Dissolution Of Quantum Dots Research Articles

Novel electrochemical PMI marker biosensor based on quantum dot dissolution using a double-label strategy

A novel and facile post-mortem interval (PMI) biosensor was fabricated using a double-label strategy to detect the glyceraldehyde 3-phosphate dehydrogenase (GAPDH) biomarker. A monoclonal anti-GAPDH antibody was immobilized on a surface label containing cadmium selenide quantum dots (CdSe QDs) on a cysteamine graphene oxide (Cys-GO) self-assembled monolayer. Glucose oxidase (GOx) was used as a signal label to conjugate with GAPDH. GAPDH recognition was achieved through the dissolution of the surface-attached CdSe QDs by hydrogen peroxide generated through GAPDH-conjugated GOx-catalyzed β-glucose oxidation. To enhance sensitivity, a competitive interaction was introduced between free and conjugated GAPDH to the active site of the anti-GAPDH antibody. The electrochemical response due to CdSe dissolution decreased proportionally with the concentration of free GAPDH. Differential pulsed voltammetry was conducted to determine the analytical characteristics of the immunosensor, including the limit of detection, linear dynamic range, target selectivity, system stability, and applicability toward the analysis of real samples.

Growth mechanisms for InAs/GaAs QDs with and without Bi surfactants

We report systematic study of growth of self-assembled InAs quantum dots (QDs) on GaAs substrate at various temperatures with and without exposure of bismuth surfactants. Results show that the coalescence amongst InAs QDs is considerably inhibited by the exposure of bismuth flux during growth in the temperature range from 475 to 500 °C, leading to improved dot uniformity and a modified dot density. The mechanism of the suppression effect by bismuth surfactants on the strain-induced islanding through inhibiting the indium adatom mobility and the evaporation rate on the surface kinetically is thus clarified for the growth of InAs QDs. The photoluminescence peak wavelength for the InAs QDs with Bi exposure red shifted slightly due to the suppression of Bi atoms on the QD dissolution during the capping process at higher temperatures. Moreover, by investigating the temperature-dependent quenching processes with and without Bi exposure, it is observed that the the weak carrier confinement occurred in QDs with the presence of Bi caused broadness in the linewidths.

Size and shape tunability of self-assembled InAs/GaAs nanostructures through the capping rate

The practical realization of epitaxial quantum dot (QD) nanocrystals led before long to impressive experimental advances in optoelectronic devices, as well as to the emergence of new technological fields. However, the necessary capping process is well-known to hinder a precise control of the QD morphology and therefore of the possible electronic structure required for certain applications. A straightforward approach is shown to tune the structural and optical properties of InAs/GaAs QDs without the need for any capping material different from GaAs or annealing process. The mere adjust of the capping rate allows controlling kinetically the QD dissolution process induced by the surface In-Ga intermixing taking place during overgrowth, determining the final metastable structure. While low capping rates make QDs evolve into more thermodynamically favorable quantum ring structures, increasing capping rates help preserve the QD height and shape, simultaneously improving the luminescence properties. Indeed, a linear relationship between capping rate and QD height is found, resulting in a complete preservation of the original QD geometry for rates above ∼2.0 ML s−1. In addition, the inhibition of In diffusion from the QDs top to the areas in between them yields thinner WLs, what could improve the performance of several QD-based optoelectronic devices.

Aggregation, Sedimentation, Dissolution, and Bioavailability of Quantum Dots in Estuarine Systems.

To understand their fate and transport in estuarine systems, the aggregation, sedimentation, and dissolution of CdSe quantum dots (QDs) in seawater were investigated. Hydrodynamic size increased from 40 to 60 nm to >1 mm within 1 h in seawater, and the aggregates were highly polydispersed. Their sedimentation rates in seawater were measured to be 4-10 mm/day. Humic acid (HA), further increased their size and polydispersity, and slowed sedimentation. Light increased their dissolution and release of dissolved Cd. The ZnS shell also slowed release of Cd ions. With sufficient light, HA increased the dissolution of QDs, while with low light, HA alone did not change their dissolution. The benthic zone in estuarine systems is the most probable long-term destination of QDs due to aggregation and sedimentation. The bioavailability of was evaluated using the mysid Americamysis bahia. The 7-day LC50s of particulate and dissolved QDs were 290 and 23 μg (total Cd)/L, respectively. For mysids, the acute toxicity appears to be from Cd ions; however, research on the effects of QDs should be conducted with other organisms where QDs may be lodged in critical tissues such as gills or filtering apparatus and Cd ions may be released and delivered directly to those tissues.

New insights into ROS dynamics: a multi-layered microfluidic chip for ecotoxicological studies on aquatic microorganisms

Reactive oxygen species (ROS) play an important role in the life of every cell, including cellular defense and signaling mechanisms. Continuous and quantitative ROS sensing can provide valuable information about the cell state, but it remains a challenge to measure. Here, we introduce a multi-layered microfluidic chip with an integrated optical sensor for the continuous sensitive detection of extracellular hydrogen peroxide (H2O2), one of the most stable ROS. This platform includes hydraulically controlled microvalves and microsieves, which enable the precise control of toxicants and complex exposure sequences. In particular, we use this platform to study the dynamics of toxicity-induced ROS generation in the green microalga Chlamydomonas reinhardtii during short-term exposures, recovery periods, and subsequent re-exposures. Two cadmium-based toxicants with distinct internalization mechanisms are used as stress inducers: CdSe/ZnS quantum dots (Qdots) and ionic cadmium (Cd2+). Our results show the quantitative dynamics of ROS generation by the model microalga, the recovery of cell homeostasis after stress events and the cumulative nature of two consecutive exposures. The dissolution of quantum dots and its possible influence on toxicity and H2O2 depletion is discussed. The obtained insights are relevant from ecotoxicological and physiological perspectives.

Genotoxic capacity of Cd/Se semiconductor quantum dots with differing surface chemistries.

Quantum dots (QD) have unique electronic and optical properties promoting biotechnological advances. However, our understanding of the toxicological structure–activity relationships remains limited. This study aimed to determine the biological impact of varying nanomaterial surface chemistry by assessing the interaction of QD with either a negative (carboxyl), neutral (hexadecylamine; HDA) or positive (amine) polymer coating with human lymphoblastoid TK6 cells. Following QD physico-chemical characterisation, cellular uptake was quantified by optical and electron microscopy. Cytotoxicity was evaluated and genotoxicity was characterised using the micronucleus assay (gross chromosomal damage) and the HPRT forward mutation assay (point mutagenicity). Cellular damage mechanisms were also explored, focusing on oxidative stress and mitochondrial damage. Cell uptake, cytotoxicity and genotoxicity were found to be dependent on QD surface chemistry. Carboxyl-QD demonstrated the smallest agglomerate size and greatest cellular uptake, which correlated with a dose dependent increase in cytotoxicity and genotoxicity. Amine-QD induced minimal cellular damage, while HDA-QD promoted substantial induction of cell death and genotoxicity. However, HDA-QD were not internalised by the cells and the damage they caused was most likely due to free cadmium release caused by QD dissolution. Oxidative stress and induced mitochondrial reactive oxygen species were only partially associated with cytotoxicity and genotoxicity induced by the QD, hence were not the only mechanisms of importance. Colloidal stability, nanoparticle (NP) surface chemistry, cellular uptake levels and the intrinsic characteristics of the NPs are therefore critical parameters impacting genotoxicity induced by QD.

Selective chemical vaporization of exogenous tellurium for characterizing the time-dependent biodistribution and dissolution of quantum dots in living rats

Quantum dots (QDs) are generally toxic as a result of their heavy metal content.

Fluorescence Reports Intact Quantum Dot Uptake into Roots and Translocation to Leaves of Arabidopsis thaliana and Subsequent Ingestion by Insect Herbivores

We explored the impact of quantum dot (QD) coat characteristics on NP stability, uptake, and translocation in Arabidopsis thaliana, and subsequent transfer to primary consumers, Trichoplusia ni (T. ni). Arabidopsis was exposed to CdSe/CdZnS QDs with three different coatings: Poly(acrylic acid-ethylene glycol) (PAA-EG), polyethylenimine (PEI) and poly(maleic anhydride-alt-1-octadecene)-poly(ethylene glycol) (PMAO-PEG), which are anionic, cationic, and relatively neutral, respectively. PAA-EG-coated QDs were relatively stable and taken up from a hydroponic medium through both Arabidopsis leaf petioles and roots, without apparent aggregation, and showed generally uniform distribution in leaves. In contrast, PEI- and PMAO-PEG-coated QDs displayed destabilization in the hydroponic medium, and generated particulate fluorescence plant tissues, suggesting aggregation. PAA-EG QDs moved faster than PEI QDs through leaf petioles; however, 8-fold more cadmium accumulated in PEI QD-treated leaves than in those exposed to PAA-EG QDs, possibly due to PEI QD dissolution and direct metal uptake. T. ni caterpillars that fed on Arabidopsis exposed to QDs had reduced performance, and QD fluorescence was detected in both T. ni bodies and frass, demonstrating trophic transfer of intact QDs from plants to insects. Overall, this paper demonstrates that QD coat properties influence plant nanoparticle uptake and translocation and can impact transfer to herbivores.

Capping layer growth rate and the optical and structural properties of GaAsSbN-capped InAs/GaAs quantum dots

Changing the growth rate during the heteroepitaxial capping of InAs/GaAs quantum dots (QDs) with a 5 nm-thick GaAsSbN capping layer (CL) strongly modifies the QD structural and optical properties. A size and shape transition from taller pyramids to flatter lens-shaped QDs is observed when the CL growth rate is decreased from 1.5 to 0.5 ML/s. This indicates that the QD dissolution processes taking place during capping can be controlled to some extent by the GaAsSbN CL growth rate, with high growth rates allowing a complete preservation of the QDs. However, the dissolution processes are shown to have a leveling effect on the QD height, giving rise to a narrower size distribution for lower growth rates. Contrary to what could be expected, these effects are opposite to the strong blue-shift and improvement of the photoluminescence (PL) observed for higher growth rates. Nevertheless, the PL results can be understood in terms of the strong impact of the growth rate on the Sb and N incorporation into the CL, which results in lower Sb and N contents at higher growth rates. Besides the QD-CL band offsets and QD strain, the different CL composition alters the band alignment of the system, which can be transformed to type-II at low growth rates. These results show the key role of the alloyed CL growth parameters on the resulting QD properties and demonstrate an intricate correlation between the PL spectra and the sample morphology in complex QD-CL structures.

Stability of core/shell quantum dots—role of pH and small organic ligands

The improvement of knowledge about the toxicity and even processability, and stability of quantum dots (QD) requires the understanding of the relationship between the QD binding head group, surface structure, and interligand interaction. The scanned stripping chronopotentiometry and absence of gradients and Nernstian equilibrium stripping techniques were used to determine the concentration of Cd dissolved from a polyacrylate-stabilized CdTe/CdS QD. The effects of various concentrations of small organic ligands such as citric acid, glycine, and histidine and the roles of pH (4.5-8.5) and exposure time (0-48 h) were evaluated. The highest QD dissolution was obtained at the more acidic pH in absence of the ligands (52%) a result of the CdS shell solubility. At pH 8.5 the largest PAA ability to complex the dissolved Cd leads to a further QD solubility until the equilibrium is reached (24% of dissolved Cd vs. 4% at pH 6.0). The citric acid presence resulted in greater QD dissolution, whereas glycine, an amino acid, acts against QD dissolution. Surprisingly, the presence of histidine, an amino acid with an imidazole functional group, leads to the formation of much strong Cd complexes over time, which may be non-labile, inducing variations in the local environment of the QD surface.

Chemically differentiating ascorbate-mediated dissolution of quantum dots in cell culture media

To investigate the dynamic dissolution of quantum dots (QDs) in cell culture media, in this study we constructed an online automatic analytical system comprising a sequential in-tube solid phase extraction (SPE) device and an inductively coupled plasma (ICP) mass spectrometer. By means of selectively extracting QDs and cadmium ions (Cd(2+)) onto the interior surface of the polytetrafluoroethylene (PTFE) tube, this novel SPE device could be used to determine the degree of QD dissolution through a simple adjustment of sample acidity. To the best of our knowledge, this study is the first to exploit PTFE tubing as a selective SPE adsorbent for the online chemical differentiation of QDs and Cd(2+) ions with the goal of monitoring the phenomenon of QD dissolution in complicated biological matrices. We confirmed the analytical reliability of this system through comparison of the measured Cd-to-QD ratios to the expected values. When analyzing QDs and Cd(2+) ions at picomolar levels, a temporal resolution of 8 min was required to load sufficient amounts of the analytes to meet the sensitivity requirements of the ICP mass spectrometer. To demonstrate the practicability of this developed method, we measured the dynamic variations in the Cd-to-QD705 ratio in the presence of ascorbate as a physiological stimulant to generate reactive oxygen species in cell culture media and trigger the dissolution of QDs; our results suggest that the ascorbate-induced QD dissolution was dependent on the time, treatment concentration, and nature of the biomolecule.

Ligand-Controlled Growth of ZnSe Quantum Dots in Water during Ostwald Ripening

A strong ligand effect was observed for the aqueous-phase growth of ZnSe quantum dots (QDs) in the Ostwald ripening (OR) stage. The QDs were made by injecting Se monomer at room temperature followed by a ramp to 100 °C. The ramp produced a second, more gradual increase in the concentrations of both Zn and Se monomers fed by the dissolution of QDs below the critical size. The dissolution process was followed using measurements of the mass of Zn in QDs and in the supernatant by inductively coupled plasma optical emission spectroscopy (ICP-OES). Despite the flux of monomers, there was little growth in the QDs of average size based on UV-vis absorption spectra, until the temperature reached 100 °C, when there was a period of rapid growth followed by a period of linear growth. The linear growth stage is the result of OR as the total mass of Zn in QDs and in the solvent remained constant. The growth data were fit to a continuum model for the limiting case of surface reaction control. The rate is proportional to the equilibrium coefficient for ligand detachment from the QD surface. The ligand 3-mercaptopropionic acid (MPA) was the most tightly bound to the surface and produced the lowest growth rate of (1.5-2) × 10(-3) nm/min in the OR stage, whereas thiolactic acid (TLA) was the most labile and produced the highest growth rate of 3 × 10(-3) nm/min. Methyl thioglycolate (MTG) and thioglycolic acid (TGA) produced rates in between these values. Ligands containing electron-withdrawing groups closer to the S atom and branching promote growth, whereas longer, possibly bidendate, ligands retard it. Mixed ligand experiments confirmed that growth is determined by ligand bonding strength to the QD. Photoluminescence spectroscopy showed that the more labile the ligand, the more facile the repair of surface defects during the exposure of the QDs to room light.

Bioavailability, Toxicity, and Bioaccumulation of Quantum Dot Nanoparticles to the AmphipodLeptocheirus plumulosus

Understanding the relative toxicities of different modes of nanoparticle exposure as compared with their dissolved metal ions are emerging areas in ecotoxicology. Here, we report on bioavailability, toxicity, and bioaccumulation of carboxyl-functionalized CdSe/ZnS quantum dots (QDs) to the amphipod Leptocheirus plumulosus exposed to equivalent Cd concentrations via dissolved Cd, QDs in water, or QDs in algal food. Both modes of QD exposure were accumulated to greater extent than dissolved Cd. Exposure to QDs via algae resulted in high amphipod mortality. Cadmium and Se in amphipods exposed to QDs in water were highly correlated and spatially localized within the amphipod. In contrast, when exposed to QDs via algae the metals were more disperse and not highly correlated suggesting QD dissolution and resultant metal ion toxicity. This study suggests QDs are accumulated to a greater extent than the dissolved ion and could lead to trophic transfer. QDs ingested with algae are bioavailable and result in toxicity, which is not observed in the absence of algae.

Oxidative dissolution of polymer-coated CdSe/ZnS quantum dots under UV irradiation: Mechanisms and kinetics

To advance the knowledge of environmental fate of nanomaterials, we systematically investigated the dissolution of polymer-coated CdSe/ZnS quantum dots (QDs) under UV (254 nm) irradiation. The environmental effects (i.e., irradiation intensity, dissolved oxygen, temperature, and humic acid), as well as the coating effects on dissolution kinetics of QDs were investigated. Our results showed that higher irradiation intensity and temperature increased ion release rates (Cd2+, SeO42−, and Zn2+), whereas the different polymer coatings varied the dissolution rates. The absence of dissolved oxygen inhibited the dissolution of QDs, and we further demonstrated that the dissolution was a photo-oxidative process involved superoxide radical formation. Humic acid had a twofold effect on dissolution due to its photosensitization and photoabsorption for UV irradiation. Finally, an empirical kinetic law was proposed to interpret the above environmental effects. This study lays groundwork to better understand the environmental fate of QDs.

Bioaccumulation and Effects of CdTe/CdS Quantum Dots on Chlamydomonas reinhardtii – Nanoparticles or the Free Ions?

In order to properly assess the environmental risk of engineered nanoparticles (ENP), it is necessary to determine their fate (including dissolution, aggregation, and bioaccumulation) under representative environmental conditions. CdTe/CdS quantum dots (QD), such as those used in medical imaging, are known to release Cd(2+) due (mainly) to the dissolution of their outer shell. In this study, Chlamydomonas reinhardtii was exposed to either a soluble Cd salt or QD at similar concentrations of total Cd. Free Cd concentrations were measured using the Absence of Gradients and Nernstian Equilibrium Stripping technique. QD dissolution increased with decreasing pH and with increasing QD concentration. When exposed to QD, bioaccumulation was largely accounted for by dissolved Cd. Nonetheless, QD were shown to be taken up by the cells and to provoke unique biological effects. Whole transcriptome screening using RNA-Seq analysis showed that the free Cd and the QD had distinctly different biological effects.

InAs/GaAs quantum dot capping in kinetically limited MOVPE growth regime

InAs/GaAs quantum dot (QD) properties can be significantly influenced by the growth conditions of the QD capping layer. We have studied the effect of a group III partial pressure in the reactor on the QD capping process and on the QD photoluminescence when the capping layer is grown under the kinetically limited regime. Two types of capping layers were prepared: GaAs and InGaAs. The GaAs capping layer growth rate decrease did not influence QD dissolution, but increased the dissolution of big hillocks. Influence of the GaAs capping layer thickness on QD photoluminescence is also demonstrated. The composition of the ternary strain reducing InGaAs capping layer can be considerably changed depending on the V/III ratio under kinetically limited growth.

InAs/GaAs quantum dot structures emitting in the 1.55 μm band

We investigated different capping layers covering InAs quantum dot structures grown on GaAs substrates by metalorganic vapor phase epitaxy in order to receive strong photoluminescence at 1.55 μm. Atomic force microscopy showed that uncovered InAs quantum dots are 4–5 nm high lenses with a broad luminescence peaking at 1.43 μm. The GaAs capping improves the quantum dot homogeneity but quantum dot dissolution causes blue shift of emitted light. On the other hand, proper engineering of InGaAs strain reducing layer allows to shift the photoluminescence maximum to 1.55 μm. Analysis of photoluminescence and microscopy data supported by calculation of quantum dot electron states shows that this is caused both by the change of the electronic-barrier structure and by the increase of the height of the overgrown quantum dots.

Improved capping layer growth towards increased stability of InGaN quantum dots

AbstractImprovements in GaN capping layer growth towards increased stability of InGaN quantum dots (QDs) will be presented. Surface quality is enhanced with changing the carrier gas during GaN capping layer growth from nitrogen to hydrogen. Without a dissolution of QDs it is possible to increase the temperature to 1050 °C applying a temperature ramp during growth of GaN.Dot stacks with an uniform and up to room temperature bright photoluminescence emission spectra is reported qualifying the structure to be incorporated into LED and laser structures. Finally, the evolution of the nucleation layer into flat and broad island‐like structures will be explained by the spinodal decomposition model (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of rapid thermal annealing on the noise properties of InAs∕GaAs quantum dot structures

Self-assembled InAs quantum dots (QDs) were grown by molecular beam epitaxy (MBE) on n+-GaAs substrates, capped between 0.4μm thick n-type GaAs layers with electron concentration of 1×1016cm−3. The effect of rapid thermal annealing at 700°C for 60s on the noise properties of the structure has been investigated using Au∕n-GaAs Schottky diodes as test devices. In the reference sample without containing QDs, the noise spectra show a generation-recombination (g-r) noise behavior due to a discrete energy level located about 0.51eV below the conduction band edge. This trap is ascribed to the M4 (or EL3) trap in GaAs MBE layers, related to a chemical impurity-native defect complex. In the structure with embedded QDs, the observed g-r noise spectra are due to a midgap trap level ascribed to the EL2 trap in GaAs, which is related to the InAs QDs dissolution due to the thermal treatment.

Nanovehicles Based Bioassay Labels

AbstractIn this article, we review recent advances of our group in nanoparticle labels based bioassay. Apoferritin and silica nanoparticles have been used as nanovehicles to load large amount of markers for highly sensitive bioassay. Markers loaded apoferritin, apoferritin‐templated metallic phosphate nanoparticles, and poly[guanine] coated silica nanoparticles have been prepared, characterized and used as labels for highly sensitive bioassay of protein and DNA. Dissociation and reconstitution characteristics at different pH as well as the special cavity structure of apoferritin nanovehicle provides a simple and convenient route to prepare versatile nanoparticle labels and avoid the complicated and tedious synthesis process of conventional nanoparticle labels. The optical and electrochemical characteristics of the prepared nanoparticle labels are easily controlled by loading different optical or electrochemical markers. Additionally, the use of apoferritin nanovehicle as template for synthesis of metallic phosphate nanoparticle labels offers fast route to prepare uniform‐size metallic nanoparticle labels for electrochemical bioassay and avoids the traditional harsh dissolution conditions to dissolve metallic nanoparticle tags (that is, the strong‐acid dissolution of quantum dots and gold nanoparticles) during the stripping analysis step. Silica nanoparticle has also been used as nanovehicle to carry thousands of poly[guanine] tracers, which was used to enhance the oxidation current of Ru(bpy)$\rm{ {_{3}^{2+}}}$, resulting in enhanced sensitivity of electrochemical immunoassay. The new nanovehicle‐based labels have been used for highly sensitive electrochemical detection of DNA and protein biomarkers, such as tumor necrosis factor‐alpha (TNF‐α). The high sensitivity and selectivity make these labels a useful addition to the armory of nanoparticle‐based bioassay. The new nanovehicles based labels hold great promise for multiplex protein and DNA detection and for enhancing the sensitivity of other bioassays.