CdSe/ZnS quantum dots based fluorescence quenching method for determination of paeonol

Aqueous polymethylmethacrylate (PMMA)-capped CdSe/ZnS quantum dots were used as fluorescence probes for paeonol determination. Based on the fluorescence quenching of aqueous CdSe/ZnS quantum dots caused by paeonol, a simple, sensitive and rapid method was developed. Under the optimal conditions, with excitation and emission wavelengths at 350 nm and 620 nm, respectively, the calibration plot of F0-F with concentration of paeonol was linear in the range of 25.04-175.2 mg L(-1) with correlation coefficient of 0.9986. The limit of detection was 0.017 mg L(-1). The concentration of paeonol in paeonol ointment was determined by the proposed method and the result agreed with the claimed value. Furthermore, the possible fluorescence quenching mechanism was discussed.     

Optical properties of CdSe and CdSe/ZnS quantum dots dispersed in solvents of different polarity

Original organic capping TOPO/TOP groups of CdSe and CdSe/ZnS quantum dots (QDs), from mother solution were replaced with 2_mercaptoethanol, which was chosen as model compound, in order to achieve water solubility. Obtained water dispersions of CdSe and CdSe/ZnS QDs were characterized by UV/VIS absorption and luminescence techniques. Luminescence measurements revealed that bare cores are very sensitive to surface capping, transfer into water diminished emission intensity. Core/shell, CdSe/ZnS, QDs are much more resistant to changes of the capping and solvent, and significant part of emission intensity was preserved in water. The article is published in the original.     

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.     

通过S2-中间态将CdSe量子点有机配体转化为ZnS保护层及其器件光伏特性

通过S2-中间态将有机配体三辛基氧膦(TOPO)转化为ZnS保护层,显著改善了CdSe量子点(QDs)器件的转换效率.配体交换后的傅里叶变换红外(FTIR)光谱结果表明,有机配体已被S2-离子配体取代;离子反应后的X射线光电子能谱(XPS)结果表明S2-离子配体反应生成了ZnS,紫外-可见(UV-Vis)吸收光谱结果表明量子点溶液吸收峰位没有发生明显改变,透射电子显微镜(TEM)结果表明配体交换后量子点粒径减小.电化学阻抗谱(EIS)结果表明光照条件下有机配体转化为ZnS保护层后TiO2/QDs/电解质界面电阻减少,证明该条件下正向电子传输增强;强度调制光电压谱(IMVS)和强度调制光电流谱(IMPS)结果表明电子寿命和扩散速度增加.相比于有机配体,形成ZnS保护层后的量子点敏化太阳能电池(QDSC)效率由0.98%提高到1.75%,相对提高了1.78倍.     

Molecularly imprinted polymeric film on semiconductor nanoparticles analyte detection by quantum dot photoluminescence

Incorporation of semiconductor nanoparticles into molecularly imprinted polymer provides a sensor material which can be easily shaped and with better selectivity because the bound template would quench the photoluminescence (PL) emission of quantum dots significantly. In this work, artificial receptors of various templates were synthesized with functional monomers such as methacrylic acid (MAA), semiconductor like CdSe/ZnS core-shell derivatized with 4-vinylpyridine and ethylene glycol dimethacrylic acid as the cross-linker. The quenching of photoluminescence emissions is presumably due to the fluorescence resonance energy transfer between quantum dots and template molecules. The photoluminescence emission is unaffected upon incubation of analyte with the blank control polymer.     

Studies of optical and structural properties of CdSe/polymer nanocomposites: evidence of charge transfer and photostability

In this work, the role of conducting [poly (p-phenylinevinylene) (PPV)] and nonconducting (polystyrene) polymers on the properties of their respective composites with CdSe quantum dots of varied sizes has been investigated. The emission and structural properties of polymer–CdSe composites are found to be dependent on the crystallite size and morphology of CdSe nanocrystallites. Smaller CdSe quantum dots (size, ∼5 nm) ensures efficient charge transfer process across polymer–CdSe interface as evident by almost complete quenching of photoluminescence (PL) emission as compared to larger CdSe quantum dots (size, ∼7 nm). Presence of residual trioctylphosphine (TOP)/ tri-n-octylphosphine-oxide (TOPO) species and agglomeration of particles act as a hindrance for quenching of emission and hence charge transfer for larger CdSe nanocrystallites. Emission studies indicated an increased conjugation length for PPV polymers in different solvents (toluene, pyridine) and in solid state. Nonconducting polymer polystyrene shows charge transfer across polymer–CdSe interface as well. However, polystyrene polymer has a shorter chain length, which ensures maximum coverage on the surface of CdSe nanocrystallites and provides better photostability to CdSe QDs within the polymer matrix as compared to that for PPV–CdSe nanocomposites.     

Surface passivation of luminescent colloidal quantum dots with poly(dimethylaminoethyl methacrylate) through a ligand exchange process

Polydimethylaminoethyl methacrylate (PDMAEMA) was used as a multidentate ligand to modify the surface of CdSe/ZnS core-shell colloidal quantum dots in toluene with trioctylphosphine oxide (TOPO) as the surface ligand. Adsorption of PDMAEMA was accompanied by release of TOPO. The process is free of agglomeration, and the modified nanocrystals become soluble in methanol. The photoluminescence properties are well-preserved in either toluene or methanol.     

Novel b-cyclodextrin modified quantum dots as fluorescent probes for polycyclic aromatic hydrocarbons (PAHs)

Water-soluble CdSe/ZnS quantum dots (QDs) were prepared via a simple sonochemical procedure using b-cyclodextrin (CD) as surface coating agent. The QDs displayed a sensitive emission enhancement for anthracene over other related polycyclic aromatic hydrocarbons, and the detection limit was around 1.6 10 8 mol/L.     

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.     

Novel β-cyclodextrin modified quantum dots as fluorescent probes for polycyclic aromatic hydrocarbons (PAHs)

Water-soluble CdSe/ZnS quantum dots (QDs)were prepared via a simple sonochemical procedure using β-cyclodextrin (CD)as surface coating agent.The QDs displayed a sensitive emission enhancement for anthracene over other related polycyclic aromatic hydrocarbons,and the detection limit was around 1.6 × 10-8 mol/L.     

Bio serves nano: biological light-harvesting complex as energy donor for semiconductor quantum dots

Light-harvesting complex (LHCII) of the photosynthetic apparatus in plants is attached to type-II core-shell CdTe/CdSe/ZnS nanocrystals (quantum dots, QD) exhibiting an absorption band at 710 nm and carrying a dihydrolipoic acid coating for water solubility. LHCII stays functional upon binding to the QD surface and enhances the light utilization of the QDs significantly, similar to its light-harvesting function in photosynthesis. Electronic excitation energy transfer of about 50% efficiency is shown by donor (LHCII) fluorescence quenching as well as sensitized acceptor (QD) emission and corroborated by time-resolved fluorescence measurements. The energy transfer efficiency is commensurable with the expected efficiency calculated according to F?rster theory on the basis of the estimated donor-acceptor separation. Light harvesting is particularly efficient in the red spectral domain where QD absorption is relatively low. Excitation over the entire visible spectrum is further improved by complementing the biological pigments in LHCII with a dye attached to the apoprotein; the dye has been chosen to absorb in the "green gap" of the LHCII absorption spectrum and transfers its excitation energy ultimately to QD. This is the first report of a biological light-harvesting complex serving an inorganic semiconductor nanocrystal. Due to the charge separation between the core and the shell in type-II QDs the presented LHCII-QD hybrid complexes are potentially interesting for sensitized charge-transfer and photovoltaic applications.     

Role of surface modification of colloidal CdSe quantum dots on the properties of hybrid organic–inorganic nanocomposites

In this work, tri-octyl phosphine/tri-octyl phosphine oxide (TOPO)-capped cadmium selenide (CdSe) quantum dots (QDs) of varied sizes (5–9 nm), prepared by varying the input Cd:Se precursor ratio using chemical route, were dispersed in conducting polymer matrices viz. poly[2-methoxy, 5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly(3-hexylthiophene) (P3HT). By using a binary solvent mixture (pyridine–chloroform), homogeneous dispersion of CdSe nanocrystals in polymers (MEH-PPV, P3HT) could be realized. The properties of the resulting dispersions could be tailored by the composition and concentration of QDs in polymer. The emission and structural properties of polymer–CdSe nanocomposites are found to be dependent on the crystallite size and morphology of CdSe nanocrystallites. An effective quenching of photoluminescence emission in the polymer nanocomposite was observed for smaller CdSe quantum dots (size ∼6 nm) as compared to larger CdSe quantum dots (size ∼9 nm), thus ensuring efficient charge transfer process across the polymer–CdSe interface in the former case. The incomplete quenching, particularly for MEH-PPV:CdSe nanocomposites, could be as a result of insufficient coverage of polymers on the surface of CdSe nanocrystallites, mainly due to phase segregation for TOPO-stripped CdSe nanocrystallites. The superior morphology and optical properties of polymer nanocomposite (P3HT:CdSe QDs) could play a pivotal role for the realization of effective charge separation and transport in hybrid solar cells.     

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。     

Synthesis and characterizations of ultra-small ZnS and Zn(1-x)Fe(x)S quantum dots in aqueous media and spectroscopic study of their interactions with bovine serum albumin

This work reports a new experimental methodology for the synthesis of ultra small zinc sulfide and iron doped zinc sulfide quantum dots in aqueous media. The nanoparticles were obtained using a simple procedure based on the precipitation of ZnS in aqueous solution in the presence of 2-mercaptoethanol as a capping agent, at room temperature. The effect of Fe(3+) ion concentration as dopant on the optical properties of ZnS was studied. The size of quantum dots was determined to be about 1nm, using scanning tunneling microscopy. The synthesized nanoparticles were characterized by X-ray diffraction, UV-Vis absorption and photoluminescence emission spectroscopies. The presence and amount of iron impurity in the structure of Zn((1-x))Fe(x)S nanocrystals were confirmed by atomic absorption spectrometry. A blue shift in band-gap of ZnS was observed upon increasing incorporation of Fe(3+) ion in the iron doped zinc sulfide quantum dots. The photoluminescence investigations showed that, in the case of iron doped ZnS nanoparticles, the emission band of pure ZnS nanoparticles at 427nm shifts to 442nm with appearance of a new sharp emission band around 532nm. The X-ray diffraction analysis indicated that the iron doped nanoparticles are crystalline, with cubic zinc blend structure, having particle diameters of 1.7±022nm. Finally, the interaction of the synthesized nanoparticles with bovine serum albumin was investigated at pH 7.2. The UV-Vis absorption and fluorescence spectroscopic methods were applied to compare the optical properties of pure and iron doped ZnS quantum dots upon interaction with BSA. It was proved that, in both cases, the fluorescence quenching of BSA by the quantum dots is mainly a result of the formation of QDs-BSA complex in solution. In the steady-state fluorescence studies, the interaction parameters including binding constants (K(a)), number of binding sites (n), quenching constants ( [Formula: see text] ), and bimolecular quenching rate constants (k(q)) were determined at three different temperatures and the results were then used to evaluate the corresponding thermodynamic parameters ΔH, ΔS and ΔG.     

Synthesis of quantum dot-tagged submicrometer polystyrene particles by miniemulsion polymerization

Submicrometer fluorescent polystyrene (PS) particles have been synthesized via miniemulsion polymerization using CdSe/ZnS core-shell quantum dots (QDs). The influence of QD concentration, QD coating (either trioctylphosphine oxide (TOPO)-coated or vinyl-functionalized), and surfactant concentration on the polymerization kinetics and the photoluminescence properties of the prepared particles has been analyzed. Polymerization kinetics were not altered by the presence of QDs, whatever their surface coating. Latexes exhibited particle sizes ranging from 100 to 350 nm, depending on surfactant concentration, and a narrow particle size distribution was obtained in all cases. The fluorescence signal of the particles increased with the number of incorporated TOPO-coated QDs. The slight red shift of the emission maximum was correlated with phase separation between PS and QDs, which occurred during the polymerization, locating the QDs in the vicinity of the particle/water interface. QD-tagged particles displayed higher fluorescence intensity with TOPO-coated QDs compared to those with the vinyl moiety. The obtained fluorescent particles open up new opportunities for a variety of applications in biotechnology.     

ZnS∶Co半导体量子点的制备及其光电化学性质

采用低温水热技术,分别以柠檬酸(CA)和巯基丙酸(MPA)为稳定剂,在70℃的水相中合成了单分散的,粒子尺寸约为4 nm的ZnS∶Co半导体量子点.研究了稳定剂、Co2+掺杂剂及其掺杂量对掺杂量子点发光性能和结构的影响.XRD结果表明,Co2+离子主要掺杂在量子点表面,对主体ZnS晶格没有影响.当采用MPA为稳定剂,掺杂量为5%(摩尔分数)时,掺杂量子点的荧光发射强度最高;而同样掺杂量下采用CA为稳定剂时,量子点的荧光发射强度有所下降.循环伏安研究显示,与空白ZnS量子点相比,Co2+离子的掺杂在ZnS的禁带中形成杂质能级,相应地,ZnS∶Co量子点的吸收边发生红移.与未掺杂ZnS量子点相比,掺杂量子点具有较少的表面非辐射复合中心,因而荧光发射强度显著提高.     

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.     

Photoinduced fluorescence enhancement in mono- and multilayer films of CdSe/ZnS quantum dots: dependence on intensity and wavelength of excitation light

Photoinduced fluorescence enhancement (PFE) behavior in mono- and multilayer films of CdSe/ZnS core/shell quantum dots (QDs) on glass substrates was investigated using various intensities and wavelengths of excitation light. CdSe/ZnS QDs capped with tri-n-octylphosphine oxide (TOPO) were produced using colloidal chemical synthesis, and mono- and multilayer QD films were fabricated on glass substrates by spin coating. The fluorescence quantum yield (QY) of the QD monolayer was greatly enhanced by continuous irradiation in a dry nitrogen atmosphere, whereas the QD multilayer showed a small enhancement of the QY or fluorescence intensity decay. In addition, the shorter the excitation wavelength, the more pronounced the PFE. The rate of increase of the QY increased with decreasing excitation intensities at any wavelength. These dependences were observed in both mono- and multilayer films. Our results suggest that the photoejection of electrons to the substrate is the origin of PFE. Assuming the charging effect of electrons trapped in the substrate, a phenomenological model is proposed to explain all of the experimental results, that is, the dependence on the intensity and wavelength of excitation light and the qualitative difference in PFE behavior between mono- and multilayer films.     

Temperature-dependent photoluminescence of water-soluble quantum dots for a bioprobe

The photoluminescence of water-soluble CdSe/ZnS core/shell quantum dots is found to be temperature-dependent: as temperature arising from 280 K to 351 K, the photoluminescence declines with emission peak shifting towards the red at a rate of ∼0.11 nm K⁻¹. And the studies show that the photoluminescence of water-soluble CdSe/ZnS quantum dots with core capped by a thinner ZnS shell is more sensitive to temperature than that of ones with core capped by a thicker one. That is, with 50% decrement of the quantum yield the temperature of the former need to arise from 280 K to 295 K, while the latter requires much higher temperature (315.6 K), which means that the integrality of shell coverage is a very important factor on temperature-sensitivity to for the photoluminescence of water-soluble CdSe/ZnS quantum dots. Moreover, it is found that the water-soluble CdSe quantum dots with different core sizes, whose cores are capped by thicker ZnS shells, possess almost the same sensitivity to the temperature. All of the studies about photoluminescence temperature-dependence of water-soluble CdSe/ZnS core/shell quantum dots show an indispensable proof for their applications in life science.     

Layer-by-layer quantum dot constructs using self-assembly methods

We describe the creation of CdSe/ZnS quantum dot assemblies using layer-by-layer construction strategies, using self-assembly. In the first approach, a dithiol linker was used to make multilayers of CdSe/ZnS quantum dots, while in the second biotin- and streptavidin-conjugated CdSe/ZnS quantum dots were used to make multilayer constructs. Both the chemical bonding nature and fluorescence spectroscopic properties of quantum dot films were characterized using X-ray photoelectron spectroscopy (XPS) and fluorescence spectroscopy.