Fluorescein and rhodamine B modified mesoporous silica particles were synthesized by post-grafting and co-condensation approaches. The materials exhibited different pore size distributions, particle shapes and sizes. The materials were characterized by nitrogen sorption, scanning electron microscopy and fluorescence spectroscopy. The Förster resonance energy transfer between the selected dye pair was explored for the different materials by exposure to various concentrations of gaseous ammonia. A logarithmic increase in rhodamine B emission with increasing ammonia concentration was observed for both post-grafted and co-condensed materials. The dye accessibility by ammonia gas in the silica framework of mesoporous materials was evaluated by using a flow cell gas sensor setup built in-house. Response to ammonia gas and recovery with nitrogen gas are explained by comparing the structure properties and dye loading of the materials. The post-grafted dye modified silica showed better performance in terms of reversibility and recovery.