Photoabsorption and Resonance Energy Transfer Phenomenon in CdTe�Protein Bioconjugates: An Insight into QD�Biomolecular Interactions

Luminescent quantum dots (QDs) possess unique photophysical
properties, which are advantageous in the development of new generation robust
fluorescent probes based on Forster resonance energy transfer (FRET) phenomena.
Bioconjugation of these QDs with biomolecules create hybrid materials having unique
photophysical properties along with biological activity. The present study is aimed at
characterizing QD bioconjugates in terms of optical behavior. Colloidal CdTe QDs
capped with 3-mercaptopropionic acid (MPA) were conjugated to different proteins
by the carbodiimide protocol using N-(3-dimethylaminopropyl)-N-ethylcarbodiimide
hydrochloride (EDC) and a coupling reagent like N-hydroxysuccinimide (NHS). The
photoabsorption of these QD�protein bioconjugates demonstrated an effective
coupling of electronic orbitals of constituents. A linear variation in absorbance of
bioconjugates at 330 nm proportionate to conjugation suggests a covalent attachment
as confirmed by gel electrophoresis. A red shift in the fluorescence of bovine serum
albumin (BSA) due to conjugation inferred a decrease in Stokes shift and solvent
polarization effects on protein. A proportionate quenching in BSA fluorescence
followed by an enhancement of QD fluorescence point toward nonradiative dipolar
interactions. Further, reduction in photobleaching of BSA suggests QD�biomolecular
interactions. Bioconjugation has significantly influenced the photoabsorption spectrum of QD bioconjugates suggesting the
formation of a possible protein shell on the surface of QD. The experimental result suggests that these bioconjugates can be
considered nanoparticle (NP) superstructures for the development of a new generation of robust nanoprobes.