A red-emitting gold nanoclusters was prepared by using the bovine serum albumin(BSA) and a ratiometric fluorescent pH probe was prepared by using the BSA-capped AuNCs and FITC.Results showed that the fluorescence intensity of FITC was gradually enhanced with pH changing from acid to alkali condition,while the fluorescence intensity of AuNCs was constant.The probe displayed a fine linear relationship towards pH,with the corresponding linear regression equation of Y=1.72X-8.8 and the relative standard deviation of 0.994 when the pH changed from 5.5 to 8.0.The probe had good reversibility when the pH was between 6.0 and 9.0.In addition,the PK15 cells were cultivated with the pH probe.Result of MTT assay showed that the pH probe was almost no toxicity to cells.