Molecularly engineered silica surfaces with an assembled porphyrin monolayer as optical NO2 molecular recognizers

The assembly of the porphyrin 5,10,15-tri-(p-dodecanoxyphenyl)-20-(p-hydroxyphenyl) porphyrin (P) with SiO2 surfaces was studied with regard to its use as NO2 sensor based on optical recognition. The assembly called P-AM was synthesized by 3 successive steps. Fused SiO2 substrates were first cleaned and dried. The substrates were then immediately emersed in a n-heptane soln. of the chemisorptive siloxane, trichloro[4-(chloromethyl)phenyl] silane, to afford a monolayer of the coupling agent (CA). The silylated substrates were finally immersed in a DMF soln. of the P Na salt. N 1s angle-resolved X-ray photoelectron spectra of the P-AM showed features accounting for the 2 kinds of N in the free porphyrin base. The UV/Vis spectrum of the P-AM showed high quality signals and found counterparts in that of P in THF. Both showed a characteristic sharp Soret band at 425 nm. The NO2 sensing capability tested in soln. showed an evident intensity decrease of the Soret band parallel to the increase of 2 addnl. bands at 452 and 685 nm after 10 s of NO2 bubbling. Upon NO2 gas exposure the UV/vis spectra of the P-AM showed the disappearance of the Soret band whereas a new band at 462 appeared.