Hierarchically Imprinted Porous Films for Rapid and Selective

ARTICLE pubs.acs.org/Langmuir

Hierarchically Imprinted Porous Films for Rapid and Selective Detection of Explosives Wei Zhu,† Shengyang Tao,† Cheng-an Tao,† Weina Li,† Changxu Lin,† Ma Li,‡ Yuquan Wen,‡ and Guangtao Li*,† †

Key Lab of Organic Optoelectronic and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China ‡ State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China

bS Supporting Information ABSTRACT: On the basis of the combination of colloidal and mesophase templating, as well as molecular imprinting, a general and effective approach for the preparation of hierarchically structured trimodal porous silica films was developed. With this new methodology, controlled formation of well-defined pore structures not only on macroand mesoscale but also on microscale can be achieved, affording a new class of hierarchical porous materials with molecular recognition capability. As a demonstration, TNT was chosen as template molecule and hierarchically imprinted porous films were successfully fabricated, which show excellent sensing properties in terms of sensitivity, selectivity, stability, and regeneracy. The pore system reported here combines the multiple benefits arising from all length scales of pore size and simultaneously possesses a series of distinct properties such as high pore volume, large surface area, molecular selectivity, and rapid mass transport. Therefore, our described strategy and the resulting pore systems should hold great promise for various applications not only in chemical sensors, but also in catalysis, separation, adsorption, or electrode materials.

’ INTRODUCTION Incorporation of different length-scale pores into one composite material in a controlled manner provides the possibility of producing pore systems that combine the multiple benefits from the different pore size regimes, including high pore volume, large surface area, molecule selectivity, and improved diffusion properties. Such hierarchical porous materials from molecular to the macroscale occur widely in nature, and exhibit the capability of performing highly sophisticated functions.1 However, using the synthetic approach, a controlled arrangement of pore structures in an interconnected form on multiple scales, in particular, on all three scales from microscale over mesoscale to macroscale, is a challenging issue,2
4 but has attracted significant attention owing to its important role in the systematic study of structure
property relationships and numerous applications in catalysis, separation, adsorption, electrode materials, or chemical sensors.5
9 Recent studies mainly described the construction of hierarchical bimodal pore systems; various materials with micro-macroporous, micro-mesoporous, meso-macroporous, or small meso
large mesoporous structures have been fabricated.2
4,10
16 In contrast, the creation of trimodal porous materials with hierarchically organized micro-meso
macroporosity of adjustable size and well-defined shape on all length scales are limited.2
4,17
21 Smarsly and coworkers reported the fabrication of hierarchical trimodal porous silica materials by using polymer latex spheres and two types of novel surfactants (KLE copolymer and imidazolium-based ionic liquid) as r 2011 American Chemical Society

templates, showing well-defined macropores (360 nm) with interconnected small (2
3 nm) and large mesopores (12 nm) in the macropore walls.17,18 Anderson's group prepared micro-meso
macroporous silica by polystyrene sphere templating and cooperative assembly of inorganic sol
gel species with amphiphilic triblock copolymers, where the microporosity in the samples was induced by a large fraction of the PEO chains embedded in the silica matrix.19 To the best of our knowledge, there are no reports on the generation of porous materials with hierarchically organized micro-meso
macroporosity of adjustable size and well-defined shape on all three length scales. The previously reported trimodal porous materials either lacked well-defined pore shapes (one type of pores was usually interparticles spaces) or pore dimensions.2
4,19 In addition, the developed methods were essentially applied to the preparation of materials in the monolith or powder forms, whereas thin films with hierarchical porosity are much less described.22 The thin film configuration is, however, of importance and highly desirable for various applications. Moreover, although the need for such structured materials is often stated as a paradigm, quite few papers have been published to demonstrate or explore the advanced applications based on functional trimodal porous materials. Received: March 21, 2011 Revised: June 2, 2011 Published: June 06, 2011 8451

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Scheme 1. Schematic Illustration of the Construction of Hierarchically Imprinted Hybrid Silica Film

In this paper, by combing colloidal crystal templating, mesophase templating, and molecular imprinting, namely, a triple imprinting approach, we report the synthesis of micro-meso
macro trimodal hybrid silica films not only with precise control of pore size and morphology on all three length scales, but also endowed with molecular selectivity. The concept of our multilevel imprinting results in the formation of interwoven differently sized pores within the silica matrix, each with a specific function (see Scheme 1). On the microporous level, the removal of the template molecule from the imprinting complex leaves cavities complementary to the size, shape, and interaction sites of the imprint that exhibit molecular recognition. These pores give the material enhanced selectivity for the given molecule. On the mesoporous level, the removal of supramolecular aggregate of the surfactant results in the formation of well-defined cylindrical mesopores that give the silica film an overall porosity including large surface areas and excellent molecule transport kinetics. Finally, on the macroporous level, the removal of synthetic latex spheres produces a controlled three-dimensionally interconnected macroporosity that renders the film further enhanced mass transport and thus improved accessibility to recognition sites. This combination of efficient mass transport, high surface area, and molecular selectivity should make such materials ideal candidates for extensive applications, for example, for the development of high-performance chemical sensors. To demonstrate the great potential of this triple imprinting methodology, 2,4,6-trinitrotoluene (TNT, a common land-mine explosive) was chosen as template/target molecule in this work, and a new kind of highly sensitive chemosensors for rapid and selective detection of TNT were developed based on this TNT-imprinted trimodal porous films.

’ EXPERIMENTAL SECTION Chemicals. 1,5-Dihydroxynaphthalene (Aldrich), 1,2-bis(trimethoxysilyl)ethane (BTME, Aldrich), Pluronic F127 (Aldrich), Pt(0) catalyst (Aldrich), 7-amino-4-methylcoumarin (AMC, Aldrich), and (3-isocyanatopropyl)triethoxysilane (IPTES, Aldrich) were used as received. 3Bromopropene, ethenyl-benzene, and hydrogen chloride were purchased from Beijing Chemical Reagents Company and used without further purification. Mill-Q water was used in whole experiments. Glass slides (3 cm 3 cm) were used as substrates. Prior to use, glass slides were sonicated in ethanol and water in turn for 15 min, and dried under a stream of N2. Characterization. 1H NMR spectra were obtained using a JEOL JNM
ECA300 at 300 MHz. MS data were obtained by means of a Finnigan MAT 112b and a Finnigan MAT 711. The UV
vis absorption spectra were recorded using a Perkin-Elmer UV/vis Lambda35 spectrometer. The fluorescence emission measurements were carried out using a fluorescence spectrometer (Perkin-Elmer LS55). The glass slide coated with silica film was placed in a quartz cuvette. The emission data were collected in the region 300
400 nm using excitation wavelength of 235 nm. X-ray diffraction (XRD) spectra were recorded on a D/max-RB (Japan, Rigaku) diffractometer with monchromatized Cu KR radiation (λ = 0.15418 nm), operating at 40 kV and 120 mA. Data were obtained with a scanning rate of 4.0° min
1. To get TEM images of the prepared silica films, silica samples were scratched off from the glass substrates and dispersed in ethanol by sonication for 15 min. A drop of the dispersed particles was then deposited onto a carbon-coated Cu grid and examined using Hitachi 800 microscope operating at 200 kV. To get SEM images of the prepared silica film, the film was examined using a LEO1530 scanning electron microscope (SEM) using Au
C coated samples. Synthesis of 1,5-Bis (allyloxy)naphthalene. 3.2 g (20 mmol) naphthalene-1,5-diol was dissolved in 40 mL acetonitrile, and then 2.5 g (44 mmol) KOH was added. After stirring at RT for 20 min, 4.4 mL 8452

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Langmuir (48 mmol) allyl bromide was added. The resulting reaction mixture was stirred for further 6 h at RT. The solution was filtered and the filtrate was concentrated. After the addition of 50 mL methanol, the obtained solution was kept at 0 °C for 12 h. The resulting faint yellow crystal was recrystallized twice by hexane, affording pure product with white color (yield: 85%). 1H NMR (300 MHz, CDCl3): δ 4.69 (br, 4H, -OCH2-), 5.32 (d  d, J = 10.29 Hz, 2H, dCH2), 5.54 (d  d, J = 11.03 Hz, 2H, dCH2), 6.17 (m, 2H, -CH2CHd), 6.84 (m, 2H, 2
Ar-H), 7.35 (m, 2H, 3
Ar-H), 7.89 (d, J = 8.25 Hz, 2H, 4
Ar-H). EIMS (m/z) = 240.1.

Synthesis of 1,5-Bis(3-(triethoxysilyl)propoxy)naphthalene. 1.68 g (7 mmol) 1,5-bis (allyloxy) naphthalene was dissolved in 15 mL dry toluene, and then 3.38 mL (18 mmol) triethoxysilane and 0.2 mL (0.014 mmol) Pt (0) catalyst were added under stirring. The resulting mixture was refluxed at 80 °C for 24 h. After concentration in vacuo, the crude product was purified by chromatography using hexane/ethyl acetate (50:1) as the eluent, affording the desired crystal product with brown color (yield: 80%). 1H NMR (300 MHz, CDCl3): δ 0.88 (m, 4H, -SiCH2-), 1.24 (t, J = 7.2 Hz, 18H, -CH3), 2.05 (m, 4H, -SiCH2CH2-), 3.85 (q, J = 6.87 Hz, 12H, -OCH2CH3-), 4.11 (t, J = 6.54 Hz, 4H, -OCH2-), 6.82 (d, J = 7.56 Hz, 2H, 2
Ar-H), 7.33 (t, J = 7.89 Hz, 2H, 3
Ar-H), 7.84 (d, J = 8.25 Hz, 2H, 4
Ar-H). EIMS (m/z) = 568.8. Synthesis of Triethoxysilylated Coumarin. 300 mg (1.72 mmol) of AMC was dissolved in 60 mL of dry THF. After 30 min of stirring, 8.49 g (34.4 mmol) of IPTES was added, and the mixture was refluxed for 20 h. After the mixture was cooled to room temperature, the solvent was removed by evaporation. The precipitated powder was dissolved in 30 mL of dry CH2Cl2, and next, 50 mL of hexane was poured gently into the CH2Cl2 solution to precipitate the product. After the mixture was allowed to sit for 1 day, the product was recovered with suction filtration and washed with abundant hexane. This precipitation/purification scheme was repeated twice. 1H NMR (300 MHz, MeOD): δ 0.67 (t, 2H, SiCH2), 1.23 (t, 9H, CH3 in OEt), 1.66 (quin, 2H, middle CH2 in propylene), 2.44 (s, 3H, aromCH3), 3.31 (quar, 2H, N-CH2), 3.83 (quar, 6H, OCH2), 6.15 (s, 1H, aromH), 7.51 (s, 1H, arom-H), 7.27 (d, 1H, arom-H), 7.61 (d, 1H, arom-H), 7.58 (s, 1H, arom-NH-). Preparation of PS Colloidal Crystal Templates. Monodispersed polystyrene (PS) colloidal microspheres with a diameter of 510 nm were synthesized according to the reported procedure. 80 μL aqueous solution of PS particles (3 wt %) were dropped on a clean glass substrate (3  3 cm2), and under controlled humidity (80% RH), a welldefined PS crystal array with close-packed face centered cubic arrangement formed through evaporation induced self-assembly process. The obtained colloidal crystals were used as templates for the fabrication of hierarchically imprinted trimodal pore films.

Synthesis of TNT-Imprinted Organosilica Precursor to Form Macro
Meso
Micropore Films (1). 5 mg (8.80 

10
3 mmol) naphthalene-bridged silane (BTPN) and 1 mg (4.40  10
3 mmol) TNT were dissolved in 2.93 mL anhydrous ethanol. After stirring at RT for 2 h, 3.54 g (10 mmol) BTME, 0.38 g (21 mmol) water, and 12.3 μL (0.07 M) HCl were added, and the resulting reaction mixture was refluxed at 60 °C for 90 min. Then, 1.25 mL water and 65.4 μL HCl (1 M) were added for a further 15 min stirring. The formed sol was further diluted with ethanol and mixed with nonionic surfactant F127. The final molar ratio of the reactants was 1 BTME/8.8  10
4 BTPN/4.4  10
4 TNT/217.6 EtOH/9.1 H2O/6.5  10
3 HCl/0.019 F127.

Synthesis of TNT-Imprinted Organosilica Precursor to Form Macro
Meso
Micropore Films (2). 2 mg (4.74  10
3 mmol)

coumarin
silane (TSPCU) and 3.23 mg (14.22  10
3 mmol) TNT were dissolved in 2.49 mL anhydrous ethanol. After stirring at RT for 2 h, 1.04 g (5 mmol) TEOS, 0.73 g (41 mmol) water, and 105 μL (1 M) HCl, 6.18 mL anhydrous ethanol were added, and the resulting reaction mixture was refluxed at 60 °C for 30 min. Then, 0.26 g F127 and 6.18 mL anhydrous ethanol were added for a further 30 min stirring. The final molar ratio of the

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reactants was 1 TEOS/9.48  10
4 TSPCU/2.84  10
3 TNT/51 EtOH/ 9.2 H2O/2.1  10
2 HCl/2.26  10
2 F127.

’ RESULTS AND DISCUSSION Thin films with molecular-imprinted trimodal pore structure were prepared by the sol
gel deposition of imprinting-complex containing mesoporous silica material inside a colloidal crystal followed by the removal of all used templates. In the present case, uniform polystyrene spheres (PS, 510 nm) were used to obtain a close-packed colloidal assembly by conventional horizontal deposition. Naphthalene-bridged silane (BTPN) was synthesized as a functional monomer for the formation of TNT-imprinted complex, arising from strong charge-transfer interaction between electronsufficient naphthalene and electron-deficient TNT (Scheme 1). For better molecule imprinting, 1,2-bis(trimethoxysilyl)ethane (BTME) was chosen as cross-linker, and Pluronics (PEO-PPO-PEO) copolymer F127 was employed as structure-directing agent for the creation of mesoporous structure in the macropore walls. Briefly, BTPN and TNT in given molar ratios were first mixed in ethanol and stirred at RT for two hours. Then, an ethanol solution containing BTME, water, F127, and HCl was added under stirring and the final molar ratio of the reactants was 1 BTME/8.8  10
4 BTPN/4.4  10
4 TNT/271.6 EtOH/9.1 H2O/6.5  10
3 HCl/0.019 F127. After refluxing at 60 °C for 1.5 h, the resulting sol was diluted with ethanol two times and infiltrated into PS colloidal crystal voids until they became transparent. The obtained film was aged in oven (60 °C) for eight days to increase the stability of the formed porous framework. Finally, the used templates (PS spheres, F127, and TNT) were extracted by toluene and ethanol. Since organic dye molecules were embedded in silica skeleton and used as reporters for sensing application, a simple calcination approach is not appropriate for the removal of templates in our case. Figure 1A displays the SEM image of the top view of the formed PS crystal array on glass substrate, showing a close-packed face centered cubic (FCC) arrangement over a range of several micrometers. The inverse replication of this ordered structure led to ca. 5 μm thickness with a periodic macroporous structure throughout the entire sample and interconnecting windows of about 145 nm (Figure 1B). The macropore size is about 392 nm with a wall thickness of 116 nm. TEM images (Figure 1C,D) reveal that the disordered wormlike mesopore network extended throughout the whole wall skeletons of the hybrid macroporous silica. Consistent with this result, the XRD pattern displayed a broad diffraction peak at 0.62° typical for wormlike mesostructure (Figure 2A). It is believed that, besides the formed windows arising from the contact areas between template spheres, these mesochannels connecting adjacent macropores through the silica walls could further facilitate rapid diffusion of molecules to a large surface area with minimal pressure buildup. It should be noted that, due to the confinement of colloidal spheres, the phase behavior of F127 inside the colloidal crystal is different from its behavior in a bulky material (ordered cubic mesopore), affording different mesopore structure. Nitrogen adsorption
desorption experiments further provide the pore characteristics of the hierarchically structured material. As shown in Figure 2B, two hysteresis loops clearly appeared. The first loop at 0.4
0.9, which corresponded to the capillary condensation within mesopores, was generated by the polymer surfactant F127, and the second loop at 0.9
1.0 may be formed by the rough structure on the macropore wall surface. The BET surface area, the pore volume, and the pore size by adsorption branch are calculated to be 418 m2/g, 0.55 cm3/g, and 10.9 nm, respectively. Supporting Information 8453

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Figure 1. SEM images of (A) PS colloidal crystal template and (B) the hierarchically imprinted hybrid silica film after the extraction of all used templates; TEM images showing (C) overview of hierarchically porous silica film and (D) the detailed structure of the macroporous walls of the film.

Figure S2 shows the micropore size distribution curve which is derived from HK method with maximum pore size around 0.62 nm, and the micropore surface area in the prepared porous material is 74.6 m2/g. The correct incorporation of the naphthalene monomer into synthesized silica films and the formation of TNT-imprinted microcavities were proven by UV/vis and fluorescence measurements. Figure S1A in Supporting Information shows the UV/vis spectrum of the silica films. Clearly, the characteristic absorption bands of naphthalene units are observable positions (300, 312, 328 nm) comparable to those of the naphthalene monomer in CH2Cl2. In agreement with this result, these films also exhibit the same photoluminescence behavior as that of the used naphthalene dye in CH2Cl2 solvent (Supporting Information Figure S1B). More importantly, we found that, due to the existence of a strong charge-transfer interaction between oxygen-donated naphthalene with electron-deficient molecules,23 a possible sandwich or paired arrangement of the naphthalene units with TNT molecule formed in the walls of mesopores (Scheme 1), evidenced by the occurrence of the charge-transfer complex absorption at 525 nm (Supporting Information Figure S1C). After the removal of TNT molecules using toluene, this characteristic band in as-synthesized hybrid silica films disappeared. Upon exposure to TNT atmosphere, the observed band again occurred. This process is completely reversible (inset in Supporting Information Figure S1C). Although it is impossible to observe microporosity using microscopy, the performed spectroscopic studies indirectly revealed the formation of the imprinted micropores in the prepared films. This result together with the meso
macroporous structure mentioned above clearly indicates that, on all length scales, the controlled fabrication of a hybrid film with interwoven and hierarchically organized micro-meso
macroporous structure is achieved in this work based on a triple template approach. In addition, we also found that, different from traditional molecular-imprinted bulky materials, the incorporation of hierarchical meso
macroporous structure in materials

made the extraction and rebinding of template molecules from or to the formed cavities much more easy and fast, due to the greatly improved mass transport as well as the location of imprinted sites only within several nanometers. The combination of the unique properties of macropore, mesopore, and imprinted micropore should provide a new kind of platform for advanced applications, accompanied with a series of unprecedented superiority. The fluorescence response of the hierarchically imprinted hybrid films to the vapors of nitro-containing aromatics (TNT or DNT) was ascertained by inserting the prepared films into glass vials (10 mL) at room temperature containing 1.5 g solid analytes and cotton gauze, which prevents the direct contact of silica film from analyte and helps to maintain a constant saturated vapor pressure.24 After exposing the silica film deposited on glass slides for a given period of time, the fluorescence spectra were measured immediately at excitation wavelength of 235 nm. The analyte’s equilibrium saturated vapor pressures are assumed to be similar to documented values (TNT 10 ppb; DNT 180 ppb). Figure 3A shows the fluorescence intensity evolution of naphthalene-doped trimodal porous silica film upon exposure to saturated TNT vapor (10 ppb) over elapsing time. Over 50% fluorescence quenching happened in the first 30 s, and after 120 s, 85% fluorescence had been quenched. It is interesting to note that the fluorescence response to TNT is strongly dependent on the type of the structure of silica film, and with increased porous hierarchy, the quenching efficiency is considerably improved, as expected. Silica film with bimodal meso- and imprinted microporous structure exhibits very efficient fluorescence response capability relative to imprinted monomodal film (Figure 3B). For example, bimodal film shows about 55% quenching in 60 s, while tightly cross-linked imprinted film shows only 2% in comparable time. With the introduction of interconnected macropores into the abovementioned bimodal film, the quenching efficiency reached 73% in 60 s. These results indicate that an appropriate combination of macropore and mesopore to achieve high molecule permeability and high density of interaction sites (sensing elements) is one of the 8454

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Figure 2. (A) XRD pattern of the hierarchically imprinted silica film; (B) N2 adsorption
desorption isotherms of the synthesized trimodal porous materials. Inset is pore size distribution (adsorption branch).

key points for obtaining high sensitivity. The trimodal film produced here already exhibits a much higher sensitivity to TNT vapor than those of the conventional conjugated polymers, e.g., poly(phenylenevinylene) derivatives.25 An extensive literature study already exists concerning the development of chemical sensors for explosive detection.26
35 However, how to selectively detect a given explosive has been a challenge in this field. In this work, the molecular imprinting technique was employed to create micropores, and the resulting nanocavities endow the prepared trimodal pore system with molecular recognition properties. Figure 4 displays the comparison of sensing behavior of TNT-imprinted pore systems and nonimprinted control samples toward TNT and its structural analogue DNT. Clearly, the molecular imprinting process affords materials having a preferred affinity for TNT and leads to a remarkable increase of quenching efficiency of TNT over DNT (by 85% and 42% for bimodal and trimodal pore systems, respectively), although the latter provides about 20 times higher vapor concentration. Molecular imprinting is a well-established technique to mimic antibody functions. Extraction of the imprint molecules leaves a binding pocket with preorganized interaction sites and compatible size/shape to the imprint. The structure of functional monomer and cross-linker as well as the polymerization conditions directly determines the features of the resulting pockets and thus their recognition properties. In fact, when urea-functionalized dye was used as a functional monomer instead of the BTPN and BTME described above, the imprinted porous film with considerably improved

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Figure 3. (A) Fluorescence quenching of the hierarchically imprinted silica film upon exposure to TNT vapor (10 ppb). (B) Time-dependent fluorescence quenching of (a) imprinted trimodal porous film, (b) imprinted bimodal porous film, and (c) tightly cross-linked imprinted film.

selectivity was achieved as shown in Figure 5. These results are consistent with conventional imprinted technology, clearly indicating that the choice of functional monomer and cross-linker as well as the polymerization conditions directly determines the performance of the imprinted materials. It can be anticipated that after the optimization of preparation conditions the imprinted films with much better selectivity should be achievable. In this work, different PS colloidal crystals were used as templates for the fabrication of TNT-imprinted trimodal porous silica films. Probably due to the presence of large windows between macropores and, hence, a facile mass transport, the effect of the thickness of the hierarchically structured films on their sensing properties is not obvious (Supporting Information Figure S3A) and is not the key point in the fabricating procedure of the sensor. In addition, as a practically applicable chemosensor, easy regeneration is highly desirable after use. In this respect, the prepared hybrid films were also examined. Supporting Information Figure S3B shows representatively the recovery property of these sensory materials. When the quenched film is immersed in toluene for four hours at RT or washed several times, the fluorescence of the silica film can be completely recovered. After several recycles, the fluorescence of the silica film remains nearly unchanged. The lifetime of these sensors is greatly elongated by this method, and can be used repeatedly. In addition, we also found that the optical properties of the synthesized mesoporous materials remain unchanged even after storage in air for two months. It is worth mentioning that, benefiting from this hierarchically imprinted structure, the detection limit of the sensory films can be extended from ppb to ppt range (Supporting Information Figure S4). 8455

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Figure 4. Comparison of the fluorescence quenching efficiency (%) of the imprinted silica films with nonimprinted counterparts: (A,B) imprinted film with bimodal structure; (A0 ,B0 ) imprinted film with trimodal structure.

Figure 5. Comparison of the fluorescence quenching efficiency (%) of the imprinted silica films with nonimprinted counterparts: (A,B) imprinted film with trimodal structure; (C) schematic illustration of TNT imprinting based on urea-functionalized dye.

’ CONCLUSIONS In conclusion, based on the combination of colloidal and mesophase templating as well as molecular imprinting, a general and effective approach for the preparation of hierarchically structured trimodal porous silica films was developed. With this new methodology, controlled formation of well-defined pore structures not only on macro- and mesoscale but also on microscale can be achieved, affording a new class of ordered interconnected porous materials with molecular recognition

capability. As a demonstration, TNT was chosen as a template molecule and hierarchically imprinted porous films were successfully fabricated, which show excellent sensing properties in terms of sensitivity, selectivity, stability, and regeneracy. In fact, compared to the previous multimodal systems, the pore system reported here combines the multiple benefits arising from all length scales of pore size and simultaneously possesses a series of distinct properties such as high pore volume, large surface area, molecular selectivity, and rapid mass transport. Therefore, we 8456

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Langmuir believe that our described method and the resulting pore systems should hold great promise for various applications not only in chemical sensors, but also in catalysis, separation, adsorption, and electrode materials.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed UV
vis spectra, fluorescence spectra, and other extensive figures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: (þ86) 10-6279-2905. Fax: (þ86) 10-6279-2905.

’ ACKNOWLEDGMENT The authors thank the financial support from the National Science Foundation of China (No. 50873051 and 20533050), MOST (2007AA03Z307), and Transregional Project (TRR61).

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dx.doi.org/10.1021/la201055b |Langmuir 2011, 27, 8451–8457