Thickness dependence of the toxic gas response in EHO Langmuir–Blodgett films prepared by ultra-fast deposition

5,10,15,20-Tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO) has previously been identified as a potential functional material for the gas sensing industry. It has been shown to exhibit fast optical response and recovery when exposed to NO2 in N2 carrier gas at very low concentrations (<5 ppm). The thickness dependence of the speed of response and recovery has been studied. LB films of this material have been fabricated using the ultra-fast deposition technique (1000 mm/min). Multilayer films ranging from 1 to 20 excursions have been produced and comparisons have been made regarding their average t50 and t90 (the time taken for 50% and 90% of the overall response to occur, respectively). A hypothesis has been suggested to explain the experimental data in terms of surface response and bulk diffusion properties.