As a typical plant virus which has biocompatibility and high transfection efficiency,tobacco mosaic virus (TMV) has shown broad application potential in drug or gene delivery field.Elucidating its intracellular trafficking is of great importance in investigation of its cytotoxicity,targeting site,and delivery efficiency,and is advantageous to designing new TMV-based drug delivery systems with different targets.By taking advantage of the regulated pH value of different organelles in a mammalian cell,we exploit a pH detection strategy to investigate the intracellular trafficking pathway of TMV.Here,we report a single-wavelength excited ratiometric fluorescent pH probe.This probe is constructed by simultaneously coupling pH-sensitive fluorescein isothiocyanate (FITC) and pH-insensitive rhodamine B isothiocyanate (RBIRC) onto the inner surface of TMV.The fluorescence intensity ratio of FITC to RBITC excited at 488 nm responds specifically towards pH value over other interferential agents.By taking use of this single-wavelength excited ratiometric pH probe and confocal laser scanning microscopy,it is shown that the endocytosed TMV is located in a pH decreasing microenvironment and eventually enters lysosomes.This work may provide important guidance on construction ofTMV-based nano carriers.