In this work, metal free and zinc tetraphenylporphyrin films were employed as nitrogen dioxide (NO2) gas sensors. The films were vacuum evaporated and the sensor response was evaluated as changes in the optical absorption spectra, hydrophobic properties and conductivity at different gas concentrations. From UV-Vis results, important changes in the absorption peaks were observed after gas exposure. The morphology of the films before and after gas interaction was obtained by using scanning electron and atomic force microscopy. The films morphology showed a degradation after gas adsorption for the metal free system but gas entrapment for the zinc porphyrin film. In order to elucidate the gas adsorption phenomena, density functional theory calculations were also performed. Here, it was observed that the porphyrin chemical structure not only affects the gas coordination sites which affect the porphyrin electronic distribution and packing arrangement, but also, determines the gas detection mechanism for sensing applications.
Nowadays, most of the current commercially available detectors of volatile organic compounds (VOC) have been fabricated by using metal oxide semiconductors, and they employ detection techniques such as photoionization, electrochemistry and non-dispersive infrared and thermal methods, which require several detection steps, expensive materials and complex signal generators [
While phthalocyanines have been most commonly employed as chemical sensors [
Because of their distinctive bright colors, porphyrin species exhibit very distinct absorption bands in the visible spectrum due to electronic transitions between HOMO and LUMO states [
In addition, the resulting conductivity of these films during gas exposure, not only depends on the molecular structure (metallic center and functional groups), but also on the analyte specie. Oxidizing gases such as NO2 have attracted considerable attention since they can be found in the atmosphere, and are considered highly toxic with very harmful effects for all life forms. As pollutant, NO2 intervene in the conversion of different NOx species in the atmosphere, reacting with sunlight to produce ozone. Moreover, NOx species are involved in many biological processes and biomimetic catalytic reactions which are important at the cellular level [
Although previous works suggested that the sensing characteristics of porphyrin films remain active over long periods with short recovery times, little evidence has been given on the effects of the gas to the porphyrin films. Furthermore, some speculation has been done regarding the binding of the analytes to the molecules, but so far, little work has been done to determine the exact location and nature of this binding. Therefore, time-dependent density functional theory (TD-DFT) has been employed to investigate the fundamental electronic and optical properties of porphyrin molecules exposed to NO2. Previous calculations on Iron(II) porphyrin systems indicated that porphyrins mediate the reduction of nitrate into nitric oxide through two different pathways involving the N-nitro and O-nitro-Fe(II) porphyrin complexes [
Finally, gas sensors can be extremely sensitive to the environment, and therefore, small changes on the film interface could produce changes in the surface energy, and therefore, the sensing response. For this reason, contact angle measurements were carried out before and after gas exposure, in order to investigate changes in the hydrophobic properties of the films.
So, in this work, we employed metal-free meso-tetraphenylporhyrin and zinc-tetraphenylporphyrin films to investigate their capacity to detect NO2 by studying changes in the adsorption spectra, morphology, contact angle and the in-situ conductivity along the films. Since most of the work related to the detection of volatile compounds has been devoted to the detection limits by using thin porphyrin films [
5, 10, 15, 20-tetraphenylporphyrin (H2TPP) and Zinc-5, 10, 15, 20-tetraphenylporphyrin (ZnTPP) compounds were purchased from Sigma-Aldrich (97%) and used without further purification. The films were deposited with the physical vapour deposition technique onto quartz and indium tin oxide (ITO) substrates for the optical and electrical characterizations, respectively, by using the VCM600 system from Norm Electronics Ltd working at 10−6 mbar. Spectroscopic measurements of the films were carried out by a UV-Vis Blue-wave miniature StellarNet spectrometer before and after gas absorption in the 400 - 1000 nm wave range. The morphology of the films was investigated with a field emission scanning electron microscope JSM-7800F from JEOL, working at 2 kV with the gentle beam mode in order to avoid damage to the films. Furthermore, atomic force microscopy images of the films interface were acquired with a JEOL JSPM-4210 microscope in the tapping mode. Both characterizations were performed before and after gas exposure to investigate morphological changes. The presence of the gas was also established with the energy dispersive spectroscopy technique by using elemental analysis with an Aztec energy standard microanalysis system. Conductive measurements of the films were acquired in a homemade device inside a gas chamber in order to investigate the in-situ conductivity of the films during was exposure. For this purpose, a dual tip setup connected to a power supply (Keithley 2400 SourceMeter) was employed. The conductive tips were placed in contact with the film surface at a constant DC current of 1 mA, and the voltage variation during gas exposure was obtained as a function of time. The NO2 gas was produced by the chemical reaction of a high purity (99.9%) copper wire and regent grade concentrated nitric acid 70% from Sigma-Aldrich to obtain different gas concentrations (20 ppm, 40 ppm and 80 ppm) assuming a 100% conversion. The reactor container was connected to the 0.637 L conductive measurement chamber. The gas was introduced at a constant flow rate to maintain the same experimental conditions in all cases. All experiments were carried out at room temperature.
On the other hand, density functional theory (DFT) calculations were performed in order to investigate the molecular interactions between porphyrin and gas molecules by using the hybrid B3LYP/6-31(d,p) basis set, since it has provided good agreement with experimental results. For this study, all molecular geometries were fully optimized with the Gaussian 09 program package. For the simulations, two different configurations for each porphyrin system were considered: a) the interaction of one nitrogen dioxide group at different positions on the porphyrin molecules, i.e. NO2-porphyrin, and b) the interaction of two porphyrin molecules and one nitrogen dioxide group, i.e. porphyrin-NO2-porphyrin. In the later case, the porphyrin self-assembled structure was employed to provide not only relevant information regarding the possible packing of the molecules within the film, but also the interaction of the NO2 molecule within the stacked porphyrin layers.
In
ppm exposure, the surface became more homogeneous but at higher magnification (not shown here), a more regular granular interface can be seen. In addition, the texture of these films after gas exposure is the same as the original interface, so, we can infer that these features do not change the original characteristics of the initial film. This result is different from the previous image (sem 1), which suggests that a different gas-film interaction mechanism took place.
In
On the other hand, for the film containing the porphyrin with the zinc central metal atom, the surface aspect is completely different after gas exposure (
The optical absorption spectra of the films before and after NO2 gas exposure for the a) metal-free and b) zinc porphyrine films are shown in
The spectra of the ZnTPP film,
Soret peak decreased its intensity as the gas concentration increased, while the new peaks between 650 - 800 nm showed the opposite trend. Another important feature of these plots is that the position of the new Soret and other peaks remained at nearly the same wavelength regardless of the gas concentration value.
Some authors have suggested that the red shift is due to changes in the aggregation state of the molecules within the film [
The in-situ and in-plane conductivity response of the metal-free and zinc porphyrin films for the various NO2 concentrations is shown in
concentration augments in both cases. As reference, the non-gas curve was included which showed a null response over time, as expected. The most important aspect of these plots is the magnitude of the saturation voltage between the two films. The conductive values are larger in the metal free system for all gas concentrations. In addition, the voltage response is faster in the former case as well as the saturation voltage. Similar results have been reported by Richardson et al. when using Langmuir-Blodgett metal-free porphyrin films [
Finally, contact angle measurements were performed before and after gas exposure (80 ppm) for both films in order to investigate changes in the surface properties. In
gas exposure, the contact angle decreased by 17 degrees for the metal free and 21 degrees for the metal film. Although the metal free film showed the lowest contact angle change, it also exhibited the highest surface roughness. Since both initial angles were lower than 90˚, it was expected a larger decrease of the contact angle for the metal free film due to the roughness increase, but the angle value was lower in this case. Since both, the chemical and morphologic properties affect the wettability of surfaces, from the roughness results, we assume that the chemical transformation rather than the roughness has a larger influence on the hydrophobic behavior of the new surface.
The optimized structures of the H2TPP, ZnTPP and gas molecules were used as starting configuration for the gas interaction. In both cases, the porphyrin ring exhibited a flat plane and the peripheral phenyl substituents were tilted 66.33˚ with respect to the average plane of the macrocycles, in agreement with other works [
By simulating the interaction between the free metal porphyrin molecule and one NO2 group, the most stable configuration was found with the NO2 molecule 2.8 Å above the porphyrin ring. The nitrogen atom pointed down towards the macrocycle, and it was located nearly above one of the inner nitrogen groups (7.67˚ from the perpendicular direction towards the macrocycle plane). The oxygen atoms lied 0.54 Å above the nitrogen plane at an angle of 7.67˚ pointing away from the macrocycle center towards the α position in the macrocycle according to the fisher nomenclature. The internal ∠ O-N-O angle was 132.05˚ as seen in
The ZnTPP-NO2 system, shows a planar ring where the most stable geometry corresponded to the coordination between one oxygen atom and the zinc metallic center in a cis-O-nitrito configuration, similar to other metal porphyrin systems interacting with NO2 groups [
The interaction among two porphyrins and one NO2 molecule provided a great amount of information regarding not only the configuration of the molecules in a stacking-like arrangement, but also the disruption of this configuration when a gas molecule was introduced. The ground energy configuration of
two metal free and zinc molecular systems was optimized previous to the gas insertion. For the metal free system, the two molecules exhibited a semi parallel arrangement, where the shortest perpendicular distance between both molecules was 4.38 Å. In addition, the system showed a shift between molecular cores of 6.82 Å (center-center distance) in addition to an in-plane rotation of 33.8˚ due to the presence of the external benzene groups. One of the molecules exhibited a planar configuration similar to the single metal free molecule, but the other, exhibited a slightly out-of-plane core saddle-shaped distortion, which can be attributed to the tetraphenyl porphyrin ring interacting with one of the external phenyl groups of the other molecule. This arrangement showed changes in the dihedral angles between the mean plane of the porphyrins and the peripheral phenyl groups that ranged between 61.28˚ to 90˚, being the perpendicular orientation of one of the phenyl groups on top of the other molecule [
On the other hand, the lowest energy configuration of two Zinc tetraporphyrin molecules showed a columnar-like arrangement, where one of the Zinc centers is slightly shifted from the vertical alignment. The distance between Zinc-Zinc atoms was 6.08 Å, whereas the horizontal shifting distance between Zinc-Zinc atoms was 3.13 Å.
In this arrangement, both molecules show planar configurations with the Zinc atoms perfectly aligned within the ring planes. In addition, the dihedral angles of the external phenyl groups did not show any change from the ground state molecular conformation. This two-molecular configuration was used as a initial arrangement for the gas interaction. The nitrogen dioxide molecule was introduced at different position in between the porphyrins molecules and the lowest energy configuration is shown in
in between the two porphyrins cores in a N-nitro configuration towards one of the porphyring rings. The nitrogen atom pointed towards one of the zinc atoms at a distance of 2.16 Å, while one of the oxygens point towards the opposite zinc atom with a separation of 2.10 Å. The ∠ O-N-O was 119.02˚. The binding energy for this arrangement was −18.50 kJ/mol.
In a different simulation, not shown here, two NO2 molecules were introduce between the two ZnTPP molecules and each coordinate to a different central metal atom. The two nitrogen dioxide groups remained closed, and the distance and alignment between the porphyrin molecules were hardly disrupted, suggesting an preferent coordination between the gas and the porphyrin metal centers.
Finally, preliminary results for the UV-Vis calculated spectra for the H2TPP molecule showed that the small bathochromic shift of the Soret peak after gas interaction is related to the distortion of the electronic structure of the macrocycle ring due to the ring deformation, whereas the appearance of the 670 nm peak is due to the absorption of the NO2 species to meso-carbon positions. As the NO2 concentration increased, more molecules coordinate to meso-carbon atoms increasing the peak size as observed in the experimental UV-Vis spectra. On the other hand, for the Zn metal porphyrin system, the large bathochromic shif of the Soret peak is due to the coordination of NO2 groups to the zinc metal center. In addition, we found distinct peaks between 520 nm and 540 nm, which are very close to the 521 nm experimental value.
Although previous results suggested that porphyrin films exposed to NO2 can recover over time or by exposing the films to elevated temperatures [
On the other hand, the film with the molecules coordinated to the zinc central atom showed the progressive absorption of NO2 gas in between the different layers of the molecular film, which in addition, provided a better stability of the molecular arrangement. From SEM and AFM results, we noticed that the film interface remained undamaged, although a swelling of the film is observed in agreement with DFT simulations.
When analyzing the absorption spectra of the H2TPP and ZnTPP films after gas exposure, bathochromic shifts of the Soret peaks were observed in both cases, although the largest difference was obtained in the later case. From the DFT simulations, the arrangement with the H2TPP molecules exhibited very little distortion of the individual macrocycle rings. In contrast, the presence of the gas in between the two Zn porphyrin molecules, not only modified the macrocycles and Zn atoms distances, but also distorted the planarity of both molecules affecting the π conjugation system contributing to the large red shift. On the other hand, it has been observed that the presence of NO2 either as gas or in substituted porphyrins molecules show the presence of peaks above the 600 nm wavelength, which is consistent with our results. Furthermore, as the gas concentration increased in our experiments, the size of the long wavelength peaks increased too, which reflects the amount of gas in the system. From our findings, shorter peaks were observed for the surface gas interaction, while larger peaks were seen for the system with gas trapped within the film.
The in-plane conductivity results showed a larger voltage increase for the metal free system and we noticed a constant saturation voltage over the time of the experiment regardless of the surface degradation due to the thickness of the films. Finally, the contact angle plots showed a decrease in the hydrophobic nature of the interface after gas exposure due to competitive changes in roughness and surface chemical transformation.
In this work, metal-free and zinc tetraphenyl porphyrin films were employed as gas NO2 gas sensors. In both cases, an irreversible degradation of the films was observed, which was most important for the metal free system due to the attack of the porphyrin ring by NO2 species. For the metal porphyrin film, the metal mediated the absorption of the gas in between the film layers by trapping NO2 gas molecules inside the molecular arrangement. These results were consistent with experimental UV-Vis, SEM, AFM and conductivity results. By using DFT and TD-DFT computer simulations, we were able to validate the experimental findings for the gas interaction mechanisms for each system. Finally, porphyrin films are sensitivity materials for the detection of NO2 and by using evaporation techniques, it is possible to select the mechanism for gas interaction as well as the detection time and life span of the sensor by modifying the presence of the central metal atom and the thickness of the molecular film.
MR gratefully acknowledges DGAPA-PAPIIT project number IN108017 and DGTIC-UNAM project number LANCAD-UNAM-DGTIC-289. JM Rivera acknowledges scholarship from CONACYT-Mexico. MR and YA Wang acknowledge Westgrid, Canada. Authors acknowledge Dr. Carlos Magana and Samuel Tehuacanero C. for SEM technical assistance and Mr. Arturo Martinez for technical support.
The authors declare no conflicts of interest regarding the publication of this paper.
Rivera, M., Rivera, J.M., Amelines-Sarria, O. and Wang, Y.A. (2018) Different Interaction Mechanisms of Evaporated Porphyrin Films Exposed to NO2. Advances in Materials Physics and Chemistry, 8, 441-457. https://doi.org/10.4236/ampc.2018.811030