Optical pH meter by means of a porphyrin monolayer covalently assembled on a molecularly engineered silica surface

Engineering of inorganic surfaces by covalent bonding of organic molecules represents an interesting approach to the synthesis of hybrid inorganic/organic nanomaterials. Few works concerning appropriately functionalized porphyrin molecules bound to different substrates have been already reported and, usually, in the perspective of sensors fabrication. Some studies regarding optical pH sensors based on thin films containing different dyes have already been reported. Among these, only a few papers regard the application of porphyrins as pH sensors and none of them deal with a covalently assembled porphyrin monolayer. In this context, we focused our interest on the 5,10,15-{p-[9-methoxy-3-(oxyethylene)]oxyphenyl}-20-( p-hydroxy-phenyl) porphyrin, a chromophore that shows a very high molar absorbance coefficient and a good affinity toward [H3O+]. The presence of only one hydroxyl group in the peripheral position of the porphyrin allows an univocal covalent linkage to the substrate. Moreover, the presence of the 3-methoxy-3-(oxyethylene) groups, covalently bound in the remaining peripheral positions of the porphyrin, increases its hydrophilic character. There was, therefore, enough motivation to embark on the fabrication of a Porphyrin monolayer, covalently assembled on a silica substrate, and to study its pH sensing behavior for acid solutions. It has proven to be highly sensitive to acid pH (sensitivity ) 0.1 pH unit), and, therefore, represents a well-suited fast and reversible optical acid pH meter. An advantage of the present system with respect to compact multilayer films is the fact that no residual UV-vis signals due to not interacting inner dye layers are observed.