Fluorescent Aptasensor Based on Aggregation-Induced Emission

Dec 3, 2013 - Recently, a great variety of aggregation-induced emission (AIE)-active molecules has been utilized to design bioprobes for label-free fl...
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Fluorescent Aptasensor Based on Aggregation-Induced Emission Probe and Graphene Oxide Xing Li, Ke Ma, Shoujun Zhu, Shiyu Yao, Zhaoyang Liu, Bin Xu,* Bai Yang, and Wenjing Tian* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun, Jilin 130012, P. R. China S Supporting Information *

ABSTRACT: Recently, a great variety of aggregation-induced emission (AIE)-active molecules has been utilized to design bioprobes for label-free fluorescent turn-on aptasensing with high sensitivity. However, due to nonspecific binding interaction between aptamer and AIE probe, these AIE-based aptasensors have nearly no selectivity, thereby significantly limiting the development. In this work, a 9,10distyrylanthracene with two ammonium groups (DSAI) is synthesized as a novel AIE probe, and the fluorescent aptasensor based on DSAI and graphene oxide (GO) is developed for selective and sensitive sensing of targeted DNA and thrombin protein. Given its AIE property and high selectivity and sensitivity, this aptasensor can be also exploited to detect other targets.

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derivatives, are nonemissive in the molecularly dissolved states, while give highly fluorescent emission when they are in the aggregate states.9,10 In recent years, a great variety of AIE-active molecules has been utilized to design the supersensitive bioprobes11,12 for label-free fluorescent turn-on sensing of aptamers, which are synthetic oligonucleotides that could be widely used as recognition elements for highly specific binding affinity toward a variety of targets, including complementary oligonucleotides, special small bio/organic/inorganic molecules, drugs, metal ions, proteins, and even cell surfaces.13 These AIE-active probes are sensitive and cost-effective and do not suffer from an aggregation-caused quenching (ACQ) effect. The AIE-based aptasensors utilize ssDNA aptamers as recognition elements and AIE-active probes as fluorescent indicators. The AIE-active bioprobes, such as silole with a quaternary ammonium moiety, are almost nonluminescent in water,14 but when single-stranded DNA (ssDNA) aptamers are added, the aggregation complex of AIE-active bioprobes and aptamers are formed based on the electrostatic interaction between the ammonium cation in AIE-active bioprobes and the backbone phosphate anions in ssDNA aptamer and the hydrophobic interaction between aryl rings in AIE-active bioprobes and nucleosides in ssDNA aptamer, leading to the remarkable fluorescent enhancement of the AIE-active bioprobes. However, the AIE-based aptasensors have nearly no selectivity. For instance, when AIE aptasensor is used for sensing of targeted complementary ssDNA, the fluorescence of AIE aptasensor will be enhanced after the addition of targeted

luorescent biosensors have received great attention because of their high sensitivity and easy operations.1 On the basis of electron transfer (ET), charge transfer (CT), and energy transfer (ET) mechanisms, various organic molecules, conjugated polymers, and inorganic quantum dots have been developed as fluorescent probes for biosensing.2 It is wellknown that fluorescent emissions of many fluorophores are quenched at high concentrations or in an aggregated state (aggregation-caused quenching or ACQ), due to the formation of such detrimental species as excimers and exciplexes.3,4 The notorious ACQ effect has compelled many fluorescent probes to operate in a fluorescent “turn-off” mode with a limited scope of practical applications. It is therefore necessary to develop fluorescent probes that do not suffer from ACQ effect. Although various approaches have been employed to interfere with luminogen aggregation, the attempts have had only limited breakthrough. In 2001, an intriguing aggregationinduced emission (AIE) phenomenon exactly opposite to the ACQ effect was reported by Tang’s group.5 Different from the notorious ACQ fluorophores, AIE-active fluorophores are nonemissive in the molecularly dissolved states, while give highly fluorescent emission when they are in the aggregate states. Experimental and theoretical investigations indicated that the intramolecular rotations in the AIE molecules would deactivate the corresponding excited states, thus making them nonemissive in the respective solutions. The intramolecular steric interactions are blocked in the aggregated states and herein their emissions are enhanced.4,6,7 So far, the concept of AIE has been developed to be a new emerging signal principle for the design of fluorescent probes.8 Typical AIE-active fluorophores, such as tetraphenylethene (TPE), silacyclopentadiene (silole), 9,10-distyrylanthracene (DSA), and their © XXXX American Chemical Society

Received: August 15, 2013 Accepted: December 3, 2013

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Scheme 1. (a) Synthesis of DSAI. (b) Schematic Description of Selective Fluorescent Aptasensor Based on DSAI/GO Probe

interaction between GO and ssDNA and dsDNA, herein, we have synthesized a novel cationic DSA derivative (DSAI) as the AIE-active probe and developed the AIE-based aptasensor for label-free fluorescent turn-on aptasensing with high sensitivity and selectivity based on DSAI and GO. Given its easy operation, low cost, and good sensitivity and selectivity, the aptasensor can be extended to other interested targets. The sensing mechanism is demonstrated in Scheme 1. DSAI is synthesized as the model probe (Scheme 1a), which possesses two positive charges, thus showing good water solubility. Besides, DSAI possess a nonplanar conformational DSA structure. They show weak fluorescence in their solutions but intense emission in their aggregate states. The DSA moiety is the key factor of the AIE property due to the restricted intramolecular torsion between the 9,10-anthylene core and the vinylene segment. Herein, it has weak fluorescence in the molecularly dispersed state or aqueous solution because of the free intramolecular torsional motion of nonplanar DSA in the aqueous solution, leading to fast nonradiative relaxation, and decreases the photoluminescence quantum yield sharply.9 Upon addition of ssDNA aptamer, the aggregation complex of DSAI and ssDNA aptamer (DSAI/ssDNA aptamer complex) is formed due to the electrostatic interactions between the ammonium cation in DSAI and backbone phosphate anions in aptamer and the hydrophobic interaction between aryl rings in DSAI and nucleosides in ssDNA aptamer.14 Accordingly, the fluorescence of the complex would be expected to be enhanced obviously owing to the restriction of intramolecular rotation of DSAI in the aggregated state, resulting in the closure of the nonradiative decay channel. When the GO is added, the adsorptive binding of ssDNA aptamer to GO guarantees the close proximity of DSAI/ssDNA aptamer complex to GO, and the following fluorescence resonance energy transfer (FRET)

complementary ssDNA or the interferential mismatched ssDNA, because the aggregation complex can be formed between the AIE-active bioprobes and either targeted complementary ssDNA or interferential mismatched ssDNA due to the electrostatic and hydrophobic interactions between the AIE-active bioprobes and ssDNA.14,15 Accordingly, the aptasensor cannot distinguish the targeted complementary DNA from the interferential mismatch DNA. Therefore, it is highly desired for the construction of AIE-based aptasensor with high selectivity. Recently, graphene oxide (GO), a novel two-dimensional one-atom-thick nanosheet, has been widely used in various chemical, material, and biomedical fields, because of its unique electronic, thermal, mechanical, and optical properties.16 Because of the strong adsorption and fluorescent quenching ability, GO was identified as an efficient platform for fluorescent biosensing.17 Especially, ssDNA could adsorb on the surface of GO, whereas it desorbed from the surface of GO when hybridized with its complementary strand and formed doublestranded DNA (dsDNA) or hybridized with its targeted protein and formed G-quadruplex DNA (G4).18,19 Take dsDNA for example, there is obviously a different adsorption interaction between GO and ssDNA and dsDNA, due to the strong π−π stacking interaction between the ring structures in nucleobases of ssDNA and the hexagonal cells of GO as well as the weak interaction between dsDNA and GO.20 Thus, the aggregation complex based on AIE-active bioprobes and ssDNA will be adsorbed on GO and those based on AIE-active bioprobes and dsDNA can be removed from GO. The fluorescence of the aggregation complex based on AIE-active bioprobes and ssDNA will be quenched by GO, while the aggregation complex based on AIE-active bioprobes and dsDNA can avoid the fluorescent quenching by GO. Inspired by the different B

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Raman spectra show that the G band at 1597 cm−1 and a D band at 1351 cm−1 are observed, which is related to the characteristic carbon structure. The CC/C−C and carbonhydroxy groups as well as carbonyl groups are further proved by the XPS of the GO, because GO can adsorb organic probes, as a result of π−π stacking interaction between aromatic fluorophores and GO.21 Additionally, GO can act as a quencher of fluorophores due to the nonradiative transfer of electronic transference from fluorophore excited states to the π system of GO.20 The morphology of GO, DSAI-GO complex, and P1DSAI-GO complex is characterized by atomic force microscopy (AFM). As displayed in Figure 1a, the thickness of GO is about

from DSAI to GO should occur, leading to high efficient fluorescent quenching of the DSAI/ssDNA aptamer complex.19 In the presence of the targeted complementary ssDNA, the binding between the ssDNA aptamer and targeted complementary ssDNA will alter the conformation of ssDNA aptamer and form dsDNA, which reduces the π−π stacking and hydrogen bonding interaction between GO and aptamers. Meanwhile, DSAI still adsorbs on the established dsDNA and stays away from GO, and the fluorescence of DSAI/dsDNA complex will be recovered and enhanced gradually. On the contrary, if the interferential mismatched ssDNA is added, the ssDNA aptamer cannot be changed into dsDNA and still adsorb on the surface of GO; thus, there is no obvious fluorescence recovery of the DSAI/ssDNA aptamer complex. The strong fluorescence of DSAI/ssDNA aptamer/targeted complementary ssDNA complex and weak fluorescence of DSAI/ssDNA aptamer/interferential mismatched ssDNA complex on the GO platform in the aqueous solution demonstrate that the targeted complementary ssDNA can be recognized effectively. To realize our design, we first investigate the AIE feature of DSAI or, in other words, the feasible fluorescent enhancement effect of DNA. As shown in Figure S1, Supporting Information, DSAI shows weak emission in water, while its suspensions in water/acetonitrile (CH3CN) mixtures with high CH3CN fractions are highly emissive. When adding a large amount of CH3CN into its water solution, the nonsolvent CH3CN causes DSAI molecules to aggregate and luminesce, indicating DSAI is typical of an AIE luminogen.7 Besides, we investigate the ssDNA-induced self-assembled aggregation of DSAI; unlabeled ssDNA aptamer (P1) is chosen as the probe ssDNA aptamer. As shown in Figure S2, Supporting Information, the fluorescent emission is dramatically enhanced with the addition of P1, which indicates the aggregation complex of DSAI and P1 is formed; the fluorescence of DSAI/P1 will be decreased with the addition of S1 nuclease (Figures S3 and S4, Supporting Information), because P1 is the substrate for S1 nuclease and can be effectively cleaved by S1 nuclease, and the ssDNA fragmentation could not induce aggregation of DSAI effectively.12 The variation of absorption spectra of DSAI in the absence and presence of P1 supports the above assumption. As shown in Figure S5, Supporting Information, the buffer solution of DSAI (10.0 μM) without P1 shows the absorption peak at 410 nm, and when P1 (10.0 μM) is added, the absorption peak is red-shifted to 432 nm, indicating the selfassembled aggregation of DSAI. After addition of S1 nuclease (50 U mL−1), the absorption peak is blue-shifted back to 410 nm, which indicates that the change of the absorption peak should be due to the disaggregation of DSAI. Herein, DSAI is a sensitive, AIE-active, and water-soluble bioprobe for ssDNA aptamer. Then, we use the P1-DSAI-GO complex-based aptasensor for the recognition of targeted complementary ssDNA(T1). The specific selectivity of the P1-DSAI-GO complex is determined by examining the fluorescent value of P1-DSAIGO complex toward T1 and interferential mismatched ssDNA (M1). GO is prepared according to the modified Hummers method. The infrared (IR) and Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) are performed to determine the chemical structure of GO (Figure S6, Supporting Information). The IR spectra indicate that it contains −COOH, epoxyl, and −OH groups, and all these oxygenbased groups endow the aqueous sensor application of GO.

Figure 1. AFM height image of GO (a) and P1-DSAI-GO complex (b).

1.0 nm. Upon addition of DSAI, the height of DSAI-GO complex in AFM is about 1.2 nm (Figure S7, Supporting Information), indicating that DSAI molecules were homogenously adsorbed on the surface of GO without any aggregation. When P1 was added into DSAI-GO complex, the height of the complex (Figure 1b) increased to 5.3 nm, indicating that the ssDNA is adsorbed on the surface of GO. Figure 2 shows the fluorescent emission spectra of DSAI (10.0 μM) in the absence and presence of GO at different conditions. Without GO, the fluorescent spectrum of DSAI shows weak fluorescent emission in Tris buffer solution (Figure 2a, curve1). After the addition of P1, the fluorescent emission could be enhanced because of the ssDNA-induced aggregation of DSAI and the formed aggregation DSAI/P1 complex (Figure 2a, curve 2). When the same amount of M1 and T1 is added into DSAI/P1 complex, respectively, the more aggregated DSAI/ssDNA aptamer complex is formed, and large fluorescent enhancement will be recorded. There is a little difference in the fluorescent intensity between M1 (Figure 2a, curve 3) and T1 (Figure 2a, curve 4), and maybe the difference is due to the different bases sequence between T1 and M1. The fluorescent intensity of T1 (Figure 2a, curve 4) is weaker than M1 (Figure 2a, curve 3), indicating that the AIE-active aptasensor could not distinguish the targeted complementary ssDNA from the interferential mismatched ssDNA. However, the AIE-active aptasensor could selectively recognize the T1 with the help of GO. As shown in Figure 2b, the fluorescence of DSAI could be quenched effectively when in the presence of GO in the buffer solution (Figure 2b, curve 1). Moreover, by virtue of the fluorescent quenching capability of GO, the background signal of fluorescence is very low, which ultimately leads to efficient visual detection. When P1 or other ssDNA is added, it could C

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Figure 2. Fluorescence emission spectra of DSAI in the absence (a) and presence (b) of GO at different conditions: (1) DSAI in buffer; (2) DSAI + P1 (200 nM); (3) DSAI + P1 +M1 (200 nM); (4) DSAI + P1 + T1 (200 nM). Experimental conditions: 10.0 μM DSAI, 200 nM P1, 10.0 μg/mL GO in 20 mM Tris-acetate, pH = 7.2 buffer solutions. λex = 405 nm.

Figure 3. TB-DSAI-GO complex as a fluorescent sensor for thrombin. (a) Fluorescence spectra for analyzing different concentrations of thrombin. (b) Plot of fluorescent peak intensity at 522 nm on the concentration of thrombin. The error bars represent the standard deviations of three repetitive measurements. Inset: Expanded low thrombin concentration region of the calibration curve. Experimental conditions: 10.0 μM DSAI, 200 nM TB, 10.0 μg/mL GO in 20 mM Tris-acetate, pH = 7.2 buffer solutions. λex = 405 nm.

mismatched M1 demonstrated that the targeted complementary ssDNA can be recognized selectively. Besides, the selectivity of the sensor described herein has also been determined by examining the fluorescent intensity of P1DSAI-GO complex toward target complementary T1 and the single-base mismatch ssDNA (SM1). As shown in Figure S10, Supporting Information, the fluorescent intensity obtained upon addition of 200 nM SM1 is the same as the fluorescent intensity obtained upon addition of 200 nM T1 on P1-DSAI complex. This result indicated that the AIE probes cannot distinguish single mismatches. However, when GO is added, the fluorescent intensity of SM1-P1-DSAI-GO complex is about 84% of fluorescent intensity of T1-P1-DSAI-GO complex, showing that the AIE probe can be a useful fluorescent indicator for the sensing of a single-base in DNA with the help of GO. Therefore, this AIE aptasensor can differentiate single mismatches, which offered the opportunity to allow single-nucleotide polymorphism (SNPs) analysis. Although this DNA-sequence signal specificity is lower than the report of labeled DNA-based aptasensor,20 the significant fluorescent enhanced sensing mode, label-free property, and AIE feature of DSAI probe can offer additional advantages of this analytical approach. The fluorescent analysis for different concentrations of targeted ssDNA ranging from 0 to 200 nM is tested, as shown in Figure S11, Supporting Information. As discussed above, the initial Tris buffer solution of the P1-DSAI-GO complex shows weak fluorescence. However, the fluorescent enhancement is observed as the concentration of T1 increases,

not induce the enhancement of emission of DSAI because of the strong adsorption interaction between GO and P1 (Figure 2b, curve 2, and Figure S8, Supporting Information). There is also not an obvious fluorescent enhancement after the addition of M1 (Figure 2b, curve 3). However, when T1 is added, P1 is hybridized to form a duplex dsDNA, and a dramatic fluorescent increase is observed (Figure 2b, curve 4). The adsorption interactions between DSAI/dsDNA complex and GO are very weak, and the DSAI/dsDNA complex can leave GO and avoid the fluorescent quenching of GO; thus, the fluorescence of DSAI/dsDNA complex can be regained and enhanced simultaneously. This conclusion of the formation of duplex dsDNA is further confirmed by circular dichroism (CD), as shown in Figure S9, Supporting Information. In the presence of P1, it has a weak positive Cotton effect peak around 276 nm that is due to the weak base stacking and a weak negative Cotton effect peak around 247 nm that is due to the weak helicity, which represent the single-stranded structure. However, the CD spectrum of P1 exhibits a strong positive Cotton effect at 276 nm corresponding to base stacking, and a strong negative Cotton effect of DNA helicity around 247 nm is observed when T1 is mixed, revealing the formation of dsDNA.22 Instead, when interferential mismatched M1 is added, the ssDNA, including P1 and M1, cannot form into the duplex dsDNA because M1 is not the complementary DNA of P1. Therefore, the DSAI/ssDNA complex will adsorb on GO and the fluorescence of DSAI/ssDNA complex can be quenched by GO. The obvious different fluorescent changes with the addition of target complementary T1 and interferential D

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DSAI-GO complex could be used as aptasensor for target protein with high selectivity and sensitivity. In conclusion, we have developed the AIE-based aptasensor for highly selective and sensitive sensing of targeted DNA and protein with the aid of GO. The AIE-active aptasensor can avoid the notorious ACQ effect and sophisticated fluorophore labeling process of fluorescent probe. Besides, GO has a high fluorescent quenching efficiency, greatly reducing the background signal of fluorescence in the aptasensor. Moerever, most nanoplatform/aptamer-based sensing technology uses labeled fluorescent probe as the indicator; the unlabeled AIE probe will exhibit more application in biological tissue. Additionally, to further optimize the performance of aptasensors, some impacts on the sensitivity and selectivity will be studied in detail, such as using various GO with different oxidation degrees and various AIE probes. Given its simplicity, easy operation, low cost, and good sensitivity and selectivity, this aptasensor can be extended to detect other targets, like heavy ions, drugs, proteins, and even cells.

which indicates that the ssDNA gradually converts into the dsDNA. In addition, the separation between the dsDNA and GO occurred after the presence of T1, resulting in the invalidation of the quenching effect of GO. The inset in Figure S11, Supporting Information, outlines the relationship between the fluorescent intensity at 520 nm and the concentration of T1. The linear range is observed from 25 to 200 nM. To illustrate the generality of this aptasensor, we analyze DSAI/GO-based aptasensor for protein. In this work, we choose thrombin as the model protein, which has great importance in molecular biology.23 An unlabeled thrombin aptamer (TB) is selected as the thrombin recognition elements aptamer and DSAI as the fluorescent indicator. There is no fluorescent enhancement when the TB is added into the DSAI/ GO complex (Figure S12, Supporting Information). Then, we choose TB-DSAI-GO complex as the aptasensor for thrombin. As shown in Figure 3, when thrombin is added into the TBDSAI-GO complex, a dramatic fluorescent increase is observed. It is reasonable that single-strand TB aptamers on graphene will combine with thrombin to form the G4 structure because of the high affinity between aptamers and thrombin.18 The formed quadruplex-thrombin complex releases from GO, and the DSAI probe gathered on the quadruplex-thrombin complex exhibits the obvious fluorescence. The detection limit can reach as low as about 3 ng/mL (Figure 3b, the inset). Therefore, aptamerDSAI-GO complex is also can serve as the aptasensor for targeted thrombin. To test the selectivity of our aptasenor for thrombin analysis, other common proteins are adopted in place of thrombin. Figure 4 shows fluorescence spectra of TB-DSAI-GO complex

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental procedures, general procedure for the synthesis of DSAI, characterization of GO, and fluorescent titration. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by 973 Program (2013CB834702), the Natural Science Foundation of China (Nos. 21074045, 21204027, 21221063), the Research Fund for the Doctoral Program of Higher Education of China (20120061120016), and the Project of Jilin Province (20100704).

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Figure 4. Relative peak fluorescence (F/F0) of TB-DSAI-GO complex for different proteins with the same concentration (1 μg/mL). Experimental conditions: 10.0 μM DSAI, 200 nM TB, 10.0 μg/mL GO in 20 mM Tris-acetate, pH = 7.2 buffer solutions. λex = 405 nm, λem = 520 nm.

REFERENCES

(1) Demchenko, A. P. Introduction to fluorescence sensing; Springer: New York, 2009. (2) Tu, D.; Liu, L.; Ju, Q.; Liu, Y.; Zhu, H.; Li, R.; Chen, X. Angew. Chem., Int. Ed. 2011, 50, 6306−6310. Wu, J.; Liu, W.; Ge, J.; Zhang, H.; Wang, P. Chem. Soc. Rev. 2011, 40, 3483−3495. Jung, J. H.; Cheon, D. S.; Liu, F.; Lee, K. B.; Seo, T. S. Angew. Chem., Int. Ed. 2010, 49, 5708−5711. Sauer, M. Angew. Chem., Int. Ed. 2003, 42, 1790−1793. Shu, X.; Liu, Y.; Zhu, J. Angew. Chem., Int. Ed. 2012, 51, 11006−11009. Haener, R.; Biner, S. M.; Langenegger, S. M.; Meng, T.; Malinovskii, V. L. Angew. Chem., Int. Ed. 2010, 49, 1227−1230. Huang, J.; Wu, Y.; Chen, Y.; Zhu, Z.; Yang, X.; Yang, C. J.; Wang, K.; Tan, W. Angew. Chem., Int. Ed. 2011, 50, 401−404. (3) Wang, M.; Zhang, G.; Zhang, D.; Zhu, D.; Tang, B. Z. J. Mater. Chem. 2010, 20, 1858. (4) Ding, D.; Li, K.; Liu, B.; Tang, B. Z. Acc. Chem. Res. 2013, 46, 2441−2453. (5) Luo, J.; Xie, Z.; Lam, J. W.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. (Cambridge) 2001, 1740−1741. (6) Hong, Y.; Lam, J. W.; Tang, B. Z. Chem. Commun. (Cambridge) 2009, 4332−4353.

with different proteins at equal concentrations. When tested with bovine serum albumin (BSA), papain (Pap), pepsin (Pep), trypsin (Try), lysozyme (Lyso), and thrombin each at a concentration of 1 μg/mL, TB-DSAI-GO complex shows a much lower fluorescent response to these control proteins because the control proteins cannot convert the single-stranded TB aptamer into the G4 stucture and the fluorescence of DSAI still be quenched by GO. However, after the addition of thrombin with the same concentration, TB-DSAI-GO complex shows obvious fluorescence bacause the TB aptamer can form G4 struture with the targeted thrombin, which released the TBDSAI from GO, and regain the fluorescence of the aggregated DSAI. This result clearly demonstrates that unlabeled aptamerE

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(7) Hong, Y.; Lam, J. W.; Tang, B. Z. Chem. Soc. Rev. 2011, 40, 5361−5388. (8) Lu, H.; Xu, B.; Dong, Y.; Chen, F.; Li, Y.; Li, Z.; He, J.; Li, H.; Tian, W. Langmuir 2010, 26, 6838−6844. Li, C.; Wu, T.; Hong, C.; Zhang, G.; Liu, S. Angew. Chem., Int. Ed. 2012, 51, 455−459. (9) He, J.; Xu, B.; Chen, F.; Xia, H.; Li, K.; Ye, L.; Tian, W. J. Phys. Chem. C 2009, 113, 9892−9899. (10) Lu, H.; Su, F.; Mei, Q.; Zhou, X.; Tian, Y.; Tian, W.; Johnson, R. H.; Meldrum, D. R. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 890− 899. Hong, Y.; Meng, L.; Chen, S.; Leung, C. W. T.; Da, L.-T.; Faisal, M.; Silva, D.-A.; Liu, J.; Lam, J. W. Y.; Huang, X.; Tang, B. Z. J. Am. Chem. Soc. 2012, 134, 1680−1689. Leung, C. W. T.; Hong, Y.; Chen, S.; Zhao, E.; Lam, J. W. Y.; Tang, B. Z. J. Am. Chem. Soc. 2013, 135, 62−65. Liu, Y.; Deng, C.; Tang, L.; Qin, A.; Hu, R.; Sun, J. Z.; Tang, B. Z. J. Am. Chem. Soc. 2011, 133, 660−663. Shi, H.; Kwok, R. T. K.; Liu, J.; Xing, B.; Tang, B. Z.; Liu, B. J. Am. Chem. Soc. 2012, 134, 17972− 17981. Wang, J.; Mei, J.; Hu, R.; Sun, J. Z.; Qin, A.; Tang, B. Z. J. Am. Chem. Soc. 2012, 134, 9956−9966. (11) Hong, Y.; Häuβler, M.; Lam, J. W.; Li, Z.; Sin, K. K.; Dong, Y.; Tong, H.; Liu, J.; Qin, A.; Renneberg, R. Chem.Eur. J. 2008, 14, 6428−6437. Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009, 4332−4353. Huang, J.; Wang, M.; Zhou, Y.; Weng, X.; Shuai, L.; Zhou, X.; Zhang, D. Bioorg. Med. Chem. 2009, 17, 7743−7748. Hong, Y.; Xiong, H.; Lam, J. W. Y.; Häußler, M.; Liu, J.; Yu, Y.; Zhong, Y.; Sung, H. H.; Williams, I. D.; Wong, K. S. Chem.Eur. J. 2010, 16, 1232−1245. Wang, M.; Zhang, G.; Zhang, D.; Zhu, D.; Tang, B. Z. J. Mater. Chem. 2010, 20, 1858−1867. Xu, J.-P.; Song, Z.-G.; Fang, Y.; Mei, J.; Jia, L.; Qin, A. J.; Sun, J. Z.; Ji, J.; Tang, B. Z. Analyst 2010, 135, 3002−3007. Shen, J.-S.; Li, D.-H.; Ruan, Y.-B.; Xu, S.-Y.; Yu, T.; Zhang, H.-W.; Jiang, Y.-B. Luminescence 2012, 27, 317−327. Li, X.; Ma, K.; Lu, H.; Xu, B.; Wang, Z.; Zhang, Y.; Gao, Y.; Yan, L.; Tian, W. Anal. Bioanal.Chem. 2013, 1−8. (12) Li, X.; Xu, B.; Lu, H.; Wang, Z.; Zhang, J.; Zhang, Y.; Dong, Y.; Ma, K.; Wen, S.; Tian, W. Anal. Methods 2013, 5, 438−441. (13) Michaud, M.; Jourdan, E.; Villet, A.; Ravel, A.; Grosset, C.; Peyrin, E. J. Am. Chem. Soc. 2003, 125, 8672−8679. (14) Wang, M.; Zhang, D.; Zhang, G.; Tang, Y.; Wang, S.; Zhu, D. Anal. Chem. 2008, 80, 6443−6448. (15) Hong, Y.; Häuβler, M.; Lam, J. W. Y.; Li, Z.; Sin, K. K.; Dong, Y.; Tong, H.; Liu, J.; Qin, A.; Renneberg, R.; Tang, B. Z. Chem.Eur. J. 2008, 14, 6428−6437. (16) Xie, W. Y.; Huang, W. T.; Li, N. B.; Luo, H. Q. Chem. Commun. 2012, 48, 82−84. Zhang, Y.; Liu, Y.; Zhena, S. J.; Huang, C. Z. Chem. Commun. 2011, 47, 11718−11720. Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M. Nat. Chem. 2010, 2, 1015−1024. Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282−286. Xu, X.; Huang, J.; Li, J.; Yan, J.; Qin, J.; Li, Z. Chem. Commun. 2011, 47, 12385. Xu, X.; Li, J.; Li, Q.; Huang, J.; Dong, Y.; Hong, Y.; Yan, J.; Qin, J.; Li, Z.; Tang, B. Z. Chem.Eur. J. 2012, 18, 7278−7286. Li, X.; Zhu, S.; Xu, B.; Ma, K.; Zhang, J.; Yang, B.; Tian, W. Nanoscale 2013, 5, 7776−7779. Liu, Z.; Liu, L.; Li, H.; Dong, Q.; Yao, S.; Kidd, A. B.; Zhang, X.; Li, J.; Tian, W. Sol. Energy Mater. Sol. Cells 2012, 97, 28−33. (17) Lu, C.-H.; Yang, H.-H.; Zhu, C.-L.; Chen, X.; Chen, G.-N. Angew. Chem., Int. Ed. 2009, 48, 4785−4787. (18) Chang, H.; Tang, L.; Wang, Y.; Jiang, J.; Li, J. Anal. Chem. 2010, 82, 2341−2346. (19) Dong, H.; Gao, W.; Yan, F.; Ji, H.; Ju, H. Anal. Chem. 2010, 82, 5511−5517. (20) He, S.; Song, B.; Li, D.; Zhu, C.; Qi, W.; Wen, Y.; Wang, L.; Song, S.; Fang, H.; Fan, C. Adv. Funct. Mater. 2010, 20, 453−459. (21) Balapanuru, J.; Yang, J.-X.; Xiao, S.; Bao, Q.; Jahan, M.; Polavarapu, L.; Wei, J.; Xu, Q.-H.; Loh, K. P. Angew. Chem., Int. Ed. 2010, 49, 6549−6553. (22) Tu, J.; Cai, C.; Ma, Y.; Luo, L.; Weng, C.; Chen, X. Spectrochim. Acta, Part A 2012, 99, 1−6. (23) Coughlin, S. R. Nature 2000, 407, 258−264.

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