Attached Quantum Dots Research Articles

A combination of pollen mosaics on pollinators and floral handedness facilitates the increase of outcross pollen movement

Darwin devoted an entire book to style and stamen polymorphisms, exemplifying the importance of pollen movement efficiency as a selective agent on floral form.1 However, after its publication, his interest was piqued by a description of floral handedness2 or enantiostyly.3 Todd2 described how left- and right-handed Solanum rostratum flowers have styles deflected to the left and right, respectively. Darwin4 wrote to Todd for seeds so that he could "…have the pleasure of seeing the flowers and experimenting on them," but he died just days later on 19 April 1882. More than a century elapsed before the first experiments demonstrated that handedness leads to high rates of outcrossing.5,6 By attaching quantum dots to pollen grains, we tracked pollen movement in Wachendorfia paniculata, which has one stamen on the same side of the style and two deflected in the opposite direction. We found that handedness leads to outcrossing because left- and right-handed morphs place most of their pollen on different sides of the pollinators. However, the partial separation of stamens and style also results in two-dimensional pollen quality mosaics on each side of carpenter bee pollinators, generating hotspots and coldspots of outcrossed pollen. Similar mosaics were not found on honeybee pollinators. Outcrossed pollen receipt was much higher than expected because stigmatic positions are fine-tuned to match the outcross pollen hotspots on carpenter bees. Exploitation of these pollen mosaics enables plants to increase the probability of between-morph (i.e., disassortative), outcross pollen movement beyond the expectations of enantiostyly.

Handed Flowers Pick High Quality Mates from Pollinators

Darwin devoted an entire book to stigma and anther polymorphisms, which exemplified the importance of pollen movement efficiency as a selective agent on floral form. However, five years after its publication, his interest was piqued by a description of floral handedness or enantiostyly. Todd described how left handed Solanum rostratum flowers have stigmas deflected to the left and anthers deflected to the right, while the opposite is true for right-handed flowers. In his final scientific correspondence, Darwin wrote to Todd for seeds so that he could … have the pleasure of seeing the flowers and experimenting on them. Darwin died days later on 19 April 1882. More than a century would elapse before the first experiments demonstrated that handedness leads to high rates of outcrossing [5, 6]. By attaching quantum dots to pollen grains, we tracked pollen movement, revealing the mechanisms behind high outcrossing rates in a plant with partial handedness (a majority of anthers deflected to one side). Handedness leads to outcrossing, because left- and right-handed morphs place most of their pollen on different sides of the pollinators. However, we also found that partial handedness results in pollen quality mosaics on each side of pollinators, comprising hotspots and coldspots of outcrossed pollen. Outcrossed pollen receipt was much higher than expected because stigmatic positions are fine-tuned to match the outcross pollen hotspots. Exploitation of these pollen quality mosaics suggests that like animals, female choice in plants can occur before or during the act of mating.

Colloidal Synthesis Path to 2D Crystalline Quantum Dot Superlattices.

By combining colloidal nanocrystal synthesis, self-assembly, and solution phase epitaxial growth techniques, we developed a general method for preparing single dot thick atomically attached quantum dot (QD) superlattices with high-quality translational and crystallographic orientational order along with state-of-the-art uniformity in the attachment thickness. The procedure begins with colloidal synthesis of hexagonal prism shaped core/shell QDs (e.g., CdSe/CdS), followed by liquid subphase self-assembly and immobilization of superlattices on a substrate. Solution phase epitaxial growth of additional semiconductor material fills in the voids between the particles, resulting in a QD-in-matrix structure. The photoluminescence emission spectra of the QD-in-matrix structure retains characteristic 0D electronic confinement. Importantly, annealing of the resulting structures removes inhomogeneities in the QD-QD inorganic bridges, which our atomistic electronic structure calculations demonstrate would otherwise lead to Anderson-type localization. The piecewise nature of this procedure allows one to independently tune the size and material of the QD core, shell, QD-QD distance, and the matrix material. These four choices can be tuned to control many properties (degree of quantum confinement, quantum coupling, band alignments, etc.) depending on the specific applications. Finally, cation exchange reactions can be performed on the final QD-in-matrix, as demonstrated herein with a CdSe/CdS to HgSe/HgS conversion.

Mercury Chalcogenide Nanoplatelet-Quantum Dot Heterostructures as a New Class of Continuously Tunable Bright Shortwave Infrared Emitters.

Despite broad applications in imaging, energy conversion, and telecommunications, few nanoscale moieties emit light efficiently in the shortwave infrared (SWIR, 1000-2000 nm or 1.24-0.62 eV). We report quantum-confined mercury chalcogenide (HgX, where X = Se or Te) nanoplatelets (NPLs) can be induced to emit bright (QY > 30%) and tunable (900-1500+ nm) infrared emission from attached quantum dot (QD) "defect" states. We demonstrate near unity energy transfer from NPL to these QDs, which completely quench NPL emission and emit with a high QY through the SWIR. This QD defect emission is kinetically tunable, enabling controlled midgap emission from NPLs. Spectrally resolved photoluminescence demonstrates energy-dependent lifetimes, with radiative rates 10-20 times faster than those of their PbX analogues in the same spectral window. Coupled with their high quantum yield, midgap emission HgX dots on HgX NPLs provide a potential platform for novel optoelectronics in the SWIR.

GUV Based Mimicry of Dendritic Spine Morphology Permits to Test Hypotheses on Ltp and Learning

Memory and learning are believed to be regulated by the strength of the connections in the synapses of the 100 billion neurons in the human brain. In the synapse the signal is transmitted between the presynaptic axon and the postsynaptic dendrite by neurotransmitters. The postsynaptic site is usually a protrusion on the dendrite called dendritic spine. Receptors in the membrane of the dendritic spine define the strength of a synapse. Thus the strength and stability of synapses are stored in the dendritic spine and are thought to be dependent on spine morphology (Yuste & Bonnhoeffer, 2001). Therefore the mushroom shape of mature dendritic spines is predicted to function as a receptor trap, locally increasing receptor density.We developed a mimetic system to investigate dendritic spine morphology and its effects on receptor confinement and diffusion. Giant unilamellar vesicles (GUV's) are made from phospholipids using electroswelling. To mimic the mushroom morphologies of dendritic spines, a micromanipulator is used to pull membrane tubes from the GUV lipid bilayer.Diffusion is measured by attaching quantum dots to lipids or receptors in the GUV membrane using biotin-streptavidin bonds. Particle tracking is then used to determine diffusion in the GUV membrane, and escape times from GUV to tube.Results show a strong dependence of GUV and tube size on escape times, confirming the idea that connection strengths of mushroom shaped dendritic spines are much stabler than the strengths of stubby shaped dendritic spines.

Fast Monolayer Adsorption and Slow Energy Transfer in CdSe Quantum Dot Sensitized ZnO Nanowires

A method for CdSe quantum dot (QD) sensitization of ZnO nanowires (NW) with fast adsorption rate is applied. Photoinduced excited state dynamics of the quantum dots in the case of more than monolayer coverage of the nanowires is studied. Transient absorption kinetics reveals an excitation depopulation process of indirectly attached quantum dots with a lifetime of ~4 ns. Photoluminescence and incident photon-to-electron conversion efficiency show that this process consists of both radiative e-h recombination and nonradiative excitation-to-charge conversion. We argue that the latter occurs via interdot energy transfer from the indirectly attached QDs to the dots with direct contact to the nanowires. From the latter, fast electron injection into ZnO occurs. The energy transfer time constant is found to be around 5 ns.

Covalent Quantum dot Receptor Linkage via the acyl Carrier Protein for Single-Molecule Tracking, Internalization, and Trafficking Studies

Here we describe a labeling technique for the covalent linkage of quantum dots to transmembrane receptors for single-molecule tracking. Our method combines the acyl carrier protein (ACP) technique with coenzyme A (CoA)-functionalized quantum dots to covalently attach quantum dots to ACP fusions of receptor proteins. The advantages of this approach include: (i) the use of a smaller attachment linker than in many other quantum dot-labeling systems; (ii) the ability to achieve a reliable 1:1 fluorophore-to-receptor labeling stoichiometry; (iii) the specificity of the method; and (iv) the covalent nature of the quantum dot linkage. We demonstrate the general suitability of this technique in single-molecule tracking, internalization, and trafficking studies by imaging two different transmembrane receptors in living cells.

Attaching quantum dots to HER2 specific phage antibodies

This work presents the results of the attachment of Qdot 655 ITK TM amino (PEG) quantum dots (QDs) (Invitrogen) and CdTe QDs (provided by Institute of Materials Science, VAST) to HER2 (Human Epidermal growth factor Receptor 2) specific phage antibodies (Abs) (provided by Institute of Biotechnology, VAST) in solution. The QDs were attached to the phage display specific HER2 Abs to form a complex QD–Ab. The QDs and complex QD–Ab were characterized by UV-VIS spectroscopy, transmission electron microscopy (TEM) and fluorescence microscopy. The fluorescence images show the QDs conjugated to the phage. Due to the QDs attaching to the surface, the phage dimensions were amplified, so its shape could be observed by optical microscopy. The complex QD–Ab was stable and lasted for a month. The results illustrate the value of the HER2 phage–QD complex as a cancer detection platform.

Quantum dot enhancement of bacteriorhodopsin-based electrodes

Nanoscale sensing arrays utilizing the unique properties of the optical protein bacteriorhodopsin and colloidal semiconductor quantum dots are being developed for toxin detection applications. This paper describes an innovative method to activate bacteriorhodopsin-based electrodes with the optical output of quantum dots, producing an enhanced electrical response from the protein. Results show that the photonic emission of CdSe/ZnS quantum dots is absorbed by the bacteriorhodopsin retinal and initiates the proton pumping sequence, resulting in an electrical output from a bacteriorhodopsin-based electrode. It is also shown that activated quantum dots in sub-10 nm proximity to bacteriorhodopsin further amplify the photovoltaic response of the protein by approximately 23%, compared to without attached quantum dots, suggesting direct energy transfer mechanisms beyond photonic emission alone. The ability of quantum dots to activate nanoscale regions on bacteriorhodopsin-based electrodes could allow sub-micron sensing arrays to be created due to the ability to activate site-specific regions on the array.

Neurexin/Neuroligin Interaction Kinetics Characterized by Counting Single Cell-Surface Attached Quantum Dots

We report what to our knowledge is a new method to characterize kinetic rates between cell-surface-attached adhesion molecules. Cells expressing specific membrane receptors are surface-labeled with quantum dots coated with their respective ligands. The progressive diminution in the total number of surface-diffusing quantum dots tracked over time collectively reflects intrinsic ligand/receptor interaction kinetics. The probability of quantum dot detachment is modeled using a stochastic analysis of bond formation and dissociation, with a small number of ligand/receptor pairs, resulting in a set of coupled differential equations that are solved numerically. Comparison with the experimental data provides an estimation of the kinetic rates, together with the mean number of ligands per quantum dot, as three adjustable parameters. We validate this approach by studying the calcium-dependent neurexin/neuroligin interaction, which plays an important role in synapse formation. Using primary neurons expressing neuroligin-1 and quantum dots coated with purified neurexin-1β, we determine the kinetic rates between these two binding partners and compare them with data obtained using other techniques. Using specific molecular constructs, we also provide interesting information about the effects of neurexin and neuroligin dimerization on the kinetic rates. As it stands, this simple technique should be applicable to many types of biological ligand/receptor pairs.

Electrochemically Controlled Surface Plasmon Enhanced Fluorescence Response of Surface Immobilized CdZnSe Quantum Dots

Cd0.1Zn0.9Se quantum dots with fluorescent emission centered on 614 nm were covalently coupled to a 11-amino-1-undecanethiol monolayer self-assembled on a gold surface. A 594 nm laser was used to excite surface plasmons in the gold film and the resulting surface plasmon enhanced fluorescence of the quantum dots was measured with a photomultiplier. The application of negative potentials (versus Ag/AgCl) led to a decrease in the surface plasmon enhanced fluorescence signal, the fluorescence signal recovered if the cell potential was returned to 0 V or open circuit. These results show that the fluorescence emission of attached quantum dots can be tuned by the application of an electrical potential in an aqueous environment, which may be relevant to quantum dot applications in biosensing.

Peptide-mediated surface-immobilized quantum dot hybrid nanoassemblies with controlled photoluminescence

Combinatorially selected peptides and peptide–organic conjugates were used as linkers with controlled structural and organizational conformations to attach quantum dots (QDs) at addressable distances from a metal surface. This study demonstrates an approach towards nanophotonics by integrating inorganic, organic, and biological constructs to form hybrid nanoassemblies through template-directed self-assembly. Peptide–organic-linked QD arrays showed stronger fluorescence than peptide-linked QD arrays. We attribute this difference primarily to the increased number density of QDs on peptide–organic-linked QD arrays.

Templated self-assembly of quantum dots from aqueous solution using proteinscaffolds

Short, histidine-containing peptides can be conjugated to lysine-containing protein scaffoldsto controllably attach quantum dots (QDs) to the scaffold, allowing for generic attachmentof quantum dots to any protein without the use of specially engineered domains.This technique was used to bind quantum dots from aqueous solution to bothchicken IgG and cowpea mosaic virus (CPMV), a 30 nm viral particle. Thesequantum dot–protein assemblies were studied in detail. The IgG–QD complexeswere shown to retain binding specificity to their antigen after modification. TheCPMV–QD complexes have a local concentration of quantum dots greater than3000 nmol ml−1, and show a 15% increase in fluorescence quantum yield over free quantum dots insolution.

Conductance control in quantum wires by attached quantum dots

AbstractThe electronic transport of two quantum dots side‐coupled to a quantum wire is studied by means of the two impurity Anderson Hamiltonian. The conductance is found to be a superposition of a Fano and a Breit–Wigner resonances as a function of the Fermi energy, provided the gate voltages of the quantum dots are slightly different. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum dots as luminescent probes in biological systems

This review describes the recent progress made in exploiting the light emitting properties of quantum dots as luminescent probes for the investigation of non-covalent interactions between two or more biological molecules. The properties of quantum dots and conventional fluorescent probes are compared and methods for attaching quantum dots to biomolecules examined. Such attachment generally involves two stages: quantum dot capping/coating and subsequent covalent or non-covalent linkage to the biomolecule of interest. Both are addressed. Finally, the use of quantum dots in biological assays is exemplified and the future roles of quantum dots discussed.

Quantum computing with quantum dots on quantum linear supports

Motivated by the recently demonstrated ability to attach quantum dots to polymers at well-defined locations, we propose a condensed-phase analog of the ion-trap quantum computer: a scheme for quantum computation using chemically assembled semiconductor nanocrystals attached to a linear support. The linear support is either a molecular string (e.g., DNA) or a nanoscale rod. The phonon modes of the linear support are used as a quantum-information bus between the dots. Our scheme offers greater flexibility in optimizing material parameters than the ion-trap method, but has additional complications. We discuss the relevant physical parameters, provide a detailed feasibility study, and suggest materials for which quantum computation may be possible with this approach. We find that Si is a potentially promising quantum-dot material, already allowing a 5--10-qubit quantum computer to operate with an error threshold of ${10}^{\ensuremath{-}3}.$

Discontinuity in the phase evolution of electron transport in a quantum channel with attached quantum dots

In a recent phase experiment by Schuster et al. [Nature 385, 417 (1997)], an abrupt phase drop of \ensuremath{\pi} was observed when the electron transmission through a quantum dot embedded in a quantum channel vanished. We show, both analytically and numerically, that this phase discontinuity is an intrinsic property of single electron transport: It arises from interference, known as Fano resonance, between two different transmission channels belonging to a localized state in the dot and a continuous state of the quantum channel. This phase behavior should also be seen in the same type of scattering resonances in other physical systems.