徐 鑒孫幼紅張 慧

(1南京曉莊學(xué)院生物化工與環(huán)境工程學(xué)院，南京 211171)

(2南京造幣有限公司技術(shù)研發(fā)中心，南京 211100)

溶液中選擇識別Hg(Ⅱ)的基于羅丹明的熒光傳感器

徐 鑒1孫幼紅1張 慧＊,2

(1南京曉莊學(xué)院生物化工與環(huán)境工程學(xué)院，南京 211171)

(2南京造幣有限公司技術(shù)研發(fā)中心，南京 211100)

設(shè)計合成了一種可在含水體系中選擇性識別Hg2+的熒光傳感器(化合物1)，該化合物中同時包含了作為信號輸出基團(tuán)的羅丹明6G和作為鍵合基團(tuán)的2-取代吡啶。其結(jié)構(gòu)得到了1H NMR，ESI-HRMS和單晶X-ray衍射分析的確認(rèn)。在混合的水體系中，該化合物對二價汞離子表現(xiàn)出高選擇性和高靈敏度的熒光和顯色傳感，體系的發(fā)光伴隨著汞離子的加入而得以增強，顏色由無色變成粉紅色。熒光滴定、ESI-HRMS和Job曲線分析都顯示該傳感器分子和汞離子形成1∶1的配合物。

羅丹明類傳感器；Hg（Ⅱ）識別；熒光選擇性；水體系

0 Introduction

Mercury contamination is widespread[1]and its toxicity has been recognized for a long time[2].Once introduced into the marine environment,inorganic Hg2+was converted into methylmercury,which is even more neurotoxic and related to the environmental and biological system[3].The fluorescence sensing of Hg（Ⅱ）is an intriguing challenge since the mercury ion,like many other heavy and transition metal ions,generally acts as a fluorescence quencher[4],which is a disadvantage for a high signal output.Furthermore,the shortcoming of fluorescence quenching in aqueous solution prevent most of the sensors from practicalapplications.In recent years,the rhodamine-based fluorescent chemsensors for the recognition of Hg（Ⅱ）have attracted considerable attention due to their excellent photophysical properties[5].In general,these sensors are non-fluorescent and colorless,whereas ring-opening of these compounds gives rise to a pink or red color and strong fluorescence emission in visible spectral range.It is suggested that this ringopening process is caused by acid or complexation with metal ions.In 2007,our group presented the first X-ray single crystal structure with a mercury ion clearly showed the ring-opened structure ofthe rhodamine derivative[6].However,this process hasn′t been studied extensively and only a few Hg2+chemsensors are available to date.

In this context,we reporta new turn-on rhodamine-based chemsensor (compound 1),which can selectively recognize Hg2+in a mixed aqueous environment.Our design is based on the ring-opening of 1 upon complexation with Hg（Ⅱ） (Scheme 1).In this system,for the convenience of practical application,the ratio of water was increased from 50%to 75%compared to our previous work[6],and no buffer solutions were used during the detection.

1 Experimental section

1.1 General

All the starting chemicals were purchased from commercial sources.All solvents were analytic grade and used as received for synthesis unless otherwise stated.The precursor of 1,N-(rhodamine-6G)lactamethylenediamine was prepared according to the method described in the literature[7].1 is a new compound.The synthetic procedure and the characterization data are thus provided in the following experimental section.1H NMR spectroscopic measurements were recorded on a Bruker AM-500 NMR spectrometer,using TMS(SiMe4)as an internal reference at room temperature.ESI-HRMS analyses were conducted on a Bruker Daltonics BioTOF Ⅲ mass spectrometer.

1.2 Synthesis of compound 1

A mixture of N-(rhodamine-6G)lactam-ethylenediamine(0.45 g,1.0 mmol),2-pyridinecarbaldehyde(0.11 g,1.0 mmol)and one drop of acetic acid in 50 mL distilled ethanol was stirred at reflux for 4 h.The resulted yellow solution was distilled under reduced pressure to ～5 mL.After diluted with ethyl ether,the mixture was washed with water once.The product gradually formed as light yellow crystals in the solution and was collected by filtration,dried under reduced pressure.Yield:0.36 g,66%.1H NMR(500 MHz,d6-DMSO,ppm):δ:8.58(d,J=4.5 Hz,1H),7.93(s,1H),7.76～7.82(m,3H),7.49～7.53(m,2H),7.42(t,J=6.0 Hz,1H),7.00 (d,J=7.0 Hz,1H),6.28 (s,2H),6.07(s,2H),5.05(t,J=5.5 Hz,2H),3.25～3.31(m,4H),3.13 (t,J=6.5 Hz,4H),1.83 (s,6H)1.21 (t,J=7.5 Hz,6H)ppm.ESI-MS:m/z calcd.for [C34H36N5O2]+546.2869,found 546.2855(100%).

1.3 Photophysical measurements

All of the photophysical measurements were carried out in acetonitrile/water (V/V=1/4)solutions unless otherwise noted.All the solvents for the measurments were purchased from commercial sources and further purified by the standard methods.All the metal ion solutions for the selectivity and competition experiments were prepared from the corresponding nitrate or perchloride salts.The fluorescence spectra were determined on AB-series 2 luminescence spectrometer.The UV-Vis spectra were obtained on a Shimmadzu 3100 spectrophotometer in a 1 cm quartzcuvette at room temperature.

1.4 X-ray structure analysis

The single crystalof1 suitable forX-ray diffraction studies was grown from an acetonitrile/water mixed solution.Single-crystal X-ray diffraction data for compound 1 were collected on a Bruher SmartApex CCD diffractometer with graphitemonochro-mated Mo Kα (λ=0.071 073 nm)using the SMART programs.The structures were solved by direct methods and refined on F2by full-matrix leastsquares methods with SHELXTL version 5.1.All nonhydrogen atoms ofcompound 1 were refined anisotropically.Hydrogen atoms were localized in their calculation positions and refined by using the riding model.Crystallographic data and parameters for data collection and refinement are summarized in Table 1.

CCDC:717917.

Table 1 Crystallographic data for compound 1

2 Results and discussion

2.1 Structure determination

Compound 1 was facilely synthesized from rhodamine 6G by a two-step reaction.The crystal structure of 1 clearly displays the unique spirolactam-ring formation(Fig.1).Such a special structure makes 1 colorless and non-fluorescent in solutions.The two planes of the spiro of the rhodamine molecule are in a mutually vertical position,which were agreed well with those found in related rhodamine-contained compounds[7-8].

2.2 Emission properties

Compound 1 exhibits a weak emission at～553 nm in a CH3CN/H2O solution.Upon addition of Hg2+,the emission intensity increased dramatically(Fig.2).According to our previous studies[6],this increasing can be ascribed to the ring opening of the spirolactam,which is induced by the complexation with Hg（Ⅱ）.The inset of Fig.2 presents the fluorescent titration profile of 1 with Hg(ClO4)2.The non-linear fitting of the titration curve with a correlation coefficient R2of 0.9925 suggests a 1:1 stoichiometry for the 1·Hg complex.The calculated stability constant Ksis 2.5×105L·mol-1in such an aqueous media.

The fluorescence responses of 1 to various metal ions are illustrated in Fig.3.To the CH3CN/H2O solution of 1 (1.0 ×10-5mol·L-1)was adding 2 equivalent of different metal ions respectively and the emission spectra were measure by excitation at 494 nm.Except Hg2+,no significant spectral changes of 1 occurred in the presence of alkali,alkaline-earth or transition metal ions,such as Li+,Na+,K+,Ca2+,Mg2+,Fe2+,Co2+,Ni2+,Cu2+,Zn2+,Cd2+,Pb2+,Ag+and Mn2+.In other words,1 displays the unique selectivity for Hg2+,which is probably due to several combined influences,such as the large radius of Hg2+,the amide deprotonation ability of Hg2+[9]and/or the suitable coordination geometry conformation of the bischelating receptor.

Achieving high selectivity for Hg（Ⅱ） over a complicated background of potential competing species is stilla challenge in Hg（Ⅱ） sensor development[10].To further study the selectivity of 1,a competition measurement was carried out by the subsequent addition of 2 equivalent of Hg2+to the solution containing 1 and 10 equivalent of other metal ions,respectively.The results are illustrated in Fig.4.The excess competitive cations did not lead to any significantfluorescentchanges.Moreover,in the presence of excess competitive cations,the Hg2+ion still resulted in the similar fluorescent changes.All this reveals that the luminescent responding of 1 for Hg2+is unaffected in a background of 5-fold excess of alkali,alkaline earth or transition metal ions.

2.3 Electronic absorption spectroscopy

1 exhibits no obvious absorption band above 400 nm in 1∶4 (V/V)CH3CN/H2O solution at a concentration of 1.0×10-5mol·L-1,suggesting the spirolactam form of 1.Upon addition of Hg2+,a new band at～530 nm appeared and developed remarkably (Fig.5).Meanwhile,the color of the solution changed from colorless to pink,which indicated the formation of the ring-opened structure of 1 upon Hg（Ⅱ） bonding.Thus,probe 1 can also serve as a naked-eye indicator for Hg2+in acetonitrile aqueous media.

2.4 Metal-binding studies

The metal-binding properties of 1 were further investigated by UV-Vis spectroscopies and ESI-HRMS analyses.To determine the stoichiometry for the 1·Hg complex,the Job′s plot analysis was carried out with a total concentration of 2.0×10-4mol·L-1(Fig.6).The inflection point at 0.5 supports a 1:1 bonding mode,which is consistent with the result of non-linear fitting of the emission titration plots.

Solid evidence for the 1·Hg complex was provided by the comparison between the ESI-MS spectrum of Hg-free compound 1 and that of 1 with 1.2 eq.of Hg2+.As shown in Fig.7,the strongest peak at m/z 745.24 ascribed to [Hg·1]22+was clearly observedwhen 1.2 equivalent of Hg2+was added to the solution of 1,while Hg-free sensor 1 exhibited only a peak at m/z 546.29 corresponding to[1+H]+.

3 Conclusion

In summary,a rhodamine-based fluorescent sensor for the detection of Hg（Ⅱ）with a high selectivity in aqueous media was prepared and structurally characterized.The current sensor presents a significant emission enhancementand acolorchangefrom colorless to pink upon addition of Hg2+ion.These photophysical changes are due to the ring-opening of the spirolactam induced by Hg（Ⅱ） binding.A “1∶1”model Hg（Ⅱ）complex was suggested.This compound 1 may be useful for the applications in Hg（Ⅱ）sensing.

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A Rhodamine-Based Fluorescent Chemsensor for Hg（Ⅱ）with High Selectivity in Solution

XU Jian1SUN You-Hong1ZHANG Hui＊,2
(1School of Biochemical and Environmental Engineering,Nanjing Xiaozhuang University,Nanjing 211171)
(2Technological Research and Development Department，Nanjing Mint Co.,Ltd.,Nanjing 211100)

A fluoresencent chemsensor(compound 1)for the detection of Hg2+ion was designed and synthesized.Thissensorwasprepared by incorporating the well-known Rhodamine 6G fluorophore and the 2-pyridinecarbaldehyde binding unit into one molecule.The structure of 1 was fully characterized by1H NMR,ESIHRMS and X-ray crystallographic analyses.The emission properties of compound 1 were investigated in a mixed aqueous environment.Upon addition of Hg2+ion,the emission increased accompanied by a color change from colorless to pink.It exhibited a high selectivity for Hg（Ⅱ）even over a complicated background of excess other metal ions as potential competing species.The metal-binding properties of 1 were studied by fluorescent titration,ESIHRMS and Job′s plot analysis.All the studies suggest a 1∶1 stoichiometry for the 1·Hg complex.CCDC:717917.

rhodamine-based chemsensor;detection of Hg（Ⅱ）;fluorescent selectivity;aqueous media

O614.24+3

A

1001-4861(2010)12-2189-06

2010-08-01。收修改稿日期：2010-10-15。

＊通訊聯(lián)系人。E-mail：sharp_zh@163.com

徐 鑒，男，35歲，實驗師；研究方向：功能配位化學(xué)。