Highly Fluorescent Aggregates Modulated by Surfactant Structure and

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Highly Fluorescent Aggregates Modulated by Surfactant Structure and Concentration Defeng Yu,† Qun Zhang,† Chunxian Wu,† Yingxiong Wang,† Lihua Peng,‡ Deqing Zhang,‡ Zhibo Li,§ and Yilin Wang*,† Key Laboratory of Colloid and Interface Science, Organic Solids Laboratory, and State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ReceiVed: March 26, 2010; ReVised Manuscript ReceiVed: May 11, 2010

The effects of anionic surfactants on the aggregation-induced emission (AIE) feature of cationic M-silole molecules have been studied. The electrostatic binding of M-silole with the surfactants greatly promotes the aggregation of the mixtures. The M-silole/surfactant aggregates at 1:1 charge ratio exhibit the maximum fluorescence intensity. Excess surfactant molecules will distribute the M-silole molecules into different micelles and weaken the fluorescence. The fluorescence intensity of the mixed M-silole/surfactant aggregates can be effectively modulated by choosing different surfactants. The gemini surfactants display a much stronger ability of enhancing fluorescence intensity than do the single-chain surfactants. Especially, the gemini surfactant with benzene rings shows the best performance in enhancing fluorescence of M-silole due to both the strongest aggregation ability and the π-π interaction with M-silole. Introduction High emission efficiency of fluorescent materials is one of the key factors to developing organic light-emitting diodes in practical applications.1,2 Generally, fluorescence is strengthened by confining nonradiative decay of fluorescent dyes, such as intermolecular interaction,3 intramolecular charge transfer,4,5 intramolecular torsional and rotational motion,6-8 etc. The deactivation of nonradiative decay can be achieved by changing the outside environments of dyes. A number of fluorescent molecules exhibit aggregation-induced emission (AIE) properties; that is, enhanced fluorescence emission efficiency is obtained for these molecules in an aggregate state as compared to in a solution state. The nonradiative channel via intramolecular vibrational, torsional, and rotational motion of the AIE molecules can be restricted in an aggregate state, and thus the fluorescence emission of these molecules can be turned on or enhanced.9 Obviously, intermolecular interaction between fluorescent dyes and other molecules may be applied to confine the nonradiative decay of dyes and hence enhance the fluorescence emission efficiency. Surfactants normally form micelles, vesicles, and other kinds of aggregates,10 and therefore could be good choices to modulate the aggregation of ionic fluorescent dyes in water and to adjust the fluorescence intensity. First, ionic water-soluble dye molecules are generally amphiphilic, so they tend to be solubilized into surfactant micelles.11-14 Second, ionic surfactant molecules can be bound with oppositely charged dyes through electrostatic interaction, and then form mixed aggregates together.15 Actually, the interaction between ionic dyes and surfactants has attracted much attention in many fields such as biochemistry, analytical chemistry, and photosensitization.15-17 However, few references reported the induced fluorescence enhancement of dye molecules by surfactants.13,14,18-20 Monomers of dye molecules were bound * To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn. † Key Laboratory of Colloid and Interface Science. ‡ Organic Solids Laboratory. § State Key Laboratory of Polymer Physics and Chemistry.

to micelles when surfactant concentration was above critical micelle concentration (CMC), and seldom exhibited strong fluorescence.13,14,18 Hachisako et al.13,14 reported that doublechain anionic peptide surfactants were able to induce intense fluorescence emission by forming extremely hydrophobic sites to incorporate stilbazolium-based compact hemicyanine dyes as monomers. The binding of the dye molecules to the hydrophobic core of the surfactant aggregates led to the increased planarity of dye molecules and minimized the role of vibrationally coupled nonradiative deactivation of the excited state, and hence induced higher fluorescence. Additionally, the monomers of thiacarbocyanine and its related derivatives were found to be solubilized into SDS, CTAB, and Triton X-100 micelles, and hence the fluorescence was slightly enhanced. H-aggregates of thiacyanine induced by aerosol-OT below CMC also exhibited 8-10 times stronger fluorescence than did the pure dye aqueous solution. On the basis of these observations, it is concluded that the intermolecular interaction between dyes and surfactants may effectively lower the intramolecular motion and decrease the nonradiative decay. In this Article, fluorescence enhancement of 1-methyl-1-(3ammonium)propyl-2,3,4,5-tetraphenyl-silole iodine (M-silole) (Scheme 1) has been studied by interaction with different surfactants. M-silole is a kind of AIE dye, and this kind of silolebased dye molecules has attracted much attention because of their distinctively high emission efficiency in aggregate or solid state.6-9,21-29 Thus, constructing aggregates with cationic Msilole and anionic surfactants may provide an effective and facile approach to obtain high fluorescence emission systems. To understand how surfactants with different aggregation ability and functional groups influence the fluorescence emission, single-chain surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS), and gemini surfactants 1,3-bis(N-dodecyl-N-propyl sulfonate sodium)-propane (12-3-12(SO3)2) and O,O′-bis[4-(1,1-dimethyl-3,3,3-tri-methylpropyl)-p-benzene sulfonate sodium]-1,3-dietherpropane (C8BC3C8B), are selected (Scheme 1). As compared to conventional single-chain surfactants, gemini surfactants have

10.1021/jp102742a  2010 American Chemical Society Published on Web 06/24/2010

Highly Fluorescent Aggregates SCHEME 1: Chemical Structures of M-Silole and the Surfactants

lower CMCs and much stronger aggregation ability.30-32 In contrast with SDS and 12-3-12(SO3)2, SDBS and C8BC3C8B have additional benzene rings. It is expected that benzene rings will have π-π interaction with dye molecules in the M-silole/ surfactant aggregates and hence could influence the fluorescence performance of the aggregates. Here, the variation of fluorescence intensity of M-silole with the surfactant concentration in aqueous solution has been systematically studied. The systems display the maximum around equal charge ratio of M-silole/ surfactant. Among the studied surfactants, the gemini surfactant with benzene rings, that is, C8BC3C8B, induces the strongest fluorescence enhancement of M-silole, which is almost 150 times higher than that of pure M-silole aqueous solution. Experimental Section Materials. 1-Methyl-1-(3-ammonium)propyl-2,3,4,5-tetraphenyl-silole iodine (M-silole) was synthesized as described previously.27 Sodium dodecyl sulfate (SDS) with g99% purity was purchased from Aldrich. Cetyltrimethyl ammonium bromide (CTAB) with g99% purity was purchased from Beijing Xinjingke Bio-Technology Co., Ltd. Sodium dodecyl benzene sulfonate (SDBS), with g95% purity, was purchased from Aladdin Reagent Co., Ltd., which was recrystallized from water/ ethanol before use. 1,3-Bis(N-dodecyl-N-propyl sulfonate sodium)propane (12-3-12(SO3)2) and O,O′-bis[4-(1,1,3,3-tetramethylbutane)-o-benzene sulfonate sodium]-1,3-dietherpropane (C8BC3C8B) (see Supporting Information) were synthesized and purified in our lab.33 Quinine hemisulfate monohydrate with 99% purity was purchased from Alfa Aesar. Milli-Q water (18 MΩ cm-1) was used throughout. UV-Vis and Fluorescence Experiments. Spectra were recorded in quart cuvettes (path length 1 cm) within 1-2 min of the sample preparation at 25 °C. The absorption spectra were recorded by a SHIMADZU UV 1601PC spectrometer, while the fluorescence spectra were measured using a Hitachi model F-4500 spectrofluorometer with an excitation wavelength of 365 nm. Isothermal Titration Microcalorimetry (ITC). The calorimetric measurements were conducted using a TAM 2277-201 microcalorimetric system (Thermometric AB, Ja¨rfa¨lla, Sweden) with a stainless steel sample cell of 1 mL at 25.00 ( 0.01 °C. The cell was initially loaded with 700-800 µL pure water or 1 × 10-4 M M-silole solution. Concentrated surfactant solution

J. Phys. Chem. B, Vol. 114, No. 27, 2010 8935 was injected into the sample cell via a 500 µL Hamilton syringe controlled by a 612 Thermometric Lund pump. A series of injections were made until the desired concentration range had been covered. The system was stirred at 50 rpm with a gold propeller. The observed enthalpy (∆Hobs) was obtained by integration over the peak of each injection in the plot of heat flow P against time t. Dynamic Light Scattering (DLS). Measurements were carried out at 25.0 ( 0.5 °C using an LLS spectrometer (ALV/ SP-125) with a multi-τ digital time correlator (ALV-5000). A solid-state He-Ne laser (output power of 22 mW at λ ) 632.8 nm) was used as a light source, and the measurement was conducted at a scattering angle of 90°. The freshly prepared samples were injected into a 7 mL glass bottle. The correlation function of scattering data was analyzed via the CONTIN method to obtain the distribution of diffusion coefficients (D) of the solutes, and then the apparent equivalent hydrodynamic radius (Rh) was determined using the Stokes-Einstein equation Rh ) kT/6πηD, where k is the Boltzmann constant, T is the absolute temperature, and η is the solvent viscosity. Confocal Laser Scanning Microscopy (CLSM). Optical images of aggregates in water were obtained using a Leica TCS SP confocal system (Leica, Germany) equipped with a 100× oil immersion objective with a numerical aperture of 1.4. The mercury lamp (330-385 nm) and 405 nm lasers were chosen for excitation. ζ-Potential Measurements. The ζ-potential measurements were performed at 25.0 ( 0.1 °C using a Malvern Zetasizer Nano-ZS instrument (ZEN3600, Malvern Instruments, Worcestershire, UK) equipped with a 4 mW He-Ne laser at a 633 nm wavelength. Cryogenic Transmission Electron Microscopy (CryoTEM). CryoTEM samples were prepared in a controlled environment vitrification system (CEVS) at 28 °C.34 A micropipet was used to load 5 µL of the M-silole/surfactant solution onto a lacey support TEM grid that was held with tweezers. The excess solution was blotted with a piece of filter paper, resulting in the formation of thin films suspending the mesh holes. After waiting for about 10 s to relax any stress induced during blotting, the samples were quickly plunged into a reservoir of liquid ethane (cooled by liquid nitrogen) at its melting temperature. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined with a JEM 2200FS TEM (200 keV) at about -174 °C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatan multiscan CCD and processed with Digital Micrograph. Scanning Electron Microscopy (SEM). SEM measurements were conducted with an S-4800 (HITACHI, Japan) scanning electron microscope. A drop of the sample solution was placed onto a cleaned silica wafer and frozen by liquid nitrogen, then dried in a vacuum, and sputtered with gold at last. X-ray Diffraction (XRD). Self-supported cast films were prepared by dispersing the aqueous solutions of aggregates onto precleaned slides and then air-drying at room temperature. Reflection XRD studies were carried out with an X-ray diffractometer (Rigaku model D/MAX2500). The X-ray beam was generated with a Cu anode at 40 kV and 200 mA, and the wavelength of the KR1 beam was 1.5406 Å. The X-ray beam was directed to the edge of the film, and the scanning 2θ was recorded from 1° to 15°, using a step width of 0.01°. Results and Discussion M-silole was dispersed in water by ultrasonic method, and a transparent solution (1 × 10-4 M) was observed by naked eye.

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Yu et al.

Figure 1. Optical microscope image and DLS measurement of the size distribution of 1 × 10-4 M M-silole solution.

TABLE 1: CMC Values of the Surfactants in Aqueous Solutions without M-Silole Measured by ITC CMC (mM)

Figure 2. Plot of I/I0 versus the surfactant concentration (Cs) at 25 ( 1 °C, where I refers to the fluorescence intensity of the mixtures of 1 × 10-4 M M-silole with SDS, 12-3-12(SO3)2, SDBS, or C8BC3C8B at 487 nm (λex ) 357 nm), and I0 refers to the fluorescence intensity of M-silole without any surfactants. Inset: Photos of (A) pure 1 × 10-4 M M-silole solution, (B) mixture of 1 × 10-4 M M-silole and 1 × 10-4 M SDS, and (C) mixture of 1 × 10-4 M M-silole and 1.5 × 10-2 M SDS under 365 nm UV light illumination.

As shown in Figure 1, globular aggregates are observed by optical microscopy, and the average hydrodynamic radius of the aggregates is ∼470 nm by DLS. An absorption band at 357 nm and an emission band at 487 nm are observed in the absorption and emission spectra (Figure SI1). Although M-silole molecules are dispersed in water in the form of globular aggregates, the M-silole aqueous solution shows rather weak fluorescence under the irradiation of mercury lamp (330-385 nm). Considering strong electrostatic repulsion among the intermolecular ammonium groups, M-silole molecules may arrange more loosely in the aggregates, and thus the fluorescence of M-silole molecules cannot be effectively enhanced. With the addition of SDS, 12-3-12(SO3)2, SDBS, and C8BC3C8B, the variations of fluorescent intensity of the M-silole aqueous solutions demonstrate a similar trend (Figure 2). The corresponding fluorescence spectra are shown in the Supporting Information (Figure SI2). As the surfactant concentration increases, the relative emission intensity (I/I0) increases first and reaches a maximum, then decreases gradually, and levels off at last. Even for SDS, showing the lowest fluorescence enhancement among the four surfactants, the differences of the fluorescence intensities at the three different concentrations of before, at, and beyond the maximum can be easily distinguished by naked eye under UV light (365 nm) illumination (inset of Figure 2). The maximum I/I0 values are 19, 40, 53, and 145 for

SDS

12-3-12(SO3)2

SDBS

C8BC3C8B

9.48

0.074

1.93

12.5

SDS, 12-3-12(SO3)2, SDBS, and C8BC3C8B, respectively. Especially, the I/I0 value remains very high within a very wide concentration range of C8BC3C8B. Using quinine hemisulfate monohydrate as a reference, the fluorescence quantum yield of pure M-silole solution is only 0.0035, while the maximum fluorescence quantum yields are improved to 0.052, 0.108, 0.077, and 0.225 in the presence of SDS, 12-3-12(SO3)2, SDBS, and C8BC3C8B, respectively. Obviously, C8BC3C8B displays the most significant fluorescence enhancing ability to M-silole. It is noted that the ratio of the single chain surfactant (SDS and SDBS) to M-silole is about 1:1, while the ratio of the gemini surfactant (12-3-12(SO3)2 and C8BC3C8B) to M-silole is nearly 1:2 when the I/I0 values reach the maxima. That is to say, the fluorescence intensity of M-silole is the strongest when the charge ratio of the negatively charged headgroups of the surfactants to the positive ammonium headgroups of M-silole fulfills 1:1 stoichiometry. As a comparison, the fluorescence intensity of M-silole solution with the addition of cationic surfactant CTAB was also measured (Figure SI3). The result shows that CTAB hardly influences the fluorescence intensity of M-silole. Clearly, the electrostatic binding between oppositely charged surfactants and fluorescent dyes is one of the key factors to induce fluorescence enhancement. The absorption spectra and the maximum absorption wavelength of the M-silole/surfactant aqueous solutions at the surfactant concentrations of (A) before fluorescence enhancement, (B) at the maximum of fluorescence enhancement, and (C) beyond the fluorescence enhancement are shown in the Supporting Information (Figure SI4 and Table SI1). As compared to the pure M-silole solution, the maximum absorption of the M-silole/surfactant solutions at the maximum fluorescence enhancement, that is, at equal M-silole/surfactant charge ratio, shows a 9-20 nm red shift, and then the absorption peaks are blue-shifted back beyond the CMC. This means that the surfactant binding significantly changes the microenvironment of M-silole molecules with the occurrence of the fluorescence enhancement. To know the relationship between the fluorescence enhancement and the aggregation behavior of the surfactants, the CMC values of the four surfactants (Table 1) in aqueous solutions without M-silole were determined by ITC. The ITC curves are shown in the Supporting Information (Figure SI5). Obviously, the most significant fluorescence enhancements do not take place

Highly Fluorescent Aggregates

Figure 3. The size distributions of 1 × 10-4 M M-silole solutions with 1 × 10-4 M SDS and SDBS, and 5 × 10-5 M 12-3-12(SO3)2 and C8BC3C8B at 25.0 °C.

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Figure 5. The observed enthalpy ∆Hobs of the surfactant monomer solution of 1 mM SDS, 0.05 mM 12-3-12(SO3)2, 1 mM SDBS, and 1 mM C8BC3C8B (below CMC) being titrated into 1 × 10-4 M M-silole solutions against the final surfactant concentration (Cs) at 25.00 °C.

Figure 4. Confocal laser scanning image of 1 × 10-4 M M-silole solution with 1 × 10-4 M SDS under mercury lamp illumination.

at the CMCs of these four surfactants but far below the CMC values. Because the fluorescence enhancements of AIE dyes are normally induced by the aggregation, the present results suggest that the M-silole molecules and the surfactant molecules have formed aggregates before the CMC values of these surfactants. To know the aggregation behavior of the surfactants with M-silole, the size distributions of the M-silole/surfactant solutions at equal charge ratio have been studied by DLS. Figure 3 shows that all of the systems display one peak and the hydrodynamic radii are about 133, 384, 72, and 115 nm for M-silole with SDS, 12-3-12(SO3)2, SDBS, and C8BC3C8B, respectively. It was mentioned above that the hydrodynamic radius of M-silole self-assemblies is ∼470 nm. Apparently, M-silole self-assemblies are converted to small aggregates upon the addition of surfactants. The mixed M-silole/surfactant aggregates show strong fluorescence as convincingly shown by the confocal laser image in Figure 4, where many fluorescing dots move irregularly in water. These M-silole/surfactant aggregates at 1:1 charge ratio are generated mainly through electrostatic interaction, which is confirmed by the following ITC results. The observed enthalpy changes (∆Hobs) of the monomer solution of the surfactants (below CMC) being titrated into the 1 × 10-4 M M-silole solution are plotted against the final surfactant concentration (Cs) in Figure 5. Very large exothermic enthalpy changes are observed just upon the addition of the surfactants, which are characteristic of electrostatic interaction between oppositely charged molecules. As the surfactant concentration increases,

Figure 6. SEM images of the aggregates of (a) M-silole/SDS, (b) M-silole/12-3-12(SO3)2, (c) M-silole/SDBS, and (d) M-silole/ C8BC3C8B.

the enthalpy changes gradually become small and finally reach zero around the equal charge ratio of M-silole/surfactant. As to 12-3-12(SO3)2, because its CMC (0.074 mM) is very low, the monomer concentration is too low so that the interaction of M-silole with the 12-3-12(SO3)2 monomers can only be observed at very low surfactant concentration in the calorimetric curve. Zeta potential measurements further convince that the M-silole/ surfactant aggregates at 1:1 charge ratio are close to electrical neutralization (Supporting Information Table S2). The morphologies of the aggregates at 1:1 M-silole/surfactant charge ratio imaged by SEM and TEM are shown in Figures 6 and 7 and Figure SI9. The SEM and TEM samples in Figure 6 and Figure SI9 were prepared by the freeze-drying method.35 All of the M-silole/surfactant aggregates show similar spherical morphology. The CryoTEM image in Figure 7 indicates these spherical aggregates are solid spheres. Further XRD result of the M-silole/C8BC3C8B aggregates (Figure SI10) presents regular and repeated thickness of about 2.3 nm, suggesting that the solid aggregates may be constructed by the bilayer structure of M-silole and the surfactant. Obviously, hydrophobic interaction between the alkyl chains of the surfactants favors this arrangement. Several references have reported such layer structures in dye-surfactant systems.36,37 At the equal charge ratio of M-silole/surfactant, the M-silole/surfactant bilayers may

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Figure 7. CryoTEM image of M-silole/C8BC3C8B aggregates.

Figure 8. DLS measurements of the size distributions of 1 × 10-4 M M-silole solutions with 1.5 × 10-2 M SDS, 5 × 10-3 M SDBS, 2 × 10-3 M 12-3-12(SO3)2, and 1.5 × 10-2 M C8BC3C8B at 25.0 °C.

easily accumulate in three dimensions through layer-by-layer approach, leading to the solid spheres. These bilayer-by-bilayer structures are similar to the solid-like vesicles reported previously.37 Unfortunately, the bilayer-by-bilayer structures in the solid spheres have not been observed from the CryoTEM images. The probable reason is that M-silole and the surfactants may be closely packed due to the charge neutralization of the

Yu et al. M-silole/surfactant mixtures and the strong π-π interaction among the M-silole molecules or between the M-silole and the surfactants with benzene ring. Thus, it is difficult to observe the inner bilayer-by-bilayer structure of the solid spheres from the TEM images. Another possibility is that the M-silole/ surfactant aggregates may not form the ordered bilayer-bybilayer structure, but form a complex structure with the aid of hydrophobic interaction among the hydrocarbon chains of the surfactants as well as π-π interaction among the M-silole molecules or between the M-silole and the surfactants with benzene ring. Beyond the maximum area in the curves of Figure 2, the fluorescence intensity of the M-silole/surfactant mixtures starts to decrease and levels off, accompanied by a blue shift of the absorption maximum of M-silole. The DLS result (Figure 8) indicates that the large aggregates are disaggregated into small aggregates with an increase in the amount of surfactant micelles. M-silole molecules then may be distributed into different micelles. The most possible situation is that the hydrophobic part of M-silole molecules may be irregularly located in the hydrophobic region of micelles and the ammonium groups are bound with the anionic surfactant headgroups at the micelle-water interface. Thus, the π-π interaction among M-silole molecules is disrupted, lowering the fluorescence intensity of the systems. In summary, the fluorescence intensity of AIE molecule M-silole can be effectively adjusted by choosing different surfactants and different M-silole/surfactant ratios. The possible mechanism is shown in Figure 9. With the addition of the surfactants, cationic M-silole molecules are bound with anionic surfactants through electrostatic interaction, and then the Msilole/surfactant mixtures start to aggregate, being driven by hydrophobic interaction between the alkyl chains of the surfactants and the π-π interaction between M-silole molecules. They may form the ordered bilayer-by-bilayer structure or the complex structure. The M-silole molecules arrange more closely in the aggregates, and hence the intramolecular motion is effectively restricted. Moreover, very strong aromatic π-π interaction between the M-silole molecules is generated in the aggregation. Thus, the fluorescence intensity of M-silole is greatly enhanced. Because the gemini surfactants effectively reduce the electrostatic repulsion among headgroups by linking the headgroups with a spacer, they have strong hydrophobic interaction due to their two hydrocarbon chains. This significantly enhances the aggregation ability of the gemini surfactants

Figure 9. The probable morphology changes of M-silole/surfactant aggregates.

Highly Fluorescent Aggregates with M-silole molecules. So the aggregates formed by the gemini surfactants arrange more closely than those by single chain surfactants. Therefore, the gemini surfactants display much stronger ability of enhancing the fluorescence intensity of M-silole than do the single-chain surfactants. Besides the strong aggregation ability, the gemini surfactant C8BC3C8B with the benzene rings attached to the headgroups can interact with the M-silole molecules through π-π interaction. Thus, the π-π interaction among M-silole molecules, C8BC3C8B molecules, and between them generates widespread π-π interaction in the aggregates. Therefore, C8BC3C8B exhibits the strongest ability of enhancing the fluorescence of M-silole. With excess surfactants beyond the CMC, the charged aggregates tend to break into small micelles due to electrostatic repulsion, and M-silole molecules are distributed into different surfactant micelles. Thus, the π-π interaction of M-silole molecules is disrupted, leading to the decrease of the fluorescence intensity. Conclusion The AIE effect of anionic single-chain and gemini surfactants with and without benzene rings on M-silole has been studied. Although M-silole molecules self-assemble into large aggregates in aqueous solution, the aggregates exhibit very weak fluorescence because the electrostatic repulsion among the ammonium groups compels the silole molecules to array loosely. With the addition of the four surfactants studied, the formed M-silole/ surfactant aggregates show strong fluorescence intensity. The aggregates with equal charge ratio of M-silole to surfactant exhibit the maximum fluorescence intensity. Because gemini surfactants have much stronger aggregation ability than single chain surfactants, the gemini surfactants show much stronger ability to enhance the fluorescence intensity of M-silole. Besides, the surfactants containing benzene rings can interact with M-silole molecules through the π-π interaction, and the π-π interaction among M-silole and the surfactant molecules may strict the intramolecular motion of M-silole more significantly. Therefore, the gemini surfactant C8BC3C8B with benzene rings exhibits the best performance of enhancing the fluorescence intensity of M-silole, and the I/I0 value reaches 145 at the maximum point. Besides, the M-silole/surfactant ratio is one of the key factors to enhance the fluorescence. The mixture at 1:1 charge ratio exhibits the strongest aggregation ability and the resultant strongest fluorescence intensity. However, excess surfactant molecules cause the M-silole molecules to be distributed into different micelles, and then the π-π interaction among M-silole molecules is disrupted. Thus, the fluorescence of M-silole is weakened. This work provides a strategy to choose surfactants to enhance the fluorescence intensity of ionic dyes. Oppositely charged surfactants with strong aggregation ability and aromatic rings would play the best performance in enhancing fluorescence. Surfactants were used to improve the sensitivity of conjugated polyelectrolyte-based biosensors previously.38,39 The present work should be helpful for choosing suitable surfactants to greatly improve the sensitivity of conjugated polyelectrolyte-based biosensors. Acknowledgment. This work was supported by the Chinese Academy of Sciences, the National Natural Science Foundation of China, and the National Basic Research Program of China (Grants 20633010, 2005cb221300). We thank the State Key Laboratory of Polymer Physics and Chemistry for DLS. Supporting Information Available: Synthesis and characterization data of C8BC3C8B; absorption and emission spectra

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