In this study, a broad range pH sensor was synthesized by loading the pH sensitive dye, fluorescein isothiocyanate (FITC), and a reference dye, rhodamine B isothiocyanate (RITC), into the mesostructure of hollow mesoporous silica nanoparticles (HMSNs) synthesized using a co-condensation method. Compared to a pH sensor based on the same pair of dyes on conventional mesoporous silica nanoparticles (MSNs), this dual-labeled pH sensor based on HMSNs shows a larger pH sensitive range, between 4.5 and 8.5, because of its broad surface curvature distribution which has a strong effect on the apparent pKa values of the FITC dye. The hollow mesoporous silica–dye nanoparticles were used to monitor intracellular pH via a ratiometric fluorescence method. The confocal images demonstrated the capacity of this broad range sensor which can differentiate simultaneously the local pH between various environments, for example, in the medium (pH = 7.2), cytosol (pH ∼ 7) and the endosome–lysosome pathway (pH = 4–5.5).
