Free-Base Carboxyphenyl Porphyrin Films Using a TiO(2) Columnar Matrix: Characterization and Application as NO(2) Sensors

The anchoring effect on free-base carboxyphenyl porphyrin films using TiO(2) microstructured columns as a host matrix and its influence on NO(2) sensing have been studied in this work. Three porphyrins have been used: 5-(4-carboxyphenyl)10,15,20-triphenyl-21H,23H-porphyrin (MCTPP); 5,10,15,20-tetrakis(4-carboxyphenyl)-21H,23H-porphyrin (p-TCPP); and 5,10,15,20-tetrakis(3-carboxyphenyl)-21H,23H-porphyrin (m-TCPP). The analysis of UV-Vis spectra of MCTPP/TiO(2), p-TCPP/TiO(2) and m-TCPP/TiO(2) composite films has revealed that m-TCPP/TiO(2) films are the most stable, showing less aggregation than the other porphyrins. IR spectroscopy has shown that m-TCPP is bound to TiO(2) through its four carboxylic acid groups, while p-TCPP is anchored by only one or two of these groups. MCTPP can only be bound by one carboxylic acid. Consequently, the binding of p-TCPP and MCTPP to the substrate allows them to form aggregates, whereas the more fixed anchoring of m-TCPP reduces this effect. The exposure of MCTPP/TiO(2), p-TCPP/TiO(2) and m-TCPP/TiO(2) films to NO(2) has resulted in important changes in their UV-Vis spectra, revealing good sensing capabilities in all cases. The improved stability of films made with m-TCPP suggests this molecule as the best candidate among our set of porphyrins for the fabrication of NO(2) sensors. Moreover, their concentration-dependent responses upon exposure to low concentrations of NO(2) confirm the potential of m-TCPP as a NO(2) sensor.