Highly sensitive chemosensor for Cu(II) and Hg(II) based on the tripodal rhodamine receptor

Sensors and Actuators B 141 (2009) 506–510

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Highly sensitive chemosensor for Cu(II) and Hg(II) based on the tripodal rhodamine receptor Xi Zeng a,b,∗ , Lei Dong a,b , Chong Wu a,b , Lan Mu a,b , Sai-Feng Xue a,b , Zhu Tao a,b a b

School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550003, PR China Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Guizhou University, Guiyang 550025, PR China

a r t i c l e

i n f o

Article history: Received 24 March 2009 Received in revised form 7 July 2009 Accepted 9 July 2009 Available online 18 July 2009 Keywords: Chemosensor Tripodal Rhodamine Cu(II) Hg(II)

a b s t r a c t A new flexible tripodal compound 1 linked with three rhodamine groups as fluorophores and recognition sites was synthesized and its sensing behavior toward metal ions was investigated by UV–vis and fluorescence spectroscopy methods. It exhibited excellent selectivity for Cu(II) over miscellaneous metal ions including Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Zn(II), Cd(II), Al(III), Mg(II), Ca(II), Sr(II), Na(I), and K(I). While the fluorescence of the 1-Cu(II) complex could be enhanced significantly upon the addition of Hg(II) compound 1 may therefore be applicable as an OFF-ON fluorescent chemosensor for Cu(II) and Hg(II). © 2009 Elsevier B.V. All rights reserved.

1. Introduction Heavy and transition metal ions play an important role in fundamental physiological processes in organisms ranging from bacteria to mammals. It is critical to understand their environmental benefits and risks to human and sensitive ecological receptors. Among transition metal ions, after iron and zinc, copper is the third most abundant, essential heavy metal ion present in human body [1,2]. However, the copper ion is toxic at levels exceeding cellular needs, and it is capable of displacing other essential metal ions that act as co-factors in enzyme-catalyzed reactions [3,4]. Numerous efforts have been devoted to developing fluorescent chemosensors to detect copper ions. Recently, many Cu(II) selective fluorescent chemosensors that display “turn-off” (fluorescence quenching) or “turn-on” (fluorescence enhancement) responses have been proposed [5–9]. It is well-known that the structural transition between spirocyclic and open-ring forms of rhodamine derivatives can be used to construct OFF-ON fluorescent chemosensors [10–15]. The tripod ligand is an excellent self-assembling scaffold to synthesize various supramolecular complexes by functional recognition specific ions. These flexible ligands can adopt a variety of conformations and bind in a number of coordination modes accord-

∗ Corresponding author. Tel.: +86 851 362 4031; fax: +86 851 362 0906. E-mail address: [email protected] (X. Zeng). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.07.013

ing to the geometric requirements of the metal ions, and may offer organometallic frameworks with different structures [16–19]. In recent years, tripodal ligands have been developed as synthetic precursors to colorimetric or fluorescence chemosensors. For example, complexation of tripodal ligands with Cu(II) and Zn(II) formed the mono- and dinuclear complexes [20]. Utilizing steric crowding in tripodal ligands can lead to a reduction of the chelationenhanced fluorescence in complexes of the Zn(II) and Cd(II), which results in quenching of the fluorescence [21]. Singh et al. reported a Cu(II) complex of a flexible tripodal receptor as a highly selective fluorescent probe for iodide [22]. The photoluminescence of tripodal ligand-lanthanide complexes in solution and in solid state has also been investigated [23]. In this manuscript, we present the design and synthesis of a new flexible tripodal compound, which is linked with three rhodamine groups as fluorophores and selectively recognizes Cu(II) over other metal ions. This compound has an intense visible color and fluoresces with a linear response in neutral buffered media. Rhodamine B hydrazide and tripodal aldehyde, when applied to the nucleophilic addition–reduction reaction, form a Schiff base with a yield of 78% without requiring further purification (Scheme 1). Compound 1, a white powder, has lower solubility in methanol and is readily soluble in chloroform. The structures of 1 and of other intermediate products were confirmed by 1 H NMR, 13 C NMR, and MS data (Supporting Information, Fig. S1–S5). To the best of our knowledge, this is the first tripodal fluorescent chemosensor for Cu(II) with three linked rhodamine fluorophores.

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Scheme 1. Synthesis of compound 1 and the proposed bound structure of 1-Cu(II).

2. Experimental Fluorescence spectra measurements were acquired on a Varian Cary Eclipse fluorescence spectrophotometer, equipped with a xenon discharge lamp and a 1 cm quartz cell. The absorbance spectra were acquired using an Amersham Biosciences Ultrospec 5300 pro UV–vis spectrophotometer. NMR spectra were recorded at 293 K on a Varian Nova-400 spectrometer. IR spectra were acquired on a Bruker Vertex 70 FT-IR spectrometer. All of the experiments were carried out at room temperature. ESI MS spectra were obtained on a HP LC-MSD spectrometer bypassing the LC step. All of the reagents were purchased from commercial suppliers (Chengdu Chemical Reagent Co., China; Aldrich and Alfa Aesar Chemical Co. Ltd.). The solutions of the metal ions were prepared from their nitrate or chloride salts. All of the chemicals used in this work were of analytical grade and were used without further purification. Double distilled water was used throughout the experiment. A Rhodamine B derivative 1 stock solution (1.00 × 10−4 mol L−1 ) was prepared in a 100 mL volumetric flask, by dissolving 18.7 mg of 1 in ethanol, and diluting to the mark with ethanol. The Cu(II) stock solution (2.00 × 10−3 mol L−1 ) was prepared in a 100 mL volumetric flask, by dissolving 48.3 mg of Cu(NO3 )2 ·3H2 O in ethanol, and then diluting to the mark with ethanol. The other metal ions were prepared in ethanol solutions (2.00 × 10−3 mol L−1 ). A Tris–HCl buffer stock solution in ethanol (1.20 × 10−2 mol L−1 , pH 6.8) was prepared with 0.012 mol L−1 Tris and titrating in the proper amount of HCl. 3. Results and discussion The appropriate pH conditions for successful operation of 1 as a sensor or chemodosimeter were evaluated (Supporting

Information, Fig. S6). At acidic conditions (pH < 5.0), an obvious enhancement of color and fluorescence appeared, likely due to ring opening of the rhodamine subsequent to strong protonation [24]. At pH > 5.0, neither the color enhancement nor the fluorescence (excited at 557 nm) characteristics of rhodamine B could be observed, which suggested that 1 was insensitive to pH values > 5.0. This also implied that the rhodamine moiety adopted a spirocyclic form. The characteristic peak at 66.05 ppm of 1 in its 13 C NMR spectrum (Supporting Information, Fig. S4) also supports this hypothesis [25]. Compound 1 showed stability in both acidic and neutral solutions (Supporting Information, Fig. S7). The addition of Cu(II) to a solution of 1 led to a fluorescence enhancement over a comparatively wide pH range (5.0–9.0), which is attributed to Cu(II)-induced opening of the rhodamine ring. Similar results also arises from the UV–vis spectra. The pH 6.8 Tris–HCl buffer in ethanol–water (v/v = 8/1) was selected for these spectroscopic investigations. Fig. 1 showed detailed absorption and fluorescence changes in 1 upon gradual titration of Cu(II). The absorption spectra of 1 (50 ␮M) in the Tris–HCl buffer exhibited only a very weak band above 500 nm. Upon addition of <10 equiv. of Cu(II), the absorbance was significantly enhanced by the appearance of a new peak at 557 nm. However, the addition of >10 equiv. of Cu(II) led to a blue-shift of the absorption band (Fig. 1a). Monitoring the UV–vis spectrum during Cu(II) titration showed a clear change from colorless to pink. On the other hand, the fluorescence titration spectra of 1, showed a new peak at 577 nm that increased with the addition of Cu(II) (Fig. 1b), which was accompanied by an emission change from colorless to orange. These results implied that 1 can serve as a “naked-eye” chemosensor selective for Cu(II) in neutral buffer media. Under optimal conditions, the linear regime for the fluorescence intensity response was between 0.6 and 100 ␮M,

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Fig. 1. (a) Spectrophotometric titration of 1 (10 ␮M) in ethanol–water (8/1, v/v, Tris–HCl buffer, pH 6.8) solution upon the addition of Cu(II) at 557 nm. From bottom to top: 0–30 equiv. Cu(II). Inset: the Job’s plot [Cu(II)+1 = 80 ␮M]. (b) Spectrofluorimetric titration of 1 (10 ␮M) under the same conditions. Inset: the Job’s plot. Excitation and emission were performed at 557/577 nm, with 5 nm excitation and emission slits.

and the detection limit for Cu(II), based on the definition by IUPAC [26] (CDL = 3Sb m−1 ), was as low as 0.15 ␮M from 10 blank solutions. Meanwhile, the linear range for the absorption response was between 20 and 400 ␮M, and the detection limit was found to be 0.30 ␮M from 10 blank solutions (Supporting Information, Fig. S8). The fluorescence intensity and UV–vis absorbance were found to be saturated after adding 1 equiv. of Cu(II) to 1, which revealed that no binding past a 1:1 stoichiometry was possible. This result was also confirmed by the Job’s plot (Fig. 1 inset). The association constant for Cu(II)-1 complex was estimated to be 5.8 × 104 L mol−1 from the fluorescence measurements and 5.9 × 104 L mol−1 from the UV–vis measurements. Another important feature of a chemodosimeter is its high selectivity towards the target compound over the other competitive cations. The spectral response of 1 to various common interfering cations and its selectivity for Cu(II) are illustrated in Fig. 2. Variations in the UV–vis and fluorescence spectra of 1 caused by various metal ions including Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Zn(II), Hg(II), Cd(II), Al(III), Mg(II), Ca(II), Sr(II), Na(I), and K(I) were measured in ethanol–water (8/1, v/v, Tris–HCl buffer, pH 6.8) solutions. Excitation and emission were performed at 557/577 nm. No significant spectral changes of 1 occurred in the presence of cations such as Cr(III), Mn(II), Fe(III), Zn(II), Hg(II), Cd(II), Al(III), Mg(II), Ca(II), Sr(II), Na(I), and K(I). Ni(II) and Co(II) caused a negligible effect compared to Cu(II) in the absorption spectra. The fluorescence enhancement and remarkable color change over other competitive cations indicated that 1 has outstanding selectivity for Cu(II).

The selectivity of 1 for Cu(II) in the presence of other cations was tested. Increases in the absorbance and fluorescence intensities resulting from the addition of the Cu(II) ion were not influenced by the subsequent addition of miscellaneous cations, except for Hg(II) (Supporting Information, Fig. S9). The addition of Hg(II) caused a gradual increase in the absorbance and fluorescence intensity of the 1-Cu(II) complex (Supporting Information, Fig. S10). To investigate the mercury enhancement mechanism, IR spectra of 1, the 1-Cu(II) complex, the 1-Hg(II) complex and the 1-Cu(II)Hg(II) complex were measured (Fig. 3). The addition of 1 equiv. of Hg(II) did not affect the amide carbonyl absorption of 1 at 1689 cm−1 . However, the amide carbonyl absorption drastically shifted to lower frequency (1590 cm−1 ) upon the addition of 1 equiv. of Cu(II). This supported the notion that the amide carbonyl oxygen of 1 is involved in the coordination of metal cations [10,27]. A proposed binding structure of 1 to Cu(II) is shown in Scheme 1. Upon the addition of Hg(II) to the 1-Cu(II) complex, the absorption at 1590 cm−1 was enhanced, but no new absorptions were observed. To further clarify the coordination behavior, 1 H NMR spectra were acquired (Fig. 4). Protons on the rhodamine moiety (Ha , Hb , Hc , Hd , He , Hf , and Hg ) of 1 shifted downfield (ı = 0.061, 0.062, 0.064, 0.053, 0.077, and 0.074 ppm, respectively) and the peaks broadened upon addition of Cu(II). This was due to the decrease in electron density of the rhodamine moiety, which indicated that Cu(II) ions coordinated to the amide carbonyl groups of 1. This coordination led to the spirocycle opening and changes to the absorption and emission spectra. Upon addition of Hg(II), there was no change in the shapes of the peaks. Protons on the benzene moiety (Hh , Hi , and Hj ) and methoxy moiety (Hl ) display downfield shifts (ı = 0.063, 0.063, 0.048, and 0.106 ppm, respectively), and protons on the CH N moiety (Hk ) display upfield shifts (ı = 0.076 ppm), which indicated that the benzene and CH N moieties are involved in Hg(II) coordination, and that 1 still exists in

Fig. 2. (a) Absorbance spectra of 1 (20 ␮M) in ethanol–water (8/1, v/v, Tris–HCl buffer, pH 6.8) solution in the presence of 400 ␮M of different cations. (b) Fluorescence spectra of 1 (10 ␮M) in the presence of 200 ␮M of different cations under the same conditions. The excitation and emission were performed at 557/577 nm with 5 nm excitation and emission slits.

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Fig. 3. IR spectra of (a) 1 (5 mM), (b) 1 (5 mM) with Cu(II) (5 mM), (c) 1 (5 mM) with Cu(II) (5 mM) and Hg(II) (5 mM) and (d) 1 (5 mM) with Hg(II) (5 mM). The IR spectra were acquired at room temperature.

the spirocycle form. It was shown that 1-Cu(II) and 1-Hg(II) complexes via different reaction mechanisms. When Hg(II) is added to the 1-Cu(II) complex, conjugation is enhanced, which enhanced the absorption and emission properties, and raised the fluorescence quantum yield from 0.38 to 0.68 (using Rhodamine-B with Ф = 0.89

Table 1 Parameters obtained from the photophysical characterizations of 1, 1-Cu(II) and 1Cu(II)-Hg(II). Parameters

1

1-Cu(II)

1-Cu(II)-Hg(II)

Absorption maxima (nm) Emission maxima (nm) Excitation maxima (nm) Fluorescence quantum yields Stock-shift (nm)

556 565 542 – 23

557 577 557 0.38 20

558 579 557 0.68 22

in ethanol as a reference [28,29]). This result indicated a synergistic effect between Hg(II) and the 1-Cu(II) complex. A trimer complex of 1-Cu(II)-Hg(II) was likely formed. Parameters obtained from the photophysical characterizations of 1, 1-Cu(II) and 1-Cu(II)-Hg(II) are summarized in Table 1. Additionally, it was observed that the addition of diethylenetriamine to a solution containing the Cu(II)1 complex, caused both the pink color and orange fluorescence to immediately disappear. This indicated that the coordination of 1 with the metal ions was reversible (Supporting Information, Fig. S11) [30–34]. 4. Conclusion In conclusion, a new tripodal rhodamine fluorescent chemosensor has been developed, which exhibited prominent absorption and fluorescence enhancements upon Cu(II) addition with particular selectivity and excellent sensitivity, and is suitable for “naked-eye” detection. The spectral response of 1 toward Cu(II) was demonstrated to be reversible and robust against interference from coexisting metal ions, except for Hg(II), which significantly enhanced the absorbance and emission. The results of IR and 1 H NMR experiments showed that 1-Cu(II) and 1-Hg(II) bind through different reaction mechanisms, and showed evidence for a synergistic effect between the Hg(II) and 1-Cu(II) complex. Compound 1 and the 1-Cu(II) complex may therefore be applicable as OFF-ON fluorescent chemosensors for Cu(II) and Hg(II) respectively. Supporting information available Synthesis, reproductions of spectra and additional spectra. Acknowledgments Fig. 4. Partial 1 H NMR spectra of 1 measured in CDCl3 /CD3 CN (4/1, v/v): (a) without metal cations and with (b) 1 equiv. Hg(II), (c) 1 equiv. Cu(II), (d) 1 equiv. Cu(II) and 1 equiv. Hg(II).

This work was supported by the Foundation of the Governor of Guizhou Province China (No. 200617) and the Talented Person Foundation of Guizhou University (No. 2007039). We also thank

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the anonymous reviewers for helpful suggestions, some of which were directly incorporated into the text. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2009.07.013. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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Biographies Xi Zeng received a chemistry BS degree in 1982 from the Guizhou University, China. Currently he is a professor in the school of Chemistry and Chemical Engineering, Guizhou University. His research interests are supramolecular chemistry and chemosensors. Lei Dong is currently working toward an MS degree in Analytical Chemistry at Guizhou University. Her research interest is analytical chemistry. Chong Wu is currently working toward an MS degree in applied chemistry at Guizhou University. His research interests are supramolecular chemistry and chemosensors. Lan Mu obtained a PhD degree in 2007 from University of Guizhou, China. Currently, she is a professor in the School of Chemistry and Chemical Engineering, Guizhou University. Her research interest is analytical chemistry. Sai-Feng Xue is a professor in the Institute of Applied Chemistry, Guizhou University, and her research interest is supramolecular chemistry. Zhu Tao obtained a PhD degree in 2001 from University of New South Wales, Australia. Currently, he is professor in the institute of Applied Chemistry, Guizhou University. His research interest is supramolecular chemistry.