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Abstract: Acidity (pH) is one of the most important factors for the stability of paper arts. In this study, a new fluorescent nanosensor was developed for measurement surface pH of paper. The nanosensor is based on the chitosan-modified silica nanoparticles, where Ru(bpy)3 2+ (bpy=2,2′-bipyridyl) was doped in the nanoparticles as reference dye and pH sensitive fluorescein isothiocyanate (FITC) were used as pH indicator. The nanocomposite was prepared by the modified reverse microemulsion method, and characterized by TEM, XPS and Uv–vis spectra. The pH nanosensor showed the linear dynamic range from 5.5 to 8.0. The dynamic performances of the nanosensor on the paper fiber showed the nanosensor was suitable for sensing in-situ surface pH of paper fiber because it could actively infiltrate into the surface of paper fiber. We demonstrated one practical application of the nanosensor for detection of surface pH in the precious art sample by integrating our nanosensor into the paper fiber. Simply washing by water, the nanosensor could be easily removed from the measured precious art sample without any damage after measurement. Taken together, this work established a modular approach to pH nanosensor design for detection of surface pH in precious art sample. Chitosan-modified nanosensor could also extend to detect pH of specific analytic samples. [Copyright &y& Elsevier]