Stable Entrapment of Small Molecules Bearing Sulfonate Groups in

In Final Form: April 6, 2001 ... When these film assemblies were covered with additional layers of ... of ISA technique and the in situ photoreaction ...
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Langmuir 2001, 17, 4035-4041

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Stable Entrapment of Small Molecules Bearing Sulfonate Groups in Multilayer Assemblies Junqi Sun, Tao Wu, Bo Zou, Xi Zhang,* and Jiacong Shen Key Lab of Supramolecular Structure and Spectroscopy of Ministry of Education, Department of Chemistry, Jilin University, Changchun 130023, People’s Republic of China Received January 5, 2001. In Final Form: April 6, 2001 A way to realize the stable entrapment of small molecules bearing sulfonate groups in the multilayer assemblies was reported. First, the sulfonate-containing small molecules were alternately assembled with photoreactive polycation diazo resins (DAR) to form electrostatically held multilayer films. Then these films were exposed under UV irradiation to form covalently attached ones due to the in situ photoreaction between diazonium and sulfonate goups. In this way, the stability of the small molecules in the multilayer was improved to some extent. When these film assemblies were covered with additional layers of photoactive diazo resins/poly(4-styrene sulfonate) films, the stability of the films could be improved further. By making full use of the incompleteness of the reaction between DAR and small molecules, good permeability of the film could be realized, which guarantees these kinds of films in finding potential application in chemically modified electrodes in which films with both stability and permeability are required. These kinds of films were characterized in detail by UV-vis spectroscopy, cyclic voltammetry, X-ray diffraction measurements, and solvent etching experiments.

Introduction To find a way to realize the stable assembly of small dye molecules in layered structure has always been a hot point in supramolecular chemistry due to its potential application in various fields.1 Since the development of the ionic self-assembly (ISA) technique, first introduced by Decher in 1991,2 it has become an effective way to realize the three-dimensionally layered assembly of charged small molecules. Small organic molecules, which contain usually no less than two charged groups, are eligible to employ the ISA technique to realize their layered assemblies. They can alternately assemble with polyions, nanoparticles, or other types of small molecules. We reported the incorporation of oligo-charged molecules, including a homodicationic bolaform molecule of pyC6BPC6py3 and two different homotetraanionic diskshaped molecules of porphyrins and phthalocyanine derivatives 4 into multilayer assemblies, several years ago. Kunitake et al.,5 Li et al.,6 Rubner et al.,7 and Mo¨hwald et al. 8 reported also the incorporation of different dyes into layered structures by using this method. Although the ISA technique is an effective way to realize the assembly of dyes, it has been discovered that thus assembled dyes are not very satisfactory in the sense of stability. Mo¨hwald et al. 8 studied in detail the effect of (1) (a) Laschewsky, A.; Wischerhoff, E.; Kauranen, M.; Persoons, A. Macromolecules 1997, 30, 8304. (b) Lui, M.; Kira, A.; Nakahara, H. J. Phys. Chem. 1996, 100, 20138. (c) Araki, K.: Wagner, M. J.; Wrighton, M. S. Langmuir 1996, 12, 5393. (d) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (2) (a) Decher, G.; Hong, J.-D. Makromol. Chem. Macromol. Symp. 1991, 46, 321. (b) Decher, G. Science 1997, 277, 1232. (3) Zhang, X.; Gao, M. L.; Kong, X. X.; Sun, Y. P.; Shen, J. C. J. Chem. Soc., Chem. Commun. 1994, 1055. (4) Sun, Y. P.; Zhang, X.; Sun, C. Q.; Wang, B.; Shen, J. C. Chem. Commun. 1996, 2379. (5) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (6) Lu¨tt, M.; Fitzsimmons, M. B.; Li, D. Q. J. Phys. Chem. B 1998, 102, 400. (7) Yoo, D.; Lee, J.-K.; Rubner, M. F. Mater. Res. Soc. Symp. Proc. 1996, 413, 395. (8) Linford, M. R.; Auch, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 178.

salt concentration on the dye extraction during the organic dyes and polyions assembly process. They ascribed the extraction of dye, when reimmersing the film into the counter-polyion solution, to equilibrium between the dyes adsorbed on the surface and existing in solution. Part of the dyes could be etched out because the binding sites are so few that the interaction of the dyes with polyions in solution is stronger than with those on substrate. To make full use of the excellent properties of dye molecules, stable assemblies of dyes are sometimes required. How to improve the stability of dye molecules in these assemblies appears to be a challenge for chemists. Recently, we reported a new way to fabricate covalently attached multilayer structures by exploiting the simplicity of ISA technique and the in situ photoreaction in the film induced by UV light.9 Diazo resins are a type of diazoniumcontaining polycation which can alternately assemble with sulfonate containing small molecules or polyions. Upon UV irradiation or by heating, the diazonium and sulfonate groups in the neighboring layers can undergo reaction to form sulfonate ester.10 Consequently, the ionic interaction between the layers is converted to covalent bonds. Thus the stability of the film improves greatly after UV irradiation, compared with that before UV irradiation. Herein, as a continuous work, we further developed this method in an attempt to realize the stable entrapment of organic dyes bearing sulfonate groups. Covalently attached multilayer assemblies were fabricated by exposing the assemblies of DAR and a variety of organic dyes with sulfonate groups ranging from two to four per molecule. These assemblies were characterized in detail by means of UV-vis spectroscopy, cyclic voltammogram measurement, and X-ray diffraction. In all cases, more stable dyecontaining multilayer assemblies were achieved compared with those without UV irradiation. A potential application (9) (a) Sun, J. Q.; Wu, T.; Sun, Y. P.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Cao, W. X. Chem. Commun. 1998, 1853. (b) Sun, J. Q.; Wang, Z. Q.; Sun, Y. P.; Zhang, X.; Shen, J. C. Chem. Commun. 1999, 693. (10) (a) Cao, W. X.; Ye, S. J.; Cao, G. S.; Zhao, C. Macromol. Rapid Commun. 1997, 18, 983. (b) Patai, S. The Chemistry of Diazonium and Diazo Groups; John Wiley & Sons: Chichester-New York-BrisbaneToronto, 1978; p 351.

10.1021/la010012d CCC: $20.00 © 2001 American Chemical Society Published on Web 05/26/2001

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Chart 1. Structures of Building Blocks Used for Multilayer Assembly: (a) DAR; (b) tpps4; (c) CuTsPc; (D) SNAN: (E) BST

of these kinds of films in chemically modified electrodes (CMEs) has been anticipated when electroactive small molecules are combined due to the high stability of the covalently attached assemblies and the possibility to make them permeable by etching out the unreacted molecules. Experimental Section Materials. The following chemicals were used as supplied: Poly(diallyldimethylammonium chloride) noted PDDA, poly(sodium 4-styrene sulfonate), noted PSS, 5,10,15,20-tetraphenyl21H,23H-porphine-P,P′,P′′,P′′′-tetrasulfonic acid, tetrasodium, noted tpps4, and sodium (phthalocyaninetetrasulfonato)copper, noted CuTsPc, were all from Aldrich. 2-(4-Sulfo-1-naphthyl azo)1,8-dihydroxy-3,6-naphthalene disulfonic acid (trisodium salt), noted SNAN, was purchased from Beijing Chemical Co. Bathophenanthrolinedisulfonic acid disodium salt trihydrate, noted BST, was from Merck. The number of sulfonate groups per molecule varied from two to four. (3-Mercaptopropyl)trimethoxysilane (MPTS) was from ACROS Organics. 3-Mercapto-1propanesulfonate (MPS) was from Aldrich. All chemicals were used without further purification. Polycationic diazo resins (DAR) was kindly provided by Professor Weixiao Cao (College of Chemistry and Molecular Engineering, Peking University, Beijing).10a The molecular weight (Mn) of the diazo resins was ca. 2640. The structures of polycations and dyes used are shown in Chart 1. Deionized water was used throughout. Instrument and Measurement. UV-vis spectra were obtained using a Shimadzu 3100 UV-vis-NIR spectrophotometer. X-ray diffraction was carried out on a Rigaku X-ray diffractometer (D/max γA, using Cr KR radiation at a wavelength of 2.2896 Å). Cyclic voltammogramms was measured on 2000 electrochemical analysis system, produced by the Analytical Instrument Center, Changchun Institute of Applied Chemistry, Chinese Academy of Science. Process To Produce Covalently Attached Dye-Containing Multilayer Assemblies. A quartz wafer was immersed in a fresh piranha solution (v/v ) 1:3, 30% H2O2/98% H2SO4) and heated until no bubbles were released. After the wafer was rinsed with plentiful water and dried, the substrates were immersed in aqueous cationic solution with 0.9 vol % PDDA for 20 min to obtain a positively charged surface. Alternatively, the substrate could also be immersered in 1 × 10-5 M (3-mercaptopropyl)trimethoxysilane (MPTS) from benzene for 6 h to form a self-

assembled monolayer terminated with -SH functional groups at the exposed surface.11 The substrate was first sonicated in chloroform for a short time to remove the physically adsorbed MPTS, after which the -SH groups were oxidized with 30% H2O2/ HOAc (v/v ) 1:5) in situ into sulfonic acid groups using the method reported in the literature.11 ITO (indium tin oxide) covered glass, used as an electrode, was sonicated in ethanol, acetone, toluene, and 2-propanol for 10 min, respectively, and then washed with copious water and immersed in 0.1 mM 3-mercapto-1-propanesulfonate solution for 2 h to get a sulfonate-containing surface. In general, the fabrication of covalently attached multilayer films involves two steps: ISA assembly and the post-UV irradiation. Take the fabrication of covalent tpps4/DAR on a positively charged substrate for example. The substrate was dipped alternately into aqueous solutions of tpps4 (0.1 mg/mL) for 10 min and DAR (1 mg/ml) for 20 min, with intermediate water rinsing and N2 drying. Multilayer assemblies could be formed by repeating these two steps in a cyclic fashion. Next, the above-fabricated films were exposed under UV irradiation for a given time to ensure the photoreaction being proceeded completely. In this way, the covalently attached multilayer assemblies were obtained. The deposition process of the films was conducted in the dark to avoid decomposition of the DAR.

Results and Discussion Preparation of Covalently Attached Multilayer Assemblies. UV-vis spectroscopy was used to follow the ionic self-assembly process. Figure 1a shows the UV-vis absorption spectra of eight bilayers of DAR/tpps4 assembled on a quartz slide. The absorbance at 380 nm was attributed to the π-π* transition of a diazonium group, while the absorbance at 426 nm corresponds to the Soret band of porphyrin. It was found that both absorbances at 380 and 426 nm increased linearly with the number of layers after the first two layers. This demonstrated a progressive deposition process. The UV-vis spectroscopy also indicates that the Soret band of porphyrin in the DAR/tpps4 multilayer films is red-shifted by ca. 12 nm compared with that of solution, which should result from (11) Liu, J. F.; Zhang, L. G.; Gu, N.; Ren, J. Y.; Wu, Y. P.; Lu, Z. H.; Mao, P. S.; Chen, D. Y. Thin Solid Films 1998, 327-329, 176.

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Figure 1. UV-vis absorption spectra of multilayer assemblies of (a) DAR/tpps4, (b)DAR/CuTsPc, (c) DAR/SNAN, and (d) DAR/ BST. From the lower to upper, the number of bilayers is two, four, six, and eight. Inset shows absorption at (a) 380 and 426 nm, (b) 380 and 232 nm, (c) 380 and 535 nm, and (d) 380 and 284 nm vs the number of bilayers.

the formation of aggregates of chromophores in the films. The assemblies of DAR with other sulfonate-containing dyes were also conducted and the deposition processes monitored with UV-vis spectroscopy are shown in Figure 1 b-d. In all cases, linear increase of the absorbance with the deposited layers was obtained. Owing to the well-known photoreaction of diazonium and sulfonate groups, the above assembled films were irradiated with a 30-W mercury lamp at a distance of 20 cm. Take the eight-bilayer DAR/tpps4 film for example, Figure 2 shows the changes in UV-vis spectra of these films with different lengths of irradiation. From this figure, it can be clearly seen that the absorbance at 380 nm decreases dramatically due to the decomposition of the diazonium groups. Concomitantly, the absorbance at 290 nm increases gradually. As a result, an isosbestic point at 332 nm appears. It was found that within 15 min, the decomposition proceeded completely. At the same time, the absorbance of the Soret band of tpps4 at around 426 nm also decreases gradually without changing the peak position and shape. Actually, the absorbance of a diazonium group peaks around 380 nm, extending from 330 to 480 nm. So the absorption at 426 nm is contributed mainly by the Soret band of tpps4 but overlapping partly with band edge of diazonium groups. Therefore the decrease

Figure 2. UV-vis absorption spectra of eight bilayers of DAR/ tpps4 upon irradiation with UV light for (a) 0, (b) 2, (c) 4, (d) 8, and (e) 15 min.

at around 426 nm is due to the cleavage of diazonium groups. The fact that the peak position and shape of the

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Scheme 1. A Simple Illustration for the Photoreaction of DAR and tpps4 in One Bilayer

Table 1. Stability Test of Small Molecules Bearing Sulfonate Groups in the Multilayer Assemblies of DAR/Dyes fraction of desorbed multilayer assemblies

a

b

c

d

absorbance (nm)

(DAR+tpps4)*8+DAR (DAR+CuTsPc)*8+DAR (DAR+BST)*8+DAR (DAR+SNAN)*8+DAR

84 71 47 70

91 83 72 80

5.6 6.8 8 7

11 12 12 23

426 232 284 535

a Immersion for 5 min (before UV irradiation). b Sonicated for 5 min (before UV irradiation). c Immersion for 5 min (after UV irradiation). d Sonicated for 5 min (after UV irradiation).

Soret band of tpps4 does not change shows that no or little conformational change took place during the reaction, which was consistent with the following result of polarized UV-vis spectroscopy. Considering the obvious change of the UV-vis spectra of the eight-bilayer DAR/tpps4 films before and after UV irradiation, a reaction should take place in situ in the assembly. First, upon UV irradiation, the diazonium group decomposed with releasing N2 and the cationic phenyl group formed within the former DAR chain;10 then an SN1 nucleophilic displacement reaction between sulfonate and cationic phenyl groups occurred to form the final sulfonate esters, as shown in Scheme 1. This reaction was confirmed by IR spectroscopy and reported before.9 The reaction leads to the formation of sulfonate ester bonds between the neighboring layers of DAR and tpps4, which substitutes the original ionic bonds. In this sense, the film stability should improve greatly after UV irradiation. Table 1 represents the stability comparison of films consisting of DAR and sulfonate-containing dyes before and after UV irradiation. These results were obtained by solvent etching the corresponding films in a ternary mixture of H2ODMF-ZnCl2 (3:5:2, w/w/w) and calculating the portion of the desorbed dyes, which could be easily determined by means of UV-vis spectroscopy. The ternary solvent was chosen because of the high solubility of DAR/dyes complex in it. As can be clearly seen from the tabulated values (Table 1), the stability of the films after UV irradiation improved greatly compared with those before UV irradiation. Take the (DAR+tpps4)*8+DAR film for example, the Soret band absorbance at 426 nm was used to calculate the desorbed portion of tpps4. Before UV irradiation, about

84% of the deposited tpps4 could be etched out after 5 min of immersion in ternary solvent. After sonication in ternary solvent again for another 5 min, the desorbed portions reached 91%. But after UV irradiation, the corresponding portions etched out by immersing 5 min and further sonicating another 5 min in ternary solvent became 5.6% and 11%, respectively. Both values from UV-irradiated film are far less than those without UV irradiation, indicating an improved stability of the film after UV irradiation. In literature, it is showed that about 50% of the dye molecule can be desorbed even for a less strong etching solvent.8 Polarized UV-vis spectroscopy was used to study the orientation of the planar porphyrin in the multilayer films before and after photoreaction. In both cases, no detectable changes were found when the mutilayer films were examined at normal incidence using polarized light with its electric vector parallel or perpendicular to the plane of incidence, indicating no in-plane anisotropy existing. When the film was examined at 30° incidence under the similar process, an obvious change of the spectra could be observed both before and after UV irradiation as shown in Figure 3. The orientation angle of the porphyrin plane was calculated by using the method described before in our literature.3 Herein, we presume the optical indices of the films before and after UV irradiation are the same and take the value of 1.5. The angles between the normal line of plane x and the substrate, i.e., θ, in average calculated for before and after UV irradiation films is 53° and 49°. In other words, the oriented angle between the porphyrin plane and horizon of the substrate are 53° and 49°, respectively, which showed slight conformational change of porphyrin in multilayer films before and after UV irradiation. Thickness measurements by X-ray diffraction were also performed on two kinds of 12-bilayer DAR/tpps4 film after UV irradiation. The difference of these two kinds of films is the ionic strength of DAR solution, with one containing 0.5 M NaCl and the other without any inorganic salt added. The concentrations of DAR (1 mg/mL) and tpps4 (0.1 mg/ mL) were kept constant in both cases. As shown in Figure 4, a series of well-resolved Kiessig fringes are found in both kinds of films, which proves the homogeneousness of the films. The existence of only Kiessig fringes but no Bragg peaks was similar to typical polycation (cation)/

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Figure 3. Polarized UV-vis spectra of 12-bilayer DAR/tpps4 films before (TEb and TMb) and after (TEa and TMa) UV irradiation for TE and TM polarizations. Incident angle ) 30°. Figure 5. Cyclic voltamgrams (CVs) of three-bilayer DAR/ tpps4 modified ITO electrode (a) before and (b) after UV irradiation in 0.1 M KCl solution containing 1 mM Fe(CN)63-. Scan rate: 100 mV/s.

Figure 4. X-ray diffraction of 12-bilayer DAR/tpps4 film deposited on quartz slides: (a) DAR/1.0 mg/mL dissolved in 0.5 M NaCl; (b) DAR/1.0 mg/mL, with the concentration of tpps4 set at 0.1 mg/mL in both cases.

polyanion (anion) assemblies based on electrostatic interaction,12 indicating an interpenetration existing between the neighboring layers. The thickness of the 12bilayer DAR/tpps4 films with and without salt was calculated to be 21.5 and 18.4 nm, respectively. This corresponds to 17.9 and 15.3 Å per bilayer. Investigation of Film Permeability to Fe(CN)63-. Self-assembled monolayers (SAMs) on gold electrodes can easily block the electrochemical communication of charged inorganic compounds, such as Fe(CN)63-/4- and Ru(NH3)62+/3+, with the electrode.13 The direct electron transfer between the electrode and the electroactive inorganic compound requires access of the compound to the electrode surface through diffusion. Any factors blocking the diffusion process can lead to the decrease of the reductive and oxidative currents in cyclic voltammogrammetry (CV). In this sense, electrochemistry will (12) Lo¨sche, M.; Schmitt, J.; Decher, G..; Bouwman, W. G..; Kjaer, K. Macromolecules 1998, 31, 8893. (13) Nakashima, N.; Deguchi, Y. Bull. Chem. Soc. Jpn. 1997, 70, 767.

provide an initial assessment of film permeability and serve as a sensitive probe of structural changes caused by the photoreaction in our systems studied. Here Fe(CN)63was used as a probe in cylic voltammetry measurements to detect the structural changes of the film caused by UV irradiation. A cleaned ITO electrode was first immersed in an aqueous solution of 3-mercapto-1-propanesulfonate (0.1 mM) for 2 h to get a self-assembled monolayer. Then, the resulting electrode was deposited with several bilayers of DAR/tpps4 with the tpps4 as the outmost layer. Figure 5 shows the CVs of the electrode covered with three bilayers of DAR/tpps4 (noted ITO/MPS/(DAR+tpps4)*3) (a) before and (b) after UV irradiation in 0.1 M KCl solution containing 1 mM Fe(CN)63-. Before UV irradiation, quasireversible anodic and cathodic waves of Fe(CN)63-/4appear at 0.310 and 0.078 V, respectively. The peak to peak separation value (232 mV) is larger than the theoretical one (60 mV) of the reversible one-electron redox reaction, which must result from both the nature of tightly packed film and some slight blocking effect of the negatively charged sulfonate groups toward the same negatively charged redox probe of Fe(CN)63-/4-. After UV irradiation, the electrochemical response of Fe(CN)63-/4is greatly suppressed. This means the diffusion of Fe(CN)63-/4- within the film became difficult after photoreaction. It is supposed that two reasons should be responsible for this change. On one hand, after UV irradiation, most of the ionic bonds were converted to be covalent ones, which leads to the shrinkage of the film due to the partial elimination of the electrostatic repulsion in the film. So a densely compacted film should result and make its permeability poor. On the other hand, after UV irradiation, the hydrophobicity of the film should increase compared to that before irradiation because the UV decomposed DAR became highly hydrophobile. The hydrophobicity should also have a negative influence on the diffusion of Fe(CN)63-/4- in the film. Photoreaction changed the film more impermeable. As for a six-bilayer tpps4/DAR covered ITO electrode, after UV irradiation, the electron communication between the ITO electrode and Fe(CN)63-/4- could be blocked almost completely. The impermeability of the film makes it a

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Sun et al. Table 2. Stability Test of DAR/tpps4 Assemblies When Covered with Additional Layers of DAR/PSS as the Outmost Layers multilayer assemblies

% desorbeda

% desorbedb

(DAR+tpps4)*3 + (DAR+PSS)*1 (DAR+tpps4)*3 + (DAR+PSS)*2 (DAR+tpps4) + (DAR+PSS)*3

7 1 0

14.8 10.9 1.6

a Immersion for 5 min (after UV irradiation). b Sonicated for 5 min (after UV irradiation).

Figure 6. Cyclic voltamgrams (CVs) of a (DAR+tpps4)*3 + DAR-modified ITO electrode (a) after UV irradiation, (b) after sonication for 5 min in ternary solvent, and (c) after another 5 min sonication in ternary solvent. Conditions: 0.5 M KCl solution containing 1 mM Fe(CN)63-, scan rate: 100 mV/s.

good candidate for corrosion prevention but not good as multilayer-film-based sensors. As for a sensor, it requires the film to be both stable and permeable. Film fabrication in this way could improve the film stability greatly but decrease the permeability at the same time. So the question is: can this technique be used in constructing a multilayer-film-based sensor? As pointed out before, this technique is unable to stabilize all the small molecules in the film. A small fraction of already assembled dye molecules could be desorbed by means of solvent etching. This should provide a way to solve the problem of poor permeability of the film. If part of the molecules assembled in the film could be etched out, there should leave some very small holes in the film, which could facilitate the film permeability. To prove the possibility of this idea, a three-bilayer DAR/tpps4 film was deposited on an ITO electrode (noted ITO/MPS/(DAR+tpps4)*3+DAR). After UV irradiation, this electrode was scanned in 0.1 M KCl solution containing 1 mM Fe(CN)63-. As shown in Figure 6a, the peak currents are very small. After this electrode was sonicated in ternary solvent for 5 min, the peak currents increase obviously (Figure 6b). Another 5 min of sonication increased the peak currents a little bit again but not as large as the first sonication did. This also means that the left tpps4 in the film is stable enough to resist further solvent etching. The possibility to covalently attach electroactive species in the multilayer assemblies, combined with the high permeability caused by solvent etching will make this technique a powerful candidate for the fabrication of stable chemically modified electrodes (CMEs). It is believed that peameability of the resulted film could be controlled easily by many factors, e.g., by controlling the reaction ratio, trapping some larger template molecules, or replacing DAR with diazonium-containing copolymers. Further Ways To Improve the Stability of the Dyes Assembled. The fact that the photoreaction used in this technique failed to stabilize each small molecule in the film still limited the application of this technique when a high stable film was required. Three types of films were prepared in an attempt to find a way to further improve

the stability of the films: film I, quartz/(DAR+tpps4)*6+DAR; film II, quartz/(DAR+tpps4)*10+DAR; film III, quartz/(DAR+tpps4)*20+DAR. So the only difference is the number of bilayer of DAR/tpps4. Then these three kinds of films were sonicated in the same ternary solvent and the desorption of tpps4 was monitored with UV-vis spectroscopy until no desorption could be found. The amount of tpps4 desorbed in each film was calculated to be 32.7%, 18.5% and 9.2% for film I, II, and III, respectively. It seems that with the increase of the number of layers, the amount of desorbed tpps4 decreases. This result is meaningful because it tells that the nearer the tpps4 is to the substrate, the less possibility it is to be etched out. In other words, only the outmost layers are vulnerable to solvent etching. Combining the fact that polyelectrolyte has more binding sites than small molecules and the resulted polycation/polyanion film is much more stable in general, it is expected that when additional layer of photoreactive polymeric layers are covered on the top of the dye-containing film, the desorption of small molecules should be prevented effectively. To show the feasibility of this assumption, the following assemblies were produced: (I)(DAR+tpps4)*3+(DAR+PSS)*1,(II)(DAR+tpps4)*3+(DAR+PSS)*2, and (III) [(DAR+tpps4)+(DAR+PSS)]*3. Each film includes three layers of tpps4. The reason to use DAR/PSS film is that PSS can react easily with DAR upon UV irradiation to form more stable covalently attached film. 9a After UV irradiation, each kind of film was etched in ternary solvent to investigate its stability, as shown in Table 2. For film I, about 14.8% of the assembled tpps4 could be etched out after 5 min sonication. While for film II, to which an additional layer of PSS/DAR is added compared with film I, the etched portion decreased to be 10.9%. But the most efficient way to improve the stability is the assemblies in film III, in which DAR/tpps4 was alternated with DAR/PSS. The desorbed tpps4 is much less, only 1.6%. In this arrangement, each layer of small molecules was cover with an additional layer of DAR/PSS and thus was fully stabilized. Here we should point out that, with the number of tpps4 film decreased, the desorbed portion will increase compared with the thicker film (e.g., eight-bilayer DAR/tpps4). So the stability results of film I, film II, and film III cannot be compared directly with the eight-bilayer film listed in Table 1. It has been proven that by covering the small molecule-containing film with additional layers of DAR/sulfonate polyanion film, the stability of the resulted film can be improved further. Conclusion In this paper, by exploiting the simplicity of ionic selfassembly (ISA) technique and the in situ photoreaction between sulfonate and diazonium groups, a way to realize the stable entrapment of small dye molecules in multilayer matrix in the form of dey/DAR has been developed. A solvent etching experiment showed that the small dye molecules entrapped in this way were very stable. Due to the incompleteness of the reaction employed in this technique, it provides two possibilities which enriches the

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applications of this technique: One is that by covering or incorporating (DAR+dye)*n film with additional layers of DAR/PSS film, the stability of the dyes assembled in the film could be improved further. The other is that by solvent etching, holes could be induced to facilitate the inlet and outlet of ions/substrate through the film and make it find application in chemically modified electrodes (CMEs) due to the combined stability and permeability of the final film assemblies. This way to fabricate stable multilayer film containing small dye molecules is characterized by simplicity and effectiveness.

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Acknowledgment. The work is supported by Ministry of Science and Technology, National Natural Science Foundation of China, Ministry of Education, and the major State Basic Research Development Program (Grant No. G2000078102) P.R. China. The authors also owe their thanks to Professor Sun Changqing for cyclic voltammetry measurements.

LA010012D