A novel thiazol-based ratiometric dye for the detection of local pH values is synthesized, and its properties are characterized by a combination of optical spectroscopy, solid-state NMR and DNP (dynamic nuclear polarization)-enhanced solid-state NMR. This novel dye covers a completely different sensitivity range with its acidic pKa value of 3.5 compared to other established dyes for ratiometric pH detection, such as SNARF. The dye is grafted to the surfaces of mesoporous silica materials, which enables, for the first time, direct in situ measurements of the local pH values in silica mesopores by a simple UV-vis spectroscopy method. The obtained results, which are in good agreement with previous indirect techniques, indicate a background electrolyte-dependent pKa shift of at least one pH unit under nanoconfined conditions compared to the pKa of the dye in bulk solution.

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