In this work,zwitterionic polymer(polyzwitterion)coated nanodiamonds(ND)with tumorous pH responsiveness were prepared for efficient application of commercial NDs in tumor cells imaging.Poly(carboxybetaine methacrylate)was firstly grafted on the pristine NDs(PCBMA-@-NDs)by surface-initiated reversible addition-fragmentation chain transfer(RAFT)polymerization.To achieve pH responsiveness and activated interaction with tumor cells,PCBMA-@-NDs were subsequently modified with benzene sulfonamide(PCBSA-@-NDs)via one step carbodiimide chemistry.The surface polymer functionalization was characterized by FTIR,1H NMR and TGA,and PCBMA-@-NDs and PCBSA-@-NDs showed great dispersibility,enhanced fluorescence intensity and excellent antifouling property in contrast to pristine NDs.Moreover,PCBSA-@-NDs was able to switch zwitterionic surface(at pH 7.4)reversibly into positive charge(at pH 6.5)via protonated or deprotonated acylsulfonamide.As expected,PCBSA-@-NDs demonstrated higher cell affinity and better imaging performance than those of zwitterionic NDs at tumor slight acid environment,proven by fluorescence microscopy and flow cytometry.Overall,we provide a feasible and valuable strategy to construct smart NDs,thus encouraging the application of cost-effective fluorescence nanomater-ials in biomedical fields.