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Naphthalenediols: A new class of novel fluorescent chemosensors for selective sensing of Cu2+ and Ni2+ in aqueous solution
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Naphthalenediols: A new class of novel fluorescent chemosensors for selective sensing of Cu2 þ and Ni2 þ in aqueous solution R. Kavitha, T. Stalin
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S0022-2313(14)00600-0 http://dx.doi.org/10.1016/j.jlumin.2014.10.029 LUMIN12959
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Journal of Luminescence
Received date: 18 April 2014 Revised date: 10 October 2014 Accepted date: 13 October 2014 Cite this article as: R. Kavitha, T. Stalin, Naphthalenediols: A new class of novel fluorescent chemosensors for selective sensing of Cu2 þ and Ni2 þ in aqueous solution, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2014.10.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Naphthalenediols: A new class of novel fluorescent chemosensors for selective sensing of Cu2+ and Ni2+ in aqueous solution R.Kavitha and T.Stalin* Department of Industrial Chemistry, Alagappa University, Karaikudi- 630 003, Tamilnadu, India. Abstract Naphthalenediols(NDs) such as 1,5-naphthalenediol (1,5-ND) and 2,7-Naphthalenediol (2,7-ND) are found to be potent chemosensor for selective sensing of Cu2+ and Ni2+ ions respectively. The sensitivity and selectivity of chemosensors have been studied by incorporating NDs into -Cyclodextrin (-CD). The chemosensor behavior of NDs towards various transition metals is studied under selective pH medium. In 1,5-ND, the colorimetric chemosensor behavior has been identified and demonstrated in aqueous medium for the selective detection of Cu2+ without any modification, likewise 2,7-ND also having sensing ability towards Ni2+. The chemosensor probe 1,5-ND showed selective chromogenic behavior towards Cu2+ ions by changing the colour from pink to purple. No significant colour change is observed upon the addition of other metal ions. The sensing property of naphthalenediols towards metal cations has been studied using UV-Vis and fluorescence spectral technique. Keywords: -Cyclodextrin, Naphthalenediols, Chemosensor, Inclusion complex
* Corresponding author (Dr. T.Stalin) E-mail address: [email protected] Tel: +91 9944266475 Fax: +91 4565 225202
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1. Introduction The interest in the development of highly sensitive probes for metal ion analysis is the current thrust in the active research field. Metal ions are playing essential roles in biology by serving as essential cofactors in the processes such as respiration, growth, gene transcription, enzymatic reactions and immune function. The transition metals can exist in many different forms within the cells, associated with low molecular weight species such as amino acids or glutathione, from which the metal ion could be released by changes in the cellular environment. Along with other transition metal ions, copper and nickel are essential trace elements in biological systems such as respiration, biosynthesis and metabolism [1]. An undersupply of these metal ions leads to deficiency and an oversupply result in toxic effects. At higher concentration, copper causes liver damage in infants and nickel causes lung injury, allergy and carcinogenesis [2]. The detection and selective monitoring of these metal ions in environmental and clinical analysis has thereof gained significant importance. Currently, highly sensitive instrumentations have been applied for the detection of metal pollutants such as atomic absorption spectroscopy (AAS) [3], voltammetry [4], inductively coupled plasma mass spectroscopy (ICP-MS) [5] and potentiometry [6]. Although those methods are very sensitive and accurate, they lean to be costeffective, technically complex, time-consuming and does not permit high throughput analysis. Among the various detection methods available, UV-Vis and fluorescence spectroscopy still remain the most frequently used modes, due to their high sensitivity and easy operational use. Therefore, during the recent past, good amount of work on sensing of Cu2+ and Ni2+ by variety of synthesized colorimetric/fluorescent probes have been reported [7-21]. However, there is still and intense demand for new efficient Cu2+ colorimetric chemosensor, especially those can work in aqueous solution with high selectivity and sensitivity [22–24]. The development of novel
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colorimetric chemosensor for the rapid and convenient detection of Cu2+ is more attractive. Recently, selective luminescent probes based on benzothiadiazoyl-triazoyl cyclodextrin and zinc compound containing symmetrical Schiff base ligand for Ni2+ [25] and coumarin Schiff-base for Ni2+ as a colorimetric sensor [26] were also investigated. Based on Cu2+ promoted oxidation reaction, a new naphtol derivative as selective colorimetric and fluorescent chemosensor for Cu2+ was also reported recently [27]. Dihydroxy naphthalene compounds having two hydroxyl groups which are substituted in the naphthalene ring system. It has numerous positional isomers differing by the location of the hydroxyl group. The different positions provide various chemical structures which offer important roles to each characteristic. They are used directly in making several dyes and are converted into numerous corresponding amines, esters, ethers and carboxylic derivatives as well as into numerous sulfo- and nitro-group substituted (mono-, di and tri) compounds. They find extensive applications in making dyes, pigments, fluorescent whiteners, tanning agents, antioxidants and antiseptics. NDs may be regarded as a singular template for building up various derivatives due to the specific arrangement of the hydroxyl group. 1,5-Naphthalenediol (abbreviated as 1,5-ND) and 2,7-Naphthalenediol (abbreviated as 2,7-ND) are applied as an intermediate for organic synthesis, especially for crown ether and naphthalene sulfonic acid series used to make colorant compounds. These two naphthalenediols such as 1,5-ND and 2,7ND are used in this chemosensor study. To the best of our knowledge, there is no report on NDs in sensing of metal ions. We are the first to report the NDs as a fluorescent chemosensor for sensing of metal cations. NDs, an important class of organic compounds, its derivatives absorb visible light and have a wide range of applications as said above. One of the derivatives of NDs such as 1,5-Naphthalenediol (1,5-
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ND) used in this study produce distinct colour in alkaline medium and its pKa value in water is 9.80 for neutral-monoanion equilibrium. Due to its high absorption coefficient and possibility of naked eye detection of anionic form, 1,5-ND has been selected as a scaffold to recognize cations in aqueous medium. However, 1,5-ND and 2,7-ND having poor solubility in water and it is not possible to function in aqueous solution. One of the best methods to improve their solubility and to operate in an aqueous medium is making inclusion complex with cyclodextrins (CDs). For substances with low water solubility, inclusion in CDs may improve their physicochemical properties such as solubility, stability and spectroscopic properties [28,29]. Among the three types of CDs, -Cyclodextrin is the best host for many guests, due to its right size. Previously we have reported the inclusion phenomena of series of organic compounds with -cyclodextrin, its inclusion behavior, stoichiometry and mechanism [30-32]. Herein, we wish to report the simple and cost-effective chemosensor consisting of NDs and -Cyclodextrin (-CD). Using -CD, NDs forms hydrophobic inclusion complexes that made these compounds to operate in an aqueous medium. We have found that the inclusion complex of NDs with -CD as potential chemosensor for cations of biological interest such as Cu2+ and Ni2+ in aqueous solution. The compound 1,5-ND show a selective binding toward Cu2+ as was expressed by a red-shift in the UV-Vis absorption spectra and a specific fluorescence quenching behavior. In this respect, no significant response to other metal cations was observed. The changes in the UV-Vis and fluorescence spectra that were observed in the presence of Cu2+ were attributed to the formation of a metal-ligand complex. Addition of Cu2+ to the solution of 1,5-ND in -CD medium resulted in a significant colour change. It should be noted that this particular behavior was exclusive for Cu2+ which allowing its detection even in the presence of other metal cations.
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2. Experimental 2.1. Chemicals and instruments -Cyclodextrin (-CD), 1,5-Naphthalenediol (1,5-ND) and 2,7-Naphthalenediol (2,7-ND) were purchased from Himedia Chemical Reagents Company and used without further purification. Solutions of metal ions were prepared from prechlorate salts of respective metals and were dissolved in distilled water. Triply distilled water was used to prepare all solutions. The UV–Vis spectra (absorption spectral measurements) was carried out with
Shimadzu UV-
2401PC double-beam spectrophotometer, fluorescence measurements were made using a Jasco FP-880 spectrofluorimeter, the pH values in the range 1.0–12.0 were measured on Elico pH meter LI-120 and FT-IR was recorded using Nicolet 380. 2.2. Preparation of stock solutions for UVand Fluorescence studies The recognition between naphthalenediols and different metal cations were investigated by UV–Vis and fluorescence spectroscopy in aqueous solution. The stock solution of -CD (12×103mol dm3) was prepared using buffer solutions. The stock solution of NDs in ethanol and metal ions were at a concentration of 1×102 M. All absorption spectral studies were carried out in pure triply distilled water at room temperature. Solutions in the pH range 1.0–12.0 were prepared by adding the appropriate amount of NaOH and H3PO4. A modified Hammett’s acidity scale (H0) [33] for the solutions below pH~2 (using a H2SO4– H2O mixture) and Yagil basicity scale (H) [34] for solutions above pH~12 (using a NaOH–H2O mixture) were employed. The solutions were prepared just before taking measurements. For colorimetric titrations, the stock solutions (5 x 10-5 M) of all metal ions were prepared in aqueous solution and used. To analyse the effect of metal through UV-Vis and fluorescence spectroscopy, various concentration of
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selective metal cations (5x10-9 to 5x10-5 M) were prepared in 10 mL volumetric flask and shaken thoroughly. All the absorption and emission spectra were recorded at 30±100C. 3. Results and discussion 3.1. Effect of pH Chemosensors are usually influenced by pH of the media in the detection of metal ions. The influence of operative pH value upon the sensitivities of chemosensor is an extremely important one. The absorption and fluorescence spectra of NDs are examined at different pH and the relevant data are given in Table 1 and Fig.1. In 1,5-ND and 2,7-ND, the position of the hydroxyl groups are ’ and ’ respectively and they are placed symmetrically. In the pH range 1-9, observed absorption maxima resembles the spectra observed in non aqueous solvents and thus can be assigned to neutral species [35-36]. The absorption maxima of NDs are observed at 331.5 nm(1,5-ND) and 328.0 nm(2,7-ND) could be due to exists of the neutral form of NDs. When the increase on acid strength (
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delocalized over the two benzene rings and formed an extended conjugation (Scheme 1). The monoanionic absorption band (532.5nm) is further red shifted into 539.5 nm in H_ 12.99, it clearly indicates the formation of dianion in 1,5-ND. Like 1,5-ND, the similar trend was also observed in 2,7-ND for the formation of monoanion. But the monoanoinic form of 2,7-ND produces a smaller red shift (328.0 to 342.5 nm) when compared to1,5-ND. Unlike 1,5-ND, here in the negative charge on oxygen atom of hydroxyl group is delocalized over the carbon atoms of naphthalene moiety and could not form the extended conjugation, instead 2,7-ND forms stable monoanion. From pH~12, the peak at 249.0 nm alone is red shifted to 263.5 nm. The red shift observed in the absorption spectrum is due to the formation of dianion which is formed by the removal of proton from ’ hydroxyl group. Further increase in pH from H_ 12.9 to H_ 15.2, the peak at 263.5 nm is progressively blue shifted into 245.0 nm. It can be explained through the resonance structures of mono and dianion of 2,7-ND. The fluorescence spectra of both compounds in various pH are also measured and shown in Fig. 1. The intensity of a single emission band of 1,5-ND with the maximum at 347.0 nm decreasing with the increase in pH from 1 to 8, while the dual fluorescence is observed for 2,7ND at 342.0 and 401.0 nm. The dual fluorescence in 2,7-ND implies that both neutral and monoanionic forms are in equilibrium. When the pH is increased from 8 to 11, the fluorescence maxima of 1,5-ND is red shifted to 416.0 nm, whereas in 2,7-ND the peak at 401.0 nm is red shifted to 407.5 nm. This red shifted spectrum indicates that deprotonation should occur in the hydroxyl group. Further increase in pH from 12 to H_ 15.20, a single broad peak at 412.0 nm for 1,5-ND and 402.0 nm for 2,7-ND is obtained. This blue shifted maximum is due to the formation of dianion and the dual emission band becomes a single emission band indicates that the stable
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dianion is formed at above pH~12. Hence, there are three prototropic species such as neutral, monoanion and dianion existed for both 1,5-ND and 2,7-ND, confirmed through UV-Vis and fluorescence emission spectrum. The acidity constant at ground state (pKa) and excited state (pKa*) values of both molecules for neutral-monoanion equilibrium are determined spectrophotometrically and its values are given in Table 1, but the acidity constant values for monoanion-dianion equilibrium could not be determined, because there was no constancy in the isobestic points. Though 1,5-ND and 2,7-ND is an extremely weak acid in its ground state (pKa=9.7 and 10.4), when excited it loses one proton and its pKa* values being 2.1 and 2.9 respectively. These values clearly indicate that both naphthalenediols are stronger acid in excited state than in ground state. It is due to the intramolecular charge transfer (ICT) in the excited state of the NDs, from the hydroxyl oxygen to the aromatic ring. Further, it was subsequently realized that the ICT effect must be even larger in the conjugate anionic base. Thus, it is the excess stabilization of the excited anionic base that renders the excited state deprotonation reaction more exothermic, leading to enhanced photoacidity. In order to find the potential application as chemosensor, NDs is studied under -CD medium to promote the sensitivity and selectivity of chemosensors. The -CD can include the variety of guest molecules to form hydrophobic inclusion complexes which made these chemosensors operate in an aqueous medium. From the pH effect, the anionic form of 1,5-ND is pink color, and may have more interactions with the metal cations, which motivated us to test the ability of neutral and monoanion of NDs in pH~7 and 10 buffer medium respectively. 3.2. Spectral response of naphthalenediols (1,5-ND & 2,7-ND) towards different metal ions 3.2.1. UV-Vis spectral studies
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The ability of neutral and moanionic forms of 1,5-ND and 2,7-ND to detect metal cations in aqueous solution are tested in the presence of various metal cations such as Ni2+, Cd2+,Co2+, Hg2+,Zr4+, Zn2+,Cu2+,Cu+, Pb2+, Bi3+ and Mg2+ in pH~7 and 10 buffer solution of -CD medium [37]. Fig.2 shows the absorption spectra of 1,5-ND and 2,7-ND in pH~7 of -CD medium and it implies that the neutral form of both compounds could not have any significant interaction with metal cations. When compared to the neutral form, the anionic form having more interaction with metal cations for both 1,5-ND and 2,7-ND and hence all the sensing studies are carried out in pH~10 buffer medium (anionic form). Fig.3 showed the UV-Vis absorption spectra of 1,5-ND and 2,7-ND in aqueous -CD medium of pH~10 with the different test metal ions. The spectra obtained for 1,5-ND in the absence and presence of above cations showed that the absorption spectrum had almost no change in the presence of these metal cations and exhibited a main peak at 532.5 nm. However, the addition of Cu2+ causes the spectra of 1,5-ND to shift to longer wavelength (550.0 nm). It is important to note that the appearance of a new strong and red absorption peak at 550.0 nm created by the addition of Cu2+, while resulted in a naked eye color change from pink to purple (Fig. 4). No color change is observed with other metal cations. The absorption spectra of free 1,5-ND exhibited a band at 532.5 nm showed a large bathochromic shift to 550.0 nm with absorption tail extending to about 650 nm, indicating that Cu2+ has strong interaction with 1,5-ND. These results indicated that 1,5-ND can be used as a colorimetric probe for Cu2+ and the distinct color change with Cu2+ indicated that the 1,5-ND is a potential probe to function as a visible chemosensor for this ion. It is important that the absorption of the system is enhanced by -CD which increasing the sensitizing effect on the determination of Cu2+ and in CD system, the solution remained clear and transparent for a long time. Furthermore, use of 1,5ND for sensing studies is extended to the various concentrations of Cu2+ (5x10-9 to 5x10-5 M) in
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the presence of -CD. Thus, the concentration of Cu2+ is increased; the absorption intensity of the corresponding peak also increased (Fig.5). These results clearly demonstrate the sensing ability of 1,5-ND for Cu2+ cation in water. To investigate the probable complexation of 1,5-ND with Cu2+, the method of continuous variations (Job’s plot) is obtained, which clearly suggested the formation of 1:1 stoichiometry between 1,5-ND and Cu2+ (Fig.5b). The total concentration of 1,5-ND and Cu2+ is fixed at a constant value (10 M), and then molar fraction of Cu2+ varied continuously. As shown in Fig.5b, the 1,5-ND-Cu2+ complex exhibits a maximum fluorescence emission when the molecular fraction of Cu2+ is about 0.5, indicating that a 1:1 stoichiometry is more possible for the binding mode of 1,5-ND and Cu2+. Moreover the association constant of 1,5-ND:Cu2+ complex will further estimate to be 1.1 x 104 M-1, assuming a 1:1 stoichiometry by the Benesi-Hildebrand method [38] (Fig.5c). Unlike 1,5-ND, the response of 2,7-ND towards different metal cations were assorted and new results were found. The addition of Cd2+,Co2+, Hg2+,Zr4+, Zn2+,Cu2+,Cu+,Bi3+ and Mg2+ to 2,7-ND caused no changes in spectrum, but only the addition of Ni2+ caused a significant change in the spectrum i.e., bathochromic shift. Comparing the response of these diols suggests that the position of substituent playing major role in the detection of metal ions. Moreover, the sensitivity and binding response of 2,7-ND with Ni2+ were studied by the various concentrations of Ni2+ (5x10-9 to 5x10-5 M) (Fig.6c). When the concentration of Ni2+ is increased, the intensity of the red shifted peak also increased. Job’s plot analysis revealed that 2,7-ND interact with Ni2+ in 1:1 stoichiometry. As like 1,5-ND, 2,7-ND also follows the 1:1 complex with the metal ion. Further, the binding affinity of 2,7-ND for Ni2+ was estimated from Benesi-Hildebrand plots. The association constant (Ka) has been found to be 1.75 x 104 M-1. The Ka value of Ni2+ complex is
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higher than the Cu2+ complex, higher the association constant value represents the more stability of metal complex. 3.2.2. Fluorescence spectral studies Fluorescence spectroscopic studies were also employed to study the cation recognition properties of naphthalenediols. The fluorescence spectrum of the chemosensor 1,5-ND was recorded in the range of 230-470 nm upon excitation at 300 nm. It is observed that in addition of Cu2+ ions quenched the fluorescence of 1,5-ND (45 % of reduction in intensity) (Fig. 7a). The experimental results indicated that the quenching efficiency, increased with an increase in the concentration of Cu2+ ions at a fixed concentration of 1,5-ND (Fig.7b). Fluorescence intensity data were analyzed according to the Stern-Volmer law [39]. F0/F=1/Ksv[Q] Where F0 is emission intensity of 1,5-ND in the absence of quencher (Q), F is emission intensity of 1,5-ND at quencher concentration [Q]. The linearity of Stern-Volmer plot (Inset of Fig.7b) suggested that only one quenching mechanism is operative and the quenching is bimolecular and dynamic [40,41]. The Stern-Volmer quenching constant for 1,5-ND with Cu2+ is 5.8x105. In bimolecular liquid systems, the fluorescence intensity is hindered due to several mechanisms, such as static and dynamic quenching, excimer and exciplex formation, charge transfer, energy transfer process, etc [42-46]. Based on the absorption results and Job’s plot, Cu2+ bound to 1,5ND and forming a 1:1 stoichiometric complex. Thus, it can be assumed that the quenching of fluorescence of 1,5-ND by Cu2+ ion is a static quenching mode due to the formation of nonfluorescent complex in the ground state. Moreover, Cu2+ is a well-known strong quencher because it is a good electron scavenger on account of its d9 electronic structure. Quenching by this type of substance most likely involves the donation of an electron from the fluorophore to
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the quencher [47]. In pH~10, the fluorophore (1,5-ND) exists in an anionic form which donate an electron to Cu2+ ion. In this regard, the energy and electron-transfer processes cannot be ruled out during the fluorescence quenching of 1,5-ND by Cu2+ in an aqueous solution. Similarly, the metal ion interaction of 2,7-ND with Ni2+ is further confirmed by fluorescence spectroscopic measurements (Fig.8). The emission peak at 407.5 nm in the spectrum of 2,7-ND enhanced upon the addition of Ni2+ in aqueous -CD medium. It is important to note that the fluorescent enhancement observed in 2,7-ND is highest and behaves in a different manner than 1,5-ND. The selectivity of 2,7-ND for Ni2+ is further followed by fluorescence titration studies under similar conditions. To assess the sensitivity, varying concentrations of Ni2+ were added to the solution of 2, 7-ND and the intensity of emission peak increases with increase in Ni2+ concentration. The maximum increase of fluorescence emission at 407.5 nm is found to be 40%, which was an agreement with the data of 1:1 complex between 2,7-ND and Ni2+ from Job’s plot. The enhancement of fluorescence was attributed to the introduction of Ni2+ and consequent occurrence of the strong complexation with 2, 7-ND, resulting in decreased non-radiative decay of the excited-state and increased radiative decay. The remarkable increase of the fluorescence intensity is due to the formation of stable complexes between 2,7-ND and Ni2+ in the presence of -CD medium. 3.2.3. The possible binding mechanism The possible mechanism of interaction between metal cations (Cu2+ and Ni2+) and naphthalenediols is proposed by considering the changes in UV/Vis and fluorescence spectra. In fact, among the various metals studied, Cu2+ ions gave the effective color change with 1,5-ND, therefore an effective coordination between the metal and the oxygen atom of 1,5-ND could be expected. The interaction of Cu2+ with 1,5-ND causes the disappearance of the band at 532.5 nm
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and the emergence of a new red shifted band that probably arises from a charge-transfer transition, in which orbital of the metal centers are involved. Compared with other transition metal ions, Cu2+ has a high thermodynamic affinity for typical N, O-chelate ligands and fast metal-to-ligand binding kinetics [48], which could account for the selectivity of the system to Cu2+. The chelation induced fluorescence quenching is probably induced by redox active Cu2+ ions via ligand-to-metal electron or energy transfer mechanisms [49]. The decrease in fluorescence intensity is attributed to electron transfer between 1,5-ND and Cu2+. The compound 2,7-ND in the presence of Ni2+ produce a red shift in absorption spectra and the enhancement of emission intensity in fluorescence spectra. These changes in fluorescence intensity can be attributed to the nature of the cations where paramagnetic cations are known to cause a fluorescence quenching while diamagnetic transition metal ions can cause fluorescence enhancement. Both Cu2+ and Ni2+ ions are paramagnetic, hence Cu2+ quench the fluorescence spectra of 1,5-ND. Even though Ni2+ is paramagnetic, it produces an increase in emission intensity of 2,7-ND. The increase in intensity is attributed to the formation of stable complexes between 2,7-ND and Ni2+. Form Job’s plot analysis, both metal cations forming 1:1 complex with chemosensor. The formation of 1:1 complex involves through the O(-) ion of chemosensor and a secondary OH group of -CD in the upper rim. This is the only probable mechanism for both complexes of naphthalenediols with metal cations. The mode of binding is further confirmed by FT-IR spectroscopy. We measured FT-IR spectra of naphthalenediols and its complex with metal cations to further examine the binding site. The newly formed band in the spectra of the complex at 584.2 cm-1(Fig.9a) and 578.2 cm-1 (Fig.9b) are assigned to Cu-O and Ni-O vibrations respectively. It confirmed the formation of a complex between metal and chemosensor (1,5-ND/2,7-ND). In
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addition to the new band, the characteristic peaks of 1,5-ND and 2,7-ND are shifted to higher frequency value. In 1,5-ND, the peaks observed at 1377 cm-1 (C-O stretching), 3288 cm-1 (O-H stretching), 1597 cm-1 (C=C stretching) are
not shifted but their intensities were largely
decreased and the peaks become broadened in the metal complex. In the case of 2,7-ND the peaks observed at 1386 cm-1 (C-O stretching), 3219 cm-1 (O-H stretching), 1627 cm-1 (C=C stretching) are shifted in the metal complex to 1380 cm-1, 3372 cm-1, 1635 cm-1 respectively. When the FT-IR spectra of 2,7-ND is compared with 1,5-ND, the peaks were shifted to larger value, in particular OH stretching and C=C stretching frequencies. Thus, in accordance with the 1;1 stoichiometry, 1,5-ND and 2,7-ND are likely to chelate Cu2+ and Ni2+ via its O(-) atom and OH group of -CD. Based on the spectroscopic data (UV-Vis, Fluorescence and IR spectra), it is confirmed that the O(-) atom of 1,5-ND coordinate with Cu2+ ions. The proposed structure of the complex is shown in Scheme 2. To realize the practical utility of these chemosensors towards Cu2+/Ni2+ over other competitive species, the competitive experiments were conducted in the presence of Cu2+/Ni2+ mixed with movalent, divalent and trivalent metal cations (Fig. 10). As can be seen from the Fig.10a, the presence of Cu2+ and another metal ion had no significant interference with the quenching created by Cu2+ ions. Similarly, as shown in Fig.10b the competing metal ions exhibited only a small or no interference with the detection of Ni2+ ions. It is noticeable that the competitive metal ions did not lead to any significant changes in emission intensity when compared with that without other metal ions besides Cu2+ and Ni2+. In addition, no obvious interference was noted in its emission intensity with the presence of other metal cations. Based on the UV-Vis and more specifically the fluorescence spectra, it can be concluded that the response of 1,5-ND and 2,7-ND towards metal ions in the presence of -CD is influenced by the
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position of substituent on the ring. These results implied that the selectivity of chemosensor 1,5ND and 2,7-ND towards Cu2+ and Ni2+ respectively was remarkable. 4. Conclusions In summary, we have demonstrated the sensing behavior of inclusion complexes of 1,5ND and 2,7-ND towards Cu2+ and Ni2+ respectively. The newly advanced simple chemosensor has been identified and used as it exists without any modifications for the detection of metal cations. 1,5-ND a novel colorimetric sensor for transition metal cation (Cu2+) and binding of this cation causes an appreciable change in the visible region of the spectrum which can be detected by naked-eye. 1,5-ND functioned both as chromogenic and fluorogenic probe for Cu2+ with high selectivity and sensitivity. In the presence of Cu2+ in aqueous solution, the distinct colour change from pink to purple colour was obtained. In 2,7-ND, there is no such color change upon the addition of Ni2+. The compound 2,7-ND in the presence of Ni2+ produce a red shift in absorption spectra and the enhancement of emission intensity in fluorescence spectra. The detection limit for both cations by this method was 10-8 M. References [1] Y.H. Zhang, H.S. Zhang, X.F. Guo, H. Wang, Microchem. J. 89 (2008) 142. [2] C.Bo, Z.Ping, Anal. Bioanal. Chem. 381, (2005) 986. [3] M.S.Chanm, S.D.Huang, Talanta 51 (2000) 373. [4] C.Collado-Sanchez, J.Perz-Pena, M.D.Gelado-Caballero, J.A.Herrera-Melia, J.J.HernadezBrito, Anal. Chim. Acta 320 (1996) 19. [5] J.Wu, E.A.Boyle, Anal. Chem. 69 (1997) 2464. [6] C.W.K.Chow, S.D.Kolev, D.E.Davey, D.E.Mulcahy, Anal. Chim. Acta 330 (1996) 79. [7] S. Goswami, D. Sen, N.K. Das, Org. Lett. 12 (2010) 856.
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[25] G.B. Li, H.C. Fang, Y.P. Cai, Z.Y. Zhou, P.K. Thallapally, J. Tian, Inorg. Chem. 49 (2010) 7241. [26] L.Wang, D.Ye, D.Cao, Spectrochimica Acta Part A 90 (2012) 40. [27] Y. K. Jang, U. C. Nam, H. L. Kwon, I. H. Hwang, C. Kim, Dyes and Pigments 99 (2013) 6. [28] J. Sun, X.S. Zhu, M. Wu, J. Fluoresc. 17 (2007) 265. [29] M.Y. Xu, S.Z. Wu, F. Zeng, C.M. Yu, Langmuir 26 (2010) 4529. [30] T.Stalin, K.Srinivasan, K.Kayalvizhi, K.Sivakumar, Spectrochim. Acta Part A 79 (2011) 169. [31] T.Stalin, K.Srinivasan, J.Vaheethabanu, P.Manisankar, J. of Molec. Struct. 987 (2011) 214. [32] T.Stalin, K.Srinivasan, K.Sivakumar,Spectrochim. Acta Part A 94 (2012) 89. [33] M.J.Jorgenson, D.R.Harter, J. Am. Chem. Soc. 85 (1963) 878. [34] G.Yagil, J. Phys. Chem. 71 (1967) 1034. [35] T.Stalin, R.Anitha Devi, N.Rajendiran, Spectrochim. Acta. 61A (2005) 2495. [36] M.Shanmugam,
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[44] H.Pal,D.K.Palit,T.Mukherjee.,J.P.Mittal,Chem.Phys.Lett.173 (1990) 354. [45] P.K.Behera,A.K.Mishra, J. Photochem. Photobiol. A 71 (1993) 115. [46] H.Zeng, G.Durocher, J.Lumin. 63 (1995) 75. [47] M.D.P.D.Costa, W.A.P.A.Jayasinghe, J.Photochem.Photobiol. A162(2004) 591. [48] [48] Z. Liang, Z. Liu, L. Jiang, Y. Gao, Tetrahedron Lett. 48 (2007) 1629. [49] S.H. Mashraqui, M. Chandiramani, R. Betkar, K. Poonia, Tetrahedron Lett. 51 (2010) 1306.
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Table captions Table 1 Various prototropic maxima (Absorption and emission spectra) and pKa values of 1,5-ND and 2,7-ND in -CD medium.
Table 1
1,5-ND
2,7-ND
Species max Neutral (pH~1.0-9.0) Monoanion (pH~9.5-11.0) Dianion (pH~11.5-H_14.02)
max emission (nm)
(nm)
max
max emission (nm)
(nm)
347.0
328.0 313.0 286.0 239.5
342.0 401.0
532.5 331.5
416.0
342.5 258.5
407.5
539.5 333.5
412.0
343.0 260.5
402.0
331.5 317.0 300.0
pKa 9.7
10.4
10.1
10.9
pKa*
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Figure captions Fig.1 Absorption and emission spectra of (a) 1,5-ND and (b) 2,7-ND (Conc. 2x10-5M) in various pH solutions. Fig.2 UV-Vis absorption spectrum of (a) 1,5-ND and (b) 2,7-ND (5x10-5M) in the presence of various cations in pH~ 7 buffer of -CD aqueous solutions. Fig.3 UV-Vis absorption spectrum of (a) 1,5-ND and (b) 2,7-ND (5x10-5M) in the presence of various cations in pH~ 10 buffer of -CD aqueous solutions. Fig.4 UV-Vis absorption spectrum of 1,5-ND (5x10-5M) a) without Cu2+ b) in the presence of Cu2+ Fig.5 (a)UV-Vis spectrum of 1,5-ND (5x10-5M) upon the addition of Cu2+(5x10-9M-5x10-5M from a to j) in aqueous buffer solution (b) Job plot examined between Cu2+ and 1,5-ND (c) Benesi-Hildebrand Plot of 1/A-A0 Vs 1/[Cu2+] Fig.6 (a)UV-Vis spectrum of 2,7-ND (5x10-5M) upon the addition of Ni2+ (5x10-9M-5x10-5M from a to j) in aqueous buffer solution (b) Job’s plot examined between Ni2+ and 1,5-ND (c) Benesi-Hildebrand Plot of 1/A-A0 Vs 1/[Ni2+] Fig.7 (a) Fluorescence spectrum of 1,5-ND (5x10-5M) without Cu2+ and in the presence of Cu2+ (b) Fluorescence spectrum of 1,5-ND (5x10-5M) upon the addition of Cu2+(1μM to 15μM) (Inset: Stern-Volmer plot) Fig.8 (a) Fluorescence spectrum of 2,7-ND (5x10-5M) without Ni2+ and in the presence of Ni2+ (b) Fluorescence spectrum of 2,7-ND (5x10-5M) upon the addition of Ni2+(1μM to 15μM) Fig.9 FT-IR spectrum of (a) 1,5-ND and 1,5-ND:Cu2+ complex, (b) 2,7-ND and 2,7-ND:Ni2+ Complex. Note: Solids of the metal complex grown from aqueous solution are used Fig.10 Fluorescence responses of (a) 1,5-ND to 105 M Cu2+ and (b)2,7-ND to Ni2+ in presence of other selected metal cations (105 M) (concentration of 1,5-ND/2,7-ND:105 M and CD:12x10-3M)
20
Research highlights Naphthalenediols such as 1,5-naphthalenediol and 2,7-Naphthalenediol are acts as a chemosensor for selective sensing of Cu2+ and Ni2+ ions The binding constants were calculated The sensitivity and selectivity of chemosensors have been studied by incorporating 1,5-ND/2,7-ND into -Cyclodextrin UV-Vis and fluorescence spectral technique
21
Fig.1
Fig.2
Fig.3
Fig. 4
Fig.5
Fig.6
Fig.7
Fig.8
Fig.9
Fig.10
Scheme Captions
Scheme 1 Prototropic equilibria (neutral-monoanion-dianion) of (A)1,5-ND and (B) 2,7ND in aqueous and β-CD medium Scheme 2 The proposed structure of 1:1 metal complex of 1,5-ND with Cu2+ and 2,7-ND with Ni2+
Scheme 1
(A) 1,5-ND
OH
O-
O
O
pH-10
-H+ OH
OH
+OH
Extended Conjugation
Monoanion
Neutral
-H
+
>pH-11 O-
O- Dianion
(B) 2,7-ND
HO
O
OH pH-11
HO
O- HO
HO
O
(-)
O
-H+
Neutral HO
(-)
O
(-) O HO
HO (-) Monoanion
O (-)
-H+ >pH-11
-O
O-
Dianion
Scheme 2 Cu2+ OH O-
O-
Cu2+ HO
HO
1,5- ND
1,5- ND:Cu2+
(Fluorescence ON)
(Fluorescence OFF)
Ni2+ O-
O-
Ni2+
OH 2,7- ND (Weak Fluorescence)
OH 2,7- ND:Ni2+ (Strong Fluorescence)
OH
Naphthalenediols: A new class of novel fluorescent chemosensors for selective sensing of Cu2 þ and Ni2 þ in aqueous solution R. Kavitha, T. Stalin
www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(14)00600-0 http://dx.doi.org/10.1016/j.jlumin.2014.10.029 LUMIN12959
To appear in:
Journal of Luminescence
Received date: 18 April 2014 Revised date: 10 October 2014 Accepted date: 13 October 2014 Cite this article as: R. Kavitha, T. Stalin, Naphthalenediols: A new class of novel fluorescent chemosensors for selective sensing of Cu2 þ and Ni2 þ in aqueous solution, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2014.10.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Naphthalenediols: A new class of novel fluorescent chemosensors for selective sensing of Cu2+ and Ni2+ in aqueous solution R.Kavitha and T.Stalin* Department of Industrial Chemistry, Alagappa University, Karaikudi- 630 003, Tamilnadu, India. Abstract Naphthalenediols(NDs) such as 1,5-naphthalenediol (1,5-ND) and 2,7-Naphthalenediol (2,7-ND) are found to be potent chemosensor for selective sensing of Cu2+ and Ni2+ ions respectively. The sensitivity and selectivity of chemosensors have been studied by incorporating NDs into -Cyclodextrin (-CD). The chemosensor behavior of NDs towards various transition metals is studied under selective pH medium. In 1,5-ND, the colorimetric chemosensor behavior has been identified and demonstrated in aqueous medium for the selective detection of Cu2+ without any modification, likewise 2,7-ND also having sensing ability towards Ni2+. The chemosensor probe 1,5-ND showed selective chromogenic behavior towards Cu2+ ions by changing the colour from pink to purple. No significant colour change is observed upon the addition of other metal ions. The sensing property of naphthalenediols towards metal cations has been studied using UV-Vis and fluorescence spectral technique. Keywords: -Cyclodextrin, Naphthalenediols, Chemosensor, Inclusion complex
* Corresponding author (Dr. T.Stalin) E-mail address: [email protected] Tel: +91 9944266475 Fax: +91 4565 225202
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1. Introduction The interest in the development of highly sensitive probes for metal ion analysis is the current thrust in the active research field. Metal ions are playing essential roles in biology by serving as essential cofactors in the processes such as respiration, growth, gene transcription, enzymatic reactions and immune function. The transition metals can exist in many different forms within the cells, associated with low molecular weight species such as amino acids or glutathione, from which the metal ion could be released by changes in the cellular environment. Along with other transition metal ions, copper and nickel are essential trace elements in biological systems such as respiration, biosynthesis and metabolism [1]. An undersupply of these metal ions leads to deficiency and an oversupply result in toxic effects. At higher concentration, copper causes liver damage in infants and nickel causes lung injury, allergy and carcinogenesis [2]. The detection and selective monitoring of these metal ions in environmental and clinical analysis has thereof gained significant importance. Currently, highly sensitive instrumentations have been applied for the detection of metal pollutants such as atomic absorption spectroscopy (AAS) [3], voltammetry [4], inductively coupled plasma mass spectroscopy (ICP-MS) [5] and potentiometry [6]. Although those methods are very sensitive and accurate, they lean to be costeffective, technically complex, time-consuming and does not permit high throughput analysis. Among the various detection methods available, UV-Vis and fluorescence spectroscopy still remain the most frequently used modes, due to their high sensitivity and easy operational use. Therefore, during the recent past, good amount of work on sensing of Cu2+ and Ni2+ by variety of synthesized colorimetric/fluorescent probes have been reported [7-21]. However, there is still and intense demand for new efficient Cu2+ colorimetric chemosensor, especially those can work in aqueous solution with high selectivity and sensitivity [22–24]. The development of novel
2
colorimetric chemosensor for the rapid and convenient detection of Cu2+ is more attractive. Recently, selective luminescent probes based on benzothiadiazoyl-triazoyl cyclodextrin and zinc compound containing symmetrical Schiff base ligand for Ni2+ [25] and coumarin Schiff-base for Ni2+ as a colorimetric sensor [26] were also investigated. Based on Cu2+ promoted oxidation reaction, a new naphtol derivative as selective colorimetric and fluorescent chemosensor for Cu2+ was also reported recently [27]. Dihydroxy naphthalene compounds having two hydroxyl groups which are substituted in the naphthalene ring system. It has numerous positional isomers differing by the location of the hydroxyl group. The different positions provide various chemical structures which offer important roles to each characteristic. They are used directly in making several dyes and are converted into numerous corresponding amines, esters, ethers and carboxylic derivatives as well as into numerous sulfo- and nitro-group substituted (mono-, di and tri) compounds. They find extensive applications in making dyes, pigments, fluorescent whiteners, tanning agents, antioxidants and antiseptics. NDs may be regarded as a singular template for building up various derivatives due to the specific arrangement of the hydroxyl group. 1,5-Naphthalenediol (abbreviated as 1,5-ND) and 2,7-Naphthalenediol (abbreviated as 2,7-ND) are applied as an intermediate for organic synthesis, especially for crown ether and naphthalene sulfonic acid series used to make colorant compounds. These two naphthalenediols such as 1,5-ND and 2,7ND are used in this chemosensor study. To the best of our knowledge, there is no report on NDs in sensing of metal ions. We are the first to report the NDs as a fluorescent chemosensor for sensing of metal cations. NDs, an important class of organic compounds, its derivatives absorb visible light and have a wide range of applications as said above. One of the derivatives of NDs such as 1,5-Naphthalenediol (1,5-
3
ND) used in this study produce distinct colour in alkaline medium and its pKa value in water is 9.80 for neutral-monoanion equilibrium. Due to its high absorption coefficient and possibility of naked eye detection of anionic form, 1,5-ND has been selected as a scaffold to recognize cations in aqueous medium. However, 1,5-ND and 2,7-ND having poor solubility in water and it is not possible to function in aqueous solution. One of the best methods to improve their solubility and to operate in an aqueous medium is making inclusion complex with cyclodextrins (CDs). For substances with low water solubility, inclusion in CDs may improve their physicochemical properties such as solubility, stability and spectroscopic properties [28,29]. Among the three types of CDs, -Cyclodextrin is the best host for many guests, due to its right size. Previously we have reported the inclusion phenomena of series of organic compounds with -cyclodextrin, its inclusion behavior, stoichiometry and mechanism [30-32]. Herein, we wish to report the simple and cost-effective chemosensor consisting of NDs and -Cyclodextrin (-CD). Using -CD, NDs forms hydrophobic inclusion complexes that made these compounds to operate in an aqueous medium. We have found that the inclusion complex of NDs with -CD as potential chemosensor for cations of biological interest such as Cu2+ and Ni2+ in aqueous solution. The compound 1,5-ND show a selective binding toward Cu2+ as was expressed by a red-shift in the UV-Vis absorption spectra and a specific fluorescence quenching behavior. In this respect, no significant response to other metal cations was observed. The changes in the UV-Vis and fluorescence spectra that were observed in the presence of Cu2+ were attributed to the formation of a metal-ligand complex. Addition of Cu2+ to the solution of 1,5-ND in -CD medium resulted in a significant colour change. It should be noted that this particular behavior was exclusive for Cu2+ which allowing its detection even in the presence of other metal cations.
4
2. Experimental 2.1. Chemicals and instruments -Cyclodextrin (-CD), 1,5-Naphthalenediol (1,5-ND) and 2,7-Naphthalenediol (2,7-ND) were purchased from Himedia Chemical Reagents Company and used without further purification. Solutions of metal ions were prepared from prechlorate salts of respective metals and were dissolved in distilled water. Triply distilled water was used to prepare all solutions. The UV–Vis spectra (absorption spectral measurements) was carried out with
Shimadzu UV-
2401PC double-beam spectrophotometer, fluorescence measurements were made using a Jasco FP-880 spectrofluorimeter, the pH values in the range 1.0–12.0 were measured on Elico pH meter LI-120 and FT-IR was recorded using Nicolet 380. 2.2. Preparation of stock solutions for UVand Fluorescence studies The recognition between naphthalenediols and different metal cations were investigated by UV–Vis and fluorescence spectroscopy in aqueous solution. The stock solution of -CD (12×103mol dm3) was prepared using buffer solutions. The stock solution of NDs in ethanol and metal ions were at a concentration of 1×102 M. All absorption spectral studies were carried out in pure triply distilled water at room temperature. Solutions in the pH range 1.0–12.0 were prepared by adding the appropriate amount of NaOH and H3PO4. A modified Hammett’s acidity scale (H0) [33] for the solutions below pH~2 (using a H2SO4– H2O mixture) and Yagil basicity scale (H) [34] for solutions above pH~12 (using a NaOH–H2O mixture) were employed. The solutions were prepared just before taking measurements. For colorimetric titrations, the stock solutions (5 x 10-5 M) of all metal ions were prepared in aqueous solution and used. To analyse the effect of metal through UV-Vis and fluorescence spectroscopy, various concentration of
5
selective metal cations (5x10-9 to 5x10-5 M) were prepared in 10 mL volumetric flask and shaken thoroughly. All the absorption and emission spectra were recorded at 30±100C. 3. Results and discussion 3.1. Effect of pH Chemosensors are usually influenced by pH of the media in the detection of metal ions. The influence of operative pH value upon the sensitivities of chemosensor is an extremely important one. The absorption and fluorescence spectra of NDs are examined at different pH and the relevant data are given in Table 1 and Fig.1. In 1,5-ND and 2,7-ND, the position of the hydroxyl groups are ’ and ’ respectively and they are placed symmetrically. In the pH range 1-9, observed absorption maxima resembles the spectra observed in non aqueous solvents and thus can be assigned to neutral species [35-36]. The absorption maxima of NDs are observed at 331.5 nm(1,5-ND) and 328.0 nm(2,7-ND) could be due to exists of the neutral form of NDs. When the increase on acid strength (
6
delocalized over the two benzene rings and formed an extended conjugation (Scheme 1). The monoanionic absorption band (532.5nm) is further red shifted into 539.5 nm in H_ 12.99, it clearly indicates the formation of dianion in 1,5-ND. Like 1,5-ND, the similar trend was also observed in 2,7-ND for the formation of monoanion. But the monoanoinic form of 2,7-ND produces a smaller red shift (328.0 to 342.5 nm) when compared to1,5-ND. Unlike 1,5-ND, here in the negative charge on oxygen atom of hydroxyl group is delocalized over the carbon atoms of naphthalene moiety and could not form the extended conjugation, instead 2,7-ND forms stable monoanion. From pH~12, the peak at 249.0 nm alone is red shifted to 263.5 nm. The red shift observed in the absorption spectrum is due to the formation of dianion which is formed by the removal of proton from ’ hydroxyl group. Further increase in pH from H_ 12.9 to H_ 15.2, the peak at 263.5 nm is progressively blue shifted into 245.0 nm. It can be explained through the resonance structures of mono and dianion of 2,7-ND. The fluorescence spectra of both compounds in various pH are also measured and shown in Fig. 1. The intensity of a single emission band of 1,5-ND with the maximum at 347.0 nm decreasing with the increase in pH from 1 to 8, while the dual fluorescence is observed for 2,7ND at 342.0 and 401.0 nm. The dual fluorescence in 2,7-ND implies that both neutral and monoanionic forms are in equilibrium. When the pH is increased from 8 to 11, the fluorescence maxima of 1,5-ND is red shifted to 416.0 nm, whereas in 2,7-ND the peak at 401.0 nm is red shifted to 407.5 nm. This red shifted spectrum indicates that deprotonation should occur in the hydroxyl group. Further increase in pH from 12 to H_ 15.20, a single broad peak at 412.0 nm for 1,5-ND and 402.0 nm for 2,7-ND is obtained. This blue shifted maximum is due to the formation of dianion and the dual emission band becomes a single emission band indicates that the stable
7
dianion is formed at above pH~12. Hence, there are three prototropic species such as neutral, monoanion and dianion existed for both 1,5-ND and 2,7-ND, confirmed through UV-Vis and fluorescence emission spectrum. The acidity constant at ground state (pKa) and excited state (pKa*) values of both molecules for neutral-monoanion equilibrium are determined spectrophotometrically and its values are given in Table 1, but the acidity constant values for monoanion-dianion equilibrium could not be determined, because there was no constancy in the isobestic points. Though 1,5-ND and 2,7-ND is an extremely weak acid in its ground state (pKa=9.7 and 10.4), when excited it loses one proton and its pKa* values being 2.1 and 2.9 respectively. These values clearly indicate that both naphthalenediols are stronger acid in excited state than in ground state. It is due to the intramolecular charge transfer (ICT) in the excited state of the NDs, from the hydroxyl oxygen to the aromatic ring. Further, it was subsequently realized that the ICT effect must be even larger in the conjugate anionic base. Thus, it is the excess stabilization of the excited anionic base that renders the excited state deprotonation reaction more exothermic, leading to enhanced photoacidity. In order to find the potential application as chemosensor, NDs is studied under -CD medium to promote the sensitivity and selectivity of chemosensors. The -CD can include the variety of guest molecules to form hydrophobic inclusion complexes which made these chemosensors operate in an aqueous medium. From the pH effect, the anionic form of 1,5-ND is pink color, and may have more interactions with the metal cations, which motivated us to test the ability of neutral and monoanion of NDs in pH~7 and 10 buffer medium respectively. 3.2. Spectral response of naphthalenediols (1,5-ND & 2,7-ND) towards different metal ions 3.2.1. UV-Vis spectral studies
8
The ability of neutral and moanionic forms of 1,5-ND and 2,7-ND to detect metal cations in aqueous solution are tested in the presence of various metal cations such as Ni2+, Cd2+,Co2+, Hg2+,Zr4+, Zn2+,Cu2+,Cu+, Pb2+, Bi3+ and Mg2+ in pH~7 and 10 buffer solution of -CD medium [37]. Fig.2 shows the absorption spectra of 1,5-ND and 2,7-ND in pH~7 of -CD medium and it implies that the neutral form of both compounds could not have any significant interaction with metal cations. When compared to the neutral form, the anionic form having more interaction with metal cations for both 1,5-ND and 2,7-ND and hence all the sensing studies are carried out in pH~10 buffer medium (anionic form). Fig.3 showed the UV-Vis absorption spectra of 1,5-ND and 2,7-ND in aqueous -CD medium of pH~10 with the different test metal ions. The spectra obtained for 1,5-ND in the absence and presence of above cations showed that the absorption spectrum had almost no change in the presence of these metal cations and exhibited a main peak at 532.5 nm. However, the addition of Cu2+ causes the spectra of 1,5-ND to shift to longer wavelength (550.0 nm). It is important to note that the appearance of a new strong and red absorption peak at 550.0 nm created by the addition of Cu2+, while resulted in a naked eye color change from pink to purple (Fig. 4). No color change is observed with other metal cations. The absorption spectra of free 1,5-ND exhibited a band at 532.5 nm showed a large bathochromic shift to 550.0 nm with absorption tail extending to about 650 nm, indicating that Cu2+ has strong interaction with 1,5-ND. These results indicated that 1,5-ND can be used as a colorimetric probe for Cu2+ and the distinct color change with Cu2+ indicated that the 1,5-ND is a potential probe to function as a visible chemosensor for this ion. It is important that the absorption of the system is enhanced by -CD which increasing the sensitizing effect on the determination of Cu2+ and in CD system, the solution remained clear and transparent for a long time. Furthermore, use of 1,5ND for sensing studies is extended to the various concentrations of Cu2+ (5x10-9 to 5x10-5 M) in
9
the presence of -CD. Thus, the concentration of Cu2+ is increased; the absorption intensity of the corresponding peak also increased (Fig.5). These results clearly demonstrate the sensing ability of 1,5-ND for Cu2+ cation in water. To investigate the probable complexation of 1,5-ND with Cu2+, the method of continuous variations (Job’s plot) is obtained, which clearly suggested the formation of 1:1 stoichiometry between 1,5-ND and Cu2+ (Fig.5b). The total concentration of 1,5-ND and Cu2+ is fixed at a constant value (10 M), and then molar fraction of Cu2+ varied continuously. As shown in Fig.5b, the 1,5-ND-Cu2+ complex exhibits a maximum fluorescence emission when the molecular fraction of Cu2+ is about 0.5, indicating that a 1:1 stoichiometry is more possible for the binding mode of 1,5-ND and Cu2+. Moreover the association constant of 1,5-ND:Cu2+ complex will further estimate to be 1.1 x 104 M-1, assuming a 1:1 stoichiometry by the Benesi-Hildebrand method [38] (Fig.5c). Unlike 1,5-ND, the response of 2,7-ND towards different metal cations were assorted and new results were found. The addition of Cd2+,Co2+, Hg2+,Zr4+, Zn2+,Cu2+,Cu+,Bi3+ and Mg2+ to 2,7-ND caused no changes in spectrum, but only the addition of Ni2+ caused a significant change in the spectrum i.e., bathochromic shift. Comparing the response of these diols suggests that the position of substituent playing major role in the detection of metal ions. Moreover, the sensitivity and binding response of 2,7-ND with Ni2+ were studied by the various concentrations of Ni2+ (5x10-9 to 5x10-5 M) (Fig.6c). When the concentration of Ni2+ is increased, the intensity of the red shifted peak also increased. Job’s plot analysis revealed that 2,7-ND interact with Ni2+ in 1:1 stoichiometry. As like 1,5-ND, 2,7-ND also follows the 1:1 complex with the metal ion. Further, the binding affinity of 2,7-ND for Ni2+ was estimated from Benesi-Hildebrand plots. The association constant (Ka) has been found to be 1.75 x 104 M-1. The Ka value of Ni2+ complex is
10
higher than the Cu2+ complex, higher the association constant value represents the more stability of metal complex. 3.2.2. Fluorescence spectral studies Fluorescence spectroscopic studies were also employed to study the cation recognition properties of naphthalenediols. The fluorescence spectrum of the chemosensor 1,5-ND was recorded in the range of 230-470 nm upon excitation at 300 nm. It is observed that in addition of Cu2+ ions quenched the fluorescence of 1,5-ND (45 % of reduction in intensity) (Fig. 7a). The experimental results indicated that the quenching efficiency, increased with an increase in the concentration of Cu2+ ions at a fixed concentration of 1,5-ND (Fig.7b). Fluorescence intensity data were analyzed according to the Stern-Volmer law [39]. F0/F=1/Ksv[Q] Where F0 is emission intensity of 1,5-ND in the absence of quencher (Q), F is emission intensity of 1,5-ND at quencher concentration [Q]. The linearity of Stern-Volmer plot (Inset of Fig.7b) suggested that only one quenching mechanism is operative and the quenching is bimolecular and dynamic [40,41]. The Stern-Volmer quenching constant for 1,5-ND with Cu2+ is 5.8x105. In bimolecular liquid systems, the fluorescence intensity is hindered due to several mechanisms, such as static and dynamic quenching, excimer and exciplex formation, charge transfer, energy transfer process, etc [42-46]. Based on the absorption results and Job’s plot, Cu2+ bound to 1,5ND and forming a 1:1 stoichiometric complex. Thus, it can be assumed that the quenching of fluorescence of 1,5-ND by Cu2+ ion is a static quenching mode due to the formation of nonfluorescent complex in the ground state. Moreover, Cu2+ is a well-known strong quencher because it is a good electron scavenger on account of its d9 electronic structure. Quenching by this type of substance most likely involves the donation of an electron from the fluorophore to
11
the quencher [47]. In pH~10, the fluorophore (1,5-ND) exists in an anionic form which donate an electron to Cu2+ ion. In this regard, the energy and electron-transfer processes cannot be ruled out during the fluorescence quenching of 1,5-ND by Cu2+ in an aqueous solution. Similarly, the metal ion interaction of 2,7-ND with Ni2+ is further confirmed by fluorescence spectroscopic measurements (Fig.8). The emission peak at 407.5 nm in the spectrum of 2,7-ND enhanced upon the addition of Ni2+ in aqueous -CD medium. It is important to note that the fluorescent enhancement observed in 2,7-ND is highest and behaves in a different manner than 1,5-ND. The selectivity of 2,7-ND for Ni2+ is further followed by fluorescence titration studies under similar conditions. To assess the sensitivity, varying concentrations of Ni2+ were added to the solution of 2, 7-ND and the intensity of emission peak increases with increase in Ni2+ concentration. The maximum increase of fluorescence emission at 407.5 nm is found to be 40%, which was an agreement with the data of 1:1 complex between 2,7-ND and Ni2+ from Job’s plot. The enhancement of fluorescence was attributed to the introduction of Ni2+ and consequent occurrence of the strong complexation with 2, 7-ND, resulting in decreased non-radiative decay of the excited-state and increased radiative decay. The remarkable increase of the fluorescence intensity is due to the formation of stable complexes between 2,7-ND and Ni2+ in the presence of -CD medium. 3.2.3. The possible binding mechanism The possible mechanism of interaction between metal cations (Cu2+ and Ni2+) and naphthalenediols is proposed by considering the changes in UV/Vis and fluorescence spectra. In fact, among the various metals studied, Cu2+ ions gave the effective color change with 1,5-ND, therefore an effective coordination between the metal and the oxygen atom of 1,5-ND could be expected. The interaction of Cu2+ with 1,5-ND causes the disappearance of the band at 532.5 nm
12
and the emergence of a new red shifted band that probably arises from a charge-transfer transition, in which orbital of the metal centers are involved. Compared with other transition metal ions, Cu2+ has a high thermodynamic affinity for typical N, O-chelate ligands and fast metal-to-ligand binding kinetics [48], which could account for the selectivity of the system to Cu2+. The chelation induced fluorescence quenching is probably induced by redox active Cu2+ ions via ligand-to-metal electron or energy transfer mechanisms [49]. The decrease in fluorescence intensity is attributed to electron transfer between 1,5-ND and Cu2+. The compound 2,7-ND in the presence of Ni2+ produce a red shift in absorption spectra and the enhancement of emission intensity in fluorescence spectra. These changes in fluorescence intensity can be attributed to the nature of the cations where paramagnetic cations are known to cause a fluorescence quenching while diamagnetic transition metal ions can cause fluorescence enhancement. Both Cu2+ and Ni2+ ions are paramagnetic, hence Cu2+ quench the fluorescence spectra of 1,5-ND. Even though Ni2+ is paramagnetic, it produces an increase in emission intensity of 2,7-ND. The increase in intensity is attributed to the formation of stable complexes between 2,7-ND and Ni2+. Form Job’s plot analysis, both metal cations forming 1:1 complex with chemosensor. The formation of 1:1 complex involves through the O(-) ion of chemosensor and a secondary OH group of -CD in the upper rim. This is the only probable mechanism for both complexes of naphthalenediols with metal cations. The mode of binding is further confirmed by FT-IR spectroscopy. We measured FT-IR spectra of naphthalenediols and its complex with metal cations to further examine the binding site. The newly formed band in the spectra of the complex at 584.2 cm-1(Fig.9a) and 578.2 cm-1 (Fig.9b) are assigned to Cu-O and Ni-O vibrations respectively. It confirmed the formation of a complex between metal and chemosensor (1,5-ND/2,7-ND). In
13
addition to the new band, the characteristic peaks of 1,5-ND and 2,7-ND are shifted to higher frequency value. In 1,5-ND, the peaks observed at 1377 cm-1 (C-O stretching), 3288 cm-1 (O-H stretching), 1597 cm-1 (C=C stretching) are
not shifted but their intensities were largely
decreased and the peaks become broadened in the metal complex. In the case of 2,7-ND the peaks observed at 1386 cm-1 (C-O stretching), 3219 cm-1 (O-H stretching), 1627 cm-1 (C=C stretching) are shifted in the metal complex to 1380 cm-1, 3372 cm-1, 1635 cm-1 respectively. When the FT-IR spectra of 2,7-ND is compared with 1,5-ND, the peaks were shifted to larger value, in particular OH stretching and C=C stretching frequencies. Thus, in accordance with the 1;1 stoichiometry, 1,5-ND and 2,7-ND are likely to chelate Cu2+ and Ni2+ via its O(-) atom and OH group of -CD. Based on the spectroscopic data (UV-Vis, Fluorescence and IR spectra), it is confirmed that the O(-) atom of 1,5-ND coordinate with Cu2+ ions. The proposed structure of the complex is shown in Scheme 2. To realize the practical utility of these chemosensors towards Cu2+/Ni2+ over other competitive species, the competitive experiments were conducted in the presence of Cu2+/Ni2+ mixed with movalent, divalent and trivalent metal cations (Fig. 10). As can be seen from the Fig.10a, the presence of Cu2+ and another metal ion had no significant interference with the quenching created by Cu2+ ions. Similarly, as shown in Fig.10b the competing metal ions exhibited only a small or no interference with the detection of Ni2+ ions. It is noticeable that the competitive metal ions did not lead to any significant changes in emission intensity when compared with that without other metal ions besides Cu2+ and Ni2+. In addition, no obvious interference was noted in its emission intensity with the presence of other metal cations. Based on the UV-Vis and more specifically the fluorescence spectra, it can be concluded that the response of 1,5-ND and 2,7-ND towards metal ions in the presence of -CD is influenced by the
14
position of substituent on the ring. These results implied that the selectivity of chemosensor 1,5ND and 2,7-ND towards Cu2+ and Ni2+ respectively was remarkable. 4. Conclusions In summary, we have demonstrated the sensing behavior of inclusion complexes of 1,5ND and 2,7-ND towards Cu2+ and Ni2+ respectively. The newly advanced simple chemosensor has been identified and used as it exists without any modifications for the detection of metal cations. 1,5-ND a novel colorimetric sensor for transition metal cation (Cu2+) and binding of this cation causes an appreciable change in the visible region of the spectrum which can be detected by naked-eye. 1,5-ND functioned both as chromogenic and fluorogenic probe for Cu2+ with high selectivity and sensitivity. In the presence of Cu2+ in aqueous solution, the distinct colour change from pink to purple colour was obtained. In 2,7-ND, there is no such color change upon the addition of Ni2+. The compound 2,7-ND in the presence of Ni2+ produce a red shift in absorption spectra and the enhancement of emission intensity in fluorescence spectra. The detection limit for both cations by this method was 10-8 M. References [1] Y.H. Zhang, H.S. Zhang, X.F. Guo, H. Wang, Microchem. J. 89 (2008) 142. [2] C.Bo, Z.Ping, Anal. Bioanal. Chem. 381, (2005) 986. [3] M.S.Chanm, S.D.Huang, Talanta 51 (2000) 373. [4] C.Collado-Sanchez, J.Perz-Pena, M.D.Gelado-Caballero, J.A.Herrera-Melia, J.J.HernadezBrito, Anal. Chim. Acta 320 (1996) 19. [5] J.Wu, E.A.Boyle, Anal. Chem. 69 (1997) 2464. [6] C.W.K.Chow, S.D.Kolev, D.E.Davey, D.E.Mulcahy, Anal. Chim. Acta 330 (1996) 79. [7] S. Goswami, D. Sen, N.K. Das, Org. Lett. 12 (2010) 856.
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[8] N. Narayanaswamy, T. Govindaraju, Sens. Actuators, B: Chemical 161 (2012) 304. [9] R. Kumar, V. Bhalla, M. Kumar, Tetrahedron 64 (2008) 8095. [10] J. Cody, C.J. Fahrni, Tetrahedron 60 (2004) 11099. [11] S. Devaraj, D. Saravanakumar, M. Kandaswamy, Sens. Actuators, B: Chemical 136 (2009) 13. [12] Y. Zheng, J. Orbulescu, X. Ji, F.M. Andreopoulos, S.M. Pham, R.M. Leblanc, J. Am.Chem. Soc. 125 (2005) 2680. [13] M. Yu, M. Shi, Z. Chen, F. Li, X. Li, Y. Gao, J. Xu, H. Yang, Z. Zhou, T. Yi, C. Huang, Chem. Eur. J. 14 (2008) 6892. [14] K.M.K. Swamy, S. Ko, S.K. Kwon, H.N. Lee, Chem. Commun. (2008) 5915. [15] H.S. Jung, P.S. Kwon, J.W. Lee, J.L. Kim, C.S. Hong, J.W. Kim, S. Yan, J.Y. Lee, J.H. Lee, T. Joo, J.S. Kim, J. Am. Chem. Soc. 131 (2009) 2008. [16] M. Shivaprasad, T. Govindaraju, Mater. Technol. 26 (2011) 168. [17] N. Shao, Y. Zhang, S. Cheung, R. Yang, W. Chan, T. Mo, K. Li, F. Liu, Anal. Chem.77 (2005) 7294. [18] Y. Zhou, F. Wang, Y. Kim, S. Kim, J. Yoon, Org. Lett. 11 (2009) 4442. [19] D. Maity, T. Govindaraju, Chem. Eur. J. 17 (2011) 1410. [20] D. Maity, A.K. Manna, D. Karthigeyan, T.K. Kundu, S.K. Pati, T. Govindaraju, Chem. Eur. J. 17 (2011) 11152. [21] A.K. Jain, V.K. Gupta, B.B. Sahoo, L.P. Singh, Anal. Proc. 32 (1995) 99. [22] M. Zhao, X. Yang, S. He, L. Wang, Sens. Actuators, B: Chemical 135 (2009) 625. [23] T. Li, Z. Yang, Y. Li, Z. Liu, G. Qi, B. Wang, Dyes Pigmen. 88 (2011) 103. [24] R. Tang, K. Lei, K. Chen, H. Zhao, J. Chen, J. Fluores. 21 (2011) 141.
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[25] G.B. Li, H.C. Fang, Y.P. Cai, Z.Y. Zhou, P.K. Thallapally, J. Tian, Inorg. Chem. 49 (2010) 7241. [26] L.Wang, D.Ye, D.Cao, Spectrochimica Acta Part A 90 (2012) 40. [27] Y. K. Jang, U. C. Nam, H. L. Kwon, I. H. Hwang, C. Kim, Dyes and Pigments 99 (2013) 6. [28] J. Sun, X.S. Zhu, M. Wu, J. Fluoresc. 17 (2007) 265. [29] M.Y. Xu, S.Z. Wu, F. Zeng, C.M. Yu, Langmuir 26 (2010) 4529. [30] T.Stalin, K.Srinivasan, K.Kayalvizhi, K.Sivakumar, Spectrochim. Acta Part A 79 (2011) 169. [31] T.Stalin, K.Srinivasan, J.Vaheethabanu, P.Manisankar, J. of Molec. Struct. 987 (2011) 214. [32] T.Stalin, K.Srinivasan, K.Sivakumar,Spectrochim. Acta Part A 94 (2012) 89. [33] M.J.Jorgenson, D.R.Harter, J. Am. Chem. Soc. 85 (1963) 878. [34] G.Yagil, J. Phys. Chem. 71 (1967) 1034. [35] T.Stalin, R.Anitha Devi, N.Rajendiran, Spectrochim. Acta. 61A (2005) 2495. [36] M.Shanmugam,
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[44] H.Pal,D.K.Palit,T.Mukherjee.,J.P.Mittal,Chem.Phys.Lett.173 (1990) 354. [45] P.K.Behera,A.K.Mishra, J. Photochem. Photobiol. A 71 (1993) 115. [46] H.Zeng, G.Durocher, J.Lumin. 63 (1995) 75. [47] M.D.P.D.Costa, W.A.P.A.Jayasinghe, J.Photochem.Photobiol. A162(2004) 591. [48] [48] Z. Liang, Z. Liu, L. Jiang, Y. Gao, Tetrahedron Lett. 48 (2007) 1629. [49] S.H. Mashraqui, M. Chandiramani, R. Betkar, K. Poonia, Tetrahedron Lett. 51 (2010) 1306.
18
Table captions Table 1 Various prototropic maxima (Absorption and emission spectra) and pKa values of 1,5-ND and 2,7-ND in -CD medium.
Table 1
1,5-ND
2,7-ND
Species max Neutral (pH~1.0-9.0) Monoanion (pH~9.5-11.0) Dianion (pH~11.5-H_14.02)
max emission (nm)
(nm)
max
max emission (nm)
(nm)
347.0
328.0 313.0 286.0 239.5
342.0 401.0
532.5 331.5
416.0
342.5 258.5
407.5
539.5 333.5
412.0
343.0 260.5
402.0
331.5 317.0 300.0
pKa 9.7
10.4
10.1
10.9
pKa*
19
Figure captions Fig.1 Absorption and emission spectra of (a) 1,5-ND and (b) 2,7-ND (Conc. 2x10-5M) in various pH solutions. Fig.2 UV-Vis absorption spectrum of (a) 1,5-ND and (b) 2,7-ND (5x10-5M) in the presence of various cations in pH~ 7 buffer of -CD aqueous solutions. Fig.3 UV-Vis absorption spectrum of (a) 1,5-ND and (b) 2,7-ND (5x10-5M) in the presence of various cations in pH~ 10 buffer of -CD aqueous solutions. Fig.4 UV-Vis absorption spectrum of 1,5-ND (5x10-5M) a) without Cu2+ b) in the presence of Cu2+ Fig.5 (a)UV-Vis spectrum of 1,5-ND (5x10-5M) upon the addition of Cu2+(5x10-9M-5x10-5M from a to j) in aqueous buffer solution (b) Job plot examined between Cu2+ and 1,5-ND (c) Benesi-Hildebrand Plot of 1/A-A0 Vs 1/[Cu2+] Fig.6 (a)UV-Vis spectrum of 2,7-ND (5x10-5M) upon the addition of Ni2+ (5x10-9M-5x10-5M from a to j) in aqueous buffer solution (b) Job’s plot examined between Ni2+ and 1,5-ND (c) Benesi-Hildebrand Plot of 1/A-A0 Vs 1/[Ni2+] Fig.7 (a) Fluorescence spectrum of 1,5-ND (5x10-5M) without Cu2+ and in the presence of Cu2+ (b) Fluorescence spectrum of 1,5-ND (5x10-5M) upon the addition of Cu2+(1μM to 15μM) (Inset: Stern-Volmer plot) Fig.8 (a) Fluorescence spectrum of 2,7-ND (5x10-5M) without Ni2+ and in the presence of Ni2+ (b) Fluorescence spectrum of 2,7-ND (5x10-5M) upon the addition of Ni2+(1μM to 15μM) Fig.9 FT-IR spectrum of (a) 1,5-ND and 1,5-ND:Cu2+ complex, (b) 2,7-ND and 2,7-ND:Ni2+ Complex. Note: Solids of the metal complex grown from aqueous solution are used Fig.10 Fluorescence responses of (a) 1,5-ND to 105 M Cu2+ and (b)2,7-ND to Ni2+ in presence of other selected metal cations (105 M) (concentration of 1,5-ND/2,7-ND:105 M and CD:12x10-3M)
20
Research highlights Naphthalenediols such as 1,5-naphthalenediol and 2,7-Naphthalenediol are acts as a chemosensor for selective sensing of Cu2+ and Ni2+ ions The binding constants were calculated The sensitivity and selectivity of chemosensors have been studied by incorporating 1,5-ND/2,7-ND into -Cyclodextrin UV-Vis and fluorescence spectral technique
21
Fig.1
Fig.2
Fig.3
Fig. 4
Fig.5
Fig.6
Fig.7
Fig.8
Fig.9
Fig.10
Scheme Captions
Scheme 1 Prototropic equilibria (neutral-monoanion-dianion) of (A)1,5-ND and (B) 2,7ND in aqueous and β-CD medium Scheme 2 The proposed structure of 1:1 metal complex of 1,5-ND with Cu2+ and 2,7-ND with Ni2+
Scheme 1
(A) 1,5-ND
OH
O-
O
O
pH-10
-H+ OH
OH
+OH
Extended Conjugation
Monoanion
Neutral
-H
+
>pH-11 O-
O- Dianion
(B) 2,7-ND
HO
O
OH pH-11
HO
O- HO
HO
O
(-)
O
-H+
Neutral HO
(-)
O
(-) O HO
HO (-) Monoanion
O (-)
-H+ >pH-11
-O
O-
Dianion
Scheme 2 Cu2+ OH O-
O-
Cu2+ HO
HO
1,5- ND
1,5- ND:Cu2+
(Fluorescence ON)
(Fluorescence OFF)
Ni2+ O-
O-
Ni2+
OH 2,7- ND (Weak Fluorescence)
OH 2,7- ND:Ni2+ (Strong Fluorescence)
OH