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. 2020 Mar 17;5(12):6299–6308. doi: 10.1021/acsomega.9b03612

A Calixarene Promotes Disaggregation and Sensing Performance of Carboxyphenyl Porphyrin Films

Rubén Flores-Sánchez †, Francisco Gámez †,‡,*, Tânia Lopes-Costa †, José María Pedrosa †,*
PMCID: PMC7114168  PMID: 32258864

Abstract

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The aggregation of a free base porphyrin, meso-tetrakis(4-carboxyphenyl)porphyrin and its Zn(II) derivative have been studied at the air/water interface in the presence of a p-tert-butylcalyx[8]arene matrix. The mixed Langmuir films were obtained either by premixing the compounds (cospreading) or by sequential addition. The negative deviation from the additivity rule of the cospread films is indicative of a comparatively good miscibility that was further confirmed by Brewster angle microscopy. The images of the cospread mixed films showed a more homogeneous morphology in comparison with those of pure porphyrin that is attributed to a deeper and earlier self-aggregation state at the interface of the latter. These results were similar for both porphyrins and revealed the disaggregating effect of the calixarene matrix. The orientation and association of the porphyrins were studied by UV–visible reflection spectroscopy at the interface. A different aggregation behavior can be inferred from the resulting spectra, and a higher orientational freedom was observed when the molecules were less aggregated in mixed cospreaded films. The disaggregating effect was retained when the films were transferred to solid supports as demonstrated by UV–visible spectroscopy. Finally, the potential use of these Langmuir–Blodgett films as optical gas sensors was tested against ammonia and amine vapors. The changes in the spectrum in the presence of the volatile compounds are higher for the Zn-porphyrin. The presence of calixarene enhances the sensor response due to the higher accessibility of volatiles to disaggregated porphyrins in the mixed films. The resulting changes were mapped into a numerical matrix that can be transformed into a color pattern to easily discriminate among these gases.

Introduction

Research into porphyrins and derivatives deposited on Langmuir–Blodgett (LB) films for optical gas sensing continues attracting attention in defense and environmental applications.1−12 The reason behind this interest relies on the modification of their electronic structure and, consequently, the UV–visible (UV–vis) spectrum in the presence of the analyte.4 This electronic structure emerges as a consequence of the conjugation of the electrons among the aromatic rings and can be tuned by precise modification of the peripheral positions or by changing the coordinating metal (if any) of the central hemo core. Hence, when the gas molecules interact with a specific porphyrin, there is usually a charge transfer between the macrocycle of the porphyrin and the gas molecule that provokes a selective and monitorable change in the UV–vis spectra. It then makes porphyrin, particularly metalloporphyrins, eligible as good “sensing materials”.13−23 However, the main problem, which carries the structure of the porphyrin on LB films, is their tendency to self-aggregate into big clusters when the intermolecular interactions exceed the effect of the hydrophobic forces and/or steric hindrance. This aggregation causes shifts and broadenings in the spectra of the transferred LB films and hinders the access of the gas analyte molecules to the molecular binding sites, with the subsequent decrease of the response capacity of the films. Furthermore, it often makes LB films not uniform as a result of unsatisfactory reproducibility in Langmuir layer transfer from sample to sample.7 Among the possibilities established in the literature, there are several studies aiming at minimizing the aggregation of the porphyrin film by adding a calixarene matrix to form mixed films and enhance the porphyrin organization and uniformity of the LB films.3,4,7,12,24,25 Calixarenes are macrocycles made up of a varied number of bridged phenol units with a peculiar and useful supramolecular behavior.26 They often exhibit a good amphiphilic balance between the lower and upper rim and therefore form high quality monolayers at the air/water interface and stable and reproducible LB films.27 The disaggregating effect provoked by these molecules has been also well-established for some complex bare and branched porphyrins5 and is probably caused by a mixed action of the dilution and a cage-like process that strongly affects to the porphyrin organization. In summary, embedding porphyrins into a calixarene matrix seems to improve the homogeneity of the porphyrin in the mixed films at the interface and increases the porosity on the surface in LB layers. This effect allows a better access of gas molecules through their cavity network to the binding sites of the porphyrin, which subsequently leads to a faster response under analyte exposure.3,7

In particular, carboxyphenyl porphyrins have demonstrated good sensitivity for the optical detection of different gases and volatile organic compounds.22,28,29 However, unlike other porphyrin families, no attempts to improve their structural properties when assembled in LB films have been reported to the best of our knowledge. In this paper, the ability of a calixarene to disaggregate two different tetracarboxyphenyl (TCPP) porphyrins has been demonstrated: the free base and its Zn-metallated analogue. Specifically, the porphyrins explored in this work are the 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (henceforth, p-TCPP) and Zn(II)-5,10,15,20-tetra(4-carboxyphenyl)porphyrin (henceforth, p-ZnTCPP). In particular, the effect of adding p-tert-butylcalyx[8]arene (C8A) to the porphyrin films at the air/water interface has been studied, extracting information from the pressure–area (π-A) isotherm supported with Brewster angle microscopy images as well as the effect of the calixarene matrix in the mobility and aggregation of the molecules at the interface measured by reflection spectroscopy. All macrocycle compounds are shown in Figure 1. The application of these optical tools to demonstrate and visualize the disaggregation process constitutes one of the main novel aspects of the present work. The effect of the addition of calixarene in the porphyrin LB film properties by using UV–vis spectroscopy to measure the aggregation degree after the transference was also considered. Finally, we focused on the study of the sensing response of the solid LB films to vapors of ammonia and volatile amines. The importance of the aggregation diminution and the presence of metal in the porphyrin will also be discussed in terms of the relative spectral changes upon exposure to an example analyte. The spectral changes of the optimized mixed films containing the Zn derivative under exposure to the target analytes are mapped into a color barcode-like images that are very useful not only for an easy visualization of the sensing response but also for its future implementation in sensor arrays.

Figure 1.

Figure 1

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Molecular structures of the porphyrins (a) p-TCPP, (b) p-ZnTCPP, and of (c) the calixarene C8A.

Results And Discussion

Surface Pressure Area Isotherms

It is known that if the porphyrin is embedded in a calixarene matrix, the interaction between the porphyrin molecules is clearly reduced, thus improving the non-aggregated organization at the interface.5,24,26,34,35 Here, we will study this effect in p-TCPP and its Zn-coordinated derivative. First, we have measured the Langmuir isotherms of pure and mixed films. Typical isotherms of an ideal amphiphile comprise a narrow molecular area range where gas/expanded–liquid/condensed–liquid phase transitions occur before a small diminution of the molecular area leads to the collapse of the monolayers. Figure 2 shows the π–A isotherms of pure p-TCPP and C8A films at the air/water interface used as references to compare with the isotherms of p-TCPP mixed films represented on the same graph. In the case of p-TCPP (solid line), the limiting area is ∼43 Å2 per molecule, which is in close agreement with the literature.36 Considering that the molecular area of p-TCPP at a flat orientation is ∼230 Å2,37 the low limiting area obtained in this work suggests that the macrocycles are tilted with respect to the water surface and/or on a stacked aggregation as a consequence of the interaction between the macrocycles at the interface, resulting in low-quality films. These results for p-TCPP films agree with the early self-aggregation observed in monolayers of other porphyrins reported in previous works.25,26,38 In the case of C8A (dotted line), the extrapolated area to 0 mN m–1 is ∼164 Å2. This extended area in dichloromethane is coincident with that published elsewhere30 and is lower than that expected for a flat orientation of the macrocycle with respect to the water surface. In particular, calculated cross sections ranging from 140 to 322 Å2 were reported depending on the type of molecular conformation and orientation. A pleated-loop conformation perpendicularly oriented with respect to the interface was proposed in that study.

Figure 2.

Figure 2

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Surface pressure–area isotherms of p-TCPP (black solid line), C8A (blue short dash line), p-TCPP/C8A-C (xp-TCPP = 0.6, black dot line; xp-TCPP = 0.8, back dash–dot line), and p-TCPP/C8A-S (xp-TCPP = 0.6, grey dash–dot–dot line) recorded at 20 °C. Similarly, results for p-ZnTCPP (red dash line) and p-ZnTCPP/C8A-C (xp-TCPP = 0.6, red dash–dot line) are included. The isotherms of mixed films are expressed per molecule.

For clarifying the effect of the calixarene in the mixed films, the isotherms are plotted in Figure 2 with respect to the total number of molecules. The molecular areas of the mixed films (both for cospread p-TCPP/C8A-C and sequential p-TCPP/C8A-S films, see the Experimental Section for details) fall in between those of p-TCPP and C8A isotherms and are closer to the molecular area than that of the major component. The isotherm expansion increases with the porphyrin content. A first insight into the miscibility of both components in the mixed films can be withdrawn from its collapse pressure. As can be observed, the collapse pressure for p-TCPP/C8A-C diminishes with the porphyrin fraction, which is indicative of miscibility. For p-TCPP/C8A-S, however, the collapse occurs prematurely, and it has been measured that the corresponding surface pressure is irrespective of the porphyrin content. These latter results are indicative of a segregation of the porphyrin among the calixarene matrix that leads to a squeezing-out process at a high pressure that provokes an untimely collapse of the films. The marked collapse in the p-TCPP/C8A-S films is observed even at the visual level. In fact, when this mixed monolayer was being compressed, the appearance of cohesion of different patches of C8A and p-TCPP moieties could be directly checked. Further quantitative insight into the important miscibility will be provided in the following.

The analysis of the behavior of mixed monolayers is generally undertaken in terms of the fulfillment of the additivity rule, which establishes that, in a binary immiscible mixture, the following expression is obeyed at a fixed π value:3,39

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where Ai and xi are the molecular area and molar fractions of the component i, respectively, and A12 is the average molecular area in the mixture. The excess area Aexc is defined for non-ideal monolayers as follows:

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Since A12 is linear on xi as long as the molecules are immiscible or ideally miscible, negative excess areas (or negative deviations of the additivity rule) indicate that the interaction between C8A and p-TCPP is stronger than those between C8A/C8A and p-TCPP/p-TCPP giving rise to a special molecular organization by the filling of the empty spaces or changes in the molecular packing as a result of the interlocking of the different components.31 In other words, the mixing Gibbs free energy ΔGmix = ∫0πAexcdπ is positive. The degree of fulfilling of this rule has been assessed by using the experimental molecular areas of pure C8A and p-TCPP obtained at each pressure and plotted in Figure 2 as a function of the molar fraction of p-TCPP.33

According to eq 1, the expected area for an ideally immiscible p-TCPP/C8A mixed monolayers at different surface pressures (0, 5, and 20 mN m–1) is calculated and plotted in Figure 3. As can be observed, the experimental molecular areas A12 for p-TCPP/C8A-C mixed monolayers are below the linear correlation defined by the ideal behavior of immiscible films, while the values measured for p-TCPP/C8A-S mixed monolayers are close to the ideal linearity. So, these experiments clearly indicate that the molecules of C8A and p-TCPP do not interact in the sequential spreading method. Consequently, there is a phase separation with a molecular organization controlled by the calixarene moieties. On the other hand, when the two components are mixed by cospreading, the resulting film is, at least, partly miscible.6 It should be remarked that the negative deviation from the additivity rule is shorter as the fraction of p-TCPP increases. In particular, for xp-TCPP = 0.8, there are almost no deviation at any pressure, indicating that the content of this component is so high that no miscibility occurs. This effect occurs with other porphyrins found in the literature and suggests that the molecular organization is controlled by the C8A molecules.4,6 A similar behavior has been observed in the p-ZnTCPP/C8A-C film with xp-TCPP = 0.6, whose differences between the experimental and ideal areas are −53.7 ± 4.7, −11.5 ± 2.3, and −1.8 ± 0.2 Å2 at 20, 5, and 0 mN m–1, respectively. Hence, we have selected the films with xp-TCPP = 0.6 for the rest of the experiments. These films are expected to contain enough porphyrin amounts for an enhanced sensing behavior without any premature collapse combined with a reasonable miscibility between the components.

Figure 3.

Figure 3

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A12 – xp-TCPP diagrams of mixed p-TCPP/C8A-C (solid squares) and p-TCPP/C8A-S (open squares) films at three selected surface pressures 0, 5, and 20 mN m–1. Straight lines indicate the ideal behavior as evaluated by eq 1 and the experimental molecular areas of the isolated components monolayers. Error bars are calculated from at least three replicate measurements on independent samples.

Brewster Angle Microscopy (BAM)

Although the above results seem to provide much information about the importance of C8A in the mixed monolayers, the isotherms alone do not give definitive information about the molecular organization. Therefore, it is necessary to support these results by images of the film surface obtained by BAM. This technique allows us to visualize directly the morphology of the films during their formation at the air/water interface. Therefore, changes in the molecular density, film thickness, or orientation inside the film are monitored in the film reflectivity.4,5 Selected BAM images of the pure C8A monolayer are shown in the right column of Figure 4. It can be noticed that after spreading (and before any compression), two different regions can be observed: a large condensed domain, which corresponds to higher density areas (bright region) that coexist with a gas-phase region associated to the lowest-density areas of the monolayer (darker regions). The difference of brightness is due to the differential reflectivity associated with the thickness of these regions. Upon compression, the condensed domains increasingly dominate the images, while the gas phase gradually disappears. At higher pressures, the film becomes a homogeneous and bright condensed phase. The bright spots on solid domains are small mountain-shaped clusters of molecules that are hard to keep in focus with the rest of the monolayer.38 These results agree with previous published images of C8A monolayers.25,38

Figure 4.

Figure 4

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Brewster angle microscope images of p-TCPP, C8A, p-TCPP/C8A-C (xp-TCPP = 0.8 and 0.6), and p-TCPP/C8A-S (xp-TCPP = 0.6) at the air/water interface under compression. The corresponding surface pressure and surface area (per molecule) are indicated in each image. Image size: 430 μm width.

In the case of pure p-TCPP shown in the left column of Figure 4, BAM images show very similar features to those obtained for other porphyrins.40 When the porphyrin is spread on the surface in this concentration regime, it tends to form big domains with irregular edges and multiple bright points over a black surface that is associated with the water subphase, indicating that just after spreading, the molecules tend to form big aggregates and to self-associate in clusters, leaving part of the surface uncovered (Figure 4a). As the monolayer is being compressed, the big domains start to get closer. However, there still appear black regions and bright spots, especially in the edges of the domains and in the regions of cohesion. Finally, at high pressures, the film covers the entire surface, but the bright points persist, showing a nonhomogeneous surface unlike the case of the C8A monolayer. These results support that the aggregation of p-TCPP at the interface occurs as a result of the π–π interaction between the macrocycles even before compression.40 BAM images for the mixed monolayer of p-TCPP/C8A-S are shown in the fourth column of Figure 4. After the deposition of the C8A solution and before compression, the image is similar to that of the pure C8A monolayer as expected (first image). After the spreading of the p-TCPP solution, the film breaks down, and some channels are opened with a visible dark subphase. At high pressures, the homogeneity of the first image is lost, showing the characteristic surface of the pure p-TCPP film instead. These images suggest that there is no miscibility between the components, and a surface formed by different zones of pure C8A and p-TCPP monolayers appears as a consequence of the segregation between the components. It has also been checked that adding the C8A solution after p-TCPP (not shown) leads to similar images.

BAM images of mixed p-TCPP/C8A-C (xp-TCPP = 0.6) films show a different appearance as result of the spreading method (third column of Figure 3). At π = 0 mN m–1, the presence of C8A reduces the formation of the clusters observed in the p-TCPP BAM images, and, as the pressure is rising, light and dark regions appear uniformly across the surface, and the black regions associated with the subphase are not present. Although a certain degree of aggregation still appears, the surface seems to be more homogeneous than p-TCPP/C8A-S films. Thus, BAM images prove the higher homogeneity and miscibility of the cospread mixed monolayers as it was previously interpreted by the isotherms and the additivity rule. As can be seen in the second column of Figure 4, the increment of the porphyrin content in the mixed monolayer induced a decrease of the miscibility. Analogous effects are observed for the p-ZnTCPP as shown in Figure S1. Further confirmation of this behavior and additional details on the organization and association of the p-TCPP molecules at the interface as well as the influence of the C8A molecules in the mixed monolayers can be obtained by measuring the reflection spectrum during the compression.38

Reflection Spectroscopy at the Air/Water Interface

Direct evidence of the presence of porphyrins and their aggregation behavior at the interface is obtained here by reflection spectroscopy. This technique detects only those chromophore molecules at the air/water interface due to the reflection enhanced effect (ΔR) originated for the absorption and serves to infer the molecular organization in the film. In this case, we have used the values of the standardized reflection spectra, measured as ΔRs = ΔR/ΔRmax where ΔRmax is the maximum ΔR value, although the original spectra grow in all cases under compression as a consequence of the increasing molecular density. Some representative spectra have been measured in films of the two porphyrins (both in the absence and presence of calixarene) for a surface pressure range spanning from high to low molecular areas as shown in Figure 5. Also, the solution spectra are shown for comparison (dotted line). The typical spectra of the porphyrins in solution consist of an intense Soret band along with four associated weaker Q-bands.38 In general, there is a red shift and a widening of the Soret band with respect to the solution where the porphyrin molecules exist as monomers because of the presence of aggregates. However, the spectra of the films of the two porphyrins in the absence of calixarene (top plots in Figure 5) are clearly more aggregated when compared to the corresponding mixed films (bottom plots in Figure 5). In particular, p-TCPP (Figure 5a) shows wider Soret bands with a shoulder at longer wavelengths, while p-ZnTCPP exhibits the most shifted ones (Figure 5c).3,41 The bathochromic shift of the Soret band is ascribed to aggregates of type J (in which the molecular planes are displaced with respect to the stacking direction), while the increment of the spectra width can be attributed to aggregates of a higher order. These results clearly demonstrate the disaggregating effect of the calixarene matrix in the mixed films whose corresponding spectra (Figure 5b,d) show a maximum of the Soret band very near to that of the solution spectrum. On the other hand, the shape of the Soret bands in the different films remains in most cases approximately unaltered under compression, suggesting that the proportion of the different species of aggregates remains constant during this process in agreement with the self-aggregation observed in the BAM results. The exception embodied by the p-ZnTCPP/C8A films, where a hypsochromic shoulder progressively grows upon compression, could be indicative of the stacking of the porphyrins in the form of H-aggregates (face-to-face orientation). Since this aggregation occurs upon compression, and in comparison with the behavior of the p-TCPP-containing films, the presence of the calixarene effect on the porphyrin disaggregation becomes more important in the p-ZnTCPP case. Regarding orientation changes of the porphyrin molecules, further information can be obtained by normalizing the original reflection spectra (see the Experimental Section for details). Normalized reflection spectra of p-TCPP and p-TCPP/C8A-C (xp-TCPP = 0.6) and the metallic counterparts films under compression are represented in Figure S2.

Figure 5.

Figure 5

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Standardized reflection spectra of pristine and mixed porphyrin films considered in this work. The surface areas (per porphyrin molecule) and surface pressures of each spectrum are labeled in the figure. The spectrum of the porphyrins in the solution is also included as dotted lines. Images (a) and (b) stand for p-TCPP and p-TCPP/C8A-C films, while images (c) and (d) correspond to p-ZnTCPP and p-ZnTCPP/C8A-C films, respectively. The molar fraction of the calixarene in the mixed films is fixed at xp-TCPP = 0.8.

The values of the normalized reflection clearly diminish in all cases upon compression. Assuming that there is no loss of molecules into the subphase, this diminution indicates that the porphyrin aggregates are changing their orientation as the surface area is reduced, specifically to a more tilted orientation. This effect is more pronounced in the p-ZnTCCP-C8A mixed films. All these observations together with the narrower Soret band in the p-ZnTCPP/C8A indicate that this films are the more disaggregated ones and hence will be more adequate for sensing purposes. This point will be treated in the next subsection.

UV–Vis Spectroscopy of Langmuir–Blodgett Films and Sensing Response

Figure 6a,d shows the UV–vis spectra of pristine LB of the pure porphyrins and its mixture with C8A at xp-TCPP = 0.6. The spectra of the porphyrins in the chloroform solution were also included as a reference (dotted line), with a maximum absorbance for the Soret band at 420 and 429 nm for p-TCPP and p-ZnTCPP, respectively. In comparison, the LB films of the pure and mixed porphyrin show the maximum wavelengths of the Soret band at 424 nm for p-TCPP and p-TCPP/C8A, 437 nm for p-ZnTCPP and 431 nm for p-ZnTCPP/C8A. Again, the spectra of the LB films are wider and red-shifted with respect to the solution; a fact that suggests that the pre-aggregation that occurred at the interface is maintained when the films are transferred onto a glass substrate. In this sense, the influence of the C8A molecules can be also observed from the results of the spectra of the LB films. It can be observed that although the Soret band is also red-shifted with respect to the solution, the peak is narrower, and its shape is more similar to the monomeric form, suggesting that the aggregation degree is smaller than in the pure LB film. A systematic influence of the C8A is given in Figure S3. In that figure, the wavelength difference of the Soret band of mixed films with respect to the solution is measured as a function of the C8A fraction for a number of calixarene molar fractions. The higher the content in C8A is, the closer to the absorption wavelength of the monomeric form will be. Moreover, it should be remarked that this experiment was repeated several times, resulting in a low reproducibility in the intensity of the unmixed porphyrin LB film spectra due to the partially covered surface coming as a consequence of a bad deposition. However, the mixed LB films covered the entire surface, and hence, the spectra are much more reproducible.

Figure 6.

Figure 6

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Left column: (a) UV–vis spectra of p-TCPP: solution (dotted line), LB film (dashed line), and mixed LB film (continuous line). (b) Changes after exposure to butylamine vapors. (c) Squared difference spectra for both p-TCPP LB films. Right column: same as left column but for p-ZnTCPP. Specifically, (d) UV–vis spectra of the p-ZnTCPP: solution (dotted line), LB film (dashed line), and mixed LB film (continuous line). (e) Changes after exposure to butylamine vapors. (f) Squared difference spectra for both p-ZnTCPP LB films. The bottom image represents the color-encoded image of the response of the LB toward butylamine. See text for details.

This diminution in the aggregation of the porphyrins in the presence of a calixarene matrix has demonstrated to improve the optical sensitivity towards analytes as a consequence of the higher accessibility of the molecules to the active site of the porphyrin.3 Here, experiments exposing these LB films to butylamine vapors, obtained by bubbling a dry N2 gas stream through the liquid analyte at constant temperature, were carried out to assess the contribution of the calixarene to the sensing activity of the porphyrins. The normalized spectra of the four LB films (p-TCPP, p-ZnTCPP and their mixed counterparts) before and after the exposure are shown in Figure 6b,e. Since aggregation in pure porphyrin films is known to be detrimental for gas sensing,40 the spectral change upon exposure in the mixed LB films is more prominent due to the disaggregating effect of the calixarene molecules. In particular, the spectral changes are more intense for the metallated porphyrin, in good agreement with the reported sensitivity to amine vapors of porphyrins that are coordinated with Zn.12,22,42,43 Moreover, the p-ZnTCPP spectrum after exposure (black dashed line) shows a shift of less than 6 nm with respect to the same film before exposure (black solid line), while the wavelength of maximum absorbance of the p-ZnTCPP/C8A after exposure shows a red shift of 15.5 nm in comparison with its UV–vis spectrum before exposure (red solid line). This is indicative of the disaggregating effect of the calixarene and the positive effect for gas sensing.

A more quantitative determination of the difference of response can be done by taking the squared difference spectra (Figure 6c,f) by subtracting the normalized spectrum captured prior to any exposure from that measured after the addition of the butylamine.44 The difference spectrum of the p-ZnTCPP/C8A-C (xp-TCPP = 0.6) LB film shows the largest absorbance change that becomes almost twice the response for the p-ZnTCPP film. This behavior is also observed for some other analytes as will be discussed below. Moreover, the representation of these changes in a color map, as explained in the Experimental Section, gives rise to a clear recognition pattern (see the image at the bottom of Figure 6) where these results are observed at a glance. Therefore, we have selected these films (p-ZnTCPP/C8A) to study its sensitivity and selectivity toward an ensemble of volatile amines and ammonia vapors. The raw spectral results are presented in Figure S4 where the marked different response of the films to each vapor is demonstrated. This method encloses this information into a single recognition pattern that is found to be characteristic of each analyte. Remarkably, in spite of the slight difference in the unexposed spectra, the RMS of the results are within 5%. The complete set of results are shown in Figure 7 where it is clearly observed that butylamine and 2-butylamine, even with relatively low vapor pressures of 10.9 and 18 kPa, respectively, are the analytes that produce the highest values of change. The exposure to ammonia or tributylamine leads to a comparatively lower response. These clear results indicate a selective behavior of the LB films against volatile vapors. Future studies will be focused on the analysis of the sensing capability of mixed LB films including other TCPP derivatives, more analytes, and the kinetics of the response and recovery after the gas exposure.

Figure 7.

Figure 7

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Identification patterns obtained upon exposure of p-ZnTCPP/C8A LB films to basic vapors. The color scale goes from red (no change) to blue (for the highest change detected). Relative vapor pressures at room temperature (scaled to ammonia value) are written in parentheses after the analyte name.

Conclusions

The effect of embedding carboxyphenyl porphyrins into a calixarene matrix on the Langmuir and Langmuir–Blodgett films are explored in both cospread and sequential spreading methods. It has been shown that the calixarene promotes the disaggregation of the associated structures of the carboxyphenyl porphyrins. In particular, the mixed monolayers of p-TCPP/C8A and p-ZnTCPP/C8A prepared by cospreading lead to more homogeneous films than those obtained with the pure porphyrins. We found the best porphyrin/calixarene proportion by exploring the specific intermolecular interactions and the type of aggregation with the help of optical tools. In particular, BAM imaging provides a direct visualization of the disaggregation of the porphyrin in the presence of the calixarene because of the subtle balance between subphase/porphyrin/calixarene interactions, which is compositional-dependent. Further insight was obtained by reflection spectroscopy. The aggregation was demonstrated to be mostly of type J, and the disaggregated effect of the calixarene is more important in the p-ZnTCPP case. Also, there is higher reproducibility of the absorption spectra obtained for the mixed LB films in comparison with those obtained with the pure porphyrins films, indicating a better quality of the mixed monolayers. Hence, the presence of the disaggregating macrocycle determines the subsequent performance of the Langmuir–Blodgett films as gas sensors. This performance is shown to be higher in the metallated porphyrin, in agreement with some results published for some carboxyphenyl porphyrins, and is strongly enhanced in the presence of the calixarene.

Experimental Section

Chemicals

The porphyrins 5,10,15,20-tetra(4-carboxyphenyl)porphyrin and Zn(II)-5,10,15,20-tetra(4-carboxyphenyl)porphyrin were obtained from Frontier Scientific. p-tert-butylcalyx[8]arene (C8A) and 1,1,1,3,3,3-hexamethyldisilazane (HDMS) were obtained from Sigma Aldrich. All macrocycle compounds were used without further purification. Water was purified using a Millipore Direct-Q system (18 MΩ cm).

Methods

Isotherms were recorded in a NIMA 302LL Langmuir trough equipped with two removable Teflon barriers with a total area of 300 cm2. The temperature was kept at 294 ± 0.5 K with a thermostat enclosure. The absence of surface-active contaminants was verified by compressing the bare water subphase, obtaining values of surface pressure of less than 0.1 mN m–1. Films were prepared by dropwise spreading of an appropriate volume of the solutions of porphyrins and porphyrin/calixarene mixture over the surface with a microsyringe. Specifically, the spreading solutions of p-TCPP and p-ZnTCPP were prepared by dissolving 1.5 mg of the porphyrin in 5 mL of a dichloromethane/methanol (1:1) mixture. The solution of C8A was prepared by dissolving 1.5 mg of C8A in 5 mL of dichloromethane. The monolayers of pure compounds were obtained by a dropwise deposition of the solutions with a microsyringe (Hamilton, 250 μL) onto a clean water surface. The π–A isotherm of the mixed monolayers of p-TCPP/C8A were obtained at different C8A volume ratios following two methods: (i) predissolving the components (cospreading, C) and (ii) sequential spreading of the components onto the air/water interface (S). In the latter, 10 min are left between the spreading of the compounds to ensure the evaporation of the solvent of the first deposited solution. After evaluating the miscibility of the porphyrin in the calixarene matrix in the p-TCPP films (see the Surface Pressure Area Isotherms section), the mixed monolayers p-ZnTCPP were prepared only by the cospreading method. Hereinafter, for instance, a mixed p-TCPP/C8A films prepared by the cospreading method will be denoted as p-TCPP/C8A-C, and similarly, p-TCPP/C8A-S will represent the same film prepared with the sequential spreading.

After spreading, the solvent was allowed to evaporate for 15 min before measurement. The compression was started using two symmetrically moving barriers with a barrier speed set to 15 cm2 min–1 (6.6, 6.5, 22, 8.2, and 8 Å2 per molecule and minute for p-TCPP, p-ZnTCPP, C8A, p-TCPP/C8A, and p-ZnTCPP/C8A, respectively). The surface pressure π is defined as π = γ0 – γ where γ0 is the surface tension of the air/water interface and γ is the surface tension in the presence of the amphiphilic mixture. π was measured assuming a zero contact angle with a dynamometric sensor of Wilhelmy-type with a 10 mm wide strip of Whatmans’ Chr1 chromatography paper at the same time as the molecular area A is evaluated. Measurements were done in at least three independent samples in order to evaluate the standard deviations.

Brewster angle microscopy was used to obtain additional information on the molecular organization of the monolayer at the air/water interface. The images were obtained by using Nanofilm (now Accurion) BAM2plus LE (lateral resolution: 2 μm). The image-processing software allows the reduction of interference fringes and noise and improves the contrast by scaling the brightness of the images.

Reflection spectroscopy measurements were performed with an Accurion RefSpec2 equipment with a spectral range of 220–1000 nm mounted on a 702BAM Langmuir trough with a total area of 982 cm2. A sensor unit collimates the light to the sample surface and focuses the reflected light into the fibers that guide it to the spectrometer. A sample shutter is controlled via electronics holding a mirror that reflects the light directly to the detector fiber. It serves as a static reference to account for any lamp drift. The setup is completed with a black plate located at the bottom of the Langmuir trough to eliminate stray light (absorbing and reflecting transmitted light out of the sensor). Details on the reflection spectrometer have been described elsewhere.31,32 The difference of reflectivity, ΔR, of the film-covered water surface is so-determined. The reflection spectra were then normalized to the same surface density of porphyrin by multiplying ΔR by the surface area extracted from the π–A isotherms (i.e., ΔRnorm = AΔR). As porphyrins are the chromophore molecule in these set of experiments, the surface area is calculated per unit of porphyrin molecule. A single spectrum measurement takes less than 4 s to be performed. In each experiment, a number of reflection spectra were taken manually at different surface area values while the isotherms were being recorded. A series of about 30 spectra were taken under compression at surface areas ranging from gas to compressed-liquid regions.

Hydrophobic glass slides of dimensions 26 × 10 mm2 immersed in HDMS for 24 h were washed with methanol and heated to 110 °C in an oven. These slides where then used to support LB films and were immersed into the subphase before the film spreading. Films of p-TCPP, p-TCPP/C8A-C, p-TCPP/C8A-S, p-ZnTCPP, and p-ZnTCPP/C8A were compressed at 20 mN m–1 and stabilized for 30 min once this target pressure is reached. Then, a controlled dipper deposited 20 layers of Y-type by subsequent cycles of withdrawal and immersion through the interface at a speed of 5 mm min–1 and 10 mm min–1 for pure porphyrin monolayers and mixed monolayers, respectively. The numbers of layers is chosen following the recommendations of ref3 for similar compounds. Transfer ratios were found to lie in the 0.85–0.95 range for pure monolayers. Mixed films reached higher values close to 1 in most cases.

A purpose-built gas testing chamber was used to assess the gas-sensitivity optical properties of the transferred Langmuir–Blodgett films. The gas stream was directed into the gas chamber that held the samples. A USB4000 Ocean Optics fiber spectrometer incorporating a Toshiba 3648-element linear CCD array detector was used to record the visible absorption spectra of the sample over the wavelength range 350–850 nm. Data were collected in presence and absence of the volatile compound at 293 K.26,33 Once the spectrum remains constant, UV–vis absorption spectra were recorded using a less noisy Cary 100 UV–vis spectrophotometer.

Finally, we have followed the procedure detailed in ref22 with slight modifications to generate an easy-to-read color patterns set from the obtained spectra. For each kind of film and analyte, the spectrum of the exposed film was subtracted from the non-exposed one after normalization. The resulting difference spectra were squared to avoid negative values and maximize the differences. All the squared difference spectra for each analyte in the Soret band region were put together and converted into an m × n matrix where m is the wavelength and n is the number of spectra used (one per analyte or one per porphyrin). The matrix was then represented as a color image using Origin 2015 software. For each type of film, the resulting image colors range from red (for points with null squared difference absorbance) to blue (maximum change points).

Acknowledgments

Funding from Ministry of Economy, Industry and Competitivity of Spain (MINECO) under projects MAT2014-57652-C2-R and PCIN-2015-169-C02 (under the project M-Era-NET/0005/2014) are gratefully acknowledged. Funding from the Operative Programme FEDER-Andalucia through project P12 FQM-2310 also contributed to the present research.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03612.

  • Additional measurements and figures (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b03612_si_001.pdf (5MB, pdf)

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Supplementary Materials

ao9b03612_si_001.pdf (5MB, pdf)

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