Functionalization of siliceous materials, part 4: immobilization of fluorinated dyes for optical chemical sensor applications

Abstract

Porous glass (PG) particles are ideal supports in developing optical gas sensors as they combine fast mass transfer in macropores with large specific surface areas which are predestined for the deposition of indicator molecules. The well-established PG material is chemically stable and can be post-synthetically functionalized for introducing specific surface properties. Thus, surface functionalization with 3-mercaptopropyltrimethoxysilane (MPTMS) or 3-aminopropyl-triethoxysilane (APTES) provides the basis for the covalent coupling of the indicator molecule platinum(II)-5,10,15,20-meso-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin (PtTFPP) to the sensor matrix, reducing the risk of migration, aggregation, and leaching of the fluorescent dye. Nucleophilic substitution of a fluorine atom of the pentafluorophenyl PtTFPP groups by amino (APTES) and thiol (MPTMS) groups enables a covalent linkage of the oxygen indicator PtTFPP to the PG surface. The spectroscopic detection of the PtTFPP-silica bonding by solid-state ¹³C CP MAS NMR spectroscopy has turned out to be rather difficult due to very low indicator amounts. As an inexpensive alternative for PtTFPP, hexafluorobenzene C₆F₆ was used successfully for the spectroscopic proof of covalent bonds between fluorophenyl groups of an indicator dye and amino- or mercapto-silane cross-linker molecules. The PtTFPP/MPTMS- and the PtTFPP/APTES-modified PG sensor particles showed no leaching in organic solvents and have been applied for oxygen sensing up to oxygen pressures of 400 mbar p_O2. The PG sensor particles show non-linear Stern–Volmer calibration plots and it has been found that the covalent bonding of PtTFPP via APTES linkage onto silica surfaces is better suited for various sensor applications.