Fast, reversible optical sensing of NO2 using 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine assemblies

The optical absorbance spectrum of LB film assemblies of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO) is highly sensitive to low concentrations of NO2 in nitrogen. LB films prepared at much faster than conventional deposition rates (similar to 1000 mm min(-1)) yield t(50) response and recovery times of 25 s and 33 s respectively and show a sensitivity of 60% relative absorbance change (at 430 nm) for 4.4 ppm NO2. The morphology of these films is revealed using atomic force microscopy to contain isolated micron-size domains which are themselves composed of grains of several nm in diameter. This unconventional 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 EHO film shows a gradually diminishing optical response as its temperature is increased, resulting from the shift in the adsorption-desorption equilibrium towards desorption as expected. The UV-visible spectrum recovers fully after exposure to NO2. The rate of recovery is slow at room temperature but can be accelerated dramatically with gentle heating (similar to 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.