Chinese Chemical Letters  2014, Vol.25 Issue (07):1065-1068   PDF    
A new rhodamine-based fl uorescent chemosensor for mercury in aqueous media
Yao Liua, Er-Bing Yanga, Rui Hana, Di Zhanga, Yong Yea,b , Yu-Fen Zhaoa,b,c    
a Phosphorus Chemical Engineering Research Center of Henan Province, the College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China;
b Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100086, China;
c Key Lab Chem Biol, Fujian Prov. Coll. Chem. & Chem. Engn., Xiamen University, Xiamen 361005, China
Abstract: A new fluorescent "on-off" chemosensor for Hg2+ initiated by a derivative of rhodamine B was designed and synthesized. Compound 1 exhibited high sensitivity and selectivity for Hg2+ over other commonly coexistent metal ions in aqueous media. Upon the addition of Hg2+, the spirocyclic ring of probe is opened and a significant enhancement of visible color and fluorescence in the range of 500-600 nm is observed. The colorimetric and fluorescent response to Hg2+ can be conveniently detected by the naked eye, which provides a facile method for visual detection of Hg2+. From the molecular structure and spectral results of 1, an irreversible, hydrolysis, desulfurization reaction mechanism is proposed.
Key words: Fluorescent probe     Rhodamine     Mercury     On-off     Chemosensor    
1. Introduction

During the recent two decades,the design and synthesis of compounds for sensing relevant environmentally and biologically important ionic species,particularly for heavy metal and transition metal cations,are currently of great interest [1]. Numerous excellent works have focused on the selective and sensitive detection of transition metal ions (such as,Cu2+,Pb2+,Zn2+and Fe3+) [2]. Governmental restrictions on the levels of residual heavy metals in end products are also very strict. Therefore,detection of Hg2+ levels has attracted tremendous attention. Many probes for Hg2+ have been proposed thus far [3]. However,most of the probes act only in organic media. Consequently,there is an urgent need to develop chemical sensors capable of detecting the presence of Hg2+ in aqueous media. As everyone knows,rhodamine derivatives are non-fluorescent and colorless,whereas ring-opening of the corresponding spirolactam gives rise to strong fluorescence emission and a pink color [4]. Recently,some rhodamine-based fluorescent probes were reported for the detection of Hg2+ metal ions [5]. In this paper,we report a new rhodamine-based derivative,turn-on fluorescent sensor 1 (Scheme 1) for Hg2+ , which shows very high sensitivity and selectivity toward Hg2+ over other metal ions. 2. Experimental

All the materials for synthesis were purchased from commercial suppliers and used without further purification. The solutions of metal ions were prepared from their nitrate salts,except for FeCl2, CrCl3and MnCl2. The use of Fe(NO3)3and FeCl3yielded nearly the same results. Rhodamine B derivative (2) was synthesized according to the literature [6]. A Hitachi F-4500 spectrofluorimeter was used for fluorescence measurements. The absorption spectra were recorded with a Techcomp UV-8500 spectrophotometer (Shanghai,China). The NMR spectra were measured on a Bruker AMX-400 spectrometer at 400 MHz in CDCl3. Elemental analyses were carried out with a Flash EA 1112 instrument. Mass spectra were acquired in positive ion mode using a Bruker ESQUIRE 3000 ion trap spectrometer equipped with a gas nebulizer probe, capable of analyzing ions up tom/z6000.

Synthesis of 1: Benzoyl chloride (0.16 g,1.1 mmol) was slowly added to the mixture of 2 (0.5 g,1.0 mmol) and triethylamine (1 mL) in 10 mL CH2Cl2at 0°C. After the addition,the mixture was stirred at room temperature for 4 h. The solvent was removed and the residue was purified by silica gel column chromatography with CH2Cl2/MeOH (15/1,v/v) as eluent to afford 1,yield 85%. Mp 116- 118°C; 1H NMR (400 MHz,CDCl3): δ8.17-8.19 (m,1H),7.92 (s, 1H),7.84 (d,2H,J= 7.2 Hz),7.40-7.52 (m,5H),7.10-7.13 (m,1H), 6.34-6.41 (m,2H),6.24 (d,2H,J= 8.8 Hz),3.95(s,2H),3.32-3.37 (m,8H),1.17-1.20 (m,12H); ESI-MS: m/z 605.6 [M+H]+; Anal. calcd. for C37H40N4O2S: C 73.48,H 6.67,N 9.26,found: C 73.42,H 6.69,N 9.30. 3. Results and discussion

Compound 1 was facilely synthesized from rhodamine B according to procedures in the literature and obtained as light orange crystals. As shown in Scheme 1,compound 1 was prepared in high yield by reacting 2 with benzoyl chloride in the presence of triethylamine. Its structure has been confirmed using 1H NMR,ESI mass spectrometry,and elemental analysis. Although 1 is a derivative of rhodamine B,it forms a nearly colorless solution in either water or absolute ethanol,indicating that the spirocyclic form exists predominantly. Besides,neither the color nor the fluorescence (excited at 530 nm) characteristics of rhodamine could be observed for1in ethanol-water (4:1,v/v),suggesting that the spirocyclic form was still preferred. Addition of mercuric ion to the solution of1causes instantaneous development of a pink color and a strong fluorescence (Fig. 1). This observation shows that the mercury-induced,ring-opening reaction takes place rapidly at room temperature.

Scheme 1. The synthesis of title compound.

Fig. 1. Color change of the neutral buffer solution of1in the presence of metal ions.

To further investigate the interaction of Hg2+ and 1,an ultraviolet photometric titration experiment was carried out. An increasing,linear absorption intensity of 1 could be observed with increasing Hg2+ concentration accompanied by color changes from colorless to pink. To determine the stoichiometry of the Hg-ligand complex,Job’s method for absorbance measurement was applied. Keeping the sum of the initial concentration of Hg2+ and 1 at 100mmol/L,the molar ratio of Hg2+ was varied from 0 to 2. A plot of [Hg2+ ]/{[Hg2+ ]+[1]}versusthe molar fraction of Hg2+ was provided in Fig. 2. It showed that the [Hg2+ ]/{[Hg2+ ]+[1]} value went through a maximum at a molar fraction of 0.5,indicating a 1:1 stoichiometry of the Hg2+ to 1 in the complex.

Fig. 2. Plots according to the method for continuous variations,indicating the 1:1 stoichiometry for1-Hg2+ (the total concentration of1and Hg2+ is 100mmol/L).

Fig. 3 shows the absorption spectra of 1 in the presence of various metal ions in ethanol-water. When no metal ion was added to the solution of 1(10mmol/L),almost no absorption above 562 nm could be observed,whereas a significant enhancement of the characteristic absorption of rhodamine B emerged soon after Hg2+ was injected into the solution. There was a large enhancement factor (34-fold) of absorbance at λmax= 562 nm upon the addition of 20 equiv. of Hg2+ . Other cations of interest gave no response (Fig. 4).

Fig. 3. Changes in the absorption spectra of1in the presence of different metal ions in ethanol-water (4:1,v/v).

Fig. 4. Change in the absorbance at 562 nm of1(10mmol/L) in presence of 20 equiv. of various different metal ions in ethanol-water (4:1,v/v).

Changes in the fluorescence properties of 1 caused by other metal ions,including Mg2+ ,K+,Co2+,Cu2+,Mn2+,Ba2+,Pb2+,Na+,Ca2+,Cr3+,Ag+,Ni2+,Cd2+,Zn2+,Fe2+,Li+and Fe3+were also measured. Fluorescence spectra of solutions of 1(10 umol/L), recorded within 5 min after the addition of 1 equiv. of each of these metal ions,are displayed in Fig. 5. Only Hg2+ ion promote significant fluorescence intensity changes,while other metal ions did not cause any changes under identical conditions. In addition, the enhancement in fluorescence intensity resulting from addition of Hg2+ is not influenced by subsequent addition of other metal ions. Finally,while the colorless to pink color change associated with the reaction of1with Hg2+ is readily detected visually,no significant color changes are promoted by other metal ions. This interesting feature reveals that 1can serve as a selective ‘‘nakedeye’’ chemosensor for Hg2+ .

Fig. 5. Fluorescence spectra of1(10mmol/L) in ethanol-water (4:1,v/v) in the presence of 1 equiv. of Mg2+,K+,Co2+,Cu2+,Mn2+,Ba2+,Pb2+,Na+,Ca2+,Hg2+ ,Zn2+, Ni2+,Cd2+,Ag+,Fe2+,Li+,Cr3+and Fe3+Ex: 560 nm,Em: 580 nm,slit: 5.

Generally,one of the most important and useful applications for a fluorescent chemosensor is the detection of metal ions,especially for heavy metals. The limit of the chemosensor for Hg2+ ion has been tested. When 1 was employed at 0.1mmol/L in EtOH/H2O (4:1,v/v),the fluorescent intensity of 1 was proportional to the added concentration of Hg2+ (Fig. 6). The detection limit was measured to be 1.0 ppb,establishing 1 was capable of distinguishing safe and toxic levels of Hg2+ in drinking water according to the U.S. (EPA) standard (2 ppb).

Fig. 6. The fluorescence intensity at 580 nm of compound 1(0.1umol/L) as a function of the Hg2+ concentration (0-20 ppb) in ethanol-water (4:1,v/v).

Both UV-vis and fluorescence data lead to a significant OFF-ON signal. From the molecular structure and spectral results of 1 ,itis concluded that the reaction mechanism should involve two steps. First,the addition of the Hg2+ ion induced the N atom of amide to attack the C atom of C55S bond,and thus a ring opening of the spirolactam of rhodamine took place. Second,a water-promoted hydrolysis followed. After the removal of HgS and phthalic acid,an intramolecular guanylation took place. Finally,a cyclic product 5 was formed through an irreversible hydrolysis,desulfurization reaction,as depicted in Scheme 2. In order to prove this reaction mechanism of the present system,the reaction products of 1 with Hg2+ were subjected to electrospray ionization mass spectral analyses. In ESI-MS,a major ion peaks were detected atm/z471.4 (Fig. 7). It was characterized as the ring opened product of rhodamine derivative,indicating the generation of 5 as a final product.

Scheme 2. Proposed mechanism.

Fig. 7. ESI-MS of 1 +Hg2+ .
4. Conclusion

In conclusion,we synthesized a new rhodamine-based fluorescent probe for Hg2+ . The colorimetric and fluorescent response to Hg2+ can be conveniently detected by the naked eye,which provides a facile method for visual detection of Hg2+ . The selectivity of this system for Hg2+ over other metal ions is excellent. Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 20972143,21375113) and Program for New Century Excellent Talents in University (No. NCET-11-0950).

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