English

• 1.   Introduction

  According to National Institute for Occupational Safety and Health (NIOSH), the concentration of H₂S immediately dangerous to life or health (IDLH) is 100 mg/L, and the recommended exposure limit (REL) is 10 mg/L for a maximum duration of 10 min.^[1] Nonetheless, H₂S is recognized as three important physical signal gas along with carbon monoxide (CO) and nitric oxide (NO) by scientists, ^[2-4] which is very important to the regulation of physiology and pathology. In the central nervous system, the biological concentration of H₂S is in the range of 50~160 μmol/L, and the sulfide level in plasma blood is in the range of 10~100 μmol/L.^[5-6] Some common diseases are related to the disorder of regulation of endogenous H₂S, such as Alzheimer's disease, Down syndrome, diabetes mellitus and cirrhosis.^[7-9] Therefore, an efficient method for detecting H₂S sensitively and selectively is highly anticipated.

  The traditional techniques used for the detection of H₂S include electrochemical, ^[10] gas chromatography, ^[11] and sulfide precipitation methods.^[12] These methods often require complicated sample preparation and tissue or cell destruction.^[13-14] Up to now, the fluorescent probe technique has drawn great attentions due to their simplicity, high sensitivity as well as real-time detection.^[15-17] Several fluorescent probes for H₂S have been reported with different reaction mechanisms to trap H₂S, such as azide reduction, ^[18-19] copper sulfide precipitation^[20] and/or nucleophilic addition.^[21-22] Compared with other fluorescent probes, the long-wavelength fluorescent probes (λ_em＝600~900 nm) have shown excellent performance and become a promising tool in biological imaging applications as a result of its unique properties, such as deeper tissues penetration, ^[23-24] less injure to biological samples^[25-26] and lower interference from background auto-fluorescence in living systems.^[27-32] In our common fluorophores, rhodamine is an ideal mode to construct fluorescent enhancement probes due to the unique OFF-ON mechanism based on the spiro-ring and excellent structural property, such as better light stability, higher sensitivity and higher fluorescence quantum yield.^[33-34] Unfortunately, rhodamine has a shorter wavelength of emission (λ_em < 600 nm), ^[35-36] it is necessary to modify rhodamine to increase the emission wavelength for the detection of active species in biological systems.

  Herein, a novel long-wavelength H₂S fluorescent probe SR based on seminaphthorhodafluor dye was successfully designed and synthesized. The spiro-ring of probe SR was opened and the fluorophore was appeared by the thiolysis reaction on dinitrophenyl ether.^{[37, 38]} The synthetic route was shown in Scheme 1, and the structure was confirmed by ¹H NMR and HRMS spectra.

  2.   Results and discussion

  2.1   Response time study of probe SR for H₂S detection

  To gain the appropriate time for the probe to detect H₂S, the relationship between the fluorescent intensity and various reaction times was explored. As shown in Figure 1, the fluorescence intensity/absorbance was enhanced gradually as time went on, and the fluorescence intensity/absorbance did not change after 5 min. It indicated that probe SR had a short response time to H₂S and could be used for real-time detection of H₂S.

  Figure 1

  

  Figure 1.  Response time study of probe SR for H₂S detection at 635 nm for solutions of probe SR (10 μmol/L) following addition of H₂S (20 μmol/L) in PBS (20.0 mmol/L, pH 7.4, 5 % CH₃CH₂OH) solution (λ_ex＝550 nm)

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  Scheme 1

  

  Scheme 1.  Synthetic route of probe SR

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  2.2   Properties of probe SR for H₂S detection

  In order to examine the recognition properties of probe SR for H₂S, the titration experiments were conducted. The fluorescence and absorption spectra of probe SR with gradual concentrations of H₂S (0~30 μmol/L) were recorded in Figures 2(a) and 2(b). The probe showed weak fluorescent intensity and absorption in solution without H₂S. After probe SR was treated with H₂S, owning to the opening of the spiro-ring of seminaphthorhodafluor in the presence of H₂S with the elimination of 2, 4-dinitrobenzene group, an obviously fluorescence emission peak was emerged at 635 nm (absorbance increased at 590 nm).

  Figure 2

  

  Figure 2.  (a) Fluorescence spectra of probe SR (10 μmol/L) with different concentrations of H₂S (0~30 μmol/L) in PBS (20.0 mmol/L, pH 7.4, 5% CH₃CH₂OH) solution (λ_ex＝550 nm), and (b) absorbance spectra of probe SR (10 μmol/L) with different concentrations of H₂S (0~30 μmol/L) (λ_ex＝550 nm)

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  To investigate the linear relationship between fluorescence intensity/absorption and the concentration of H₂S, fluorescence intensity and absorption were processed by using the principle of least squares. The linear equation between the fluorescence intensity and H₂S concentration was as y＝17.59x+59.21 (R²＝0.991), as shown in Figure 3(a). The linear equation between the absorbance and H₂S concentration was as y＝0.0123x+0.0754 (R²＝0.9842), which was shown in Figure 3(b). These gave a satisfying working curve with a linear range of 2~10 μmol/L and the detection limit was 0.50/1.00 μmol/L. Taken together, the results indicated that probe SR could detect H₂S quantitatively with high sensitivity in simulated organisms.

  Figure 3

  

  Figure 3.  (a) Fluorescence intensity at 635 nm of probe SR (10 μmol/L) with different concentrations of H₂S (2~10 μmol/L) in PBS (20.0 mmol/L, pH 7.4, 5% CH₃CH₂OH) solution (λ_ex＝550 nm), and (b) absorption at 590 nm of probe SR (10 μmol/L) with different concentrations of H₂S (2~10 μmol/L)

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  2.3   Selectivity and competition studies of probe SR

  Selectivity is an important index to detect the performance of the fluorescent probe, in order to validate the selectivity of probe in practice, various biologically relevant species were tested, such as biothiols (Cys, Lys, GSH), anions (${\rm{NO}}_{\rm{3}}^ - $, CN^－, I^－, Br^－, Cl^－, ${\rm{HCO}}_{\rm{3}}^ - $ SCN^－, ${\rm{SO}}_{\rm{4}}^{2 - }$, ${{\rm{S}}_{\rm{2}}}{\rm{O}}_{\rm{3}}^{2 - }$, ${\rm{SO}}_{\rm{3}}^{2 - }$), metal ions (Zn²⁺, Ca²⁺, Mg⁺, K⁺, Na⁺), reactive oxygen and nitrogen species (NO, ${\rm{O}}_{\rm{2}}^ - $, H₂O₂, OCl^－). As shown in Figure 4, the fluorescence intensity/absorption responses of the probe to various species was shown in the first row, and the response of the probe to H₂S under the coexistence of H₂S and other species was shown in the second row. Only the addition of H₂S induced obvious optical changes while addition of other species did not lead to any significant optical enhancement even at high concentrations. When various species (above mentioned) existed alone, the probe had basically no response to but had a good response to H₂S. When H₂S was coexisted with the above species, the response of probe SR to H₂S was not significantly affected. The results showed that probe SR had an excellent anti-interference ability and could be used to detect H₂S in complex systems.

  Figure 4

  

  Figure 4.  Fluorescence intensity (a) and absorption (b) of probe SR detects various species in PBS (20.0 mmol/L, pH 7.4, 5% CH₃CH₂OH) solution

  In the first row: probe SR+H₂S, Cys, Lys, GSH, NO, H₂O₂, OCl^－, ${\rm{O}}_{\rm{2}}^ - $, SCN^－, ${\rm{SO}}_{\rm{4}}^{2 - }$, ${{\rm{S}}_{\rm{2}}}{\rm{O}}_{\rm{3}}^{2 - }$, ${\rm{SO}}_{\rm{3}}^{2 - }$, Zn²⁺, Ca²⁺, Mg⁺, K⁺, Na⁺, ${\rm{NO}}_{\rm{3}}^ - $, CN^－, I^－, Br^－, Cl^－, ${\rm{HCO}}_{\rm{3}}^ - $. In the second row: probe SR+H₂S+other ions (Cys, Lys, GSH, NO, H₂O₂, OCl^－, ${\rm{O}}_{\rm{2}}^ - $, SCN^－, ${\rm{SO}}_{\rm{4}}^{2 - }$, ${{\rm{S}}_{\rm{2}}}{\rm{O}}_{\rm{3}}^{2 - }$, ${\rm{SO}}_{\rm{3}}^{2 - }$, Zn²⁺, Ca²⁺, Mg⁺, K⁺, Na⁺, ${\rm{NO}}_{\rm{3}}^ - $, CN^－, I^－, Br^－, Cl^－, ${\rm{HCO}}_{\rm{3}}^ - $). λ_ex＝550 nm

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  2.4   Mechanism study of probe SR

  In order to analyze the interaction mechanism between probe SR and H₂S, probe SR (0.012 g, 0.02 mmol) was dissolved in 5 mL of ethanol, and H₂S (0.50 mmol) was added to the solution. The mixture was stirred at 25 ℃ for 1 h and concentrated under vacuum, then the crude product was purified by silica gel column chromatography solid separation (methanol/dichloromethane, V:V＝1:30) to obtain a magenta solid compound (0.004 g, 0.01 mmol, 50%). The main thiolysis product was subjected to HRMS, which was shown in Figure 5(b).

  Figure 5

  

  Figure 5.  HRMS of compound 2 (a) and the thiolysis product of probe SR (b)

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  As shown in Figure 5, HRMS of the resulting product (a peak of 438.1767) was consistent with compound 2 (HRMS calcd for C₂₈H₂₄NO₄ 438.1705, found 438.1715). This result further confirmed that the sulfur hydrolysis of H₂S resulted in the removal of 2, 4-dinitrobenzene of probe SR (Scheme 2). Then the spiro-ring of seminaphthorhodafluor was open and the fluorophore of compound 3 was appeared.

  2.5   Detection of H₂S with filter papers

  To expand the practical application of fluorescent probe SR, filter papers as sensors was made to detect H₂S in real water samples. As shown in Figure 6, when the filter papers with probe SR detected H₂S at different concentrations, the concentrations of H₂S were as following: 0, 1, 10, 10², 10³, 10⁴ μmol/L. The color of the filter papers changed from blue to red under the 365 nm ultraviolet lamp. It indicated that filter papers with probe SR showed simple and visual response to H₂S in real water samples.

  Figure 6

  

  Figure 6.  Detection of H₂S with filter papers under the 365 nm ultraviolet lamp

  The concentrations of H₂S from left to right were showed as follows: 0, 1, 10, 10², 10³, 10⁴ μmol/L

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  2.6   Detection of H₂S with hydrogels

  As far as we know, there are few reports of fluorescent gels being used as sensors to detect active species. In order to explore the application of probe hydrogel in the field of real-time environmental monitoring, the fluorescent hydrogels were prepared for the detection of H₂S. As shown in Figure 7, when the hydrogel with probe SR was in contact with H₂S solution, the color of the hydrogels changed from colorless to red under natural light, which was shown in Figure 7(a). The fluorescence changed from weak fluorescence to red under the 365 nm ultraviolet lamp, which was shown in Figure 7(b). The results indicated that the formation of intelligent probe hydrogel has a rapid and visual reaction to H₂S. The fluorescent hydrogels could be processed into a novel H₂S sensing film, and using this film as the core to make an H₂S detection device, which is expected to realize the detection of H₂S leakage in chemical plants.

  Figure 7

  

  Figure 7.  (a) Detection of H₂S with hydrogels under natural light. (b) Detection of H₂S with hydrogels under the 365 nm ultraviolet lamp

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  Scheme 2

  

  Scheme 2.  Proposed response mechanism for H₂S

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  2.7   Visualization of H₂S in Hela cells

  Inspired by these desired fluorescence properties, the bioimaging applications of probe SR for H₂S detection in biological systems were studied (Figure 8). Hela cells were pretreated with 20.0 μmol/L probe SR for 0.5 h and then incubated with 40.0 μmol/L H₂S in the same culture medium. The cells untreated with H₂S showed no fluorescence while the cells treated with H₂S displayed strong fluorescence by contrast. The preliminary experiments in Hela cells demonstrated that the probes were well suited for in vivo imaging of H₂S in biological systems.

  Figure 8

  

  Figure 8.  Fluorescent images of Hela cells incubated with 20.0 μmol/L probe SR (a, b) and then further incubated with 40.0 μmol/L H₂S for 0.5 h (c, d)

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  3.   Conclusions

  In the experiments, a novel long-wavelength H₂S fluorescent probe based on the thiolysis reaction on dinitrophenyl ether was designed and synthesized. The structure of probe and intermediates were characterized by ¹H NMR and HRMS spectra. The fluorescence intensity/absorption results confirmed that the probe could be used for the quantitative detection of H₂S with high selectivity and sensitivity. Moreover, the probe had been used in filter paper and hydrogel for the rapid and convenient detection of H₂S. It provided a potential tool for biological ion tracking and real-time environmental monitoring.

  4.   Experimental section

  4.1   Materials and instruments

  All chemicals were obtained from commercial suppliers and used without further purification. NaHS used as a hydrogen sulfide source in all experiments was dissolved in water. ¹H NMR spectra were recorded on a Varian INOVA-400 MHz spectrometer using tetramethylsilane (TMS) as an internal standard. Mass spectra were measured by electron spray ionization (ESI) using a Brukermicro TOF-QIIESI-Q-TOF LC/MS/MS spectrometer. Fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer equipped with a xenon discharge lamp. UV-vis absorption spectra were measured in 1.0 cm path length quartz cuvettes on a Shimadzu UV-1700 spectrophotometer.

  4.2   Synthesis of probe SR

  Compound 2 were synthesized according to the literature.^{[39] 1}H NMR(Chloroform-d₆, 400 MHz) δ: 1.16 (t, J＝7.0 Hz, 6H), 3.33 (q, J＝7.2 Hz, 4H), 6.44 (dd, J＝9.3, 2.5 Hz, 1H), 6.58 (d, J＝2.6 Hz, 1H), 6.72 (d, J＝8.8 Hz, 1H), 6.87 (d, J＝9.0 Hz, 1H), 6.95 (d, J＝2.4 Hz, 1H), 7.01 (d, J＝8.8 Hz, 1H), 7.09 (dd, J＝9.1, 2.4 Hz, 1H), 7.10~7.15 (m, 1H), 7.62 (tt, J＝7.3, 5.8 Hz, 2H), 8.16~8.18 (m, 1H), 8.25 (d, J＝9.0 Hz, 1H). HRMS (ESI) calcd for C₂₈H₂₄NO₄ 438.1700, found 438.1715.

  Compound 2 (0.438 g, 1.00 mmol), 2, 4-dinitrobromo- benzene (0.246 g, 1.00 mmol) and potassium carbonate (0.350 g) were dissolved in 10 mL of dry N, N-dimethyl- formamide (DMF), and the mixture was stirred at 90 ℃ for 4 h. After cooling to ambient temperature, the magenta precipitate was obtained. The precipitate was filtered and added to water (50 mL). The crude product was purified by silica gel column chromatography solid separation (methanol/dichloromethane, V:V＝1:250) to afford 0.244 g of SR (9-(2, 4-dinitrophenoxy)-N, N-diethylrhodol), 40.40% yield. orange solid, m.p. 192.8 ℃; ¹H NMR (Chloroform-d, 400 MHz) δ: 1.26 (t, J＝7.0 Hz, 6H), 3.45 (q, J＝7.1 Hz, 4H), 6.51 (d, J＝8.7 Hz, 1H), 6.71 (d, J＝8.7 Hz, 2H), 6.86 (d, J＝8.6 Hz, 1H), 7.15 (d, J＝9.2 Hz, 1H), 7.20~7.22 (m, 1H), 7.42~7.46 (m, 2H), 7.53 (d, J＝2.3 Hz, 1H), 7.66 (td, J＝7.2, 1.4 Hz, 2H), 8.08~8.10 (m, 1H), 8.36 (dd, J＝9.3, 2.7 Hz, 1H), 8.78 (d, J＝9.1 Hz, 1H), 8.91 (d, J＝2.7 Hz, 1H); ¹³C NMR (Chloroform-d, 100 MHz) δ: 169.5, 155.8, 153.4, 152.9, 152.3, 147.6, 141.9, 139.9, 135.4, 134.9, 129.7, 129.0, 128.9, 127.1, 126.1, 126.0, 125.0, 124.0, 122.3, 122.2, 122.1, 119.7, 119.12, 117.1, 113.6, 44.56, 12.53. HRMS calcd for C₃₄H₂₆N₃O₈ [M]⁺ 604.1714, found 604.1715.

  4.3   Preparation of probe solution

  The stock solution (10 μmol/L) of probe SR was prepared by dissolving directly in the ethanol, the various test species stock solution (20 μmol/L) of HCO₃^－, Cl^－, Br^－, I^－, CN^－, NO₃^－, SO₃^2－, S₂O₃^2－, SO₄^2－, SCN^－, O₂^－, OCl^-, Zn²⁺, Mg⁺, Ca²⁺, Na⁺, K⁺, Cys, Lys, GSH, NO and H₂O₂ were prepared in distilled water. In the titration experiments, 0.5 mL of 200 μmol/L probe SR and different volumes of H₂S solution were filled to 10 mL with PBS (20.0 mmol/L, pH 7.4, 5% CH₃CH₂OH) solution. In the interference experiment, 0.5 mL of 200 μmol/L H₂S, 1 mL of 200 μmol/L testing species and 0.5 mL of probe SR (200 μmol/L) were mixed and filled to 10 mL with PBS (20.0 mmol/L, pH 7.4, 5% CH₃CH₂OH) solution. The fluorescence spectra of each solution were measured after 5 min. The emission spectra were obtained by using the excitation wavelength of 550 nm, and both excitation and emission have a slit width of 5 nm.

  4.4   Detection of H₂S with filter papers

  A filter paper was cut to 1 cm×6 cm and immersed in the solution of probe SR (10 μmol/L, 25 mL). The filter papers were taken out from the solution, dried in air and then immersed in different H₂S concentrations. The concentrations of H₂S were as following: 1, 10, 10², 10³, 10⁴ mol/L. After 10 min, the color changes of filter papers were observed.

  4.5   Detection of H₂S with hydrogels

  According to the literature, ^[40] acrylic acid was selected as monomer, Pluronic F127 diacrylate (F127DA) as macro-cross-linkers and α-ketoglutaric acid as photoinitiator to form polyacrylate hydrogels under ultraviolet light. Two acrylic hydrogels, named hydrogel A and hydrogel B, were prepared. Then the hydrogels soaked in the probe solution (10 μmol/L) until saturated. Hydrogel B taken out from the solution soaked in H₂S solution (10 μmol/L) for comparing with hydrogel A. After 20 min, the color changes of hydrogels A and B were observed.

  Supporting Information Characterization spectra (HRMS, ¹HNMR and ¹³CNMR) of compounds 1~2 and probe SR. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/.

  
  
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  Figure 1  Response time study of probe SR for H₂S detection at 635 nm for solutions of probe SR (10 μmol/L) following addition of H₂S (20 μmol/L) in PBS (20.0 mmol/L, pH 7.4, 5 % CH₃CH₂OH) solution (λ_ex＝550 nm)

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  Scheme 1  Synthetic route of probe SR

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  Figure 2  (a) Fluorescence spectra of probe SR (10 μmol/L) with different concentrations of H₂S (0~30 μmol/L) in PBS (20.0 mmol/L, pH 7.4, 5% CH₃CH₂OH) solution (λ_ex＝550 nm), and (b) absorbance spectra of probe SR (10 μmol/L) with different concentrations of H₂S (0~30 μmol/L) (λ_ex＝550 nm)

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  Figure 3  (a) Fluorescence intensity at 635 nm of probe SR (10 μmol/L) with different concentrations of H₂S (2~10 μmol/L) in PBS (20.0 mmol/L, pH 7.4, 5% CH₃CH₂OH) solution (λ_ex＝550 nm), and (b) absorption at 590 nm of probe SR (10 μmol/L) with different concentrations of H₂S (2~10 μmol/L)

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  Figure 4  Fluorescence intensity (a) and absorption (b) of probe SR detects various species in PBS (20.0 mmol/L, pH 7.4, 5% CH₃CH₂OH) solution

  In the first row: probe SR+H₂S, Cys, Lys, GSH, NO, H₂O₂, OCl^－, ${\rm{O}}_{\rm{2}}^ - $, SCN^－, ${\rm{SO}}_{\rm{4}}^{2 - }$, ${{\rm{S}}_{\rm{2}}}{\rm{O}}_{\rm{3}}^{2 - }$, ${\rm{SO}}_{\rm{3}}^{2 - }$, Zn²⁺, Ca²⁺, Mg⁺, K⁺, Na⁺, ${\rm{NO}}_{\rm{3}}^ - $, CN^－, I^－, Br^－, Cl^－, ${\rm{HCO}}_{\rm{3}}^ - $. In the second row: probe SR+H₂S+other ions (Cys, Lys, GSH, NO, H₂O₂, OCl^－, ${\rm{O}}_{\rm{2}}^ - $, SCN^－, ${\rm{SO}}_{\rm{4}}^{2 - }$, ${{\rm{S}}_{\rm{2}}}{\rm{O}}_{\rm{3}}^{2 - }$, ${\rm{SO}}_{\rm{3}}^{2 - }$, Zn²⁺, Ca²⁺, Mg⁺, K⁺, Na⁺, ${\rm{NO}}_{\rm{3}}^ - $, CN^－, I^－, Br^－, Cl^－, ${\rm{HCO}}_{\rm{3}}^ - $). λ_ex＝550 nm

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  Figure 5  HRMS of compound 2 (a) and the thiolysis product of probe SR (b)

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  Figure 6  Detection of H₂S with filter papers under the 365 nm ultraviolet lamp

  The concentrations of H₂S from left to right were showed as follows: 0, 1, 10, 10², 10³, 10⁴ μmol/L

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  Figure 7  (a) Detection of H₂S with hydrogels under natural light. (b) Detection of H₂S with hydrogels under the 365 nm ultraviolet lamp

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  Scheme 2  Proposed response mechanism for H₂S

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  Figure 8  Fluorescent images of Hela cells incubated with 20.0 μmol/L probe SR (a, b) and then further incubated with 40.0 μmol/L H₂S for 0.5 h (c, d)

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