Cyanostilbene mercury ion fluorescent probe compound and preparation method thereof

Abstract

The invention relates to a cyano-stilbene mercury ion fluorescent probe compound and a preparation method thereof. The molecular structure of the compound is shown in the specification.

Description

Technical Field

[0001] This invention belongs to the field of fluorescent probes, and relates to a fluorescent probe compound that selectively recognizes metal ions, its preparation method, and its application. Background Technology

[0002] Mercury is a globally distributed and highly toxic heavy metal that causes serious environmental health problems due to its persistence, long-distance migration, and significant bioaccumulation potential. Unlike many other heavy metals, mercury can exist in the environment and in organisms in a variety of chemical forms, including elemental mercury (Hg). 0 Inorganic mercury (e.g., Hg) 2+ ) and organic mercury species, such as methylmercury (CH3Hg) + In particular, inorganic mercury (mainly in the form of Hg) 2+ Mercury (in its various forms) is widely present in water, soil, and atmosphere, and its main sources are human activities such as fossil fuel combustion, non-ferrous metal smelting, chlor-alkali production, and improper disposal of mercury-containing products.

[0003] Even in trace amounts, mercury ions (Hg) 2+ Mercury ions also pose serious threats to ecosystems and human health. Once inside aquatic systems, they are converted into methylmercury by microorganisms in sediments, a more toxic substance that accumulates through the food chain. This process ultimately leads to irreversible damage to the central nervous system, kidneys, and immune system of higher organisms, including humans. Therefore, developing rapid, sensitive, and selective methods for detecting mercury ions is crucial for environmental monitoring, food safety, and public health protection. While standard laboratory techniques such as atomic absorption / emission spectroscopy (AAS / AES) and inductively coupled plasma mass spectrometry (ICP-MS) offer high sensitivity and accuracy, their high cost, complex sample preparation processes, and reliance on skilled operators limit their application in field and real-time monitoring.

[0004] To overcome the aforementioned limitations, chemical sensors based on optical or electrical signals have attracted considerable attention in recent years due to their advantages such as low cost, fast response, ease of operation, and real-time analysis potential. Among them, fluorescence-based sensors exhibit extremely high sensitivity by monitoring fluorescence "on" or ratio changes caused by analyte binding. In various recognition modes, high-affinity organic small molecule probes show great potential due to their well-defined structures, ease of synthesis, and tunable properties. However, existing mercury ion fluorescent probes still face several shortcomings: in terms of selectivity, they are susceptible to interference from other metal ions; common "on" probes have high background signals and are prone to false positives; in terms of performance, most probe responses are irreversible, making it difficult to monitor concentration fluctuations in real time; furthermore, their anti-interference ability in complex real-world samples is generally weak, hindering the translation from laboratory research to practical applications. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a method for preparing a cyanobistyl styrene mercury ion fluorescent probe compound. Hg is added to the probe. 2+ Within the next 15 minutes, the fluorescence will increase by about 7 times, which can be used as a fluorescent probe to recognize mercury ions.

[0006] This invention designs and synthesizes a novel intramolecular charge transfer (ICT) fluorescent probe, named HPTA, based on the dicyandiphenylacrylonitrile backbone. This probe achieves Hg control through the coordination of the secondary amino group and sulfur atom of the 2-thiocaprolactam (2TH) moiety with mercury ions. 2+ This invention provides highly selective and sensitive "triggered" fluorescence detection. It systematically evaluates the sensing performance of HPTIA, including its sensitivity (detection limit), selectivity, response time, and possible recognition mechanisms. Furthermore, the probe has been successfully applied to Hg in real solid and liquid samples. 2+ The results show that it has good application prospects in analyzing complex environmental matrices and provides a feasible strategy for developing efficient on-site detection techniques for heavy metal ions.

[0007] This invention is achieved through the following technical solutions.

[0008] The cyanobrythryl styrene mercury ion fluorescent probe compound (HPTIA) described in this invention has the following molecular structure:

[0009]

[0010] The synthetic route of the cyanobrythryl styrene mercury ion fluorescent probe compound (HPTIA) described in this invention is as follows:

[0011]

[0012] Specifically, the preparation method of the cyanobrythryl styrene mercury ion fluorescent probe compound (HPTIA) of the present invention includes the following steps:

[0013] Step (1): 4-hydroxyphenylacetonitrile, a moderately strong base, and terephthalaldehyde monodiethyl acetal or terephthalaldehyde in a molar ratio of 1:0.1-5:1-3 are refluxed in a condensation reaction solvent at a reaction temperature of 60°C. o C~100 o C, with a reaction time of 3 h to 12 h, yields the intermediate product FHCS in 60% to 90% yield.

[0014] Step (2): FHCS, 2-thiocaprolactam, and a moderately strong base are refluxed in the condensation reaction solvent at a reaction temperature of 80°C. o C~130 o C, the reaction time is 8 h to 24 h, to obtain the probe HPTIA with a yield of 71% to 89%. The molar ratio of FHCS, 2-thiocaprolactam and sodium hydroxide is 1:1 to 3:0.5 to 4.

[0015] The aforementioned moderately strong base includes, but is not limited to, any one of pyridine, magnesium hydroxide, potassium carbonate, cesium carbonate, sodium hydroxide, or potassium hydroxide.

[0016] The solvent for the condensation reaction includes, but is not limited to, any one of methanol, ethanol, benzene, toluene, N,N-dimethylformamide, dimethyl sulfoxide, or acetic acid.

[0017] The cyanobrythril-based mercury ion fluorescent probe (HPTIA) described in this invention has the following characteristics:

[0018] The fluorescent probe HPTIA enables fluorescence "off-on" detection of mercury ions.

[0019] Within the mercury ion concentration range of 0-100 μM, the fluorescence intensity of the fluorescent probe HPTIA showed a linear relationship with the mercury ion concentration, with the linear equation being y=10.631471x + 39.67736, R²=0.9930. The calculated detection limit for mercury ions was 138 nmol / L.

[0020] The fluorescent probe HPTIA responds to mercury ions in 15 minutes.

[0021] The fluorescent probe HPTIA can detect mercury ions in tap water. The recovery rate of mercury ions in actual water samples is 97.35%~103.21%, and the relative standard deviation is 0.31~5.49%.

[0022] The principle of this invention:

[0023] When the fluorescent probe HPTA prepared in this invention interacts with mercury ions, the mercury ions complex with the hydroxyl group and the sulfur atom of the 2-TH group in the probe molecule, which weakens the original photoinduced electron transfer effect, thereby enhancing the fluorescence effect of the system and ultimately realizing the detection of mercury ions.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The fluorescent probe HPTIA prepared in this invention enables green and reversible fluorescence detection of mercury ions. Its innovation lies in its rapid detection process; simultaneously, the probe molecule, through the synergistic effect of hydroxyl groups and thioamides, achieves a highly efficient 1:1 coordination mode, significantly reducing the amount of probe required. This not only lowers detection costs but also embodies advanced green and environmentally friendly concepts from principle to practice.

[0026] The fluorescent probe HPTA prepared by this invention has a response time of only 15 minutes to mercury ions, which results in faster detection speed and improved detection efficiency. Attached Figure Description

[0027] Figure 1 This invention prepares the fluorescent probe intermediate FHCS. 1 H NMR spectrum.

[0028] Figure 2 This invention prepares the fluorescent probe intermediate FHCS. 13 C10 NMR spectrum.

[0029] Figure 3 This invention prepares the fluorescent probe HPTIA. 1 H NMR spectrum.

[0030] Figure 4 This invention prepares the fluorescent probe HPTIA. 13 C10 NMR spectrum.

[0031] Figure 5 The present invention provides the FT-IR spectrum of the fluorescent probe HPTIA.

[0032] Figure 6 Selective recognition of metal ions by the fluorescent probe HPTIA.

[0033] Figure 7 The effect of coexisting metal ions on the recognition of mercury ions by the fluorescent probe HPTIA.

[0034] Figure 8 Linear relationship of fluorescence response of fluorescent probe HPTIA to different concentrations of mercury ions.

[0035] Figure 9Job's plot of the fluorescent probe HPTIA against mercury ions.

[0036] Figure 10 Response time diagram of the fluorescent probe HPTIA to mercury ions.

[0037] Figure 11 Diagram of the reversible cycle number of mercury ions for the fluorescent probe HPTIA. Detailed Implementation

[0038] The present invention will be further illustrated by the following embodiments. The technical solution of the present invention is not limited to the specific embodiments listed below, but also includes any combination of the specific embodiments. Its scope of protection is not limited to the embodiments described below.

[0039] Example 1: The preparation method of the fluorescent probe HPTIA in this example is carried out according to the following steps:

[0040] Step (1): Under a nitrogen atmosphere, 2.663 g (20 mmol) of 4-hydroxyphenylacetonitrile and 4.165 g (20 mmol) of terephthalaldehyde monodiethyl acetal were dissolved in 50 mL of methanol. Then, 0.112 g (2 mmol) of potassium hydroxide was added, and the mixture was 65 mL. o The mixture was stirred and refluxed for 6 hours. The color of the mixture gradually turned brown. Thin-layer chromatography showed that all starting materials were consumed. After the reaction was complete, the mixture was cooled to room temperature, and the solvent was evaporated under reduced pressure. 50 mL of 1 mol / L NH₄Cl solution was added, forming a precipitate. The precipitate was filtered and purified by recrystallization in methanol / water (1:1, v / v). After drying, compound FHCS was collected as a pale yellow solid in 85% yield (60%).

[0041] Step (2): Dissolve compound FHCS (500 mg, 1 equivalent) and 2-thiocaprolactam (1.159 g, 1.08 equivalent) in acetic acid (13.5 mL). Add sodium hydroxide (0.698 g, 4 equivalent) to the solution, and then heat at 120°C under a nitrogen atmosphere. o The reaction was carried out under reflux at C. The reaction progress was monitored by thin-layer chromatography. After the reaction was complete (24 hours), the reaction mixture was poured into ice water. The resulting brown precipitate was filtered off to obtain the crude product, Probe. This crude compound was further purified by column chromatography (ethyl acetate / hexane, 25:75 v / v). Yield: 71%.

[0042] Example 2: The preparation method of the fluorescent probe HPTIA in this example is carried out according to the following steps:

[0043] Step (1): Under a nitrogen atmosphere, 2.663 g (20 mmol) of 4-hydroxyphenylacetonitrile and terephthalaldehyde monodiethyl acetal (6.248 g (30 mmol) were dissolved in 50 mL of ethanol. Then, 0.4 g (10 mmol) of sodium hydroxide was added, and the mixture was 60 mL. o Stirred and refluxed at C for 3 hours. The mixture gradually turned brown. Thin-layer chromatography showed that all starting materials were consumed. After the reaction was complete, the mixture was cooled to room temperature, and the solvent was evaporated under reduced pressure. 50 mL of 1 mol / L NH4Cl solution was added, forming a precipitate. The precipitate was filtered and purified by recrystallization in methanol / water (1:1, v / v). After drying, compound FHCS was collected as a pale yellow solid in 85% yield (79%).

[0044] Step (2): Dissolve compound FHCS (500 mg, 1 equivalent) and 2-thiocaprolactam (1.609 g, 1.5 equivalent) in dimethyl sulfoxide (13.0 mL). Add potassium hydroxide (0.056 g, 0.5 equivalent) to the solution, and then heat at 130°C under a nitrogen atmosphere. o The reaction was carried out under reflux at C. The reaction progress was monitored by thin-layer chromatography. After the reaction was complete (14 hours), the reaction mixture was poured into ice water. The resulting brown precipitate was filtered off to obtain the crude product, Probe. This crude compound was further purified by column chromatography (ethyl acetate / hexane, 25:75 v / v). Yield: 89%.

[0045] Example 3: The preparation method of the fluorescent probe HPTIA in this example is carried out according to the following steps:

[0046] Step (1): Under a nitrogen atmosphere, 2.663 g (20 mmol) of 4-hydroxyphenylacetonitrile and terephthalaldehyde monodiethyl acetal (8.330 g (40 mmol)) were dissolved in 50 mL of dimethyl sulfoxide. Then, cesium carbonate (6.500 g (20 mmol) was added to the solution, and the mixture was 100 mL. o Stirred and refluxed at C for 8 hours. The mixture gradually turned brown. Thin-layer chromatography showed that all starting materials were consumed. After the reaction was complete, the mixture was cooled to room temperature, and the solvent was evaporated under reduced pressure. 50 mL of 1 mol / L NH4Cl solution was added, forming a precipitate. The precipitate was filtered and purified by recrystallization in methanol / water (1:1, v / v). After drying, compound FHCS was collected as a pale yellow solid in 85% yield (90%).

[0047] Step (2): Dissolve compound FHCS (500 mg, 1 equivalent) and 2-thiocaprolactam (2.254 g, 2.1 equivalent) in toluene (13.5 mL). Add magnesium hydroxide (0.464 g, 4 equivalent) to the solution, and then heat at 110 °C under a nitrogen atmosphere. o The reaction was carried out under reflux at C. The reaction progress was monitored by thin-layer chromatography. After the reaction was complete (18 hours), the reaction mixture was poured into ice water. The resulting brown precipitate was filtered off to obtain the crude product, Probe. This crude compound was further purified by column chromatography (ethyl acetate / hexane, 25:75 v / v). Yield: 85%.

[0048] Example 4: The preparation method of the fluorescent probe HPTIA in this example is carried out according to the following steps:

[0049] Step (1): Under a nitrogen atmosphere, 4-hydroxyphenylacetonitrile (2.663 g, 20 mmol) and terephthalaldehyde (6.700 g, 50 mmol) were dissolved in 50 mL of N,N-dimethylformamide. Then potassium carbonate (3.450 g, 25 mmol) was added, and the mixture was 80 mL. o Stirred and refluxed at C for 12 hours. The mixture gradually turned brown. Thin-layer chromatography showed that all starting materials were consumed. After the reaction was complete, the mixture was cooled to room temperature, and the solvent was evaporated under reduced pressure. 50 mL of 1 mol / L NH4Cl solution was added, forming a precipitate. The precipitate was filtered and purified by recrystallization in methanol / water (1:1, v / v). After drying, compound FHCS was collected as a pale yellow solid in 85% yield (82%).

[0050] Step (2): Dissolve compound FHCS (500 mg, 1 equivalent) and 2-thiocaprolactam (2.576 g, 2.4 equivalent) in benzene (15.0 mL). Add cesium carbonate (0.276 g, 1 equivalent) to the solution, and then heat at 80°C under a nitrogen atmosphere. o The reaction was carried out under reflux at C. The reaction progress was monitored by thin-layer chromatography. After the reaction was complete (9 hours), the reaction mixture was poured into ice water. The resulting brown precipitate was filtered off to obtain the crude product, Probe. This crude compound was further purified by column chromatography (ethyl acetate / hexane, 25:75 v / v). Yield: 76%.

[0051] Example 5: The preparation method of the fluorescent probe HPTIA in this example is carried out according to the following steps:

[0052] Step (1): Under a nitrogen atmosphere, 4-hydroxyphenylacetonitrile (2.663 g, 20 mmol) and terephthalaldehyde (8.040 g, 60 mmol) were dissolved in 60 mL of toluene. Then, magnesium hydroxide (1.740 g, 100 mmol) was added, and the mixture was 90 mL. o Stirred and refluxed at C for 10 hours. The mixture gradually turned brown. Thin-layer chromatography showed that all starting materials were consumed. After the reaction was complete, the mixture was cooled to room temperature, and the solvent was evaporated under reduced pressure. 50 mL of 1 mol / L NH4Cl solution was added, and a precipitate formed. The precipitate was filtered and purified by recrystallization in methanol / water (1:1, v / v). After drying, compound FHCS was collected as a pale yellow solid in 85% yield (76%).

[0053] Step (2): Dissolve compound FHCS (500 mg, 1 equivalent) and 2-thiocaprolactam (3.219 g, 3.0 equivalent) in ethanol (13.5 mL). Add potassium carbonate (0.828 g, 3 equivalent) to the solution, and then heat at 80°C under a nitrogen atmosphere. o The reaction was carried out under reflux at C. The reaction progress was monitored by thin-layer chromatography. After the reaction was complete (20 hours), the reaction mixture was poured into ice water. The resulting brown precipitate was filtered off to obtain the crude product, Probe. This crude compound was further purified by column chromatography (ethyl acetate / hexane, 25:75 v / v). Yield: 82%.

[0054] The NMR IR spectrum of the fluorescent probe HPTIA is as follows: Figure 1-5 As shown.

[0055] Example 6: Preparation of the fluorescent probe HPTIA solution, performed according to the following steps:

[0056] Accurately weigh 34.7 mg of the fluorescent probe HPTIA and prepare a 1.0 × 10⁻⁶ solution with methanol. - ² mol / L Mother liquor A;

[0057] Take 500 μL of a concentration of 1.0 × 10⁻⁶. - Solution A with a concentration of 2 mol / L was diluted to 50 mL with methanol to obtain a solution with a concentration of 1.0 × 10⁻⁶ mol / L. - 4 1 mol / L of the fluorescent probe HPTIA solution.

[0058] Example 7: Selective recognition of metal ions by the fluorescent probe HPTIA, performed according to the following steps:

[0059] Take 2 mL of the fluorescent probe HPTIA solution and add 1 eq. of Ca.2+ Fe 3+ Sn 2+ Mn 2+ Ba 2+ Cu 2+ Fe 2+ Al 3+ Zn 2+ Ag + Gd 3+ Nd 3+ Y 3+ Tb 4+ Pr 3+ Ce 4+ Eu 3+ The fluorescence intensity of the aqueous solution was measured at an excitation wavelength of 365 nm, and the results are as follows. Figure 6 As shown, the fluorescence of the system was significantly enhanced after the addition of mercury ions, while the fluorescence intensity did not change significantly when other metal ions were added, indicating that the fluorescent probe HPTA solution can specifically recognize mercury ions.

[0060] Example 8: The anti-interference performance of the fluorescent probe HPTIA for mercury ion recognition was demonstrated by the following steps:

[0061] Add 1 eq. of Ca to the fluorescent probe HPTIA solution sequentially. 2+ Fe 3+ Sn 2+ Mn 2+ Ba 2+ Cu 2+ Fe 2+ Al 3+ Zn 2+ Ag + Gd 3+ Nd 3+ Y 3+ Tb 4+ Pr 3+ Ce 4+ Eu 3+ An aqueous solution of Hg was used, and its fluorescence intensity was recorded at an excitation wavelength of 365 nm. Then, 1 eq. of Hg was added sequentially. 2+ In aqueous solution, the changes in fluorescence intensity were observed and recorded, and the results are as follows: Figure 6 , 7 As shown, the addition of mercury ions to each system resulted in a significant enhancement of fluorescence. Therefore, the fluorescent probe HPTIA exhibits good anti-interference capability for the detection of mercury ions.

[0062] Example 9: The detection limit of the fluorescent probe HPTIA for mercury ions was determined by the following steps:

[0063] Take 20 μL of a solution with a concentration of 1.0 × 10⁻⁶. -4 A solution of the fluorescent probe HPTIA at a concentration of 1 mol / L was prepared by adding mercury ion solutions of different concentrations, and finally diluted to 2 mL with methanol solution. The fluorescence intensity was then measured, and the results are as follows: Figure 8 As shown, when the mercury ion concentration is within the range of 1 μM to 100 μM, the fluorescence intensity continuously increases with increasing mercury ion concentration. Furthermore, within the range of 1 μM to 100 μM, the fluorescence intensity of the probe HPTA exhibits a good linear relationship with the mercury ion concentration, with the fitting equation being y = 10.631471x + 39.67736, R² = 0.9930. Based on the formula for calculating the detection limit 3σ / k, the detection limit of the fluorescent probe HPTA for mercury ions is calculated to be 187 nmo¹ / L, indicating that the fluorescent probe HPTA can achieve trace detection of mercury ions with good detection sensitivity.

[0064] Example 10: The coordination ratio of the fluorescent probe HPTA to mercury ions was determined according to the following steps:

[0065] Maintain the total concentration of the fluorescent probe HPTIA and mercury ions in the detection system at 1 x 10⁻⁶. -6 With the mol / L ratio kept constant, the equivalence ratio of the fluorescent probe HPTIA to mercury ions was varied, and the fluorescence intensity was measured separately. A Job's Plot was then plotted, and the results are shown below. Figure 9 As shown, the fluorescence intensity reaches an inflection point when the mole fraction of mercury ions is 0.48, indicating that the coordination ratio between the fluorescent probe HPTA and mercury ions is 1:1.

[0066] Example 11: The response time of the fluorescent probe HPTIA to mercury ions was determined according to the following steps:

[0067] Take 20 μL of a solution with a concentration of 1.0 × 10⁻⁶. -4 Add 20 μL of 1 x 10 mol / L fluorescent probe HPTIA solution to a solution containing 1 x 10 mol / L. -4 A mol / L aqueous solution of mercury ions was diluted with methanol to 2 mL. The fluorescence intensity was measured every three seconds, and the results were recorded as follows: Figure 10 As shown, the fluorescence intensity of the system stabilized after approximately 15 minutes following the addition of mercury ions. This indicates that the probe HPTA can rapidly recognize mercury ions.

[0068] Example 12: The number of reversible cycles of the fluorescent probe HPTIA for mercury ions was determined by the following steps:

[0069] The experiment tested the reversibility of a 100 μM mercury ion fluorescent probe by alternately and incrementally adding mercury ions (Hg²⁺) and sulfide ions (S²⁻). The specific procedure was as follows: First, a series of solutions with a constant probe concentration of 100 μM were prepared. The first solution was a pure probe solution as the fluorescence baseline; the second solution added Hg²⁺ to the first solution to achieve a final concentration of 100 μM; the third solution added S²⁻ to the second solution to also achieve a final concentration of 100 μM. Subsequent samples followed this pattern, adding Hg²⁺ or S²⁻ alternately and at equal concentrations to the previous sample, increasing the final concentration of the target ion by 100 μM each time. The reversible response performance of the probe was visually confirmed by measuring the cyclical change in fluorescence intensity of this series of samples during the "on-off-on-off" cycle. The results were recorded as follows: Figure 11 As shown. Example 13: Application of the fluorescent probe HPTIA in the detection of mercury ions in actual water samples.

[0070] To investigate the potential application of the fluorescent probe HPTIA in a real-world environment, laboratory tap water was selected and pretreated: the water sample was centrifuged at 4000 rpm for 10 min, filtered using a 0.45 μm filter, and mercury ion solutions with concentrations of 25 μmol / L, 50 μmol / L, and 75 μmol / L were prepared. These different concentrations of mercury ion solutions were added to the HPTIA solutions, and the fluorescence emission peak intensity of the probe at 530 nm was measured at an excitation wavelength of 365 nm. The concentration of the mercury ion solution was calculated by substituting this value into the following equation. The detection results are shown in Table 1.

[0071] y = 10.631471x + 39.67736

[0072] Where X is the mercury ion concentration and Y is the fluorescence emission peak intensity.

[0073] Table 1. Detection of mercury ions using the fluorescent probe HPTIA in actual water samples.

[0074]

[0075] As shown in Table 1, the recovery rate of mercury ions in actual water samples was 97.35%–103.21%, with a relative standard deviation of 0.31–5.49%. The measured mercury ion concentration had a very small error compared to the corresponding spiked concentration. These results indicate that the fluorescent probe HPTIA prepared in this invention has good accuracy in detecting mercury ions in actual water samples, and can quantitatively detect mercury ions in the range of 0–100 μmol / L, demonstrating good practical performance.

Claims

1. A cyanobrythryl-based mercury ion fluorescent probe compound, characterized in that, The molecular structure is as follows:

2. The method for preparing the cyanobistryl mercury ion fluorescent probe compound according to claim 1, characterized in that, Includes the following steps: Step (1): 4-hydroxyphenylacetonitrile, a moderately strong base, and terephthalaldehyde monodiethyl acetal or terephthalaldehyde in a molar ratio of 1:0.1-5:1-3 are refluxed in a condensation reaction solvent at a reaction temperature of 60°C. o C~100 o C, with a reaction time of 3 h to 12 h, yields the intermediate product FHCS; Step (2): FHCS, 2-thiocaprolactam, and a moderately strong base are refluxed in the condensation reaction solvent at a reaction temperature of 80°C. o C~130 o C, the reaction time is 8 h to 24 h, to obtain probe HPTA, wherein the molar ratio of FHCS, 2-thiocaprolactam and sodium hydroxide is 1:1 to 3:0.5 to 4; The aforementioned moderately strong base includes any one of pyridine, magnesium hydroxide, potassium carbonate, cesium carbonate, sodium hydroxide, or potassium hydroxide; The solvent for the condensation reaction includes any one of methanol, ethanol, benzene, toluene, N,N-dimethylformamide, dimethyl sulfoxide, or acetic acid.

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