10.1021/ol300867e r 2012 American Chemical Society Published on Web 05/09/2012
ORGANIC
LETTERS
2012
Vol. 14, No. 10
2564–2567
Novel Thieno-imidazole Based Probe for
Colorimetric Detection of Hg
2
þ
and
Fluorescence Turn-on Response of Zn
2
þ
Rudrakanta Satapathy, Yen-Hsing Wu, and Hong-Cheu Lin*
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30049, Taiwan (ROC)
Received April 5, 2012
ABSTRACT
Novel thieno-imidazole based polymer P showed both colorimetric and ratiometric detections of Hg2þas well as fluorometric detection of Zn2þvia fluorescence turn-on response with augmented lifetime. Its model polymer M did not show any such sensing capability under similar conditions, which further confirmed the unique sensitivity of P toward Hg2þand Zn2þvia the chelation of metal ions to both “S” and “N” heteroatoms.
Owing to the ghastly immunotoxic, genotoxic, neuro-toxic, malnutrition, and digestive disorder effects, mercury is considered to be a highly dangerous element in the environment, which can be easily bioaccumulated.1Zinc, a pervasive second most essential transition element in the human body after iron, is involved in numerous biological processes, such as cellular metabolism, neurotransmission, signal transduction, gene expression, apoptosis, as well as pathological processes in many diseases (including Alzhei-mer’s disease, epilepsy, ischemic stroke, and so forth).2It is thus imperative to develop analytic and detective meth-ods for sensitive sensing of Zn2þand Hg2þ. Unlike other biological transition metal ions, such as Fe2þ, Mn2þ, and Cu2þions, both Zn2þand Hg2þions are spectroscopically
and magnetically neutral and thus do not give any signals due to their 3d104s0(Zn2þ) and 5d106s0(Hg2þ) electronic configurations. Thus common analytic techniques, such as Mossbauer, nuclear magnetic resonance (NMR), and elec-tron paramagnetic resonance (EPR), are unable to detect this archetypal metal ion in biological systems. Further-more, fluorescence offers significant advantages over other methods for detection of these metal ions because of
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(2) (a) Que, E. L.; Domaille, D. W.; Chang, C. J. Chem. Rev. 2008, 108, 1517 and references therein. (b) Frederickson, C. J.; Koh, J. Y.; Bush, A. I. Nat. Rev. Neurosci. 2005, 6, 449. (c) Scrimgeoura, A. G.; Stahl, C. H.; McClung, J. P.; Marchitelli, L. J.; Young, A. J. J. Nutri. Biol. 2007, 18, 813.
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simplicity, nondestructive character, high sensitivity, and instantaneous response.3Consequently, noninvasive fluor-escence technique is a mode of choice for metal ion imagining.4
The furthermost advantage of fluorescence “turn-on” sensors related to “turn-off” sensors is the ease of detecting low-concentration contrast relative to a “dark” back-ground, which reduces the possibility of false positive signals and enhances the sensitivity, as demonstrated by numerous studies.5 The intramolecular charge transfer (ICT)-based fluorescence mechanism has been widely used to illustrate fluorescence turn-on and turn-off behaviors for various sensory materials.6Furthermore, ratiometric fluorescent probes have immense significance because they permit signal ratio corresponding to more than one wave-length and provide built-in correction for environmental effects.7The naked eye perceived color change would be beneficial for instantaneous visual sensing.
Numerous probes have been reported for sensing of Hg2þ, for example, bis(N-methylindolyl)methane,8a amino-acid,8b,c indole,8d sulfonamide,8e triazole-based dansyl,8f methionine,8g and rhodamine.8h Likewise several probes have been designed for Zn2þ sensing, such as dipicoly-lamine,9a,b DPA,9c TPEN,9d and Schiff base derivatives.9e Since the sulfur atom is considered to be a ‘‘soft’’ donor atom, it can act as a chelating agent and also increase the sensor’s affinity and selectivity for binding transition metal cations.
Herein, we synthesized thieno-imidazole based polymer P (Scheme 1), with neighboring nitrogen and sulfur het-eroatoms as chelating sites for metal ions. This was found to be a suitable design for colorimetric detection of Hg2þ and fluorometric detection of Zn2þ. 3,30-Bithiopene was prepared from 3-bromothiophene by treating with n-BuLi and CuCl2, which was further acylated without any Lewis
acid to produce compound 3. Then, it was coupled with 4-((2-ethylhexyl)oxy)benzaldehyde by refluxing in the pre-sence of acetic acid and NH4OAc to obtain compound 4.
N-Alkylation of 4 by hexyliodide and K2CO3 yielded
compound 5, which was brominated by NBS to obtain monomer 6. Finally, Grignard polymerization of mono-mer 6 by CH3MgBr and NiDPPCl2produced polymer P.
To compare the selectivity of polymer P toward metal ions,
model polymer M (where only a single nitrogen atom in the imidazole ring is available for binding) was synthesized. Synthesis of model polymer M is depicted in Supplemen-tary Scheme S1.
To verify the sensing abilities of metal ions, polymer P was titrated over a wide range of metal ions, such as Naþ, Kþ, Ba2þ, Ca2þ, Ni2þ, Cuþ, Co2þ, Agþ, Zn2þ, and Hg2þ (Supplementary Figure S1). Originally, two absorption maxima at 248 and 303 nm appeared in polymer P. Upon the sequential addition of Hg2þ, a significant change in absorption pattern was observed. As shown in Figure 1a, the final intensities of the peak at 248 nm increased ca. 4.9-fold, and the peak at 303 nm decreased ca. 50%. Further-more, a new peak appeared at 395 nm and increased almost 9.7-fold. Two clear isosbestic points at 281 and 367 nm were obtained upon titration with Hg2þ. The intensities at 305 and 248 nm (Figure 1b) as well as 395 and 305 nm (Figure 1c) were compared, which showed the intensity changed almost linearly with the concentration of Hg2þ. The color of the polymer solution changed from colorless to yellow, which could be easily detected by the naked eye. However, other metal ions did not show such alteration in absorption pattern. A Job plot was plotted to find the stoichiometry during the binding of Hg2þ(Supplementary Figure S2). The variation of absorption intensities at 385 and 305 nm (A385 A305) as a function of molar ratio
XM= [Hg2þ]/{[Hg2þ] þ [P]} showed 1:1 stoichiometry. 1
H NMR titration was further conducted to elucidate the binding mode, and the NMR titration spectra of monomer 6 (in d-THF) upon the addition of 01.1 equiv of Hg2þ(in D2O) are depicted in Figure 2. Notable downfield shifts of
peaks corresponding to the aromatic protons of monomer 6 were observed upon the gradual addition of Hg2þ. However, no further changes in 1H NMR signals were observed at higher equivalents of Hg2þ. This result is consistent with 1:1 binding stoichiometry of Hg2þ with
Scheme 1. Synthesis of Polymer P
(5) (a) Rurack, K.; Kollmannsberger, M.; Resch-Genger, U.; Daub, J. J. Am. Chem. Soc. 2000, 122, 968. (b) McQuade, D. T.; Hegedus, A. H.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 12389.
(6) (a) Kim, J. S.; Quang, D. T. Chem. Rev. 2007, 107, 3780. (b) Zhang, J. F.; Lim, C. S.; Bhuniya, S.; Cho, B. R.; Kim, J. S. Org. Lett. 2011, 13, 1190. (7) Yuan, L.; Lin, W.; Chen, B.; Xie., Y. Org. Lett. 2012, 14, 432. (8) (a) Kaur, P.; Kaur, S.; Singh, K Org. Biomol. Chem. 2012, 10, 1497. (b) Yang, M. H.; Thirupathi, P.; Lee, K. H. Org. Lett. 2011, 13, 5028. (c) Lohani, C. R.; Kim, J. M.; Lee, K. H. Tetrahedron 2011, 67, 4130. (d) Kaur, P.; Kaur, S.; Singh, K.; Sharma, P. R.; Kaur, T. Dalton Trans. 2011, 40, 10818. (e) Joshi, B. P.; Lohani, C. R.; Lee, K. H. Org. Biomol. Chem. 2010, 8, 3220. (f) Neupane, L. N.; Kim, J. M.; Lohani, C. R.; Lee., K. H. J. Mater. Chem. 2012, 22, 4003. (g) Yang, M. H.; Lohani, C. R.; Cho, H.; Lee, K. H. Org. Biomol. Chem. 2011, 9, 2350. (h) Kumar, M.; Kumar, N.; Bhalla, V.; Singh, H.; Sharma, P. R.; Kaur., T. Org. Lett. 2011, 13, 1422.
(9) (a) Zhang, J. F.; Kim, S.; Han, J. H.; Lee, S. J.; Pradhan, T.; Cao, Q. Y.; Lee, S. J.; Kang, C.; Kim, J. S. Org. Lett. 2011, 13, 5294. (b) Xu, Y.; Pang., Y. Dalton Trans. 2011, 40, 1503. (c) Xu, Y.; Pang., Y. Chem. Commun. 2010, 46, 4070. (d) Hanaoka, K.; Muramatsu, Y.; Urano, Y.; Terai, T.; Nagano., T. Chem.;Eur. J. 2010, 16, 568. (e) Wang, L.; Qin, W.; Tang, X; Dou, W.; Liu., W. J. Phys. Chem. A 2011, 115, 1609.
2566 Org. Lett., Vol. 14, No. 10, 2012
polymer P. This result is consistent with 1:1 binding stoichiometry of Hg2þwith polymer P. The binding con-stant for binding of Hg2þwas calculated to be 0.74 105 from Supplementary Figure S3.
Model polymer M (where the binding site is only nitro-gen heteroatom in the imidazole ring) showed two absorp-tion maxima at 267 and 290 nm. Polymer M was titrated with Hg2þunder the similar condition as that of polymer P. However, unlike polymer P, no additional peak ap-peared and the absorption pattern was not changed sub-stantially (Supplementary Figure S4). This may be attributed to the susceptibility of Hg2þ to be bound between “S” and “N” of thio-imidazole unit in polymer P. Thus, it is concluded that the thio-imidazole based probe is a suitable design for the colorimetric detection of Hg2þ.
Similarly, the fluorescence patterns of polymer P upon the addition of various metal ions under similar conditions are depicted in Supplementary Figure S5, and their histo-grams are shown in Supplementary Figure S6. As observed in Figure 3, upon the addition of Zn2þ, the fluorescence intensity increased ca. 10-fold, which was the most sig-nificant change in fluorescence signals compared with the other metal ions. The lone-pair electrons in the thieno-imidazole receptor undergo rapid electron transfer, which causes the week fluorescence of P. In the presence of suitable cationic analytes (here Zn2þ), the nonbonding electrons get coordinated and thus the ICT effect is inhibited, which resulted in the strong fluorescence of P. Thus, in the presence of Zn2þ, P showed a remarkable increase in the intensity of the emission band with a chela-tion enhanced fluorescence factor (CHEF)10= 10.5. Again, it resulted in a 14.3-fold enhancement of the quantum yield for polymer P (fromΦ = 0.03 to Φ = 0.43). From the fluorescence binding isotherm, the association constant was calculated to be Ka= 5.14 10
6
M1. To find the interference of other back ground cations, we conducted the titrations by using a dual metal system (Supplementary Figure S7). The enhanced fluorescence of polymer P in the presence of Zn2þwas not substantially perturbed by the background cations. To gain insight into the binding of Zn2þwith the repeating unit of P,1H NMR titration was carried out by the gradual addition of Zn2þ(in D2O) to
monomer 6 (in d-THF). As shown in Figure 4, a momen-tous upfield shift of peaks corresponding to the aromatic protons of monomer 6 was observed upon the sequential addition of Hg2þ(01.1 equiv). Fluorometric titration of Zn2þwas conducted for model polymer M to investigate the effect of Zn2þ. Model polymer M did not show any significant fluorescence enhancement in the presence of Zn2þunder similar conditions (see Supplementary Figure S8). As shown in Table 1, the quantum yield of M revealed only a 2.25-fold increase (fromΦ = 0.04 to Φ = 0.09), and
Figure 2.1H NMR spectra (aromatic region) of monomer 6 upon the addition of 01.1 equiv of Hg2þwrt [6].
Figure 3. Fluorescence spectral response of P (1.1 105M) in THF/H2O (1/1) acquired by the sequential addition of 010 equiv of Zn2þ. Inset: Intensity of P as a function of equiv of Zn2þadded.
Figure 1. (a) Absorption spectral changes of P (1.1 105M) in THF/H2O (1:1) upon titration with (01.1 104M) of Hg
2þ. Upper inset: absorption pattern from 275 to 600 nm. Lower inset: color change upon the addition of Hg2þ. Absorption spectral ratios (b) A305/A248and (c) A395/A205as a function of equivalents of Hg2þadded.
(10) Satapathy, R.; Wu, Y. H.; Lin, H. C. Chem. Commun.2012, DOI: 10.1039/C2CC31131C.
Org. Lett., Vol. 14, No. 10, 2012 2567
correspondingly the CHEF was increased just up to 2.2 times. Furthermore, the association constant of poly-mer P with Zn2þwas calculated as 5.14 106, which is almost 3.8 103times higher than that of polymer M. To elucidate the effect of polymerization, we further carried out the fluorescence titration of compound 5 with Zn2þ. Upon the addition of equivalent amounts of Zn2þ, the fluorescence intensities of compound 5 were sequentially increased merely up to 4 times (Supplementary Figure S9). Thus, in contrast to the small molecular analogue 5 (CHEF = 3.8), the increased sensitivity of polymer P (CHEF = 10.5) is due to the molecular ion effect with a larger number of binding sites in polymer P.
Time-resolved fluorescence measurements10 were per-formed for both polymer P and its Zn2þ-complex probed at 390 nm (excited at 375 nm), as shown in Figure 5. In the absence of Zn2þ, a single exponential fitting for the fluores-cence lifetime of polymer P was estimated as 0.49 ns, which corresponded to the lifetime of the S1 state. Upon the addi-tion of Zn2þ, the fluorescence lifetime corresponding to the S1 state was elevated to 2.96 ns. No remarkable change in fluo-rescence lifetime of model polymer M was observed under similar conditions (see Table 1 and Supplementary Figure S10).
Upon the progressive addition of Zn2þ, two oxidation peaks Ox1and Ox2of polymer P were anodically shifted
and disappeared, respectively (see Supplementary Figure S11), and the half-wave oxidation potential (E1/2) of Ox1
(appeared at 1520 mV originally) was shifted to 1200 mV with ΔE1/2 = 320 mV. This indicated a less favorable
oxidation process for polymer P due to the presence of close proximity of a positively charged Zn cation center. Furthermore, the HOMO level of polymer P was lowered (from5.36 to 5.56), and its LUMO level was increased (from 3.92 to 3.42) after complexation with Zn2þ. Thus, the obstruction of the reductive as well as oxidative ICT caused the enhancement of the fluorescent intensity.
To check the affinities of different anions toward a P-Zn complex, we added various anions, such as F, Cl, Br, I, S2, NO3, Ac, PO43, and SCN, to the solution of
P-Zn complex. Surprisingly, unlike other anions, upon the addition of I, the color of the solution changed from colorless to yellow (Supplementary Figure S12a). The pro-gressive appearance of a new peak at 375 nm was observed upon the sequential addition of I(Supplementary Figure S12b). Thus, P-Zn complex is susceptible for the colorimetric detection of I ions. Reversible association of Zn2þ with polymer P was achieved by the addition of Na2-EDTA as a
suitable counter ligand (Supplementary Figure S13). There-fore, the onoffon fluorescence etiquette was successfully achieved via the alternate addition of Zn2þand Na2-EDTA
for 5 successive cycles (Supplementary Figure S14).
In summary, the naked-eye detection of Hg2þ was achieved by polymer P with a thieno-imidazole backbone. Furthermore, polymer P showed a fluorescence turn-on response with an enhanced fluorescence lifetime in the presence of Zn2þ. Its model polymer M could not show any such remarkable sensitivity toward Hg2þand Zn2þ, which confirmed the unique sensitivity of P via the chela-tion of metal ions to both “S” and “N” heteroatoms. Moreover, the colorimetric detection of Iwas selectively accomplished by P-Zn complex over other anions.
Acknowledgment. Financial support of this project was provided by the National Science Council of Taiwan (ROC) through NSC 99-2113-M-009-006-MY2.
Supporting Information Available. Synthetic proce-dures, Figures S1S12, and NMR spectral data. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 4.1H NMR spectra (aromatic region) of monomer 6 upon the addition of 01.1 equiv of Zn2þwrt [6].
Table 1. Quantum Yields, Fluorescence Lifetimes, Association Constants, and CHEF Values of Polymers P and M upon the Addition of Zn2þ Φa lifetimeb K ac CHEFd P 0.03 0.49 NA NA P þ Zn2þ 0.43 2.96 5.14 106 10.5 M 0.04 0.34 NA NA M þ Zn2þ 0.09 0.37 1.35 103 2.26 a
Quantum yields calculated using tryptophan (in H2O) as a standard. bObtained from time-resolved fluorescence measurements.cAssociation
constants calculated from the slopes of binding isotherms.dCHEF is defined as I/I0, where I is the maximum emission intensity of the receptormetal
complex and I0is the maximum emission intensity of the free receptor.
Figure 5. Time-resolved fluorescence spectral responses for polymer P before and after the addition of Zn2þ.
![Figure 2. 1 H NMR spectra (aromatic region) of monomer 6 upon the addition of 0 1.1 equiv of Hg 2 þ wrt [6].](https://thumb-ap.123doks.com/thumbv2/9libinfo/7699729.144653/3.910.83.436.101.510/figure-nmr-spectra-aromatic-region-monomer-addition-equiv.webp)
