Systematic study on FRET-pair functionalization of mesoporous thin films for correlation of pH-sensing and ionic mesopore accessibility

Ordered mesoporous silica materials (OMS) are relevant in various applications such as separation, catalysis, or drug delivery. Thereby their performance is mainly dependent on mesopore accessibility and molecular transport within mesopores which is strongly influenced by the spatial confinement. This is especially relevant in case of charged mesopore walls and thus electrostatically controlled transport. Therefore, it is highly important to monitor charged states of functionalized mesopores. In this study we adapt pH-probing strategies, known from pH-monitoring e.g. in cells, to mesoporous thin films aiming to monitor mesopore charge and correlate this to mesopores ionic permselectivity. Therefore, grafting of a FRET dye pair, consisting of a pH-sensitive dye (fluorescein isothiocyanate (FITC)) and a pH-insensitive reference dye (rhodamine B isothiocyanate (RITC)), to the mesopore wall is optimized. The grafted FITC-RITC FRET pair allows to monitor mesopore “pH” over a broad range of up to six pH-units after optimization of FRET pair functionalization. Thereby, “pH” monitoring is performed based on absorption and fluorescence readout using confocal microscopy baring the potential of localized detection. The detected pH-dependent absorption and fluorescence emission is in good agreement with the pH-dependent ionic mesopore permselectivity, and thus with the influence of mesopore confinement on ionic permselectivity as detected by cyclic voltammetry. In addition, this study not only shows the possibility of using FRET dye pairs for monitoring charged states in mesopores but as well the influence of these probing dye molecules on the mesopores characteristics, such as ionic permselectivity, indicating the need of optimizing functionalization with probe molecules.