Determination of pH values has a vital influence in many chemical and biological processes. To accurately determine pH values, we fabricated a highly photostable ratiometric fluorescent pH−sensitive nanosensor by staining the core of mesoporous silica nanoparticle with a rhodol dye and chemically labelling its outer shell with a rhodamine derivative dye. The two dyes possess opposite pH−responding directions, which increases the band of the signal change. There is a nine−fold change in fluorescence intensity ratios when the solution pH changes from 3.0 and 9.0. Meanwhile, the nanosensors displayed yellowish emission in low pH value, orange emission in mid pH value, and reddish emission in high pH value, which can be readily inspected by bare eyes. Last but not least, excellent photostability and reversibility features make the nanosensors useful for the continuous measuring of pH with high accuracy.
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