Xu-Mei Fu

A colorimetric and fluorescent dual-channel chemosensor (G1) with a simple structure was designed and synthesized to identify Hg

 in an aqueous solution with high selectivity and sensitivity. After adding Hg

, the color of G1 in the dimethyl sulfoxide (DMSO)/H

O (2:8, v/v) solution changed from colorless to a distinct rose red, whereas the fluorescence changed from colorless to bright orange under ultraviolet (UV) light. The detection limit on the fluorescence response of G1 to Hg

 mol/L. The results of the Job’s plot suggested that the combined stoichiometry between G1 and Hg

 and ethylenediaminetetraacetic acid (EDTA), the fluorescence process can be reversed and almost no fluorescence loss. In addition, G1-based test strips were produced and were able to qualitatively detect Hg

With the development of science and technology, mercury (Hg) is widely used in medical, military, and electrical applications (1–3). Hg

 is easily accumulated in the human body, and its great toxicity lies in its strong ability to bind to thiol groups and amino groups in proteins and enzymes (4,5), which can further cause vision loss, mental disorders, movement disorders, tumor formation, and other diseases (6–8). After mercury-containing compounds are introduced into the ecosystem, their strong migration and low degradation pollutes the environment (9–11). The maximum allowable limit of mercury ions in drinking water that was stipulated by the World Health Organization is 6 ng/mL (12–14).

In recent years, more fluorescent sensors, nanoparticles, and chemical probes have been developed for mercury (II) detection because of their simple operation, high sensitivity and selectivity, and short detection time (15–22). However, most of the functional groups of fluorescent probes are easily oxidized and contain sulfur (23–26), which is highly toxic (27–34). Therefore, it is of great significance to develop an effective and rapid detection method for mercury ions.

Our research group has studied mercury-detecting probes for a long time (35–43). 

 is a type of fluorescent probe luminescent group with high fluorescence quantum efficiency, favorable water solubility, and a high absorption coefficient (44–46). In the absence of Hg(II), rhodamine was present in the nonfluorescent spiral state. However, with the addition of Hg(II), the color changed, the fluorescence emission was enhanced, and the spiral ring opened. Because of these observations when Hg(II) is added to rhodamine, it makes rhodamine an ideal signal group for detecting Hg(II) (47,48). In this paper, we designed a probe, G1, based on a rhodamine-benzaldehyde derivative, and systematically investigated the recognition performance toward metal ions. After adding Hg

, the G1-based ring went through an opening process, the solution color changed from colorless to red, and the fluorescence changed from a closed to an open state. Rhodamine B acted more effectively in Hg

 detection because of its long excitation wavelength, good light stability, high fluorescence quantum yield, good water solubility, and less toxicity compared to other rhodamines. The results showed that G1 was an open fluorescence sensor with high sensitivity and high selectivity to detect Hg

. The test paper based on the probe G1 was constructed, and that was able to successfully detect Hg

All reagents and solvents in the experiment were purchased from the market without further purification. Perchlorate salt of cations (Cu

) were purchased from Alfa-Aesar Chemical Reagent Co. and stored in a vacuum desiccator. Melting points were performed on an X-4 digital melting point apparatus. The ultraviolet-visible (UV-vis) absorption spectra and fluorescence emission spectra was carried out by a UV-2550 UV-vis absorption spectrometer and an RF-5301 fluorescence spectrometer using a 1-cm quartz cell. Data from 

C NMR were recorded using a Varian Mercury-400BB A nuclear magnetic resonance (NMR) spectrometer and the measured mass spectrometry (MS) data was recorded using a ZAB-HS mass spectrometer instrument.

Compound 1 was obtained by one-step condensation (49), using rhodamine B (1.92 g, 4 mmol) and hydrazine hydrate (2 mL, 4 mmol) in methanol (20 mL). The reaction was refluxed 75 °C for 6 h. After the reaction was completed, it was cooled to room temperature. When the pH was adjusted to 8 with a sodium hydroxide (NaOH) solution, a large amount of solid precipitation occurred. The compound was then washed three to four times with distilled water to produce a pale pink compound in 81% yield, melting point: 213–215 °C.

Compound 1 (0.92 g, 2 mmol) and 2-bromo-3-hydroxy-4-methoxybenzaldehyde (0.46 g, 2 mmol) were dissolved in ethanol (20 mL), and the solution was stirred and refluxed for 6 h. After the mixture was cooled to room temperature and the precipitate was washed with distilled water, the mixture was filtered and the precipitate was recrystallized with DMF-H

O to give a crimson compound in 77% yield. As shown in Scheme 1, the melting point was between 234–236 °C.

General Procedure for UV-vis and Fluorescence Spectroscopy Experiments

In UV-vis and fluorescence experiments, G1 and several cations (Cu

) were prepared in DMSO solution. At the same time, the subjects in all experiments were kept at a con- stant concentration (2.0 × 10

 M). The UV-vis absorption spectrum and the fluorescence spectrum were measured using a UV-2550 UV-vis absorption spectrometer and a RF-5301 fluorescence spectrometer, respectively.

The procedure was as follows: 5 mg of ligand G1 was dissolved in an nucleus magnetic tube with DMSO-d

, the latter being added to the former in the corresponding amount. Then, the measurement was carried out in a Varian Mercury-400BB NMR apparatus (Table I).

After adding 50 equivalents of various cations (Cu


v/v) solution at room temperature, we observed these solutions changing. The results showed that a wide absorption peak appeared at 562 nm on the UV-vis absorption spectrum. However, no significant change was observed after adding other cations to the G1 solution. Meanwhile, it was observed that the G1 solution changed from colorless to rose red after the addition of Hg

FIGURE 1: (a) UV-vis spectra of G1 (2.0 × 10

 M) in the presence of 50 equivalent various anions in a DMSO:H

O (2:8, v/v) solution. Inset: Color changes of G1 (2.0 × 10

 M) with various anions (50 equivalent) in the DMSO:H

O (2:8, v/v) solution. (b) Fluorescence spectrum of G1 (2.0 × 10

The fluorescence spectrum showed that G1 in a DMSO/H

O (2:8, v/v) solution had almost no fluorescence under 437 nm excitation. With the addition of Hg

 to the G1 solution, the fluorescence intensity at 578 nm was significantly enhanced and a change in fluorescence color from colorless to bright orange could be observed. At the same time, other cations (Cu

) didn’t cause any significant change in the fluorescence emission of the G1 solution. The results suggested that the G1 solution was able to uniquely identify Hg

 titration experiment at room temperature to investigate the change in the response of G1 (2.0 × 10

 M) to the ring opening of the rhodamine derivatives in the DMSO/H

O (v/v =2:8) solution. As shown in Figure 2, the Hg

 (0.1 M) gradually added to the DMSO/H2O (2:8, v/v) solution of G1, and the changes in UV-vis absorption spectrum and fluorescence spectrum were observed. In the UV-vis absorption spectrum, as Hg

 increases from 0 to 15.2 equivalents, the absorption peak at 562 nm gradually increases. Figure 4 shows the change in fluorescence emission spectra with the addition of Hg

. At the excitation at 400 nm, when the concentration of Hg

 increased from 0 to 24.1 equivalents, the fluorescence intensity of the host gradually enhanced. 

FIGURE 2: (a) Absorbance spectra titration of G1 (2.0 × 10

 M) in the presence of different concentrations of Hg

O (v/v= 2:8). (b) Fluorescence intensity titration of G1 (2.0 × 10

FIGURE 4: (a) Fluorescence detection limit spectra of G1 in a DMSO:H

O (v/v =2:8) solution upon adding an increasing concentration of Hg

To further verify the selectivity of the sensor G1, we studied the recognition ability of the host G1 to Hg

 in the presence of a series of cations (Cu

) by recording the changes in the UV-vis spectra and the fluorescence spectrum. As shown in Figure 3, we found that the absence of Hg

, no significant spectral changes were observed after the addition of other cations to G1. However, when Hg

 was added to the solution, they all showed significant spectral changes. The experimental results showed that other cations had considerable influence on the recognition of Hg

 by G1 in the UV-vis spectra and fluorescence spectrum.

FIGURE 3: (a) Absorbance spectra changes for G1 (2.0 × 10

 M) in the presence of selected metal ions in DMSO:H

O (v/v =2:8) solution. (b) Fluorescence spectra of G1 (2.0 × 10

 M) in the presence of selected metal ions in a DMSO:H

Figure 4 shows that the fluorescence intensity of G1 changes almost linearly with the increase of the Hg

 concentration. As calculated on the basis of 3SB/S (where 

 is the standard deviation of the blank solution and 

 is the slope of the calibration curve) (50), the detection limit was down to 6.09 × 10

 M. To determine the stoichiometric ratio of G1 and Hg

, we set the total concentration of the host and guest as a fixed value, and a Job’s plot was drawn. We changed the ratio of the host and guest in the solution before measuring the fluorescence spectrum. The result showed that the stoichiometric ratio of G1:Hg

To explore the mechanism of sensor G1 for Hg

H NMR titration was carried out. As shown in Figure 5, G1 has a single peak at 8.64 ppm and 9.55 ppm in DMSO-d

, which corresponds to−CH=N and−OH, respectively. After Hg

 was added, the −OH peak of the G1 gradually disappeared. In addition, the proton peak in the benzene ring also had a splitting phenomenon because of the electron density change caused by the ring opening of the spirolactam of the G1. Based on these results, we showed the possible reaction mechanism between G1 and Hg

FIGURE 5: (a) The proposed reaction mechanism between G1 and Hg

H NMR spectrum changes of different amounts of Hg

To test the effect of pH (2–12) on the recognition process of G1 in the DMSO/H

 to the different pH values of the G1 buffer solution for pH experiments. When adding Hg

 to G1 buffer solutions of different pH values, it was found that the host G1 recognized Hg

 in the pH range of 4–9. It indicated that the recognition of ions with the host G1 is affected by pH (Figure 6).

FIGURE 6: Corresponding fluorescence intensity of G1 to Hg

We studied the repeatability of the sensor and added ethylenediaminetetraacetic acid (EDTA) to the G1-Hg

mixture to test and identify whether the process was reversible. As shown in Figure 7, the fluorescence of the G1-Hg

 solution restored to the initial state after EDTA was added, indicating that the complexation process of the main body G1 Hg

 was reversible. Because of the strong complexing ability of EDTA of Hg

, the sensor demetalized and spironolamide ring regeneration led to the fluorescence quenching again after EDTA and Hg

 complexed. The fluorescence of sensor G1 can be switched “Off-On-Off” repeatedly, with little fluorescence loss during this process.

FIGURE 7: The reversibility of the fluorescence of G1 controlled by alternating additions of EDTA and Hg

We made test strip based on the host G1, which could quickly detect Hg

 in the actual environment. We dipped the prepared test paper into a DMSO solution of G1 (0.1 mol/L), and dried it in a vacuum drying oven. As shown in Figure 8, when Hg

 aqueous solution was added to the prepared test strip, the obvious color change of the test strip was clearly observed, with strong red fluorescence being observed under the UV lamp. Therefore, the prepared test strip was able to quickly and efficiently detect Hg

FIGURE 8: Photographs of G1 (0.1 M) on test strips (a) only G1, (b) after immersion into water solutions with Hg

, (c) only G1 under the UV lamp, and (d) after immersion into water solutions with Hg

 under a UV lamp at room temperature and irradiation under UV light at 365 nm.

We further verified that G1 detects the Hg

 content in the water environment. We added 0–5 ppb of Hg

 to the G1 (0.1 mol/L) solution. When the concentration reached 2 ppm (the maximum allowable level of mercury ions in US Environmental Protection Agency drinking water), the fluorescence intensity of the sensor G1 increased by three times (Figure 9).

FIGURE 9: Linear fluorescence response of G1 (0.1 M) in water samples. The response (F) is normalized to the emission of the free sensor (F

Through analyzing the water samples, we verified the practicability of the sensor. Under the same conditions, we used the fluorescence method to determine the Hg

 content in the tap water sample, and repeated the test three times. As shown in Table II, the Hg

 concentration determined by the experiment was similar to the added amount concentration, and the recovery rate was approximately 100%. The relative standard deviation (RSD) was less than 3.90%, indicating that the sensor G1 was accurate and feasible to measure the mercury content in environmental samples.

We designed and synthesized a convenient, fast and efficient sensor G1, which can selectively recognize Hg

 in a wide pH range with dual-channel. The detection limit of the sensor G1 for Hg

 and EDTA, the fluorescence pro- cess can be reversed, and the switching process can be repeated many times with almost no fluorescence loss. The mechanism of G1 recognition was discussed by means of 

H NMR, and it was believed that the reaction mechanism between G1 and Hg

 was because of the ring opening of the spironolac- tone ring of rhodamine. In addition, G1-based test strips were prepared, which could be used as colorimetric and fluorescence measurement tools for the rapid detection of Hg

. The qualitative determination of mercury ions in tap water has been tested successfully, and the feasibility of G1 in practical application has been verified. We believe that G1 has potential application potential as a Hg

We gratefully acknowledge the support of the National Nature Science Foundation of China (No. 21467012).

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Articles

A simple colorimetric and fluorescent dual-channel chemosensor was designed and synthesized to identify Hg2+ in an aqueous solution with demonstrated high selectivity and sensitivity.

A Novel High Selectivity Fluorimetric and Colorimetric Probe Based on Rhodamine B Hydrazide Derivatives for Detecting Hg2+ in Aqueous Media

 fluorescent probe, HNT, was synthesized by 1-hydrazine methyl-2-naphthol and 2,3,4-trihydroxybenzaldehyde. CN

 has stronger basicity and a superior hydrogen-bonding ability than other anions. Therefore, CN

 can deprotonate naphthalene hydroxyl in fluorescent probe HNT, which induced the bright yellow fluorescence that was quenched before changing the color of the solution from yellow to orange. This color change indicates that fluorescent probe HNT showed selective recognition of CN

 and was not interfered with by other anions. HNT can quantitatively detect CN

 M. The manufactured HNT test strip could also quickly detect CN

, which indicates that HNT has certain application value.

Cyanogen has attracted extensive attention because of its outstanding virulence and practicability among many anions. The toxicity of CN

 can cause serious harm to the environment and living organisms (1). When living organisms ingest a certain amount of CN

 through respiration, their digestive tract, or the surface of their skin, CN

 will inhibit the activity of various enzymes in cells, causing symptoms such as vomiting, convulsions, and even central respiratory failure and death (2–4). Cyanide is also widely used in industrial production to synthesize polymers fibers and resins, extract gold, and synthesize various herbicides, which increases the possibility of life being harmed by CN

 (5–7). The United States and the World Health Organization (WHO) stipulates that the maximum content of CN

 in drinking water should not exceed 1.9 μmol/L (8). Given this regulation, the synthesis of a fluorescent probe with a high sensitivity, low manufacturing cost, short response time, and specific recognition of CN

, is significant to the environment and organisms. In the literature, probes mainly detect CN

 through mechanisms such as addition, hydrogen bonding, and deprotonation (9–13). Among them, deprotonation uses CN with strong basicity to undergo deprotonation reaction with the hydroxyl or imino group in the probe to change the electron distribution in the probe molecule and generate corresponding photoelectrochemical changes. Currently, the probe is the most effective method for detecting CN

 concentration (14–18). Our team is also very interested in such probes (19–28).

In this work, 2-hydroxy-1-naphthaldehyde derivatives were used as luminescent groups because of their good spectral characteristics (14,29,30), which improves the sensitivity of these probes. To ensure the probe has better selectivity and recyclability, we designed a probe based on naphthaldehyde (31–33). After the probe reacted with CN

, the molecule was deprotonated because of the strong basicity of CN

. The fluorescence and UV spectrum of the molecule changed simultaneously, the color of the solution changed from yellow to orange, and the fluorescence was quenched at λ

= 365 nm. The results show that the HNT flourescence probe can selectively recognize CN

. The detection limit of the HNT probe for cyanide has been as low as 6.93 × 10

 M. In addition, we have made a test strip for the HNT probe, which can quickly identify and detect the contents of water samples. According to the mechanism of the above process, we verified it by 

H nuclear magnetic resonance (NMR), UV-vis, and other different methods. Besides that, we also made density functional theory (DFT) calculations.

All reagents and solutions are commercially available and analytically pure without further purification. The experimental water was double distilled and prepared by a standard method. The tetrabutyl ammonium salt (F

) of the anion were analytically pure and purchased from Alfa-Aesar Chemical Reagent Co. and stored in a vacuum dryer.

UV-vis spectra were measured on a Shimadzu UV-2550 UV spectrometer. The fluorescence emission spectrum was recorded on a Shimadzu RF-5301 fluorescence spectrometer; 

H NMR used tetramethylsilane (TMS, δ grade) as the internal standard and was measured at 600 MHz on a Mercury-400BB spectrometer. The melting point was measured on the X-4 digital micro melting point meter. Electrospray ionization–mass spectrometry (ESI-MS) was measured on a ZAB-HS instrument.

Synthesis and Characterization of Probe HNT

The intermediate 1-hydrazine methyl-2-naphthol was synthesized according to the method in the literature (34). 2-hydroxy-1-naphthalaldehyde hydrazine (0.9309 mg, 5 mM) and 2,3,4-trihydroxybenzaldehyde (0.7706 mg, 5 mM) were added to a round bottom flask with dimethylformamide (DMF) (25 mL) as the solvent and refluxed under stirring at 120 °C for 7 h before being cooled to room temperature, filtered to obtain a yellow solid, and crystallized with water ethanol to obtain a pure product (Scheme 1). The yield was 72% (m.p. > 300 °C). 

H-NMR spectral conditions and results are as follows: H-NMR (600 MHz, DMSO-d

H). IR(KBr; cm−): 3442 (s), 1625 (s), 1390 (m), 1353 (m), 1320 (m), 1177 (m), 993 (w), 840 (w), 786 (m), 742 (w); 

SCHEME 1: The synthetic procedure for sensor HNT.

The synthesis steps of fluorescence probe HNT were introduced in detail, and its structure has been characterized by 1H NMR, infrared (IR) spectroscopy, and ESI–MS. Next, the influence of different anions (F

 M) on the spectral changes of chemical probe were studied by UV spectroscopy and fluorescence spectroscopy.

After adding 50 equivalents of different anions (F

), a dimethyl sulfoxide (DMSO):water (1:1, v:v) solution containing the fluorescence probe HNT was utilized as the host to fully react and observe the changes of the solution. The results showed that the color of the solution containing the HNT probe changed from bright yellow to orange only after CN

 was added. At the same time, the corresponding absorption spectrum data showed that the strong absorption peak at 375 nm disappeared, and a new absorption peak appeared at 428 nm. However, other anions (F

) did not cause any significant change in the absorption spectrum of the HNT solution. (Figure 1a).

FIGURE 1: (a) Absorbance luminosity change spectrum of various anions added to sensor HNT solution (2.0 × 10

 M). Inset: Photographs of other anions and CN

 color changes added to HNT solution under visible light. (b) The fluorescence intensity change spectrum of various anions added to sensor HNT solution (2.0 × 10

 color changes added to HNT solution under 365 nm light.

The dimethyl sulfoxide:water (1:1, v:v) solution containing the host HNT was excited at 365 nm of the UV lamp. The emission peak at 533 nm was significantly reduced only when the CN

 ions were added (Figure 1b). Under the 365 nm of lamp, the solution was observed to change from bright yellow to colorless. When other anions were present in solution, the fluorescence color and emission intensity of the host HNT had no obvious change.

To verify the anti-interference ability of the probe HNT, we investigated the recognition ability of the probe HNT to CN

) by recording the UV-vis spectrum and fluorescence spectrum. As shown in Figure 2a, there was no obvious photochemical change in dimethyl sulfoxide:water (1:1, v:v) solution with other anions and host HNT. When CN

 was added to each group of solutions, the UV-vis spectrum and emission intensity were similar to the solutions, which contain CN

 only. The results showed that the existence of other competitive ions had an inconsiderable effect on the specific detection of CN

FIGURE 2: (a) Absorbance spectrum change diagram of adding 20 equivalents of various anions and 20 equivalents of CN

 = 428 nm). (b) The fluorescence intensity change diagram of adding 20 equiv. of various cations and 20 equivalents of CN

 was further studied by UV-vis and fluorescence titration. As shown in Figure 3a, once the concentration of CN

 increased from 0.0 to 8.2 equivalents, the absorption peak at 375 nm decreased and at 428 nm it increased gradually. With the increased CN

 concentration, the emission intensity at 533 nm decreased gradually under a 365 nm excitation wave. When the concentration of CN

 increases to 12.4 equivalents, the fluorescence emission intensity is the weakest (Figure 3b).

FIGURE 3: (a) Absorption spectrum of HNT (2.0 × 10

O (1:1, v/v) solution. Inset: Diagram of the relationship between absorbance and the number of equivalents of CN

. (b) Fluorescence spectra of HNT (2.0 × 10

A novel hydrazone-based CN- fluorescent probe, HNT, is described here, and a study was done to see if it could show selective recognition of CN- ions.

A Highly Selective Colorimetric and “Turn-Off” Fluorimetric Probe Based on Unsymmetrical Naphthalene Formaldehyde Derivatives for the Detection of CN− in Aqueous Media