Microwave-Assisted Aqueous Synthesis of Highly Luminescent Carboxymethyl Chitosan-Coated CdTe/CdS Quantum Dots as Fluorescent Probe for Live Cell Imaging

This paper describes the development of a simplified and rapid method for the aqueous synthesis of quantum dots (QDs) with CdTe cores and gradient CdS external shells (CdTe/CdS QDs) aided by microwave irradiation. Several synthesis parameters, such as molar ratio of reagents, pH, reaction temperature, and reaction time, were studied in details. Under the optimized conditions, highly effective CdTe/CdS QDs could be synthesized in aqueous phase in only 15 min. In order to improve the biocompatibility of the CdTe/CdS QDs, these QDs were then interacted with carboxymethyl chitosan (CMC) so as they could be used as fluorescent probes in the aqueous phase. With the incorporation of CMC, the stability of modified QDs was found to have improved significantly (from 4 months to more than 10 months at room temperature). The photoluminescence quantum yield (PLQY) of the modified QDs could reach 75%, other parameters include a full width at half maximum of the emission (FWHM) spectrum as 40 ~ 60 nm, and an average size, estimated from electron microscopic images, as 3.5 nm. As fluorescent probes, these modified QDs were successfully used for imaging live Madin–Darby canine kidney (MDCK) cells, in which the preliminary results indicated that these modified QDs demonstrated good biocompatibility and showed promising applications for bio-labeling and imaging.     

Fluorescence Property of ZnO Nanoparticles and the Interaction with Bromothymol Blue

We synthesized ZnO quantum dots (QDs) simply in alcoholic solution, and investigated the interaction between ZnO QDs and bromothymol blue. The structural, morphological, size and spectral properties of ZnO QDs were studied. It was found that ZnO QDs were spherical nanoparticles in the crystal structure, and the average diameter of ZnO QDs was about 4.8 nm. The excitation and emission peaks were located at 346 nm and 520 nm, respectively, which were obtained on a common fluorophotometer. The quantum yield of ZnO QDs was obtained by using quinine sulfate as a reference reagent. In addition, the fluorescence of ZnO QDs can be quenched by bromothymol blue, and the quenching mechanism was proposed in a dynamic quenching mode.     

Well-crystallized zinc oxide quantum dots with narrow size distribution

In this study, pulsed laser ablation, online annealing, and following size classification using a differential mobility analyzer (DMA) were employed to fabricate quantum dots (QDs) of zinc oxide (ZnO). The irregularly shaped ZnO particles were obtained at annealing temperature less than 873 K, which gradually transformed into spherical QDs with increasing the annealing temperature. Finally, ZnO QDs with narrow size distribution having spherical shapes were successfully obtained at temperatures above 1173 K under the DMA classification at a nominal size of 10 nm. TEM observation demonstrated that the ZnO QDs obtained by this process were well-crystallized single crystallites with a wurtzite structure. Further, ZnO QDs with average sizes in the range of 4.8–8.1 nm were successfully fabricated by reducing the specified sizes of DMA. These features of the fabricated ZnO QDs are favorable for investigation of intrinsic quantum size effect in ZnO.     

Liposome encapsulation of fluorescent nanoparticles: Quantum dots and silica nanoparticles

Quantum dots (QDs) and silica nanoparticles (SNs) are relatively new classes of fluorescent probes that overcome the limitations encountered by organic fluorophores in bioassay and biological imaging applications. We encapsulated QDs and SNs in liposomes and separated nanoparticle-loaded liposomes from unencapsulated nanoparticles by size exclusion chromatography. Fluorescence correlation spectroscopy was used to measure the average number of nanoparticles inside each liposome. Results indicated that nanoparticle-loaded liposomes were formed and separated from unencapsulated nanoparticles by using a Sepharose gel. As expected, fluorescence self-quenching of nanoparticles inside liposomes was not observed. Each liposome encapsulated an average of three QDs. These studies demonstrated that nanoparticles could be successfully encapsulated into liposomes and provided a methodology to quantify the number of nanoparticles inside each liposome by fluorescence correlation spectroscopy.     

Cadmium sulfide quantum dots stabilized by castor oil and ricinoleic acid

Castor oil and ricinoleic acid (an isolate of castor oil) are environmentally friendly bio-based organic surfactants that have been used as capping agents to prepare nearly spherical cadmium sulfide quantum dots (QDs) at 230, 250 and 280 °C. The prepared quantum dots were characterized by Ultra violet–visible (UV–vis), Photoluminescence (PL), Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM) and X-ray diffraction (XRD) giving an overall CdS QDs average size of 5.14±0.39 nm. The broad XRD pattern and crystal lattice fringes in the HRTEM images showed a hexagonal phase composition of the CdS QDs. The calculated/estimated average size of the prepared castor oil capped CdS QDs for various techniques were 4.64 nm (TEM), 4.65 nm (EMA), 5.35 nm (UV–vis) and 6.46 nm (XRD). For ricinoleic acid capped CdS QDs, the average sizes were 5.56 nm (TEM), 4.78 nm (EMA), 5.52 nm (UV–vis) and 8.21 nm (XRD). Optical properties of CdS QDs showed a change of band gap energy from its bulk band gap of 2.42–2.82 eV due to quantum size confinement effect for temperature range of 230–280 °C. Similarly, a blue shift was observed in the photoluminescence spectra. Scanning electron microscope (SEM) observations show that the as-synthesized CdS QDs structures are spherical in shape. Fourier transform infra-red (FTIR) studies confirms the formation of castor oil and ricinoleic acid capped CdS QDs.     

Mechanistic Aspects of Quantum Dot Based Probing of Cu (II) Ions: Role of Dendrimer in Sensor Efficiency

Selective quenching of luminescence of quantum dots (QDs) by Cu²⁺ ions vis-à-vis other physiologically relevant cations has been reexamined. In view of the contradiction regarding the mechanism, we have attempted to show why Cu²⁺ ions quench QD-luminescence by taking CdS and CdTe QDs with varying surface groups. A detailed study of the solvent effect and also size dependence on the observed luminescence has been carried out. For a 13% decrease in particle diameter (4.3 nm →3.7 nm), the quenching constant increased by a factor of 20. It is established that instead of surface ligands of QDs, conduction band potential of the core facilitates the photo-induced reduction of Cu (II) to Cu (I) thereby quenching the photoluminescence. Taking the advantage of biocompatibility of dendrimer and its high affinity towards Cu²⁺ ions, we have followed interaction of Cu²⁺-PAMAM and also dendrimer with the CdTe QDs. Nanomolar concentration of PAMAM dendrimer was found to quench the luminescence of CdTe QDs. In contrast, Cu²⁺-PAMAM enhanced the fluorescence of CdTe QDs and the effect has been attributed to the binding of Cu²⁺-PAMAM complex to the CdTe particle surface. The linear portion of the enhancement plot due to Cu²⁺-PAMAM can be used for determination of Cu²⁺ ions with detection limit of 70 nM.     

Non-invasive Near Infrared Fluorescence Imaging of CdHgTe Quantum Dots in Mouse Model

Near infrared CdHgTe quantum dots (QDs) acted as biomarker for in vivo imaging were synthesized in aqueous solution. The size and the fluorescence wavelength of the synthesized quantum dots can be arbitrary manipulated by using different refluxing time. In particular, the fluorescence wavelength was extended to near infrared range (700∼900 nm), which make the in vivo imaging possible. Meanwhile, the characteristics, such as morphology, size, spectra, stability and toxicity were investigated. The dynamic bio-distribution, clearance from blood, liver and intestine in living animal were in vivo monitored by a NIR imaging system. The circulation of CdHgTe QDs in living mice was addressed semi-quantitatively according to the changes of fluorescence intensity. The high stability as well as high fluorescence intensity makes QDs particular interested candidates for in vivo imaging studies.     

Synthesis,biocompatibility and luminescence properties of quantum dots conjugated with amino acid-functionalized β-cyclodextrin

A series of CdSe and CdSe/CdS quantum dots (QDs) labeled with amino acid-modified β-cyclodextrin (β-CD) was prepared by a simple ultrasonic method. These amino acid-modified β-CD-coated QDs are very soluble and stable in biological buffer. They also have high colloidal stability and strong optical emission properties that are similar to those of untreated tri-n-octylphosphine oxide (TOPO)-coated QDs. The quantum yields (QYs) of these amino acid-modified β-CD-coated CdSe and CdSe/CdS QDs in biological buffer were found to be very high. In particular, the QYs of the positively charged l-His-β-CD-coated CdSe/CdS QDs were as high as 33.5±1.8%. In addition, the fluorescence lifetime of these QDs was also very long in PBS solutions as determined by frequency domain spectroscopy. For example, the lifetime of l-His-β-CD-coated CdSe/CdS QDs was 8.6 ns. The in vitro cytotoxicity of these QDs in ECV-304, SH-SY5Y and HeLa cells was found to be lower. l-His-β-CD-coated CdSe/CdS QDs were the least cytotoxic (IC₅₀ 95.6±3.2 mg mL^?1 in ECV-304 cells after 48 h). The flow cytometry results show that the positively charged amino acid led to a considerable increase in biocompatibility of QDs. This may be attributed to the presence of an amino acid-modified β-CD outer layer, which enhanced the biocompatibility.     

Preparation and purification of l-cysteine capped CdTe quantum dots and its self-recovery of degenerate fluorescence

l-cysteine capped CdTe quantum dots (QDs) were prepared in aqueous solution by a simple and efficient method, showing many advantages such as short synthesis period, the broaden range of starting pH value and the wide fluorescence emission wavelength range. A novel purification process was designed to remove excess Cd²⁺ which has potential cytotoxicity for bio-analysis. Three-dimensional fluorescence charts of pre- and post-purification showed that the purified QDs were of better luminescent performance. The prepared QDs were of cubic crystal structure with an average size of 2-6 nm, which were characterized by XRD and HRTEM. It is confirmed by IR spectra that the l-cysteine ligands were conjugated with CdTe cores via covalent bond. The degenerate fluorescence of QDs can be self-recovered in the presence of l-cysteine without other processing steps.     

Photosensitization of multiwalled carbon nanotube scaffolds with ZnO quantum dots for photovoltaic applications

In this article, we have decorated multiwalled carbon nanotubes (MWCNTs) scaffold with ZnO quantum dots (QDs, size in the range of 2.9–4.5 nm) and investigated their prospects for photovoltaic applications. ZnO QDs, in the present study, work as photosensitizer instead of electron transporting media as used in recent conventional strategic solar cells. ZnO QDs/MWCNTs composite shows an increased visible absorbance and quenching of the broad visible emission at around ~560 nm, while only ZnO QDs exhibit a strong visible emission. An efficient electron–hole separation facilitates an increase in the short-circuit current. These results show a possibility of developing a nontoxic, ZnO QDs sensitized MWCNTs composite-based photovoltaic solar cell.     

Preparation of Highly Stable and Photoluminescent Cadmium-Free InP/GaP/ZnS Core/Shell Quantum Dots and Application to Quantitative Immunoassay

Indium phosphide (InP) quantum dots (QDs) are ideal substitutes for widely used cadmium-based QDs and have great application prospects in biological fields due to their environmentally benign properties and human safety. However, the synthesis of InP core/shell QDs with biocompatibility, high quantum yield (QY), uniform particle size, and high stability is still a challenging subject. Herein, high quality (QY up to 72%) thick shell InP/GaP/ZnS core/shell QDs (12.8 ± 1.4 nm) are synthesized using multiple injections of shell precursor and extension of shell growth time, with GaP serving as the intermediate layer and 1-octanethiol acting as the new S source. The thick shell InP/GaP/ZnS core/shell QDs still keep high QY and photostability after transfer into water. InP/GaP/ZnS core/shell QDs as fluorescence labels to establish QD-based fluorescence-linked immunosorbent assay (QD-FLISA) for quantitative detection of C-reactive protein (CRP), and a calibration curve is established between fluorescence intensity and CRP concentrations (range: 1–800 ng mL⁻¹, correlation coefficient: R² = 0.9992). The limit of detection is 2.9 ng mL⁻¹, which increases twofold compared to previously reported cadmium-free QD-based immunoassays. Thus, InP/GaP/ZnS core/shell QDs as a great promise fluorescence labeling material, provide a new route for cadmium-free sensitive and specific immunoassays in biomedical fields.     

Structural,morphological and optical properties of PEDOT:PSS/QDs nano-composite films prepared by spin-casting

This paper describes the structural, morphological and optical properties of the nano-composite of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and quantum dots (QDs). The ZnSe and CdSe QDs have been synthesized, with the aid of Mercaptoacetic acid (MAA), by a colloidal method with an average size of ~5 to 7 nm. QDs have been embedded in PEDOT:PSS using a simple solution processing approach and has been deposited as thin films by spin coating technique. The QDs embedded PEDOT:PSS enhances the light absorption spectra of samples, prominently in terms of absorption intensity which may consequently improve sensitivity of the optoelectronic devices.     

A sol-gel method for preparing ZnO quantum dots with strong blue emission

ZnO quantum dots (QDs) with strong blue emission have been successfully synthesized by sol-gel method, and their crystal structures, sizes, and photoluminescence properties were characterized by X-ray diffractometer, scanning electron microscope, and ultraviolet-visible spectroscopy. It has been found that ZnO QDs had a hexagonal wurtzite crystal structure, and their average diameter was about 16.0-32.2 nm. Both the reaction time and temperature were found to have a strong influence on the average size and photoluminescence properties of ZnO QDs. Longer reaction time and higher reaction temperature resulted in larger average size for ZnO QDs. It has been shown that at reaction temperature 60 °C the emission intensity for ZnO QDs increased first with reaction time before 7 h and then decreased after 7 h. For the same reaction time 7 h, ZnO QDs synthesized at 60 °C showed the strongest emission intensity. It was found that annealing in nitrogen, vacuum, and air all resulted in an increase of the size of ZnO QDs and a reduction in their photoluminescence. The dependence of the size and properties of ZnO QDs on the reaction parameters as well as the annealing conditions has been discussed.     

In vivo Monitoring of Organ-Selective Distribution of CdHgTe/SiO2 Nanoparticles in Mouse Model

CdHgTe/SiO₂ nanoparticles were prepared by SiO₂ capping on the surface of CdHgTe QDs. The characteristics, such as optical spectra, photostability, size and cell toxicity were investigated. The dynamic distribution of CdHgTe/SiO₂ nanoparticles was in vivo monitored by near infrared fluorescence imaging system. CdHgTe/SiO₂ nanoparticles acted as a novel fluorescence probe have a maximum fluorescence emission of 785 nm and high photo-stability. The hydrodynamic diameter of CdHgTe/SiO₂ nanoparticles could be adjusted to 122.3 nm. Compared to CdHgTe QDs, inhibitory effects of CdHgTe/SiO₂ nanoparticles on proliferation of HCT116 cells decreased to a certain extent. CdHgTe/SiO₂ nanoparticles had their specific dynamic distribution behavior, which provided new perspectives for bio-distribution of nanoparticles.     

Self-assembled SiGe quantum dots embedded in Ge matrix by Si ion implantation and subsequent annealing

We have fabricated SiGe quantum dots (QDs) by means of a two-step Si ion implantation followed by thermal rapid thermal annealing (RTA) method. SiGe QDs with the 4–6 nm diameter are formed uniformly in the near-surface region of Ge substrate. The RTA processes are performed at 800 and 900 °C for 15 s, respectively. Both experimental and theoretical analysis indicates that the higher temperature (900 °C) RTA can enhance the growth of SiGe QDs. Two photoluminescence peaks are observed near 572 and 581 nm at room temperature. The mechanism of the luminescence from SiGe QDs is discussed.     

Folate Conjugated CdHgTe Quantum Dots with High Targeting Affinity and Sensitivity for In vivo Early Tumor Diagnosis

CdHgTe-folate conjugates, acting as novel active-targeting fluorescence probes, were prepared by covalent conjugation of CdHgTe QDs and folic acid. Their characteristics, such as optical spectra, stability and cancer cell targeting were investigated in detail. The fluorescence wavelength of CdHgTe-folate conjugates was 790 nm and a full width at half-maximum (FWHM) of them was 50–70 nm. Their fluorescence stability could satisfy the need of long and continuous fluorescence imaging. The in vivo dynamic bio-distribution of CdHgTe-folate conjugates in S180 tumor beard mouse model was monitored by a NIR imaging system. The resultes indicated that CdHgTe-folate conjugates targeted to tumor effectively. The high fluorescence intensity together with targeting effect makes CdHgTe-folate conjugates promising candidates for imaging, monitoring and early diagnosis of cancer at molecular and cell level.     

Phase transfer of agglomerated nanoparticles: deagglomeration by adsorbing grafted molecules and colloidal stability in polymer solutions

We aimed to develop liposomes for loading both cisplatin and quantum dots (QDs) for both drug delivery and bioimaging. The resultant quantum-dot-liposomes (QDLs) with cisplatin were characterized using dynamic light scattering, transmission electron microscopy (TEM), encapsulation efficiencies, and fluorescence intensity. QDLs composed of CdSe or CdSe/ZnS QDs represented a size of about 100?nm. The QDLs were prepared at a high QD loading efficiency of nearly 100?%. Most QDs were located within the liposomal bilayers as evidenced by TEM. Slow and sustained cisplatin release from QDLs was achieved. The cellular uptake of QDLs demonstrated effective internalization and significant fluorescence in melanoma cells. The signal derived from QDLs could be observed by different wavelength settings. The cisplatin-containing QDLs revealed higher cytotoxic activity compared to an equal dose of free cisplatin. CdSe/ZnS QDLs were intravenously administered to mice, and the biodistribution was observed with an in vivo imaging system. Significant fluorescence signal and cisplatin accumulation were detected in the brain and skin, as verified by ex vivo imaging and drug distribution. Liposomal inclusion could reduce the reticuloendothelial system uptake of QDs and cisplatin. QDLs evaluated in this study represent a new potential method for theranostic purposes.     

复合荧光CdSe量子点-脂质体的制备与表征

通过脂质体方法成功地将三辛基氧化膦(TOPO)包覆的CdSe发光量子点从非极性有机溶剂转移到生物相容性的水溶液中.分别通过透射电镜(TEM)、荧光Mapping图像,以及光致发光(PL)光谱进行表征.TEM照片显示制备的CdSe核量子点为球形,具有良好的单分散特性,平均粒径约为3nm.CdSe-脂质体复合体的平均尺寸大约20nm,TEM清楚地显示了CdSe量子点被诱捕在脂质体中.荧光Mapping显示了CdSe-脂质体复合体的发光强度分布.脂质体方法转移TOPO包覆的CdSe量子点,借助了磷脂的双分子链与CdSe表面的TOPO配体之间的疏水相互作用,在CdSe的第一配体层外部形成第二配体层,保留了CdSe的存在环境,光致发光光谱表明,量子点-脂质复合体基本保持了CdSe核量子点的发射效率.     

Efficient two-photon absorption of CdSe-CdS/ZnS core-multishell quantum dots under the excitation of near-infrared femtosecond pulsed laser

We report that two-photon absorption (TPA) properties of semiconductor CdSe-core CdS/ZnS-multishell quantum dots (QDs) in toluene under excitation of femtosecond laser at 800 nm. The results show efficient TPA process and large TPA cross section of three types of size QDs, which is 1900, 5710, and 16060 GM (1 GM = 10⁻⁵⁰ cm⁴ s photon⁻¹), respectively. TPA cross section dramatically increases with increased core size, showing a strong size-dependence effect. Furthermore, two-photon excitation (TPE) fluorescence intensity not only depends on TPA capacity, but also relies on improved quantum yield resulting from passivation of QD surface by different coated monolayers (MLs). These facts in combination with the narrow fluorescence bandwidth make these QDs as promising probes for multicolor two-photon microscopy.     

Formation of InAs quantum dots at ultrahigh growth rates

We fabricated quantum dot (QD) structures at ultrahigh growth rates. Smaller fluctuations in QD size were observed when they were grown at a rate of 1.0 ML/s under conventional growth conditions (growth temperature of 500 °C and As₄ flux of 9×10⁻⁶ Torr). For QDs grown at high rates, growth interruption played an important role in the fabrication of QD structures; this was confirmed by carrying out reflection high-energy electron diffraction. Photoluminescence for QDs grown at high and low growth rates, with growth interruption and with low-temperature capping was observed at around 1250 nm at room temperature, indicating that high-quality QDs can be fabricated by employing high growth rates.