核-壳结构CdSe/ZnS量子点的制备、硫醇修饰及其光学性能

采用高温有机相包覆技术制备了CdSe/ZnS核壳结构量子点材料,考察了包覆量对量子点材料的光学性能的影响,研究了含脂肪链和芳香基的双硫醇分子1,4-苯二甲硫醇和1,8-辛二硫醇对于具有核-壳结构的CdSe/ZnS量子点材料的修饰作用,考察了修饰作用对于量子点的量子效率和荧光强度等光学性能的影响.实验结果表明:随着硫化锌包覆量的增加,量子点的量子效率及其荧光发射强度明显提高;硫醇的修饰能显著增强量子点的发光强度,随着硫醇浓度的增加,其发光性能增强,但是达到一定程度后,光学性能基本不随硫醇浓度的变化而变化.根据固体核磁共振等实验结果推测:硫醇分子可能部分替代了量子点体系中的正三辛基氧膦配体,稳定了量子点体系,对量子点起修饰保护作用,从而提高了量子点的光学性能.     

水溶性的CdSe/CdS/ZnS量子点的合成及表征

用L-半胱氨酸盐(Cys)作为稳定剂,合成了水溶性的双壳结构的CdSe/CdS/ZnS半导体量子点。吸收光谱和荧光光谱结果表明,双壳结构的CdSe/CdS/ZnS纳米微粒比单一的CdSe核纳米粒子和单核壳结构的CdSe/CdS纳米粒子具有更优异的发光特性。用透射电子显微镜(TEM)、ED、XRD、XPS和FTIR等方法对CdSe核和双壳层的CdSe/CdS/ZnS纳米微粒的结构、分散性及形貌分别进行了表征。     

Design and evaluation of polymer matrices for the encapsulation of CdSe/ZnS quantum dots in photonic nanocomposite thin films

A systematic approach and a new scheme for the evaluation of the as–is encapsulation of CdSe/ZnS core/shell quantum dots into polymer matrices is proposed, aiming to the implementation of thin film photonic integrated structures. Work focuses on quantum dots capped by hexadecylamine and trioctylphosphine oxide with no ligand exchange or other intermediate processing steps involved. The polymers studied include poly(methyl–methacrylate) (PMMA), polystyrene and acrylic polymers incorporating long alkyl chains, which are expected to promote the compatibility of the quantum dot ligands to that of the polymer chains. In this approach, the variation of photoluminescence properties of the nanocomposite thin films is measured versus increased concentration of the quantum dots, so as to evaluate the suitability of each polymer structure as an efficient host. Furthermore, the refractive index of the quantum dots/polymer nanocomposite thin films are also estimated using white light reflectance spectroscopy data, as appropriate for the integration of photonic devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 552–560     

Quantum Dot Photoluminescence Quenching by Cr(III) Complexes. Photosensitized Reactions and Evidence for a FRET Mechanism

Reported are quantitative studies of the energy transfer from water-soluble CdSe/ZnS and CdSeS/ZnS core/shell quantum dots (QDs) to the Cr(III) complexes trans-Cr(N(4))(X)(2)(+) (N(4) is a tetraazamacrocycle ligand, X(-) is CN(-), Cl(-), or ONO(-)) in aqueous solution. Variation of N(4), of X(-), and of the QD size and composition allows one to probe the relationship between the emission/absorption overlap integral parameter and the efficiency of the quenching of the QD photoluminescence (PL) by the chromium(III) complexes. Steady-state studies of the QD PL in the presence of different concentrations of trans-Cr(N(4))(X)(2)(+) indicate a clear correlation between quenching efficiency and the overlap integral largely consistent with the predicted behavior of a F?rster resonance energy transfer (FRET)-type mechanism. PL lifetimes show analogous correlations, and these results demonstrate that spectral overlap is an important consideration when designing supramolecular systems that incorporate QDs as photosensitizers. In the latter context, we extend earlier studies demonstrating that the water-soluble CdSe/ZnS and CdSeS/ZnS QDs photosensitize nitric oxide release from the trans-Cr(cyclam)(ONO)(2)(+) cation (cyclam = 1,4,8,11-tetraazacyclotetradecane) and report the efficiency (quantum yield) for this process. An improved synthesis of ternary CdSeS core/shell QDs is also described.     

Modified ligand-exchange for efficient solubilization of CdSe/ZnS quantum dots in water: a procedure guided by computational studies

One of the methods to render CdSe/ZnS core-shell quantum dots(QDots) water-soluble is to functionalize the surface with carboxylate groups by the use of heterobifunctional ligands such as 3-mercaptopropionic acid, where the thiolic end binds onto the outer ZnS shell. However, currently available ligand-exchange procedures starting with TOPO-capped quantum dots often lead to significant loss of quantum yields and poor stability of the colloids in water. As part of our efforts to overcome these problems, we used computational methods to understand the nature of binding between alkyl thiols and ZnS wurtzite surfaces. Guided by the computational results, we modified the ligand-exchange method and increased the reactivity of 3-mercaptopropionic acid toward the ZnS surface in chloroform. The functionlization reaction required only mild reaction conditions and led to QDot nanoparticles that were individually dispersed in water with good colloidal stability. Importantly, the photoluminescence performance of the QDots was highly preserved.     

水溶性的高荧光CdTe/CdSeⅡ型核壳量子点的合成与表征

用L-半胱氨酸(L-cysteine)作为稳定剂,以制备的CdTe量子点为核模板,水相合成了具有近红外发光的Ⅱ型核壳CdTe/CdSe半导体量子点。实验考察了合成温度,核模板的尺寸和组分比等因素对合成高质量的CdTe/CdSe量子点的影响。用紫外-可见吸收和荧光光谱研究了合成的量子点的光学性质。在优化的合成条件下,荧光发射光谱在586～753nm范围连续可调,荧光量子产率高达68%;通过X-射线衍射(XRD),X射线光电子能谱(XPS)和透射电镜(TEM)对合成的Ⅱ型核壳CdTe/CdSe量子点进行了结构和形貌表征。     

Colloidal CdSe quantum dot electroluminescence: ligands and light-emitting diodes

We examine the effects of surface ligand exchange on the performance of hybrid organic/inorganic light emitting diodes (LEDs) that use colloidal nanocrystal quantum dots as emissive centers. Using a series of primary alkylamines with different alkane chain lengths, we exchange the native surface ligands on a series of CdSe/CdZnS/ZnS core/shell/shell nanocrystal quantum dots and compare the differences in photoluminescence and electroluminescence efficiency of the emissive quantum dot layer. We fabricate LEDs made with octadecylamine-, octylamine-, and butylamine-exchanged quantum dots. We find that the differences in electroluminescence efficiency of the devices are not always proportional to the photoluminescence quantum efficiency of the quantum dots. We discuss this trend both in terms of the competing needs of high photoluminescence efficiency and good charge injection and energy transfer, and also in terms of the different processability and film morphology arising from the use of nanoparticles passivated with shorter ligands. Correspondence: David S. Ginger, Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, USA     

Super Sensitization: Grand Charge (Hole/Electron) Separation in ATC Dye Sensitized CdSe,CdSe/ZnS Type‐I,and CdSe/CdTe Type‐II Core–Shell Quantum Dots

Ultrafast charge‐transfer dynamics has been demonstrated in CdSe quantum dots (QD), CdSe/ZnS type‐I core–shell, and CdSe/CdTe type‐II core–shell nanocrystals after sensitizing the QD materials by aurin tricarboxylic acid (ATC), in which CdSe QD and ATC form a charge‐transfer complex. Energy level diagrams suggest that the conduction and valence band of CdSe lies below the LUMO and the HOMO level of ATC, respectively, thus signifying that the photoexcited hole in CdSe can be transferred to ATC and that photoexcited ATC can inject electrons into CdSe QD, which has been confirmed by steady state and time‐resolved luminescence studies and also by femtosecond time‐resolved absorption measurements. The effect of shell materials (for both type‐I and type‐II) on charge‐transfer processes has been demonstrated. Electron injection in all the systems were measured to be <150 fs. However, the hole transfer time varied from 900 fs to 6 ps depending on the type of materials. The hole‐transfer process was found to be most efficient in CdSe QD. On the other hand, it has been found to be facilitated in CdSe/CdTe type‐II and retarded in CdSe/ZnS type‐I core–shell materials. Interestingly, electron injection from photoexcited ATC to both CdSe/CdTe type‐II and CdSe/ZnS type‐I core–shell has been found to be more efficient as compared to pure CdSe QD. Our observation suggests the potential of quantum dot core–shell super sensitizers for developing more efficient quantum dot solar cells.     

Forming water-soluble CdSe/ZnS QDs using amphiphilic polymers, stearyl methacrylate/methylacrylate copolymers with different hydrophobic moiety ratios and their optical properties and stability

The amphiphilic stearyl methacrylate/methylacrylic acid copolymers (PSMs) were used as phase transfer reagents to convert CdSe/ZnS core-shell quantum dots (QDs) in chloroform to water-soluble PSMs-coated quantum dots (PSM-QDs). The optical properties and stability of PSM-QDs were influenced by the hydrophobic moiety ratios of PSMs, the PSM/QDs mass/volume ratio and the reaction time. The resulting PSM-QDs on optimum reaction conditions retained 60% of the photoluminescence value of the original CdSe/ZnS QDs in chloroform. The carboxylate-based PSM-QDs survived UV irradiation in air for at least 15 days. Upon UV irradiation, the PSM-QDs became about 2 times brighter than the original CdSe/ZnS QDs in chloroform, and the UV-brightened PL can retain the brightness for at least several months. Experimental results further confirmed the stability of PSM-QDs against strong acid, photochemical and thermal treatments. In addition to good performance of PSM-QDs, the synthesis of PSM and the corresponding water-soluble QDs is relatively simple.     

Quantum dot fluorescence as a function of alkyl chain length in aqueous environments

In this work, we examine the dependence of the fluorescence quantum yield of water-soluble CdSe/ZnS quantum dots on the local environment. The hydrophobicity of the local environment was modified by using different alkyl chain lengths in a set of oligo-ethylene glycols. Our results show that the quantum yield of CdSe/ZnS quantum dots is highest for the longest alkyl chain length, suggesting that a more hydrophobic environment is beneficial for generating bright, water-soluble quantum dots.     

Sequential injection immunoassay for human bone morphogenic protein-7 using an immunoreactor immobilized with anti-human bone morphogenic protein-7 antibody–CdSe/ZnS quantum dot conjugates

The detection of human bone morphogenic protein-7 (BMP-7) was achieved using a sequential injection immunoassay (SIIA) system. The SIIA system is based on the binding between BMP-7 and anti-human BMP-7 (AbBMP7)–CdSe/ZnS quantum dot (QD) conjugates immobilized onto a glass disk or an optical fiber, using fluorescence detection at excitation and emission wavelengths of 470 nm and 580 nm, respectively. The AbBMP7–QD conjugates were prepared by conjugating anti-human BMP-7 antibody (AbBMP7) to hydrophilic CdSe/ZnS core/shell quantum dots (QDs). The SIIA system was fully automated using software written in the LabVIEW™ development environment. The analytical performance of the SIIA system was characterized with a number of variables such as carrier flow rate and elution buffer. Under partially optimized operating conditions, the SIIA system had a linear calibration graph at up to 10.0 ng mL⁻¹ BMP-7 (R² ≥ 0.975) and a sample frequency of two samples per hour. The SIIA system with an optical fiber immunosensor was used to detect and quantify BMP-7 in spiked real samples obtained from a biological process with recoveries in the range of 95–102%.     

Single-source precursor route for overcoating CdS and ZnS shells around CdSe core nanocrystals

We reported a facile route for overcoating CdS and ZnS shells around colloidal CdSe core nanocrystals. To synthesize such double shelled core/shell nanocrystals, first, CdSe core nanocrystals were prepared in a much “greener” and cheap route, which did not involve the use of hazardous and expensive trioctylphosphine. Then, a low-cost and labor-saving route was adopted for the CdS and ZnS shell growth with the use of thermal decomposition of commercial available air stable single-source precursors cadmium diethyldithio-carbamate and zinc diethyldithiocarbamate in a non-coordinating solvent at intermediate temperatures. Powder X-ray diffraction patterns and transmission electron microscopy images confirm the epitaxial growth of the shell in the core/shell nanocrystals. The photoluminescence quantum yield of the resulting CdSe/CdS/ZnS core/shell nanocrystals can be as high as 90% in organic media and up to 60% after phase transfer into aqueous media. By varying the size of CdSe cores, the emission wavelength of the obtained core/shell nanostructures can span from 554 to 636 nm.     

Preparation and characterization of thiacalix[4]arene coated water-soluble CdSe/ZnS quantum dots as a fluorescent probe for Cu2+ ions

Highly fluorescent water-soluble CdSe/ZnS (core/shell) quantum dots (QDs) as a fluorescent Cu2+ ion probe were synthesized using thiacalix[4]arene carboxylic acid (TCC) as a surface coating agent. Hydrophobic trioctylphosphine oxide (TOPO) capped CdSe/ZnS QDs were overcoated with TCC in tetrahydrofuran at room temperature, and deprotonation of the carboxyl groups of TCC resulted in the formation of water-soluble QDs. The surface structure of the QDs was characterized by using transmission electron microscopy (TEM) and fluorescence correlation spectroscopy (FCS). TEM images showed that TCC-coated QDs were monodispersed with the particle size (core-shell moiety) of approximately 5 nm. Hydrodynamic diameter of the TCC-coated QDs was determined to be 8.9 nm by FCS, showing that the thickness of the surface organic layer of the QDs was approximately 2 nm. These results indicate that the surface layer of TCC-coated QDs forms a bilayer structure consisting of TOPO and TCC molecules. TCC-coated CdSe/ZnS QDs were highly fluorescent (quantum yield, 0.21) compared to the QDs surface-modified with mercaptoacetic acid and mercaptoundecanoic acid. Fluorescence of the TCC-coated QDs was effectively quenched by Cu2+ ions even in the presence of other transition metal ions such as Cd2+, Zn2+, Co2+, Fe2+, and Fe3+ ions in the same solution. The Stern-Volmer plot for the fluorescence quenching by Cu2+ ions showed a linear relationship up to 30 microM of Cu2+ ions. The ion selectivity of TCC-coated QDs was determined by measurements of fluorescence responses towards biologically important transition metal ions (50 microM) including Fe2+, Fe3+, Co2+>Zn2+, Cd2+. The fluorescence of TCC-coated QDs was almost insensitive to other biologically important ions such as Na+, K+, Mg2+, and Ca2+, suggesting that TCC-coated QDs can be used as a fluorescent Cu2+ ion probe for biological samples. A possible quenching mechanism by Cu2+ ions was also discussed on the basis of a Langmuir-type adsorption isotherm.     

Interplay between Auger and ionization processes in nanocrystal quantum dots

We study the interplay between Auger effects and ionization processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence decay rate on excitation density. Our results suggest that Auger recombination accelerates ionization processes that lead to the occupation of dark, nonemissive nanocrystal states. A model is proposed in the quantized Auger regime describing these experimental observations and providing an estimate of the Auger assisted ionization rates.     

Enhancement of the photoluminescence of CdSe quantum dots during long-term UV-irradiation: privilege or fault in life science research?

The present study describes an impressive enhancement of the photoluminescence (PL) intensity of low-temperature synthesized CdSe nanocrystals (75 degrees C) during long-term UV-irradiation. The integrated PL-intensity of CdSe core and CdSe/ZnS core/shell nanocrystals, dispersed in chloroform, enhanced about 3 and 6 times, respectively, during 9 h exposure to UV-light, without any significant changes in the characteristic absorbance spectra and shifting of PL-spectra. After termination of the irradiation a comparatively slow photobleaching was detected with tau(1/2) = 6 h for CdSe core and tau(1/2) = 14 h for CdSe/ZnS core/shell nanocrystals. The most impressive was the effect of UV-irradiation on the photoluminescence of water-soluble CdSe nanocrystals. The integrated PL-intensity enhanced about 10 times during 11 h exposure to UV-light and the improved PL-intensity was preserved during 3 days after termination of the irradiation without any significant photobleaching. The results are discussed in the context of application of CdSe nanocrystals as novel fluorophores in life science experiments.     

Aqueous synthesis of glutathione-capped CdTe/CdS/ZnS and CdTe/CdSe/ZnS core/shell/shell nanocrystal heterostructures

Here we demonstrate the aqueous synthesis of colloidal nanocrystal heterostructures consisting of the CdTe core encapsulated by CdS/ZnS or CdSe/ZnS shells using glutathione (GSH), a tripeptide, as the capping ligand. The inner CdTe/CdS and CdTe/CdSe heterostructures have type-I, quasi-type-II, or type-II band offsets depending on the core size and shell thickness, and the outer CdS/ZnS and CdSe/ZnS structures have type-I band offsets. The emission maxima of the assembled heterostructures were found to be dependent on the CdTe core size, with a wider range of spectral tunability observed for the smaller cores. Because of encapsulation effects, the formation of successive shells resulted in a considerable increase in the photoluminescence quantum yield; however, identifying optimal shell thicknesses was required to achieve the maximum quantum yield. Photoluminescence lifetime measurements revealed that the decrease in the quantum yield of thick-shell nanocrystals was caused by a substantial decrease in the radiative rate constant. By tuning the diameter of the core and the thickness of each shell, a broad range of high quantum yield (up to 45%) nanocrystal heterostructures with emission ranging from visible to NIR wavelengths (500-730 nm) were obtained. This versatile route to engineering the optical properties of nanocrystal heterostructures will provide new opportunities for applications in bioimaging and biolabeling.     

Single quantum dot-micelles coated with silica shell as potentially non-cytotoxic fluorescent cell tracers

The present study describes a stabilization of single quantum dot (QD) micelles by hydrophobic silica precursors and an extension of the silica layer to form a silica shell around the micelle. The obtained product consists of up to 92% of single nanocrystals (CdSe, CdSe/ZnS, or CdSe/ZnSe/ZnS quantum dots) in the silica micelles, coated with silica shell. The thickness of silica shell could vary, starting from 3 to 4 nm. Increasing the shell thickness increases the photoluminescent characteristics of QDs in aqueous solution. The silica-shelled single CdSe/ZnS QD micelles possess a high quantum yield in aqueous solution, a controlled small size, sharp photoluminescence spectra (fwhm approximately 30 nm), an absence of aggregation, and a high transparency. The presence of a hydrophobic layer between the QD and silica shell ensures an incorporation of other hydrophobic molecules (with interesting properties) in the close proximity of nanocrystal. Thus, it is possible to combine the characteristics of hybrid material with the priority of small size. The nanoparticles are amino functionalized and ready for conjugation. A comparatively good biocompatibility is demonstrated. The nanoparticles show ability for intracellular delivery and are noncytotoxic during long-term incubation with viable cells in the absence of light exposure, which makes them appropriate for cell tracing and drug delivery.     

Ultrafast Carrier Dynamics in CdSe/CdS/ZnS Quantum Dots

The intra- and inter-band relaxation dynamics of CdSe/CdS/ZnS core/shell/shell quantum dots are investigated with the aid of time-resolved nonlinear transmission spectra which are obtained using femtosecond pump-probe technique. By selectively exciting the core and shell carrier, the dynamics are studied in detail. Carrier relaxation is found faster in the conduction band of the CdS shell (about 130 fs) than that in the conduction band of the CdSe core (about 400 fs). From the experiments it is distinctly demonstrated the existence of the defect states in the interface between the CdSe core and the CdS shell, indicating thatultrafast spectroscopy might be a suitable tool in studying interface and surface morphology properties in nanosystems.     

Synthesis of Multifunctional Mn‐Doped CdTe/ZnS Core/Shell Quantum Dots

The synthesis of a novel water‐soluble Mn‐doped CdTe/ZnS core‐shell quantum dots using a proposed ultrasonic assistant method and 3‐mercaptopropionic acid (MPA) as stabilizer is descried. To obtain a high luminescent intensity, post‐preparative treatments, including the pH value, reaction temperature, reflux time and atmosphere, have been investigated. For an excellent fluorescence of Mn‐doped CdTe/ZnS, the optimal conditions were pH 11, reflux temperature 100°C and reflux time 3 h under N₂ atmosphere. While for phosphorescent Mn‐doped CdTe/ZnS QDs, the synthesis at pH 11, reflux temperature 100°C and reflux time 3 h under air atmosphere gave the best strong phosphorescence. The characterizations of Mn‐doped CdTe/ZnS QDs were also identified using AFM, IR, powder XRD and thermogravimetric analysis. The data indicated that the photochemical stability and the photoluminescence of CdTe QDs are greatly enhanced by the outer inorganic ZnS shell, and the doping Mn²⁺ ions in the as‐prepared quantum dots contribute to strong luminescence. The strong luminescence of Mn‐doped CdTe/ZnS QDs reflected that Mn ions act as recombination centers for the excited electron‐hole pairs, attributing to the transition from the triplet state (⁴T₁) to the ground state (⁶A₁) of the Mn²⁺ ions. All the experiments demonstrated that the surface states played important roles in the optical properties of Mn‐doped CdTe/ZnS core‐shell quantum dots.     

Use of Cdse/ZnS quantum dots for sensitive detection and quantification of paraquat in water samples

Based on the highly sensitive fluorescence change of water-soluble CdSe/ZnS core-shell quantum dots (QD) by paraquat herbicide, a simple, rapid and reproducible methodology was developed to selectively determine paraquat (PQ) in water samples. The methodology enabled the use of simple pretreatment procedure based on the simple water solubilization of CdSe/ZnS QDs with hydrophilic heterobifunctional thiol ligands, such as 3-mercaptopropionic acid (3-MPA), using microwave irradiation. The resulting water-soluble QDs exhibit a strong fluorescence emission at 596 nm with a high and reproducible photostability. The proposed analytical method thus satisfies the need for a simple, sensible and rapid methodology to determine residues of paraquat in water samples, as required by the increasingly strict regulations for health protection introduced in recent years. The sensitivity of the method, expressed as detection limits, was as low as 3.0 ng L⁻¹. The lineal range was between 10–5 × 10³ ng L⁻¹. RSD values in the range of 71–102% were obtained. The analytical applicability of proposed method was demonstrated by analyzing water samples from different procedence.