Characterization, optical recognition behavior, sensitivity, and selectivity of silica surfaces functionalized with a porphyrin monolayer

The optical gas recognition capability of a covalently self-assembled monolayer of 5,10,15-tri-{p-dodecanoxyphenyl}-20-(p-hydroxyphenyl) porphyrin molecules on silica substrates was studied. The following analytes have been investigated: NO, CO, CH4, H-2, NH3, HCl, CHCl3, C2H5OH, CH3OH, pyridine, tetrahydrofurane, triethylamine, and dimethyl form ami de. The self-assembled porphyrin monolayer appears highly sensitive to 1 ppm of NO2 in both anhydrous and humid conditions. The selectivity of the self-assembled porphyrin monolayer with respect to other analytes was also examined and present data show that the presence of CO, CH4, H-2, and NH3 does not influence its UV-vis spectrum. Many common solvents slowly affect the position of the Soret band. The presence of HCl vapors results in a broad band extending over the entire 440-500 nm range while the starting Soret disappears. UV-vis measurements on a n-hexane 1.0(.)10(-5) solution of the 2,3,7,8,1 2,13,17,18-octaethyl-21H,23H-porphine, that does not contain phenyl substituents in the meso positions, show no changes in the optical spectrum upon NO2 interaction. It emerges that aromatic substituents in the meso positions play a crucial role in determining the optical sensing properties.