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 2, CO, CH 4, H 2, NH 3, HCl, CHCl 3, C 2 H 5 OH, CH 3 OH, pyridine, tetrahydrofurane, triethylamine, and dimethylformamide. The self-assembled porphyrin monolayer appears highly sensitive to 1 ppm of NO 2 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, CH 4, H 2, and NH 3 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 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine, that does not contain phenyl substituents in the meso positions, show no changes in the optical spectrum upon NO 2 interaction. It emerges that aromatic substituents in the meso positions play a crucial role in determining the optical sensing properties.