Visual Detection of Copper(II) Ions Based on an Anionic

Letter pubs.acs.org/ac

Visual Detection of Copper(II) Ions Based on an Anionic Polythiophene Derivative Using Click Chemistry Zhiyi Yao,†,§ Yanbo Yang,†,§ Xueliang Chen,‡ Xianping Hu,‡ Li Zhang,‡ Lei Liu,† Yuliang Zhao,† and Hai-Chen Wu*,† †

Key Laboratory for Biomedical Effects of Nanomaterials & Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, China S Supporting Information *

ABSTRACT: We have developed a novel approach for the rapid visual detection of Cu2+ based on an anionic polythiophene derivative (sodium poly(2-(4-methyl-3thienyloxy)propanesulfonate, PMTPS) using click chemistry. The method relies on the disassembly of PMTPS aggregates in the presence of cationic surfactant through electrostatic and hydrophobic interactions. In the assay of Cu2+ detection, a cationic surfactant was formed via a click reaction catalyzed by copper(I), which was derived in situ from copper(II) and promoted the disassembly of PMTPS aggregates leading to the distinct solution color change from purple to yellow. This polymer probe has excellent sensitivity and selectivity for Cu2+ with a detectable range in the micromolar regime by naked eyes and can be used for monitoring Cu2+ concentrations below the safety limit in real-world samples.

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4MeTs) are one kind of conjugated polyelectrolytes (CPEs) whose chain conformational changes are very sensitive to external stimuli. They have attracted much attention in recent years due to their promising applications in colorimetric sensing of various analytes.15−18 Especially, the fluorometric and colorimetric dual output modes of these sensors provide distinct advantages of high sensitivity, easy visualization, and real-time in situ responses in sensing applications.19−21 Previous studies have established that the mechanism of colorimetric sensing is attributed to the analyte-induced aggregation of the polythiophene backbone.22−25 Interestingly, certain surfactants can form ionic complexes with P3RO4MeTs through electrostatic and hydrophobic interactions and cause the disassembly of P3RO-4MeTs aggregates. This rapid color changing process, so-called surfactochromism, has been used in the sensitive detection of different surfactants.26−29 We envisioned that the combination of this procedure and click chemistry could be used for rapid detection of Cu2+ in water (Scheme 1). In this strategy, we first synthesized an anionic P3RO-4MeT derivative, sodium poly(2-(4-methyl-3thienyloxy)propanesulfonate) (PMTPS) as the colorimetric probe which alone exists in an aggregation state. Second, we split the surfactant molecule into two parts: a long-chain alkyl azide (1-azidododecane, AD) and a positively charged ammonium compound with a terminal alkynyl group (N,N,Ntrimethylprop-2-yn-1-ammonium bromide, TAB). In the absence of Cu+, addition of the mixture of AD and TAB into

opper, a common heavy metal, is one of the essential trace elements in the human body due to its important roles in various biological processes.1 However, long-term exposure to excess copper ions (Cu2+) is highly toxic to organisms and the human body; thereby, Cu2+ is becoming one of the major components of the environmental pollutants, especially in drinking water.2,3 To date, numerous methods have been developed for the detection of Cu2+, including inductively coupled plasma mass spectroscopy (ICPMS) and atomic absorption/emission spectroscopy (AAS/AES),4 electrochemical detection,5 sensors based on the surface plasmon resonance6 and surface-enhanced Raman scattering techniques,7 nanomaterial-based probes such as gold and silver nanoparticles,8 quantum dots,9 and fluorescent receptors specifically designed for Cu2+.10−12 Although each method has distinct advantages in certain situations, there are still some limitations, including tedious sample pretreatment, requiring sophisticated and expensive equipment, and excitation and emission in the UV region, etc. Therefore, it is still necessary to develop new methods that could realize rapid and real-time detection of Cu2+ with simple instruments. Especially, it is highly expected to establish a platform for monitoring Cu2+ in water by the naked eye below the safety limit according to the “Guidelines for Drinking-Water Quality of World Health Organization (WHO)”. Fluorescent approaches for the detection of Cu2+ have been generally designed based on the excellent optoelectronic properties of conjugated polymers (CPs),13 while only a few studies of colorimetric detection of Cu2+ with CPs were reported where the sensitivity and responding time still need to be improved.14 Poly(3-alkoxy-4-methylthiophene)s (P3RO© XXXX American Chemical Society

Received: May 7, 2013 Accepted: June 6, 2013

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Scheme 1. Chemical Structure of PMTPS and Schematic Illustration of the Formation of AD-TAB and the Proposed Sensing Mechanism for the Detection of Cu2+

1). This large spectral shift (by 150 nm) and the disappearance of vibronic bands signified the dissociation of PMTPS aggregates30 and confirmed the role of Cu2+ in inducing the series of changes. To substantiate the above observations, we conducted another two experiments. First, we run the click reaction between AD and TAB catalyzed by Cu2+ under the aforementioned conditions and purified the final product (Supporting Information, Figures S5 and S6). After the structure of the cationic surfactant, AD-TAB, was confirmed by spectroscopic characterizations, we added it to the solution of PMTPS and recorded the absorption spectra. As expected, when increasing amount of AD-TAB was added to the solution, the absorption spectra of PMTPS exhibited similar characteristics to that observed for PMTPS-kit in the presence of Cu2+ (Supporting Information, Figure S7). Second, in order to gain more insight into the mechanism of the disassembly of PMTPS aggregates induced by AD-TAB, we compared the spectra of PMTPS in the presence of cetyltrimethylammonium bromide (CTAB), sodium dodecylbenzenesulfonate (SDBS), and octyltrimethylammonium bromide (OTAB) (Supporting Information, Figures S8−S10). It was found that PMTPS had strong interactions with CTAB but showed only weak responses to the presence of SDBS and OTAB. This result corroborated that the electrostatic interactions together with hydrophobic interactions between PMTPS and AD-TAB played key roles in promoting the dissociation of PMTPS aggregates. These two experiments provided further support that Cu2+ could be reduced by SA in the kit mixture to afford Cu+, which catalyzed the click reaction between AD and TAB. The in situ formed amphiphilic molecules bound with PMTPS and thus promoted the disassembly of PMTPS aggregates leading to the distinct solution color change from purple to yellow. The specificity of PMTPS toward Cu2+ based on this approach was evaluated by examining the absorption spectra of PMTPS-kit in HEPES buffer in the presence of various other metal ions, including Cd2+, Pb2+, Hg2+, Co2+, Ni2+, Zn2+, Fe3+, Ag+, K+, Na+, Ca2+, and Mg2+ at concentrations of 100 μM, which was 20 times higher than Cu2+ (Supporting Information, Figure S11). It was found that the addition of those metal ions

the solution of PMTPS would not result in any changes. However, a trace amount of Cu+ could catalyze the click reaction between AD and TAB to afford an amphiphilic molecule. These newly formed molecules could promote the disassembly of the PMTPS aggregates and immediately lead to a solution color change from purple to yellow (Scheme 1). Since Cu+ is usually generated in situ by the reduction of Cu2+ with sodium ascorbate (SA), the proposed strategy would be a very useful method for visual detection of Cu2+ in water. Water-soluble PMTPS was prepared following the previously reported procedure (Supporting Information, Figures S1− S4).30 It exhibits a purple color in aqueous media. Figure 1

Figure 1. Absorption spectra of PMTPS, PMTPS-kit, and PMTPS-kitCu2+ in 10 mM HEPES buffer (pH 7.4) containing 5% (v/v) ethanol. The kit mixture includes AD (2.0 × 10−4 M), TAB (1.0 × 10−3 M), and SA (1.0 × 10−4 M); [PMTPS] = 1.0 × 10−4 M.

shows the absorption spectra of PMTPS, PMTPS-kit (kit including AD, TAB, and SA) and PMTPS-kit-Cu2+. The absorption maximum of PMTPS in HEPES {2-[4-(2hydroxyethyl)-1-piperazinyl]ethanesulfonic acid} buffer (10 mM, pH 7.4, containing 5% (v/v) ethanol) appears around 534 nm along with two broad shoulders at 500 and 577 nm, indicating that PMTPS adopts a planar conformation which facilitates the aggregation of polythiophene backbones.30 Upon addition of the kit mixture to PMTPS solution, the absorption maximum hardly shifted; whereas, the introduction of various amounts of Cu2+ to the PMTPS-kit solution caused a blue-shift of the absorption maximum gradually to 384 nm with a prominent solution color change from purple to yellow (Figure B

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did not change the spectra of PMTPS except that Hg2+ caused an enhancement of the baseline. However, all the solutions remained purple in color as that of PMTPS-kit. Interestingly, the introduction of Cu2+ (final concentration 5.0 μM) into the PMTPS-kit solution caused the solution color to change from purple to yellow (λmax = 384 nm) in less than 5 min. This provides a rapid and convenient method for visual detection of Cu2+ in aqueous solutions (Figure 2). In addition, we used the

Notably, this detection limit is about 1 order of magnitude lower than the limit defined by the safety standards of WHO. Therefore, this approach holds the potential for the practical monitoring of excess Cu2+ in water sources. Prompted by the outstanding sensing performance of this system, we conducted absorbance measurements of PMTPS-kit in the presence of interfering anions and cations to verify the feasibility of the application of PMTPS in practical Cu2+ detection. Figure S13 in the Supporting Information compares the absorption spectra of the PMTPS-kit responding to Cu2+ in the presence of environmental anions such as NO3−, Cl−, Br−, HCO3−, SO42− and HPO42−. It was shown from Figure 4 that

Figure 4. Relative absorbance of PMTPS-kit, control (PMTPS-kitCu2+) and control in the presence of various environmental anions as indicated in 10 mM HEPES buffer (pH 7.4, 5% (v/v) ethanol). [PMTPS] = 0.1 mM, [Cu2+] = 5.0 μM, [anions] = 1.0 mM.

Figure 2. Relative absorbance (A) and photograph (B) of PMTPS-kit in the presence of Cu2+ and other metal ions in 10 mM HEPES buffer (pH = 7.4, 5% (v/v) ethanol). [PMTPS] = 100 μM, [Cu2+] = 5 μM, [other ions] = 100 μM.

the introduction of these anions in the millimolar range, 200 times higher than that of Cu2+, did not influence the responses of this system for Cu2+ detection. Another similar test was performed in the presence of a mixture of various metal ions at the concentrations of allowable high limit set by the WHO standards. The results showed that the presence of certain concentrations of metal ions had negligible effects on the visual detection of Cu2+ (Figure 5). To further illustrate the practical

value (I − I0) to quantify the selectivity of the probe toward Cu2+, where I0 and I are the ratios of the absorbance at 384 nm to the absorbance at 534 nm, A384/A534, in the absence and presence of metal ions, respectively. It is obvious that all the metal ions tested did not interfere with the detection of Cu2+, most likely due to the unique catalytic capacity of Cu+ for click reaction (Figure 2). To check the detection limit of the visual sensing of Cu2+, we compared the solution color of PMTPS-kit with gradually increasing amounts of Cu2+ in HEPES buffer (Figure 3). It was

Figure 5. Absorption spectra (left) and photographs (right) of matrix control (PMTPS-kit-metal ions mixture) in the absence and presence of Cu2+ (5.0 μM). Metal ion matrix includes Na+ (0.869 mM), K+ (45.9 μM), Ca2+ (63.3 μM), Mg2+ (14.5 μM), Co2+ (19.0 μM), Cd2+ (0.026 μM), Pb2+ (0.048 μM), Hg2+ (0.015 μM), Ag+ (0.46 μM), Zn2+ (0.77 μM), Fe3+ (35.7 μM), and Ni2+ (1.2 μM). The concentrations of metal ions were referenced to the safety standards of WHO.

Figure 3. Photographs of PMTPS-kit (1.0 × 10−4 M) in the presence of various amounts of Cu2+ in 10 mM HEPES buffer (pH 7.4, 5% (v/ v) ethanol). The concentrations of Cu2+ are indicated on the caps of the bottles (μM).

found that when the concentration of Cu2+ was equal to or greater than 3.0 μM, the solution color of PMTPS-kit completely changed from purple to yellow within 30 min. Thus, the lower limit for the visual detection of Cu2+ with PMTPS-kit could be set at 3 μM, which is slightly higher than the detection limit by fluorescence measurements (0.8 μM, Supporting Information, Figure S12). This value is among the lowest reported for the detection of Cu2+ with naked eyes.

applications of this approach, several spiked samples including tap water (Beijing) and environmental water (Taihu Lake, Wuxi) were tested using this system. The concentrations of Cu2+ in tap water (TW) and Taihu Lake (TL) blank samples were determined by ICPMS: for TW, [Cu2+] = 21.9 nM; for TL, [Cu2+] = 43.8 nM. With references to the WHO standard, C

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20 μM of Cu2+ was spiked into a TW sample and a TL sample, respectively. Figure 6 showed the absorption spectra and the

color of the probe solution in the blank and spiked samples. It was very certain that the presence of spiked Cu2+ caused a large blue-shift of 150 nm in absorption spectra and dramatic solution color changes from purple to yellow in both tested samples. These results demonstrated that the PMTPS probe could be practically useful in rapid monitoring of Cu2+ from different water sources below the safety limit by WHO standards. In conclusion, we have developed a novel approach for the visual detection of Cu2+ in aqueous media based on an in situ click reaction and subsequent disassembly of PMTPS aggregates. Compared with previous methods based on conjugated polymers, this approach shows rapid responses and excellent selectivity and sensitivity toward Cu2+, with a detectable range in the micromolar regime by naked eyes. This is among the lowest values ever reported for the limit of visual detection of Cu2+. We expect that the present approach could be not only used in monitoring Cu2+ concentrations in different water sources but also extended to colorimetric sensing in bioassays.

ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental procedures and NMR and additional spectral data. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Valko, M.; Rhodes, C.; Moncol, J.; Izakovic, M.; Mazur, M. Chem.-Biol. Interact. 2006, 160, 1. (2) Nies, D. H. Appl. Microbiol. Biotechnol. 1999, 51, 730. (3) Flemming, C. A.; Trevors, J. T. Water, Air, Soil Pollut. 1989, 44, 143. (4) (a) Caroli, S.; Alimonti, A.; Petrucci, F.; Horvath, Z. Anal. Chim. Acta 1991, 248, 241. (b) Wu, J. F.; Boyle, E. A. Anal. Chem. 1997, 69, 2464. (c) Tokalioglu, S.; Kartal, S.; Elci, L. Anal. Chim. Acta 2000, 413, 33. (d) Deng, B. Y.; Wang, Y. Z.; Zhu, P. C.; Xu, X. S.; Ning, X. Anal. Chim. Acta 2010, 683, 58. (5) Liu, A.-C.; Chen, D.-c.; Lin, C.-C.; Chou, H.-H.; Chen, C.-h. Anal. Chem. 1999, 71, 1549. (6) Lee, S.; Choi, I.; Hong, S.; In Yang, Y.; Lee, J.; Kang, T.; Yi, J. Chem. Commun. 2009, 6171. (7) Sarkar, S.; Pradhan, M.; Sinha, A. K.; Basu, M.; Pal, T. Chem. Eur. J. 2012, 18, 6335. (8) (a) Zhou, Y.; Wang, S.; Zhang, K.; Jiang, X. Angew. Chem., Int. Ed. 2008, 47, 7454. (b) Ma, Y.-r.; Niu, H.-y.; Zhang, X.-l.; Cai, Y.-q. Chem. Commun. 2011, 47, 12643. (9) Zhu, A.; Qu, Q.; Shao, X.; Kong, B.; Tian, Y. Angew. Chem., Int. Ed. 2012, 51, 7185. (10) Krämer, R. Angew. Chem., Int. Ed. 1998, 37, 772. (11) Kovács, J.; Rödler, T.; Mokhir, A. Angew. Chem., Int. Ed. 2006, 45, 7815−7817. (12) Jo, J.; Lee, H. Y.; Liu, W.; Olasz, A.; Chen, C.-H.; Lee, D. J. Am. Chem. Soc. 2012, 134, 16000. (13) (a) Xing, C.; Shi, Z.; Yu, M.; Wang, S. Polymer 2008, 49, 2698. (b) Zhu, L.; Yang, C.; Zhong, C.; Xu, L.; Qin, J. Polymer 2008, 49, 3716. (c) An, L.; Liu, L.; Wang, S. Biomacromolecules 2009, 10, 454. (d) An, L.; Wang, S. Chem.-Asian J. 2009, 4, 1196. (e) Lv, F.; Feng, X.; Tang, H.; Liu, L.; Yang, Q.; Wang, S. Adv. Funct. Mater. 2011, 21, 845. (f) Zhu, C.; Liu, L.; Yang, Q.; Lv, F.; Wang, S. Chem. Rev. 2012, 112, 4687. (14) (a) Yao, Z.; Huang, B.; Hu, X.; Zhang, L.; Li, D.; Guo, M.; Zhang, X.; Yuan, H.; Wu, H.-C. Analyst 2013, 138, 1649. (b) Xu, Q.; Lee, K. M.; Wang, F.; Yoon, J. J. Mater. Chem. 2011, 21, 15214. (15) (a) Ho, H.-A.; Boissinot, M.; Bergeron, M. G.; Corbeil, G.; Doré, K.; Boudreau, D.; Leclerc, M. Angew. Chem., Int. Ed. 2002, 41, 1548. (b) Ho, H.-A.; Leclerc, M. J. Am. Chem. Soc. 2004, 126, 1384. (c) Tang, Y.; Feng, F.; He, F.; Wang, S.; Li, Y.; Zhu, D. J. Am. Chem. Soc. 2006, 128, 14972. (16) Sommers, C. D.; Mans, D. J.; Mecker, L. C.; Keire, D. A. Anal. Chem. 2011, 83, 3422. (17) Li, C.; Numata, M.; Takeuchi, M.; Shinkai, S. Angew. Chem., Int. Ed. 2005, 44, 6371. (18) Yao, Z.; Feng, X.; Hong, W.; Li, C.; Shi, G. Chem. Commun. 2009, 4696. (19) Yao, Z.; Feng, X.; Li, C.; Shi, G. Chem. Commun. 2009, 5886. (20) Yao, Z.; Bai, H.; Li, C.; Shi, G. Chem. Commun. 2011, 47, 7431. (21) Li, C.; Shi, G. ACS Appl. Mater. Interfaces 2013, DOI: 10.1021/ am400009d. (22) Ho, H.-A.; Najari, A.; Leclerc, M. Acc. Chem. Res. 2008, 41, 168. (23) Li, C.; Numata, M.; Takeuchi, M.; Shinkai, S. Chem.-Asian J. 2006, 1, 95. (24) Yao, Z.; Li, C.; Shi, G. Langmuir 2008, 24, 12829. (25) Yao, Z.; Bai, H.; Li, C.; Shi, G. Chem. Commun. 2010, 46, 5094. (26) Yao, Z.; Li, Y.; Li, C.; Shi, G. Chem. Commun. 2010, 46, 8639. (27) An, Y.; Bai, H.; Li, C.; Shi, G. Soft Matter 2011, 7, 6873. (28) Li, Y.; Bai, H.; Li, C.; Shi, G. ACS Appl. Mater. Interfaces 2011, 3, 1306. (29) Evans, R. C.; Knaapila, M.; Willis-Fox, N.; Kraft, M.; Terry, A.; Burrows, H. D.; Scherf, U. Langmuir 2012, 28, 12348. (30) Chayer, M.; Faïd, K.; Leclerc, M. Chem. Mater. 1997, 9, 2902.

Figure 6. Absorption spectra (left) and photographs (right) of PMTPS-kit before and after the addition of 20.0 μM Cu2+ in tap water (TW, solid line) and Taihu Lake water (TL, dashed line). [PMTPSkit] = 0.1 mM.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions §

Z. Yao and Y. Yang contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grants 21204089 and 21175135), the National Basic Research Program of China (973 Program, Grant 2010CB933600), and the “100 Talents” Program of the Chinese Academy of Sciences. D

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