Bioconjugates of luminescent CdSe-ZnS quantum dots with an engineered two-domain protein G for use in fluoro-immunoassays

Colloidal semiconductor quantum dots (QDs) seem suitable for labeling certain biomolecules for use in fluorescent tagging applications, such as fluoro-immunoassays. Compared to organic dye labels, QDs are resistant to photo-degradation, and these luminescent nanoparticles have size-dependent emission spectra spanning a wide range of wavelengths in the visible and near IR. We previously described an electrostatic self-assembly approach for conjugating highly luminescent colloidal CdSe-ZnS core-shell QDs with engineered two-domain recombinant proteins. Here we describe the application of this approach to prepare QD conjugates with the beta2 immunoglobulin G (IgG) binding domain of streptococcal protein G (PG) appended with a basic lucine zipper attachment domain (PG-zb). We also demonstrate that the QD/PG conjugates retain their ability to bind IgG antibodies, and that a specific antibody coupled to QD via the PG functional domain efficiently binds its antigen. These preliminary results indicate that electrostatically self-assembled QD/PG-zb/IgG bioconjugates can be used in fluoro-immunoassays.