Journal of Sensors
Volume 2018 (2018), Article ID 7954839, 7 pages
https://doi.org/10.1155/2018/7954839

Research Article

Fluorescent Rosamine/TiO₂ Composite Films for the Optical Detection of NO₂

María G. Guillén,¹ Belén Suárez,¹ Javier Roales,¹ Francisco Gámez,¹ Alejandro P. Vargas,¹ Francisco G. Moscoso,¹ Tânia Lopes-Costa,¹ Carla Queirós,² Ana M. G. Silva,² and José M. Pedrosa¹

¹Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Ctra. Utrera Km. 1, 41013 Sevilla, Spain
²LAQV/REQUIMTE, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, R. Campo Alegre, 4169-007 Porto, Portugal

Correspondence should be addressed to Javier Roales; jroabat@upo.es and José M. Pedrosa; jmpedpoy@upo.es

Received 30 March 2017; Accepted 8 February 2018; Published 29 March 2018

Academic Editor: Yasuko Y. Maruo

Copyright © 2018 María G. Guillén et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Two rosamine derivatives were used as fluorescent sensors for the detection of NO₂, a toxic and oxidant gas whose presence in populated areas needs to be controlled. Both compounds shared the same molecular structure but had different peripheral substituents: a carboxylic acid and an amino group. Transparent nanocrystalline TiO₂ films were prepared by screen printing and used as substrates, where the rosamines were incorporated by simple immersion into their respective solutions to form composite films. According to the molecular structures of the rosamines, the anchoring to the substrates was proposed to be by either covalent bonding and electrostatic interaction, or only electrostatic interaction, and was determined by the different substituents in each rosamine. Upon their exposure to increasing concentrations of NO₂, both types of composite films showed intense and fast spectral changes, and the speed of response was related to the concentration of the gas. The anchoring mode and the electrophilic effect of the substituents determined the better sensing capability and the faster response shown by the carboxylic derivative in all cases.

1. Introduction

Xanthene dyes are a family of compounds characterized by an intense absorption and fluorescence that has been widely employed for a variety of technological applications [1–5]. Some of these compounds, such as the well-known rhodamines, have been largely used for the fabrication of solar cells or sensing devices [6–8], and their intense fluorescence emission makes them appropriate to be used as efficient probes for a number of applications [9–12]. Dye-based sensors, either based on UV-Vis or fluorescence spectroscopies, have proved to be a valid alternative to traditional gas identification techniques, such as gas chromatography, because of their lower operational cost and simplicity. Among them, those based on fluorescence usually feature better sensitivity and robustness justifying its choice over absorption spectroscopy [13].

Although possible, the use of fluorescent sensors in solution is not a practical approach. Most applications require the dye molecules to be supported on a solid substrate, especially those focused on gas sensing. Techniques such as spin coating or Langmuir-Blodgett allow the deposition of molecular films onto a solid substrate. However, the stability of these films is not always satisfying given the little or null chemical interaction between the adsorbates and the substrate [14], and, in the latter case, the procedure is particularly time-consuming. An important challenge derived from the deposition of dyes onto solid substrates is their strong tendency to form aggregates, which are known to be detrimental for gas sensing purposes [14]. Aggregation of dye molecules results in important changes in their photophysical properties, such as the broadening of the absorption bands because of the coexistence of various species, resulting in less-defined peaks and poorer monitoring of the spectral changes induced by the sensed analyte and limiting their use as optical probes [15–17]. At the same time, high molecular aggregation hinders the access of gas analytes to the active recognition sites of the dyes, causing a delay in their spectral response [14]. Functionalized substrates with specific compositions and structures are among the preferred options to deal with these drawbacks [18–25]. Among them, the preparation of TiO₂ substrates following diverse methodologies has been used in the past for solar cell and gas sensing purposes, proving their suitability for the covalent anchoring of dyes with carboxylic groups [16, 26] or for the attachment by electrostatic interaction [27]. Specifically, nanocrystalline TiO₂ films prepared by screen printing have shown promising features in terms of transparency and porosity, besides their low cost and simplicity of fabrication.

Here, we use transparent nanocrystalline TiO₂ films prepared by screen printing as substrates for the incorporation of two rosamine derivatives featuring a carboxylic acid and an amino group, respectively. The anchoring of carboxylic rosamines to inorganic substrates prepared by physical vapor deposition has been previously studied with promising gas sensing results [28]. Our main objectives are to compare the film formation capabilities of both rosamines regarding their different molecular structures and to analyze the gas sensing properties of the rosamine/TiO₂ composites towards NO₂ gas. The detection of this toxic gas has attracted much attention given its elevated toxicity and participation in the formation of other pollutants. This is of particular concern in urban areas, where it can be found in dangerous concentrations as a result of the combustion of fossil fuels. We hypothesize that (i) the different substituents of both rosamines will be determinant in their respective anchoring to the substrates and (ii) it may result in different gas sensing responses.

2. Materials and Methods

2.1. Chemicals

The optimized synthesis of [9-(4-carboxyphenyl)-6-diethylamino-3-xanthenylidene]-diethylamine chloride (RosCOOH, Figure 1(a)) and [9-(4-aminophenyl)-6-diethylamino-3-xanthenylidene]-diethylamine chloride (RosNH₂, Figure 1(b)) is described elsewhere [29]. Their molecular structures present strong similarities to the well-known rhodamine B, and hence their optical properties in dichloromethane solution are similar to those of this compound [29]. Dichloromethane was supplied by Sigma-Aldrich (Madrid, Spain) and was used as received.

(a)

(b)

Figure 1: Molecular structures of (a) RosCOOH and (b) RosNH₂.

2.2. Fabrication of Rosamine/TiO₂ Composite Films

Nanocrystalline TiO₂ films were prepared on glass substrates with Dyesol 18NR-T (Dyesol, Queanbeyan, Australia), a paste containing TiO₂ nanoparticles with an average size of 20 nm. We followed the screen printing procedure, which consisted in the application of one layer of the TiO₂ paste through a 43T mesh screen that led to the formation of a transparent TiO₂ film of 0.16 cm² active surface. Then, the substrates were sintered for 30 min at 500°C. The average thickness of these films was found to be about 1.8 μm according to scanning electron microscopy measurements (not shown). Incorporation of rosamines into the TiO₂ films was accomplished by simple immersion of the samples in 3.6 μM dichloromethane solution containing either RosCOOH or RosNH₂ for 24 h at room temperature. After this, the films were rinsed with dichloromethane for 10 min to remove any dye molecules that were not incorporated into the TiO₂ matrix and allowed to air-dry.

2.3. Spectroscopy and NO₂ Exposure

UV-visible absorption spectra were recorded using a Cary 100UV-Vis spectrophotometer (Agilent, Madrid, Spain). Photoluminescence spectra and sensing kinetics were recorded with a Hitachi F-7000 Fluorescence Spectrophotometer (Hitachi High Technologies, Krefeld, Germany). For the exposure to NO₂, rosamine/TiO₂ composite films were inserted in a purpose-modified sealable fluorescence cuvette with a gas inlet and an outlet. The flow rates of gases were controlled using two Bronkhorst F-201FV mass flow controllers (Bronkhorst High-Tech BV, Ruurlo, The Netherlands). In order to remove any possible contaminating gases, dry N₂ was flushed into the cuvette before introducing the sample. After inserting the sample in the cuvette, a constant dry N₂ flow was kept to prevent contamination during thermal stabilization. A constant flow of NO₂ (5, 10, 20, 30, or 50 ppm) was obtained from a 50 ppm NO₂ cylinder from Abelló Linde (Cádiz, Spain) and its subsequent dilution with dry N₂ from the same supplier. In all cases, the gas flow rate entering the cuvette was 1 L/min. The gas mixture was introduced into the cuvette until complete saturation of the rosamine. All exposures were performed at room temperature.

3. Results and Discussion

3.1. Spectroscopic Characterization of RosCOOH/TiO₂ and RosNH₂/TiO₂ Composite Films

UV-Vis absorption spectra of dichloromethane solutions of RosCOOH (3.6 μM) and RosNH₂ (3.6 μM) showed their monomer bands at 563 nm and 551 nm, respectively (Figure 2). A vibronic shoulder was located at 525 nm for RosCOOH and 514 nm for RosNH₂. The highly diluted conditions ensured that the rosamine molecules were in their monomeric forms. Once anchored to TiO₂, each rosamine experienced different spectral changes. The absorption bands in the RosCOOH/TiO₂ film appeared broadened (full width at half maximum [fwhm] for the film: 52 nm, fwhm for the solution: 29 nm) and blueshifted (9 nm) with respect to the solution spectrum (Figure 2). Such modifications in the rosamine spectrum indicate that H-aggregation, or face-to-face stacking [30, 31], would be occurring between RosCOOH molecules in the film as a result of π-π interactions. The spectrum of RosNH₂/TiO₂ showed less broadened absorption bands than those of the carboxylic rosamine (fwhm for the film: 43 nm, fwhm for the solution: 28 nm) and was shifted towards higher wavelengths (5 nm). In this case, the spectral shifting would be caused by J-aggregation of RosNH₂ molecules [32].

(a)

(b)

Figure 2: UV-Vis absorption spectra of (a) RosCOOH and (b) RosNH₂ in dichloromethane solution (dashed line) and anchored to nanocrystalline TiO₂-based composite films (solid line).

To obtain more insight into the aggregation process occurring between rosamine molecules in the films, we compared the fluorescence spectrum of each of the rosamine films with their respective solution (Figure 3). When anchored to the TiO₂, RosCOOH spectrum showed mainly its monomeric form, with a slight redshift (1 nm) with respect to its solution spectrum due to a minimal contribution of J-aggregates. The presence of H-aggregates already identified in the absorption spectrum would not induce shifts in fluorescence [28, 33, 34], hence the absence of further shifting in the fluorescence spectrum of this rosamine. On the other hand, RosNH₂/TiO₂ film spectrum featured a redshift (7 nm) with respect to its solution spectrum, which was attributed to the presence of J-aggregates as they are characterized by a shifting towards higher wavelengths of the bands in the fluorescence spectrum [28, 33, 34]. This is in agreement with the information obtained from the analysis of the absorption spectrum of the film.

(a)

(b)

Figure 3: Photoluminescence spectra of (a) RosCOOH/TiO₂ film (λex = 530 nm) and RosCOOH dichloromethane solution (3.6 μM) and (b) RosNH₂/TiO₂ film (λex = 525 nm) and RosNH₂ dichloromethane solution (3.6 μM).

The different types of aggregation between rosamines once anchored to the TiO₂ molecules in the film and their implications on their respective spectra can be explained in terms of the type of interaction between rosamine and TiO₂, taking into account that the transition dipole moments for both rosamines are located along the xanthene ring. The binding of RosCOOH to TiO₂ has been extensively discussed elsewhere [28]. Briefly, the molecular structure of RosCOOH allows its anchoring to TiO₂ by covalent bonding (through the carboxylic group) and by electrostatic interaction (through the ammonium group). With this arrangement, RosCOOH would be chemically anchored to the surface via its phenyl group, which would act as a lever that lifts up the xanthene ring due to the additional electrostatic interaction. This tilting would favor the π-π stacking and therefore the formation of H-aggregates. On the other hand, the anchoring of RosNH₂ molecules would only be possible through electrostatic interaction with the film surface. This type of interaction between positively charged dye molecules and TiO₂ films has been studied in previous works [27]. In this case, RosNH₂ would be anchored by only one point to the substrate, allowing a wide variety of angles between rosamine and substrate and increasing the possibility of formation of J-aggregates. An excellent discussion about aggregation states in xanthene derivatives alongside an illustrative scheme of possible geometric dispositions can be found in Martínez et al. [35].

The presence of both H- and J-aggregation may have implications in the gas sensing capabilities of the composite films. The effects of molecular aggregation caused by π-π interactions on gas sensing have been previously discussed in the literature [14]. Besides limiting the access of analyte molecules into the film, molecular aggregation results in broadened absorption bands with less defined peaks that leads to poor monitoring of the spectral changes induced by the sensed analyte. The use of fluorophores allows the use of photoluminescence to monitor the changes induced by their exposure to analytes, with the advantage of being less sensitive to aggregation, yet more sensitive to the changes induced on the fluorophores. In our case, the presence of aggregates was not intense enough to avoid the interaction with gas molecules or to lead to the formation of large clusters that would prevent the use of UV-Vis spectroscopy. However, the analysis of the gas sensing capabilities of rosamine/TiO₂ composite films would benefit from the higher sensitivity and robustness of photoluminescence. Hence, we focused on the modifications produced on the emission spectra of our composite films by their exposure to different concentrations of NO₂.

3.2. Sensing Response to NO₂

Both RosCOOH/TiO₂ and RosNH₂/TiO₂ composite films showed intense photoluminescence once excited at a wavelength (λex) of 530 nm and 525 nm, respectively (Figure 4), which is a common feature of rhodamine derivatives [13, 36]. It is proved that the binding to the substrate did not produce a substantial quenching of the typical emission of this kind of fluorophore. Upon their exposure to 20 ppm NO₂, the emission spectra of both composite films experienced an intense decrease leading to the almost complete loss of their photoluminescence (Figure 4). The amount of quenching at saturation was similar in both cases, around 84% for RosCOOH and 78% for RosNH₂. Such changes indicate that rosamines anchored to a solid matrix of TiO₂ represent a good choice for the detection of NO₂.

(a)

(b)

Figure 4: Photoluminescence spectra of (a) RosCOOH/TiO₂ (λex = 530 nm) and (b) RosNH₂/TiO₂ (λex = 525 nm) composite films before and after their exposure to 20 ppm NO₂. Insets: normalized photoluminescence (PL) spectra for each of the rosamines before and after their exposure to NO₂.

A slight redshift was found in the fluorescence maxima of both RosCOOH/TiO₂ and RosNH₂/TiO₂ after NO₂ exposure (Figure 4, inset). We attributed this shifting to the less responsiveness featured by J-aggregates. Assuming that films of both rosamines were partially composed by J-aggregates, as elucidated previously, they would not interact with NO₂ as fast and intensely as the rosamine monomers. After the quenching of the fluorescence of the monomers, the fluorescence spectra of the rosamines would primarily be composed by the remaining emission of the J-aggregates, reshaping the spectra towards a redshifted one. This shifting was found to be more intense in RosNH₂/TiO₂ films than in RosCOOH/TiO₂ films, which has been attributed to the higher proportion of J-aggregates in the former.

Given the strong electron-withdrawing nature of NO₂ and the electron-donating character of rosamines, the most plausible mechanism for the changes induced on the rosamines would be an oxidation through a charge transfer process from the electron-rich xanthene group to the oxidant gas. Ohyama et al. [37] reported that rhodamine B fluorescence is quenched after exposure to NO₂, due to the aforementioned oxidative nature of this gas. The strong similarities between rhodamine B and the rosamines used in this work support the idea that the same process may be occurring in our sensor. Similar interactions have been found between NO₂ and other dyes with rich π-electron systems [38, 39].

The exposure of RosCOOH/TiO₂ and RosNH₂/TiO₂ composite films to 20 ppm NO₂ also resulted in the decrease of the absorption bands in their UV-Vis spectra. The decrease was found to be 52% for RosCOOH/TiO₂ and 49% for RosNH₂/TiO₂, confirming the better sensitivity of photoluminescence and its choice over UV-Vis spectroscopy. After their exposure to NO₂, the composite films were flushed with dry N₂ to attempt their recovery, but the changes induced on the rosamine spectra were found to be irreversible. Such behavior suggests the use of our composite films as single-use NO₂ sensors, which would be a plausible approach given their low cost and relative ease of fabrication. Other recovery strategies are currently a subject of further research.

In order to further analyze the sensing capabilities of RosCOOH/TiO₂ and RosNH₂/TiO₂ composite films towards NO₂, we analyzed the speed of response of their exposure to increasing concentrations of the toxic gas in the range 5–50 ppm by monitoring the emission at the wavelength of maximum change (577 nm for RosCOOH and 590 nm for RosNH₂, Figure 5). Both rosamines showed a fast response towards NO₂, as indicated by the slope of their kinetics. This can be attributed to the high responsiveness of our rosamines and to the elevated porosity of the TiO₂ substrates that would allow a fast diffusion of the gas molecules inside the film and their contact with the active sites of the rosamines. The slopes of the response of RosCOOH/TiO₂ to each of the NO₂ concentrations were higher than those corresponding to RosNH₂/TiO₂, indicating that the carboxylic rosamine responded significantly faster than the amine derivative in all cases, being the differences more remarkable for the exposures to lower concentrations of NO₂. In both cases, the slopes of the response increased with NO₂ concentration, that is, the response times were concentration dependent. These results suggest that a calibration of the composite film response within the desired range of concentrations would allow its use for quantification purposes. After a time that varied according to the rosamine and NO₂ concentration, the signal stabilized with a horizontal slope, indicating that there were no more available active sites for the gas molecules to interact with the dyes.

Figure 5: Kinetics of the exposure of RosCOOH/TiO₂ (monitored at 577 nm, λex = 530 nm) and RosNH₂/TiO₂ (monitored at 590 nm, λex = 525 nm) composite films to 5, 20, and 50 ppm NO₂.

In light of the different responses observed for both composite films, we proceeded to quantify their speed of response towards different concentrations of NO₂. We calculated t₅₀, which is the time taken for the signal to reach 50% of its maximum change, and found that it decreased with NO₂ concentration and that in all cases it was lower for RosCOOH/TiO₂ than for RosNH₂/TiO₂ composite films (Figure 6). We attribute these differences in the sensing properties to different charge densities in both rosamines. RosCOOH is linked to TiO₂ forming a carboxilate allowing for the xanthene ring to preserve a higher negative charge density than its amino-derivatized counterpart. A higher negative charge density would mean higher availability of electro-deficient compounds as is the case of NO₂, to be attracted by the aromatic core. As a result, the changes induced on RosCOOH upon exposure to the oxidant gas would occur faster, hence improving its performance with respect to RosNH₂.

Figure 6: Response time (t₅₀) corresponding to the exposure of RosCOOH/TiO₂ and RosNH₂/TiO₂ composite films to NO₂. Error bars indicate ±σ (standard deviation). The response time of RosCOOH anchored to microcolumnar (μc-) SiO₂ once exposed to 50 ppm NO₂ is included for comparison.

In a previous work, RosCOOH was used to prepare composite films based on microcolumnar (μc-) SiO₂ following the same infiltration procedure as the one shown here. Microcolumnar TiO₂ was also studied but omitted here due to poor gas sensing results. RosCOOH/μc-SiO₂ films were exposed to 50 ppm NO₂, leading to spectral changes similar to those obtained in this work. However, the analysis of the t₅₀ revealed a slightly slower speed of response than when using nanocrystalline TiO₂ as substrates (t₅₀ = 350 s for RosCOOH/TiO₂, t₅₀ = 360 s for RosCOOH/μc-SiO₂). It is worth mentioning at this point that both substrates differ highly in their preparation procedures. Microcolumnar SiO₂ films were prepared by glancing angle physical vapor deposition while nanocrystalline TiO₂ was made by the screen printing procedure, being the latter significantly faster and simpler and requiring less sophisticated laboratory equipment. Hence, the moderately faster response of nanocrystalline RosCOOH/TiO₂ composite films alongside the higher simplicity of their preparation justifies its choice over microcolumnar RosCOOH/SiO₂ composite films.

4. Conclusions

Two rosamine derivatives, containing either a carboxylic acid or an amino group as peripheral substituent, were successfully incorporated into transparent nanocrystalline TiO₂ films. The anchoring of the dyes to the substrates was found to be compatible with either chemical binding and electrostatic interaction, or only electrostatic interaction, according to the different substituents in each rosamine. The exposure of the composite films to NO₂ resulted in intense and fast spectral changes, which were attributed to a charge transfer process from the electron-rich xanthene group to the oxidant gas. The speed of response, calculated through the t₅₀ parameter, was related to the concentration of the gas. This indicated a concentration-dependent behavior and the possibility of quantification of the gas concentration by calibrating the response. The binding of the sensing molecules to the substrates was found to be determinant for the sensing capabilities of each of the composite films. The carboxylic-derivatized rosamine showed a faster response than the amino derivatized in all cases, and this behavior was attributed to a higher negative density of charge in the former that would enhance its spectral response to NO₂ gas. These results are similar to those previously obtained using microcolumnar-SiO₂ composite films, with the main advantage of featuring an easier substrate fabrication.

Disclosure

This work is part of the PhD thesis of María G. Guillén entitled “Development of optical sensors based on thin films of fluorescent organic materials for the detection of toxic gases” (July 2017, University Pablo de Olavide, Sevilla, Spain).

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this article.

Acknowledgments

The Oporto group thanks Fundação para a Ciência e a Tecnologia ((FCT)/Ministério da Ciência, Tecnologia e Ensino Superior) for the financial support to the UID/QUI/50006/2013-POCI/01/0145/FERDER/007265 (Laboratório Associado para a Química Verde/REQUIMTE) through national funds and cofinanced by FEDER, under the PT2020 Partnership Agreement, and to the Project M-ERA-NET/0005/2014. The group from Sevilla gratefully acknowledges funding from the Ministry of Economy and Competitiveness (MINECO) under Projects MAT2014-57652-C2-2-R and PCIN-2015-169-C02-02 (under the Project M-Era-NET/0005/2014). Funding from the Operative Programme FEDER-Andalucia through Project P12 FQM-2310 also contributed to the present research.

References

X. Chen, T. Pradhan, F. Wang, J. S. Kim, and J. Yoon, “Fluorescent chemosensors based on spiroring-opening of xanthenes and related derivatives,” Chemical Reviews, vol. 112, no. 3, pp. 1910–1956, 2012.

P. Yang, G. Wirnsberger, H. C. Huang et al., “Mirrorless lasing from mesostructured waveguides patterned by soft lithography,” Science, vol. 287, no. 5452, pp. 465–467, 2000.

R. Sasai, N. Iyi, T. Fujita et al., “Luminescence properties of rhodamine 6G intercalated in surfactant/clay hybrid thin solid films,” Langmuir, vol. 20, no. 11, pp. 4715–4719, 2004.

F. Marlow, M. D. McGehee, D. Zhao, B. F. Chmelka, and G. D. Stucky, “Doped mesoporous silica fibers: a new laser material,” Advanced Materials, vol. 11, no. 8, pp. 632–636, 1999.

D. Tleugabulova, Z. Zhang, Y. Chen, M. A. Brook, and J. D. Brennan, “Fluorescence anisotropy in studies of solute interactions with covalently modified colloidal silica nanoparticles,” Langmuir, vol. 20, no. 3, pp. 848–854, 2004.

G. Aragay, J. Pons, and A. Merkoçi, “Recent trends in macro-, micro-, and nanomaterial-based tools and strategies for heavy-metal detection,” Chemical Reviews, vol. 111, no. 5, pp. 3433–3458, 2011.

H. N. Kim, M. H. Lee, H. J. Kim, J. S. Kim, and J. Yoon, “A new trend in rhodamine-based chemosensors: application of spirolactam ring-opening to sensing ions,” Chemical Society Reviews, vol. 37, no. 8, pp. 1465–1472, 2008.

L. Bahadur and P. Srivastava, “Efficient photon-to-electron conversion with rhodamine 6G-sensitized nanocrystalline n-ZnO thin film electrodes in acetonitrile solution,” Solar Energy Materials & Solar Cells, vol. 79, no. 2, pp. 235–248, 2003.

K. H. Drexhage, “Fluorescence efficiency of laser dyes,” Journal of Research of the National Bureau of Standards Section A: Physics and Chemistry, vol. 80A, no. 3, pp. 421–428, 1976.

M. Beija, C. A. M. Afonso, and J. M. G. Martinho, “Synthesis and applications of rhodamine derivatives as fluorescent probes,” Chemical Society Reviews, vol. 38, no. 8, pp. 2410–2433, 2009.

Y.-Q. Sun, J. Liu, X. Lv, Y. Liu, Y. Zhao, and W. Guo, “Rhodamine-inspired far-red to near-infrared dyes and their application as fluorescence probes,” Angewandte Chemie International Edition, vol. 51, no. 31, pp. 7634–7636, 2012.

Y. Koide, M. Kawaguchi, Y. Urano et al., “A reversible near-infrared fluorescence probe for reactive oxygen species based on Te-rhodamine,” Chemical Communications, vol. 48, no. 25, pp. 3091–3093, 2012.

J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer-Verlag New York Inc., New York, NY, USA, 2006.

J. M. Pedrosa, C. M. Dooling, T. H. Richardson et al., “Influence of molecular organization of asymmetrically substituted porphyrins on their response to NO₂ gas,” Langmuir, vol. 18, no. 20, pp. 7594–7601, 2002.

G. A. Schick, I. C. Schreiman, R. W. Wagner, J. S. Lindsey, and D. F. Bocian, “Spectroscopic characterization of porphyrin monolayer assemblies,” Journal of the American Chemical Society, vol. 111, no. 4, pp. 1344–1350, 1989.

J. Roales, J. M. Pedrosa, M. Cano et al., “Anchoring effect on (tetra)carboxyphenyl porphyrin/TiO₂ composite films for VOC optical detection,” RSC Advances, vol. 4, no. 4, pp. 1974–1981, 2014.

J. Roales, J. M. Pedrosa, M. G. Guillén et al., “Optical detection of amine vapors using ZnTriad porphyrin thin films,” Sensors and Actuators B: Chemical, vol. 210, pp. 28–35, 2015.

P. S. Liu and K. M. Liang, “Functional materials of porous metals made by P/M, electroplating and some other techniques,” Journal of Materials Science, vol. 36, no. 21, pp. 5059–5072, 2001.

G. E. Fryxell, “The synthesis of functional mesoporous materials,” Inorganic Chemistry Communications, vol. 9, no. 11, pp. 1141–1150, 2006.

R. Schöllhorn, “Intercalation systems as nanostructured functional materials,” Chemistry of Materials, vol. 8, no. 8, pp. 1747–1757, 1996.

K. Matsukawa, “Development of photo-functional materials from organic/inorganic nano-hybrids,” Journal of Photopolymer Science and Technology, vol. 18, no. 2, pp. 203–210, 2005.

M. R. Bockstaller, R. A. Mickiewicz, and E. L. Thomas, “Block copolymer nanocomposites: perspectives for tailored functional materials,” Advanced Materials, vol. 17, no. 11, pp. 1331–1349, 2005.

E. A. Abou Neel, D. M. Pickup, S. P. Valappil, R. J. Newport, and J. C. Knowles, “Bioactive functional materials: a perspective on phosphate-based glasses,” Journal of Materials Chemistry, vol. 19, no. 6, pp. 690–701, 2009.

J. H. Van Esch and B. L. Feringa, “New functional materials based on self-assembling organogels: from serendipity towards design,” Angewandte Chemie International Edition, vol. 39, no. 13, pp. 2263–2266, 2000.

S.-W. Tam-Chang and L. Huang, “Chromonic liquid crystals: properties and applications as functional materials,” Chemical Communications, no. 17, pp. 1957–1967, 2008.

J. Rochford, D. Chu, A. Hagfeldt, and E. Galoppini, “Tetrachelate porphyrin chromophores for metal oxide semiconductor sensitization: effect of the spacer length and anchoring group position,” Journal of the American Chemical Society, vol. 129, no. 15, pp. 4655–4665, 2007.

P. Castillero, J. R. Sánchez-Valencia, M. Cano et al., “Active and optically transparent tetracationic porphyrin/TiO₂ composite thin films,” ACS Applied Materials & Interfaces, vol. 2, no. 3, pp. 712–721, 2010.

M. G. Guillén, F. Gámez, B. Suárez et al., “Preparation and optimization of fluorescent thin films of rosamine-SiO2/TiO2 composites for NO2 sensing,” Materials, vol. 10, no. 12, p. 124, 2017.

I. C. S. Cardoso, A. L. Amorim, C. Queirós et al., “Microwave-assisted synthesis and spectroscopic properties of 4-substituted rosamine fluorophores and naphthyl analogues,” European Journal of Organic Chemistry, vol. 2012, no. 29, pp. 5810–5817, 2012.

G. De Miguel, M. T. Martín-Romero, J. M. Pedrosa et al., “Dis-aggregation of an insoluble porphyrin in a calixarene matrix: characterization of aggregate modes by extended dipole model,” Physical Chemistry Chemical Physics, vol. 10, no. 11, pp. 1569–1576, 2008.

M.-S. Choi, “One-dimensional porphyrin H-aggregates induced by solvent polarity,” Tetrahedron Letters, vol. 49, no. 49, pp. 7050–7053, 2008.

G. De Miguel, M. Pérez-Morales, M. T. T. M. T. Martín-Romero et al., “J-aggregation of a water-soluble tetracationic porphyrin in mixed LB films with a calix[8]arene carboxylic acid derivative,” Langmuir, vol. 23, no. 7, pp. 3794–3801, 2007.

F. López Arbeloa, V. Martínez Martínez, T. Arbeloa, and I. López Arbeloa, “Photoresponse and anisotropy of rhodamine dye intercalated in ordered clay layered films,” Journal of Photochemistry and Photobiology C: Photochemistry Reviews, vol. 8, no. 2, pp. 85–108, 2007.

W. Rettig, B. Strehmel, S. Schrader, and H. Seifert, Applied Fluorescence in Chemistry, Biology and Medicine, Springer Berlin Heidelberg, Berlin, Heidelberg, 2012.

V. Martínez Martínez, F. López Arbeloa, J. Bañuelos Prieto, T. Arbeloa López, and I. López Arbeloa, “Characterization of rhodamine 6G aggregates intercalated in solid thin films of laponite clay. 1. absorption spectroscopy,” The Journal of Physical Chemistry B, vol. 108, no. 52, pp. 20030–20037, 2004.

R. Haugland, Handbook of Fluorescent Probes and Research Products, Molecular Probes, Inc., Eugene, OR, USA, 2002.

T. Ohyama, Y. Y. Maruo, T. Tanaka, and T. Hayashi, “Fluorescence-intensity changes in organic dyes impregnated in porous glass on exposure to NO₂,” Sensors and Actuators B: Chemical, vol. 59, no. 1, pp. 16–20, 1999.

J. Roales, J. M. Pedrosa, M. G. Guillén et al., “Free-base carboxyphenyl porphyrin films using a TiO₂ columnar matrix: characterization and application as NO₂ sensors,” Sensors, vol. 15, no. 12, pp. 11118–11132, 2015.

J. M. Pedrosa, C. M. Dooling, T. H. Richardson et al., “The optical gas-sensing properties of an asymmetrically substituted porphyrin,” Journal of Materials Chemistry, vol. 12, no. 9, pp. 2659–2664, 2002.