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S2- and its applications
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 146–153
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
A large Stokes shift, sequential, colorimetric fluorescent probe for sensing Cu2+/S2- and its applications
T
Ji-Zhen Lia, Tao-Hua Lengb, , Zhi-Qiang Wangc, Li Zhoua, Xue-Qing Gongc, Yong-Jia Shena, ⁎ Cheng-Yun Wanga, ⁎
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China b National Food Quality Supervision and Inspection Center (Shanghai), Shanghai Institute of Quality Inspection and Technical Research, Shanghai 200233, PR China c Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
ARTICLE INFO
ABSTRACT
Keywords: Copper ions Sulfide Large Stokes shift Colorimetric Near-infrared DFT calculations
Copper ions (Cu2+) and sulfide (S2−) are important markers in many physiologies and pathological processes. In this work, a new near-infrared fluorescent probe 1 for colorimetric and sequential detection of Cu2+/S2− was designed and developed. The probe showed a rapid (less than 1 min), highly selective and sensitive response toward copper ions. Notably, the probe could also be applied to detect S2− through reversible formation-separation of complex 1-Cu2+ and CuS with a large Stokes shift of 234 nm. The detection limit for Cu2+ and S2− was found to be 1.8 × 10-8 M and 1.5 × 10-8 M, respectively. Furthermore, the binding stoichiometry between 1 and Cu2+ was found to be 1:1, the binding mode was also demonstrated using density functional theory (DFT) calculations and contrastive compound research. In addition, the probe was successfully applied in real water samples assay for the detection of Cu2+, and the strip papers experiments also showed that probe 1 can be used to detect Cu2+ and S2−.
1. Introduction Copper, as the third most abundant transition metal ions, is an important trace mineral element in organisms [1], plays key roles in biological and environmental fields [2]. However, unregulated amounts of copper may destroy the metabolic balance of physiological systems, and induce many health issues, such as Parkinson’s [3] and Wilson’s [4] diseases. In addition, copper is also a significant pollutant due to its widespread use in industry and agriculture. Therefore, the development of efficient methods for copper (II) detection may be very meaningful for the human health and environment. Compared with other traditional methods [5,6], fluorescent probes have attracted increasing attention on account of their high sensitivity and selectivity [7], real-time detection with rapid respond time [8] and potential biological applications [9]. To date, many fluorescence probes for copper detection have been reported over the several years [10,11]. More interestingly, among these probes, some reactive sulfur species, such as glutathione (GSH), cysthine (Cys), homocysteine (Hcy) and sulfide anion (S2−) could also be monitored via the probe-copper complex [12–16]. As a toxic substance and environmental pollutant,
⁎
S2− is mainly generated in natural gas purification, petroleum refining, sewage treatment and paper-making [17]. Sulfide anions can also combine with H+ under acidic conditions, generating more toxic H2S or HS-, which not only pollutes the atmosphere but also causes organ’s damage when exposing to high concentration of H2S [18,19]. Until now, a number of fluorescence probes for its detection have been designed and developed. [20–22]. However, most of these probes are mainly based on the organic reaction like the cleavage of sulfonate or dinitrophenyl ester [23], Michael addition [24,25], and thiolysis of the 7-nitro-1,2,3-benzoxadiazole (NBD) amine or ester bond [26,27]. These reactions are plagued by some serious limitations like synthesis requires strict conditions and relatively time-consuming procedures, which may limit their applications in analytical and biological conditions. Herein, based on the previous works reported by our group [28], we synthesized a new dicyanoisophorone-based derivative (probe 1, Scheme 1). The dicyanoisophorone moiety was chosen as a fluorophore because of its large π-conjugation system and long emission wavelength in NIR region [29], and the salicyladehyde azine was introduced as schiff base unit due to its high affinity for cations and excited state intra-molecular proton transfer phenomenon (ESIPT) characteristics
Corresponding authors. E-mail addresses: [email protected] (T.-H. Leng), [email protected] (C.-Y. Wang).
https://doi.org/10.1016/j.jphotochem.2019.01.006 Received 22 August 2018; Received in revised form 5 January 2019; Accepted 6 January 2019 Available online 11 January 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 146–153
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Scheme 1. The synthetic route of probe 1 and compound 6.
[30]. We envisioned that the probe could induce a large Stokes shift while bonding with metal ions. As expected, probe 1 displayed high sensitivity and selectivity toward Cu2+. What’s more, the complex 1Cu2+ also exhibited good selectivity and sensitivity for S2− with a distinct fluorescence enhancement and large Stokes shift of 234 nm, which was much longer than reported previously (Table S1).
2.3. Theoretical calculation The density functional theory (DFT) calculations were performed by the B3LYP method with the 6–31 G(d) basis set using the Gaussian 09 package. During the calculations, no geometry constraint or symmetry was imposed. To guarantee that all the structures studied were genuine minima on the potential energy surface, frequency calculations were performed at the same theoretical level.
2. Experiments section 2.1. Materials and instruments
3. Results and discussion
All reagents and solvents were purchased from commercial suppliers and analytical grade. The UV–vis spectra were recorded on a Varian Cary 500 spectrophotometer using a 1 cm path length quartz cell, and the fluorescence spectra were recorded on CARY Eclipse Spectrophotometer. 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on a Bruker AM-400 spectrometer in DMSO-d6 and CDCl3 with tetramethylsilane (TMS) as an internal standard. High resolution mass spectrometry data were recorded on a Waters LCT Permier XE spectrometer. The measurements of pH were done using a pH-10C digital pH meter. Melting points were obtained in melting-point tubes using an SGWX-4 apparatus.
3.1. Design and synthesis of probe Since schiff-base fluorescent probes have been widely used in metal ions detection owing to their high selectivity, sensitivity, reliability and special complexation sites toward metal ions accompanied with obvious optical signal changes [33,34], the well-designed probe 1 was synthesized by the condensation of salicyladehyde azine (4) and a dicyanoisophorone derivative (3), and the intermediate products 3 and 4 were synthesized in good yields by the methods reported in the literatures [31,32]. To explore the influence of molecular structure on the recognition of metal ions, we synthesized a contrastive compound 6. All the detailed characterization data determined by 1H and 13C NMR, HRMS were presented in supporting information (S1-S14).
2.2. Synthesis of probe 1 Synthetic route of probe 1 is shown in Scheme 1. Compound 3 and compound 4 were synthesized according to the previous works [31,32]. Compound 3 (100 mg, 0.3 mmol) and compound 4 (41 mg, 0.3 mmol) were dissolved in 10 mL EtOH, and a drop of acetic acid was added as catalyst. Then the mixture was heated to 80℃ and reacted for 12 h under N2 atmosphere. After cooling, the solution was poured into ice water (50 mL), the crude precipitate was collected by filtration and washed with water for three times. After being dried under vacuum, the crude product was further purified by silica column chromatography (petroleum ether/dichloromethane = 1/1, v/v) to give the pure product as a yellow solid (105 mg, 80% yield). Mp: 242.2–243.5 °C. 1H NMR (d6-DMSO, 600 MHz) δ: 11.46 (s, 1H, -OH), 11.09 (s, 1H, -OH), 9.03 (s, 1H, -CH = N-), 8.97 (s, 1H, -N = CH-), 8.07 (s, 1H, Ar-H), 7.80 (d, J = 8.8 Hz, 1H, Ar-H), 7.79 (d, J = 7.6 Hz, 1H, Ar-H), 7.43 (d, J = 8.4 Hz, 1H, Ar-H), 7.39-7.30 (m, 2H, −CH = CH-), 7.04 (d, J = 8.8 Hz, 1H, Ar-H), 7.02-6.96 (m, 2H, Ar-H), 6.86 (s, 1H, −CH=), 2.62 (s, 2H, −CH2-), 2.56 (s, 2H, −CH2-), 1.03 (s, 6H, -2CH3). ESIHRMS: Calc. Mass for [C27H24N4O2+H]+: 437.1978; found 437.1976.
3.2. Solvent effect on the probe 1 We initially investigated the optical behaviour of probe 1 in common organic solvents including toluene, dichlormethane, ethyl acetate, tetrahydrofuran, ethanol, acetonitrile, DMF and DMSO. The absorption and fluorescence spectra of probe 1 were shown in Fig. S15 and the corresponding data were summarized in Table S2. As depicted, the absorption maxima showed a small range of movement with the change of solvent polarity. However, fluorescence spectra showed pronounced solvent-polarity dependent changes in the difference solvents. In the toluene, there was very weak fluorescence emission could be observed. It was then found that a large fluorescence enhancement in DMSO-water (3:1, v/v) solvent (Fig. S15B). The changes of the fluorescence spectrum of probe 1 in different ratios of DMSO-water were also monitored (Fig. S15D). As a result, DMSO-water (2:1, v/v) was selected as the ideal solvent media for this work. 147
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Fig. 1. (A) UV–vis spectral changes of probe 1 (10 μm) in the presence of different concentrations of Cu2+ (0–2.5 equiv). Inset: colour changes of probe (Left) and probe-Cu2+ (Right) under daylight; (B) UV–vis spectral changes of probe 1 (10 μm) in the presence of various other metal ions (10 equiv) and Cu2+ (2.5 equiv). All the tests were carried out in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES, pH = 7.4).
3.3. Spectral studies of the probe 1 with Cu2+
Additionally, 1:1 bonding mode was confirmed by a Job’s plot analysis (Fig. S18). To examine the selectivity of probe 1 toward Cu2+, we performed the fluorescence studies of probe 1 in the presence of various other cations. As shown in Fig. 2B, no obvious changes could be observed except for Cu2+, which displayed a distinct quenching of fluorescence intensity with probe 1. Moreover, the anti-interference experiments have also been conducted by adding Cu2+ (2.5 equiv) to the solution of probe 1 containing interfering metal ions (2.5 equiv) (Fig. S19). In the presence of these competitive species, the fluorescence intensity of probe 1 could be also completely quenched by Cu2+, which demonstrated that coexistence metal ions did not cause substantial interference in Cu2+ detection.
First, the absorption and fluorescence properties of this probe with Cu2+ were investigated in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES, pH = 7.4). As shown in Fig. 1A, free probe 1 exhibited a strong absorption band at 425 nm (ε = 34856 M−1 cm-1) which was corresponding to the S0-S4 type of transition (see TD-DFT calculation results, table S3). In continuous addition of Cu2+(0–2.5 equiv), the maximum peak at 425 nm decreased gradually. At the same time, a new absorption band formed at 475 (ε = 32467 M−1 cm-1) nm and an isosbestic point appeared at 440 nm, which indicated a new stable complex between probe 1 and Cu2+ was formed. Besides, a distinct colour change from light yellow to brown could be observed by naked eye (Fig. 1A insert). Considering the coordination properties of schiff-base to metal ions such as zinc, aluminum, iron, and mercury, we performed the selective experiments of probe 1 towards other metal ions, including K+, Na+, Ag+, Zn2+, Ni2+, Pb2+, Cd2+, Mg2+, Hg2+, Fe2+, Ba2+, Ca2+, Co2+, Cr3+, Fe3+, and Al3+ (Fig. 1B). Upon the addition of 10 equiv of each cation, the probe 1 showed almost no or slightly changes in absorbance spectra. In contrast, only Cu2+ induced a remarkable red shift of absorbance spectra, indicating that probe 1 featured an excellent selectivity to Cu2+. To further study the Cu2+-responsive sensitivity of probe 1, we performed the fluorescence titration experiments. As shown in Fig. 2A, the probe displayed a long wavelength emission band at 659 nm, the addition of Cu2+ (0–25 μm) leads to a gradually decreased in fluorescence intensity. This change might be attributed to the chelation-enhanced fluorescence quenching (CHEQ) effect [35] and the paramagnetic property of Cu2+. What’s more, we also found that the fluorescence intensity of probe 1 showed a good linearity (R = 0.9946) to Cu2+ concentrations in the range of 0–10 μm. The detection limit (3σ/K) of probe 1 for Cu2+ was calculated to be 1.8 × 10−8 M (Fig. S16), which is much lower than 1.3 μM allowed by environmental protection agency (EPA), and the binding constant (K) was calculated as 1.71 × 103 M-1 by using Benesi-Hildebrand equation (Fig. S17).
3.4. pH effect studies The effect of pH ranging from 2.0 to 10.0 on the fluorescence response of probe 1 to Cu2+ was investigated in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES). As shown in Fig. 3 and Fig. S20, probe 1 displayed stable emission intensity in pH range 2.0-8.0. However, the fluorescence intensity of probe 1 could not be quenched by Cu2+ in the acid region (pH ≤ 5.0), which might be due to the protonation of the two nitrogen atoms and hydroxyl groups of probe 1, hence the ability to chelate with copper decreases. Again, the decreasing fluorescence intensity of probe 1 in the basic region (pH > 8.0) might due to the deprotonation of the two hydroxyl groups. Therefore, probe 1 can be used as a reliable copper sensor in pH 6-8. 3.5. DFT calculations To further investigate the photophysical properties of probe 1 and 1-Cu2+, DFT calculations were performed by using the Gaussian 09 program. The HOMO and LUMO orbitals of probe 1 and 1-Cu2+ were listed in Fig. 4 and Table S4. In the probe, the energy gaps between HOMO and LUMO orbitals were measured to be 2.97 eV. Besides, the α
Fig. 2. (A) Fluorescence spectra changes of probe 1 (10 μm) upon addition of Cu2+ (0–25 μm). Inset: photograph of probe 1 (Left) and probe-Cu2+ (Right) on excitation 365 nm using a UV lamp; (B) Fluorescence spectra of probe 1 (10 μm) with different metal ions (Cu2+ 25 μm, other metal ions 100 μm). λex = 425 nm. All the tests were carried out in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES, pH = 7.4).
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Fig. 3. The structure changes of probe 1 in the acidic and basic pH.
and β orbital of 1-Cu2+ were respectively 2.66 eV and 1.79 eV. These HOMO and LUMO energy gaps decrease in the 1-Cu2+ indicated that the energy required for the excitation of electron reduced as compared to the free probe, and caused a red shift in the absorption wavelength, which was consistent with the experimental results.
added, the UV–vis absorbance and fluorescence of 1-Cu2+ could be largely restored. These results demonstrated that the binding of 1-Cu2+ and S2− was chemically reversible. The fluorescence emission spectral titration experiments of 1-Cu2+ with S2− were subsequently studied. As shown in Fig. 5B, upon addition of S2− to the solution of 1-Cu2+, the emission intensity at 659 nm exhibited a remarkable increase. Fig.S22 showed a good linear relationship (R2 = 0.9953) and the detection limit was calculated as 1.5 × 10-8 M. All the spectra data were recorded within one minute after S2- was added, and the intensity became stable over an extended period of time (Fig. S23), indicating that complex 1-Cu2+ could act as a real-time and stable probe for S2- detection. To determine the selectivity of complex 1-Cu2+, we examined the UV–vis absorption and fluorescence spectra toward various other anions such as F−, Cl−, Br−, I−, CN−, AcO−, PO43-, HPO42-, H2PO4−, SCN−, SO42-, SO32-, HSO4−, HSO3−, NO3−. As shown in Fig. S24, when complex 1-Cu2+ was treated with the S2- (3 equiv), dual responses of absorbance spectra and fluorescence intensity could be obtained. Upon adding other anions (3 equiv), no significant change was observed except CN−, which caused slightly increase of fluorescence and blue shift
3.6. Spectral studies of complex 1-Cu2+ with S2− On the basis of Cu2+ has strongly binding affinities to sulfide, we envisioned that the complex 1-Cu2+ could be used as an excellent turnon probe to further detect S2−. To confirm this assumption, we examined the absorbance and fluorescence response of the complex 1Cu2+ towards S2−. As shown in Fig. 5A, 1-Cu2+ exhibited an absorption band at 475 nm. After adding 3 equiv S2−, the maximum peak was gradually blue-shifted, and the spectra eventually matched the original probe 1. Besides, the colour of solution 1-Cu2+ turned into the initial yellow, indicating the recovery of probe 1 from the 1- Cu2+ complex through the chelation of S2- with copper. The reversible behavior of S2− complexation with 1-Cu2+ was subsequently confirmed by EDTA titration. As shown in Fig. S21, when 1 equiv of EDTA solution was
Fig. 4. Frontier molecular orbital profiles of probe 1 and 1-Cu2+ based on DFT calculations.
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Fig. 5. (A) UV–vis spectral changes of 1-Cu2+ (10 μm) in the presence of different concentrations of S2− (0–30 μm.); (B) Fluorescence spectra changes of 1-Cu2+ (10 μm) upon addition of S2- (0–30 μm). λex = 425 nm. All the tests were carried out in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES, pH = 7.4).
of absorbance. Furthermore, in the presence of these anions, the red emission of probe 1 could be largely restored by subsequent addition of S2- (Fig. S25). These results obviously showed that the complex 1-Cu2+ can selectively detect S2-. The effect of pH ranging from 2.0 to 10.0 on the fluorescence response of complex 1-Cu2+ with S2− was also investigated (Fig. S26). The emission intensity was significantly enhanced when pH was above 5, which indicated that the complex 1-Cu2+ could be used for the detection of S2− in neutral and alkaline conditions.
on these three coordination bonds, Mayer bond order analysis of the probe-Cu2+ compound was conducted through the Multiwfn program [36]. The values involving Cu2+ are listed in Table S5. As is known to all, the value size of the Mayer bond order is positively correlated with the strength of the bond. If the value approximates zero, it means there is no or almost no bonding between two atoms. From the above data, we can clearly see that the Mayer bond orders of O38-Cu56, O55-Cu56, N54-Cu56 are much larger than those of N40-Cu56 and C39-Cu56 which are close to zero. Therefore, it can be concluded that these two CueO coordinate bonds and one CueN coordinate bond are the dominant bonds of the probe bonding to Cu2+. To further confirm the binding of Cu2+ through these three atoms of probe 1, we synthesized a contrastive compound 6 to mimic the binding sites of probe 1. Compound 6 has the similar structure as probe 1 in most parts except one hydroxyl group at the ortho-position of the schiff base unit was removed. As a consequence, compound 6 showed nearly no emission even in pure DMSO solution when excited at 420 nm (ε = 68,685 M−1 cm−1). In contrast, probe 1 showed a strong fluorescence at 659 nm with excitation at 425 nm. Then, we carried out the titration and selectivity experiments of compound 6 toward Cu2+. However, the fluorescence and absorbance spectra of compound 6 showed no change upon addition of Cu2+ ions (Fig. S27). From these studies, it is clearly that the two deprotonated phenolate groups and one nitrogen atom of probe 1 binding with the Cu2+. Since photo-induced electron (PET) is generally accepted as a mechanism for the “turn-on’’ fluorescent probe. To verify whether the mechanism for the detection of S2− has undergone a PET process, we calculated the HOMO–LUMOs energies of accepter and donor. As illustrated in Fig. 7 and Scheme 2, in the presence of Cu2+, the HOMO (donor, E=−5.813 eV) has higher energy than of the acceptor (E= −6.355 eV), therefore, upon excitation of complex 1-Cu2+, the electron can be transferred from the HOMO (donor) to the low-energy HOMO of the acceptor and quenches the fluorescence. Upon the addition of S2−, the Cu2+ ions are captured by S2- to produce stable CuS and release free probe 1. Meanwhile, the energy of the receptor HOMO is lowered to −5.957 eV, inhibiting the electron transfer from the HOMO donor (E= −5.957 eV) to acceptor HOMO (E= −5.303 eV), and it leads to an enhancement of the fluorescence. These results agree with the PET mechanism reported for metal complexes [37–39].
3.7. Mechanism studies With the fact that binding ratio between 1 and Cu2+ was 1:1 stoichiometry, we explained how the probe binds the Cu2+ through the DFT calculations. The optimized geometries of the probe 1 and 1-Cu2+ complex were shown in Fig. 6. As can be seen, there were two intramolecular hydrogen bonds (HBs) between the phenolic hydroxyls and the nearby nitrogen atom. In the 1-Cu2+ complex, the phenol group rotated and it combined with Cu2+ through two obvious Cu-O coordinate bonds and one Cu-N coordinate bond. In order to determine whether the binding of Cu2+ mainly depends
3.8. Reversibility of the probe 1 for Cu2+ and S2− For many practical applications, a good recyclability and reusability is necessary. As shown in Fig. S28, the reversible experiments of probe 1 was repeated 5 times by the modulation of Cu2+ and S2− addition, no obvious emission intensity attenuation at 659 nm could be observed, indicating that probe 1 could be used as a reversible ON-OFF-ON sensor for Cu2+ and S2- detection.
Fig. 6. The optimized structures of the probe and the probe-Cu2+ compound (C: grey, H: white, N: blue, O: red, Cu: pink).
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Fig. 7. S2− recognition mechanism based on photo-induced electron transfer (PET).
Considering the toxicity of Cu2+ in the environment, we examined the ability of probe 1 to quantitatively detect Cu2+ in real water samples. All the water samples were collected from laboratory tap water and river water (Qingchun River, East China University of Science and Technology, Shanghai, China), and filtered through a membrane before used. Then a given volume (5 μM, 10 μM, 15 μM, 20 μM, 25 μM) of Cu2+ aqueous solutions were added into the test solution (DMSO: H2O = 2:1, v/v). We tested the fluorescent response of probe a to all these water samples at 659 nm and used distilled water as a control experiment. As shown in Fig. 9, the results displayed a good consistency between water samples and distilled water, and the recovery of Cu2+ was between 82˜104% (Table 1). These results suggested that probe 1 has potential application for quantitative detection of Cu2+ in real water samples. 4. Conclusions Scheme 2. Proposed sensing mechanism of probe 1 for Cu2+ and S2−.
In summary, we have designed and synthesized a highly selective and sensitive fluorescent probe 1 that could detect Cu2+ and H2S sequentially and reversibly. The probe exhibited remarkable colour change from light yellow to brown and significant fluorescence quenching in the presence of Cu2+ with a detection limit of 1.8 × 10−8 M. In addition, the binding mode was also demonstrated using density functional theory (DFT) calculations and contrastive compound research. Notably, based on a complex 1-Cu2+ formation-separation process and CuS formation, the complex of 1-Cu2+ ensemble could be used as a turn-on type probe for H2S detection with a large Stokes shift of 234 nm, and the detection limit reached as low as 1.5 × 10−8 M.
3.9. Applications in real samples To examine the practical applications of probe 1, the test strips were prepared by immersing filter paper into the solutions of probe 1 and 1Cu2+, and then drying in air. As shown in Fig. 8, the filter papers written with Cu2+ and S2− solutions displayed significant changes in the colour and fluorescence intensity. What’s more, the intensity of the coloured writings also changed with the different concentrations of Cu2+.
Fig. 8. The colour and fluorescence change of the strip papers in the presence of Cu2+ (0, 10 μm, 20 μm) and S2− (0, 20 μm) under daylight (left) and UV lamp (right).
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Fig. 9. Fluorometric determination of Cu2+ in three water samples by probe 1 (10 μM) in DMSO/H2O solution (2:1, v/v). λex = 425 nm.
References
Table 1 Determination of Cu2+ concentration in water samples. Sample
Cu2+ spiked (μM)
Cu2+ recovereda (μM)
Recovery (%)
Qingchun River
5 10 15 20 25 5 10 15 20 25
4.135 ± 0.012 10.332 ± 0.009 15.236 ± 0.007 20.488 ± 0.025 24.898 ± 0.002 4.612 ± 0.002 10.103 ± 0.01 15.233 ± 0.004 20.775 ± 0.05 24.277 ± 0.045
82.7 103.3 101.5 102.4 99.5 92.2 101.0 101.5 104.3 97.1
Tap water
a
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Average of three determinations.
Furthermore, probe 1 was successfully applied to detect Cu2+ and S2- in strip papers and real water samples. Therefore, the probe has potential applications in environmental and real industrial samples for Cu2+ and S2- detection. Acknowledgments The authors are grateful to financial support from the Natural Science Foundation of Shanghai (16ZR1408000), and the National Natural Science Foundation of China (No. 21576087). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2019.01. 006. 152
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
A large Stokes shift, sequential, colorimetric fluorescent probe for sensing Cu2+/S2- and its applications
T
Ji-Zhen Lia, Tao-Hua Lengb, , Zhi-Qiang Wangc, Li Zhoua, Xue-Qing Gongc, Yong-Jia Shena, ⁎ Cheng-Yun Wanga, ⁎
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China b National Food Quality Supervision and Inspection Center (Shanghai), Shanghai Institute of Quality Inspection and Technical Research, Shanghai 200233, PR China c Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
ARTICLE INFO
ABSTRACT
Keywords: Copper ions Sulfide Large Stokes shift Colorimetric Near-infrared DFT calculations
Copper ions (Cu2+) and sulfide (S2−) are important markers in many physiologies and pathological processes. In this work, a new near-infrared fluorescent probe 1 for colorimetric and sequential detection of Cu2+/S2− was designed and developed. The probe showed a rapid (less than 1 min), highly selective and sensitive response toward copper ions. Notably, the probe could also be applied to detect S2− through reversible formation-separation of complex 1-Cu2+ and CuS with a large Stokes shift of 234 nm. The detection limit for Cu2+ and S2− was found to be 1.8 × 10-8 M and 1.5 × 10-8 M, respectively. Furthermore, the binding stoichiometry between 1 and Cu2+ was found to be 1:1, the binding mode was also demonstrated using density functional theory (DFT) calculations and contrastive compound research. In addition, the probe was successfully applied in real water samples assay for the detection of Cu2+, and the strip papers experiments also showed that probe 1 can be used to detect Cu2+ and S2−.
1. Introduction Copper, as the third most abundant transition metal ions, is an important trace mineral element in organisms [1], plays key roles in biological and environmental fields [2]. However, unregulated amounts of copper may destroy the metabolic balance of physiological systems, and induce many health issues, such as Parkinson’s [3] and Wilson’s [4] diseases. In addition, copper is also a significant pollutant due to its widespread use in industry and agriculture. Therefore, the development of efficient methods for copper (II) detection may be very meaningful for the human health and environment. Compared with other traditional methods [5,6], fluorescent probes have attracted increasing attention on account of their high sensitivity and selectivity [7], real-time detection with rapid respond time [8] and potential biological applications [9]. To date, many fluorescence probes for copper detection have been reported over the several years [10,11]. More interestingly, among these probes, some reactive sulfur species, such as glutathione (GSH), cysthine (Cys), homocysteine (Hcy) and sulfide anion (S2−) could also be monitored via the probe-copper complex [12–16]. As a toxic substance and environmental pollutant,
⁎
S2− is mainly generated in natural gas purification, petroleum refining, sewage treatment and paper-making [17]. Sulfide anions can also combine with H+ under acidic conditions, generating more toxic H2S or HS-, which not only pollutes the atmosphere but also causes organ’s damage when exposing to high concentration of H2S [18,19]. Until now, a number of fluorescence probes for its detection have been designed and developed. [20–22]. However, most of these probes are mainly based on the organic reaction like the cleavage of sulfonate or dinitrophenyl ester [23], Michael addition [24,25], and thiolysis of the 7-nitro-1,2,3-benzoxadiazole (NBD) amine or ester bond [26,27]. These reactions are plagued by some serious limitations like synthesis requires strict conditions and relatively time-consuming procedures, which may limit their applications in analytical and biological conditions. Herein, based on the previous works reported by our group [28], we synthesized a new dicyanoisophorone-based derivative (probe 1, Scheme 1). The dicyanoisophorone moiety was chosen as a fluorophore because of its large π-conjugation system and long emission wavelength in NIR region [29], and the salicyladehyde azine was introduced as schiff base unit due to its high affinity for cations and excited state intra-molecular proton transfer phenomenon (ESIPT) characteristics
Corresponding authors. E-mail addresses: [email protected] (T.-H. Leng), [email protected] (C.-Y. Wang).
https://doi.org/10.1016/j.jphotochem.2019.01.006 Received 22 August 2018; Received in revised form 5 January 2019; Accepted 6 January 2019 Available online 11 January 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. The synthetic route of probe 1 and compound 6.
[30]. We envisioned that the probe could induce a large Stokes shift while bonding with metal ions. As expected, probe 1 displayed high sensitivity and selectivity toward Cu2+. What’s more, the complex 1Cu2+ also exhibited good selectivity and sensitivity for S2− with a distinct fluorescence enhancement and large Stokes shift of 234 nm, which was much longer than reported previously (Table S1).
2.3. Theoretical calculation The density functional theory (DFT) calculations were performed by the B3LYP method with the 6–31 G(d) basis set using the Gaussian 09 package. During the calculations, no geometry constraint or symmetry was imposed. To guarantee that all the structures studied were genuine minima on the potential energy surface, frequency calculations were performed at the same theoretical level.
2. Experiments section 2.1. Materials and instruments
3. Results and discussion
All reagents and solvents were purchased from commercial suppliers and analytical grade. The UV–vis spectra were recorded on a Varian Cary 500 spectrophotometer using a 1 cm path length quartz cell, and the fluorescence spectra were recorded on CARY Eclipse Spectrophotometer. 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on a Bruker AM-400 spectrometer in DMSO-d6 and CDCl3 with tetramethylsilane (TMS) as an internal standard. High resolution mass spectrometry data were recorded on a Waters LCT Permier XE spectrometer. The measurements of pH were done using a pH-10C digital pH meter. Melting points were obtained in melting-point tubes using an SGWX-4 apparatus.
3.1. Design and synthesis of probe Since schiff-base fluorescent probes have been widely used in metal ions detection owing to their high selectivity, sensitivity, reliability and special complexation sites toward metal ions accompanied with obvious optical signal changes [33,34], the well-designed probe 1 was synthesized by the condensation of salicyladehyde azine (4) and a dicyanoisophorone derivative (3), and the intermediate products 3 and 4 were synthesized in good yields by the methods reported in the literatures [31,32]. To explore the influence of molecular structure on the recognition of metal ions, we synthesized a contrastive compound 6. All the detailed characterization data determined by 1H and 13C NMR, HRMS were presented in supporting information (S1-S14).
2.2. Synthesis of probe 1 Synthetic route of probe 1 is shown in Scheme 1. Compound 3 and compound 4 were synthesized according to the previous works [31,32]. Compound 3 (100 mg, 0.3 mmol) and compound 4 (41 mg, 0.3 mmol) were dissolved in 10 mL EtOH, and a drop of acetic acid was added as catalyst. Then the mixture was heated to 80℃ and reacted for 12 h under N2 atmosphere. After cooling, the solution was poured into ice water (50 mL), the crude precipitate was collected by filtration and washed with water for three times. After being dried under vacuum, the crude product was further purified by silica column chromatography (petroleum ether/dichloromethane = 1/1, v/v) to give the pure product as a yellow solid (105 mg, 80% yield). Mp: 242.2–243.5 °C. 1H NMR (d6-DMSO, 600 MHz) δ: 11.46 (s, 1H, -OH), 11.09 (s, 1H, -OH), 9.03 (s, 1H, -CH = N-), 8.97 (s, 1H, -N = CH-), 8.07 (s, 1H, Ar-H), 7.80 (d, J = 8.8 Hz, 1H, Ar-H), 7.79 (d, J = 7.6 Hz, 1H, Ar-H), 7.43 (d, J = 8.4 Hz, 1H, Ar-H), 7.39-7.30 (m, 2H, −CH = CH-), 7.04 (d, J = 8.8 Hz, 1H, Ar-H), 7.02-6.96 (m, 2H, Ar-H), 6.86 (s, 1H, −CH=), 2.62 (s, 2H, −CH2-), 2.56 (s, 2H, −CH2-), 1.03 (s, 6H, -2CH3). ESIHRMS: Calc. Mass for [C27H24N4O2+H]+: 437.1978; found 437.1976.
3.2. Solvent effect on the probe 1 We initially investigated the optical behaviour of probe 1 in common organic solvents including toluene, dichlormethane, ethyl acetate, tetrahydrofuran, ethanol, acetonitrile, DMF and DMSO. The absorption and fluorescence spectra of probe 1 were shown in Fig. S15 and the corresponding data were summarized in Table S2. As depicted, the absorption maxima showed a small range of movement with the change of solvent polarity. However, fluorescence spectra showed pronounced solvent-polarity dependent changes in the difference solvents. In the toluene, there was very weak fluorescence emission could be observed. It was then found that a large fluorescence enhancement in DMSO-water (3:1, v/v) solvent (Fig. S15B). The changes of the fluorescence spectrum of probe 1 in different ratios of DMSO-water were also monitored (Fig. S15D). As a result, DMSO-water (2:1, v/v) was selected as the ideal solvent media for this work. 147
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Fig. 1. (A) UV–vis spectral changes of probe 1 (10 μm) in the presence of different concentrations of Cu2+ (0–2.5 equiv). Inset: colour changes of probe (Left) and probe-Cu2+ (Right) under daylight; (B) UV–vis spectral changes of probe 1 (10 μm) in the presence of various other metal ions (10 equiv) and Cu2+ (2.5 equiv). All the tests were carried out in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES, pH = 7.4).
3.3. Spectral studies of the probe 1 with Cu2+
Additionally, 1:1 bonding mode was confirmed by a Job’s plot analysis (Fig. S18). To examine the selectivity of probe 1 toward Cu2+, we performed the fluorescence studies of probe 1 in the presence of various other cations. As shown in Fig. 2B, no obvious changes could be observed except for Cu2+, which displayed a distinct quenching of fluorescence intensity with probe 1. Moreover, the anti-interference experiments have also been conducted by adding Cu2+ (2.5 equiv) to the solution of probe 1 containing interfering metal ions (2.5 equiv) (Fig. S19). In the presence of these competitive species, the fluorescence intensity of probe 1 could be also completely quenched by Cu2+, which demonstrated that coexistence metal ions did not cause substantial interference in Cu2+ detection.
First, the absorption and fluorescence properties of this probe with Cu2+ were investigated in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES, pH = 7.4). As shown in Fig. 1A, free probe 1 exhibited a strong absorption band at 425 nm (ε = 34856 M−1 cm-1) which was corresponding to the S0-S4 type of transition (see TD-DFT calculation results, table S3). In continuous addition of Cu2+(0–2.5 equiv), the maximum peak at 425 nm decreased gradually. At the same time, a new absorption band formed at 475 (ε = 32467 M−1 cm-1) nm and an isosbestic point appeared at 440 nm, which indicated a new stable complex between probe 1 and Cu2+ was formed. Besides, a distinct colour change from light yellow to brown could be observed by naked eye (Fig. 1A insert). Considering the coordination properties of schiff-base to metal ions such as zinc, aluminum, iron, and mercury, we performed the selective experiments of probe 1 towards other metal ions, including K+, Na+, Ag+, Zn2+, Ni2+, Pb2+, Cd2+, Mg2+, Hg2+, Fe2+, Ba2+, Ca2+, Co2+, Cr3+, Fe3+, and Al3+ (Fig. 1B). Upon the addition of 10 equiv of each cation, the probe 1 showed almost no or slightly changes in absorbance spectra. In contrast, only Cu2+ induced a remarkable red shift of absorbance spectra, indicating that probe 1 featured an excellent selectivity to Cu2+. To further study the Cu2+-responsive sensitivity of probe 1, we performed the fluorescence titration experiments. As shown in Fig. 2A, the probe displayed a long wavelength emission band at 659 nm, the addition of Cu2+ (0–25 μm) leads to a gradually decreased in fluorescence intensity. This change might be attributed to the chelation-enhanced fluorescence quenching (CHEQ) effect [35] and the paramagnetic property of Cu2+. What’s more, we also found that the fluorescence intensity of probe 1 showed a good linearity (R = 0.9946) to Cu2+ concentrations in the range of 0–10 μm. The detection limit (3σ/K) of probe 1 for Cu2+ was calculated to be 1.8 × 10−8 M (Fig. S16), which is much lower than 1.3 μM allowed by environmental protection agency (EPA), and the binding constant (K) was calculated as 1.71 × 103 M-1 by using Benesi-Hildebrand equation (Fig. S17).
3.4. pH effect studies The effect of pH ranging from 2.0 to 10.0 on the fluorescence response of probe 1 to Cu2+ was investigated in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES). As shown in Fig. 3 and Fig. S20, probe 1 displayed stable emission intensity in pH range 2.0-8.0. However, the fluorescence intensity of probe 1 could not be quenched by Cu2+ in the acid region (pH ≤ 5.0), which might be due to the protonation of the two nitrogen atoms and hydroxyl groups of probe 1, hence the ability to chelate with copper decreases. Again, the decreasing fluorescence intensity of probe 1 in the basic region (pH > 8.0) might due to the deprotonation of the two hydroxyl groups. Therefore, probe 1 can be used as a reliable copper sensor in pH 6-8. 3.5. DFT calculations To further investigate the photophysical properties of probe 1 and 1-Cu2+, DFT calculations were performed by using the Gaussian 09 program. The HOMO and LUMO orbitals of probe 1 and 1-Cu2+ were listed in Fig. 4 and Table S4. In the probe, the energy gaps between HOMO and LUMO orbitals were measured to be 2.97 eV. Besides, the α
Fig. 2. (A) Fluorescence spectra changes of probe 1 (10 μm) upon addition of Cu2+ (0–25 μm). Inset: photograph of probe 1 (Left) and probe-Cu2+ (Right) on excitation 365 nm using a UV lamp; (B) Fluorescence spectra of probe 1 (10 μm) with different metal ions (Cu2+ 25 μm, other metal ions 100 μm). λex = 425 nm. All the tests were carried out in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES, pH = 7.4).
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Fig. 3. The structure changes of probe 1 in the acidic and basic pH.
and β orbital of 1-Cu2+ were respectively 2.66 eV and 1.79 eV. These HOMO and LUMO energy gaps decrease in the 1-Cu2+ indicated that the energy required for the excitation of electron reduced as compared to the free probe, and caused a red shift in the absorption wavelength, which was consistent with the experimental results.
added, the UV–vis absorbance and fluorescence of 1-Cu2+ could be largely restored. These results demonstrated that the binding of 1-Cu2+ and S2− was chemically reversible. The fluorescence emission spectral titration experiments of 1-Cu2+ with S2− were subsequently studied. As shown in Fig. 5B, upon addition of S2− to the solution of 1-Cu2+, the emission intensity at 659 nm exhibited a remarkable increase. Fig.S22 showed a good linear relationship (R2 = 0.9953) and the detection limit was calculated as 1.5 × 10-8 M. All the spectra data were recorded within one minute after S2- was added, and the intensity became stable over an extended period of time (Fig. S23), indicating that complex 1-Cu2+ could act as a real-time and stable probe for S2- detection. To determine the selectivity of complex 1-Cu2+, we examined the UV–vis absorption and fluorescence spectra toward various other anions such as F−, Cl−, Br−, I−, CN−, AcO−, PO43-, HPO42-, H2PO4−, SCN−, SO42-, SO32-, HSO4−, HSO3−, NO3−. As shown in Fig. S24, when complex 1-Cu2+ was treated with the S2- (3 equiv), dual responses of absorbance spectra and fluorescence intensity could be obtained. Upon adding other anions (3 equiv), no significant change was observed except CN−, which caused slightly increase of fluorescence and blue shift
3.6. Spectral studies of complex 1-Cu2+ with S2− On the basis of Cu2+ has strongly binding affinities to sulfide, we envisioned that the complex 1-Cu2+ could be used as an excellent turnon probe to further detect S2−. To confirm this assumption, we examined the absorbance and fluorescence response of the complex 1Cu2+ towards S2−. As shown in Fig. 5A, 1-Cu2+ exhibited an absorption band at 475 nm. After adding 3 equiv S2−, the maximum peak was gradually blue-shifted, and the spectra eventually matched the original probe 1. Besides, the colour of solution 1-Cu2+ turned into the initial yellow, indicating the recovery of probe 1 from the 1- Cu2+ complex through the chelation of S2- with copper. The reversible behavior of S2− complexation with 1-Cu2+ was subsequently confirmed by EDTA titration. As shown in Fig. S21, when 1 equiv of EDTA solution was
Fig. 4. Frontier molecular orbital profiles of probe 1 and 1-Cu2+ based on DFT calculations.
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Fig. 5. (A) UV–vis spectral changes of 1-Cu2+ (10 μm) in the presence of different concentrations of S2− (0–30 μm.); (B) Fluorescence spectra changes of 1-Cu2+ (10 μm) upon addition of S2- (0–30 μm). λex = 425 nm. All the tests were carried out in DMSO-HEPES buffer (2:1, v/v, 20 mM HEPES, pH = 7.4).
of absorbance. Furthermore, in the presence of these anions, the red emission of probe 1 could be largely restored by subsequent addition of S2- (Fig. S25). These results obviously showed that the complex 1-Cu2+ can selectively detect S2-. The effect of pH ranging from 2.0 to 10.0 on the fluorescence response of complex 1-Cu2+ with S2− was also investigated (Fig. S26). The emission intensity was significantly enhanced when pH was above 5, which indicated that the complex 1-Cu2+ could be used for the detection of S2− in neutral and alkaline conditions.
on these three coordination bonds, Mayer bond order analysis of the probe-Cu2+ compound was conducted through the Multiwfn program [36]. The values involving Cu2+ are listed in Table S5. As is known to all, the value size of the Mayer bond order is positively correlated with the strength of the bond. If the value approximates zero, it means there is no or almost no bonding between two atoms. From the above data, we can clearly see that the Mayer bond orders of O38-Cu56, O55-Cu56, N54-Cu56 are much larger than those of N40-Cu56 and C39-Cu56 which are close to zero. Therefore, it can be concluded that these two CueO coordinate bonds and one CueN coordinate bond are the dominant bonds of the probe bonding to Cu2+. To further confirm the binding of Cu2+ through these three atoms of probe 1, we synthesized a contrastive compound 6 to mimic the binding sites of probe 1. Compound 6 has the similar structure as probe 1 in most parts except one hydroxyl group at the ortho-position of the schiff base unit was removed. As a consequence, compound 6 showed nearly no emission even in pure DMSO solution when excited at 420 nm (ε = 68,685 M−1 cm−1). In contrast, probe 1 showed a strong fluorescence at 659 nm with excitation at 425 nm. Then, we carried out the titration and selectivity experiments of compound 6 toward Cu2+. However, the fluorescence and absorbance spectra of compound 6 showed no change upon addition of Cu2+ ions (Fig. S27). From these studies, it is clearly that the two deprotonated phenolate groups and one nitrogen atom of probe 1 binding with the Cu2+. Since photo-induced electron (PET) is generally accepted as a mechanism for the “turn-on’’ fluorescent probe. To verify whether the mechanism for the detection of S2− has undergone a PET process, we calculated the HOMO–LUMOs energies of accepter and donor. As illustrated in Fig. 7 and Scheme 2, in the presence of Cu2+, the HOMO (donor, E=−5.813 eV) has higher energy than of the acceptor (E= −6.355 eV), therefore, upon excitation of complex 1-Cu2+, the electron can be transferred from the HOMO (donor) to the low-energy HOMO of the acceptor and quenches the fluorescence. Upon the addition of S2−, the Cu2+ ions are captured by S2- to produce stable CuS and release free probe 1. Meanwhile, the energy of the receptor HOMO is lowered to −5.957 eV, inhibiting the electron transfer from the HOMO donor (E= −5.957 eV) to acceptor HOMO (E= −5.303 eV), and it leads to an enhancement of the fluorescence. These results agree with the PET mechanism reported for metal complexes [37–39].
3.7. Mechanism studies With the fact that binding ratio between 1 and Cu2+ was 1:1 stoichiometry, we explained how the probe binds the Cu2+ through the DFT calculations. The optimized geometries of the probe 1 and 1-Cu2+ complex were shown in Fig. 6. As can be seen, there were two intramolecular hydrogen bonds (HBs) between the phenolic hydroxyls and the nearby nitrogen atom. In the 1-Cu2+ complex, the phenol group rotated and it combined with Cu2+ through two obvious Cu-O coordinate bonds and one Cu-N coordinate bond. In order to determine whether the binding of Cu2+ mainly depends
3.8. Reversibility of the probe 1 for Cu2+ and S2− For many practical applications, a good recyclability and reusability is necessary. As shown in Fig. S28, the reversible experiments of probe 1 was repeated 5 times by the modulation of Cu2+ and S2− addition, no obvious emission intensity attenuation at 659 nm could be observed, indicating that probe 1 could be used as a reversible ON-OFF-ON sensor for Cu2+ and S2- detection.
Fig. 6. The optimized structures of the probe and the probe-Cu2+ compound (C: grey, H: white, N: blue, O: red, Cu: pink).
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Fig. 7. S2− recognition mechanism based on photo-induced electron transfer (PET).
Considering the toxicity of Cu2+ in the environment, we examined the ability of probe 1 to quantitatively detect Cu2+ in real water samples. All the water samples were collected from laboratory tap water and river water (Qingchun River, East China University of Science and Technology, Shanghai, China), and filtered through a membrane before used. Then a given volume (5 μM, 10 μM, 15 μM, 20 μM, 25 μM) of Cu2+ aqueous solutions were added into the test solution (DMSO: H2O = 2:1, v/v). We tested the fluorescent response of probe a to all these water samples at 659 nm and used distilled water as a control experiment. As shown in Fig. 9, the results displayed a good consistency between water samples and distilled water, and the recovery of Cu2+ was between 82˜104% (Table 1). These results suggested that probe 1 has potential application for quantitative detection of Cu2+ in real water samples. 4. Conclusions Scheme 2. Proposed sensing mechanism of probe 1 for Cu2+ and S2−.
In summary, we have designed and synthesized a highly selective and sensitive fluorescent probe 1 that could detect Cu2+ and H2S sequentially and reversibly. The probe exhibited remarkable colour change from light yellow to brown and significant fluorescence quenching in the presence of Cu2+ with a detection limit of 1.8 × 10−8 M. In addition, the binding mode was also demonstrated using density functional theory (DFT) calculations and contrastive compound research. Notably, based on a complex 1-Cu2+ formation-separation process and CuS formation, the complex of 1-Cu2+ ensemble could be used as a turn-on type probe for H2S detection with a large Stokes shift of 234 nm, and the detection limit reached as low as 1.5 × 10−8 M.
3.9. Applications in real samples To examine the practical applications of probe 1, the test strips were prepared by immersing filter paper into the solutions of probe 1 and 1Cu2+, and then drying in air. As shown in Fig. 8, the filter papers written with Cu2+ and S2− solutions displayed significant changes in the colour and fluorescence intensity. What’s more, the intensity of the coloured writings also changed with the different concentrations of Cu2+.
Fig. 8. The colour and fluorescence change of the strip papers in the presence of Cu2+ (0, 10 μm, 20 μm) and S2− (0, 20 μm) under daylight (left) and UV lamp (right).
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Fig. 9. Fluorometric determination of Cu2+ in three water samples by probe 1 (10 μM) in DMSO/H2O solution (2:1, v/v). λex = 425 nm.
References
Table 1 Determination of Cu2+ concentration in water samples. Sample
Cu2+ spiked (μM)
Cu2+ recovereda (μM)
Recovery (%)
Qingchun River
5 10 15 20 25 5 10 15 20 25
4.135 ± 0.012 10.332 ± 0.009 15.236 ± 0.007 20.488 ± 0.025 24.898 ± 0.002 4.612 ± 0.002 10.103 ± 0.01 15.233 ± 0.004 20.775 ± 0.05 24.277 ± 0.045
82.7 103.3 101.5 102.4 99.5 92.2 101.0 101.5 104.3 97.1
Tap water
a
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