Metal Ion-Sensing Polymer in the Weak Binding Monomer Regime

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2009, 113, 8214–8217 Published on Web 05/21/2009

Metal Ion-Sensing Polymer in the Weak Binding Monomer Regime Xingqiang Liu and Jin Zhu* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, Nanjing UniVersity, Nanjing 210093, China ReceiVed: March 31, 2009; ReVised Manuscript ReceiVed: May 07, 2009

The field of conjugated polymer-based metal ion sensing has evolved rapidly with many systems developed via fluorescence quenching mechanism. Thus far, demonstrated polymer architectures invariably entail the main-chain or side-chain incorporation of strong binding sites. We show herein that extraordinary metal ion sensing systems could be created with monomer units in the weak binding regime. Poly(thienylene phenylene) exhibits highly selective fluorescence quenching response toward an environmentally toxic species, Hg2+. Our results offer important clues for expanding the chemical space of intelligent polymer systems. Chemoresponsive conjugated polymers have received significant attention due to their importance in the sensing of chemical species.1 The design of this class of polymers typically involves the incorporation of main-chain or side-chain functional units that show strong binding capacity toward the targets of interest.2 With respect to signal transduction, fluorescence quenching has increasingly become the method of choice for achieving high-sensitivity assay results. The high sensitivity derives from the ability to create enhanced electronic communication, energy migration, and signal gains in response to the binding of analytes.1b Indeed, numerous polymer systems with metal ion-sensing capability have been constructed using this strategy. Despite the tremendous progress, one significant drawback inherent in the strong binding monomer-derived polymer structures is the lack of selectivity, as exemplified by the widely adopted bipyridine-based frameworks.2 Herein, we report on the metal ion-sensing properties of a polymer architecture with monomer units in the weak binding regime. Poly(thienylene phenylene) exhibits highly selective fluorescence quenching response toward an environmentally toxic species, Hg2+. Our findings clearly demonstrate that strong monomer binding sites are not necessarily an essential part of an efficient detection system. Architecture control, a key determinant of the properties of chemical structures and efficacies of functional sensors, has been the center of our research focus.3 We start our investigation with Hg2+ due to its inherent ability to bind biological molecules, inactivate cell functions, and cause damaging diseases. Prior to this work, various benzene- and thiophene-derived fluorescent sensing schemes have been implemented through the integration of external binding sites.1b,4 The external binding site-based design originates from the perception that interaction between Hg2+ and arenes is weak, even for a variety of heteroatomcontaining structures.5a However, we noticed that the existence of charge transfer complex, albeit with low association constant (e.g., K ) 1.5 for benzene; higher K for electron-donating group* To whom correspondence should be addressed. Fax: (+) 86 25 -83317761. E-mail: [email protected].

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SCHEME 1: Molecular Structures of P1-P6

derivatized benzene), was experimentally confirmed earlier by Kochi et al.5b We hypothesize that the stability of such a charge transfer complex could be enhanced in a properly configured conjugated polymer so that static quenching sites could be generated. Initial efforts were focused on thienylene-phenylene alternating copolymers,6 a system with ease of structural modifications and distinct physical characteristics of the comprising units (e.g., high and low bandgaps). The results reported herein indeed confirmed our postulation that effective binding and thus efficient sensing could occur on a conjugated polymer, even though the constituent monomer units only exhibit weak coordinating capability. Previously documented methods for the preparation of poly(thienylene phenylene) include Stille coupling and Suzuki coupling etc.7 We have carried out the polymerization between difunctionalized Grignard reagents and organic halides by using a catalytic amount of Ni(dppp)Cl2. Compared with other strategies, this method features shortened synthetic steps, as outlined in Scheme 1 and Supporting Information, S1-S2. The methyl side chain (1b) is investigated based on its prior utility in the generation of thermochromic, ionochromic and biochromic poly(thiophene) structures.8 Indeed, optical properties of conjugated polymers are dictated by a delicate balance between repulsive steric interactions and attractive interchain/intrachain interactions.8a Alkoxy side chains (4a and 4b) are inspected by virtue of their common use in enhancing the solubility and processability of polymers. All polymers were obtained in high yields (76-88%) by reacting the Grignard reagent (2 mmol) and organic halide (2  2009 American Chemical Society

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Figure 1. Emission spectra of a CH2Cl2 solution of P1 (3.0 × 10 M) after the addition of Hg2+. [Hg2+] ) (0, 0.01, 0.02, 0.03, 0.13, 0.23, 0.33, 1.33, 2.33, 3.33, 4.33, 5.33, 6.33, 7.33, 8.33, 9.33) × 10-4 M. Excitation wavelength: 385 nm.

mmol) in THF under Ar atmosphere using 2 mol % Ni(dppp)Cl2 as the catalyst. 1H NMR and size exclusion chromatography characterization provide conclusive evidence for the wellcontrolled incorporation of both monomers within target structures (Supporting Information, Figure S3 and Table S1). Considering the fact that the electronic structure of a conjugated polymer is primarily determined by 7-13 repeat units,1a essential properties associated with the alternating structure can be well manifested by the synthesized polymers. Our properly selected synthetic scheme ensures the solubility of all polymers in various solvents, a key requirement for sensing applications. The assynthesized polymers exhibit varied chain rigidity and thermal stability in accordance with the monomers incorporated (Supporting Information, Figures S4 and S5, Table S1). P1 exhibits a strong absorption band at 367 nm, which is associated with π-π* transition of the conjugated segment (Supporting Information, Figures S6 and S7, Table S2). The maximum absorption wavelengths (λmax) of π-conjugated polymers are dictated by both the degree of conjugation and conjugation length. As the methyl side chain is removed from the polymer, a red shift of λmax (399 nm) is observed for P2, reflecting the existence of enhanced effective conjugation. As expected, extended delocalization could also be achieved by fusion of a heterocycle with the phenylene unit, as manifested by the location of λmax at 464 nm for P6. All polymers synthesized through the Ni(dppp)Cl2-catalyzed coupling procedure are emissive in the visible region under UV irradiation (365 nm). Therefore, whereas P1 and P5 are bluish green and P2 and P3 are yellowish green, P4 and P6 appear blue and red, respectively. Thus fluorescent properties of the polymers could be modulated in a wide range through the attachment of different chemical moieties to the conjugated backbone. The distinct luminescent colors are also manifested in the emission spectra (Supporting Information, Figure S7). In the first set of experiments, the ion-sensing capability of P1 was tested by challenging a dilute CH2Cl2 solution of polymer with various metal ions (in a small amount of CH3OH). Indeed, the addition of spectroscopically silent Hg2+ into P1 leads to the appearance of a new absorption peak at 525 nm and correspondingly persistent, slightly reddish color, indicative of the formation of stable charge transfer complex (Supporting Information, Figure S8). Significantly, fluorescence of the polymer is also effectively quenched by Hg2+ (Figure 1). The emission intensity decreases to about 47% of the initial value at a concentration of 2.33 × 10-4 M. Carefully repeated experiments show that quenching response could be detected at a mercury concentration as low as 5 × 10-8 M (10 ppb) (Supporting Information, Figure S9). Although like many reported probes,9,10 our system requires the use of an organic solvent. However, for aqueous samples from rivers, ground-

Figure 2. Fluorescence quenching degrees (1 - F/F0) of a CH2Cl2 solution P1 (3.0 × 10-6 M) in the presence of various metal ions (8.33 × 10-4 M). F and F0 are taken as the fluorescence intensity at 483 nm.

water, cells, and so forth the assay can be performed in two steps: the first is the evaporation of water, and the second step is the addition of responsive polymer solution into the dried sample. In this two-step detection format, we have been able to observe fluorescence quenching at a mercury concentration as low as 1 × 10-7 M (20 ppb) (Supporting Information, Figure S10). Although several conjugated polymer-derived Hg2+ assay systems have been previously documented, they rely exclusively on the utility of nitrogen-based strong binding ligands.10 There have also been many Hg2+-sensing schemes relying on the utility of small organic molecules, nanoparticles, and biological molecules.11 Our detection limits are comparable to many of those reported probes.10,11a-c,12 Therefore, it is our belief that polymers could also be a useful addition to this expanding class of Hg2+-responsive materials.10-13 It is anticipated that the sensing sensitivity of our system could be further improved through facile increase of molecular weight and therefore excited state sampling domain.1b An extensive survey of metal ions suggests that P1 is also, to a much lesser extent, responsive to Ag+ (Figure 2 and Supporting Information, Figure S11). The fluorescence quenching by Hg2+ is about three times as effective as that by Ag+, thus affording us with a highly selective detection system. An empirical diagonal rule has been proposed for diagonally adjacent elements of the second and third periods of the periodic table. Therefore, presumably the stronger interaction between Ag+ and conjugated polymers, as compared to non-Hg2+ cations, leads to higher fluorescence quenching response. The stronger interaction might also hold for Cu2+. From a practical point of view, the synthesis of our polymer is much simpler than many other existing sensing structures.10b In addition, small organic sensing molecules usually require at least comparable synthetic steps and more complex functional group manipulations.12b To gain further insight into the sensing mechanism, polymers P2-P6 with distinct chemical structures (e.g., side chain bulkiness, heteroatom binding site) are examined. Polymers P2-P5 exhibit similar responses to Hg2+ in the absorption spectra, and analogous to P1 a longer wavelength charge transfer absorption band invariably appears (Supporting Information, Figures S12-S15). Variation on the side chains only slightly alters the sensing efficacy of poly(thienylene phenylene) (Figure 3 and Supporting Information, Figures S16-19). It is noteworthy that the maximum quenching attainable by P4 and P5, polymers without alkoxy side chain on the benzene moiety, is not as high as that achieved by P1, P2, and P3, consistent with the fact that electron-donating oxygen atom enhances the charge transfer capability. This also excludes polymer aggregation-based mechanism because under such a circumstance, one would expect a lower quenching efficiency from bulky side chain-modified polymers P1, P2, and P3. Intuitively, one would expect more potent response via the introduction of coordination-capable heterocycle into the system. Contrary to this, P6 displays less

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Letters National Basic Research Program of China (2007CB925103), and the Program for New Century Excellent Talents in University (NCET-06-0451).

Figure 3. Fluorescence quenching degrees (1 - F/F0) of a CH2Cl2 solution of P1-P6 (3.0 × 10-6 M) in the presence of Hg2+ (8.33 × 10-4 M). F and F0 are taken as the fluorescence intensity at the peak emission wavelength for each individual polymer.

efficient quenching response toward Hg2+ (Figure 3 and Supporting Information, Figure S21). Difference in the stability of charge transfer complex is likely to be the primary reason leading to such a loss of efficacy, as evidenced by the less prominent longer wavelength spectroscopic feature for P6. In addition, compared with P1, responses from numerous other metal ions are more significant, demonstrating the detrimental effect exerted by a strong binding site (Supporting Information, Figures S22 and S23). Most notably, quenching efficiency from Ag+ is only slightly lower than Hg2+ (Supporting Information, Figures S23 and S24), which could cause substantial interference in practice. Taken together, we have demonstrated a polymer architecture that exhibits selective sensing response toward Hg2+. The selectivity derives from the formation of a stable charge transfer complex between the conjugated polymer and Hg2+, as compared with other metal cations. The weak interactions5 observed in the arene systems are enhanced through the creation of extended conjugation structures. Although arene molecules generally display transient charge transfer bands with Hg2+ at low temperatures,5 stable complexes could be observed with our conjugated polymers. This confirms our hypothesis that the stability of the charge transfer complex could be enhanced through the proper configuration of the polymer. One phenomenon that deserves comment is that inner filter effect from certain salts could result in apparent decrease of the polymer fluorescence intensity. For example, a percentage of the excitation light could be effectively absorbed by the addition of Fe3+, leading to a reduced amount of emitted photons (Supporting Information, Figures S25-S27). However, by tuning the polymer architecture, one can change the absorption, excitation, and emission properties and reduce the inner filter effect, as observed in P6. Sensing specificity in the presence of large excess amount of other metal ions is a constant challenge for all detection platforms. This nonquenching-based, fluorescence intensity reduction mechanism should be an issue for all molecular sensing systems that have excitation/emission overlap with the absorption from Fe3+.12a,b In summary, we have reported the utility of poly(thienylene phenylene) in the efficient and selective sensing of metal ions. Ion-specific response of this class of polymers suggests that external recognition sites, often incorporated through laborintensive, multistep chemical synthesis, are not indispensible. Selectivity in the fluorescent quenching sensors could benefit from the exclusive utility of weak binding monomer units. The fundamentally distinct, polymer-based sensing principle (in contrast to the monomer-derived schemes) demonstrated herein offers important clues for expanding the structural space of intelligent chemical systems. Acknowledgment. J.Z. acknowledges support from the National Natural Science Foundation of China (20604011), the

Supporting Information Available: The synthesis and structures of P1-P6; 1H NMR of monomers; 13C NMR of monomers; 1H NMR of polymers P1-P6; DSC diagrams and TGA traces of polymers P1-P6; molecular weights and Tg, Td of P1-P6; optical data of polymers P1-P6; absorption and emission spectra of P1-P6; UV-vis absorption and emission spectra of P2-P6 after the addition of Hg2+ and other cations; fluorescence quenching degrees (1 - F/F0) of P1-P6 after the addition of Hg2+ and other cations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537–2574. (b) Thomas, S. W., III; Joly, G. D.; Swager, T. M. Chem. ReV. 2007, 107, 1339–1386. (2) See, for example: (a) Wang, B.; Wasielewski, M. R. J. Am. Chem. Soc. 1997, 119, 12–21. (b) Liu, B.; Yu, W.-L.; Pei, J.; Liu, S.-Y.; Lai, Y.-H.; Huang, W. Macromolecules 2001, 34, 7932–7940. (c) Kim, I.-B.; Dunkhorst, A.; Gilbert, J.; Bunz, U. H. F. Macromolecules 2005, 38, 4560– 4562. (d) Liu, Y.; Zhang, S.; Miao, Q.; Zheng, L.; L Zong,. Cheng, Y. Macromolecules 2007, 40, 4839–4847. (3) See, for example: (a) Tian, L; Shu, X.; Zhu, J. AdV. Mater. 2007, 19, 4548–4551. (b) Qiu, F.; Jiang, D.; Ding, Y.; Zhu, J.; Huang, L. L. Angew. Chem., Int. Ed. 2008, 47, 5009–5012. (4) See, for example: (a) Zhang, Y.; Murphy, C. B.; Jones, W. E., Jr. Macromolecules 2002, 35, 630–636. (b) Ding, A.-L.; Pei, J.; Yu, W.-L.; Lai, Y.-H.; Huang, W. Thin Solid Films 2002, 417, 198–201. (c) Pei, J.; Ding, A.-L.; Yu, W.-L.; Lai, Y.-H. Macromol. Rapid Commun. 2002, 23, 21–25. (5) (a) Steckel, J. A. Chem. Phys. Lett. 2005, 409, 322–330. (b) Lau, W.; Kochi, J. K. J. Am. Chem. Soc. 1986, 108, 6720–6732. (c) Lau, W.; Huffman, J. C.; Kochi, J. K. J. Am. Chem. Soc. 1982, 104, 5515–5517. (6) (a) Shin, S.-H; Park, J.-S.; Park, J.-W.; Kim, H. K. Synth. Met. 1999, 102, 1060–1062. (b) Song, S.-Y.; Shim, H.-K. Synth. Met. 2000, 111112, 437–439. (c) Fahlman, M.; Birgersson, J.; Kaeriyama, K.; Salaneck, W. R. Synth. Met. 1995, 75, 223–228. (7) (a) Bao, Z.; Chan, W.; Yu, L. Chem. Mater. 1993, 5, 2–3. (b) Jayakannan, M.; van Dongen, J. L. J.; Janssen, R. A. J. Macromolecules 2001, 34, 5386–5393. (c) Pelter, A.; Jenkins, I.; Jones, D. E. Tetrahedron 1997, 53, 10357–10400. (8) (a) Leclerc, M. AdV. Mater. 1999, 11, 1491–1498. (b) Ho, H.-A.; Boissinot, M.; Bergeron, M. G.; Corbeil, G.; Dore´, K.; Boudreau, D.; Leclerc, M. Angew. Chem., Int. Ed. 2002, 41, 1548–1551. (c) Dufresne, G.; Bouchard, J.; Bellette, M.; Durocher, G.; Leclerc, M. Macromolecules 2000, 33, 8252–8257. (9) (a) Guo, X.; Qian, X.; Jia, L. J. Am. Chem. Soc. 2004, 126, 2272– 2273. (b) Yang, Y.-K.; K.-J.; Yook, Tae, J. J. Am. Chem. Soc. 2005, 127, 16760–16761. (c) Ros-Lis, J. V.; Marcos, M. D.; Martinez-Manez, R.; Rurack, K.; Soto, J. Angew. Chem., Int. Ed. 2005, 44, 4405–4407. (d) Hennrich, G.; Walther, W.; Resch-Genger, U.; Sonnenschein, H. Inorg. Chem. 2001, 40, 641–644. (10) (a) Tang, Y.; He, F.; Yu, M.; Feng, F.; An, L.; Sun, H.; Wang, S.; Li, Y.; Zhu, D. Macromol. Rapid Commun. 2006, 27, 389–392. (b) Zou, Y.; Wan, M.; Sang, G.; Ye, M.; Li, Y. AdV. Funct. Mater. 2008, 18, 2724– 2732. (c) Li, C.; Zhou, C.; Zheng, H.; Yin, X.; Zuo, Z.; Liu, H.; Li, Y. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 1998–2007. (11) (a) Lee, J.-S.; Han, M. S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 4093–4096. (b) Wu, D.; Descalzo, A. B.; Weik, F.; Emmerling, F.; Shen, Z.; You, X.-Z.; Kurack, K. Angew. Chem., Int. Ed. 2008, 47, 193–197. (c) Zhu, X.-J.; Fu, S.-T.; Wong, W.-K.; Guo, J.-P.; Wong, W.-Y. Angew. Chem., Int. Ed. 2006, 45, 3150–3154. (d) Zhang, X.; Xiao, Y.; Qian, X. Angew. Chem., Int. Ed. 2008, 47, 8025–8029. (e) Li, D.; Wieckowska, A.; Willner, I. Angew. Chem., Int. Ed. 2008, 47, 3927–3931. (f) Liu, J.; Lu, Y. Angew. Chem., Int. Ed. 2007, 46, 7587–7590. (g) Hollenstein, M.; Hipolito, C.; Lam, C.; Dietrich, D.; Perrin, D. M. Angew. Chem., Int. Ed. 2008, 47, 4346–4350. (12) (a) Caballero, A.; Martinez, R.; Lloveras, V.; Ratera, I.; VidalGancedo, J.; Wurst, K.; Tarraga, A.; Molina, P.; Veciana, J. J. Am. Chem. Soc. 2005, 127, 15666–15667. (b) Yoon, S.; Albers, A. E.; Wong, A. P.; Chang, C. J. J. Am. Chem. Soc. 2005, 127, 16030–16031. (c) Descalzo, A. B.; Martinez-Manez, R.; Radeglia, R.; Rurack, K.; Soto, J. J. Am. Chem. Soc. 2003, 125, 3418–3419. (13) For sensing platforms based on molecular imprinting and optical methods, see, for example: (a) Xie, Z.-H.; Guo, L.-Q.; Lin, X.-C.; Chen,

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