Improving the selectivity of a free base tetraphenylporphyrin based gas sensor for NO2 and carboxylic acid vapors

Langmuir-Schaefer (LS) films of a free base porphyrin 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO) are used in conjunction with a PMMA and carboxylic acid substituted calix[8]arene barrier layers deposited on top of the porphyrin films to enhance the selectivity of this class of vapor sensor. Initially, the monolayer properties of EHO without a barrier layer are investigated by reporting Π-A isotherms and the effect of transfer pressure on the gas sensing mechanism is studied. Optimal surface pressures for transferring the EHO films onto glass substrates and the number of layers to be transferred for gas sensing applications are presented. The prospect of tuning these vapor sensors in order to differentiate between large and small analyte molecules is investigated by applying overlying barrier layers which consists of a PMMA:calix[8]arene mixtures. EHO LS films with different barrier layers are exposed to NO2, acetic acid, butyric acid and hexanoic acid vapors. EHO films without any barrier layer are found to be highly sensitive to all four analytes studied; but the respective response speed depends on the vapor being sensed. Upon addition of the PMMA:calix[8]arene barrier layers on top of the EHO films this selectivity is further enhanced. The PMMA:calix[8]arene barrier layer acts as a size selective layer and the degree of selectivity is determined by the relative quantity of calix[8]arene molecules in the selective layer. For all the sensors tested recovery is achieved by gently heating, and the reversibility of sensor material is shown to be very good.