Multi-wavelength intermittent photoluminescence of single CdSe quantum dots

A single chromophore detection using video-microscopy is one of the latest methodologies to reveal unique characteristics, which could not be obtained from ensemble measurements. Among many kinds of subjects, dynamic optical properties observed in colloidal semiconductor nanoparticles are attractive and important not only for the basis of photo-physics but also for application studies, e.g. biological labeling, electronic devices. In this study, fluorescence video-microscopy was performed on cadmium selenide (CdSe) quantum dots (QDs) spin-coated on a glass substrate. From single CdSe QDs detection, emissions at wavelengths separated over 60 nm were observed for the first time. This spectral feature was attributed to the existence of double-emissive relaxation processes in CdSe QDs. Photoluminescence intermittency was also observed both from relaxation processes. Fluorescence video-microscopy, which was advanced in biology, can be applicable for the real-time monitoring of dynamic properties in semiconductor photo-physics.     

Formation of water-soluble and biocompatible TOPO-capped CdSe quantum dots with efficient photoluminescence

Hollow hydroxyapatite (HA) microspheres (diameter = 100–800 μm) were prepared by reacting solid Li₂O–CaO–B₂O₃ glass spheres in 0.25 M K₂HPO₄ solution at 37°C. The influence of subsequent heating on the microstructure, surface area, and compressive strength of the HA microspheres was evaluated using scanning electron microscopy, the BET method, and nano-mechanical testing. The surface area and rupture strength of the as-prepared microspheres were 135 m²/g and 1.6 ± 0.6 MPa, respectively. On heating for 8 h at 600°C, the surface area decreased to 27 m²/g, but there was no increase in the compressive strength (1.7 ± 0.4 MPa). Heating to 800°C (8 h) resulted in a marked decrease in the surface area (to 2.6 m²/g) and a sharp increase in the compressive strength (to >35 ± 8 MPa). These hollow HA microspheres may be useful as devices for drug or protein growth factor delivery or as scaffolds for engineered tissues.     

Formation of water-soluble and biocompatible TOPO-capped CdSe quantum dots with efficient photoluminescence

In this work, polysorbate surfactants with same functional groups but with varying molecular masses (Tween-80, Tween-40 and Tween-20) in different concentrations (0.1% to 20% w/w) were used to study the effect of the length of the surfactant chain on the luminescence of the entrapped TOPO-capped CdSe nanocrystals. Various phospholipids with different functional headgroups such as ethylene glycol (-PEG) and amine (-NH2) were used to improve biocompatibility and provide sites for bioconjugation respectively. It is understood that that the hydrophobic ends of the surfactant binds with the water repelling groups of the cap layer, thus modifying the CdSe cap layer and making it soluble in aqueous media. It was observed that the PL emission intensity of CdSe increases with increase in concentration of Tween-series surfactant unlike in the case of thiol-coated CdSe nanoparticles. However, higher PL intensity was obtained in the case of stoichiometric CdSe with Tween-40 corresponding to 20% w/w. The efficient PL sustainability of water-soluble CdSe QD's can be attributed to the simpler chain structure of Tween-40 surfactant resulting in better passivation of the micelle.     

Amphoteric CdSe nanocrystalline quantum dots

The nanocrystal quantum dot (NQD) charge states strongly influence their electrical transport properties in photovoltaic and electroluminescent devices, optical gains in NQD lasers, and the stability of the dots in thin films. We report a unique electrostatic nature of CdSe NQDs, studied by electrophoretic methods. When we submerged a pair of metal electrodes, in a parallel plate capacitor configuration, into a dilute solution of CdSe NQDs in hexane, and applied a DC voltage across the pair, thin films of CdSe NQDs were deposited on both the positive and the negative electrodes. Extensive characterizations including scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) and Raman studies revealed that the films on both the positive and the negative electrodes were identical in every respect, clearly indicating that: (1)?a fraction (<1%) of the CdSe NQDs in free form in hexane solution are charged and, more importantly, (2)?there are equal numbers of positive and negative CdSe NQDs in the hexane solution. Experiments also show that the number of deposited dots is at least an order of magnitude higher than the number of initially charged dots, indicating regeneration. We used simple thermodynamics to explain such amphoteric nature and the charging/regeneration of the CdSe NQDs.     

Noncovalent attachment of CdSe quantum dots to single wall carbon nanotubes

Noncovalent attachment of CdSe quantum dots (QDs) to single wall carbon nanotubes (SWNTs) through an intermediary 1-pyrenebutyric acid N-hydroxy-succinimide ester (PBASE) molecule has been performed. The ligand exchange process from trioctylphosphine oxide (TOPO)-capped CdSe to the 4-aminothiophenol (ATP) ligand is supported by solvent solubility, NMR spectroscopy, and IR spectroscopy, with an estimated molecular efficiency > 50:1. Noncovalent coupling of the PBASE molecule causes a redshift in the SWNT interband electronic transitions, consistent with a π–π interaction that promotes electron delocalization. TEM analysis after chemical coupling of the CdSe–ATP QDs to the PBASE–SWNTs shows an abundant coverage of QDs along the SWNT bundles. Raman spectra (1.96 eV excitation) of PBASE–SWNTs and the noncovalent product demonstrate that each of the major Raman modes (RBM, D-, G-, or G′-bands) is unaltered by the noncovalent interaction with PBASE or attachment of CdSe QDs, indicating that the structural integrity of the SWNTs is maintained. However, upshifts in the Raman modes are observed, the largest being for the G′-band, indicating charge transfer between the SWNTs and attached CdSe QDs.     

Continuous distribution of emission states from single CdSe/ZnS quantum dots

The photoluminescence dynamics of colloidal CdSe/ZnS/streptavidin quantum dots were studied using time-resolved single-molecule spectroscopy. Statistical tests of the photon-counting data suggested that the simple "on/off" discrete state model is inconsistent with experimental results. Instead, a continuous emission state distribution model was found to be more appropriate. Autocorrelation analysis of lifetime and intensity fluctuations showed a nonlinear correlation between them. These results were consistent with the model that charged quantum dots were also emissive, and that time-dependent charge migration gave rise to the observed photoluminescence dynamics.     

Bias-induced photoluminescence quenching of single colloidal quantum dots embedded in organic semiconductors

We demonstrate reversible quenching of the photoluminescence from single CdSe/ZnS colloidal quantum dots embedded in thin films of the molecular organic semiconductor N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) in a layered device structure. Our analysis, based on current and charge carrier density, points toward field ionization as the dominant photoluminescence quenching mechanism. Blinking traces from individual quantum dots reveal that the photoluminescence amplitude decreases continuously as a function of increasing forward bias even at the single quantum dot level. In addition, we show that quantum dot photoluminescence is quenched by aluminum tris(8-hydroxyquinoline) (Alq3) in chloroform solutions as well as in thin solid films of Alq3 whereas TPD has little effect. This highlights the importance of chemical compatibility between semiconductor nanocrystals and surrounding organic semiconductors. Our study helps elucidate elementary interactions between quantum dots and organic semiconductors, knowledge needed for designing efficient quantum dot organic optoelectronic devices.     

Extracellular biosynthesis of biocompatible CdSe quantum dots

An extracellular biosynthesis method has been developed to prepare cadmium selenide (CdSe) quantum dots (QDs) with strong fluorescence emission by incubating cheap Cd and Se inorganic salts with Escherichia coli (E.coli) bacteria. Ultraviolet–visible absorption spectra, photoluminescence (PL) spectra, and high‐resolution transmission electron microscopy analysis showed that the biosynthesised CdSe QDs have an average size of 3.1 nm, the excellent optical properties with fluorescence emission around 494 nm, and the good crystallinity. It was found that addition of 80 mg of mercaptosuccinic acid resulted in the formation of CdSe QDs with highest PL intensity. Furthermore, Fourier‐transform infrared spectra of as‐synthesised CdSe QDs confirmed the presence of a surface protein capping layer. The biosynthesised CdSe QDs were incorporated into the yeast cells as illustrated by laser confocal scanning microscopy images, showing a great potential in bio‐imaging and bio‐labelling application.Inspec keywords: microorganisms, molecular biophysics, fluorescence, visible spectra, nanofabrication, nanobiotechnology, proteins, cellular biophysics, nanostructured materials, wide band gap semiconductors, cadmium compounds, semiconductor quantum dots, II‐VI semiconductors, transmission electron microscopy, photoluminescence, optical microscopy, ultraviolet spectra, Fourier transform infrared spectra, biological techniques, semiconductor growthOther keywords: biocompatible CdSe quantum dots, extracellular biosynthesis method, cadmium selenide quantum dots, high‐resolution transmission electron microscopy analysis, biosynthesised CdSe QDs, Fourier‐transform infrared spectra, Escherichia coli, ultraviolet‐visible absorption spectra, PL intensity, fluorescence emission, photoluminescence spectra, optical properties, surface protein capping layer, laser confocal scanning microscopy images, bioimaging, biolabelling application, yeast cells, f mercaptosuccinic acid, CdSe     

Synthesis and photoluminescence of bright water-soluble CdSe/ZnS quantum dots overcoated by hybrid organic shell

Photoluminescence properties from water soluble CdSe/ZnS QDs encapsulated with hybrid trioctylphosphine-poly(acrylamide-co-acrylic acid)-ethanolamine (TOPO-PSMA-EA) shell have been investigated. It was found that PL efficiency of CdSe/ZnS QDs in water was increased 5–30% after introducing PSMA-EA polymers to encapsulate CdSe/ZnS-TOPO QDs. Higher PSMA concentrations were found to enhance the PL efficiency of QDs up to 1.8 folds, which is ascribed to a better packing and passivation of the TOPO-PSMA-EA shell over the QDs. Time-resolved photoluminescence suggested that the mean lifetime of photoexcited carriers in the water-soluble CdSe/ZnS-TOPO-PSMA-EA QDs elongated 2–17 ns compared with that of uncoated samples, indicating that PL quenching defects were effectively removed for CdSe/ZnS QDs with hybrid TOPO-PSMA-EA shell.     

Synthesis and photoluminescence of ZnS quantum dots

Single-phase zinc sulphide (ZnS) quantum dots were synthesized by a chemical method. The influence of the pH value of the Zn(CH3COO)2 solution on the size and photoluminescence properties of the ZnS quantum dots was evaluated. X-ray power diffraction, transmission electron microscopy, and ultraviolet-visible spectroscopy were used to characterize the structure, size, surface states, and photoluminescence properties of ZnS quantum dots. The results showed that the crystal structure of ZnS quantum dots was a cubic zinc blende structure, and their average diameter was about 3.0 nm. ZnS quantum dots with good distribution and high purity were obtained. A strong broad band centered at about 320 nm was observed in the excitation spectrum of ZnS quantum dots. Their emission spectrum peaking at about 408 nm, was due mostly to the trap-state emission. The relative integrated emission intensity of ZnS quantum dots decreased as the pH value of the Zn(CH3COO)2 solution increased, which could be ascribed to the increase in average diameter of the ZnS quantum dots as the pH value of Zn(CH3COO)2 solution increased.     

Sensitization of ZnO nanorods with CdSe quantum dots

We have optimized the low-temperature growth of aligned arrays of zinc oxide nanorods of controlled length and diameter on conductive substrates. Varying the solution concentration and growth time, we were able to tune the nanorod diameter and length in the ranges 40–600 nm and 0.5–15 μm, respectively. The grown zinc oxide nanorods were photosensitized with CdSe quantum dots (QDs) in an oleic shell, which was replaced by pyridine. We studied the optical and transport properties of the ZnO nanorod arrays, with and without CdSe QDs on their surface. The current-voltage characteristics of the ZnO nanorod arrays with CdSe QDs are significantly influenced by illumination with light at a wavelength under the absorption band of the QDs, which points to effective interaction between the QDs and ZnO matrix.     

Low temperature synthesis of colloidal CdSe quantum dots

In this study, the CdSe nanocrystals were prepared in phenyl ether and octyl amine to investigate the variations of their size, bandgap energy, and photoluminescence with growth time and temperature. The sizes of the CdSe nanocrystals were measured using High Resolution Transmission Electron Microscopy (HRTEM), and found to be nearly monodisperse for relatively low growth temperature, 130 degrees C. Their optic properties were characterized by photoluminescence measurements, which showed that the colors of the nanocrystals could be controlled. The bandgap energies of the nanocrystals were calculated theoretically and found to be in accord with quantum confinement theory. This synthetic method requires only a cheap solvent and offers good reproducibility at a lower price.     

The influence of capping thioalkyl acid on the growth and photoluminescence efficiency of CdTe and CdSe quantum dots

The influence of thioalkyl acid ligand was evaluated during aqueous synthesis at 100?°C and under hydrothermal conditions (150?°C) of CdTe and CdSe quantum dots (QDs). Experiments performed with 3-mercaptopropionic acid (MPA), 6-mercaptohexanoic acid (MHA) and 11-mercaptoundecanoic acid (MUA) demonstrated that the use of MHA and MUA allowed for the preparation of very small nanoparticles (0.6-2.5?nm) in carrying out the reaction under atmospheric pressure or in an autoclave and that the photophysical properties of QDs were dependent on the ligand and on the synthesis conditions. The influence of various experimental conditions, including the Te-to-Cd ratio, temperature, and precursor concentration, on the growth rate of CdTe or CdSe QDs has been systematically investigated. The fluorescence intensities of CdTe QDs capped with MPA, MHA, or MUA versus pH were also found to be related to the surface coverage of the nanoparticles.     

Fluorescence thermal antiquenching of liposome encapsulated CdSe quantum dots

The green emission semiconductor CdSe quantum dots are successfully encapsulated with lecithoid molecules and transferred into aqueous solution. The liposome-encapsulated CdSe maintain similar emission spectrum properties to free CdSe quantum dots. Fluorescence thermal antiquenching is investigated for the liposome-encapsulated CdSe when the temperature is increased from 20 degrees C to 80 degrees C. The reason of the fluorescence enhancement with increasing the temperature is that the vesicle structure of liposome-encapsulated CdSe becomes the CdSe micell structure over the phase transition temperature of the liposome vesicles, and the corresponding structure variation inducing surface reconstruction of CdSe quantum dots.     

CdSe and CdSe/ZnS quantum dots for the detection of C-reactive protein

In this study, a method for the detection of C-reactive protein (CRP) using CdSe and CdSe/ZnS quantum dots (QDs) is proposed. CdSe and CdSe/ZnS core-shell QDs are synthesised by using 2-mercaptosuccinic acid (MSA) as a capping agent. These QDs were then subjected to various characterisation studies, namely X-ray diffraction and transmission electron microscope for size and structure, Fourier transform infrared spectroscopy for the confirmation of functional groups, ultraviolet–visible absorption and fluorescence spectroscopy for optical characteristics and dynamic light scattering for hydrodynamic changes of QDs. Two biochemical mixtures were developed: one by mixing blood serum containing CRP and CdSe-phosphorylethanolamine (PEA) and the other by mixing blood serum with CdSe/ZnS-PEA. When these mixtures are observed for fluorescence due to interaction of QDs with CRP, a correlation between changes in fluorescence for different concentrations of CRP is noted. The result demonstrates that CRP can be detected with the help of QDs without using any antibodies.     

CdSe量子点的合成、功能化及生物应用

量子点是一种新型荧光纳米材料,具有独特而优良的荧光性质,近年来受到研究者的广泛关注。文章综述蛋白质、抗体、肽类以及DNA等对CdSe量子点(CdSe QDs)的表面功能化作用,以及CdSe QDs在生物传感分析中的重要研究进展。具体介绍CdSe量子点的多种合成方法(包括有机相合成、水相合成等),蛋白质、抗体、肽类、DNA利用共价键或静电作用对CdSe量子点修饰方法,以及其在生物医学标记与成像、生物传感、药物载送以及癌症治疗等领域的相关应用,最后针对现有研究的不足进行展望。希望通过对CdSe量子点全方位总结与概述,在一定程度上帮助科研工作者快速、准确了解其相关性质与研究进展。     

Recombination dynamics in CdTe/CdSe type-II quantum dots

Recombination dynamics in CdTe/CdSe core-shell type-II quantum dots (QDs) has been investigated by time-resolved photoluminescence (PL) spectroscopy. A very long PL decay time of several hundred nanoseconds has been found at low temperature, which can be rationalized by the spatially separated electrons and holes occurring in a type-II heterostructure. For the temperature dependence of the radiative lifetime, the linewidth and the peak energy of PL spectra show that the recombination of carriers is dominated by delocalized excitons at temperatures below 150?K, while the mixture of delocalized excitons, electrons and holes overwhelms the process at higher temperature. The binding energy of delocalized excitons obtained from the temperature dependence of the non-radiative lifetime is consistent with the theoretical value. The energy dependence of lifetime measurements reveals a third power relationship between the radiative lifetime and the radius of QDs, the light of which can be shed by the quantum confinement effect. In addition, the radiative decay rate is found to be proportional to the square root of excitation power, arising from the change of wavefunction overlap of electrons and holes due to the band bending effect, which is an inherent character of a type-II band alignment.     

Shell distribution on colloidal CdSe/ZnS quantum dots

Scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) was used to determine the chemical distribution of semiconductor shell material around colloidal core-shell CdSe/ZnS quantum dots (QDs). EELS signals from positions around the QD indicate a well-defined shell of ZnS surrounding the CdSe core, but the distribution of the shell material is highly anisotropic. This nonuniformity may reflect the differences in chemical activity of the crystal faces of the core QD and implies a nonoptimal QD surface passivation.     

Ligand-mediated modification of the electronic structure of CdSe quantum dots

X-ray absorption spectroscopy and ab initio modeling of the experimental spectra have been used to investigate the effects of surface passivation on the unoccupied electronic states of CdSe quantum dots (QDs). Significant differences are observed in the unoccupied electronic structure of the CdSe QDs, which are shown to arise from variations in specific ligand-surface bonding interactions.     

Probing the cytotoxicity of CdSe quantum dots with surface modification

The cytotoxicity of CdSe quantum dots (QDs) with surface modification was reported first in the paper. CdSe QDs were incorporated into poly (d, l) lactide (PLA) nanoparticles and then surface modified with Fluronic® 68 (F-68), cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), respectively. Three different particle sizes and zeta potential of the surface modified CdSe QDs were produced using a nano-precipitation method. The cytotoxicity of the surface modified CdSe QDs was evaluated in HepG2 cell model with MTT viability assay. The results showed that the cytotoxicity of the surface modified CdSe QDs in vitro was dependent on the surface properties. Surface modification with F-68 and SDS could lessen the cytotoxicity of CdSe QDs, while surface modification with CTAB showed significant cell damage. CdSe QDs surface modified with F-68 were injected into mice and the fluorescence images in viscus were obtained. The results suggested that CdSe QDs surface modified with F-68 have low cytotoxicity and good potential for biological labeling and imaging applications.