- Email: [email protected]
Fluorometric and naked-eye detectable dual signaling chemodosimeter for hypochlorite
Sensors and Actuators B 204 (2014) 741–745
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fluorometric and naked-eye detectable dual signaling chemodosimeter for hypochlorite Shyamaprosad Goswami a,∗ , Sibaprasad Maity a,b , Annada C. Maity a , Avijit Kumar Das a a b
Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India Department of Applied Sciences, Haldia Institute of Technology, Hatiberia, Haldia, West Bengal 721657, India
a r t i c l e
i n f o
Article history: Received 26 March 2014 Received in revised form 6 August 2014 Accepted 7 August 2014 Available online 18 August 2014 Keywords: Phenanthroline Diaminomaleonitrile Fluorometric Naked-eye Chemodosimeter
a b s t r a c t Phenanthroline dialdehyde appended sensor (PDS) has been designed and synthesized which displays an excellent selectivity as hypochlorite sensor in mixed aqueous medium. Its selectivity and sensitivity is established through hypochlorite promoted de-diaminomaleonitrile reaction causing naked eye recognizable color change from yellow to colorless as well as remarkable fluorescence turn on by the disrupted intra-molecular charge transfer mechanism but in presence of different analytes like H2 O2 , HS− , NO3 − , NO2 − , hydrazine, CN− , F− , and Cl− , no such characteristic change is observed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Molecular oxygen (O2 ) is biologically significant to all aerobic organisms including humans. During wide variety of physiological processes, the body produces a number of species derived from oxygen. One class of these species, referred to as ‘reactive oxygen species (ROS)’, includes the superoxide, hydroxyl and peroxyl radical, hydrogen peroxide, singlet oxygen (1 O2 ) and hypochlorous acid/hypochlorite. The ROS are produced endogenously from oxygen mainly through the mitochondrial respiration process [1]. An important member of ROS hypochlorous acid, which is approximately half dissociated into its conjugate base (ClO− ) at physiological condition, are strong oxidizing agents employed in various organic syntheses [2] and used as disinfectant and household bleach [3]. Endogenous HOCl, which are produced from the myeloperoxidase (MPO)-mediated peroxidation of chloride ions in activated phagocytic leukocytes including neutrophils, monocytes, and macrophages, play important roles in the human immune defence system, and contribute to the destruction of invading bacteria and pathogens [4]. Although HOCl contributes to the destruction of bacteria in living organisms, the overproduced HOCl causes oxidative stress through the oxidation of bio-molecules,
∗ Corresponding author. Tel.: +91 3326684561/62/63/63; fax: +91 3326682916. E-mail addresses: [email protected], [email protected] (S. Goswami), [email protected] (S. Maity). http://dx.doi.org/10.1016/j.snb.2014.08.024 0925-4005/© 2014 Elsevier B.V. All rights reserved.
such as lipids, proteins and DNA, and numerous disorders such as inflammatory diseases, atherosclerosis, respiratory distress, cardiovascular diseases, rheumatoid arthritis, cancer and renal disease [5–8]. Owing to its significance in human health and disease, the elucidation of the biological functions of hypochlorite has become an important area of research. One of the major obstacles to understand the roles that these species play is the lack of suitable methods for detecting ROS in vivo that is caused by their very short lifetimes and the presence of various antioxidants in cells. Synthetic fluorescent probes are the most powerful tools for the detection of this ClO− , owing to their inherent advantages including greater sensitivity, fast response time and simplicity of implementation, offering application methods not only for in vitro assays but also for in vivo imaging studies. These probes also have the advantage of facile visualization of intracellular dynamics and high-resolution localization of bio-molecules of interest. Thus biological relevant findings inspire us to develop sensitive and specific probes for detecting HOCl in both water samples and living systems. Most fluorescent probes are abiotic supramolecular systems that commonly bind analytes by non-covalent interactions, such as hydrogen bonding, electrostatic attractions and coordination phenomena. Recent continuing demands for improving sensitivity and selectivity have inspired us toward fascinating chemodosimeter that has been designed using chemical events. Few fluorescent probes for hypochlorite have been developed recently, based upon the strong oxidation property [9–17]. Although these reported chemosensors have demonstrated reasonable selectivity for HOCl
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Scheme 1. Synthetic route to PDS.
over other ROS, easily synthesizable fluorescent probes of better sensitivity and reactivity for HOCl are still required for the biological imaging applications. Herein, we report a dual-signaling chemodosimeter which can combine the sensitivity of fluorescence with the convenience and esthetic appeal of a colorimetric assay for hypochlorite. It is well known that photoluminescence of a fluorophore can be partially or completely quenched through intramolecular charge transfer (ICT), when an electron donating group remains conjugated with it. Sometimes fluoro-ionophore is designed in such a way that after being bound to cation the electron donating group loses its electron donating ability, hence ICT ceases making the ionophore highly fluorescent [18]. But here, instead of suppressing ICT, we have such a way designed our present sensor where the electron donating group has been detached, through certain chemical reaction promoted by specific analytes from parent fluorophoric moiety to rejuvenate its original fluorescence intensity. In this context we have designed phenanthroline diimine appended sensor (PDS) (Scheme 1).
Fig. 1. UV–Vis absorption spectra of PDS upon titration with NaOCl in CH3 CN H2 O (6:4 v/v, 10 mM HEPES, pH 7.4).
Anal calcd for C22 H12 N10 : 63.30% C, 3.14% H, 33.56% N; found: 63.41% C, 3.26% H, 33.51% N. 2.2. Spectroscopic measurements For UV–Vis and fluorescence titrations, stock solution of the sensor PDS was prepared in DMSO and diluted with CH3 CN H2 O (6:4 v/v, 10 mM HEPES, pH 7.4) to get C = 2 × 10−5 M solution. The solution of the guest anion like NaOCl was prepared (2 × 10−4 ml−1 ) in pure water or CH3 CN. The original volume of the PDS solution was 2 ml. Solutions of the sensor of various concentrations and increasing concentrations of anions were prepared separately. The spectra of these solutions were recorded by means of UV–Vis methods and fluorescence method.
2. Experimental
3. Results and discussion
The chemicals and solvents were purchased from Sigma–Aldrich Chemicals Private Limited and were used without further purification. 1 H NMR spectra were recorded on Brucker 400 MHz instruments. NMR titration was carried out in d6 -DMSO solvent on 400 MHz instrument. For NMR spectra, d6 -DMSO was used as solvent with TMS as an internal standard. Chemical shifts are expressed in ␦ units and 1 H–1 H and 1 H–C coupling constants in Hz. UV–Vis titration was performed on a JASCO UV-V530 spectrophotometer and fluorescence titration was done using PerkinElmer LS 55 fluorescence spectrophotometer with a fluorescence cell of 10 mm path.
The sensing properties of PDS (C = 2 × 10−5 M) were investigated by means of UV–Vis and fluorescence titration in CH3 CN H2 O solution (6:4 v/v, 10 mM HEPES, pH 7.4). The absorption spectra of PDS show the absorption band centered at 395 nm (Fig. 1). Upon addition of sodium hypochlorite (C = 2 × 10−4 M) as the source of OCl− , the absorption band of PDS which by itself peaked at 395 nm gradually weakened accompanied by the visually detectable distinct color change from yellow to colorless. To have some idea about the selectivity of PDS toward OCl− we had carried out the absorption titration with different analytes like
2.1. Synthesis of PDS (a) 1,10-Phenanthroline-2,9-dicarbaldehyde was prepared through oxidation of 2,9-dimethyl-1,10-phenanthroline by SeO2 in refluxing dioxane in presence of 2 drops of water for 12 h. (b) PDS was then prepared through simple Schiff’s base condensation reaction between 1,10-phenanthroline-2,9-dicarbaldehyde and diaminomaleonitrile in refluxing ethanol in presence of two drops of acetic acid as 70% yield. The PDS was characterized by 1 H NMR and HRMS. 1 H NMR (d -DMSO, 400 MHz): ␦ (ppm): 8.72 (d, 2H, J = 8.20 Hz), 6 8.54 (d, 2H, J = 8.30 Hz), 8.47 (s, 2H), 8.37 (s, 4H), 8.03 (d, 2H, J = 6.10 Hz). HRMS (M + Na)+ : calcd for 439.1702, found 439.1785.
Fig. 2. UV–Vis spectra of PDS upon addition of different analytes (5 equiv.) in CH3 CN H2 O(6:4 v/v, 10 mM HEPES, pH 7.4).
S. Goswami et al. / Sensors and Actuators B 204 (2014) 741–745
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OCl
/μM Fig. 3. Changes of absorbance of PDS (C = 2 × 10−5 M) as a function of [ClO− ] (C = 2 × 10−4 M) at 395 nm.
H2 O2 , HS− , NO3 − , NO2 − , hydrazine, CN− , F− , and Cl− , but no such characteristic change was observed in absorption spectra (Fig. 2). The yellow color of the PDS solution remained unchanged in the presence of those competing species; whereas it became colorless only in presence of hypochlorite only and these changes are naked eyes detectable. It is also revealed that minimum 7 M of ClO− can be detected by using 20 M of PDS solution in UV–Vis titration (Fig. 3). Fluorescence is a powerful tool to detect ions owing to its operational simplicity and high sensitivity [19–21]. So, the fluorescence emission spectra were also measured in CH3 CN H2 O solution (6:4 v/v, 10 mM HEPES, pH 7.4) with excitation at 370 nm. PDS (C = 2 × 10−5 M) itself displayed very weak fluorescence peaked at 447 nm, because of efficient ICT from NH2 of diaminomaleonitrile moiety to phenanthroline moiety. When NaOCl (C = 2 × 10−4 M) was added to the PDS solution, the fluorescence intensity significantly enhanced by about 10 folds with blue shift showing a broad band peaked nearly at 420 nm (Fig. 4). The specificity of PDS was also determined through fluorescence titration, as shown in Fig. 5, nearly no fluorescence intensity changes were observed in emission spectra with H2 O2 , HS− , NO3 − , NO2 − , hydrazine, CN− , F− , and Cl− . Thus through unique absorption and emission signaling, PDS can selectively sense OCl− , suppressing other competing analytes. The fluorescence enhancement may be due to the suppression of ICT phenomenon in presence of hypochlorite. The detection limit of
Fig. 4. Changes in fluorescence spectra of PDS (C = 2 × 10−5 M) upon successive addition of NaOCl (C = 2 × 10−4 M) upto 0–5 equiv. in CH3 CN H2 O (6:4 v/v, 10 mM HEPES, pH 7.4).
Fig. 5. Fluorescence spectra of PDS upon addition of different guest analytes (5 equiv.) in CH3 CN H2 O (6:4 v/v, 10 mM HEPES, pH 7.4).
/μM Fig. 6. Changes of fluorescence intensity of PDS (C = 2 × 10−5 M) as a function of [ClO− ] (C = 2 × 10−4 M) at 420 nm.
PDS as a fluorescence sensor for the hypochlorite was determined to be 3.3 M using 20 M of PDS (Fig. 6). The molar absorption coefficient of PDS is 6.6 × 104 M−1 cm−1 at 395 nm. After addition of OCl− ion, fluorescence quantum yield increases 83-fold (Ф/Ф0 = 0.664/0.008 = 83, max (em) = 447 nm). To find out the nature of interaction between PDS and OCl− , for which
Fig. 7. Partial 1 H NMR spectrum of PDS in d6 -DMSO with addition of different equiv. of NaOCl.
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References
Scheme 2. Sensing of hypochlorite by PDS.
there occurs suppression of internal charge transfer causing fluorescence ‘turn on’, we performed 1 H NMR titration (Fig. 7). It reveals that presence of NaOCl in the PDS solution, causes disappearance of signal at 8.47 ppm ( CH N proton) and appearance of a new signal at 10.3 ppm ( CHO proton) which are supposed to arise due to hypochlorite mediated conversion of imine CH N group to CHO group. These results confirm the generation of phenanthroline dialdehyde (1) through de-diaminomaleonitrile reaction (Scheme 2) in PDS, which occurred in presence of OCl− . This phenomenon is also supported by the disappearance of broad singlet at 8.37 ppm which is due to NH2 protons in PDS, which is supposed to resonate at upfield now due to loss of its conjugation through de-diaminomaleonitrile reaction. 4. Conclusion In conclusion, a highly selective and sensitive chemodosimeter, phenanthroline based diimine sensor (PDS), for sensing of hypochlorite in aqueous medium has been designed and synthesized. The interactions between PDS and hypochlorite, showing naked eye detectable color change from yellow to colorless, were studied by fluorescence spectroscopy and 1 H NMR titration. Experimental observations reveal its rapid response, high sensitivity and excellent selectivity only in presence of hypochlorite which catalyzes a chemical reaction, resulting in the modification of PDS with concomitant change in optical characteristics. But no other analytes like H2 O2 , HS− , NO3 , NO2 − , hydrazine, CN− , F− , and Cl− could respond. The fluorescence enrichment is proposed to occur through the de-diaminomaleonitrile reaction of PDS, catalyzed by OCl− , due to which ICT mechanism is interrupted by, breaking donor and acceptor linkage and consequently fluorescence is turned on through the generation of phenanthroline dialdehyde. Acknowledgements DST and CSIR (Govt. of India) are gratefully acknowledged by the authors for financial support and fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2014.08.024.
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Biographies Shyamaprosad Goswami completed his B.Sc. degree in chemistry (1st Class Honors) in 1972 from Presidency College, University of Calcutta. He obtained his M.Sc degree (organic specialization) in 1974 and Ph.D. from Calcutta University 1983. Then he
S. Goswami et al. / Sensors and Actuators B 204 (2014) 741–745 joined Post Doctorate Research Associate in U.S.A with Prof. Andy D. Hamilton, ViceChancellor, Oxford University, U.K., Prof. E.C. Taylor at Princeton University, USA. His research interest fields are molecular recognition and supramolecular chemistry, organic synthesis, methodology and medicinal chemistry. Sibaprasad Maity was born on 2nd January, 1980. He obtained his B.Sc. in 2000 from Midnapore College and M.Sc. Degree in chemistry (organic specialization) in 2002 from Vidyasagar University, West Bengal, India. He had qualified Graduate Aptitude Test in engineering and CSIR–NET in 2002. Then he joined as lecturer in chemistry under Department of Applied Sciences, Haldia Institute of Technology, Haldia, Purba Medinipur, West Bengal, in the year of 2003. During this journey he is also working under supervision of Prof. Goswami at Department of Chemistry, BESU, Howrah, India for his doctoral work. His area of research interest includes molecular recognition, organic synthesis and methodology. Annada C. Maity was born on 2nd November, 1979. He obtained his B.Sc. in 2000 from Midnapore College and M.Sc. Degree in Chemistry (organic specialization)
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in 2002 from Vidyasagar University, West Bengal, India. He had qualified Graduate Aptitude Test in Engineering and CSIR–UGC NET in 2002. Then he joined ‘The Goswami Group’ Dept. of Chemistry, BESU for his Doctoral work under the supervision of Professor Shyamaprosad Goswami, and was awarded Ph.D. (in synthetic organic chemistry) in 2009. His area of research interest includes organic synthesis, methodology and molecular recognition. Avijit Kumar Das was born at Lakshipari, Paschim Medinipur, West Bengal, India on the 14th March, 1988. He received his B.Sc. Degree with Honors in chemistry in 2008 from Midnapore College, Vidyasagar University, West Bengal, India. He obtained his M.Sc. degree in chemistry with organic specialization in 2010 from Bengal Engineering and Science University, Shibpur, Howrah, West Bengal, India. He had qualified the all India (CSIR–UGC) NET held in June, 2011. Then he joined ‘The Goswami Group’ Dept. of Chemistry, BESU for his doctoral work under the supervision of Professor Shyamaprosad Goswami in 2011. His areas of research interest include molecular recognition, supramolecular chemistry, organic synthesis and methodology.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fluorometric and naked-eye detectable dual signaling chemodosimeter for hypochlorite Shyamaprosad Goswami a,∗ , Sibaprasad Maity a,b , Annada C. Maity a , Avijit Kumar Das a a b
Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India Department of Applied Sciences, Haldia Institute of Technology, Hatiberia, Haldia, West Bengal 721657, India
a r t i c l e
i n f o
Article history: Received 26 March 2014 Received in revised form 6 August 2014 Accepted 7 August 2014 Available online 18 August 2014 Keywords: Phenanthroline Diaminomaleonitrile Fluorometric Naked-eye Chemodosimeter
a b s t r a c t Phenanthroline dialdehyde appended sensor (PDS) has been designed and synthesized which displays an excellent selectivity as hypochlorite sensor in mixed aqueous medium. Its selectivity and sensitivity is established through hypochlorite promoted de-diaminomaleonitrile reaction causing naked eye recognizable color change from yellow to colorless as well as remarkable fluorescence turn on by the disrupted intra-molecular charge transfer mechanism but in presence of different analytes like H2 O2 , HS− , NO3 − , NO2 − , hydrazine, CN− , F− , and Cl− , no such characteristic change is observed. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Molecular oxygen (O2 ) is biologically significant to all aerobic organisms including humans. During wide variety of physiological processes, the body produces a number of species derived from oxygen. One class of these species, referred to as ‘reactive oxygen species (ROS)’, includes the superoxide, hydroxyl and peroxyl radical, hydrogen peroxide, singlet oxygen (1 O2 ) and hypochlorous acid/hypochlorite. The ROS are produced endogenously from oxygen mainly through the mitochondrial respiration process [1]. An important member of ROS hypochlorous acid, which is approximately half dissociated into its conjugate base (ClO− ) at physiological condition, are strong oxidizing agents employed in various organic syntheses [2] and used as disinfectant and household bleach [3]. Endogenous HOCl, which are produced from the myeloperoxidase (MPO)-mediated peroxidation of chloride ions in activated phagocytic leukocytes including neutrophils, monocytes, and macrophages, play important roles in the human immune defence system, and contribute to the destruction of invading bacteria and pathogens [4]. Although HOCl contributes to the destruction of bacteria in living organisms, the overproduced HOCl causes oxidative stress through the oxidation of bio-molecules,
∗ Corresponding author. Tel.: +91 3326684561/62/63/63; fax: +91 3326682916. E-mail addresses: [email protected], [email protected] (S. Goswami), [email protected] (S. Maity). http://dx.doi.org/10.1016/j.snb.2014.08.024 0925-4005/© 2014 Elsevier B.V. All rights reserved.
such as lipids, proteins and DNA, and numerous disorders such as inflammatory diseases, atherosclerosis, respiratory distress, cardiovascular diseases, rheumatoid arthritis, cancer and renal disease [5–8]. Owing to its significance in human health and disease, the elucidation of the biological functions of hypochlorite has become an important area of research. One of the major obstacles to understand the roles that these species play is the lack of suitable methods for detecting ROS in vivo that is caused by their very short lifetimes and the presence of various antioxidants in cells. Synthetic fluorescent probes are the most powerful tools for the detection of this ClO− , owing to their inherent advantages including greater sensitivity, fast response time and simplicity of implementation, offering application methods not only for in vitro assays but also for in vivo imaging studies. These probes also have the advantage of facile visualization of intracellular dynamics and high-resolution localization of bio-molecules of interest. Thus biological relevant findings inspire us to develop sensitive and specific probes for detecting HOCl in both water samples and living systems. Most fluorescent probes are abiotic supramolecular systems that commonly bind analytes by non-covalent interactions, such as hydrogen bonding, electrostatic attractions and coordination phenomena. Recent continuing demands for improving sensitivity and selectivity have inspired us toward fascinating chemodosimeter that has been designed using chemical events. Few fluorescent probes for hypochlorite have been developed recently, based upon the strong oxidation property [9–17]. Although these reported chemosensors have demonstrated reasonable selectivity for HOCl
742
S. Goswami et al. / Sensors and Actuators B 204 (2014) 741–745
Scheme 1. Synthetic route to PDS.
over other ROS, easily synthesizable fluorescent probes of better sensitivity and reactivity for HOCl are still required for the biological imaging applications. Herein, we report a dual-signaling chemodosimeter which can combine the sensitivity of fluorescence with the convenience and esthetic appeal of a colorimetric assay for hypochlorite. It is well known that photoluminescence of a fluorophore can be partially or completely quenched through intramolecular charge transfer (ICT), when an electron donating group remains conjugated with it. Sometimes fluoro-ionophore is designed in such a way that after being bound to cation the electron donating group loses its electron donating ability, hence ICT ceases making the ionophore highly fluorescent [18]. But here, instead of suppressing ICT, we have such a way designed our present sensor where the electron donating group has been detached, through certain chemical reaction promoted by specific analytes from parent fluorophoric moiety to rejuvenate its original fluorescence intensity. In this context we have designed phenanthroline diimine appended sensor (PDS) (Scheme 1).
Fig. 1. UV–Vis absorption spectra of PDS upon titration with NaOCl in CH3 CN H2 O (6:4 v/v, 10 mM HEPES, pH 7.4).
Anal calcd for C22 H12 N10 : 63.30% C, 3.14% H, 33.56% N; found: 63.41% C, 3.26% H, 33.51% N. 2.2. Spectroscopic measurements For UV–Vis and fluorescence titrations, stock solution of the sensor PDS was prepared in DMSO and diluted with CH3 CN H2 O (6:4 v/v, 10 mM HEPES, pH 7.4) to get C = 2 × 10−5 M solution. The solution of the guest anion like NaOCl was prepared (2 × 10−4 ml−1 ) in pure water or CH3 CN. The original volume of the PDS solution was 2 ml. Solutions of the sensor of various concentrations and increasing concentrations of anions were prepared separately. The spectra of these solutions were recorded by means of UV–Vis methods and fluorescence method.
2. Experimental
3. Results and discussion
The chemicals and solvents were purchased from Sigma–Aldrich Chemicals Private Limited and were used without further purification. 1 H NMR spectra were recorded on Brucker 400 MHz instruments. NMR titration was carried out in d6 -DMSO solvent on 400 MHz instrument. For NMR spectra, d6 -DMSO was used as solvent with TMS as an internal standard. Chemical shifts are expressed in ␦ units and 1 H–1 H and 1 H–C coupling constants in Hz. UV–Vis titration was performed on a JASCO UV-V530 spectrophotometer and fluorescence titration was done using PerkinElmer LS 55 fluorescence spectrophotometer with a fluorescence cell of 10 mm path.
The sensing properties of PDS (C = 2 × 10−5 M) were investigated by means of UV–Vis and fluorescence titration in CH3 CN H2 O solution (6:4 v/v, 10 mM HEPES, pH 7.4). The absorption spectra of PDS show the absorption band centered at 395 nm (Fig. 1). Upon addition of sodium hypochlorite (C = 2 × 10−4 M) as the source of OCl− , the absorption band of PDS which by itself peaked at 395 nm gradually weakened accompanied by the visually detectable distinct color change from yellow to colorless. To have some idea about the selectivity of PDS toward OCl− we had carried out the absorption titration with different analytes like
2.1. Synthesis of PDS (a) 1,10-Phenanthroline-2,9-dicarbaldehyde was prepared through oxidation of 2,9-dimethyl-1,10-phenanthroline by SeO2 in refluxing dioxane in presence of 2 drops of water for 12 h. (b) PDS was then prepared through simple Schiff’s base condensation reaction between 1,10-phenanthroline-2,9-dicarbaldehyde and diaminomaleonitrile in refluxing ethanol in presence of two drops of acetic acid as 70% yield. The PDS was characterized by 1 H NMR and HRMS. 1 H NMR (d -DMSO, 400 MHz): ␦ (ppm): 8.72 (d, 2H, J = 8.20 Hz), 6 8.54 (d, 2H, J = 8.30 Hz), 8.47 (s, 2H), 8.37 (s, 4H), 8.03 (d, 2H, J = 6.10 Hz). HRMS (M + Na)+ : calcd for 439.1702, found 439.1785.
Fig. 2. UV–Vis spectra of PDS upon addition of different analytes (5 equiv.) in CH3 CN H2 O(6:4 v/v, 10 mM HEPES, pH 7.4).
S. Goswami et al. / Sensors and Actuators B 204 (2014) 741–745
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OCl
/μM Fig. 3. Changes of absorbance of PDS (C = 2 × 10−5 M) as a function of [ClO− ] (C = 2 × 10−4 M) at 395 nm.
H2 O2 , HS− , NO3 − , NO2 − , hydrazine, CN− , F− , and Cl− , but no such characteristic change was observed in absorption spectra (Fig. 2). The yellow color of the PDS solution remained unchanged in the presence of those competing species; whereas it became colorless only in presence of hypochlorite only and these changes are naked eyes detectable. It is also revealed that minimum 7 M of ClO− can be detected by using 20 M of PDS solution in UV–Vis titration (Fig. 3). Fluorescence is a powerful tool to detect ions owing to its operational simplicity and high sensitivity [19–21]. So, the fluorescence emission spectra were also measured in CH3 CN H2 O solution (6:4 v/v, 10 mM HEPES, pH 7.4) with excitation at 370 nm. PDS (C = 2 × 10−5 M) itself displayed very weak fluorescence peaked at 447 nm, because of efficient ICT from NH2 of diaminomaleonitrile moiety to phenanthroline moiety. When NaOCl (C = 2 × 10−4 M) was added to the PDS solution, the fluorescence intensity significantly enhanced by about 10 folds with blue shift showing a broad band peaked nearly at 420 nm (Fig. 4). The specificity of PDS was also determined through fluorescence titration, as shown in Fig. 5, nearly no fluorescence intensity changes were observed in emission spectra with H2 O2 , HS− , NO3 − , NO2 − , hydrazine, CN− , F− , and Cl− . Thus through unique absorption and emission signaling, PDS can selectively sense OCl− , suppressing other competing analytes. The fluorescence enhancement may be due to the suppression of ICT phenomenon in presence of hypochlorite. The detection limit of
Fig. 4. Changes in fluorescence spectra of PDS (C = 2 × 10−5 M) upon successive addition of NaOCl (C = 2 × 10−4 M) upto 0–5 equiv. in CH3 CN H2 O (6:4 v/v, 10 mM HEPES, pH 7.4).
Fig. 5. Fluorescence spectra of PDS upon addition of different guest analytes (5 equiv.) in CH3 CN H2 O (6:4 v/v, 10 mM HEPES, pH 7.4).
/μM Fig. 6. Changes of fluorescence intensity of PDS (C = 2 × 10−5 M) as a function of [ClO− ] (C = 2 × 10−4 M) at 420 nm.
PDS as a fluorescence sensor for the hypochlorite was determined to be 3.3 M using 20 M of PDS (Fig. 6). The molar absorption coefficient of PDS is 6.6 × 104 M−1 cm−1 at 395 nm. After addition of OCl− ion, fluorescence quantum yield increases 83-fold (Ф/Ф0 = 0.664/0.008 = 83, max (em) = 447 nm). To find out the nature of interaction between PDS and OCl− , for which
Fig. 7. Partial 1 H NMR spectrum of PDS in d6 -DMSO with addition of different equiv. of NaOCl.
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References
Scheme 2. Sensing of hypochlorite by PDS.
there occurs suppression of internal charge transfer causing fluorescence ‘turn on’, we performed 1 H NMR titration (Fig. 7). It reveals that presence of NaOCl in the PDS solution, causes disappearance of signal at 8.47 ppm ( CH N proton) and appearance of a new signal at 10.3 ppm ( CHO proton) which are supposed to arise due to hypochlorite mediated conversion of imine CH N group to CHO group. These results confirm the generation of phenanthroline dialdehyde (1) through de-diaminomaleonitrile reaction (Scheme 2) in PDS, which occurred in presence of OCl− . This phenomenon is also supported by the disappearance of broad singlet at 8.37 ppm which is due to NH2 protons in PDS, which is supposed to resonate at upfield now due to loss of its conjugation through de-diaminomaleonitrile reaction. 4. Conclusion In conclusion, a highly selective and sensitive chemodosimeter, phenanthroline based diimine sensor (PDS), for sensing of hypochlorite in aqueous medium has been designed and synthesized. The interactions between PDS and hypochlorite, showing naked eye detectable color change from yellow to colorless, were studied by fluorescence spectroscopy and 1 H NMR titration. Experimental observations reveal its rapid response, high sensitivity and excellent selectivity only in presence of hypochlorite which catalyzes a chemical reaction, resulting in the modification of PDS with concomitant change in optical characteristics. But no other analytes like H2 O2 , HS− , NO3 , NO2 − , hydrazine, CN− , F− , and Cl− could respond. The fluorescence enrichment is proposed to occur through the de-diaminomaleonitrile reaction of PDS, catalyzed by OCl− , due to which ICT mechanism is interrupted by, breaking donor and acceptor linkage and consequently fluorescence is turned on through the generation of phenanthroline dialdehyde. Acknowledgements DST and CSIR (Govt. of India) are gratefully acknowledged by the authors for financial support and fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2014.08.024.
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Biographies Shyamaprosad Goswami completed his B.Sc. degree in chemistry (1st Class Honors) in 1972 from Presidency College, University of Calcutta. He obtained his M.Sc degree (organic specialization) in 1974 and Ph.D. from Calcutta University 1983. Then he
S. Goswami et al. / Sensors and Actuators B 204 (2014) 741–745 joined Post Doctorate Research Associate in U.S.A with Prof. Andy D. Hamilton, ViceChancellor, Oxford University, U.K., Prof. E.C. Taylor at Princeton University, USA. His research interest fields are molecular recognition and supramolecular chemistry, organic synthesis, methodology and medicinal chemistry. Sibaprasad Maity was born on 2nd January, 1980. He obtained his B.Sc. in 2000 from Midnapore College and M.Sc. Degree in chemistry (organic specialization) in 2002 from Vidyasagar University, West Bengal, India. He had qualified Graduate Aptitude Test in engineering and CSIR–NET in 2002. Then he joined as lecturer in chemistry under Department of Applied Sciences, Haldia Institute of Technology, Haldia, Purba Medinipur, West Bengal, in the year of 2003. During this journey he is also working under supervision of Prof. Goswami at Department of Chemistry, BESU, Howrah, India for his doctoral work. His area of research interest includes molecular recognition, organic synthesis and methodology. Annada C. Maity was born on 2nd November, 1979. He obtained his B.Sc. in 2000 from Midnapore College and M.Sc. Degree in Chemistry (organic specialization)
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in 2002 from Vidyasagar University, West Bengal, India. He had qualified Graduate Aptitude Test in Engineering and CSIR–UGC NET in 2002. Then he joined ‘The Goswami Group’ Dept. of Chemistry, BESU for his Doctoral work under the supervision of Professor Shyamaprosad Goswami, and was awarded Ph.D. (in synthetic organic chemistry) in 2009. His area of research interest includes organic synthesis, methodology and molecular recognition. Avijit Kumar Das was born at Lakshipari, Paschim Medinipur, West Bengal, India on the 14th March, 1988. He received his B.Sc. Degree with Honors in chemistry in 2008 from Midnapore College, Vidyasagar University, West Bengal, India. He obtained his M.Sc. degree in chemistry with organic specialization in 2010 from Bengal Engineering and Science University, Shibpur, Howrah, West Bengal, India. He had qualified the all India (CSIR–UGC) NET held in June, 2011. Then he joined ‘The Goswami Group’ Dept. of Chemistry, BESU for his doctoral work under the supervision of Professor Shyamaprosad Goswami in 2011. His areas of research interest include molecular recognition, supramolecular chemistry, organic synthesis and methodology.