Aggregation Of Quantum Dots Research Articles

Optical Sensors Based on II-VI Quantum Dots.

Fundamentals of quantum dots (QDs) sensing phenomena show the predominance of these fluorophores over standard organic dyes, mainly because of their unique optical properties such as sharp and tunable emission spectra, high emission quantum yield and broad absorption. Moreover, they also indicate no photo bleaching and can be also grown as no blinking emitters. Due to these properties, QDs may be used e.g., for multiplex testing of the analyte by simultaneously detecting multiple or very weak signals. Physico-chemical mechanisms used for analyte detection, like analyte stimulated QDs aggregation, nonradiative Förster resonance energy transfer (FRET) exhibit a number of QDs, which can be applied in sensors. Quantum dots-based sensors find use in the detection of ions, organic compounds (e.g., proteins, sugars, volatile substances) as well as bacteria and viruses.

The in situ ligand exchange linker-assisted assembly of oil-soluble CdSe quantum dots to TiO2 films

Recently, the pre-synthesized oil-soluble quantum dots (QDs) have triggered great interests due to advantages of monodispersity, stability and easy synthesis. Aggregation of QDs is the reason for low loading and poor efficiency during the in situ ligand exchange linker-assisted assembly process of preparing photovoltaic devices. In this work, we modify TiO2 films with 4-mercaptobenzoic acid (MBA) as linkers and adjust modification time (MT) of MBA on the surface of TiO2 films. As the MT increases from 0 to 6 min, even 360 min, loading amount of QDs and photovoltaic performance improve first and then decrease. The results of line scan in cross-sectional view of photoanodes show that QDs uniformly distribute throughout the short-MT films thickness, while obvious aggregation occur in the surface of long-MT films. The best device with 3 min of MBA modification exhibits an optimal efficiency of 5.58%.

Solvothermal Synthesis, Characterization and Photocatalytic activity of ZnO Nanoparticle

A simple and efficient strategy of one–pot synthesis of ZnO photocatalyst was developed using solvothermal synthesis and heat treatment of ZnCl2 with NaOH solutions in polyethylene glycol at a low temperature led to the formation of ZnO nanoparticles consisting of the aggregates of uniform-sized quantum dots. Several analytical techniques including X-ray diffraction (XRD) reveals the hexagonal wurtzite structure of ZnO, FT-IR, and UV-VIS has been used to characterize the resulting ZnO photocatalyst.

The effect of PEG functionalization on the in vivo behavior and toxicity of CdTe quantum dots.

CdTe quantum dots (QDs) are considered a potential toxic substance because they contain metal ions. However, most toxicology data are derived from in vitro studies or limited in vivo analysis and may not reflect in vivo responses and biodistribution. Proper modification is one of the most widely used routes to reduce the toxicity of QDs. Herein, we demonstrated the role of polyethylene glycol (PEG) in decreasing the toxicity of QDs by studying the animal survival, clinical biochemistry, organ histology, biodistribution and oxidative stress in thioglycolic acid (TGA)- and mercapto-acetohydrazide (TGH)-stabilized CdTe QD (TGA/TGH-CdTe QD)-treated groups. Via the histology, transmission electron microscopy (TEM) and biodistribution results, it was found that the QDs mainly accumulated in the liver and kidney at 7 days post-injection, and obvious tissue damage was also observed in the bare TGA/TGH-CdTe QD group. Based on the evaluation of oxidative stress in the liver and kidney, the indicators exhibited an obvious variation with a high dose of TGA/TGH-CdTe QDs. In contrast, the QD aggregation decreased in the liver and kidney with no clear physiological index variation after PEG functionalization. Thus, PEG plays an important role in decreasing the toxicity of the CdTe QDs, and both the accumulation of cadmium and oxidative stress variation instead of an isolation factor are responsible for the in vivo toxicity of these QDs.

Cucurbituril-mediated quantum dot aggregates formed by aqueous self-assembly for sensing applications

Self-assembled nanoparticles have important applications in energy systems, optical devices and sensors, via the formation of aggregates with controlled interparticle spacing. Here we report aqueous self-assembly of rigid macrocycle cucurbit[7]uril (CB[7]) and fluorescent quantum dots (QDs), and demonstrate the potential of the system for efficient energy transfer and sensing of small molecules.

Ultrabroad-band, red sufficient, solid white emission from carbon quantum dot aggregation for single component warm white light emitting diodes with a 91 high color rendering index.

An ultrabroad-band solid white emission from carbon quantum dot aggregation with a full width at half maximum over 200 nm throughout the entire visible light window and, even better, with a sufficient red component is first reported. UV pumped WLEDs are fabricated by integrating phosphors onto a UV-LED chip, and they show favorable warm white light characteristics with CIE coordinates of (0.42, 0.38) and a 91 high color rendering index.

White-Light-Emitting Melamine-Formaldehyde Microspheres through Polymer-Mediated Aggregation and Encapsulation of Graphene Quantum Dots.

Graphene quantum dot (GQD) encapsulated melamine‐formaldehyde (MF) polymer microspheres with uniform particle size and tunable high‐quality white‐light emissions are prepared via a polymer‐mediated GQD assembly and encapsulation strategy. In solution, GQDs are first aggregated with MF prepolymer through electrostatic interaction and further encapsulated inside the microspheres formed by polymerization of MF prepolymer under acid catalysis and heating. During this process, the aggregated GQDs are fixed in the MF polymer matrix with their emission extended from blue to full visible range, presenting bright white luminescence under ultraviolet excitation. The prepared white‐light‐emitting GQD‐MF microspheres exhibit uniform morphology with an average particle size of 2.0 ± 0.08 µm and their luminescence properties are effectively regulated by the doping concentration of GQDs in the MF polymer matrix. A series of white‐light‐emitting GQD‐MF microspheres with quantum yields from 0.83 to 0.43, Commission Internationale de L'Eclairage coordinates from (0.28, 0.28) to (0.33, 0.32), and color rendering index from 0.75 to 0.88 are obtained with excellent photostability and thermal stability. By dispersing the GQD‐MF microspheres in cross‐linked polydimethylsiloxane matrix, flexible film with dual functions of high‐quality white‐light‐emitting and light diffusion is obtained and successfully applied for white light‐emitting diode fabrication.

Composite Structures with Emissive Quantum Dots for Light Enhancement

AbstractSince their inception, quantum dots have proven to be advantageous for light management applications due to their high brightness and well‐controlled absorption, scattering, and emission properties. As quantum dots become commercially available at large scale, the need for robust, stable, and flexible optical components continues to drive the development of robust and flexible quantum dot composite materials. In this review, after a thorough introduction to quantum dots, discussion delves into methods for fabricating quantum dot loaded composite optical elements such as thin films, microfabricated patterns, and microstructures. The importance of surface chemistry and ligand engineering, host matrixes, wet processing, and unique patterning methodologies is presented by considering photostability, aggregation, and phase separation of quantum dots in corresponding composites. With regard to prospective optical applications of quantum dot materials, emphasis is placed on light emitting and guiding composite materials for lasing applications, specifically whispering gallery mode‐based photonic microsystems. These developments will enable novel flexible, portable, and miniaturized optoelectronic devices such as light‐emitting diodes, flexible pixelated displays, solar cells, large‐area microwaveguides, omnidirectional micromirrors, optical metasurfaces, and directional microlasers.

S-doped carbon dots capped ZnCdTe quantum dots for ratiometric fluorescence sensing of guanine

Quantum dots (QDs)-based ratiometric fluorescence probe have drawn extensive attention in recent years. However, intrinsic dual-emitting QDs were rarely reported, and most of them were based on the metal doped-QDs. Herein, an intrinsic dual-emitting ZnCdTe QDs were synthesized by using fluorescence S doped-carbon dots (CDs) as capping ligands for the first time in this work, two emission peaks (derived from CDs and QDs respectively) were observed in CDs-ZnCdTe QDs and the probe showed excellent stability and optical properties. Moreover, we found that the fluorescence emission of QDs could be quenched by guanine via excited state electron transfer or the aggregation of QDs, but the emission of CDs remained stable, thus we successfully developed a ratiometric probe for the determination of guanine. Under optimized conditions, the detection limit of guanine was 0.076 μmol⋅L−1 (3σ/k), with a linear range from 0.25 to 40.0 μmol⋅L−1. The smart ratiometric fluorescence probe was demonstrated for the detection of guanine in DNA samples with satisfactory recoveries ranging from 96.95 to 102.50%, indicating it was a promising biosensor for highly selectively and sensitively determination of guanine.

On-off-on luminescent pyrophosphate probe based on the use of Mn-doped ZnS quantum dots and using Eu(III) as a mediator.

A selective phosphorescent on-off-on probe with long decay lifetime has been designed for the detection of pyrophosphate ions (PPi). The detection scheme is based on the use of europium(III)-modulated Mn(II)-doped ZnS quantum dots capped with N-acetyl-L-cysteine. Both the aggregation of quantum dots and electron transfer induced by Eu(III) ions cause phosphorescence to be quenched ("off" state). Phosphorescence is, however, restored on addition of PPi to the system ("on" state). The effect is attributed to the removal of Eu(III) from the carboxy groups on the surface of the quantum dots owing to the stronger interaction between PPi and Eu(III). A linear relationship exists between phosphorescence intensity (best measured atexcitation/emission wavelengths of316/594 nm)and PPi concentration in the 400nM to 6000nM with a detection limit of 145nM. An additional attractive feature is provided by the long-lived phosphorescence (1920μs) of the quantum dots. It can be used to eliminate interference by short-lived fluorescence in biological samples by performing time resolved measurements. The probe was applied to the determination of PPi in spiked in urine samples and gave recoveries in the range from 98 to 105% with RSDs of <2.0%. Graphical abstract Schematic of a long-lived phosphorescent on-off-on probe for the sensitive and selective detection of pyrophosphate ions (PPi). It is based on the use ofEu(III)-modulated Mn(II)-doped ZnS quantum dots (QDs). Phosphorescence isquenched of QDs after the addition of Eu3+but restored after the addition of PPi.

Controlling the Catalytic and Optical Properties of Aggregated Nanoparticles or Semiconductor Quantum Dots Using DNA-Based Constitutional Dynamic Networks.

Nucleic acid-based constitutional dynamic networks (CDNs) attract growing interest as a means to mimic complex biological networks. The triggered stabilization of the CDNs allows the dictated guided reversible reconfiguration and re-equilibration of the CDNs to other CDN configurations, where some of the constituents are up-regulated, while other constituents are down-regulated. Although substantial progress in controlling the adaptive dynamic control of the compositions of networks by means of auxiliary triggers was demonstrated, the use of CDNs as active ensembles for controlling chemical functionalities is still a challenge. We report on the assembly of signal-triggered CDN systems that guide the switchable aggregation of Au nanoparticles (NPs), thereby controlling their plasmonic properties and their catalytic functions (Au NPs-catalyzed oxidation of l-DOPA to dopachrome). In addition, we demonstrate that the triggered and orthogonal up-regulation and down-regulation of the constituents of the CDNs leads to the dictated aggregation of different-sized CdSe/ZnS quantum dots (QDs), cross-linked by K+-ion-stabilized G-quadruplex units. The incorporation of hemin into the G-quadruplex cross-linking units yields horseradish peroxidase-mimicking DNAzyme units that catalyze the generation of chemiluminescence via the oxidation of luminol by H2O2. The resulting chemiluminescence stimulates the chemiluminescence resonance energy transfer (CRET) process to the QDs, resulting in the luminescence of the two-sized QDs. By the application of appropriate triggers, the CDN-dictated up-regulation and down-regulation of the different-sized QDs aggregates are demonstrated, and the control over the photophysical functions of the different-sized QDs, by means of the CDNs, is highlighted.

A sensitive assay of telomerase activity based on the controllable aggregation of quantum dots

In order to overcome the shortcomings of traditional methods for telomerase activity detection of time-consuming and susceptibility to interferences in cell extracts, here, we develop a semiconductor quantum dots-streptavidin conjugates (QDs-SA)-based biosensor for sensitive detection of telomerase activity. In the absence of telomerase, the aggregation and thus the fluorescence quenching of QDs-SA can be observed in buffer solution due to charge shielding effect. In the presence of telomerase and biotin-dATP, QDs-SA will bind to telomerase elongation products due to the high affinity between the biotin and streptavidin, protecting the QDs-SA from aggregation and retaining their strong fluorescence. This sensor exhibited good fluorescence responses toward telomerase activity from HeLa cells in the range of 40–1000 cells with a detection limit of 13 cells or A549 cells within the linear range of 20–500 cells with a detection limit of 9 cells. Furthermore, the sensor can be used to efficiently monitor the evolution of telomerase activity in living cells, demonstrating great potential in tumor diagnosis and inhibitor drug screening.

Ultrasensitive free bilirubin detection in whole blood via counting quantum dots aggregates at single nanoparticle level

Here we report a versatile strategy for detecting bilirubin by harnessing the cationic polymer polyethylenimine induced aggregation of human serum albumin-modified quantum dots. These aggregates are easily discriminated from the individual QD by single molecule spectral imaging microscopy and correlated to the bilirubin concentration. The proposed method can selectively detect bilirubin with a detection limit as low as 0.6 nM. Remarkably, this method has been successfully applied to the trace detection of free bilirubin in human whole blood and serum samples with satisfactory results.

Energy transfer-enhanced photocatalytic reduction of protons within quantum dot light-harvesting–catalyst assemblies

Excitonic energy transfer (EnT) is the mechanism by which natural photosynthetic systems funnel energy from hundreds of antenna pigments to a single reaction center, which allows multielectron redox reactions to proceed with high efficiencies in low-flux natural light. This paper describes the use of electrostatically assembled CdSe quantum dot (QD) aggregates as artificial light harvesting-reaction center units for the photocatalytic reduction of H+ to H2, where excitons are funneled through EnT from sensitizer QDs (sQDs) to catalyst QDs (cQDs). Upon increasing the sensitizer-to-catalyst ratio in the aggregates from 1:2 to 20:1, the number of excitons delivered to each cQD (via EnT) per excitation of the system increases by a factor of nine. At the optimized sensitizer-to-catalyst ratio of 4:1, the internal quantum efficiency (IQE) of the reaction system is 4.0 ± 0.3%, a factor of 13 greater than the IQE of a sample that is identical except that EnT is suppressed due to the relative core sizes of the sQDs and cQDs. A kinetic model supports the proposed exciton funneling mechanism for enhancement of the catalytic activity.

Microwave-Assisted Heating Method toward Multicolor Quantum Dot-Based Phosphors with Much Improved Luminescence.

Solid-state highly photoluminescent quantum dot (QD)-based phosphors attract great scientific interests as color converters because of an increasing demand for white-light-emitting devices. Herein, a microwave-assisted heating method is presented to fabricate multicolor QD-based phosphors within 30 s through microwave-assisted heating of the mixture of QDs and sodium silicate aqueous solution. In the composites, the formed cross-linked networks not only play as a matrix to prevent QD aggregation in solid state but also cause the variation of the refractive index around QDs and the QD surface optimization, which contributes to good stabilities and twice enhancement in photoluminescence quantum yields (69%) compared with the initial QD aqueous solution (33%). Using the QD-based phosphors as color conversion layers, white-light-emitting diodes were realized with controllable color temperature, high color purity, and high color-rendering index (90.3), which show a great potential in display and illumination. Furthermore, the luminescence lifetime of the QD-based phosphors is less than 25 ns. The potential application of the QD-based phosphors in visible light communication was also demonstrated, with the modulation bandwidth achieving 42 MHz.

Ultrasensitive determination of mercury ions (Ⅱ) by analysis of the degree of quantum dots aggregation

In this work, an ultrasensitive fluorescence strategy on counting the degree of aggregation (DOA) of QDs aggregates was developed for Hg2+ detection, which is based on thymine-Hg2+-thymine (T-Hg2+-T) coordination chemistry. Oligonucleotides containing 12 thymines are modified on the surface of QDs which could protect QDs effectively from aggregation in the absence of Hg2+. While in the presence of Hg2+, QDs will be made to attach with each other by the structure of “T-Hg2+-T” formed, and then aggregated in the sensing system. The DOA is acquired easily by recording the entire process of the blue shifting and photobleaching of QDs aggregates. First-order spectrum of each QD in the aggregates can be released from the original overlapped spectral image of the aggregates in sequence under continuous illumination because QDs exhibit the asynchronous spectra blue shifting. Employed 0.3 nM functional QDs in the sensing system, an ultralow detection limit of 4.6 pM was reached by the counting DOA-based sensing strategy. Besides, the sensor also shows high selectivity against other 12 metal ions even at high concentrations. At last, the application of the sensing strategy for river water shows that the real samples can be worked well using this method.

Luminous composite ultrathin films of CdTe quantum dots/silk fibroin co-assembled with layered doubled hydroxide: Enhanced photoluminescence and biosensor application

Quantum dots (QDs) luminescent films are extensively applied to optoelectronics and optical devices. However, QDs aggregation results in the quenching of their fluorescence property which limits their practical applications to a greater extent. In order to resolve this issue, 3-mercaptopropionic acid (3-MPA) functionalized Cadmium Tellurium (CdTe) QDs were stabilized by silk fibroin (SB) and co-assembled with layered doubled hydroxide (LDH) to form (QDs@SF/LDH)n ultrathin films (UTFs) via the layer-by-layer (LBL) technique. UV–Vis absorption and fluorescence spectroscopy showed a stepwise and normal growth of the films upon increasing the number of deposition cycles. XRD and AFM studies confirmed the formation of a periodic layered structure and regular surface morphology of the thin films. As compared to (CdTe QDs/LDH)nUTFs, the (CdTe QDs@SF/LDH)nUTFs displayed fluorescence enhancement and longer fluorescent lifetime, both in solid states and aqueous solutions. Furthermore compared with the solution state, the fluorescence enhancement of SF-RC and SF-β are, respectively, 7 times and 17 times in the (CdTe QDs@SF/LDH)n UTFs, indicating that the LDH nanosheets favor the fluorescence enhancement effect on the CdTe QDs@SF. The fabricated materials displayed fluorescence response to a biological molecule such as immune globulin, lgG. Thus, the (CdTe QDs@SF/LDH)n UTFs has a potential to be used as biosensor.

Interaction of carboxylated CdSe/ZnS quantum dots with fish embryos: Towards understanding of nanoparticles toxicity

Due to colloidal instability even with protective coatings, nanoparticles tend to aggregate in complex environments and possibly interact with biota. In this study, visualization of quantum dots (QDs) interaction with rainbow trout (Oncorhynchus mykiss) embryos was performed. Studies on zebrafish (Danio rerio) and pearl gourami (Trichogaster leerii) embryos have shown that QDs interact with embryos in a general manner and their affects are independent on the type of the embryo. It was demonstrated that carboxylated CdSe/ZnS QDs (4 nM) were aggregating in accumulation media and formed agglomerates on the surface of fish embryos under 1–12 days incubation in deep-well water. Detailed analysis of QDs distribution on fish embryos surface and investigation of the penetration of QDs through embryo's membrane showed that the chorion protects embryos from the penetration through the chorion and the accumulation of nanoparticles inside the embryos. Confocal microscopy and spectroscopy studies on rainbow trout embryos demonstrated that QDs cause chorion damage, due to QDs aggregation on the surface of chorion, even the formation of the agglomerates at the outer part of the embryos and/or with the mucus were detected. Aggregation of QDs and formation of agglomerates on the outer part of the embryo's membrane caused the intervention of the aggregates to the chorion and even partially destroyed the embryo's chorion. The incorporation of QDs in chorion was confirmed by two methods: in living embryos from a 3D reconstruction view, and in slices of embryos from a histology view. The damage of chorion integrity might have adverse effects on embryonic development. Moreover, for the first time the toxic effect of QDs was separated from the heavy metal toxicity, which is most commonly discussed in the literature to the toxicity of the QDs.

AC-dielectrophoretic force assisted fabrication of conducting quantum dot aggregates in the electrical breakdown-induced CNT nanogap

In this paper, we fabricated quantum dot (QD) aggregates at desired locations using dielectrophoretic (DEP) forces induced in the carbon nanotube (CNT) nanogap created by Joule heating-induced electrical breakdown. Nanogaps with a size of at least 20–30 nm can be effectively fabricated in the ambient condition, and fabrication yield can be monitored through in-situ electrical signal without post morphological analysis. The geometry of CNT electrodes with high aspect ratio as well as the gap size of the electrodes to a few tens of nanometers scale enabled the derivation of sufficiently high DEP forces that facilitate the trapping of QD in the CNT nanogap. Above all, we were able to fabricate a conducting crack-free QD aggregates by exchanging the ligands on the surface of the QDs in the presence of a DEP force and this approach showed the possibility of being applied as a QD based optoelectronic devices.

Cd-free Cu–Zn–In–S/ZnS quantum dots@SiO2 multiple cores nanostructure: preparation and application for white LEDs

The work reports the fabrication of Cu doped Zn–In–S (CZIS) alloy quantum dots (QDs) using dodecanethiol and oleic acid as stabilizing ligands. With the increase of doped Cu element, the photoluminescence (PL) peak is monotonically red shifted. After coating ZnS shell, the PL quantum yield of CZIS QDs can reach 78%. Using reverse micelle microemulsion method, CZIS/ZnS QDs@SiO2 multi-core nanospheres were synthesized to improve the colloidal stability and avoid the aggregation of QDs. The obtained multi-core nanospheres were dispersed in curing adhesive, and applied as a color conversion layer in down converted light-emitting diodes. After encapsulation in curing adhesive, the newly designed LEDs show artifically regulated color coordinates with varying the weight ratio of green QDs and red QDs, and the concentrations of these two types of QDs. Moreover, natural white and warm white LEDs with correlated color temperature of 5287, 6732, 2731, and 3309 K can be achieved, which indicates that CZIS/ZnS QDs@SiO2 nanostructures are promising color conversion layer material for solid-state lighting application.