadmium (II) acetate,tin (II) chloride, lead (II) acetate, mercury (II) nitrate, manganese (II)acetate, chromium (III) chloride, and analytical grade solvents suchas acetonitrile (CH3CN) and ethanol (EtOH) were purchased fromSigma Aldrich and used as such. Shimadzu UV-2600 UV–vis spec-trophotometer was used to record UV–visible spectra using quartzcell with 1 cm path length. Fluorescence emission spectra were re-corded in a Shimadzu RF-5301 PC spectrofluorophotometer at ascan rate of 500 nm/slit width with Ex: 10 nm Em: 10 nm. Excita-tion wavelength set was 300 nm. 5 � 10�5 M solution of R1, R2 inCH3CN and 1.5 � 10�3 M solutions of the cations in H2O were pre-pared. 0.2 eq. (10 lL) – 2 eq. (100 lL) of guest solution was addedto 3 ml of R1 and R2 taken in the UV cuvette.
Synthesis and characterization of sensors R1 and R2
Sensor R1: N-[(1E)-1H-pyrrol-2-ylmethylene] benzene-1,4-diamine1,4-Phenylene diamine (0.5 g, 1 mmol) was stirred with pyrrole
2-carboxaldehyde (0.435 g, 1 mmol) in dry dichloromethane(DCM) at 25 �C for 20 h. The reaction mass was concentrated undervacuum. The residue was triturated with n-hexane and filtered. Thissolid was again triturated with DCM, filtered and dried to yield pure(R1). (60% yield). 1H NMR (d ppm, 400 MHz, DMSO-d6): 11.4 (1H, s),8.3 (1H, s), 6.9–7.1 (3H, m), 6.3 (3H, m), 6.1 (1H, m), 5.0 (d, 2H) 13CNMR (d ppm, 100 MHz, DMSO-d6): 149.5, 149.1, 130.7, 123.7,121.5, 116.2, 109.6 IR (KBr, cm�1): 3658, 3232, 3108, 2972, 3043,2897, 2558, 2742, 2562, 2403, 1689, 1625, 1550, 1490, 1452, 1419,1334, 1311, 1246, 1203, 1164, 1012 LCMS: m/z 262.9, 185.8.
2-Nitro-1,4-phenylene diamine (0.6 g, 1 mmol) was refluxedwith pyrrole 2-carboxaldehyde (0.435 g, 1 mmol) in ethanol
(10 ml) for 16 h. Reaction mixture was concentrated under vacuumand the residue was triturated with n-hexane. The residue was fil-tered and dried to yield pure (2) (73% yield) 1H NMR (d ppm,400 MHz, DMSO-d6): 11.6 (1H, s), 8.3 (1H, s), 7.7 (1H, d), 7.4 (3H,m) 7.0 (1H, d), 6.9 (1H, d), 6.6 (1H, d), 6.1 (1H, d). 13C NMR (dppm, 100 MHz, DMSO-d6): 148.6, 144.3, 140.0, 130.5, 130.1,129.9, 123.6, 120.0, 116.0, 115.7, 109.6. IR (KBr, cm�1): 3480,3429, 3395, 3254, 3119, 2892, 1772, 1634, 1596, 1557, 1503,1451, 1369, 1339, 1249, 1095 LCMS: m/z 231.0.
Results and discussion
Cation sensing by colorimetric analysis
Receptors 1 and 2 (Fig. 1) were synthesized by simple conden-sation of pyrrole-2-carboxaldehyde with 1,4-phenylene diamineand 2-nitro-1,4-phenylene diamine respectively in good yields fol-lowing the procedure reported by us and well characterized by 1H,13C NMR, LCMS, FT IR and UV–vis spectroscopic techniques [36]. R1and R2 are very good sensor for biologically important anions likeF� and �OH ions. R1 turned from colorless to yellow and R2showed dramatic color change from yellow to permanganate colorin presence of F� and HO� ions without any interference fromother anions. To extend the scope of the R1 and R2, its sensitivitytowards metal ions in complete aqueous medium was probed.We investigated the recognition ability of the R1 and R2(5 � 10�5 M in CH3CN) by naked-eye colorimetric experimentsfor transition metal ions such as Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+,Pb2+, Hg2+, Mn2+, Sn2+ and Cr3+. When the metal ion solution inH2O (1.5 � 10�3 M) was added to the R1, the color of the solutionchanged from colorless to pale orange for Fe3+, olive green forCu2+, yellow for Hg2+ and green for Cr3+ as seen in Fig. 2a. Thepromising feature of the present sensor systems is that it gives dif-ferent color for different metal ions. Thus a single receptor cansense multi metal ions without any overlap. The receptor solutiondoes not show any visible color changes even with a large excess ofother metal ions like Co2+, Ni2+, Zn2+, Cd2+, Pb2+, Mn2+ and Sn2+.Upon the addition of metal ions into R2, the color was changedfrom colorless to dark yellow for Fe3+, pale olive for Cu2+ and paleyellow for Hg2+ and Cr3+ (Fig. 2b). The color change can be attrib-uted to the complex formation. Thus R1 and R2 are able to senseFe3+, Cu2+, Hg2+ and Cr3+ in aqueous medium, physiologically suit-able condition which is an added advantage.
Optical spectroscopic studies
The binding interaction studies or R1 and R2 in CH3CN againstcations of environmental relevance, such as Fe3+, Co2+, Ni2+, Cu2+,Zn2+, Cd2+, Pb2+, Hg2+, Mn2+, Sn2+ and Cr3+ shows sensitive responseto Fe3+, Cu2+, Hg2+ and Cr3+ in H2O. The change in the UV–vis absor-bance spectrum of R1 and R2 due to the addition of 200 lL of metal
Fig. 2a. Color changes of R1 (5 � 10�5 M soln in CH3CN) before and after the addition of 200 lL (2 eq.) of Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+, Mn2+, Sn2+ and Cr3+
(1.5 � 10�3 M soln in H2O) ions respectively.
Fig. 2b. Color changes of R2 (5 � 10�5 M soln in CH3CN) before and after the addition of 200 lL (2 eq.) of Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+, Mn2+, Sn2+ and Cr3+
(1.5 � 10�3 M soln in H2O) ions respectively.
250 300 350 400 450 500 550 6000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Abs
orba
nce
Wavelength (nm)
R1 R1+Fe3+
R1+Co2+
R1+Ni2+
R1+Cu2+
R1+Zn2+
R1+Cd2+
R1+Pb2+
R1+Hg2+
R1+Sn2+
R1+Mn2+
R1+Cr3+
Fig. 3a. UV–vis spectra of R1 (5 � 10�5 M, in CH3CN) upon titration with aqueoussolution of cations (R, R + Fe3+, R + Co2+, R + Ni2+, R + Cu2+, R + Zn2+, R + Cd2+,R + Pb2+, R + Hg2+, R + Mn2+, R + Sn2+ and R + Cr3+). R1 Fe(III) Co(II) Ni(II) Cu(II) Zn(II) Cd(II) Pb(II) Hg(II) Mn(II) Sn(II) Cr(III)
0.00
0.15
0.30
0.45
0.60
0.75
0.90
Abs
orba
nce
Fig. 3b. The relative absorbance of R1 at 450 nm with various metal ions (200 lL).
430 D. Udhayakumari, S. Velmathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 428–435
cations in H2O is shown in Fig. 3a and 3c. With the stepwise addi-tion of (0–200 lL) Fe3+, Hg2+ and Cr3+ ions to the R1, the band at350 nm declined and a new band formed at 425 nm. With theincreasing concentration of Cu2+, the band at 350 nm disappearedalong with progressive appearance of new band at 500 nm. Theisosbestic points at 374 nm (Fe3+), 397 nm (Cu2+), 362 nm (Hg2+)and 381 nm (Cr3+) were indicating the presence of unique complexin equilibrium with the R1 (see Supporting information). On suc-cessive addition of Fe3+, Hg2+ and Cr3+ to the R2 the band at350 nm reduced and a new band formed at 400 nm. The incremen-tal addition of Cu2+ into R2 resulted in the red shift of the 350 nmband to 400 nm. The new band formation may be due to the ligandto metal charge transfer complex formation (LMCT) and is respon-sible for the changes of color which helped in the ‘naked eye’detection of Fe3+, Cu2+, Hg2+ and Cr3+. The UV–vis change suggeststhat the receptor (R1 and R2) moiety is involved in complexation
with metal ions. The relative absorbance of receptors (R1 and R2)with all metal ions shows the selectivity of Fe3+ ions over other me-tal ions (Fig. 3b and 3d).
Binding constant, Job’s plot and detection limit studies
From the UV–vis spectroscopic measurements the associationconstants (Kapp) of R1 and R2 with Fe3+, Cu2+, Hg2+ and Cr3+ wasdetermined using Benesi–Hildebrand equation [37]. R2 showshigher binding constant than the R1 with metal ions. This maybe due to the presence of electron withdrawing group (–NO2) init. Job’s plot studies were done to find out the stoichiometry ofthe complex formed between the receptors and cations and itwas found that the complex formed was in 2:1 stoichiometries.The detection limit of metal ions by R1 and R2 was calculated from
0.0
0.2
0.4
0.6
0.8
1.0
1.2A
bsor
banc
e
Wavelength (nm)
R2 R2+Fe3+
R2+Co2+
R2+Ni2+
R2+Cu2+
R2+Zn2+
R2+Cd2+
R2+Pb2+
R2+Hg2+
R2+Mn2+
R2+Sn2+
R2+Cr3+
250 300 350 400 450 500 550
Fig. 3c. UV–vis spectra of R2 (5 � 10�5 M, in CH3CN) upon titration with aqueoussolution of cations (R, R + Fe3+, R + Co2+, R + Ni2+, R + Cu2+, R + Zn2+, R + Cd2+,R + Pb2+, R + Hg2+, R + Mn2+, R + Sn2+ and R + Cr3+).
Fig. 3d. The relative absorbance of R2 at 425 nm with various metal ions (200 lL).
D. Udhayakumari, S. Velmathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 428–435 431
the plot of UV–vis absorbance versus metal ions concentration.From the plot, detection limit was calculated using the formula3r/m. Where ‘r’ is the standard deviation and ‘m’ is the slope ofthe straight line. Thus the R1 and R2 can be utilized as an efficientFe3+, Cu2+, Hg2+ and Cr3+ ion sensor at micromolar level. Accordingto World Health Organization (WHO-1984), 0.1 mg/l of iron, 1 mg/lof copper and 0.001 mg/l of mercury of present in drinking water.Our receptor R1 and R2 detect iron, copper and mercury in micro-molar levels (10�6 M/L) which is lower or equal to maximum per-mitted amount of iron, copper and mercury in drinking waterdefined by World Health Organization. The binding constant, stoi-chiometry and detection limit values are listed in Table 1.
Fluorescence spectroscopic study
Generally, fluorescence emission spectroscopy is more sensitivetoward small changes that affect the electronic properties ofmolecular receptors. The fluorescent response of R1 to Fe3+, Cu2+,Hg2+ and Cr3+ was further investigated with fluorescence spectrum.R1 shows, emission band at 450 nm. Fluorescence emissionchanges of R1 in the presence of all cations like Fe3+, Co2+, Ni2+,
Cu2+, Zn2+, Cd2+, Pb2+, Hg2+, Mn2+, Sn2+ and Cr3+ is shown inFig. 4a. Fig. 4b shows the decrease in the emission intensity ob-served upon the addition of Fe3+, Cu2+, Hg2+ and Cr3+. Fig. 4c illus-trates the fluorescence emission changes of R2 upon the addition ofvarious metal ions as aqueous solutions. The emission band wasobserved at 425 nm in the fluorescence spectrum of R2. Additionof Fe3+, Cu2+, Hg2+ and Cr3+ ions decreased the fluorescence inten-sity of R2. The response of the fluorescence spectra upon addingother ions to R2 was investigated. Other ions gave no distinct re-sponses in H2O. After the incremental addition of Fe3+, Cu2+, Hg2+
and Cr3+ ions, the emission of R1 and R2 is markedly quenched(see Supporting information). Fig. 4d shows the relative intensityof R1 at 400 nm with various metal ions (200 lL). R1 and R2 maythus be used as a ‘‘turn-off’’ molecular probe towards metal ions.In the case of anions with R1 and R2, the enhanced red shift oc-curred due to the increased basicity of the anions. It is logical to ex-pect for an increase in acidity of R2 upon electronic excitation inthe anion bound complex, such that proton transfer might takeplace from R2 to anions to account for the anion emission at450 nm [36]. But in the case of cations with R1 and R2, shows fluo-rescence quenching, is might be due to the photoinduced electrontransfer mechanism.
Fluorescence quenching is measured quantitatively with theStern–Volmer equation. The calculated Stern–Volmer constantKsv (M�1) and non-linear regression correlation coefficient of R1and R2 with Fe3+, Cu2+, Hg2+ and Cr3+ are shown in Table 2. As seenin Table 2, all the non-linear regression coefficients wereapproximately 0.8, which suggests a non-linear correlation. Thisnon-linear correlation between the quencher concentration andFo/F[Q] is in accordance with the Stern–Volmer equation. The non-linearities in the Stern–Volmer plots are interpreted in terms of thesphere of action defined in the static quenching model. If the fluo-rophore, F, and quencher, Q, associate in the ground state formingnon-fluorescent complexes, FQ, then true dynamic quenchingoccurs.
Sensing in the presence of competing ions
The R1 and R2 exhibits colorimetric and fluorescence sensingfor anions like F�, HO� and cations like Fe3+, Cu2+, Hg2+ and Cr3+
ions in the presence of other anions and cations. In both R1 andR2 the binding ability of Fe3+ is higher than the Cu2+, Hg2+ andCr3+. In the present study, they show strong binding for Fe3+ bycomplexation mode, hence it becomes important to study thebinding ability of the receptor in the presence of competing anions,cations and vice versa. In order to examine the sensing ability of R1and R2 in the presence of competing anion, titrations were carriedout in two ways. The sensing ability of R1 and R2 in the presence ofcompeting anions can be clearly depicted by the schematic repre-sentation shown in Fig. 5. First, we titrated R1 and R2 with F� andHO� in the presence of 200 lL (2 eq.) of metal ions, Mn+. Second wetitrated with Mn+ in the presence of 200 lL (2 eq.) of anions.
We chose Fe3+ and F�, HO� for the dual sensing experiment. Thespectral changes are shown in supporting information. As can beseen, initial titration of R1 with two equivalents of Fe3+, the absorp-tion band at 350 nm decrease in the presence of Fe3+ and a newband is observed at 425 nm which may be due to ligand to metalcharge transfer complex formation. Addition of F� (0–2 equiv.) re-sults in the reduction of the intensity of the 425 nm due to the for-mation of Fe:F ion pair in solution as has been recently observed insome cases [38–40]. The same titration method is followed for Fe3+
and OH�. In the second experiment, which is carried out in thepresence of 200 lL (2 eq.) of F�, addition of Fe3+ ion leads to a grad-ual reduction of the intensity of the 350 nm band due to thesequestering of F� ion to form an ion pair with Fe3+ ion in solution.The intensity of the band at 425 nm increased which may be due to
Table 1Binding constant, stoichiometry ratio and detection limit for R1 and R2 with metalions.
Receptor + ions Binding constant(Kapp)
Stoichiometry Detection limit(M/L)
R1 + Fe3+ 8.9 � 105 2:1 0.12 � 10�6
R1 + Cu2+ 4.6 � 104 2:1 0.14 � 10�6
R1 + Hg2+ 3.5 � 104 2:1 0.68 � 10�7
R1 + Cr3+ 1.2 � 104 2:1 0.14 � 10�6
R2 + Fe3+ 9.4 � 105 2:1 0.12 � 10�6
R2 + Cu2+ 1.9 � 105 2:1 0.23 � 10�6
R2 + Hg2+ 4.2 � 104 2:1 0.15 � 10�6
R2 + Cr3+ 5.0 � 104 2:1 0.52 � 10�6
400 450 500 550 6000
20
40
60
80
100
120
140
160 R1 R1+Fe3+
R1+Co2+
R1+Ni2+
R1+Cu2+
R1+Zn2+
R1+Cd2+
R1+Pb2+
R1+Hg2+
R1+Sn2+
R1+Mn2+
R1+Cr3+
Inte
nsit
y
Wavelength (nm)
Fig. 4a. Emission spectra of R1 (5 � 10�5 M, in CH3CN) upon titration with aqueoussolution of cations (R, R + Fe3+, R + Co2+, R + Ni2+, R + Cu2+, R + Zn2+, R + Cd2+,R + Pb2+, R + Hg2+, R + Mn2+, R + Sn2+ and R + Cr3+).
Fig. 4b. The relative intensity of R1 at 425 nm with various metal ions (200 lL).
400 450 500 550 6000
10
20
30
40
50
60
70
80
90 R2 R2+Fe3+
R2+Co2+
R2+Ni2+
R2+Cu2+
R2+Zn2+
R2+Cd2+
R2+Pb2+
R2+Hg2+
R2+Sn2+
R2+Mn2+
R2+Cr3+
Inte
nsit
y
Wavelength (nm)
Fig. 4c. Emission spectra of R2 (5 � 10�5 M, in CH3CN) upon titration with aqueoussolution of cations (R, R + Fe3+, R + Co2+, R + Ni2+, R + Cu2+, R + Zn2+, R + Cd2+,R + Pb2+, R + Hg2+, R + Mn2+, R + Sn2+ and R + Cr3+).
432 D. Udhayakumari, S. Velmathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 428–435
the interaction between R1 and Fe3+ ions (see Supportinginformation).
The above experiments were also carried out with R2 in thepresence of Fe3+, F� ions and vice versa. When 200 lL of Fe3+ ionswere added to R2, the absorbance band at 350 nm decreased withappearance of a new band at 400 nm due to the formations of R2–Fe (III) complex. On titration with F� (0–2 equiv.), the intensity of
the absorption band at 400 nm decreased due to the formation ofFe:F ion pair in solution. In the presence of 200 lL of F� ion, addi-tion of Fe3+ ions results in the reduction of the intensity of the350 nm band suggesting sequestering of the F� ion to form anion pair with Fe3+ ion in solution and enhancement in the400 nm band which might be due to complex formation of R2 withFe3+ ions (see Supporting information). The competing ion studyproves that Fe3+ ion has a strong binding with R1 and R2 even inpresence of anions like F� and HO�.
The binding constant value of the R1 with F� is 5.28 � 103, R1with HO� is 2.62 � 103 and R2 with F� is 2.08 � 104, R2 with
NHN
NH2
R
HNN
H2N
R
Fe
NHN
NH2
R
Fe3+
N-N
NH2
R
H+A-
F- and OH-
A= F- or OH-
-F- and OH-Fe: F- or OH- ion pair
Fe3+
F- and OH-
AB
Fig. 5. Possible structures of complexes formed between receptors (R1 and R2) and F� and �OH/Fe3+ ions.
NH
NNH2
R
HN
N
H2N
R
Mn+
NH
NNH2
R
Mn+=Cu2+, Hg2+& Cr3+
- Cu/ Hg/ CrFe: Cu/Hg/Cr ion pair
Fe3+
Cu2+, Hg2+ &Cr3+
AB
Fe3+
NH
NNH2
R
HN
N
H2N
R
Fe
Fig. 6. Possible structures of complexes formed between receptors (R1 and R2) and Fe3+/other cations.
Fig. 7a. Color changes observed upon the addition of various cations to R1–Fe3+ complex. (From left to the right: R1, R1–Fe3+, R1–Fe3++Co2+, R1–Fe3+ + Ni2+, R1–Fe3+ + Cu2+,R1–Fe3+ + Zn2+, R1–Fe3+ + Cd2+, R1–Fe3+ + Pb2+, R1–Fe3+ + Hg2+, R1–Fe3+ + Mn2+, R1–Fe3+ + Sn2+, R1–Fe3+ + Cr3+).
Fig. 7b. Color changes observed upon the addition of various cations to R2–Fe3+ complex. (From left to the right: R2, R2–Fe3+, R2–Fe3+ + Co2+, R2–Fe3+ + Ni2+, R2–Fe3+ + Cu2+,R2–Fe3+ + Zn2+, R2–Fe3+ + Cd2+, R2–Fe3+ + Pb2+, R2–Fe3+ + Hg2+, R2–Fe3+ + Mn2+, R2–Fe3+ + Sn2+, R2–Fe3+ + Cr3+).
D. Udhayakumari, S. Velmathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 428–435 433
250 300 350 400 450 500 550 6000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Abs
orba
nce
Wavelength (nm)
R1 R1+Fe3+
R1+2eq Fe3++2 eq Co2+
R1+2eq Fe3++2 eqNi2+
R1+2eq Fe3++2 eqCu2+
R1+2eq Fe3++2 eqZn2+
R1+2eq Fe3++2 eqCd2+
R1+2eq Fe3++2 eqPb2+
R1+2eq Fe3++2 eqHg2+
R1+2eq Fe3++2 eqMn2+
R1+2eq Fe3++2 eqSn2+
R1+2eq Fe3++2 eqCr3+
Fig. 8a. UV–vis spectra of various cations to R1–Fe3+ complex. (From left to theright: R1, R1–Fe3+, R1–Fe3+ + Co2+, R1–Fe3+ + Ni2+, R1–Fe3+ + Cu2+, R1–Fe3+ + Zn2+,R1–Fe3+ + Cd2+, R1–Fe3+ + Pb2+, R1–Fe3+ + Hg2+, R1–Fe3+ + Mn2+, R1–Fe3+ + Sn2+, R1–Fe3+ + Cr3+).
200 250 300 350 400 450 500 550 6000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Abs
orba
nce
Wavelength (nm)
R2 R2+Fe3+
R2+2eq Fe3++2 eq Co2+
R2+2eq Fe3++2 eqNi2+
R2+2eq Fe3++2 eqCu2+
R2+2eq Fe3++2 eqZn2+
R2+2eq Fe3++2 eqCd2+
R2+2eq Fe3++2 eqPb2+
R2+2eq Fe3++2 eqHg2+
R2+2eq Fe3++2 eqMn2+
R2+2eq Fe3++2 eqSn2+
R2+2eq Fe3++2 eqCr3+
Fig. 8b. UV–vis spectra of various cations to R2–Fe3+ complex. (From left to theright: R2, R2–Fe3+, R2–Fe3+ + Co2+, R2–Fe3+ + Ni2+, R2–Fe3+ + Cu2+, R2–Fe3+ + Zn2+,R2–Fe3+ + Cd2+, R2–Fe3+ + Pb2+, R2–Fe3+ + Hg2+, R2–Fe3+ + Mn2+, R2–Fe3+ + Sn2+, R2–Fe3+ + Cr3+).
Fig. 9. Color changes observed upon the addition of corrosion sample solution to R1and R2. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)
250 300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Abs
orba
nce
Wavelength (nm)
R1 R1+200 µL Corrosion sample
250 300 350 400 450 500 550 6000.0
0.2
0.4
0.6
0.8
Abs
orba
nce
Wavelength (nm)
R2 R2+200 µL Corrosion sample
(a)
(b)
Fig. 10. UV–vis spectra of (a) R1 + corrosion sample solution and (b) R2 + corrosionsample solution.
434 D. Udhayakumari, S. Velmathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 428–435
HO� is 3.47 � 104, while the binding constant for the Fe3+ with R1is 8.9 � 105 and R2 with Fe3+ is 9.4 � 105.
Interference with metal ions
The interaction and interference of metal ions with R1 and R2was studied. Fe3+ ion shows a strong binding by complexationmode compare to other sensing metal ions like Cu2+, Hg2+
and Cr3+. The sensing ability of R1 and R2 in the presence of
interference cations can be clearly depicted by schematic represen-tation shown in Fig. 6. First we have titrated R1 with 2 eq Fe3+ inthe presence of other metal ions like Co2+, Ni2+, Cu2+, Zn2+, Cd2+,Pb2+, Hg2+, Mn2+, Sn2+ and Cr3+. There was no change in the pale or-ange color of the receptor 1.Fe3+ complex observed after the addi-tion of the other cations (Fig. 7a). These results further confirmedby UV–vis spectroscopic method. With 200 lL of Fe3+ ions, R1shows a new band at 420 nm, due to the formation of R1–Fe3+
complex. Further titration with other metal cations, there is slightdecrease in absorption band at 420 nm (Fig. 8a). The similar resultwas observed for R2.Fe3+ complex in the presence of other interfer-ence metal ions (Figs. 7b and 8b). From this study it can be inferredthat Fe3+ ion has a strong binding with R1 and R2 even in the pres-ence of other sensing metal ions.
Real sample analysis
To extent the real sample application of the R1 and R2, we didthe qualitative analysis for Fe3+ ion in the corrosion sample. Thecorrode iron sample (2 mg) was dissolved in water: HCl mixture(9:1) and 5 � 10�3 M of R1 and R2 in CH3CN. Once the corrosionsample solution was added to the R1 and R2, the color was sud-denly changed from colorless to pale green color for R1 and greenfor R2 (Fig. 9). The absorbance of R1 was measured with and with-out addition of corrosion sample solution, after the addition of cor-rosion sample solution to R1, the absorbance at 420 nm wasobserved. These indicating the presence of Fe3+ ion in corrosionsample forming a complex with R1 (Fig. 10a). The similar resultsare observed in the presence of R2 with corrosion sample solution(Fig. 10b).
D. Udhayakumari, S. Velmathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 428–435 435
Conclusion
In summary, convenient and highly sensitive chemosensorSchiff bases (R1 and R2) were synthesized and their chemosensingproperties were investigated in aqueous analyte solution. R1 andR2 can be used as a sensitive visual chemosensor for Fe3+, Cu2+,Hg2+ and Cr3+ in water in presence of other metal ions by distinctcolor change. R1 and R2 shows higher binding constant with 2:1stoichiometry. R1 and R2 when bind with metal ions, significantfluorescence quenching was observed. This may be due to the pho-toinduced electron transfer mechanism. Fluorescence quenchingconstant was determined using Stern–Volmer plot. To the best ofour knowledge, this could be an excellent example of a molecularsensor capable of detecting multiple metal ions by different colorresponse with a micromolar detection level.
Acknowledgement
The authors thank to DRDO (ERIP/ER/1006004/M/01/1333 da-ted 23-05-2011) for financial assistance in the form of a majorsponsored project.
Appendix A. Supplementary material
UV–vis, fluorescent and competing ions titrations of receptors 1and 2 with different metal ions are available. Supplementary dataassociated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/j.saa.2013.11.083.
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