Gas sensing properties of porphyrin assemblies prepared using ultra-fast LB deposition

The UV–vis absorbance spectrum of LB film assemblies of 5,10,15,20-tetrakis(3,4-bis[2-ethylhexyloxyphenyl])-21H,23H-porphine is very sensitive to low concentrations of NO2. LB films prepared at very high deposition rates (∼1000 mm min−1) yield t50 response and recovery times of 25 and 33 s, respectively, and show a sensitivity of 60% relative absorbance change (at 430 nm) for 4.4 ppm NO2. Atomic force microscopy shows that the morphology of these films is characterised by isolated micron-size domains, which are themselves composed of grains of several nm in diameter. This unconventional LB structure leads to a useful sensing material as a result of the molecular functionality of the porphyrin coupled to the enhanced surface area of the porous film assembly. The optical response of the EHO gradually decreases as its temperature is increased, resulting from the shift in the adsorption-desorption equilibrium towards desorption. An activation energy for adsorption of 0.68 eV is obtained. The spectrum recovers fully after exposure to NO2 and the rate of recovery can be accelerated dramatically with gentle heating (∼350 K) for a few seconds. The concentration dependence of the optical response over the range 0.8–4.4 ppm follows a Langmuir model.