Engineering inorganic surfaces by covalent bonding of organic molecules represents an interesting approach to the synthesis of hybrid organic/inorganic nanomaterials, in the perspective of fabrication of highly sensitive gas sensors for environmental monitoring.
Both porphyrin and phthalocyanine thin films offer attracting challenges for the easy monitoring at ppm level on the basis of their semiconducting and also optical properties.
We focused our interest on the 5,10,15-tri-{p-dodecanoxyphenyl}-20-(p-hydroxyphenyl) porphyrin and 5,10,15-tri-[p-(w-methoxy-polyethylenoxy)phenyl]-20-(p-hydroxyphenyl)porphyrin (called P), chromophores (see Scheme 1) that show a very high molar absorbance coefficient (3.3×10^5) and a good affinity toward NO2. The combination of these features makes the porphyrin very efficient as an optical gas-sensor. The presence of only one hydroxyl group in the peripheral position of the porphyrin allows a univocal covalent linkage to the substrate. Moreover, the steric hindrance due to the three long substituents (both aliphatic and polyethylene glycolic), covalently bounded in the remaining peripheral positions of the porphyrin, could prevent the well-known aggregation due to stacking interactions. In particular, we have covalently linked a P monolayer on silica substrates, obtaining the so called P-AM (Scheme 1).
Silica substrates have been characterized by means of different techniques, such as static and dynamic water contact angle measurements, angle resolved X-ray photoelectron spectra (XPS) and UV-visible spectroscopy. The NO2 sensing capability of the porphyrin system was first tested in THF solution and then directly on P-AM surfaces.
In the following, we discuss the results obtained by means of UV-Vis spectroscopy. The UV-vis spectrum (Figure 1) of the THF porphyrin solution, after 10 s of bubbling of 5 ppm NO2 in a N2 gas stream shows an evident intensity decrease of the Soret and Q-bands parallel to the increase of two additional bands at 452 and 685 nm.
This result is in accordance with previously reported data on similar systems and indicates reversible oxidation of the porphyrin.
Cycles of 50 fast UV-Vis scans (scan frequency 4 s, scan rate 50 nm/s) in the 400-500-nm range of the P-AM were carried out during an in situ continuous leaking of a NO2/N2 gas stream at 1 ppm (flow rate 100 sccm). Figure 2 shows the evolution of the UV-vis absorbance spectra of the P-AM upon gas exposure. It becomes evident that the Soret band at 425 nm progressively disappears whereas a new band at 462 nm grows upon NO2.
The collected results demonstrated high efficiency and good selectivity of the P-AM systems in the reversible recognition of NO2, higher than those reported in the literature.
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