In the present study, Au nanoparticle based surface energy transfer (SET) has been used to measure conformational changes in proteins. A significant photoluminescence (PL) quenching (91-97%) of tryptophan intensities of bovine serum albumin (BSA) protein is observed in the presence of Au nanoparticles, and the measured distances (r) between the donor (tryptophan) and the acceptor (Au nanoparticle) are 27.0, 22.9, and 25.7 Å for E, N, and B forms of BSA protein, respectively. Results indicate that Au nanoparticle quenches BSA fluorescence mainly through a static quenching mechanism. Analysis suggests that binding constant and bound/unbound ratio varies with changing the conformation of protein. The PL quenching of dye varies from 47.2 to 86.6% with changing the conformation of protein without changing the radiative rate of dye. The measured distances (d) between the donor (dye) and the acceptor (Au nanoparticle) are 116.5, 76.1, and 86.4 Å for E, N, and B forms of BSA protein, respectively, using the efficiency of surface energy transfer (SET) which follows 1/d 4 distance dependence. The estimated radii of different conformations of the protein nicely match with the reported values of hydrodynamic radii of different conformations of BSA protein. Therefore, such bioconjugated Au nanoparticle based surface energy transfer should have great potentials for optical-based molecular ruler.

We demonstrated the synthesis of LaPO 4 :Er:Yb-doped nanoparticles/nanorods and LaPO 4 :Er@Yb core-shell nanoparticles/nanorods by a solution-based technique. The mechanism related to morphology control of LaPO 4 : Er:Yb nanorods/nanoparticles is proposed and discussed. Bright-green (550 nm) and red (670 nm) emission were observed due to the transitions 2 H 11/2 + 4 S 3/2 f 4 I 15/2 and 4 F 9/2 f 4 I 15/2 , respectively. The experimental data for 550-and 670-nm emission bands of doped nanoparticle/nanorod and core-shell nanoparticles/nanorods have been fit with a straight line with a slope of ∼2, which confirms the two-photon absorption process. The enhancement of upconversion emission of LaPO 4 :Er:Yb-doped nanoparticles and LaPO 4 :Er@Yb core-shell nanoparticles/nanorods are mainly due to modifications of surface-related effects. It is found that the tensile strain increases from +1.0% to +1.9% with changing the shape from nanoparticle to nanorod and reversal of the lattice strain (compressive) is obtained for coated nanoparticle/ nanorod. It is worth mentioning that the lattice strain varies with changing the shape and surface coating on nanocrystals and the upconversion emission intensity increases with decreasing the tensile lattice strain and it increases with increasing compressive strain. Analysis suggests that the lattice strain plays an important role in modification of the upconversion properties of the rare-earth-doped nanocrystals.

In this report, we present a self-driven chemical process to design exclusive Au/CdSe pentapod heterostructures with Au core and CdSe arms. We have analyzed these heterostructures using high-resolution transmission electron microscope (HRTEM), high angle annular dark field-scanning transmission electron microscopic (HAADF-STEM), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) studies. Microscopic studies suggest that pentapod arms of CdSe are nucleated on the (111) facets of Au and linearly grown only along the [001] direction. From the XPS study, the shifting of peak positions in the higher binding energy region for Au/CdSe heterostructures compared to Au nanoparticles has been found which indicates the charge transfer from CdSe to Au in heterostructures. The steady state and time resolved spectroscopic studies unambiguously confirm the electron transfer from photoexcited CdSe to Au, and the rate of electron transfer is found to be 3.58×10⁸ s⁻¹. It is interesting to note that 87.2% of R6G dye is degraded by the Au/CdSe heterostructures after 150 min UV irradiation, and the apparent rate constant for Au/CdSe heterostructures is found to be 0.013 min⁻¹. This new class of metal-semiconductor heterostructures opens up new possibilities in photocatalytic, solar energy conversion, photovoltaic, and other new emerging applications.

Formation of heteroepitaxy and designing different-shaped heterostructured nanomaterials of metal and semiconductor in solution remains a frontier area of research. However, it is evident that the synthesis of such materials is not straightforward and needs a selective approach to retain both metal and semiconductor identities in the reaction system during heterostructure formation. Herein, the epitaxial growth of semiconductor CdSe on selected facets of metal Au seeds is reported and different shapes (flower, tetrapod, and core/shell) hetero-nanostructures are designed. These results are achieved by controlling the reaction parameters, and by changing the sequence and timing for introduction of different reactant precursors. Direct evidence of the formation of heteroepitaxy between {111} facets of Au and (0001) of wurtzite CdSe is observed during the formation of these three heterostructures. The mechanism of the evolution of these hetero-nanostructures and formation of their heteroepitaxy with the planes having minimum lattice mismatch are also discussed. This shape-control growth mechanism in hetero-nanostructures should be helpful to provide more information for establishing the fundamental study of heteroepitaxial growth for designing new nanomaterials. Such metal-semiconductor nanostructures may have great potential for nonlinear optical properties, in photovoltaic devices, and as chemical sensors.

In the present study, we demonstrate a pronounced effect on the photoluminescence (PL) quenching and shortening of decay time of CdSe quantum dots (QDs) during interaction with Au nanoparticles in a Au−BSA conjugated CdSe QD system. A systematic blue shift of the excitonic band of CdSe QDs and the red shifting of a plasmon band of Au nanoparticles are observed in a Au−BSA conjugated CdSe QDs system. Strong evidence of size dependent efficient resonance energy transfer between CdSe QDs and Au nanoparticles is observed. The PL quenching values are 60%, 40%, and 30% for 5.0 nm CdSe QDs, 5.4 nm CdSe QDs, and 5.8 nm CdSe QDs, respectively. The energy transfer efficiencies are 40.9%, 30%, and 19.2% for 5.0 nm CdSe, 5.4 nm CdSe, and 5.8 nm CdSe QDs, respectively. Using the FRET process, the measured distances (d) between the donor and acceptor are 95.3, 102.2, and 110.3 Å for Au−BSA conjugated 5.0 nm CdSe, Au−BSA conjugated 5.4 nm CdSe, and Au−BSA conjugated 5.8 nm CdSe QDs, respectively. These results are well matched with the structural estimated value, transmission electron microscopy data, and data from dipole approximation. Such energy transfer between QDs and Au nanoparticles provides a new paradigm for design of an optical based molecular ruler for the application in chemical sensing.