Colloidal Graphene as a Transducer in Homogeneous Fluorescence

Article pubs.acs.org/est

Colloidal Graphene as a Transducer in Homogeneous FluorescenceBased Immunosensor for Rapid and Sensitive Analysis of Microcystin-LR Meng Liu, Huimin Zhao, Shuo Chen, Hongtao Yu, and Xie Quan* School of Environmental Science and Technology, Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), Dalian University of Technology, Dalian, 116024, China S Supporting Information *

ABSTRACT: Herein, we reported the assembly of colloidal graphene (CG) and microcystin (MC)-LR-DNA bioconjugates to develop a homogeneous competitive fluorescence-based immunoassay for rapid and sensitive detection of MC-LR in water samples. Initially, the MC-LR-DNA probe was quickly adsorbed onto the CG surface through the strong noncovalent π−π stacking interactions and can be effectively quenched benefiting from the high quenching efficiency of CG. In contrast, the competitive binding of anti-MCLR with MC-LR-DNA destroyed the graphene/MC-LR-DNA interaction, thus resulting in the restoration of fluorescence signal. This signal transduction mechanism made it possible for analysis of the target MC-LR. Taking advantage of the colloidal nature of the as-prepared graphene, the assay was carried out in homogeneous solution throughout, which avoided numerous immobilization, incubation, and washing steps that were necessary to traditional heterogeneous immunoassays, thereby reducing the whole assay time (within less than 35 min) and allowing a much better antigen−antibody interaction. Moreover, due to the direct competitive mode, the assay did not involve any antibody labeling or modification process, which would be beneficial to preserve the binding affinity of antigen−antibody. Under optimal conditions, the proposed immunosensor can be applied for quantitative analysis of MC-LR with a detection limit of 0.14 μg/L, which satisfied the World Health Organization (WHO) provisional guideline limit of 1 μg/L for MC-LR in drinking water, thus providing a powerful tool for rapid and sensitive monitoring of MC-LR in environmental samples.

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fields of environmental monitoring, food safety, and toxicity assessment. The traditional techniques for MC-LR detection are mainly focused on high-performance liquid chromatography (HPLC) combined with either an ultraviolet−visible detector or mass spectrometry (MS) (using ion trap, triple quadrupole and matrix-assisted laser desorption ionization time-of-flight as detectors), which realize their excellent performances at the expense of time, cost, and tedious procedures for sample pretreatment or preconcentration. Bioassays using whole organism or cell provide a simple method for screening MCs or assessing their cytoxicity but suffer from low sensitivity and lack of specificity.6 In addition, biochemical methods based on the protein phosphatase inhibition (PPI) assays allow the sensitive determination of MCs in water through radioisotopic, colorimetric, or electrochemical responses,7,8 but the results of which are likely to be affected by matrix effects or other MCs derivates.7

INTRODUCTION

Cyanobacterial blooms in surface waters pose a serious threat to human and environmental health worldwide due to the liberation of intracellular toxic metabolite, microcystins (MCs), one of the most commonly detected cyclic heptapeptide cyanotoxins with five constant amino acids and two L-amino acids variants,1 which can cause acute live damage to mammals at high doses or even render tumor promotion following lowlevel chronic exposure originated from their potent inhibition of eukaryotic serine/threonine phosphatases.2,3 In addition, the danger of MCs is greatly amplified by the fact that some variants can be accumulated in an organism through the biological chain. To date, over 80 MCs have been isolated and identified from freshwater cyanobacteria genera with low LD50 values, among which microcystin-LR (MC-LR) is the most toxic congener with the LD50 of 43 μg/kg,4 lower than that of potassium cyanide. On the basis of its acute toxicity, the World Health Organization (WHO) has set a guideline value of 1 μg/ L for MC-LR in drinking water.5 In this regard, it is highly necessary and urgent to develop a facile, robust, and real-time method for the determination of MC-LR in surface water with high sensitivity and selectivity to meet the requirements in the © 2012 American Chemical Society

Received: Revised: Accepted: Published: 12567

July 16, 2012 October 15, 2012 October 31, 2012 October 31, 2012 dx.doi.org/10.1021/es3028583 | Environ. Sci. Technol. 2012, 46, 12567−12574

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reactivity for other MCs variants.29 A piezoelectric cantileverbased immunosensor also enabled MC-LR quantification with a wide dynamic range of 1 ng/L to 100 μg/L.30 Despite these achievements, further simplifying the preparation, optimization, and operation procedures in the immunosensor system still remains a great challenge. Moreover, similar to traditional ELISA, most nanomaterial-assisted immunosensors and labelfree immunoassays for MC-LR typically adopt the heterogeneous competitive approach that was carried out in a modified solid-phase substrate, which means to first fabricate a sensing interface through the immobilization of recognition elements or targets that can effectively separate the “invasion” and then transfer the signal. While this heterogeneous assay format is inherently sensitive, due to the possible separation of unbound molecules from bound molecules, the requirement of multiple immobilization or coating steps, blocking procedures, and numerous incubation and wash cycles make the assay somewhat time-consuming and labor intensive, even resulting in loss of the stability and activity of immobilized immunoreagents. Meanwhile, the solid-phase assay usually suffers from false-positive signals as a result of the nonspecific adsorption of molecules. In this respect, developing a facile direct competitive immunosensor in homogeneous solution phase that avoids modified or labeled antibody and immobilization substrate to realize the rapid and highly sensitive detection of MC-LR in water samples should be of great interest and significance. Not only does this simplify the experimental operation and save whole analysis time to a large extent, but also it helps to retain the high binding affinity of the biorecognition system and improve the precision and reproducibility during the sensing. Unfortunately, no attempt, so far, has been made to exploit such a homogeneous direct competitive assay mode. In recent years, two-dimensional graphene-based materials with their amazing features have emerged as promising transducers in diverse analytical devices.31−34 As noted, the πrich conjugation surface domains allow graphene to interact with biomolecules, such as DNA molecules through noncovalent π−π stacking interactions and hydrophobic interactions,35 thus laying a powerful basis for the design of novel biosensors. Furthermore, it is possible to produce homogeneous colloidal suspensions of graphene sheets in aqueous solvent through controlling the functional groups on the sheets, opening up numerous opportunities to utilize colloidal graphene for achieving extended functionalities. In this study, taking advantage of the remarkable graphene-DNA nucleobases interaction in a solution phase, as well as the antigen−antibody binding specificity, we present a facile and robust homogeneous fluoroimmunoassay for rapid and sensitive detection of MC-LR in water samples. To the best of our knowledge, such homogeneous immunoassay for quantitative determination of MC-LR has not been reported so far.

To overcome these limitations, the past years witnessed great progress in immunoassays for direct or indirect MC-LR analysis in environmental samples due to their specificity, sensitivity, ease-of-use, and reliability, including enzyme-linked immunosorbent assays (ELISA), enzyme or dye-linked immunosensor, and immunostrip through the coupling of polyclonal antibodies, monoclonal antibodies, or recombinant antibody fragments as recognition elements,9 via fluorescent,10,11 chemiluminescent,12 colorimetric,13 or electrochemical14−16 signals. For example, a rapid one-step fluorescent immunostrip system was developed for sensitive detection of MCs within 15 min.11 In particular, a monoclonal antibody (Clone MC8C10) that is highly specific to MC-LR against the other MCs (crossreactivity 2 h >40 min >75 min

0.03 0.032 0.1 0.03 0.05

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0.03

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electrochemical electrochemical colorimetric

>10 min >1 h 15−20 min

0.1 0.5 0.03−0.6

14 16 21

antibody-Au NPs antibody-Alexa Fluor 647 antibody

colorimetric fluorescence

35 min 15 min

0.5 ∼0.047

23 11

fluorescence

15 min

∼0.095

11

antibody

fluorescence

12 h) and blocking and washing steps in the heterogeneous assay.

In order to further improve the sensitivity, a “pre-mixing” protocol for MC-LR detection was performed. In contrast to the “simultaneous-mixing” method, where the antibody and the target were added to the probe simultaneously, we first allowed target MC-LR to incubate with anti-MC-LR (with a resultant concentration of 3 mg/L) in PBS solution (20 mM, containing 50 mM NaCl, pH 7.4) and then followed by mixing the above obtained solution with MC-LR-DNA/CG probe before recording the fluorescence signal. In this approach, the binding process between the target MC-LR and the anti-MC-LR is preferentially separated from detection and can be performed well under optimal conditions, thereby making it possible to amplify the signal. To confirm this scenario, we first optimized the incubation time for the formation of the target−antibody immunocomplex to obtain the best sensing performance. It was observed that the fluorescence changes of the sensor gradually increased until the incubation time was up to 15 min (Figure S12A, Supporting Information), and further prolonging the incubation time did not obviously enhance the response (Figure S12B, Supporting Information). Under optimized conditions, the proposed assay was performed for the quantitative analysis of MC-LR. Figure 2C demonstrated that the kinetics of fluorescence enhancement was dependent on the

fluorescence signal of FAM was recovered, thereby offering a powerful basis for fluorescent MC-LR assays. MC-LR Immunoassay. For MC-LR analysis, a homogeneous direct competitive immunoassay was then conducted in solution using the MC-LR-DNA/CG platform coupled timedependent fluorescence measurement. Once standard MC-LR solutions and fixed specific anti-MC-LR antibody were added to the MC-LR-DNA/CG system, the MC-LR in the incubation solution was expected to compete with the adsorbed MC-LRDNA on the CG surface to bind the limited binding sites of the anti-MC-LR antibody, which would produce a target concentration-dependent fluorescence signal, thus providing the basis for quantitative detection. As shown in Figure 2A, one observed that the kinetics of fluorescence enhancement was decreased with the increasing MC-LR concentration in the range of 0−500 μg/L. The dependence of the fluorescence intensity on the standard MC-LR concentration was illustrated in Figure 2B, based on which a quantitative assay for MC-LR can be achieved. The detection limit (defined as 3σ/slope, with σ being the standard deviation of the blank samples) of the immunosensor was calculated to be 0.93 μg/L, which met the standard of the WHO requirements for MC-LR content in drinking water (1 μg/L). 12571

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target concentration ranging from 0 to 200 μg/L. Note that the developed approach can effectively avoid the competition between the MC-LR-DNA probe and the target toward antibody binding, thus leading to the fast formation of equilibrium state of the antigen−antibody reaction occurring on CG surface, which then exhibited a relative narrow dynamic range of MC-LR detection. Deduced from the dose−response curve in Figure 2B, a detection limit of 0.14 μg/L can be obtained, indicating a sensitivity improvement of about 6-fold in comparison with that of the “simultaneous-mixing” assay. Table 1 summarized the analytical performance of heterogeneous immunoassay, homogeneous immunosensor, and immunostrip technique for the detection of MC-LR. The results indicated that the sensitivity achieved in the present sensing system is lower than that of the original heterogeneous immunoassay based on the indirect competition format and is comparable to other heterogeneous sensing platforms that used direct competition mode.10,12−20,22,24,26 Nonetheless, as the assay process was carried out in homogeneous solution, this design avoided the need for multiple time-consuming immobilization or coating, incubation, and washing steps in common heterogeneous immunoassays relying on either indirect or direct competition manner, thus reducing the whole assay time to less than 35 min which would provide a much more rapid and convenient approach for the quantitative analysis of MC-LR compared with traditional heterogeneous immunoassays. Furthermore, the major advantage over homogeneous immunoassay and immunostrip technique based on indirect competitive mode was the absence of complex antibody labeling or modification, which would allow a much better antibody−antigen interaction, thus realizing the facile and sensitive detection of MC-LR in water samples. Note that, although a direct competitive immunostrip system was developed for sensitive detection of MCs within 15 min,11 large biomolecules (BSA) used for labeling MC-LR might interfere with the antibody−antigen interaction derived from the small size of the MC-LR in comparison with that of biomolecules, thus causing the poor reproducibility and precision when using an antibody-immobilized system.11 In contrast, the covalent coupling of MC-LR to 5′ ends of the small DNA oligomers in our proposed method can be effectively controlled, which would facilitate the antibody−targets interaction during the sensing. The specificity (CR, cross-reactivity) of the assay was evaluated using other MCs variants (MC-YR, MC-RR, MCLW) and nodularin toxins (Figure S13, Supporting Information). The results showed that the CR values of MC8C10 antibody to the other [4-arginine] microcystins including MCRR and MC-YR were below 14%. Nodularin, a cyclic pentapeptide, containing an arginine followed the amino acid Adda, indicated a relative low cross-reactivity (3.7%). Other MCs such as MC-LW were not recognized (Table S1, Supporting Information). The results were in line with the previously reports, which demonstrated that monoclonal antibody MC8C10 exhibited high specificity for those [4arginine] microcystins, especially for MC-LR.10,12,17 Thus, this assay provided the potential to quantify the MC-LR levels in environmental water samples. In the following, we were devoted to evaluate the matrix effects on the proposed assay due to the fact that the field samples were mainly surface water. Time-dependent fluorescence changes against standard MC-LR spiked in bottled water, filtered lake water (obtained from Xishan lake), tap

water, and PBST buffer (containing 8 mM Na2HPO4, 2 mM KH2PO4, 10 mM KCl, 140 mM NaCl, 0.05% Tween-20, pH 7.4) were recorded at 37 °C. With the increasing MC-LR concentration in spiked water samples, the kinetics of fluorescence enhancement was accordingly decreased (Figure S14, Supporting Information), similar to the results observed in PBS buffer. In addition, the standard dose−response curves can be obtained, irrespective of the test matrixes, with the corresponding detection limits (determined by 3σ/slope, with σ being standard deviation of the blank samples) of 1.40, 0.91, 0.84, and 0.41 μg/L for MC-LR in bottled water, lake water, tap water, and PBST buffer, respectively (Figure S15, Supporting Information). Note that PBST buffer samples showed higher sensitivity than that of the PBS buffer, which could be ascribed to the higher binding interactions between MC-LR and antiMC-LR. Collectively, these results implied that the developed assay was suitable for the detection of microcystin-LR in real water samples. Analysis of Water Samples. To evaluate the practicality of the present method, the homogeneous fluorescence-based immunoassay was applied to detect the MC-LR level in tap water and lake water. The water samples collected were simply filtered and showed no MC-LR present (analyzed by LC/MS). The analytical results for the samples spiked with 1−100 μg/L of standard MC-LR were given in Table S2, Supporting Information. One observed that the developed assay displayed the recoveries ranging from 93.4% to 110% for the real samples along with a relative standard deviation (RSD) of around 8%, which can match the obtained values from LC/MS (Table S2, Supporting Information, the results were calculated from the standard calibration curves in Figure S16, Supporting Information), thus confirming the potential applicability of the homogeneous fluorescence-based immunoassay for the quantification of MC-LR. In summary, we demonstrated here a promising homogeneous fluorescence-based immunoassay by taking advantage of the unique graphene/DNA nucleobase interaction as well as the remarkable antigen−antibody binding interaction, which can realize the rapid and sensitive detection of MC-LR in water samples. It was worth noting that the present approach possessed several features and advantages that were not available in the traditional immunoassay for MC-LR. First of all, the assay was conducted in homogeneous solution throughout, avoiding numerous coating, incubation, and washing steps, thus allowing a much better antibody−antigen binding and a rapid on-site analysis. Second, the immunoassay was based on a direct competitive format, without the need for antibody labeling or modification, thus simplifying the assay procedures, which would be beneficial to preserve the binding affinity between MC-LR and anti-MC-LR antibody. Finally, due to the excellent quenching efficiency of the graphene-based nanoplatform, the present immunoassay provided a low background signal and little background fluctuation, thereby offering a high signal-to-background ratio and an acceptable sensitivity for MC-LR. Moreover, we envision that the novel principles presented here could open new opportunities for the development of other homogeneous competitive fluorescencebased immunoassays for analysis of small molecules or pollutants in the biological and environmental fields in the future. 12572

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tographic assay system for the quantification of microcystins. Environ. Sci. Technol. 2003, 37 (9), 1899−1904. (12) Long, F.; Shi, H. C.; He, M.; Sheng, J. W.; Wang, J. F. Sensitive and rapid chemiluminescence enzyme immunoassay for microcystinLR in water samples. Anal. Chim. Acta 2009, 649 (1), 123−127. (13) Lindner, P.; Molz, R.; Yacoub-George, E.; Dürkop, A.; Wolf, H. Development of a highly sensitive inhibition immunoassay for microcystin-LR. Anal. Chim. Acta 2004, 521 (1), 37−44. (14) Zhang, F.; Yang, S. H.; Kang, T. Y.; Cha, G. S.; Nam, H.; Meyerhoff, M. E. A rapid competitive binding nonseparation electrochemical enzyme immunoassay (NEEIA) test strip for microcystin-LR (MCLR) determination. Biosens. Bioelectron. 2007, 22 (7), 1419−1425. (15) Campàs, M.; Marty, J. L. Highly sensitive amperometric immunosensors for microcystin detection in algae. Biosens. Bioelectron. 2007, 22 (6), 1034−1040. (16) Lotierzo, M.; Abuknesha, R.; Davis, F.; Tothill, I. E. A membrane-based ELISA assay and electrochemical immunosensor for microcystin-LR in water samples. Environ. Sci. Technol. 2012, 46 (10), 5504−5510. (17) Sheng, J. W.; He, M.; Shi, H. C. A highly specific immunoassay for microcystin-LR detection based on a monoclonal antibody. Anal. Chim. Acta 2007, 603 (1), 111−118. (18) Wei, Q.; Zhao, Y. F.; Du, B.; Wu, D.; Cai, Y. Y.; Mao, K. X.; Li, H.; Xu, C. X. Nanoporous PtRu alloy enhanced nonenzymatic immunosensor for ultrasensitive detection of microcystin-LR. Adv. Funct. Mater. 2011, 21 (21), 4193−4198. (19) Wang, L.; Chen, W.; Xu, D.; Shim, B.; Zhu, Y.; Sun, F.; Liu, L.; Peng, C.; Jin, Z.; Xu, C.; Kotov, N. A. Simple, rapid, sensitive, and versatile SWNT-paper sensor for environmental toxin detection competitive with ELISA. Nano Lett. 2009, 9 (12), 4147−4152. (20) Zhang, J.; Lei, J. P.; Xu, C. L.; Ding, L.; Ju, H. X. Carbon nanohorn sensitized electrochemical immunosensor for rapid detection of microcystin-LR. Anal. Chem. 2010, 82 (3), 1117−1122. (21) Wang, L. B.; Zhu, Y. Y.; Xu, L. G.; Chen, W.; Kuang, H.; Liu, L. Q.; Agarwal, A.; Xu, C. L.; Kotov, N. A. Side-by-side and end-to-end gold nanorod assemblies for environmental toxin sensing. Angew. Chem., Int. Ed. 2010, 49 (32), 5472−5475. (22) Zhu, Y. Y.; Xu, L. G.; Ma, W.; Chen, W.; Yan, W. J.; Kuang, H.; Wang, L. B.; Xu, C. L. G-quadruplex DNAzyme-based microcystin-LR (toxin) determination by a novel immunosensor. Biosens. Bioelectron. 2011, 26 (11), 4393−4398. (23) Lawton, L. A.; Chambers, H.; Edwards, C.; Nwaopara, A. A.; Healy, M. Rapid detection of microcystins in cells and water. Toxicon 2010, 55 (5), 973−978. (24) Loyprasert, S.; Thavarungkul, P.; Asawatreratanakul, P.; Wongkittsuksa, B.; Limsakul, C.; Kanatharana, P. Label-free capacitive immunosensor for microcystin-LR using self-assembled thiourea monolayer incorporated with Ag nanoparticles on gold electrode. Biosens. Bioelectron. 2008, 24 (1), 78−86. (25) Ma, W.; Chen, W.; Qiao, R.; Liu, C.; Yang, C.; Li, Z.; Xu, D.; Peng, C.; Jin, Z.; Xu, S.; Wang, L. Rapid and sensitive detection of microcystin by immunosensor based on nuclear magnetic resonance. Biosens. Bioelectron. 2009, 25 (1), 240−243. (26) Yu, H. W.; Jang, A.; Kim, L. H.; Kim, S. J.; Kim, I. S. Bead-based competitive fluorescence immunoassay for sensitive and rapid diagnosis of cyanotoxin risk in drinking water. Environ. Sci. Technol. 2011, 45 (18), 7804−7811. (27) Long, F.; He, M.; Zhu, A.; Song, B. D.; Sheng, J. W.; Shi, H. C. Compact quantitative optic fiber-based immunoarray biosensor for rapid detection of small analytes. Biosens. Bioelectron. 2010, 26 (1), 16−22. (28) Herranz, S.; Marazuela, M. D.; Moreno-Bondi, M. C. Automated portable array biosensor for multisample microcystin analysis in freshwater samples. Biosens. Bioelectron. 2012, 33 (1), 50− 55. (29) Herranz, S.; Bocková, M.; Marazuela, M. D.; Homola, J.; Moreno-Bondi, M. C. An SPR biosensor for the detection of

ASSOCIATED CONTENT

S Supporting Information *

Chemicals and reagents; preparation of graphene oxide and graphene; preparation and characterization of MC-LR-DNA probe; characterization of CG; fluorescence response of the MC-LR-DNA probe in the presence of CG; selectivity of the MC-LR-DNA/CG platform; fluorescence response of MC-LRDNA/CG with different lengths; fluorescence response of MCLR-DNA/CG at different incubation times for the formation of MC-LR-antibody; cross-reactivity of MCs variants and nodularin; fluorescence response of the sensor in different matrixes; analytical results for the determination of MC-LR in water samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; fax: +86-411-84706140. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2011CB936002) and Program for Changjiang Scholars and Innovative Research Team in University (IRT0813).

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REFERENCES

(1) Kull, T. P. J.; Backlund, P. H.; Karlsson, K. M.; Meriluoto, J. A. O. Oxidation of the cyanobacterial hepatotoxin microcystin-LR by chlorine dioxide: Reaction kinetics, characterization, and toxicity of reaction products. Environ. Sci. Technol. 2004, 38 (22), 6025−6031. (2) Nishiwaki-Matsushima, R.; Ohta, T.; Nishiwaki, S.; Suganuma, M.; Kohyama, K.; Ishikawa, T.; Carmichael, W. W.; Fujiki, H. Liver tumor promotion by the cyanobacterial cyclic peptide toxin microcystin-LR. J. Cancer Res. Clin. Oncol. 1992, 118 (6), 420−424. (3) MacKintosh, C.; Beattie, K. A.; Klumpp, S.; Cohen, P.; Codd, G. A. Cyanobacterial microcystin-LR is a potent and specific inhibitor of protein phosphatases 1 and 2A from both mammals and higher plants. FEBS Lett. 1990, 264 (2), 187−192. (4) Gupta, N.; Pant, S. C.; Vijayaraghavan, R.; Rao, P. V. L Comparative toxicity evaluation of cyanobacterial cyclic peptide toxin microcystin variants (LR, RR, YR) in mice. Toxicology 2003, 188 (2− 3), 285−296. (5) WHO. Guidelines for Drinking-Water Quality, 2nd ed.; addendum to Vol. 2, Health Criteria and Other Supporting Information; World Health Organization: Geneva, Switzerland, 1998. (6) Sangolkar, L. N.; Maske, S. S.; Chakrabarti, T. Methods for determining microcystins (peptide hepatotoxins) and microcystinproducing cyanobacteria. Water Res. 2006, 40 (19), 3485−3496. (7) Rapala, J.; Erkomaa, K.; Kukkonen, J.; Sivonen, K.; Lahti, K. Detection of microcystins with protein phosphatase inhibition assay, high-performance liquid chromatography-UV detection and enzymelinked immunosorbent assay: Comparison of methods. Anal. Chim. Acta 2002, 466 (2), 213−231. (8) Campàs, M.; Szydlowska, D.; Trojanowicz, M.; Marty, J. L. Towards the protein phosphatase-based biosensor for microcystin detection. Biosens. Bioelectron. 2005, 20 (8), 1520−1530. (9) Metcalf, J. S.; Codd, G. A. Analysis of cyanobacterial toxins by immunological methods. Chem. Res. Toxicol. 2003, 16 (2), 103−112. (10) Long, F.; He, M.; Zhu, A. N.; Shi, H. C. Portable optical immunosensor for highly sensitive detection of microcystin-LR in water samples. Biosens. Bioelectron. 2009, 24 (8), 2346−2351. (11) Kim, Y. M.; Oh, S. W.; Jeong, S. Y.; Pyo, D. J.; Choi, E. Y. Development of an ultrarapid one-step fluorescence immunochroma12573

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microcystins in drinking water. Anal. Bioanal. Chem. 2010, 398 (6), 2625−2634. (30) Ding, Y.; Mutharasan, R. Highly sensitive and rapid detection of microcystin-LR in source and finished water samples using cantilever sensors. Environ. Sci. Technol. 2011, 45 (4), 1490−1496. (31) Guo, S. J.; Dong, S. J. Graphene and its derivative-based sensing materials for analytical devices. J. Mater. Chem. 2011, 21 (46), 18503− 18516. (32) Morales-Narváez, E.; Merkoçi, A. Graphene oxide as an optical biosensing platform. Adv. Mater. 2012, 24 (25), 3298−3308. (33) Song, Y.; Wei, W.; Qu, X. Colorimetric biosensing using smart materials. Adv. Mater. 2011, 23 (27), 4215−4236. (34) Kochmann, S.; Hirsch, T.; Wolfbeis, O. S. Graphenes in chemical sensors and biosensors. Trends Anal. Chem. 2012, 39, 87− 113. (35) Varghese, N.; Mogera, U.; Govindaraj, A.; Das, A.; Maiti, P. K.; Sood, A. K.; Rao, C. N. R. Binding of DNA nucleobases and nucleosides with graphene. ChemPhysChem 2009, 10 (1), 206−210. (36) Swathi, R. S.; Sebastian, K. L. Resonance energy transfer from a dye molecule to graphene. J. Chem. Phys. 2008, 129, 054703. (37) Liu, M.; Zhao, H.; Chen, S.; Yu, H.; Quan, X. Interface engineering catalytic graphene for smart colorimetric biosensing. ACS Nano 2012, 6 (4), 3142−3151. (38) Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 2008, 3 (2), 101−105. (39) Zhou, Y.; Bao, Q. L.; Tang, L. A. L.; Zhong, Y. L.; Loh, K. P. Hydrothermal dehydration for the “green” reduction of exfoliated graphene oxide to graphene and demonstration of tunable optical limiting properties. Chem. Mater. 2009, 21 (13), 2950−2956. (40) Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M. Graphene oxide as a chemically tunable platform for optical applications. Nat. Chem. 2010, 2 (12), 1015−1024.

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