Spectroscopic and Microscopic Characterization and Behavior of an Optical pH Meter Based on a Functional Hybrid Monolayer Molecular System: Porphyrin Molecules Covalently Assembled on a Molecularly Engineered Silica Surface

A covalently assembled monolayer of 5,10,15-tri{ p -[9-methoxy-tri(ethyleneoxy)]phenyl} 20 ( p -hydroxyphenyl)porphyrin molecules on a silica substrate was fabricated and characterized by X-ray photoelectron spectra, AFM images, and UV−vis measurements. In addition, its [H 3 O + ] optical sensing behavior was studied and compared to that of the parent water-soluble star polymer 5,10,15,20-tetrakis{ p- [ω-methoxy-poly(ethyleneoxy)]phenyl}porphyrin having ∼40 repetitive units in each of the poly(oxyethylene) branches. Upon immersion in increasingly acidic solutions, the UV−vis absorbance spectra of the porphyrin-assembled monolayer show the gradual disappearance of the Soret band at 427.2 nm and the progressive increase of a new band at 459.1 nm. In strongly acidic solutions (pH 1.0), the spectrum is substantially dominated by the new band and remains unchanged even with more acidic solutions. The simple washing with water and drying under a N 2 gas stream totally recovers the starting Soret signal. The presence of Mn(II), Co(II), Ni(II), Cd(II), or Pb(II) ions in water solution does not interfere with the behavior of the present optical acid pH meter. In contrast, the presence of Hg(II), Cu(II), or Zn(II) ions changes the pH-metering performance due to formation of metal−porphyrin complexes as revealed by UV−vis and MALDI-TOF measurements of metal-containing solutions of the similar 5,10,15,20-tetrakis{ p- [ω-methoxy-poly(ethyleneoxy)]phenyl}porphyrin with ∼17 repetitive units in each poly(oxyethylene) branch.