Energy transfer: Visualizing Resonance Energy Transfer in Supramolecular Surface Patterns of β‐CD‐Functionalized Quantum Dot Hosts and Organic Dye Guests by Fluorescence Lifetime Imaging (Small 24/2010)

Detection of an analyte via supramolecular host–guest binding and quantum dot (QD)‐based fluorescence resonance energy transfer (FRET) signal transduction mechanism is demonstrated. Surface patterns consisting of CdSe/ZnS QDs functionalized at their periphery with β‐cyclodextrin (β‐CD) were obtained by immobilization of the QDs from solution onto glass substrates patterned with adamantyl‐terminated poly(propylene imine) dendrimeric “glue.” Subsequent formation of host–guest complexes between vacant β‐CD on the QD surface and an adamantyl‐functionalized lissamine rhodamine resulting in FRET was confirmed by fluorescence microscopy, spectroscopy, and fluorescence lifetime imaging microscopy (FLIM).     

Ligand replacement-induced fluorescence switch of quantum dots for ultrasensitive detection of organophosphorothioate pesticides

The development of a simple and on-site assay for the detection of organophosphorus pesticed residues is very important for food safety and exosystem protection. This paper reports the surface coordination-originated fluorescence resonance energy transfer (FRET) of CdTe quantum dots (QDs) and a simple ligand-replacement turn-on mechanism for the highly sensitive and selective detection of organophosphorothioate pesticides. It has been demonstrated that coordination of dithizone at the surface of CdTe QDs in basic media can strongly quench the green emission of CdTe QDs by a FRET mechanism. Upon the addition of organophosphorothioate pesticides, the dithizone ligands at the CdTe QD surface are replaced by the hydrolyzate of the organophosphorothioate, and hence the fluorescence is turned on. The fluorescence turn on is immediate, and the limit of detection for chlorpyrifos is as low as ～0.1 nM. Two consecutive linear ranges allow a wide determination of chlorpyrifos concentrations from 0.1 nM to 10 μM. Importantly, the fluorescence turn-on chemosensor can directly detect chlorpyrifos residues in apples at a limit of 5.5 ppb, which is under the maximum residue limit allowed by the U.S. Environmental Protection Agency. The very simple strategy reported here should facilitate the development of fluorescence turn-on chemosensors for chemo/biodetection.     

Near-infrared-emitting colloidal Ag<Subscript>2</Subscript>S quantum dots excited by an 808 nm diode laser

Thioglycolic acid (TGA)-coated colloidal Ag₂S quantum dots (QDs) emitting in the near-infrared (NIR) region upon excitation by an 808 nm diode laser were synthesized. The observed photoluminescence (PL) was attributed to the presence of ligand-modified Ag₂S on the QD surfaces and could be easily controlled by a simple dilution process due to the concentration-dependent surface structure of the colloidal QDs. Upon dilution of the solution, the PL intensity initially increased before later decreasing, with a blueshift being observed in the PL spectra. These phenomena can be accounted for by the aggregation of QDs due to a decrease in the content of ligand-modified Ag₂S on the QD surfaces upon dilution, which in turn affected the fluorescence resonance energy transfer (FRET), and re-emission of the surface energy level.     

Visualizing resonance energy transfer in supramolecular surface patterns of β-CD-functionalized quantum dot hosts and organic dye guests by fluorescence lifetime imaging

Detection of an analyte via supramolecular host-guest binding and quantum dot (QD)-based fluorescence resonance energy transfer (FRET) signal transduction mechanism is demonstrated. Surface patterns consisting of CdSe/ZnS QDs functionalized at their periphery with β-cyclodextrin (β-CD) were obtained by immobilization of the QDs from solution onto glass substrates patterned with adamantyl-terminated poly(propylene imine) dendrimeric "glue." Subsequent formation of host-guest complexes between vacant β-CD on the QD surface and an adamantyl-functionalized lissamine rhodamine resulting in FRET was confirmed by fluorescence microscopy, spectroscopy, and fluorescence lifetime imaging microscopy (FLIM).     

Fluorescence Lifetime Imaging and FRET‐Induced Intracellular Redistribution of Tat‐Conjugated Quantum Dot Nanoparticles through Interaction with a Phthalocyanine Photosensitiser

The interaction of Tat‐conjugated PEGylated CdSe/ZnS quantum dots (QD) with the amphiphilic disulfonated aluminium phthalocyanine photosensitiser is investigated in aqueous solution and in a human breast cancer cell line. In aqueous solution, the QDs and phthalocyanine form stable nanocomposites. Using steady‐state and time‐resolved fluorescence measurements combined with singlet oxygen detection, efficient Förster resonance energy transfer (FRET) is observed with the QDs acting as donors, and the phthalocyanine photosensitiser, which mediates production of singlet oxygen, as acceptors. In cells, the Tat‐conjugated QDs localise in lysosomes and the QD fluorescence lifetimes are close to values observed in aqueous solution. Strong FRET‐induced quenching of the QD lifetime is observed in cells incubated with the nanocomposites using fluorescence lifetime imaging microscopy (FLIM). Using excitation of the QDs at wavelengths where phthalocyanine absorption is negligible, FRET‐induced release of QDs from endo/lysosomes is confirmed using confocal imaging and FLIM, which is attributed to photooxidative damage to the endo/lysosomal membranes mediated by the phthalocyanine acceptor.     

基于量子点的分子灯塔探针的制备及其在DNA探针中的应用

根据荧光共振能量转移理论合成出一种新颖的分子灯塔探针.由于CdTe量子点（QD s）的荧光发射光谱与DABCYL的紫外-可见吸收光谱有很好的重叠性,所以此种探针采用CdTe量子点作为能量给体,DABCYL作为能量受体.通过水相法合成出直径为2.5 nm的CdTe量子点,并且在偶联剂1-乙基-3-（3-二甲基氨丙基）碳二亚氨盐酸盐（EDC）作用下,与5-′NH2-DNA-DABCYL连接得到了分子灯塔探针.实验发现探针的荧光强度相比CdTe-DNA有明显的下降,最大能量转移效率为68.3%,表明CdTe QD s和DABCYL之间发生了荧光共振能量转移.结果表明,此种探针体系对于互补DNA及其变种有着很好的特异性,且其检测极限为5.170×10^-9mol/L.     

Nanobodies and Nanocrystals: Highly Sensitive Quantum Dot‐Based Homogeneous FRET Immunoassay for Serum‐Based EGFR Detection

Semiconductor quantum dot nanocrystals (QDs) for optical biosensing applications often contain thick polyethylene glycol (PEG)‐based coatings in order to retain the advantageous QD properties in biological media such as blood, serum or plasma. On the other hand, the application of QDs in Förster resonance energy transfer (FRET) immunoassays, one of the most sensitive and most common fluorescence‐based techniques for non‐competitive homogeneous biomarker diagnostics, is limited by such thick coatings due to the increased donor‐acceptor distance. In particular, the combination with large IgG antibodies usually leads to distances well beyond the common FRET range of approximately 1 to 10 nm. Herein, time‐gated detection of Tb‐to‐QD FRET for background suppression and an increased FRET range is combined with single domain antibodies (or nanobodies) for a reduced distance in order to realize highly sensitive QD‐based FRET immunoassays. The “(nano)²” immunoassay (combination of nanocrystals and nanobodies) is performed on a commercial clinical fluorescence plate reader and provides sub‐nanomolar (few ng/mL) detection limits of soluble epidermal growth factor receptor (EGFR) in 50 μL buffer or serum samples. Apart from the first demonstration of using nanobodies for FRET‐based immunoassays, the extremely low and clinically relevant detection limits of EGFR demonstrate the direct applicability of the (nano)^2‐ assay to fast and sensitive biomarker detection in clinical diagnostics.     

Quantum dot-fluorescent protein pairs as novel fluorescence resonance energy transfer probes

Fluorescence resonance energy transfer (FRET) characteristics, including the efficiency, donor-acceptor distance, and binding strength of six fluorescent protein (FP)-quantum dot (QD) pairs were quantified, demonstrating that FPs are efficient acceptors for QD donors with up to 90% quenching of QD fluorescence and that polyhistidine coordination to QD core-shell surface is a straightforward and effective means of conjugating proteins to commercially available QDs. This provides a novel approach to developing QD-based FRET probes for biomedical applications.     

Surface plasmon enhanced energy transfer between donor and acceptor CdTe nanocrystal quantum dot monolayers

Surface plasmon enhanced Fo?rster resonant energy transfer (FRET) between CdTe nanocrystal quantum dots (QDs) has been observed in a multilayer acceptor QD-gold nanoparticle-donor QD sandwich structure. Compared to a donor-acceptor QD bilayer structure without gold nanoparticles, the FRET rate is enhanced by a factor of 80 and the Fo?rster radius increases by 103%. Furthermore, a strong impact of the donor QD properties on the surface plasmon mediated FRET is reported.     

Size dependent electron transfer from CdTe quantum dots linked to TiO2 thin films in quantum dot sensitized solar cells

In this present study, we demonstrate the size dependent charge transfer from CdTe quantum dots (QDs) into TiO₂ substrate and relate this charge transfer to the actual behavior of a CdTe sensitized solar cell. CdTe QDs was synthesized using mercaptopropionic acid as the capping agent. The conduction band offset for TiO₂ and CdTe QDs indicates thermodynamically favorable band edge positions for smaller QDs for the electron-transfer at the QD–TiO₂ interface. Time-resolved emission studies were carried out for CdTe QD on glass and CdTe QD on TiO₂ substrates. Results on the quenching of QD luminescence, which relates to the transfer kinetics of electrons from the QD to the TiO₂ film, showed that at the smaller QD sizes the transfer kinetics are much more rapid than at the larger sizes. I–V characteristics of quantum dot sensitized solar cells (QDSSC) with different sized QDs were also investigated indicating higher current densities at smaller QD sizes consistent with the charge transfer results. The maximum injection rate constant and photocurrent were obtained for 2.5 nm CdTe QDs. We have been able to construct a solar cell with reasonable characteristics (V_oc = 0.8 V, J_sc = 1 mA cm⁻², FF = 60%, η = 0.5%).     

Fluorescent Silkworm Silk Prepared via Incorporation of Green, Yellow, Red, and Near-Infrared Fluorescent Quantum Dots

Novel fluorescent silkworm silks incorporated with CdTe quantum dots (QDs) have been prepared via electrostatic assembly using a polyelectrolyte (PE) as a fixer. The bright fluorescence of the silks/QDs obtained could be tuned from green to near-infrared (NIR) by changing the QD size, thereby enabling the fluorescence to penetrate through an opaque pig skin of ~3.5 mm in thickness. A large amount of the QDs were not only deposited on the surface of the silk, but also found permeated into the silk. The silk incorporated with just one PE/QDs/PE layer exhibited a smooth surface. However, the presence of more PE/QDs/PE layers in the silk resulted in an increased surface roughness. The mechanical properties of the silks incorporated with one PE/QDs/PE layer were close to those of pure silks, but the maximum pull force and elastic limit elongation at break of a single silk thread decreased with increasing number of PE/QDs/PE layers. Although a very small amount of Cd²⁺ and Te^2- ions were released from the silks/QDs in neutral and alkaline aqueous solutions, nearly no QD were released from these fluorescent silks. The fluorescence intensity of the silk/QDs was stable in neutral and alkaline aqueous solutions, but significantly decreased in acidic solution. The simple preparation method of the silks/QDs reported in this paper is both highly efficient and low cost, and the fluorescent silks obtained are anticipated to be of great interest for fabricating multicolored fluorescent cloths and anticounterfeiting labels, as well as being useful for other bio- or physico-related applications.     

Surface characterization of GSH-CdTe quantum dots

The surface characterization of CdTe QDs synthesized by a novel procedure using glutathione (GSH), low temperatures (60–90 °C) and K₂TeO₃ as the –Te precursor is reported. Fluorescence of the produced QDs is stable in the pH range 6–13 and QDs inside eukaryotic cells are highly fluorescent. The surface composition of GSH-CdTe QDs with different spectroscopic properties and particle size distributions was determined by XPS. The XPS analysis indicated that the QDs are essentially CdTe, although all nanoparticles contain 12–24% of CdO (and in one case also TeO₂). GSH decomposes with reaction time releasing small amounts of S⁻² ions that react with Cd(Te) to yield Cd(Te)S in a smaller amount than that of CdTe. Finally, the use of QDs in fluorescence mediated immunodetection of bacterial pathogens has been evaluated.     

Exploring feasibility of multicolored CdTe quantum dots for in vitro and in vivo fluorescent imaging

We report the use of novel multicolored CdTe quantum dots (QDs) as fluorophores for biological fluorescence imaging. The CdTe QDs were prepared to exhibit emission wavelengths in the green, yellow, and red range by using trifluoroacetic acid (TFA), L-cysteine and thioglycolic acid (TGA) as surface stabilizers, respectively. The particles have good water solubility and photostability. Fluorescence imaging potential was evaluated in vitro and in vivo using a multispectral Maestro CRI Fluorescence Imaging system. The results show that different colored CdTe QDs allow sensitive detection simultaneously or separately both in vitro and in vivo against background fluorescence. The studies indicate that CdTe QDs can provide alternative fluorescent probes for biological imaging.     

Ultrasensitive Pb2+ detection by glutathione-capped quantum dots

Water-soluble and stable quantum dots (QDs), CdTe and CdZnSe, are applied for ultrasensitive Pb(2+) detection. These QDs are capped with glutathione (GSH) shells. GSH and its polymeric form, phytochelatin, are employed by nature to detoxify heavy metal ions. As a result of specific interaction, the fluorescence intensity of GSH-capped QDs is selectively reduced in the presence of heavy metal ions such as Pb(2+). The detection limit of Pb(2+) is found to be 20 nM due to the superior fluorescence properties of QDs. Detailed studies by spectroscopy, microscopy, and dynamic light scattering show that competitive GSH binding of Pb(2+) with the QD core changed both the surface and photophysical properties of the QDs. Fluorescence of QDs is quenched, and QD aggregation occurs. Coupling the GSH-capped QDs with a high-throughput detection system, we have developed a simple scheme for quick and ultrasensitive Pb(2+) detection without the need for additional electronic devices. In the presence of ionic mixtures, our system is still capable of Pb(2+) detection with a detection limit as low as 40 nM. The system only becomes less sensitive when the ionic mixture is present at a very high concentration (i.e., > or =50 microM).     

Using layer-by-layer assembly to fabricate NaLa(MoO4)2@CdTe core–shell microspheres with high fluorescence

The development of fluorescent materials with low energy consumption, low cost and desirable optical properties is needed for the perspective of practical application. Here, functional NaLa(MoO₄)₂@CdTe core–shell microspheres with high fluorescence were prepared by layer-by-layer self-assembly technique. Through the consecutive electrostatic adsorption of charged cetyltrimethyl ammonium bromide and CdTe quantum dots (QDs), the uniform and regular multilayer shell of CdTe QDs was synthesized. The NaLa(MoO₄)₂@CdTe microspheres exhibited improved photoluminescence intensity and stability of red emission, compared with that of the CdTe QDs powder, and the fluorescence enhancement mechanism were investigated. The CdTe QDs multilayer shell is expected to supersede the Eu³⁺ ion for producing a novel red phosphor.     

Cytotoxicity of cadmium-containing quantum dots based on a study using a microfluidic chip

There is a lack of reliable nanotoxicity assays available for monitoring and quantifying multiple cellular events in cultured cells. In this study, we used a microfluidic chip to systematically investigate the cytotoxicity of three kinds of well-characterized cadmium-containing quantum dots (QDs) with the same core but different shell structures, including CdTe core QDs, CdTe/CdS core–shell QDs, and CdTe/CdS/ZnS core-shell-shell QDs, in HEK293 cells. Using the microfluidic chip combined with fluorescence microscopy, multiple QD-induced cellular events including cell morphology, viability, proliferation, and QD uptake were simultaneously analysed. The three kinds of QDs showed significantly different cytotoxicities. The CdTe QDs, which are highly toxic to HEK293 cells, resulted in remarkable cellular and nuclear morphological changes, a dose-dependent decrease in cell viability, and strong inhibition of cell proliferation; the CdTe/CdS QDs were moderately toxic but did not significantly affect the proliferation of HEK293 cells; while the CdTe/CdS/ZnS QDs had no detectable influence on cytotoxicity with respect to cell morphology, viability, and proliferation. Our data indicated that QD cytotoxicity was closely related to their surface structures and specific physicochemical properties. This study also demonstrated that the microfluidic chip could serve as a powerful tool to systematically evaluate the cytotoxicity of nanoparticles in multiple cellular events.     

Room‐Temperature Triple‐Ligand Surface Engineering Synergistically Boosts Ink Stability,Recombination Dynamics,and Charge Injection toward EQE‐11.6% Perovskite QLEDs

Developing low‐cost and high‐quality quantum dots (QDs) or nanocrystals (NCs) and their corresponding efficient light‐emitting diodes (LEDs) is crucial for the next‐generation ultra‐high‐definition flexible displays. Here, there is a report on a room‐temperature triple‐ligand surface engineering strategy to play the synergistic role of short ligands of tetraoctylammonium bromide (TOAB), didodecyldimethylammonium bromide (DDAB), and octanoic acid (OTAc) toward “ideal” perovskite QDs with a high photoluminescence quantum yield (PLQY) of >90%, unity radiative decay in its intrinsic channel, stable ink characteristics, and effective charge injection and transportation in QD films, resulting in the highly efficient QD‐based LEDs (QLEDs). Furthermore, the QD films with less nonradiative recombination centers exhibit improved PL properties with a PLQY of 61% through dopant engineering in A‐site. The robustness of such properties is demonstrated by the fabrication of green electroluminescent LEDs based on CsPbBr₃ QDs with the peak external quantum efficiency (EQE) of 11.6%, and the corresponding peak internal quantum efficiency (IQE) and power efficiency are 52.2% and 44.65 lm W^?1, respectively, which are the most‐efficient perovskite QLEDs with colloidal CsPbBr₃ QDs as emitters up to now. These results demonstrate that the as‐obtained QD inks have a wide range application in future high‐definition QD displays and high‐quality lightings.     

Heterostructures with CdTe/ZnTe quantum dots for single photon emitters grown by molecular beam epitaxy

We report on the molecular beam epitaxy (MBE) of heterostructures with CdTe/ZnTe quantum dots (QDs) with relatively low surface density, which could be used as single-photon emitters. The QDs were formed on the surface of a 3.1- to 4.5-monolayer-thick two-dimensional strained CdTe layer by depositing amorphous Te layer and its fast thermal desorption. Subsequent thermal annealing of the surface with QDs in the absence of external Te flux led to strong broadening and short-wavelength shift of the QD photoluminescence (PL) peak. Measurement of the micro-PL spectra of individual CdTe/ZnTe quantum dots in fabricated mesastructures with a diameter of 200—1000 nm allowed estimation of the QD surface density as ~10¹⁰ cm^–2.     

Electrically Pumped White‐Light‐Emitting Diodes Based on Histidine‐Doped MoS2 Quantum Dots

MoS₂ quantum dots (QDs)‐based white‐light‐emitting diodes (QD‐WLEDs) are designed, fabricated, and demonstrated. The highly luminescent, histidine‐doped MoS₂ QDs synthesized by microwave induced fragmentation of 2D MoS₂ nanoflakes possess a wide distribution of available electronic states as inferred from the pronounced excitation‐wavelength‐dependent emission properties. Notably, the histidine‐doped MoS₂ QDs show a very strong emission intensity, which exceeds seven times of magnitude larger than that of pristine MoS₂ QDs. The strongly enhanced emission is mainly attributed to nitrogen acceptor bound excitons and passivation of defects by histidine‐doping, which can enhance the radiative recombination drastically. The enabled electroluminescence (EL) spectra of the QD‐WLEDs with the main peak around 500 nm are found to be consistent with the photoluminescence spectra of the histidine‐doped MoS₂ QDs. The enhanced intensity of EL spectra with the current increase shows the stability of histidine‐doped MoS₂ based QD‐WLEDs. The typical EL spectrum of the novel QD‐WLEDs has a Commission Internationale de l'Eclairage chromaticity coordinate of (0.30, 0.36) exhibiting an intrinsic broadband white‐light emission. The unprecedented and low‐toxicity QD‐WLEDs based on a single light‐emitting material can serve as an excellent alternative for using transition metal dichalcogenides QDs as next generation optoelectronic devices.     

Temperature‐Dependent Exciton and Trap‐Related Photoluminescence of CdTe Quantum Dots Embedded in a NaCl Matrix: Implication in Thermometry

Temperature‐dependent optical studies of semiconductor quantum dots (QDs) are fundamentally important for a variety of sensing and imaging applications. The steady‐state and time‐resolved photoluminescence properties of CdTe QDs in the size range from 2.3 to 3.1 nm embedded into a protective matrix of NaCl are studied as a function of temperature from 80 to 360 K. The temperature coefficient is found to be strongly dependent on QD size, with the highest sensitivity obtained for the smallest size of QDs. The emission from solid‐state CdTe QD‐based powders is maintained with high color purity over a wide range of temperatures. Photoluminescence lifetime data suggest that temperature dependence of the intrinsic radiative lifetime in CdTe QDs is rather weak, and it is mostly the temperature‐dependent nonradiative decay of CdTe QDs which is responsible for the thermal quenching of photoluminescence intensity. By virtue of the temperature‐dependent photoluminescence behavior, high color purity, photostability, and high photoluminescence quantum yield (26%–37% in the solid state), CdTe QDs embedded in NaCl matrices are useful solid‐state probes for thermal imaging and sensing over a wide range of temperatures within a number of detection schemes and outstanding sensitivity, such as luminescence thermochromic imaging, ratiometric luminescence, and luminescence lifetime thermal sensing.