Diffusion Of Quantum Dots Research Articles

Poly(maleic anhydride) Shell Modified with Negatively and Positively Charged Groups to Control Zeta Potential and Hydrodynamic Size of Encapsulated Quantum Dots at Variable pH

AbstractWe prepared water‐soluble quantum dots (QDs) with tunable ξ‐potential varied from −38 to +20 mV and controlled sensitivity to pH level. Poly(maleic anhydride‐alt‐1‐tetradecene) (PMAT) was modified with bifunctional molecules containing sulfate, sulfonate, phosphate, phosphonate, and quaternary ammonium in different proportions. The sulfonate shell of QDs leads to a stable negative ξ‐potential in the biological pH range from 4.5 to 9.5, while modification with quaternary ammonium groups gives QDs with positive and pH‐independent charge. If nearly half of carboxyl groups of PMAT were modified with quaternary ammonium groups, ξ‐potential begins pH‐dependent with the isoelectric point at pH≈5.5. To obtain pH‐independent, neutrally charged QDs, the rest of carboxyl groups can be converted into sulfonate groups at a 1 : 1 ratio; sulfonate‐quaternary ammonium QDs show high colloidal stability in solutions with high ionic strength. In addition, encapsulated QDs show an unusual correlation between their hydrodynamic size and ζ‐potential: when ζ‐potential approaches zero, the hydrodynamic size markedly increases due to a decrease in the electrostatic repulsion component of the QDs diffusion coefficient.

Quasi-Two-Dimensional Diffusion in Adherent Cells Revealed by Three-Dimensional Single Quantum Dot TrackingThis work was supported by the National Natural Science Foundation of China (Grant Nos. 11674383, 11874415, 21991133, and 11774407), the National Key Research and Development Program (Grant No. 2016YFA0301500), the Youth Innovation Promotion Association of CAS (Grant No. 2019006), and the Fundamental Research Funds for the Central Universities (Grant

Intracellular diffusion is critical for molecule translocation in cytoplasm and mediates many important cellular processes. Meanwhile, the diffusion dynamics is affected by the heterogeneous cytoplasm. Previous studies on intracellular diffusion are mainly based on two-dimensional (2D) measurements under the assumption that the three-dimensional (3D) diffusion is isotropic. However, the real behaviors of 3D diffusion of molecules in cytoplasm are still unclear. Here, we have built a 3D single-particle tracking (SPT) microscopy and studied the 3D diffusion of quantum dots (QDs) in adherent A549 cells. Notably, we found that the intracellular diffusion of QDs is quasi-2D, with the axial motion being severely confined. Further investigations demonstrated that disrupting the cytoskeleton component or endoplasmic reticulum (ER) does not alter the quasi-2D diffusion pattern, although ER reduces the diffusion rates and slightly relieves the constraint in the axial diffusion. The preferred quasi-2D diffusion is quite robust and attributed to the complex cytoarchitectures in the flat adherent cells. With the aid of 3D SPT method, the quasi-2D diffusion in cells was revealed, shedding new light on the physical nature of cytoplasm.

Dynamics of Intermittent Delayed Emission in Single CdSe/CdS Quantum Dots

Bright and fast fluorescence makes semiconductor nanocrystals, or quantum dots (QDs), appealing for applications ranging from biomedical research to display screens. However, a few percent of their fluorescence intensity is surprisingly slow. Research into this “delayed emission” has been scarce, despite undesired consequences for some applications and potential opportunities for others. Here, we characterize the dynamics of delayed emission exhibited by individual CdSe/CdS core/shell QDs and correlate these with changes in the emission spectrum. The delayed-emission intensity from a single QD fluctuates strongly during an experiment of several minutes and is thus not always “on”, implying that control over delayed emission may be possible. Periods of bright delayed emission correlate with red-shifted emission spectra. This behavior is consistent with exciton polarization by fluctuating electric fields due to diffusing surface charges, which have been known to cause spectral diffusion in QDs. Our findings thus provide a stepping stone for future efforts to control delayed emission.

NanoPaint: A Tool for Rapid and Dynamic Imaging of Membrane Structural Plasticity at the Nanoscale.

Single-particle tracking with quantum dots (QDs) constitutes a powerful tool to track the nanoscopic dynamics of individual cell membrane components unveiling their membrane diffusion characteristics. Here, the nano-resolved population dynamics of QDs is exploited to reconstruct the topography and structural changes of the cell membrane surface with high temporal and spatial resolution. For this proof-of-concept study, bright, small, and stable biofunctional QD nanoconstructs are utilized recognizing the endogenous neuronal cannabinoid receptor 1, a highly expressed and fast-diffusing membrane protein, together with a commercial point-localization microscope. Rapid QD diffusion on the axonal plasma membrane of cultured hippocampal neurons allows precise reconstruction of the membrane surface in less than 1 min with a spatial resolution of tens of nanometers. Access of the QD nanoconstructs to the synaptic cleft enables rapid 3D topological reconstruction of the entire presynaptic component. Successful reconstruction of membrane nano-topology and deformation at the second time-scale is also demonstrated for HEK293 cell filopodia and axons. Named "nanoPaint," this super-resolution imaging technique amenable to any endogenous transmembrane target represents a versatile platform to rapidly and accurately reconstruct the cell membrane nano-topography, thereby enabling the study of the rapid dynamic phenomena involved in neuronal membrane plasticity.

Implementation of Metallic Vertical Interconnect Access in Hybrid Intercalated Graphene/Quantum Dot Photodetector for Improved Charge Collection

Colloidal quantum dots (QDs) are of great interest in optoelectronic and photovoltaic devices with low-cost processing, strong light absorption, and size tunable direct band gap. However, their limited carrier mobility and short diffusion length limit efficient charge collection and transport. The short diffusion length in QD solid films, 100–200 nm, limits their thickness to t≈200–300 nm, resulting in poor absorption in the near-infrared, lambda>800 nm, wasting part of sunlight and reducing power conversion efficiency. Recently, a novel architecture based on multiple graphene monolayers (Gr) intercalated inside QD films was reported to improve charge extraction beyond QDs diffusion length. The intercalated graphene layers ensure efficient charge collected in QD films thicker than the diffusion length. However, this architecture still fails to collect carriers from the QDs when the thickness is >200 nm due to the poor vertical conductivity of the devices. Herein, we present the fabrication, optimization and implementation of intercalated devices with vertical interconnecting contacts, increasing carrier collection and photocurrent, aiming to develop a novel architecture for improved photodetection and photovoltaics with QDs. First, we analyze the individual roles of Gr and QDs, studying the evolution of light absorption, photocurrent, and conductivity as successive QD and Gr layers are added. We find the optimal interspacing between graphene layers in the intercalated system, aiming for the best compromise between light absorption and efficient charge collection. Our main contribution is the implementation of vertical interconnect access (VIAs) to each graphene layer to ensuring efficient charge transfer from Gr to the gold electrical contacts for efficient current collection. We show that for 850 nm wavelength illumination, photocurrent of intercalated devices show a 10 fold improvement over devices without VIAs. We also use a back-gate voltage to monitor Fermi level shift in Gr and charge transfer from QDs to Gr. The intercalated configuration with VIAs contacts herein presented is a significant improvement in charge collection for QD optoelectronic applications as well as a promising architecture to enhance the power conversion efficiency for QD solar cells.

Position-Dependent Three-Dimensional Diffusion in Nematic Liquid Crystal Monitored by Single-Particle Fluorescence Localization and Tracking.

Anisotropic mass diffusion in liquid crystals (LCs) is important from the point of both basic LC physics and their applications in optoelectronic devices. We use super-resolution fluorescence microscopy with astigmatic imaging to track 3D diffusion of quantum dots (QDs) in an ordered nematic LC. The method allowed us to evaluate the diffusion coefficients independently along the three spatial axes as well as to determine the absolute position of the QD with respect to the cell wall. We found variations of the diffusion coefficient along the different directions across the cell thickness and explained these as being due to changes of a tilt angle of the LC director. Close to the surface, the diffusion is slowed down due to the confinement effect of the cell wall. Overall, the QD diffusion is much slower than expected for a corresponding particle size. This phenomenon is suggested to originate from reorientation of the LC director in the vicinity of the particle.

Distribution of polyethylene glycol coated quantum dots in mice skin

The distribution of nanoparticles (NP) in an organism is an important issue for developing NP-based drug delivery systems and for general nanotoxicology. The knowledge of NP localisation in the skin is crucial for the optimisation of NP behaviour in vivo. Therefore, we have used semiconductor quantum dots (QD) to investigate their biodistribution in the skin by means of confocal fluorescence microscopy after subcutaneous injection. The results obtained showed that the diffusion of QD in the dermis is limited by basement membrane and dense connective tissue fibres, which resulted in negligible QD penetration into the epidermis, hair follicles, sebaceous and sweat glands, nerves and blood vessels. Low permeation of QD through the tissues results in slow clearance and raises the risks of potential immune, inflammatory and cytotoxic responses. The study reveals the significance of the tissue architecture for the interstitial and intracellular migration patterns of non-functionalised QD.

Charge influence of liposome on transdermal delivery efficacy

To investigate the charge influence of the drug delivery system, cholesterol analogues, either with cationic, 4-cholesterocarbonyl-4′-(N,N,N-triethylamine butyloxyl bromide) azobenzene (CAB), or the neutral, 4-cholesterocarbonyl-4′-(N,N-diethylamine butyloxyl) azobenzene (ACB) were synthesized and combined in liposomal membranes respectively. CdTe quantum dots (QDs) were loaded into the vesicles as the fluorescence probe to trace the transdermal delivery processing on the nude mouse dorsal skin. Cellular uptake of QDs by NIH 3T3 cells was also studied. The result showed that the cationic liposome facilitated the internalization of loaded QDs into cells and distributed in cytoplasm area within a 1 h incubation. Moreover, in contrast to neutrally charged liposomes, the cationic liposome promoted transdermal delivery of loaded QDs through the skin. In 2 h, cationic liposomes delivered QDs to the upper epidermis, whereas QDs in neutrally charged liposomes still remained on the skin surface which had no obvious facilitation. The retentions of neutrally charged liposome loaded QDs in the epidermis after 8 h, were similar to the naked QDs which were used as control. On the other hand, although in low amounts, the cationic liposomes delivered QDs into vascular dermis at 12 h, helped the diffusion of QDs to the dermal layer.

Physico-chemical role of CdSe/ZnS quantum dots in the photo-polymerization process of acrylate composite materials

The work presented in this paper is devoted to the physico-chemical role of quantum dots (QDs) in the photopolymerization of composite materials. We show first that the photoexcitation of CdSe/ZnS semiconductor quantum dots can initiate the polymerization of an acrylate monomer in association with a tertiary amine and in the absence of a photoinitiator. But by using a holographic technique, among the others, the gratings recorded in such composite materials are not stable and relax once the light interference pattern is switched off. This behaviour is attributed to a very low degree of conversion of monomers, which does not prevent the post-exposure diffusion of monomers and nanoparticles. The comparison with gratings obtained in the presence of a photosensitizer dye (higher hologram quality and stability) highlights the key importance of using such dye in the polymerization process. The gratings recorded in the complete syrup, containing the dye and QDs which differ by their size and UV-visible absorption spectra, exhibit an additional refractive index modulation. The latter results from the spatial distribution of QDs and the change in the kinetics of polymerization. We further focus on understanding the fall of the polymerization rate owing to the presence of QDs. This decrease may result from the trapping of the tertiary amine (co-initiator) at the surface of those QDs, which reduces the rate of decay of the dye and, consequently, the polymerization rate. Under such conditions, the diffusion of QDs and monomers is favoured and their spatial redistribution is enhanced. These features contribute to modify the physical properties of the gratings.

Anomalous Power Laws of Spectral Diffusion in Quantum Dots: A Connection to Luminescence Intermittency

We show that the wandering of transition frequencies in colloidal quantum dots does not follow the statistics expected for ordinary diffusive processes. The trajectory of this anomalous spectral diffusion is characterized by a sqrt[t] dependence of the squared deviation. The behavior is reproduced when the electronic states of quantum dots are assumed to interact with environments such as, for example, an ensemble of two-level systems, where the correlation times are distributed according to a power law similar to the one generally attributed to the dot's luminescence intermittency.

Tuning single GaAs quantum dots in resonance with a rubidium vapor

We study single GaAs quantum dots with optical transitions that can be brought into resonance with the widely used D2 transitions of rubidium atoms. We achieve resonance by Zeeman or Stark shifting the quantum dot levels. We discuss an energy stabilization scheme based on the absorption of quantum dot photoluminescence in a rubidium vapor. This offers a scalable means to counteract slow spectral diffusion in quantum dots.

Particle Tracking Single Protein-Functionalized Quantum Dot Diffusion and Binding at Silica Surfaces

We evaluate commercial QD585 and QD605 streptavidin-functionalized quantum dots (QDs) for single-particle tracking microscopy at surfaces using total internal reflectance fluorescence and measure single QD diffusion and nonspecific binding at silica surfaces in static and flow conditions. The QD diffusion coefficient on smooth, near-ideal, highly hydroxylated silica surfaces is near bulk-solution diffusivity, as expected for repulsive surfaces, but many QD trajectories on rougher, less-than-ideal surfaces or regions display transient adsorptions. We attribute the binding to defect sites or adsorbates, possibly in conjunction with protein conformation changes, and estimate binding energies from the transient adsorption lifetimes. We also assess QD parameters relevant to tracking, including hydrodynamic radius, charge state, signal levels, blinking reduction with reducing solutions, and photoinduced blueing and bleaching.

Photoinduced Film Formation of Colloidal CdSe Quantum Dots

Tri-n-octylphosphine oxide (TOPO)-capped colloidal cadmium selenide (CdSe) quantum dots (QDs) in octane formed a QD film under laser irradiation. The film was formed at a constant rate of deposition after an incubation period. The deposition rate increased linearly with increasing laser intensity and QD concentration. Addition of TOPO molecules into the suspension significantly retarded film formation. The TOPO molecules are thought to desorb from the QD surface under laser irradiation because well-capped QDs with TOPO molecules did not form a film. We propose that absorbed photons induce desorption of TOPO molecules from QDs and destabilized QDs are formed in the suspension. The formed unstable QDs attach to the substrate, resulting in the formation of a QD film. A model is proposed to analyze the kinetics of film formation. It consists of three steps: the diffusion of QDs from the bulk suspension, the photoinduced desorption reaction of TOPO molecules, and the mass transport of unstable QDs onto the sub...