Optical Response of Mesoporous Synthetic Opals to the Adsorption of

Langmuir 2008, 24, 2779-2784

2779

Optical Response of Mesoporous Synthetic Opals to the Adsorption of Chemical Species Yuri Yamada,* Tadashi Nakamura, and Kazuhisa Yano Toyota Central Research & DeVelopment Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ReceiVed August 4, 2007. In Final Form: December 3, 2007 We have demonstrated the fabrication of a colloidal crystalline array (synthetic opal) from monodispersed mesoporous silica spheres (MMSS) and the control of its optical response simply by changing the amount of benzene vapor adsorbed into the pores of MMSS. It was revealed that the refractive index of the colloidal crystal of MMSS showed an 11.7% increase by taking advantage of benzene adsorption, and thereby, the structural color changed reversibly. We also conducted the same measurement on silica spheres without mesopores and observed no change in the refractive index or the structural color. This optical response gives rise to the possibility of using MMSS colloidal crystals not only for controlling light reflection but also as sensing devices based on color change due to vapor adsorption. We have also incorporated an organic dye, the porphyrin derivative R,β,χ,δ,-tetrakis(1-methylpyridinium4-yl)porphyrin F-toluenesulfonate (TMPyP), into the pores of MMSS. By adopting an electrophoretic deposition process in ethanol, periodic arrays fabricated from TMPyP-MMSS conjugates with absolute ζ-potentials near zero were obtained. The Bragg diffraction peak of the colloidal crystalline array shifted to longer wavelengths due to an increase in the refractive index with increasing amounts of TMPyP adsorbed in the pores. The current work demonstrates the new possibility of creating colloidal crystals from MMSS with mesopores filled with various kinds of adsorbates to control the optical response effectively.

1. Introduction Since Yablonovich1 and John2 first introduced the concept of photonic crystals, much attention has been focused on these fascinating structures. Photonic crystals are spatially periodic structures that display unique optical properties, evident in the way they can be designed to confine and control the propagation of light due to the presence of photonic band gaps. Because of these unique properties, these materials open up the possibility of utilizing them in many useful applications, such as optical communication, switching, sensing, and lasing.3-5 To fabricate high-quality three-dimensional periodic structures, various techniques, such as microfabrication of semiconductors, holography, and self-assembly, have been employed.6-10 Among these techniques, self-assembly of monodispersed colloidal spheres has been widely adopted because this provides a simple and cost-effective route to the creation of three-dimensionally ordered structures. These structures are called colloidal crystals or synthetic opals. Up to now, colloidal crystalline arrays have been studied by many research groups, and a few reports have been published concerning the fabrication of newly synthesized spheres.11,12 However, most of the effort has been limited to the fabrication of polystyrene latex or Sto¨ber silica spheres. * To whom correspondence should be addressed. E-mail: e4610@ mosk.tytlabs.co.jp. (1) Yablonovich, E. Phys. ReV. Lett. 1987, 58, 2059. (2) John, S. Phys. ReV. Lett. 1987, 58, 2486. (3) Ruhl, T.; Spahn, P.; Hermann, C.; Jamois, C.; Hess, O. AdV. Funct. Mater. 2006, 16, 885. (4) Newton, M. R.; Bohaty, A. K.; White, H.; Zharov, S. I. J. Am. Chem. Soc. 2005, 127, 7268. (5) Shkunov, M. N.; Vardeny, Z. V.; DeLong, M. C.; Polson, R. C.; Zakhidov, A. A.; Baughman, R. H. AdV. Funct. Mater. 2002, 12, 21. (6) Masuda, Y.; Itoh, T.; Itoh, M.; Koumoto, K. Langmuir 2004, 20, 55588. (7) Lu, Y.; Yin, Y. D.; Gates, B.; Xia, Y. N. Langmuir 2001, 17, 6344. (8) Li, H. L.; Dong, W. T.; Bongard, H. J.; Marlow, F. J. Phys. Chem. B 2005, 109, 9939. (9) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (10) Kim, M. H.; Im, S. H.; Park, O. O. AdV. Funct. Mater. 2005, 15, 1329. (11) Jeong, U.; Kim, J. U.; Xia, Y. N. Nano Lett. 2005, 5, 937. (12) Liddell, C. M.; Summers, C. J. AdV. Mater. 2003, 15, 1715.

Recently, we described the synthesis of highly monodispersed mesoporous silica spheres (denoted as MMSS hereafter) that contain ordered starburst mesopores with hexagonal regularity.13 Colloidal structures based on mesoporous materials are very interesting structures, which have attracted the attention of many researchers.14,15 In our previous report, we succeeded in fabricating a colloidal crystal and demonstrated a novel strategy for controlling light reflection on the colloidal crystal by taking advantage of the adsorption properties of the MMSS.16 By simply introducing water vapor into the crystalline array, the structural color and reflection spectra could be reversibly altered due to the adsorption and desorption of water vapor in the mesoporous channels. For comparison, reflection spectra were also collected from silica spheres without mesopores (we represent these particles by the acronym NSS). Although these counterpart materials did not have mesopores, a small shift in the peak of the reflection spectrum was observed. This could be accounted for by the high hydrophilicity of the silanol groups on the surface of the silica spheres. Additionally, some research groups have reported the observation of changes in the structural color of colloidal crystals made from an aqueous dispersion of NSS during the drying process.17 Another research group has demonstrated that there is a relationship between the elimination of internal water from NSS and a reduction in the refractive index.18 Thus, our strategy for controlling light reflection from colloidal crystals by taking advantage of the adsorption properties is novel, but the combination of silica spheres and water is not innovative. On the other hand, we have made a first step toward the practical use of monodispersed mesoporous silica spheres by incorporating (13) Yano, K.; Fukushima, Y. J. Mater. Chem. 2003, 13, 2577. (14) Villaescusa, L. A.; Mihi, A.; Rodrı´guez, I.; Garcı´a-Bennett, E.; Mı´guez, H. J. Phys. Chem. B 2005, 109, 19643. (15) Yang, S. M.; Coombs, N.; Ozin, G. A. AdV. Mater. 2000, 12, 1940. (16) Yamada, Y.; Nakamura, T.; Ishii, M.; Yano, K. Langmuir 2006, 22, 2444. (17) Ishii, M.; Nakamura, H.; Nakano, H.; Tsukigase, A.; Harada, M. Langmuir 2005, 21, 5367. (18) Garcia-Santamaria, F.; Miguez, H.; Ibisate, M.; Meseguer, F.; Lopez, C. Langmuir 2002, 18, 1942.

10.1021/la702400f CCC: $40.75 © 2008 American Chemical Society Published on Web 01/30/2008

2780 Langmuir, Vol. 24, No. 6, 2008

iron oxide in the mesopores.19 Owing to their high pore volume, our newly synthesized silica spheres are potential hosts for the inclusion not only of inorganic oxides but also of metal complexes and organic dyes. Organic dyes are becoming an increasingly important class of materials in medical science, biology, and optics. In our previous work,20 the porphyrin derivatives of R,β,χ,δ,-tetrakis(1-methylpyridinium-4-yl)porphyrin F-toluenesulfonate (TMPyP) and an electron acceptor, methyl viologen MV2+, were separately incorporated within the mesopores of MMSS and within the interlayers of photocatalytic titania nanosheets, respectively. We successfully laminated these composite films and showed a unique photoinduced electron transfer through the heterogeneous interface between them. The TMPyP-MMSS conjugate was dispersed in ethanol and spincoated onto quartz substrates, but it turned out that the obtained film was not actually built up from three-dimensional colloidal crystalline arrays. To obtain a hexagonally close-packed structure, we also attempted self-assembly from an aqueous dispersion of the TMPyP-MMSS conjugate. However, an ordered structure was not observed in either case. Two reasons can be speculated why three-dimensional colloidal crystalline arrays were not fabricated from the TMPyP-MMSS conjugate. One possible reason is related to the low surface charge (ξ-potential) of the TMPyP-MMSS conjugate. In general, colloidal crystalline arrays are obtained by electrostatic interactions between charged monodispersed spheres in an aqueous dispersion. The ξ-potential of Sto¨ber silica (NSS)21 and polystyrene latex22 is about -40mV, and, with such a high absolute ξ-potential, ordered colloidal crystalline arrays can be fabricated easily from these particles. On the contrary, the ξ-potential of MMSS in which the mesopores are filled with the cationic dye TMPyP falls to nearly zero. This implies that the electrostatic interaction is very low and that the particles tend to agglomerate due to the action of attractive van der Waals forces.23 A second possible reason is the high water solubility of TMPyP. Although MMSS can act as an ideal host for various materials including water-soluble organic dyes, we cannot completely discount the possibility that these dyes elute out from the mesopores of the MMSS during the process of colloidal assembly. Such excess leachates would play a role in concentrating the ionic strength throughout the dispersion, only to fail in the fabrication of a synthetic opal structure. Therefore, it remains a challenging goal to fabricate ordered colloidal crystalline arrays from MMSS with mesopores filled with watersoluble materials, such as TMPyP. Here, we report on changes observed in the reflection spectra from a colloidal crystal of silica spheres derived from benzene adsorption. The use of benzene in this context has not been reported previously. The higher refractive index of benzene compared to that of water offers the possibility of utilizing benzene to induce a higher refractive index for colloidal crystals fabricated from MMSS than can be achieved using water. We also demonstrate the fabrication of a synthetic opal structure from a TMPyP-MMSS conjugate and consequently investigate the relationship between the reflection spectra and the amount of TMPyP adsorbed in the pores of the MMSS. 2. Experimental Section 2.1. General. R,β,χ,δ,-Tetrakis(1-methylpyridinium-4-yl)porphyrin F-toluenesulfonate (TMPyP, Tokyo Kasei) of extra pure grade (19) Nakamura, T.; Yamada, Y.; Yano, K. J. Mater. Chem. 2006, 16, 2417. (20) Yui, T.; Kobayashi, Y.; Yamada, Y.; Tsuchino, T.; Yano, K.; Kajino, T.; Fukushima, Y.; Torimoto. T.; Inoue, H.; Takagi, K. Phys. Chem. Chem. Phys. 2006, 8, 4585. (21) de Kerchove, A. J.; Elimelech, M. Langmuir 2005, 21, 6462. (22) Rasmusson, M.; Wall, S. J. Colloid Interface Sci. 1999, 312, 209. (23) Liu, J. W.; Luijten, E. Phys. ReV. Lett. 2004, 93, 247802.

Yamada et al. and benzene (Wako Inc.) were used without further purification. Highly monodispersed mesoporous silica spheres (MMSS) were prepared from hexadecyltrimethylammonium bromide and tetramethoxysilane in accordance with our previous report.13 In this study, we used MMSS with diameters of 480 and 560 nm (Figure S1 in the Supporting Information). The silica spheres without mesopores (NSS) were purchased from Nippon Shokubai Co., Ltd. 2.2. Characterization. The absorption spectra were measured on a Shimadzu spectrophotometer MPS-2400 (Kyoto, Japan). Nitrogen adsorption isotherms were measured at 77 K with a Quantachrome Autosorb-1 instrument. The specific surface areas were calculated by the Brunauer-Emmet-Teller (BET) method with adsorption data ranging from P/P0 ) 0.05 to P/P0 ) 0.15, and the pore size distribution curves were analyzed by the Barrett-Joyner-Halenda (BJH) method.24 Scanning electron microscopy (SEM) images were taken with a S-3600N microscope (Hitachi High-Technologies Corporation) at an acceleration voltage of 10 kV. The sample surfaces were coated with gold prior to observation. Benzene adsorption isotherms were studied using a Belsorp-18 apparatus at 298 K in a static benzene vapor system. Prior to measurement, the samples were immersed in water for 24 h and then evacuated at room temperature. The transmittance spectrum was collected on a UV3600 spectrophotometer (Shimadzu). The surface electrical potential (ζ-potential) of the colloidal spheres was measured by a microscope electrophoresis method using a Microtec Nichion ZEECOM2000 instrument. 2.3. Fabrication of a Synthetic Opal Structure Made of MMSS. To fabricate a synthetic opal from MMSS with a diameter of 480 nm, we adopted the use of a fluidic cell.17 The cell was composed of two flat glass substrates with two spacers sandwiched between them. The cell had two openings with the same thickness as the spacer. A fluid reservoir was connected to one of the openings. The glass substrates were cleaned in a UV/ozone cleaner (Nippon Laser) for 20 min. To obtain a well-ordered colloidal crystal, we prepared a 10 wt % aqueous dispersion of MMSS. Before injection into the cell, continuous sonication was maintained for at least 3 h. A welldispersed solution of MMSS was injected into the reservoir and allowed to penetrate into the space within the cell by capillary force. A close-packed colloidal crystalline array grew gradually from the vicinity of the other opening, and the sample was dried by solvent evaporation at room temperature. 2.4. Preparation of TMPyP-MMSS Conjugates. MMSS with a diameter of 560 nm were chosen to form the TMPyP-MMSS conjugates which were prepared as follows. A total of 40 mg of MMSS was added to 800 µL of TMPyP dissolved in deionized water (0-30 mM). The suspension was then shaken overnight at ambient temperature to reach adsorption equilibrium. The conjugate was collected by centrifugation and dried at room temperature. The amount of TMPyP adsorbed into the pores of the MMSS was determined spectrophotometrically by measuring the absorbance at 442 nm of the supernatant solutions after centrifugation, which is the characteristic wavelength of the absorption band of TMPyP. 2.5. Fabrication of Synthetic Opals Made from the TMPyPMMSS Conjugate. Synthetic opals made from the TMPyP-MMSS conjugates were fabricated by an electrophoretic deposition (EPD) process. EPD was performed at a constant voltage of 7 V, employing indium tin oxide (ITO) anodic and cathodic electrodes with a working area of 2 cm × 2 cm. The electrodes were fixed in parallel to each other, and the distance between the electrodes was 0.5 cm. Before deposition, the ITO glass substrates were washed under sonication in acetone for 30 min. The TMPyP-MMSS conjugate was well dispersed in ethanol (ca. 5 wt %). After deposition, the coated ITO slides were withdrawn and dried at room temperature. 2.6. Angle-Resolved Reflection Spectroscopy. To determine the refractive index of the crystalline arrays, angle-resolved reflection spectra were measured. To increase the measurement accuracy, the incident angle θ between the beam and the normal to the surface of the film was varied from 12° to 34° with intervals of 2° and the (24) Barrett, E. P.; Joyner, L. G.; Halenda, P. H. J. Am. Chem. Soc. 1951, 73, 373.

Optical Response of Mesoporous Synthetic Opals

Langmuir, Vol. 24, No. 6, 2008 2781

Figure 1. Adsorption isotherms of nitrogen (a) and benzene (b) in MMSS, and nitrogen (c) and benzene (d) in NSS. The inset in (a) is a pore size distribution plot. scattered light was collected in the Bragg configuration. The interplanar spacing d and the effective refractive index neff were determined by fitting the Bragg condition to the plotted data. The Bragg condition is given by eq 1. λpeak ) 1.633d(neff2 - sin2 θ)1/2

(1)

To remove the water from between the particles and from the mesopores, the film was evacuated for 5 h at 323 K. To measure the reflection spectrum under benzene vapor, the colloidal film was placed in a glass container, into which nitrogen containing benzene vapor was introduced. Measurements were conducted at two different benzene vapor pressures (P/P0 ) 0, 1).

3. Results and Discussion 3.1. Benzene Adsorption in MMSS and NSS Synthetic Opals. In our previous report, we succeeded in showing a reversible change in the reflection spectra simply by introducing water vapor into a MMSS synthetic opal. In this study, we report on changes in the reflection spectra derived from benzene adsorption. Figure 1 shows nitrogen and benzene adsorption isotherms for MMSS and NSS. In the case of MMSS, both isotherms are type IV,25 which is typical for mesoporous materials. The pore size distribution is very narrow (inset), and the pore diameter calculated from the nitrogen adsorption isotherm is 20 Å. The specific surface area and the pore volume were 1010 m2 g-1 and 0.6 cm3 g-1, respectively. Although benzene is hydrophobic, the P/P0 value at which the amount of adsorbed benzene increases steeply because capillary condensation is very low, and the amount adsorbed reaches more than 0.5 g g-1. Examination of both isotherms indicates that MMSS have an ordered mesoporous structure. (25) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pirotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.

SEM images of MMSS synthetic opal display a close-packed face-centered cubic (fcc) structure. (Figure S2 in the Supporting Information). It should be mentioned that some voids and cracks are observed in these films. These were caused both by capillary forces during the drying process and by mechanical forces when peeling the film from the glass substrate to prepare the SEM sample. The transmission spectrum (Figure S3 in the Supporting Information) exhibits a stop band. The SEM images and the transmission spectrum indicate that the MMSS synthetic opal is a relatively high quality one. We conducted angle-resolved reflection spectroscopy measurements under different benzene vapor pressures for both MMSS and NSS synthetic opals (Figures S4 and S5 in the Supporting Information). The benzene vapor pressures were 0 and 1 for conditions A and B, respectively. From examination of the benzene adsorption isotherm on MMSS (Figure 1), it would be safe to say that the pores of the MMSS were almost completely filled with benzene for condition B. Figure 2 shows the relationship between the peak of the reflection spectrum, λpeak, and the incident angle θ for the two conditions. In both conditions, the reflection spectra displayed shifts to shorter wavelengths with increasing incident angle θ. When the spectra were collected under condition B for the MMSS synthetic opal (a), λpeak shifted to longer wavelengths. In our previous report, we found that the reflection spectra of colloidal crystalline arrays fabricated from MMSS could be changed dramatically just by controlling the amount of water vapor adsorbed into the mesopores of the MMSS. The reason why such a dramatic change occurs can be explained as follows. If the colloidal crystalline array has a face-centered cubic (fcc) structure, the filling fraction is assumed to be 0.74 for the sample. In this case, the effective refractive index neff can be calculated from eqs 2 and 3,26,27 where nm is the refractive index of the (26) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266. (27) Yamasaki, T.; Tsutsui, T. Appl. Phys. Lett. 1998, 72, 1957.

2782 Langmuir, Vol. 24, No. 6, 2008

Yamada et al.

Figure 2. Wavelength of reflection peak versus incident angle of light for MMSS (a) and NSS (b) synthetic opals measured for two different conditions.

medium (air) and ni and Vi are the refractive indices and volume fractions of the components within the MMSS, respectively, which in our previous report were silica, water, and air. Equation 3 suggests that nMMSS can be easily controlled by varying the amount of adsorbate adsorbed into the mesopores.

neff2 ) 0.26nm2 + 0.74nMMSS2 nMMSS2 )

∑i ni2Vi

(2) (3)

In this study, benzene vapor was chosen as the adsorbate instead of water vapor. Following the same reasoning used in the case of water vapor, the shift in λpeak for the MMSS synthetic opal structure can be interpreted as being due to the benzene vapor adsorption properties of the mesopores of MMSS. We should also emphasize the reversible change in the structural color of the MMSS synthetic opal observed during the benzene adsorption-desorption cycle. This phenomenon is attributed to secondary diffraction occurring in the visible region, which was also seen in the case of water vapor in our previous report. This result strongly suggests that the synthetic opal of MMSS can also be expected to be used as a sensing device based on color change due to vapor adsorption. On the contrary, for the NSS synthetic opal, the data for condition A coincided almost exactly with those for condition B. That is, no shift in λpeak was observed. We can clarify this phenomenon from the adsorption characteristics of NSS. From Figure 1b, it is clear that NSS adsorbs neither nitrogen nor benzene, indicating that the spheres do not have mesopores, nor do they have the ability to adsorb benzene into ordered pores like MMSS. This difference in adsorption capacities between MMSS and NSS is considered to affect the results of the angle-reflection spectra measurements. Therefore, this qualitative comparison strongly supports the notion that benzene vapor adsorption occurs only in the mesopores of MMSS. When water vapor was chosen as an adsorbate (as shown in our previous report), the NSS adsorbed water vapor of about 0.2 g g-1, causing λpeak to shift toward a longer wavelength, although

the shift in λpeak was about half that observed for MMSS. It seems that NSS has disordered micropores, because a calcination process was not included in the procedure for synthesizing NSS. According to Wells et al.,28 after calcination in air at 1073 K for 3 h or longer, internal silanol groups condense to form siloxane bridges, and the NSS are found to be nonporous. Consequently, it is believed that the disordered micropores in NSS contribute to the adsorption of water vapor. It seems that water vapor only can be adsorbed into the micropores of NSS. We explain this phenomenon by the differences between the kinetic diameters, which are 2.65, 3.6, and 5.85 Å for water, nitrogen, and benzene, respectively.29 Because water has the smallest kinetic diameter, it can penetrate into the micropores of NSS. Conversely, it seems that benzene vapor is an appropriate candidate for this kind of study, because it can be used to emphasize the differences in adsorption properties (along with the optical properties) of silica spheres with ordered mesopores (MMSS) and those without mesopores (NSS). In this study, the refractive indices can be obtained by two methods. The first is by applying the Bragg equation (eq 1). From eq 1, the particle diameter d and the effective refractive index neff (B) can be determined. The second method employs eqs 2 and 3. From these equations and the benzene vapor adsorption properties, the effective refractive index neff (A) can be calculated. We used known refractive index values, nair ) 1.00, nbenzene ) 1.50, and nsilica ) 1.46,30 and a density of 1.8 cm3 g-1 for MMSS. In condition A, benzene was not introduced into the cell, so in this case the components within the MMSS were silica and air. Conversely, in the case of condition B, the pores of the MMSS were almost completely filled with benzene, so we must add this new component (benzene) to the calculation. The calculated values of neff (A) are 1.18 and 1.35 for conditions A and B, respectively, and are in good agreement with the measured values, given by neff (B), which are 1.17 and 1.31. The refractive index neff (B) for condition B shows an 11.7% increase compared to that for condition A. In our previous report, we found that the change in neff (B) between conditions A and B equated to an 8.5% increase in water vapor. We can interpret this difference by adopting the refractive indices of benzene (1.50) and water (1.33). In the case of NSS, the increase in neff (B) was limited to only 3% with water vapor, and furthermore, when benzene vapor was used as the adsorbate, no increase at all was observed. In the case of MMSS, the mean pore sizes, pore volume, and even their polarity can be tuned to suit many different kinds of adsorbate. This indicates that it is possible to use a diverse range of materials to control the light reflection from the MMSS synthetic opals. At the same time, taking advantage of this optical response, the MMSS synthetic opals are also very promising for use as sensing devices. 3.2. TMPyP Adsorption for MMSS Synthetic Opals. TMPyP is one of the cationic porphyrin derivatives and can be introduced efficiently into the mesopores of MMSS via interactions with silanol groups on the surface. Therefore, controlling the amount of TMPyP adsorbed into the pores of MMSS is relatively easy and provides a convenient way of investigating the relationship between the reflection spectrum and the amount of bulky organic dye adsorbed into the pores. The adsorption isotherm of TMPyP is shown in Figure 3. Adsorption proceeded efficiently before (28) Wells, J. D.; Koopal, L. K.; de Keizer, A. Colloids Surf., A 2000, 166, 171. (29) Oumi, Y.; Kakinaga, Y.; Kodaira, T.; Teranishi, T.; Sano, T. J. Mater. Chem. 2002, 13, 181. (30) McComb, D. W.; Treble, B. M.; Smith, C. J.; De La Rue, R. M.; Johnson, N. P. J. Mater. Chem. 2001, 11, 143.

Optical Response of Mesoporous Synthetic Opals

Langmuir, Vol. 24, No. 6, 2008 2783

Figure 3. Adsorption isotherm of TMPyP into pores of MMSS.

Figure 5. SEM images of synthetic opals from TMPyP-MMSS conjugates; images (a), (b), and (c) correspond to samples B, C, and D, respectively. (c) is a macroscopic image of (b).

Figure 4. Changes in nitrogen adsorption isotherms for TMPyPMMSS conjugates in which various amounts of TMPyP (A, 0 mg; B, 6.6 mg; C, 25.0 mg; D, 31.5 mg) were adsorbed in the pores. Table 1. ζ-Potentials and Refractive Indices of TMPyP-MMSS Conjugates sample

TMPyP adsorbed (mg/100 mg of MMSS)

ζ-potential (mV)

neff

A B C D

0 6.6 25.0 31.5

-38.1 -14.9 -7.64 -5.21

1.17 1.22 1.26 1.28

reaching a constant level. The maximum amount of TMPyP is 31.5 mg at an equilibrium concentration of 13 mM. Figure 4 shows nitrogen adsorption isotherms of TMPyPMMSS conjugates. With increasing amounts of TMPyP, the amount of adsorbed nitrogen decreases gradually as shown by curves A-D. In curve A, the adsorbed amount of nitrogen increases slightly at lower pressures and increases sharply at around P/P0 ) 0.2 due to capillary condensation. With increasing amounts of adsorbed TMPyP, the relative pressure at which capillary condensation occurs shifts to lower pressures and the nitrogen uptake by TMPyP-MMSS conjugates drops off in comparison with that for MMSS (curve A). From the nitrogen adsorption measurements, pore volumes and specific surface areas were calculated (Table S1 in the Supporting Information). As is clear from the above results, an increase in the amount of TMPyP adsorbed into the pores of the MMSS is accompanied by a decrease in the pore volume and specific surface area. These results strongly support the assumption that the pores of the MMSS were substantially filled with TMPyP. The self-assembly of monodispersed colloidal particles is one of the most commonly used techniques for preparing colloidal crystals. To accomplish this technique, it is essential that charged colloidal particles form a uniform and steady suspension by electrostatic interactions. It is suggested that the ζ-potential of colloidal particles gives an indication of the stability of the colloidal dispersion. The values of the ζ-potentials of MMSS and TMPyP-MMSS conjugates are shown in Table 1. The ζ-potential of MMSS is about -40 mV, and this high absolute value would encourage MMSS to form a stable colloidal

dispersion. Conversely, with increasing adsorbed amounts of TMPyP in the mesopores of the MMSS, the absolute ζ-potential gradually approaches zero. In the case of sample D, the ζ-potential was about -5.2 mV, so that its electrostatic repulsion was significantly weaker than that of MMSS. In fact, we were unable to obtain stable dispersions from these TMPyP-MMSS conjugates. To compensate for the low surface charge of the TMPyPMMSS conjugates, the EPD process, which is a promising technique for the assembly of charged particles from colloidal suspensions,31-33 was adopted. To prevent TMPyP eluting out from the mesopores of the MMSS in the EPD process, the conjugates were dispersed in ethanol. Figure 5 shows SEM images of synthetic opals obtained from TMPyP-MMSS conjugates in which various amounts of TMPyP were adsorbed in the pores of MMSS. Hexagonally well-ordered arrays were observed in all samples, and most of the structures were divided into domains caused by capillary forces during the drying process. However, the ordered domain sizes were large enough to enable us to measure their reflection spectra. In this research, by adopting an EPD process in an ethanol solution, we have successfully demonstrated a method for obtaining synthetic opals from TMPyP-MMSS conjugates. This method can be applied to make opal structures from MMSS filled with various kinds of compounds that decrease the ζ-potential through adsorption or that have high water solubility. The results obtained here signify the possibility of extending this to other functional materials, thus fabricating colloidal crystals from MMSS with mesopores filled with other materials. Figure 6 shows the relationship between the incident angle θ and λpeak of opal structures from TMPyP-MMSS conjugates. It is clear that, by increasing the amount of TMPyP adsorbed into the pores of the MMSS, λpeak shifts to a longer wavelength. We observed a peak shift of about 150 nm between samples A and D. By fitting the formula for Bragg’s diffraction condition (eq 1) to these plotted data, their refractive indices could be determined (Table 1). It is obvious that the refractive indices of the colloidal crystals of TMPyP-MMSS conjugates can be tuned from 1.16 to 1.28 according to the amount of TMPyP absorbed in the mesopores. We believe that this is the first report in which colloidal crystalline arrays have been fabricated by applying the EPD (31) Kanamura, K.; Hamagami, J. Solid State Ionics 2004, 172, 303. (32) Wang, Y. C.; Leu, I. C.; Hon, M. H. J. Mater. Chem. 2002, 12, 2439. (33) Holgado, M.; Garcı´a-Santamarı´a, F.; Blanco, A.; Ibisate, M.; Cintas, A.; Mı´guez, H.; Serna, C. J.; Molpeceres, C.; Requena, J.; Mifsud, A.; Meseguer, F.; Lopez, C. Langmuir 1999, 15, 4701.

2784 Langmuir, Vol. 24, No. 6, 2008

Figure 6. Wavelength of reflection peak versus incident angle of light for synthetic opals made from TMPyP-MMSS conjugates.

process to monodispersed mesoporous silica spheres in which the pores were filled with adsorbates. From the results, we assume that not only water or benzene vapor but also bulky organic dyes such as TMPyP can be adsorbed in the mesopores of MMSS to efficiently control the optical response of synthetic opals. A study is underway to obtain synthetic opals from MMSS in which the mesopores have been filled with various kinds of organic dyes with stop bands in the visible light region.

4. Conclusions In this study, we have demonstrated the fabrication of a colloidal crystalline array (synthetic opal) from monodispersed mesoporous silica spheres (MMSS) and the control of its optical response simply by changing the amount of benzene vapor adsorbed in the pores of the MMSS. By taking advantage of benzene adsorption, the refractive index of the colloidal crystalline array

Yamada et al.

of MMSS shows an 11.7% increase, thereby reversibly changing the structural color. From this optical response, it is expected that the MMSS opal structure can be used not only for controlling light reflection but also as a sensing device based on color change due to vapor adsorption. This phenomenon was not observed for colloidal crystalline composed of silica spheres without mesopores. We have also succeeded in fabricating periodic arrays from TMPyP-MMSS conjugates with absolute ζ-potentials near zero. These conjugates were fabricated using an electrophoretic deposition process in ethanol. We have also reported on the optical properties of these arrays as estimated by angle-resolved reflection spectroscopy. The Bragg diffraction peak shifted to longer wavelengths according to the amount of TMPyP adsorbed in the mesopores. This red shift was due to an increase in the refractive index. We are convinced that the current work suggests new possibilities for the creation of synthetic opals from MMSS with mesopores filled with various kinds of compounds. Furthermore, it was also revealed that, using our novel strategy, not only vapors but also bulky materials could be adsorbed into the mesopores of MMSS to effectively control the optical response of synthetic opals. Acknowledgment. The authors are grateful for Dr. Masahiko Ishii for thoughtful and insightful discussions. This research was partially supported by the Japan Society for the Promotion of Science and a Grant-in-Aid for Scientific Research (B), 17310079. Supporting Information Available: SEM images of MMSS and MMSS synthetic opal, angle-resolved reflection spectra measured from MMSS and NSS opal structures, transmittance spectrum, and physicochemical properties of TMPyP-MMSS conjugates. This material is available free of charge via the Internet at http://pubs.acs.org LA702400F