An upconversion nanoparticle-based fluorescence resonance energy transfer system for effectively sensing caspase-3 activity.

We report a new fluorescence resonance energy transfer (FRET) sensing platform for the sensitive detection of caspase-3 activity in vitro and in cells using NaGdF4:Yb3+,Er3+@NaGdF4 upconversion nanoparticles (UCNPs) as the energy donor and Rhodamine B (RB) as the energy acceptor. The phosphorylated RB-modified peptide containing a caspase-3 cleavage site and cell-penetrating peptide (CPP) motif (sequence, (RB)-DEVDGGS(p)GCGT(p)GRKKRRQRRRPQ) is immobilized on the UCNP surface via the strong coordination interaction between Gd3+ ions with phosphate. After the cleavage of DEVD by caspase-3, the RB is released from the UCNP surface and the reduced upconversion luminescence (UCL) is recovered. Under the optimum conditions, the recovery ratio of the UCL is linearly dependent on the caspase-3 concentration within the range of 0.01 to 1000 pg mL-1 and with a limit of detection (LOD) of 0.01 pg mL-1 (S/N = 3). In particular, the as-proposed UCNP-based FRET sensing platform has reasonable selectivity which is successfully employed to monitor caspase-3 activity in drug-induced apoptosis of HeLa cells.