Fast optical NO sub 2 gas sensing of porphyrin Langmuir-Blodgett films

A porphyrin sensor for the detection of NO sub 2 gas in the 0.1-5.0 ppm range has been developed. The sensing characteristics of 5,10,15,20-tetrakis(3,4,-bis[-ethylhexyloxy]phenyl)-21H,23H-porphine (EHO) Langmuir-Blodgett (LB) films are shown to be dependent on a range of parameters including surface pressure, film thickness and deposition speed. This research has led to exceptionally fast response times with the best t sub 5 sub 0 values of approx 15s achieved with relatively large absorbance changes of typically 60%. The kinetics of the sensing response have been simulated using the Elovichian model and shown to exhibit Langmuirian and Arrhenius behaviour. Ultra-fast LB deposition (500-1000 mm min sup - sup 1) has been developed as a means of both decreasing sample production time and increasing the surface area - volume ratio for enhanced gas response. This technique maintains orientational symmetry and high transfer ratio, yet displays a highly disordered porous structure. The 'bed of nails' texture is proposed as the primary enabler of the high sensitivities. Kinetics studies at different temperatures for both response and recovery have led to an Arrhenius plot which yields an activation energy of approx 0.7 eV. This is comparable to the values found in other work for related molecules. The ability of these films to fully recover is also interesting as very few organic systems have been shown to possess this quality. The two-stage nature of the recovery process has been attributed to fast desorption from surface sites followed by slower diffusion from the bulk material. Indeed approx 70 % of recovery has been shown to occur in a relatively short period of time - adding weight to the 'sponge like' structure theory. The Langmuir concentration studies have shown this material to be suitable for low concentration applications ranging from 0.1 - 5.0 ppm. This coupled to the fast rate of response and recovery has made these materials attractive to industry for l