A dansyl-based fluorescent probe for turn-off and turn-on detection of Hg 2+ in a full water system

doi:10.21203/rs.3.rs-1960827/v1

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A dansyl-based fluorescent probe for turn-off and turn-on detection of Hg 2+ in a full water system

https://doi.org/10.21203/rs.3.rs-1960827/v1

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A dasnyl-based fluorescent probe, 2-((5-dimethylamino)naphthalene-1- sulfonamido)acetic acid (DNSG), was synthesized from dansyl chloride and glycine. In HEPES buffer (20 mM, pH 6.7), DNSG could detect Hg²⁺ by fluorescence quenching. The maximal fluorescence intensity showed a good linear relationship with the Hg²⁺ concentration (0 ~ 80 µM). The detection limit was 0.33 µM. With the help of silk fibroin (0.076 g/L), DNSG could detect Hg²⁺ by fluorescence enhancement and blueshift. The linear concentration range for Hg²⁺ was 0 ~ 160 µM with a detection limit of 0.81 µM. DNSG could detect Hg²⁺ quickly and reversibly with excellent anti-interference ability and further could assay environmental water sample efficiently. The sensing mechanism was examined by Job’s plot, mass spectrum and ¹H NMR analysis.

Dasnyl

Fluorescent probe

Hg2+

Silk fibroin

Full water system

Mercury is of concern because highly toxic mercury ions and organic mercury cause serious human health and ecological problems. It has been reported that Hg²⁺ can cause irreversible cognitive and motion disorders, even at very low concentrations [1–3]. Thus, it is necessary to develop effective analytical methods for concise and accurate detection of trace amounts of Hg²⁺ [4]. Fluorescent probes with the advantages of high sensitivity, easy operation and nondestructiveness have been particularly attractive in the detection of heavy or transition metal ions [5–11]. However, most fluorescent probes have poor water solubility, and their detection must be implemented in organic solvent media. Fluorescent probes with good water solubility are urgently needed. Furthermore, one probe can usually respond to the analyte by fluorescence quenching or fluorescence enhancement. There are few fluorescent probes that can detect one analyte by both fluorescence quenching and fluorescence enhancement.

Dansyl is a significant fluorophore suitable for fluorescent probes because of its high fluorescence quantum yield and large Stokes shift [12–17]. To date, most dansyl-based probes detect metal ions by fluorescence quenching [18–22], while turn-on fluorescent probes have higher sensitivity and more application fields. Therefore, developing turn-on dansyl-based fluorescent probes is of great significance. Priyanka et al. reported that the dansyl amino acid probe could detect Hg²⁺ by fluorescence enhancement after the addition of bovine serum albumin (BSA) [23], which suggests that the sensitivity of the probe to substrate can be changed by introducing some assisted components to the detection system.

Here, we attached glycine to a dansyl moiety to achieve a fluorescent probe (DNSG) with a simple structure and good water solubility. Similar to most reported dansyl-based fluorescent probes, DNSG could detect Hg²⁺ through fluorescence quenching. Then, inspired by the literature, silk fibroin (SF) prepared from the cocoons of B. mori silkworm in our lab was introduced to the testing system and the expected result that DNSG could detect Hg²⁺ by fluorescence enhancement achieved.

2.1. Synthesis of the probe

Materials and apparatus can be found in Supplementary materials. As shown in Scheme 1, dansyl chloride (0.1 g, 0.37 mmol) was dissolved in acetone (10 mL), and sodium bicarbonate (0.59 g, 7.06 mmol) aqueous (10 mL) solution with glycine (0.13 g, 1.77 mmol) was added slowly. The resulting mixture was reacted for 2 h at room temperature [24]. Then, the solvent was evaporated to dry on a rotary evaporator. HCl aqueous (1 M) was added dropwise to adjust the pH to 4.5, and ethyl acetate (15 mL⋅3) was used to extract the product in aqueous solution. The organic layer was collected, and the solvent was evaporated on a rotary evaporator. The product was purified using column chromatography on silica and eluted with methanol/dichloromethane (1/10, v/v). The dansyl-based fluorescent probe DNSG was obtained as a brown solid (23 mg, 19. %). ¹H NMR (400 M, DMSO-d₆, δ/ppm): 8.44 (d, J = 7.9 Hz, 1H, d), 8.29 (d, J = 8.8 Hz, 1H, g), 8.13 (d, J = 7.0 Hz, 1H, e), 7.59 (dd, J = 13.6, 7.9 Hz, 2H, f, c), 7.25 (d, J = 7.4 Hz, 1H, b), 3.53 (s, 2H, i), 2.82 (s, 6H, a) (see Supplementary materials, Fig. S1). ¹³C NMR (300 M, DMSO-d₆, δ/ppm): 171.19 (1C, m), 152.25 (1C, c), 136.92 (1C, h), 130.22 (1C, i), 130.06 (1C, e), 130.01 (1C, k), 128.88 (1C, f), 128.76 (1C, g), 124.45 (1C, j), 120.17 (1C, d), 116.04 (1C, b), 46.09 (3C, a, l) (Fig. S2). HRMS (m/z): calculated for C₁₄H₁₆N₂O₄S 308.0831, found 309.0928 [M + H]⁺, 331.0721 [M + Na]⁺ (Fig. S3). IR (KBr) cm^− 1: 3278 (OH), 2940 (CH₃), 1700 (C = O), 1592, 1469, 1415 (ArH) (Fig. S4). Elemental analysis: calculated for C₁₄H₁₆N₂O₄S: C, 54.53, N, 9.08, H, 5.23; found: C, 54.15, N, 8.86, H, 5.61.

2.2. Preparation of the silk fibroin solution

The cocoons of B. mori silkworm were degummed three times in 0.05% (wt) Na₂CO₃ solution at 98 ~ 100 ℃ for 30 min and then washed completely with DI water to remove the sericin proteins on the outermost surface. After drying at 60 ℃ for at least 6 h, the silk fibers were dissolved in LiBr solution (9.5 mol/L) in a 15:100 bath ratio (w/v) for 40 min, followed by dialysis against running DI water to remove the salts and small molecules for 3 ~ 4 d and then further purification by centrifugation to obtain the final silk fibroin (SF) solution.

2.3. UV–Vis absorption and fluorescence test

DNSG and metal salts were dissolved in deionized (DI) water to form 1 mM and 10 mM stock solutions, respectively. When DNSG acted as a quenched probe, a stock solution of DNSG (100 µL) and one of the ion stock solutions (200 µL) were added into a 10 mL volumetric flask and diluted with HEPES buffer solution (20 mM, pH 6.7) for the selectivity study. In the titration experiment, 100 µL stock solution of DNSG was mixed with a certain amount of the Hg²⁺ stock solution and diluted to 10 mL with HEPES buffer solution (20 mM, pH 6.7). When DNSG was used as an enhanced probe, silk fibroin (200 µL, 0.076 g/L) was added to the volumetric flask before metal ions, and the rest of the operation was the same as that of the quenched probe. The spectra of the mixed solutions were recorded after standing for 5 min. For fluorescence measurement, the excitation wavelength was 330 nm, and the slit width was 5 nm.

2.4. Real sample assay

In the real sample assay [25–26], pond water (after filtration) or tap water from Dushu Lake Campus of Soochow University was used instead of DI water to prepare HEPES buffer solution. Then, 100 µL of a stock solution of DNSG and 60 (80 or 100) µL of a stock solution of Hg²⁺ were introduced. The mixed solution was diluted to volume with HEPES buffer solution (20 mM, pH 6.7), and the fluorescence spectra were recorded. The concentration of Hg²⁺ in real water samples was calculated from the linear relationship between the maximal fluorescence intensity of DNSG and the concentration of Hg²⁺.

3.1. DNSG as a turn-off fluorescent probe for Hg²⁺

3.1.1. Selectivity of DNST to Hg²⁺

The UV–vis absorption response of DNSG to various metal ions was measured (Fig. 1) first in HEPES buffer solution (20 mM, pH 6.7). Among K⁺, Na⁺, Mg²⁺, Fe³⁺, Cu²⁺, Zn²⁺, Cr³⁺, Fe²⁺, Ca²⁺, Pb²⁺, Hg²⁺, Ni²⁺, Mn²⁺, Co²⁺ and Cd²⁺, most of the ions had no apparent effects on the absorption of DNSG, except that Fe³⁺ slightly strengthened the absorption. This result indicated that the UV–vis absorption spectra of DNSG did not obviously respond to the investigated metal ions in HEPES buffer solution (20 mM, pH 6.7).

Then, the fluorescence spectra of DNSG upon the addition of metal ions were measured. As shown in Fig. 2, the fluorescent signal of DNSG was strong upon excitation at 330 nm. The addition of Hg²⁺ induced a great decrease in fluorescence intensity at 552 nm accompanied by the disappearance of the yellow fluorescence, while other ions caused no significant changes in the fluorescent spectra of DNSG. Therefore, the fluorescent spectrum of DNSG was highly selective and sensitive to Hg²⁺.

3.1.2. Fluorescence intensity of DNSG versus concentration of Hg²⁺

The titration of DNSG (10 µM) with Hg²⁺ was performed. As shown in Fig. 3, upon the gradual addition of Hg²⁺ (0 ~ 80 µM), the yellow fluorescence of DNSG solutions decreased. The maximum fluorescence intensity and the concentration of Hg²⁺ presented a good linear relationship with a correlation coefficient of 0.9932. The evaluated detection limit for Hg²⁺ was 0.33 µM (Calculation method: see Supplementary materials). Therefore, DNSG could be used for the quantitative detection of Hg²⁺ by fluorescence quenching.

3.1.3. Effects of competing ions

To examine the selectivity of the probe for Hg²⁺ in a complex background with potentially competing species, the fluorescence quenching of DNSG with Hg²⁺ was investigated in the presence of other metal ions. As shown in Fig. 4, the maximum fluorescence intensity of the DNSG/Hg²⁺ solutions with and without one of the competing ions showed no significant differences, which indicated that the competitive ions had negligible disturbance to the detection of Hg²⁺. This observation suggests that DNSG has good anti-interference ability for detecting Hg²⁺.

3.1.4. Time response of DNSG to Hg²⁺

The time dependence of DNST for sensing Hg²⁺ was examined. Upon addition of Hg²⁺, the fluorescence intensity at 552 nm quenched at once and stayed at a very weak level during the tested time (Fig. 5). These results showed that DNSG could be a reliable quickly responsive probe for Hg²⁺.

3.2. DNSG as a turn-on fluorescent probe for Hg²⁺

3.2.1. Selectivity of DNST to Hg²⁺

To enhance the sensitivity of the DNSG probe, we applied SF to the test system. As shown in Fig. 6, adding SF (0.076 g/L) into HEPES buffer solution (20 mm, pH 6.7) of DNSG caused no variation in the UV–vis absorption spectrum of DNSG. When metal ions were added, only Hg²⁺ induced a very weak absorption peak at approximately 280 nm, and other ions caused the absorption spectra of DNSG with silk fibroin to drift up or down in the UV region.

Next, we studied the fluorescent response of DNSG after the addition of SF to various ions. K⁺, Na⁺, Mg²⁺, Fe³⁺, Cu²⁺, Zn²⁺, Cr³⁺, Fe²⁺, Ca²⁺, Pb²⁺, Hg²⁺, Ni²⁺, Mn²⁺, Co²⁺ and Cd²⁺ were added to HEPES buffer solution (20 mM, pH 6.7) of DNSG with SF. As Fig. 7 shows, only Hg²⁺ strengthened the fluorescence by 1.4-fold, accompanied by a 52 nm blueshift. The fluorescence of the DNSG, DNSG/SF and DNSG/SF/Hg²⁺ solutions changed from light yellow to yellow to blue. Other ions showed no pronounced effects on the fluorescence spectra. The results demonstrated that DNSG could detect Hg²⁺ by fluorescence enhancement and blueshift.

3.2.2. Fluorescence intensity of DNSG versus concentration of Hg²⁺

Fluorescence titration was conducted, and the results are shown in Fig. 8. The emission intensity increased gradually with increasing concentrations of Hg²⁺ (0 ~ 200 µM), and the wavelength had a 52 nm blueshift. There was a good linear relationship (R² = 0.9859) between the fluorescence intensity and Hg²⁺ concentration in the range of 0 ~ 160 µM. The detection limit was 0.81 µM.

3.2.3. Effects of competing ions

To research the influence of coexisting ions on the detection of Hg²⁺, K⁺, Na⁺, Mg²⁺, Fe³⁺, Cu²⁺, Zn²⁺, Cr³⁺, Fe²⁺, Ca²⁺, Pb²⁺, Ni²⁺, Mn²⁺, Co²⁺ and Cd²⁺ were added to the DNSG/SF/Hg²⁺ solution respectively, and fluorescence spectra were recorded. As shown in Fig. 9, the competitive ions showed negligible disturbance to the detection of Hg²⁺ by both maximal fluorescence intensity and maximal fluorescence wavelength. This observation suggested that DNSG had good anti-interference ability for detecting Hg²⁺ as a turn-on fluorescent probe.

3.2.4. Time response of DNSG to Hg²⁺

As an important factor for evaluating the probe in practical sensing, the response time toward Hg²⁺ was tested. Clearly, the addition of Hg²⁺ to the HEPES buffer solution of DNSG/SF led to an immediate and obvious fluorescence enhancement and blueshift of the peaks. Moreover, the fluorescence intensity and the shift of the peaks remained constant over a relatively wide period of time. Hence, DNSG is a reliable quickly responsive turn-on fluorescent probe for Hg²⁺.

3.3. Real sample analysis

To test the practical application of DNSG, a standard addition method to determine Hg²⁺ in pond water and tap water samples was applied. The Hg²⁺ content in all samples was calculated by the linear equation from fluorescence titration. The results are displayed in Table 1.

Table 1

Recovery of Hg²⁺ in pond water and tap water.
Method		Hg²⁺added /µM	Hg²⁺ found /µM	Recovery /%	RSD (n = 3) /%
Turn-off	Pond water	60	57.3	95.5	1.4
	Pond water	80	77.7	97.1	0.7
	Tap water	60	58.6	97.7	1.2
	Tap water	80	78.5	98.1	0.6
Turn-on	Pond water	60	56.8	94.7	0.7
	Pond water	100	100.8	100.8	2.6
	Tap water	60	57.2	95.3	5.3
	Tap water	100	98.3	98.3	0.2

From Table 1, it can be seen that the detected concentrations of Hg²⁺ were all close to that of the added ones whether DNSG was a turn-off or a turn-on fluorescent probe. As a quenched probe, the recovery was between 95.5% and 98.1%. The relative standard deviation (RSD) of the three measurements was between 0.6 and 1.4%. As an enhanced probe, the recovery was between 95.3% and 100.8%. The RSDs of the three measurements were between 0.2 and 5.3%. It is apparent that DNSG is a concise, accurate and effective fluorescent probe for Hg²⁺ that can detect Hg²⁺ in multiple ways.

3.4. Sensing mechanism exploration

3.4.1. DNSG as a turn-off fluorescent probe for Hg²⁺

The reaction mechanism of DNSG when it was a quenched probe was studied first. Job’s plot confirmed that DNSG and Hg²⁺ were 1:1 stoichiometric ratio (Fig. 11) with a binding constant of 3.63×10⁵ M^− 1 (Calculation method: see Supplementary materials).

Then, reversible experiment was performed. As shown in Fig. 12, Hg²⁺ weakened the fluorescence intensity of DNSG in HEPES buffer solution (20 mM, pH 6.7). Upon addition of EDTA to the solution of DNSG/Hg²⁺, the fluorescence spectra almost returned to the original state in the absence of Hg²⁺, and the result confirmed that the Hg²⁺ recognition process was reversible.

The mass spectrum of the DNSG/Hg²⁺ solution was determined as shown in Fig. 13, where m/z 511.3212 is the molecular ion peak of [DNSG + Hg²⁺+H⁺], and 533.3331 is the molecular ion peak of [DNSG + Hg²⁺+Na⁺], indicating that DNSG can complex with Hg²⁺ at a 1:1 ratio.

Considering Job’s plot, reversible experiment and mass spectrum, we depicted a sensing mechanism of turn-off DNSG for Hg²⁺ in Scheme 2.

3.4.2. DNSG as a turn-on fluorescent probe for Hg²⁺

The reaction mechanism of DNSG as an enhanced probe was also studied. Job’s plot experiment indicated a 1:2 stoichiometry for DNSG and Hg²⁺ (Fig. 14). The binding constant was 1.10×10⁸ M^− 2.

As shown in Fig. 15, the fluorescence intensity of DNSG in HEPES buffer solution (20 mM, pH 6.7) was weakly enhanced after the addition of SF. With the addition of Hg²⁺, the fluorescence intensity increased, and the wavelength blueshifted. Upon addition of EDTA to the solution of DNSG/SF/Hg²⁺, the fluorescence spectra almost returned to the original state in the absence of Hg²⁺, and the result confirmed that the Hg²⁺ recognition process was reversible.

¹H NMR spectra of DNSG, DNSG/SF and DNSG/SF/Hg²⁺ were collected and compared (Fig. 16). Except widening the proton peak of H₂O, the addition of SF to DNSG caused no obvious changes in the proton signals of DNSG, which indicated that DNSG did not react with SF. The reason that proton signals of SF were not observed may be due to the low concentration of SF and the solvent effect of D₂O. After adding Hg²⁺, the proton peaks of the naphthalene ring (7.63 ~ 8.46 ppm) and tertiary amino N(CH₃)₂ (3.07 ppm) in the ¹H NMR spectrum of DNSG/SF/Hg²⁺ were downfield, indicating that the nitrogen atom in N(CH₃)₂ probably coordinated with Hg²⁺ in the presence of SF. The peak of H₂O further widened, indicating that H₂O joined the complex. The peak of CH₂ (3.51 ppm) greatly reduced, indicating that its adjacent imino (NH) and carbonyl (C = O) groups participated in the chelation of Hg²⁺.

According to Job’s plot, reversible experiment and ¹H NMR spectra, we depicted a sensing mechanism of DNSG for Hg²⁺ with the assistance of SF in Scheme 3.

In summary, we reported a fluorescent probe (DNSG) synthesized from dansyl chloride and glycine. DNSG can detect Hg²⁺ not only by fluorescence quenching but also by fluorescence enhancement with the help of silk fibroin. The detection can be carried out in a full water system reversibly. DNSG showed many advantages, such as simple synthesis, good water solubility, high selectivity and sensitivity, fast response speed and strong anti-interference ability. 1:1 and 1:2 complex formed between DNSG and Hg²⁺ in the turn-off and turn-on detection respectively. Moreover, DNSG can be applied in determination of Hg²⁺ in environmental water samples. Thus, DNSG is a desired concise, accurate and effective multichannel fluorescent probe for Hg²⁺.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21074085), Key Laboratory of Polymeric Materials Design and Synthesis for Biomedical Function (Soochow University), and the National Key R&D Program of China (2019YFA0905200)

Supplementary materials

Supplementary materials associated with this article can be found in the online version.

Ethical Approval

Not applicable.

Competing interests

There are no competing interests to declare.

Authors' contributions

Chunming Sha did most research work and wrote the first draft of the manuscript.

Zhengkun Li supplemented some data.

Shengzhou Lu prepared silk fibroin.

Xiaolong Hu collated and checked references.

Dongmei Xu contributed ideas, supervised the work, revised and completed the manuscript.

Funding

Not applicable.

Availability of data and materials

Not applicable.

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Schemes 1 to 3 are available in Supplementary Files section.

No competing interests reported.

Supplementarymaterials.doc
Graphicalabstract.docx
Scheme1.png
Scheme 1. Synthetic route of DNSG.
Scheme2.png
Scheme 2. Proposed sensing mechanism for DNSG as a turn-off fluorescent probe for Hg²⁺.
Scheme3.png
Scheme 3. Proposed sensing mechanism for DNSG as a turn-on fluorescent probe for Hg²⁺.

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You are reading this latest preprint version

A dansyl-based fluorescent probe for turn-off and turn-on detection of Hg 2+ in a full water system

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Synthesis of the probe

2.2. Preparation of the silk fibroin solution

2.3. UV–Vis absorption and fluorescence test

2.4. Real sample assay

3. Results And Discussion

3.1. DNSG as a turn-off fluorescent probe for Hg²⁺

3.1.1. Selectivity of DNST to Hg²⁺

3.1.2. Fluorescence intensity of DNSG versus concentration of Hg²⁺

3.1.3. Effects of competing ions

3.1.4. Time response of DNSG to Hg²⁺

3.2. DNSG as a turn-on fluorescent probe for Hg²⁺

3.2.1. Selectivity of DNST to Hg²⁺

3.2.2. Fluorescence intensity of DNSG versus concentration of Hg²⁺

3.2.3. Effects of competing ions

3.2.4. Time response of DNSG to Hg²⁺

3.3. Real sample analysis

3.4. Sensing mechanism exploration

3.4.1. DNSG as a turn-off fluorescent probe for Hg²⁺

3.4.2. DNSG as a turn-on fluorescent probe for Hg²⁺

4. Conclusion

Declarations

References

Schemes

Additional Declarations

Supplementary Files

Status:

Version 1

A dansyl-based fluorescent probe for turn-off and turn-on detection of Hg 2+ in a full water system

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Synthesis of the probe

2.2. Preparation of the silk fibroin solution

2.3. UV–Vis absorption and fluorescence test

2.4. Real sample assay

3. Results And Discussion

3.1. DNSG as a turn-off fluorescent probe for Hg2+

3.1.1. Selectivity of DNST to Hg2+

3.1.2. Fluorescence intensity of DNSG versus concentration of Hg2+

3.1.3. Effects of competing ions

3.1.4. Time response of DNSG to Hg2+

3.2. DNSG as a turn-on fluorescent probe for Hg2+

3.2.1. Selectivity of DNST to Hg2+

3.2.2. Fluorescence intensity of DNSG versus concentration of Hg2+

3.2.3. Effects of competing ions

3.2.4. Time response of DNSG to Hg2+

3.3. Real sample analysis

3.4. Sensing mechanism exploration

3.4.1. DNSG as a turn-off fluorescent probe for Hg2+

3.4.2. DNSG as a turn-on fluorescent probe for Hg2+

4. Conclusion

Declarations

References

Schemes

Additional Declarations

Supplementary Files

Status:

Version 1

3.1. DNSG as a turn-off fluorescent probe for Hg²⁺

3.1.1. Selectivity of DNST to Hg²⁺

3.1.2. Fluorescence intensity of DNSG versus concentration of Hg²⁺

3.1.4. Time response of DNSG to Hg²⁺

3.2. DNSG as a turn-on fluorescent probe for Hg²⁺

3.2.1. Selectivity of DNST to Hg²⁺

3.2.2. Fluorescence intensity of DNSG versus concentration of Hg²⁺

3.2.4. Time response of DNSG to Hg²⁺

3.4.1. DNSG as a turn-off fluorescent probe for Hg²⁺

3.4.2. DNSG as a turn-on fluorescent probe for Hg²⁺