Fluorescence enhancement and quenching of Eu<SUP>3+</SUP> ions by Au–ZnO core-shell and Au nanoparticles

We investigate the enhancement and quenching of Eu<SUP>3+</SUP> emission in presence of Au–ZnO core-shell nanoparticles and Au nanoparticles. The quenching of Eu<SUP>3+</SUP>-emission in presence of Au nanoparticles is caused by change of nonradiative rate due to energy transfer. However, the enhancement of Eu<SUP>3+</SUP>-emission in presence of Au–ZnO core-shell nanoparticles is due to local field, which modifies the radiative and nonradiative rate. Enhancement and quenching of Eu<SUP>3+</SUP> emission reflect the change in environment of Eu<SUP>3+</SUP> from Au nanoparticles to Au–ZnO core-shell nanoparticles which is confirmed by the Judd–Ofelt parameter (Ω<SUB>2</SUB>) analysis

Efficient resonance energy transfer from dye to Au@SnO<SUB>2</SUB> core–shell nanoparticles

The present study reports the shell thickness dependence fluorescence resonance energy transfer between Rhodamine 6G dye and Au@SnO2 core–shell nanoparticles. There is a pronounced effect on the PL quenching and shortening of the lifetime of the dye in presence of Au@SnO2 core–shell nanoparticles. The calculated energy transfer efficiencies from dye to Au@SnO2 are 64.4% and 78.3% for 1.5 nm and 2.5 nm thickness of shell, respectively. Considering the interactions of single acceptor and multiple donors, the calculated average distances (rn) are 75.8 and 71.5 Å for 1.5 nm and 2.5 nm thick core–shell Au@SnO2 nanoparticles, respectively

Au@ZnO core−shell nanoparticles are efficient energy acceptors with organic dye donors

The present study highlights the efficient fluorescence resonance energy transfer from Rhodamine 6G dye to Au@ZnO core−shell nanoparticle by steady state and time-resolved spectroscopy. The calculated energy transfer efficiencies from dye to nanoparticles are 41.3, 52.6, and 72.6% for Au, mixture of Au and ZnO, and core−shell Au@ZnO nanoparticles, respectively. There is a pronounced effect on the PL quenching and a shortening of the lifetime of the dye in the presence of Au@ZnO core−shell nanoparticle which is associated with high charge storage capacity. The nonradiative decay rates are 2.80 × 108, 3.90 × 108 and 7.67 × 108 s-1 for pure Au, mixture of Au and ZnO and core- shell Au@ZnO nanoparticles, respectively, indicating the resonance energy transfer process. The calculated Förster distances (R0) are 135.0 and 144.4 Å for Au, and core- shell Au@ZnO nanoparticles, respectively and corresponding the calculated distances (d) between the donor and acceptor are 143.05, and 123.5 Å. Considering the interactions of one acceptor and several donors, the calculated average distances (rn) between the donor and acceptor are 89.2 and 77.2 Å for Au and core- shell Au@ZnO nanoparticles, respectively. However, the distances between the donor and acceptor are 88.2 and 67.6 Å for Au and core- shell Au@ZnO nanoparticles, respectively, using the efficiency of surface energy transfer which follows a 1/d4 distance dependence between donor and acceptor. On the basis of these finding, we may suggest that surface energy transfer process has a more reasonable agreement with experimental finding

Formation of heteroepitaxy in different shapes of Au-CdSe metal-semiconductor hybrid nanostructures

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

Metal conjugated semiconductor hybrid nanoparticle-based fluorescence resonance energy transfer

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

Quenching of confined C480 dye in the presence of metal-conjugated γ-Cyclodextrin

Here we demonstrate the designing of new optical-based materials having nanotubular γ-CD aggregates linked by coumarin 480 dyes: Au nanoparticles for light harvesting system. It is found that the fluorescence quenching of the coumarin 480 dye increases from 79 to 99% due to confinement of the dye inside γ-CD. The formation of the nanotubular structure of C480/CD complex is confirmed by fluorescence anisotropy decay and TEM study. The average decay times of the C480 dye inside γ-CD in the absence and presence of attached Au nanoparticles are 4.77 and 1.91 ns, respectively, and the rate constant of ET is found to be 3.13 × 10<SUP>8</SUP> s<SUP>-1</SUP>

Core-size-dependent catalytic properties of bimetallic Au/Ag core–shell nanoparticles

Bimetallic core–shell nanoparticles have recently emerged as a new class of functional materials because of their potential applications in catalysis, surface enhanced Raman scattering (SERS) substrate and photonics etc. Here, we have synthesized Au/Ag bimetallic core–shell nanoparticles with varying the core diameter. The red-shifting of the both plasmonic peaks of Ag and Au confirms the core–shell structure of the nanoparticles. Transmission electron microscopy (TEM) analysis, line scan EDS measurement and UV–vis study confirm the formation of core–shell nanoparticles. We have examined the catalytic activity of these core–shell nanostructures in the reaction between 4-nitrophenol (4-NP) and NaBH4 to form 4-aminophenol (4-AP) and the efficiency of the catalytic reaction is found to be increased with increasing the core size of Au/Ag core–shell nanocrystals. The catalytic efficiency varies from 41.8 to 96.5% with varying core size from 10 to 100 nm of Au/Ag core–shell nanoparticles, and the Au100/Ag bimetallic core–shell nanoparticle is found to be 12-fold more active than that of the pure Au nanoparticles with 100 nm diameter. Thus, the catalytic properties of the metal nanoparticles are significantly enhanced because of the Au/Ag core–shell structure, and the rate is dependent on the size of the core of the nanoparticles

Size dependent resonance energy transfer between semiconductor quantum dots and dye using FRET and kinetic model

In the present study, we demonstrate the size dependent resonance energy transfer from CdSe QDs (donor) to Nile Red dye (acceptor) using steady state and time-resolved spectroscopy. A strong evidence of size dependent efficient resonance energy transfer between CdSe QDs and dye molecules is observed. Using the Förster theory, the calculated energy transfer efficiencies from QD to dye are 8.4, 13.8, and 51.2% for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe, respectively. A stochastic model for the kinetics of energy transfer from CdSe QDs to Nile Red dye molecules has been proposed to understand the interaction between excited states of CdSe QDs with dye molecules. By analyzing time-resolved fluorescence decay curves of CdSe QDs in the absence and in the presence of Nile Red dye, the values of the rate constant (kq) for energy transfer per one dye molecule and the efficiency (ϕET) of quenching have been calculated. The estimated energy transfer rates are 0.002, 0.016, and 0.038 ns−1 for 2.4 nm CdSe, 2.9 nm CdSe, and 3.3 nm CdSe QDs, respectively, which are well matched with FRET data

Hybrid colloidal Au-CdSe pentapod heterostructures synthesis and their photocatalytic properties

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 3.58×10<SUP>8</SUP> s<SUP>-1</SUP>. 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<SUP>-1</SUP>. This new class of metal-semiconductor heterostructures opens up new possibilities in photocatalytic, solar energy conversion, photovoltaic, and other new emerging applications

Photophysical properties of Au-CdTe hybrid nanostructures of varying sizes and shapes

We design well-defined metal-semiconductor nanostructures using thiol-functionalized CdTe quantum dots (QDs)/quantum rods (QRs) with bovine serum albumin (BSA) protein-conjugated Au nanoparticles (NPs)/nanorods (NRs) in aqueous solution. The main focus of this article is to address the impacts of size and shape on the photophysical properties, including radiative and nonradiative decay processes and energy transfers, of Au-CdTe hybrid nanostructures. The red shifting of the plasmonic band and the strong photoluminescence (PL) quenching reveal a strong interaction between plasmons and excitons in these Au-CdTe hybrid nanostructures. The PL quenching of CdTe QDs varies from 40 to 86 % by changing the size and shape of the Au NPs. The radiative as well as the nonradiative decay rates of the CdTe QDs/QRs are found to be affected in the presence of both Au NPs and NRs. A significant change in the nonradiative decay rate from 4.72×10<SUP>6</SUP> to 3.92×10<SUP>10</SUP> s<SUP>-1</SUP> is obtained for Au NR-conjugated CdTe QDs. It is seen that the sizes and shapes of the Au NPs have a pronounced effect on the distance-dependent energy transfer. Such metal-semiconductor hybrid nanostructures should have great potentials for nonlinear optical properties, photovoltaic devices, and chemical sensors