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The influence of excitation wavelength and embedding media on fluorescence blinking statistics of 4 nm x 20 nm cadmium selenide (CdSe) nanorods is investigated. Photon antibunching (PAB) experiments confirm nonclassical emission from single CdSe nanorods that exhibit a radiative lifetime of 26 +/- 13 ns. The blinking data show behaviors that can be categorized into two classes: excitation near the energy of the band gap and at energies exceeding 240 meV above the band gap. Excitation at the band gap energy (lambda >or= 560 nm) results in more pronounced "on" time probabilities in the distribution of "on" and "off" events, while those resulting from excitation exceeding the band gap by 240 meV or more (lambda <or= 560 nm) are 200 times less likely to display continuous "on" fluorescence persisting beyond 4 s. The "off" time statistics are also sensitive to the excitation wavelength, showing a similar, but inversely correlated, effect. To understand better the excitation-wavelength dependence, fluorescence measurements are obtained for single nanorods deposited both on a bare microscope coverslip and embedded in 1-ethyl-3-methylimidizolium bis(trifluoromethylsulfonyl)imide room-temperature ionic liquid (RTIL). The embedding RTIL medium has a significant influence on the resultant fluorescence statistics only when the excitation energy exceeds the 240 meV threshold. The results are explained by a threshold to access nonemissive trap states, attributed to self-trapping of hot charge carriers at the higher photon excitation energies.
ACS Nano 2008 Oct 28
PMID:Excitation-wavelength dependence of fluorescence intermittency in CdSe nanorods. 1920 61

Recent advances and progress in nanobiotechnology have demonstrated many nanoparticles (NPs) as potential and novel drug delivery vehicles, therapeutic agents, and contrast agents and luminescent biological labels for bioimaging. The emergence of new biomedical applications based on NPs signifies the need to understand, compare, and manage their cytotoxicity. In this study, we demonstrated the use of high-content screening assay (HCA) as a universal tool to probe the cytotoxicity of NPs and specifically cadmium telluride quantum dots (CdTe QDs) and gold NPs (Au NPs) in NG108-15 murine neuroblastoma cells and HepG2 human hepatocellular carcinoma cells. Neural cells represent special interest for NP-induced cytotoxicity because the optical and electrical functionalities of materials necessary for neural imaging and interfacing are matched well with the properties of many NPs. In addition, the cellular morphology of neurons is particularly suitable for automated high content screening. HepG2 cells represent a good model for high content screening studies since they are commonly used as a surrogate for human hepatocytes in pharmaceutical studies. We found the CdTe QDs to induce primarily apoptotic response in a time- and dosage-dependent manner and produce different toxicological profiles and responses in undifferentiated and differentiated neural cells. Au NPs were found to inhibit the proliferation and intracellular calcium release of HepG2 cells.
ACS Nano 2008 May
PMID:High-content screening as a universal tool for fingerprinting of cytotoxicity of nanoparticles. 1920 90

We perform in this paper a kinetic study of the photoelectrochemical responses of nanoporous thin photoactive films. The films were fabricated by by a layer-by-layer assembly of positively charged polyelectrolytes (poly-l-Lysine, pLys) and negatively charged semiconductor nanoparticles (NPs) on a carboxylic acid terminated alkanethiol-modified gold electrode. Two types of NPs were used to build uniform films: cadmium selenide (CdSe) and cadmium selenide/cadmium sulfide core/shell (CdSe@CdS). Large photocathodic and photoanodic currents were recorded for CdSe and CdSe@CdS sensitized films, respectively. A theoretical model of the photocurrent responses was developed to analyze the kinetics of photoinduced processes and coupled reactions, showing that the multilayer films behave as homogeneous nanoporous semiconducting photoelectrodes.
ACS Nano 2008 May
PMID:Nanoporous photocathode and photoanode made by multilayer assembly of quantum dots. 1920 96

Centrifugation is an increasingly important technique for nanomaterial processing. Here, we examine this process for gold, cadmium selenide, and iron oxide nanocrystals using an analytical ultracentrifuge. Such data provide an accurate measure of the sedimentation coefficients for these materials, and we find that this parameter has a significant dependence on the size and surface coating. Conventional models for particle sedimentation cannot capture the behavior of these nanocrystals unless the density of the nanocrystals is described by a size-dependent term that accounts for both the inorganic core and the organic coating. Using this modification in the particle sedimentation framework, it is possible to estimate sedimentation coefficients from information about the nanocrystal core and surface coating dimensions. Such data are useful in choosing the speeds for a centrifugation process and are particularly important when bimodal nanocrystal distributions are present.
ACS Nano 2008 Feb
PMID:Size-dependent sedimentation properties of nanocrystals. 1920 32

In this paper, we report the successful use of non-cadmium-based quantum dots (QDs) as highly efficient and nontoxic optical probes for imaging live pancreatic cancer cells. Indium phosphide (core)-zinc sulfide (shell), or InP/ZnS, QDs with high quality and bright luminescence were prepared by a hot colloidal synthesis method in nonaqueous media. The surfaces of these QDs were then functionalized with mercaptosuccinic acid to make them highly dispersible in aqueous media. Further bioconjugation with pancreatic cancer specific monoclonal antibodies, such as anticlaudin 4 and antiprostate stem cell antigen (anti-PSCA), to the functionalized InP/ZnS QDs, allowed specific in vitro targeting of pancreatic cancer cell lines (both immortalized and low passage ones). The receptor-mediated delivery of the bioconjugates was further confirmed by the observation of poor in vitro targeting in nonpancreatic cancer based cell lines which are negative for the claudin-4-receptor. These observations suggest the immense potential of InP/ZnS QDs as non-cadmium-based safe and efficient optical imaging nanoprobes in diagnostic imaging, particularly for early detection of cancer.
ACS Nano 2009 Mar 24
PMID:Imaging pancreatic cancer using bioconjugated InP quantum dots. 1924 45

The effective water dispersion of highly uniform nanoparticles synthesized in organic solvents is a major issue for their broad applications. In an effort to overcome this problem, iron oxide and cadmium selenide nanocrystals were surrounded by lipid bilayers to create stable, aqueous dispersions. The core inorganic particles were originally generated in oleic acid and 1-octadecene. When these organic solutions were mixed with water and a sparing amount of excess fatty acid, up to 70% of the nanoparticles transferred into the aqueous phase. This simple approach was applied to two different nanocrystal types, and nanocrystal diameters ranging from 5 to 15 nm. In all cases, the resulting materials were stable, nonaggregated suspensions that retained their original magnetic and optical properties. The phase transfer efficiency is maximum when very little oleic acid is added (e.g. 0.2 w/w %). At higher concentrations, above the critical micelle concentration, the formation of micelles begins to compete with bilayer generation leading to less effective phase transfer. Unlike other approaches for water dispersion that rely on amphiphiles with significant water solubility, the fatty acids used in this work are only sparingly soluble in water. As a result, there is minimal dynamic exchange between free and bound surface agents and the resulting aqueous solutions contain little residual free organic carbon. Thermogravimetric analysis (TGA) confirmed the presence of bilayers around the nanocrystal cores. The particle size, size distribution, process yield, and colloidal stability were found using a suite of methods including transmission electron microscopy, small angle X-ray scattering, dynamic light scattering, inductively coupled plasma-optical emission spectroscopy, and ultraviolet-visible spectroscopy. Bilayer-nanocrystal complexes possess many of the same size-dependent features as the original materials, and as such offer new avenues for exploring and exploiting the interface between nanocrystals and biology.
ACS Nano 2009 Aug 25
PMID:Bilayers as phase transfer agents for nanocrystals prepared in nonpolar solvents. 1959 66

The dynamics of the CdSe nanorod synthesis reaction have been studied, giving attention to the kinetics of magic-sized clusters (MSCs) that form as intermediates in the overall reaction. The MSCs have a distinct absorption peak, and the kinetics of this peak give insight into the overall reaction mechanism. In these studies, the reaction mixture consists primarily of Cd(phosphonate)(2) and trioctyl phosphine selenium in a solution of trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO). We find that the rate at which precursors react to form CdSe monomers and the rates at which monomers react to form nanoparticles can be varied by changing the chemistry of the reaction mixture. Decreasing the TOP concentration decreases the extent to which selenium is bound, both in the precursors and on the particles' surfaces, and thereby increases both the precursor to monomer and monomer to particle reaction rates. Decreasing the phosphonate concentration decreases the extent to which phosphonate binds cadmium in the precursors and on the surface of the nanoparticles, also increasing the rates of both reactions. This is also accomplished by the addition of inorganic acids which protonate the phosphonates. The presence of inorganic acids (impurities) is the primary reason that the overall synthesis reaction is faster in solutions made with technical grade rather than purified TOPO. The TOP and phosphonic acid concentrations are coupled because excess phosphonic acids react with TOP, forming TOPO and less strongly binding species, specifically phosphinic acids, phosphine oxides, and phosphines.
ACS Nano 2010 Mar 23
PMID:Role of magic-sized clusters in the synthesis of CdSe nanorods. 2019 41

Morphology, that is, the study of form comprising shape, size, and structure, is important for materials research in general. For nanostructured materials, popularly known as nanomaterials, morphology has a special significance since form, in this case, dictates physical and chemical properties. Unlike bulk materials, properties of nanomaterials are strongly correlated to form. Here, we present a novel strategy for the synthesis of morphology-controlled segmented CdSe semiconductor nanowires based on a straightforward sweep voltammetry approach of preprogrammed characteristics. It was found here that, by simply and simultaneously modulating the basic parameters of each cyclic voltammetry cycle during the nanowire growth process, scan rate, and cycle potential range, we can achieve a precise control over the morphology of the semiconductor material segment, density, and dimensions, obtained after each voltammetric cycle. The morphology of CdSe segments was found to be controlled by the extent of co-deposition of metal cadmium together with the deposition of CdSe. Thus "dense" CdSe segments and "nondense" segments can be achieved in the absence and presence of cadmium metal co-deposition, respectively. Accompanied by the density modulation achieved by the potential range applied, it was also observed that a fine control over each segment's length, varying between few tenths to few hundred nanometers, can be achieved by simple altering the scan rate of each cycle along the wire. Also, we propose a simple mechanism that accounts for the formation of segments of controlled morphology. This is the first report on the synthesis of "segmented" CdSe nanowires of controlled morphology, density, and length of each segment, by simple single-step cycle voltammetry preprogrammed sequences from a single electrodeposition solution. In addition, this novel strategy may be applied for the synthesis of additional analogue semiconductor materials of importance (e.g., CdS, CdTe, etc.). This segmented nanowire's synthetic route is remarkably fast and simple, leading to a high encoding capacity with a large number of distinguishable signatures.
ACS Nano 2010 Apr 27
PMID:Electrochemical synthesis of morphology-controlled segmented CdSe nanowires. 2033 56

A strategy for the rational design of a novel colorimetric sensor based on dithioerythritol-modified gold nanoparticles for the selective recognition of Hg2+ in aqueous media is presented. This approach relies on the combination of gold nanoparticles with Hg2+ through sulfur-Hg2+-sulfur interaction. The gold nanoparticles showed high selectivity toward Hg2+ with binding-induced red shift in the absorption spectra, with no response to major interfering cations such as Pb2+, Cd2+, and Cu2+ in the presence of ethylenediamine tetraacetic acid. The system responds to Hg2+ with a detection limit of 100 nM and might open a new avenue for the development of Hg2+ sensing probes.
ACS Appl Mater Interfaces 2010 Jan
PMID:Highly sensitive gold nanoparticle-based colorimetric sensing of mercury(II) through simple ligand exchange reaction in aqueous media. 2035 48

Cadmium telluride (CdTe) tetrapods are synthesized with varying aspect ratios through multiple injections of the Te precursor, which provides an excellent means of controlling and tailoring the optical properties of the tetrapods. The self-assembly of CdTe tetrapods at the air/water interface is explored using the Langmuir-Blodgett (LB) technique due to potential use in solar cells arising from the intriguing tetrapod shape that improves charge transport and the optimum band gap energy of CdTe that enhances light absorption. Interestingly, the Langmuir isotherm shows two pressure plateau regions: one at approximately 10 mN/m with the other at the high surface pressure of approximately 39 mN/m. LB deposition at various pressures allows the discernment of the unique two-dimensional packing alluded in the isotherm. By placing CdTe at the air/water interface, it is revealed in the deposition that the tetrapods experienced a dewetting phenomenon, forming a ribbon structure at the onset of surface pressure with a height corresponding to the length of one tetrapod arm. With the increase of surface pressure, the ribbons widen to an eventual large-scale percolated network pattern. The packing density of tetrapods is successfully manipulated by controlling the surface pressure, which may find promising applications in optoelectronic devices.
ACS Nano 2010 Apr 27
PMID:Self-assembly of CdTe tetrapods into network monolayers at the air/water interface. 2035 90


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