<p>There is an intensive demand for developing personalized precision chemotherapy of osteosarcoma (OS) because chemotherapy is the first-line strategy for treatment of OS, and the conventional OS chemotherapeutic drugs can lead to severe side effects. In this study, a fluorescence resonance energy transfer (FRET)-based ratiometric fluorescence nanosensor, UCNP@PDA@5-TAMRA-pep, has been developed to detect caspase-3 activity with relatively high sensitivity (with a limit of detection of 0.82 ng mL^-1) and good specificity through conjugation of TAMRA modified caspase-3 peptide substrates (5-TAMRA-pep, which includes a caspase-3 cleavage site DEVD) on polydopamine (PDA) coated upconversion nanoparticle (UCNP@PDA). The TAMRA and PDA are employed as the donor and acceptor of FRET system, respectively, while UCNP is used as the internal reference. In the presence of caspase-3, the fluorescence of TAMRA can be recovered by cleavage of 5-TAMRA-pep, while the upconversion luminescence (UCL) intensity of UCNP is kept as constant. The UCNP@PDA@5-TAMRA-pep has been successfully used to accurately evaluate cisplatin’s (cis-Pt’s) concentration-dependent apoptotic effect on mouse-bearing MG-63 tumor model through in site monitoring caspase-3 activity.</p> Graphical Abstract <p></p>