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The atomic pair distribution function (PDF) analysis of X-ray powder diffraction data has been used to study the structure of small and ultra-small CdSe nanoparticles. A method is described that uses a wurtzite and zinc-blende mixed phase model to account for stacking faults in CdSe particles. The mixed-phase model successfully describes the structure of nanoparticles larger than 2 nm yielding a stacking fault density of about 30%. However, for ultrasmall nanoparticles smaller than 2 nm, the models cannot fit the experimental PDF showing that the structure is significantly modified from that of larger particles and the bulk. The observation of a significant change in the average structure at ultra-small size is likely to explain the unusual properties of the ultrasmall particles such as their white light emitting ability.
Individual ultrasmall CdSe nanocrystals have recently been found to emit white light, but the ultimate origin of the phenomenon has remained elusive. Here we use a combination of state-of-the-art experiment and theory to show that excitation sets the ultrasmall nanocrystals into a fluxional state. Their energy gaps vary continuously on a femtosecond time scale, so that even an individual nanocrystal can emit across the entire visual range. In addition, we observe the outer layers of the larger monochromatic emitting nanocrystals to be fluxional. Such fluxionality should be considered when optimizing nanocrystals for applications. Thus, small is indeed different, but ultrasmall is different yet again.
A simple treatment method using formic acid has been found to increase the fluorescence quantum yield of ultrasmall white light-emitting CdSe nanocrystals from 8% to 45%. Brighter white-light emission occurs with other carboxylic acids as well, and the magnitude of the quantum yield enhancement is shown to be dependent on the alkyl chain length. Additionally, the nanocrystal luminescence remains enhanced relative to the untreated nanocrystals over several days. This brightened emission opens the possibility for even further quantum yield improvement and potential for use of these white-light nanocrystals in solid-state lighting applications.
A fluorescent magnetic hybrid imaging nanoprobe (HINP) was fabricated by the conjugation of superparamagnetic Fe3O4 nanoparticles and visible light emitting (∼600 nm) fluorescent CdTe/CdS quantum dots (QDs). The assembly strategy used the covalent linking of the oxidized dextran shell of magnetic particles to the glutathione ligands of QDs. The synthesized HINP formed stable water-soluble colloidal dispersions. The structure and properties of the particles were characterized by transmission electron and atomic force microscopy, energy dispersive x-ray analysis and inductively coupled plasma optical emission spectroscopy, dynamic light scattering analysis, optical absorption and photoluminescence spectroscopy, and fluorescent imaging. The luminescence imaging region of the nanoprobe was extended to the near-infrared (NIR) (∼800 nm) by conjugation of the superparamagnetic nanoparticles with synthesized CdHgTe/CdS QDs. Cadmium, mercury based QDs in HINP can be easily replaced by novel water-soluble glutathione stabilized AgInS2/ZnS QDs to present a new class of cadmium-free multimodal imaging agents. The observed NIR photoluminescence of fluorescent magnetic nanocomposites supports their use for bioimaging. The developed HINP provides dual-imaging channels for simultaneous optical and magnetic resonance imaging.
We report the observation of broad-spectrum fluorescence from single CdSe nanocrystals. Individual semiconductor nanocrystals typically have a narrower emission spectrum than that of an ensemble. However, our experiments show that the ensemble white-light emission observed in ultrasmall CdSe nanocrystals is the result of many single CdSe nanocrystals, each emitting over the entire visible spectrum. These results indicate that each white-light-emitting CdSe nanocrystal contains all the trap states that give rise to the observed white-light emission.
Highly fluorescent CdSe quantum dots (qdots) can serve as a platform for tethering multiple copies of a receptor-targeted ligand, affording study of how the level of multivalency affects receptor binding. We previously showed that qdots conjugated with long PEG chains terminated by muscimol, a known GABA(C) agonist, exhibit specific binding to the surface membrane of GABA(C) receptor-expressing Xenopus oocytes. The present report addresses the effect of varying the number, i.e., valency, of muscimol- (M-) terminated PEG chains attached to the qdot on binding of the resulting conjugate to GABA(C) receptors. M-PEG-qdots of differing muscimol valency were prepared by conjugating AMP-CdSe/ZnS qdots with muscimol-terminated and methylamine-terminated PEG chains in proportions designed to yield varying percentages of muscimol-terminated chains among the total approximately 150-200 chains bound to the qdot. The investigated valencies represented 0%, approximately 25%, approximately 50%, and 100% loading with muscimol (preparations termed M-PEG-qdot0, M-PEG-qdot25, M-PEG-qdot50, and M-PEG-qdot100, respectively. Binding of a given conjugate to surface membranes of GABA(C) receptor-expressing oocytes was analyzed by quantitative fluorescence microscopy following defined incubation with approximately 30 nM of the conjugate. With 5-20 min incubation, the fluorescence signal resulting from incubation with M-PEG-qdot25 exceeded, by approximately 6-fold, the fluorescence level obtained with M-PEG-qdot preparations that lacked muscimol-terminated chains (M-PEG-qdot0). M-PEG-qdot50 yielded a net signal roughly similar to that of M-PEG-qdot25, and that produced by M-PEG-qdot100 exceeded, by approximately 30-50%, those for M-PEG-qdot25 and M-PEG-qdot50. The time course of changes in oocyte surface membrane fluorescence resulting from the introduction of and removal of M-PEG-qdots in the medium bathing the oocyte indicated only a modest dependence of both binding and wash-out kinetics on muscimol valency. The results demonstrate a dependence of the binding activity of the M-PEG-qdot conjugates on muscimol valency, presumably reflecting higher GABA(C) avidity and/or affinity of the muscimol at high valency, and provide insight on the interactions of membrane receptor proteins with qdot conjugates containing multiple copies of a receptor-targeting ligand.
We report white light-emitting diodes fabricated with ultrasmall CdSe nanocrystals, which demonstrate electroluminescence from a size of nanocrystals (<2 nm) previously thought to be unattainable. These LEDs have excellent color characteristics, defined by their pure white CIE color coordinates (0.333, 0.333), correlated color temperatures of 5461-6007 K, and color rendering indexes as high as 96.6. The effect of high voltage on the trap states responsible for the white emission is also described.
Functionalization of highly fluorescent CdSe/ZnS core-shell nanocrystals (quantum dots, qdots) is an emerging technology for labeling cell surface proteins. We have synthesized a conjugate consisting of approximately 150-200 muscimols (a GABA receptor agonist) covalently joined to the qdot via a poly(ethylene glycol) (PEG) linker (approximately 78 ethylene glycol units) and investigated the binding of this muscimol-PEG-qdot conjugate to homomeric rho1 GABAC receptors expressed in Xenopus oocytes. GABAC receptors mediate inhibitory synaptic signaling at multiple locations in the central nervous system (CNS). Binding of the conjugate was analyzed quantitatively by determining the fluorescence intensity of the oocyte surface membrane in relation to that of the surrounding incubation medium. Upon 5- to 10-min incubation with muscimol-PEG-qdots (34 nM in qdot concentration), GABAC-expressing oocytes exhibited a fluorescent halo at the surface membrane that significantly exceeded the fluorescence of the incubation medium. This halo was absent following muscimol-PEG-qdot treatment of oocytes lacking GABAC receptors. Incubation of the oocyte with free muscimol (100 microM-5 mM), PEG-muscimol (500 microM), or GABA (100 microM - 5 mM) substantially reduced or eliminated the fluorescence halo produced by muscimol-PEG-qdots, and the removal of GABA or free muscimol led to a recovery of muscimol-PEG-qdot binding. Unconjugated qdots and PEG-qdots that lacked conjugated muscimol neither exhibited significant binding activity nor diminished the subsequent binding of muscimol-PEG-qdots. The results indicate that muscimol joined to qdots via a long-chain PEG linker exhibits specific binding activity at the ligand-binding pocket of expressed GABAC receptors, despite the presence of both the long PEG linker and the sterically bulky qdot.
Alloy nanocrystals provide an additional degree of freedom in selecting desirable properties for nanoscale engineering because their physical and optical properties depend on both size and composition. We report the pyrolytic synthesis of homogeneously alloyed CdS(x)Se(1-x) nanocrystals in all proportions. The nanocrystals are characterized using UV-visible absorption spectroscopy, transmission electron microscopy, X-ray diffractrometry, and Rutherford backscattering spectrometry to determine precisely structure, size, and composition. The dependence of band gap on nanocrystal size and composition is elucidated, yielding a bowing constant of 0.29, in agreement with bulk values. In addition, the morphology of the resultant nanocrystals can be altered by changing the reaction conditions, generating structures ranging from homogeneous, spherical nanocrystals to one-dimensional gradient nanorods.