Hydrogenated soy phosphatidylcholine (HSPC), cholesterol (CHOL), 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino(polyethylene glycol)-2000] (DSPE-PEG2000-Amine), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(Lissamine rhodamine B sulfonyl) (Rhod-PE) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[folate(polyethylene glycol)-5000] (DSPE-PEG5000-Folate) were obtained from Avanti Polar Lipids (Alabaster, AL). 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid mono (N-hydroxysuccinimide ester) (DOTA-NHS-ester) and gadolinium (1,4,7,10-tetraazacyclododecane)-1,4,7,10-tetraacetate (DOTA-Gd) were obtained from Macrocyclics (Dallas, TX, USA). Europium chloride (EuCl3), praseodymium chloride (PrCl3) and cerium nitrate (Ce(NO3)3) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Ce-DOTA was synthesized in the lab. All other chemical were used as received. All buffer solutions were prepared with ultrapure grade water.