Dual-Function Fluorescent Probe for Detection of Hydrogen Sulfide and Water Content in Dimethyl Sulfoxide.

To detect hydrogen sulfide (H2S) and water content in dimethyl sulfoxide, the fluorescent probe (Probe 1) was used, as it not only detects H2S but also detects the water content. After H2S was added into Probe 1, the intensity of fluorescence increased and was up to 1300 times. In case the H2S concentration was in the range 0-20 μM, it was able to be detected by Probe 1, and the limit of detection was 0.851 nM. When Probe 1 and H2S underwent a reaction, the solution color had some changes. These colors changed in terms of the concentration changes of H2S, ranging from colorless to yellow. The Probe 1 test paper only needed to be exposed to hydrogen sulfide gas for 20 s for the color change to occur. Besides, Probe 1-H2S was used to detect water content in dimethyl sulfoxide which ranged from 0 to 100%. The color change of the solution was opposite to that of H2S, ranging from yellow to colorless.

Introduction

Hydrogen sulfide (H2S) is a typical active sulfide compound.[1,2] H2S is of great importance physiologically and pathophysiologically.[3,4] Increased H2S content may increase the risk of various diseases, including ischemic disease,[5] atherosclerosis,[6] tumor,[7] diabetes,[8] and cardiac hypertension.[9] As H2S has a characteristic rotten egg smell, excessive H2S may be detrimental to the flavor of wine.[10,11] Commonly, water is regarded as an impurity in organic solvents. The water content plays an important role in chemical reactions and industrial fields.[12]

The Karl Fischer titration is often adopted for the detection of water content in organic solvents.[13] At present, by means of a resort to fluorescent probes, water in organic solvents has been traced and analyzed.[13] A slew of ways have been used to detect H2S, ranging from high-performance liquid chromatography[14] and gas chromatography[15] to nanomaterial-based sensors.[16,17] Nowadays, fluorescent probes have enticed people as they can be effective in a short time, highly sensitive, easy in operation, and convenient for observation.[18−28] A lot of H2S-oriented fluorescent probes owing to their good nucleophilic activity have emerged, such as probes involving the nucleophilic substitution reactions with 2,4-dinitrobenzenesulfonate, 7-nitro-1,2,3-benzoxadiazole, and 2,4-dinitrobenzene.[29−38] For H2S detection, plenty of organic fluorescent probes with double sites had been used in aqueous medium and living cells.[39−43] However, the dual-function fluorescent probes for the detection of water content in dimethyl sulfoxide (DMSO) and detection of H2S have not been reported.

In light of the above issue, a fresh probe (Probe 1) is explored in the paper with a view to detect water content in DMSO and to detect H2S. Naphthalene is taken as fluorophore and 2,4-dinitrobenzenesulfonate and aldehyde is selected for the reaction group. The one-time function relationship between luminescent intensity of Probe 1–H2S and water content is conductive to the detection of the water content in DMSO.

Results and Discussion

Probe Prepared

The esterification reaction of compound 1 with compound 2 is used to generate Probe 1 (Scheme ). The synthesis is not complicated and this is true for purification under the reaction of recrystallization as well. To confirm Probe 1, 13C nuclear magnetic resonance (NMR), EA, and 1H NMR were used (Figures S1 and S2, Supporting Information).

Scheme 1

Synthetic Route of Probe 1

Sensing Properties of Probe 1 toward H2S

The fluorescence response of Probe 1 to H2S in DMSO (Figure a), CH3CN (Figure b), and C2H5OH (Figure c) solutions was verified. The fluorescence intensity was detected at 0, 0.5, and 1–30 min after H2S (200 μM, 20 μL) was added. The fluorescence signal increased rapidly from 0 to 0.5 min, decreased gradually from 0.5 min, and reached an equilibrium at 11 min in DMSO, 20 min in CH3CN, and 26 min in C2H5OH (Figure d). The fluorescence intensity of Probe 1 increased by almost 1300-fold in DMSO after H2S was added. Probe 1 in DMSO had a faster reaction time and greater fluorescence intensity changes than in CH3CN and C2H5OH.

Figure 1

(a) Time response of Probe 1 (10 μM) added to H2S (200 μM) in DMSO at 25 °C, λex = 425 nm; (b) in CH3CN; (c) in C2H5OH; and (d) time response of Probe 1 added to H2S in different solvents. Repeated three times.

DMSO, CH3CN, and C2H5OH solutions make a difference in the water content. In these solutions, the fluorescence response of Probe 1–H2S (10 μM Probe 1 + 200 μM H2S) was verified. After Probe 1 (20 μL, 10 μM) and H2S (20 μL, 200 μM) were added to a DMSO–water system (2 mL), the luminescent intensity was reduced with the increasing water content and the optical emission spectra were red-shifted (Figure a). There was a one-time function relationship between Probe 1–H2S with the water level (R2 = 0.9963; Figure b). As the concentration of H2O increased (0, 20, 40, 60, 80, and 100%), there was a change from yellow to colorless. The observer could find out this under natural light (Figure c). Regarding the 365 nm ultraviolet lamp, the color changed from a light blue to blue-green color (Figure d). From these data, it could be seen that the detection of H2O content in DMSO within the range of 0–100% can be completed by the Probe 1 + H2S system. In the CH3CN–water and C2H5OH–water systems, the fluorescence intensity did not show a concentration-dependent change with the increasing water content (Figures S3a,b and S4a,b, Supporting Information). The color gradually changed from yellow to colorless, and it could be seen by the naked eye (Figure S3c, Supporting Information). Under an ultraviolet lamp (365 nm), the color gradually changed from light blue to blue-green color (Figure S4c, Supporting Information). As a result, DMSO was chosen as the most suitable solution for Probe 1 toward H2S, and the best reaction time was 11 min.

Figure 2

(a) Fluorescent spectra of Probe 1 with H2S in DMSO which led to the moisture level increase, λex = 425 nm; (b) plot of the fluorescence intensity of Probe 1–H2S with the moisture level; (c) photograph of Probe 1–H2S in various moisture level DMSO under ambient light; (d) under 365 nm UV light.

When H2S was gradually added (0, 2, 4, 6, 8, 10, 12, 14, 16, and 18 μM), the maximal ultraviolet absorption peak of Probe 1 gradually increased (Figure a), which was linearly and positively dependent on the concentration of H2S (R2 = 0.9972; Figure b,c). At 487 nm, the luminescent intensity was increased 1300 times. There was a one-time function relationship between luminescent intensity and H2S concentration (R2 = 0.9991; Figure d). The limit of detection (LOD) was 0.851 nM (S/N = 3). With the rising H2S concentration, the solution color changed to yellow and then deepened. This was seen under natural light by researchers. When a 365 nm ultraviolet lamp was used, there was an increase in the fluorescence intensity and there was color change from colorlessness to light blue (Figure f).

Figure 3

(a) Ultraviolet absorption spectrum of Probe 1 with H2S (0, 2, 4, 6, 8, 10, 12, 14, 16, and 18 μM); (b) plot of the ultraviolet absorption intensity of Probe 1 with H2S (0–18 μM); (c) fluorescence intensity of Probe 1 with H2S (0, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 μM); (d) plot of fluorescence intensity of Probe 1 with H2S (0–20 μM); repeated three times. (e) Photograph of Probe 1 solutions with an addition of H2S (0, 2, 4, 6, 8, and 10 μM) under natural light; (f) under UV light (365 nm).

To examine the selectivity of Probe 1 for H2S, some compounds and ions, such as hydrazine, Hcy, PhSH, ammonia solution, Cys, H2O2, GSH, NH4+, Na+, Br–, K+, SO32–, Mg2+, Cl–, Ca2+, F–, and I–, were tested. None of these competitors (10 μM) led to a significant fluorescent response by Probe 1. Moreover, with an addition of H2S to the solutions with hydrazine, Hcy, PhSH, ammonia solution, Cys, H2O2, GSH, NH4+, Na+, Br–, K+, SO32–, Mg2+, Cl–, Ca2+, F–, and I–, a competition experiment was carried out. The experiment showed that the luminescent intensities of Probe 1–H2S and Probe 1 + H2S + competitors were almost the same (Figure ). After comparison, it could be found that Probe 1 is an excellent selective tool to detect H2S. Another finding from the experiment was that there was no negative action from NH2NH2, PhSH, Cys, Hcy, and GSH.

Figure 4

Changes in fluorescence intensity of Probe 1 (10 μM) after an addition of 1 equiv for each. (1, blank; 2, NH2NH2; 3, PhSH; 4, Cys; 5, Hcy; 6, GSH; 7, HSO3–; 8, SO32–; 9, NH3; 10, H2O2; 11, NH4+; 12, F–; 13, I–; 14, Br–; 15, Cl–; 16, Na+; 17, Mg2+; 18, Ca2+; 19, K+. 10 μM for H2S). Repeated three times.

Reaction Mechanism

Likely, two reaction paths may contribute to the response mechanism. Because of the primary response mechanism, the nucleophile substitution reaction of Probe 1 with H2S occurred and yielded compound 3 and compound 4 which was turned to compound 1 after protonation in a direct manner (Scheme , route a). Under the action of the secondary response mechanism, the nucleophilic addition reaction of Probe 1 with H2S was done and brought about compound 7 which involved in the nucleophile substitution reaction of compound 7 with H2S and yielded compound 6 (Scheme , route b). To identify the mechanism, 1H NMR, HMBC, HPLC, and MS were carried out.

Scheme 2

Proposed Mechanism for Probe 1–H2S

With 1H NMR titration methods, the response mechanism between Probe 1 and H2S was determined. As 0.5 equiv H2S was added, proton signals at 10.02 ppm (Hb), 10.40 ppm (Hc), 6.10 (H1) ppm, and 5.16 ppm (H2) appeared. As the concentration of H2S increased to 1 equivalent and 2 equiv, the proton signal at 10.15 ppm (Ha) disappeared and the signals at 5.90 (H3) ppm and 5.10 ppm (H4) appeared (Figure S5, Supporting Information). The proton signal of Hc disappeared because O–H changed to O–Na. The proton signals of Hb, Hc, H3, and H4 were confirmed by HMQC (Figures S6 and S7, Supporting Information). As 0.5 equi H2S was added to compound 1, the proton signals at 6.10 (H1) ppm and 5.16 ppm (H2) did not appear, which indicates that H2S could not participate in the nucleophilic addition reaction with compound 1, so compound 1 could not react with H2S to compound 5 (Figure S8, Supporting Information).

The ratio between route a and route b had been calculated using 1H NMR (Figure S5, Supporting Information) of Probe 1–H2S (1:0.5), the ratio accounting for 1/0.36. Therefore, the primary response mechanism was the nucleophile substitution reaction of 2,4-dinitrobenzenesulfonate with H2S; the secondary response mechanism was the nucleophilic addition reaction of aldehyde with H2S and then nucleophile substitution reaction of 2,4-dinitrobenzenesulfonate with H2S.

HPLC was used for the verification of the response mechanism. With the addition of 1 equiv H2S, the peak of compound 1 appeared and Probe 1 disappeared, and three new peaks appeared (Figure S9, Supporting Information). By performing MS, it could be seen that there was a peak at m/z = 171.1855, and this was related to the compound 1 formation (Figure S10, Supporting Information). The peak at m/z = 199.1293 was related to the compound 3 formation (Figure S11, Supporting Information). The peak at m/z = 206.2636 was related to the compound 6 formation (Figure S12, Supporting Information). The peak at m/z = 458.1605 (M + Na) was related to compound 7 formation (Figure S13, Supporting Information).

The capture of hydrogen by the oxygen negative ions of compound 4 to give compound 1 was proved by the fluorescence intensity of compound 1 and compound 4 (Figure S14, Supporting Information). The fluorescence intensity of compound 1 was weak in water (−OH) but strong in pH 7.4 buffer solution (−O–). This capture was also proved by the fluorescence intensity of Probe 1 with H2S first increasing and then decreasing (Figures d and 4b). The data laid a foundation for the detection of H2O content in DMSO via the Probe 1–H2S system because its fluorescence intensity was decreased as the H2O content increased.

Detection of H2S

To demonstrate the applicability of Probe 1, the ability performance of Probe 1 for detecting H2S in water samples was demonstrated and investigated. Water from the Yellow River, tap water, and mineral water were added to Probe 1 solution, and the concentration of H2S was determined in these water samples. Next, different amounts of H2S (10 and 12 μM) were added and the recovery values ranged from 98.17 to 102.00% (Table ). From the data, it could be found that Probe 1 had capability to detect H2S.

Table 1

Detection of H2S in Water

sample	H2S level found (μmol)	added (μmol)	found (μmol)	recovery (%)	RSD (%; n = 3)
mineral water	0 ± 0.000	10.00	10.00	100	0.9
		12.00	12.00	100	1.0
tap water	0 ± 0.018	10.00	9.95	99.50	1.0
		12.00	11.78	98.17	2.8
Yellow River water	0.235 ± 0.003	10.00	10.43	102	0.7
		12.00	12.26	100.2	0.4

Application of Probe 1

The test strips were prepared using our previous approach.[23] The test strip was directly exposed to H2S gas. After 20 s, the color of test paper changed to yellow (Figure a). Under an ultraviolet lamp (365 nm), there was obvious luminescence (Figure b).

Figure 5

(a) Photograph of a test strip exposed to hydrogen sulfide gas under skylight; (b) photograph of a test strip exposed to H2S gas under UV light (365 nm); (c) photograph of Probe 1 (10 μM, 20 μL) added to H2S solution (0, 0.01, 0.06, 0.1, 1, and 10 mM) under ambient light; and (d) photograph of Probe 1 added to H2S solution (0, 0.01, 0.06, 0.1, 1, and 10 mM) under 365 nm UV light.

After Probe 1 (20 μL, 10 μM) was added to real hydrogen sulfide solution samples (2 mL), the sample solutions changed from colorless to yellow (Figure c). Under a 365 nm ultraviolet lamp, it changed from colorless to yellowish green (Figure d). The mutation point of color was the third sample (0.06 mM, 2 mg/L). This concentration (2 mg/L) is a low limit value of H2S solution for the bathing therapy. All the above data indicate that Probe 1 had capability to detect H2S solution.

Conclusions

In summary, a dual-function fluorescent probe (Probe 1) was prepared to detect H2S and water content in DMSO. The recognition mechanism of probe to H2S was confirmed. The discriminative detection of H2S was achieved by comparing and analyzing the difference in the reaction time and solution color of Probe 1–H2S. The fluorescence intensity of Probe 1 was increased by almost 1300-fold when H2S was added. The detection range of Probe 1 to H2S was 0–20 μM and the LOD was 0.851 nM. Via the reaction with H2S with different concentrations, Probe 1 brought color changes from colorlessness to yellow. The color deepened little by little. The atmosphere was observed by the researcher under natural light. The vital finding was that the color of the Probe 1 test paper went through change from colorlessness to yellow when it was subjected to H2S gas for 20 s in a direct manner. Furthermore, the water content in DMSO was determined by Probe 1–H2S in the range 0–100% and the color change of the solution was opposite to that of H2S, ranging from yellow to colorless.

Experimental Section

Materials

The chemicals glutathione (GSH, 99%), triethylamine (99%), cysteine (Cys, 99%), 2,4-dinitrobenzene sulfonyl chloride (98%), homocysteine (Hcy), hydrazine hydrate (Hydrazine, 64%), 6-hydroxy-2-naphthaldehyde (98%), sodium thiophenolate (PhSNa, 90%), and chloroform (CHCl3) were purchased from Beijing Innochem Company. Analytes sodium sulfide (Na2S), ammonia solution (NH3), sodium hydrogen sulfite (NaHSO3), calcium chloride anhydrous (CaCl2), sodium sulfide (NaCl), hydrogen peroxides (H2O2), fluoride (NaF), sodium sulfite (Na2SO3), potassium iodide (KI), sodium bromide (NaBr), ammonium chloride (NH4Cl), magnesium sulfate anhydrous (MgSO4), and DMSO were obtained from Beijing Innochem Company.

Instruments

With the internal standard of TMS, the Bruker AV 300 MHz NMR spectrometer was used for the NMR spectra in the experiment. A Hitachi F-4600 luminescence spectrometer was used to have a record of luminescence intensity spectra.

Synthesis of Probe 1

The mixture of 6-hydroxy-2-naphthaldehyde (compound 1; 0.50 g, 2.90 mmol), N(C2H5)3 (0.30 g, 3.0 mmol), and CHCl3 (10 mL) was stirred at a flask. Compound 2 (1.55 g, 5.80 mmol) in CHCl3 (10 mL) was slowly added and stirred in ice bath for 0.5 h. The reaction mixture was subsequently heated to 40 °C for 4 h. The filter was used to clean the precipitate which was to be recrystallized from chloroform so that a white-colored solid Probe 1 was produced.

1H NMR (300 MHz, DMSO-d6): δ 10.16 (s, 1H), 9.14 (d, J = 2.2 Hz, 1H), 8.66 (s, 1H), 8.59 (dd, J = 8.7, 2.3 Hz, 1H), 8.29 (dd, J = 8.9, 2.4 Hz, 2H), 8.13 (d, J = 8.6 Hz, 1H), 7.97 (dd, J = 8.6, 1.3 Hz, 1H), 7.94 (d, J = 2.2 Hz, 1H), 7.47 (dd, J = 9.0, 2.4 Hz, 1H). 13C NMR (75 MHz, DMSO-d6): δ 193.29, 152.07, 148.65, 136.68, 135.05, 134.44, 134.13, 133.10, 131.76, 129.71, 128.02, 124.29, 122.18, 121.65, 120.56. Anal. Calcd for C17H10N2O8S: C, 50.75; H, 2.51; N, 6.96. Found: C, 50.63; H, 2.40; N, 6.92.