Peptide-functionalized upconversion nanoparticles-based FRET sensing platform for Caspase-9 activity detection in vitro and in vivo

In the present work, peptide-functionalized upconversion@silica nanoparticle (termed as UCNP@SiO 2 @Cy5-pep)-based fluorescence resonance energy transfer (FRET) sensing platform has been fabricated for the detection of caspase-9 activity in vitro and in vivo. A Cy5 labeled peptide containing specific motif LEHD for caspase-9 cleavage was designed and conjugated with UCNP@SiO 2 through covalent attachment . The red upconversion luminescence (UCL) emission of UCNP@SiO 2 can be quenched by Cy5 while the green UCL emission of UCNP@SiO 2 remains undisturbed. After the cleavage of LEHD by caspase-9, the Cy5 departed from UCNP@SiO 2 surface, resulting in recovery of red UCL emission of UCNP@SiO 2 . The UCNP@SiO 2 @Cy5-pep has been successfully used to monitoring the changes of caspase-9 activity levels in apoptotic cancerous cells (MG-63 and SW480) by cisplatin-induction. Under same experimental conditions, it is found that the intracellular caspase-9 activity level of cisplatin-treated MG-63 cells is higher than that of cisplatin-treated SW480. This result is consistent with that of commercial caspase-9 activity kits. The UCL signal intensity ratio of red emission to green emission of UCNP (termed as R/G) is linearly dependent on the amount of MG-63 cells within the range of 5 × 10 3 to 1 × 10 6  cells (i.e., 0.5–100 U mL −1 caspase-9) with a limit of detection (LOD) of 675 cells (i.e., 0.068 U mL −1 caspase-9). Furthermore, the practicability of UCNP@SiO 2 @Cy5-pep is demonstrated by detection of caspase-9 activities in tumor tissues in vivo, and satisfactory results are obtained.