Photostability comparison of CdTe and CdSe/CdS/ZnS quantum dots in living cells under single and two-photon excitations

The photostability is an outstanding feature of quantum dots (QDs) used as fluorescence probes in biological staining and cell imaging. To find out the related factors in the QD photostability, the photobleaching of naked CdTe QDs and BSA coated CdSe/CdS/ZnS QDs in human hepatocellular carcinoma (QGY) cells and human nasopharynx carcinoma (KB) cells were studied under single photon excitation (SPE) and two-photon excitation (TPE). In these two cell lines the cellular QDs were irradiated by a 405 nm continuous wave laser for SPE or an 800 nm femto-second (fs) laser for TPE. The QD photobleaching with the irradiation time was found to fit a biexponential decay. The fast decay plays a dominant role in the bleaching course and thus can be used as the parameter to quantitatively evaluate the QD photostability. The TPE decreased the QD photobleaching as compared to SPE. The BSA coated core/shell QDs had improved the photostability up to 4-5 times than the naked QDs due to the shielding effect of the QD shell. Therefore, it is better to use core/shell structured QDs as the fluorescence probe combining with a TPE manner for those long-term monitoring studies.     

Thiol-Capped CdTe Quantum Dots with Two-Photon Excitation for Imaging High Autofluorescence Background Living Cells

To effectively image living cells with quantum dots (QDs), particularly for those cells containing high content of native fluorophores, the two-photon excitation (TPE) with a femto-second 800 nm laser was employed and compared with the single-photon excitations (SPE) of 405 nm and 488 nm in BY-2 Tobacco (BY-2-T) and human hepatocellular carcinoma (QGY) cells, respectively. The 405 nm SPE produced the bright photoluminescence (PL) signals of cellular QDs but also induced a strong autofluorescence(AF) from the native fluorophores like flavins in cells. The AF occupied about 30% and 13% of the total signals detected in QD imaging channel in the BY-2-T and QGY cells, respectively. With the excitation of 488 nm SPE, the PL signals were lower than those excited with the 405 nm SPE, although the AF signals were also reduced. The 800 nm TPE generated the best PL images of intracellular QDs with the highest signal ratio of PL to AF, because the two-photon absorption cross section of QDs is much higher than that of the native fluorophores. By means of the TPE, the reliable cellular imaging with QDs, even for the cells having the high AF background, can be achieved.     

Spectroscopic study of oscillator strength and radiative decay time of colloidal CdSe quantum dots

Characterization of samples of cadmium selenide quantum dots (CdSe) QDs dissolved in toluene colloidal solutions at a concentration of 1.4 mg/ml was carried out through UV–Vis absorption and photoluminescence (PL) spectroscopy. The size-dependent absorption and red-shifted PL emission peak wavelengths could be tuned between 510–576 and 545–606 nm respectively. Optical absorption spectral measurements yielded CdSe QDs having diameters about ~ 2.44–3.69 nm with energy gaps 2.32–2.08 eV which are higher than the bulk CdSe (1.74 eV) reminiscent of quantum confinement. This is found to be in good agreement with the semi-empirical pseudopotential model. In addition, the first excitonic absorption transition 1S_(e)1S_3/2(h) oscillator strength and the corresponding fluorescence radiative decay time of CdSe QDs are assessed using relevant Einstein relations for absorption and emission in a two-level system. The elaborated calculations would anticipate that the transition oscillator scale with the CdSe QD radius as ~ R^2.54. Correspondingly, the calculated radiative decay times decrease from 56.4 to 23.2 ns which scale with CdSe QDs radius as ~ R^?2.155 in fairly good agreement with experimental values reported in the literature.     

In Situ Self-Assembly of Quantum Dots at the Plasma Membrane Mediates Energy Transfer-Based Activation of Channelrhodopsin

The use of nanoparticle (NP) bioconjugates to control the activity of membrane ion channels has recently emerged as a new paradigm for the activation of electrically excitable cells. An NP-based strategy is reported for the specific activation of channelrhodopsin C1V1 (ChR-C1V1) expressed in the plasma membrane of HEK 293T/17 cells. Hydrophilic CdSe/ZnS core–shell semiconductor quantum dots (QDs) are self-assembled to the exofacial face of recombinantly expressed ChR-C1V1 by metal affinity-driven interaction of the QD ZnS shell with an N-terminal hexahistidine tag displayed on ChR-C1V1. This configuration enables the Förster resonance energy transfer (FRET)-based excitation and activation of the 11-cis-retinal moiety of ChR-C1V1 using the QD as a light harvesting transducer/energy donor. It is shown that the specific laser-induced opening of the ChR-C1V1 channel wherein the photoexcited QD (405 nm excitation, 530 nm emission) iteratively activates ChR-C1V1 channels as confirmed using the voltage-sensitive dye (VSD) bis-(1,3-diethylthiobarbituric acid)trimethine oxonol (DiSBAC ₂ (3)). In the absence of the QD transducer, excitation of ChR-C1V1-expressing cells at 405 nm results in no activation of ChR-C1V1. The results demonstrate the ability to controllably interface QDs with living cells for the activation of ChR membrane proteins and detail a new NP-bioconjugate hybrid system for the specific activation of ion channels.     

InAs/GaAs单量子点中电子/空穴自旋弛豫

利用分子束外延制备了三种类型量子点样品,它们分别是:未掺杂样品、n型Si调制掺杂样品和p型Be调制掺杂样品。在5 K温度下,采用共聚焦显微镜系统,测量了单量子点的光致发光谱和时间分辨光谱, 研究了单量子点中三种类型激子(本征激子、负电荷激子和正电荷激子)的电子/空穴自旋翻转时间。它们的自旋翻转时间常数分别为: 本征激子的自旋翻转时间约16 ns, 正电荷激子中电子的自旋翻转时间约2 ns, 负电荷激子中空穴的自旋翻转时间约50 ps。     

半导体自组织量子点量子发光机理与器件

介绍了自组织量子点单光子发光机理及器件研究进展.主要内容包括：半导体液滴自催化外延GaAs纳米线中InAs量子点和GaAs量子点的单光子发光效应、自组织InAs/GaAs量子点与分布布拉格平面微腔耦合结构的单光子发光效应和器件制备,单量子点发光的共振荧光测量方法、量子点单光子参量下转换实现的纠缠光子发射、单光子的量子存储效应以及量子点单光子发光的光纤耦合输出芯片制备等.     

Determining the effective density of airborne nanoparticles using multiple charging correction in a tandem DMA/ELPI setup

The cysteamine-modified hyaluronic acid (HA) polymer was employed to coat quantum dots (QDs) through a convenient one-step reverse micelle method, with the final QDs hydrodynamic size of around 22.6 nm. The HA coating renders the QDs with very good stability in PBS for more than 140 days and resistant to large pH range of 2–12. Besides, the HA-coated QDs also show excellent fluorescence stability in BSA-containing cell culture medium. In addition, the cell culture assay indicates no significant cytotoxicity for MD-MB-231 breast cancer cells, and its targeting ability to cancer receptor CD44 has been demonstrated on two breast cancer cell lines. The targeting mechanism was further proved by the HA competition experiment. This work has established a new approach to help solve the stability and toxicity problems of QDs, and moreover render the QDs cancer targeting property. The current results indicate that the HA polymer-coated QDs hold the potential application for both in vitro and in vivo cancer imaging researches.     

Tracking the Dynamics of Single Quantum Dots: Beating the Optical Resolution Twice

Using particle tracking routines the location of single point light sources can be determined with an accuracy of a few nanometers. By using quantum dots (QDs) emitting at different wavelengths, the measurement of the distance between these point light sources, which are closer than the optical resolution of 200-nm, was achieved. The nanocrystals have major advantages over conventional chromophores in higher quantum yield, more photostability, and the possibility of different emission wavelengths by an excitation with a single wavelength. The colocalization of two single QDs at the least distance of 40-nm can be measured with a standard deviation of 5-nm and a time resolution of 117-ms using one excitation wavelength.     

Manufacturing of Volumetric Glass–Based Composites with Single‐ and Double‐QD Doping

Quantum dot (QD)‐based light‐emitting materials are gaining increased attention because of their easily tunable optical properties desired for various applications in biology, optoelectronics, and photonics. However, few methods can be used to manufacture volumetric materials doped with more than one type of QD other than QD‐polymer hybrids, and they often require complicated preparation processes and are prone to luminescence quenching by QD aggregation and separation from the matrix. Here, simultaneous doping of a volumetric glass‐based nanocomposite with two types of QDs is demonstrated for the first time in a single‐step process using the nanoparticle direct doping method. Glass rods doped with CdTe, CdSe/ZnS, or co‐doped with both QDs, are obtained. Photoluminescence and lifetime experiments confirm temperature‐dependent double emission with maxima at 596 and 720 nm with mean lifetimes up to 16 ns, as well as radiative energy transfer from the short wavelength–emitting QDs to the long wavelength–emitting QDs. This approach may enable the simple and cost‐efficient manufacturing of bulk materials that produce multicolor luminescence with cascade excitation pumping. Applications that could benefit from this include broadband optical fiber amplifiers, backlight systems in LCD screens, high‐power LEDs, or down‐converting solar concentrators used to increase the efficiency of solar panels.     

White light-emitting quantum dot diodes and tuning of luminescence processes

A novel white light-emitting diode based on a large Stokes shift (~200 nm) and using pure green light-emitting CdSeS quantum dots (QDs) with an Ag/ZnSnO/QDs/spiro-TPD/ITO structure has been fabricated in which ZnSnO and spiro-TPD are served as the electron and hole transport layer, respectively. The large Stokes shift of the CdSeS QDs excludes potentially Förster resonance energy transfer process, which allows spiro-TPD to act as both an emitter and hole transport layer. The devices exhibit a wide EL spectrum consisting of three components: blue emission from spiro-TPD, green emission from QD band–band recombination, and red emission from QD surface-state recombination. We further found that as the intensity ratios among these three components vary with bias the color of the QD light-emitting diodes is tunable. The device displays a good white light-emitting characteristic with CIE coordinates of (0.281, 0.384) at an appropriate bias.     

Development and Investigation of Ultrastable PbS/CdS/ZnS Quantum Dots for Near‐Infrared Tumor Imaging

Achieving bright, reliable, robust, and stable probes for in vivo imaging is becoming extremely urgent for the cancer imaging research community. To date very few works have reported on elucidating in the varied and chemically complex biological milieu. The authors report detailed investigations of the synthesis of near‐infrared, water dispersive, strongly luminescent, and highly stable PbS/CdS/ZnS core/shell/shell quantum dots (QDs). These QDs are extremely stable, they could keep their initial morphology, dispersion status, and photoluminescence (PL) in phosphate buffered saline buffer for as long as 14 months. The QDs also show excellent photostability and could keep ≈80% of their initial PL intensity after 1 h continuous, strong UV illumination. More interestingly, they show negligible toxicity to cultured cells even at high QDs concentration. Given these outstanding properties, the QDs are explored for in vivo, tumor imaging in mice. With one order of magnitude lower QD concentration (0.04 mg mL^–1), significantly weaker laser intensity (0.04 W cm^–2 vs ≈1 W cm^–2), and considerably shorter signal integration time (≤1 ms vs hundreds of ms) as compared to the best reported rare earth doped nanoparticles, the QDs show high emission intensity even at injection depth of ≈2.5 mm.     

Electrochemical characterization of core@shell CoFe2O4/Au composite

Quantum dots (QDs) have received considerable attention due to their unique optical and electrical properties. Although substantial research has focused on the potential applications and toxicological impacts of QDs, far less effort has been directed toward understanding their fate and transport in the environment. In this work, the effect of four coatings, polyethylene glycol functionalized polymer (PEGP), carboxyl derivatized polymer (COOHP), linoleic acid (LA), and polyacrylic acid-octylamine (PAA-OA), on the transport and retention of QDs in porous media were evaluated under environmentally relevant conditions. Aqueous QD suspensions (ca. 10 nM) were introduced into water-saturated columns packed with 40–50 mesh Ottawa sand at a pore-water velocity of 7.6 m/day. At an ionic strength (IS) of 3 mM and pH of 7, PEGP-coated QDs were completely retained within the column, while more than 60 % of COOHP-coated QDs were transported through a column run under identical conditions. When PAA-OA and LA were used as coatings, effluent QD recoveries increased to more than 65 and 89 % of the injected mass, respectively. Additionally, a decrease in pH from 9.5 to 5.0, or an increase of IS from 0 to 30 mM reduced the eluted mass of PAA-OA-coated QDs by more than 2 and 15 times, respectively. The relative mobility of coated QDs (LA > PAA-OA > COOHP > PEGP) was consistent with total interaction energy profiles between QDs and sand surfaces calculated based on Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. At an IS of 3 mM (NaCl) and pH 7, a linear correlation was obtained between the fraction of eluted QDs and the magnitude of the primary interaction energy barrier. These findings demonstrate the strong dependence of QD transport on coating type and indicate that interaction energies based on DLVO theory can be used to predict the relative mobility of QDs in porous media.     

Effects of a longitudinal magnetic field on spin injection and detection in InAs/GaAs quantum dot structures

Effects of a longitudinal magnetic field on optical spin injection and detection in InAs/GaAs quantum dot (QD) structures are investigated by optical orientation spectroscopy. An increase in the optical and spin polarization of the QDs is observed with increasing magnetic field in the range 0-2?T, and is attributed to suppression of exciton spin depolarization within the QDs that is promoted by the hyperfine interaction and anisotropic electron-hole exchange interaction. This leads to a corresponding enhancement in spin detection efficiency of the QDs by a factor of up to 2.5. At higher magnetic fields, when these spin depolarization processes are quenched, the electron spin polarization in anisotropic QD structures (such as double QDs that are preferably aligned along a specific crystallographic axis) still exhibits a rather strong field dependence under non-resonant excitation. In contrast, such a field dependence is practically absent in more 'isotropic' QD structures (e.g.?single QDs). We attribute the observed effect to stronger electron spin relaxation in the spin injectors (i.e.?wetting layer and GaAs barriers) of the lower-symmetry QD structures, which also explains the lower spin injection efficiency observed in these structures.     

Photoluminescence of InAs Self-Organized Quantum Dots Formation on InP Substrate by MOCVD

In this letter, we present results of photoluminescence (PL) emission from single-layer and multilayer InAs self-organized quantum dots (QDs), which were grown on (001) InP substrate. The room temperature PL peak of the single-layer QDs locates at 1608 nm, and full width at half-maximum (FWHM) of the PL peak is 71 meV. The PL peak of the multilayer QDs locates at 1478 nm, PL intensity of which is stronger than that of single-layer QDs. The single-layer QD PL spectra also display excited state emission and state filling as the excitation intensity is increased. Low temperature PL spectra show a weak peak between the peaks of QDs and wetting layer (WL), which suggests the recombination between electrons in the WL and holes in the dots.     

High‐Responsivity Photodetectors Based on Formamidinium Lead Halide Perovskite Quantum Dot–Graphene Hybrid

Highly performance photodetector requires a wide range of responses of the incident photons and converts them to electrical signals efficiently. Here, a photodetector based on formamidinium lead halide perovskite quantum dots (e.g., FAPbBr₃ QDs)–graphene hybrid, aiming to take the both advantages of the two constituents. The FAPbBr₃ QD–graphene layer not only benefits from the high mobility and wide spectral absorption of the graphene material but also from the long charge carrier lifetime and low dark carrier concentration of the FAPbBr₃ QDs. The photodetector based on FAPbBr₃ QD–graphene hybrid exhibits a broad spectral photoresponse ranging from 405 to 980 nm. A photoresponsivity of 1.15 × 10⁵AW⁻¹ and an external quantum efficiency as high as 3.42 × 10⁷% are obtained under an illumination power of 3 µW at 520 nm wavelength. In detail, a high responsivity is achieved in 405–538 nm, while a relatively low but fast response is observed in 538–980 nm. The photoelectric conversion mechanism of this hybrid photodetector is investigated in the view of built‐in electric field from the QD–graphene contact which improves the photoconductive gain.     

Scanning photoluminescent spectroscopy of bioconjugated quantum dots

We report on the application of the bio-conjugated quantum dots (QDs) for a “sandwich” enzyme-linked immunosorbent assay (ELISA) cancer testing technique. Quantum dot ELISA detection of the cancer PSA antigen at concentrations as low as 0.01 ng/ml which is ～50 times lower than the classic “sandwich” ELISA was demonstrated. Scanning photoluminescence (PL) spectroscopy was performed on dried ELISA wells and the results compared with the same QD samples dried on a solid substrate. We confirmed a “blue” up to 37 nm PL spectral shift in a case of QDs conjugated to PSA antibodies. Increasing of the “blue” spectral shift was observed at lower PSA antigen concentrations. The results can be used to improve sensitivity of “sandwich” ELISA cancer antigen detection.     

Temperature dependence of optical properties of InAs quantum dots grown on GaAs(113)A and (115)A substrates

We have investigated the temperature dependence of photoluminescence (PL) peak position of InAs self-assembled quantum dots (QDs) grown on GaAs(11N)A (N = 3, 5) substrates. The interband transition energy is calculated by the resolution of the 3D Schrödinger equation for a parallelepipedic InAs QD, with a width of about 8 nm and a height around 3 nm. Experimentally, it was found that the PL spectra quenches at about 160 K. In addition, the full width at half maximum (FWHM) has an abnormal evolution with varying temperature. The latter effect maybe due to the carrier repopulation between QDs. The disorientation of the GaAs substrate and the low width of terraces which was presented in the high index surfaces have an important contribution in the PL spectra. Despite the non-realist chosen shape of QD and the simplest adopted model, theoretical and experimental results revealed a clear agreement.     

Generation of MoS<Subscript>2</Subscript> quantum dots by laser ablation of MoS<Subscript>2</Subscript> particles in suspension and their photocatalytic activity for H<Subscript>2</Subscript> generation

MoS₂ quantum dots (QDs) have been obtained in colloidal suspensions by 532 nm laser ablation (7 ns fwhp/pulse, 50 mJ/pulse) of commercial MoS₂ particles in acetonitrile. High-resolution transmission electron microscopy images show a lateral size distribution from 5 to 20 nm, but a more homogeneous particle size of 20 nm can be obtained by silica gel chromatography purification in acetonitrile. MoS₂ QDs obtained by laser ablation are constituted by 3–6 MoS₂ layers (1.8–4 nm thickness) and exhibit photoluminescence whose λ_PL varies from 430 to 530 nm depending on the excitation wavelength. As predicted by theory, the confinement effect and the larger periphery in MoS₂ QDs increasing the bandgap and having catalytically active edges are reflected in an enhancement of the photocatalytic activity for H₂ generation upon UV–Vis irradiation using CH₃OH as sacrificial electron donor due to the increase in the reduction potential of conduction band electrons and the electron transfer kinetics.     

Optimum Quantum Yield of the Light Emission from 2 to 10 nm Hydrosilylated Silicon Quantum Dots

Optimizing the light‐emitting efficiency of silicon quantum dots (Si QDs) has been recently intensified by the demand of the practical use of Si QDs in a variety of fields such as optoelectronics, photovoltaics, and bioimaging. It is imperative that an understanding of the optimum light‐emitting efficiency of Si QDs should be obtained to guide the design of the synthesis and processing of Si QDs. Here an investigation is presented on the characteristics of the photoluminescence (PL) from hydrosilylated Si QDs in a rather broad size region (≈2–10 nm), which enables an effective mass approximation model to be developed, which can very well describe the dependence of the PL energy on the QD size for Si QDs in the whole quantum‐confinement regime, and demonstrates that an optimum PL quantum yield (QY) appears at a specific QD size for Si QDs. The optimum PL QY results from the interplay between quantum‐confinement effect and surface effect. The current work has important implications for the surface engineering of Si QDs. To optimize the light‐emission efficiency of Si QDs, the surface of Si QDs must be engineered to minimize the formation of defects such as dangling bonds at the QD surface and build an energy barrier that can effectively prevent carriers in Si QDs from tunneling out.     

Nanostars on a fiber facet with near field enhancement for surface-enhanced Raman scattering detection

A 4-pointed gold nanostar is proposed to form the array on a fiber facet to achieve a greatly enhanced near field intensity for Surface-Enhanced Raman Scattering (SERS) detection. The proposed gold nanostar array has a Surface Plasmon Resonance (SPR) peaked at a wavelength of ～650 nm with up to 45 times electric field intensity enhancement compared with the state-of-the-art nanorod design. It has a wideband SPR field enhancement spanned from 600 to 720 nm, which covers the wavelengths for both the excitation light (632.8 nm) and the Raman signal of the analytes (675–706 nm); With symmetrical structure it forms four hot spots in every unit cell and can detect best for light polarized horizontal or perpendicular to the waist of the nanostars. It also could be altered to tune the SPR and allows the fiber sensor to resonate at different wavelengths, as demonstrated by an example at 533 nm. All the above features make the gold nanostar-based compact and portable fiber sensor an attractive solution for SERS detection.