Langmuir–Schaefer films of five different free base tetraphenylporphyrins for optical-based gas sensing of NO2

The Langmuir–Schaefer (LS) film-forming properties of five different free base tetraphenylporphyrins are investigated. The five porphyrins vary in that the phenyl groups in each have different functional groups attached. The influence of pre-exposing the porphyrin solutions to NO2 prior to LS film formation is also studied. Atomic force microscopy (AFM) has been employed to examine the film morphology and stability at increasing temperature. The results have then been used to explain the different sensitivities for the porphyrins investigated when used as optical-based NO2 sensors. The different functional groups on the porphyrins investigated are shown to have only minor effects upon the absorption spectra but dramatically alter both the surface pressure/area per molecule isotherm and the morphology of the films formed by LS deposition. The subsequent film morphologies have a strong influence upon their sensitivity when used as optical-based NO2 sensors. Upon gentle heating the morphology of the films changes which results in a degradation of their sensitivity to NO2.