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Quinolone based chemosensor for the naked-eye and spectrophotometric detection of Cu2+ in aqueous media
Inorganic Chemistry Communications 49 (2014) 59–62
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Quinolone based chemosensor for the naked-eye and spectrophotometric detection of Cu2+ in aqueous media Samadhan R. Patil a, Jitendra P. Nandre a, Prashant A. Patil a,b, Shilpa Bothra c, Suban K. Sahoo c, Antonin Klasek d, Julián Rodríguez-López e, Promod P. Mahulikar a, Umesh D. Patil a,⁎ a
School of Chemical Sciences, North Maharashtra University, P. B. No. 80, Jalgaon 425 001, M.S., India S.S.V.P.S's L. K. Dr. P. R. Ghogrey Science College, Dhule 424 001, India Department of Applied Chemistry, S. V. National Institute Technology, Surat 395 007, Gujarat, India d Department of Chemistry, Faculty of Technology, Tomas Bata University, CZ-762 72 Zlin, Czech Republic e Facultad de Ciencias y Tecnologías Químicas, Universidad de Castilla-La Mancha, Avda. Camilo José Cela, 10, 13071 Ciudad Real, Spain b c
a r t i c l e
i n f o
Article history: Received 21 August 2014 Received in revised form 14 September 2014 Accepted 17 September 2014 Available online 18 September 2014 Keywords: Chemosensor Naked-eye detection Cu2+ ion Intramolecular charge transfer DFT
a b s t r a c t A new 4-hydroxy-2-quinolone based chemosensor (BPHTQ-1) was designed for the spectrophotometric and naked-eye detection of Cu2+. The chemosensor displayed a high selectivity and sensitivity towards Cu2+ in the presence of other competitive metal cations in pure aqueous media. The Cu2+ recognition furnished a distinguishable color change of BPHTQ-1 from colorless to yellow with a significant hyperchromic shift at 300 nm. The S and O atoms of BPHTQ-1 provided a 1:1 binding scaffold for the recognition of Cu2+ ion with a high binding affinity of 19,338 M−1 and a detection limit of 1.39 μM, which is quite low compared with World Health Organization (WHO) reports. © 2014 Elsevier B.V. All rights reserved.
The development of chemosensors for the detection of transition metal cations is an active area of research due to their potential application as diagnostic tools in the medical, physiological and environmental field [1–7]. Transition metal cations are well known for their important roles in many biological and environmental processes. Among the various transition metal cations, copper is the third most abundant transition element in the human body and plays an important role in various physiological processes like hemoglobin biosynthesis, dopamine production, nerve function regulation, gene expression, bone development, and the functional as well as structural enhancement of proteins [8–11]. Cu2+ also plays a vital role as a catalytic co-factor for a variety of metallo-enzymes and transcriptional events such as superoxide dismutase, cytochrome c oxidase and tyrosinase [12,13]. Apart from the biological and environmental importance, copper is widely used in the metal, pharmaceutical and agrochemical industries for making alloys, electrical wires, batteries, drugs, machine parts and fertilizers [14,15]. However, the excess of Cu2+ ion has serious harmful effect on the living systems. The over accumulation in human being leads to various neurodegenerative diseases such as Alzheimer's disease, prion disease, Wilson's disease, kidney damage, Menkes disease, gastrointestinal disorders, amyotrophic sclerosis, lipid metabolism, and inflammatory ⁎ Corresponding author. E-mail address: [email protected] (U.D. Patil).
http://dx.doi.org/10.1016/j.inoche.2014.09.021 1387-7003/© 2014 Elsevier B.V. All rights reserved.
disorders [16–19]. Therefore, the rapid and easy detection of Cu2+ is very important in environmental and biological systems. Recently, the naked-eye detection method has gained an immense interest because of its simplicity, low cost, and ability to detect Cu2+ up to micro/submicromolar levels without involving any sophisticated costly instruments [20–23]. Herein, as a part of our efforts in the field of analyte recognition [7,21,24–26], a simple and easy-to-prepare receptor, bis(4-hydroxy-2-quinolone-3-yl)sulfide (BPHTQ-1, Scheme 1), as an efficient colorimetric sensor for the highly selective detection of Cu2+ by visible color change from colorless to yellow and a significant hyperchromic shift at 300 nm in the UV–Vis absorption study. For these experiments, the receptor was taken in DMSO solution but the cations were added from 100% water medium. To the best of our knowledge, this sensor represents the first example with a significant hyperchromic shift of ΔA = 1.19 in the presence of Cu2+ ion. The receptor BPHTQ-1 was synthesized as previously reported in the literature (Scheme 1) [27]. Then, the chemosensing behavior of BPHTQ-1 with group-I, II and III metal ions (Ba2 +, Ca2 +, Cs+, Li+, Mg2 +, Na+, K+, Al3 +, Sr2 +, etc.) and transition/heavy metal ions (Ag+, Co2 +, Cu2+, Cr3 +, Fe2 +, Cd2+, Fe3 +, Mn2 +, Ni2 +, Pb2 +, Zn2 +, Hg2 +, etc.) was investigated by naked-eye detection and UV–Vis absorption spectroscopy methods. In the naked-eye experiment, the qualitative recognition ability of Cu2+ ions was identified from the perceptible visual color change of
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Scheme 1. The preparation of chemosensor BPHTQ-1.
Fig. 1. Naked-eye detection of Cu2+ ion in the presence of other metal cations under visible light. Only the Cu2+ ion (5 eq., 1 mL, 1 × 10−2 M in water) gives the visual color change for BPHTQ-1 (2 mL, 1 × 10−3 M in DMSO) from colorless to clear yellow.
BPHTQ-1 solution from colorless to yellow (Fig. 1). To perform this naked-eye experiments, 5 equivalents of various cations including Cu2+ (1 mL, 1 × 10−2 M, in H2O) were added to a solution of BPHTQ1 (2 mL, 1 × 10−3 M, in DMSO). No noticeable color change of the receptor was observed with other tested cations. The naked-eye results inspired us to undertake the systematic spectroscopic investigation on the cation recognition ability of BPHTQ-1, which was performed at a lower concentration of 5 × 10−5 M in DMSO. The absorption spectrum of BPHTQ-1 showed a broad absorption band between 250-350 nm due to π–π* and/or n–π* electronic transition. Upon addition of 5 equivalents of different cations (50 μL, 1 × 10−2 M, in H2O), only the Cu2+ perturbs the absorption spectrum effectively, whereas the other cations failed to affect the absorption spectrum of BPHTQ-1 (Fig. 2). The addition of Cu2+ ions resulted in the appearance of an intense and sharp absorption band at 300 nm. In addition, the detection of Cu2+ by BPHTQ-1 in aqueous medium was free from the interference of other competitive metal cations (Fig. S1), which indicates that this sensor could detect the Cu2+ ion selectively with high sensitivity over a range of twenty other tested competing metal cations.
The chemosensing ability and binding mechanism of BPHTQ-1 towards Cu2+ were investigated by absorption titration with Cu2+. As shown in Fig. 3, upon sequential addition of Cu2+ (0–5 equivalents), the absorption band centered at ca. 300 nm underwent a steady
Fig. 3. Changes in the absorption spectra of BPHTQ-1 (2 mL, 5 × 10−5 M, in DMSO) upon addition of 1–5 equiv. Cu2+ ions (0–500 μL, 1 × 10−3 M, in H2O).
Fig. 2. Absorption spectral changes of BPHTQ-1 (2 mL, 5 × 10−5 M in DMSO) upon addition of 5 equivalents of various metal cations.
Fig. 4. Benesi–Hilderbrand plot of chemosensor BPHTQ-1 with Cu2+ ion for the evaluation of the association constant.
S.R. Patil et al. / Inorganic Chemistry Communications 49 (2014) 59–62
Fig. 5. Job's plot showing the 1:1 binding of BPHTQ-1 to Cu2+.
increase in the absorption intensity. From the titration data, the limit of detection (LOD) and limit of quantification (LOQ) of the receptor BPHTQ-1 were calculated by using the equations: LOD = (3.3 × standard deviation) / slope and LOQ = (10 × standard deviation) / slope. The relative standard deviation was obtained by recording the absorption measurements of ten blank samples. Using the calibration curve (Fig. S2), the LOD and LOQ were estimated to be 1.39 μM and 4.22 μM, respectively. Next, the association constant, Ka, was evaluated graphically by plotting 1/ΔA against 1/[Cu2+] (Fig. 4). The data was linearly fitted according to the Benesi–Hildebrand equation and the Ka value was obtained from the slope and the y-intercept of the line. The Ka value obtained for BPHTQ-1·Cu2+ complex was found to be 19,338 M− 1. The reversibility of the chemosensor was also examined. The interaction between BPHTQ-1 (2 mL, 5 × 10−5 M in DMSO) and Cu2+ (10 μL, 1 × 10−2 M in H2O) was fully reversible upon introduction of 4
61
equivalents of EDTA (40 μL, 1 × 10−2 M in water), which could be identified from the restore of the absorption spectrum of BPHTQ-1 (Fig. S3). This reversibility process was repeated three times with same results. The 1:1 binding stoichiometry for the complexation between BPHTQ1 and Cu2+ was determined from Job's plot (Fig. 5) and mole ratio plot (Fig. S4). In addition, a linear dependence of absorbance at 300 nm was observed as a function of Cu2+ concentration (Fig. S2), which also indicates the 1:1 binding stoichiometry between BPHTQ-1 and Cu2+. Further evidence for the formation of a 1:1 complex between BPHTQ-1 and Cu2+ was obtained from the MALDI-TOF mass spectrometry analysis of an equimolar mixture of BPHTQ-1 and copper perchlorate hexahydrate in DMSO using DHB (2,5-dihydroxybenzoic acid) as the matrix (Fig. S5). Well-defined and highly resolved peaks observed at m/z 567.2 and 505.2 could be assigned to the molecular cation [BPHTQ-1·Cu]+ (calc. m/z 567.0) and [BPHTQ-1·H]+ (calc. m/z 505.1) respectively, with an excellent agreement between the theoretical and experimental isotope distributions. The use of THAP (2,4,6-trihydroxyacetophenone) as the matrix gave an extra peak at m/z 630.4 that corresponds to [BPHTQ-1–2H·2Cu]+ (Fig. S6). This peak also appeared when a 1:5 M mixture of BPHTQ-1: Cu(ClO4)2·6H2O was analyzed using different matrices (Fig. S7); once again with an excellent match for the isotope distributions. Further, in an attempt to confirm the binding mechanism of Cu2+ with BPHTQ-1, we carried out 1H NMR titration experiments in which the spectra were recorded by adding successive amounts of copper perchlorate hexahydrate (0, 0.2, 0.5, 1.0 equiv.) to a solution of BPHTQ-1 in DMSO-d6. The addition of paramagnetic Cu2+ resulted in broadening of the signals and the chemical shifts for all signals remained unaffected. Therefore, no conclusions could be extracted from these experiments. It was expected that the chemosensor BPHTQ-1 should interact with the Cu2+ by the donation of the lone electron pairs located on the oxygen and sulfur atoms to a vacant orbital on the metal ion (Fig. S8). This electron donation or charge transfer resulted in the color change from colorless to clear yellow. This binding mode was finally supported by density functional theory (DFT) analysis of
Fig. 6. DFT computed (a) optimized structure of receptor BPHTQ-1 and its complex with Cu2+, and the (b) LUMO and (c) HOMO diagrams of BPHTQ-1 and its BPHTQ-1·Cu2+ complex in the gas phase.
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BPHTQ-1 and its BPHTQ-1·Cu2+ complex. The study of the charge transfer process during the formation of the complex was performed by applying the B3LYP functional, and the basis sets 6-31G** (for C, H, N and O atoms) and LANL2DZ (for Cu atom) available in the computational code Gaussian 09W [28]. Among the various possible structures, the energetically most favorable, optimized structures for BPHTQ-1 and its complex with Cu2+ are shown in Fig. 6. The complexation of BPHTQ-1 with Cu2+ resulted in lowering the interaction energy (Eint = Ecomplex − Ereceptor − ECu2+) by −165.08 kcal/mol, which indicates the formation of a stable complex. Furthermore, the band gap between the HOMO and LUMO of BPHTQ-1 was lowered upon complexation with Cu2+, which may be responsible for the appearance of yellow coloration for the BPHTQ-1·Cu2+ complex (Fig. 1). The analysis of the HOMO and LUMO diagrams of BPHTQ-1 and its complex with Cu2+ clearly indicates that the intramolecular charge transfer (ICT) occurred between the receptor and Cu2+. In conclusion, we have developed a new chemosensor for the selective and specific detection of Cu2+ ion in aqueous media. The recognition of Cu2+ by BPHTQ-1 gave rise to a color change from colorless to yellow and significant hyperchromic shift in the UV–Vis absorption study. The high selectivity of the sensor BPHTQ-1 towards Cu2+ taken from a pure water medium in the detection study and the detection limit down to 1.39 μM would make the chemosensor BPHTQ-1 a promising candidate for the qualitative and quantitative detection of Cu2+ ions. Considering the versatility, we believe that the sensor BPHTQ-1 could be prompted for many practical applications in chemical, environmental and biological systems for the colorimetric detection of Cu2+.
Acknowledgment S. R. Patil is thankful to DST, New Delhi, India, for the financial assistant under INSPIRE fellowship. A. Klasek thanks Mrs. H. Geržova (Faculty of Technology, Tomas Bata University in Zlín) for the technical help. J. Rodríguez-López also thanks the Ministerio de Economía y Competitividad (Spain)/FEDER (EU) for the financial support — project BFU2011-30161-C02-02.
Appendix A. Supplementary material Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2014.09.021.
References [1] A.P. de Silva, D.B. Fox, A.J.M. Huxley, T.S. Moody, Coord. Chem. Rev. 205 (2000) 41. [2] U.E. Spichiger-Keller (Ed.), Chemical Sensors and Biosensors for Medical and Biological Applications, Wiley–VCH, Weinheim, Germany, 1998. [3] A.W. Czarnik, Fluorescent Chemosensors for Ion and Molecular Recognition, American Chemical Society, Washington D.C., 1993 [4] P.B. Tchounwou, W.K. Ayensu, N. Ninashvili, D. Sutton, Environ. Toxicol. 18 (2003) 149. [5] V. Amendola, L. Fabbrizzi, M. Licchelli, C. Mangano, P. Pallavicini, L. Parodi, A. Poggi, Coord. Chem. Rev. 649–669 (1999) 190. [6] L. Prodi, F. Bolletta, M. Montalti, N. Zaccheroni, Coord. Chem. Rev. 205 (2000) 59. [7] J. Nandre, S. Patil, V. Patil, F. Yu, L. Chen, S. Sahoo, T. Prior, C. Redshaw, P. Mahulikar, U. Patil, Biosens. Bioelectron. 61 (2014) 612. [8] E.D. Harris, J. Trace Elem. Exp. Med. 14 (2001) 207. [9] C. Andreini, L. Banci, I. Bertini, A. Rosato, J. Proteome Res. 7 (2008) 209. [10] E.L. Que, D.W. Domaille, C. Chang, J. Chem. Rev. 108 (2008) 1517. [11] Z. Xu, K.-H. Baek, H. Kim, J. Cui, X. Qian, D.R. Spring, I. Shin, J. Yoon, J. Am. Chem. Soc. 132 (2010) 601. [12] K.D. Karlin, Science 261 (1993) 701. [13] H. Tapiero, D.M. Townsend, K.D. Tew, Biomed. Pharmacother. 57 (2003) 386. [14] A.K. Jain, R.K. Singh, S. Jain, J. Raisoni, Trans. Met. Chem 33 (2008) 243. [15] P.G. Georgopoulos, A. Roy, M.J. Yonone-Lioy, R.E. Opiekun, P.J. Lioy, J. Toxicol. Environ. Health 4 (2001) 341. [16] D. Strausak, J.F.B. Mercer, H.H. Dieter, W. Stremmel, G. Multhaup, Brain Res. Bull. 55 (2001) 175. [17] E. Gaggelli, H. Kozlowski, D. Valensin, G. Valensin, Chem. Rev. 106 (2006) 1995. [18] G. Muthaup, A. Schlicksupp, L. Hess, D. Beher, T. Ruppert, C.L. Masters, K. Beyreuther, Science 271 (1996) 1406. [19] R.A. Løvstad, BioMetals 17 (2004) 111. [20] S.-P. Wu, K.-J. Du, Y.-M. Sung, Dalton Trans. 39 (2010) 4363. [21] S.R. Patil, J.P. Nandre, D. Jadhav, S. Bothra, S.K. Sahoo, M. Devi, C.P. Pradeep, P.P. Mahulikar, U.D. Patil, Dalton Trans. 43 (2014) 13299. [22] N.R. Chereddy, S. Janakipriya, P.S. Korrapati, S. Thennarasu, A.B. Mandal, Analyst 138 (2013) 1130. [23] C. Yu, J. Zhang, R. Wanga, L. Chen, Org. Biomol. Chem. 8 (2010) 5277. [24] M. Chandel, S.M. Roy, D. Sharma, S.K. Sahoo, A. Patel, P. Kumari, R.S. Dhale, K.S.K. Ashok, J.P. Nandre, U.D. Patil, J. Lumin. 154 (2014) 515. [25] D. Sharma, A. Moirangthem, S.K. Sahoo, A. Basu, S.M. Roy, R.K. Pati, K.S.K. Ashok, J.P. Nandre, U.D. Patil, RSC Adv. 4 (2014) 41446. [26] J. Nandre, S. Patil, P. Patil, S. Sahoo, C. Redshaw, P. Mahulikar, U.D. Patil, J. Fluoresc. (2014), http://dx.doi.org/10.1007/s10895-014-1438-4. [27] O. Rudolf, V. Mrkvicka, A. Lycka, M. Rouchal, A. Klasek, Helv. Chim. Acta 95 (2012) 1352. [28] H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, F. Bloino, G. Zheng, G.J. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.N. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision A.1, Gaussian, Inc., Wallingford CT, 2009.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Quinolone based chemosensor for the naked-eye and spectrophotometric detection of Cu2+ in aqueous media Samadhan R. Patil a, Jitendra P. Nandre a, Prashant A. Patil a,b, Shilpa Bothra c, Suban K. Sahoo c, Antonin Klasek d, Julián Rodríguez-López e, Promod P. Mahulikar a, Umesh D. Patil a,⁎ a
School of Chemical Sciences, North Maharashtra University, P. B. No. 80, Jalgaon 425 001, M.S., India S.S.V.P.S's L. K. Dr. P. R. Ghogrey Science College, Dhule 424 001, India Department of Applied Chemistry, S. V. National Institute Technology, Surat 395 007, Gujarat, India d Department of Chemistry, Faculty of Technology, Tomas Bata University, CZ-762 72 Zlin, Czech Republic e Facultad de Ciencias y Tecnologías Químicas, Universidad de Castilla-La Mancha, Avda. Camilo José Cela, 10, 13071 Ciudad Real, Spain b c
a r t i c l e
i n f o
Article history: Received 21 August 2014 Received in revised form 14 September 2014 Accepted 17 September 2014 Available online 18 September 2014 Keywords: Chemosensor Naked-eye detection Cu2+ ion Intramolecular charge transfer DFT
a b s t r a c t A new 4-hydroxy-2-quinolone based chemosensor (BPHTQ-1) was designed for the spectrophotometric and naked-eye detection of Cu2+. The chemosensor displayed a high selectivity and sensitivity towards Cu2+ in the presence of other competitive metal cations in pure aqueous media. The Cu2+ recognition furnished a distinguishable color change of BPHTQ-1 from colorless to yellow with a significant hyperchromic shift at 300 nm. The S and O atoms of BPHTQ-1 provided a 1:1 binding scaffold for the recognition of Cu2+ ion with a high binding affinity of 19,338 M−1 and a detection limit of 1.39 μM, which is quite low compared with World Health Organization (WHO) reports. © 2014 Elsevier B.V. All rights reserved.
The development of chemosensors for the detection of transition metal cations is an active area of research due to their potential application as diagnostic tools in the medical, physiological and environmental field [1–7]. Transition metal cations are well known for their important roles in many biological and environmental processes. Among the various transition metal cations, copper is the third most abundant transition element in the human body and plays an important role in various physiological processes like hemoglobin biosynthesis, dopamine production, nerve function regulation, gene expression, bone development, and the functional as well as structural enhancement of proteins [8–11]. Cu2+ also plays a vital role as a catalytic co-factor for a variety of metallo-enzymes and transcriptional events such as superoxide dismutase, cytochrome c oxidase and tyrosinase [12,13]. Apart from the biological and environmental importance, copper is widely used in the metal, pharmaceutical and agrochemical industries for making alloys, electrical wires, batteries, drugs, machine parts and fertilizers [14,15]. However, the excess of Cu2+ ion has serious harmful effect on the living systems. The over accumulation in human being leads to various neurodegenerative diseases such as Alzheimer's disease, prion disease, Wilson's disease, kidney damage, Menkes disease, gastrointestinal disorders, amyotrophic sclerosis, lipid metabolism, and inflammatory ⁎ Corresponding author. E-mail address: [email protected] (U.D. Patil).
http://dx.doi.org/10.1016/j.inoche.2014.09.021 1387-7003/© 2014 Elsevier B.V. All rights reserved.
disorders [16–19]. Therefore, the rapid and easy detection of Cu2+ is very important in environmental and biological systems. Recently, the naked-eye detection method has gained an immense interest because of its simplicity, low cost, and ability to detect Cu2+ up to micro/submicromolar levels without involving any sophisticated costly instruments [20–23]. Herein, as a part of our efforts in the field of analyte recognition [7,21,24–26], a simple and easy-to-prepare receptor, bis(4-hydroxy-2-quinolone-3-yl)sulfide (BPHTQ-1, Scheme 1), as an efficient colorimetric sensor for the highly selective detection of Cu2+ by visible color change from colorless to yellow and a significant hyperchromic shift at 300 nm in the UV–Vis absorption study. For these experiments, the receptor was taken in DMSO solution but the cations were added from 100% water medium. To the best of our knowledge, this sensor represents the first example with a significant hyperchromic shift of ΔA = 1.19 in the presence of Cu2+ ion. The receptor BPHTQ-1 was synthesized as previously reported in the literature (Scheme 1) [27]. Then, the chemosensing behavior of BPHTQ-1 with group-I, II and III metal ions (Ba2 +, Ca2 +, Cs+, Li+, Mg2 +, Na+, K+, Al3 +, Sr2 +, etc.) and transition/heavy metal ions (Ag+, Co2 +, Cu2+, Cr3 +, Fe2 +, Cd2+, Fe3 +, Mn2 +, Ni2 +, Pb2 +, Zn2 +, Hg2 +, etc.) was investigated by naked-eye detection and UV–Vis absorption spectroscopy methods. In the naked-eye experiment, the qualitative recognition ability of Cu2+ ions was identified from the perceptible visual color change of
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S.R. Patil et al. / Inorganic Chemistry Communications 49 (2014) 59–62
Scheme 1. The preparation of chemosensor BPHTQ-1.
Fig. 1. Naked-eye detection of Cu2+ ion in the presence of other metal cations under visible light. Only the Cu2+ ion (5 eq., 1 mL, 1 × 10−2 M in water) gives the visual color change for BPHTQ-1 (2 mL, 1 × 10−3 M in DMSO) from colorless to clear yellow.
BPHTQ-1 solution from colorless to yellow (Fig. 1). To perform this naked-eye experiments, 5 equivalents of various cations including Cu2+ (1 mL, 1 × 10−2 M, in H2O) were added to a solution of BPHTQ1 (2 mL, 1 × 10−3 M, in DMSO). No noticeable color change of the receptor was observed with other tested cations. The naked-eye results inspired us to undertake the systematic spectroscopic investigation on the cation recognition ability of BPHTQ-1, which was performed at a lower concentration of 5 × 10−5 M in DMSO. The absorption spectrum of BPHTQ-1 showed a broad absorption band between 250-350 nm due to π–π* and/or n–π* electronic transition. Upon addition of 5 equivalents of different cations (50 μL, 1 × 10−2 M, in H2O), only the Cu2+ perturbs the absorption spectrum effectively, whereas the other cations failed to affect the absorption spectrum of BPHTQ-1 (Fig. 2). The addition of Cu2+ ions resulted in the appearance of an intense and sharp absorption band at 300 nm. In addition, the detection of Cu2+ by BPHTQ-1 in aqueous medium was free from the interference of other competitive metal cations (Fig. S1), which indicates that this sensor could detect the Cu2+ ion selectively with high sensitivity over a range of twenty other tested competing metal cations.
The chemosensing ability and binding mechanism of BPHTQ-1 towards Cu2+ were investigated by absorption titration with Cu2+. As shown in Fig. 3, upon sequential addition of Cu2+ (0–5 equivalents), the absorption band centered at ca. 300 nm underwent a steady
Fig. 3. Changes in the absorption spectra of BPHTQ-1 (2 mL, 5 × 10−5 M, in DMSO) upon addition of 1–5 equiv. Cu2+ ions (0–500 μL, 1 × 10−3 M, in H2O).
Fig. 2. Absorption spectral changes of BPHTQ-1 (2 mL, 5 × 10−5 M in DMSO) upon addition of 5 equivalents of various metal cations.
Fig. 4. Benesi–Hilderbrand plot of chemosensor BPHTQ-1 with Cu2+ ion for the evaluation of the association constant.
S.R. Patil et al. / Inorganic Chemistry Communications 49 (2014) 59–62
Fig. 5. Job's plot showing the 1:1 binding of BPHTQ-1 to Cu2+.
increase in the absorption intensity. From the titration data, the limit of detection (LOD) and limit of quantification (LOQ) of the receptor BPHTQ-1 were calculated by using the equations: LOD = (3.3 × standard deviation) / slope and LOQ = (10 × standard deviation) / slope. The relative standard deviation was obtained by recording the absorption measurements of ten blank samples. Using the calibration curve (Fig. S2), the LOD and LOQ were estimated to be 1.39 μM and 4.22 μM, respectively. Next, the association constant, Ka, was evaluated graphically by plotting 1/ΔA against 1/[Cu2+] (Fig. 4). The data was linearly fitted according to the Benesi–Hildebrand equation and the Ka value was obtained from the slope and the y-intercept of the line. The Ka value obtained for BPHTQ-1·Cu2+ complex was found to be 19,338 M− 1. The reversibility of the chemosensor was also examined. The interaction between BPHTQ-1 (2 mL, 5 × 10−5 M in DMSO) and Cu2+ (10 μL, 1 × 10−2 M in H2O) was fully reversible upon introduction of 4
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equivalents of EDTA (40 μL, 1 × 10−2 M in water), which could be identified from the restore of the absorption spectrum of BPHTQ-1 (Fig. S3). This reversibility process was repeated three times with same results. The 1:1 binding stoichiometry for the complexation between BPHTQ1 and Cu2+ was determined from Job's plot (Fig. 5) and mole ratio plot (Fig. S4). In addition, a linear dependence of absorbance at 300 nm was observed as a function of Cu2+ concentration (Fig. S2), which also indicates the 1:1 binding stoichiometry between BPHTQ-1 and Cu2+. Further evidence for the formation of a 1:1 complex between BPHTQ-1 and Cu2+ was obtained from the MALDI-TOF mass spectrometry analysis of an equimolar mixture of BPHTQ-1 and copper perchlorate hexahydrate in DMSO using DHB (2,5-dihydroxybenzoic acid) as the matrix (Fig. S5). Well-defined and highly resolved peaks observed at m/z 567.2 and 505.2 could be assigned to the molecular cation [BPHTQ-1·Cu]+ (calc. m/z 567.0) and [BPHTQ-1·H]+ (calc. m/z 505.1) respectively, with an excellent agreement between the theoretical and experimental isotope distributions. The use of THAP (2,4,6-trihydroxyacetophenone) as the matrix gave an extra peak at m/z 630.4 that corresponds to [BPHTQ-1–2H·2Cu]+ (Fig. S6). This peak also appeared when a 1:5 M mixture of BPHTQ-1: Cu(ClO4)2·6H2O was analyzed using different matrices (Fig. S7); once again with an excellent match for the isotope distributions. Further, in an attempt to confirm the binding mechanism of Cu2+ with BPHTQ-1, we carried out 1H NMR titration experiments in which the spectra were recorded by adding successive amounts of copper perchlorate hexahydrate (0, 0.2, 0.5, 1.0 equiv.) to a solution of BPHTQ-1 in DMSO-d6. The addition of paramagnetic Cu2+ resulted in broadening of the signals and the chemical shifts for all signals remained unaffected. Therefore, no conclusions could be extracted from these experiments. It was expected that the chemosensor BPHTQ-1 should interact with the Cu2+ by the donation of the lone electron pairs located on the oxygen and sulfur atoms to a vacant orbital on the metal ion (Fig. S8). This electron donation or charge transfer resulted in the color change from colorless to clear yellow. This binding mode was finally supported by density functional theory (DFT) analysis of
Fig. 6. DFT computed (a) optimized structure of receptor BPHTQ-1 and its complex with Cu2+, and the (b) LUMO and (c) HOMO diagrams of BPHTQ-1 and its BPHTQ-1·Cu2+ complex in the gas phase.
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BPHTQ-1 and its BPHTQ-1·Cu2+ complex. The study of the charge transfer process during the formation of the complex was performed by applying the B3LYP functional, and the basis sets 6-31G** (for C, H, N and O atoms) and LANL2DZ (for Cu atom) available in the computational code Gaussian 09W [28]. Among the various possible structures, the energetically most favorable, optimized structures for BPHTQ-1 and its complex with Cu2+ are shown in Fig. 6. The complexation of BPHTQ-1 with Cu2+ resulted in lowering the interaction energy (Eint = Ecomplex − Ereceptor − ECu2+) by −165.08 kcal/mol, which indicates the formation of a stable complex. Furthermore, the band gap between the HOMO and LUMO of BPHTQ-1 was lowered upon complexation with Cu2+, which may be responsible for the appearance of yellow coloration for the BPHTQ-1·Cu2+ complex (Fig. 1). The analysis of the HOMO and LUMO diagrams of BPHTQ-1 and its complex with Cu2+ clearly indicates that the intramolecular charge transfer (ICT) occurred between the receptor and Cu2+. In conclusion, we have developed a new chemosensor for the selective and specific detection of Cu2+ ion in aqueous media. The recognition of Cu2+ by BPHTQ-1 gave rise to a color change from colorless to yellow and significant hyperchromic shift in the UV–Vis absorption study. The high selectivity of the sensor BPHTQ-1 towards Cu2+ taken from a pure water medium in the detection study and the detection limit down to 1.39 μM would make the chemosensor BPHTQ-1 a promising candidate for the qualitative and quantitative detection of Cu2+ ions. Considering the versatility, we believe that the sensor BPHTQ-1 could be prompted for many practical applications in chemical, environmental and biological systems for the colorimetric detection of Cu2+.
Acknowledgment S. R. Patil is thankful to DST, New Delhi, India, for the financial assistant under INSPIRE fellowship. A. Klasek thanks Mrs. H. Geržova (Faculty of Technology, Tomas Bata University in Zlín) for the technical help. J. Rodríguez-López also thanks the Ministerio de Economía y Competitividad (Spain)/FEDER (EU) for the financial support — project BFU2011-30161-C02-02.
Appendix A. Supplementary material Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2014.09.021.
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