Figure 1. Cationic liposomes (CL) formulations labeled with indocyanine green (ICG) or Oregon green (OG) were internally quenched. Liposomes composed of 60 mol% 1,2 dioleoyl-3-trimethylammonium propane (DOTAP), 40 - x mol% 1,2 dioleoyl-sn-glycero-3-phosphocholine (DOPC), and x mol% ICG-DOPE or OG-DOPE were prepared and diluted 1:50 (CL-ICG) or 1:2,000 (CL-OG). Intraliposomal quenching reduces the fluorescence in the liposomal membrane at concentrations of more than 0.5 mol% ICG or 5 mol% OG. The fluorescence signal of ICG is ten times lower than that of OG if the different dilutions are taken into account.