Plasmonically Induced Energy Flow in Monodisperse Quantum Dot Solids

We studied the excitation-power-dependent red shift and broadening of the emission spectra of monodisperse CdSe/ZnS quantum dot solids when they are in close proximity of gold metallic nanoparticles. Our results suggest that these features are the signs of plasmonic enhancement of the interdot energy transfer in such solids leading to (a) efficient funneling of excitons to the locations where quantum dots with large CdSe cores are and (b) near complete depletion of excitons in regions where the quantum dots with incomplete shells or/and small core sizes are located. We studied the impacts of the heat generated by the metallic nanoparticles and discussed the effects of excitation-power-dependent photoionization rates. The uneven spatial distribution of excitons in monodisperse quantum dot solids in the presence of metallic nanoparticles suggests that plasmonic fields can drive significant spatial migration of energy in such structures.     

Fluorescence Quenching of CdTe Nanocrystals by Bound Gold Nanoparticles in Aqueous Solution

Water-soluble gold nanoparticles with an average diameter of 5 nm were prepared with carboxylic acid terminated thiol ligands. These ligands contain zero to eight methylene moieties. CdTe nanocrystals with an average diameter of 5 nm were synthesized with aminoethanethiol capping. These nanocrystals displayed characteristic absorption and emission spectra of quantum dots. The amine terminated CdTe nanocrystals and carboxylic-acid-terminated gold nanoparticles were conjugated in aqueous solution at pH 5.0 by electrostatic interaction, and the conjugation was monitored with fluorescence spectroscopy. The CdTe nanocrystals were significantly quenched upon binding with gold nanoparticles. The quenching efficiency was affected by both the concentration of gold nanoparticles in the complex and the length of spacer between the CdTe nanocrystal and Au nanoparticle. The observed quenching was explained using Förster resonance energy transfer (FRET) mechanism, and the Förster distance was estimated to be 3.8 nm between the donor–acceptor pair.     

Plasmonic Control of Spontaneous Emission of Quantum Dots in Sub-Wavelength Photonic Templates

Finite difference time domain (FDTD) simulations were performed on two different plasmonic sub-wavelength photonic templates embedded with CdSe quantum dots. Tunable loading of these templates with plasmonic nano antenna allowed control of the emission from the embedded quantum dots. We discuss how large loading of nano antenna can effectively control the optical density of states for the quantum dots leading to enhancement of their radiative decay rates as observed in experiments. On the other hand, at low level of loading, while FDTD fails to capture the observed enhancement of decay rates in experiment, an alternative mechanism is suggested to exist in such cases. Thus, subtle interplay of multiple mechanisms engineered by appropriate placement and loading of plasmonic nano antenna in such templates is demonstrated as an effective method to control optical density of states and hence spontaneous emission of embedded quantum dots.     

Synthesis,Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.     

Luminescence Enhancement of Carboxyl-Coated CdTe Quantum Dots by Silver Nanoparticles

Metallic nanoparticles (NPs) have been used to improve the sensibility of biosensors and bioassays either by enhancing radiative emission or inducing quenching process on fluorescent probes. The aim of this research was to study the interaction of silver and silver-pectin NPs with water-dispersed carboxyl-coated cadmium telluride (CdTe) quantum dots (QDs). Metallic NPs were observed to change the emission of these fluorophores through local field effects. In a solution-base platform, an increase of 82 % was observed for the CdTe emission due to the interaction of QDs and silver-pectin NPs. QDs interaction with silver NPs without pectin was also investigated and a smaller emission enhancement of 20 % was detected. We observed that the NPs’ nature and QDs’ surface charge and concentration are important parameters for NPs-QDs interaction. Moreover, the presence of the pectin polymer shows to be a key component to the observed fluorescence enhancement.     

Energy transfer between CdSe/ZnS core/shell quantum dots and fluorescent proteins

Fluorescent proteins from the green fluorescent protein (GFP) family interact strongly with CdSe/ZnS quantum dots. Photoluminescence of GFP5 is suppressed by red-emitting CdSe/ZnS quantum dots with high efficiency in a pH-dependent manner. The elevated degree of quenching, around 90%, makes it difficult to analyze the remaining signal, and it is not clear yet whether FRET is the reason behind the quenching. When the donor is a green-emitting CdSe/ZnS quantum dot and the acceptor is the HcRed1 protein, it is possible to detect quenching of the donor and sensitized emission from the acceptor. It was verified that the sensitized emission has the low anisotropy characteristic of FRET. The present characterization identifies donor-acceptor pairs formed by fluorescent proteins and CdSe/ZnS quantum dots that are suitable for the exploration of cellular events. These donor-acceptor pairs take advantage of the exceptional photochemical properties of quantum dots allied with the unique ability of fluorescent proteins to act as gene-based fluorescent probes.     

Mortalin imaging in normal and cancer cells with quantum dot immuno-conjugates

Quantum dots are the nanoparticles that are recently emerging as an alternative to organic fluorescence probes in cell biology and biomedicine, and have several predictive advantages. These include their ⑴broad absorption spectra allowing visualization with single light source, ⑵exceptional photo-stability allowing long term studies and ⑶narrow and symmetrical emission spectrum that is controlled by their size and material composition. These unique properties allow simultaneous excitation of different size of quantum dots with a single excitation light source, their simultaneous resolution and visualization as different colors. At present there are only a few studies that have tested quantum dots in cellular imaging. We describe here the use of quantum dots in mortalin imaging of normal and cancer cells. Mortalin staining pattern with quantum dots in both normal and cancer cells mimicked those obtained with organic florescence probes and were considerably stable.     

Comparative study of the quenching of core and core-shell CdSe quantum dots by binding and non-binding nitroxides.

The quenching of core and core-shell CdSe quantum dots by TEMPO and 4-amino-TEMPO has been examined using steady state fluorescence spectroscopy. The efficiency of quenching is strongly dependent on the nanoparticle size, the binding properties of the nitroxide, and the presence or not of a protective shell, ZnS in our systems. The shell reduces the quenching efficiency significantly only in the case of binding nitroxides, such as 4-amino-TEMPO. Downward quenching plots revealing bimodal quenching characterize the Stern-Volmer plots obtained for 4-amino-TEMPO. The slope characteristic of the low concentrations regime depends strongly on the presence of a shell. For example, for particles with a 2.4 nm core, emitting at 525 nm the concentrations of 4-amino-TEMPO required to reduce the emission to one half are 0.65 microM and 0.08 mM for core and core-shell nanoparticles, respectively. Surprisingly, in the high concentration regime, a single Stern-Volmer slope of about 4000 M-1 seems to accommodate all systems. We speculate that this value is characteristic of the exchange of TOPO ligands by 4-amino-TEMPO.     

Quenching of Fluorescence from CdSe/ZnS Nanocrystal QDs Near Copper Nanoparticles in Aqueous Solution

Significant quenching of fluorescence from CdSe/ZnS nanocrystal quantum dots (QDs) coated with mercaptoundecanoic ligands has been realized by copper nanoparticles (NPs). (a) Static quenching in the electrostatic association between the CdSe/ZnS QDs and cetyltrimethylammonium bromide-coated Cu NPs and (b) dynamic quenching of the same nanocrystals by polyvinylpyrrolidone-coated Cu NPs were studied. In both cases, the quenching of fluorescence from the CdSe/ZnS nanocrystals is sensitive to nanomolar concentrations of the copper NPs, and the quenching efficiency increases as spectral overlap between the CdSe/ZnS emission and the copper nanoparticle absorption increases. This suggests that the observed quenching is a result of energy transfer processes. These findings open new avenues for the utilization of Cu NPs in energy transfer-based applications.     

Autometallographic tracing of quantum dots

A short clarifying view of how semiconductor quantum dots (QDs) can be made visible in tissue sections by autometallographic (AMG) silver enhancement and how the introduction of AMG enhanceable gold nanoparticles into isolated cells can be used to follow the fate of these marked cells in organisms and cell cultures. As the AMG approach for visualizing quantum dots is extremely sensitive, QDs less than one nanometer can be made visible at both LM and EM levels.     

Fluorescent (Au@SiO2)SiC Nanohybrids: Influence of Gold Nanoparticle Diameter and SiC Nanoparticle Surface Density

Gold@silica core–shell nanoparticles were prepared with various gold core diameters (ranging from 20 to 150 nm) and silica thicknesses (ranging from 10 to 30 nm). When the gold diameter is increased, the size dispersion became larger, leading to a broader plasmon band. Then, silicon carbide (SiC) nanoparticles were covalently immobilized onto silica to obtain hybrid (Au@SiO₂) SiC nanoparticles. The absorption properties of these hybrid nanoparticles showed that an excess of SiC nanoparticles in the dispersion can be identified by a strong absorption in the UV region. Compared to SiC reference samples, a blue shift of the fluorescence emission, from 582 to 523 nm, was observed, which was previously attributed to the strong surface modification of SiC when immobilized onto silica. Finally, the influence of several elaboration parameters (gold diameter, silica thickness, SiC concentration) on fluorescence enhancement was investigated. It showed that the highest enhancements were obtained with 10 nm silica thickness, low concentration of SiC nanoparticles, and surprisingly, with a 20-nm gold core diameter. This last result could be attributed to the broad plasmon band of big gold colloids. In this case, SiC emission strongly overlapped gold absorption, leading to possible quenching of SiC fluorescence by energy transfer.     

In vivo tumor targeting and spectroscopic detection with surface-enhanced Raman nanoparticle tags

We describe biocompatible and nontoxic nanoparticles for in vivo tumor targeting and detection based on pegylated gold nanoparticles and surface-enhanced Raman scattering (SERS). Colloidal gold has been safely used to treat rheumatoid arthritis for 50 years, and has recently been found to amplify the efficiency of Raman scattering by 14-15 orders of magnitude. Here we show that large optical enhancements can be achieved under in vivo conditions for tumor detection in live animals. An important finding is that small-molecule Raman reporters such as organic dyes were not displaced but were stabilized by thiol-modified polyethylene glycols. These pegylated SERS nanoparticles were considerably brighter than semiconductor quantum dots with light emission in the near-infrared window. When conjugated to tumor-targeting ligands such as single-chain variable fragment (ScFv) antibodies, the conjugated nanoparticles were able to target tumor biomarkers such as epidermal growth factor receptors on human cancer cells and in xenograft tumor models.     

141 DNA origami as a platform for assembly of nanophotonic elements

Achieving DNA-functionalized semiconductor quantum dots (QDs) that are robust enough to be compatible with the DNA nanotechnology that withstand precipitation at high temperature and ionic strength is a challenge. Here we report a method that facilitates the synthesis of stable core/shell (1–20 monolayers) QD-DNA conjugates in which the end part (5–10 nucleotides) of the phosphorothiolated oligonucleotides is embedded within the shell of the QD. These reliable QD-DNA conjugates exhibit excellent chemical, colloidal and photonic stability over a wide pH range (4–12) and at high salt concentrations (>100?mM Na⁺ or Mg²⁺), bright fluorescence emission with quantum yields of upto 70%, and broad spectral tunability with emission ranging from UV to NIR (360–800?nm). The assembly of these different QDs into DNA origami in a well-controlled pattern was demonstrated (Deng, Samanta, Nangreave, Yan, & Liu, 2012). We also used DNA origami as a platform to co-assemble a gold nanoparticle with 20?nm diameter (AuNP) and an organic fluorophore (TAMRA) and studied the distance dependent plasmonic interactions between the particle and the dye using steady state fluorescence and lifetime measurements. Greater fluorescence quenching was found at smaller inter-particle distances, which was accompanied by an enhancement of the decay rate. We further fabricated 20?nm and 30?nm AuNP homodimers with different inter-particle distances using DNA origami scaffolds and positioned a Cy3 fluorophore between the AuNPs in both the assemblies. Up to 50% enhancement of the Cy3 fluorescence quantum efficiency was observed for the dye between the 30?nm AuNPs. These results are in good agreement with the theoretical simulations (Pal et al., 2013).     

Spectroscopic investigation of alloyed quantum dot‐based FRET to cresyl violet dye

Quantum dots (QDs), bright luminescent semiconductor nanoparticles, have found numerous applications ranging from optoelectronics to bioimaging. Here, we present a systematic investigation of fluorescence resonance energy transfer (FRET) from hydrophilic ternary alloyed quantum dots (CdSeS/ZnS) to cresyl violet dye with a view to explore the effect of composition of QD donors on FRET efficiency. Fluorescence emission of QD is controlled by varying the composition of QD without altering the particle size. The results show that quantum yield of the QDs increases with increase in the emission wavelength. The FRET parameters such as spectral overlap J(λ), Förster distance R₀, intermolecular distance (r) , rate of energy transfer k_T (r), and transfer efficiency (E) are determined by employing both steady‐state and time‐resolved fluorescence spectroscopy. Additionally, dynamic quenching is noticed to occur in the present FRET system. Stern–Volmer (K_D) and bimolecular quenching constants (k_q) are determined from the Stern–Volmer plot. It is observed that the transfer efficiency follows a linear dependence on the spectral overlap and the quantum yield of the donor as predicted by the Förster theory upon changing the composition of the QD. Copyright © 2015 John Wiley & Sons, Ltd.     

Plasmonic Enhancement of Fluorescence and Raman Scattering by Metal Nanotips

We report modifications to the optical properties of fluorophores in the vicinity of noble metal nanotips. The fluorescence from small clusters of quantum dots has been imaged using an apertureless scanning near-field optical microscope. When a sharp gold tip is brought close to the sample surface, a strong distance-dependent enhancement of the quantum dot fluorescence is observed, leading to a simultaneous increase in optical resolution. These results are consistent with simulations of the electric field and fluorescence enhancement near plasmonic nanostructures. Highly ordered periodic arrays of silver nanotips have been fabricated by nanosphere lithography. Using fluorescence lifetime imaging microscopy, we have created high-resolution spatial maps of the lifetime components of vicinal fluorophores; these show an order of magnitude increase in decay rate from a localized volume around the nanotips, resulting in a commensurate enhancement in the fluorescence emission intensity. Spatial maps of the Raman scattering signal from molecules on the nanotips shows an enhancement of more than five orders of magnitude.     

Rapid DNA detection by interface PCR on nanoparticles

A novel, rapid DNA detection method based on fluorescence quenching of quantum dots (QDs) by gold nanoparticles (GNPs) through polymerase chain reaction (PCR) was developed. In proof-of-concept experiments, the length of the amplicon DNA ranging from 152 to 1003 base pairs (bp) could be determined based on quenched fluorescence intensity with 136 bp as the lower limit of effective range. And the real sample detections were also achieved successfully by this developed method. Therefore, this DNA detection method has the potential to be the powerful gene diagnostic tool.     

Sensitization enhancement of europium in ZnSe/ZnS core/shell quantum dots induced by efficient energy transfer

Eu‐doped ZnSe:/ZnS quantum dots (formed as ZnSe:Eu/ZnS QDs) were successfully synthesized by a two‐step wet chemical method: nucleation doping and epitaxial shell growing. The sensitization characteristics of Eu‐doped ZnSe and ZnSe/ZnS core/shell QD are studied in detail using photoluminescence (PL), PL excitation spectra (PLE) and time‐resolved PL spectroscopy. The emission intensity of Eu ions is enhanced and that of ZnSe QDs is decreased, implying that energy was transferred from the excited ZnSe host materials (the donor) to the doped Eu ions (the acceptor). PLE reveals that the ZnSe QDs act as an antenna for the sensitization of Eu ions through an energy transfer process. The dynamics of ZnSe:Eu/ZnS core/shell quantum dots with different shell thicknesses and doping concentrations are studied via PL spectra and fluorescence lifetime spectra. The maximum phosphorescence efficiency is obtained when the doping concentration of Eu is approximately 6% and the sample showed strong white light under ultraviolet lamp illumination. By surface modification with ZnS shell layer, the intensity of Eu‐related PL emission is increased approximately three times compared with that of pure ZnSe:Eu QDs. The emission intensity and wavelength of ZnSe:Eu/ZnS core/shell quantum dots can be modulated by different shell thickness and doping concentration. The results provide a valuable insight into the doping control for practical applications in laser, light‐emitting diodes and in the field of biotechnology. Copyright © 2014 John Wiley & Sons, Ltd.     

DNA monolayer on gold substrates characterized by nanoparticle labeling and scanning force microscopy

Monolayers of single-stranded DNA on gold substrates were studied by scanning force microscopy. Complementary DNA probes labeled by gold nanoparticles were applied for contrast enhancement. Substrate regions modified with DNA could be visualized in a highly specific manner. The influence of the solution concentration on the surface density of adsorbed nanoparticles could be visualized. Because individual label particles can be easily detected, this labeling technique opens the way for characterization of DNA monolayers with a lateral resolution in the nanometer range.     

Plasmon-Assisted Enhancement and Tuning of Optical Properties in <Emphasis Type="Italic">β-</Emphasis>In<Subscript>2</Subscript>S<Subscript>3</Subscript> Quantum Dots

Exciton-plasmon coupling can significantly modify the spectral response of semiconductor quantum dots in a metal nanoparticle-semiconductor complex system. β-In₂S₃ quantum dots of size ～3 nm and Ag nanospheres of size ～100 nm were synthesized by chemical route and coated over glass substrates. In the strong coupling regime, the plasmons are shown to mediate indirect Coulomb interaction between the quantum dots. In the proximity of Ag plasmons, the excitonic binding energy of the β-In₂S₃ quantum dots increases by ～500 meV, indicating that the interaction potential between the quantum dots is positive and repulsive in nature. This interaction also leads to strong coupling of the defect levels in the SQD complex. The defect emission wavelength can be enhanced by an order of 10² or shifted from red region (～650 nm) to green (～550 nm) by controlling the plasmon-induced defect level coupling. The experimental observation demonstrates one of the theoretically predicted consequences of exciton-plasmon interaction. This work demonstrates the possibility of harnessing the potential of the two complimentary systems (semiconductor quantum dots and metal nanoparticles) to achieve controllable emission and absorption properties for fabrication of nano plasmonic devices.     

Facile synthesis and photophysical characterization of luminescent CdTe quantum dots for Forster resonance energy transfer based immunosensing of staphylococcal enterotoxin B

Aqueous phase synthesis of CdTe quantum dots (QDs) with surface functionalization for bioconjugation remains the best approach for biosensing and bioimaging applications. We present a facile aqueous phase method to prepare CdTe QDs by adjusting precursor and ligand concentrations. CdTe QDs had photoluminescence quantum yield up to ≈33% with a narrow spectral distribution. The powder X‐ray diffraction profile elucidated characteristic broad peaks of zinc blende cubic CdTe nanoparticles with 2.5–3 nm average crystalline size having regular spherical morphology as revealed by transmission electron microscopy. Infra‐red spectroscopy confirmed disappearance of characteristic absorptions for –SH thiols inferring thiol coordinated CdTe nanoparticles. The effective molar concentration of 1 : 2.5 : 0.5 respectively for Cd²⁺/3‐mercaptopropionic acid/HTe^– at pH 9 ± 0.2 resulted in CdTe quantum dots of 2.2–3.06 nm having band gap in the range 2.74–2.26 eV respectively. Later, QD₅₂₃ and QD₆₀₁ were used for monitoring staphylococcal enterotoxin B (SEB; a bacterial superantigen responsible for food poisoning) using Forster resonance energy transfer based two QD fluorescence. QD₅₂₃ and QD₆₀₁ were bioconjugated to anti‐SEB IgY antibody and SEB respectively according to carbodiimide protocol. The mutual affinity between SEB and anti‐SEB antibody was relied upon to obtain efficient energy transfer between respective QDs resulting in fluorescence quenching of QD₅₂₃ and fluorescence enhancement of QD₆₀₁. Presence of SEB in the range 1–0.05 µg varied the rate of fluorescence quenching of QD₅₂₃, thereby demonstrating efficient use of QDs in the Forster resonance energy transfer based immunosensing method by engineering the QD size. Copyright © 2012 John Wiley & Sons, Ltd.