Accessibility to gases of dye molecules in hybrid surfactant-silica mesophases

3010

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.

ACCESSIBILITY TO GASES OF DYE M O L E C U L E S IN HYBRID SURFACTANT-SILICA M E S O P H A S E S O n i d a , B. l, Borello, L. 1, Fiorilli, So1, Barolo, C. 2, E d l e r , K.J. 3, O t e r o Are~in, Co 4 and G a r r o n e , E. 1 1Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, l0129 Torino, Italy. E-mail: [email protected] 2Dipartimento di Chimica Generale ed Organica Applicata, Corso Massimo d'Azeglio, 48 - 10125 Torino. 3Department of Chemistry, University of Bath, Bath BA2 7AY, UK. 4Departamento de Quimica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain.

ABSTRACT Two types of CTABr-silica mesophases have been prepared, one of them a powder with hexagonal structure containing Congo Red, another one a film with cubic structure containing Disperse Red 1. In the former case the dye has been occluded via co-entrapping, whereas in the latter it has been covalently anchored to the silica matrix via a co-condensation route. In both systems, the dye molecules proved to be accessible to NH3, HC1 and (CH3)3N in the gas phase, notwithstanding the presence of micelles in the mesophase, which results to be fully permeable to such molecules. Keywords: hybrid mesophase, dye, MCM-41, optical sensor INTRODUCTION Since the discovery of the surfactant-based strategy of preparation [1, 2], mesoporous materials have been studied for application in several fields. Recently, dye-containing mesoporous and mesostructured materials have attracted interest for optical applications, such as NLO and laser materials, photochromic materials and optical sensors [3, 4]. For these purposes, the as-synthesized materials still containing organic templates may be used [3]. As a recent example, Scott et al. have investigated the energy transfer between coumarin 485 and pyrrometene 567 in a mesostructured silica film [5]. Concerning sensing applications, previous work dealt with materials free from surfactant [3, 4], characterized by highly uniform porosity allowing facile diffusion of molecules. We are currently investigating the possibility of using, for sensing purposes, mesostructured systems still containing the micellar phase, because a hybrid surfactant-silica system may open new possibilities with respect to the surfactant-free material, in that the micellar phase could act as a sort of membrane, allowing the tailoring of sensor properties. Along this line, Rottman at al. [6, 7] have shown that co-entrapment of surfactants and pH indicators in sol-gel matrices leads to materials with a new pK~ with respect to the pH indicator in solution: this allows the tuning of acidity constants for several dyes. Moreover, the surfactant-silica system is prepared in a one-step synthesis, thus avoiding the surfactant removal process which may be critical for the production of materials as films or monoliths, the forms most suitable for sensing applications. For both optoelectronic and sensing applications, permeability of the micellar phase and accessibility of guest molecules from the outer environment are critical factors. In a previous paper [8] we have shown that Congo Red (CR, scheme 1) embedded in a surfactant-containing MCM-41 mesophase is accessible to HCI and NH3 from a gas phase. In the present work we extend this study to other two dye-containing mesophases: i) the same system prepared as a powder at a different pH (i.e. through a different mechanism [9]); ii) a different system prepared as a film, with the dye covalently anchored to the silica matrix [10, 11]. To these purposes CR has been embedded in SBA-3 [8], and Disperse Red 1 (DR1) has been covalently bonded to the silica matrix of a cetyltrimethylammonium bromide (CTABr)-containing mesostructured film.

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EXPERIMENTAL CR-containing SBA-3 For the synthesis of CR-containing SBA-3 (CR-SBA-3), the standard recipe for SBA-3 preparation has been followed [9], adding CR to the acidic synthesis solution before adding TEOS. All reagents were from Sigma-Aldrich, analytical grade. Bi-distilled water was from Carlo Erba. The composition of the synthesis mixture (molar ratio) was: 1 TEOS: 0.12 CTABr: 9.2 HCI: 130 H20:0.01 CR. The mixture has been stirred for 1 hour, then the product was filtered. Similarly to what observed for CR-MCM-41 [8], CR was quantitatively incorporated into the material and the filtrate was colourless. The powder was strongly coloured in blue, the colour of protonated CR, because of the low pH of the synthesis mixture. Washing with water did not lead to any dye leaching, confirming the incorporation of CR into the micelles [6, 8]. Instead, change of powder colour toward a red hue, i. e. the colour of deprotonated CR, has been observed, because of the decrease of H30 + concentration, upon washing. A CR-free SBA-3 sample has been also prepared for comparison. Powders have been dried at room temperature and characterized by means of XRD (Philips X'pert, CuK~ radiation) and Diffuse Reflectance UV-visible spectroscopy (Varian Cary 500).

DRl-containing mesostructured film Covalent bonding of DR1 to the inner surface of a mesostructured silica film has been carried out via the one-step co-condensation routte [10]. To this purpose DR1 has been prepared following the classical procedure and then modified introducing the triethoxysilane functionality according to reference 12, so obtaining the species DR1UPTEOS depicted in Scheme 2 [12]. Synthesis of the film was carried out following a procedure partially inspired by reference 13. A prehydrolysed solution was prepared by stirring for 24 h at room temperature an ethanol solution containing TEOS, DR1UPTEOS, water and HC1. A second solution obtained by dissolution of CTAB in ethanol was then added to the prehydrolysed solution, together with an additional amount of HC1 and water. Typical molar ratios in the final solution were: 1 TEOS: 20 EtOH: 4 9 10.3 HCI: 5 H20:0.1 CTAB: 1 9 1 0 -4 DR1UPTEOS. The templated film was prepared by depositing drops of the solution (after 30 minutes) on glass slides, pre-cleaned with acetone. The system was aged at 343 K for lh. The mesostructured film thus obtained (hereafter DR1-MF) was homogeneously red coloured and trasparent.

3012 Structural characterization was carried out by means of a home-made small-angle scattering reflectometer with X-Ray source (W). Optical properties have been studied by means of UV-Visible spectroscopy in the transmission mode (Varian Cary 500). R E S U L T S AND D I S C U S S I O N

Powder form Figure 1 compares XRD patterns of CR-SBA-3 and SBA-3, revealing for both the hexagonal structure of the mesophase. The (100) peak in the former case appears at a slightly larger angle (inset), from which a value of 4.26 nm calculated for the unit cell lattice parameter, which results smaller than that calculated for SBA-3 (4.35 nm). Though definitely small, this difference is worth of note, because an opposite effect has been observed with the CR-containing MCM-41, where incorporation of CR molecules in the mesophase causes an increase of the cell parameter [8], i.e. an enlargement of the hydrophobic portion of the micelle. ~CR-SBA-3 SBA-3

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Figure I. XRD patterns of CR-SBA-3 (solid curve) and SBA-3 (broken curve). Inset: magnification of the low-angles range (dl00 peak). The different synthesis pH, acidic for SBA-3 and basic for MCM-41 brings about a different mechanism in the mesophase assembly, describable in terms of direct (IS +) and mediated (I+X-S+) silica (I) and surfactant (S) interactions in MCM-41 and SBA-3, respectively (X- representing the Br-surfactant counterion) [9]. The decrease in the lattice parameter upon incorporation of CR in the case of SBA-3 mesophases may be accounted for considering the positive charge of the silica surface in this case, which most probably directly interacts with the negative sulfonate groups of the dye (see scheme 1). This causes CR molecules to locate at the surfactant-silica interface, intercalating surfactant molecules during micelle assembly. This leads to an increase of the average distance between polar heads, i. e. an enlargement of the hydrophilic area of the micelle, and consequently to a decrease in micelle diameter [14], similarly to what observed when dye molecules act as co-surfactants [15]. Figure 2a reports the UV-visible spectrum of CR-SBA-3 (broken curve): two bands are seen at 510 and 624 nm due to the n-n* transition of the chromophore in the protonated and deprotonated form, respectively.

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Figure 2. UV-visible spectra of CR-SBA-3. Section a: as such (broken curve) and after exposure to NH3 (solid curve). Section b: after exposure to NH3 (curve 1); after exposure to HC1 (curve 2); after re-exposure to NH3 (curve 3); after re-exposure to HC1 (curve 4). Exposure of the sample to gaseous NH3 yields a new spectrum (solid curve) were only the band at 510 nm is observed, indicating that all protonated CR molecules transferred a proton to ammonia. The ~nax of deprotonated CR embedded in SBA-3 is higher than that observed for CR in CR-MCM-41 (495 nm) [8]. The difference in Lmax is due to a different chromophore environment [6, 17, 18], the larger wavelenght in CR-SBA-3 being indicative of a more polar environment than in CR-MCM-41 [4, 17, 18]. This is in full agreement with the direct interaction silica-dye (I+CR-), which implies CR molecules entirely located at the hydrophilic domain of the mesophase in CR-SBA-3, at variance with the CR-MCM-41 case, where most probably sulfonate groups interact with the cationic heads of surfactant molecules. These in turn interact with the negatively charged silica wall, whereas the rest of the molecule, i.e. aromatic rings, amino and azo groups, is embedded in the micelle [8]. Subsequent exposure of CR-SBA-3 to HC1 yields an immediate change in colour from red to blue: accordingly the spectrum 2 of Figure 2b is obtained, where the intense band due to the n-n* transition of protonated CR is observed. After re-exposure to NH3, the spectrum of deprotonated CR is obtained (curve 3) and subsequent re-exposure to HC1 yields again the spectrum of protonated CR (curve 4). This cycle may be repeated several times. It is concluded that, similarly to what observed for CR-MCM-41, CR in CR-SBA-3 may be reached by NH3 and HC1 molecules, showing that the mesophase is permeable to these molecules in gas phase. The same experiment has been carried out with trimethylamine and corresponding spectra are reported in Figure 3: CR molecules are fully accessible also to amine molecules.

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Film form Figure 4 reports the low-angle X-ray scattering pattern of DR I-MF: three main peaks are observed (labelled with asterisks), indicating the ordered structure of the mesophase. The first two peaks suggest a cubic structure, Im3m being the most probable space group [ 19, 20]. I

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Figure 5. UV visible spectra of DR1-MF. Section a, curve 1" as such; curve 2" after exposure to HC1; curve 3: after exposure to NH3. Upon exposure of the film to gaseous HC1 the band is red-shifted and becomes narrower and more intense (curve 2), as a consequence of protonation at the azo group [17, 23]. The band becomes also more structured, probably because of a modification in the structure of vibrational sub-bands [24]. After exposure to ammonia, the original spectrum is restored (curve 3). Again, this cycle may be repeated several times. The same was observed by using trymethylamine instead of ammonia. As before, the above data prove that also when the chromophore is anchored to the silica matrix of a surfactant-containing film, it is accessible to molecules in gas phase from the outer environment. Note however that, for device manufacturing, thin film systems with anchored sensing molecules offer advantages over dispersed powders, because of enhanced stability and handling facilities. CONCLUSIONS CTABr-silica ordered mesophase prepared at an acidic pH in both powder and film forms are permeable to HC1, NH3 and trymethylamine from a gas phase, as witnessed by the accessibility of occluded dyes (CR and DR1) to these molecules. In the case of DR1, the accessibility has been proved for the dye covalently bonded to the silica matrix. The results suggest the possibility of using such systems for preparing optical chemical sensors, taking advantage of the micellar phase to tailor sensors properties. On the other hand, the observed permeability is a feature which has to be taken into account when considering the potential use of dye-doped mesosotructured films for optical devices, such as dye lasers, optical switching or non linear optical applications in more general terms.

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