LETTER pubs.acs.org/Langmuir
Bolaform Supramolecular Amphiphiles as a Novel Concept for the Buildup of Surface-Imprinted Films Jiawei Zhang,† Yiliu Liu,† Guanglu Wu,† Monika Sch€onhoff,‡ and Xi Zhang*,† †
Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China ‡ Institute of Physical Chemistry, University of M€unster, 48149 M€unster, Germany
bS Supporting Information ABSTRACT: Stable multilayer films were fabricated on the basis of the alternating layer-by-layer assembly of a two-component bolaform supramolecular amphiphile and diazoresins, followed by photochemical cross-linking of the structure. UVvisible spectroscopy and atomic force microscopy revealed a uniform deposition process. Moreover, one component of the supramolecular amphiphile can be removed from the multilayer films after cross-linking between the second component and the diazoresin. The release and uptake of the imprinted supramolecular amphiphile component are shown to be reversible. Furthermore, uptake experiments of different molecules show the selectivity of the imprinted sites for the template molecule. Thus, surface-imprinted films can be formed by employing dissociable two-component supramolecular amphiphiles. This research reveals that supramolecular amphiphiles can be used as a novel concept for the construction of multilayer films, and it also provides a new method of generating surface-imprinted multilayers.
’ INTRODUCTION The layer-by-layer (LbL) technique has proven to be a powerful method of constructing multilayer thin films with predesigned composition and versatile functions.1 Although it did not become popular when it was first reported by Iler,2 it has attracted more and more attention because of its simplicity, universality, and precise control of the nanostructures after being rediscovered by Decher and co-workers.3 Besides conventional LbL assembly methods based on electrostatic interactions,4,5 hydrogen bonds,6,7 hostguest interactions,8,9 or charge-transfer interactions,10,11 unconventional LbL methods1217 have been developed, which include self-assembly in solution as well as LbL assembly at solidliquid interfaces. Many building blocks, such as water-insoluble12 or single-charge molecules,16 which cannot be assembled by the conventional method of LbL assembly, can be assembled by unconventional LbL assembly. As a result, new multilayer nanoarchitectures are formed and new functions are endowed. Among various functions, molecular imprinting in LbL films18 has attracted considerable interest because of its potential applications in biorecognition,19 drug delivery,20 and so forth. Supramolecular amphiphiles refer to amphiphiles formed on the basis of noncovalent interactions.21 Besides the simple preparation procedure, the molecular architectures formed by supramolecular amphiphiles are tunable as opposed to conventional amphiphiles; therefore, supramolecular amphiphiles have proved to be promising candidates for building stimuli-responsive materials.2224 In our previous work, we discussed the generation of imprinted sites for porphyrin (Por) in a photo-cross-linked r 2011 American Chemical Society
polyelectrolyte multilayer film,25 and the interaction between Por and the imprinted sites was due to the electrostatic interaction of the template molecules and polymeric constituents. In this letter, bolaform supramolecular amphiphiles have been applied as building blocks to prepare multilayer films by alternating the LbL assembly between supramolecular amphiphiles and a photosensitive agent, diazoresin (DAR).26 Under UV irradiation, one component of the supramolecular amphiphiles can react with DAR to form a stable cross-linked film, and the other component of the supramolecular amphiphiles can be removed, generating a surfaceimprinted film with special binding sites for one component. This research is of great significance because it demonstrates that supramolecular amphiphiles can be new building blocks for LbL assembly. Moreover, it provides a new way to fabricate surfaceimprinted films.
’ EXPERIMENTAL SECTION Materials. 8-Hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) and 3-mercaptopropionic acid were purchased from SigmaAldrich. 5,10,15,20-Tetrakis (4-trimethylammoniophenyl) porphyrin tetra(p-toluenesulfonate) (Por) was purchased from Acros Chemical Company. DAR with a number-average molecular weight (Mn) of about 2640 g mol1 was kindly provided by Prof. Yuping Dong (College of Material Science & Engineering, Beijing Institute of Technology). Received: July 1, 2011 Revised: August 3, 2011 Published: August 04, 2011 10370
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Scheme 1. Synthesis Route to diMV
Figure 1. (a) Schematic illustration of the formation of the supramolecular amphiphile. (b) Aqueous solutions of HPTS, the HPTSdiMV complex, and diMV. (c) UVvis absorption spectra and (d) fluorescence emission spectra of HPTS and HPTSdiMV solutions at concentrations of HPTS and diMV of 50 and 25 μM (pH 10), respectively. Quartz slides were purchased from Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences. Instruments. UVvis spectra were recorded with a Hitachi UVvis 3010 spectrophotometer. Steady-state fluorescence spectra were obtained using a Hitachi F-7000 apparatus equipped with a Xe 900 lamp with an excitation wavelength of 340 nm. The slit widths of excitation and emission were 5 and 2.5 nm, respectively. 1H NMR spectra were obtained on a Jeol JNM-ECA300 spectrometer operating at 300 MHz for protons. The isothermal titration calorimetry (ITC) experiment was carried out using a Microcal VP-ITC apparatus. The measurements were performed in distilled water (pH 10) at 298.15 K. HPTS (0.2 mM) was in the sample cell, and diMV (2.0 mM) was in the injection syringe. Cyclic voltammograms were obtained with an Autolab PGSTAT12 in a conventional three-electrode electrochemical cell. Atomic force microscopy (AFM) images were recorded on a Multimode Nanoscope IV (Veeco). Imaging was performed in tapping mode in air using silicon cantilevers (200—300 kHz).
Synthesis of 10 ,100 -(Butane-1,4-diyl)-bis-(1-methyl-4, 40 -bipyridine-1,10 -diium) Dibromide Diiodide (diMV). The synth-
esis route to obtaining diMV is shown in Scheme 1. 1,4-Dibromobutane and excess 4,40 -bipyridine were stirred in CH3CN at 70 °C for 24 h. The
precipitate that was produced was filtered and washed with CH3CN and diethyl ether to obtain 1 (yield 90%). Compound 1 and iodomethane were stirred in DMF at 90 °C, and the precipitate that was produced after 6 h was filtered and washed with CH3CN and then recrystallized in water to obtain the final product in 85% yield. 1H NMR (300 MHz, d6-DMSO, 25 °C, TMS): δ 9.40 (4H, d), 9.28 (4H, d), 8.82 (4H, d), 8.76 (4H, d), 4.75 (4H, t), 4.41 (6H, s), 2.06 (4H, m). Substrate Preparation. Quartz slides were treated with hot piranha solution (7:3 v/v 98% H2SO4/30% H2O2) for 1 h and then rinsed carefully with distilled water and dried with nitrogen. Caution! Piranha solution is extremely corrosive, and appropriate safety precautions should be utilized. For the electrochemical measurement, gold electrodes (model CHI 101, 2 mm diameter) were used as the working electrode. The gold electrodes were first mechanically polished with 1, 0.3, and 0.05 μM γAl2O3, washed with distilled water, and then electrochemically scanned in a 2 mM K3[Fe(CN)6] and 0.1 M KCl solution by potential scanning between 0.05 and 0.55 V at a scanning rate of 0.1 V/s until a reproducible cyclic voltammogram was obtained. Subsequently, the electrodes were rinsed with distilled water and dried with nitrogen, followed by immersion into an ethanol solution of 3-mercaptopropionic 10371
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Figure 2. ITC data for diMV binding with HPTS.
Figure 3. (a) Schematic representation of the LbL assembly of DAR and HPTSdiMV. (b) UVvis spectra monitoring the LbL assembly of a (DAR/ HPTSdiMV)5DAR multilayer film (i.e., five bilayers of DAR and HPTSdiMV plus an extra DAR layer). (Inset) Absorbance at 380 nm vs the number of layers deposited. (c) UVvis spectra of a (DAR/HPTSdiMV)5DAR multilayer film before and after UV irradiation. (d) AFM image of a (DAR/ HPTSdiMV)5DAR multilayer film after UV irradiation (3 3 μm2). acid (5 mM) for 12 h to generate a negative-charge-bearing electrode surface. After that, the gold electrodes were rinsed thoroughly with ethanol and dried with nitrogen before further characterization. SupramolecularAmphiphile Formation. The bolaform supramolecular amphiphile was prepared by mixing equivalent volumes of aqueous solutions of HPTS (100 μM, pH 10) and diMV (50 μM, pH 10), followed by ultrasound treatment (20 min), and the molecular complex that was formed has the stoichiometry (diMV)1(HPTS)2.
Multilayer Formation and Photoreaction. The quartz slides prepared as described above were first immersed in a DAR aqueous solution (0.5 g L1) for 10 min, followed by rinsing with distilled water and drying with nitrogen. The slides were then immersed in an HPTS diMV aqueous solution (HPTS = 50 μM, pH 10) for 10 min, followed by the same rinsing and drying cycle. By repeating both immersion processes alternately, films of five bilayers and a DAR layer as the outmost layer were prepared. The multilayer films were irradiated to form cross-linked films using a 100 W mercury lamp (300 s irradiation), 10372
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Figure 4. (a) Cyclic voltammograms of the gold electrodes modified by (DAR/HPTSdiMV)5DAR multilayer films in the presence of 2 mM Ru(NH3)63+ and 0.1 M KCl (1) after UV irradiation, (2) after the removal of diMV from the film, and (3) after immersion into a 1 g/L diMV solution for 30 min. (Inset) Current variation, ΔI, during the cyclic loading and unloading of diMV to and from a (DAR/HPTSdiMV)5DAR-modified electrode. (b) Cyclic voltmmograms of a reference film of (DAR/ HPTS)5DAR (1) after UV irradiation and (2) after treatment with a mixed solution of H2O/dimethylformamide/ZnCl2. and the distance between the lamp and the sample was about 10 cm. LbL assembly on gold electrodes was performed in the same way, except that 0.1 M KCl was employed in both aqueous solutions of DAR and HPTSdiMV. For control experiments, the multilayer films were formed on gold electrodes in a similar way, except that the aqueous solution contained only HPTS (HPTS = 50 μM, pH 10) instead of HPTSdiMV. Electrochemical Measurements. Electrochemical measurements were performed in a conventional three-electrode glass electrochemical cell at ambient temperature. The working electrode was a gold electrode modified with (DAR/HPTSdiMV)5DAR multilayer films, the auxiliary electrode was a platinum electrode, and the reference electrode was an Ag/AgCl (3 M KCl) electrode. A solution of 2 mM K3[Fe(CN)6] and 0.1 M KCl was used as the negative indicator, and a solution of 2 mM [Ru(NH3)6]Cl3 and 0.1 M KCl was used as the positive indicator. Prior to the measurements, the electrolyte solutions were purged for 30 min with nitrogen, and a constant flow of nitrogen was applied above the liquid phase during the electrochemical measurements. Removal and Rebinding of diMV. diMV was released from crosslinked imprinted films by ultrasonic agitation of the substrate for 20 min in a mixed solvent of H2O/dimethylformamide/ZnCl2 (3:5:2 w/w/w). The time-dependent reloading of diMV to the above substrate was carried out by immersing the substrate in a diMV solution with various concentrations from 0.01 to 1 g L1 (0.1 M KCl), and then it was electrochemically scanned in a 2 mM [Ru(NH3)6]Cl3 and 0.1 M KCl solution by potential scanning between 0.4 and 0.15 V at a scanning rate of 0.1 V/s.
’ RESULTS AND DISCUSSION Formation of a Supramolecular Amphiphile Based on Electrostatic and Charge-Transfer Interactions. In this study,
diMV containing two viologen moieties is used as the electron
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acceptor and HPTS is used as the electron donor (Figure 1a). As shown in Figure 1b, a transparent yellow solution can be readily prepared by the direct mixing of a transparent green solution of HPTS and a colorless solution of diMV in water, indicating the formation of a HPTSdiMV complex. Further evidence for the formation of supramolecular amphiphiles was obtained from UVvis absorption and fluorescence emission spectroscopy. The UVvis spectrum of the complex in Figure 1c exhibits a red shift of the band at about 450 nm compared with the spectrum of HPTS. In addition, fluorescence quenching is observed after the complexation, as shown in Figure 1d. These data suggested that supramolecular amphiphiles are indeed formed on the basis of combined electrostatic and charge-transfer interactions. We wondered if diMV and HPTS can form a 1:2 complex in aqueous solution with electrostatic and charge-transfer interactions as the driving forces.27 We have employed ITC to answer this question. As shown in Figure 2, the binding modes of diMV and HPTS are confirmed by ITC experiments, and diMV is found to bind with HPTS in a 1:2 molar ratio. The binding constant is 3.4 106 L mol1. The stronger binding facilitates the formation of the supramolecular amphiphile. Supramolecular Amphiphile as Building Block for LbL Assembly. The process of LbL assembly of DAR and HPTSdiMV is schematically illustrated in Figure 3a. The HPTSdiMV layer may, however, be less ordered than depicted in the scheme because of the interaction with the disordered polymer chains of DAR. UVvis absorption spectroscopy has been employed to monitor the assembly process of the multilayer films. The absorbance at 380 nm (A380) is attributed to the ππ* transition of the diazonium groups of DAR (Figure 3b).28 A linear increase in A380 is observed with the number of layers, which indicates a regular deposition process (Figure 3b). The stepwise assembly of the multilayer film was also confirmed by FT-IR spectra (Figure S1). As you will note from Supporting Information Figure S2, without diMV, HPTS cannot be assembled well with DAR in a layer-by-layer fashion. After deposition, the LbL films were stabilized by UV-irradiation-induced cross-linking. As shown in Figure 3c, the absorbance band at 380 nm, which corresponds to diazonium groups, disappears after UV irradiation, and a new band due to the ester linkage appears at around 290 nm, which demonstrates that the photoreaction occurs between the diazonium and the sulfonate groups or hydroxyl groups (Scheme S1).29,30 The surface morphology of the multilayer films was also investigated by atomic force microscopy (AFM); see Figure 3d. It can be concluded that the surface of the film is rather smooth, with a roughness on the order of 10 nm. Buildup of Surface-Imprinted Films. Cyclic voltammetry was employed to monitor the release and reloading of diMV. For release, the cross-linked film on a gold electrode was treated with a mixed solvent of H2O/dimethylformamide/ZnCl2 (3:5:2 w/ w/w),25 which is an effective solvent for destroying electrostatic interactions. Before treatment by the mixed solvent, no redox response of the films was observed for positively charged Ru(NH3)63+ (Figure 4a, curve 1), indicating that the gold electrode is covered with compact multilayer films. After film treatment with the mixed solvent, a significant redox signal is observed (Figure 4a, curve 2), implying that diMV is removed from the cross-linked LbL film and Ru(NH3)63+ can access the surface of the gold electrode. diMV can be reloaded into the LbL film by 30 min of immersion in a 1 g/L solution of diMV, as demonstrated by the almost vanishing redox signal (Figure 4a, curve 3). These loading and unloading processes can be repeated for several 10373
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molar fraction of template molecules was lower than 20%. This limit was due to either the insolubility of the template molecules or the low grafting ratio of template molecules to the polymeric constituents. By taking advantage of supramolecular amphiphiles, surface-imprinted films with a much higher loading capacity of template molecules can be achieved, thus providing a new approach to generating surface-imprinted films with a higher density of imprinted sites.
’ CONCLUSIONS A bolaform supramolecular amphiphile has been successfully employed as a building block to construct surface-imprinted LbL films with diazoresins. The as-prepared multilayer films have exhibited good selectivity with respect to the imprinted molecule because of electrostatic and charge-transfer interactions. In the future, other interactions by which supramolecular amphiphiles might be assembled, such as hydrogen bonding, hostguest recognition, and ππ interactions, could be applied to form surface-imprinted films, which would expand the scope of the building blocks and the application of surface molecularly imprinted multilayer films. Figure 5. (a) Time-dependent binding of diMV onto the imprinted (DAR/HPTSdiMV)5DAR film after diMV removal by the mixed solvent of H2O/dimethylformamide/ZnCl2 at different concentration of diMV. (b) Time-dependent binding of Por (1.2 mM), MV (1.2 mM), and diMV (1.2 mM) to the imprinted (DAR/HPTSdiMV)5DAR film. ΔI0 and ΔI are current variations in cyclic voltammograms in the absence and presence of the guest molecules, respectively.
cycles: the inset in Figure 4a shows the current variation ΔI, defined as the maximum peak current minus the minimum peak current in the voltage increase and decrease curves, respectively, in a series of loading and release cycles. For curves 1 and 3, where no maximum or minimum occurs, the current values were evaluated at the voltage value where the maximum and minimum, respectively, occur in curve 2. The reversible variation of ΔI with subsequent loading and release cycles suggests that the multilayer films may be used as surface-imprinted films for reversible uptake and release. Moreover, for reference film (DAR/ HPTS)5DAR without imprinted sites, no significant change in the redox response was observed, as expected (Figure 4b). To understand the kinetics of the adsorption of diMV into the surface-imprinted films, the time-dependent reloading of diMV into the imprinted sites was monitored by cyclic voltammetry at different bulk concentrations of diMV. ΔI0 and ΔI are the current variations in the absence and presence of diMV, respectively. As shown in Figure 5a, the loading of diMV is concentrationdependent, and the loading process reaches equilibrium at about 15 min. In contrast, if the unloaded multilayer films were immersed in the aqueous solution of MV or Por, then only a minor change in the redox signal was observed in comparison with the reloading of diMV; see Figure 5b. This indicates that the binding between diMV and the imprinted sites is specific. Thus, systems imprinted on the basis of supramolecular amphiphiles show very good properties, in particular, those concerning their reversibility and selectivity. There is, however, even a further great advantage of this new concept of imprinting over imprinting strategies demonstrated earlier. In earlier systems, on the basis of the conjugation of template molecules and polymer chains in solution, such as Por or Theo systems,20,25 the
’ ASSOCIATED CONTENT
bS
Supporting Information. FT-IR spectra of DAR, HPTS, and diMV. The photoreaction between DAR and HPTS. UV vis spectral monitoring of the LbL assembly of a (DAR/ HPTS)DAR multilayer film and (DAR/DHP-diMV)DAR multilayer film. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the Natural Science Foundation of China (20974059 and 50973051), the National Basic Research Program of China (2007CB808000), DFG-NSFC Transregio SFB (TRR61), the Science Foundation of China Postdoctor (20110490369), and the Tsinghua University Initiative Scientific Research Program (2009THZ02230) for financial support. We acknowledge the help of Mr. Jisheng Yu at Tsinghua University with ITC experiments and Mr. Peng Han at Tsinghua University with IR experiments. We thank Prof. Liyan Wang of Jilin University for useful suggestions. ’ REFERENCES (1) Decher, G.; Schlenoff, J. B. Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, Germany, 2002. (2) Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569. (3) Decher, G.; Hong, J. D. Makromol. Chem. Macromol. Symp. 1991, 46, 321. (4) Zhang, X.; Shen, J. C. Adv. Mater. 1999, 11, 1139. (5) Schmitt, J.; Gr€unewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; L€osche, M. Macromolecules 1993, 26, 7058. (6) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (7) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. 10374
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(8) Crespo-Biel, O.; Dordi, B.; Reinhoudt, D. N.; Huskens, J. J. Am. Chem. Soc. 2005, 127, 7594. (9) Suzuki, I.; Egawa, Y.; Mizukawa, Y.; Hoshi, T.; Anzai, J. Chem. Commun. 2002, 164. (10) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1998, 14, 2768. (11) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (12) Ikeda, A.; Hatano, T.; Shinkai, S.; Akiyama, T.; Yamada, S. J. Am. Chem. Soc. 2001, 123, 4855. (13) Artyukhin, A. B.; Bakajin, O.; Stroeve, P.; Noy, A. Langmuir 2004, 20, 1442. (14) Sch€utte, M.; Kurth, D. G.; Linford, M. R.; C€olfen, H.; M€ohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2891. (15) Fabianowski, W.; Roszko, M.; Brodzi~nska, W. Thin Solid Films 1998, 329, 743. (16) Chen, H.; Zeng, G. H.; Wang, Z. Q.; Zhang, X.; Peng, M. L.; Wu, L. Z.; Tung, C. H. Chem. Mater. 2005, 17, 6679. (17) Ma, N.; Zhang, H. Y.; Song, B.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2005, 17, 5065. (18) Gauczinski, J.; Liu, Z.; Zhang, X.; Sch€onhoff, M. Langmuir 2010, 26, 10122. (19) Niu, J.; Liu, Z. H.; Fu, L.; Shi, F.; Ma, H. W.; Ozaki, Y.; Zhang, X. Langmuir 2008, 24, 11988. (20) Guan, G. J.; Liu, R. Y.; Wu, M. H.; Li, Z.; Liu, B. H.; Wang, Z. Y.; Gao, D. M.; Zhang, Z. P. Analyst 2009, 134, 1880. (21) Zhang, X.; Wang, C. Chem. Soc. Rev. 2011, 40, 94. (22) Wang, C.; Chen, Q. S.; Xu, H. P.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2010, 22, 2553. (23) Wang, C.; Guo, Y. S.; Wang, Y. P.; Xu, H. P.; Zhang, X. Chem. Commun. 2009, 5380. (24) Jeon, Y. J.; Bharadwaj, P. K.; Choi, S. W.; Lee, J. W.; Kim, K. Angew. Chem., Int. Ed. 2002, 41, 4474. (25) Shi, F.; Liu, Z.; Wu, G. L.; Zhang, M.; Chen, H.; Wang, Z. Q.; Zhang, X.; Willner, I. Adv. Funct. Mater. 2007, 17, 1821. (26) Shimazu, K. Photogr. Sci. Eng. 1973, 17, 33. (27) Wang, C.; Guo, Y. S.; Wang, Y. P.; Xu, H. P.; Wang, R. J.; Zhang, X. Angew. Chem., Int. Ed. 2009, 48, 8962. (28) Sun, J. Q.; Wu, T.; Liu, F.; Wang, Z. Q.; Zhang, X.; Shen, J. C. Langmuir 2000, 16, 4620. (29) Sun, J. Q.; Wu, T.; Sun, Y. P.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Cao, W. X. Chem. Commun. 1998, 1853. (30) Cao, T. B.; Wei, F.; Yang, Y. L.; Huang, L.; Zhao, X. S.; Cao, W. X. Langmuir 2002, 18, 5186.
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