Improvement of optical gas sensing using LB films containing a water insoluble porphyrin organized in a calixarene matrix

Abstract

A water insoluble porphyrin 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO) that is highly sensitive to low concentrations of NO₂ in nitrogen has been organized at the air–water interface using a host calix[8]arene (C8A) matrix. Different composition ratios of C8A have been studied at the air–water interface to select the optimal composition for obtaining a less aggregated porphyrin in the mixed films. An understanding of the interaction between EHO and C8A has been provided by means of a miscibility study. Mixed monolayers do not follow the additivity rule and the negative deviation from it depends on the molar fraction of the porphyrin in the mixture. Surface pressure–area (π–A) plots expressed per C8A molecule indicate the presence of the porphyrin at the air–water interface at low surface pressure while under further compression the hydrophobic EHO segregates above the C8A matrix. Brewster angle microscopy images of the mixed C8A : EHO monolayers show that the distribution of the EHO molecules at the air–water interface in the C8A matrix is more homogeneous than that observed for the pure EHO monolayer. The reflection spectra reveal the EHO aggregation in the mixed monolayers, although the C8A matrix in those films modifies such aggregation. The effect of the new organization for the EHO molecules in the mixed films is clearly manifested by the enhancement of response (faster), reproducibility (higher), sensitivity (higher) and temperature (wide range, being interesting for industrial applications) during exposure to NO₂, in comparison with those results previously obtained for the pure EHO film.