Optimization of mixed Langmuir-Blodgett films of a water insoluble porphyrin in a calixarene matrix for optical gas sensing

The gas sensing capabilities of Langmuir-Blodgett (LB) mixed films of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO) and p-tert-butylcalix[8]arene (C8A) have been studied in this work. Although EHO is known to be very sensitive to NO₂ gas, this study demonstrates that the C8A matrix improves the sensing properties of the porphyrin molecules in the solid state. After the exposure to NO₂ and the subsequent recovery, the UV-vis spectrum of a C8A:EHO film shows no aggregation of the porphyrin. In atomic force microscopy (AFM) images, C8A:EHO films appear with sharper surfaces than those made of pure EHO, allowing a better accessibility of the gas molecules to the active binding sites. Multilayer LB films of the C8A:EHO system ranging from 2 to 40 layers have been prepared to study their response to NO₂ by UV-vis spectroscopy, and their kinetics reveal an important thickness dependence. Through the analysis of AFM images, it has been found that the surface roughness increases until the sample reaches 20 layers and then remains almost constant, which is related to the response time. The optimum film thickness has been found to be 20 layers, for which both the speed of response and the surface roughness are maximum.