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.     

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.     

Surface modification of CdSe quantum dots for biosensing applications: Role of ligands

For an optimum performance of colloidal nanocrystal devices for a variety of applications such as photonic devices, solar cells and biological labelling, the determining factors are the nanocrystal surface and size. In this work, these two factors have been tuned via wet chemistry to tailor the material properties: The absorption and emission spectra have been tailored by choice of the nanocrystal size; nanocrystal shape by surface modification and photoluminescence (PL) efficiency determined by surface traps, has been tuned via appropriate selection of the nanocrystal capping ligands. Here, we have shown that through ligand-exchange process, the surface of the CdSe quantum dots (QDs) can be modified by replacing the longer-chain ligands of conventional trioctyl phosphine oxide (TOPO) or oleic acid (OA) with shorter-chain ligand of butyl amine. This imparts colloidal stability and water solubility to CdSe QDs for its potential applications in biosensors and biological imaging. It is conjectured that crystallite sizes, oxidation potential of CdSe QDs and stereochemical compatibility of ligands (TOPO, oleic acid and butyl amine) greatly influences the photophysics and photochemistry of CdSe QDs.     

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.     

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.     

Capillary electrophoresis method for the characterization and separation of CdSe quantum dots

This paper presents a simple and rapid methodology to separate and characterize free CdSe quantum dots (QDs) in aqueous medium by capillary electrophoresis (CE). First, we describe a controlled derivatization procedure to obtain water-soluble QDs through noncovalent interactions. This derivatization methodology was based on the formation of a complex between the QDs and several types of surfactants to enhance the hydrophilicity and stability of the CdSe QDs. The surfactants used to achieve the surface functionalization were trioctylphosphine oxide/trioctylphosphine (TOPO/TOP) and sodium dodecyl sulfate (SDS). Different CdSe QDs core sizes were synthesized as function of the nanocrystals growing time and then subjected to controlled coating. These free QDs were separated by capillary zone electrophoresis (CZE) based on the differences in the charge-to-mass ratio of the QDs-TOPO/TOP-SDS complexes, and the detection was carried out with UV-vis and laser-induced fluorescence (LIF) techniques obtaining detection limits 5 times lower with CE-LIF. Under the optimal working conditions, four different-sized QDs were successfully separated whose average sizes were 3.1, 3.6, 4.3, and 4.9 nm, and the size distribution was less than 7% for all of them [calculated from the full width at half-maximum (fwhm) of the fluorescence spectra and confirmed by high-resolution transmission electron microscopy (HTEM)]. Therefore, we were able to separate QDs that differ in only 0.5 nm in diameter and 19 nm in fluorescence emission maximum. This corresponds to the better resolution achieved in the analysis of these kinds of nanoparticles. Finally, a correlation between the migration times plus or minus peak width and the core sizes plus or minus size distribution was established.     

Studies of interaction of amines with TOPO/TOP capped CdSe quantum dots: Role of crystallite size and oxidation potential

This work reports the interaction of aliphatic (triethyl amine, butyl amine) and aromatic amines (PPD, aniline) with CdSe quantum dots of varied sizes. The emission properties and lifetime values of CdSe quantum dots were found to be dependent on the oxidation potential of amines and crystallite sizes. Smaller CdSe quantum dots (size 5 nm) ensure better surface coverage of amines and hence higher quenching efficiency of amines could be realized as compared to larger CdSe quantum dots (size 14 nm). Heterogeneous quenching of amines due to the presence of accessible and inaccessible set of CdSe fluorophores is indicated. PPD owing to its lowest oxidation potential (0.26 V) has been found to have higher quenching efficiency as compared to other amines TEA and aniline having oxidation potentials 0.66 and >1.0 V, respectively. Butyl amine on the other hand, plays a dual role: its post-addition acts as a quencher for smaller and enhances emission for larger CdSe quantum dots, respectively. The beneficial effect of butyl amine in enhancing emission intensity could be attributed to enhance capping effect and better passivation of surface-traps.     

A novel method for the preparation of water-soluble and small-size CdSe quantum dots

A novel method for the preparation of water-soluble and small-size CdSe quantum dots has been reported under high-intensity ultrasonic irradiation. The as-prepared products have been characterized by absorption spectra, X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM).     

Dipolar structures in colloidal dispersions of PbSe and CdSe quantum dots

We show by cryogenic transmission electron microscopy that PbSe and CdSe nanocrystals of various shapes in a liquid colloidal dispersion self-assemble into equilibrium structures that have a pronounced dipolar character, to an extent that depends on particle concentration and size. Analyzing the cluster-size distributions with a one-dimensional (1D) aggregation model yields a dipolar pair attraction of 8-10 kBT at room temperature. This accounts for the long-range alignment of the crystal planes of individual nanocrystals in self-assembled superstructures and for anisotropic nanostructures grown via oriented attachment.     

Identification and control of the origin of photoluminescence from silicon quantum dots

We present a detailed investigation into the origin of photoluminescence (PL) from silicon quantum dots in hydrogenated amorphous silicon nitride annealed in oxygen ambient. On the basis of structural characterization, temperature-dependent PL, time-resolved PL, and PL excitation spectra, we identify that the luminescence of the oxidized samples originates from the localized exciton radiative recombination via the surface states related to Si-N or Si-O-Si bonds. In combination with the results due to annealing in argon and hydrogen environments, we have further shown that control of the origin of the PL can be realized by modifying the radiative defect density through annealing treatment.     

Metal ion (silver, cadmium and zinc ions) modified CdS quantum dots for ultrasensitive copper ion sensing

Metal ion (Ag(+), Cd(2+), Zn(2+)) modified CdS quantum dots (QDs) were synthesized and used for Cu(2+) sensing. Modification by these metal ions could enhance the PL intensity of CdS QDs with the extent of the PL enhancement being related to the concentration of the metal ions. Different metal ion (Ag(+), Cd(2+), Zn(2+)) modified CdS QDs also showed different analytical characteristics for Cu(2+) sensing. In particular, Ag( + ) modified CdS QDs showed greatly enhanced sensitivity for Cu(2+) determination than did the unmodified CdS QDs. A limit of detection (LOD) of 2.0 × 10(-10) M was obtained for Ag(+) modified CdS QDs, which is the lowest LOD obtained using QDs as fluorescence probes for Cu(2+) sensing. This study demonstrates the important role of surface state of QDs in fluorescence sensing.     

Size tuning of MAA capped CdSe and CdSe/CdS quantum dots and their stability in different pH environments

The present work reports synthesis of mercaptoacetic acid capped CdSe nanoparticles soluble in water at different temperatures and with different precursor ratios. This enabled to tune the particle size of QDs from 2.7 to 5.8 nm. The particles consist of nanocrystals; with mixed phase, hexagonal wurtzite as well as sphalerite cubic and are luminescent with quantum yield 10%. The quantum yield up to 20% has been obtained by growing a shell of CdS over the CdSe. HR-TEM images, XRD patterns and the photoluminescence excitation spectra shows epitaxial growth of CdS shell over CdSe and with average size 3.2 ± 1.2 and 4.7 ± 1.2 nm for CdSe and CdSe/CdS quantum dots respectively. FT-IR spectrum and the negative zeta potential value together confirms the attachment of mercaptoacetic acid to the QD surface, where the carboxylic acid group is facing towards solvent and provides stability due to electrostatic hindrance. Further, the QDs are checked for their stability and the luminescence in environments of different pH (4–11 pH). Both CdSe and CdSe/CdS agglomerate with total elimination of fluorescence for 4 pH medium, and no shift in the fluorescence emission peak observed for the 6–9 pH, therefore QDs can be applicable as the fluorescence tags in this specific range of pH.     

Cyto-/genotoxic effect of CdSe/ZnS quantum dots in human lung adenocarcinoma cells for potential photodynamic UV therapy applications

Quantum dots (QDs) are luminescent nanoparticles (NPs) with promising potential in numerous medical applications, but there remain persistent human health and safety concerns. Although the cytotoxic effects of QDs have been extensively investigated, their genotoxic effects remain under-explored. This study scrutinized the cyto- and genotoxic effects of QDs with a Cadmium selenide/Zinc sulfide (CdSe/ZnS) core/shell, and suggests comprehensive guidelines for the application of QDs in cancer therapy. QDs were used to treat A549 cells in the presence and absence of ultraviolet A/B (UVA/UVB) irradiation. QD-induced cell death was evaluated by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), apoptosis, and lactate dehydrogenase (LDH) assays, as well as by real-time PCR analysis of differential mRNA levels of genes, such as ataxia telangiectasia mutated (ATM), p53, and caspase-9, involved in apoptosis. The genotoxic effect of CdSe/ZnS QDs was measured in human cancer cells, for the first time, by comet and micronucleus assays. Treatment with CdSe/ZnS QDs and UVB irradiation resulted in the most severe extent of cell death, indicating strong induction of phototoxicity by CdSe/ZnS QDs in the presence of UVB. Both apoptotic and necrotic cell death were observed upon QDs and UVB combined treatment. The induction of Olive tail moments and micronuclei formation was also most significant when CdSe/ZnS QDs and UVB irradiation were combined. Our results on the genotoxic effect and mechanistic details of CdSe/ZnS QD-induced cell death suggest that UVB irradiation is the most effective method for increasing the potency of QDs during photodynamic cancer therapy.     

Large blue shift of the biexciton state in tellurium doped CdSe colloidal quantum dots

The exciton-exciton interaction energy of tellurium doped CdSe colloidal quantum dots is experimentally investigated. The dots exhibit a strong Coulomb repulsion between the two excitons, which results in a huge measured biexciton blue shift of up to 300 meV. Such a strong Coulomb repulsion implies a very narrow hole wave function localized around the defect, which is manifested by a large Stokes shift. Moreover, we show that the biexciton blue shift increases linearly with the Stokes shift. This result is highly relevant for the use of colloidal QDs as optical gain media, where a large biexciton blue shift is required to obtain gain in the single exciton regime.     

Surface functionalized cadmium telluride quantum dots for the optical detection and determination of herbicides

An optical sensor for detection of herbicides was developed through the functionalization of CdTe quantum dots (CdTe-QDs) with cysteamine hydrochloride. The functionalized CdTe-QDs was characterized with various physicochemical methods such as X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, energy dispersive X-ray analysis, Ultraviolet–visible and photoluminescence spectroscopies. The optical band gap of the functionalized CdTe-QDs as calculated by using Tauc plot was 3.75 eV. It was found that the fluorescence intensity of the functionalized CdTe-QDs quenched linearly in the presence of different herbicides according to the Stern–Volmer equation. Thus, the functionalized CdTe-QDs can be used as simple, rapid, inexpensive, and optical sensitive sensor for practical detection of herbicides.     

A study of specific features of the electronic spectrum of quantum dots in CdSe semiconductor

Monolayers of CdSe/CdS/ZnS quantum dots (QDs) formed on the aqueous subphase and transferred to solid substrates by the Langmuir–Blodgett method have been studied. The samples obtained were examined by transmission electron microscopy, atomic-force microscopy, and scanning tunnel microscopy. The structure of the QD monolayer obtained on the substrate was analyzed. Specific features of the electronic spectrum of the quantum objects formed in the samples under study were determined.     

Optimization of band structure and quantum-size-effect tuning for two-photon absorption enhancement in quantum dots

The two-photon absorption, 2PA, cross sections of PbS quantum dots, QDs, are theoretically and experimentally investigated and are shown to be enhanced with increasing quantum confinement. This is in contrast to our previous results for CdSe and CdTe QDs where the reduced density of states dominated and resulted in a decrease in 2PA with a decrease in QD size. Qualitatively this trend can be understood by the highly symmetric distribution of conduction and valence band states in PbS that results in an accumulation of allowed 2PA transitions in certain spectral regions. We also measure the frequency nondegenerate 2PA cross sections that are up to five times larger than for the degenerate case. We use a k·p four-band envelope function formalism to model the increasing trend of the two-photon cross sections due to quantum confinement and also due to resonance enhancement in the nondegenerate case.     

Synthesis, nonlinear optical properties and photoluminescence of ZnSe quantum dots in stable solutions

ZnSe quantum dots (QDs) were synthesized by using a novel oleic acid-controlled hydrothermal route. The as-synthesized QDs were easily dispersed in nonpolar solvents to form highly stable homogenous solutions, on the basis of which their optical properties were systematically investigated. It was found that the QDs had multiple optical properties of both obvious optical nonlinearity with a frequency-doubled Nd:YAG laser (532 nm) used as inducing beam and a strong photoluminescence emission at ca. 458 nm, suggesting that they have potential applications in complex functional optical devices.