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

Abstract

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 (∼1000 mm min−1) yield t50 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 (∼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.