Detection and imaging of cathepsin L in cancer cells using the aggregation of conjugated polymer dots and magnetic nanoparticles

New technique for cathepsin L detection and imaging was developed via controlling an aggregation between magnetic nanoparticles (MNPs) and conjugated polymer dots (CPdots). By changing the electrostatic surface charges of MNPs, the techniques provided two protocols: 1) fluorescence quenching and spin-spin relaxation (T₂)-based magnetic resonance imaging (MRI); 2) fluorescence recovery for fluorescent imaging. By adding cathepsin L, the fluorescence and MRI relaxivity were altered by the formation of aggregates of CPdots and MNPs, inducing fluorescence quenching by MNP as well as a change in T₂ relaxation. In addition, nonfluorescent, pre-aggregated nanohybrid (CPdots and MNPs) can be dissolved in the presence of cathepsin L, resulting in turn-on fluorescent detection and the imaging of cathepsin L in cancer cells and in mice with tumors. In all cases, the MNPs played important roles as the fluorescence quencher and as the MRI-signal generator. The surface of CPdots was covered with poly(L-lysine) (pLys) for the aggregation of CPdots and MNPs, in which pLys was specifically degraded by the enzymatic action of cathepsin L. The CPdots systems showed high sensitivity and selectivity toward cathepsin L, by which these systems can be considered as potential candidates for cathepsin L detection and imaging.