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.     

Characterization and 2D self-assembly of CdSe quantum dots at the air-water interface

Langmuir film properties, UV-vis spectroscopy, epifluorescence microscopy, and transmission electron microscopy were used to study CdSe quantum dots (QDs) in 2D. By combining these results, it was possible to determine the molar absorptivity, limiting nanoparticle area, luminescence property, and arrangement of the QDs in the monolayer films at the air-water interface. Either trioctylphosphine oxide (TOPO) or 1-octadecanethiol (ODT) stabilized the QDs. The data collected reveal that TOPO forms close-packed monolayers on the surface of the QDs and that ODT-stabilized QDs undergo alkyl chains interdigitation. It was also found that varying the nanoparticle size, nature of surfactant, surface pressure, and mixed monolayers could help engineer the 2D self-assembly of the QDs at the air-water interface. Of practical importance is the transfer of these monolayer films onto hydrophilic or hydrophobic solid substrates, which could be successfully accomplished via the Langmuir-Blodgett film deposition technique.     

Controlled self-assembly of hydrophobic quantum dots through silanization

We demonstrate the formation of one-, two-, and three-dimensional nanocomposites through the self-assembly of silanized CdSe/ZnS quantum dots (QDs) by using a controlled sol-gel process. The self-assembly behavior of the QDs was created when partially hydrolyzed silicon alkoxide monomers replaced hydrophobic ligands on the QDs. We examined systematically self-assembly conditions such as solvent components and QD sizes in order to elucidate the formation mechanism of various QD nanocomposites. The QD nanocomposites were assembled in water phase or on the interface of water and oil phase in emulsions. The partially hydrolyzed silicon alkoxides act as intermolecules to assemble the QDs. The QD nanocomposites with well-defined solid or hollow spherical, fiber-like, sheet-like, and pearl-like morphologies were prepared by adjusting the experimental conditions. The high photoluminescence efficiency of the prepared QD nanocomposites suggests partially hydrolyzed silicon alkoxides reduced the surface deterioration of QDs during self-assembly. These techniques are applicable to other hydrophobic QDs for fabricating complex QD nanocomposites.     

Multi-analyte homogenous immunoassay based on quenching of quantum dots by functionalized graphene

We propose a homogenous multi-analyte immunoassay based on the quenching of quantum dot (QD) fluorescence by means of graphene. Two QDs with emission maxima at 636 and 607 nm were bound to antibodies selective for mouse or chicken immunoglobulins, respectively, and graphene functionalized with carboxylic moieties was employed to covalently link the respective antigen. The antibody-antigen interaction led graphene close enough to QDs to quench the QD fluorescence by resonance energy transfer. The addition of free antigens that competed with those linked to graphene acted as a “turn-on” effect on QD fluorescence. Fluorescence emitted by the two QDs could be recorded simultaneously since the QDs emitted light at different wavelengths while being excited at the same wavelength and proved to be linearly correlated with free antigen concentration. The developed assay allows measuring both antigens over 2–3 orders of magnitude and showed estimated limits of detection in the nanomolar range. This approach is thus a promising universal strategy to develop homogenous immunoassays for diverse antigens (cells, proteins, low-molecular-mass analytes) in a multi-analyte configuration.     

FTIR-ATR spectroscopic determination of the distribution of surfactants in latex films

FTIR-ATR (Fourier Transform Infra-Red-Attenuated Total Reflection) has been used to analyze the surface composition of coalesced acrylic latex films. The behavior of two anionic surfactants has been characterized. It has been found that surfactant distribution depends on the nature of the surfactant. A comparison between the normalized absorbance in transmission and in reflection has shown an enrichment of surfactants at the surfaces of films with a coalescence time of 3 days. The surfactant concentration at the film-air interface is higher than at the film substrate interface. A concentration gradient exists through the film thickness. In addition, the incompatible surfactant migrates towards the interface as coalescence proceeds.     

Stamping transfer of a quantum dot interlayer for organic photovoltaic cells

An organophilic cadmium selenide (CdSe) quantum dot (QD) interlayer was prepared on the active layer in organic solar cells by a stamping transfer method. The mother substrate composed of a UV-cured film on a polycarbonate film with strong solvent resistance makes it possible to spin-coat QDs on it and dry transfer onto an active layer without damaging the active layer. The QD interlayers have been optimized by controlling the concentration of the QD solution. The coverage of QD particles on the active layer was verified by TEM analysis and fluorescence images. After insertion of the QD interlayer between the active layer and metal cathode, the photovoltaic performances of the organic solar cell were clearly enhanced. By ultraviolet photoelectron spectroscopy of CdSe QDs, it can be anticipated that the CdSe QD interlayer reduces charge recombination by blocking the holes moving to the cathode from the active layer and facilitating efficient collection of the electrons from the active layer to the cathode.     

Preparation of multilayered CdSe quantum dot sensitizers by electrostatic layer-by-layer assembly and a series of post-treatments toward efficient quantum dot-sensitized mesoporous TiO2 solar cells

A multilayer of CdSe quantum dots (QDs) was prepared on the mesoporous surface of a nanoparticulate TiO(2) film by a layer-by-layer (LBL) assembly using the electrostatic interaction of the oppositely charged QD surface for application as a sensitizer in QD-sensitized TiO(2) solar cells. To maximize the absorption of incident light and the generation of excitons by CdSe QDs within a fixed thickness of TiO(2) film, the experimental conditions of QD deposition were optimized by controlling the concentration of salt added into the QD-dissolved solutions and repeating the LBL deposition a few times. A proper concentration of salt was found to be critical in providing a deep penetration of QDs into the mesopore, thus leading to a dense and uniform distribution throughout the whole TiO(2) matrix while anchoring the oppositely charged QDs alternately in a controllable way. A series of post-treatments with (1) CdCl(2), (2) thermal annealing, and (3) ZnS-coating was found to be very critical in improving the overall photovoltaic properties, presumably through a better connection between QDs, effective passivation of QD's surface, and a high impedance of recombination, which were proved by transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS) experiments. With a proper post-treatment of multilayered QDs as a sensitizer, the overall power conversion efficiency in the CdSe QD-sensitized TiO(2) solar cells could reach 1.9% under standard illumination condition of simulated AM 1.5G (100 mW/cm(2)).     

Langmuir-Blodgett thin films of quantum dots: synthesis, surface modification, and fluorescence resonance energy transfer (FRET) studies

We describe herein studies on as-prepared hydrophobic ZnS-CdSe quantum dots (QDs) at the air-water interface. Surface pressure-area (pi-A) isotherms have been used to study the monolayer behavior. Uniform, lamellar multilayer thin films of QDs were deposited by the Langmuir-Blodgett (LB) technique. The role of two different surfactant systems commonly employed in the synthesis of these QDs (trioctylphosphine oxide-octadecylamine (TOPO-ODA) system and trioctylphosphine oxide-tetradecylphosphonic acid (TOPO-TDPA) system) on the monolayer behavior and the quality of thin films produced has been investigated. The thin films were characterized by quartz crystal microgravimetry (QCM), contact angle measurements, fluorescence spectroscopy, and transmission electron microscopy (TEM). These QD films were further modified by an amphiphilic polymer, poly(maleic anhydride-alt-1-tetradecene) (PMA). The hydrophobic interaction between the polymers and the surfactants attached to the QDs drove the self-assembly process. The carboxylic acid functional groups in the polymer were also used to immobilize avidin. We have demonstrated a proof of concept for the biosensing strategy wherein the avidin-coated QD films attracted biotinylated gold nanoparticles, resulting in fluorescence resonance energy transfer (FRET) quenching of the thin films.     

Triplet harvesting in poly(9‐vinylcarbazole) and poly(9‐(2,3‐epoxypropyl)carbazole) doped with CdSe/ZnS quantum dots encapsulated with 16‐(N‐carbazolyl) hexadecanoic acid ligands

Semiconductor quantum dots (QDs) can be used as alternative for transition metal complexes to harvest the nonemissive triplet excitons in organic light‐emitting diodes (OLEDs). In search for a QD‐based OLED material generating blue emission, poly(9‐vinylcarbazole) (PVK) and poly(9‐(2,3‐epoxypropyl) carbazole) (PEPK) are chosen as host for blue‐emitting CdSe/ZnS core/shell QDs. The QDs are encapsulated with 16‐(N‐carbazolyl) hexadecanoic acid (C16), a ligand terminated by a carbazole moiety. As alternative for PVK, PEPK, where the lower molecular weight and less extensive excimer formation could promise a better film formation and more extensive exciton hopping, is explored. The efficiencies of singlet ( ) and triplet ( ) energy transfer to the C16 capped QDs are estimated by combining stationary photoluminescence spectra and fluorescence decays of pristine polymer films with those of polymer films doped with the QDs. At a loading of 30 wt % of the QDs, increases from 12 ± 1% in PVK to 41 ± 2% in PEPK while increases from 37 ± 22% in PVK to 72 ± 48% in PEPK. The investigation of the film morphology by atomic force microscopy confirms that the main factor limiting the triplet transfer efficiency in the PVK matrix is the clustering of the C16 capped QDs. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 539–551     

A Ligand System for the Flexible Functionalization of Quantum Dots via Click Chemistry

We present a novel ligand, 5‐norbornene‐2‐nonanoic acid, which can be directly added during established quantum dot (QD) syntheses in organic solvents to generate “clickable” QDs at a few hundred nmol scale. This ligand has a carboxyl group at one terminus to bind to the surface of QDs and a norbornene group at the opposite end that enables straightforward phase transfer of QDs into aqueous solutions via efficient norbornene/tetrazine click chemistry. Our ligand system removes the traditional ligand‐exchange step and can produce water‐soluble QDs with a high quantum yield and a small hydrodynamic diameter of approximately 12 nm at an order of magnitude higher scale than previous methods. We demonstrate the effectiveness of our approach by incubating azido‐functionalized CdSe/CdS QDs with 4T1 cancer cells that are metabolically labeled with a dibenzocyclooctyne‐bearing unnatural sugar. The QDs exhibit high targeting efficiency and minimal nonspecific binding.     

Reversible surface electronic traps in PbS quantum dot solids induced by an order-disorder phase transition in capping molecules

The electronic properties of semiconductor quantum dots (QDs) are critically dependent on the nature of the ligand molecules on their surfaces. Here we show the reversible formation of surface electronic trap states in the model system of solid thin films of PbS QDs capped with thiol molecules. As the temperature was increased from cryogenic to room temperature, we discovered a phase transition in the fluorescence spectra from excitonic emission to trap emission. The critical temperature (T(c)) of the phase transition scales with molecular length and in each case is close to the bulk melting temperature of the capping molecules. We conclude that an order-disorder transition in the molecular monolayer above T(c) introduces surface mobility and the formation of a disordered atomic lead layer at the QD/capping molecule interface, leading to electronic trap formation.     

Highly photoluminescent multilayer QD-glass films prepared by LbL self-assembly

A novel and facile preparation method for layer-by-layer (LbL) self-assembled films incorporating quantum dots (QDs) and having intense photoluminescence (PL) from blue to red is presented. Functional sol-gel-derived glass layers prepared by the hydrolysis of 3-aminopropyltrimethoxysilane (APS) or 3-mercaptopropyltrimethoxysilane (MPS) have been used as a linkage between QD layers. Absorption, PL spectroscopy, transmission electron microscopy, and atomic force microscopy were employed for characterization, which revealed that the QDs in the prepared films had a nearly close-packed coverage and were not aggregated. The PL efficiencies of the QDs (CdTe or ZnSe, both are thioglycolic acid-stabilized) dispersed in the films were roughly half that of the initial colloidal solutions but reached 24% before a refractive index correction. The thickness of the red-emitting film with 10 CdTe QD layers was approximately 50 nm. The concentration of QDs in the film derived from the first absorption peak was approximately 0.01 M. Because the PL starts to show a red shift, the obtained concentration is practically the ultimate one in the glass matrix. The mercapto, amino, and carboxyl groups play important roles in LbL self-assembling processes.     

Influence of Oleic Acid and Electrolyte on the Interfacial Dilational Properties of Surfactant/Polymer Systems at the Decane-Water Interface

The dilational properties of partly hydrolyzed polyacrylamide (HPAM) and 4,5-diheptyl-2-propylbenzene sulfonate (377) mixed systems in the absence or presence of electrolyte or oleic acid at the oil-water interface have been described by means of the oscillating barriers method and the interfacial tension relaxation method. The polymer plays different roles in influencing the nature of polymer-surfactant adsorbed layers at different surfactant concentrations. At low surfactant concentration, the addition of polymer perhaps weakens the “entanglement” of long alkyl chains, which decreases strikingly the dilational modulus of the adsorbed layer. At high surfactant concentration, the addition of the polymer increases the dilational modulus due to the hydrophobic interactions between polymer and surfactant molecules. On the case of adding electrolyte, the frequency dependence of dilational modulus increases due to the enhancement of exchange process of surfactant molecules and bivalent cation has more obvious effect than Na ion. Oleic acid plays dual roles in controlling interfacial dilational properties of mixed adsorption films: a small quantity of oleic acid increases the dilational modulus by forming densely packed mixed adsorption layer with surfactant molecules, while the superfluous addition of oleic acid could decrease the dilational modulus mainly due to the weakening of the “entanglement” among long alkyl chains in surfactant molecules.     

硫脲修饰ZnSe:Cu量子点的水相合成及荧光特性

以3-巯基丙酸为稳定剂在水相中合成了Cu掺杂的ZnSe量子点(QDs), 并利用硫脲(CH₄N₂S)对其进行表面修饰, 制备出核壳结构的ZnSe:Cu/ZnS 量子点. 制得的量子点呈闪锌矿结构, 尺寸约为5 nm, 有较好的分散性, 其荧光发射峰在460 nm左右. 经CH₄N₂S修饰后, 量子点表面形成了宽禁带的ZnS包覆层, 将电子和空穴限域在了ZnSe:Cu 核内, 减少了表面发生非辐射复合的载流子, 显著提高了量子点的荧光强度. 与Na₂S、硫代乙酰胺(TAA)等常用硫源相比, 以CH₄N₂S为硫源制得的ZnSe:Cu/ZnS 量子点壳层厚度可控, 表面钝化效果更好, 显示出更佳的荧光效率和稳定性. ZnSe:Cu/ZnS 量子点经过紫外线照射后消除了表面的悬空键, 进一步提高了其量子产率, 最终获到了具有较好荧光性质的ZnSe:Cu/ZnS量子点.     

Preparation of liposomes loaded with quantum dots, fluorescence resonance energy transfer studies, and near-infrared in-vivo imaging of mouse tissue

We report on a simple, fast and convenient method to engineer lipid vesicles loaded with quantum dots (QDs) by incorporating QDs into a vesicle-type of lipid bilayer using a phase transfer reagent. Hydrophilic CdTe QDs and near-infrared (NIR) QDs of type CdHgTe were incorporated into liposomes by transferring the QDs from an aqueous solution into chloroform by addition of a surfactant. The QD-loaded liposomes display bright fluorescence, and the incorporation of the QDs into the lipid bilayer leads to enhanced storage stability and reduced sensitivity to UV irradiation. The liposomes containing the QD were applied to label living cells and to image mouse tissue in-vivo using a confocal laser scanning microscope, while NIR images of mouse tissue were acquired with an NIR fluorescence imaging system. We also report on the fluorescence resonance energy transfer (FRET) that occurs between the CdTe QDs (the donor) and the CdHgTe QDs (the acceptor), both contained in liposomes. Based on these data, this NIR FRET system shows promise as a tool that may be used to study the release of drug-loaded liposomes and their in vivo distribution.

Triggered aggregation of PbS nanocrystal dispersions; towards directing the morphology of hybrid polymer films using a removable bilinker ligand

Hybrid polymer films consist of quantum dots (QDs) dispersed in a polymer matrix. A key fundamental challenge that is hindering their optimisation in optoelectronic devices such as hybrid solar cells is overcoming uncontrolled aggregation of the QDs. In an effort to direct aggregation, and trigger self-assembly, we added a bilinker ligand (1,2-ethanedithiol) to dispersed PbS QDs in polymer solutions prior to film deposition by spin casting. Turbidity studies of the PbS QD/1,2-ethanedithiol dispersions enabled a relationship to be established between the extent of 1,2-ethanedithiol-triggered QD aggregation and the nominal fractional coverage of the QDs by 1,2-ethanedithiol. The extent of aggregation (and self-assembly) increased with nominal fraction coverage. Above a value of about 1.0 QD aggregation increased substantially. TEM images showed that at low 1,2-ethanedithiol concentrations triggered assembly of network-like QD structures occurred. At high 1,2-ethanedithiol concentrations the QDs self-assembled into more-ordered micrometre-sized crystals. The results suggest that 1,2-ethanedithiol decreases the inter-QD separation in dispersion as a result of rapid ligand exchange and this process results in QD aggregation as well as self-assembly. The assembled QD structures were successfully trapped within polymer films by spin casting of PbS QD/1,2-ethanedithiol dispersions containing added polystyrene or polytriarylamine.     

Characterization of temperature dependent modifier effects in SFC using linear solvation energy relationships

Summary Linear solvation energy relationships (LSERs) are used to probe the changes in mobile and stationary phase properties of a carbon dioxide-based mobile phase and a polymeric stationary phase under near-critical conditions. Four mobile phase modifiers are compared with respect to dipolarity/polarizability, hydrogen bond donating and accepting ability, and other intermolecular interactions as a function of temperature. As temperature nears the mixture critical point, the differences in these properties between the mobile and stationary phases change to reflect the growing heterogeneity in mobile phase component distribution at the chromatographic interface. The stationary phase loses many of its original characteristics and takes on characteristics typical of the mobile phase modifier due to preferential adsorption of the modifier at the surface of the stationary phase.     

Interfacial rheology of protein–surfactant mixtures

The distribution of proteins and surfactants at fluid interfaces (air–water and oil–water) is determined by the competitive adsorption between the two types of emulsifiers and by the nature of the protein–surfactant interactions, both at the interface and in the bulk phase, with a pronounced impact on the interfacial rheological properties of these systems. Therefore, the interfacial rheology is of practical importance for food dispersion (emulsion or foam) formulation, texture, and stability. In this review, the existence of protein–surfactant interactions, the mechanical behaviour and/or the composition of emulsifiers at the interface are indirectly determined by interfacial rheology of the mixed films. The effect on the interfacial rheology of protein–surfactant mixed films of the protein, the surfactant, the interface and bulk compositions, the method of formation of the interfacial film, the interactions between film forming components, and the displacement of protein by surfactant have been analysed. The last section tries to understand the role of interfacial rheology of protein–surfactant mixed films on food dispersion formation and stability. The emphasis of the present review is on the interfacial dilatational rheology.     

Substrate- and time-dependent photoluminescence of quantum dots inside the ultrathin polymer LbL film

The photoluminescence of CdSe/ZnS quantum dots (QDs) in different configurations at solid surfaces (glass, silicon, PDMS, and metals) is considered for three types of organization: QDs directly adsorbed on solid surfaces, separated from the solid surface by a nanoscale polymer film with different thickness, and encapsulated into a polymer film. The complete suppression of photoluminescence for QDs on conductive metal surfaces (copper, gold) indicated a strong quenching effect. The temporal variation of the photoluminescent intensity on other substrates (glass, silicon, and PDMS) can be tuned by placing the nanoscale (3-50 nm) LbL polymer film between QDs and the substrate. The photooxidation and photobleaching processes of QD nanoparticles in the vicinity of the solid surface can be tuned by proper selection of the substrate and the dielectric nanoscale polymer film placed between the substrate and QDs. Moreover, the encapsulation of QD nanoparticles into the polymer film resulted in a dramatic initial increase in the photoemission intensity due to the accelerated photooxidation process. The phenomenon of enhanced photoemission of QDs encapsulated into the ultrathin polymer film provides not only the opportunity for making flexible, ultrathin, QD-containing polymer films, transferable to any microfabricated substrate, but also improved light emitting properties.     

Effect of the eluent pH and acidic modifiers in high-performance liquid chromatography retention of basic analytes

The retention of ionogenic bases in liquid chromatography is strongly dependent upon the pH of the mobile phase. Chromatographic behavior of a series of substituted aniline and pyridine basic compounds has been studied on C18 bonded silica using acetonitrile-water (10:90) as the eluent with different pHs and at various concentrations of the acidic modifier counter anions. The effect of different acidic modifiers on solute retention over a pH range from 1.3 to 8.6 was studied. Ionized basic compounds showed increased retention with a decrease of the mobile phase pH. This increase in retention was attributed to the interaction with counter anions of the acidic modifiers. The increase in retention is dependent on the nature of the counter anion and its concentration in the mobile phase. It was shown that altering the concentration of counter anion of the acidic modifier allows the optimization of the selectivity between basic compounds as well as for neutral and acidic compounds.