InP-Bovine Serum Albumin Conjugates as Energy Transfer Probes.

The use of indium phosphide (InP) quantum dots (QDs) as biological fluorophores is limited by the low photoluminescence quantum yield (ϕ) and the lack of effective bioconjugation strategies. The former issue has been addressed by introducing a strain relaxing intermediate shell such as ZnSe, GaP etc. that significantly enhances the ϕ of InP. Herein, we present an effective strategy for the conjugation of emissive InP/GaP/ZnS QDs with a commonly used globular protein, namely bovine serum albumin (BSA), which generate colloidally stable QD bioconjugates, labeled as InP-BSA and demonstrate its use as energy transfer probes. The conjugate contains one protein per QD, and the circular dichroism spectra of BSA and InP-BSA exhibit similar fractions of α-helix and β-sheet, reflective of the fact that the secondary structure of the protein is intact on binding. More importantly, the fluorescence polarization studies corroborate the fact that the bound protein can hold a variety of chromophoric acceptors. Upon selectively exciting the InP-BSA component in the presence of bound chromophores, a reduction in the emission intensity of the donor is observed with a concomitant increase in emission of the acceptor. Time-resolved investigations further confirm an efficient nonradiative energy transfer from InP-BSA to the bound acceptors.