Research Advances on Apoptosis Caused by Quantum Dots

Recently, quantum dots (QDs) have been widely applied in biological and biomedical fields such as cell labeling, living tissue imaging, and photodynamic therapy because of their superior optical properties. Meanwhile, the potential biological negative effects and/or toxic effects of QDs have become increasingly important, especially the cytotoxicity caused by QDs. One of the common cytotoxicity when living organisms are treated with QD is apoptosis, where many attempts have been made to explain the mechanisms of apoptosis caused by QDs’ use. One of the mechanisms is the production of cadmium ion (Cd²⁺) and reactive oxygen species (ROS). Excess generation of ROS will result in oxidative stress that would mediate apoptosis. Furthermore, the activation of cell death receptors and mitochondria-dependent such as B cell lymphoma 2 (Bcl-2) family and the caspase family could onset apoptosis. Signal transduction such as some classical signal pathways of PI3K-AKT, NF-E2-related factor 2 (Nrf2)-antioxidant response element (ARE), mitogen-activated protein kinases (MAPKs), and nuclear factor kappa B (NF-κB) also plays an important role in the regulation of apoptosis. Several ways to reduce the apoptotic rate have been introduced, such as surface modification, controlling, the dose, size, and exposure time of QDs as well as using antioxidants or inhibitors. In this review, we attempted to review the most recent findings associated with apoptosis caused by QDs so as to provide some guidelines for a safer QD application in the future.     

Targeting of Embryonic Stem Cells by Peptide-Conjugated Quantum Dots

Background

Targeting stem cells holds great potential for studying the embryonic stem cell and development of stem cell-based regenerative medicine. Previous studies demonstrated that nanoparticles can serve as a robust platform for gene delivery, non-invasive cell imaging, and manipulation of stem cell differentiation. However specific targeting of embryonic stem cells by peptide-linked nanoparticles has not been reported.

Methodology/Principal Findings

Here, we developed a method for screening peptides that specifically recognize rhesus macaque embryonic stem cells by phage display and used the peptides to facilitate quantum dot targeting of embryonic stem cells. Through a phage display screen, we found phages that displayed an APWHLSSQYSRT peptide showed high affinity and specificity to undifferentiated primate embryonic stem cells in an enzyme-linked immunoabsorbent assay. These results were subsequently confirmed by immunofluoresence microscopy. Additionally, this binding could be completed by the chemically synthesized APWHLSSQYSRT peptide, indicating that the binding capability was specific and conferred by the peptide sequence. Through the ligation of the peptide to CdSe-ZnS core-shell nanocrystals, we were able to, for the first time, target embryonic stem cells through peptide-conjugated quantum dots.

Conclusions/Significance

These data demonstrate that our established method of screening for embryonic stem cell specific binding peptides by phage display is feasible. Moreover, the peptide-conjugated quantum dots may be applicable for embryonic stem cell study and utilization.     

Investigating Fluorescence Quenching of ZnS Quantum Dots by Silver Nanoparticles

Water dispersible zinc sulfide quantum dots (ZnS QDs) with an average diameter of 2.9 nm were synthesized in an environment friendly method using chitosan as stabilizing agent. These nanocrystals displayed characteristic absorption and emission spectra having an absorbance edge at 300 nm and emission maxima (λ _emission) at 427 nm. Citrate-capped silver nanoparticles (Ag NPs) of ca. 37-nm diameter were prepared by modified Turkevich process. The fluorescence of ZnS QDs was significantly quenched in presence of Ag NPs in a concentration-dependent manner with K _sv value of 9 × 10⁹ M⁻¹. The quenching mechanism was analyzed using Stern–Volmer plot which indicated mixed nature of quenching. Static mechanism was evident from the formation of electrostatic complex between positively charged ZnS QDs and negatively charged Ag NPs as confirmed by absorbance study. Due to excellent overlap between ZnS QDs emission and surface plasmon resonance band of Ag NPs, the role of energy transfer process as an additional quenching mechanism was investigated by time-resolved fluorescence measurements. Time-correlated single-photon counting study demonstrated decrease in average lifetime of ZnS QDs fluorescence in presence of Ag NPs. The corresponding F?rster distance for the present QD–NP pair was calculated to be 18.4 nm.     

CdSe/ZnS-SA量子点细胞毒性的初步研究

目的：探讨链亲和素修饰的CdSe/ZnS核壳结构量子点（CdSe/ZnS-SA）对稳定转染pcDNA3.1/APP595/596质粒的人胚肾（HEK293）细胞的短期毒性作用.方法：将CdSe/ZnS-SA量子点与稳定转染pcD-NA3.1/APP595/596质粒的HEK293细胞共培育,在倒置荧光显微镜下观察细胞形态学变化;MTT法测定细胞活性;流式细胞术检测细胞凋亡率.结果：终浓度为2.5 nmol/L-20 nmol/L的CdSe/ZnS-SA量子点与HEK293细胞分别共育8h、16h、24 h后,细胞的形态无明显改变;终浓度为2.5 nmol/L-25 nmol/L的CdSe/ZnS-SA量子点与HEK293细胞分别共育8h、16 h、24 h,各处理组与对照组间,各处理组间的吸光度值、细胞凋亡率差异均无统计学意义（P＞0.05）.结论：一定浓度范围的CdSe/ZnS-SA量子点在短期内对稳定转染pcDNA3.1/APP595/596质粒的HEK293细胞无明显的毒性作用,具有较好的生物相容性.     

Luminescence Enhancement of Carboxyl-Coated CdTe Quantum Dots by Silver Nanoparticles

Metallic nanoparticles (NPs) have been used to improve the sensibility of biosensors and bioassays either by enhancing radiative emission or inducing quenching process on fluorescent probes. The aim of this research was to study the interaction of silver and silver-pectin NPs with water-dispersed carboxyl-coated cadmium telluride (CdTe) quantum dots (QDs). Metallic NPs were observed to change the emission of these fluorophores through local field effects. In a solution-base platform, an increase of 82 % was observed for the CdTe emission due to the interaction of QDs and silver-pectin NPs. QDs interaction with silver NPs without pectin was also investigated and a smaller emission enhancement of 20 % was detected. We observed that the NPs’ nature and QDs’ surface charge and concentration are important parameters for NPs-QDs interaction. Moreover, the presence of the pectin polymer shows to be a key component to the observed fluorescence enhancement.     

Interparticle Coupling Effects of Two Quantum Dots System on the Transport Properties of a Single Plasmon

Plasmonics - Transport properties of a single plasmon interacting with two quantum dots (QDs) system coupled to one-dimensional surface plasmonic waveguide are investigated theoretically via the...     

Temperature Driven Plasmon-Exciton Coupling in Thermoresponsive Dextran-Graft-PNIPAM/Au Nanoparticle/CdTe Quantum Dots Hybrid Nanosystem

The temperature-driven plasmon-exciton coupling in thermoresponsive dextran-graft-PNIPAM/Au nanoparticle/CdTe quantum dot (D-g-PNIPAM/Au NPs/CdTe QDs) hybrid nanosystem was studied. A significant (0.84 eV) splitting of the absorption peak was observed in the absorption spectrum of the nanosystem, which reflects the fact of formation of plexcitons, occurring due to strong plasmon-exciton coupling. An increasing with time plasmonic enhancement of the photoluminescence of CdTe QDs was revealed, as a result of the penetration of quantum dots into the volume of the D-g-PNIPAM/Au NP hybrid nanosystem and bonding to it. The heating–cooling cycle of the aqueous solution of the studied nanosystem leads to a reversible quenching-recovery alteration of the QD photoluminescence. The quenching was rationalized as a result of an increased probability of nonradiative resonance energy transfer (RET) from CdTe QDs to Au NPs, which occurs due to shortening of the NP-QD distance, caused by shrinking of the macromolecule due to cooling-induced lower critical solution temperature phase transition. Increasing the NP-QD distance in the heating stage recovers the QD PL intensity. The observed effect opens up opportunities for the controlled reversible temperature-driven tuning of the photoluminescence intensity of D-g-PNIPAM/Au NP/CdTe QD nanosystem, which is highly important for its potential use in photonics and biomedical applications.

     

Uptake of CdSe and CdSe/ZnS Quantum Dots into Bacteria via Purine-Dependent Mechanisms

Quantum dots (QDs) rendered water soluble for biological applications are usually passivated by several inorganic and/or organic layers in order to increase fluorescence yield. However, these coatings greatly increase the size of the particle, making uptake by microorganisms impossible. We find that adenine- and AMP-conjugated QDs are able to label bacteria only if the particles are <5 nm in diameter. Labeling is dependent upon purine-processing mechanisms, as mutants lacking single enzymes demonstrate a qualitatively different signal than do wild-type strains. This is shown for two example species, one gram negative and one gram positive. Wild-type Bacillus subtilis incubated with QDs conjugated to adenine are strongly fluorescent; very weak signal is seen in mutant cells lacking either adenine deaminase or adenosine phosphoribosyltransferase. Conversely, QD-AMP conjugates label mutant strains more efficiently than the wild type. In Escherichia coli, QD conjugates are taken up most strongly by adenine auxotrophs and are extruded from the cells over a time course of hours. No fluorescent labeling is seen in killed bacteria or in the presence of EDTA or an excess of unlabeled adenine, AMP, or hypoxanthine. Spectroscopy and electron microscopy suggest that QDs of <5 nm can enter the cells whole, probably by means of oxidative damage to the cell membrane which is aided by light.     

Preparation and Biological Effect of Nucleotide-Capped CdSe/ZnS Quantum Dots on Tetrahymena thermophila

In this paper, we described the preparation and characterization of different types of modified CdSe/ZnS quantum dots (QDs) and explored the biological effects of QDs with different surface modifications on the whole growth of unicellular protozoan Tetrahymena thermophila BF(5) using a thermal activity monitor air isothermal microcalorimeter. Our results demonstrated that adenosine 5'-monophosphate (AMP) showed stronger interaction with QDs than other types of nucleotide. AMP-QDs could stimulate the growth of T. thermophila while mercaptoacetic acid-capped CdSe/ZnS quantum dots inhibited it. In addition, the population density determination and fluorescence imaging of T. thermophila BF(5) also confirmed the results obtained from microcalorimetry. It is believed that this approach will provide a more convenient methodology for the kinetics and thermodynamics of microorganism when coexisting with QDs in real time, and all of which are very significant to understanding the effect of QDs to organism.     

量子点的近场激发荧光特性研究

近场光学显微镜具有nm量级的空间分辨率,量子点(quantum dots,QDs)荧光探针具有激发谱宽、发射谱线窄、荧光强度高、抗光漂白和稳定性高等优点,两者结合用于生物大分子的成像探测和识别具有广泛的应用前景。用近场光学显微镜对链霉亲和素偶联的QDs进行近场荧光激发,并对其荧光发射特性和光稳定性进行研究,结果表明:近场光学显微镜nm量级的空间分辨率,可以同时观察到了QDs的单体、二聚体和三聚体;QDs的荧光发射强度高,近场荧光像对比度好,单量子点的荧光半高宽达到25nm;对一定入射波长的单色激发光,QDs的近场荧光强度随着激发功率密度的增加线性增加,并很快趋于稳定。与传统的荧光染料如异硫氰酸荧光素相比,QDs的稳定性非常好,在激发功率密度为300W/cm2的近场辐射下,量子点的荧光强度超过6h基本保持不变,其抗光漂白能力远远高于普通荧光染料。     

Controlling the Luminescence of Gold Quantum Dots by the Plasmonic Effect of Silver Nanoprisms

Plasmonics - Plasmonic nanostructures can be used to control the photoluminescence properties of various emitting materials. In this work, an efficient plasmon-induced energy transfer (quenching)...     

Integration and Oligomerization of Bax Protein in Lipid Bilayers Characterized by Single Molecule Fluorescence Study

Bax is a pro-apoptotic Bcl-2 family protein. The activated Bax translocates to mitochondria, where it forms pore and permeabilizes the mitochondrial outer membrane. This process requires the BH3-only activator protein (i.e. tBid) and can be inhibited by anti-apoptotic Bcl-2 family proteins such as Bcl-x_L. Here by using single molecule fluorescence techniques, we studied the integration and oligomerization of Bax in lipid bilayers. Our study revealed that Bax can bind to lipid membrane spontaneously in the absence of tBid. The Bax pore formation undergoes at least two steps: pre-pore formation and membrane insertion. The activated Bax triggered by tBid or BH3 domain peptide integrates on bilayers and tends to form tetramers, which are termed as pre-pore. Subsequent insertion of the pre-pore into membrane is highly dependent on the composition of cardiolipin in lipid bilayers. Bcl-x_L can translocate Bax from membrane to solution and inhibit the pore formation. The study of Bax integration and oligomerization at the single molecule level provides new evidences that may help elucidate the pore formation of Bax and its regulatory mechanism in apoptosis.     

Quantum Dots Reveal Shifts in Organic Nitrogen Uptake by Fungi Exposed to Long-Term Nitrogen Enrichment

Anthropogenic nitrogen (N) enrichment can alter N dynamics associated with decomposing plant litter. However, it is unclear to what extent these alterations occur via microbial effects (e.g., changes in gene regulation, physiology, or community composition) versus plant litter effects (e.g., changes in composition of N and C compounds). To isolate microbial effects from plant litter effects, we collected plant litter from long-term N fertilized and control plots, reciprocally inoculated it with microbes from the two treatments, and incubated it in a common field setting for three months. We used quantum dots (QDs) to track fungal uptake of glycine and chitosan. Glycine is a relatively simple organic N compound; chitosan is more complex. We found that microbial and litter origins each contributed to a shift in fungal uptake capacities under N fertilization. Specifically, N fungi preferred glycine over chitosan, but control fungi did not. In comparison, litter effects were more subtle, and manifested as a three-way interaction between litter origin, microbial origin, and type of organic N (glycine versus chitosan). In particular, control fungi tended to target chitosan only when incubated with control litter, while N fungi targeted glycine regardless of litter type. Overall, microbial effects may mediate how N dynamics respond to anthropogenic N enrichment in ecosystems.     

Early Events in the Fusarium verticillioides-Maize Interaction Characterized by Using a Green Fluorescent Protein-Expressing Transgenic Isolate

The infection of maize by Fusarium verticillioides can result in highly variable disease symptoms ranging from asymptomatic plants to severe rotting and wilting. We produced F. verticillioides green fluorescent protein-expressing transgenic isolates and used them to characterize early events in the F. verticillioides-maize interaction that may affect later symptom appearance. Plants grown in F. verticillioides-infested soil were smaller and chlorotic. The fungus colonized all of the underground parts of a plant but was found primarily in lateral roots and mesocotyl tissue. In some mesocotyl cells, conidia were produced within 14 to 21 days after infection. Intercellular mycelium was detected, but additional cells were not infected until 21 days after planting. At 25 to 30 days after planting, the mesocotyl and main roots were heavily infected, and rotting developed in these tissues. Other tissues, including the adventitious roots and the stem, appeared to be healthy and contained only a small number of hyphae. These results imply that asymptomatic systemic infection is characterized by a mode of fungal development that includes infection of certain tissues, intercellular growth of a limited number of fungal hyphae, and reproduction of the fungus in a few cells without invasion of other cells. Development of visibly rotted tissue is associated with massive production of fungal mycelium and much less organized growth.     

Modification of Spontaneous Emission Rates of Self-assembled CdSe Quantum Dots by Coupling to Hybrid Optical Nanoantennas

Plasmonics - The spontaneous emission of a light source can be modified by tailoring its local density of optical states. Indeed, this concept has been commonly utilized to enhance the spontaneous...     

Characterization of Eag1 Channel Lateral Mobility in Rat Hippocampal Cultures by Single-Particle-Tracking with Quantum Dots

Voltage-gated ion channels are main players involved in fast synaptic events. However, only slow intracellular mechanisms have so far been described for controlling their localization as real-time visualization of endogenous voltage-gated channels at high temporal and spatial resolution has not been achieved yet. Using a specific extracellular antibody and quantum dots we reveal and characterize lateral mobility as a faster mechanism to dynamically control the number of endogenous ether-a-go-go (Eag)1 ion channels inside synapses. We visualize Eag1 entering and leaving synapses by lateral diffusion in the plasma membrane of rat hippocampal neurons. Mathematical analysis of their trajectories revealed how the motion of Eag1 gets restricted when the channels diffuse into the synapse, suggesting molecular interactions between Eag1 and synaptic components. In contrast, Eag1 channels switch to Brownian movement when they exit synapses and diffuse into extrasynaptic membranes. Furthermore, we demonstrate that the mobility of Eag1 channels is specifically regulated inside synapses by actin filaments, microtubules and electrical activity. In summary, using single-particle-tracking techniques with quantum dots nanocrystals, our study shows for the first time the lateral diffusion of an endogenous voltage-gated ion channel in neurons. The location-dependent constraints imposed by cytoskeletal elements together with the regulatory role of electrical activity strongly suggest a pivotal role for the mobility of voltage-gated ion channels in synaptic activity.