Optochemical ammonia gas sensing properties of meso-substituted porphyrin derivatives immobilized Nafion film on glass slide

We have synthesized free base porphyrin derivatives with different substituents at meso position meso-tetrakis(4-nitrophenyl)porphyrin (MTNP), meso-tetrakis(4-pyridyl)porphyrin, 5,15-bis(ferrocenyl)-10,20-bis(4-nitrophenyl)porphyrin and 5,15-bis(4-nitrophenyl)-10,20-bis(3,5-dimethoxy-phenyl)porphyrin) and incorporated them into Nafion (Nf) film coated on glass slide to examine the ammonia gas sensing properties by optochemical method. The Soret band of the porphyrin derivatives was red shifted when they incorporated into Nf film in contrast to dichloromethane. The cause for the red shift was due to the protonation of free base porphyrin by the sulfonic acid groups of the Nf film coated on glass slide. When the porphyrin incorporated Nf film was exposed to ammonia gas, the Soret band was blue shifted. Among the different substituted porphyrin derivatives, Nf-MTNP was more sensitive toward ammonia gas. When Nf-MTNP film was exposed to ammonia gas, the Soret band was shifted from 447 nm to 424 nm. The concentration of ammonia gas was monitored from the absorbance changes at 424 nm. The detection limit of the Nf-MTNP sensing element toward ammonia gas was found to be 0.025 ppm.