Conjugation of spin labeled proteins to fluorescent quantum dots

Biocompatible nanomaterials functionalised with proteins have a high potential for diagnostics and therapy. Fluorescent quantum dots covalently coupled to functional proteins are of particular interest. The covalent binding to the functionalised ligands of the quantum dot polymer coating is a critical step because it has to be most efficient without affecting the protein conformation and function. In the present study we report on the application of EPR spectroscopy in the investigation of the bioconjugation process of fluorescent quantum dots. Our particular interest is the quantification of potential binding sites on the nanoparticle coating. After spin labeling, we determined 80+/-7 binding sites on an amino-functionalised quantum dot. Moreover, EPR spectroscopy allowed to quantify the amount of spin labeled recombinant human annexin V coupled to a carboxyl-functionalised quantum dot.