The present invention discloses a solid lipid nanoparticle (SLN) comprising: a) a solid lipid core comprising at least a glyceride and/or at least a fatty acid; b) a mixture of amphiphilic components forming a shell around said core a); c) an alkaline-earth complex with a compound of formula I and/or II: d) at least a fluorescent dye selected from: a cyanine fluorescent dye and/or a polyetherocyclic compound selected from: coumarin, pyrano, quinoline, pyranoquinoline, indole and pyranoindole derivates in acid form or a pharmaceutically acceptable salt thereof. These nanoparticles allow a prolonged blood circulation half-life, show enhanced photostability and improved fluorescence signal. The dye is preserved from degradation and improves the fluorescent quantum yield.