Excited-state lifetime assay for protein detection on gold colloids-fluorophore complexes

The interaction of the surface plasmons of gold nanoparticles (NP) a few nanometers in size with fluorophores can be used to engineer their fluorescence properties. This possibility can be exploited in principle to obtain nanodevices for protein-protein recognition. We studied different types of constructs based on gold NPs on which derivatives of fluorescein were bound. The interaction of this fluorophore with the gold surface plasmon resonances, mainly occurring through quenching, affects its excited-state lifetime that is measured by fluorescence burst analysis in standard solutions. The binding of proteins to the gold NPs through antigen-antibody recognition further modifies the dye excited-state lifetime. This change can therefore be used to measure the protein concentration. The data reported here indicate that one can measure the concentration of bovine serum albumine in solution with an apparent limit of detection of 5 +/- 2 pM.