Phase engineering of hydrophobic meso-environments in silica

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Phase engineering of hydrophobic meso-environments in silica particles for technical performance enrichment Zilong Guo, Zhenzhen Huang, Yanfang Wang, Dayang Wang, Ming-Yong Han, and Wensheng Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01040 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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Phase engineering of hydrophobic mesoenvironments in silica particles for technical performance enrichment Zilong Guo†, Zhenzhen Huang†, Yanfang Wang†, Dayang Wang†, Ming-Yong Han‡, and Wensheng Yang†,* †State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China. ‡Institute of Materials Research and Engineering, 2 Fusionopolis Way, Singapore 138634. KEYWORDS: Mesoscopic environment; Hydrophobic; Mesoporous silica; Micelles; Phase modulation

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ABSTRACT

Hexadecyltrimethylammonium bromide (CTAB) was utilized to template the growth of mesoporous silica particles via ammonia-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) in the reaction solutions with varied volume fractions of ethanol (fR). The use of 9,10-bis(phenylethynyl) anthracene (BPEA) as a fluorescence probe unraveled a clear difference in interior structure between the CTAB micelles confined at different fR. At fR of 0.3, the confined CTAB micelles consisted of regularly and densely packed alkane chains, which created crystalline interiors, in which the doped BPEA molecules were effectively isolated in the monomeric form and well protected against aggressive attack from the surrounding environment. At fR of 0.4 or 0.5, the confined CTAB micelles consisted of less regularly but densely packed alkane chains, created glassy interiors, which enabled reversible aggregation of the doped BPEA in response to the surrounding environmental change, for instance the ethanol content in the particle dispersion. At fR of 0.6 or 0.7, the confined CTAB micelles consisted of loosely packed alkane chains, created amorphous interiors, which offered sufficiently large free spaces to facilitate the material exchange with the surrounding environment, evidenced by noticeable intake of the Pyronin Y molecules present in the particle dispersion. The revealed phase modulation of the interiors of surfactant micelles, confined in the pores of mesoporous particles, from crystalline to glassy and amorphous structures, which were scarcely reported in literature, will inspire rational design of mesoporous silica particles with desired technical performance according to the purposes of the practical application.


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INTRODUCTION The existence of hydrophobic mesodomains is a common characteristic for the self-assembled amphiphilic molecules (e.g. surfactants, lipids, and copolymers as well as proteins) in aqueous media.1-7 Primarily acting as physical cross-links, the hydrophobic moieties of amphiphilic molecules pack together by considerably strong, hydrophobic forces, which not only retain the structural stability of the molecular self-assemblies but also play a key role in modulating the phase transition of the molecular self-assemblies and the physicochemical behaviors associated thereof. As one dramatic example, the biological functions of cell membranes are critically sensitive to the interior fluidity of the hydrophobic chains in the lipid bilayers.8-10 Up to date, however, it remains problematic to translate the biological inspirations into advanced material design, though they hold significant implications in various applications such as drug delivery across biological barriers and desalination via separation of water from salts in seawater.11-14 The self-assemblies of amphiphilic molecules are dynamically stable, both their exterior morphologies and interior structures undergo noticeable phase transition upon subtle alteration in the surrounding environment, which causes great difficulty in correlating the packing structures with the overall properties of the molecular self-assemblies.15-19 To address these concerns, the present work aimed to study the packing structures of the hydrophobic interiors of the selfassemblies (i.e. micelles) of amphiphilic hexadecyltrimethylammonium bromide (CTAB), confined in the mesopores of silica particles. Here we utilized CTAB micelles as templates for growth of mesoporous silica particles at different volume fractions of ethanol in reaction media (fR). Our results demonstrated that the hydrophobic interior structures of the confined CTAB micelles were transformed from densely and regularly packed, crystalline phase to densely but less regularly packed, glassy phase and to loosely and randomly packed, amorphous phase upon


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the increase of fR from 0.3 to 0.7, while the mesopores incurred a tiny size reduction from 2.4 to 2.0 nm. The revealed phase modulation enabled us to finely modulate the mobility of targeted molecules in CTAB micelle-filled mesopores together with their interaction with the surrounding environment and, in turn, to achieve the technical performance of mesoporous silica particles according to the purposes of applications. A myriad of surfactants have been utilized to template growth of mesoporous materials in the past decade.20-23 Up to date, however, most of research activities focus on tailoring the exterior morphology of surfactant micelles to diversify the dimension and shape of pores of targeted mesoporous materials, while little attention is paid on the packing structures of the hydrophobic cores of the surfactant micelles confined in the resulting mesopores.24-26 The phase transition of surfactant micelles in solution remains an active and important topic in the research field related to soft matter. The prevailing speculation of surfactant micelles confined in mesopores is that the surfactant aliphatic tails densely and regularly pack together into a (quasi-)crystalline structure, thus securing the structural stability of the surfactant micelles in reaction media and in turn endorsing their template role during the growth of mesopores. The adjustment of the packing structures of the hydrophobic interiors of surfactant micelles is commonly executed by altering the chemical nature of surfactants, for instance by introducing unsaturated double bonds or benzyl rings to their aliphatic tails to disrupt the packing density and irregularity.27 Here 9,10bis(phenylethynyl)anthracene (BPEA), which can be well incorporated into mesoporous materials via hydrophobic interactions,28-30 was utilized as a fluorescence probe to scrutinize the packing density and regularity of hydrophobic interior structures of CTAB micelles confined in the pores of mesoporous silica particles, obtained at different fR. It was unravelled that a small increase of 0.1 in fR resulted in noticeable structural modulation of the hydrophobic interiors of


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the confined CTAB micelles from crystalline phase to glassy phase and to amorphous phase, which remains little touched in literature by far.

EXPERIMENT SECTION Materials. BPEA (C30H18, MW: 378.46), Pyronin Y (C17H19N2O⋅Cl, MW: 302.80) and CTAB (C16H33(CH3)3NBr, MW: 364.45) were purchased from TCI, Sangon Biotech (Shanghai) Co., Ltd., and Sinopharm Chemical Reagent Co., Ltd., respectively. Ethanol (C2H5OH, MW: 46.07), ammonium hydroxide (NH3⋅H2O, MW: 35.05, 25 %) and TEOS (Si(OC2H5)4, MW:208.33, freshly distilled prior to use) were obtained from Beijing Chemical Int. Pure water with a resistivity of 18 MΩ⋅cm-1 (Pall Pure Lab Plus) was used in all the experiments. Characterizations. Transmission electron microscopy (TEM) images were measured with a JEOL-2010 microscope operating at 100 kV. TEM samples were prepared by dropping and drying the aliquots of silica particle dispersions onto copper grids coated with Formvar film. UVvisible absorption spectra were taken on a Shimadzu UV-1800 absorption spectrophotometer. Emission spectra were collected on a Shimadzu RF-5301PC spectrofluorophotometer, while emission decays were collected on a DeltaFlex modular fluorescence lifetime system. Fourier transform infrared (FTIR) spectra were recorded using a Thermo Nicolet 6700 FTIR spectrometer. The N2 sorption isotherms of silica particles after the extraction of surfactants were measured on a Micromeritics ASAP 2010 Accelerated Surface Area and Porosimetry System. Prior to the test, the silica particles were treated in an ethanol solution of hydrochloric acid at 80 o

C for 16 h (the solution was prepared by mixing 1 mL of 12 M HCl aqueous solution into 100

mL of ethanol). After centrifugation (12,000 rpm, 15 min), the collected silica particles were dried in vacuo at 30 oC for 12 h and then degassed at 250 oC for 6 h. Small-angle X-ray


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scattering (SAXS) was performed using an Anton-Parr SAXSess small-angle X-ray scattering system (Cu Kα radiation, 40 kV, 35 mA). Preparation of BPEA-doped silica particles. Typically, 5 mL of 1×10-2 M CTAB solution in water, 2 mL of 5×10-5 M BPEA solution in ethanol, and 50 µL of as-received ammonia were added sequentially and mixed uniformly under magnetic stirring. The total volume of solution was then adjusted to 10 mL by adding a mixture of ethanol and water with fR values ranging from 0.3 to 0.8. Subsequently, 50 µL of TEOS was added under magnetic stirring, followed by incubation under maganetic stirring (200 rmp) at 25 oC for 6 h. Upon centrifugation at 12,000 rpm for 15 min, the resulting BPEA-doped silica particles were collected, while the supernatants were used to determine the doping efficiency of BPEA into silica particles by means of UV– visible absorption spectroscopy. After repetitive filtration and washing with water for several times, free CTAB and BPEA molecules were thoroughly removed, and the purified BPEA-doped silica particles were dispersed in 10 mL of water (3.8 mg/mL), then dried in vacuo at 30 oC for 2 h for further characterizations. Photostability of BPEA-doped silica particles in fluorescence microscopy imaging. The images were obtained with a reflected fluorescence system from Olympus. The samples were excited by 1000 W Mercury lamp with a band pass filter BP460-495. The emission channel was equipped with band pass filter BA510-550. The fluorescence images were recorded by a microscope digital camera DP80 from Olympus. Dye adsorption of BPEA-doped silica particles in aqueous solution. 38 mg BPEA-doped silica particles were dispersed in the 10 mL (0.01 mg/mL) Pyronin Y aqueous solution and 1 mL aliquots of the solution were taken out at different time, the silica particles were separated out by centrifugation (15,000 rpm, 1 min) and then redispersed in 1 mL pure water to observe the color


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of the collected particles under UV lamp, and the remianed supernatants were used to determine the amount of Pyronin Y adosrbed by the silica particles. Photoluminescence behavior of BPEA-doped silica particles in response to the ethanol content of the surrounding media. The aqueous suspensions of as-prepared BPEA-doped silica particles (2 mL, 3.8 mg/mL) were centrifuged to collect the particles. After drying for 2 h in a vacuum oven, the dried particles were redispersed into 2 mL of water/ethanol mixtures with different ݂R values with the aid of 1 min sonication. The emission colors of resulting suspensions were characterized by digital camera. The aliquots of the aqueous dispersions of as-prepared BPEA-doped silica particles (1 µL, 38 mg/mL) were one-by-one dropped on a stain steel plate to create patterns for photoluminescence anti-counterfeiting applications.

RESULTS AND DISCUSSION BPEA was the fluorescent probe of choice for two major reasons (Figure 1a). Firstly, since both water and ethanol are poor solvents for BPEA due to its highly apolar nature, BPEA molecules were preferentially localized in the hydrophobic interiors of CTAB micelles formed in the reaction media of ethanol/water mixture before and after the templated growth of mesoporous silica particles. Secondly, BPEA molecules are preferentially present in the form of monomer in solution to display a characteristic green emission color in the range of 450 – 550 nm with the strongest emission at 480 nm (Figure S1). The rationale behind is the dynamic rotation of the two acetylenic bonds of individual BPEA molecules to prevent neighboring BPEA molecules from moving close to each other, inhibiting their aggregation. As such, BPEA molecules remain well dissolved in good solvent or precipitated when poor solvent such as ethanol was added. Upon the addition of more ethanol, the BPEA aggregates were kinetically captured in viscous


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solution of a good solvent such as dodecane to present orange emission color with the intensity maximum at 600 nm (Figure S1). Taking this into account, one may expect the appearance of BPEA aggregates with characteristic orange emission color in the fairly viscous media because the rotation of their two acetylenic bonds is efficiently suppressed. This encouraged us to utilize BPEA as a fluorescent probe to interrogate the difference in packing density and regularity of the hydrophobic interiors of CTAB micelles confined in the pores of mesoporous silica particles obtained at different fR. Prior to this study on the hydrophobic interior structures of confined CTAB micelles, BPEA was used to probe the phase transition of hexadecane with temperature, since hexadecane is an alkane chain of sixteen carbon atoms, which is identical to the hydrophobic tail of CTAB and therefore acts as a proper model to establish the correlation between the fluorescence behavior of BPEA and the packing structure of alkane chains. Since hexadecane is a good solvent for BPEA and its melting point is ca. 18 oC, the solution of BPEA in hexadecane at room temperature (ca. 25 oC) presents characteristic green emission color of BPEA monomer (Figure 1b). That monomeric emission color of BPEA remains after its hexadecane solution is rapidly solidified in liquid nitrogen at ca. –196 oC (Figure 1c). At such a low temperature, hexadecane is frozen into its crystalline phase,31 so the mobility of BPEA molecules present therein is reduced significantly not to cause any aggregation, regardless of the strong tendency of aggregation driven by considerably reduced solubility of BPEA. When the frozen hexadecane solution of BPEA are warmed up at ca. 4 oC, still below the melting point of hexadecane (ca. 18 oC), for 1 h, the transition of hexadecane from crystalline to the glassy phase is expected. In response to the phase transition, the orange emission color, characteristic of BPEA aggregates, is visible (Figure 1d). That suggests that the BPEA molecules become sufficiently mobile while the rotation of their


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two acetylenic bonds are still efficiently inhibited in glassy hexadecane phase where the alkane chains are less regularly but densely packed together. Figure 1e shows complete recovery of the characteristic monomeric green emission color of BPEA when its frozen hexadecane solution is warmed up to melt at ca. 25 oC for 1 h, which should be a result of both increase in BPEA solubility and rotation flexibility of two acetylenic bonds of BPEA in liquid hexadecane.

Figure 1. (a) Schematic depiction of the molecular structure of BPEA, in which the dynamic rotation of its two acetylenic bonds is highlighted. (b-e) Photos of a droplet (100 µM) of the hexadecane solution of BPEA (100 µM) dropped on glass slides at ca. 25 oC (b), which are subsequently frozen at ca.–196 oC by liquid nitrogen (c) and then warmed at ca. 4 oC (c) and 25 oC (d), respectively, for 1 h. Scale bars in all the images were 2 mm.

Built on the revealed correlation between the emission color of BPEA and the packing structure of alkane chains, we incorporated BPEA molecules into the hydrophobic interiors of CTAB micelles confined in mesopores by introducing them in the reaction media where the CTAB micelles were utilized to template the growth of silica particles at fR in the range of 0.3 – 0.8. Average sizes of the as-prepared silica particles increased from 138 nm to 630, 883, 1080 and 1140 nm (Figure S2), correspondingly, their pore sizes decreased slightly from 2.4 nm to


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2.3, 2.2, 2.1 and 2.0 nm and their BET surface areas decreased from 1326 m2g-1 to 1286, 1079, 1068 and 1063 m2g-1 with the increase of fR from 0.3 to 0.4, 0.5, 0.6, and 0.7 (Figure S3), which, as suggested in literature, can be attributed to an increase in solubility of CTAB molecules and in turn decrease in number and size of CTAB micelles in the reaction media with the increase of fR.32,33 Note that mesoporous silica particles were hardly produced at fR > 0.7, no mesoporous character was identifiable in the pore size distribution curve and the BET surface area of the silica particles further decreased to 728 m2g-1 (Figure S3). When BPEA molecules were introduced into the reaction media, the loading efficiency of BPEA in the as-prepared

Figure 2. (a) Photos of the Eppendorf tubes containing the dispersions mesoporous silica particles prepared at fR ranging from 0.3 to 0.7. (b, c) Photos of the Eppendorf tubes containing the sediments of the mesoporous silica particles collected via centrifugation of the corresponding particle dispersions, shown in (a), taken before (b) and after (c) drying the particle sediments in vacuo. All the photos are taken under irradiation of a 365 nm UV lamp. (d, e) Emission spectra of the dispersions (d) and vacuo-dried sediments (e) of the silica particles obtained at the different fR, shown in (a) and (c) respectively. The excitation wavelength is set at 365 nm. The inset of (d) shows the plot of the intensity ratio of the emission bands centered at 480 and 600 nm (I480/I600) versus fR. The inset in (e) shows the plot of the position of the maximum of the characteristic emission band of the BPEA monomers versus fR.


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mesoporous particles was as high as ca. 98% in the fR range of 0.3 – 0.5, while was noticeably reduced to ca. 90%, ca. 70% and ca. 1.2%, respectively, with the decrease of fR to 0.6, 0.7 and 0.8 (Figure S4). The low doping efficiency (1.2%) at fR of 0.8 suggested the absence of CTAB micelles in the reaction solution, which are necessary to template the growth of mesoporous silica and incorporate the dye via hydrophobic interactions. Taken into account small decrease in both pore size and BPEA loading efficiency with an increase in fR, the mesoporous silica particles especially obtained at the fR in the range of 0.3–0.5 have CTAB micelles with comparable sizes (volumes) and, thus, comparable BPEA content within their mesopores. When BPEA molecules are introduced to the reaction media for CTAB micelle-templated growth of mesoporous silica particles, dispersions (Figure 2a) and sediments (Figure 2b) of the resulting silica particles show almost identical emission colors, indicative of successful loading of apolar BPEA molecules into the silica particles, namely, within the C16-alkane chain interiors of the CTAB micelles confined in the particle mesopores. The emission color of the resulting BPEA-doped particles is noticeably dependent on the fR, it is green at fR of 0.3, greenish-yellow at fR of 0.4, and orange at fR of 0.5 and above, respectively. In the corresponding emission spectra of the particle dispersions, the characteristic emission bands of BPEA monomers in the range of 450–500 nm are substantially weakened with the increase of fR from 0.3 to 0.4 and above, which is concomitant with the appearance of a noticeable emission band of BPEA aggregates centered at 600 nm at fR ≥ 0.4 (Figure 2d). The intensity ratio of the emissions at 480 and 600 nm (I480/I600, inset in Figure 2d, representing the aggregation degree of the BPEA molecules), decreased sharply from 11.5 to 2.8 and 0.4 when the fR increased from 0.3 to 0.4 and 0.5, and then decreased slightly to 0.2 and 0.1 when the fR further increased to 0.6 and 0.7. These clearly indicated that the CTAB micelles formed at fR of 0.3, embody fairly densely and


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regularly packed C16-alkane chains than those formed at fR ≥ 0.4. Upon being dried in vacuo, surprisingly, the BPEA-doped particles exhibit green emission color again, characteristic of BPEA monomers, regardless of fR at which they are produced (Figure 2c), due to the increased solubility of BPEA in the alkane chains after removal of ethanol. However, the dissection of the emission spectra of the vacuo-dried particles reveals a perceivable blue shift of the characteristic emission of the BPEA emissions, from 485 nm to 480 nm with the increase of fR from 0.4 to 0.7 (Figure 2e and the inset). That blue shift may be attributed to the rise in free spaces present in the C16-alkane chain interiors of the confined CTAB micelles with the increase of fR from 0.4 to 0.7,34 which is therefore expected to enable the CTAB micelles intake more ethanol from the reaction media uptake, and the BPEA molecules doped in the CTAB micelles can move closer to each other to aggregate upon the ethanol uptake.35,36 After being redispersed in ethanol/water mixtures, the dried BPEA-doped mesoporous silica particles show a noticeable emission color change with the volume fraction of ethanol in the dispersion media (fD) increasing, but the sensitivity of the particle emission color to the fD strongly hinges on the fR at which the particles are produced (Figure 3). After centrifugation of the particles, no emission color of the BPEA was observable in the supernatants (Figure S5), meaning no dye was released into the solution during the redispersing process. The particles obtained at fR of 0.5 readily change their emission color from green to greenish-yellow and finally to orange, respectively, with the fD increasing to 0.10 and 0.15 (Figure 3c) and so do the particles obtained at fR of 0.4 with the fD increasing to 0.15 and 0.20 (Figure 3b). In contrast, the particles, obtained at fR of 0.3, 0.6, and 0.7, show a greenish-yellow emission color only at fD > 0.30 (Figure 3a, 3d, and 3e). These results further endorse the difference in the packing density between the CTAB micelles confined in the particle mesopores at fR ranging from 0.4 to 0.7.


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Figure 3. (a-e) Photos of the dispersions of BPEA-doped mesoporous silica particles, prepared at fR of 0.3 (a), 0.4 (b), 0.5 (c), 0.6 (d), and 0.7 (e), in ethanol/water mixtures. The fD values of the ethanol/water mixtures for the particle redispersion are varied from 0.00 to 0.05, 0.10, 0.15, 0.20, 0.25, 0.30 and 0.35, which are marked on the photos of the corresponding particle dispersions. (f) Photos of three types of the dot arrays with shape of J, L, and U letters on a stain steel plate, which are made of BPEA-doped mesoporous silica particles produced at fR of 0.3, 0.4 and 0.5. The photos are shot before (upper panel) and after the stain steel plate is immersed into the ethanol/water mixture with fD of 0.35 for 1 min (lower panels). The photos are taken under irradiation of a 365 nm UV lamp.

Infrared spectra were recorded to further assess the packing structures of the CTAB micelles confined in the mesoporous silica particles obtained at different fR. The anti-symmetric C-H stretch of CTAB incurred small but noticeable blue shift band from 2920 cm-1 at fR of 0.3 to 2924 cm-1 at fR of 0.4 and then to 2925 cm-1 at fR ≥ 0.5 (Figure S6), which was indicative of a reduction in packing density of the C16-alkane chains of the CTAB micelles confined in the particle mesopores at fR increasing from 0.3 to 0.5.37-39 Furthermore, the emission decay study of the as-prepared BPEA-doped mesoporous silica particles revealed a reduction in fluorescence lifetime from 4.1 ns to 3.8, 3.4, 2.9 and 2.8 ns with the increase of fR from 0.3 to 0.4, 0.5, 0.6 and


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0.7 (Figure S7), which was indicative of a reduction in packing rigidity of the C16- alkane chains of the CTAB micelles confined in the resulting particle mesopores.40 SAXS patterns of the particles prepared at the fR of 0.3 presented a peak at a q value of 1.5 nm-1, which shifted to 1.6 and 1.7 nm-1 and became weaker when the fR increased to 0.4 and ≥ 0.5 (Figure S8), which were indicative of the reduced pore sizes and less densely packing of the aliphatic chains in the CTAB micelles.41 Taken together, the CTAB micelles confined in mesoporous silica particles can be readily categorized into three types (Scheme 1). (i) The first type of CTAB micelles, confined in the particle mesopores at fR of 0.3, have crystalline interiors consisting of regularly and densely packed C16-alkane chains (Scheme 1a), reminiscent of hexadecane at ca. –196 oC (Figure 1c). In these crystalline interiors not only the mobility of the doped BPEA molecules but also the intake of ethanol from the surrounding media are effectively suppressed. The BPEA molecules doped therein may incur perceivable aggregation only at a fairly high fD, at which ethanol commences to diffuse into the confined CTAB micelles from the surrounding to loosen the regularly packed alkane chains (Figure 3a). Inspired by the resemblance of the BPEA emission color to that observed in hexadecane at ca. 4 oC (Figure 1d), (ii) the second type of CTAB micelles, confined in the particle mesopores at fR of 0.4 and 0.5, have glassy interiors consisting of less regularly but densely packed C16-alkane chains (Scheme 1b), which create some free spaces to effectively push up the intake of ethanol from the surrounding media and the mobility of the doped BPEA molecules but, at the same time, to effectively suppress the rotation of the BPEA acetylenic bonds, thus enabling aggregation of the doped BPEA molecules in sharp response to the surrounding fD (Figure 3b and 3c). As indicated in Figure 2e, Figure S6 and S7, the CTAB micelles, confined in the particle mesopores at fR of 0.4, have the packing density in their glassy interiors slightly larger than those confined in the mesopores at fR of 0.5, which accounts for the


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Scheme 1. Schematic illustration of the crystalline (a), glassy (b) and amorphous (c) phases of the C16-alkane chain interiors of CTAB micelles, confined in the pores of as-prepared mesoporous particles produced at fR of 0.3, 0.5 and 0.7, respectively. According to the micellar interior structures, yet are illustrated the aggregation of the BPEA molecules doped in the CTAB micelles and the particle emission color change associated thereof in response to the fD of the surrounding environment.

delayed response of the BPEA aggregation in the former interiors to the fD (Figure 3b). That is further corroborated by the fact that upon 1 min dipping of BPEA-doped mesoporous silica particles into ethanol/water mixtures with fD of 0.35, the emission color turns out to be orange and greenish-yellow, respectively for particles obtained at fR of 0.5 and 0.4, while the original green emission color remains hardly changed for the particles obtained at fR of 0.3 (Figure 3f). (iii) The third type of CTAB micelles, confined in the particle mesopores at fR of 0.6 and 0.7, have amorphous interiors consisting of loosely packed C16-alkane chains in the interior (Scheme 1c), which should be plausibly at fairly large fR as a result of an increase in the solubility of CTAB molecules in ethanol/water mixtures. These amorphous interiors can absorb more ethanol from the surrounding and, in turn, facilitate the BPEA molecules doped therein to move closer to


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each other in response to the ethanol intake. However, they should be poorly effective in suppressing of the rotation of the BPEA acetylenic bonds, which results in significantly reduced response of BPEA aggregation to the fD (Figure 3d and 3e). To further clarify the difference in the interior structure of the confined CTAB micelles, BPEAdoped mesoporous silica particles obtained at different fR were tested against photobleaching. Upon exposure to 1000 W mercury lamp, as shown in Figure 4a, 30 s irradiation causes complete vanishing of the emission of the particles obtained at fR of 0.7, the emission of the particles obtained at fR of 0.5 become rather weak but perceivable, which disappears after another 30 s irradiation. In contrast, the particles obtained at fR of 0.3 retain 40% of the initial emission intensity after 60 s irradiation. The difference in photobleaching resistance is clearly in line with the decrease of the packing density of the confined CTAB micelles with the fR increasing from 0.3 to 0.7. The improved photobleaching resistance of BPEA-doped mesoporous silica particles obtained at fR of 0.3 underlines that the crystalline interior structures of the CTAB micelles provide a good protection of the BPEA molecules doped inside against aggressive attacks imposed by the surrounding environment, which bear some resemblance to the stabilization role of the hydrophobic β-barrel cores in green fluorescent proteins.42 Furthermore, as-prepared BPEA-doped mesoporous silica particles were placed in the aqueous solution of Pyronin Y, a cationic dye that can intercalate and stain RNA red. After 15 min incubation and separation of the silica particles by centrifugation, as shown in Figure 4b, redispersion of the particles obtained at fR of 0.7 showed yellow emission as a result of overlay of the emission color of the Pyronin Y and BPEA, indicating the noticeable intake of Pyronin Y (3.8 mg/g), while the redispersions of the particles obtained at fR of 0.3 and 0.5 kept the original green emission, meaning few Pyronin Y molecules (

Phase engineering of hydrophobic meso-environments in silica

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