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Accessibility of dye molecules embedded in surfactant-silica hybrid materials in both powder and film forms
Sensors and Actuators B 100 (2004) 107–111
Accessibility of dye molecules embedded in surfactant-silica hybrid materials in both powder and film forms L. Borello a , B. Onida a , C. Barolo b , K.J. Edler c , C. Otero Areán d , E. Garrone a,∗ a
Dipartimento di Scienze dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129 Torino, Italy Dipartimento di Chimica Generale ed Organica Applicata, Università di Torino Corso Massimo d’Azeglio, 48-10125 Torino, Italy c Department of Chemistry, University of Bath, Bath BA2 7AY, UK d Departamento de Qu´ımica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain
b
Available online 21 February 2004
Abstract Two CTABr-silica mesophases have been prepared, one of them as a powder with hexagonal structure containing Congo Red, the other one as 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 and HCl in gas phase, notwithstanding the presence of micelles in the mesophase, which results to be fully permeable to such molecules. © 2004 Elsevier B.V. All rights reserved. Keywords: Ordered mesophase; Dye; Hybrid material
1. 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 the former purposes, the as-synthesized materials still containing organic templates may be used, showing improved characteristics with respect to sol–gel glasses [3]. 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. In fact, 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 et al. [5,6] have shown that co-entrapment of surfactants and pH indicators in sol–gel matrices leads to material with a new pKi with respect to the pH indicator in solution, allowing the tuning of acidity constants for several dyes. ∗
Corresponding author. Tel.: +39-011-5644661; fax: +39-011-5644699. E-mail address: [email protected] (E. Garrone). 0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.12.030
Moreover, the surfactant-silica system is prepared in a one-step synthesis, avoiding the surfactant removal process which may be a critical step for the production of materials as films or monoliths, i.e. in forms suitable for sensing application. The permeability to gases of hybrid surfactant-silica materials is vital in view of this application. In a previous paper [7] we have shown that Congo Red (CR, Scheme 1) embedded in a surfactant-containing MCM-41 mesophase is accessible to HCl and NH3 in the gas phase. In the present work we extend this study to two dyecontaining mesophases—(i) a powder formed at different pH (i.e. through a different mechanism [8]); (ii) a film, the form most suitable for applications, with the dye covalently anchored to the silica matrix [9,10]. To these purposes CR has been embedded in SBA-3 [7], and Disperse Red 1 (DR1) has been covalently bonded to the silica matrix of a cetyltrimethylammonium bromide (CTABr)-containing mesostructured film. 2. Experimental 2.1. 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 [8], adding CR to the acidic synthesis solution before adding TEOS. All reagents were from Sigma-Aldrich,
L. Borello et al. / Sensors and Actuators B 100 (2004) 107–111
SO3 Na
Na O3S N
N
N
N
NH2
H 2N Scheme 1.
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 HCl:130 H2 O:0.01 CR. The mixture has been stirred for 1 h, then the product was filtered. Similarly to what observed for CR-MCM-41 [7], CR was quantitatively incorporated in 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 [5,7]. Instead, change of powder colour toward a red hue, i. e. the colour of deprotonated CR, has been observed, because of the decrease of H3 O+ concentration upon washing. The powder has been dried at room temperature and then characterized by means of XRD (Philips X’pert, Cu K␣ radiation), Diffuse Reflectance UV-Vis spectroscopy (Varian Cary 500) and FTIR spectroscopy (Bruker Equinox 55, equipped with MCT detector). 2.2. DR1-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 route [9]. To this purpose DR1 has been prepared following the classical procedure and then modified introducing the triethoxysilane functionality according to reference [11], thus obtaining the species depicted in Scheme 2, named DR1UPTEOS [11]. Synthesis of the film was carried out following a procedure partially inspired by reference [12]. A prehydrolysed solution was prepared by stirring for 24 h at room temperature an ethanol solution containing TEOS, DR1UPTEOS, water and HCl. A second solution obtained by dissolution of CTAB in ethanol was then added to the prehydrolysed solution, together with an additional amount of HCl and water. Typical molar ratios in the final solution were—1 TEOS:20 EtOH:4 × 10−3 HCl:5 H2 O:0.1 CTAB:1 × 10−4 DR1UPTEOS. The templated film was prepared by de-
positing drops of the solution (after 30 min) on glass slides, pre-cleaned with acetone. The system was aged at 343 K for 1 h. The mesostructured film thus obtained (hereafter DR1-MF) was homogeneously red coloured and trasparent. 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-Vis spectroscopy in the transmission mode (Varian Cary 500).
3. Results and discussion 3.1. CR-SBA-3 Fig. 1 shows the XRD pattern of CR-SBA-3, revealing the hexagonal structure of the mesophase. A d100 value of 3.75 nm was calculated, which corresponds to a unit cell parameter of 4.33 nm. Fig. 2a reports the corresponding UV-Vis spectrum (broken curve)—two bands are seen at 510 and 624 nm due to the –∗ transition of the chromophore in the protonated and deprotonated form, respectively. 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 λmax of deprotonated CR embedded in SBA-3 is higher than that observed for CR in CR-MCM-41 (495 nm) [7]. The two mesophases differ for the synthesis pH, acidic for SBA-3 and basic for MCM-41 (accordingly, CR-MCM-41 is red [7]). The different pH brings about a different mechanism in the mesophase assembly, describable in terms of direct (I− S+ ) and mediated (I+ X− S+ ) silica (I) and surfactant (S) interactions in MCM-41 and SBA-3, respectively, (X− representing the surfactant counterion, i.e. Br− in the present case) [8].
Intensity
108
O N O
Si(OEt) 3
N N
N
H N O O
Scheme 2.
2
4
6
2θ Fig. 1. XRD pattern of CR-SBA-3.
8
10
L. Borello et al. / Sensors and Actuators B 100 (2004) 107–111
109
2.5
2.0
3 4
1 2.0
2
Kubelka-Munk
Kubelka-Munk
1.5
1.0
1.5
1.0
0.5
0.5
0.0 0.0 400
(a)
500
600
700
800
400
900
wavelength (nm)
500
600
700
800
900
wavelength (nm)
(b)
Fig. 2. UV-Vis spectra of CR-SBA-3. (a) As such (broken curve) and after exposure to NH3 (solid curve). (b) After exposure to NH3 (curve 1); after exposure to HCl (curve 2); after re-exposure to NH3 (curve 3); after re-exposure to HCl (curve 4).
CR-SBA-3 may be reached by NH3 and HCl molecules, showing that the mesophase is permeable to these molecules in gas phase. 3.2. DR1-MF Fig. 3 reports the low-angle scattering pattern of DR1-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 [16,17]. 1
0.1
Counts
A difference in λmax may be due to a different state of chromophore aggregation [13], as well as to a different chromophore environment [5,14,15]. Indeed, a shoulder above 500 nm, present in the spectrum of CR-MCM-41, has been previously tentatively ascribed [8] to some amount of ordered dye aggregates, in the head-to-tail stacking arrangement (J dimers) [13]. On this basis, it could be inferred that a higher amount of ordered aggregates are present in CR-SBA-3 than in CR-MCM-41. On the other hand, the shorter wavelength in CR-MCM-41 may be due to a less polar environment for the chromophore than in the present case [4,14,15]. Considering that the presence of anionic sulphonic groups causes at least this part of the molecule to sit at the hydrophilic region of the mesophase, i.e. the silica-surfactant interface, differences in the structure of this region, i.e. I+ X− S+ in contrast to I− S+ , may cause different polarity of the chromophore environment. At the moment it is not possible to assess which is the cause of the differences of CR optical properties in the two mesophases; further studies are in progress. Subsequent exposure of CR-SBA-3 to HCl yields an immediate change in colour from red to blue: accordingly the spectrum 2 of Fig. 2b is obtained, where the intense band due to the –∗ 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 HCl 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
* 0.01
*
1E-3
*
1E-4
0.1
0.2
0.3
0.4
0.5
-1
Q (A ) Fig. 3. Small-angle scattering pattern of DR1-MF.
0.6
110
L. Borello et al. / Sensors and Actuators B 100 (2004) 107–111
1
case of DR1, the accessibility has been proved for the dye covalently bonded to the silica matrix. The results suggest the possibility of using these systems to prepare optical chemical sensors, taking advantage of the micellar phase to tailor sensors properties [5,6]. 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.
3
References
2
Kubelka-Munk
0.015
0.010
0.005
0.000 350
400
450
500
550
600
650
Wavelength (nm) Fig. 4. UV visible spectra of DR1-MF. Curve 1, as such; curve 2, after exposure to HCl; curve 3, after exposure to NH3 .
Fig. 4 shows the UV-Vis spectrum of DR1-MF (curve 1). A single nearly symmetric band at 512 nm is observed, due to the –∗ charge transfer electronic transition typical of the push–pull chromophore. No evidence of significant aggregation is visible [18,19], in agreement with the low concentration of dye in the system. Upon exposure of the film to gaseous HCl the band is red-shifted and becomes narrower and more intense (curve 2), as a consequence of protonation at the azo group [14,20]. The band becomes also more structured, probably because of a modification in the structure of vibrational sub-bands [21]. After exposure to ammonia, the original spectrum is restored (curve 3). Again, this cycle may be repeated several times. As before, the above data prove that also when the chromophore is anchored to the silica matrix of a surfactantcontaining film, it is accessible to molecules in gas phase from the outer environment. Note however that, for device manufacturing, thin film systems having anchored sensing molecules offer advantages over disperse powders, because of enhanced stability and handling facilities.
4. Conclusions CTABr-silica ordered mesophases prepared at acidic pH in both powder and film forms are permeable to HCl and NH3 in the gas phase, as witnessed by the accessibility of occluded dyes (CR and DR1) to these molecules. In the
[1] T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, The preparation of alkyltrimethylammonium-kamenite complexes and their conversion to microporous materials, Bull. Chem. Soc. Jpn. 63 (1990) 988. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Ordered mesoporous moleculars sieves synthesized by a liquid-crystal template mechanism, Nature 359 (1992) 710. [3] B.J. Scott, G. Wirnsberger, G.D. Stucky, Mesoporous and mesostructured materials for optical applications, Chem. Mater. 13 (2001) 3140, and references therein. [4] G. Schulz-Ekloff, D. Wöhrle, B. van Duffel, R.A. Schoonheydt, Chromophores in porous silica and minerals: preparation and optical properties, Microporous Mesoporous Mater. 51 (2002) 91, and references therein. [5] C. Rottman, G. Grader, Y. De Hazan, S. Melchior, D. Avnir, Surfactant-induced modification of dopants reactivity in sol–gel matrixes, J. Am. Chem. Soc. 121 (1999) 8533. [6] C. Rottman, D. Avnir, Getting a library of activities from a single compound: tunability and very large shifts in acidity constants induced by sol–gel entrapped micelles, J. Am. Chem. Soc. 123 (2001) 5730. [7] B. Onida, B. Bonelli, L. Flora, F. Geobaldo, C. Otero Arean, E. Garrone, Permeability of micelles in surfactant-containing MCM-41 silica as monitored by embedded dye molecules, Chem. Commun. (2001) 2216. [8] Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schuth, G.D. Stucky, Generalized synthesis of periodic surfactant/inorganic composite materials, Nature 368 (1994) 317. [9] D.J. Macquarrie, Direct preparation of organically modified MCMtype materials. Preparation and characterization of aminopropylMCM and 2-cyanoethyli-MCM, Chem. Commun. (1996) 1961. [10] C.E. Fowler, B. Lebeau, S. Mann, Covalent coupling of an organic chomophore into functionalized MCM-41 mesophase by templatedirected co-condensation, Chem. Commun. (1998) 1825. [11] F. Chaput, D. Riehl, Y. Lévy, J.P. Boilot, Azo oxide gels for optical storage, Chem. Mater. 5 (1993) 589. [12] D. Grosso, F. Babonneau, P.-A. Albouy, H. Amenitsch, A.R. Balkenende, A. Brunet-Bruneau, J. Rivory, An in situ study of mesostructured CTAB-silica film formation during dip coating using time resolved SAXS and interferometry measurements, Chem. Mater. 14 (2002) 931. [13] T.M. Cooper, M.O. Stone, Investigation of self-assembly upon formation of an electrostatic complex of Congo Red and a helical peptide, Langmuir 14 (1998) 6662. [14] J. Griffiths, Colour and Constitution of Organic Molecules, Academic Press, London, 1976. [15] R.A. Edwards, R.W. Woody, Molecular orbital calculations of the optical properties of Congo Red and Cibacron Blue and their complexes with proteins, J. Phys. Chem. 87 (1983) 1329.
L. Borello et al. / Sensors and Actuators B 100 (2004) 107–111 [16] C.H. Macgillaury, E.G.R. Rieck (Eds.), International Tables for Xray Crystallography, vol. III, Kynock Press, Birmingham, England, 1968. [17] D. Zhao, P. Yang, N. Melosh, Feng, B.F. Chmelka, G.D. Stucky, Continous mesoporous silica film with highly ordered large pore structures, Adv. Mat. 10 (1998) 1380. [18] D. Riehl, F. Chaput, Y. Lévy, J.-P. Boilot, F. Kajzar, P.-A. Chollet, Second-order optical nonlinearities of azo chromophores covalently attached to a sol–gel matrix, Chem. Phys. Lett. 245 (1995) 36.
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[19] I.G. Marino, D. Bersani, P.P. Lottici, Holographic gratings in DR1doped sol–gel silica and ORMOSILs thin Film, Opt. Mater. 15 (2001) 279. [20] H. Zollinger, Syntheses, properties and applications of organic dyes and pigments, Color Chemistry, VCH, Weinheim, 1991, pp. 130– 137. [21] T. Stoyanova, S. Stoyanov, L. Antonov, V. Petrova, Ammonium– azonium tautomerism in N,N-dialchylaminoazo dyes. Part I: general considerations, Dyes Pigments 31 (1996) 1.
Accessibility of dye molecules embedded in surfactant-silica hybrid materials in both powder and film forms L. Borello a , B. Onida a , C. Barolo b , K.J. Edler c , C. Otero Areán d , E. Garrone a,∗ a
Dipartimento di Scienze dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129 Torino, Italy Dipartimento di Chimica Generale ed Organica Applicata, Università di Torino Corso Massimo d’Azeglio, 48-10125 Torino, Italy c Department of Chemistry, University of Bath, Bath BA2 7AY, UK d Departamento de Qu´ımica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain
b
Available online 21 February 2004
Abstract Two CTABr-silica mesophases have been prepared, one of them as a powder with hexagonal structure containing Congo Red, the other one as 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 and HCl in gas phase, notwithstanding the presence of micelles in the mesophase, which results to be fully permeable to such molecules. © 2004 Elsevier B.V. All rights reserved. Keywords: Ordered mesophase; Dye; Hybrid material
1. 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 the former purposes, the as-synthesized materials still containing organic templates may be used, showing improved characteristics with respect to sol–gel glasses [3]. 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. In fact, 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 et al. [5,6] have shown that co-entrapment of surfactants and pH indicators in sol–gel matrices leads to material with a new pKi with respect to the pH indicator in solution, allowing the tuning of acidity constants for several dyes. ∗
Corresponding author. Tel.: +39-011-5644661; fax: +39-011-5644699. E-mail address: [email protected] (E. Garrone). 0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.12.030
Moreover, the surfactant-silica system is prepared in a one-step synthesis, avoiding the surfactant removal process which may be a critical step for the production of materials as films or monoliths, i.e. in forms suitable for sensing application. The permeability to gases of hybrid surfactant-silica materials is vital in view of this application. In a previous paper [7] we have shown that Congo Red (CR, Scheme 1) embedded in a surfactant-containing MCM-41 mesophase is accessible to HCl and NH3 in the gas phase. In the present work we extend this study to two dyecontaining mesophases—(i) a powder formed at different pH (i.e. through a different mechanism [8]); (ii) a film, the form most suitable for applications, with the dye covalently anchored to the silica matrix [9,10]. To these purposes CR has been embedded in SBA-3 [7], and Disperse Red 1 (DR1) has been covalently bonded to the silica matrix of a cetyltrimethylammonium bromide (CTABr)-containing mesostructured film. 2. Experimental 2.1. 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 [8], adding CR to the acidic synthesis solution before adding TEOS. All reagents were from Sigma-Aldrich,
L. Borello et al. / Sensors and Actuators B 100 (2004) 107–111
SO3 Na
Na O3S N
N
N
N
NH2
H 2N Scheme 1.
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 HCl:130 H2 O:0.01 CR. The mixture has been stirred for 1 h, then the product was filtered. Similarly to what observed for CR-MCM-41 [7], CR was quantitatively incorporated in 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 [5,7]. Instead, change of powder colour toward a red hue, i. e. the colour of deprotonated CR, has been observed, because of the decrease of H3 O+ concentration upon washing. The powder has been dried at room temperature and then characterized by means of XRD (Philips X’pert, Cu K␣ radiation), Diffuse Reflectance UV-Vis spectroscopy (Varian Cary 500) and FTIR spectroscopy (Bruker Equinox 55, equipped with MCT detector). 2.2. DR1-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 route [9]. To this purpose DR1 has been prepared following the classical procedure and then modified introducing the triethoxysilane functionality according to reference [11], thus obtaining the species depicted in Scheme 2, named DR1UPTEOS [11]. Synthesis of the film was carried out following a procedure partially inspired by reference [12]. A prehydrolysed solution was prepared by stirring for 24 h at room temperature an ethanol solution containing TEOS, DR1UPTEOS, water and HCl. A second solution obtained by dissolution of CTAB in ethanol was then added to the prehydrolysed solution, together with an additional amount of HCl and water. Typical molar ratios in the final solution were—1 TEOS:20 EtOH:4 × 10−3 HCl:5 H2 O:0.1 CTAB:1 × 10−4 DR1UPTEOS. The templated film was prepared by de-
positing drops of the solution (after 30 min) on glass slides, pre-cleaned with acetone. The system was aged at 343 K for 1 h. The mesostructured film thus obtained (hereafter DR1-MF) was homogeneously red coloured and trasparent. 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-Vis spectroscopy in the transmission mode (Varian Cary 500).
3. Results and discussion 3.1. CR-SBA-3 Fig. 1 shows the XRD pattern of CR-SBA-3, revealing the hexagonal structure of the mesophase. A d100 value of 3.75 nm was calculated, which corresponds to a unit cell parameter of 4.33 nm. Fig. 2a reports the corresponding UV-Vis spectrum (broken curve)—two bands are seen at 510 and 624 nm due to the –∗ transition of the chromophore in the protonated and deprotonated form, respectively. 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 λmax of deprotonated CR embedded in SBA-3 is higher than that observed for CR in CR-MCM-41 (495 nm) [7]. The two mesophases differ for the synthesis pH, acidic for SBA-3 and basic for MCM-41 (accordingly, CR-MCM-41 is red [7]). The different pH brings about a different mechanism in the mesophase assembly, describable in terms of direct (I− S+ ) and mediated (I+ X− S+ ) silica (I) and surfactant (S) interactions in MCM-41 and SBA-3, respectively, (X− representing the surfactant counterion, i.e. Br− in the present case) [8].
Intensity
108
O N O
Si(OEt) 3
N N
N
H N O O
Scheme 2.
2
4
6
2θ Fig. 1. XRD pattern of CR-SBA-3.
8
10
L. Borello et al. / Sensors and Actuators B 100 (2004) 107–111
109
2.5
2.0
3 4
1 2.0
2
Kubelka-Munk
Kubelka-Munk
1.5
1.0
1.5
1.0
0.5
0.5
0.0 0.0 400
(a)
500
600
700
800
400
900
wavelength (nm)
500
600
700
800
900
wavelength (nm)
(b)
Fig. 2. UV-Vis spectra of CR-SBA-3. (a) As such (broken curve) and after exposure to NH3 (solid curve). (b) After exposure to NH3 (curve 1); after exposure to HCl (curve 2); after re-exposure to NH3 (curve 3); after re-exposure to HCl (curve 4).
CR-SBA-3 may be reached by NH3 and HCl molecules, showing that the mesophase is permeable to these molecules in gas phase. 3.2. DR1-MF Fig. 3 reports the low-angle scattering pattern of DR1-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 [16,17]. 1
0.1
Counts
A difference in λmax may be due to a different state of chromophore aggregation [13], as well as to a different chromophore environment [5,14,15]. Indeed, a shoulder above 500 nm, present in the spectrum of CR-MCM-41, has been previously tentatively ascribed [8] to some amount of ordered dye aggregates, in the head-to-tail stacking arrangement (J dimers) [13]. On this basis, it could be inferred that a higher amount of ordered aggregates are present in CR-SBA-3 than in CR-MCM-41. On the other hand, the shorter wavelength in CR-MCM-41 may be due to a less polar environment for the chromophore than in the present case [4,14,15]. Considering that the presence of anionic sulphonic groups causes at least this part of the molecule to sit at the hydrophilic region of the mesophase, i.e. the silica-surfactant interface, differences in the structure of this region, i.e. I+ X− S+ in contrast to I− S+ , may cause different polarity of the chromophore environment. At the moment it is not possible to assess which is the cause of the differences of CR optical properties in the two mesophases; further studies are in progress. Subsequent exposure of CR-SBA-3 to HCl yields an immediate change in colour from red to blue: accordingly the spectrum 2 of Fig. 2b is obtained, where the intense band due to the –∗ 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 HCl 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
* 0.01
*
1E-3
*
1E-4
0.1
0.2
0.3
0.4
0.5
-1
Q (A ) Fig. 3. Small-angle scattering pattern of DR1-MF.
0.6
110
L. Borello et al. / Sensors and Actuators B 100 (2004) 107–111
1
case of DR1, the accessibility has been proved for the dye covalently bonded to the silica matrix. The results suggest the possibility of using these systems to prepare optical chemical sensors, taking advantage of the micellar phase to tailor sensors properties [5,6]. 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.
3
References
2
Kubelka-Munk
0.015
0.010
0.005
0.000 350
400
450
500
550
600
650
Wavelength (nm) Fig. 4. UV visible spectra of DR1-MF. Curve 1, as such; curve 2, after exposure to HCl; curve 3, after exposure to NH3 .
Fig. 4 shows the UV-Vis spectrum of DR1-MF (curve 1). A single nearly symmetric band at 512 nm is observed, due to the –∗ charge transfer electronic transition typical of the push–pull chromophore. No evidence of significant aggregation is visible [18,19], in agreement with the low concentration of dye in the system. Upon exposure of the film to gaseous HCl the band is red-shifted and becomes narrower and more intense (curve 2), as a consequence of protonation at the azo group [14,20]. The band becomes also more structured, probably because of a modification in the structure of vibrational sub-bands [21]. After exposure to ammonia, the original spectrum is restored (curve 3). Again, this cycle may be repeated several times. As before, the above data prove that also when the chromophore is anchored to the silica matrix of a surfactantcontaining film, it is accessible to molecules in gas phase from the outer environment. Note however that, for device manufacturing, thin film systems having anchored sensing molecules offer advantages over disperse powders, because of enhanced stability and handling facilities.
4. Conclusions CTABr-silica ordered mesophases prepared at acidic pH in both powder and film forms are permeable to HCl and NH3 in the gas phase, as witnessed by the accessibility of occluded dyes (CR and DR1) to these molecules. In the
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