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Biomedical subjects

Wolfgang J Parak

Publications and source records attributed to Wolfgang J Parak.

At least 19 recordsLinked to original sources
PubMed2006

Electron-hole dynamics in CdTe tetrapods.

We present transient absorption studies with femtosecond time resolution on the electron-hole dynamics in CdTe tetrapod nanostructures. Electron-hole pairs are generated by optical excitation in the visible spectral range, and an immediate bleach and induced absorption signal are observed. The relaxation dynamics to the lowest excitonic state is completed in about 6 ps. Experiments with polarized excitation pulses give information about the localization of the excited-state wave functions. The influence of the nanocrystal shape on the optical properties of CdTe nanoparticles is discussed.

Anisotropy↗
PubMed2006

Multiple wurtzite twinning in CdTe nanocrystals induced by methylphosphonic acid.

Branching in semiconductor nanocrystals, which leads to tetrapods and to more complex architectures, is the subject of intensive investigation. Here we support the model according to which branching in CdTe nanocrystals is driven by the formation of multiple wurtzite twins. This is in contrast to previous models for this material. We found that twinning, as well as anisotropic growth, can be triggered by the presence of suitable molecules, such as for instance methylphosphonic acid. In the case of CdTe nanocrystals, we designed a robust growth scheme in which the variation of a single parameter (the concentration of methylphosphonic acid in solution) leads to the controlled formation of nanocrystals with shapes ranging from spheres to anisotropic structures with varying level of branching, as both twinning and anisotropic growth are progressively favored. We believe that these concepts can be extended to other nanocrystal systems.

Journal Article↗
PubMed2006

Combined atomic force microscopy and optical microscopy measurements as a method to investigate particle uptake by cells.

We propose a combination of atomic force microscopy (AFM) and optical microscopy for the investigation of particle uptake by cells. Positively and negatively charged polymer microcapsules were chosen as model particles, because their interaction with cells had already been investigated in detail. AFM measurements allowed the recording of adhesion forces on a single-molecule level. Due to the micrometer size of the capsules, the number of ingested capsules could be counted by optical microscopy. The combination of both methods allowed combined measurement of the adhesion forces and the uptake rate for the same model particle. As a demonstration of this system, the correlation between the adhesion of positively or negatively charged polymer microcapsules onto cell surfaces and the uptake of these microcapsules by cells has been investigated for several cell lines. As is to be expected, we find a correlation between both processes, which is in agreement with adsorption-dependent uptake of the polymer microcapsules by cells.

Adhesiveness↗
PubMed2006

Multiple particle tracking in 3-D+t microscopy: method and application to the tracking of endocytosed quantum dots.

We propose a method to detect and track multiple moving biological spot-like particles showing different kinds of dynamics in image sequences acquired through multidimensional fluorescence microscopy. It enables the extraction and analysis of information such as number, position, speed, movement, and diffusion phases of, e.g., endosomal particles. The method consists of several stages. After a detection stage performed by a three-dimensional (3-D) undecimated wavelet transform, we compute, for each detected spot, several predictions of its future state in the next frame. This is accomplished thanks to an interacting multiple model (IMM) algorithm which includes several models corresponding to different biologically realistic movement types. Tracks are constructed, thereafter, by a data association algorithm based on the maximization of the likelihood of each IMM. The last stage consists of updating the IMM filters in order to compute final estimations for the present image and to improve predictions for the next image. The performances of the method are validated on synthetic image data and used to characterize the 3-D movement of endocytic vesicles containing quantum dots.

Algorithms↗
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

Magnetic targeting and cellular uptake of polymer microcapsules simultaneously functionalized with magnetic and luminescent nanocrystals.

By using a flow channel system for modeling the bloodstream in the circulatory system and by locally creating a magnetic field gradient caused by a permanent magnet, we demonstrate specific trapping of polymer capsules simultaneously functionalized with two types of nanoparticles--magnetic and luminescent nanocrystals. In the regions where the capsules were trapped by the magnetic field, drastically increased uptake of capsules by cells has been observed. The uptake of capsules by cells could be conveniently monitored with a fluorescence microscope by the luminescence of CdTe nanocrystals that had been embedded into the shells of the capsules. Our experiments envisage the feasibility of magnetic targeting of polymer capsules loaded by pharmaceutical agents to pathogenic parts of a tissue.

Breast Neoplasms↗
PubMed2005

Nanoengineered polymer capsules: tools for detection, controlled delivery, and site-specific manipulation.

We present the concept of multifunctional nanoengineered polymer capsules and outline their applications as new drug delivery systems or supramolecular toolboxes containing, for example, enzymes capable of converting nontoxic prodrugs into toxic drugs at a designated location. Such functionalized nanocontainers offer a wide range of applications including enzymatic catalysis, controlled release, and directed drug delivery in medicine due to their multifunctionality. The unique advantage of capsules in comparison to other systems is that they can be functionalized or loaded simultaneously with the above-mentioned components, thus permitting multifunctional processes in single cells.

Animals↗
PubMed2005

Fluorescent nanocrystals as colloidal probes in complex fluids measured by fluorescence correlation spectroscopy.

The diffusion properties of fluorescent colloidal CdSe and CdSe/ZnS nanocrystals (QDs) with different hydrophilic coatings were characterized in complex fluids such as actin solutions using fluorescence correlation spectroscopy (FCS). The hydrodynamic radii of the QDs were determined both in organic solvents and water. Attention was given to the potential artifacts arising from the fluorescence properties of the QDs. With increasing excitation intensities, the apparent particle concentration and diffusion times are overestimated if using a simple diffusion model. This can be explained by a numerical simulation. The diffusion behavior of QDs in actin networks of different concentrations was determined to demonstrate the potential use of nanocrystals as probes in soft biological matter. The decreasing diffusion coefficient of the nanocrystals with increasing actin concentration results in an intrinsic polymer viscosity of 0.12+/-0.02 ml mg(-1), in accordance with literature values.

Actins↗
PubMed2005

Cytotoxicity of colloidal CdSe and CdSe/ZnS nanoparticles.

Cytotoxicity of CdSe and CdSe/ZnS nanoparticles has been investigated for different surface modifications such as coating with mercaptopropionic acid, silanization, and polymer coating. For all cases, quantitative values for the onset of cytotoxic effects in serum-free culture media are given. These values are correlated with microscope images in which the uptake of the particles by the cells has been investigated. Our data suggest that in addition to the release of toxic Cd(2+) ions from the particles also their surface chemistry, in particular their stability toward aggregation, plays an important role for cytotoxic effects. Additional patch clamp experiments investigate effects of the particles on currents through ion channels.

Animals↗
PubMed2005

Selective growth of PbSe on one or both tips of colloidal semiconductor nanorods.

PbSe nanocrystals with rock-salt structure are grown on the tips of colloidal CdS and CdSe nanorods. The facets of wurtzite rods provide a substrate with various degrees of reactivity for the growth of PbSe. The presence of dangling Cd bonds may explain subtle differences between nonequivalent facets resulting in the selective nucleation of PbSe only on one of the two tips of each CdS rod. This approach has the potential to facilitate the fabrication of heterostructures with tailored optical and electronic properties.

Adsorption↗
PubMed2005

The role of metal nanoparticles in remote release of encapsulated materials.

Laser mediated remote release of encapsulated fluorescently labeled polymers from nanoengineered polyelectrolyte multilayer capsules containing gold sulfide core/gold shell nanoparticles in their walls is observed in real time on a single capsule level. We have developed a method for measuring the temperature increase and have quantitatively investigated the influence of absorption, size, and surface density of metal nanoparticles using an analytical model. Experimental measurements and numerical simulations agree with the model. The treatment presented in this work is of general nature, and it is applicable to any system where nanoparticles are used as absorbing centers. Potential biomedical applications are highlighted.

Coated Materials, Biocompatible↗
PubMed2004

Three-dimensional measurements of the pressure distribution in artificial joints with a capacitive sensor array.

A spherically folded capacitive pressure sensor array is introduced and characterized. By placing the sensor array between the ball and the cavity of artificial joints, the pressure distribution within the joint was recorded with spatial resolution for different size matching between the ball and the cavity, for different directions of loading and for joints with incomplete cavities. The performance of the sensor array is analyzed, possible fields of application as well as its limitations are discussed.

Biosensing Techniques↗
PubMed2003

Room-temperature single-nucleotide polymorphism and multiallele DNA detection using fluorescent nanocrystals and microarrays.

We report two cDNA microarray-based applications of DNA-nanocrystal conjugates, single-nucleotide polymorphism (SNP) and multiallele detections, using a commercial scanner and two sets of nanocrystals with orthogonal emissions. We focus on SNP mutation detection in the human p53 tumor suppressor gene, which has been found to be mutated in more than 50% of the known human cancers. DNA-nanocrystal conjugates are able to detect both SNP and single-base deletion at room temperature within minutes, with true-to-false signal ratios above 10. We also demonstrate microarray-based multiallele detection, using hybridization of multicolor nanocrystals conjugated to two sequences specific for the hepatitis B and hepatitis C virus, two common viral pathogens that inflict more than 10% of the population in the developing countries worldwide. The simultaneous detection of multiple genetic markers with microarrays and DNA-nanocrystal conjugates has no precedent and suggests the possibility of detecting an even greater number of bacterial or viral pathogens simultaneously.

Alleles↗
PubMed2003

Quantum dot-based cell motility assay.

Motility and migration are measurable characteristics of cells that are classically associated with the invasive potential of cancer cells, but in vitro assays of invasiveness have been less than perfect. We previously developed an assay to monitor cell motility and migration using water-soluble CdSe/ZnS nanocrystals; cells engulf the fluorescent nanocrystals as they crawl across them and leave behind a fluorescent-free trail. We show here that semiconductor nanocrystals can also be used as a sensitive two-dimensional in vitro invasion assay. We used this assay to compare the behavior of seven different adherent human cell lines, including breast epithelial MCF 10A, breast tumor MDA-MB-231, MDA-MB-435S, MCF 7, colon tumor SW480, lung tumor NCI H1299, and bone tumor Saos-2, and observed two distinct behaviors of cancer cells that can be used to further categorize these cells. Some cancer cell lines demonstrate fibroblastic behaviors and leave long fluorescent-free trails as they migrate across the dish, whereas other cancer cells leave clear zones of varying sizes around their periphery. This assay uses fluorescence detection, requires no processing, and can be used in live cell studies. These features contribute to the increased sensitivity of this assay and make it a powerful new tool for discriminating between non-invasive and invasive cancer cell lines.

Carcinogenicity Tests↗