A Fluorescent Chemodosimeter for Live-Cell Monitoring of Aqueous

Dec 21, 2015 - Aqueous sulfides are emerging signaling agents implicated in various pathological and physiological processes. The development of sensi...
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A Fluorescent Chemodosimeter for Live-Cell Monitoring of Aqueous Sulfides Shiguo Wang,† Suying Xu,*,† Gaofei Hu,† Xilin Bai,† Tony D. James,‡ and Leyu Wang*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, Beijing University of Chemical Technology, Beijing 100029, P. R. China ‡ Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, United Kingdom S Supporting Information *

ABSTRACT: Aqueous sulfides are emerging signaling agents implicated in various pathological and physiological processes. The development of sensitive and selective methods for the sensing of these sulfides is therefore very important. Herein, we report that the as-synthesized 1-oxo-1H-phenalene-2,3-dicarbonitrile (OPD) compound provides promising fluorescent properties and unique reactive properties toward aqueous sulfides. It was found that OPD showed high selectivity and sensitivity toward Na2S over thiols and other inorganic sulfur compounds through a sulfide involved reaction which was confirmed by high-resolution mass spectroscopy (HRMS) and nuclear magnetic resonance (NMR) results. The fluorescence intensity increases linearly with sulfide concentration in the range of 1.0−30 μM with a limit of detection of 52 nM. This novel fluorescent probe was further exploited for the fluorescence imaging sensing of aqueous sulfide in HeLa cells.

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spatial resolution.24,25 On the basis of reductive properties and the metal sulfide complexation features of aqueous sulfide, many fluorescent probes26−36 were fabricated by means of sulfide-involved reactions. Ma group’s designed a ratiometric fluorescence probe for H2S by incorporating an azido group into cresyl violet.37 The presence of sulfide could reduce the electron-withdrawing azido group to electron-donating amine group, which could significantly affect the overall electron density distribution, leading to a spectroscopic alteration. Yang group constructed a target and location dual-controlled probe, which selectively display fluorescence response to H2S only under lysosomal pH environment.38 Alternatively, the strong nucleophilicity properties of sulfide were also extensively explored. Sulfide involved nucleophilic substitution24,39,40 and Michael addition29,41 were frequently utilized in designing sulfide probes. Another important feature of sulfide is that it is easily formed precipitation or complex with metal cations. On the basis of such properties, a series of H2S sensors by

uminescence probes including upconversion nanoparticles (UCNPs),1−4 quantum dots (QDs),5−8 and organic dyes (ODs)9−13 have been widely investigated and utilized for biochemical sensing, tumor imaging,14 and cell imaging. Recently, the fluorescence sensing and imaging of some typical disease related agents such as H2O2, dopamine, and H2S has drawn increasing interests.15−21 Hydrogen sulfide (H2S), which is highly water-soluble exists as diprotonated (H2S), monoanion (HS−), and dianion (S2−) forms and has long been considered a toxic environmental pollutant.22 Meanwhile, recent findings have revealed that aqueous sulfides are emerging as signaling agents implicated in various pathological and physiological processes including regulation of inflammation, protection against ischemia/reperfusion injury, inhibition of insulin signaling, mediation of neurotransmission, apoptosis and oxygen sensing.23 Thus, abnormal H2S levels are important indicators for central nervous system diseases and therefore, real-time monitoring of H2S fluctuation is desirable. At this point, fluorescent sensing and imaging technique have been considered as one of the promising strategies for quantification of aqueous sulfides and providing an understanding of the metabolic processes owing to its high sensitivity and high © XXXX American Chemical Society

Received: November 5, 2015 Accepted: December 21, 2015

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DOI: 10.1021/acs.analchem.5b04194 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry employing copper containing complex42,43 or metal−organic framework44 have been developed. To date, despite many different methods being developed for H2S recognition, exploration of simple new facile and selective strategies is still desirable. As a novel fluorescent molecule with excellent spectroscopic and chemical properties, 1-oxo-1H-phenalene-2,3-dicarbonitrile (OPD), whose structure has been revised recently,32,45,46 has been found to be susceptible to be attacked by nucleophiles such as thiols, primary and secondary amines through so-called nucleophilic aromatic substitution of hydrogen (SNArH) reactions.47 Herein, our findings indicated that the OPD compound has a pronounced response toward sodium sulfide and were demonstrated to be able to selectively and sensitively recognize sulfide through a non-SNArH reaction. It was found that addition of sodium sulfide would induce immediate turnon fluorescence, accompanied by color changes from bright yellow to pink (Scheme 1). The resultant species were carefully

Characterization. 1-Oxo-1H-phenalene-2,3-dicarbonitrile (OPD) was synthesized following previously reported work,46 which was purified by chromatography and confirmed by nuclear magnetic resonance (NMR, Figure S11) and mass spectroscopy (MS, Figure S12) (corresponding spectra were presented in Supporting Information). 1H NMR (600 MHz, DMSO-d6): δ: 8.72 (1H, d, J = 8.0 Hz), 8.68 (1H, d, J = 7.4 Hz), 8.64 (1H, d, J = 8.1 Hz), 8.43 (1H, d, J = 7.3 Hz), 8.07 (1H, t, J = 7.7 Hz), 7.98 (1H, t, J = 7.7 Hz). 13C NMR (150 MHz, DMSO-d6): δ = 177.6, 138.3, 137.7, 134.4, 132.7, 131.8, 131.4, 129.0, 128.0, 127.4, 126.1, 122.3, 119.8, 114.0, 113.5. HRMS (ESI+; solvent CH3CN) ESI-MS m/z: [M + H]+ calcd for [C15H7N2O]+ 231.0558; measured, 231.0553. The resulting product produced by addition of Na2S into OPD was confirmed both by 1H NMR and 13C NMR and then by HRMS (Figures S13 and S8). The volume of 50 μL of Na2S D2O solution (0.15 M) was added into 950 μL of OPD solution in DMSO-d6 (2.5 mM). 1H NMR (600 MHz, DMSOd6): δ 8.21 (2H, dd, Jo = 7.2 Hz, Jm = 1.1 Hz), 8.12 (2H, dd, Jo = 8.2 Hz, Jm = 1.1 Hz), 7.65 (2H, dd, Jo1 = 7.2 Hz, Jo2 = 8.2 Hz). 13 C NMR (150 MHz, DMSO-d6): δ = 179.8, 131.7, 131.5, 129.6, 127.7, 126.1, 126.0, 121.0. DEPT-135 NMR (150 MHz, DMSO-d6): δ = 131.6 (CH), 126.1 (CH), 126.0 (CH), the other disappeared peaks compared to the 13C spectrum are assigned to quaternary carbon signals. HRMS (ESI−, solvent CH3CN) ESI-MS m/z: [M − H]−, [C15H5N2O2]− calcd for 245.0351, measured 245.0373. General Procedures for Fluorescence and UV−Visible Measurements. UV−vis absorption studies were conducted in a mixture solution of DMSO/H2O (70:30, v/v). A volume of 2.0 mL of the solution containing 50 μM OPD was first introduced to a quartz cell, following additions of 10 μL of Na2S stock solution (30 mM). The kinetic investigations were carried out by measuring both absorption and fluorescence intensities of the resulting mixture at different time intervals. The fluorescence intensities were recorded with an excitation wavelength of 465 nm and emission wavelength ranging from 470 to 750 nm. The responses of interferences were performed by adding 3 equiv of different anions and thiols (150 μM) to each sample. It was noted that optical responses in the presence of 5 mM of GSH were also observed. Tris buffers with different pH values were chosen for investigation of the pH effect. The limit of detection (LOD) was determined by adding sulfides over a series of concentrations into OPD samples. Cell Fluorescence Imaging. HeLa cells were cultured in Dulbecco’s modified eagle medium (DMEM) with 10% of heat-inactivated fetal bovine serum (FBS) and 100 μg/mL of penicillin/streptomycin overnight at 37 °C under a 5% CO2 and 95% relative humidity atmosphere. A final number of 2 × 104 cells were seeded into each well in a 96-well plate, which were divided into two groups: one group was incubated with OPD probe in the presence of Na2S and the other group was used as the control group, which was incubated merely with OPD probe under identical conditions. A final concentration of 10 μM OPD was added into wells, which were incubated for 30 min then the original media was discarded and filled with fresh media. Afterward, Na2S solution with a final concentration of 30 μM was added and incubated for 1 h prior to cell imaging. Confocal laser scanning microscopy images were acquired on a TCS SP5 confocal microscopes system (Leica) with an excitation wavelength at 543 nm.

Scheme 1. Schematic Illustration of the Reaction of OPD Probe with Na2Sa

Inset shows the color changes of OPD probe (50 μM) before and after addition of Na2S (150 μM) in DMSO−water (70:30, v:v). a

investigated and confirmed by nuclear magnetic resonance (NMR) and high-resolution mass spectroscopy (HRMS) techniques. The OPD probe showed high selectivity toward Na2S over thiols and other anions with a limit of detection (LOD) of 52 nM in the linear range of 1.0−30 μM. Additionally, we successfully demonstrated the applicability of OPD for fluorescence imaging and sensing of Na2S in living cells.

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EXPERIMENTAL SECTION Reagents and Chemicals. Na2HPO4·12H2O, NaH2PO4· 2H2O, NaOH, CuSO4·5H2O, NaF, Na3PO4, NaOAc, Na2CO3, NaHCO3, NaCl, KBr, KI, Na2SO4, KHSO4, Na2SO3, Na2S2O3, NaNO2, NaNO3, NaIO4, Na2S·9H2O, HCl, glutathione (GSH), L-cysteine, ethanol, methanol, acetonitrile, dichloromethane (DCM), and 2-amino-2-(hydroxymethyl)-1,3-propanediol (Tris, (HOCH2)3CNH2) were supplied by Beijing Chemical Factory. All chemicals were at least of analytical grade and used as supply without further purification. Instrumentation. 1H and 13C NMR spectra were recorded with a Bruker AV600 NMR spectrometer (1H frequency is 600.13 MHz). Fluorescence spectra were conducted in quartz cuvettes on a Hitachi F-4600 fluorescence spectrophotometer at room temperature. The absorption measurements were carried out on a Shimadzu UV-3600 spectrophotometer with a spectral window range of 190−3600 nm. All cell imaging experiments were conducted on a TCS SP5 confocal microscopes system (Leica) with excitation wavelength at 543 nm. B

DOI: 10.1021/acs.analchem.5b04194 Anal. Chem. XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Spectroscopic Studies of OPD as A Na2S Fluorescent Probe. The absorption spectra and fluorescence studies of OPD were initially recorded in a mixture solution (DMSO/ water = 70:30). The free OPD displays an absorption centered at 440 nm (Figure 1A); however, upon addition with Na2S, two

Encouraged by these observations, the effect of buffer and pH on the absorption and fluorescence properties of OPD treated with Na2S were then measured by recording the intensity changes. It was found that OPD in DMSO−Tris buffer resulted in the maximum fluorescent intensity (Figure S3). Therefore, both the UV−vis absorption and fluorescence changes of OPD toward Na2S were investigated in Tris buffer (containing 70% DMSO). With respect to the pH effect, the OPD was stable under different pH values; whereas, after addition of Na2S, both the absorption and fluorescence varied a lot when the pH is lower than 4.5, indicating the resulting species are sensitive to acidic conditions (Figure S4). Therefore, in view of the potential biological applications of OPD, all the experiments were performed at pH 7.4. LOD and Linear Range. The capability of OPD for recognizing aqueous sulfide was investigated by monitoring fluorescent intensities after adding various concentrations of S2−. Under optimal conditions, the fluorescence intensities increased linearly with the concentration of Na2S (Figure 2). A linear range from 1.0 to 30 μM was obtained with a limit of detection of 52 nM (R2 = 0.9903), indicating that OPD has high sensitivity toward aqueous sulfide. To illustrate the good selectivity of this newly developed system, a series of anions and thiols were evaluated. Under

Figure 1. (A) Evolution of absorption spectra after addition of Na2S; (B) absorption changes along with time at 531 and 660 nm. The concentrations of OPD and Na2S were 50 μM and 150 μM, respectively. Absorption spectra in part A were recorded in DMSO− water (7:3, v/v).

new absorption bands centered at 531 and 660 nm (Figure 1A) appeared corresponding to an apparent color change from yellow-green to orange (Scheme 1 inset). The absorption band at 531 nm gradually increased and reached a plateau after 1 h (Figure 1B). Notably, the absorption at 660 nm increased quickly, and subsequently decreased over extended periods of time. Additionally, these changes were accompanied by a color change from yellow to pale green, then to pink (Figure S1). Consistently, free OPD probe has a very weak fluorescence at 505 nm and after being treated with Na2S, a red-shift to 571 nm and significant enhancement of fluorescence was observed (Figure S2). Therefore, we hypothesized that the addition of Na2S into OPD solution results in the rapid formation of strong fluorescent product, which makes OPD a potential fluorescent probe for aqueous sulfide.

Figure 2. (A) Fluorescence spectral changes of OPD (10 μM) in the presence of different concentrations of Na2S (0−500 μM) in DMSO− Tris buffer (7:3, v/v, pH 7.4, 20 mM Tris buffer). Excitation, 465 nm; emission, 571 nm. (B) Fluorescence intensities at 571 nm with various concentrations of Na2S and the linear range. C

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Analytical Chemistry identical conditions, addition of 3 equiv of F−, PO43−, HPO42−, H2PO4−, Ac−, OH−, CO32−, HCO3−, Cl−, Br−, I−, SO42−, HSO4−, SO32−, S2O32−, NO2−, NO3−, and IO4− produced negligible fluorescence as compared with that of aqueous sulfide (Figure 3). As for the commonly encountered thiols,

Figure 4. 1H NMR spectra of OPD probe (black) and the product (green).

substituted by HS− groups, leaving the other three on the skeleton (possible reaction routes were shown in Figure S7, route 1). Such observation leads us to explore a new reaction route other than the SNArH reaction. The HS− group might alternatively proceed under a completely different reaction, after which the resulting adduct has a symmetric structure (Figure S7, route 2). Such assumption was further demonstrated by the high-resolution mass spectrometry (HRMS) results (Figure S8) which showed that the adduct compound has a molecular weight of 245.0373, consistent with the product obtained by possible reaction route 2. Though this assumption was implicated using the NMR as well as HRMS results, it was quite hard to understand why the resulting adduct did not contain sulfur atoms after addition of aqueous sulfide. Perhaps that hydroxide group plays a dominate role during this reaction. We further carried out a series of experiments to explore the possible reasons. Instead of Na2S, NaOH was added to OPD solution in the DMSO−water (v:v, 7:3), it was found that absorption spectrum of OPD at 440 nm blue-shifted to 417 nm and increased along with time (Figure S9A) and there hardly had any absorption around 531 and 660 nm when compared to that observed with Na2S. Such observations indicate that there were different processes occurring for the addition of NaOH and Na2S, which also indicated that the HS− indeed plays an important role in the reaction. Alternatively, we added Na2S to OPD solution in pure DMSO. The absorption at 440 nm almost stayed unchanged and a new peak at 660 nm developed. Yet, there was no new absorption peak at 531 nm (Figure S9B). These results indicate that the appearance of absorption at 660 nm was induced by sulfide, while the absorption at 531 nm was related with water or hydroxide. Therefore, we concluded that the optical changes of OPD after adding Na2S in a DMSO−water system involved both H2O and Na2S. Another interesting phenomenon is that air plays an important role during the reaction as illustrated in absorption spectra (Figure 5) and fluorescence spectra (Figure S10). When adding Na2S into OPD solution under N2 atmosphere, only absorption at 660 nm came out without appearance of absorption peak at 531 nm (Figure 5). In addition, when leaving the solution into open air again, the teal blue color would turn into pink color and the absorption at 660 nm disappeared with the absorption at 531 nm coming out. Whereas in the open air, the absorption at 660 nm would increase quickly then decrease, followed by the increase of absorption at 531 nm (Figure 1B). It appears that without

Figure 3. Fluorescence enhancement in the presence of thiols and other anions in DMSO−Tris (7:3, v/v, pH 7.4, 20 mM). Bars represent relative responses at 571 nm. Data were recorded at 60 min after addition of thiols or anions. λex/λem: 465 nm/571 nm. The concentration of OPD was 50 μM and the interferences were 3 equiv of that for OPD, (for GSH, 3 equiv and 100 equiv were both used). Legend: from left to right (1) blank (OPD), (2) F−, (3) PO43−, (4) HPO42−, (5) H2PO4−, (6) Ac−, (7) OH−, (8) CO32−, (9) HCO3−, (10) Cl−, (11) Br−, (12) I−, (13) SO42−, (14) HSO4−, (15) SO32−, (16) S2O32−, (17) NO2−, (18) NO3−, (19) IO4−, (20) S2−, (21) GSH, (22) GSH (5 mM), (23) L-Cys.

even 5 mM of GSH (100 equiv) produced only a slight increase of fluorescence. However, the presence of L-Cys was a potential interference since 3 equiv of L-Cys results in a considerable enhancement of fluorescence. However, addition of L-Cys induces a color change from bright yellow to purple, which is different from that induced by addition of sulfide, as shown in Figure S5. Therefore, the potential interference of L-Cys could be overcome by using a colorimetric assay. These results demonstrated that OPD could be employed for specific recognition of aqueous sulfide. Exploration of the Mechanism. The reaction between OPD and sulfide was also monitored by the NMR technique. Previous studies claimed that the OPD with strong electronwithdrawing groups has a highly electron-deficient nature, thus susceptible to nucleophilic aromatic substitution reactions (SNArH).38,45,46 Initially, we presume similar reaction would occur smoothly by the attack of HS− group. Surprisingly, judging from the NMR results, the original six sets of proton signals on the aromatic rings of OPD merely appears as three sets of protons after being treated with Na2S, as shown in Figure 4. The three sets of proton signals including two doublets and one triplet lie in the region from 7.6 to 8.3 ppm, suggesting the protons attaching to the aromatic backbone. The 13 C NMR clearly indicates that original carbon backbone still remains. The distortionless enhancement by polarization transfer-135 (DEPT-135) NMR results (shown in Figure S6) found the signals at 131.6, 126.1, and 126.0 ppm, further implying that all the carbons are quaternary carbon except for three −CH− carbons. Judging from NMR results, it seems there are three protons that were substituted. However, it is quite unlikely that there are three protons that have been D

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conclusion that sulfide (providing HS−) played an important role to induce the double bond addition reaction and was then hydrolyzed quickly by OH− to form the final adduct with a highly symmetrical structure. Visualizing Na2S in Living Cells. As mentioned above, OPD showed little fluorescence response to other anions and biological thiols, which made the OPD probe promising for specific recognition of aqueous sulfide in live cells. Initially, the HeLa cells were treated with OPD. As a control, fluorescent confocal images of HeLa cells treated with and without Na2S were both taken and compared. It clearly indicated that the probe displayed negligible fluorescence without Na2S (Figure 6A−C), while the one treated with Na2S displayed significantly

Figure 5. Absorption spectra and color changes of OPD treated with Na2S under different conditions. OPD solution (green-yellow), OPD treated with Na2S under N2 atmosphere for 30 min (teal blue), and then in air (red).

oxygen, the reaction procedure was stopped in half way and putting back to the open air would make the procedures moving on. With respect to fluorescence, the fluorescent intensity of OPD treated with Na2S under N2 atmosphere is lower than that under air (Figure S10). However, once this solution was further exposed to air, the fluorescence of the mixture solution gradually recovered to the level of that treated directly in air. On the basis of these results, a new reaction route was proposed, as shown in Scheme 2. Unlike the previously

Figure 6. Confocal microscopy images of aqueous sulfide in HeLa cells after incubation with OPD probe (10 μM) at 37 °C (λex = 543 nm) treated without (A−C) and with (D−F) Na2S. (A) Bright field image of cells incubated with OPD for 30 min, (D) bright field image of cells incubated with OPD for 30 min and then Na2S for another 60 min, (B, E) fluorescent images of cells shown in panels A and D, respectively; panels C and F are overlay images of the bright field and fluorescence images, respectively.

Scheme 2. Proposed Mechanism of Sulfide-Involved Reaction for OPD

enhanced fluorescence (Figure 6D−F). These data demonstrate the potential of OPD as a sensitive fluorescent probe for specific recognition of aqueous sulfides.

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CONCLUSIONS In summary, a new Na2 S-responsive probe has been successfully developed through a novel sulfide-catalyzed reaction. Such sulfide-catalyzed reactions have not been observed before and offer a facile and simple strategy to construct fluorescent probes for sulfides with high sensitivity and selectivity. In addition, this probe also demonstrated its applicability for recognizing sulfide in cells, which offers great potential for designing novel fluorescent probes to provide precise information on dynamics distribution of intracellular sulfides.

reported routes involving nucleophilic groups such as thiols and amines attacking the aromatic ring, in this case the reductive properties of aqueous sulfide leads to the reduction of the double bond. The addition of the HS− group to the electronpoor CC double bond produces I, which then undergoes isomerization and rearrangement to afford II. Form II is unstable and would quickly precede ring-expansion procedures together with oxidation under the help of oxygen to form III. The formation of intermediate III could also be detected by MS results (Figure S8), giving a molecule weight of 261.03. Under aqueous conditions, the III would be easily hydrolyzed through attacking of the CS bond by hydroxide group, followed by elimination to afford the final product, verified both by HRMS and NMR. Thus, we came to an important

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b04194. Color changes (Figure S1) and fluorescence spectra (Figure S2) of OPD treated with Na2S, fluorescence intensity of OPD toward to Na2S in different buffer (Figure S3), the effect of different pH on absorption E

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changes (Figure S4), color changes of OPD treated with typical interfering analytes (Figure S5), DEPT-135 NMR of resulting product treated with Na2S (Figure S6), possible reaction routes for OPD reacting with Na2S (Figure S7), ESI-MS spectra of adduct (Figure S8), absorption spectral changes of OPD treated with NaOH or Na2S in pure DMSO (Figure S9), fluorescence spectra of OPD treated with Na2S under different conditions (Figure S10), NMR and ESI-MS of OPD probe (Figure S11, S12), and NMR of the resulting product (Figure S13) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge for the financial support by the National Natural Science Foundation of China (Grants 21475007, 21275015, 21505003). We also thank the support from the “Innovation and Promotion Project of Beijing University of Chemical Technology”, the “Public Hatching Platform for Recruited Talents of Beijing University of Chemical Technology”, and the High-Level Faculty Program of Beijing University of Chemical Technology (Grant buctrc201507).

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DOI: 10.1021/acs.analchem.5b04194 Anal. Chem. XXXX, XXX, XXX−XXX