Interdependence of ICD rates in paired quantum dots on geometry

Using state‐of‐the‐art antisymmetrized multiconfiguration time‐dependent Hartree (MCTDH) electron dynamics calculations we study the interdependence of the intermolecular Coulombic decay (ICD) process on the geometric parameters of a doubly‐charged paired quantum dot (PQD) model system in the framework of the effective mass approximation (EMA). We find that ICD displays a maximum rate for a certain geometry of the electron‐emitting quantum dot, which is simultaneously dependent on both the distance between the quantum dots as well as the photon‐absorbing quantum dot's geometry. The rate maximum is shown to be caused by the competing effects of polarization of electron density and Coulomb repulsion. The ICD rate‐maximized PQD geometry in GaAs QDs yields a decay time of 102.39 ps. It is given by two vertically‐aligned cylindrical QDs with radii of 14.42 nm separated by 86.62 nm. The photon absorbing QD then has a height of 46.59 nm and the electron emitting QD a height of 16.33 nm. © 2017 Wiley Periodicals, Inc.     

Turn‐On Luminescent Probes for the Real‐Time Monitoring of Endogenous Hydroxyl Radicals in Living Cells

The utilization of semiconductor quantum dots (QDs) as optical labels for biosensing and biorecognition has made substantial progress. However, the development of a suitable QD‐based luminescent probe that is capable of detecting individual reactive oxygen species (ROS) represents a great challenge, mainly because the fluorescence of QDs is quenched by a wide variety of ROS. To overcome this limitation, a novel QD‐based turn‐on luminescent probe for the specific detection of ^.OH has been designed, and its application in monitoring the endogenous release of ^.OH species in living cells is demonstrated. Metal citrate complexes on the surfaces of the QDs can act as electron donors, injecting electrons into the LUMO of the QDs, while ^.OH can inject holes into the HOMO of the QDs. Accordingly, electron–hole pairs are produced, which could emit strong luminescence by electron–hole recombination. Importantly, this luminescent probe does not respond to other ROS.     

High‐density quantum dots composites and its photolithographic patterning applications

With the growing interest in quantum dots (QDs), many applications are emerging recently. In particular, the display industry has shown widespread interest in using QDs as the next generation colorants. One application is to replace conventional color filters with QD‐based color conversion films to significantly improve color purity and luminous efficiency. However, QD blending which is capable of photolithographic patterning is a very challenging problem due to its low dispersion property and aggregations in polar medias. Herein, we report a photo‐patternable QD dispersion that can produce fine patterns through a photolithography process. First, the QDs dispersed in a nonpolar solvent, for example, chloroform or hexane, were separated and dried to obtain a QD powder. And then, the dispersion characteristics of the QD powders were investigated after mixing commercial dispersants and UV curable oligomers. Furthermore, the QD dispersion was investigated up to 30 wt.% of QDs by mixing with various commercial additives. We have studied the optical property changes of QDs during the photocuring process and the heating process prior to actual application. And, we have studied the surface characteristics of the fine QDs patterns after patterning process. As a result, it was confirmed that QDs are able to be well dispersed up to 30 wt.%.     

Stabilizer‐Free Poly(lactide‐co‐glycolide) Nanoparticles Conjugated with Quantum Dots as a Potential Carrier Applied in Human Mesenchymal Stem Cells

We report that human mesenchymal stem cells (hMSCs) were successfully labeled with poly(lactide‐co‐glycolide) nanoparticles (PLGA NPs) surface‐conjugated quantum dots (QDs) (PLGA‐QD NPs) via endocytosis pathway. These NPs were not toxicity even treated with PLGA‐QD NPs at high concentrations for at least four weeks. Besides, PLGA‐QD NPs‐labeled hMSCs did not change their proliferation and differentiation capability toward the cell fates of adipocytes, osteocytes, and chrondrocytes. It's known that PLGA has been widely employed to act as delivery carrier which encapsulates drugs and releases them under a controlled way. Currently, we have also demonstrated that FITC‐loaded PLGA‐QD NPs degraded in hMSCs to achieve intracellular release of FITC. The aim of this research is to investigate viability, proliferation and differentiation capability and the potential for gene delivery of MSCs labeled with PLGA‐QD NPs. In addition to PLGA‐QD NPs, QDs alone were used to serve as a control set for comparison.     

DNA‐Templated Assembly of a Heterobivalent Quantum Dot Nanoprobe For Extra‐ and Intracellular Dual‐Targeting and Imaging of Live Cancer Cells

Quantum dots (QDs) hold great promise for the molecular imaging of cancer because of their superior optical properties. Although cell‐surface biomarkers can be readily imaged with QDs, non‐invasive live‐cell imaging of critical intracellular cancer markers with QDs is a great challenge because of the difficulties in the automatic delivery of QD probes to the cytosol and the ambiguity of intracellular targeting signals. Herein, we report a new type of DNA‐templated heterobivalent QD nanoprobes with the ability to target and image two spatially isolated cancer markers (nucleolin and mRNA) present on the cell surface and in the cell cytosol. Bypassing endolysosomal sequestration, this type of QD nanoprobes undergo macropinocytosis following the nucleolin targeting and then translocate to the cytosol for mRNA targeting. Fluorescence resonance energy transfer (FRET) based confocal microscopy enables unambiguous signal deconvolution of mRNA‐targeted QD nanoprobes inside cancer cells.     

Electronic and Magnetic Properties of Encapsulated MoS2 Quantum Dots: The Case of Noble Metal Nanoparticle Dopants

With the rise of 2D materials, such as graphene and transition metal dichalcogenides, as viable materials for numerous experimental applications, it becomes more necessary to maintain fine control of their properties. One expedient and efficacious technique to regulate their properties is surface functionalization. In this study, DFT calculations are performed on triangular MoS₂ quantum dots (QDs) either partially or completely doped with nanoparticles (NPs) of the noble metals Au, Ag, and Pt. The effects of these dopants on the geometry, electronic properties, magnetic properties, and chemical bonding of the QDs are investigated. The calculations show that the structural stability of the QDs is reduced by Au or Ag dopants, whereas Pt dopants have a contrasting effect. The NPs diminish the metallicity of the QD, the extent of which is contingent on the number of NPs adsorbed on the QD. However, these NPs exert distinctly disparate charge transfer effects—Ag NPs n‐dope the QDs, whereas Au and Pt NPs either n‐ or p‐dope. The molecular electrostatic potential maps of the occupied states show that metallic states are removed from the doping sites. Notwithstanding the decrease of magnetization in all three types of hybrid QD, the distribution of spin density in the Pt‐doped QD is inherently different from that in the other QDs. Bond analyses using the quantum theory of atoms in molecules and the crystal orbital Hamilton population suggest that bonds between the Pt NPs and the QDs are the most covalent and the strongest, followed by the Au?QD bonds, and then Ag?QD bonds. The versatility of these hybrid QDs is further examined by applying an external electric field in the three orthogonal orientations, and comparing their properties with those in the absence of the electric field. There are two primary observations: 1) dopants at the tail, head and tail, and in the fully encased configuration are most effective in modifying the distribution of metallic states if the electric field is absent, and 2) the metallic states in these aforementioned QDs are generally insensitive to the electric field. Conversely, the asymmetric electric effects on the charge transfer in these QDs have to be carefully monitored to allow finer control of their structural stability. This study aptly demonstrates the value of noble metal dopants for manipulating the properties of MoS₂ QDs, and shows the versatility of these hybrid QDs as tunable nanodevices. This notably extends the functionality of these nanostructures for applications such as catalysis and nanoelectronics.     

Tuning Optical Properties of Si Quantum Dots by π‐Conjugated Capping Molecules

The absorption and photoluminescence (PL) properties of silicon quantum dots (QDs) are greatly influenced by their size and surface chemistry. Herein, we examined the optical properties of three Si QDs with increasing σ–π conjugation length: octyl‐, (trimethylsilyl)vinyl‐, and 2‐phenylvinyl‐capped Si QDs. The PL photon energy obtained from as‐prepared samples decreased by 0.1–0.3 eV, while the PL excitation (PLE) extended from 360 nm (octyl‐capped Si QDs) to 400 nm (2‐phenylvinyl‐capped Si QDs). A vibrational PL feature was observed in all samples with an energy separation of about 0.192±0.013 eV, which was explained based on electron–phonon coupling. After soft oxidization through drying, all samples showed blue PL with maxima at approximately 410 nm. A similar high‐energy peak was observed with the bare Si QD sample. The changes in the optical properties of Si QDs were mainly explained by the formation of additional states arising from the strong σ–π conjugation and QD oxidation.     

Near‐Infrared‐Light‐Triggered Antimicrobial Indium Phosphide Quantum Dots

The emergence of multidrug‐resistant (MDR) pathogens represents one of the most urgent global public health crises. Light‐activated quantum dots (QDs) are alternative antimicrobials, with efficient transport, low cost, and therapeutic efficacy, and they can act as antibiotic potentiators, with a mechanism of action orthogonal to small‐molecule drugs. Furthermore, light‐activation enhances control over the spatiotemporal release and dose of the therapeutic superoxide radicals from QDs. However, the limited deep‐tissue penetration of visible light needed for QD activation, and concern over trace heavy metals, have prevented further translation. Herein, we report two indium phosphide (InP) QDs that operate in the near‐infrared and deep‐red light window, enabling deeper tissue penetration. These heavy‐metal‐free QDs eliminate MDR pathogenic bacteria, while remaining non‐toxic to host human cells. This work provides a pathway for advancing QD nanotherapeutics to combat MDR superbugs.     

Current Advances in Quantum‐Dots‐Based Photoelectrochemical Immunoassays

As a newly developed technique, photoelectrochemical (PEC) immunoassays have attracted great attention in recent years because of their low cost and desirable sensitivity. Because the detection signal originates from the photoelectric conversion of photoelectric materials, the appearance and application of quantum dots (QDs), which possess unique photophysical properties and regulated optoelectronic characteristics, has taken the development of PEC immunoassays to new heights. This review concisely introduces the general mechanism of QDs‐based photoelectric conversion for immunoassays and summarizes the current advances in QD applications in immunoassays. Given that signal strategies and photoactive materials are the key elements in PEC biosensor systems, we comprehensively highlight the state‐of‐the‐art signaling strategies and various applications of QDs in PEC immunoassays to introduce advances in QDs‐based PEC immunoassays. Finally, challenges and future developmental trends are briefly discussed     

A Brief Overview of Some Physical Studies on the Relaxation Dynamics and Förster Resonance Energy Transfer of Semiconductor Quantum Dots

This article highlights some physical studies on the relaxation dynamics and Förster resonance energy transfer (FRET) of semiconductor quantum dots (QDs) and the way these phenomena change with size, shape, and composition of the QDs. The understanding of the excited‐state dynamics of photoexcited QDs is essential for technological applications such as efficient solar energy conversion, light‐emitting diodes, and photovoltaic cells. Here, our emphasis is directed at describing the influence of size, shape, and composition of the QDs on their different relaxation processes, that is, radiative relaxation rate, nonradiative relaxation rate, and number of trap states. A stochastic model of carrier relaxation dynamics in semiconductor QDs was proposed to correlate with the experimental results. Many recent studies reveal that the energy transfer between the QDs and a dye is a FRET process, as established from 1/d⁶ distance dependence. QD‐based energy‐transfer processes have been used in applications such as luminescence tagging, imaging, sensors, and light harvesting. Thus, the understanding of the interaction between the excited state of the QD and the dye molecule and quantitative estimation of the number of dye molecules attached to the surface of the QD by using a kinetic model is important. Here, we highlight the influence of size, shape, and composition of QDs on the kinetics of energy transfer. Interesting findings reveal that QD‐based energy‐transfer processes offer exciting opportunities for future applications. Finally, a tentative outlook on future developments in this research field is given.     

Effect of Surface Modification on Semiconductor Nanocrystal Fluorescence Lifetime

Semiconductor nanocrystals, namely, quantum dots (QDs), present a set of unique photoluminescence properties, which has led to increased interest in using them as advantageous alternatives to conventional organic dyes. Many applications of QDs involve surface modification to enhance the solubility or biocompatibility of the QDs. One of the least exploited properties of QDs is the very long photoluminescence lifetime that usually has complex kinetics owing to the effect of quantum confinement. Herein, we describe the effect of different surface modifications on the photoluminescence decay kinetics of QDs. The different surface modifications were carefully chosen to provide lipophilic or water‐soluble QDs with either positive or negative surface net charges. We also survey the effect on the QD lifetime of several ligands that interact with the QD surface, such as organic chromophores or fluorescent proteins. The results obtained demonstrate that time‐resolved fluorescence is a useful tool for QD‐based sensing to set the basis for the development of time‐resolved‐based nanosensors.     

Macroscopic Responsive Liquid Quantum Dots Constructed via Pillar[5]arene‐Based Host‐Guest Interactions

Liquid quantum dots (QDs) have been used as a fluorescent films sensor. Constructing a macroscopic, responsive, liquid QD system for lysine (Lys) is a challenging task. To achieve a selective macroscopic response towards Lys, herein we present a new strategy for integrating host–guest chemistry into a liquid QD system. Water‐soluble pillar[5]arene WP5 was designed and synthesized as a host. WP5 was introduced onto the surface of PEG1810‐modified QDs by host–guest interactions to obtain liquid WP5‐1810‐QDs. The interaction between WP5 and Lys is stronger than that between WP5 and PEG‐1810, causing WP5 to be released from the 1810‐QDs surface in the presence of Lys, resulting in macroscopic fluorescence quenching. This smart material shows promise in amino acid sensing and separation.     

Preparation of High‐Performance Water‐Soluble Quantum Dots for Biorecognition through Fluorescence Resonance Energy Transfer

An improved method for the synthesis of high‐performance and water‐soluble quantum dots (QDs) involving the encapsulation of mercaptosuccinic acid coated QDs (MSA‐QDs) with poly(diallyldimethylammonium chloride) (PDDA) followed by their direct photoactivation with fluorescent radiation near 295 K to yield PDDA‐coated QDs (PDDA‐QDs) has been demonstrated. The quantum yield (QY) of the PDDA‐QDs was significantly improved from 0.6 (QY of MSA‐QDs) to 48 %. By using this synthetic strategy, highly photoluminescent PDDA‐QDs of varied size were readily prepared. The surface properties of PDDA‐QDs and MSA‐QDs were extensively characterized. The highly luminescent and positively charged PDDA‐QDs serve as a useful and convenient tool for protein adsorption. With a Δ⁵‐3‐ketosteroid isomerase adsorbed PDDA‐QD complex, the biorecognition of steroids was demonstrated through the application of fluorescent resonance energy transfer.     

Charge Transport between Coupling Colloidal Perovskite Quantum Dots Assisted by Functional Conjugated Ligands

Long alkyl‐chain capping ligands are indispensable for preparing stable colloidal quantum dots. However, its insulating feature blocks efficient carrier transport among QDs, leading to inferior performance in light‐emitting diodes (LEDs). The trade‐off between conductivity and colloidal stability of QDs has now been overcome. Methylamine lead bromide (MAPbBr₃) QDs with a conjugated alkyl‐amine, 3‐phenyl‐2‐propen‐1‐amine (PPA), as ligands were prepared. Owing to electron cloud overlapping and the delocalization effect of conjugated molecules, the conductivity and carrier mobility of PPA‐QDs films increased almost 22 times over that of OA‐QD films without compromising colloidal stability and photoluminescence. PPA‐QDs LEDs exhibit a maximum current efficiency of 9.08 cd A^?1, which is 8 times of that of OA‐QDs LEDs (1.14 cd A^?1). This work provides critical solution for the poor conductivity of QDs in applications of energy‐related devices.     

Hot‐Hole Extraction from Quantum Dot to Molecular Adsorbate

Ultrafast thermalized and hot‐hole‐transfer processes have been investigated in CdSe quantum dot (QD)/catechol composite systems in which hole transfer from photoexcited QDs to the catechols is thermodynamically favorable. A series of catechol derivatives were selected with different electron‐donating and ‐withdrawing groups, and the effect of these groups on hole transfer and charge recombination (CR) dynamics has been investigated. The hole‐transfer time was determined using the fluorescence upconversion technique and found to be 2–10 ps depending on the molecular structure of the catechol derivatives. The hot‐hole‐transfer process was followed after monitoring 2S luminescence of CdSe QDs. Interestingly, hot‐hole extraction was observed only in the CdSe/3‐methoxycatechol (3‐OCH₃) composite system owing to the higher electron‐donating property of the 3‐methoxy group. To confirm the extraction of the hot hole and to monitor the CR reaction in CdSe QD/catechol composite systems, ultrafast transient absorption studies have been carried out. Ultrafast transient‐absorption studies show that the bleach recovery kinetics of CdSe QD at the 2S excitonic position is much faster in the presence of 3‐OCH₃. This faster bleach recovery at the 2S position in CdSe/3‐OCH₃ suggests hot‐hole transfer from CdSe QD to 3‐OCH₃. CR dynamics in CdSe QD/catechol composite systems was followed by monitoring the excitonic bleach at the 1S position and was found to decrease with free energy of the CR reaction.     

Characterization and separation of semiconductor quantum dots and their conjugates by capillary electrophoresis

Semiconductor quantum dots (QDs) are very important luminescent nanomaterials with a wide range of potential applications. Currently, QDs as labeling probes are broadly used in bioassays, including immunoassay, DNA hybridization, and bioimaging, due to their excellent physical and chemical properties, such as broad excitation spectra, narrow and size‐dependent emission profiles, long fluorescence life time, and good photostability. The characterization of QDs and their conjugates is crucial for their wide bioapplications. CE has become a powerful tool for the separation and characterization of QDs and their conjugates. In this review, some CE separation models of QDs are first introduced, mainly including CZE, CGE, MEKC, and ITP. And then, some key applications, such as the measurements of size, surface charge, and concentration of QDs and the characterization of QDs conjugates (e.g. QD–protein, QD–DNA, QD–small molecule), are also described. Finally, future perspectives are discussed.     

Immobilization of Distinctly Capped CdTe Quantum Dots onto Porous Aminated Solid Supports

Immobilization of quantum dots (QDs) onto solid supports could improve their applicability in the development of sensing platforms and solid‐phase reactors by allowing the implementation of reusable surfaces and the execution of repetitive procedures. As the reactivity of QDs relies mostly on their surface chemistry, immobilization could also limit the disruption of solution stability that could prevent stable measurements. Herein, distinct strategies to immobilize QDs onto porous aminated supports, such as physical adsorption and the establishment of chemical linking, were evaluated. This work explores the influence of QD capping and size, concentration, pH, and contact time between the support and the QDs. Maximum QD retention was obtained for physical adsorption assays. Freundlich and Langmuir isotherms were used to analyze the equilibrium data. Gibbs free energy, enthalpy, and entropy were calculated and the stability of immobilized QDs was confirmed.     

Assembly and separation of semiconductor quantum dot dimers and trimers

Repeated precipitation of colloidal semiconductor quantum dots (QD) from a good solvent by adding a poor solvent leads to an increasing number of QD oligomers after redispersion in the good solvent. By using density gradient ultracentrifugation we have been able to separate QD monomer, dimer, and trimer fractions from higher oligomers in such solutions. In the corresponding fractions QD dimers and trimers have been enriched up to 90% and 64%, respectively. Besides directly coupled oligomers, QD dimers and trimers were also assembled by linkage with a rigid terrylene diimide dye (TDI) and separated again by ultracentrifugation. High-resolution transmission electron micrographs show that the interparticle distances are clearly larger than those for directly coupled dots proving that the QDs indeed are cross-linked by the dye. Moreover, energy transfer from the QDs to the TDI "bridge" has been observed. Individual oligomers (directly coupled or dye-linked) can be readily deposited on a substrate and studied simultaneously by scanning force and optical microscopy. Our simple and effective scheme is applicable to a wide range of ligand stabilized colloidal nanoparticles and opens the way to a detailed study of electronic coupling in, e.g., QD molecules.     

Role of Emissive and Non‐Emissive Complex Formations in Photoinduced Electron Transfer Reaction of CdTe Quantum Dots

Bimolecular photoinduced electron transfer (PET) from excited state CdTe quantum dot (QD*) to an electron deficient molecule 2,4‐dinitrotoluene (DNT) is studied in toluene. We observed two types of QD‐DNT complex formations; (i) non‐emissive complex, in which DNT is embedded deep inside the surface polymer layer of QD and (ii) emissive complex, in which DNT molecules are attached to QDs but approach to the QD core is shielded by polymer layer. Because of its non‐emissive nature, the lifetime of QD is not affected by dark complex formation, though the steady‐state emission is greatly quenched. However, emissive complex formation causes both, lifetime and steady‐state emission quenching. In our fitting model, consideration of Poisson distribution of the attached quencher (DNT) molecules at QD surface enables a comprehensive fitting to our time resolved data. QD‐DNT complex formation was confirmed by an isothermal titration calorimetry (ITC) study. Fitting to the time resolved data using a stochastic kinetic model shows moderate increase (0.05 ns^?1 to 0.072 ns^?1) of intrinsic quenching rate with increasing the QD particle size (from ≈3.2 nm to ≈5.2 nm). Our fitting also reveals that the number of DNT molecules attached to a single QD increases from ≈0.1–0.2 to ≈1.2–1.7, as the DNT concentration is increased from ≈1 mm to 17.5 mm . Complex formation at higher quencher concentration assures that the observed PET kinetics is a thermodynamically controlled process where solvent diffusion has no role on it.     

Photocatalysis with Quantum Dots and Visible Light: Selective and Efficient Oxidation of Alcohols to Carbonyl Compounds through a Radical Relay Process in Water

Selective oxidation of alcohols to aldehydes/ketones has been achieved with the help of 3‐mercaptopropionic acid (MPA)‐capped CdSe quantum dot (MPA‐CdSe QD) and visible light. Visible‐light‐prompted electron‐transfer reaction initiates the oxidation. The thiyl radical generated from the thiolate anion adsorbed on a CdSe QD plays a key role by abstracting the hydrogen atom from the C−H bond of the alcohol (R¹CH(OH)R²). The reaction shows high efficiency, good functional group tolerance, and high site‐selectivity in polyhydroxy compounds. The generality and selectivity reported here offer a new opportunity for further applications of QDs in organic transformations.