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A novel fluorescence sensor for dual sensing of Hg2+ and Cu2+ ions
Journal of Photochemistry and Photobiology A: Chemistry 353 (2018) 19–25
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
A novel fluorescence sensor for dual sensing of Hg2+ and Cu2+ ions Yi-Wun Siea , Chia-Lin Lia , Chin-Feng Wanb , Hongbin Yanc , An-Tai Wua,* a
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan Department of Medical Applied Chemistry, Chung Shan Medical University, Taichung City 40201,Taiwan c Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON L2S 3A1, Canada b
A R T I C L E I N F O
Article history: Received 16 August 2017 Received in revised form 2 November 2017 Accepted 5 November 2017 Available online 6 November 2017 Keywords: Fluorescence enhancement Blue shift Colorimetric
A B S T R A C T
A novel fluorescence probe (L) was prepared and its sensing behavior toward metal ions was investigated. Probe L exhibited colorimetric response toward Hg2+ and fluorescence enhancement with an obvious blue shift toward Cu2+ in CH3CN/H2O (v/v, 9:1). The association constant for probe L-Hg2+ and probe LCu2+ in CH3CN/H2O (v/v, 9:1) was determined as 1.5 104 M 1 and 5.3 108 M 2, respectively. The detection limit of probe L for the analysis of Hg2+ and Cu2+ was determined as 22.69 ppb and 155.53 ppb, respectively. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With recent development in ion sensors, the design and synthesis of new chemosensors for the efficient detection of trace metal ions remain as an important research topic in environmental investigations [1]. Generally, industrial waste water, not only shows a single metal pollution, always contains a variety of heavy metal ions at the same time. Thus, development of new receptors for the detection of different analytes simultaneously is emerging as an area of great interest [2], since such system would lead to faster analytical processing and potential cost reductions. Among various important metal ions, Cu2+ is a significant environmental pollutant and also plays a critical role in various biological processes [3–5]. The accumulation of excess amounts of Cu2+ in humans or dysregulation of copper homeostasis can cause a series of severe diseases such as Alzheimer's and Parkinson's diseases [6]. On the other hand, Hg2+ and its derivatives are significant environmental pollutants with inherent high toxicity. In the marine environment, Hg2+ is converted by bacteria into methylmercury, a highly potent neurotoxin, which accumulates in humans through the food chain [7]. While some fluorescent probes for dual detection of Hg2+ and Cu2+ have been reported [8–18], these probes require complicated synthesis, and are usually poorly soluble in water and lack sensitivity. Therefore, for practical applications, simple and sensitive fluorescent probes for the simultaneous detection of Hg2+ and Cu2+ metal ions are still in great demand.
* Corresponding author. E-mail address: [email protected] (A.-T. Wu). https://doi.org/10.1016/j.jphotochem.2017.11.003 1010-6030/© 2017 Elsevier B.V. All rights reserved.
Recently, the development of fluorescence ratiometric probes for metal ions has attracted much attention since they allow for the measurement of emission intensities at two different wavelengths [19–24]. However, development of ratiometric probes for Cu2+ is a challenge due to its inherent paramagnetic nature, and hence, the complexation with Cu2+ generally results in quenching of the fluorescence intensity of the probe [25–30]. FRET (fluorescence resonance energy transfer)-based fluorescent probes for Cu2+ ions have been reported, but are less common compared with Hg2+. FRET (fluorescence resonance energy transfer)-based fluorescent probes for Cu2+ ions have been reported, but are less common compared with Hg2+. In this regard, the use of guest-induced FRET mechanism should be an efficient approach to design ratiometric fluorescence probes. Ramaiah et al. reported a fluorescent probe based on dansyl and naphthalimide as the FRET (fluorescence resonance energy transfer) pair for the ratiometric fuorescent detection of Cu2+ [31]. They also reported a FRET-based probe for Cu2+ by using the combination of indole and rhodamine moieties [32]. Until now, only a few examples can selectively senses Hg2+ and 2+ Cu through two different channels: display color change and “turn-on” or ratiometric fluorescent changes in emission spectra [33]. Herein, we synthesized a new fluorescent probe (L), which could selectively detect Hg2+ and Cu2+ based on different fluorescence mechanisms among a series of ions. Probe L can also be used for selective sensing of Hg2+ and Cu2+ based on the distinct color changes in real samples application. In this probe L, two fluorophores, i.e. indole and dansyl, were conjugated as a donor– acceptor pair for FRET. Probe L can be readily prepared by the reaction of 1H-indole-2-carbohydrazide with dansyl chloride, as
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Scheme 1. Synthesis of probe L.
Fig. 3. The color changes of probe L (20 mM) upon addition of 5 equiv. of Hg2+ and Cu2+ in H2O/CH3CN = 1/9 (v/v), observed under UV light (lex. = 365 nm, T = 28 C).
Fig. 1. UV/vis spectra of probe L (20 mM) recorded in H2O/CH3CN = 1/9 (v/v) after addition of 5 equiv. of various metal ions.
Fig. 4. Fluorescence emission spectra of probe L (20 mM) in the presence of 5 equiv. of various metal ions in H2O/CH3CN = 1/9 (v/v) (lex. = 300 nm, T = 28 C).
Fig. 2. The color change of probe L (20 mM) upon addition of 5 equiv. of various metal ions in H2O/CH3CN = 1/9 (v/v), observed by naked eyes.
Fig. 5. Fluorescence spectra of probe L (20 mM) in H2O/CH3CN = 1/9 (v/v) upon addition of increasing concentrations of Hg2+ (lex. = 300 nm, T = 28 C).
Y.-W. Sie et al. / Journal of Photochemistry and Photobiology A: Chemistry 353 (2018) 19–25
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were recorded under electron impact (EI) or electron spray interface (ESI) conditions with a UV–vis spectra were recorded by using Jasco V630 spectrophotometer with a diode array detector, and the resolution was set at 1 nm. Fluorescence spectroscopy was measured on a Jasco FP-8300 fluorescence spectrophotometer. 2.2. Synthesis of probe L
Fig. 6. Fluorescence spectra of probe L (20 mM) in H2O/CH3CN = 1/9 (v/v) upon addition of increasing concentrations of Cu2+ (lex. = 300 nm, T = 28 C).
shown in Scheme 1. The structure of probe L was confirmed by 1H and 13C NMR and mass spectroscopic data (Figs. S1–S3). 2. Materials and instrumental methods
To a solution of 1H-indole-2-carbohydrazide (0.10 g, 0.57 mmol) in ethanol (20 mL) was added 5-(dimethylamino)naphthalene-1sulfonyl chloride (0.165 g, 0.613 mmol). The mixture was stirred under reflux for 12 h. After cooling to room temperature, ethanol was evaporated completely and the crude product was purified by column chromatography (silica, hexane: ethyl acetate 10:1, v/v) to give yellow solid (70%). M.p: 220 C; 1H NMR (300 MHz, DMSO-d6) d: 11.55 (s, 2H), 10.62 (s, 2H), 10.25 (s, 2H), 8.44 (t, J = 9 Hz, 4H), 8.17 (dd, 2H), 7.58 (m, 6H), 7.33 (d, J = 8.4 Hz, 2H), 7.25 (d, J = 7.5 Hz, 2H), 7.18 7.03 (m, 4H), 6.99 (t, J = 7.2 Hz, 2H), 2.83 (s, 6H); 13C NMR (75.4 MHz, DMSO-d6) d: 45.15, 103.81, 112.26, 114.96, 119.91, 120.00, 121.78, 123.45, 123.82, 126.83, 128.62, 128.70, 129.10, 129.67, 129.91, 130.05, 135.29, 136.57, 151.11, 160.49; HRMS (EI): Calcd for C21H20N4O3S (M+), m/z 408.1256; found m/z 408.1250. 3. Results and discussion
2.1. Experimental section
3.1. The absorption and fluorescence studies of probe L toward various metal ions
All reagents were obtained from Sigma-Aldrich and were used without further purification. Analytical thin-layer chromatography was performed using silica gel 60 F254 plates (Merck). The 1H and 13 C NMR spectra were recorded with a Bruker AM 300 spectrometer at 300 and 75.4 MHz, respectively. Chemical shifts are given in ppm with residual DMSO as reference. Mass spectra
The sensing properties of the probe L for cations were evaluated by UV/vis and fluorescence measurements in CH3CN/H2O (9:1, v/v) in the presence of various metal ions in their perchlorate salts: Li+, Na+, K+, Ca2+, Mn2+, Hg2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Pb2+, Cd2+, Zn2+ and Al3+. As shown in Fig. 1, probe L showed a major absorption
Scheme 2. The proposed mechanism for the reaction of probe L with (a) Hg2+ and (b) Cu2+.
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major band at 380 nm. Meanwhile, the solution of probe L showed a color change from colorless to light yellow, which could be detected by the naked-eye (Fig. 2). In the presence of Cu2+, the absorption spectra of probe L showed a blue shift, with a major band at 290 nm. The solution of probe L showed a color change from light yellow to dark blue that could be detected under UV lamp (Fig. 3). The sensing phenomena were also monitored by fluorescence spectroscopy. As shown in Fig. 4, the free probe L displayed a fluorescence emission band at 550 nm with medium intensity, which indicated the existence of FRET from the excited state of the indole chromophore to the dansyl unit. Upon addition of Hg2+, the fluorescence of probe L was quenched. Upon addition of Cu2+, however, probe L exhibited a prominent fluorescence enhancement with a blue shift relative to the original emission band of 550 nm. 3.2. Fluorescence titration and binding studies
Fig. 7. Time dependent experiments of (a) Hg2+; (b) Cu2+ ion (lex. = 300 nm, T = 28 C).
band at 300 nm (indole ring) with a shoulder at 350 nm (dansyl group). For the free units of probe L, the absorption spectrum of indole ring was showed an absorption band at 292 nm. The absorption spectrum of dansyl group was showed two absorption bands at 264 and 392 nm, respectively (Fig. S4). In the presence of Hg2+, the absorption spectra of probe L red shifted, showing a
To further investigate the sensing properties of probe L, fluorescence titration of probe L with Hg2+ and Cu2+ were performed. With the addition of increasing amounts of Hg2+ to a solution of probe L in CH3CN/H2O (v/v, 9:1), the fluorescence intensity of probe L at 550 nm was quenched gradually, as shown in Fig. 5. After the addition of 3 equivalents of Hg2+, the fluorescence intensity was quenched almost completely (99%). The result indicated that probe L could form a stable complex with Hg2+. On the other hand, the fluorescence titration of probe L with Cu2+ showed a regular decrease in the intensity of FRET-mediated emission from the dansyl moiety at 550 nm with the increase in concentration of Cu2+ ions. Meanwhile, a concomitant increase in the emission intensity of the indole chromophore at 380 nm with an isoemissive point at 502 nm was observed, as shown in Fig. 6. Further, additions of 3 equivalents of Cu2+ ions resulted in the complete quenching of the FRET-mediated emission, with a concomitant ca. 5.8-fold increase in the fluorescence intensity at 380 nm (Fig. S5). The selective quenching of FRET-mediated emission at 550 nm upon addition of the Cu2+ indicate that these ions form complex with the probe L involving the sulfonamide group of the dansyl moiety, carbazide of the indole and indole itself. As a consequence, the dansyl moiety becomes incapable of quenching the excited state of the indole chromophore, resulting in the restoration of indole emission, thereby facilitating the detection of Cu2+. The significant “turn on” intensity with a blue shift of ca. 170 nm allows for the fluorescence ratiometric detection of Cu2+ ions by probe L. The association constant for probe L-Hg2+ and probe L-Cu2+ in CH3CN/H2O (v/v, 9:1) was determined as
Fig. 8. The color changes in different sources of water containing Hg2+, observed by naked eyes.
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Fig. 9. The color changes in different sources of water containing Cu2+, observed under UV light (lex. = 365 nm, T = 28 C).
1.5 104 M 1 and 5.3 108 M 2 by the Stern-Volmer equation (Fig. S6) and Hill plot (Fig. S7), respectively. The detection limit of probe L for the analysis of Hg2+ and Cu2+ was determined as 22.69 ppb and 155.53 ppb, respectively. The linear relationship of the fluorescence titration suggests a 1:1 stoichiometry of probe L to Hg2+, which is evident in the Job plot (Fig. S8). The formation of 1:1 complex between probe L and Hg2+ was further confirmed by the appearance of a peak at m/z 685 in the mass spectrum, assignable to [probe L + Hg2+ + CH3CN + 2H2O 2H+] (Fig. S9). On the other hand, a Job plot indicated a 1:2 stoichiometric complex of probe L with Cu2+ (Fig. S10). In addition, the formation of 1:2 complex between probe L and Cu2+ was further confirmed by the appearance of a peak at m/z 647 in the mass spectrum, assignable to [probe L + 2Cu2+ + ClO4 + H2O 3H+] (Fig. S11). Scheme 2 summarizes the proposed reaction between probe L and Hg2+ and Cu2+. Such a distinctive binding of probe L results in the selective and sensitive detection of both Hg2+ and Cu2+. 3.3. Application in real samples From the standpoint of practical applications, real-time determination of analytes is highly desirable. In this respect, the fluorescence intensity of probe L in CH3CN/H2O (v/v, 9:1) at 550 nm upon addition of 5.0 equivalents of Hg2+ and 380 nm upon addition of Cu2+ was monitored over time. As shown in Fig. 7, the fluorescence intensity plateaued within one minute after addition
Table 1 Probe L recoveries (%) obtained from analysis of water sample spiked with Hg2+. Sample
Added (mM)
Observed (mM)
Recovery (%)
Drinking water
6.0 12.0 20.0
5.5 12.4 18.5
91.7 103.3 92.5
of Hg2+ and Cu2+. The practical application of probe L for selective sensing of Hg2+ or Cu2+ in different source of water such as tap, drink, ditch, lake and ground water was also demonstrated. Hg (ClO4)2 and Cu(ClO4)2 were first dissolved in water of the above source, followed by the addition of probe L into each water sample. All the water samples containing Hg2+ showed clear color change from colorless to bright yellow, which is readily observed by the naked-eye (Fig. 8). On the other hand, addition of probe L into each water sample with Cu2+ clearly led to color change from bright yellow to blue under a UV lamp (Fig. 9). These results indicated that probe L can be used for selective sensing of Hg2+ and Cu2+ based on the distinct color changes in real samples. In addition, the effect of pH on the emission bands of the probe L–Hg2+ or probe L–Cu2 + complex in CH3CN/H2O (v/v, 9:1) solution was also examined. It was found that the fluorescence intensity of probe L–Hg2+ or probe L–Cu2+ in CH3CN/H2O (v/v, 9:1) solution showed a significant emission band in the pH range 4–9 (Fig. S12), suggesting that probe L could be applied in the analysis of environmental aqueous samples with a relatively wide pH range. 3.4. Reversible and competition experiments Reversibility is a desired feature in developing novel sensor for practical application. To examine the reversibility of probe L toward Hg2+ or Cu2+ in CH3CN/H2O (9:1, v/v), a solution of EDTA (20 mM) was added to the complex solution of receptor L with Hg2+ or Cu2+. When EDTA was added to the probe L-Hg2+ solution, the intensity of fluorescence at 550 nm was recovered more than 80%. Meanwhile, the color of the solution changed back to the original colorless instantly. These changes were fully reversible as the addition of EDTA reversed the spectroscopic as well as color changes (Fig. S13a). These results suggest that probe L has the
Table 2 Probe L recoveries (%) obtained from analysis of water samples spiked with Cu2+. Sample
Added (mM)
Observed (mM)
Recovery (%)
Tap water
4.0 12.0 20.0
3.9 10.7 19.3
97.5 89.1 96.5 Fig. 10. 1H NMR titration plots of probe L with Hg2+ in CD3CN.
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potential to be fabricated into reversible devices to sense Hg2+. On the other hand, when EDTA was added to the probe L-Cu2+ solution, no obvious change in the intensity of fluorescence was observed (Fig. S13b). The result suggests that probe L is an irreversible probe for the recovery of Cu2+. In addition, the recoveries were investigated by spiking the Hg2+ and Cu2+ standard at three levels (6, 12 and 20 mM), respectively, and analyzing three replicates for each concentration. As shown in Tables 1 and 2, recoveries were in the range of 89.1–103.3%, indicating the probe L was greatly applicable for the effective preconcentration and quantitative determination of trace Hg2+ or Cu2+ in real water samples. The selectivity of probe L toward Hg2+ and Cu2+ was further ascertained by competition experiments. As shown in Figs. S14– S15, probe L was treated with Hg2+ or Cu2+ in the presence of other metal ions of the same concentration. Relatively low interference was observed for the detection of Hg2+ in the presence of other ions. Thus, probe L can be used as a selective colorimetric sensor toward Hg2+ in the presence of most competing ions. For the detection of Cu2+, probe L shows good selectivity in the presence of most metal ions, whereas fluorescence quenching was observed in the presence of Hg2+, most probably due to the displacement of Cu2 + by Hg2+ from the L-Cu2+ complex. 3.5. 1H NMR titration experiments The 1H NMR titration experiments of probe L in CD3CN were further carried out before and after addition of different amounts of Hg2+. As shown in Fig. 10, upon the addition of 1.0 equivalents of Hg2+, the three doublet peaks at 7.51, 7.58 and 6.10 ppm were shifted downfield to 7.60, 7.84 and 7.90 ppm, respectively. In addition, three doublet peaks at 8.22, 8.42 and 8.58 ppm were shifted downfield to 8.34, 8.42 and 9.12 ppm, respectively. The three NH signals only slightly shifted. These changes of the chemical shifts indicated that probe L could form a stable complex with Hg2+. Based on the above results, we propose that an intramolecular hydrogen bond is present between carbazide of indole group and sulfonamide in probe L. The presence of Hg2+ or Cu2+ might destroy the intramolecular hydrogen bond, leading to fluorescence quenching. 4. Conclusion In summary, we have designed and synthesized a highly selective and sensitive chemosensor, probe L, for the detection of Hg2+ and Cu2+. Probe L exhibited a good selectivity toward Hg2+, leading to fluorescence quenching. This probe also showed an excellent selectivity toward Cu2+, resulting in fluorescence enhancement accompanied by a blue shift in emission. Furthermore, probe L can be utilized for selective sensing of Hg2+ and Cu2+ based on the distinct color changes in real samples. Acknowledgement Ministry of Science and Technology, R. O. C. (grant numbers: 106-2113-M-018-002). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jphotochem.2017.11.003.
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Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
A novel fluorescence sensor for dual sensing of Hg2+ and Cu2+ ions Yi-Wun Siea , Chia-Lin Lia , Chin-Feng Wanb , Hongbin Yanc , An-Tai Wua,* a
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan Department of Medical Applied Chemistry, Chung Shan Medical University, Taichung City 40201,Taiwan c Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON L2S 3A1, Canada b
A R T I C L E I N F O
Article history: Received 16 August 2017 Received in revised form 2 November 2017 Accepted 5 November 2017 Available online 6 November 2017 Keywords: Fluorescence enhancement Blue shift Colorimetric
A B S T R A C T
A novel fluorescence probe (L) was prepared and its sensing behavior toward metal ions was investigated. Probe L exhibited colorimetric response toward Hg2+ and fluorescence enhancement with an obvious blue shift toward Cu2+ in CH3CN/H2O (v/v, 9:1). The association constant for probe L-Hg2+ and probe LCu2+ in CH3CN/H2O (v/v, 9:1) was determined as 1.5 104 M 1 and 5.3 108 M 2, respectively. The detection limit of probe L for the analysis of Hg2+ and Cu2+ was determined as 22.69 ppb and 155.53 ppb, respectively. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With recent development in ion sensors, the design and synthesis of new chemosensors for the efficient detection of trace metal ions remain as an important research topic in environmental investigations [1]. Generally, industrial waste water, not only shows a single metal pollution, always contains a variety of heavy metal ions at the same time. Thus, development of new receptors for the detection of different analytes simultaneously is emerging as an area of great interest [2], since such system would lead to faster analytical processing and potential cost reductions. Among various important metal ions, Cu2+ is a significant environmental pollutant and also plays a critical role in various biological processes [3–5]. The accumulation of excess amounts of Cu2+ in humans or dysregulation of copper homeostasis can cause a series of severe diseases such as Alzheimer's and Parkinson's diseases [6]. On the other hand, Hg2+ and its derivatives are significant environmental pollutants with inherent high toxicity. In the marine environment, Hg2+ is converted by bacteria into methylmercury, a highly potent neurotoxin, which accumulates in humans through the food chain [7]. While some fluorescent probes for dual detection of Hg2+ and Cu2+ have been reported [8–18], these probes require complicated synthesis, and are usually poorly soluble in water and lack sensitivity. Therefore, for practical applications, simple and sensitive fluorescent probes for the simultaneous detection of Hg2+ and Cu2+ metal ions are still in great demand.
* Corresponding author. E-mail address: [email protected] (A.-T. Wu). https://doi.org/10.1016/j.jphotochem.2017.11.003 1010-6030/© 2017 Elsevier B.V. All rights reserved.
Recently, the development of fluorescence ratiometric probes for metal ions has attracted much attention since they allow for the measurement of emission intensities at two different wavelengths [19–24]. However, development of ratiometric probes for Cu2+ is a challenge due to its inherent paramagnetic nature, and hence, the complexation with Cu2+ generally results in quenching of the fluorescence intensity of the probe [25–30]. FRET (fluorescence resonance energy transfer)-based fluorescent probes for Cu2+ ions have been reported, but are less common compared with Hg2+. FRET (fluorescence resonance energy transfer)-based fluorescent probes for Cu2+ ions have been reported, but are less common compared with Hg2+. In this regard, the use of guest-induced FRET mechanism should be an efficient approach to design ratiometric fluorescence probes. Ramaiah et al. reported a fluorescent probe based on dansyl and naphthalimide as the FRET (fluorescence resonance energy transfer) pair for the ratiometric fuorescent detection of Cu2+ [31]. They also reported a FRET-based probe for Cu2+ by using the combination of indole and rhodamine moieties [32]. Until now, only a few examples can selectively senses Hg2+ and 2+ Cu through two different channels: display color change and “turn-on” or ratiometric fluorescent changes in emission spectra [33]. Herein, we synthesized a new fluorescent probe (L), which could selectively detect Hg2+ and Cu2+ based on different fluorescence mechanisms among a series of ions. Probe L can also be used for selective sensing of Hg2+ and Cu2+ based on the distinct color changes in real samples application. In this probe L, two fluorophores, i.e. indole and dansyl, were conjugated as a donor– acceptor pair for FRET. Probe L can be readily prepared by the reaction of 1H-indole-2-carbohydrazide with dansyl chloride, as
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Scheme 1. Synthesis of probe L.
Fig. 3. The color changes of probe L (20 mM) upon addition of 5 equiv. of Hg2+ and Cu2+ in H2O/CH3CN = 1/9 (v/v), observed under UV light (lex. = 365 nm, T = 28 C).
Fig. 1. UV/vis spectra of probe L (20 mM) recorded in H2O/CH3CN = 1/9 (v/v) after addition of 5 equiv. of various metal ions.
Fig. 4. Fluorescence emission spectra of probe L (20 mM) in the presence of 5 equiv. of various metal ions in H2O/CH3CN = 1/9 (v/v) (lex. = 300 nm, T = 28 C).
Fig. 2. The color change of probe L (20 mM) upon addition of 5 equiv. of various metal ions in H2O/CH3CN = 1/9 (v/v), observed by naked eyes.
Fig. 5. Fluorescence spectra of probe L (20 mM) in H2O/CH3CN = 1/9 (v/v) upon addition of increasing concentrations of Hg2+ (lex. = 300 nm, T = 28 C).
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were recorded under electron impact (EI) or electron spray interface (ESI) conditions with a UV–vis spectra were recorded by using Jasco V630 spectrophotometer with a diode array detector, and the resolution was set at 1 nm. Fluorescence spectroscopy was measured on a Jasco FP-8300 fluorescence spectrophotometer. 2.2. Synthesis of probe L
Fig. 6. Fluorescence spectra of probe L (20 mM) in H2O/CH3CN = 1/9 (v/v) upon addition of increasing concentrations of Cu2+ (lex. = 300 nm, T = 28 C).
shown in Scheme 1. The structure of probe L was confirmed by 1H and 13C NMR and mass spectroscopic data (Figs. S1–S3). 2. Materials and instrumental methods
To a solution of 1H-indole-2-carbohydrazide (0.10 g, 0.57 mmol) in ethanol (20 mL) was added 5-(dimethylamino)naphthalene-1sulfonyl chloride (0.165 g, 0.613 mmol). The mixture was stirred under reflux for 12 h. After cooling to room temperature, ethanol was evaporated completely and the crude product was purified by column chromatography (silica, hexane: ethyl acetate 10:1, v/v) to give yellow solid (70%). M.p: 220 C; 1H NMR (300 MHz, DMSO-d6) d: 11.55 (s, 2H), 10.62 (s, 2H), 10.25 (s, 2H), 8.44 (t, J = 9 Hz, 4H), 8.17 (dd, 2H), 7.58 (m, 6H), 7.33 (d, J = 8.4 Hz, 2H), 7.25 (d, J = 7.5 Hz, 2H), 7.18 7.03 (m, 4H), 6.99 (t, J = 7.2 Hz, 2H), 2.83 (s, 6H); 13C NMR (75.4 MHz, DMSO-d6) d: 45.15, 103.81, 112.26, 114.96, 119.91, 120.00, 121.78, 123.45, 123.82, 126.83, 128.62, 128.70, 129.10, 129.67, 129.91, 130.05, 135.29, 136.57, 151.11, 160.49; HRMS (EI): Calcd for C21H20N4O3S (M+), m/z 408.1256; found m/z 408.1250. 3. Results and discussion
2.1. Experimental section
3.1. The absorption and fluorescence studies of probe L toward various metal ions
All reagents were obtained from Sigma-Aldrich and were used without further purification. Analytical thin-layer chromatography was performed using silica gel 60 F254 plates (Merck). The 1H and 13 C NMR spectra were recorded with a Bruker AM 300 spectrometer at 300 and 75.4 MHz, respectively. Chemical shifts are given in ppm with residual DMSO as reference. Mass spectra
The sensing properties of the probe L for cations were evaluated by UV/vis and fluorescence measurements in CH3CN/H2O (9:1, v/v) in the presence of various metal ions in their perchlorate salts: Li+, Na+, K+, Ca2+, Mn2+, Hg2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Pb2+, Cd2+, Zn2+ and Al3+. As shown in Fig. 1, probe L showed a major absorption
Scheme 2. The proposed mechanism for the reaction of probe L with (a) Hg2+ and (b) Cu2+.
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major band at 380 nm. Meanwhile, the solution of probe L showed a color change from colorless to light yellow, which could be detected by the naked-eye (Fig. 2). In the presence of Cu2+, the absorption spectra of probe L showed a blue shift, with a major band at 290 nm. The solution of probe L showed a color change from light yellow to dark blue that could be detected under UV lamp (Fig. 3). The sensing phenomena were also monitored by fluorescence spectroscopy. As shown in Fig. 4, the free probe L displayed a fluorescence emission band at 550 nm with medium intensity, which indicated the existence of FRET from the excited state of the indole chromophore to the dansyl unit. Upon addition of Hg2+, the fluorescence of probe L was quenched. Upon addition of Cu2+, however, probe L exhibited a prominent fluorescence enhancement with a blue shift relative to the original emission band of 550 nm. 3.2. Fluorescence titration and binding studies
Fig. 7. Time dependent experiments of (a) Hg2+; (b) Cu2+ ion (lex. = 300 nm, T = 28 C).
band at 300 nm (indole ring) with a shoulder at 350 nm (dansyl group). For the free units of probe L, the absorption spectrum of indole ring was showed an absorption band at 292 nm. The absorption spectrum of dansyl group was showed two absorption bands at 264 and 392 nm, respectively (Fig. S4). In the presence of Hg2+, the absorption spectra of probe L red shifted, showing a
To further investigate the sensing properties of probe L, fluorescence titration of probe L with Hg2+ and Cu2+ were performed. With the addition of increasing amounts of Hg2+ to a solution of probe L in CH3CN/H2O (v/v, 9:1), the fluorescence intensity of probe L at 550 nm was quenched gradually, as shown in Fig. 5. After the addition of 3 equivalents of Hg2+, the fluorescence intensity was quenched almost completely (99%). The result indicated that probe L could form a stable complex with Hg2+. On the other hand, the fluorescence titration of probe L with Cu2+ showed a regular decrease in the intensity of FRET-mediated emission from the dansyl moiety at 550 nm with the increase in concentration of Cu2+ ions. Meanwhile, a concomitant increase in the emission intensity of the indole chromophore at 380 nm with an isoemissive point at 502 nm was observed, as shown in Fig. 6. Further, additions of 3 equivalents of Cu2+ ions resulted in the complete quenching of the FRET-mediated emission, with a concomitant ca. 5.8-fold increase in the fluorescence intensity at 380 nm (Fig. S5). The selective quenching of FRET-mediated emission at 550 nm upon addition of the Cu2+ indicate that these ions form complex with the probe L involving the sulfonamide group of the dansyl moiety, carbazide of the indole and indole itself. As a consequence, the dansyl moiety becomes incapable of quenching the excited state of the indole chromophore, resulting in the restoration of indole emission, thereby facilitating the detection of Cu2+. The significant “turn on” intensity with a blue shift of ca. 170 nm allows for the fluorescence ratiometric detection of Cu2+ ions by probe L. The association constant for probe L-Hg2+ and probe L-Cu2+ in CH3CN/H2O (v/v, 9:1) was determined as
Fig. 8. The color changes in different sources of water containing Hg2+, observed by naked eyes.
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Fig. 9. The color changes in different sources of water containing Cu2+, observed under UV light (lex. = 365 nm, T = 28 C).
1.5 104 M 1 and 5.3 108 M 2 by the Stern-Volmer equation (Fig. S6) and Hill plot (Fig. S7), respectively. The detection limit of probe L for the analysis of Hg2+ and Cu2+ was determined as 22.69 ppb and 155.53 ppb, respectively. The linear relationship of the fluorescence titration suggests a 1:1 stoichiometry of probe L to Hg2+, which is evident in the Job plot (Fig. S8). The formation of 1:1 complex between probe L and Hg2+ was further confirmed by the appearance of a peak at m/z 685 in the mass spectrum, assignable to [probe L + Hg2+ + CH3CN + 2H2O 2H+] (Fig. S9). On the other hand, a Job plot indicated a 1:2 stoichiometric complex of probe L with Cu2+ (Fig. S10). In addition, the formation of 1:2 complex between probe L and Cu2+ was further confirmed by the appearance of a peak at m/z 647 in the mass spectrum, assignable to [probe L + 2Cu2+ + ClO4 + H2O 3H+] (Fig. S11). Scheme 2 summarizes the proposed reaction between probe L and Hg2+ and Cu2+. Such a distinctive binding of probe L results in the selective and sensitive detection of both Hg2+ and Cu2+. 3.3. Application in real samples From the standpoint of practical applications, real-time determination of analytes is highly desirable. In this respect, the fluorescence intensity of probe L in CH3CN/H2O (v/v, 9:1) at 550 nm upon addition of 5.0 equivalents of Hg2+ and 380 nm upon addition of Cu2+ was monitored over time. As shown in Fig. 7, the fluorescence intensity plateaued within one minute after addition
Table 1 Probe L recoveries (%) obtained from analysis of water sample spiked with Hg2+. Sample
Added (mM)
Observed (mM)
Recovery (%)
Drinking water
6.0 12.0 20.0
5.5 12.4 18.5
91.7 103.3 92.5
of Hg2+ and Cu2+. The practical application of probe L for selective sensing of Hg2+ or Cu2+ in different source of water such as tap, drink, ditch, lake and ground water was also demonstrated. Hg (ClO4)2 and Cu(ClO4)2 were first dissolved in water of the above source, followed by the addition of probe L into each water sample. All the water samples containing Hg2+ showed clear color change from colorless to bright yellow, which is readily observed by the naked-eye (Fig. 8). On the other hand, addition of probe L into each water sample with Cu2+ clearly led to color change from bright yellow to blue under a UV lamp (Fig. 9). These results indicated that probe L can be used for selective sensing of Hg2+ and Cu2+ based on the distinct color changes in real samples. In addition, the effect of pH on the emission bands of the probe L–Hg2+ or probe L–Cu2 + complex in CH3CN/H2O (v/v, 9:1) solution was also examined. It was found that the fluorescence intensity of probe L–Hg2+ or probe L–Cu2+ in CH3CN/H2O (v/v, 9:1) solution showed a significant emission band in the pH range 4–9 (Fig. S12), suggesting that probe L could be applied in the analysis of environmental aqueous samples with a relatively wide pH range. 3.4. Reversible and competition experiments Reversibility is a desired feature in developing novel sensor for practical application. To examine the reversibility of probe L toward Hg2+ or Cu2+ in CH3CN/H2O (9:1, v/v), a solution of EDTA (20 mM) was added to the complex solution of receptor L with Hg2+ or Cu2+. When EDTA was added to the probe L-Hg2+ solution, the intensity of fluorescence at 550 nm was recovered more than 80%. Meanwhile, the color of the solution changed back to the original colorless instantly. These changes were fully reversible as the addition of EDTA reversed the spectroscopic as well as color changes (Fig. S13a). These results suggest that probe L has the
Table 2 Probe L recoveries (%) obtained from analysis of water samples spiked with Cu2+. Sample
Added (mM)
Observed (mM)
Recovery (%)
Tap water
4.0 12.0 20.0
3.9 10.7 19.3
97.5 89.1 96.5 Fig. 10. 1H NMR titration plots of probe L with Hg2+ in CD3CN.
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potential to be fabricated into reversible devices to sense Hg2+. On the other hand, when EDTA was added to the probe L-Cu2+ solution, no obvious change in the intensity of fluorescence was observed (Fig. S13b). The result suggests that probe L is an irreversible probe for the recovery of Cu2+. In addition, the recoveries were investigated by spiking the Hg2+ and Cu2+ standard at three levels (6, 12 and 20 mM), respectively, and analyzing three replicates for each concentration. As shown in Tables 1 and 2, recoveries were in the range of 89.1–103.3%, indicating the probe L was greatly applicable for the effective preconcentration and quantitative determination of trace Hg2+ or Cu2+ in real water samples. The selectivity of probe L toward Hg2+ and Cu2+ was further ascertained by competition experiments. As shown in Figs. S14– S15, probe L was treated with Hg2+ or Cu2+ in the presence of other metal ions of the same concentration. Relatively low interference was observed for the detection of Hg2+ in the presence of other ions. Thus, probe L can be used as a selective colorimetric sensor toward Hg2+ in the presence of most competing ions. For the detection of Cu2+, probe L shows good selectivity in the presence of most metal ions, whereas fluorescence quenching was observed in the presence of Hg2+, most probably due to the displacement of Cu2 + by Hg2+ from the L-Cu2+ complex. 3.5. 1H NMR titration experiments The 1H NMR titration experiments of probe L in CD3CN were further carried out before and after addition of different amounts of Hg2+. As shown in Fig. 10, upon the addition of 1.0 equivalents of Hg2+, the three doublet peaks at 7.51, 7.58 and 6.10 ppm were shifted downfield to 7.60, 7.84 and 7.90 ppm, respectively. In addition, three doublet peaks at 8.22, 8.42 and 8.58 ppm were shifted downfield to 8.34, 8.42 and 9.12 ppm, respectively. The three NH signals only slightly shifted. These changes of the chemical shifts indicated that probe L could form a stable complex with Hg2+. Based on the above results, we propose that an intramolecular hydrogen bond is present between carbazide of indole group and sulfonamide in probe L. The presence of Hg2+ or Cu2+ might destroy the intramolecular hydrogen bond, leading to fluorescence quenching. 4. Conclusion In summary, we have designed and synthesized a highly selective and sensitive chemosensor, probe L, for the detection of Hg2+ and Cu2+. Probe L exhibited a good selectivity toward Hg2+, leading to fluorescence quenching. This probe also showed an excellent selectivity toward Cu2+, resulting in fluorescence enhancement accompanied by a blue shift in emission. Furthermore, probe L can be utilized for selective sensing of Hg2+ and Cu2+ based on the distinct color changes in real samples. Acknowledgement Ministry of Science and Technology, R. O. C. (grant numbers: 106-2113-M-018-002). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jphotochem.2017.11.003.
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