Preparation and Optimization of Fluorescent Thin Films of Rosamine-SiO2/TiO2 Composites for NO2 Sensing

materials Article Preparation and Optimization of Fluorescent Thin Films of Rosamine-SiO2/TiO2Composites for NO2Sensing María G. Guillén 1, Francisco Gámez 1, Belén Suárez 1, Carla Queirós 2, Ana M. G. Silva 2, Ángel Barranco 3, Juan Ramón Sánchez-Valencia 3, José María Pedrosa 1 and Tânia Lopes-Costa 1,∗ 1 Departmento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Sevilla 41013, Spain; [email protected] (M.G.G.); [email protected] (F.G.); [email protected] (B.S.); [email protected] (J.M.P.) 2REQUIMTE-LAQV, UCIBIO Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, R. Campo Alegre, Porto 4169-007, Portugal; [email protected] (C.Q.); [email protected] (A.M.G.S.) 3Instituto de Ciencia de Materiales de Sevilla, Universidad de Sevilla-CSIC, Américo Vespucio 49, Sevilla 41092, Spain; [email protected] (Á.B.); [email protected] (J.R.S.-V.) *Correspondence: [email protected]; Tel.: +34-954-977-363 Academic Editor: Shlomo Berger Received: 7 December 2016; Accepted: 23 January 2017; Published: 31 January 2017 Abstract: The incorporation of a prototypical rosamine fluorescent dye from organic solutions into transparent and microstructured columnar TiO 2 and SiO 2 (MO 2 ) thin films, prepared by evaporation at glancing angles (GAPVD), was evaluated. The aggregation of the adsorbed molecules, the infiltration efficiency and the adsorption kinetics were studied by means of UV-Vis absorption and fluorescence spectroscopies. Specifically, the infiltration equilibrium as well as the kinetic of adsorption of the emitting dye has been described by a Langmuir type adsorption isotherm and a pseudo second order kinetic model, respectively. The anchoring mechanism of the rosamine to the MO 2 matrix has been revealed by specular reflectance Fourier transform infrared spectroscopy and infiltration from aqueous solutions at different pH values. Finally, the sensing performance towards NO 2 gas of optimized films has been assessed by following the changes of its fluorescence intensity revealing that the so-selected device exhibited improved sensing response compared to similar hybrid films reported in the literature. Keywords: rosamine dyes; porous columnar semiconductors; gas sensors 1. Introduction Xanthene dyes comprise a set of molecular architectures that possess particularly efficient absorptive and fluorescence properties. In combination with specific singularities acquired upon selective functionalization of the xanthene moiety, their applications have been progressively extended among a large landscape of technological areas [ 1 – 6 ]. For instance, the sensitizing effect provoked by the environment in rhodamine fluorophores has been widely employed as part of hybrid materials in fields as dye sensitized solar cells or chemosensing [ 7 – 11 ]. Moreover, their intense fluorescence emission allows their use as fluorescent probes, laser dyes, pigments, fluorescent standards or detection of reactive organic species [ 12 – 15 ]. These molecules are also widely used in biotechnology—for example, in the study of the membranes fluidity, the structure and dynamics of micelles, imaging in cells, and also in sensors for the detection of mercury, copper, iron and chromium in cells [16–19]. Materials 2017,10, 124; doi:10.3390/ma10020124 www.mdpi.com/journal/materials Materials 2017,10, 124 2 of 15 Although for some of these applications the dye is used in solution, most of them require this fluorescent probe to be anchored to a solid surface [ 20 – 26 ]. From a molecular viewpoint, establishing optimal values for the dye concentration, pH and solvent will determine the dominant chemical forms in the equilibrium, where cationic, zwitterionic, covalent dimers and π - π aggregates can exist. However, despite these cautions, when transferring to a solid matrix, the external potential represented by the solid walls in its pores makes the dyes even more prone to aggregation. This aggregation-inductive effect strongly challenges some practical goals by masking their solution-optimized emission properties when transferred to a rigid substrate. In this direction, materials science put at our disposal a wide plethora of functionalized substrates with predefined compositions and micro and nanostructures that present the adequate optical and electronic properties for very specific applications [ 27 – 38 ]. Remarkably, physical vapor deposition (PVD) methods allow us a very subtle and independent control of the structural and morphological parameters of the films, becoming an appropriate choice for the fabrication of hybrid materials [ 39 ]. Specifically, glancing angle physical vapor deposition (GAPVD) generates mainly columnar structures with micro and mesostructured porous surfaces [ 40 – 43 ] very suitable for the infiltration and incorporation of active molecules into photonic devices. Additionally, since the core of these rhodamine dyes is constituted by a planar xanthene ring 3,6-disubstituted by in-plane amino groups and a nearby perpendicular 2 0 -carboxyphenyl ring in position 9, a pH-dependent behavior of the optical properties emerge as a consequence of the equilibrium between a fluorescent open ring and a non-fluorescent spirolactone forms [ 44 ]. Hence, in some circumstances, this feature goes hand-in-hand with the steric hindrance of the ring that preserves the anchoring to the substrate by the carboxylic substituent. Transposition of the carboxylic group from the 2 0 to the 4 0 positions suppresses these effects to a large extent. The resulting ensemble of molecules obtained within this positional change is denominated rosamine and its optimized synthetic procedure has been reported elsewhere [ 45 ]. The similarities between rhodamines and rosamines make them candidates for similar applications avoiding the mentioned chemical challenges. An innovative aspect of the present work is the coupling of both aspects by incorporating the rosamine shown in Figure 1into non-dispersive TiO 2 and SiO 2 thin films prepared by GAPVD as hosting matrices. Taking into account the chemical structure of this rosamine, it can be anchored to the MO 2 surface by electrostatic interaction between its ammonium group and the negatively charged surface of the oxide, or by chemical binding through its carboxylic group (well known in the field of dye sensitized solar cells). The spectroscopic properties of the films and the thermodynamics and kinetics of the dye incorporation process were evaluated. Their fluorescence response will be finally tested for the case of the best-behaved composite against the toxic NO 2 gas which constitutes a very dangerous air pollutant for both environmental and human health [46]. Figure 1. Schematic structure of the rosamine B molecular ion. Materials 2017,10, 124 3 of 15 2. Results and discussion 2.1. Spectroscopic Characterization of RosB/MO2Composites: Molecular Aggregation The RosB absorption spectrum in CH 2 Cl 2 is constituted by a monomer band at 564 nm and a vibronic shoulder (traditionally called dimer band) at 527 nm (see Figure S1 in the Supplementary Material Section). The difference in the monomer-dimer absorbance peaks is consistent with their rhodamine fluorophore analogue [ 47 ]. Prior to further measurements, the use of CH 2 Cl 2 is justified here because, besides presenting a positive solvatochromic effect that is not of interest for this work, it disfavors the aggregation with respect to aqueous solutions, mainly due to the smaller dielectric sandwiching effect and the absence of intermolecular hydrogen bonding in the former solvent. This fact is clearly shown in Figure S1 in the Supplementary Material section, where the absorbance spectra of a diluted solution of RosB in water and in organic CH2Cl2solvents are compared. The higher value of the aggregate/monomer ratio in the aqueous solution is apparent as expected. However, considering that the shape of the excitation spectra was always found to be very similar to that of their absorption spectra, this fact has been understood as the existence of a single type of molecules/aggregates with a spectrum characterized by two bands. These bands occur at shorter and longer wavelengths in relation with the maximum absorption of the monomer and are labeled as H and J bands, respectively [ 10 ]. An additional interpretation to this phenomenon can be mentioned: the aggregates can produce a similar emission to that of the monomers. Regardless of the specific choice for the interpretation of the data, changes in the absorption profile can be unequivocally attributed to different ensembles of supramolecular arrangements. Hence, we will keep the classical first order exciton theory (ET) for the explanation of the experimental evidence. In this theory, these bands are attributed to the absorption of the dyes in dimeric forms with different mutual orientations labeled also as H and J (or Scheibe) dimers. These preformed aggregates play an important role in the emission properties of the supported composite. It is reported that dimers constitute efficient non radiative quenchers of the fluorescence by an internal conversion from charged-transfer states in their fundamental ground to excited singlet followed by an intersystem crossing to the charge-transfer triplet states [ 48 ]. In the way the preformed aggregates (mostly dimers) contribute to the total quenching upon deposition, it will be a factor to take into account in the choice of the optimum conditions for the highly emitting devices stated in the title. Although the Stokes shift (i.e., the difference between the maximum wavelengths of absorbance and fluorescence) clearly increases with RosB concentration (see Figure S2), no appreciable shape changes appear in the absorption spectrum of RosB in CH 2 Cl 2 within the concentration range explored in this study. This fact ascribed to reabsorption phenomena has been described in dyes with overlapping emission/absorption spectra due to the absence of dipole moment parallel to the long axis [ 49 ]. In any case, the quenching of the fluorescence emission with the dye concentration shows a well-defined behavior that could be visualized by means of the ratio between the fluorescence intensity and the absorbance at the excitation wavelength (fluorescence efficiency) as shown in Figure S3. In view of the comments that we have given above, this diminution in solution cannot be associated to an enhancement of the aggregation but to reabsorption. In Figure 2, the absorption spectra of the RosB infiltrated in MO 2 films at an infiltration time of 24 h from different bulk concentrations is compared with the spectra of the CH 2 Cl 2 solution at 3.5 µ M for the sake of visual facilitation, considering that no appreciable aggregation contribution to the total amount of molecules in organic solution was observed. A general aspect of the adsorbed rosamine spectrum is that the absorption bands are broadened because of the interaction with the inorganic matrix and the formation of H aggregates as revealed by the progressive growth of the shoulder at ∼ 527 nm with the concentration of the RosB in the infiltrating solution. The stacking of RosB into non-covalent complexes is enhanced in TiO 2 as observed in the comparison of both panels of Figure 2as a consequence of the influence of the environment micropolarity and microacidity. This occurrence can be attributed to the higher acidic character of the surface hydroxyl groups in Materials 2017,10, 124 4 of 15 silanol with respect to titanol and to the charge compensation process that is the leading contributing effect to self-aggregation. Given the similarities with rhodamine B, rosamine B should own a pK a value of ∼ 3 [ 50 ]. A zwitterionic form is then expected in this solvent allowing for a oneor two-sided anchoring to the surface charges of the matrices. Nevertheless, the point of zero charge ( pzc ) differs from ∼ 2.4 for SiO 2 to ∼ 5.5 for TiO 2 [ 43 , 51 ], and, hence, the charge balance between MO 2 and the zwitterionic form of RosB would be differently equilibrated in the matrices, and the anchoring will also be unlike between them. This aspect will be discussed below by means of vibrational spectroscopy and infiltration of the matrices with aqueous solutions at different pH values. S i O 2 107.0 µM 7 1 .7 µM 3 5 .8 µM 1 7 .9 µM 1 2 .5 µM 3 .6 µ M W a v e l e n g t h ( n m ) S o l u t i o n 450 500 550 600 650 700 A b s o r b a n c e ( s c a l e d ) T i O 2 Figure 2. Absorption spectra for composite RosB/MO 2 thin films prepared by infiltration of CH 2 Cl 2 solutions at different concentrations. The infiltration time was 24 h in all cases. The corresponding spectrum in solution is plotted as dashed lines for the sake of comparison. In the limit of the diluted dye concentration, a broad distribution at longer wavelengths clearly emerges upon adsorption, which, according to ET, can be attributed to head-to-tail J -aggregates. The relative monomer/oligomer population decreases when the concentration increases (metachromatic effect) and also broadens and shifts to shorter wavelengths, which, provided π interactions with the free electrons of oxygen atoms in the surface are of minor importance in non-doped oxides [ 52 ], is indicative of an augmentation of the aggregation number of non-fluorescent sandwich-like H-aggregates. Additionally, the blueshift of the monomer band in SiO 2 (547 nm) is indicative of a more basic character of the environment created by the residual silanol groups and non-interacting amino groups within the pores of the amorphous silica columns with respect to the CH 2 Cl 2 solution [ 53 ]. Remarkably, this band seems to have acquired a more structured shape with a new band at 555 nm. We tentatively assigned this feature to the existence of micro and mesoporous in the nanocolumns that will be detailed further along in this discussion. A plausible contribution to the splitting in the energy levels of the protonated and zwitterionic forms of the dye that are usually ill-resolved should also be mentioned [ 54 ]. H -dimerization is less accentuated, and a smaller aggregation number can be inferred from the nearly constant frequency of the dimer band. Therefore, a priori, rosamine infiltrated in SiO 2 will provide better results in relation with its photoluminescence efficiency properties despite it throwing smaller absorbance intensity in comparison with TiO 2 because the rosamine load is also smaller in these samples. These aspects are treated in detail in the following. Materials 2017,10, 124 5 of 15 2.2. Spectroscopic Characterization of RosB/MO2Composites: Adsorption Equilibrium and Kinetics Adsorption isotherms have been derived from the above results. The absorption band is integrated as a measure of the total amount of RosB molecules attached to MO 2 , i.e., the molecular surface concentration Γ [ 55 ]. The experiments were carried out by infiltrating RosB into the films during 24 h for a set of dye solutions with different concentrations as shown in Figure 3. The shape of the measured data indicates a high increment for small RosB concentrations to reach a high value at a solution concentration of around 40 µ M and saturation is reached for more concentrated solutions. The so-obtained data are fitted to a Langmuir function of the form: A=CLb[RosB] 1+b[RosB], (1) where A represents the integrated absorption band. The additional CL and b parameters stand for the adsorption capacity and affinity parameter in the Langmuir model. It should be mentioned that the fitting to Freundlich, Dubinin–Radushevich or Temkin models does not result in such an accurate reproduction of these points (R 2≥ 0.99). The obtained parameters would put in front of the optimum performance for sensing to nanocolumnar TiO 2 films, with a double number of adsorption sites and a higher affinity of the dye (in a factor 6 with respect to SiO 2 ). However, most parts of the absorption band in TiO 2 is due to non-fluorescent H -aggregates, and, hence, a notable quenching of the fluorescence properties will dominate these composites as we have commented on above. 0 20 40 60 80 100 120 0 5 1 0 1 5 2 0 2 5 3 0 T i O 2 S i O 2 A b s o r b a n c e P e a k A r e a R o s B c o n c e n t r a t i o n (µM) Figure 3. Adsorption behavior for RosB infiltrated in MO 2 films. The corresponding fit to a Langmuir-type function is denoted with a continuous line while experimental data are labeled with circles. The temporal behavior of the infiltration of the dye molecules into the columnar MO 2 films has also been studied. The obtained experimental data is divided into a fast growth followed by a much slower process while approaching a saturation value Γ∞ (see Figure S4 in the Supplementary Material section). This pattern can be adjusted with the pseudosecond order adsorption kinetics model that has been tested for chemisorption of dyes into inorganic matrices [ 56 ], and it is supposed to account for the surface chemical bonds and physical diffusion steps. The model is the result of solving the following rate equation: dΓ(t) dt =k2[Γ∞−Γ(t)]2(2) that can be straighforwardly integrated with the appropriate boundary condition (Γ(0) = 0) to get: 1 Γ∞−Γ(t)=1 Γ∞ +k2t, (3) Materials 2017,10, 124 6 of 15 with k2 the pseudosecond order adsorption rate. This equation can be linearized in several ways, and we have chosen [57]: t A(t)=1 k2A2 ∞ +1 A∞ t(4) by considering again that the integrated absorption band Ais proportional to Γ. The result is plotted in Figure 4for the 35.8 µ M solution in the whole time range considered, with R 2≥ 0.999. Similar fitting parameters are obtained for the two matrices, with ( A∞ , k2 ) = (25.3, 0.0042) and (22.6, 0.0038) for TiO 2 and SiO 2 , respectively, in units of [ A∞ ] = nm and [ k2 ] = min −1 . Together with the Langmuir fitting, it implies that these MO 2 columnar microstructures exhibit a good infiltration capacity but also an excellent surface functionality for chemical adsorption and a good porous structure for the diffusive accessibility of the incoming rosamine molecules to the active adsorption sites. 0 50 100 1400 1500 1600 0 2 4 6 6 0 7 0 8 0 T i O 2 S i O 2 T i m e / A b s o r b a n c e P e a k A r e a T i m e ( m i n ) Figure 4. Linearized plot of the temporal evolution of the adsorption of RosB molecules infiltrated in MO 2 films. The corresponding fit is denoted with a continuous line while experimental data are labeled with circles. 2.3. Spectroscopic Characterization of RosB/MO2Composites: Anchoring As mentioned above, the molecular structure of RosB enables its anchoring to MO 2 by chemical (through the carboxylic group) or electrostatical (through the amonium group) bounds. In Figure 5, the specular reflectance Fourier Transformed Infrared (FTIR) spectra of RosB and MO 2 precursors have been compared with the bounded RosB in order to analyze the spectral changes in the region of the carbonyl and hydroxyl groups. Only the RosB bands with a major percentage (more than 50%) of potential energy distribution of normal modes of vibration located in the ethylamine (A), phenyl ring with the COOH group (P) or xanthene ring (X) groups are labeled according to the assignment reported in Ref. [ 58 ]. First of all, the strong absorption of the M-O-M asymmetric (transverse-optic and longitudinal-optic modes) stretching band clouds the signals coming from bounded RosB and hence this region is shadowed in the figure. In the 3300–3900 cm −1 region, there is the superposition of the hydrogen bond network of silanol and titanol groups and to the symmetric and antisymmetric OH modes of water coordinated to M 4+ cations [ 59 ]. Finally the bending motion of the adsorbed H 2 O molecules appears at ∼ 1640 and ∼ 1710 cm −1 for TiO 2 and SiO 2 , respectively [ 60 , 61 ]. The comparison of the RosB precursor with the bounded RosB spectra sheds light on to the mechanistic pathway of its surface anchoring. In the SiO 2 substrate, the bands associated with the amine modes (A) disappear, at least partly, while the shape of the OH region remains unaltered with respect to the untreated substrate. This fact indicates an electrostatic anchoring of RosB to SiO 2 and is in agreement with the absence of solid data supporting chemical bounds in carboxylic-dyes attached to silica matrices. On the contrary, in TiO 2 , both types of bands (A and P) suffer modification upon adsorption. Particularly, the bands Materials 2017,10, 124 7 of 15 at ∼ 1490, ∼ 1400 and ∼ 1180 cm −1 that are assigned to C=O, C-O and C-OH vibration become much weaker in favor of two new bands (marked with an asterisk) that correspond to the symmetric and asymmetric CO − 2 vibrations. These changes in the bands associated to P-vibrations have been reported to be compatible with chelating and/or bidentate binding modes of the carboxylate groups on the TiO 2 surface. Overall, it seems plausible that RosB is anchored to TiO2by two different ways: electrostatic or chemical bounds, which would explain the higher load derived for these films from Figure 3. P R o s B R o s B ⋅⋅⋅S i O 2 A a d s o r b e d H 2O + H 3O+ S ila n o l O H s t r e t c h i n g S i - O - S i s t r e t c h i n g S i O 2 X + A A P A + P a d s o r b e d H 2O + H 3O+ * A R o s B R o s B ⋅⋅⋅T i O 2 * 1000 1250 1500 1750 3000 3300 3600 3900 P P P T ita n o l O H s t r e t c h i n g T i - O - T i s t r e t c h i n g R e f l e c t a n c e ( s c a l e d ) W a v e l e n g t h ( c m - 1 ) T i O 2 X + A X + A P A P X + A + P X + P Figure 5. Specular reflectance Fourier Transform infrared spectra of MO 2 and RosB precursors compared with the bounded RosB · · · MO 2 in the devices. Vibration bands associated with phenyl ring (P). Amine (A) and xanthene ring (X) are indicated conveniently. See the main text for details. Complementary information can be extracted by infiltrating the dye from aqueous solutions at different pH values as reported in Figure S4 (Supplementary Information Section). The charge compensation on the inorganic matrix surface will depend on its pzc as explained above. In this sense, the immersion of a solution with a pH higher than its pzc results in a net negative charge on its surface by dissociation of the -OH (titanol or silanol) surface groups that is compensated by the cations in the double layer. With this point in mind and taking into account the pK a value of the RosB molecule, in SiO 2 , the maximum load is reached for the measured pH which is closer to the pK a . If appreciable contribution of chemical anchoring takes place in these substrates, a much more evident increase near the point of charge compensation between RosB and MO 2 would be seen. This is the case of TiO 2 where the dye load is significantly enhanced near its pzc value and decreases with the pH because a deficient charge compensation of the excessive negative charge on the surface, i.e., the net negative charge in the surface is partly compensated by the positive charge of the zwitterionic RosB, but the Materials 2017,10, 124 8 of 15 overall charge remains negative, a fact that makes difficult a new approaching of another dye molecule. Thus, it is reconfirmed that while in TiO 2 the anchoring occurs via chemical and electrostatic bonding, and this is not the case in SiO 2 where an electrostatic mechanism seems to dominate the adsorption of RosB. 2.4. Photoluminescence of RosB/MO 2 Composites: Choosing the Optimal Preparation Conditions for Enhancing the Fluorescence Signal As it has been discussed in the previous sections, whereas both monomers and aggregates are active in absorption, only the lowest excited state of dimers (the J dimers) and the monomers are fluorescent. Specific J - H conversion would require the measurement of fluorescence life times and is out of the scope of the present work. Fluorescence spectra constitute a mapping of (mainly) the monomeric state emission without the influence of the H -dimers and higher order aggregate quenching. Having this idea in mind, the interpretation of the temporal evolution of the infiltrated rosamine in MO 2 presented in Figure 6is eased. For the assistance of the subsequent explanation, the corresponding wavelength of the maximum fluorescence (589 nm) of the RosB monomer (3.5 µ M) in CH 2 Cl 2 solution is presented as a vertical line. This figure shows the temporal evolution of the RosB fluorescence infiltrated from two different concentration solutions in the two oxides. In both SiO 2 and TiO 2 films, the intensity evolution with infiltration time follows the same trend. The rosamine molecules infiltrated from a diluted solution increase their fluorescence intensity with time as a consequence of the smaller aggregation rate. If the concentration of the infiltrating solution is increased, the H -aggregation is apparent in the diminution of the intensity with the infiltration time in comparison with the diluted case, i.e., the emission is quickly quenched with infiltration time. This is also supported by the results shown in Figure 2. However, even when the fluorescence of SiO 2 films is smaller than those obtained for TiO 2 films, the first owns a comparably smaller loading capacity with respect to TiO 2 as extracted with the corresponding Langmuir fitting. This point holds for both RosB concentrations with higher emission capacity for SiO 2 , again related to the lower rosamine aggregation observed in this material when compared to TiO 2 (see Figure 2). Therefore, the films with the optimum optical properties are those constituted by a solid nanocolumnar SiO 2 infiltrated with diluted RosB solution that exhibit the higher emission capacity in relation with the number of adsorbed molecules. It seems timely to mention that higher exposition times ( ≥ 24 h) lead to a diminution of the intensity even in the diluted cases because aggregation started to grow even in the open and wide mesoporous of their external surface. The spectral details observed in the four cases can be explained by the opposing effects of aggregation and structural allocation of the dye. While J -aggregation induces a redshift of the photoluminescence bands, the fluorescence of the dye monomers adsorbed on different areas of the MO 2 structure also provokes a shift evolution from monomers infiltrated to the inner parts or micropores of the oxide to a blue-shifted peak near to the solution maximum that corresponds to the outer-placed monomers. The same effect has been described in dyes adsorbed in different smectite (clay) minerals [ 62 , 63 ] in which the fluorescence of the adsorbed molecules at the external surface of smectites occurs at a lower wavelength than the corresponding fluorescence of Rhodamine 6G in the interlamellar space. This effect seems to be due to a geometrical constraint caused by the restricted space that reinforces the π -delocalization between both chromophores by a conformational-locking in which a more parallel disposition of the phenyl ring respect to the xanthene plane is favored [ 64 ]. Additional contributions of the microenvironment changes cannot be ruled out but do not explain the observed changes by themselves. In both MO 2 films, the fluorescence intensity in the concentrated cases diminishes as a consequence of the H -aggregation, while J -aggregation induces a redshift with infiltration time as commented above. The structural effects of the inorganic matrix is negligible in these cases due to the high number of molecules filling the pores. In TiO 2 , we found a slight shift to the blue with infiltration time for the diluted RosB solution as a result of the adsorption from the inner (microporous) to the outer surface as it can be deduced from the closeness of the Materials 2017,10, 124 9 of 15 maximum fluorescence intensity with respect to the monomer emission in solution. The absence of a heterogeneous distribution of pores in SiO 2 is the reason behind the redshift in the diluted case which can be attributed to J-aggregation as in the concentrated counterpart. 0 1 2 3 4 5 6[ R o s B ] = 1 0 7 µM P h o t o l u m i n i s c e n c e i n t e n s i t y ( c o u n t s ) 550 600 650 700 750 0 4 8 1 2 1 6 2 0 5 m i n 3 5 m i n 6 5 m i n 2 4 h S i O 2 T i O 2 [ R o s B ] = 3 . 6 µM 0 1 2 3 4 5 6 550 600 650 700 7500 4 8 1 2 1 6 2 0 W a v e l e n g t h ( n m ) Figure 6. Photoluminescence spectra for composite RosB/MO 2 thin films prepared by infiltration of dichloromethane solutions at the highest and lowest concentrations considered at different infiltration times. 2.5. Sensing Response to NO2Gas As a final application of the selected SiO 2 microfilms infiltrated with diluted rosamine solution, we have tested the spectral changes provoked by an exposure to a NO 2 stream (50 ppm). The observed spectral changes upon NO 2 exposure documented in the top panel of Figure 7( ∆λ = 11 nm) demonstrate that rosamines bounded to a solid matrix constitute a good choice for gas detection. The most probable mechanism for such changes is possibly an oxidation reaction through a charge transfer process from the electron-rich xanthene ring to the oxidant gas [ 65 – 67 ]. Although the observed bathochromic shift is independent from the RosB concentration of the infiltrated solution, the intensity drop decreases with this concentration. For instance, the decrement of 23% observed for the most diluted solution is only 9% when the RosB concentration is quintupled (data not shown). This difference can be clearly explained in terms of the different aggregation state of the RosB molecules in these films: while low aggregation facilitates the analyte penetration and interaction, high molecular aggregation prevents this process [ 68 ]. The situation is even clearer when the more sensible fluorescence is tested