New hybrid optical sensors have been prepared by grafting specifically designed fluorescent, functionalised, phosphorus-containing dendrimers onto a nanocrystalline mesoporous titania thin film formed by evaporation-induced self-assembly. The structural characterisation and optical behaviour of these new fluorescent probes have been studied both in solution and after being grafted onto an inorganic network, which resulted in the discovery of improved probing selectivity in the solid state. This new hybrid sensor exhibits high sensitivity to phenolic OH moieties (especially those from resorcinol and 2-nitroresorcinol), which induce the quenching of fluorescence more efficiently in the solid state than in solution. This effect is a result of the increased spatial proximity of the fluorescent molecules, which is induced by pore confinement that makes the formation of hydrogen bonds between the hydroxyl moieties of the quenchers and the carbonyl groups of the dendrimer easier.