Occurrence of Forster Resonance Energy Transfer between Quantum Dots and Gold Nanoparticles in the Presence of a Biomolecule

In this paper the interactions of gold nanoparticles (Au NPs) and bovin serum albumin-gold nanoconjugates (BSA-GNPs) with cadmium sulfide quantum dots (CdS QDs) are investigated by using steady-state and time-resolved spectroscopic techniques under physiological conditions (pH similar to 7). From the analysis of the steady-state and time-resolved fluorescence quenching of CdS QDs in aqueous solution in the presence of BSA-GNPs it has been inferred that fluorescence resonance (Forster type) energy transfer (FRET) is primarily responsible for the quenching phenomenon. But in the presence of only Au NPs the fluorescence quenching of CdS NPs is primarily static in nature. Thus, it is apparent that, in the presence of BSA (in the case of the bionanoconjugate system), FRET becomes operative between CdS QDs and Au NPs present in the BSA-GNPs bionanoconjugate, whereas in the absence of this biomolecule direct contact between CdS and Au NPs facilitates the formation of ground-state complex. As shown from the high-resolution transmission electron microscopy (HRTEM) images of the bionanoconjugate, formation of a thin BSA layer around the Au NPs, situated at the core, inhibits the CdS QDs to come in contact with the Au NPs. In the CdS-bionanoconjugate system, CdS and Au NPs become separated by a distance of similar to 17 +/- 2 A, as observed from HRTEM measurement. It may be presumed that when Au NPs are present in the bionanoconjugate system, the system may suffer some conformational changes which facilitates the energy transfer process to occur within the CdS QDs and the Au NPs. Further investigations with similar systems would be necessary to make unequivocal assertion of this phenomenon. From the determination of the thermodynamic parameters it is apparent that the effect of van der Waals interaction is responsible for the interaction of CdS QDs with Au NPs to form ground-state complex. The effect of CdS NPs on the conformation of BSA-GNPs has been examined by analyzing CD spectra. Though the observed results demonstrate some conformational changes in the bionanoconjugate in the presence of CdS NPs, the secondary structure of the conjugate, predominantly of the a-helix, is found to retain its identity. This type of interaction between QDs and Au NPs in a protein-conjugated form provides a new insight for design and the development of FRET-based bionanosensors.