Synthesis and properties of three novel rhodamine-based fluorescent sensors for Hg<sup>2+</sup>

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Miao-Miao Hong, Ai-Feng Liu, Ying Xu, Dong-Mei Xu.Synthesis and properties of three novel rhodamine-based fluorescent sensors for Hg²⁺[J]. Chin. Chem. Lett., 2016,27(6): 989-992 复制到剪切板

Synthesis and properties of three novel rhodamine-based fluorescent sensors for Hg²⁺

Miao-Miao Hong^a,b, Ai-Feng Liu^a,b, Ying Xu^c, Dong-Mei Xu^a,b

^a College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China ;
^b Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Soochow University, Suzhou 215123, China ;
^c Testing and Analysis Center, Soochow University, Suzhou 215123, China

Received Jan 30, 2016, Received in revised form Mar 11, 2016, Accepted Mar 16, 2016, Available online Jun 25, 2016

* Corresponding author. E-mail:xdm.sd@163.com (D.-M. Xu)

Abstract: Three novel rhodamine-based Hg²⁺ fluorescent sensors were designed and synthesized. The sensors could work in semi-aqueous solutions with nearly neutral pH and showed high selectivity and sensitivity to Hg²⁺ with remarkable fluorescence enhancement. For these three sensors, the linear working range broadened (0-80, 0-100 and 0-140 mmol/L, respectively) and the sensitivity increased (7.7, 15.5 and 17.6 folds of the fluorescence enhancement and 512, 66.2 and 37.6 ppb of the detection limit) with the rising of the thiourea-unit numbers. Furthermore the sensors exhibited excellent interference immunity tomultiple environmentally and biologically relevant metal ions. Pond and tap water assay showed good practicability of the sensors. The number of the bound Hg²⁺ equaling to that of the thiourea units and the irreversible recognition process implied a new interaction way between Hg²⁺ and the sensor.

Key words: Rhodamine Polyethylenepolyamine Thiourea Fluorescent sensor Hg²⁺

1. Introduction

Mercury is the third most frequently found and second most toxic heavy metal according to the research results of the Agency for Toxic Substances and Disease Registry (ATSDR) of the USA Department of Health and Human Services ^[1]. Due to easy absorptivity,high bioaccumulation and persistence,all elemental, inorganic and organic mercury can cause human health problems such as serious congnitive and motion disorders,kidney,central nervous system and prenatal brain damages ^[2]. Hg²⁺ is the most common species of mercury pollutants. Hence,monitoring the presence of Hg²⁺ is very important.

Fluorescent sensors attract great attention in the field of heavy metal ion detection owing to being highly selective,highly sensitive,maneuverable,low-cost,and suitable for on-site and real-time signaling ^{[3, 4]}. A large number of 1,8-naphthalimide ^[5-7],rhodamine ^[8-12],BODIPY ^[13-15],coumarin ^{[16, 17]},and dansyl-based ^{[18, 19]} Hg²⁺ fluorescent sensors have been reported. However,some of them could only respond to Hg²⁺ in organic media ^{[9, 11, 13]},others suffered from poor sensing properties including selectivity,sensitivity,interference immunity and practicability ^{[8, 12, 20]}. The overall performance of the sensors still remains to improve and the relationship between the structure and the performance of the sensor has not been well established. So the study of new fluorescent Hg²⁺ sensors is of great significance.

Rhodamine dyes have the advantages of high photostability, long excitation and emission wavelength,easily modified parent structure,as well as remarkable and vivid color changes when triggered by the analyte ^{[3, 21, 22]}. N and S atoms are most effective coordination sites for Hg²⁺ ^{[9, 23-25]}. Therefore,herein we employed rhodamine B as reporter,thiourea as receptor,and polyethylenepolyamines as linker to construct novel Hg²⁺ sensors with pleasant overall performance such as aqueous working medium,favorable working pH,high sensitivity and selectivity, wide linear Hg²⁺ concentration range,strong interference immunity, and good availability.

2. Experimental

The novel fluorescent Hg²⁺ sensors with two,three and four thiourea receptors were prepared by rhodamine B (RB) and phenyl isothiocyanate (PITC) linked with diethylenetriamine (DETA), triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) respectively,as shown in Scheme 1. Their structures were fully characterized by FTIR,¹H NMR,¹³C NMR,LC-MS and elementary analysis.

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Scheme. 1. Synthetic route of the fluorescent Hg²⁺ sensors.

These three fluorescent Hg²⁺ sensors are: 1-(2-(3＇,6＇-bis(diethylamino)- 3-oxospiro[isoindoline-1,9＇-xanthen]-2-yl)ethyl)-3-phenyl- 1-(2-(3-phenylthioureido)ethyl)thiourea (RDTU),1-(2-(3＇,6＇- bis(diethylamino)-3-oxospiro[isoindoline-1,9＇-xanthen]-2- yl)ethyl)-3-phenyl-1-(2-(3-phenyl-1-(2-(3-phenylthioureido) ethyl) thioureido)ethyl)thiourea (RTTU) and 1-(2-(3＇,6＇-bis(- diethylamino)-3-oxospiro[isoindoline-1,9＇-xanthen]-2-yl)ethyl)-3- phenyl-1-(2-(3-phenyl-1-(2-(3-phenyl-1-(2-(3-phenylthioureido) ethyl)thioureido)-ethyl)thioureido)ethyl)thiourea (RPTU).

The detailed procedures and characterization of the new compounds,and the materials,apparatus and methods were described in the Supporting information (Figs. S1-S12).

3. Results and discussion

The novel fluorescent Hg²⁺ sensors were easily prepared by two step reactions between rhodamine B,polyethylenepolyamine (PEPA) and phenyl isothiocyanate (PITC). Through the quasi Click reaction between the -NH₂ and -NH groups in PEPA and the -NCS group in PITC,multiple phenylthiourea units were introduced into the sensors by a one-pot reaction under very mild conditions, which made the synthetic process high efficiency and energy saving.

The sensing properties of RDTU,RTTU and RPTU to Hg²⁺ was investigated through adding various metal ions,Na⁺,K⁺,Ca²⁺,Mg²⁺, Fe³⁺,Cu²⁺,Zn²⁺,Cr³⁺,Pb²⁺,Ni²⁺,Fe²⁺,Mn²⁺,Co²⁺,Cd²⁺ and Hg²⁺ (120 mmol/L),into the CH₃CN/HEPES buffer (pH 7.2) solutions of the sensors (20 mmol/L),and recording the absorption and fluorescence spectra of the solutions. It was found that the absorption spectra of the solutions showed no significant changes (Figs. S13-S15 in Supporting information),but the fluorescence of the solutions was enhanced dramatically by Hg²⁺ over other metal ions (Fig. 1). The results indicated that RDTU,RTTU and RPTU may be potential highly selective and sensitive turn-on fluorescent sensors for Hg²⁺. Further study showed that the optimal media for RDTU,RTTU and RPTU was CH₃CN/HEPES buffer (pH 7.2) 1:1,9:1 and 10:1 (v/v) respectively.

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Figure 1. Fluorescence spectra of the sensors in the absence and presence of various metal ions. Solvent: CH₃CN/HEPES buffer (pH 7.2, 1:1, 9:1 and 10:1 (v/v) for RDTU, RTTU and RPTU, respectively). Concentration: 20 mmol/L for sensors, 120 mmol/L for metal ions. λ_ex: 520 nm, slit width: 10 nm. Other ions: Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Cu²⁺, Zn²⁺, Cr³⁺, Pb²⁺, Ni²⁺, Fe²⁺, Mn²⁺, Co²⁺, and Cd²⁺.

The dependence of the fluorescence intensity of the sensor/Hg²⁺ solutions on the concentration of Hg²⁺ was studied. The results showed that the fluorescence intensity of the sensor/Hg²⁺ solutions first increased then level off with the rising of Hg²⁺ from 0 to 220 mmol/L (Fig. 2). The solutions of RDTU,RTTU and RPTU (20 mmol/L) with 140 mmol/L of Hg²⁺ got 7.7,15.5 and 17.6-fold fluorescence enhancement,respectively. The maximal fluorescence intensity and the concentration of Hg²⁺ ([Hg²⁺]) showed good linear relationship in the range of 0-80,0-100 and 0-140 mmol/L of [Hg²⁺] with the detection limits of 512,66.2 and 37.6 ppb,respectively. The results showed that not only RDTU, RTTU and RPTU could be used to quantitatively detect Hg²⁺,but also the linear working concentration range was broadened,and the sensitivity was raised along with the increase of the number of the NCH2CH2 and the thiourea units.

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Figure 2. The relationship between the maximal fluorescence intensity and the concentration of Hg²⁺. Solvent: CH₃CN/HEPES buffer (pH 7.2, 1:1, 9:1 and 10:1 (v/v) for RDTU, RTTU and RPTU, respectively). Concentration: 20 mmol/L for sensors. 0–220 mmol/L for Hg²⁺. λ_ex: 520 nm; slit width: 10 nm.

To examine the effects of coexistent metal ions on the detection of Hg²⁺,the environmentally and biologically relevant ions,Na⁺,K⁺, Ca²⁺,Mg²⁺,Fe³⁺,Cu²⁺,Zn²⁺,Cr³⁺,Pb²⁺,Ni²⁺,Fe²⁺,Mn²⁺,Co²⁺ and Cd²⁺ (120 mmol/L),were added to the sensor (20 mmol/L)/Hg²⁺ (120 mmol/L) CH₃CN/HEPES buffer solutions (pH 7.2) and the fluorescence spectrawere tested. As shownin Fig. 3,the competitive ions showed no pronounced influence on the fluorescence intensity of the sensor/Hg²⁺ solutions. Therefore RDTU,RTTU and RPTU have excellent anti-interference ability for Hg²⁺ detection and they are reliable Hg²⁺ fluorescent sensors.

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Figure 3. Effects of coexisting ions on the fluorescence maxima of the sensor/Hg²⁺ solutions. a, c and e: Sensor + Hg²⁺+Mn⁺, whereMn⁺: 1-Na⁺, 2-K⁺, 3-Ca²⁺, 4-Mg²⁺, 5- Fe³⁺, 6-Cu²⁺, 7-Zn²⁺, 8-Cr³⁺, 9-Pb²⁺, 10-Ni²⁺, 11-Fe²⁺, 12-Mn²⁺, 13-Co²⁺, 14-Cd²⁺. b, d and f: Sensor + Hg²⁺. Solvent: CH₃CN/HEPES buffer (pH 7.2, 1:1, 9:1 and 10:1 (v/v) for RDTU, RTTU and RPTU respectively). Concentration: 20 mmol/L for sensors, 120 mmol/L for metal ions. λ_ex: 520 nm; slit width: 10 nm.

To know their practicability,the sensors were used for the determination of Hg²⁺ in pond water and tap water. The results were displayed in Table 1. The detected concentration of Hg²⁺ was close to that of the added Hg²⁺. The recovery was 94.3%-104%, 93.5%-101% and 97.6%-105%,and the relative standard deviation (RSD) of three measurements was less than 2.00%,4.57% and 2.59% for RDTU,RTTU and RPTU,respectively. Therefore the sensors can be applied in monitoring Hg²⁺ in environmental water.

Table 1
Recovery of Hg²⁺ in pond water and tap water (n = 3). Sensor Samplea Hg²⁺ (mmol/L) Recovery

The molar fractions of Hg²⁺ (0.67,0.75 and 0.8) corresponding to the maximal fluorescent increment of the three sensor solutions in Job’s plot (Fig. 4) showed that 1:2 RDTU/Hg²⁺,1:3 RTTU/Hg²⁺ and 1:4 RPTU/Hg²⁺ complexes formed with the association constants of 5.30 × 10⁸ L² mol^-2,9.88× 10¹¹ L³ mol^-3 and 1.00 × 10¹⁶ L⁴ mol^-4,respectively. The number of the bound Hg²⁺ expectedly equaled to that of the thiourea units,indicating that the S atom in thiourea other than the N atom in NCH₂CH₂ acted as the receptor. The reason may lie in the sulphophile affinity of the Hg²⁺. Moreover,excess EDTA could not quench the fluorescence of the sensor/Hg²⁺ solutions (Fig. 5),indicating the irreversible coordination of the sensor with Hg²⁺.

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Figure 4. Job’s plot for sensor versus Hg²⁺. Total concentrations: [RDTU] + [Hg²⁺] (40 mmol/L); [RTTU] + [Hg²⁺] (90 mmol/L); [RPTU] + [Hg²⁺] (100 mmol/L). F0 and F: fluorescence maxima before and after addition of Hg²⁺, respectively. λ_ex: 520 nm; slit width: 10 nm.

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Figure 5. Reversibility of the fluorescent detection of Hg²⁺ with the sensors. 1: sensor, 2: sensor +Hg²⁺, 3: sensor + Hg²⁺ + EDTA. Solvent: CH₃CN/HEPES buffer (pH 7.2, 1:1, 9:1 and 10:1 (v/v) for RDTU, RTTU and RPTU respectively). Concentration: 20 mmol/ L for the sensors, 120 mmol/L for Hg²⁺. λ_ex: 520 nm; slit width: 10 nm.

These results could not be explained by the reversible sensing mechanism through amino/oxo/thioxo donor coordination ^{[9, 25, 26]},the irreversible sensing mechanism based on the hydrolysis ^{[27, 28]} or cyclization reaction ^[29-31],and the sandwich-stacking binding mode ^[32] reported in the references. Therefore,we conjectured that a new interaction way between Hg²⁺ and the sensor might occur.We currently could suppose Hg²⁺ was bound to the S atom in the thiourea receptor,leading to the formation of the sensor/Hg²⁺ complex aswell as the conversion of the rhodamine spirolactam to the ring-opened amide form,as shown in Scheme 2.

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Scheme. 2. Supposed sensing mechanism of the sensors for Hg²⁺.

4. Conclusion

In conclusion,we designed and synthesized three novel turn-on fluorescent sensors for Hg²⁺. They are highly selective and sensitive to Hg²⁺ in water matrix with excellent anti-interference ability, and can be applied in monitoring Hg²⁺ in environmental water samples effectively. As expected,each thiourea unit binds one Hg²⁺,resulting in the linear working ranges of the sensors broadened,and the sensitivity rose along with the increase of the thiourea receptor’s numbers. Subsequent work such as the further sensing mechanism exploration is undergoing in our laboratory.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 21074085),the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Graduate Student Innovation Training Project of Jiangsu Province (No. KYLX_1241),and the State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2016.03.027.

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Chinese Chemical Letters 2016, Vol. 27 Issue (6): 989-992	PDF