Biomedical subjects

A Paul Alivisatos

Publications and source records attributed to A Paul Alivisatos.

At least 19 recordsLinked to original sources

PubMed2007

Mechanistic study of precursor evolution in colloidal group II-VI semiconductor nanocrystal synthesis.

The molecular mechanism of precursor evolution in the synthesis of colloidal group II-VI semiconductor nanocrystals was studied using 1H, 13C, and 31P NMR spectroscopy and mass spectrometry. Tri-n-butylphosphine chalcogenides (TBPE; E = S, Se, Te) react with an oleic acid complex of cadmium or zinc (M-OA; M = Zn, Cd) in a noncoordinating solvent (octadecene (ODE), n-nonane-d20, or n-decane-d22), affording ME nanocrystals, tri-n-butylphosphine oxide (TBPO), and oleic acid anhydride ((OA)2O). Likewise, the reaction between trialkylphosphine selenide and cadmium n-octadecylphosphonic acid complex (Cd-ODPA) in tri-n-octylphosphine oxide (TOPO) produces CdSe nanocrystals, trialkylphosphine oxide, and anhydrides of n-octadecylphosphonic acid. The disappearance of tri-n-octylphosphine selenide in the presence of Cd-OA and Cd-ODPA can be fit to a single-exponential decay (kobs = (1.30 +/- 0.08) x 10-3 s-1, Cd-ODPA, 260 degrees C, and kobs = (1.51 +/- 0.04) x 10-3 s-1, Cd-OA, 117 degrees C). The reaction approaches completion at 70-80% conversion of TOPSe under anhydrous conditions and 100% conversion in the presence of added water. Activation parameters for the reaction between TBPSe and Cd-OA in n-nonane-d20 were determined from the temperature dependence of the TBPSe decay over the range of 358-400 K (deltaH++ = 62.0 +/- 2.8 kJ.mol-1, deltaS++ = -145 +/- 8 J.mol-1.K-1). A reaction mechanism is proposed where trialkylphsophine chalcogenides deoxygenate the oleic acid or phosphonic acid surfactant to generate trialkylphosphine oxide and oleic or phosphonic acid anhydride products. Results from kinetics experiments suggest that cleavage of the phosphorus chalcogenide double bond (TOP=E) proceeds by the nucleophilic attack of phosphonate or oleate on a (TOP=E)-M complex, generating the initial M-E bond.

Colloids↗

PubMed2007

Semiconductor quantum rods as single molecule fluorescent biological labels.

In this paper, we report the development of rod-shaped semiconductor nanocrystals (quantum rods) as fluorescent biological labels. Water-soluble biocompatible quantum rods have been prepared by surface silanization and applied for nonspecific cell tracking as well as specific cellular targeting. Quantum rods are brighter single molecule probes as compared to quantum dots. They have many potential applications as biological labels in situations where their properties offer advantages over quantum dots.

Fluorescent Dyes↗

PubMed2006

Controlled assembly of hybrid bulk-heterojunction solar cells by sequential deposition.

This work presents a technique to create ordered and easily characterized hybrid nanocrystal-polymer composites by sequential deposition of tetrapod-shaped cadmium telluride (CdTe) nanocrystals and poly(3-hexlythiophene). With controlled fabrication and composite morphology, these devices offer several advantages over traditional co-deposited hybrid cells and provide a model system for detailed investigation into the operation of bulk-heterojunction cells.

Journal Article↗

PubMed2006

Second harmonic generation and confined acoustic phonons in highly excited semiconductor nanocrystals.

The photo-induced enhancement of second harmonic generation and the effect of nanocrystal shape and pump intensity on confined acoustic phonons in semiconductor nanocrystals have been investigated with time-resolved scattering and absorption measurements. The second harmonic signal showed a sublinear increase of the second-order susceptibility with respect to the pump pulse energy, indicating a reduction of the effective one-electron second-order nonlinearity with increasing electron-hole density in the nanocrystals. The coherent acoustic phonons in spherical and rod-shaped semiconductor nanocrystals were detected in a time-resolved absorption measurement. Both nanocrystal morphologies exhibited oscillatory modulation of the absorption cross section, the frequency of which corresponded to their coherent radial breathing modes. The amplitude of the oscillation also increased with the level of photoexcitation, suggesting an increase in the amplitude of the lattice displacement as well.

Journal Article↗

PubMed2006

Faceting of nanocrystals during chemical transformation: from solid silver spheres to hollow gold octahedra.

We demonstrate that performing a replacement reaction on single crystalline Ag nanospheres of approximately 10 nm in diameter in an organic solvent produces hollow Au nanocrystals with an octahedral shape. Different from those Au shells made by starting with Ag particles about 1 order of magnitude larger, which largely reproduce that of the sacrificial Ag counterparts, the hollow nanocrystals obtained in this work show significant changes in the external morphology from the spherical Ag precursors. This evolution of a faceted external morphology during chemical transformation is made possible by the enhanced role of surface effects in our smaller nanocrystals. The competition between the Au atom deposition and Ag atom dissolution on various nanocrystal surfaces is believed to determine the final octahedral shape of the hollow Au nanocrystals. Simultaneous achievement of surface-mediated shape control and a hollow morphology in a one-pot, single-step synthetic procedure in this study promises an avenue to finer tuning of particle morphology, and thus physical properties such as surface plasmon resonance.

Journal Article↗

PubMed2006

The concept of delayed nucleation in nanocrystal growth demonstrated for the case of iron oxide nanodisks.

A comprehensive study of iron oxide nanocrystal growth through non-hydrolitic, surfactant-mediated thermal reaction of iron pentacarbonyl and an oxidizer has been conducted, which includes size control, anisotropic shape evolution, and crystallographic phase transition of monodisperse iron oxide colloidal nanocrystals. The reaction was monitored via in situ UV-vis spectroscopy, taking advantage of the color change accompanying the iron oxide colloid formation, allowing measurement of the induction time for nucleation. Features of the synthesis such as the size control and reproducibility are related to the occurrence of the observed delayed nucleation process. As a separate source of iron and oxygen is adopted, phase control could also be achieved by sequential injections of oxidizer.

Journal Article↗

PubMed2006

Cellular effect of high doses of silica-coated quantum dot profiled with high throughput gene expression analysis and high content cellomics measurements.

Quantum dots (Qdots) are now used extensively for labeling in biomedical research, and this use is predicted to grow because of their many advantages over alternative labeling methods. Uncoated Qdots made of core/shell CdSe/ZnS are toxic to cells because of the release of Cd2+ ions into the cellular environment. This problem has been partially overcome by coating Qdots with polymers, poly(ethylene glycol) (PEG), or other inert molecules. The most promising coating to date, for reducing toxicity, appears to be PEG. When PEG-coated silanized Qdots (PEG-silane-Qdots) are used to treat cells, toxicity is not observed, even at dosages above 10-20 nM, a concentration inducing death when cells are treated with polymer or mercaptoacid coated Qdots. Because of the importance of Qdots in current and future biomedical and clinical applications, we believe it is essential to more completely understand and verify this negative global response from cells treated with PEG-silane-Qdots. Consequently, we examined the molecular and cellular response of cells treated with two different dosages of PEG-silane-Qdots. Human fibroblasts were exposed to 8 and 80 nM of these Qdots, and both phenotypic as well as whole genome expression measurements were made. PEG-silane-Qdots did not induce any statistically significant cell cycle changes and minimal apoptosis/necrosis in lung fibroblasts (IMR-90) as measured by high content image analysis, regardless of the treatment dosage. A slight increase in apoptosis/necrosis was observed in treated human skin fibroblasts (HSF-42) at both the low and the high dosages. We performed genome-wide expression array analysis of HSF-42 exposed to doses 8 and 80 nM to link the global cell response to a molecular and genetic phenotype. We used a gene array containing approximately 22,000 total probe sets, containing 18,400 probe sets from known genes. Only approximately 50 genes (approximately 0.2% of all the genes tested) exhibited a statistically significant change in expression level of greater than 2-fold. Genes activated in treated cells included those involved in carbohydrate binding, intracellular vesicle formation, and cellular response to stress. Conversely, PEG-silane-Qdots induce a down-regulation of genes involved in controlling the M-phase progression of mitosis, spindle formation, and cytokinesis. Promoter analysis of these results reveals that expression changes may be attributed to the down-regulation of FOXM and BHLB2 transcription factors. Remarkably, PEG-silane-Qdots, unlike carbon nanotubes, do not activate genes indicative of a strong immune and inflammatory response or heavy-metal-related toxicity. The experimental evidence shows that CdSe/ZnS Qdots, if appropriately protected, induce negligible toxicity to the model cell system studied here, even when exposed to high dosages. This study indicates that PEG-coated silanized Qdots pose minimal impact to cells and are a very promising alternative to uncoated Qdots.

Cell Line↗

PubMed2006

Continuous distribution of emission states from single CdSe/ZnS quantum dots.

The photoluminescence dynamics of colloidal CdSe/ZnS/streptavidin quantum dots were studied using time-resolved single-molecule spectroscopy. Statistical tests of the photon-counting data suggested that the simple "on/off" discrete state model is inconsistent with experimental results. Instead, a continuous emission state distribution model was found to be more appropriate. Autocorrelation analysis of lifetime and intensity fluctuations showed a nonlinear correlation between them. These results were consistent with the model that charged quantum dots were also emissive, and that time-dependent charge migration gave rise to the observed photoluminescence dynamics.

Cadmium Compounds↗

PubMed2006

Two-dimensional nanoparticle arrays show the organizational power of robust DNA motifs.

The bottom-up spatial organization of potential nanoelectronic components is a key intermediate step in the development of molecular electronics. We describe robust three-space-spanning DNA motifs that are used to organize nanoparticles in two dimensions. One strand of the motif ends in a gold nanoparticle; only one DNA strand is attached to the particle. By using two of the directions of the motif to produce a two-dimensional crystalline array, one direction is free to bind gold nanoparticles. Identical motifs, tailed in different sticky ends, enable the two-dimensional periodic ordering of 5 and 10 nm diameter gold nanoparticles.

DNA↗

PubMed2006

Crystal splitting in the growth of Bi2S3.

Bi2S3 nanostructures with a sheaflike morphology are obtained via reaction of bismuth acetate-oleic acid complex with elemental sulfur in 1-octadecence. These structures may form by the splitting crystal growth mechanism, which is known to account for the morphology some mineral crystals assume in nature. By control of the synthetic parameters, different shapes are obtained, analogous to those which have been observed to occur by crystal splitting in minerals. These new and complex Bi2S3 nanostructures are characterized by transmission and scanning electron microscopy, and electron and X-ray diffraction.

Journal Article↗

PubMed2005

Air-stable all-inorganic nanocrystal solar cells processed from solution.

We introduce an ultrathin donor-acceptor solar cell composed entirely of inorganic nanocrystals spin-cast from solution. These devices are stable in air, and post-fabrication processing allows for power conversion efficiencies approaching 3% in initial tests. This demonstration elucidates a class of photovoltaic devices with potential for stable, low-cost power generation.

Journal Article↗

PubMed2005

High-temperature microfluidic synthesis of CdSe nanocrystals in nanoliter droplets.

The high-temperature synthesis of CdSe nanocrystals in nanoliter-volume droplets flowing in a perfluorinated carrier fluid through a microfabricated reactor is presented. A flow-focusing nanojet structure with a step increase in channel height reproducibly generated octadecene droplets in Fomblin Y 06/6 perfluorinated polyether at capillary numbers up to 0.81 and with a droplet:carrier fluid viscosity ratio of 0.035. Cadmium and selenium precursors flowing in octadecene droplets through a high-temperature (240-300 degrees C) glass microreactor produced high-quality CdSe nanocrystals, as verified by optical spectroscopy and transmission electron microscopy. Isolating the reaction solution in droplets prevented particle deposition and hydrodynamic dispersion, allowing the reproducible synthesis of nanocrystals at three different temperatures and four different residence times in the span of 4 h. Our synthesis of a wide range of nanocrystals at high temperatures, high capillary numbers, and low viscosity ratio illustrates the general utility of droplet-based microfluidic reactors to encapsulate nanoliter volumes of organic or aqueous solutions and to precisely control chemical or biochemical reactions.

Cadmium Compounds↗

PubMed2005

Colloidal nanocrystal synthesis and the organic-inorganic interface.

Colloidal nanocrystals are solution-grown, nanometre-sized, inorganic particles that are stabilized by a layer of surfactants attached to their surface. The inorganic cores possess useful properties that are controlled by their composition, size and shape, and the surfactant coating ensures that these structures are easy to fabricate and process further into more complex structures. This combination of features makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. Chemists are achieving ever more exquisite control over the composition, size, shape, crystal structure and surface properties of nanocrystals, thus setting the stage for fully exploiting the potential of these remarkable materials.

Journal Article↗

PubMed2005

A beamline for high-pressure studies at the Advanced Light Source with a superconducting bending magnet as the source.

A new facility for high-pressure diffraction and spectroscopy using diamond anvil high-pressure cells has been built at the Advanced Light Source on beamline 12.2.2. This beamline benefits from the hard X-radiation generated by a 6 T superconducting bending magnet (superbend). Useful X-ray flux is available between 5 keV and 35 keV. The radiation is transferred from the superbend to the experimental enclosure by the brightness-preserving optics of the beamline. These optics are comprised of a plane parabola collimating mirror, followed by a Kohzu monochromator vessel with Si(111) crystals (E/DeltaE approximately equal 7000) and W/B4C multilayers (E/DeltaE approximately equal 100), and then a toroidal focusing mirror with variable focusing distance. The experimental enclosure contains an automated beam-positioning system, a set of slits, ion chambers, the sample positioning goniometry and area detector (CCD or image-plate detector). Future developments aim at the installation of a second endstation dedicated to in situ laser heating and a dedicated high-pressure single-crystal station, applying both monochromatic and polychromatic techniques.

Journal Article↗

PubMed2005

Quantum-dot-based cell motility assay.

Because of their favorable physical and photochemical properties, colloidal CdSe/ZnS-semiconductor nanocrystals (commonly known as quantum dots) have enormous potential for use in biological imaging. In this report, we present an assay that uses quantum dots as markers to quantify cell motility. Cells that are seeded onto a homogeneous layer of quantum dots engulf and absorb the nanocrystals and, as a consequence, leave behind a fluorescence-free trail. By subsequently determining the ratio of cell area to fluorescence-free track area, we show that it is possible to differentiate between invasive and noninvasive cancer cells. Because this assay uses simple fluorescence detection, requires no significant data processing, and can be used in live-cell studies, it has the potential to be a powerful new tool for discriminating between invasive and noninvasive cancer cell lines or for studying cell signaling events involved in migration.

Adenocarcinoma↗

PubMed2005

A molecular ruler based on plasmon coupling of single gold and silver nanoparticles.

Forster Resonance Energy Transfer has served as a molecular ruler that reports conformational changes and intramolecular distances of single biomolecules. However, such rulers suffer from low and fluctuating signal intensities, limited observation time due to photobleaching, and an upper distance limit of approximately 10 nm. Noble metal nanoparticles have plasmon resonances in the visible range and do not blink or bleach. They have been employed as alternative probes to overcome the limitations of organic fluorophores, and the coupling of plasmons in nearby particles has been exploited to detect particle aggregation by a distinct color change in bulk experiments. Here we demonstrate that plasmon coupling can be used to monitor distances between single pairs of gold and silver nanoparticles. We followed the directed assembly of gold and silver nanoparticle dimers in real time and studied the kinetics of single DNA hybridization events. These "plasmon rulers" allowed us to continuously monitor separations of up to 70 nm for >3,000 s.

DNA↗

PubMed2005

First-principles modeling of unpassivated and surfactant-passivated bulk facets of wurtzite CdSe: a model system for studying the anisotropic growth of CdSe nanocrystals.

Equilibrium geometries, surface energies, and surfactant binding energies are calculated for selected bulk facets of wurtzite CdSe with a first-principles approach. Passivation of the surface Cd atoms with alkyl phosphonic acids or amines lowers the surface energy of all facets, except for the polar 000 facet. On the nonpolar facets, the most stable configuration corresponds to full coverage of surface Cd atoms with surfactants, while on the polar 0001 facet it corresponds only to a partial coverage. In addition, the passivated surface energies of the nonpolar facets are in general lower than the passivated polar 0001 facet. Therefore, the polar facets are less stable and less efficiently passivated than the nonpolar facets, and this can rationalize the observed anisotropic growth mechanism of wurtzite nanocrystals in the presence of suitable surfactants.

Journal Article↗

PubMed2005

Semiconductor nanocrystals for biological imaging.

Conventional organic fluorophores suffer from poor photo stability, narrow absorption spectra and broad emission spectra. Semiconductor nanocrystals, however, are highly photo-stable with broad absorption spectra and narrow size-tunable emission spectra. Recent advances in the synthesis of these materials have resulted in the generation of bright, sensitive, extremely photo-stable and biocompatible semiconductor fluorophores. Commercial availability facilitates their application in a variety of unprecedented biological experiments, including multiplexed cellular imaging, long-term in vitro and in vivo labeling, deep tissue structure mapping and single particle investigation of dynamic cellular processes. Semiconductor nanocrystals are one of the first examples of nanotechnology enabling a new class of biomedical applications.

Animals↗